Scaffolds for Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 10

Table 1.4 Biopolymers currently used or under investigation for biomedical application (Ikada
and Tsuji 2000 )
Polymers Structure Degradation Rate Biomedical Application

PGA Crystalline 100 % in 2-3 months Suture, soft tissue,

fracture fixation,

PLGA Amorphous 100 % in 50-100 days Oral implant, drug

delivery

PLA Semicrystalline 50 % in 1-2 years Fracture fixation, liga-

ment

PCL Semicrystalline 50 % in 4 years Augmentation Implant

Poly (orthoester) Amorphous 60 % in 50 weeks Suture, lubricant

powder, bone plate

Fig. 1.2 The chemical
structure of PHBV (Luzier
1992 )

1.4.1.1 Poly(hydroxybutyrate) Polymer and Poly(hydroxybutyrate-
co-hydroxyvalerate) Copolymer

Poly(hydroxybutyrate-co-hydroxyvalerate) PHBV copolymers are thermoplastic
polyesters. These polymers are composed of hydroxybutyrate (HB) units with
between 0 and 24 % of hydroxyvalerate (HV) units appearing randomly through-
out the polymer chain (Fig. 1.2 ) (Luzier 1992 ).

1.4.1.2 PHB and PHBV Synthesis

PHB and PHBV are produced by fermentation. A wide range of microorganisms
can be used to make PHB or PHBV. Among them, the bacterium, Alcaligenes, are
quite common in the environment and they can grow on a wide range of carbon
sources in both aerobic and anaerobic conditions. The strain Alcaligenes eutrophus
grows very efficiently on glucose and is safely handled in large quantities.
Bacteria need a carbon source, an energy source, nitrogen, phosphorus, sulfur,
trace elements, water and oxygen for balanced growth. If one nutrient (e.g., N, P, or
S) is limited, bacteria cannot produce amino acids and proteins, and they cannot grow.
These facts are exploited by PHBV production (Holmes 1982 ; Galgut et al. 1991 ).
Polymeric storage materials are usually formed when the microorganisms are exposed
to stress conditions and when their environment changes to unsuitable living condi-
tions. That is the lack or shortage of one or more essential nutrients and/or decreased
oxygen supply for aerobic species, the microorganisms produce intracellular carbon