Biomedical Engineering Reference
In-Depth Information
the molecular affinity to water and leads to a slower hydrolysis rate. It takes sev-
eral months or even years for a PLA scaffold or implant to lose mechanical integ-
rity in vitro or in vivo. To achieve intermediate degradation rates between PGA
and PLA, various lactic and glycolic acid ratios are used to synthesize PLGAs.
These polymers (PLA, PGA, and PLGAs) are among the few synthetic polymers
approved by the US Food and Drug Administration (FDA) for certain human clini-
cal applications (Zeltinger et al. 2001 ; Sherwood et al. 2002 ; Koegler and Griffith
2004 ; Lu et al. 2005 ).
There are other linear aliphatic polyesters which are also used in tissue engi-
neering research. They are poly( ε -caprolactone) (PCL) (Allen et al. 1998 ) and
poly(hydroxybutyrate) (PHB). It has been suggested that due to the lower rigidity
of PCL, it can be more appropriate for cell growth and formation of ECM than
PLLA (Zhao et al. 2004 ). It is also reported that PCL is also able to reduce the
stress shielding effect and the strength of PCL is low and not sufficient for load-
bearing application (Lowry et al. 1997 ). PCL degrades at a slower rate than PLA,
PGA, and PLGA. This slow degradation process makes PCL less attractive for
general tissue engineering applications but more attractive for long term implant
and controlled release applications. PHB is made by microorganisms via fermen-
tation and degrades very slowly because of their hydrophobic nature (Holmes
1982 ). This polymer has already been used to produce composite biomaterials for
potential bone tissue repair. Together with high biocompatibility, PHBV polymers
have degradation times much longer than other biocompatible polymers, which
can allow the PHBV scaffolds to maintain their mechanical integrity until there is
sufficient bone growth throughout the implants (Lutton et al. 2001 ). PHBV poly-
mers have been found as minimal inflammatory in long term studies of subcutane-
ous implants in mice and rats (Gogolewski et al. 1993 ). This polymer indicated
positive cell attachment and growth (Kumarasuriyar et al. 2005 ). Table 1.3 shows
the actual and possible applications of biodegradable polymers in medicine and
Table 1.4 lists synthetic biodegradable polymers currently used or under investiga-
tion for medical applications.
Table 1.3 Medical applications of bioadsorbable polymers (Ikada and Tsuji 2000 )
Function
Purpose
Examples
Bonding
Suturing
Vascular and intestinal
anastomosis
Fixation
Fracture bone fixation
Adhesion
Surgical adhesion
Closure
Covering
Wound cover, local hemostasis
Occlusion
Vascular embolization
Scaffold
Cellular proliferation
Skin and blood vessel
reconstruction
Tissue guide
Nurve reunion
Drug delivery
Sustained drug release
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