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The Wear Testing Artificial Joint Systems
Many reasons can cause the damage of joints such as aging, stress, trauma, or body weight. When the joints are damaged severely resulting in pain and even difficulties in movements, total joint arthroplasties (TJAs) are recognized as the ultimate effective treatment currently. The performance of TJAs depends on their tribological properties, and the wear particles that are generated from the tribological process play a key role in the lifetime of artificial joint system. It has been shown that wear particles induce biological responses and even cause osteolysis and bone resorption. One of polymers, polyetheretherketone (PEEK), has been introduced as bearing materials for TJAs because of its resistance to fatigue strain. Therefore, it is critical to prove whether PEEK wear particles could induce biological responses in vivo.
Materials
PEEK cylinder pins were obtained from A-SPINE Asia Co. Ltd. (batch number was SSR 0151). The pins were 6.35?mm in diameter and 25.4?mm in length with diamond turning on both end surfaces without polishing. Fetal bovine serum (FBS, Hyclone) was diluted in phosphate buffered saline (PBS) to 25%. 50% and 10% of glycerine (J.T.Baker) was prepared in distilled water. All PEEK pins were presoaked in PBS for at least 30 days so as to become completely saturated.
Wear Process
By rubbing PEEK pin with the cutting edges of the silicon surface textures, the PEEK wear particles were generated. ASTM F732 was used as a guideline. The setup of the system was described [8], and the relative motions between a PEEK pin and a cutting device were illustrated in Figure 1(b). The PEEK pin was weighed three times and mounted on the tester. Linear reciprocating wear tests were run under a nominal contact pressure of 0.6, 1.5, or 3.0?MPa, a stroke length of 19?mm, and an average sliding speed of 57.2?mm/s for 6 hours. After wear process, the PEEK pin was weighed and the wear loss was obtained after adjusting the weight change from before test.
Microfabricated Surface Textures
The silicon wafer surface with controlled asperities was prepared by photolithography patterning and etching of the bulk substrate as shown in Figure 1(c) and previously described [9]. Two-inch diameter polished type P silicon wafers with (100) orientation purchased from Summit-Tech were used as the substrate material. Wet oxidation of the silicon wafers was carried out in a glass-tube oven at 1100°C for 135 minutes to form a silicon dioxide film with a thickness of 1?μm. A pattern of rectangles with different size and aspect ratios was made on a chrome direct-writing photomask. The silicon dioxide (SiO2) surface was spin coated with a Shipley 1813 positive photoresist. The dark-featured photomask with rectangular patterns (5?μm × 5?μm, hereafter called surface S; 5?μm × 10?μm, hereafter called surface L) were then placed on the photoresist surface and exposed to an ultraviolet source in a mask aligner to decompose the surrounding polymer surface, leaving a positive rectangular pattern. The decomposed photoresist polymer was then removed in a Shipley 351 developer. The resulting positive photoresist surface was then etched first with a buffered oxide etch (BOE) solution (diluted buffer hydrofluoric acid (HF) solution) to etch away the SiO2 in a wet chemical bath. The photoresist was then removed by washing with acetone-alcohol. Subsequently, the silicon material was subjected to isotropic silicon etching (HNA etchant; liquid volume ratio HF?:?HNO3?:?CH3COOH = 8?:?75?:?17) in a wet chemical bath at room temperature. The SiO2 layer was removed after an isotropic undercutting etching process. The resulting surface features are an array of rectangular ridges with sharp edges. Finally, a layer of 5?nm Cr coating was evaporated onto the surface to increase the strength and wear resistance of the surface texture. The height of the surface textures was measured by a Mahr profilometer (Gottingen, Germany) and the feature length and width were measured from scanning electron microscopy (SEM) observations.
Isolation of the Wear Particles
PEEK particles were collected in a sterilized beaker. The collected solution was added with 6?N NaOH and shaken at 65°C for 24 hours. After centrifuging, collecting the bottom solid layer, adding new NaOH, and shaking at 65°C for 24 hours, the solution was again centrifuged at 5000?rpm at 4°C for 1 hour. The bottom layer was rinsed with 50?mL purified water, sonicated for 10 minutes, and then centrifuged at 5000?rpm at 4°C for 30 minutes. Particles were rinsed with purified water twice and then once with 95% ethanol. All the liquid was discarded, and 95% ethanol was added. After sonicating for 10 minutes, wear particles were collected on a 0.1?μm pore size membrane through a vacuum filtration process.
Analysis of the Particles
The particles collected on the filter paper were examined by using a scanning electron microscope. Micrographs of the particles were then analyzed by using image analyzer software (Scion Image, a personal computer version of NIH Image) to measure their dimensions. Measurements were made for at least 300 particles in each condition.