IMPROVING THE LONG-TERM DURABILITY OF POLYMERS USED IN BIOMEDICAL APPLICATIONS

Abstract

Hydrophobic surfaces can improve the long-term mechanical response of polymers by delaying their degradation caused by moisture absorption over time. This improvement in long-term mechanical performance can significantly increase the lifespan of polymers used in various biomedical applications, such as total joint replacement prostheses applications. Although a number of surface modification techniques have been developed over the years, such as introduction of various textures on the surface; their specific influences on hydrophobicity enhancement as well as long-term mechanical performance are yet to be fully understood. In this study, surface textures, with variation in type and geometry, are introduced on model Ultrahigh Molecular Weight Polyethylene (UHMWPE) and High Density Polyethylene (HDPE) surfaces to study the effect of surface modification on hydrophobicity and long-term mechanical performance under environmental conditions. The results show that introduction of surface textures significantly improves the hydrophobicity of model polymers. Texture length or diameter significantly affects the improvement in hydrophobicity. However, texture spacing does not have significant influence on the improvement in hydrophobicity. How this improvement in hydrophobicity facilitates improving the long-term mechanical performance under environmental conditions is investigated. The study provides useful guidelines to improve the long-term mechanical response of polymers for various applications, including biomedical applications.

1. INTRODUCTION

Improvement in hydrophobic properties of polymers has long been sought in various applications, such as in biomedical applications. The mechanical properties, such as modulus, strength and toughness, of polymers used in biomedical implants can deteriorate over time due to extended exposure to moisture environment [1, 2]. Hydrophobic polymer surfaces can provide enhanced long-term mechanical performance by delaying the degradation of polymers caused by moisture absorption over time. This improvement in long-term mechanical performance can significantly increase the lifespan of polymers used in various biomedical applications, such as total joint replacement prostheses applications.

A number of techniques have been developed over the years to improve the hydrophobicity of polymers, such as introduction of textures on polymer surfaces through laser surface texturing [3]. The degree of hydrophobicity in solid surface generally depends on surface chemistry and surface topography [4].

Hydrophobicity of a surface is characterized by the contact angle (θ) between a water droplet and the surface. A hydrophobic surface shows strong repulsion towards water, whereas a hydrophilic surface shows strong attraction towards water. If the water contact angle (WCA) is less than 90°, the liquid is considered wetting the surface and the surface is deemed as hydrophilic [5]. On the other hand, if the WCA is greater than 90°, the surface is considered hydrophobic [5]. The surface is considered superhydrophobic if the WCA is greater than 150°.

In our previous studies [6, 7], effects of various surface textures on hydrophobicity of polymers have been investigated using Ultrahigh Molecular Weight Polyethylene (UHMWPE) and High Density Polyethylene (HDPE) model systems. Results showed that introduction of surface textures on UHMWPE and HDPE surfaces in general improves hydrophobicity. However, relationship between texture dimension and hydrophobicity could not be established. Furthermore, it was shown that the theoretical models, such as Wenzel and Cassie-Baxter models [7], are unable to fully describe the improvement in hydrophobicity observed experimentally, indicating the need for developing transition state model.

The present study focuses on investigating the effects of various surface texture geometries on the hydrophobicity and long-term mechanical behavior of UHMWPE and HDPE model polymers. These two polymers have been widely used in biomedical applications, specifically in prostheses applications. Square and cylinder surface textures, both protrusion and cavity, with variation in texture dimension are used to study the influence of these surface textures on WCA, and, therefore, hydrophobicity of UHMWPE and HDPE. Long-term mechanical behaviors of these model polymers under environmental conditions are also studied.

2. EXPERIMENTAL

Surface textures with variation in type and dimension were introduced on multipurpose aluminum sheet of 5”×5”×1/8” size, using TYKMA LaserGear BOQX engraver. The desired texture dimension was achieved by varying the loop count, power, speed, and frequency parameters in the laser engraver. These textured aluminum pieces were used as dies to prepare textured polymer samples using the Hot Press.

UHMWPE powder (GUR 4120, M_w=4.7×10⁶ g/mol) and HDPE pellets (Melt Flow Index: 0.3 dg/min), generously provided by SABIC, were used to prepare the model polymer samples. To prepare the textured polymer samples, the engraved aluminum plate was sprayed with mold release and dried for 10 minutes before placing it in the oven. The oven was preheated at 350°F to preheat the aluminum plate to the same temperature. Two smooth aluminum plates, coated with Teflon, were used to apply the required temperature and pressure on the sample by placing the engraved aluminum plate and polymer powder/pellets in between these smooth plates. For UHMWPE, conditions outlined in the ISO 11542-2 [8] was used to prepare the samples. For HDPE, conditions outlined in the ISO 17855-2 [9] was used to prepare the samples. Based on our previous studies [6, 7], the texture height/depth was kept 100 μm for all the textures prepared in this study.

To measure the hydrophobicity of polymer surfaces, water contact angle (WCA) of the polymer samples was measured using a custom-built contact angle measurement setup. A pipette was used to place a 10 μl water droplet on the surface. Motorola mobile phone with a 15MP camera, attached with a macro lens, was used to capture high-resolution image focusing on the water droplet at 2X zoom. Figure 1 shows examples of the images obtained for the WCA measurements. The high-resolution images obtained through this process were analyzed to measure the WCA, using the ImageJ software. To measure the WCA, the images were first converted into grayscale in the ImageJ software. The contact angle plugin in the ImageJ software, which can trace the droplet shape to give an accurate WCA measurement, was used to analyze the water droplet images and measure the WCA. At least three WCA measurements were obtained for each sample to get an average value. Using this procedure, the WCA of the UHMWPE and HDPE smooth sample was found to be 98.4°±0.8° and 104.7°±0.6°, respectively. Therefore, both UHMWPE and HDPE used in this study can be considered hydrophobic.

FIGURE 1:

IMAGES OF WATER DROPLET FOR WATER CONTACT ANGLE MEASUREMENT: (A) SMOOTH SURFACE (WCA: 104.9°); AND (B) TEXTURED SURFACE (SQUARE PILLAR, WCA: 148.5°) OF HDPE.

Uniaxial tension test was performed on both dry and environmental conditioned samples following the ASTM D638 standard [10], using an MTS universal testing machine (MTS Criterion Model 45). The dog-bone specimens of model systems were prepared with a nominal width and thickness of 3 mm, following the ASTM D638 specimen type V. A crosshead speed of 6.35 mm/min was used for the tensile tests. The environmental conditioned samples were prepared by submerging the samples in DI water (purchased from Sigma-Aldrich) for specific amount of time. At least three samples were tested for each condition to get an average value.

3. RESULTS AND DISCUSSION

Figure 2 shows examples of 3D height images of surface texture features introduced on the polymer samples, obtained using Olympus DSX510 digital microscope. The 3D height images of textured polymer samples are analyzed using the microscope software to obtain accurate texture dimension.

FIGURE 2:

3D HEIGHT IMAGES FOR: (A) SQUARE PROTRUSION; (B) CYLINDER PROTRUSION; (C) SQUARE CAVITY; AND (D) CYLINDER CAVITY (L: LENGTH (SQUARE TEXTURE) OR DIAMETER (CYLINDER TEXTURE), S: SPACING BETWEEN THE SURFACE FEATURES, H: HEIGHT/DEPTH OF THE SURFACE FEATURES).

Figure 3 shows the percent change in WCA with respect to smooth surface with introduction of square pillars for UHMWPE. The average WCA values for each system (with coefficient of variation <1.9%), obtained from at least three WCA measurements, were used to determine the respective percent change in WCA. As can be seen in the figure, introduction of square pillar texture on UHMWPE in general improves hydrophobicity. Texture spacing does not influence the improvement in hydrophobicity when the spacing is large enough. At smaller texture spacing, in general, the smaller the pillar length (L), the higher the hydrophobicity improvement. HDPE also shows similar trend.

FIGURE 3:

PERCENT CHANGE IN CONTACT ANGLE (Δθ) WITH SQUARE PILLAR LENGTH (L) AND SPACING FOR A CONSTANT PILLAR HEIGHT FOR UHMWPE.

Figure 4 shows the percent change in WCA with respect to smooth surface with introduction of cylinder pillars for UHMWPE. The average WCA values for each system (with coefficient of variation <1.2%), obtained from at least three WCA measurements, were used to determine the respective percent change in WCA. As shown in the figure, introduction of cylinder pillar texture on UHMWPE in general improves the hydrophobicity as well. Texture spacing does not influence the improvement in hydrophobicity when the spacing is large enough. Furthermore, the smaller the texture diameter (L), the higher the hydrophobicity improvement in UHMWPE. For HDPE, similar to square protrusion, texture spacing does not influence the improvement in hydrophobicity when the spacing is large enough. Furthermore, when the texture spacing is large enough, the smaller the pillar diameter (L), the higher the hydrophobicity improvement for HDPE.

FIGURE 4:

PERCENT CHANGE IN CONTACT ANGLE (Δθ) WITH CYLINDER PILLAR DIAMETER (L) AND SPACING FOR A CONSTANT PILLAR HEIGHT FOR UHMWPE.

Figure 5 shows the percent change in WCA with respect to smooth surface with introduction of square cavity for HDPE. The average WCA values for each system (with coefficient of variation <2.2%), obtained from at least three WCA measurements, were used to determine the respective percent change in WCA. As shown in the figure, introduction of square cavity texture on HDPE in general improves the hydrophobicity as well. Texture spacing does not influence the improvement in hydrophobicity when the spacing is large enough. The smaller the square cavity length (L), the higher the hydrophobicity improvement. UHMWPE also shows similar trend; however, UHMWPE show higher improvement compared to respective HDPE systems.

FIGURE 5:

PERCENT CHANGE IN CONTACT ANGLE (Δθ) WITH SQUARE CAVITY LENGTH (L) AND SPACING FOR A CONSTANT CAVITY DEPTH/HEIGHT FOR HDPE.

Figure 6 shows the percent change in WCA with respect to smooth surface with introduction of cylinder cavity for HDPE. The average WCA values for each system (with coefficient of variation <1.4%), obtained from at least three WCA measurements, were used to determine the respective percent change in WCA. As shown in the figure, introduction of cylinder cavity texture on HDPE in general improves the hydrophobicity as well. Similar to square cavity, texture spacing does not influence the improvement in hydrophobicity when the spacing is large enough. Furthermore, similar to square cavity, the smaller the cylinder cavity diameter (L), the higher the hydrophobicity improvement. UHMWPE also shows similar trend; however, UHMWPE show higher improvement compared to respective HDPE systems.

FIGURE 6:

PERCENT CHANGE IN CONTACT ANGLE (Δθ) WITH CYLINDER CAVITY DIAMETER (L) AND SPACING FOR A CONSTANT CAVITY DEPTH/HEIGHT FOR HDPE.

Based on the hydrophobicity improvement results, as discussed earlier, the texture dimension that showed the highest improvement in hydrophobicity for each texture type (square protrusion and cavity, cylinder protrusion and cavity) was chosen to conduct the environmental conditioning study.

Figure 7 shows the effect of environmental condition on the tensile yield strength of UHMWPE. As can be seen in the figure, all the smooth/flat and textured UHMWPE systems have similar yield strength in dry condition. The yield strength of flat/smooth UHMWPE system deteriorates when submerged in the DI water for 30 days. However, all the textured UHMWPE systems still have similar yield strength after 30 days exposure to DI water, compared to the dry systems properties. Similar trend is observed for the Young’s modulus and failure strain as well where smooth/flat UHMWPE system’s Young’s modulus and failure strain decrease when submerged in the DI water for extended period whereas the textured systems show similar properties over time.

FIGURE 7:

EFFECT OF ENVIRONMENTAL CONDITIONING ON THE YIELD STRENGTH OF SMOOTH/FLAT AND TEXTURED UHMWPE SAMPLES.

The representative stress-strain curves of smooth/flat UHMWPE systems, as shown in Figure 8, show the gradual degradation of mechanical performance with extended exposure to the moisture environment. However, the textured UHMWPE systems show the same mechanical performance with extended exposure to moisture environment (the stress-strain curves are not shown to avoid redundancy). This long-term durability in mechanical performance in the textured UHMWPE systems can be attributed to the improvement in hydrophobicity achieved by introducing textures on the UHMWPE surface.

FIGURE 8:

REPRESENTATIVE STRESS-STRAIN CURVES OF SMOOTH/FLAT UHMWPE SAMPLES AT DIFFERENT ENVIRONMENTAL CONDITIONS.

Figure 9 shows the effect of environmental condition on the tensile yield strength of HDPE. As shown in the figure, all the smooth/flat and textured HDPE systems have similar yield strength in dry condition. The yield strength of flat/smooth HDPE system remains similar when exposed to DI water for 15 days, but deteriorates when submerged in the DI water for 30 days. However, all the textured HDPE systems still have similar yield strength after 30 days exposure to DI water, compared to the dry systems properties. Similar trend is observed for the Young’s modulus and failure strain as well where smooth/flat HDPE system’s Young’s modulus and failure strain reduce when submerged in the DI water for extended period whereas the textured systems show similar properties over time.

FIGURE 9:

EFFECT OF ENVIRONMENTAL CONDITIONING ON THE YIELD STRENGTH OF SMOOTH/FLAT AND TEXTURED HDPE SAMPLES.

The representative stress-strain curves of smooth/flat HDPE systems, as shown in Figure 10, show the degradation of mechanical performance with 30 days exposure to the moisture environment. However, the textured HDPE systems show the same mechanical performance with extended exposure to moisture environment (the stress-strain curves are not shown to avoid redundancy). This long-term durability in mechanical performance in the textured HDPE systems can also be attributed to the improvement in hydrophobicity achieved by introducing textures on the HDPE surface.

FIGURE 10:

REPRESENTATIVE STRESS-STRAIN CURVES OF SMOOTH/FLAT HDPE SAMPLES AT DIFFERENT ENVIRONMENTAL CONDITIONS.

4. CONCLUSION

In this study, effects of various surface texture geometries, such as square and cylinder protrusion and cavity, on the hydrophobicity and long-term mechanical behavior of UHMWPE and HDPE have been investigated. The results show that introduction of these surface textures on UHMWPE and HDPE surfaces in general improves hydrophobicity. According to the experimental results, in general, the smaller the texture length or diameter, the higher the hydrophobicity improvement. Furthermore, based on the water droplet size and texture spacing range considered in the study, texture spacing generally does not influence the improvement in hydrophobicity when the spacing is large enough. Based on the environmental conditioning study, it can be concluded that indeed the improvement in hydrophobicity can be translated to long-term durability of polymers under environment conditions. The mechanical performance of flat/smooth UHMWPE and HDPE systems starts to deteriorate with extended exposure to moisture environment, whereas the textured systems’ mechanical performance remain the same with extended exposure to moisture environment. The study provides useful guidelines to improve the long-term durability of polymers used in biomedical applications.