Biomedical Engineering Reference
In-Depth Information
Figure 6.4
Representative segmented and random poly(ester amide)s.
biocompatibility and a relatively slow degradation rate. Additionally, it has been
demonstrated that closed-cell foams of these materials can be obtained using CO
2
as a blowing agent [73]. Poly(ether ester amide) (PEEA) copolymers based on PEG,
1,4- butanediol, and dimethyl - 7,12 - diaza - 6,13 - dione - 1,18 - octadecanedioate [74]
(Figure 6.4b) have also been evaluated as scaffold materials for tissue engineering
and have proved to sustain adhesion and growth of endothelial cells.
Random PEAs can be easily obtained by a two-step procedure fi rstly described
by Castaldo
et al.
[75] (Figure 6.4c). In this work, an oligoester mixture was pre-
pared by reaction of a diol (1,6-hexanediol, 1,10-decanediol, or 1,12-dodecanediol)
with an excess of sebacoyl dichloride. Oligoesters and the excess of dichloride were
then reacted with a stoichiometric amount of the appropriate diamine
(1,6 - hexanediamine, 1,10 - decanediamine, 1,12 - dodecanediamine). Thermal prop-
erties turned out to be strongly dependent on the fi nal amide ratio, as also found
for polymers based on adipic acid, 1,6-hexanediamine, and 1,4-butanediol. PEAs
related to nylons 6 or 66 and poly(caprolactone) have also been studied and their
enzymatic degradation has been demonstrated [76, 77].
PEAs with a regular microstructure can be obtained by using different kinds of
monomers. Thus, polymers differing in the arrangement of amide and ester
groups within their molecular chain can be derived. Obviously, the amide/
methylene ratio may also be easily varied depending on the methylene content of
the involved monomers. These materials are generally crystalline and render ori-
ented fi ber X-ray diffraction patterns as well as single-crystal electron diffraction
patterns. Analysis of their structure has shown a complex unit cell arrangement
which refl ects the packing preferences of amide and ester groups [78]. Biodegrad-
able PEAs with a regular structure can be classifi ed on the basis of their main
representative chemical units:
•
Derivatives of
- hydroxy acids (polydepsipeptides). They
can combine useful properties of poly(
α
- amino acids and
α
- amino
acids)s and are considered as particularly attractive reabsorbable materials.
Polymers can be prepared by ring-opening polymerization of morpholine-2,5-
diones. However, the synthesis of these monomers is complex and polymeriza-
tion usually requires severe reaction conditions resulting in unexpected
by-products from side reactions. This, together with the usual low molecular
α
- hydroxy acid)s and poly(
α
