As it formed boundary lubrication, it is suggested that molecules
adhered to the friction surface might be the main factor for
reducing the friction coefficient. The variation of the friction
coefficient with the PVP solutions changing could support the
hypothesis of the lubrication mechanism. First, the friction coef-
ficient would remain constant since the PVP concentration reaches
up to a high level. The concentration level of the PVP bovine serum
solution was also found to be lower than that of the PVP deionized
water solution for attaining the stabilisation of friction coefficient
(i.e., 10 wt% vs. 15 wt%). These could be resulted from the amount
of the PVP molecules adhered to the friction surfaces. The amount
of the PVP molecules on the surfaces, i.e., the friction properties,
would not be influenced by the change of the PVP concentration
since the PVP molecules on the surfaces reached saturation. In
addition, the friction coefficient of the bovine serum solution
levelled off more early than that of the deionized water solution
when the PVP concentration increased. This could be explained by
the adhesion of some proteins of the bovine serum solution on the
surfaces of the friction pair. The adhesion of the proteins would
induce less available space for PVP molecules on the surfaces of
the joint prostheses, which manifested that the friction coefficient
of the PVP bovine serum solutions attained stabilisation at a lower
concentration. The adhesion of the proteins of the bovine serum
solutions also accounted for that higher friction coefficient was
obtained from the PVP bovine serum solutions than the PVP
deionized water solutions (shown in Fig. 3(d)).