Biomedical Engineering Reference
In-Depth Information
Scheme 6.4
Synthesis of starch - based polyurethanes.
Starch, which is the second largest biomass on earth, and synthetic plastics do
not mix easily. This problem can be overcome by chemically linking the synthetic
and the natural polymer. Barikani and Mohammadia [64] used the hydroxyl func-
tionality of the biopolymer and grafted a prepolymer derived from HDI and a PCL
macrodiol onto starch (Scheme 6.4). SEM micrographs confi rmed that the starch
granules were completely coated by PUR.
It was found that larger amounts of prepolymer led to an increase in hydropho-
bicity and a decrease in glass transition temperature.
Current research is aimed at combining natural products containing cellulose
with different materials like plastics to obtain new materials that can be tailored
according to fi nal use requirements. Since 1998, new biocomposite wood-
replacement panels made from wheat straw and a PUR resin have been commer-
cialized by Dow Polyurethanes [65]. The resulting biocomposite material can be
used for kitchen counters, shelving, ready- to - assemble furniture, cabinets, door
cores, and fl oor underlays.
Chitin, which is widely distributed in nature mainly as the skeletal material of
crustaceans, is structurally similar to cellulose as it has an acetamide group in
place of a hydroxyl group. Chitin-based PUR elastomers with potential as biomedi-
cal implants and tunable mechanical properties have been synthesized by step-
growth polymerization techniques using PCL and MDI [66]. The prepolymer was
extended with different mass ratios of chitin and BDO. The mechanical properties
of these polymers were improved by increasing chitin content, which furthermore
lowered the cytotoxicity of samples.
Chitosan, commercially produced by deacetylation of chitin, has several applica-
tions in the biomedical fi eld (e.g., in bandages and other hemostatic agents). A
number of graft and block PUR derivatives have been investigated: (i) PUR pre-
polymers prepared from PEG and isophorone diisocyanate have been successfully
grafted onto chitosan [67]; (ii) Xu
et al.
described a novel blood-compatible water-
