Effect of Concentrations of Bovine Serum Albumin on Tribological Response of Cobalt Chromium Femoral Head

INTRODUCTION: Nano/micro-scale tribological properties (surface roughness and frictional coefficient of engineering and biological materials) have been investigated through Atomic Force Microscopy (AFM) [1]. At the macro-scale, previous studies in the literature have reported on the effect of lubricants on friction and wear through pin-on-disk tests and there has been the general agreement that albumin plays an important role as a lubricant in the tribological behavior of hip implant materials such as UHMWPE (ultra high molecular weight polyethylene)-on-cobalt chromium [2, 3], UHMWPE-on-stainless steel [2, 4, 5], and UHMWPEon-alumina ceramic [2, 4]. At the micro-scale, however, the effect of lubricants on the tribological properties of hip implant materials has not been well identified, although micro-scale AFM measurements are highly suitable for exploring frictional effectiveness of boundary lubrication for diverse lubricants in diarthrodial joints. Therefore, the objective of the present study is to investigate the role of albumin as a boundary lubricant in the lubrication of cobalt chromium (CoCr) femoral head of total hip arthroplasty (THA) by measuring its frictional coefficients with AFM techniques. MATERIALS AND METHODS: One sample with length, width, and thickness of 10 mm, 10 mm, and 5 mm, respectively was machined from the main wear region of a CoCr femoral head after 10 years of usage which was retrieved from hip surgery for aseptic loosening of THA. The sample was cleaned in an ultrasonic bath for five minutes and glued on top of the cylindrical flat plates (1 mm thickness and 19 mm diameter) using a small amount of cyanoacrylate glue. In this study, two types of solutions were prepared as lubricants: PBS (Phosphate Buffered Saline, Sigma-Aldrich, No. P5493) as a control solution, and BSA (Bovine Serum Albumin, SigmaAldrich, No. A2153) at different protein concentrations (10 mg/ml, 20 mg/ml and 30 mg/ml) in PBS. All lubricants were stored at a temperature of 4 C. For AFM measurements with lubricants, the sample was imaged at room temperature using an AFM device (XE 70, Park Scientific Instruments, South Korea) which was placed in a sealed box and equipped with software (XEI version 1.6.5) for image processing and the calculation of the surface roughness. A rectangular silicon cantilever with a normal spring constant of KN=0.95 N/m that was integrated with a square-pyramid conical tip (curvature less than 10 nm) was used. Friction force (FL) was calculated by the multiplication of the lateral voltage signal (VLFM in V), lateral spring constant (KL in N/m), and lateral sensitivity (SL in mm/V). In the present study, the friction force was calculated from the whole image of 25μm×25μm scanned area, and the lateral sensitivity (SL) is proportional to the normal sensitivity (SN=61.14 nm/V) by the following equation