Biomedical Engineering Reference
In-Depth Information
According the carbon chain PHB is classified into two classes:
a) Short chain lengths (SCL) are comprised of monomers with 3 to 5 carbon at-
oms per repeat unit
b) Medium chain lengths (MCL) are comprised of monomers with 6 to 14 carbon
atoms in the repeat unit. The conversion of SCL to MCL resulted in a change
in physical properties from crystalline to low crystalline elastomeric thermo-
plastics.
The PHB is distinguished primarily by its physical characteristics. It produces
transparent film at a melting point higher than 130°C and is biodegradable without
residue. The PHB can be synthesized and accumulated by Gram negative and also by
wide range of Gram positive bacteria,
cyanobacteria
and
archaea
. But the extensive
cross linking and the thick peptidoglycan layer present in the gram positive species
enables the bacterium to retain cell wall and it is more difficult to disrupt such cells to
isolate the intracellular PHB. Wallen and Rohwedder (1974) reported the identifica-
tion of polyhydroxyalkanoates other than P(3HB) especially poly-3-hydroxyvalerate
P(3HV) and poly-3-hydroxyhexanoate P(3HHx). More than 80 different forms of
PHAs can be synthesized by bacteria (Lee, 1996).
ProPerties oF PhB
Chemical structure and monomer composition are the most important factors in deter-
mining the physical and material properties of a polymer. Table 2 shows the properties
of PHB in comparison of synthetic plastic.
(a) It is thermoplastic.
(b) It has biodegradability, easily degrade in natural environment
(c) Biocompatible and non-toxic
(d) Isotactic and optically pure
(e) Insoluble in water and high density
(f) Highly crystalline nature
(g) Piezoelectric
(h) Produced from renewable sources
table 2.
Properties of PHB.
Parameter
Polypropylene (pp)
PHB
Melting point Tm [
0
C]
171-186
171-182
Crystallinity [%]
65-70
65-80
Density [g cm
-3
]
0.905-0.94
1.23-1.25
Molecular weight Mw (x10-5)
2.2-7
1-8
Tensile strength [MPa]
39
40
UV resistance
poor
good
Solvent resistance
good
poor
Biodegradability
-
good
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