A soft climbing robot based on smart wood with switchable adhesion

A soft climbing robot based on smart wood with switchable adhesion | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A soft climbing robot based on smart wood with switchable adhesion Xiaodong Wang, Yaodong Li, Xiaolei Guo, Guanggui Cheng, Rui Lei, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6259286/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Jul, 2025 Read the published version in Tribology Letters → Version 1 posted 8 You are reading this latest preprint version Abstract Climbing robots have vital uses in uncharted terrain exploration, military intelligence collecting, and other areas. To address the drawbacks of classic wall-climbing robots, this study introduced a novel soft climbing robot constructed of the smart wood with switchable adhesion force. The experimental findings indicated that the soft robot implemented in this research could effectively perform climbing movements on diverse walls and sloping pavements, enabled by temperature control through cold/hot water circulation and pneumatic actuation. Further research revealed that the reversible phase transition of PNIPAM at different temperatures was the main reason for the variable adhesion force of the smart wood. Moreover, the adhesion force model developed in this work indicated that the adhesion force of the smart wood surface was mostly composed of the contact mechanics force and the capillary force. Finally, this study will offer novel insights for the design of climbing robots and advance their potential applications. Climbing robots Soft robots Smart wood PNIPAM Switchable adhesion. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1 Introduction Climbing robots have critical roles in a variety of fields, including wall-mounted equipment maintenance, reconnaissance of unexplored terrain, military information collection, and interplanetary mission execution [ 1 ]. The motion of climbing robots on vertical walls or pipes is mostly determined by the friction force generated by adsorption between the robot and the wall. Existing studies indicate that the adsorption between the climbing robot and the wall surface may arise from many aspects [ 1 – 3 ]. However, creating climbing robots capable of adsorbing various walls remains a challenge. For example, the structural deformation of the robot might be employed to hold it tightly to the vertical pipes, allowing it to climb higher. When it comes to structural deformation, soft robots are a trendy topic right now [ 4 – 6 ]. As a result, some researchers were inspired by the physiological structure and movement mode of human arms and created a soft robot based on a novel form of rigid-flexible hybrid artificial muscle called LCE-MXene-Spring (LMS) [ 7 ]. Their experiments showed that this soft robot propelled by LMS could climb on pipes of various structures. Also inspired by human arm movements, other researchers developed a soft four-legged climbing robot powered by pneumatic pressure [ 8 ]. This robot could climb pipes with different slopes and reach speeds of up to 5 mm/s [ 8 ]. Furthermore, the twisting climbing action of the snake can also inspire researchers to create novel climbing robots. It has been demonstrated that an air-driven snake-inspired soft robot can climb like a snake, even going around a bend along a rod [ 9 ]. However, the aforementioned adsorption strategy for obtaining a holding force between the robot and the pipes via deformation would be difficult to implement on the vertical wall. Then, the researchers used the magnetic attraction strategy to design the wall-climbing robot. For example, researchers have created inchworm-like climbing robots, earthworm-like climbing robots, and crawler-mounted climbing robots using permanent magnets/electromagnets [ 10 – 12 ]. In addition to magnetic fields, electrostatic fields are also used as an adsorption strategy between the climbing robot and the wall. So far, researchers have created a soft climbing robot featuring dielectric elastomer actuators and dual-pole electroadhesive pads [ 13 ]. Experimental results demonstrate that the electrostatic adsorption-based climbing robot achieves free locomotion on walls made of various construction materials [ 13 ]. Unfortunately, both climbing robots require either magnetic surfaces or high voltage (kV level) to be applied to the robot. These limitations certainly restrict the application of climbing robots. To address the aforementioned issues, the researchers explored the negative pressure/vacuum adsorption strategy. Tang et al. used the extremely soft elastomer ecoflex to create a two-legged, air-powered climbing robot with suction cups [ 14 ]. According to their findings, this pneumatic soft climbing robot using the negative pressure adsorption strategy can climb various walls in liquid and atmospheric environments. However, the adsorption strategy described above will fail on rough wall surfaces (particularly those with grooves). It is clear that there is still a long way to go in designing a climbing robot with widespread application. In fact, the way geckos climb walls in nature may inspire researchers to create climbing robots. For instance, Li et al. designed a climbing robot with adjustable adhesion by simulating the microstructure of gecko toes [ 15 ]. This type of climbing robot is capable of climbing freely on walls made of various materials [ 15 , 16 ]. As can be observed, the creation of smart materials with switchable adhesion will encourage the continued development of climbing robots. Following this line of thinking, this study will combine balsa wood and thermoresponsive PNIPAM to create an adhesion-controlled smart wood. The smart wood will be connected to soft robotic feet that can be heated and cooled with water. Finally, the climbing motion of the soft robot is achieved by telescopic deformation of the soft robot driven by pneumatic pressure. 2 Manuscript submission In this study, the motion of the soft climbing robot was determined by the thermoresponsive smart wood and the structural deformation of the proposed soft robot triggered by pneumatic pressure. The smart wood utilized was primarily constructed of the lignin-removed balsa wood and Poly (N-isopropyl acrylamide) (PNIPAM). The lignin in balsa wood was removed by soaking it in an alkaline solution at room temperature [17]. Assisted by the crosslinking agent N,N'-Methylenebisacrylamide (MBA), the lignin-removed balsa wood underwent polymerization with NIPAM monomer, resulting in the formation of smart wood, as demonstrated in Fig. 1a. After the smart wood was prepared, its surface morphology could be obtained by a scanning electron microscope (S-3400N，Hitachi，Japan), as shown in Fig. 1d. During the climbing process of the soft robot, the smart wood exhibited high adhesive properties at low temperatures and dramatically lower adhesion at high temperatures. The soft climbing robot had a similar construction to the inchworm, with front and rear feet and a center body, as illustrated in Fig. 1b. The front and rear feet were hollow cavity constructions created by metal 3D printing (ILSM 280D, Zhongrui Technology, China). The prepared smart wood would be cut into 25×25×1mm pieces and applied to the bottoms of the front and rear feet with a thermal conductivity adhesive (K-5205, kafuter, China). To make the smart wood operate at different temperatures, this study poured water at various temperatures into the front and rear feet of the robot via water pipes. A small pump (BSP 27250S, Chengdu Xinweicheng Technology Co., China) pumping water at various temperatures in several tanks provided the water supply. Furthermore, the center body was a retractable hollow elastomer manufactured by 3D printing (Objet500 Connex3，Stratasys，American) in the shape of a bellows. It was worth mentioning that this body was connected to two air pumps (TCN112-8，Chengdu Haixun Fluid Technology Co., China) by gas catheters, allowing it to work telescopically. To accomplish linear movement of the center body, the robot was be equipped with two linear metal guide wires (not shown in Fig. 1) with a diameter of 1 mm. Finally, after assembling the various elements listed above, the soft climbing robot had an overall dimension of approximately 100×25×35 mm. The assembled soft climbing robot could realize climbing movement under the cooperation of cold/hot water and the air pump, as shown in Fig. 1c. In order to explore the motion performance of the soft climbing robot, this study conducted the robot climbing experiments on diverse pavements, namely wood, paper, PVC, and glass. To compensate for the challenges faced by climbing robots using a negative pressure adsorption strategy on rough surfaces, this study also conducted climbing experiments with the soft robot on a grooved wood surface, as illustrated in Appendix A. It should be emphasized that during the experiments, the pavements above would be in different tilting states, and the tilt angle α was between 0° and 90°. Meanwhile, the pavement would be defined as a wall at a tilt angle α of 90°. Moreover, the pneumatic pressure P and the water temperature T during the climbing movements were set at 33.3-61.8 kPa and 10-46 ℃, respectively. Unless otherwise stated, the experiments in this study were conducted at room temperature with an ambient relative humidity of 65%. 3 Results 3.1 Effect of pneumatic pressure on the climbing behavior of the soft robot In this study, the elongation behavior of the soft robot is mostly determined by the pneumatic pressure P entering the center body. Thus, the climbing behavior of the soft robot under various pneumatic pressures was investigated in this work, as illustrated in Fig. 2. The results demonstrated that the soft robot could adhere securely to the vertical wood surface after cold water (10 ℃) was injected into the front and rear feet, as shown in Fig. 1 at t = 0 s. Following that, the front foot of the soft robot would be heated with hot water (46 ℃) to lower the adhesion force between the foot and the wall surface. The center body of the soft robot would be stretched under the actuation of pneumatic pressure. Since the adhesion force between the rear foot and the wall surface was greater than that between the front foot and the wall surface, the front foot of the soft robot moved upward as the middle body elongated. Subsequently, the front foot of the soft robot was cooled using cold water, while the rear foot was heated with hot water. The center body of the soft robot would shorten once the second air pump was activated. Since the adhesion force between the front foot and the wall was stronger than that between the rear foot and the wall at this stage, the rear foot would creep upward as the center body deformed. Finally, the rear foot was re-cooled by cold water, allowing the soft robot to return to its original state ( t = 0 s) and complete a single cycle of the climbing process. As depicted in Fig. 2, the soft robot successfully achieved vertical climbing on a wood surface through the above steps, demonstrating a proportional relationship between climbing displacement and applied pneumatic pressure. To enable comparison of subsequent tests, the termination line climbed by the soft robot within 600 s under the condition of P = 61.8 kPa in this section was defined as the reference line L *, as shown in Fig. 2. To objectively examine the climbing behavior of soft robots under diverse pneumatic pressure and on inclined pavement surfaces, the climbing displacement D of the soft robot under given conditions was statistically analyzed. The results in Fig. 3 showed that the displacement D of the soft robot on the vertical wood surface increased from 33 mm at P = 33.3 kPa to 72.5 mm at P = 61.8 kPa in 600 s. It can be observed that increasing pneumatic pressure will aid in the climbing displacement of the soft robot. From another perspective, when the pneumatic pressure was kept at P = 33.3 kPa, the crawling displacement D of the soft robot on the inclined wood surface rose from 33 mm at α = 90° to 55 mm at α = 0°. Apparently, the proposed soft robot could crawl farther on a horizontal surface in the same period of time. This observation was also confirmed at pneumatic pressures of 47.5 kPa and 61.8 kPa. The experimental results indicated that as the tilt angle α of the pavement decreased from 90° to 0°, the crawling displacement D of the soft robot increased from 49 mm to 66 mm at P = 47.5 kPa, and from 72.5 mm to 96 mm at P = 61.8 kPa. 3.2 Effect of temperature on the climbing behavior of the soft robot This study focuses on a novel smart wood composite, which is fabricated through the integration of wood fiber and thermoresponsive PNIPAM gel. It was reported that PNIPAM has a lower critical solution temperature (LCST) of roughly 32 °C [18]. The adhesion characteristics of PNIPAM fluctuate at different temperatures around LCST, influencing the climbing behavior of the soft robot. Specifically, the smart wood demonstrates robust adhesion at a low temperature T L ( T L LCST). As a result, the aforementioned low and high temperatures were modified in this study to investigate the influence of temperature on the climbing behavior of the soft robot. It should be noted that the climbing control approach of the soft robot in this section was identical to that in Sect. 2 (Fig. 1) and Sect. 3.1. Meanwhile, to facilitate comparative trials, the pneumatic pressure P in this section was maintained at a constant value of 61.8 kPa. According to the testing results in Fig. 4, when the smart wood was set to a low temperature of 10 °C and a high temperature of 34 °C, the soft robot could easily climb on the vertical wood wall. The statistical findings in Fig. 5 showed that under this working condition, the climbing displacement of the soft robot within 600 s was 62.5 mm. Interestingly, when the low temperature remained constant, the high temperature progressively increased from 34 °C to 46 °C, enabling the soft robot to climb on the wood wall. At this moment, the climbing displacement of the soft robot would grow to 72.5 mm within 600 s. On the other hand, the soft robot climbing experiments were also conducted at a low (high) temperature of 22 °C (46 °C). The experimental results pointed out that the climbing displacement of the soft robot was close to 64 mm. Thereafter, as the low temperature T L decreased from 22 °C to 10 °C ( T H remained constant), the climbing displacement of the soft robot would gradually increase to 72.5 mm, as shown in Fig. 4 and Fig. 5. Notably, a stick-slip phenomenon was observed during the climbing motion of the soft robot at T L = 22 °C and 19 °C, as demonstrated in Fig. 5b. These experimental results indicated an optimal operating temperature range for the soft robot. When compared to the reference line L *, it can be discovered that controlling the smart wood with lower T L and higher T H improves soft robot climbing displacement. Clearly, the temperatures employed in the preceding operations should not exceed the temperature range at which the reversible phase transition of PNIPAM occurs. 3.3 Climbing behavior of the soft robot on various pavements The adhesive characteristics of the PNIPAM gel surface are affected by the ambient temperature and the surface features of the contact object [18, 19]. Therefore, in addition to focusing on the climbing behavior of the soft robot on polar surfaces (paper, wood, PVC), this research compared the influence of a non-polar surface (glass) on the soft robot's climbing. The results of Fig. 2 and Fig. 4 suggested that the soft robot could easily climb on a vertical wood wall under the given conditions. Similarly, when the wall material was changed to paper and PVC, the soft robot was able to complete the climbing movement on the vertical wall surface, as shown in Fig. 6. Experimental comparisons showed that under consistent testing conditions, the climbing performance of the soft robot was most effective on PVC surfaces, followed by successively reduced displacement on wood and paper walls. However, when the wall material was replaced with non-polar glass, the soft robot maintained its climbing capability, as demonstrated in Fig. 6. Comparative measurements revealed that the climbing displacement of the robot on the glass surface significantly exceeded that observed on the three previously tested polar materials under identical temporal conditions. These testing findings demonstrated that the soft robot climbing on different types of walls would cause varied displacements. It was also implied that stronger adhesion between the surface and the smart wood did not consistently result in improved climbing performance or larger displacement values. Here, the displacement D of the soft robot on pavement of various materials and tilt angles was also measured. According to the testing results in Fig. 3 and Fig. 7, the climbing displacement of the soft robot on paper, wood, PVC, and glass under α = 90° was 70.5 mm, 72.5 mm, 74 mm, and 94 mm, respectively. As the tilt angle α gradually decreased from 90° to 0°, the above displacement D increased to 91 mm, 96 mm, 99.5 mm, and 120 mm, respectively. These experimental data further confirm that the soft robot used in this study can climb on a variety of materials, and the climbing displacement D will decrease with the increase of the tilt angle α . Moreover, to overcome the limitation that traditional climbing robots based on negative pressure adsorption strategies are difficult to climb on rough walls, this study conducted a climbing experiment of the designed soft robot on a grooved wood wall, as shown in Appendix A. The experimental results confirmed that, under the same test parameters, the climbing displacement D of the soft robot on the vertical wood wall with two different surface structures was close to 73.5 mm. This is a significant finding for the applications of the soft climbing robot employed in this research. 4 Discussion 4.1 Molecular structure of the smart wood and reversible phase transition of PNIPAM Chemically cross-linked PNIPAM is a gel-like polymer with limited mechanical properties. To increase the mechanical characteristics of PNIPAM for use in soft climbing robots, wood fiber was mixed with it. The molecular structure of PNIPAM is well known to have both a hydrophilic amidogroup (-CO-NH-) and a hydrophobic isopropyl (-CH(CH 3 ) 2 ) [ 18 ]. For the lignin-removed balsa wood, the main components are cellulose and a small amount of hemicellulose and lignin. Therefore, the lignin-removed balsa wood is mostly a linear polymer composed of many β-D-Glucopyranose molecules, and its molecular formula is (C 6 H 10 O 5 ) n , where n represents the degree of polymerization [ 20 ]. Due to the strong polarity of β-D-Glucopyranose, it is easy to form hydrogen bonds with PNIPAM molecules, as shown in Fig. 8 . The existence of these hydrogen bonds would create a strong binding force between wood cellulose and PNIPAM. This conclusion is supported by the findings of the steady climbing experiment of the soft robot in Sect. 3 of this study. However, the climbing motion of the soft robot relies on the reversible adhesion changes of the smart wood. In this process, the amidogroup (-CO-NH-) and isopropyl (-CH(CH 3 ) 2 ) in the PNIPAM molecule will play an important role. Therefore, this investigation used an analytical technique with variable temperature Fourier transform infrared (FTIR) spectra to characterize the essential chemical functional groups of PNIPAM, the smart wood depicted in Fig. 8 , and their transformation processes at various temperatures. The FTIR tests were performed on a ‌Nicolet iS50 analytical FTIR spectrometer (Is50, Thermo Scientific, American) in the wavenumber of 4000 − 400 cm − 1 . According to the experimental results in Fig. 9 , an absorption peak at 3420 cm − 1 was detected when the smart wood was maintained at 10 ℃ and 46 ℃. As evidenced by prior studies, this distinct absorption peak originates from the stretching vibrations of hydroxyl (-OH) groups in cellulose [ 21 ]. Similarly, an absorption at 3270 cm − 1 comes from the O–H [ 22 ]. On the other hand, the peaks at 1653 cm − 1 are related to the amidegroups of PNIPAM following the polymerization procedure [ 23 ]. At 1540 cm − 1 , the absorption peak characteristic of the N-H bond was observed (called amide II band) [ 24 ]. Furthermore, the absorption peak at 2970 cm − 1 and 1460 cm − 1 could be attributed to the asymmetric stretching vibration and asymmetric deformation of the -CH 3 groups in PNIPAM, respectively [ 21 ]. Notably, both smart wood and PNIPAM at different temperatures had the aforementioned chemical functional groups that characterize PNIPAM. These experimental findings demonstrate that the smart wood made of PNIPAM and cellulose was effectively created, and that PNIPAM and cellulose were well bonded. However, the gray region in Fig. 9 demonstrated that when the temperature decreased from 46°C to 10°C, the absorption peaks surrounding the characteristic peaks of the amidogroup would shift to the lower wavenumber. This observed shift in absorption peaks provided direct evidence that C = O and N-H groups formed hydrogen bonds with water molecules at low temperatures, while these bonds dissociated at high temperatures. Therefore, the changes in ambient temperature are capable of triggering the phase transition of PNIPAM. Based on the aforementioned analysis and experimental findings, it is not difficult to determine the climbing mechanism of the soft climbing robot employed in this study, as shown in Fig. 10 . When the front and rear feet of the soft robot were infused with cold water, the molecular structure of PNIPAM in the smart wood was stretched. At this point, the amidegroup of PNIPAM easily forms hydrogen bonds with the pavement surface comprised of polar materials, resulting in a stronger adhesion force. For non-polar pavement materials, this adhesion force will be reduced to some extent, but it will still be able to withstand the gravity of the robot. However, when the foot of the soft robot is filled with hot water, the PNIPAM molecular structure changes phase. In this case, the isopropyl group in PNIPAM reduces the adhesion force between the smart wood and the pavement surface, allowing the robot to readily disengage from the pavement. By utilizing the switchable adhesion force of the smart wood and the extension/contraction deformation of the soft robot, the robot is capable of performing climbing motions in accordance with the gaits depicted in Fig. 1 c. 4.2 Adhesion model of the smart wood at different conditions While the influence of chemical functional groups on the smart wood adhesion has been examined in Sect. 4.1 , the underlying physical mechanisms governing this adhesion phenomenon remain to be elucidated through further investigation. The adhesion force F ad is primarily composed of four fundamental components: (i) van der Waals force F van , (ii) contact mechanics force F con , (iii) capillary force F cap , and (iv) electrostatic force F ele [ 17 , 25 – 27 ]. In this study, the adhesion force of the smart wood could only be regulated by altering the temperature, thus the van der Waals and electrostatic forces could not dominate the adhesion force between the smart wood and the pavement surface. Consequently, this study primarily focused on investigating the roles of contact mechanics and capillary forces in the adhesion mechanism. The adhesion force F ad can be quantitatively determined through the application of Eqs. (1), (2), and (3) [ 25 , 26 ]. where ε is a correction factor caused by material properties, ω is the force required to separate PNIPAM per unit area from the pavement surface, A PNIPAM is the contact area between PNIPAM oozing from the surface of the smart wood and the pavement surface at different temperatures, A peak is the contact area of the microscopic contact peak between the smart wood and the pavement surface, R g = 8.31 J/(mol·K) is the ideal gas constant, T ab is the absolute temperature, RH is the relative humidity, V m = 18 cm 3 /mol is the molar volume of water. In order to get the values of the necessary parameters in the above equations and verify the validity of the proposed adhesion force model, the seepage area A PNIPAM of PNIPAM from the surface of the smart wood at various temperatures was investigated in this study. The experimental results in Fig. 11 pointed out that the area A PNIPAM of PNIPAM oozing from a certain area on the surface of the smart wood varied with ambient temperature. Specifically, A PNIPAM would steadily drop from 3.41 mm 2 at 15°C to 1.42 mm 2 at 32°C. The most intriguing observation was that the area A PNIPAM rebounded to 3.35 mm 2 as the temperature decreased from 32°C to 15°C. Based on the experimental data shown in Fig. 11 , the area A PNIPAM can be calculated from Eq. (4) using the approach of data fitting and proportional conversion. where A wood is the apparent contact area of the smart wood to be studied. Furthermore, this study also examined the (maximum) adhesion force F ad between the smart wood and various pavement surfaces at different temperatures, as illustrated in Fig. 12 a. It was demonstrated that the adhesion force F ad between the smart wood and the paper pavement could reach 442.3 mN at T = 15°C, A wood = 6.25 cm². As the temperature increased to 35°C, the adhesion force decreased to a negligible value. This temperature-dependent adhesion behavior could be consistently observed across various pavement materials. As shown in Fig. 12 a, the adhesion force F ad of the smart wood exhibited a gradual decrease from 431.5 mN, 417.3 mN, and 391.4 mN at 15°C to insignificant values at 40°C on wood, PVC, and glass substrates, respectively. These experimental results further validate the proposed mechanism of adhesion force generation in the smart wood, as discussed in Sect. 4.1 . To investigate the effect of environmental relative humidity on the adhesion properties of the smart wood, the adhesion force measurements were conducted on different pavement interfaces under controlled relative humidity conditions. Figure 12 b indicated that the adhesion force F ad of the smart wood would decrease from 442.3 mN, 431.5 mN, 417.3 mN, and 394.1 mN at 65% RH to 365.2 mN, 351.5 mN, 338.9 mN, and 326.8 mN at 95% RH on paper, wood, PVC, and glass substrates, respectively. Obviously, the relative humidity of the environment also affects the adhesion force of the smart wood. Based on the above experimental results, it was easy to calculate the specific values of the relevant parameters of Eq. (2) and Eq. (3) in this study, see Appendix B for details. On this basis, the results calculated by the theoretical model of Eq. (1) were compared with the experimental results, as shown in Fig. 12 . The comparative findings revealed that the physical model of adhesion force utilized in this study accurately predicted the adhesion force of the smart wood surface. Finally, based on the above adhesion model, the climbing results of the soft robot on different materials in Sect. 3.3 could be easily understood. When the soft robot had a large adhesion force with the pavement material, the separation between the soft robot and the pavement would be relatively difficult. Then, the distance the soft robot traveled under the same air pressure would be reduced. Therefore, the higher the adhesion force between the soft robot and the material, the slower the soft robot would travel. 5 Conclusions By mixing the lignin-removed balsa wood with PNIPAM, a type of smart wood with regulated adhesion was created. This smart wood exhibited strong and weak adhesion at ambient temperatures below and above LCST (~ 32°C), respectively. Then, this study successfully developed a novel climbing robot by integrating the fabricated smart wood with 3D-printed soft robotic feet. The testing findings indicated that the soft robot could climb 33 mm, 49 mm, and 72.5 mm on a vertical wood wall in 600 s under pneumatic pressures of 33.3 kPa, 47.5 kPa, and 61.8 kPa, respectively. As the tilt angle α of the pavement decreased from 90° to 0°, the above crawling displacement D of the soft robot would increase to 55 mm, 66 mm, and 96 mm, respectively. Simultaneously, the testing findings confirmed that reducing the low working temperature T L and raising the high working temperature T H of the smart wood would improve the climbing/crawling speed of the soft robot. Additionally, the soft robot designed in this paper was able to climb on different material surfaces. For example, under the given working condition, the soft robot could climb 70.5 mm,72.5 mm,74 mm, and 94 mm on a wall made of paper, wood, PVC, and glass. At various temperatures, the change in adhesion force of the smart wood is primarily due to the phase transition process. At low temperatures ( T < LCST), the molecular structure of PNIPAM in the smart wood is stretched. The amidegroup (-CO-NH-) of PNIPAM easily forms hydrogen bonds with the pavement surface composed of polar materials, resulting in a stronger adhesion force. For non-polar pavement materials, this adhesion force will be significantly reduced. As the temperature rises, the molecular structure of PNIPAM will agglomerate, and the isopropyl group (-CH(CH 3 ) 2 ) with hydrophobic properties will reduce the adhesion force of the smart wood. Since this adhesion force of the smart wood can only be regulated by altering the temperature, the adhesion force in this study is primarily dominated by the contact mechanics force and the capillary force. This model of the adhesion force was also supported by the experimental results in this research. Although a stronger adhesion force enables the robot to adhere to the wall, it also hinders its climbing efficiency. Therefore, while maintaining sufficient adhesion force for wall attachment, minimizing the adhesion force on the smart wood surface can significantly enhance the climbing speed of the proposed robot. Declarations Author Contributions All authors contributed to the study conception and design. The first draft of the manuscript was written by Xiaodong Wang and Yaodong Li. Data collection and analysis were performed by Xiaolei Guo, Guanggui Cheng, and Jing Hua. Method was guided by Rui Lei. Material preparation was performed by Qichen Zeng, Xucheng Wang, and Xuehai Guo. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Funding This work was supported by the Natural Science Foundation of China (52475191, 52405193, 32471791). Data Availability No datasets were generated or analyzed during the current study. Declarations Conflict of interest The authors have no competing interests to declare that are relevant to the content of this article. References Elbadawi, M., Andrikopoulos, G., Nikolakopoulos, G., Gustafsson, T.: Bio-Inspired Climbing Robots in Wet Environments: Recent Trends in Adhesion Methods and Materials. 2018 IEEE International Conference on Robotics and Biomimetics (ROBI O), pp. 2347-2353. Place (2018) Chen, S., Cao, Y., Sarparast, M., Yuan, H., Dong, L., Tan, X., et al. Soft Crawling Robots: Design, Actuation, and Locomotion. Advanced Materials Technologies 5 :1900837 (2020). Fang, Y., Wang, S., Bi, Q., Cui, D., Yan, C. Design and Technical Development of Wall-Climbing Robots: A Review. Journal of Bionic Engineering 19 :877-901 (2022). Chen, K., Li, T., Yan, T., Xie, F., Feng, Q., Zhu, Q., et al. A soft gripper design for apple harvesting with force feedback and fruit slip detection. Agriculture 12 :1802 %@ 2077-0472 (2022). Lu, E., Ma, Z., Li, Y., Xu, L., Tang, Z. Adaptive backstepping control of tracked robot running trajectory based on real-time slip parameter estimation. International Journal of Agricultural and Biological Engineering 13 :178-187 (2020). Zhang, F., Teng, S., Wang, Y., Guo, Z., Wang, J., Xu, R. Design of bionic goat quadruped robot mechanism and walking gait planning. International Journal of Agricultural and Biological Engineering 13 :32-39 (2020). Li, X., Liu, L., Huang, P., Li, B., Xing, Y., Wu, Z. A highly adaptable soft pipeline robot for climbing outside millimeter-sized pipelines. Nano Energy 134 :110566 (2025). Singh, G., Patiballa, S., Zhang, X., Krishnan, G.: A Pipe-Climbing Soft Robot. 2019 International Conference on Robotics and Automation (ICRA), pp. 8450-8456. Place (2019) Liao, B., Zang, H., Chen, M., Wang, Y., Lang, X., Zhu, N., et al. Soft Rod-Climbing Robot Inspired by Winding Locomotion of Snake. Soft Robotics 7 :500-511 (2020). Park, G., Rodrigue, H. Soft climbing robot with magnetic feet for multimodal locomotion. Scientific Reports 13 :8377 (2023). Wang, G., Li, W., Che, H. Design and analysis of anti-overturning mechanism for magnetic wall-climbing robot. Journal of Mechanical Science and Technology 38 :379-387 (2024). Zhang, W., Zhang, W., Sun, Z. A reconfigurable soft wall-climbing robot actuated by electromagnet. International Journal of Advanced Robotic Systems 18 :1729881421992285 (2021). Gu, G., Zou, J., Zhao, R., Zhao, X., Zhu, X. Soft wall-climbing robots. Science Robotics 3 :eaat2874 (2018). Tang, Y., Zhang, Q., Lin, G., Yin, J. Switchable Adhesion Actuator for Amphibious Climbing Soft Robot. Soft Robotics 5 :592-600 (2018). Li, X., Bai, P., Li, X., Li, L., Li, Y., Lu, H., et al. Robust scalable reversible strong adhesion by gecko-inspired composite design. Friction 10 :1192-1207 (2022). Sun, J., Bauman, L., Yu, L., Zhao, B. Gecko-and-inchworm-inspired untethered soft robot for climbing on walls and ceilings. Cell Reports Physical Science 4 (2023). Wang, X., Li, Y., Lei, R., Guo, X., Zhang, R. Adhesion-grasping behavior of a mechanical gripper based on thermoresponsive smart wood. Tribology International 205 :110558 (2025). Nagase, K. Thermoresponsive interfaces obtained using poly(N-isopropylacrylamide)-based copolymer for bioseparation and tissue engineering applications. Advances in Colloid and Interface Science 295 :102487 (2021). Bovaldinova, K.A., Feldstein, M.M., Sherstneva, N.E., Moscalets, A.P., Khokhlov, A.R. Thermo-switchable pressure-sensitive adhesives with strong tunable adhesion towards substrate surfaces of different hydrophilicity. Polymer 125 :10-20 (2017). Kawamoto, H., Murayama, M., Saka, S. Pyrolysis behavior of levoglucosan as an intermediate in cellulose pyrolysis: polymerization into polysaccharide as a key reaction to carbonized product formation. Journal of Wood Science 49 :469-473 (2003). Wang, J., Zhou, X., Xiao, H. Structure and properties of cellulose/poly(N-isopropylacrylamide) hydrogels prepared by SIPN strategy. Carbohydrate Polymers 94 :749-754 (2013). You, Z., Dong, Y., Li, X., Yang, P., Luo, M., Zhu, Z., et al. One-pot synthesis of multi-functional cellulose-based ionic conductive organohydrogel with low-temperature strain sensitivity. Carbohydrate Polymers 251 :117019 (2021). Zhang, F., Wu, W., Zhang, X., Meng, X., Tong, G., Deng, Y. Temperature-sensitive poly-NIPAm modified cellulose nanofibril cryogel microspheres for controlled drug release. Cellulose 23 :415-425 (2016). Gicquel, E., Martin, C., Heux, L., Jean, B., Bras, J. Adsorption versus grafting of poly(N-Isopropylacrylamide) in aqueous conditions on the surface of cellulose nanocrystals. Carbohydrate Polymers 210 :100-109 (2019). Harrison, A.J., Corti, D.S., Beaudoin, S.P. Capillary Forces in Nanoparticle Adhesion: A Review of AFM Methods. Particulate Science and Technology 33 :526-538 (2015). Jimidar, I.S.M., Kwiecinski, W., Roozendaal, G., Kooij, E.S., Gardeniers, H.J.G.E., Desmet, G., et al. Influence of Wettability and Geometry on Contact Electrification between Nonionic Insulators. ACS Applied Materials & Interfaces 15 :42004-42014 (2023). Wang, X.D., Guo, J., Chen, C., Chen, L., Qian, L.M. A simple method to control nanotribology behaviors of monocrystalline silicon. Journal of Applied Physics 119 :044304 (2016). Additional Declarations No competing interests reported. Supplementary Files Appendix.docx GraphicalAbstract.jpg Cite Share Download PDF Status: Published Journal Publication published 18 Jul, 2025 Read the published version in Tribology Letters → Version 1 posted Editorial decision: Revision requested 19 May, 2025 Reviews received at journal 15 Apr, 2025 Reviewers agreed at journal 25 Mar, 2025 Reviewers agreed at journal 23 Mar, 2025 Reviewers invited by journal 23 Mar, 2025 Editor assigned by journal 20 Mar, 2025 Submission checks completed at journal 20 Mar, 2025 First submitted to journal 19 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6259286","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":433788412,"identity":"7ad15442-1dee-496d-ac8e-49a534ecc590","order_by":0,"name":"Xiaodong Wang","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Xiaodong","middleName":"","lastName":"Wang","suffix":""},{"id":433788413,"identity":"ec765c04-d580-4897-a6e3-2d9866ab6872","order_by":1,"name":"Yaodong Li","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Yaodong","middleName":"","lastName":"Li","suffix":""},{"id":433788414,"identity":"8903ba72-f1e0-4137-bad1-7c88ef550217","order_by":2,"name":"Xiaolei Guo","email":"","orcid":"","institution":"Nanjing Forestry University","correspondingAuthor":false,"prefix":"","firstName":"Xiaolei","middleName":"","lastName":"Guo","suffix":""},{"id":433788415,"identity":"99eec8cd-87e6-45c0-81f5-1b9804f8f0e6","order_by":3,"name":"Guanggui Cheng","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Guanggui","middleName":"","lastName":"Cheng","suffix":""},{"id":433788416,"identity":"2d16eb89-f6a6-424b-b97d-2c7dba9c3d4f","order_by":4,"name":"Rui Lei","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAw0lEQVRIiWNgGAWjYBACPgbGBgYGAxswxcBDjBY2iJY0krSAwWEIRZwW9ua2zzwF5xOb+w8wPnjbxiBvTlALz8Hm2TwGtxMbZyQwG85tYzDc2UBIi0RiMzNECwObNG8bQ4LBAUJa5B+CtJxLbOw/wP6bOC0SjCAtBxIbGxLYmInTwpPYzDjHINm4cUZis+SccxKGGwhp4Wc//pjhzR872Y39hw9+eFNmI0/QFjgwbABHpgSx6oFAngS1o2AUjIJRMMIAADhvOdAjSALyAAAAAElFTkSuQmCC","orcid":"","institution":"Jiangsu University","correspondingAuthor":true,"prefix":"","firstName":"Rui","middleName":"","lastName":"Lei","suffix":""},{"id":433788417,"identity":"b658e151-3a93-444d-a710-4f25ac7d7414","order_by":5,"name":"Jing Hua","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Jing","middleName":"","lastName":"Hua","suffix":""},{"id":433788418,"identity":"0dbfc3b4-a0f5-4c4b-aaf4-c462acdbe70f","order_by":6,"name":"Qichen Zeng","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Qichen","middleName":"","lastName":"Zeng","suffix":""},{"id":433788419,"identity":"d8975eb9-747b-4450-aa1b-16b16ae23df0","order_by":7,"name":"Xucheng Wang","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Xucheng","middleName":"","lastName":"Wang","suffix":""},{"id":433788420,"identity":"b273aa61-fd2c-46ea-87c3-67a190679b2f","order_by":8,"name":"Xuehai Guo","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Xuehai","middleName":"","lastName":"Guo","suffix":""}],"badges":[],"createdAt":"2025-03-19 08:08:29","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6259286/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6259286/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11249-025-02043-7","type":"published","date":"2025-07-18T15:57:35+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":79781247,"identity":"558660d4-8beb-4914-89a9-4ba9a96d491e","added_by":"auto","created_at":"2025-04-02 15:03:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4609550,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea \u003c/strong\u003eIllustration showing the preparation of the smart wood; \u003cstrong\u003eb\u003c/strong\u003e structure diagram of the soft climbing robot from different views; \u003cstrong\u003ec\u003c/strong\u003e schematic diagram of climbing steps for the soft climbing robot; \u003cstrong\u003ed\u003c/strong\u003e scanning electron microscope (SEM) image of the smart wood surface at room temperature.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/99cee45ffd90203cb6c9780e.png"},{"id":79781964,"identity":"67d4eb07-ac0f-4966-be19-d46ed319e0e7","added_by":"auto","created_at":"2025-04-02 15:11:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":5855855,"visible":true,"origin":"","legend":"\u003cp\u003eThe climbing behavior of the soft robot on a vertical wall at various pneumatic pressures. The tilt angle \u003cem\u003eα\u003c/em\u003e was 90°, and the wall was made of wood.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/232370581ee4b8dccf942022.png"},{"id":79782205,"identity":"6f87d56a-3fb4-4234-819a-61dccba1ec2d","added_by":"auto","created_at":"2025-04-02 15:19:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4328611,"visible":true,"origin":"","legend":"\u003cp\u003eDisplacement measurements of the soft robot across inclined pavement surfaces under varying pneumatic pressures. The tilt angle \u003cem\u003eα\u003c/em\u003e was between 0° and 90°, and the pavement was made of wood. \u003cstrong\u003ea\u003c/strong\u003e \u003cem\u003eP\u003c/em\u003e = 61.8 kPa; \u003cstrong\u003eb\u003c/strong\u003e \u003cem\u003eP\u003c/em\u003e= 47.5 kPa; \u003cstrong\u003ec\u003c/strong\u003e \u003cem\u003eP\u003c/em\u003e = 33.3 kPa.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/f7706de7c920c768ece8e127.png"},{"id":79781255,"identity":"c8a98b83-cdab-49d1-9fcb-75f82ec3d028","added_by":"auto","created_at":"2025-04-02 15:03:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":7941421,"visible":true,"origin":"","legend":"\u003cp\u003eThe climbing behavior of the soft robot on a vertical wall at various temperatures. The tilt angle \u003cem\u003eα\u003c/em\u003e was 90°, the pneumatic pressure \u003cem\u003eP\u003c/em\u003e was 61.8 kPa, and the wall was made of wood.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/624fe6fc691ccf84906c1ace.png"},{"id":79781963,"identity":"a709f289-dd34-4ef0-b84b-3e1081d1b712","added_by":"auto","created_at":"2025-04-02 15:11:08","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":3957262,"visible":true,"origin":"","legend":"\u003cp\u003eDisplacement measurements of the soft robot on a vertical wall at various temperatures. The tilt angle \u003cem\u003eα\u003c/em\u003e was 90°, the pneumatic pressure \u003cem\u003eP\u003c/em\u003e was 61.8 kPa, and the wall was made of wood. \u003cstrong\u003ea\u003c/strong\u003e \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e = 10 °C; \u003cstrong\u003eb\u003c/strong\u003e \u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e = 46 °C.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/ea73175bf7fbd95992b59bd8.png"},{"id":79782207,"identity":"49ae6959-dcef-4b92-8916-07a5d6078e9f","added_by":"auto","created_at":"2025-04-02 15:19:09","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":526500,"visible":true,"origin":"","legend":"\u003cp\u003eThe climbing behavior of the soft robot on several vertical walls made of different materials. The tilt angle \u003cem\u003eα\u003c/em\u003e was 90°, the pneumatic pressure \u003cem\u003eP\u003c/em\u003e was 61.8 kPa, and the temperature of \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e) was 10 ℃ (46 ℃).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/19001aaead13bbb990203038.png"},{"id":79781286,"identity":"d6b53896-21c7-4e83-aa4c-f43c784cca82","added_by":"auto","created_at":"2025-04-02 15:03:09","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":3425519,"visible":true,"origin":"","legend":"\u003cp\u003eDisplacement measurements of the soft robot on the pavement composed of \u003cstrong\u003ea\u003c/strong\u003e glass, \u003cstrong\u003eb\u003c/strong\u003e paper and \u003cstrong\u003ec\u003c/strong\u003e PVC. The tilt angle \u003cem\u003eα\u003c/em\u003e was between 0° and 90°, the pneumatic pressure \u003cem\u003eP\u003c/em\u003e was 61.8 kPa, and the temperature of \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e) was 10 ℃ (46 ℃).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/6716321020c8c1bbada0f9f7.png"},{"id":79781967,"identity":"e0618dff-1418-49a8-89af-82469314e661","added_by":"auto","created_at":"2025-04-02 15:11:08","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":3650555,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration showing the molecular structure of cellulose and PNIPAM in the smart wood.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/022b3d67a893502449cfd0af.png"},{"id":79781965,"identity":"2c3ee6b5-e2da-4d4d-b7de-ca9dfedee869","added_by":"auto","created_at":"2025-04-02 15:11:08","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1882134,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of infrared spectrum detection between PNIPAM and smart wood at different temperatures.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/32cb98f6e8974c6435f928b3.png"},{"id":79781966,"identity":"2d0230f4-4002-489b-843a-dec22c20dbdd","added_by":"auto","created_at":"2025-04-02 15:11:08","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":3041397,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of climbing mechanism of soft robot at different temperatures. \u003cstrong\u003ea\u003c/strong\u003e The climbing diagram of the soft robot; \u003cstrong\u003eb\u003c/strong\u003e phase transition of PNIPAM.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/ebb03a1fadafcf0c647f552b.png"},{"id":79781269,"identity":"cab4c627-9ec2-4404-b33d-df8ca3af22ca","added_by":"auto","created_at":"2025-04-02 15:03:09","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":6066442,"visible":true,"origin":"","legend":"\u003cp\u003eReversible changes of a certain PNIPAM region (circled by dashed lines) on the surface of the smart wood at various temperatures. The scale bar is 0.8 mm.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/d1e0709a6d6f09cb1834e482.png"},{"id":79782208,"identity":"cfea74ea-7660-48c4-980b-7cb72b7f5034","added_by":"auto","created_at":"2025-04-02 15:19:09","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":1295234,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental and theoretical adhesion force\u003cem\u003e F\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e under different conditions: \u003cstrong\u003ea\u003c/strong\u003e adhesion force between the smart wood and different pavement materials at different temperatures (\u003cem\u003eA\u003c/em\u003e\u003csub\u003ewood\u003c/sub\u003e = 6.25 cm\u003csup\u003e2\u003c/sup\u003e, \u003cem\u003eRH\u003c/em\u003e = 65 %); \u003cstrong\u003eb\u003c/strong\u003e adhesion force between the smart wood and different pavement materials under different relative humidity (\u003cem\u003eA\u003c/em\u003e\u003csub\u003ewood\u003c/sub\u003e = 6.25 cm\u003csup\u003e2\u003c/sup\u003e, \u003cem\u003eT\u003c/em\u003e = 15 °C).\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/2eb235f48848d09c3aa53218.png"},{"id":88506162,"identity":"f8511ccb-aef1-4926-b6cc-1376c5f90852","added_by":"auto","created_at":"2025-08-07 07:31:59","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":44047420,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/3a98ccdf-ee04-4a01-837a-1c2e2d126b63.pdf"},{"id":79781961,"identity":"0420189a-924d-4f16-b992-701d5b014fea","added_by":"auto","created_at":"2025-04-02 15:11:08","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":366850,"visible":true,"origin":"","legend":"","description":"","filename":"Appendix.docx","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/d8c43002dc27bc99ee800763.docx"},{"id":79781249,"identity":"b0c5b176-61c3-45f8-a063-1583e848db0d","added_by":"auto","created_at":"2025-04-02 15:03:08","extension":"jpg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":709193,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6259286/v1/c99544bd6757ba30bb927d6d.jpg"}],"financialInterests":"No competing interests reported.","formattedTitle":"A soft climbing robot based on smart wood with switchable adhesion","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eClimbing robots have critical roles in a variety of fields, including wall-mounted equipment maintenance, reconnaissance of unexplored terrain, military information collection, and interplanetary mission execution [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The motion of climbing robots on vertical walls or pipes is mostly determined by the friction force generated by adsorption between the robot and the wall. Existing studies indicate that the adsorption between the climbing robot and the wall surface may arise from many aspects [\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. However, creating climbing robots capable of adsorbing various walls remains a challenge.\u003c/p\u003e \u003cp\u003eFor example, the structural deformation of the robot might be employed to hold it tightly to the vertical pipes, allowing it to climb higher. When it comes to structural deformation, soft robots are a trendy topic right now [\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. As a result, some researchers were inspired by the physiological structure and movement mode of human arms and created a soft robot based on a novel form of rigid-flexible hybrid artificial muscle called LCE-MXene-Spring (LMS) [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Their experiments showed that this soft robot propelled by LMS could climb on pipes of various structures. Also inspired by human arm movements, other researchers developed a soft four-legged climbing robot powered by pneumatic pressure [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. This robot could climb pipes with different slopes and reach speeds of up to 5 mm/s [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Furthermore, the twisting climbing action of the snake can also inspire researchers to create novel climbing robots. It has been demonstrated that an air-driven snake-inspired soft robot can climb like a snake, even going around a bend along a rod [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, the aforementioned adsorption strategy for obtaining a holding force between the robot and the pipes via deformation would be difficult to implement on the vertical wall. Then, the researchers used the magnetic attraction strategy to design the wall-climbing robot. For example, researchers have created inchworm-like climbing robots, earthworm-like climbing robots, and crawler-mounted climbing robots using permanent magnets/electromagnets [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In addition to magnetic fields, electrostatic fields are also used as an adsorption strategy between the climbing robot and the wall. So far, researchers have created a soft climbing robot featuring dielectric elastomer actuators and dual-pole electroadhesive pads [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Experimental results demonstrate that the electrostatic adsorption-based climbing robot achieves free locomotion on walls made of various construction materials [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Unfortunately, both climbing robots require either magnetic surfaces or high voltage (kV level) to be applied to the robot. These limitations certainly restrict the application of climbing robots.\u003c/p\u003e \u003cp\u003eTo address the aforementioned issues, the researchers explored the negative pressure/vacuum adsorption strategy. Tang et al. used the extremely soft elastomer ecoflex to create a two-legged, air-powered climbing robot with suction cups [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. According to their findings, this pneumatic soft climbing robot using the negative pressure adsorption strategy can climb various walls in liquid and atmospheric environments. However, the adsorption strategy described above will fail on rough wall surfaces (particularly those with grooves). It is clear that there is still a long way to go in designing a climbing robot with widespread application. In fact, the way geckos climb walls in nature may inspire researchers to create climbing robots. For instance, Li et al. designed a climbing robot with adjustable adhesion by simulating the microstructure of gecko toes [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. This type of climbing robot is capable of climbing freely on walls made of various materials [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. As can be observed, the creation of smart materials with switchable adhesion will encourage the continued development of climbing robots. Following this line of thinking, this study will combine balsa wood and thermoresponsive PNIPAM to create an adhesion-controlled smart wood. The smart wood will be connected to soft robotic feet that can be heated and cooled with water. Finally, the climbing motion of the soft robot is achieved by telescopic deformation of the soft robot driven by pneumatic pressure.\u003c/p\u003e"},{"header":"2 Manuscript submission","content":"\u003cp\u003eIn this study, the motion of the soft climbing robot was determined by the thermoresponsive smart wood and the structural deformation of the proposed soft robot triggered by pneumatic pressure. The smart wood utilized was primarily constructed of the lignin-removed balsa wood and Poly (N-isopropyl acrylamide) (PNIPAM). The lignin in balsa wood was removed by soaking it in an alkaline solution at room temperature [17]. Assisted by the crosslinking agent N,N'-Methylenebisacrylamide (MBA), the lignin-removed balsa wood underwent polymerization with NIPAM monomer, resulting in the formation of smart wood, as demonstrated in Fig. 1a. After the smart wood was prepared, its surface morphology could be obtained by a scanning electron microscope (S-3400N，Hitachi，Japan), as shown in Fig. 1d. During the climbing process of the soft robot, the smart wood exhibited high adhesive properties at low temperatures and dramatically lower adhesion at high temperatures.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe soft climbing robot had a similar construction to the inchworm, with front and rear feet and a center body, as illustrated in Fig. 1b. The front and rear\u0026nbsp;feet were hollow cavity constructions created by metal 3D printing (ILSM 280D, Zhongrui Technology, China). The prepared smart wood would be cut into 25×25×1mm pieces and applied to the bottoms of the front and rear feet with a thermal conductivity adhesive (K-5205, kafuter, China). To make the smart wood operate at different temperatures, this study poured water at various temperatures into the front and rear feet of the robot via water pipes. A small pump (BSP 27250S, Chengdu Xinweicheng Technology Co., China) pumping water at various temperatures in several tanks provided the water supply. Furthermore, the center body was a retractable hollow elastomer manufactured by 3D printing (Objet500 Connex3，Stratasys，American) in the shape of a bellows. It was worth mentioning that this body was connected to two air pumps (TCN112-8，Chengdu Haixun Fluid Technology Co., China) by gas catheters, allowing it to work telescopically. To accomplish linear movement of the center body, the robot was be equipped with two linear metal guide wires (not shown in Fig. 1) with a diameter of 1 mm. Finally, after assembling the various elements listed above, the soft climbing robot had an overall dimension of approximately 100×25×35 mm.\u003c/p\u003e\n\u003cp\u003eThe assembled soft climbing robot could realize climbing movement under the cooperation of cold/hot water and the air pump, as shown in Fig. 1c. In order to explore the motion performance of the soft climbing robot, this study conducted the robot climbing experiments on diverse pavements, namely wood, paper, PVC, and glass. To compensate for the challenges faced by climbing robots using a negative pressure adsorption strategy on rough surfaces, this study also conducted climbing experiments with the soft robot on a grooved wood surface, as illustrated in Appendix A. It should be emphasized that during the experiments, the pavements above would be in different tilting states, and the tilt angle \u003cem\u003eα\u003c/em\u003e was between 0° and 90°. Meanwhile, the pavement would be defined as a wall at a tilt angle \u003cem\u003eα\u003c/em\u003e of 90°. Moreover, the pneumatic pressure \u003cem\u003eP\u003c/em\u003e and the water temperature \u003cem\u003eT\u003c/em\u003e during the climbing movements were set at 33.3-61.8 kPa and 10-46 ℃, respectively. Unless otherwise stated, the experiments in this study were conducted at room temperature with an ambient relative humidity of 65%.\u003c/p\u003e"},{"header":"3 Results","content":"\u003ch2\u003e3.1 \u0026nbsp;Effect of pneumatic pressure on the climbing behavior of the soft robot\u003c/h2\u003e\n\u003cp\u003eIn this study, the elongation behavior of the soft robot is mostly determined by the pneumatic pressure \u003cem\u003eP\u003c/em\u003e entering the center body. Thus, the climbing behavior of the soft robot under various pneumatic pressures was investigated in this work, as illustrated in Fig. 2. The results demonstrated that the soft robot could adhere securely to the vertical wood surface after cold water (10 ℃) was injected into the front and rear feet, as shown in Fig. 1 at \u003cem\u003et\u003c/em\u003e = 0 s. Following that, the front foot of the soft robot would be heated with hot water (46 ℃) to lower the adhesion force between the foot and the wall surface. The center body of the soft robot would be stretched under the actuation of pneumatic pressure. Since the adhesion force between the rear foot and the wall surface was greater than that between the front foot and the wall surface, the front foot of the soft robot moved upward as the middle body elongated. Subsequently, the front foot of the soft robot was cooled using cold water, while the rear foot was heated with hot water. The center body of the soft robot would shorten once the second air pump was activated. Since the adhesion force between the front foot and the wall was stronger than that between the rear foot and the wall at this stage, the rear foot would creep upward as the center body deformed. Finally, the rear foot was re-cooled by cold water, allowing the soft robot to return to its original state (\u003cem\u003et\u003c/em\u003e = 0 s) and complete a single cycle of the climbing process. As depicted in Fig. 2, the soft robot successfully achieved vertical climbing on a wood surface through the above steps, demonstrating a proportional relationship between climbing displacement and applied pneumatic pressure. To enable comparison of subsequent tests, the termination line climbed by the soft robot within 600 s under the condition of \u003cem\u003eP\u003c/em\u003e = 61.8 kPa in this section was defined as the reference line \u003cem\u003eL\u003c/em\u003e*, as shown in Fig. 2.\u003c/p\u003e\n\u003cp\u003eTo objectively examine the climbing behavior of soft robots under diverse pneumatic pressure and on inclined pavement surfaces, the climbing displacement \u003cem\u003eD\u003c/em\u003e of the soft robot under given conditions was statistically analyzed. The results in Fig. 3 showed that the displacement \u003cem\u003eD\u003c/em\u003e of the soft robot on the vertical wood surface increased from 33 mm at \u003cem\u003eP\u003c/em\u003e = 33.3 kPa to 72.5 mm at \u003cem\u003eP\u003c/em\u003e = 61.8 kPa in 600 s. It can be observed that increasing pneumatic pressure will aid in the climbing displacement of the soft robot. From another perspective, when the pneumatic pressure was kept at \u003cem\u003eP\u003c/em\u003e = 33.3 kPa, the crawling displacement \u003cem\u003eD\u003c/em\u003e of the soft robot on the inclined wood surface rose from 33 mm at \u003cem\u003e\u0026alpha;\u003c/em\u003e = 90\u0026deg; to 55 mm at \u003cem\u003e\u0026alpha;\u003c/em\u003e = 0\u0026deg;. Apparently, the proposed soft robot could crawl farther on a horizontal surface in the same period of time. This observation was also confirmed at pneumatic pressures of 47.5 kPa and 61.8 kPa. The experimental results indicated that as the tilt angle \u003cem\u003e\u0026alpha;\u0026nbsp;\u003c/em\u003eof the pavement decreased from 90\u0026deg; to 0\u0026deg;, the crawling displacement \u003cem\u003eD\u003c/em\u003e of the soft robot increased from 49 mm to 66 mm at \u003cem\u003eP\u003c/em\u003e = 47.5 kPa, and from 72.5 mm to 96 mm at \u003cem\u003eP\u003c/em\u003e = 61.8 kPa.\u0026nbsp;\u003c/p\u003e\n\u003ch2\u003e3.2 \u0026nbsp;Effect of temperature on the climbing behavior of the soft robot\u003c/h2\u003e\n\u003cp\u003eThis study focuses on a novel smart wood composite, which is fabricated through the integration of wood fiber and thermoresponsive PNIPAM gel. It was reported that PNIPAM has a lower critical solution temperature (LCST) of roughly 32 \u0026deg;C [18]. The adhesion characteristics of PNIPAM fluctuate at different temperatures around LCST, influencing the climbing behavior of the soft robot. Specifically, the smart wood demonstrates robust adhesion at a low temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e \u0026lt; LCST) and transitions to reduced adhesion when exposed to a high temperature\u003cem\u003e\u0026nbsp;T\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e \u0026gt; LCST). As a result, the aforementioned low and high temperatures were modified in this study to investigate the influence of temperature on the climbing behavior of the soft robot. It should be noted that the climbing control approach of the soft robot in this section was identical to that in Sect. 2 (Fig. 1) and Sect. 3.1. Meanwhile, to facilitate comparative trials, the pneumatic pressure\u003cem\u003e\u0026nbsp;P\u003c/em\u003e in this section was maintained at a constant value of 61.8 kPa.\u003c/p\u003e\n\u003cp\u003eAccording to the testing results in Fig. 4, when the smart wood was set to a low temperature of 10 \u0026deg;C and a high temperature of 34 \u0026deg;C, the soft robot could easily climb on the vertical wood wall. The statistical findings in Fig. 5 showed that under this working condition, the climbing displacement of the soft robot within 600 s was 62.5 mm. Interestingly, when the low temperature remained constant, the high temperature progressively increased from 34 \u0026deg;C to 46 \u0026deg;C, enabling the soft robot to climb on the wood wall. At this moment, the climbing displacement of the soft robot would grow to 72.5 mm within 600 s. On the other hand, the soft robot climbing experiments were also conducted at a low (high) temperature of 22 \u0026deg;C (46 \u0026deg;C). The experimental results pointed out that the climbing displacement of the soft robot was close to 64 mm. Thereafter, as the low temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e decreased from 22 \u0026deg;C to 10 \u0026deg;C (\u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e remained constant), the climbing displacement of the soft robot would gradually increase to 72.5 mm, as shown in Fig. 4 and Fig. 5. Notably, a stick-slip phenomenon was observed during the climbing motion of the soft robot at \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e = 22 \u0026deg;C and 19 \u0026deg;C, as demonstrated in Fig. 5b. These experimental results indicated an optimal operating temperature range for the soft robot. When compared to the reference line \u003cem\u003eL\u003c/em\u003e*, it can be discovered that controlling the smart wood with lower \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e and higher \u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e improves soft robot climbing displacement. Clearly, the temperatures employed in the preceding operations should not exceed the temperature range at which the reversible phase transition of PNIPAM occurs.\u003c/p\u003e\n\u003ch2\u003e3.3 \u0026nbsp;Climbing behavior of the soft robot on various pavements\u003c/h2\u003e\n\u003cp\u003eThe adhesive characteristics of the PNIPAM gel surface are affected by the ambient temperature and the surface features of the contact object [18, 19]. Therefore, in addition to focusing on the climbing behavior of the soft robot on polar surfaces (paper, wood, PVC), this research compared the influence of a non-polar surface (glass) on the soft robot\u0026apos;s climbing. The results of Fig. 2 and Fig. 4 suggested that the soft robot could easily climb on a vertical wood wall under the given conditions. Similarly, when the wall material was changed to paper and PVC, the soft robot was able to complete the climbing movement on the vertical wall surface, as shown in Fig. 6. Experimental comparisons showed that under consistent testing conditions, the climbing performance of the soft robot was most effective on PVC surfaces, followed by successively reduced displacement on wood and paper walls. However, when the wall material was replaced with non-polar glass, the soft robot maintained its climbing capability, as demonstrated in Fig. 6. Comparative measurements revealed that the climbing displacement of the robot on the glass surface significantly exceeded that observed on the three previously tested polar materials under identical temporal conditions. These testing findings demonstrated that the soft robot climbing on different types of walls would cause varied displacements. It was also implied that stronger adhesion between the surface and the smart wood did not consistently result in improved climbing performance or larger displacement values.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHere, the displacement \u003cem\u003eD\u003c/em\u003e of the soft robot on pavement of various materials and tilt angles was also measured. According to the testing results in Fig. 3 and Fig. 7, the climbing displacement of the soft robot on paper, wood, PVC, and glass under \u003cem\u003e\u0026alpha;\u003c/em\u003e = 90\u0026deg; was 70.5 mm, 72.5 mm, 74 mm, and 94 mm, respectively. As the tilt angle \u003cem\u003e\u0026alpha;\u003c/em\u003e gradually decreased from 90\u0026deg; to 0\u0026deg;, the above displacement\u003cem\u003e\u0026nbsp;D\u003c/em\u003e increased to 91 mm, 96 mm, 99.5 mm, and 120 mm, respectively. These experimental data further confirm that the soft robot used in this study can climb on a variety of materials, and the climbing displacement \u003cem\u003eD\u003c/em\u003e will decrease with the increase of the tilt angle \u003cem\u003e\u0026alpha;\u003c/em\u003e. Moreover, to overcome the limitation that traditional climbing robots based on negative pressure adsorption strategies are difficult to climb on rough walls, this study conducted a climbing experiment of the designed soft robot on a grooved wood wall, as shown in Appendix A. The experimental results confirmed that, under the same test parameters, the climbing displacement \u003cem\u003eD\u003c/em\u003e of the soft robot on the vertical wood wall with two different surface structures was close to 73.5 mm. This is a significant finding for the applications of the soft climbing robot employed in this research.\u0026nbsp;\u003c/p\u003e"},{"header":"4 Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e4.1 Molecular structure of the smart wood and reversible phase transition of PNIPAM\u003c/h2\u003e\n \u003cp\u003eChemically cross-linked PNIPAM is a gel-like polymer with limited mechanical properties. To increase the mechanical characteristics of PNIPAM for use in soft climbing robots, wood fiber was mixed with it. The molecular structure of PNIPAM is well known to have both a hydrophilic amidogroup (-CO-NH-) and a hydrophobic isopropyl (-CH(CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. For the lignin-removed balsa wood, the main components are cellulose and a small amount of hemicellulose and lignin. Therefore, the lignin-removed balsa wood is mostly a linear polymer composed of many \u0026beta;-D-Glucopyranose molecules, and its molecular formula is (C\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e10\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e)\u003cem\u003en\u003c/em\u003e, where \u003cem\u003en\u003c/em\u003e represents the degree of polymerization [\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]. Due to the strong polarity of \u0026beta;-D-Glucopyranose, it is easy to form hydrogen bonds with PNIPAM molecules, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. The existence of these hydrogen bonds would create a strong binding force between wood cellulose and PNIPAM. This conclusion is supported by the findings of the steady climbing experiment of the soft robot in Sect. 3 of this study.\u003c/p\u003e\n \u003cp\u003eHowever, the climbing motion of the soft robot relies on the reversible adhesion changes of the smart wood. In this process, the amidogroup (-CO-NH-) and isopropyl (-CH(CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) in the PNIPAM molecule will play an important role. Therefore, this investigation used an analytical technique with variable temperature Fourier transform infrared (FTIR) spectra to characterize the essential chemical functional groups of PNIPAM, the smart wood depicted in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e, and their transformation processes at various temperatures. The FTIR tests were performed on a \u0026zwnj;Nicolet iS50 analytical FTIR spectrometer (Is50, Thermo Scientific, American) in the wavenumber of 4000\u0026thinsp;\u0026minus;\u0026thinsp;400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. According to the experimental results in Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e, an absorption peak at 3420 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was detected when the smart wood was maintained at 10 ℃ and 46 ℃. As evidenced by prior studies, this distinct absorption peak originates from the stretching vibrations of hydroxyl (-OH) groups in cellulose [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]. Similarly, an absorption at 3270 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e comes from the O\u0026ndash;H [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]. On the other hand, the peaks at 1653 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are related to the amidegroups of PNIPAM following the polymerization procedure [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. At 1540 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the absorption peak characteristic of the N-H bond was observed (called amide II band) [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. Furthermore, the absorption peak at 2970 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1460 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e could be attributed to the asymmetric stretching vibration and asymmetric deformation of the -CH\u003csub\u003e3\u003c/sub\u003e groups in PNIPAM, respectively [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]. Notably, both smart wood and PNIPAM at different temperatures had the aforementioned chemical functional groups that characterize PNIPAM. These experimental findings demonstrate that the smart wood made of PNIPAM and cellulose was effectively created, and that PNIPAM and cellulose were well bonded. However, the gray region in Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e demonstrated that when the temperature decreased from 46\u0026deg;C to 10\u0026deg;C, the absorption peaks surrounding the characteristic peaks of the amidogroup would shift to the lower wavenumber. This observed shift in absorption peaks provided direct evidence that C\u0026thinsp;=\u0026thinsp;O and N-H groups formed hydrogen bonds with water molecules at low temperatures, while these bonds dissociated at high temperatures. Therefore, the changes in ambient temperature are capable of triggering the phase transition of PNIPAM.\u003c/p\u003e\n \u003cp\u003eBased on the aforementioned analysis and experimental findings, it is not difficult to determine the climbing mechanism of the soft climbing robot employed in this study, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e. When the front and rear feet of the soft robot were infused with cold water, the molecular structure of PNIPAM in the smart wood was stretched. At this point, the amidegroup of PNIPAM easily forms hydrogen bonds with the pavement surface comprised of polar materials, resulting in a stronger adhesion force. For non-polar pavement materials, this adhesion force will be reduced to some extent, but it will still be able to withstand the gravity of the robot. However, when the foot of the soft robot is filled with hot water, the PNIPAM molecular structure changes phase. In this case, the isopropyl group in PNIPAM reduces the adhesion force between the smart wood and the pavement surface, allowing the robot to readily disengage from the pavement. By utilizing the switchable adhesion force of the smart wood and the extension/contraction deformation of the soft robot, the robot is capable of performing climbing motions in accordance with the gaits depicted in Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e4.2 Adhesion model of the smart wood at different conditions\u003c/h2\u003e\n \u003cp\u003eWhile the influence of chemical functional groups on the smart wood adhesion has been examined in Sect. \u003cspan class=\"InternalRef\"\u003e4.1\u003c/span\u003e, the underlying physical mechanisms governing this adhesion phenomenon remain to be elucidated through further investigation. The adhesion force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e is primarily composed of four fundamental components: (i) van der Waals force \u003cem\u003eF\u003c/em\u003e\u003csub\u003evan\u003c/sub\u003e, (ii) contact mechanics force \u003cem\u003eF\u003c/em\u003e\u003csub\u003econ\u003c/sub\u003e, (iii) capillary force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ecap\u003c/sub\u003e, and (iv) electrostatic force \u003cem\u003eF\u003c/em\u003e\u003csub\u003eele\u003c/sub\u003e [\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e]. In this study, the adhesion force of the smart wood could only be regulated by altering the temperature, thus the van der Waals and electrostatic forces could not dominate the adhesion force between the smart wood and the pavement surface. Consequently, this study primarily focused on investigating the roles of contact mechanics and capillary forces in the adhesion mechanism. The adhesion force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e can be quantitatively determined through the application of Eqs. (1), (2), and (3) [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003ewhere \u003cem\u003e\u0026epsilon;\u003c/em\u003e is a correction factor caused by material properties, \u003cem\u003e\u0026omega;\u003c/em\u003e is the force required to separate PNIPAM per unit area from the pavement surface, \u003cem\u003eA\u003c/em\u003e\u003csub\u003ePNIPAM\u003c/sub\u003e is the contact area between PNIPAM oozing from the surface of the smart wood and the pavement surface at different temperatures, \u003cem\u003eA\u003c/em\u003e\u003csub\u003epeak\u003c/sub\u003e is the contact area of the microscopic contact peak between the smart wood and the pavement surface, \u003cem\u003eR\u003c/em\u003e\u003csub\u003eg\u003c/sub\u003e = 8.31 J/(mol\u0026middot;K) is the ideal gas constant, \u003cem\u003eT\u003c/em\u003e\u003csub\u003eab\u003c/sub\u003e is the absolute temperature, \u003cem\u003eRH\u003c/em\u003e is the relative humidity, \u003cem\u003eV\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e = 18 cm\u003csup\u003e3\u003c/sup\u003e/mol is the molar volume of water.\u003c/p\u003e\n \u003cp\u003eIn order to get the values of the necessary parameters in the above equations and verify the validity of the proposed adhesion force model, the seepage area \u003cem\u003eA\u003c/em\u003e\u003csub\u003ePNIPAM\u003c/sub\u003e of PNIPAM from the surface of the smart wood at various temperatures was investigated in this study. The experimental results in Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e pointed out that the area \u003cem\u003eA\u003c/em\u003e\u003csub\u003ePNIPAM\u003c/sub\u003e of PNIPAM oozing from a certain area on the surface of the smart wood varied with ambient temperature. Specifically, \u003cem\u003eA\u003c/em\u003e\u003csub\u003ePNIPAM\u003c/sub\u003e would steadily drop from 3.41 mm\u003csup\u003e2\u003c/sup\u003e at 15\u0026deg;C to 1.42 mm\u003csup\u003e2\u003c/sup\u003e at 32\u0026deg;C. The most intriguing observation was that the area \u003cem\u003eA\u003c/em\u003e\u003csub\u003ePNIPAM\u003c/sub\u003e rebounded to 3.35 mm\u003csup\u003e2\u003c/sup\u003e as the temperature decreased from 32\u0026deg;C to 15\u0026deg;C. Based on the experimental data shown in Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e, the area \u003cem\u003eA\u003c/em\u003e\u003csub\u003ePNIPAM\u003c/sub\u003e can be calculated from Eq. (4) using the approach of data fitting and proportional conversion.\u003c/p\u003e\n \u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e\u003csub\u003ewood\u003c/sub\u003e is the apparent contact area of the smart wood to be studied.\u003c/p\u003e\n \u003cp\u003eFurthermore, this study also examined the (maximum) adhesion force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e between the smart wood and various pavement surfaces at different temperatures, as illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003ea. It was demonstrated that the adhesion force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e between the smart wood and the paper pavement could reach 442.3 mN at \u003cem\u003eT\u003c/em\u003e\u0026thinsp;=\u0026thinsp;15\u0026deg;C, \u003cem\u003eA\u003c/em\u003e\u003csub\u003ewood\u003c/sub\u003e = 6.25 cm\u0026sup2;. As the temperature increased to 35\u0026deg;C, the adhesion force decreased to a negligible value. This temperature-dependent adhesion behavior could be consistently observed across various pavement materials. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003ea, the adhesion force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e of the smart wood exhibited a gradual decrease from 431.5 mN, 417.3 mN, and 391.4 mN at 15\u0026deg;C to insignificant values at 40\u0026deg;C on wood, PVC, and glass substrates, respectively. These experimental results further validate the proposed mechanism of adhesion force generation in the smart wood, as discussed in Sect. \u003cspan class=\"InternalRef\"\u003e4.1\u003c/span\u003e. To investigate the effect of environmental relative humidity on the adhesion properties of the smart wood, the adhesion force measurements were conducted on different pavement interfaces under controlled relative humidity conditions. Figure \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003eb indicated that the adhesion force \u003cem\u003eF\u003c/em\u003e\u003csub\u003ead\u003c/sub\u003e of the smart wood would decrease from 442.3 mN, 431.5 mN, 417.3 mN, and 394.1 mN at 65% \u003cem\u003eRH\u003c/em\u003e to 365.2 mN, 351.5 mN, 338.9 mN, and 326.8 mN at 95% \u003cem\u003eRH\u003c/em\u003e on paper, wood, PVC, and glass substrates, respectively. Obviously, the relative humidity of the environment also affects the adhesion force of the smart wood. Based on the above experimental results, it was easy to calculate the specific values of the relevant parameters of Eq. (2) and Eq. (3) in this study, see Appendix B for details. On this basis, the results calculated by the theoretical model of Eq. (1) were compared with the experimental results, as shown in Fig. \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e. The comparative findings revealed that the physical model of adhesion force utilized in this study accurately predicted the adhesion force of the smart wood surface. Finally, based on the above adhesion model, the climbing results of the soft robot on different materials in Sect. \u003cspan class=\"InternalRef\"\u003e3.3\u003c/span\u003e could be easily understood. When the soft robot had a large adhesion force with the pavement material, the separation between the soft robot and the pavement would be relatively difficult. Then, the distance the soft robot traveled under the same air pressure would be reduced. Therefore, the higher the adhesion force between the soft robot and the material, the slower the soft robot would travel.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"5 Conclusions","content":"\u003cp\u003eBy mixing the lignin-removed balsa wood with PNIPAM, a type of smart wood with regulated adhesion was created. This smart wood exhibited strong and weak adhesion at ambient temperatures below and above LCST (~\u0026thinsp;32\u0026deg;C), respectively. Then, this study successfully developed a novel climbing robot by integrating the fabricated smart wood with 3D-printed soft robotic feet. The testing findings indicated that the soft robot could climb 33 mm, 49 mm, and 72.5 mm on a vertical wood wall in 600 s under pneumatic pressures of 33.3 kPa, 47.5 kPa, and 61.8 kPa, respectively. As the tilt angle \u003cem\u003eα\u003c/em\u003e of the pavement decreased from 90\u0026deg; to 0\u0026deg;, the above crawling displacement \u003cem\u003eD\u003c/em\u003e of the soft robot would increase to 55 mm, 66 mm, and 96 mm, respectively. Simultaneously, the testing findings confirmed that reducing the low working temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e and raising the high working temperature \u003cem\u003eT\u003c/em\u003e\u003csub\u003eH\u003c/sub\u003e of the smart wood would improve the climbing/crawling speed of the soft robot. Additionally, the soft robot designed in this paper was able to climb on different material surfaces. For example, under the given working condition, the soft robot could climb 70.5 mm,72.5 mm,74 mm, and 94 mm on a wall made of paper, wood, PVC, and glass.\u003c/p\u003e \u003cp\u003eAt various temperatures, the change in adhesion force of the smart wood is primarily due to the phase transition process. At low temperatures (\u003cem\u003eT\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;LCST), the molecular structure of PNIPAM in the smart wood is stretched. The amidegroup (-CO-NH-) of PNIPAM easily forms hydrogen bonds with the pavement surface composed of polar materials, resulting in a stronger adhesion force. For non-polar pavement materials, this adhesion force will be significantly reduced. As the temperature rises, the molecular structure of PNIPAM will agglomerate, and the isopropyl group (-CH(CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e) with hydrophobic properties will reduce the adhesion force of the smart wood. Since this adhesion force of the smart wood can only be regulated by altering the temperature, the adhesion force in this study is primarily dominated by the contact mechanics force and the capillary force. This model of the adhesion force was also supported by the experimental results in this research. Although a stronger adhesion force enables the robot to adhere to the wall, it also hinders its climbing efficiency. Therefore, while maintaining sufficient adhesion force for wall attachment, minimizing the adhesion force on the smart wood surface can significantly enhance the climbing speed of the proposed robot.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. The first draft of the manuscript was written by Xiaodong Wang and Yaodong Li. Data collection and analysis were performed by Xiaolei Guo, Guanggui Cheng, and Jing Hua. Method was guided by Rui Lei. Material preparation was performed by\u0026nbsp;Qichen Zeng,\u0026nbsp;Xucheng Wang,\u0026nbsp;and\u0026nbsp;Xuehai Guo. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Natural Science Foundation of China (52475191, 52405193, 32471791).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;No datasets were generated or analyzed during the current study.\u003c/p\u003e\n\u003cp\u003eDeclarations\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u003c/strong\u003e The authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eElbadawi, M., Andrikopoulos, G., Nikolakopoulos, G., Gustafsson, T.: Bio-Inspired Climbing Robots in Wet Environments: Recent Trends in Adhesion Methods and Materials. 2018 IEEE International Conference on Robotics and Biomimetics (ROBI O), pp. 2347-2353. Place (2018)\u003c/li\u003e\n\u003cli\u003eChen, S., Cao, Y., Sarparast, M., Yuan, H., Dong, L., Tan, X., et al. Soft Crawling Robots: Design, Actuation, and Locomotion. Advanced Materials Technologies\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e5\u003c/strong\u003e:1900837 (2020).\u003c/li\u003e\n\u003cli\u003eFang, Y., Wang, S., Bi, Q., Cui, D., Yan, C. Design and Technical Development of Wall-Climbing Robots: A Review. Journal of Bionic Engineering \u003cstrong\u003e19\u003c/strong\u003e:877-901 (2022).\u003c/li\u003e\n\u003cli\u003eChen, K., Li, T., Yan, T., Xie, F., Feng, Q., Zhu, Q., et al. A soft gripper design for apple harvesting with force feedback and fruit slip detection. Agriculture \u003cstrong\u003e12\u003c/strong\u003e:1802 %@ 2077-0472 (2022).\u003c/li\u003e\n\u003cli\u003eLu, E., Ma, Z., Li, Y., Xu, L., Tang, Z. Adaptive backstepping control of tracked robot running trajectory based on real-time slip parameter estimation. International Journal of Agricultural and Biological Engineering\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e13\u003c/strong\u003e:178-187 (2020).\u003c/li\u003e\n\u003cli\u003eZhang, F., Teng, S., Wang, Y., Guo, Z., Wang, J., Xu, R. Design of bionic goat quadruped robot mechanism and walking gait planning. International Journal of Agricultural and Biological Engineering \u003cstrong\u003e13\u003c/strong\u003e:32-39 (2020).\u003c/li\u003e\n\u003cli\u003eLi, X., Liu, L., Huang, P., Li, B., Xing, Y., Wu, Z. A highly adaptable soft pipeline robot for climbing outside millimeter-sized pipelines. Nano Energy \u003cstrong\u003e134\u003c/strong\u003e:110566 (2025).\u003c/li\u003e\n\u003cli\u003eSingh, G., Patiballa, S., Zhang, X., Krishnan, G.: A Pipe-Climbing Soft Robot. 2019 International Conference on Robotics and Automation (ICRA), pp. 8450-8456. Place (2019)\u003c/li\u003e\n\u003cli\u003eLiao, B., Zang, H., Chen, M., Wang, Y., Lang, X., Zhu, N., et al. Soft Rod-Climbing Robot Inspired by Winding Locomotion of Snake. Soft Robotics \u003cstrong\u003e7\u003c/strong\u003e:500-511 (2020).\u003c/li\u003e\n\u003cli\u003ePark, G., Rodrigue, H. Soft climbing robot with magnetic feet for multimodal locomotion. Scientific Reports\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e13\u003c/strong\u003e:8377 (2023).\u003c/li\u003e\n\u003cli\u003eWang, G., Li, W., Che, H. Design and analysis of anti-overturning mechanism for magnetic wall-climbing robot. Journal of Mechanical Science and Technology \u003cstrong\u003e38\u003c/strong\u003e:379-387 (2024).\u003c/li\u003e\n\u003cli\u003eZhang, W., Zhang, W., Sun, Z. A reconfigurable soft wall-climbing robot actuated by electromagnet. International Journal of Advanced Robotic Systems \u003cstrong\u003e18\u003c/strong\u003e:1729881421992285 (2021).\u003c/li\u003e\n\u003cli\u003eGu, G., Zou, J., Zhao, R., Zhao, X., Zhu, X. Soft wall-climbing robots. Science Robotics \u003cstrong\u003e3\u003c/strong\u003e:eaat2874 (2018).\u003c/li\u003e\n\u003cli\u003eTang, Y., Zhang, Q., Lin, G., Yin, J. Switchable Adhesion Actuator for Amphibious Climbing Soft Robot. Soft Robotics \u003cstrong\u003e5\u003c/strong\u003e:592-600 (2018).\u003c/li\u003e\n\u003cli\u003eLi, X., Bai, P., Li, X., Li, L., Li, Y., Lu, H., et al. Robust scalable reversible strong adhesion by gecko-inspired composite design. Friction \u003cstrong\u003e10\u003c/strong\u003e:1192-1207 (2022).\u003c/li\u003e\n\u003cli\u003eSun, J., Bauman, L., Yu, L., Zhao, B. Gecko-and-inchworm-inspired untethered soft robot for climbing on walls and ceilings. Cell Reports Physical Science\u003cstrong\u003e 4 \u003c/strong\u003e(2023).\u003c/li\u003e\n\u003cli\u003eWang, X., Li, Y., Lei, R., Guo, X., Zhang, R. Adhesion-grasping behavior of a mechanical gripper based on thermoresponsive smart wood. Tribology International\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e205\u003c/strong\u003e:110558 (2025).\u003c/li\u003e\n\u003cli\u003eNagase, K. Thermoresponsive interfaces obtained using poly(N-isopropylacrylamide)-based copolymer for bioseparation and tissue engineering applications. Advances in Colloid and Interface Science\u003cstrong\u003e 295\u003c/strong\u003e:102487 (2021).\u003c/li\u003e\n\u003cli\u003eBovaldinova, K.A., Feldstein, M.M., Sherstneva, N.E., Moscalets, A.P., Khokhlov, A.R. Thermo-switchable pressure-sensitive adhesives with strong tunable adhesion towards substrate surfaces of different hydrophilicity. Polymer\u003cstrong\u003e 125\u003c/strong\u003e:10-20 (2017).\u003c/li\u003e\n\u003cli\u003eKawamoto, H., Murayama, M., Saka, S. Pyrolysis behavior of levoglucosan as an intermediate in cellulose pyrolysis: polymerization into polysaccharide as a key reaction to carbonized product formation.\u003cem\u003e \u003c/em\u003eJournal of Wood Science\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e49\u003c/strong\u003e:469-473 (2003).\u003c/li\u003e\n\u003cli\u003eWang, J., Zhou, X., Xiao, H. Structure and properties of cellulose/poly(N-isopropylacrylamide) hydrogels prepared by SIPN strategy. Carbohydrate Polymers\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e94\u003c/strong\u003e:749-754 (2013).\u003c/li\u003e\n\u003cli\u003eYou, Z., Dong, Y., Li, X., Yang, P., Luo, M., Zhu, Z., et al. One-pot synthesis of multi-functional cellulose-based ionic conductive organohydrogel with low-temperature strain sensitivity. Carbohydrate Polymers\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e251\u003c/strong\u003e:117019 (2021).\u003c/li\u003e\n\u003cli\u003eZhang, F., Wu, W., Zhang, X., Meng, X., Tong, G., Deng, Y. Temperature-sensitive poly-NIPAm modified cellulose nanofibril cryogel microspheres for controlled drug release. Cellulose \u003cstrong\u003e23\u003c/strong\u003e:415-425 (2016).\u003c/li\u003e\n\u003cli\u003eGicquel, E., Martin, C., Heux, L., Jean, B., Bras, J. Adsorption versus grafting of poly(N-Isopropylacrylamide) in aqueous conditions on the surface of cellulose nanocrystals. Carbohydrate Polymers\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e210\u003c/strong\u003e:100-109 (2019).\u003c/li\u003e\n\u003cli\u003eHarrison, A.J., Corti, D.S., Beaudoin, S.P. Capillary Forces in Nanoparticle Adhesion: A Review of AFM Methods. Particulate Science and Technology \u003cstrong\u003e33\u003c/strong\u003e:526-538 (2015).\u003c/li\u003e\n\u003cli\u003eJimidar, I.S.M., Kwiecinski, W., Roozendaal, G., Kooij, E.S., Gardeniers, H.J.G.E., Desmet, G., et al. Influence of Wettability and Geometry on Contact Electrification between Nonionic Insulators. ACS Applied Materials \u0026amp; Interfaces\u003cem\u003e \u003c/em\u003e\u003cstrong\u003e15\u003c/strong\u003e:42004-42014 (2023).\u003c/li\u003e\n\u003cli\u003eWang, X.D., Guo, J., Chen, C., Chen, L., Qian, L.M. A simple method to control nanotribology behaviors of monocrystalline silicon. Journal of Applied Physics \u003cstrong\u003e119\u003c/strong\u003e:044304 (2016).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Climbing robots, Soft robots, Smart wood, PNIPAM, Switchable adhesion.","lastPublishedDoi":"10.21203/rs.3.rs-6259286/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6259286/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eClimbing robots have vital uses in uncharted terrain exploration, military intelligence collecting, and other areas. To address the drawbacks of classic wall-climbing robots, this study introduced a novel soft climbing robot constructed of the smart wood with switchable adhesion force. The experimental findings indicated that the soft robot implemented in this research could effectively perform climbing movements on diverse walls and sloping pavements, enabled by temperature control through cold/hot water circulation and pneumatic actuation. Further research revealed that the reversible phase transition of PNIPAM at different temperatures was the main reason for the variable adhesion force of the smart wood. Moreover, the adhesion force model developed in this work indicated that the adhesion force of the smart wood surface was mostly composed of the contact mechanics force and the capillary force. Finally, this study will offer novel insights for the design of climbing robots and advance their potential applications.\u003c/p\u003e","manuscriptTitle":"A soft climbing robot based on smart wood with switchable adhesion","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-02 15:03:03","doi":"10.21203/rs.3.rs-6259286/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-05-19T13:59:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-04-15T16:05:13+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"303872677157632390107869456342068990342","date":"2025-03-25T13:22:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"49415300848181426827521674104523140847","date":"2025-03-23T14:00:16+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-03-23T13:13:48+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-20T05:41:09+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-03-20T05:39:25+00:00","index":"","fulltext":""},{"type":"submitted","content":"Tribology Letters","date":"2025-03-19T08:02:59+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"482db3bc-0113-4a78-b4a1-600d3650238f","owner":[],"postedDate":"April 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-08-07T07:14:35+00:00","versionOfRecord":{"articleIdentity":"rs-6259286","link":"https://doi.org/10.1007/s11249-025-02043-7","journal":{"identity":"tribology-letters","isVorOnly":false,"title":"Tribology Letters"},"publishedOn":"2025-07-18 15:57:35","publishedOnDateReadable":"July 18th, 2025"},"versionCreatedAt":"2025-04-02 15:03:03","video":"","vorDoi":"10.1007/s11249-025-02043-7","vorDoiUrl":"https://doi.org/10.1007/s11249-025-02043-7","workflowStages":[]},"version":"v1","identity":"rs-6259286","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6259286","identity":"rs-6259286","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}