EFFECT OF SURFACE MODIFICATION ON LONG-TERM MECHANICAL AND TRIBOLOGICAL PERFORMANCE OF BIOMEDICAL POLYMERS

Abstract

Long-term mechanical durability of biomedical polymers is essential for their effectiveness in hip and knee joint replacement prostheses, where materials experience various stress situations in the presence of physiological fluids. However, prolonged exposure to physiological fluids can accelerate moisture absorption, leading to mechanical degradation of the polymers. This can significantly reduce the lifespan of polymers used in various biomedical applications, such as total joint replacement prostheses applications. Surface modification can improve hydrophobicity, which can delay moisture absorption and enhance the resistance to mechanical degradation. Among the various surface modification techniques, surface texturing has drawn significant attention as it does not alter the chemical properties of the material. This study investigates the role of surface texturing in improving the long-term mechanical and tribological performance of Ultrahigh Molecular Weight Polyethylene (UHMWPE) and High-Density Polyethylene (HDPE). UHMWPE and HDPE samples with square, cylindrical, and hemispherical micro-protrusions and microcavities were submerged in deionized (DI) water and compared against the mechanical properties of flat samples. Scratch and wear testing was done on flat and textured polymer samples under dry and boundary lubrication scenarios to investigate the effect of surface modification on tribological behavior of polymers. Relationships between long-term mechanical behavior and tribological performance under dry and boundary lubrication scenarios were sought. The findings highlight the crucial role of surface texturing in preserving mechanical and tribological performance under physiological conditions.

1. INTRODUCTION

Long-term mechanical durability of biomedical polymers is critical to the effectiveness of joint replacement prostheses, such as hip and knee implants. These polymers endure various stress situations while being in contact with physiological fluids, which can accelerate moisture absorption and result in mechanical degradation. This degradation process significantly diminishes the operational lifespan of materials like ultrahigh molecular weight polyethylene (UHMWPE) and high-density polyethylene (HDPE), commonly known for their low friction, high wear resistance, and excellent biocompatibility. Moisture absorption is a major issue when long-term durability is needed under moisture environments. Consequently, strategies aimed at improving the mechanical durability of these materials are of major significance for biomedical applications.

Surface modification is an effective approach to enhance the moisture resistance of UHMWPE and HDPE, thus prolonging their mechanical performance under physiological conditions. Among the various surface modification techniques, surface texturing has garnered significant attention due to its ability to modify physical properties without altering the underlying chemical composition of the material. Previous studies have shown that micro-scale surface textures can positively influence material-fluid interactions, thereby affecting hydrophobic properties and moisture absorption rates [1,2].

Research has indicated that specific factors such as the length or diameter of surface textures, the spacing between them, and the type of texture must be carefully considered to achieve optimal results in terms of hydrophobicity, moisture resistance and long-term mechanical performance [1,2]. Textured surfaces can create air pockets that reduce the contact area with moisture, thus delaying the absorption of water compared to flat surfaces. This phenomenon has been particularly evident when textures were incorporated into the surfaces of UHMWPE and HDPE samples submerged in deionized water [2]. The experimentation demonstrated promising results, confirming the hypothesis that surface texturing effectively mitigates the moisture absorption over time.

Tribological performance is another essential aspect of mechanical durability. The interaction between textured surfaces and lubricating fluids in a moist environment has considerable impact on wear resistance and friction coefficients under different lubrication scenarios. Scratch testing of flat versus textured polymer samples revealed that, in addition to improving hydrophobicity and moisture resistance, textured surfaces demonstrated a marked decrease in wear under boundary lubrication conditions [3]. This finding highlights the multifaceted benefits of employing surface modification techniques, with significant implications for enhancing the operational lifespan of joint replacement prostheses in vivo.

Moreover, tribological performance of biomedical polymers has been enhanced through the incorporation of lubricants that promote boundary lubrication [4,5]. The interaction between modified surfaces and lubricants such as lubricin or polyethylene glycol (PEG) improves the reduction of friction and wear, fostering a favorable lubrication regime that further limits mechanical degradation due to moisture exposure. In light of these promising results, surface modification via texturing not only improves moisture resistance but also addresses tribological challenges that arise in humid environments.

Building on our previous studies [1,2,6], where we established the relationship between surface texture geometry and hydrophobicity in HDPE and UHMWPE, and subsequently linked the improvements in hydrophobicity to reduced moisture absorption and improved long-term mechanical performance; the present work extends the investigation to tribological behavior. Specifically, this study evaluates how flat and textured surfaces perform under dry and boundary-lubricated conditions using scratch testing to simulate real-world contact scenarios relevant to hip and knee implants. By incorporating frictional analysis into our existing framework, this study aims to provide a more complete understanding of how surface texturing may influence not only durability but also contact mechanics under physiological conditions.

2. EXPERIMENTAL

The polymer materials, supplied by SABIC, included UHMWPE powder (GUR 4120, molecular weight: 4.7 × 10⁶ g/mol) and HDPE pellets (melt flow index: 0.3 dg/min).

To fabricate the textured polymer samples, an aluminum mold was engraved using a TYKMA LaserGear BOQX laser engraver by adjusting processing parameters such as loop count, power settings, engraving speed, and frequency. The engraved aluminum mold was coated with a silicone mold release agent and allowed to dry for 10 minutes. The mold was then preheated in an oven at 350°F to match the processing temperature. Two Teflon-coated, smooth aluminum plates were used to sandwich the engraved mold and polymer material (powder for UHMWPE, and pellets for HDPE), applying the necessary heat and pressure during hot pressing. Processing conditions followed ISO 11542–2 for UHMWPE [7] and ISO 17855–2 for HDPE [8], ensuring compliance with standardized protocols. A set of model polymer samples with protrusion and cavity textures on the surface, with texture length/diameter (L) and spacing (S) ranging from 50–400 μm for square and cylinder textures and from 100–400 μm for hemispherical textures, were prepared following the procedure. The texture dimension range was chosen based on the manufacturability of the laser engraver and our prior research on effects of surface textures on polymer hydrophobicity and moisture resistance [1,2]. Consistent with prior research, a texture height/depth of 100 μm was maintained across all samples to ensure uniformity in surface characteristics. An Olympus DSX510 digital microscope was used to obtain detailed surface information of both smooth and textured surfaces. Uniaxial tension test was performed on both dry and environmental conditioned samples, submerged in deionized (DI) water for various timeframe, following the ASTM D638 standard [9] and using an MTS universal testing machine. Detailed methodologies for the surface texturing process, digital microscopy, texture characterization, and mechanical behavior are available in our previous studies [1,2,6].

To evaluate the tribological behavior of flat and textured HDPE and UHMWPE samples, scratch testing was performed according to the ASTM D7027 standard [10] using a scratch testing machine from Surface Machine Systems. The tests employed two spherical tip sizes with diameters of 1 mm and 10 mm. Scratches were applied with a linearly increasing load of 1–30 N over a 60 mm scratch length, at a constant speed of 100 mm/s. In addition to dry testing, boundary lubrication conditions were implemented to evaluate the tribological behavior of flat and textured polymer samples under simulated fluid exposure. A thin layer of deionized (DI) water was used as the lubricating medium, chosen to simulate physiological moisture exposure common in biomedical environments while eliminating the influence of impurities during the experiments. However, it should be noted that DI water may not fully replicate the complex composition of physiological fluids, which may include electrolytes, proteins, and other biomolecules that could affect frictional behavior. For each 76.2 mm × 76.2 mm sample, approximately 290 μL of DI water was dispensed using a precision micropipette to create a uniform 50 μm thick lubrication layer. To maintain constant thickness throughout the experiment, a custom fixture was developed to hold the liquid in place. This volume was calculated based on the sample area and desired film thickness. This approach enabled a direct comparison of tribological behavior under dry and boundary lubrication conditions, assessing the effect of surface textures on frictional performance in moisture-exposed environments. At least 3 scratch tests were conducted for each system in each condition. Scratch depth was analyzed using Olympus DSX510 microscope where the depth was measured with respect to the base surface.

3. RESULTS AND DISCUSSION

Uniaxial tension tests of HDPE and UHMWPE flat and textured samples under prolonged exposure to deionized (DI) water reveal that micro-textures on the surface can improve resistance to moisture-induced degradation. After 30 and 90 days exposure to DI water, yield strength and failure strain of flat HDPE samples deteriorate progressively compared to textured samples. For UHMWPE, yield strength and failure strain of flat samples reduces compared to that of the textured samples after 30 days exposure to DI water. However, when UHMWPE samples are submerged in DI water for 90 days, both flat and textured samples show similar deterioration in yield strength and failure strain. Thus, surface micro-texturing significantly enhances the long-term mechanical durability of UHMWPE and HDPE when exposed to DI water. This long-term mechanical durability can be attributed to delayed moisture absorption enabled by improved hydrophobicity in the micro-textured surfaces, which was previously confirmed through contact angle measurements [1]. Gravimetric analysis shows that textured surfaces exhibited slower moisture absorption. Detailed analysis of the results can be found in literature [6], which summarizes our findings on effects of surface micro-texture on moisture absorption and long-term mechanical durability of polymers. Based on our previous work [1,6,11], the surface texture geometries studied in this research are as follows: for HDPE, square protrusion (L=50 μm, S=150 μm), square cavity (L=50 μm, S=50 μm), cylinder protrusion (L=50 μm, S=50 μm), cylinder cavity (L=50 μm, S=50 μm), hemispherical protrusion (L=100 μm, S=100 μm), hemispherical cavity (L=100 μm, S=100 μm); and for UHMWPE, square protrusion (L=50 μm, S=50 μm), square cavity (L=50 μm, S=50 μm), cylinder protrusion (L=50 μm, S=50 μm), cylinder cavity (L=50 μm, S=150 μm), hemispherical protrusion (L=100 μm, S=100 μm), hemispherical cavity (L=100 μm, S=100 μm). The specific geometries used in this study were selected based on their optimal hydrophobic performance as identified in [1,6,11]. This dual focus on long-term mechanical integrity and tribological behavior aims to provide a comprehensive understanding of how surface modification enhances polymer performance in simulated physiological conditions.

Figure 1(a–d) illustrates the relationship between the scratch coefficient of friction $(\text{SCoF})(=\frac{\mathit{\text{Tangential}}\mathit{\text{Load}}}{\mathit{\text{Normal}}\mathit{\text{Load}}})$ and normal load for flat and textured HDPE samples tested using 1 mm diameter tip under dry and boundary lubrication conditions. In the initial load range, significant scatter in SCoF values is observed, likely due to inertia effect. As the load increases beyond this range, the SCoF begins to follow a more stable, increasing trend. This increase is primarily attributed to progressive material deformation, where the scratch tip encounters greater resistance in front of the tip as it induces plastic deformation in the polymer. The resulting frictional response reflects the material’s resistance to tip penetration and lateral movement under load.

FIGURE 1:

SCRATCH COEFFICIENT OF FRICTION (SCOF) OF HDPE UNDER VARYING NORMAL LOAD USING 1 MM TIP, COMPARING DRY AND BOUNDARY LUBRICATION CONDITION: (A) FLAT; (B) SQUARE TEXTURE; (C) CYLINDRICAL TEXTURE; AND (D) HEMISPHERICAL TEXTURE (SC: SQUARE CAVITY; SP: SQUARE PROTRUSION; CC: CYLINDER CAVITY; CP: CYLINDER PROTRUSION; HC: HEMISPHERICAL CAVITY; HP: HEMISPHERICAL PROTRUSION).

The similarity in SCoF trends between flat and all textured surfaces (square, cylinder and hemispherical protrusion and cavities), and between dry and lubricated conditions, indicates that neither surface texturing nor the presence of a thin lubrication layer significantly alters the scratch behavior of HDPE in this testing configuration. These results suggest that the dominant factor influencing frictional performance during scratching with 1 mm ball tip is the bulk deformation behavior of the polymer, with surface features and lubrication contributing only marginal effects under increasing load.

Figure 2(a–d) presents the scratch coefficient of friction (SCoF) behavior of UHMWPE samples, both flat and textured, tested using 1 mm diameter tip under dry and boundary lubrication conditions. Similar to the trend observed for HDPE, SCoF exhibits an increasing pattern with respect to normal load. At lower loads, the ScoF values display noticeable scatter, which can be attributed to the inertia effect. As the normal load increases along the scratch length, a more uniform and narrower SCoF distribution emerges. This linear increase is indicative of enhanced contact conformity and greater material deformation, which introduces consistent resistance to the moving tip. The narrowing scatter at higher loads further supports the presence of stable material engagement and potential development of a steady-state friction regime. This behavior is consistently observed across all surface conditions flat and textured and under both dry and boundary lubrication conditions. The lack of significant divergence among different textures suggests that the specific surface features introduced do not markedly influence the scratch resistance of UHMWPE at the tested loading regime. Instead, material bulk properties and load-induced deformation mechanisms dominate the tribological response under 1 mm tip.

FIGURE 2:

SCRATCH COEFFICIENT OF FRICTION (SCOF) OF UHMWPE UNDER VARYING NORMAL LOAD USING 1 MM TIP, COMPARING DRY AND BOUNDARY LUBRICATION CONDITION: (A) FLAT; (B) SQUARE TEXTURE; (C) CYLINDRICAL TEXTURE; AND (D) HEMISPHERICAL TEXTURE (SC: SQUARE CAVITY; SP: SQUARE PROTRUSION; CC: CYLINDER CAVITY; CP: CYLINDER PROTRUSION; HC: HEMISPHERICAL CAVITY; HP: HEMISPHERICAL PROTRUSION).

Figure 3(a–d) presents the scratch coefficient of friction (SCoF) behavior of HDPE samples, flat and textured, tested under dry and boundary lubrication conditions using a 10 mm hemispherical tip. In contrast to previous results obtained with a 1 mm tip, the SCoF profiles here do not exhibit a clear increasing trend with respect to normal load. Instead, after minor fluctuations at the initial stages of loading, the SCoF stabilizes along the scratch length and remains relatively constant. This flattening of the friction profile is likely due to the large contact area induced by the 10 mm tip, which facilitates more uniform load distribution along the scratch path. As a result, material deformation beneath and in front of the tip is less and fairly constant, which reduces material resistance in front of the tip, and, thus, reducing the SCoF. Across all textures, square, cylindrical and hemispherical protrusions and cavities, no substantial difference in SCoF is observed.

FIGURE 3:

SCRATCH COEFFICIENT OF FRICTION (SCOF) OF HDPE UNDER VARYING NORMAL LOAD USING 10 MM TIP, COMPARING DRY AND BOUNDARY LUBRICATION CONDITION: (A) FLAT; (B) SQUARE TEXTURE; (C) CYLINDRICAL TEXTURE; AND (D) HEMISPHERICAL TEXTURE (SC: SQUARE CAVITY; SP: SQUARE PROTRUSION; CC: CYLINDER CAVITY; CP: CYLINDER PROTRUSION; HC: HEMISPHERICAL CAVITY; HP: HEMISPHERICAL PROTRUSION).

Figure 4 shows the average SCoF for flat and textured HDPE samples tested with a 10 mm diameter tip under dry and boundary lubrication conditions. Since the SCoFs are fairly constant, as shown in Figure 3, the average SCoF is calculated within the stable load range of 10–25 N. As can be seen in the figure, dry testing shows a higher average SCoF compared to the boundary lubrication scenario. As the 10 mm tip has larger diameter that induces minimal material deformation, this result indicates frictional behavior due to surface interaction only. As such, the lubrication reduces the average SCoF for all the samples in this case. Among all the samples under boundary lubrication scenario, square protrusion and cavity systems seem to show higher lubrication effect, with the lowest average SCoF.

FIGURE 4:

AVERAGE SCRATCH COEFFICIENT OF FRICTION (SCOF) OF HDPE FOR 10 MM TIP, COMPARING DRY AND BOUNDARY LUBRICATION CONDITION FOR FLAT AND VARIOUS TEXTURE GEOMETRIES (SP: SQUARE PROTRUSION; SC: SQUARE CAVITY; CP: CYLINDER PROTRUSION; CC: CYLINDER CAVITY; HP: HEMISPHERICAL PROTRUSION; HC: HEMISPHERICAL CAVITY).

Figure 5(a–d) presents the scratch coefficient of friction (SCoF) behavior of UHMWPE samples, both flat and textured, tested under dry and boundary lubrication conditions using a 10 mm hemispherical tip. Similar to the behavior observed in HDPE, as shown in Figure 3, the SCoF profiles of UHMWPE do not show a pronounced increasing trend with normal load. Instead, after minor variations during initial loading, the SCoF stabilizes and remains nearly constant along the scratch path. This can be attributed to the larger contact area created by the 10 mm tip, which leads to a more uniform stress distribution and reduced localized material deformation in the contact zone. Consequently, the plastic deformation is lower and more consistent compared to that of 1 mm tip. Among all textured configurations, square, cylindrical, and hemispherical protrusions and cavities, no significant variation in SCoF is observed under either dry or boundary lubrication conditions.

FIGURE 5:

SCRATCH COEFFICIENT OF FRICTION (SCOF) OF UHMWPE UNDER VARYING NORMAL LOAD USING 10 MM TIP, COMPARING DRY AND BOUNDARY LUBRICATION CONDITION: (A) FLAT; (B) SQUARE TEXTURE; (C) CYLINDRICAL TEXTURE; AND (D) HEMISPHERICAL TEXTURE (SC: SQUARE CAVITY; SP: SQUARE PROTRUSION; CC: CYLINDER CAVITY; CP: CYLINDER PROTRUSION; HC: HEMISPHERICAL CAVITY; HP: HEMISPHERICAL PROTRUSION).

Figure 6 shows the average SCoF for flat and textured UHMWPE samples tested with a 10 mm diameter tip under dry and boundary lubrication conditions. Similar to HDPE, since the SCoFs are almost constant, the average of SCoF is calculated within 10–25N stable load range. The results show that cylindrical protrusion and cavity samples show somewhat higher SCoF in the dry state, while under wet conditions, all samples show a general reduction in average SCoF. Given the larger diameter of the 10 mm tip, which induces minimal material deformation, the resulting frictional behavior is primarily governed by surface interactions.

FIGURE 6:

AVERAGE SCRATCH COEFFICIENT OF FRICTION (SCOF) OF UHMWPE FOR 10 MM TIP, COMPARING DRY AND BOUNDARY LUBRICATION CONDITION FOR FLAT AND VARIOUS TEXTURE GEOMETRIES (SP: SQUARE PROTRUSION; SC: SQUARE CAVITY; CP: CYLINDER PROTRUSION; CC: CYLINDER CAVITY; HP: HEMISPHERICAL PROTRUSION; HC: HEMISPHERICAL CAVITY).

Figure 7(a–f) shows digital microscopy images of scratch grooves on flat, and protrusion and cavity textured samples, for 1 mm ball tip (a-c) and 10 mm ball tip (d-f), respectively. With 1 mm diameter tip, the flat sample exhibits a continuous and uniform scratch path, while the protrusion-textured surface shows slight disturbances along the groove edges due to surface features. In the cavity-textured sample, interruptions in the scratch track are observed where the tip passes over recessed regions. However, for 10 mm diameter tip, the scratching process barely creates a scratch groove due to the distributed load over a larger surface area, making the scratch depth negligible compared to 1 mm tip. These images qualitatively illustrate how surface geometry influences local deformation without significantly altering overall scratch penetration. The 3D height images are used to measure scratch depth for 1 mm diameter tip from the base surface using microscope analysis software.

FIGURE 7:

MICROSCOPIC IMAGES SHOWING SCRATCH GROOVE ON HDPE SAMPLES: SCRATCHES GENERATED USING 1 MM TIP ON: (A) FLAT SURFACE, (B) PROTRUSION TEXTURE, AND (C) CAVITY TEXTURE; SCRATCHES GENERATED USING 10 MM TIP ON: (D) FLAT SURFACE, (E) PROTRUSION TEXTURE, AND (F) CAVITY TEXTURE (TOP: OPTICAL IMAGE, BOTTOM: HEIGHT IMAGE).

Figure 8 shows the variation in scratch depth with increasing normal load for HDPE samples tested under dry and boundary lubrication conditions, respectively, using 1 mm diameter tip. Across all test configurations, scratch depth increases with normal load, indicating that plastic deformation is the dominant mechanism regardless of surface texture and lubrication. This can be related to the increase in SCoF values with normal load as seen in Figure 1(a–d). In both dry (Figure 8a) and lubricated (Figure 8b) conditions, with 1 mm tip, the textured surfaces follow similar scratch depth trends compared to the flat sample, indicating that the texture type and geometry and lubrication condition have negligible effect on scratch behavior for 1 mm diameter tip.

FIGURE 8:

CHANGE IN SCRATCH DEPTH WITH NORMAL LOAD FOR HDPE TESTED USING 1 MM TIP UNDER: (A) DRY; AND (B) BOUNDARY LUBRICATION CONDITIONS (CC: CYLINDER CAVITY; CP: CYLINDER PROTRUSION; SC: SQUARE CAVITY; SP: SQUARE PROTRUSION; HC: HEMISPHERICAL CAVITY; HP: HEMISPHERICAL PROTRUSION).

Figure 9 presents the relationship between normal load and scratch depth for UHMWPE samples tested under dry and boundary lubrication conditions using 1 mm diameter tip. Across all samples, scratch depth increases with applied normal load, demonstrating a consistent plastic deformation response. This plastic deformation can be considered as confirmation for increasing SCoF with respect to normal load seen in Figure 2(a–d). In the 1 mm tip tests, seen in Figure 9, both dry (Figure 9a) and lubricated (Figure 9b) conditions show overlapping trends across flat and textured surfaces, with only minor differences in depth that are not clearly distinguishable in the plotted curves. This again indicates that texture type and lubrication condition have negligible effect on scratch behavior for 1 mm diameter tip.

FIGURE 9:

CHANGE IN SCRATCH DEPTH WITH NORMAL LOAD FOR UHMWPE TESTED USING 1 MM TIP UNDER: (A) DRY; AND (B) BOUNDARY LUBRICATION CONDITIONS (CC: CYLINDER CAVITY; CP: CYLINDER PROTRUSION; SC: SQUARE CAVITY; SP: SQUARE PROTRUSION; HC: HEMISPHERICAL CAVITY; HP: HEMISPHERICAL PROTRUSION).

Our previous work [1,2,6,11] demonstrated that surface texturing significantly enhances the hydrophobicity of HDPE and UHMWPE and moisture resistance of HDPE, and therefore, can improve the longevity of biomedical implants that use these polymers. The findings in this study show that surface texturing may not significantly alter the tribological behavior under boundary lubrication scenario, further indicating that surface texturing may improve the longevity of polymers used in biomedical implants that undergo contact scenarios. The processing technique used in this study introduces the textures on polymer surfaces without any chemical modification, preserving the original material chemistry. This is a key advantage over chemical treatments, which can compromise long-term stability. Together, these findings highlight the potential of surface texturing to extend the functional lifespan of biomedical implants.

4. CONCLUSION

This study evaluated the tribological performance of flat and textured HDPE and UHMWPE samples under dry and boundary lubrication conditions using scratch testing with 1 mm and 10 mm diameter tips. The selected surface textures, including square, cylindrical, and hemispherical protrusions and cavities, were based on previous research that identified their effectiveness in enhancing hydrophobicity.

Tribological testing revealed that the scratch coefficient of friction (SCoF) depends strongly on tip size and contact configuration. Under the 1 mm tip, both HDPE and UHMWPE samples exhibited increasing SCoF with load due to progressive material deformation. Surface texturing had limited influence on frictional behavior in this configuration, as bulk material deformation dominated the response. Conversely, the 10 mm tip induced minimal plastic deformation and produced relatively constant SCoF profiles across all loads, highlighting that friction was governed primarily by surface interactions. In this case, boundary lubrication with DI water consistently reduced SCoF for all surface types, though differences between flat and textured samples remained marginal.

Scratch depth measurements corroborated these trends for 1 mm tip, showing load-dependent increase in depth regardless of surface texture or lubrication condition. While surface texturing introduced local variations in scratch path morphology, especially under 1 mm tip, the overall deformation behavior and penetration depth remained largely unaffected across all configurations.

In conclusion, surface texturing plays a crucial role in improving the long-term mechanical durability of biomedical polymers in moist environments, but its effect on tribological performance is highly dependent on the scale of contact and loading conditions. While lubrication can reduce friction, micro-textures have a limited role in altering scratch resistance in regimes dominated by bulk or distributed contact. These findings provide valuable insights for designing surface-engineered polymers that can increase the life span of biomedical implants with future work focusing on understanding the tribological behavior under long-term exposure to physiological environments.