Biomedical Engineering Reference
In-Depth Information
(a)
O
O
CH
2
Ph
NH
O
O
N
H
O
n
O
CH
2
Ph
(b)
OOH
2
C
H
2
C
OO
O
O
O
H
N
H
H
OO
x
y
x
z
n
-
m
m
O
O
FIGURE 5.4
Chemical structure of the repeating unit of unsaturated poly(ester amide)s (AA-UPEAs).
(a) Unsaturated bonds located in the AA-UPEA backbone; (b) unsaturated bonds located as the pendant to the
AA-UPEA backbone.
saturated AA-PEAs because of the rigidity of the >C
∙
C< bonds in the backbone. There are two options
for the >C
∙
C< bonds to be located in the AA-UPEA backbone: diamide (from diacid) or diester (from
diol) segments. The AA-UPEAs based only on fumaryl, FPB, and FPH (i.e., the >C
∙
C< double bond in
the diamide segment) had higher
T
g
than those AA-UPEAs having the same double bond in the diester
segment. This is because the >C
∙
C< double bond in the diamide segment could also conjugate with
the two carbonyl groups and resulted in a higher rigidity of the polymer backbone, while the >C
∙
C<
double bond in the diester segment is isolated by the adjacent methylene group, the lack of conjugation.
Contrary to the unsaturated AA-UPEAs having double bonds in the backbone, those unsaturated
AA-UPEAs having pendant >C
∙
C< double bonds (Pang et al., 2010; Pang and Chu, 2010a,b) actually
lowers their
T
g
when compared with the corresponding saturated AA-PEAs. For example, 2-Phe-4 and
8-Phe-4 had
T
g
of 55°C and 40°C, respectively, while the Phe-based UPEA copolymers with pendant
>C
∙
C< double bonds (from 2-allyl glycine) had
T
g
values ranging from 20°C to 38°C, depending on
methylene chain length in diacid, diols, as well as the feed ratio of regular amino acid to 2-allyl glycine.
This suggested that the presence of 2-allyl glycine unit in the AA-UPEA backbone could impart additional
chain flexibility due to the increasing free volume from pendant double bonds which could act as internal
plasticizers, lowered the intermolecular interaction between copolymer chains. Therefore, more allylg-
lycine contents could result in higher chain flexibility and hence lower
T
g
values as reported (Pang et al.,
2010; Pang and Chu, 2010a,b).
hese >C
∙
C< double bonds in unsaturated AA-UPEAs also provide potential reactive sites for either
synthesizing additional derivatives or attaching biologically active agents to render biological activity to
AA-UPEAs. An example of synthesizing additional derivatives from AA-UPEAs is the reported studies
of AA-UPEA-based hydrogels via photocrosslinking with PEG diacrylate precursor (Guo and Chu, 2005;
Pang and Chu, 2010b). Figure 5.5 illustrates the scanning electron microscopic images of a AA-UPEA-
based hydrogel. Figure 5.5a is from AA-UPEAs with pendant >C
∙
C< double bonds, whereas Figure
5.5b is from AA-UPEAs with >C
∙
C< located in the AA-UPEAs backbone. These pendant or backbone
>C
∙
C< groups have also been converted into other functional groups like thiol-based -COOH by using
3-mercaptopropionic acid, -NH
3
Cl by using 2-aminoethanethiol hydrochloride, and -SO
3
Na by using
sodium-3-mercapto-1-propanesulfonate (Guo and Chu, 2010; Pang and Chu, 2010a). Figure 5.6 shows
the chemical scheme to synthesize these functional AA-PEAs via the unsaturated >C
∙
C< bonds in
the AA-UPEAs backbone (Guo and Chu, 2010). As a result, additional functional AA-PEAs could be
designed and synthesized from AA-UPEAs.
Beside the unsaturated AA-UPEA approach to provide additional functionality to AA-PEAs via their
unsaturated >C
∙
C< bonds, other efforts could also provide chemical functionality to AA-SPEAs, the
