Biomedical Engineering Reference
In-Depth Information
3.4 The staining of hydroperoxides by nitric oxide gas.
detected (Fig. 3.4; Carlsson et al., 1990; Costa et al., 1998). This hydroperoxide
index can be interpreted as an oxidation potential since the decomposition of
these hydroperoxides will eventually lead to oxidation products.
As mentioned before, free radicals initiate oxidation in UHMWPE. While most
free radical species are very mobile and can react or recombine readily, the free
radicals induced in the crystalline domains of UHMWPE commonly by radiation
(for sterilization or crosslinking) have very low mobility due to the rigidity and
spacing between crystalline chains and can persist in UHMWPE for years (Jahan
et al., 2001). These free radicals are measured using electron spin resonance
(ESR), which can provide insight into the type of free radicals as well as the
amount (Jahan, 2009). However, there exists a stable `oxygen-induced radical'
(OIR) in UHMWPE, the structure of which is yet to be determined (Fig. 3.5).
Aging methods
Since most problems associated with UHMWPE have occurred in the second
decade of service, the development of methods that can simulate the oxidative
3.5 The free radical signature of 100 kGy irradiated UHMWPE as a function of
aging time in water at 40 ëC.
