Biomedical Engineering Reference
In-Depth Information
TMDI
trimethylhexamethylene diisocyanate
WAXD
wide - angle X - ray diffraction
6.1
Chemistry and Properties of Biodegradable Polyurethanes
Polyurethane s ( PUR s) were fi rst used for industrial applications in the 1940s, but
the development of biocompatible polymers did not start until the 1960s. PURs
have since then remained one of the most popular groups of biomaterials employed
in medical devices. Toughness, durability, biocompatibility, and biostability are
some of the characteristics that make PURs interesting for a wide variety of
long-term implantable devices. However, the number of applications requiring
biodegradability instead of biostability is on the rise, and consequently also the
demand for new PURs with a controlled degradation rate.
Biodegradable PURs employed as thermoplastics are basically synthesized using
a diisocyanate, a diol, and a chain-extension agent as main raw components [1, 2]
(Tables 6.1-6.3, Figure 6.1). Although both aromatic and aliphatic diisocyanates
have an applied interest, it should be pointed out that the putative carcinogenic
nature of aromatic compounds [3, 4] is leading to an increasing use of HDI, BDI,
and LDI, whose ultimate degradation products are more likely to be nontoxic (e.g.,
lysine).
The diol component commonly chosen is a low-molecular-weight polymer with
hydroxyl end groups and a backbone that, in the case of biodegradable PURs, may
correspond to a polyether, polyester, or polycarbonate [5]. The fi rst gave rise to the
Table 6.1
Diisocyanate raw materials.
OCN - CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
- NCO
1,6-Hexamethylene diisocyanate (HDI)
Lysine methyl ester diisocyanate (LDI)
OCN - CH
2
CH
2
CH
2
CH
2
- NCO
1,4-Butylenediisocyanate (BDI)
Isophorone diisocyanate (IPDI)
trans-1,4-Cyclohexylene diisocyanate
2,2,4-Trimethylhexamethylene
diisocyanate (TMDI)
Dicyclohexylmethane diisocyanate (H12MDI)
