Scaffolds for Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 12

Table 1.6 Thermal properties and degradation products of PGA, PLLA and PHB (Holmes
1982 ; Galgut et al. 1991 )
Polymer T m (°C) T g (°C) Degradation products

PGA 225-230 35-40 Glycolic acid

PLLA 173-178 60-65 L-lactic acid

PHB 175 4 3-Hydroxybutanoic acid

Another interesting property of the PHB polymer is that they exhibit piezoelec-
tric properties. This phenomenon has physiological significance in the stimulation
of bone growth. PHB behaves similarly as poly( γ -methyl-L-glutamate) in that the
piezoelectric response is generated by the application of a shear stress to orientate
polymer crystallites (Holmes 1982 ).

1.4.1.4 Biodegradation of PHB and its Copolymers

It has been reported that, PHB is biodegradable in vivo as a subcutaneous or intra-
muscular implant (Holmes 1982 ). The ultimate biodegradation product is (R)
3-hydroxybutanoic acid which is a normal metabolite in human blood (Holmes
1982 ). The polymer itself exhibits good biocompatibility with no cytotoxic
response. PHB and its copolymers also hydrolyzed in water with the normal uni-
versal acid-base catalysis for esters. At high pH, the rate of degradation is quite
fast but the hydrolysis proceeds very slowly in neutral buffer at body tempera-
ture. The kinetics does not appear to follow first-order behavior as the reciprocal
molecular weight does not decrease linearly with time (Holmes 1982 ). The plot of
the logarithm of molecular weight versus time has been found to be nearly linear.
Moreover, the initial experiments suggested that the rate of degradation of PHB
in vivo is significantly faster than the in vitro hydrolysis rate at the same tempera-
ture and pH. Actually the non-specific esterase and lysozyme enzymes secreted by
the body's immune system catalyze the process (Holmes 1982 ). The range of bio-
deterioration of implanted films can be varied from very fast to a modest but meas-
urable resorption to virtually undetectable weight loss of fiber monofilament over
an 18-months period (Holmes 1982 ).

1.4.2 Inorganic Materials

Certain inorganic compounds have been studied for bone and other mineralized
tissue engineering research in addition to the large variety of polymeric (macro-
molecular) materials. These materials can be categorized as porous bioactive
glasses and calcium phosphates. The most frequently used within the calcium
phosphates are β -tricalcium phosphate ( β -TCP), hydroxyapatite (HA) and its