Biomedical Engineering Reference
In-Depth Information
•
The encapsulation of organic liquids in PUR nanocapsules prepared by inter-
facial miniemulsion polycondensation of isophorone diisocyanate and propan-
etriol has also been reported [52]. Encapsulation effi ciency was found to be
dependent on the water solubility, interfacial tension against water, and com-
patibility with PUR of the liquids.
6.3.3
Other Biomedical Applications
Bone cements and hydrogels are other examples of biomedical applications where
biodegradable PURs have a promising future. Guelcher
et al.
[53] studied biode-
gradable PURs as an alternative to acrylic bone cements. New PUR networks have
been prepared by two-component reactive liquid molding of low-viscosity prepoly-
mers derived from lysine polyisocyanates (lysine methyl ester diisocyanate and
lysine triisocyanate) and poly(3- caprolactone -
co
- dl - lactide -
co
- glycolide) triols.
Loh
et al.
developed thermoresponsive multiblock poly(ester urethane)s synthe-
sized from poly(
-caprolactone), PEG, and poly(propylene glycol) using
1,6 - hexamethlyene diisocyanate (HDI) as a coupling agent [54]. Bulk hydrophilicity
of the obtained copolymers could be controlled either by adjusting the composition
of the copolymer or by changing the temperature of the environment. These
materials give rise to a hydrogel-like material without using toxic crosslinking
agents. Furthermore, fi lms of these samples are thermoresponsive since they form
highly swollen hydrogel-like materials when soaked in cold water and shrink when
soaked in warm water, these changes being reversible.
ε
6.4
New Polymerization Trends to Obtain Degradable Polyurethanes
New synthetic routes are currently being studied to obtain green and sustainable
PURs [55]. New polymers should avoid diisocyanate reactants, improve chemical
recyclability, and enhance biodegradability while keeping high-performance prop-
erties of typical PURs.
6.4.1
Polyurethanes Obtained without Using Diisocynates
Several approaches can be considered although none has been used at an indus-
trial level. The fi rst representative example is the reaction between a cyclic carbon-
ate and an amine rendering the urethane bond. In particular, the polyaddition
reaction between L-lysine and a bifunctional fi ve-membered cyclic carbonate in
the presence of a strong base was described by Kihara
et al.
[56] . An alternative
route (Scheme 6.3a) was reported by Rokicki
et al.
[57], who obtained PURs by
reaction of ethylene carbonate with 1,6-hexanediamine or 1,4-butanediamine at
room temperature and without any catalyst, the authors obtained a diurethanediol
