ABSTRACT
In this study, the friction and wear behavior of ultrahigh molecular weight polyethylene (UHMWPE) were evaluated as a function of polymer crystallinity in the presence of the phospholipid dipalmitoyl phosphatidylcholine (DPPC) dissolved in ethanol. Samples of UHMWPE were separately heat treated to get high and low crystallinity samples. Degree of crystallinity was evaluated using differential scanning calorimetry. Quantitative friction and wear experiments were conducted using a custom-made microtribometer with commercially available spherical Si(3)N(4) probes in controlled and phospholipid-dissolved lubricants. The higher crystallinity sample exhibited slightly lower friction than the lower crystallinity in the control and decreased significantly when phospholipids were present. The higher crystallinity sample showed a higher wear resistance than the lower crystallinity sample during all reciprocating wear tests. DPPC acting as a lubricant had a marginal effect on the wear resistance of high crystallinity UHMWPE, whereas the low crystallinity sample became more prone to wear. Atomic force microscopy topography images and contact angle measurements of both samples before and after phospholipid exposure indicate that the higher crystallinity sample absorbed a greater density of DPPC. Increasing crystallinity is a way of escalating adsorption of surface active phospholipids onto UHMWPE to make it a more wear-resistant load-bearing material for total joint replacements.
Subject(s)
1,2-Dipalmitoylphosphatidylcholine , Materials Testing , Polyethylene , Adsorption , Molecular Weight , Stress, MechanicalABSTRACT
This paper presents experimental investigations to actively modulate the nanoscale friction properties of a self-assembled monolayer (SAM) assembly using an external electric field that drives conformational changes in the SAM. Such "friction switches" have widespread implications in interfacial energy control in micro/nanoscale devices. Friction response of a low-density mercaptocarboxylic acid SAM is evaluated using an atomic force microscope (AFM) in the presence of a DC bias applied between the sample and the AFM probe under a nitrogen (dry) environment. The low density allows reorientation of individual SAM molecules to accommodate the attractive force between the -COOH terminal group and a positively biased surface. This enables the surface to present a hydrophilic group or a hydrophobic backbone to the contacting AFM probe depending upon the direction of the field (bias). Synthesis and deposition of the low-density SAM (LD-SAM) is reported. Results from AFM experiments show an increased friction response (up to 300%) of the LD-SAM system in the presence of a positive bias compared to the friction response in the presence of a negative bias. The difference in the friction response is attributed to the change in the structural and crystalline order of the film in addition to the interfacial surface chemistry and composition presented upon application of the bias.
ABSTRACT
Tribological properties of materials used in biomedical implants critically affect the performance of the implant. A UHMWPE cup paired with a ceramic ball is a popular combination for implants due to its relatively low wear rate. In this study we investigate the effect of surface roughness of UHMWPE on the friction behavior and onset of wear in a UHMWPE/silicon nitride interface in both dry air and bovine serum environments. Microscale multi-asperity contact is examined using a ball-on-flat reciprocating microtribometer. Nanoscale single-asperity contact and surface topography are examined using atomic force microscopy. Friction was found to increase with a decrease in surface roughness of the UHMWPE sample in air, which is due to an increase in real area of contact. This trend was seen to disappear or even reverse in serum. This is due to an increase in the interfacial shear stress of the UHMWPE surface when exposed to the serum. This increase is believed to be caused by an adhered layer of protein on the UHMWPE surface.