Biomedical Engineering Reference
In-Depth Information
Directional:
Adirectional:
Parallel:
or
Antiparallel:
Mixed:
Figure 5.2
The possible orientations of
α
-AAs in the polymeric backbones.
“ tail - to - tail ” ), parallel, antiparallel, or mixed (Figure 5.2 ). These could be polymers
of other classes - polyurethanes and polyureas along with the said polyamides. To
render the polymers easily cleavable (in most cases hydrolysable), the labile chemi-
cal bonds have to be incorporated into the polymeric backbones to provide desir-
able rates of biodegradation. Preference should be given to ester bonds taking into
account both biodegradation rates and the stability (shelf life). The new polymers
comprising different types of heterolinks such as ester, urethane, urea, along with
peptide (amide) bonds, with the nonconventional orientation of
- AAs are expected
to diminish the immunogenicity of the polymers by “confusing nature” due to
“ unrecognizable ” structures of macromolecules.
α
5.2
Amino Acid - Based Biodegradable Polymers ( AABBP s)
5.2.1
Monomers for Synthesizing AABBP s
In this chapter, three classes of AABBPs containing ester bonds as biodegradable
sites are considered. These are AA-BB polycondensation polymers with noncon-
ventional “ head - to - head ” and “ tail - to - tail ” orientation of
- AAs in the polymeric
backbones - poly(ester amide) s ( PEA s), poly(ester urethane) s ( PEUR s), and
poly(ester urea)s (PEUs). The PEAs are composed of three building blocks: (i)
α
-
AAs, (ii) fatty diols, and (iii) dicarboxylic acids. They allow manipulation of polymer
properties in a wide range. PEURs and PEUs are also composed of three types of
building blocks - two blocks are (i)
α
α
- AAs and (ii) diols; however, the third block
is (iii) carbonic acid.
5.2.1.1
Key Bis - Nucleophilic Monomers
Key monomers for synthesizing all three classes of AABBPs are bis-nucleophiles
that represent dimerized
- amino acyl) - alkylene diester (tosic acid salt
of amino acid/alkylene diester, TAAD). These compounds are stable in the salt
form, commonly as di-
p
- toluenesulfonic acid (TosOH) salts. They are generally
α
- AAs - bis - (
α
