Degradation of Polyurethanes for Cardiovascular Applications - Advances in Biomaterials Science and Biomedical Applications - page 57

spaces of the precured polymers by 6 weeks, and fluorochrome analysis suggested that
this bone had started to be laid down at between 4 and 5 weeks. The polymer without
β-tricalcium phosphate (TCP) showed histological evidence of some degradation by 6
weeks with progressive increase in polymer loss by 12 and 24 weeks. The polymer with
β-TCP showed no evidence of degradation at 6 weeks and only minimal loss at 12
weeks. By 12 weeks, there had been considerable degradation of the polymers and at
week 24, polymer was completely degraded.

The in vivo degradation of segmented poly(urethane urea)s (SPUUs) with hard segments de‐
rived only from methyl 2,6-diisocyantohexanoate (LDI) and PCL, PTMC (polytrimethylene
carbonate), P(TMC-co-CL), P(CL-co-DLLA), or P(TMC-co-DLLA) as soft segment was con‐
ducted by Asplund et al. (Asplund et al., 2008). The in vivo study of SPUU-PCL using male
Sprague-Dawley rats displayed the typical foreign body response seen with most inert poly‐
meric implant materials. The reaction at 1 week thus displayed an infiltration of ED1 posi‐
tive macrophages closest to the implant surface, an outside layer of fibroblasts and some
collagen formation. At 6 weeks, the foreign body capsule had matured, displaying lower
numbers of interfacial macrophages and an increased amount of collagen in the fibrotic cap‐
sule. The thickness of the foreign body capsule was similar to the controls. These observa‐
tions seemed also to be reflected in the number of ED1 positive macrophages, as well as in
the total number of cells throughout the reactive capsule.

Hafeman et al. (Hafeman et al., 2008) synthesized polyurethane scaffolds by one-shot reac‐
tive liquid molding of hexamethylene diisocyanate trimer (HDIt) or lysine triisocyanate
(LTI) and a polyol as hardener. Trifunctional polyester polyols of 900-Da and 1,800-Da mo‐
lecular weight were prepared from a glycerol starter and 60% ε-caprolactone, 30% glycolide,
and 10% D,L-lactide monomers, and stannous octoate catalyst. Tissue response was evaluat‐
ed by subcutaneous implantation in male Sprague-Dawley rats for up to 21 days. During
this time, initial infiltration of plasma progressed to the formation of dense granulation tis‐
sue. All of the implants showed progressive invasion of granulation tissue with little evi‐
dence of an overt inflammatory response or cytotoxicity. Fibroplasia and angiogenesis
appeared to be equivalent among the different formulations. Extracellular matrix with dense
collagen fibers progressively replaced the characteristic, early cellular response. The LTI
scaffolds exhibited a greater extent of degradation at 21 days, although the incorporation of
PEG into the HDIt scaffold accelerated its degradation significantly. Degradation rates were
much higher in vivo . With time, each of the materials showed signs of fragmentation and en‐
gulfment by a transient, giant cell, foreign body response. After the remnant material was
resorbed, giant cells were no longer evident.

Khouw et al. (Khouw et al., 2000 ) reported that the foreign body response to degradable ma‐
terials differs between rats and mice. van Minnen et al., (van Minnen et al., 2008) also sug‐
gested that it is possible that the response between rats and rabbits differs as well, due to the
faster degradation in the rabbit. This may be related to differences at the enzymatic or cellu‐
lar level, but also to the highly mobile and well vascularized skin of the rabbit, as compared
to the rat.