In order to analyze the lubricating effect of the PVP solutions on
the knee joint prosthesis, the friction coefficients of all the 10 PVP
solution groups (i.e., five different concentrations of PVP in two
different solvents) were tested using a universal ball-on-disc
machine UMT-3 (Center for Tribology Inc., California, USA). Both
the Ti6Al4V ball and the UHMWPE disc were ultrasonic cleaned
firstly, and were mounted in the UMT-3 with the friction surface
immersed in the PVP solution (shown in Fig. 1). According to the
standard test method for total joint prostheses (i.e., ASTM F732), a
compression load of 2.94 N was exerted (yielding a maximum
Hertzian pressure of 29.7 MPa on the contacting surface) and a
reciprocating sliding with 15 mm stroke was performed at the
speed of 30 mm/s. It was found that the results of the friction
coefficient became stable after a long duration (shown in Fig. 2).
Therefore, the reciprocating stroke was maintained for 5 min and
the friction coefficient of each PVP solution was determined by
averaging the friction coefficient measurements in the last one
minute. The friction coefficient measuring test was performed six
times for each PVP solution