Ageing processes of biomedical polymers in the body - Durability and Reliability of Medical Polymers - page 166

or twelve (Simmons et al. , 2004) months. Naidu et al. (1997) measured
the viscoelastic properties (storage and loss modulus) of unimplanted and
retrieved silicone elastomer Silastic ® HP-100 finger implants, which had
been implanted in rabbits for four months and concluded that the moduli
did not change significantly after implantation.

8.3.3 Calcification
Calcification of polyurethans has been reported, whereby calcium deposits
have been detected on the surface of implanted polyurethans (Griesser, 1991).
For example, several studies have reported calcification of PU heart valves
(Bernacca et al. , 1995; Bernacca et al. , 1997; Mackay et al. , 1996; Wheatley
et al. , 2000) and blood pumps (Golomb et al. , 1993) and a PU acetabular
bearing surface for a hip joint (Khan et al. , 2005a; Khan et al. , 2005b) in vivo
and in vitro . It has been suggested that the calcification plays an important
role in the degradation of the properties of polyurethans because it leads
to changes in the physical properties (for example, elasticity) (Griesser,
1991) and is attributed to be the leading cause of failure in bioprosthetic
heart valves (Golomb et al. , 1993). However, in recent studies, it has been
reported that polyurethans grafted with polydimethylsiloxane (silicone)
were less likely to show signs of calcification after six months in an ovine
model (Soldani et al. , 2010) or in vitro (dabagh et al. , 2005), suggesting
that surface modification of the PU can affect the likelihood of calcification.
Another recent study on PU stent valves (Metzner et al. , 2010) concluded
that there was no evidence of calcification after implanting seven stents into
sheep for four weeks.
Furthermore, in silicone implants, calcification of silicone breast implant
capsules has been reported in previous studies (Bantick and Taggart, 1995;
Gümüs, 2009; Peters and Smith, 1995; Peters et al. , 2001; Siggelkow
et al. , 2003) and is suggested as an important factor for implant failure. A
long-term study on silicone toe implants (Smetana and Vencalkova, 2003)
also reported evidence of calcification in fifteen of its cases. Encrustation
of silicone, PU and latex urinary catheters with evidence of calcium and
magnesium deposits on the surface of the material has also been reported
(Cox et al. , 1988; desgrandchamps et al. , 1997; Hukins et al. , 1983; Talja
et al. , 1990).

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8.3.4 Degradation as an intended consequence
For some medical applications, polymer degradation may be an intended
consequence of the medical application, where a permanent device is not
required. When a biodegradable device is implanted in the body, a general
requirement is that the mechanical properties of the material should not