Effect of surface crosslinking on the Friction behavior of Tea Polyphenol stabilized Ultra-high molecular weight polyethylene

Effect of surface crosslinking on the Friction behavior of Tea Polyphenol stabilized Ultra-high molecular weight polyethylene | 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 Effect of surface crosslinking on the Friction behavior of Tea Polyphenol stabilized Ultra-high molecular weight polyethylene Adnan Ahmad, Nouman Ali Shah, Rizwan M. Gul, M. Ali Kamran, M. Aaqib Ishaq, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4341120/v2 This work is licensed under a CC BY 4.0 License Status: Posted Version 2 posted You are reading this latest preprint version Show more versions Abstract The use of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) incorporating vitamin E (VE) as an antioxidant is widely acknowledged in the field of joint implants. While VE plays a crucial role in preventing oxidation, concerns have been raised regarding its impact on crosslink density, leading to wear performance deterioration. This study proposes the hypothesis that tea polyphenols, specifically lipid-soluble epigallocatechin gallate (IsEGCG) and epigallocatechin gallate (EGCG), may counteract the typical decrease in crosslink density caused by VE, thereby enhancing wear performance. The antioxidants were integrated into UHMWPE at a concentration of 0.2 wt%, followed by surface chemical crosslinking using di-cumyl peroxide. Surface properties, including crosslink density, roughness, coefficient of friction, and wear performance, were comprehensively evaluated. The results indicate a significantly higher crosslink density in UHMWPE blended with IsEGCG and EGCG compared to VE-stabilized UHMWPE, which exhibited a 17% reduction compared to virgin UHMWPE. The coefficient of friction increased post-crosslinking, with tea polyphenol-blended UHMWPE demonstrating a relatively higher value, confirming a highly crosslinked network structure. The wear resistance of surface-crosslinked UHMWPE stabilized with tea polyphenols was markedly superior compared to UHMWPE stabilized with VE. Additionally, a substantial presence of scratches, furrows, and flakes was observed on the surface of VE-stabilized UHMWPE in contrast to tea polyphenol-stabilized UHMWPE. These findings suggest that tea polyphenols present promising alternatives to VE for enhancing the overall performance and longevity of UHMWPE-based implants. Polymer Science Biopolymers Biomaterials UHMWPE antioxidants surface crosslinking tea polyphenols wear total joint replacement Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 INTRODUCTION The escalating number of patients with joint issues has underscored the need for researchers to develop high-performance biomaterials for total joint replacement(1). Ultra-high molecular weight polyethylene (UHMWPE), with additional modifications, has emerged as the preferred material for total hip and knee replacement procedures, delivering excellent outcomes over the past few decades(2, 3). A high-performance implant is characterized by three crucial factors: wear resistance, mechanical strength, and oxidation stability. The predominant cause of joint failure lies in the wear and tear of the polymer component, a consequence of continuous movement and contact, ultimately leading to joint loosening and osteolysis(4-6). Crosslinking has gained widespread acceptance as a strategy to enhance wear resistance by establishing a network structure that minimizes surface orientation and alters surface topography, thereby influencing the release of wear debris in UHMWPE(7, 8). Radiation crosslinking has exhibited promising results in reducing UHMWPE wear in artificial joints by creating a highly crosslinked structure through high-energy ionizing gamma radiation or electron beam exposure(9). However, radiation crosslinking causes oxidation embrittlement due to the formation of free radicals reacting with oxygen, leading to degradation in the mechanical performance of the implant(10, 11). The process of radiation crosslinking generates free radicals by breaking UHMWPE bonds. While free radicals in the amorphous phase transform into a crosslinked structure, those formed in the crystalline region significantly impact the long-term oxidation resistance capabilities of the polymer(12). Chemical crosslinking has been employed for several decades to achieve a highly crosslinked structure using organic peroxides, but its oxidizing nature has impeded its application in clinical settings(13). Antioxidants are widely used to enhance the oxidation resistance of the crosslinked UHMWPE(14-16). Vitamin E (VE) is particularly favored as a natural antioxidant, offering a significant boost to oxidation resistance(17). VE donates hydrogen from the phenolic group to react with the trapped residual radicals playing a role in hindering chain scission(18). However, VE is also a free radical scavenger and it decreases the crosslinked density of the polymer, resulting in reducing the effectiveness of wear resistance of the crosslinked UHMWPE(19, 20). To mitigate this inhibitory effect on the crosslinked structure, an optimal VE content of approximately 0.2 wt% has been identified, as evaluated by Oral et al.(13). Therefore, the current research is focused to investigate an effective stabilizer along with the minimum adverse effect on the crosslinked structure. Recent research has delved into a more potent antioxidant, tea polyphenol, a natural substance found in tea, exhibiting superior resistance to oxidation compared to VE and thus considered a more favorable alternative stabilizer(21-23). Tea polyphenols, owing to their numerous phenolic hydroxyls, possess a greater capacity for hydrogen donation than VE, which comprises a single phenolic hydroxyl. Importantly, the incorporation of tea polyphenols does not impact the crosslinking process, unlike the decrease observed in VE-blended UHMWPE(22, 24). Further, the connections between friction, wear, and crystallinity are still not fully understood. Gaining insights into the influence of structure and crystallinity on friction and wear properties would be beneficial for the advancement of high-quality joint materials(25, 26). This investigation posits that the utilization of tea polyphenols as antioxidants, specifically lipid-soluble epigallocatechin gallate (IsEGCG) and epigallocatechin gallate (EGCG), instead of VE, can augment the wear resistance of UHMWPE. This substitution is anticipated to maintain an elevated crosslink density and result in heightened oxidation stability. The chosen polyphenols, IsEGCG and EGCG, were selected based on their commendable microbial inhibition and oxidation stability characteristics(21, 27). Subsequent to independent blending with UHMWPE, the blends underwent surface crosslinking employing di-cumyl peroxide (DCP). The study encompassed an assessment of crosslink density, coefficient of friction, and wear rate of the crosslinked antioxidant-blended UHMWPE. Furthermore, an analysis of the crystalline structure and surface topography was conducted. MATERIALS AND METHODS Materials Ultra-high molecular weight polyethylene (UHMWPE) with an average molecular weight of ~5 × 10 6 g/mol, were used in this study. Vitamin also known as alpha tocopherol VE was used as commercial antioxidant, while the tea polyphenols such as lipid-soluble epigallocatechin gallate (IsEGCG) and epigallocatechin gallate (EGCG) were selected based on its higher antioxidation potential for UHMWPE as shown in our previous studies. Di-cumyl peroxide (DCP) was used as a chemical crosslinking reagent. The chemical structures for all the chemicals are given in Table 1. Preparation of UHMWPE blends Antioxidants were separately added into powder form of UHMWPE at a fraction of 0.2 wt% in a round bottom flask containing 120 ml isopropyl alcohol. The blends were dried in an oven for 7 days at a temperature of 60 ℃ under a vacuum environment. It was followed by consolidation of the blends in compression molding at 20 ℃ and 10 MPa. The compression cycles of 3 min were repeated for a total of 20 min at the same temperature and pressure. It is then cooled at room temperature for 10 min under the same pressure (10 MPa). Pucks of 108 mm in diameter and 10 mm in thickness were produced for each category of blends. The pucks were further machined to a smaller disc of 40 mm diameter and 4 mm thickness which is the required dimension for the pin-on-disc tribometer. Virgin UHMWPE (UH), VE blended (VE-UH), IsEGCG blended (IsEGCG-UH) and EGCG blended UHMWPE (EGCG-UH) were the four different categories of samples prepared for the comparative analysis. For simplicity, the UHMWPE is shortened to UH. Chemical crosslinking using peroxide DCP is an organic peroxide with a melting temperature of 38 ℃ is a solid and used for achieving the surface crosslink structure. The initial step was the diffusion of the peroxide by keeping the samples in the peroxide in a 200 ml round bottom flask in an oil bath. The doping temperatures were kept at 80 ℃ and a constant nitrogen flow was ensured throughout the process for 4 hrs. Decomposition of the peroxides in the samples is a subsequent process by heating the samples at 150 ℃ in an empty flask for 4 hrs in a nitrogen environment. The experimental parameters are the best possible values previously reported by Gul et.al(28). The surface crosslinking schematic diagram is depicted in Figure 1(a). Percent weight change before and after crosslinking The weight of the samples was measured before and after the diffusion process, and at the end of the decomposition process using a weighing scale. The weight change after the diffusion step was considered as the peroxide intake. Weight changes after the decomposition process imply the decomposition of the peroxide and the crosslinked structure developed in the polymer. Crosslink density measurement by gravimetric swelling method The surface crosslink density was measured by swelling a section ( n = 4, for each group; 1mm thickness sliced from the surface and 3 × 3 mm dimension) cut from the exterior part of the crosslinked samples. The cubes swelled in xylene at 130 ℃ and for 2h to attain equilibrium status. The gravimetric swell ratio was calculated from its initial weight and the xylene uptake, which were used to measure the crosslink density by using a previously reported process as shown in Equation (1)(29). where, and V 1 = 136 cm 3 /mol. Melting point and crystallinity analysis using differential scanning calorimetry Differential scanning calorimeters were employed to generate the first melting profile of un-crosslinked UHMWPE and its blends. Samples (~5 mg) were heated from 40 to 180 ℃ at a heating rate of 10 ℃/min while ensuring the nitrogen gas atmosphere. The percent crystallinity is measured by integrating the enthalpy peak from 40 to 160 ℃ and normalizing it with the enthalpy of fusion of 100 % crystalline polyethylene (291 J/g). Measurement of co-efficient of friction by pin-on-disc (POD) tribometer Pin-on-disc oscillating tribometer was used to study the friction and wear behavior of the polymer blends. Figure 1(b) illustrates the friction and wear testing of UHMWPE samples. The pin was consisting of an aluminum oxide ball (3 mm diameter), which was tested against the disc’s samples (4 mm thickness and 40 mm diameter, n = 4) in diluted bovine serum as a natural lubricant. A 10 N static load were exerted by the pin on the moving discs (60 mm/sec, 3448 cycles) at a 3 mm radius from its center for 3 hrs. Determination of wear rate by mechanical profilometer The wear rate was calculated from the wear track created by the pin-on-disc tribometer, using a mechanical profilometer. The samples were attached to the profilometer table and the stylus was positioned near the wear track, seen through the device camera by a visual user interface. The wear track curve was generated through a contact stylus to calculate the cross-sectional area (Figure 1(c)). Finally, the wear rate was calculated from the wear track curve. Surface characterization using scanning electron microscopy Scanning electron microscope was used to investigate the morphology of the wear track. Ultra-microtome was employed for sample preparation before scanning and the samples were enclosed in a thin metal film to analyze the surfaces using electron microscopy. Surface roughness measurement The samples were cleaned in an ultrasonic cleaner and carried out for surface roughness measurement through an Accretech mechanical profilometer. The profilometer having a stylus of 4 mm was positioned in the center to move over the sample’s surfaces at a speed of 0.5 mm/sec by taking 400 mm of evaluation length. Three scans were taken for each of the samples by changing the position after every scan. Statistical analysis Student’s t -test was performed as a statistical analysis method for the two-tailed distribution of unequal variance and assigning the significance to p 3). RESULTS Weight change of UHMWPE blends The percent weight gains due to DCP diffusion into the virgin UHMWPE, VE blended, IsEGCG blended and EGCG blended UHMWPE is shown in Figure 2(a). The virgin UHMWPE and its blends lost significant weight after the decomposition process. Virgin UHMWPE showed the highest weight gain at 21% compared to IsEGCG due to the absence of antioxidants, which exist in the rest of the samples. The percentage weight loss after the decomposition is highest in virgin UHMWPE is 63.9% while the lowest is observed in EGCG blended samples, which is 40.1% as shown in Figure 2(b). Crosslink density of UHMWPE blends The crosslinked density of virgin UHMWPE and its blends with 0.2 wt% antioxidants are shown in Figure 2(c). The crosslinked density of virgin UHMWPE is 305.4 mol/m 3 (as shown by the red dashed line Figure 2(c)). Interestingly, EGCG blended UHMWPE shows a crosslink density of 309.7 mol/m 3 , which is slightly higher than that of virgin UHMWPE. On the other hand, the addition of VE into the UHMWPE blends causes a massive reduction in the crosslinked density. The crosslinked density of VE blended UHMWPE is 254 mol/m 3 with a noticeable decrease of 17%. In comparison to virgin UHMWPE, the crosslinked density of tea polyphenols blended samples remained unchanged. The crosslinked density of EGCG and IsEGCG blended UHMWPE is 295 mol/m 3 and 309 mol/m 3 respectively that show a considerable difference compared with the VE blend, rather higher than virgin UHMWPE ( p < 0.05). It has been shown previously that crosslinking create a cage mechanism and meeting points contain peroxide molecule(30). The crosslinked structure is formed at the entanglement site due to the compactness between polymer chains. The crosslinked density increase observed in EGCG and IsEGCG blended UHMWPE is a result of higher entanglement density in tea polyphenols due to its potential for the removal of free radicals. Surface Roughness Un-crosslinked and virgin UHMWPE shows the lowest surface roughens value of 0.626 µm compared with tea polyphenols blended samples, which have the highest surface roughness values of 1.094 and 1.071 µm for IsEGCG and EGCG blended UHMWPE respectively (Figure 2(d)). The surface roughness value of VE/UHMWPE is 0.76 µm. However, no significant difference in the results of the crosslinked and un-crosslinked VE/UHMWPE samples was observed, which was comparable to the crosslinked virgin UHMWPE ( p < 0.05). Melting point and crystallinity of UHMWPE blends Melting point ( T m ) and crystallinity ( X c ) of the crosslinked and antioxidants blended samples were evaluated by the first heating profiles as shown in Figure 3(a). It is observed that the melting curve widens for virgin UHMWPE and IsEGCG/UHMWPE, while it is seen sharper in VE/UHMWPE and EGCG/UHMWPE. X c of virgin UHMWPE, 0.2 wt% VE blended, 0.2 wt% IsEGCG blended and 0.2 wt% EGCG blended UHMWPE are shown in Figure 3(b). The X c of crosslinked UHMWPE is 32% in the absence of antioxidants, which is seen to be higher among all the other samples. In contrast, X c of IsEGCG blended UHMWPE is 27%, which is the lowest. The T m 0f VE/UHMWPE and IsEGCG/UHMWPE were found to be nearly identical, at 149 ℃ and 148 ℃ respectively, as shown by the highest points on the line in Figure 3(c). There was no significant difference between the T m for each blend ( p >0.05) Friction behavior of UHMWPE blends The coefficient of friction (COF) values and wear rate were evaluated to characterize the tribological behavior of UHMWPE samples blended with VE, IsEGCG and EGCG. Figure 4(a) and (b) displays the friction coefficient change of the un-crosslinked and crosslinked group of samples respectively. The friction coefficient of uncross linked VE blended UHMWPE is 0.07, taken as the lowest reference among the whole samples. The friction coefficient of virgin UHMWPE, IsEGCG blended and EGCG blended are 0.08, 0.1 and 0.103 respectively before crosslinking comparing with crosslinked samples of friction coefficient 0.109, 0.074, 0.12 and 0.17 for virgin UHMWPE, VE blended, IsEGCG blende and EGCG blended UHMWPE ( p < 0.05). The highest values are observed for the tea polyphenols blended samples; especially EGCG/ UHMWPE friction coefficient is more than double compared with VE/UHMWPE. Wear of UHMWPE blends Figure 5 displays the wear volume of crosslinked and uncross linked specimens. The wear volume of un-crosslinked virgin UHMWPE, VE/UHMWPE, IsEGCG/UHMWPE and EGCG/UHMWPE have minute differences and vary around 0.123-0.13 mm 3 . The wear volume decreases significantly after the crosslinking and the result concentrates at 0.086-0.092 mm 3 ( p < 0.05). The percentage decrease in wear after the crosslinking process is indicated by the dashed line in Figure 5. The UHMWPE blended with IsEGCG exhibited the greatest reduction in wear, measuring 32.6%, while the blend with VE showed the lowest reduction in wear, measuring 29.4% (Figure 5). Scanning Electron Microscopy Figure 6 shows the SEM images of the wear track on virgin UHMWPE samples and antioxidants blended UHMWPE samples. Figure 6 (a, c, e, g) represents the images of un-crosslinked UHMWPE blends and Figure 6 (b, d, f, h) shows the crosslinked UHMWPE blends. A noticeable difference is observed comparing the crosslinked and uncross linked samples. The scratches and furrows on the virgin UHMWPE and VE-UHMWPE are more and very deep; this shows that the wear performance of the material is poor (Figure 6 (a, b, c, d)). DISCUSSION Antioxidants have been utilized in manufacturing industries for decades, particularly in polyolefins like polyethylene. However, their application as additives to UHMWPE joint implants began only in the 21st century(16, 31, 32). Vitamin E stands out as the most favored antioxidant in UHMWPE implants, blended into the polymer powder before solidification, and subsequently subjected to a crosslinking process for effective utilization(33, 34). Nonetheless, the presence of vitamin E as an antioxidant leads to the scavenging of free radicals, resulting in a reduction in the crosslink density of vitamin E-blended UHMWPE compared to virgin UHMWPE without antioxidants(35). The crosslink density directly influences the wear of UHMWPE, which causes the early failure of joint implants(36, 37). The development of an efficient system to optimize crosslink density, wear resistance, and oxidation stability is imperative for ensuring the longevity of UHMWPE joint implants. In this research, we have introduced surface-crosslinked and tea polyphenol-blended UHMWPE, significantly enhancing the tribological performance of the implant material. Chemical crosslinking is generally carried out by blending peroxides into the resin powder(38). Our proposed method of crosslinking used a peroxide with a high decomposition temperature in order to allow peroxide diffusion into the polymer. Based on the previous study by Gul et al., we selected DCP as organic peroxide with a 1h half-life temperature of 137 ℃(28). The diffusion temperature was set to 80 ℃ far below the decomposition of the selected peroxide. Weight gain observed after the decomposition is attributed to the existence of peroxide decomposition products (Figure 2(a)). Common decomposition products for di-cumyl peroxide are acetone, methane, and cumyl alcohol(39). The weight gain after doping is a result of peroxide charging into the polymer, while almost 50-65 % of the weight gain during diffusion was lost after decomposition due to the volatility of the peroxide (Figure 2(a)). The peroxide volatile nature after decomposition is essential for the ultimate use of chemical crosslinked UHMWPE as joint implant material(40). The surface crosslinked structure is accomplished and the results are consistent with the previous research conducted for the same group of samples(21). Peroxides create free radicals by abstracting hydrogen from the UHMWPE chain during the decomposition process, leading to a crosslinked structure(41). However, the mechanical strength is undesirably depreciated due to free radicals, which act as an initiator for oxidation embrittlement(12). VE has been frequently used for oxidation stability and is approved by FDA (Food and Drug Administration) for biomedical purposes. The oxidation mechanism of VE in irradiated UHMWPE involves the donation of hydrogen, which is then scavenged by the proton from VE to prevent the cascade reactions with oxygen and hinder the formation of free radicals(42-44). The free radicals are highly prone to react with an antioxidant before achieving the crosslinking structure due to the presence of only one phenol hydroxyl resulting in a reduced crosslink density. The previously reported work is verified by the loss of crosslink density with VE blended UHMWPE (Figure 2(c)). In contrast to VE, the crosslink density of IsEGCG/UHMWPE and EGCG/UHMWPE blends are increased, approximately equaling the virgin UHMWPE (Figure 4). It is suggested that tea polyphenols contain multiple phenolic groups that can detach many phenoxy radicals from dehydrogenated IsEGCG and EGCG, which participate in crosslinking along with peroxide, therefore, improving the crosslink density. The recently reported work by Fu et. al . and Shen et.al . also corroborated the results, who concluded that natural polyphenols, gallic acid and dodecyl gallate consisting of three hydroxyl groups individually, resulted in a higher oxidation resistance in crosslink UHMWPE compared to the VE/UHMWPE(45, 46). In both the EGCG and IsEGCG blended UHMWPE samples, the percent crystallinity is reduced, particularly IsEGCG/UHMWPE blend showed a large decrease, which may reflect the creation of a crosslink network structure in comparison to virgin UHMWPE (Figure 3(b)). The results agree with several previously reported studies(47-49), that explain how crystal growth is restrained due to further controlling the chain mobility because of increasing the crosslinked network offered by tea polyphenols blended samples. IsEGCG and EGCG are intended to scavenge surplus radicals that are not taking part in the cross-linking process. Similar to the commonly used antioxidant VE, It has the ability to stabilize residual radicals by providing protons(45). Interestingly, the melting curve of Virgin UHMWPE and IsEGCG/UHMWPE was observed to be wider compared to VE/UHMWPE and EGCG/UHMWPE, indicating a broad melting range for these polymers (Figure 3(a)). In contrast, VE/UHMWPE and EGCG/UHMWPE exhibited a sharper melting curve, suggesting a more well-defined melting behavior. It is also noteworthy that the melting temperature ( T m) of VE/UHMWPE and IsEGCG/UHMWPE were found to be nearly identical, at 149 ℃ and 148 ℃, respectively. This observation suggests that the addition of VE or IsEGCG to UHMWPE has a similar effect on the melting behavior of the resulting polymer. The coefficient of friction (COF) is notably low in the initial few seconds for each of the tested samples and then increase to an equilibrium value (Figure 4 (a, b)). In the beginning, the polymer has a smooth and lubricating surface, keeping the friction coefficient at a minimum. However, as the test continues the surface becomes rough, causing an increase in the friction coefficient until it reaches an equilibrium value(50, 51). A significant increase was observed in COF values of crosslinked samples relative to uncross linked samples (Figure 4 (a, b)). The direct correlation between the coefficient of friction and crosslink density has been reported in several research studies, which is comparable to our work. As the degree of crosslinking increases, the more tangled structure of the polymer fibrils leads to a decline in the polymer chains on the outer surface, playing a role in brush-mediated lubrication, which results in an increase in the coefficient of friction(52-54). The compact crosslinked structure inhibits the motion of the UHMWPE molecular chains, which causes a rise in the shear stress or coefficient of friction in the crosslinked UHMWPE samples. The VE/UHMWPE showed a minimum coefficient of friction compared with virgin UHMWPE and tea polyphenol blended UHMWPE. A negligible increase in the coefficient of friction occurs in VE blended UHMWPE after the crosslinking process, which may suggest the crosslinking structure is not quite effective. Contrary to that, EGCG blended UHMWPE has the highest coefficient of friction before and after crosslinking. The crosslinking achieved on the surface of IsEGCG and EGCG blended UHMWPE resulted in a wear volume of about 0.086 mm 3 (Figure 5). This wear volume of tea polyphenol is nearly comparable to the wear tests conducted on a similar blended sample (test conditions remain constant), for example, 0.12 mm 3 wear volume for 0.1 and 0.3 wt% EGCG blended UHMWPE(55). Crosslinking brought an apparent decrease in wear volume in four samples. The VE blended UHMWPE resulted in the highest wear volume of 0.13 and 0.092 mm 3 before and after crosslinking respectively, in agreement with the observance of lower crosslinking density in the VE blended UHMWPE. The wear volume of tea polyphenols blended UHMWPE was found to be nearly identical to that of virgin UHMWPE, a result that is consistent with the observed crosslinking density (Figure 2(c)). The wear results corroborate the direct relation between the crosslinking density and wear resistance, which is closely linked to the failure of joint implants(15, 56). The SEM image of wear scars for different samples shows scratches, furrows and lines along the wear track (Figure 6), which suggest that abrasive wear is the leading wear mechanism caused by loaded ceramic ball(57). The groves and scratches on the virgin UHMWPE, VE blended and EGCG blended UHMWPE are very wide and deep before the crosslinking process, expressing poor wear performance (Figure 6 (a, c, g)). A rough surface with flakes appears on the wear track of VE blended UHMWPE after the crosslinking process, this shows that the wear resistance of the blend is still low and adhesive wear mechanism developed (Figure 6 (d)). The number of flakes and scratches are significantly reduced by incorporating IsEGCG and EGCG into UHMWPE, and there are negligible wear particles found on the wear track of crosslinked IsEGCG blended UHMWPE, this shows that the wear performance of tea polyphenols blended UHMWPE is highly improved. CONCLUSIONS A comparative analysis was conducted to assess the effect of surface crosslinking on the overall wear performance of natural antioxidants (VE and tea polyphenol) stabilized UHMWPE blends. The blends underwent surface crosslinking via diffusion and decomposition using di-cumyl peroxide, followed by comprehensive testing to assess surface properties such as crosslink density, surface roughness, coefficient of friction, and wear performance. It is concluded that, in comparison to VE, tea polyphenol blended UHMWPE blends resulted in a significantly higher crosslinked structure leading to an increase in crosslink density and improved wear resistance. More interestingly, the tribological performance of tea polyphenol enhanced and resulted in a low amount of wear loss in comparison to VE. These results can be beneficial to researchers for developing a novel tea polyphenol stabilized UHMWPE blends with surface crosslinked structure for total joint replacements. Declarations ACKNOWLEDGEMENTS The authors greatly acknowledge the financial support of the Higher Education Commission Grant ((Ph-II-MG-5)/PAK-TURK/R&D/HEC/2018) and Pakistan Science Foundation/National Science Foundation China Grant (PSF/NSFC-II/ENG/KP-UET (02)). References (1) Schwartz, A. M., Farley, K. X., Guild, G. N. and Bradbury, T. L., Jr. (2020), “Projections and Epidemiology of Revision Hip and Knee Arthroplasty in the United States to 2030,” J Arthroplasty 35, 6S, pp. S79-S85. (2) Patil, N. A., Njuguna, J. and Kandasubramanian, B. (2020), “UHMWPE for biomedical applications: Performance and functionalization,” European Polymer Journal 125. (3) Hussain, M., Naqvi, R. A., Abbas, N., Khan, S. M., Nawaz, S., Hussain, A., Zahra, N. and Khalid, M. W. (2020), “Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) as a Promising Polymer Material for Biomedical Applications: A Concise Review,” Polymers (Basel) 12, 2. (4) Spece, H., Schachtner, J. T., MacDonald, D. W., Klein, G. 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(2021), “Ultrahigh molecular weight polyethylene with improved crosslink density, oxidation stability, and microbial inhibition by chemical crosslinking and tea polyphenols for total joint replacements,” Journal of Applied Polymer Science 138, 43, pp. 51261-51261. (22) Ren, Y., Zhang, Z. Y., Lan, R. T., Xu, L., Gao, Y., Zhao, B., Xu, J. Z., Gul, R. M. and Li, Z. M. (2019), “Enhanced oxidation stability of highly cross-linked ultrahigh molecular weight polyethylene by tea polyphenols for total joint implants,” Mater Sci Eng C Mater Biol Appl 94, pp. 211-219. (23) Ren, Y., Wei, X., Wei, S. T., Wang, F. Y., Wang, J., Xu, J. Z., Xu, L., Gul, R. M. and Li, Z. M. (2019), “High Oxidation Stability of Tea Polyphenol-stabilized Highly Crosslinked UHMWPE Under an in Vitro Aggressive Oxidative Condition,” Clin Orthop Relat Res 477, 8, pp. 1947-1955. (24) Wang, F. Y., Ren, Y., Lan, R. T., Fu, W. Q., Chen, Z. J., Huang, S., Gul, R. M., Wang, J., Xu, J. Z. and Li, Z. M. (2021), “Controlled bacteriostasis of tea polyphenol loaded ultrahigh molecular weight polyethylene with high crosslink density and oxidation resistance for total joint replacement,” Materials Science and Engineering: C 124, pp. 112040-112040. (25) Vidya, V., Anoop, C. A., Jinan, S., Anwar, R. and Senthil Saravanan, M. S. (2020), “Tribological analysis of cross linked UHMWPE used in artificial knee replacement,” Materials Today: Proceedings 27, pp. 2748-2751. (26) Mihalko, W. M., Haider, H., Kurtz, S., Marcolongo, M. and Urish, K. (2020), “New materials for hip and knee joint replacement: What's hip and what's in kneed?,” J Orthop Res 38, 7, pp. 1436-1444. (27) Wang, F.-Y., Ren, Y., Lan, R.-T., Fu, W.-Q., Chen, Z.-J., Huang, S., Gul, R. M., Wang, J., Xu, J.-Z. and Li, Z.-M. (2021), “Controlled bacteriostasis of tea polyphenol loaded ultrahigh molecular weight polyethylene with high crosslink density and oxidation resistance for total joint replacement,” Materials Science and Engineering: C 124. (28) Gul, R. M., Fung, K., Doshi, B. N., Oral, E. and Muratoglu, O. K. (2017), “Surface cross-linked UHMWPE using peroxides,” J Orthop Res 35, 11, pp. 2551-2556. (29) Muratoglu, O. K., Bragdon, C. R., O'Connor, D. O., Jasty, M., Harris, W. H., Gul, R. and McGarry, F. (1999), “Unified wear model for highly crosslinked ultra-high molecular weight polyethylenes (UHMWPE),” Biomaterials 20, 16, pp. 1463-1470. (30) Sambasivan, S., Fischer, D. A. and Hsu, S. M. (2007), “Effect of cross-linking ultrahigh molecular weight polyethylene: Surface molecular orientation and wear characteristics,” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 25, 4, pp. 932-937. (31) Kirschweng, B., Vörös, B., Arroussi, M., Tátraaljai, D., Zsuga, M. and Pukánszky, B. (2020), “Melt stabilization of polyethylene with natural antioxidants: comparison of a natural extract and its main component,” Journal of Thermal Analysis and Calorimetry. (32) Francenia Santos-Sánchez, N., Salas-Coronado, R., Villanueva-Cañongo, C. and Hernández-Carlos, B. (2019), “Antioxidant Compounds and Their Antioxidant Mechanism,” Editon Edition. (33) Bracco, P. and Oral, E. (2011), “Vitamin E-stabilized UHMWPE for total joint implants: a review,” Clin Orthop Relat Res 469, 8, pp. 2286-2293. (34) Lerf, R., Zurbrugg, D. and Delfosse, D. (2010), “Use of vitamin E to protect cross-linked UHMWPE from oxidation,” Biomaterials 31, 13, pp. 3643-3648. (35) Xu, J. Z., Muratoglu, O. K. and Oral, E. (2019), “Improved oxidation and wear resistance of ultrahigh molecular weight polyethylene using cross-linked powder reinforcement,” J Biomed Mater Res B Appl Biomater 107, 3, pp. 716-723. (36) Cheung, A., Yan, C. H., Fu, H., Cheung, M. H., Chan, P. K. and Chiu, K. Y. (2019), “Ten- to Sixteen-Year Follow-Up of Highly Cross-Linked Polyethylene in Total Hip Arthroplasty: What Factors Affect Wear?,” J Arthroplasty 34, 9, pp. 2016-2021. (37) Singh, G., Klassen, R., Howard, J., Naudie, D., Teeter, M. and Lanting, B. (2018), “Manufacturing, oxidation, mechanical properties and clinical performance of highly cross-linked polyethylene in total hip arthroplasty,” Hip Int 28, 6, pp. 573-583. (38) Shah, N. A., Lan, R. T., Dai, R., Jiang, K., Shen, H. Y., Hong, R., Xu, J. Z., Li, L. and Li, Z. M. (2022), “Improved oxidation stability and crosslink density of chemically crosslinked ultrahigh molecular weight polyethylene using the antioxidant synergy for artificial joints,” Journal of Biomedical Materials Research Part B: Applied Biomaterials 111, 1, pp. 26-37. (39) Arkema (2013), “Luperox dialkyl peroxides. Organic peroxides,” Editon Edition, Arkema, Inc, Philadelphia. (40) Tomoyuki, N. (2006), “Organic Peroxides as Crosslinking Agents,” Gomu Kyokaishi(Journal of the Society of Rubber Industry, Japan), 79.6, pp. 298-303. (41) Kayandan, S., Doshi, B. N., Oral, E. and Muratoglu, O. K. (2018), “Surface cross-linked ultra high molecular weight polyethylene by emulsified diffusion of dicumyl peroxide,” J Biomed Mater Res B Appl Biomater 106, 4, pp. 1517-1523. (42) Lan, R.-T., Ren, Y., Wei, X., Tang, L.-Z., Shah, N. A., Xu, L., Huang, S.-S., Gul, R. M., Xu, J.-Z. and Li, Z.-M. (2021), “Synergy between vitamin E and D-sorbitol in enhancing oxidation stability of highly crosslinked ultrahigh molecular weight polyethylene,” Acta Biomaterialia 134, pp. 302-312. (43) Chen, W., Bichara, D. A., Suhardi, J., Sheng, P. and Muratoglu, O. K. (2017), “Effects of vitamin E-diffused highly cross-linked UHMWPE particles on inflammation, apoptosis and immune response against S. aureus,” Biomaterials 143, pp. 46-56. (44) Niki, E. (2014), “Role of vitamin E as a lipid-soluble peroxyl radical scavenger: in vitro and in vivo evidence,” Free Radic Biol Med 66, pp. 3-12. (45) Shen, J., Gao, G., Liu, X. and Fu, J. (2015), “Natural polyphenols enhance stability of crosslinked UHMWPE for joint implants,” Clin Orthop Relat Res 473, 3, pp. 760-766. (46) Shen, J., Liu, X. and Fu, J. (2014), “Effect of squalene absorption on oxidative stability of highly crosslinked UHMWPE stabilized with natural polyphenols,” Polymer Degradation and Stability 110, pp. 113-120. (47) George, A., Ngo, H. D. and Bellare, A. (2014), “Influence of crystallization conditions on the tensile properties of radiation crosslinked, vitamin E stabilized UHMWPE,” J Mech Behav Biomed Mater 40, pp. 406-412. (48) Banche, G., Allizond, V., Bracco, P., Bistolfi, A., Boffano, M., Cimino, A., Brach del Prever, E. M. and Cuffini, A. M. (2014), “Interplay between surface properties of standard, vitamin E blended and oxidised ultra high molecular weight polyethylene used in total joint replacement and adhesion of Staphylococcus aureus and Escherichia coli,” Bone Joint J 96-B, 4, pp. 497-501. (49) Pruitt, L. A., Ansari, F., Kury, M., Mehdizah, A., Patten, E. W., Huddlestein, J., Mickelson, D., Chang, J., Hubert, K. and Ries, M. D. (2013), “Clinical trade-offs in cross-linked ultrahigh-molecular-weight polyethylene used in total joint arthroplasty,” J Biomed Mater Res B Appl Biomater 101, 3, pp. 476-484. (50) Kang, X., Zong, X., Zhang, P., Zeng, X., Liu, Y., Yao, C., Wang, T., Feng, P. and Yang, C. (2021), “Effects of epigallocatechin gallate incorporation in UHMWPE on biological behavior, oxidative degradation, mechanical and tribological performance for biomedical applications,” Tribology International 158, January, pp. 106887-106887. (51) Singh, M. K., Ilg, P., Espinosa-Marzal, R. M., Kröger, M. and Spencer, N. D. (2016), “Effect of Crosslinking on the Microtribological Behavior of Model Polymer Brushes,” Tribology Letters 63, 2, pp. 1-9. (52) Li, A. (2013), “Structure-Property Relationships of Surface-Grafted Polymeric Architectures: From Ultra-Thin Films To Quasi-3D Polymer Assemblies,” Editon Edition. (53) Li, A., Benetti, E. M., Tranchida, D., Clasohm, J. N., Schönherr, H. and Spencer, N. D. (2011), “Surface-grafted, covalently cross-linked hydrogel brushes with tunable interfacial and bulk properties,” Macromolecules 44, 13, pp. 5344-5351. (54) Chen, M., Briscoe, W. H., Armes, S. P. and Klein, J. (2009), “Lubrication at physiological pressures by polyzwitterionic brushes,” Science (New York, N.Y.) 323, 5922, pp. 1698-1701. (55) Xu, Y. Q., Gao, Y. and Granato, D. (2021), “Effects of epigallocatechin gallate, epigallocatechin and epicatechin gallate on the chemical and cell-based antioxidant activity, sensory properties, and cytotoxicity of a catechin-free model beverage,” Food Chem 339, pp. 128060. (56) Liu, T., Esposito, C. I., Burket, J. C. and Wright, T. M. (2017), “Crosslink Density Is Reduced and Oxidation Is Increased in Retrieved Highly Crosslinked Polyethylene TKA Tibial Inserts,” Clinical orthopaedics and related research 475, 1, pp. 128-136. (57) Gürgen, S., Çelik, O. N. and Kuşhan, M. C. (2019), “Tribological behavior of UHMWPE matrix composites reinforced with PTFE particles and aramid fibers,” Composites Part B: Engineering 173, pp. 106949-106949. Tables Table 1 is available in the supplementary files section. Additional Declarations The authors declare no competing interests. Supplementary Files Table1.docx Cite Share Download PDF Status: Posted Version 2 posted You are reading this latest preprint version Show more versions 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-4341120","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":299503011,"identity":"1cbfb759-9b15-4530-97d3-6391a8c71690","order_by":0,"name":"Adnan Ahmad","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Adnan","middleName":"","lastName":"Ahmad","suffix":""},{"id":299503012,"identity":"070f1a7f-33f9-4010-b5a6-22ed1f1e24c7","order_by":1,"name":"Nouman Ali Shah","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYPACGwYGZuYGGM+AGC1pQC2MpGk5DMTEajFn7zHd8HPH+Wj+dsYGxp9tdYkN7M3bJBhq7uDUYtlzxuxm75nbuTMOMzYw87YdTmzgOVYmwXDsGU4tBjdyzG7wtt3ObQBpYWw7kNggkWMmwdhwGK+Wm3/bzuXOPwxzmPwbwlpu87YdyN0A1MLA28YMtIWHgJYzx8puy7Yl524EajnMc+6wcRtPWrFFwjE8Wo43b7v5ts0ud975wwcf/iirk+1nP7zxxoca3FpQwAFGNgYGNhArgTgNIPCHeKWjYBSMglEwcgAApZdaUk69IlwAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-9301-7597","institution":"Sun Yat sen University","correspondingAuthor":true,"prefix":"","firstName":"Nouman","middleName":"Ali","lastName":"Shah","suffix":""},{"id":299503013,"identity":"33c2c255-a75b-4f53-9712-4f2cb2f7695d","order_by":2,"name":"Rizwan M. 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(b) Friction and wear testing setup by pin-on-disc tribometer. (c)\u003cstrong\u003e \u003c/strong\u003eCross section area of the wear track was measured by a profilometer\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/329d8ec82a711681904ce24b.png"},{"id":57024062,"identity":"fd3a39c1-abf2-4f5d-9993-cf3a1dc36711","added_by":"auto","created_at":"2024-05-23 14:30:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":162212,"visible":true,"origin":"","legend":"\u003cp\u003e(a)\u003cstrong\u003e \u003c/strong\u003ePercent weight gain after doping at 80 ℃ with DCP and decomposition at 150 ℃ for virgin, VE, IsEGCG and EGCG blended UHMWPE. (b) Comparison of percent difference in weight change between different UHMWPE blends. (c) Crosslink density of VE, IsEGCG and EGCG blended UHMWPE, the red dashed line shows the crosslink density of virgin UHMWPE blends. (d) Surface Roughness of different crosslinked and un-crosslinked UHMWPE blends. (*\u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05)\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/79be3d7986cff5b5e9138683.png"},{"id":57024061,"identity":"72b9965d-9b8e-4367-a40c-1c146a33793a","added_by":"auto","created_at":"2024-05-23 14:30:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":180506,"visible":true,"origin":"","legend":"\u003cp\u003e(a) DSC melting profiles of virgin, VE, IsEGCG and EGCG blended UHMWPE, (b) Crystallinity of virgin, VE, IsEGCG and EGCG blended UHMWPE. (* \u003cem\u003ep\u003c/em\u003e\u0026lt; 0.05) (c) Melting point of virgin, VE, IsEGCG and EGCG blended UHMWPE\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/5f6acee33bab1e4fa2cad9eb.png"},{"id":57024065,"identity":"04589383-3ccf-417e-b516-0ddd8eb6cf51","added_by":"auto","created_at":"2024-05-23 14:30:06","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":252586,"visible":true,"origin":"","legend":"\u003cp\u003eCoefficient of friction for virgin, VE blended, IsEGCG blended and EGCG blended UHMWPE, (a) Un-Crosslinked, and (b) surface crosslinked\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/2c39a04791b646f6b0edb8cc.png"},{"id":57024060,"identity":"6648f827-1bcb-466e-818e-3fc1f26921e4","added_by":"auto","created_at":"2024-05-23 14:30:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":128838,"visible":true,"origin":"","legend":"\u003cp\u003eWear volume of virgin UHMWPE, VE/ UHMWPE, IsEGCG/ UHMWPE and EGCG/ UHMWPE before and after the crosslinking process.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/8c31167caabe5817a851d091.png"},{"id":57024066,"identity":"67d4e42f-3bbc-4f07-8abb-87d3953f1f45","added_by":"auto","created_at":"2024-05-23 14:30:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":555314,"visible":true,"origin":"","legend":"\u003cp\u003eMicrograph of the wear track (a) un-crosslinked virgin UHMWPE (b) crosslinked virgin UHMWPE (c) un-crosslinked VE blended UHMWPE (d) crosslinked VE blended UHMWPE (e) un-crosslinked IsEGCG blended UHMWPE (f) crosslinked IsEGCG blended UHMWPE (g) un-crosslinked EGCG blended UHMWPE (h) crosslinked EGCG blended UHMWPE\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/dd7ef22600435e8d6c88a90b.png"},{"id":57025199,"identity":"a98257be-d709-4b16-8626-511895761f45","added_by":"auto","created_at":"2024-05-23 14:46:07","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1962723,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/ff7bf2ca-6681-479e-a072-4294848016eb.pdf"},{"id":57024660,"identity":"aa0f7831-f8c3-4a72-9f89-6d2083c13162","added_by":"auto","created_at":"2024-05-23 14:38:06","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":89469,"visible":true,"origin":"","legend":"","description":"","filename":"Table1.docx","url":"https://assets-eu.researchsquare.com/files/rs-4341120/v2/7fa2cc0ba75eb8b3dceff52d.docx"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eEffect of surface crosslinking on the Friction behavior of Tea Polyphenol stabilized Ultra-high molecular weight polyethylene\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eThe escalating number of patients with joint issues has underscored the need for researchers to develop high-performance biomaterials for total joint replacement(1). Ultra-high molecular weight polyethylene (UHMWPE), with additional modifications, has emerged as the preferred material for total hip and knee replacement procedures, delivering excellent outcomes over the past few decades(2, 3). A high-performance implant is characterized by three crucial factors: wear resistance, mechanical strength, and oxidation stability. The predominant cause of joint failure lies in the wear and tear of the polymer component, a consequence of continuous movement and contact, ultimately leading to joint loosening and osteolysis(4-6).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eCrosslinking has gained widespread acceptance as a strategy to enhance wear resistance by establishing a network structure that minimizes surface orientation and alters surface topography, thereby influencing the release of wear debris in UHMWPE(7, 8). Radiation crosslinking has exhibited promising results in reducing UHMWPE wear in artificial joints by creating a highly crosslinked structure through high-energy ionizing gamma radiation or electron beam exposure(9). However, radiation crosslinking causes oxidation embrittlement due to the formation of free radicals reacting with oxygen, leading to degradation in the mechanical performance of the implant(10, 11). The process of radiation crosslinking generates free radicals by breaking UHMWPE bonds. While free radicals in the amorphous phase transform into a crosslinked structure, those formed in the crystalline region significantly impact the long-term oxidation resistance capabilities of the polymer(12).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eChemical crosslinking has been employed for several decades to achieve a highly crosslinked structure using organic peroxides, but its oxidizing nature has impeded its application in clinical settings(13). Antioxidants are widely used to enhance the oxidation resistance of the crosslinked UHMWPE(14-16). Vitamin E (VE) is particularly favored as a natural antioxidant, offering a significant boost to oxidation resistance(17). VE donates hydrogen from the phenolic group to react with the trapped residual radicals playing a role in hindering chain scission(18). However, VE is also a free radical scavenger and it decreases the crosslinked density of the polymer, resulting in reducing the effectiveness of wear resistance of the crosslinked UHMWPE(19, 20). To mitigate this inhibitory effect on the crosslinked structure, an optimal VE content of approximately 0.2 wt% has been identified, as evaluated by Oral et al.(13). Therefore, the current research is focused to investigate an effective stabilizer along with the minimum adverse effect on the crosslinked structure.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eRecent research has delved into a more potent antioxidant, tea polyphenol, a natural substance found in tea, exhibiting superior resistance to oxidation compared to VE and thus considered a more favorable alternative stabilizer(21-23). Tea polyphenols, owing to their numerous phenolic hydroxyls, possess a greater capacity for hydrogen donation than VE, which comprises a single phenolic hydroxyl. Importantly, the incorporation of tea polyphenols does not impact the crosslinking process, unlike the decrease observed in VE-blended UHMWPE(22, 24). Further, the connections between friction, wear, and crystallinity are still not fully understood. Gaining insights into the influence of structure and crystallinity on friction and wear properties would be beneficial for the advancement of high-quality joint materials(25, 26).\u003c/p\u003e\n\u003cp\u003eThis investigation posits that the utilization of tea polyphenols as antioxidants, specifically lipid-soluble epigallocatechin gallate (IsEGCG) and epigallocatechin gallate (EGCG), instead of VE, can augment the wear resistance of UHMWPE. This substitution is anticipated to maintain an elevated crosslink density and result in heightened oxidation stability. The chosen polyphenols, IsEGCG and EGCG, were selected based on their commendable microbial inhibition and oxidation stability characteristics(21, 27). Subsequent to independent blending with UHMWPE, the blends underwent surface crosslinking employing di-cumyl peroxide (DCP). The study encompassed an assessment of crosslink density, coefficient of friction, and wear rate of the crosslinked antioxidant-blended UHMWPE. Furthermore, an analysis of the crystalline structure and surface topography was conducted.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cp\u003e\u003cstrong\u003eMaterials\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUltra-high molecular weight polyethylene (UHMWPE) with an average molecular weight of ~5 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e g/mol, were used in this study. Vitamin also known as alpha tocopherol VE was used as commercial antioxidant, while the tea polyphenols such as lipid-soluble epigallocatechin gallate (IsEGCG) and epigallocatechin gallate (EGCG) were selected based on its higher antioxidation potential for UHMWPE as shown in our previous studies. Di-cumyl peroxide (DCP) was used as a chemical crosslinking reagent. The chemical structures for all the chemicals are given in Table 1.\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePreparation of UHMWPE blends\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAntioxidants were separately added into powder form of UHMWPE at a fraction of 0.2 wt% in a round bottom flask containing 120 ml isopropyl alcohol. The blends were dried in an oven for 7 days at a temperature of 60 ℃ under a vacuum environment. It was followed by consolidation of the blends in compression molding at 20 ℃ and 10 MPa. The compression cycles of 3 min were repeated for a total of 20 min at the same temperature and pressure. It is then cooled at room temperature for 10 min under the same pressure (10 MPa). Pucks of 108 mm in diameter and 10 mm in thickness were produced for each category of blends. The pucks were further machined to a smaller disc of 40 mm diameter and 4 mm thickness which is the required dimension for the pin-on-disc tribometer. Virgin UHMWPE (UH), VE blended (VE-UH), IsEGCG blended (IsEGCG-UH) and EGCG blended UHMWPE (EGCG-UH) were the four different categories of samples prepared for the comparative analysis. For simplicity, the UHMWPE is shortened to UH.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eChemical crosslinking using peroxide\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDCP is an organic peroxide with a melting temperature of 38 ℃ is a solid and used for achieving the surface crosslink structure. The initial step was the diffusion of the peroxide by keeping the samples in the peroxide in a 200 ml round bottom flask in an oil bath. The doping temperatures were kept at 80 ℃ and a constant nitrogen flow was ensured throughout the process for 4 hrs. Decomposition of the peroxides in the samples is a subsequent process by heating the samples at 150 ℃ in an empty flask for 4 hrs in a nitrogen environment. The experimental parameters are the best possible values previously reported by Gul et.al(28). The surface crosslinking schematic diagram is depicted in Figure 1(a).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePercent weight change before and after crosslinking\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe weight of the samples was measured before and after the diffusion process, and at the end of the decomposition process using a weighing scale. The weight change after the diffusion step was considered as the peroxide intake. \u0026nbsp;Weight changes after the decomposition process imply the decomposition of the peroxide and the crosslinked structure developed in the polymer.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCrosslink density measurement by gravimetric swelling method\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe surface crosslink density was measured by swelling a section (\u003cem\u003en\u003c/em\u003e = 4, for each group; 1mm thickness sliced from the surface and 3 \u0026times; 3 mm dimension) cut from the exterior part of the crosslinked samples. The cubes swelled in xylene at 130 ℃ and for 2h to attain equilibrium status. The gravimetric swell ratio was calculated from its initial weight and the xylene uptake, which were used to measure the crosslink density by using a previously reported process as shown in Equation (1)(29).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/58854_b38fc7f3db2c487f/58854_custom_files/img171647418314.png\" width=\"238\" height=\"105\"\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003ewhere,\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" width=\"238\" height=\"105\"\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eand V\u003csub\u003e1\u0026nbsp;\u003c/sub\u003e= 136 cm\u003csup\u003e3\u003c/sup\u003e/mol.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMelting point and crystallinity analysis using differential scanning calorimetry\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDifferential scanning calorimeters were employed to generate the first melting profile of un-crosslinked UHMWPE and its blends. Samples (~5 mg) were heated from 40 to 180\u0026nbsp;℃ at a heating rate of\u0026nbsp;10\u0026nbsp;℃/min while ensuring the nitrogen gas atmosphere. The percent crystallinity is measured by integrating the enthalpy peak from 40 to 160\u0026nbsp;℃ and normalizing it with the enthalpy of fusion of 100 % crystalline polyethylene (291 J/g).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMeasurement of co-efficient of friction by pin-on-disc (POD) tribometer\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePin-on-disc oscillating tribometer was used to study the friction and wear behavior of the polymer blends. Figure 1(b) illustrates the friction and wear testing of UHMWPE samples. The pin was consisting of an aluminum oxide ball (3 mm diameter), which was tested against the disc\u0026rsquo;s samples (4 mm thickness and 40 mm diameter, \u003cem\u003en\u0026nbsp;\u003c/em\u003e= 4) in diluted bovine serum as a natural lubricant. A 10 N static load were exerted by the pin on the moving discs (60 mm/sec, 3448 cycles) at a 3 mm radius from its center for 3 hrs.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDetermination of wear rate by mechanical profilometer\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe wear rate was calculated from the wear track created by the pin-on-disc tribometer, using a mechanical profilometer. The samples were attached to the profilometer table and the stylus was positioned near the wear track, seen through the device camera by a visual user interface. The wear track curve was generated through a contact stylus to calculate the cross-sectional area (Figure 1(c)). Finally, the wear rate was calculated from the wear track curve.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSurface characterization using scanning electron microscopy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eScanning electron microscope was used to investigate the morphology of the wear track. Ultra-microtome was employed for sample preparation before scanning and the samples were enclosed in a thin metal film to analyze the surfaces using electron microscopy.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSurface roughness measurement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe samples were cleaned in an ultrasonic cleaner and carried out for surface roughness measurement through an Accretech mechanical profilometer. The profilometer having a stylus of 4 mm was positioned in the center to move over the sample\u0026rsquo;s surfaces at a speed of 0.5 mm/sec by taking 400 mm of evaluation length. Three scans were taken for each of the samples by changing the position after every scan.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eStatistical analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eStudent\u0026rsquo;s \u003cem\u003et\u003c/em\u003e-test was performed as a statistical analysis method for the two-tailed distribution of unequal variance and assigning the significance to \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05 (n \u0026gt;3).\u003c/p\u003e"},{"header":"RESULTS","content":"\u003cp\u003e\u003cstrong\u003eWeight change of UHMWPE blends\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe percent weight gains due to DCP diffusion into the virgin UHMWPE, VE blended, IsEGCG blended and EGCG blended UHMWPE is shown in Figure 2(a). The virgin UHMWPE and its blends lost significant weight after the decomposition process. Virgin UHMWPE showed the highest weight gain at 21% compared to IsEGCG due to the absence of antioxidants, which exist in the rest of the samples. The percentage weight loss after the decomposition is highest in virgin UHMWPE is 63.9% while the lowest is observed in EGCG blended samples, which is 40.1% as shown in Figure 2(b).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCrosslink density of UHMWPE blends\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe crosslinked density of virgin UHMWPE and its blends with 0.2 wt% antioxidants are shown in Figure 2(c). The crosslinked density of virgin UHMWPE is 305.4 mol/m\u003csup\u003e3\u0026nbsp;\u003c/sup\u003e(as shown by the red dashed line Figure 2(c)). Interestingly, EGCG blended UHMWPE shows a crosslink density of 309.7 mol/m\u003csup\u003e3\u003c/sup\u003e, which is slightly higher than that of virgin UHMWPE. On the other hand, the addition of VE into the UHMWPE blends causes a massive reduction in the crosslinked density. The crosslinked density of VE blended UHMWPE is 254 mol/m\u003csup\u003e3\u0026nbsp;\u003c/sup\u003ewith a noticeable decrease of 17%. In comparison to virgin UHMWPE, the crosslinked density of tea polyphenols blended samples remained unchanged. The crosslinked density of EGCG and IsEGCG blended UHMWPE is 295 mol/m\u003csup\u003e3\u003c/sup\u003e and 309 mol/m\u003csup\u003e3\u003c/sup\u003e respectively that show a considerable difference compared with the VE blend, rather higher than virgin UHMWPE (\u003cem\u003ep\u0026nbsp;\u003c/em\u003e\u0026lt; 0.05). It has been shown previously that crosslinking create a cage mechanism and meeting points contain peroxide molecule(30). The crosslinked structure is formed at the entanglement site due to the compactness between polymer chains. The crosslinked density increase observed in EGCG and IsEGCG blended UHMWPE is a result of higher entanglement density in tea polyphenols due to its potential for the removal of free radicals.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSurface Roughness\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eUn-crosslinked and virgin UHMWPE shows the lowest surface roughens value of 0.626 \u0026micro;m compared with tea polyphenols blended samples, which have the highest surface roughness values of 1.094 and 1.071 \u0026micro;m for IsEGCG and EGCG blended UHMWPE respectively (Figure 2(d)). The surface roughness value of VE/UHMWPE is 0.76 \u0026micro;m. However, no significant difference in the results of the crosslinked and un-crosslinked VE/UHMWPE samples was observed, which was comparable to the crosslinked virgin UHMWPE (\u003cem\u003ep\u0026nbsp;\u003c/em\u003e\u0026lt; 0.05).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMelting point and crystallinity of UHMWPE blends\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMelting point (\u003cem\u003eT\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e) and crystallinity\u0026nbsp;(\u003cem\u003eX\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e) of the crosslinked and antioxidants blended samples were evaluated by the first heating profiles as shown in Figure 3(a). It is observed that the melting curve widens for virgin UHMWPE and IsEGCG/UHMWPE, while it is seen sharper in VE/UHMWPE and EGCG/UHMWPE. \u003cem\u003eX\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e of virgin UHMWPE, 0.2 wt% VE blended, 0.2 wt% IsEGCG blended and 0.2 wt% EGCG blended UHMWPE are shown in Figure 3(b). The \u003cem\u003eX\u003c/em\u003e\u003csub\u003ec\u0026nbsp;\u003c/sub\u003eof crosslinked UHMWPE is 32% in the absence of antioxidants, which is seen to be higher among all the other samples. In contrast, \u003cem\u003eX\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e of IsEGCG blended UHMWPE is 27%, which is the lowest. The \u003cem\u003eT\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e 0f VE/UHMWPE and IsEGCG/UHMWPE were found to be nearly identical, at 149 ℃ and 148 ℃ respectively, as shown by the highest points on the line in Figure 3(c). There was no significant difference between the \u003cem\u003eT\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e for each blend (\u003cem\u003ep\u003c/em\u003e \u0026gt;0.05)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFriction behavior of UHMWPE blends\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe coefficient of friction (COF) values and wear rate were evaluated to characterize the tribological behavior of UHMWPE samples blended with VE, IsEGCG and EGCG. Figure 4(a) and (b) displays the friction coefficient change of the un-crosslinked and crosslinked group of samples respectively. The friction coefficient of uncross linked VE blended UHMWPE is 0.07, taken as the lowest reference among the whole samples. The friction coefficient of virgin UHMWPE, IsEGCG blended and EGCG blended are 0.08, 0.1 and 0.103 respectively before crosslinking comparing with crosslinked samples of friction coefficient 0.109, 0.074, 0.12 and 0.17 for virgin UHMWPE, VE blended, IsEGCG blende and EGCG blended UHMWPE (\u003cem\u003ep\u0026nbsp;\u003c/em\u003e\u0026lt; 0.05). The highest values are observed for the tea polyphenols blended samples; especially EGCG/ UHMWPE friction coefficient is more than double compared with VE/UHMWPE.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eWear of UHMWPE blends\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 5 displays the wear volume of crosslinked and uncross linked specimens. The wear volume of un-crosslinked virgin UHMWPE, VE/UHMWPE, IsEGCG/UHMWPE and EGCG/UHMWPE have minute differences and vary around 0.123-0.13 mm\u003csup\u003e3\u003c/sup\u003e. The wear volume decreases significantly after the crosslinking and the result concentrates at 0.086-0.092 mm\u003csup\u003e3\u003c/sup\u003e (\u003cem\u003ep\u0026nbsp;\u003c/em\u003e\u0026lt; 0.05). The percentage decrease in wear after the crosslinking process is indicated by the dashed line in Figure 5. The UHMWPE blended with IsEGCG exhibited the greatest reduction in wear, measuring 32.6%, while the blend with VE showed the lowest reduction in wear, measuring 29.4% (Figure 5).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eScanning Electron Microscopy\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 6 shows the SEM images of the wear track on virgin UHMWPE samples and antioxidants blended UHMWPE samples. Figure 6 (a, c, e, g) represents the images of un-crosslinked UHMWPE blends and Figure 6 (b, d, f, h) shows the crosslinked UHMWPE blends. A noticeable difference is observed comparing the crosslinked and uncross linked samples. The scratches and furrows on the virgin UHMWPE and VE-UHMWPE are more and very deep; this shows that the wear performance of the material is poor (Figure 6 (a, b, c, d)).\u003c/p\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eAntioxidants have been utilized in manufacturing industries for decades, particularly in polyolefins like polyethylene. However, their application as additives to UHMWPE joint implants began only in the 21st century(16, 31, 32). Vitamin E stands out as the most favored antioxidant in UHMWPE implants, blended into the polymer powder before solidification, and subsequently subjected to a crosslinking process for effective utilization(33, 34). Nonetheless, the presence of vitamin E as an antioxidant leads to the scavenging of free radicals, resulting in a reduction in the crosslink density of vitamin E-blended UHMWPE compared to virgin UHMWPE without antioxidants(35). The crosslink density directly influences the wear of UHMWPE, which causes the early failure of joint implants(36, 37). The development of an efficient system to optimize crosslink density, wear resistance, and oxidation stability is imperative for ensuring the longevity of UHMWPE joint implants. In this research, we have introduced surface-crosslinked and tea polyphenol-blended UHMWPE, significantly enhancing the tribological performance of the implant material.\u003c/p\u003e\n\u003cp\u003eChemical crosslinking is generally carried out by blending peroxides into the resin powder(38). Our proposed method of crosslinking used a peroxide with a high decomposition temperature in order to allow peroxide diffusion into the polymer.\u0026nbsp;Based on the previous study by Gul et al.,\u0026nbsp;we selected DCP as organic peroxide with a 1h half-life temperature of 137\u0026nbsp;℃(28). The diffusion temperature was set to 80 ℃ far below the decomposition of the selected peroxide. Weight gain observed after the decomposition is attributed to the existence of peroxide decomposition products (Figure 2(a)). Common decomposition products for di-cumyl peroxide are acetone, methane, and cumyl alcohol(39). The weight gain after doping is a result of peroxide charging into the polymer, while almost 50-65 % of the weight gain during diffusion was lost after decomposition due to the volatility of the peroxide (Figure 2(a)). The peroxide volatile nature after decomposition is essential for the ultimate use of chemical crosslinked UHMWPE as joint implant material(40). The surface crosslinked structure is accomplished and the results are consistent with the previous research conducted for the same group of samples(21).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePeroxides create free radicals by abstracting hydrogen from the UHMWPE chain during the decomposition process, leading to a crosslinked structure(41). However, the mechanical strength is undesirably depreciated due to free radicals, which act as an initiator for oxidation embrittlement(12). VE has been frequently used for oxidation stability and is approved by FDA (Food and Drug Administration) for biomedical purposes. The oxidation mechanism of VE in irradiated UHMWPE involves the donation of hydrogen, which is then scavenged by the proton from VE to prevent the cascade reactions with oxygen and hinder the formation of free radicals(42-44). The free radicals are highly prone to react with an antioxidant before achieving the crosslinking structure due to the presence of only one phenol hydroxyl resulting in a reduced crosslink density. The previously reported work is verified by the loss of crosslink density with VE blended UHMWPE (Figure 2(c)). In contrast to VE, the crosslink density of IsEGCG/UHMWPE and EGCG/UHMWPE blends are increased, approximately equaling the virgin UHMWPE (Figure 4). It is suggested that tea polyphenols contain multiple phenolic groups that can detach many phenoxy radicals from dehydrogenated IsEGCG and EGCG, which participate in crosslinking along with peroxide, therefore, improving the crosslink density. The recently reported work by Fu \u003cem\u003eet. al\u003c/em\u003e. and Shen \u003cem\u003eet.al\u003c/em\u003e. also corroborated the results, who concluded that natural polyphenols, gallic acid and dodecyl gallate consisting of three hydroxyl groups individually, resulted in a higher oxidation resistance in crosslink UHMWPE compared to the VE/UHMWPE(45, 46).\u003c/p\u003e\n\u003cp\u003eIn both the EGCG and IsEGCG blended UHMWPE samples, the percent crystallinity is reduced, particularly IsEGCG/UHMWPE blend showed a large decrease, which may reflect the creation of a crosslink network structure in comparison to virgin UHMWPE (Figure 3(b)). The results agree with several previously reported studies(47-49), that explain how crystal growth is restrained due to further controlling the chain mobility because of increasing the crosslinked network offered by tea polyphenols blended samples. IsEGCG and EGCG are intended to scavenge surplus radicals that are not taking part in the cross-linking process. Similar to the commonly used antioxidant VE, It has the ability to stabilize residual radicals by providing protons(45). Interestingly, the melting curve of Virgin UHMWPE and IsEGCG/UHMWPE was observed to be wider compared to VE/UHMWPE and EGCG/UHMWPE, indicating a broad melting range for these polymers (Figure 3(a)). In contrast, VE/UHMWPE and EGCG/UHMWPE exhibited a sharper melting curve, suggesting a more well-defined melting behavior.\u0026nbsp;It is also noteworthy that the melting temperature (\u003cem\u003eT\u003c/em\u003em) of VE/UHMWPE and IsEGCG/UHMWPE were found to be nearly identical, at 149 ℃ and 148 ℃, respectively. This observation suggests that the addition of VE or IsEGCG to UHMWPE has a similar effect on the melting behavior of the resulting polymer.\u003c/p\u003e\n\u003cp\u003eThe coefficient of friction (COF) is notably low in the initial few seconds for each of the tested samples and then increase to an equilibrium value (Figure 4 (a, b)). In the beginning, the polymer has a smooth and lubricating surface, keeping the friction coefficient at a minimum. However, as the test continues the surface becomes rough, causing an increase in the friction coefficient until it reaches an equilibrium value(50, 51).\u0026nbsp;A significant increase was observed in COF values of crosslinked samples relative to uncross linked samples (Figure 4 (a, b)). The direct correlation between the coefficient of friction and crosslink density has been reported in several research studies, which is comparable to our work. As the degree of crosslinking increases, the more tangled structure of the polymer fibrils leads to a decline in the polymer chains on the outer surface, playing a role in brush-mediated lubrication, which results in an increase in the coefficient of friction(52-54). The compact crosslinked structure inhibits the motion of the UHMWPE molecular chains, which causes a rise in the shear stress or coefficient of friction in the crosslinked UHMWPE samples. The VE/UHMWPE showed a minimum coefficient of friction compared with virgin UHMWPE and tea polyphenol blended UHMWPE. A negligible increase in the coefficient of friction occurs in VE blended UHMWPE after the crosslinking process, which may suggest the crosslinking structure is not quite effective. Contrary to that, EGCG blended UHMWPE has the highest coefficient of friction before and after crosslinking.\u003c/p\u003e\n\u003cp\u003eThe crosslinking achieved on the surface of IsEGCG and EGCG blended UHMWPE resulted in a wear volume of about 0.086 mm\u003csup\u003e3\u003c/sup\u003e (Figure 5). This wear volume of tea polyphenol is nearly comparable to the wear tests conducted on a similar blended sample (test conditions remain constant), for example, 0.12 mm\u003csup\u003e3\u003c/sup\u003e wear volume for 0.1 and 0.3 wt% EGCG blended UHMWPE(55). Crosslinking brought an apparent decrease in wear volume in four samples. The VE blended UHMWPE resulted in the highest wear volume of 0.13 and 0.092 mm\u003csup\u003e3\u003c/sup\u003e before and after crosslinking respectively, in agreement with the observance of lower crosslinking density in the VE blended UHMWPE. The wear volume of tea polyphenols blended UHMWPE was found to be nearly identical to that of virgin UHMWPE, a result that is consistent with the observed crosslinking density (Figure 2(c)). The wear results corroborate the direct relation between the crosslinking density and wear resistance, which is closely linked to the failure of joint implants(15, 56).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe SEM image of wear scars for different samples shows scratches, furrows and lines along the wear track (Figure 6), which suggest that abrasive wear is the leading wear mechanism caused by loaded ceramic ball(57). The groves and scratches on the virgin UHMWPE, VE blended and EGCG blended UHMWPE are very wide and deep before the crosslinking process, expressing poor wear performance (Figure 6 (a, c, g)). A rough surface with flakes appears on the wear track of VE blended UHMWPE after the crosslinking process, this shows that the wear resistance of the blend is still low and adhesive wear mechanism developed (Figure 6 (d)). The number of flakes and scratches are significantly reduced by incorporating IsEGCG and EGCG into UHMWPE, and there are negligible wear particles found on the wear track of crosslinked IsEGCG blended UHMWPE, this shows that the wear performance of tea polyphenols blended UHMWPE is highly improved.\u003c/p\u003e"},{"header":"CONCLUSIONS","content":"A comparative analysis was conducted to assess the effect of surface crosslinking on the overall wear performance of natural antioxidants (VE and tea polyphenol) stabilized UHMWPE blends. The blends underwent surface crosslinking via diffusion and decomposition using di-cumyl peroxide, followed by comprehensive testing to assess surface properties such as crosslink density, surface roughness, coefficient of friction, and wear performance. It is concluded that, in comparison to VE, tea polyphenol blended UHMWPE blends resulted in a significantly higher crosslinked structure leading to an increase in crosslink density and improved wear resistance. More interestingly, the tribological performance of tea polyphenol enhanced and resulted in a low amount of wear loss in comparison to VE. These results can be beneficial to researchers for developing a novel tea polyphenol stabilized UHMWPE blends with surface crosslinked structure for total joint replacements."},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eACKNOWLEDGEMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors greatly acknowledge the financial support of the Higher Education Commission Grant ((Ph-II-MG-5)/PAK-TURK/R\u0026amp;D/HEC/2018) and Pakistan Science Foundation/National Science Foundation China Grant (PSF/NSFC-II/ENG/KP-UET (02)).\u003c/p\u003e"},{"header":"References","content":"\u003cp\u003e(1)\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Schwartz, A. M., Farley, K. X., Guild, G. N. and Bradbury, T. 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(2019), \u0026ldquo;Tribological behavior of UHMWPE matrix composites reinforced with PTFE particles and aramid fibers,\u0026rdquo; Composites Part B: Engineering 173, pp. 106949-106949.\u003c/p\u003e"},{"header":"Tables","content":"Table 1 is available in the supplementary files section."}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"University of Engineering and Technology Peshawar","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"UHMWPE, antioxidants, surface crosslinking, tea polyphenols, wear, total joint replacement","lastPublishedDoi":"10.21203/rs.3.rs-4341120/v2","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4341120/v2","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe use of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) incorporating vitamin E (VE) as an antioxidant is widely acknowledged in the field of joint implants. While VE plays a crucial role in preventing oxidation, concerns have been raised regarding its impact on crosslink density, leading to wear performance deterioration. This study proposes the hypothesis that tea polyphenols, specifically lipid-soluble epigallocatechin gallate (IsEGCG) and epigallocatechin gallate (EGCG), may counteract the typical decrease in crosslink density caused by VE, thereby enhancing wear performance. The antioxidants were integrated into UHMWPE at a concentration of 0.2 wt%, followed by surface chemical crosslinking using di-cumyl peroxide. Surface properties, including crosslink density, roughness, coefficient of friction, and wear performance, were comprehensively evaluated. The results indicate a significantly higher crosslink density in UHMWPE blended with IsEGCG and EGCG compared to VE-stabilized UHMWPE, which exhibited a 17% reduction compared to virgin UHMWPE. The coefficient of friction increased post-crosslinking, with tea polyphenol-blended UHMWPE demonstrating a relatively higher value, confirming a highly crosslinked network structure. The wear resistance of surface-crosslinked UHMWPE stabilized with tea polyphenols was markedly superior compared to UHMWPE stabilized with VE. Additionally, a substantial presence of scratches, furrows, and flakes was observed on the surface of VE-stabilized UHMWPE in contrast to tea polyphenol-stabilized UHMWPE. These findings suggest that tea polyphenols present promising alternatives to VE for enhancing the overall performance and longevity of UHMWPE-based implants.\u003c/p\u003e","manuscriptTitle":"Effect of surface crosslinking on the Friction behavior of Tea Polyphenol stabilized Ultra-high molecular weight polyethylene","msid":"","msnumber":"","nonDraftVersions":[{"code":2,"date":"2024-05-23 14:30:01","doi":"10.21203/rs.3.rs-4341120/v2","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}},{"code":1,"date":"2024-05-07 04:24:26","doi":"10.21203/rs.3.rs-4341120/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"74efc5eb-5a3d-4422-80e2-f2633f276f61","owner":[],"postedDate":"May 23rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":32318005,"name":"Polymer Science"},{"id":32318006,"name":"Biopolymers"},{"id":32318007,"name":"Biomaterials"}],"tags":[],"updatedAt":"2025-01-05T04:31:40+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-23 14:30:01","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v2","identity":"rs-4341120","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4341120","identity":"rs-4341120","version":["v2"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}