Highly Compressible Hydrogel Reinforced with Cellulose Nanocrystals for Ultrasound Scanning via Microwave-Assisted Synthesis

Highly Compressible Hydrogel Reinforced with Cellulose Nanocrystals for Ultrasound Scanning via Microwave-Assisted Synthesis | 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 Highly Compressible Hydrogel Reinforced with Cellulose Nanocrystals for Ultrasound Scanning via Microwave-Assisted Synthesis Der-Yun Cheng, Yi-Hsiang Liao, Jiashing Yu, Ying-Chih Liao This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1623970/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract In this study, a rapid fabrication method was developed to prepare hydrogel structures with high mechanical strength and low attenuation coefficient for ultrasound scanning. Poly acrylic acid (PAA) hydrogel was first prepared via a free radical polymerization approach. To shorten the process time (~ 30 minutes in water bath), microwave heating was applied to facilitate the reaction and reduce the reaction time down to 80 seconds. The produced hydrogels showed excellent elasticity but had a low compressive strength of 100 kPa. To further enhance the mechanical strengths of PAA hydrogels, cellulosenanocrystals (CNCs) were added to the precursor solution. After the microwave assisted crosslinking process, the compressive strength of the hydrogel increased to 350 kPa. Moreover, the ultimate compressive strain was enhanced from 60% to 80% with great recoverability. The PAA/CNC hydrogel has a great ultrasound trnansimission for high-quality ultrasound images comparable to conventional liquid hydrogels. To demonstrate the feasibility of the PAA/CNC hydrogel in ultrasonic medical applications, a customized ultrasound probe coat was created with a 3D printed mold and practically used in the ultrasound scanning process. Cellulose Nanocrystal PAA Hydrogel Microwave Ultrasound Transmitting Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Introduction Ultrasound scanning is a widely used medical examination technology. The scanning technology can help detect objects in human body by converting the sonic reflections into images to investigate organ conditions or to perform physical checkup. During the scanning process, a lubricating gel is usually needed to allow ultrasound probes to move smoothly and more importantly to ensure continuous contact on skin. Without using lubricating gels, scanning errors might be induced due to air gaps between the probe and skin (Carovac, Smajlovic, and Junuzovic 2011). The application of liquid gels allows good ultrasound penetrations for distinct image formation, but also causes inconvenience as the gel needs to be cleaned after therapy. To avoid this inconvenience, ultrasound transmitting coats have been proposed (Aoyagi and Hiraguri 2017): the ultrasound probe can be wrapped with a silicone coat or a plastic bag filled with water to perform ultrasound analyses without smearing liquid gels over human body. Among various materials, hydrogels catch widespread attention due to their great ultrasound transmittance and bio-compatibility. By forming hydrogen bonds between molecules, the polymeric network in hydrogels can absorb large amount of water, which allows good ultrasound transmission, and also provides good elasticity and mechanical strengths (Ahmed 2015; Chai, Jiao, and Yu 2017) at the same time. Moreover, with special nano-structure designs, hydrogels can be also stretchable and responsive to environments (Xia et al. 2013 ) for various smart biomedical applications. To obtain good ultrasonic scanning signals, the compressive motion of the probe requires a hydrogel coating with high mechanical strength and compressive recoverability (Oyen 2014 ; Vedadghavami et al. 2017 ). Among various hydrogel polymers, poly acrylic acid (PAA) is a commonly used water absorbing polymer (Lim, Ahmad, and Lazim 2015), and has been widely used as a gel coupling medium for ultrasonic scanning (Jahan et al. 2020). However, PAA hydrogels regularly exhibit low mechanical endurance for compressive or shear stresses. Fortunately, the recent research shows that addition of cellulose nanocrystals (CNCs), a notable hydrophilic crystalline nanomaterial, can bridge between PAA polymer chains to form interpenetration networks and thus enhance the mechanical strength of hydrogels (Lim et al. 2017 ; Yang et al. 2012 ; Lim et al. 2014 ; Chang, Lue, and Zhang 2008 ). Apart from physical property enhancement of PAA hydrogel, the manufacture procedure is also of critical importance. Regularly, PAA hydrogels are produced by crosslinking monomers in water at elevated temperatures. Traditionally, aqueous precursor samples are thermally heated in water or oil bath to produce hydrogel. Due to the thermal conductive limits, it usually takes hours for the samples to reach required temperature for complete crosslinking reaction. During the heating process, the non-uniform temperature profiles in the samples might also lead to defects in the thermally crosslinked samples. Thus, to increase the heating uniformity and reaction efficiency, microwave-assisted process is commonly adopted in the literature (Galema 1997). Microwave heating make use of electromagnetic rotation or vibration of polar molecules for heat generation. Because of its radiative nature, microwave heating can efficiently deliver thermal energy uniformly inside aqueous samples, and reduce the heating time from hours to minutes with guaranteed product quality (Nüchter et al. 2004 ; Dallinger and Kappe 2007 ). In this study, a formulation method will be developed to prepare PAA hydrogel via a microwave assisted process to complete the crosslinking reaction quickly within minutes. With proper CNCs and crosslinker addition, the mechanical strengths will be optimized to fulfill the compression requirements for ultrasonic scanning probe coats. The ultrasonic transmittance and bio-compatibility of the prepared hydrogels will also be investigated. Finally, three-dimensional structures made from the hydrogel will be demonstrated to show the feasibility for potential ultrasound applications. Materials And Methods Materials Acrylic acid (AA) was purchased from Emperor Chemical CO., LTD. Cellulose nanocrystals (CNCs) were provided by The University of Maine. N, N’ – methylenebisacrylamide (MBA) was purchased from Sigma-Aldrich. Ammonium persulfate (APS) was purchased from GE Healthcare Life Sciences. All chemicals were used as received without further purification. Hydrogel Synthesis AA monomer was first added with appropriate amount of CNCs and diluted with DI water and well mixed in an ultrasonic bath for 5 minutes. After mixing, MBA was added as the crosslinking agent and APS was mixed in as the reaction initiator. To ensure that the solution was completely mixed without bubbles, the solution was mixed and deaerated with a planetary centrifugal mixer (THINKY, ARE-310), and then put into a microwave oven for gelation (Fig. 1 ). Characterizations The rheological properties of the precursor solutions or hydrogels were measured by using a rheometer (TA, HR2, Thermal). The mechanical strength of hydrogel was examined by using a dynamic mechanical analyzer (JSV-H1000, ALGOL instrument CO., LTD.). The ultrasound attenuation coefficient was examined by using a water tank test. The morphology of the samples was observed by a scanning electron microscope (SEM, Nova NanoSEM 230) and an optical microscope (MICROTECH D5-SWD). Results And Discussion Gel preparation method First, the gelation temperature of the PAA precursor is determined for hydrogel preparation. The viscosity of an aqueous precursor mixture containing AA, MBA, and APS is measured with increasing temperature (Fig. 2 ). Initially, the viscosity of the precursor solution remains constant at 0.04 Pa.s and slightly increases with temperature. As the temperature reaches 70 o C, the viscosity elevates dramatically and climbs to 1000 Pa.s at 75 o C, indicating a gelation temperature at ~ 70–75 o C, which is consistent with literature reports (Umar, Naim, and Sanagi 2014). To prepare the hydrogel, a water bath of 75 o C can be used to heat the precursor solution (Fig. 3 a). Because the thermal energy is transferred mainly by conduction through the vial, the temperature increases the required temperature of 75 o C in 1200 seconds with a time constant of 582 seconds, following the typical lump capacitance model (Lachi, El Wakil, and Padet 1997). The crosslinking reaction can be much facilitated by microwave heating comparing with traditional water bath (Jovanovic and Adnadjevic 2010). In contrast to hot water bath, in the microwave heating process (Fig. 3 b), the temperature increases linearly with time due to the radiation nature of the heating mechanism. In particular, because of the large water content, the precursor solution absorbs emitted microwave energy quickly in ~ 60 s, 20 times faster than the water bath, to reach the gelation temperature. For the sake of heating efficiency, the microwave heating method are used in the following section for hydrogel preparation. Preparation of PAA hydrogel The crosslinking degree of hydrogel strongly affects the elasticity (Weber, Lopez, and Anseth 2009), and can be adjusted by changing the crosslinker/precursor ratio. To determine the optimum crosslinker/precursor ratio, samples with different crosslinker/AA ratios are prepared and crosslinked via microwave heating. Rheological properties of these samples are examined to investigate the gelling condition of the prepared hydrogels (Bromberg 1998 ; Epstein-Barash et al. 2010 ; Tian, Liu, and Li 2016 ). Regularly, damping factor (tan \(\text{δ}\) ), which is the ratio between storage and loss modulus, is commonly used as an index for gel elasticity evaluation. When tan \(\text{δ}\) value is much less than 1, the sample is expected to be elastic and presents solid nature. With a crosslinker/AA ratio of 1/120, the tan \(\text{δ}\) value is less than 1 at low frequencies but gradually increases beyond unity at frequencies higher than 10 rad/s. This partial fluidity at high frequencies suggests that crosslinking points of the sample are insufficient for rigid gel formation. As the crosslinker/AA ratio increases to 1/60 or beyond, the tan \(\text{δ}\) values remains ~ 0.1 at all frequencies, indicating sufficient crosslinking points for gel formation. Hence, to produce hydrogels with stable viscoelasticity, a crosslinker/AA ratio of 1/60 is used in the following sections (Fig. 4 a). To further ensure sufficient hydrogel formation, the rheological properties of crosslinked samples at different AA concentration are also investigated. As shown in Fig. 4 b, with 10 wt% AA content, the tan \(\text{δ}\) value increases with frequency and exceeds 1 at frequencies higher than 60 rad/s, indicating the liquid nature of the sample at high frequency. When the AA concentration is more than or equal to 12 wt%, the sample is able to remain its hydrogel structure with significant elasticity with tan \(\text{δ}\) values at ~ 0.2. Despite high water contents of these samples (Fig. 5 ) for ultrasonic transmitting efficiencies, the compressive strengths of these PAA hydrogels are less than 140 kPa, lower than the required compressive stress of 200 kPa on the probes in ultrasonic scanning processes. Therefore, further enhancement of mechanical strength is needed. Addition of cellulose nanocrystals (CNCs). To improve the compressive strength at high water content, CNCs are introduced to the precursor solution for hydrogel formation. The viscosity changes along with temperature rise after CNC addition is shown in Fig. 6 a. The viscosity increases abruptly at 80 ° C, higher than the gelation temperature of pristine PAA, indicating a different crosslinking mechanism. From literature (Spoljaric et al. 2013 ; Lim et al. 2017 ), with CNC addition, esterification reaction usually occurs between the PAA chain and CNC, and results in a higher gelation temperature. To further verify the esterification, FTIR spectra of PAA, CNC, and PAA/CNC hydrogels are compared in Fig. 6 b. The peak at 1720 cm − 1 , representing the carboxylate group (COOH) in PAA, disappears after CNC addition. Instead, a peak at 1221 cm − 1 , representing an ester group (COOC) (Chen et al. 2015 ), is observed in the PAA/CNC hydrogel. The esterified PAA/CNC composites show a great improvement in mechanical strength. As shown in Fig. 7 a, comparing to pristine PAA hydrogel, addition of only 0.7 wt% CNCs can significantly augments the compressive strength from 100 KPa to 350 KPa, indicating a more durable molecular structure after the esterification between PAA and CNC (Spoljaric et al. 2013 ). Further addition of CNCs, however, cannot increase the ultimate compressive strength, possibly because of the limited esterified linkages between CNC and PAA. To further investigate the morphological changes, the SEM analysis (Fig. 7 b) shows that the hydrogel pore size decreases from ~ 25 µm to ~ 10 µm after CNC addition. This porous hydrogel structure with smaller pore size and more uniform pore distribution thus results in higher compressibility (Xia et al. 2013 ; Lim et al. 2014 ; Wang et al. 2013 ). To ensure the durability of the PAA/CNC hydrogel during ultrasound scanning procedure, the recoverability of the hydrogels under repeatedly compression are also evaluated. A compression cycle (0–70% strain) at a strain rate of 0.2 min − 1 is used to test the recoverability of PAA/CNC hydrogel. From the results (Fig. 8 a), the stress/strain curve of the first and tenth cycles nearly overlap each other, indicating the great elasticity and recoverability of the PAA/CNC hydrogel. The tensile strength is also greatly enhanced after CNC addition. As shown in Fig. 8 b, the pristine PAA hydrogel is quite fragile and breaks at ~ 2% tensile strain. After CNC addition, the PAA/CNC hydrogel can remain elastic with a tensile strain up to 70%. Besides the enhanced mechanical strengths, the PAA/CNC hydrogel also possesses a higher shear strength of 6000 Pa than its PAA counterpart (3500 Pa). This enhanced shear strength can thus fulfill the shear requirement for the inevitably rubbing process in ultrasound scanning (Fig. 8 c). Creation of complex structures To showcase the convenience of this microwave assisted hydrogel formation method, several specific structures with complicated geometries are created with molds. As shown in Fig. 9 a, all the prepared hydrogel structures show good shape definition in details. In addition, if the structure designs are too complicated to complete with a single mold, multiple molds can be used in sequence to perform shape combination. A piece of hydrogel structure is first created and merged in another mold to construct a more complex geometry without an apparent boundary (Fig. 9 b). These examples demonstrate the capability of this microwave approach to create hydrogel structures quickly at ease. Ultrasound scanning performance Ultrasound attenuation coefficients are first evaluated to verify the ultrasound transmittence through the hydrogel for scanning signal translation. Several widely used reagent in ultrasonic therapies are tested and the results are listed in Table 1 (Kim et al. 2018 ; Ginzel, Ginzel, and Brothers 1994; Prokop et al. 2003 ). Among all tested materials, the PAA/CNC hydrogel has the minimal attenuation coefficient (0.0156 dB/cm/MHz) for ultrasound, i.e. a great ultrasound transmission due to its high water content. The value of the attenuation coefficient is also close to that of PAA hydrogel, indicating that CNC addition to PAA hydrogel has little impact on ultrasound transmitting ability. To futher demonstrate the possibility ot apply hydrogel in ultrasound scanning, an ultrasound scanning coat is created (Fig. 10 a). An untrasound scanning probe is first 3D scanned to create a CAD file, which is used to generate a 3D printed mold. After filled with precursor solution and heated by microwave, the prepared coat can fit well on the probe for ultrasound scanning. The scanning image using the PAA/CNC gel coat shows little signal noises (Fig. 10 b), and the scanning results are exactly the same as those from using a commercial pad, indicating the great transmissive effects of the PAA/CNC gel coat. To test the biocompatibility for biomedical applications, cell viability of the PAA/CNC gel are also evaluated. By using a direct contact method (Fig. 11 ), the population of live cells (green dots) is approximately the same in all samples, and nearly no dead cells (red dots) are observed. The results indicate that PAA and CNC have high biocompatibility and low toxicity. Therefore, the PAA/CNC hydrogel coat is confirmed to be secure when applied in medical procedures. Conclusion The hydrogel synthesis developed in this study provides a time-saving and convenient procedure with the assistance of microwave heating. Compared with regular hydrogel preparation process (~ 30 minutes in water bath), the microwave assited process facilitates the reaction extent and reduces the reaction time down to 80 seconds. To fulfill the requirements of high mechanical strength and low attenuation coefficient for ultrasound scanning, cellulosenanocrystals (CNCs) are added to the precursor solution to enhance the mechanical strengths of PAA hydrogels. The compressive strength of the hydrogel increases from 100 kPa to 350 kPa after the CNC addition, and the ultimate compressive strain is enhanced from 60–80% with great recoverability. The PAA/CNC hydrogel shows good biocompatibility and is highly transimisive to ultrasound. To demonstrate the feasibility of using the PAA/CNC hydrogel in ultrasonic medical applications, a customized ultrasound probe coat is created with a 3D printed mold. The coat is practically used in the ultrasound scanning process to yield high-quality ultrasound images comparable to those from conventional liquid hydrogels. These results show the capability of this microwave assisted approach to quickly prepare hydrogel structures for biomedical uses and can be further extended to other hydrogel applications. Declarations Acknowledgements This study is supported by the “Advanced Research Center for Green Materials Science and Technology” from the Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (110L9006), the Ministry of Science and Technology in Taiwan (MOST 109-2221-E-002 -064 -MY3, MOST 110-2634-F-002-043), and National Taiwan University (NTU-CC-110L891503). References Ahmed, Enas M (2015) Hydrogel: Preparation, characterization, and applications: A review. J Adv Res 6: 105-21. 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Tables Table 1 Attenuation coefficients of ultrasound transmitting agents Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 25 May, 2022 Reviewers invited by journal 22 May, 2022 Editor invited by journal 19 May, 2022 Editor assigned by journal 18 May, 2022 First submitted to journal 18 May, 2022 Editorial decision: Major revisions 17 May, 2022 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-1623970","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":107927697,"identity":"3787e8ec-160e-4d73-a731-803782b35e81","order_by":0,"name":"Der-Yun Cheng","email":"","orcid":"","institution":"National Taiwan University","correspondingAuthor":false,"prefix":"","firstName":"Der-Yun","middleName":"","lastName":"Cheng","suffix":""},{"id":107927698,"identity":"a9b98edd-276c-46e6-8b5f-3081e3d22847","order_by":1,"name":"Yi-Hsiang Liao","email":"","orcid":"","institution":"National Taiwan University","correspondingAuthor":false,"prefix":"","firstName":"Yi-Hsiang","middleName":"","lastName":"Liao","suffix":""},{"id":107927699,"identity":"d1dec6a6-7938-4526-9ca1-f6ec781cdf52","order_by":2,"name":"Jiashing Yu","email":"","orcid":"","institution":"National Taiwan University","correspondingAuthor":false,"prefix":"","firstName":"Jiashing","middleName":"","lastName":"Yu","suffix":""},{"id":107927700,"identity":"e7ac0e28-cf19-4771-a14f-25b42154bf12","order_by":3,"name":"Ying-Chih Liao","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuElEQVRIiWNgGAWjYBACPmYQWQHhSBClhQ2s5QxENZAwIEILiGBsI0kLO/PDx7zz7tQZHGA+eJuH4U9iA2GHsRkb8257JmFwgC3ZmofBgBgtDGbSvNsOA7XwmEkDteQSoYX9mzTvHJAW/m/EagEaztsAtoWNaC3FhnOOHZaceZjN2HKOgXE9QS38/Mc3PnhTc5if73jzwxtvKuSMCekAAyYeEAmOUyKiBQwYfxCpcBSMglEwCkYoAABd9y/Pb+g0JQAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-9496-4190","institution":"National Taiwan University","correspondingAuthor":true,"prefix":"","firstName":"Ying-Chih","middleName":"","lastName":"Liao","suffix":""}],"badges":[],"createdAt":"2022-05-05 02:03:48","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1623970/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1623970/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":21839003,"identity":"831e9868-bb17-4646-a519-8aeefbb6be12","added_by":"auto","created_at":"2022-05-24 16:52:10","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":51158,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of experimental procedure.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig01.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/8ef436b6296137f57eea3aec.png"},{"id":21837484,"identity":"9dd14f73-9e3e-4e47-a3f7-98a38553f14f","added_by":"auto","created_at":"2022-05-24 16:42:10","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":13674,"visible":true,"origin":"","legend":"\u003cp\u003eViscosity variation of AA precursor solution with temperature. A temperature ramping rate of 10 \u003csup\u003eo\u003c/sup\u003eC/min is used. The solution is composed of 12 wt% AA, 0.2 wt% MBA, 0.1 wt% APS, and 87.7 wt% water.\u003c/p\u003e","description":"","filename":"Fig02.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/64834e2b1ba79106b5c574aa.png"},{"id":21837487,"identity":"acef400d-7cbf-42e3-b814-0493f4af22df","added_by":"auto","created_at":"2022-05-24 16:42:10","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":96747,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of heating efficiency between \u003cstrong\u003ea\u003c/strong\u003e water bath and \u003cstrong\u003eb\u003c/strong\u003e microwave heating for a 5 g precursor solution. The composition of the precursor solution is the same as that in Fig. 2.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig03.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/4e07e87c10159fc11700b1e3.png"},{"id":21839004,"identity":"2e3e5c3c-530c-443d-88dc-83e4ca5ef074","added_by":"auto","created_at":"2022-05-24 16:52:10","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":58268,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea \u003c/strong\u003eThe tan \u0026nbsp;values of crosslinked hydrogel samples with different AA/crosslinker ratios. All the samples are prepared by heating aqueous solution consisted of 12 wt% AA in microwave oven for 80 s. \u003cstrong\u003eb\u003c/strong\u003e The tan \u0026nbsp;values of samples with different AA concentrations.\u0026nbsp;\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig04.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/6914822677fe33fde609ee1b.png"},{"id":21837493,"identity":"9e82e711-6088-4d00-b628-faf239cffd38","added_by":"auto","created_at":"2022-05-24 16:42:10","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":82026,"visible":true,"origin":"","legend":"\u003cp\u003eThe equilibrium water content and the compressive ultimate strengths of hydrogels made from different AA contents in precursor solution.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig05.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/88accc9970d7617e6cc7de29.png"},{"id":21837488,"identity":"01cfd433-0d83-4fc8-83df-a2498c973b15","added_by":"auto","created_at":"2022-05-24 16:42:10","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":75462,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Variation of viscosity of precursor solution with temperature. The PAA/CNC solution is composed of 12 wt% AA, 0.7 wt% CNCs, 0.2 wt% MBA, 0.1 wt% APS, and 87 wt% water. A temperature ramping rate of 10 \u003csup\u003eo\u003c/sup\u003eC/min is used. \u003cstrong\u003eb\u003c/strong\u003e FTIR spectra of PAA, CNC, and PAA/CNC hydrogels.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig06.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/91e80a5721b8fca977086304.png"},{"id":21838327,"identity":"09ed4d4b-f659-4590-9dc5-2249a02777b0","added_by":"auto","created_at":"2022-05-24 16:47:10","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":112333,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e The ultimate compressive strengths of PAA/CNC hydrogel with different CNC contents. \u003cstrong\u003eb\u003c/strong\u003e The SEM images of PAA and PAA/CNC (0.7wt%) hydrogels.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig07.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/14ba471f8eae81e93f0ae0b8.png"},{"id":21838331,"identity":"a8e8d3ce-a1af-47fc-af2e-1015edd23425","added_by":"auto","created_at":"2022-05-24 16:47:10","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":81024,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Recoverability of PAA/CNC hydrogel (12 wt% AA with 0.7 wt% CNCs) after 10 compression cycles. A PAA hydrogel (12 wt% AA) is compared but breaks around 100 kPa. A strain rate of 20 %/min is used. \u003cstrong\u003eb\u003c/strong\u003e Tensile strength test for PAA and PAA/CNC hydrogels. A strain rate of 20 %/min is used. \u003cstrong\u003ec\u003c/strong\u003e Comparison of shear strengths between PAA and PAA/CNC hydrogels. An angular frequency of 1 rad/s is used.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig08.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/db6a819001199cbef2ba9167.png"},{"id":21838332,"identity":"b8bcb632-369b-4cb6-8fe1-672e2cfa5d88","added_by":"auto","created_at":"2022-05-24 16:47:11","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":333867,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e 3D structures made of the PAA/CNC hydrogel from molding. The CAD files are compared to show the accurate detailed geometries. \u003cstrong\u003eb \u003c/strong\u003eDemonstrative gel structures using multi-step molding process. The precursor solution was dyed with red and blue to differentiate first and second molding process.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig09.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/b27611ba9b724ceecd3b96d5.png"},{"id":21837490,"identity":"7b0a40fc-34c0-475f-a945-b4e0db9bb288","added_by":"auto","created_at":"2022-05-24 16:42:10","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":280010,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e Mold model and the PAA/CNC hydrogel probe coat constructed by molding process with microwave heating. The hydrogel coat can fit in the ultrasound scanning probe with good accuracy. \u003cstrong\u003eb\u003c/strong\u003e Ultrasound image testing by scanning a human arm with the PAA/CNC probe coat.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Fig10.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/77bb7f2c327cd68e1d602d5c.png"},{"id":21838329,"identity":"d132b170-f58a-4254-84c3-58493f24d46a","added_by":"auto","created_at":"2022-05-24 16:47:10","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":121057,"visible":true,"origin":"","legend":"\u003cp\u003eThe live and dead cell test for \u003cem\u003ein vitro\u003c/em\u003e cytotoxicity of PAA hydrogel and PAA hydrogel with CNCs. Direct contact method is used. Live/Dead staining of 24-hour and 37 \u003csup\u003eo\u003c/sup\u003eC cultures. L929 is used and the cell density is of 15000/well with DMEM-HG as the medium.\u003c/p\u003e","description":"","filename":"Fig11.png","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/ef6d86c3864caaff0c0cfcde.png"},{"id":21839005,"identity":"d573275a-c26e-4edc-b968-b120b9a8ee92","added_by":"auto","created_at":"2022-05-24 16:52:13","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1567412,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1623970/v1/ecc791c9-b771-4555-afac-8421831fdbce.pdf"}],"financialInterests":"","formattedTitle":"Highly Compressible Hydrogel Reinforced with Cellulose Nanocrystals for Ultrasound Scanning via Microwave-Assisted Synthesis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUltrasound scanning is a widely used medical examination technology. The scanning technology can help detect objects in human body by converting the sonic reflections into images to investigate organ conditions or to perform physical checkup. During the scanning process, a lubricating gel is usually needed to allow ultrasound probes to move smoothly and more importantly to ensure continuous contact on skin. Without using lubricating gels, scanning errors might be induced due to air gaps between the probe and skin (Carovac, Smajlovic, and Junuzovic 2011). The application of liquid gels allows good ultrasound penetrations for distinct image formation, but also causes inconvenience as the gel needs to be cleaned after therapy. To avoid this inconvenience, ultrasound transmitting coats have been proposed (Aoyagi and Hiraguri 2017): the ultrasound probe can be wrapped with a silicone coat or a plastic bag filled with water to perform ultrasound analyses without smearing liquid gels over human body. Among various materials, hydrogels catch widespread attention due to their great ultrasound transmittance and bio-compatibility. By forming hydrogen bonds between molecules, the polymeric network in hydrogels can absorb large amount of water, which allows good ultrasound transmission, and also provides good elasticity and mechanical strengths (Ahmed 2015; Chai, Jiao, and Yu 2017) at the same time. Moreover, with special nano-structure designs, hydrogels can be also stretchable and responsive to environments (Xia et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) for various smart biomedical applications.\u003c/p\u003e \u003cp\u003eTo obtain good ultrasonic scanning signals, the compressive motion of the probe requires a hydrogel coating with high mechanical strength and compressive recoverability (Oyen \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Vedadghavami et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Among various hydrogel polymers, poly acrylic acid (PAA) is a commonly used water absorbing polymer (Lim, Ahmad, and Lazim 2015), and has been widely used as a gel coupling medium for ultrasonic scanning (Jahan et al. 2020). However, PAA hydrogels regularly exhibit low mechanical endurance for compressive or shear stresses. Fortunately, the recent research shows that addition of cellulose nanocrystals (CNCs), a notable hydrophilic crystalline nanomaterial, can bridge between PAA polymer chains to form interpenetration networks and thus enhance the mechanical strength of hydrogels (Lim et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Lim et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Chang, Lue, and Zhang \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eApart from physical property enhancement of PAA hydrogel, the manufacture procedure is also of critical importance. Regularly, PAA hydrogels are produced by crosslinking monomers in water at elevated temperatures. Traditionally, aqueous precursor samples are thermally heated in water or oil bath to produce hydrogel. Due to the thermal conductive limits, it usually takes hours for the samples to reach required temperature for complete crosslinking reaction. During the heating process, the non-uniform temperature profiles in the samples might also lead to defects in the thermally crosslinked samples. Thus, to increase the heating uniformity and reaction efficiency, microwave-assisted process is commonly adopted in the literature (Galema 1997). Microwave heating make use of electromagnetic rotation or vibration of polar molecules for heat generation. Because of its radiative nature, microwave heating can efficiently deliver thermal energy uniformly inside aqueous samples, and reduce the heating time from hours to minutes with guaranteed product quality (N\u0026uuml;chter et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Dallinger and Kappe \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this study, a formulation method will be developed to prepare PAA hydrogel via a microwave assisted process to complete the crosslinking reaction quickly within minutes. With proper CNCs and crosslinker addition, the mechanical strengths will be optimized to fulfill the compression requirements for ultrasonic scanning probe coats. The ultrasonic transmittance and bio-compatibility of the prepared hydrogels will also be investigated. Finally, three-dimensional structures made from the hydrogel will be demonstrated to show the feasibility for potential ultrasound applications.\u003c/p\u003e"},{"header":"Materials And Methods","content":"\u003cdiv class=\"Section2\" id=\"Sec3\"\u003e\n \u003ch2\u003eMaterials\u003c/h2\u003e\n \u003cp\u003eAcrylic acid (AA) was purchased from Emperor Chemical CO., LTD. Cellulose nanocrystals (CNCs) were provided by The University of Maine. N, N\u0026rsquo; \u0026ndash; methylenebisacrylamide (MBA) was purchased from Sigma-Aldrich. Ammonium persulfate (APS) was purchased from GE Healthcare Life Sciences. All chemicals were used as received without further purification.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec4\"\u003e\n \u003ch2\u003eHydrogel Synthesis\u003c/h2\u003e\n \u003cp\u003eAA monomer was first added with appropriate amount of CNCs and diluted with DI water and well mixed in an ultrasonic bath for 5 minutes. After mixing, MBA was added as the crosslinking agent and APS was mixed in as the reaction initiator. To ensure that the solution was completely mixed without bubbles, the solution was mixed and deaerated with a planetary centrifugal mixer (THINKY, ARE-310), and then put into a microwave oven for gelation (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec5\"\u003e\n \u003ch2\u003eCharacterizations\u003c/h2\u003e\n \u003cp\u003eThe rheological properties of the precursor solutions or hydrogels were measured by using a rheometer (TA, HR2, Thermal). The mechanical strength of hydrogel was examined by using a dynamic mechanical analyzer (JSV-H1000, ALGOL instrument CO., LTD.). The ultrasound attenuation coefficient was examined by using a water tank test. The morphology of the samples was observed by a scanning electron microscope (SEM, Nova NanoSEM 230) and an optical microscope (MICROTECH D5-SWD).\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cdiv class=\"Section2\" id=\"Sec6\"\u003e\n \u003ch2\u003eGel preparation method\u003c/h2\u003e\n \u003cp\u003eFirst, the gelation temperature of the PAA precursor is determined for hydrogel preparation. The viscosity of an aqueous precursor mixture containing AA, MBA, and APS is measured with increasing temperature (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). Initially, the viscosity of the precursor solution remains constant at 0.04 Pa.s and slightly increases with temperature. As the temperature reaches 70 \u003csup\u003eo\u003c/sup\u003eC, the viscosity elevates dramatically and climbs to 1000 Pa.s at 75 \u003csup\u003eo\u003c/sup\u003eC, indicating a gelation temperature at ~\u0026thinsp;70\u0026ndash;75 \u003csup\u003eo\u003c/sup\u003eC, which is consistent with literature reports (Umar, Naim, and Sanagi 2014). To prepare the hydrogel, a water bath of 75 \u003csup\u003eo\u003c/sup\u003eC can be used to heat the precursor solution (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea). Because the thermal energy is transferred mainly by conduction through the vial, the temperature increases the required temperature of 75\u003csup\u003eo\u003c/sup\u003eC in 1200 seconds with a time constant of 582 seconds, following the typical lump capacitance model (Lachi, El Wakil, and Padet 1997). The crosslinking reaction can be much facilitated by microwave heating comparing with traditional water bath (Jovanovic and Adnadjevic 2010). In contrast to hot water bath, in the microwave heating process (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb), the temperature increases linearly with time due to the radiation nature of the heating mechanism. In particular, because of the large water content, the precursor solution absorbs emitted microwave energy quickly in ~\u0026thinsp;60 s, 20 times faster than the water bath, to reach the gelation temperature. For the sake of heating efficiency, the microwave heating method are used in the following section for hydrogel preparation.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec7\"\u003e\n \u003ch2\u003ePreparation of PAA hydrogel\u003c/h2\u003e\n \u003cp\u003eThe crosslinking degree of hydrogel strongly affects the elasticity (Weber, Lopez, and Anseth 2009), and can be adjusted by changing the crosslinker/precursor ratio. To determine the optimum crosslinker/precursor ratio, samples with different crosslinker/AA ratios are prepared and crosslinked via microwave heating. Rheological properties of these samples are examined to investigate the gelling condition of the prepared hydrogels (Bromberg \u003cspan class=\"CitationRef\"\u003e1998\u003c/span\u003e; Epstein-Barash et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e; Tian, Liu, and Li \u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e). Regularly, damping factor (tan\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{\u0026delta;}\\)\u003c/span\u003e\u003c/span\u003e), which is the ratio between storage and loss modulus, is commonly used as an index for gel elasticity evaluation. When tan\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{\u0026delta;}\\)\u003c/span\u003e\u003c/span\u003e value is much less than 1, the sample is expected to be elastic and presents solid nature. With a crosslinker/AA ratio of 1/120, the tan\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{\u0026delta;}\\)\u003c/span\u003e\u003c/span\u003e value is less than 1 at low frequencies but gradually increases beyond unity at frequencies higher than 10 rad/s. This partial fluidity at high frequencies suggests that crosslinking points of the sample are insufficient for rigid gel formation. As the crosslinker/AA ratio increases to 1/60 or beyond, the tan\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{\u0026delta;}\\)\u003c/span\u003e\u003c/span\u003e values remains\u0026thinsp;~\u0026thinsp;0.1 at all frequencies, indicating sufficient crosslinking points for gel formation. Hence, to produce hydrogels with stable viscoelasticity, a crosslinker/AA ratio of 1/60 is used in the following sections (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea).\u003c/p\u003e\n \u003cp\u003eTo further ensure sufficient hydrogel formation, the rheological properties of crosslinked samples at different AA concentration are also investigated. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, with 10 wt% AA content, the tan \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{\u0026delta;}\\)\u003c/span\u003e\u003c/span\u003e value increases with frequency and exceeds 1 at frequencies higher than 60 rad/s, indicating the liquid nature of the sample at high frequency. When the AA concentration is more than or equal to 12 wt%, the sample is able to remain its hydrogel structure with significant elasticity with tan\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\text{\u0026delta;}\\)\u003c/span\u003e\u003c/span\u003e values at ~\u0026thinsp;0.2. Despite high water contents of these samples (Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e) for ultrasonic transmitting efficiencies, the compressive strengths of these PAA hydrogels are less than 140 kPa, lower than the required compressive stress of 200 kPa on the probes in ultrasonic scanning processes. Therefore, further enhancement of mechanical strength is needed.\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eAddition of cellulose nanocrystals (CNCs).\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003eTo improve the compressive strength at high water content, CNCs are introduced to the precursor solution for hydrogel formation. The viscosity changes along with temperature rise after CNC addition is shown in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003ea. The viscosity increases abruptly at 80\u003csup\u003e\u0026deg;\u003c/sup\u003eC, higher than the gelation temperature of pristine PAA, indicating a different crosslinking mechanism. From literature (Spoljaric et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lim et al. \u003cspan class=\"CitationRef\"\u003e2017\u003c/span\u003e), with CNC addition, esterification reaction usually occurs between the PAA chain and CNC, and results in a higher gelation temperature. To further verify the esterification, FTIR spectra of PAA, CNC, and PAA/CNC hydrogels are compared in Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003eb. The peak at 1720 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, representing the carboxylate group (COOH) in PAA, disappears after CNC addition. Instead, a peak at 1221 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, representing an ester group (COOC) (Chen et al. \u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e), is observed in the PAA/CNC hydrogel.\u003c/p\u003e\n \u003cp\u003eThe esterified PAA/CNC composites show a great improvement in mechanical strength. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003ea, comparing to pristine PAA hydrogel, addition of only 0.7 wt% CNCs can significantly augments the compressive strength from 100 KPa to 350 KPa, indicating a more durable molecular structure after the esterification between PAA and CNC (Spoljaric et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e). Further addition of CNCs, however, cannot increase the ultimate compressive strength, possibly because of the limited esterified linkages between CNC and PAA. To further investigate the morphological changes, the SEM analysis (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003eb) shows that the hydrogel pore size decreases from ~\u0026thinsp;25 \u0026micro;m to ~\u0026thinsp;10 \u0026micro;m after CNC addition. This porous hydrogel structure with smaller pore size and more uniform pore distribution thus results in higher compressibility (Xia et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lim et al. \u003cspan class=\"CitationRef\"\u003e2014\u003c/span\u003e; Wang et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e\n \u003cp\u003eTo ensure the durability of the PAA/CNC hydrogel during ultrasound scanning procedure, the recoverability of the hydrogels under repeatedly compression are also evaluated. A compression cycle (0\u0026ndash;70% strain) at a strain rate of 0.2 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is used to test the recoverability of PAA/CNC hydrogel. From the results (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ea), the stress/strain curve of the first and tenth cycles nearly overlap each other, indicating the great elasticity and recoverability of the PAA/CNC hydrogel. The tensile strength is also greatly enhanced after CNC addition. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003eb, the pristine PAA hydrogel is quite fragile and breaks at ~\u0026thinsp;2% tensile strain. After CNC addition, the PAA/CNC hydrogel can remain elastic with a tensile strain up to 70%. Besides the enhanced mechanical strengths, the PAA/CNC hydrogel also possesses a higher shear strength of 6000 Pa than its PAA counterpart (3500 Pa). This enhanced shear strength can thus fulfill the shear requirement for the inevitably rubbing process in ultrasound scanning (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ec).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec8\"\u003e\n \u003ch2\u003eCreation of complex structures\u003c/h2\u003e\n \u003cp\u003eTo showcase the convenience of this microwave assisted hydrogel formation method, several specific structures with complicated geometries are created with molds. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ea, all the prepared hydrogel structures show good shape definition in details. In addition, if the structure designs are too complicated to complete with a single mold, multiple molds can be used in sequence to perform shape combination. A piece of hydrogel structure is first created and merged in another mold to construct a more complex geometry without an apparent boundary (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003eb). These examples demonstrate the capability of this microwave approach to create hydrogel structures quickly at ease.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"Section2\" id=\"Sec9\"\u003e\n \u003ch2\u003eUltrasound scanning performance\u003c/h2\u003e\n \u003cp\u003eUltrasound attenuation coefficients are first evaluated to verify the ultrasound transmittence through the hydrogel for scanning signal translation. Several widely used reagent in ultrasonic therapies are tested and the results are listed in Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e (Kim et al. \u003cspan class=\"CitationRef\"\u003e2018\u003c/span\u003e; Ginzel, Ginzel, and Brothers 1994; Prokop et al. \u003cspan class=\"CitationRef\"\u003e2003\u003c/span\u003e). Among all tested materials, the PAA/CNC hydrogel has the minimal attenuation coefficient (0.0156 dB/cm/MHz) for ultrasound, i.e. a great ultrasound transmission due to its high water content. The value of the attenuation coefficient is also close to that of PAA hydrogel, indicating that CNC addition to PAA hydrogel has little impact on ultrasound transmitting ability.\u003c/p\u003e\n \u003cp\u003eTo futher demonstrate the possibility ot apply hydrogel in ultrasound scanning, an ultrasound scanning coat is created (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003ea). An untrasound scanning probe is first 3D scanned to create a CAD file, which is used to generate a 3D printed mold. After filled with precursor solution and heated by microwave, the prepared coat can fit well on the probe for ultrasound scanning. The scanning image using the PAA/CNC gel coat shows little signal noises (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003eb), and the scanning results are exactly the same as those from using a commercial pad, indicating the great transmissive effects of the PAA/CNC gel coat. To test the biocompatibility for biomedical applications, cell viability of the PAA/CNC gel are also evaluated. By using a direct contact method (Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e), the population of live cells (green dots) is approximately the same in all samples, and nearly no dead cells (red dots) are observed. The results indicate that PAA and CNC have high biocompatibility and low toxicity. Therefore, the PAA/CNC hydrogel coat is confirmed to be secure when applied in medical procedures.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThe hydrogel synthesis developed in this study provides a time-saving and convenient procedure with the assistance of microwave heating. Compared with regular hydrogel preparation process (~\u0026thinsp;30 minutes in water bath), the microwave assited process facilitates the reaction extent and reduces the reaction time down to 80 seconds. To fulfill the requirements of high mechanical strength and low attenuation coefficient for ultrasound scanning, cellulosenanocrystals (CNCs) are added to the precursor solution to enhance the mechanical strengths of PAA hydrogels. The compressive strength of the hydrogel increases from 100 kPa to 350 kPa after the CNC addition, and the ultimate compressive strain is enhanced from 60\u0026ndash;80% with great recoverability. The PAA/CNC hydrogel shows good biocompatibility and is highly transimisive to ultrasound. To demonstrate the feasibility of using the PAA/CNC hydrogel in ultrasonic medical applications, a customized ultrasound probe coat is created with a 3D printed mold. The coat is practically used in the ultrasound scanning process to yield high-quality ultrasound images comparable to those from conventional liquid hydrogels. These results show the capability of this microwave assisted approach to quickly prepare hydrogel structures for biomedical uses and can be further extended to other hydrogel applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis study is supported by the \u0026ldquo;Advanced Research Center for Green Materials Science and Technology\u0026rdquo; from the Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (110L9006), the Ministry of Science and Technology in Taiwan (MOST 109-2221-E-002 -064 -MY3, MOST 110-2634-F-002-043), and National Taiwan University (NTU-CC-110L891503).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAhmed, Enas M (2015) Hydrogel: Preparation, characterization, and applications: A review. 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J Mater Chem 22: 22467-80.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003e\u003cstrong\u003eTable 1 \u003c/strong\u003eAttenuation coefficients of ultrasound transmitting agents\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/69519_bce2c0439cd956a6/69519_custom_files/img1653404077.png\"\u003e\u003c/p\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":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Cellulose Nanocrystal, PAA Hydrogel, Microwave, Ultrasound Transmitting","lastPublishedDoi":"10.21203/rs.3.rs-1623970/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1623970/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, a rapid fabrication method was developed to prepare hydrogel structures with high mechanical strength and low attenuation coefficient for ultrasound scanning. Poly acrylic acid (PAA) hydrogel was first prepared via a free radical polymerization approach. To shorten the process time (~ 30 minutes in water bath), microwave heating was applied to facilitate the reaction and reduce the reaction time down to 80 seconds. The produced hydrogels showed excellent elasticity but had a low compressive strength of 100 kPa. To further enhance the mechanical strengths of PAA hydrogels, cellulosenanocrystals (CNCs) were added to the precursor solution. After the microwave assisted crosslinking process, the compressive strength of the hydrogel increased to 350 kPa. Moreover, the ultimate compressive strain was enhanced from 60% to 80% with great recoverability. The PAA/CNC hydrogel has a great ultrasound trnansimission for high-quality ultrasound images comparable to conventional liquid hydrogels. To demonstrate the feasibility of the PAA/CNC hydrogel in ultrasonic medical applications, a customized ultrasound probe coat was created with a 3D printed mold and practically used in the ultrasound scanning process.\u003c/p\u003e","manuscriptTitle":"Highly Compressible Hydrogel Reinforced with Cellulose Nanocrystals for Ultrasound Scanning via Microwave-Assisted Synthesis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-05-24 16:42:08","doi":"10.21203/rs.3.rs-1623970/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2022-05-25T04:54:34+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-05-23T01:17:28+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Cellulose","date":"2022-05-20T02:38:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-05-18T12:36:47+00:00","index":"","fulltext":""},{"type":"submitted","content":"Cellulose","date":"2022-05-18T05:45:44+00:00","index":"","fulltext":""},{"type":"decision","content":"Major revisions","date":"2022-05-17T23:15:44+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"cellulose","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cels","sideBox":"Learn more about [Cellulose](https://www.springer.com/journal/10570)","snPcode":"10570","submissionUrl":"https://submission.nature.com/new-submission/10570/3","title":"Cellulose","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"05c8ed0b-746d-4ee8-a10b-22023c63c050","owner":[],"postedDate":"May 24th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-09-14T17:35:55+00:00","versionOfRecord":[],"versionCreatedAt":"2022-05-24 16:42:08","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1623970","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1623970","identity":"rs-1623970","version":["v1"]},"buildId":"J0_U0BvcaRcwD8yVFaRlm","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}