The PEEK wear particles produced by rubbing PEEK pin in water and tested with three different contact pressures were analyzed (Table 1). We found that decreasing contact pressure resulted in a reduction of particle length, particle width, and wear rate (Figure 2(a) and Table 1). We also observed that, under the same contact pressure, rubbing PEEK pins against the surface L led to a larger particle length, width, and wear rate compared to the surface S (Figure 2(a) and Table 1). In addition, we observed that rubbing PEEK pins against surface L caused sheet-like particles while rubbing against smaller surface resulted in granular-like particles (Figures 2(b) and 2(d)).
The articulation of PEEK pins were also carried out in FBS, which is widely used in joint stimulations. Similar outcomes were observed: decreasing contact pressure caused a reduction in particle length, particle width, and the wear rate (Table 1). We observed both long fibrous and thin platelet wear particles in FBS regardless of contact pressures and surface geometries (Figures 2(c) and 2(e)) that was distinct from rubbing in water, this phenomenon only observed in more lubricating environment.
When we compared the wear rate with water and FBS against the same surface geometry, we found that the wear rate was smaller when articulation of PEEK pin was in FBS than in water regardless of the contact pressures (Figure 2(a)). Indeed, FBS contains many components such as albumin, globulin, and glucose that are similar to human synovial fluid. Interestingly, the shapes of wear particles in FBS were distinct compared with them being in water. It is possible that FBS forms a thin layer on the surfaces to increase the modulus of elasticity of PEEK; thus PEEK was harder to be broken resulting in fibrous shape of particles especially under a smaller contact pressure.
Effects of Glycerine on Wear Particle Length and Amounts:
We next investigated whether changing the concentration of a specific lubricant has effects on particle size and wear rate. Our results demonstrated that decreasing the concentration of glycerine resulted in an increase in particle length, width, and wear rate (Figure 3(a) and Table 1). We also noticed that the morphology of wear particles were both sheet-like and granular-like in 100% glycerine (Figures 3(b) and 3(c)). In contrast, the wear particle shape in 10% glycerine was uniformly sheet-like regardless of the surface geometries suggesting that 100% glycerine provided a more lubricating environment.
Glycerine is viscous liquid, and our results showed that high viscosity of glycerine caused the smallest particles and the wear rate (Table 1). It has been shown that the higher viscosity of hyaluronic acid results in fewer wear amounts and smaller particle size, and it may explain the outcomes in our observation [10]. Both rod-like and granular shape of carbon fiber reinforced-PEEK (CFR-PEEK) wear particles were identified in macrophages in human tissue retrieval studies, although the effects of particle shapes on the artificial joint system are still unclear. Therefore, the method we developed here could be rapidly generating various shapes of PEEK wear particles for the further investigation.
