Polyanhydrides - Biodegradable Polymers - page 52

prepolymer was prepared to prevent phase separation and produce homogeneous
polymeric matrix. p -Nitroaniline was incorporated in the polymer matrix by
compression-molding in the form of a disk. They were then placed in buffer
(0.1 M, 7.4 pH) and release of p -nitroaniline as well as TMA-gly was measured and
found to be similar to its linear counterpart of the polymer (TMA- gly:SA 30:70)
[41, 42] , indicating that crosslinking has little effect on the degradation behavior
of this particular polymer, possibly due to its high hydrophilicity and low degree
of crosslinking. Thus, this system gives opportunity to further evaluate the degree of
crosslinking and control over the same to produce material useful for varied
applications.
In another study by Zhang et al. [43], the effect of type of amide bonds present
in the PA backbone and its blending with polyesters like PLA on degradation has
also been studied. Polymers of N , N - bis( l - alanine) - sebacoylamide ( BSAM ) and

P(1,6 - bis[ p -carboxyphenoxy] hexane [CPH]-BSAM) were synthesized and blended
with PLA. Hydrolytic degradation of polyanhydrides and their blends with PLA
were evaluated in 0.1 M phosphate buffer pH 7.4 at 37 °C. The results indicate that
the existence of amide bonds in the main chain of polymers slow down the deg-
radation rate, and this tendency increases with the increasing amount of these.
The copolymers and their blends with PLA possess excellent physical and mechan-
ical properties, thus making them more widely used in drug delivery and nerve
regeneration.

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3.2.7
Photopolymerizable Polyanhydrides

Photocrosslinking is preferred over chemical crosslinking which utilizes chemi-
cals that can cause adverse effects. Fiber-optic cables are used to provide photons
immediately after introduction of the polymer system to the desired site via injec-
tion. The other main advantages of photoinitiated polymerizations over other
crosslinking techniques are spatial and temporal control of the polymerization
which allows the precise control of polymer formation by directing and shuttering
the light source. The reactions are rapid enough to overcome oxygen inhibition
and moisture effects and can be controlled to occur over a time frame of seconds
to minutes. Ease of fashioning and fl exibility during implantation in terms of
physical and mechanical properties of materials without major modifi cations to
the backbone chemistry, which can alter biocompatibility, is added advantage.
However, the principal limitation to more extensive use of photopolymerizations
in biotechnology and medicine is the lack of biocompatible monomers and/or
oligomers that photopolymerize to form degradable polymer networks [44, 45].
Anhydride monomers with reactive methacrylate functionalities have been
developed and used for the preparation of PAs which shows in - situ crosslinking
on exposure to light. These systems were demonstrated to be biocompatible and
were used for bone augmentation applications [46].
Shastri et al. have prepared a new family of photochemically cured PAs which
can produce semi-interpenetrating degradable networks and evaluate them for