Synthetic Biodegradable Polymers:
Biodegradable polymers offer the advantage of being able to be eliminated
from the body after fulfilling their intended use. Therefore, avoid the usual costly
and complicated procedures to remove the implants or scaffolds. It is not surprising
that biodegradable biomaterials are becoming more and more important in
biomaterials and tissue engineering field. In this chapter, some of the important
synthetic polymers will be reviewed.
Several biodegradable polymers such as polylactide (PLA), polyglycolide
(PGA), poly(glycolide-co-lactide) (PLGA), poly(dioxanone), poly(trimethylene
carbonate), poly(carbonate), and so on have been used extensively or tested on a
wide range of medical applications due to their good biocompatibility, controllable
biodegradability, and relatively good processability. PLA, PGA, and PLGA are
bioresorbable aliphatic polyesters belonging to the group of poly α-hydroxy acids.
These polymers degrade by nonspecific hydrolytic scission of their ester bonds.
1- Poly(glycolide):
Poly( glycolide) (PGA) is synthesized through the ring opening polymerization
of glycolide to yield high-molecular-weight materials. The monomer glycolide is
synthesized from the dimerization of glycolic acid.
PGA is the simplest linear aliphatic polyester. It is highly crystalline (45% 55%) with a high melting point (220°C -225°C) and a glass transition temperature
of 35°C -40°C. Because of its high degree of crystallization, it is not soluble in
most organic solvents except for highly fluorinated organic solvents such as
hexafluoroisopropanol.
PGA Fibers exhibit high strength and modulus because of its high degree of
crystallinity. But it is too stiff to be used as sutures except as braided material.
PGA was used to develop the first totally synthetic absorbable suture that has been
marketed as Dexon since the 1960s by Davis and Geek. Sutures of PGA lose about
50% of their strength after two weeks and 100% after four weeks and are
completely absorbed in 4 - 6 months.
PGA biodegrades by a combination of hydrolytic scission and enzymatic
(esterase) action producing glycolic acid which either can enter the tricarboxylic
acid (TCA) cycle to be finally excreted by the body as carbon dioxide and water or
is excreted in urine .
Glycolide can be copolymerized with other monomers, such as lactide and
trimethylene carbonate, to yield copolymer with various mechanical and
biodegradation properties for different applications.