Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 33

Table 2.3 The comparative weight loss for different biodegradable polymers given as time for
10 % weight loss at 37 °C and pH 7.4 (Amass et al. 1998 )
Polymer T 10 (h)

Vicryl ® absorbable suture
(PLLA (8 %)-co-PGA (92 %))

450 (18.8 days)

Dexon ® (PGA suture material) 550 (22.9 days)

PDS ® (poly (p-dioxanone) suture material) 1200 (50.0 days)

PHBV (20 % HV) (M w = 3 × 10 5 )

4.7 % @ 5500 (229.1 days)

PHBV (12 % HV) (M w = 3.5 × 10 5 ) 5.6 % @ 5500 (229.1 days)

PHB (0 % HV) (M w = 8 × 10 5 )

18 % @ 2500 (104.2 days)

matrix is very small, there is no internal autocatalysis or surface/center differen-
tiation. As a result, slower degradation resulted.
It was reported that the porosity of the polymer matrix is also an important fac-
tor. Lam et al. studied the influence of porosity on the degradation rate of PLLA
films in vitro and in vivo and concluded that non-porous PLLA degrades faster
than porous PLLA (Lam et al. 1994 ). This phenomenon can be attributed to the
fact that no internal autocatalysis occurred due to ionic exchange facilitated by the
porous structure in the case of porous films.
Among other factors, the influence of γ -ray irradiation which is used in the
sterilization process of medical devices has been reported. Spenlehauer et al. stud-
ied and reported that γ -ray irradiation can decrease the molecular weight of PLLA
and PLGA microspheres dramatically (Spenlehauer et al. 1989 ).
It was also noted that no major difference were observed between the degrada-
tion rates of PLLA in pH = 7.4 phosphate buffer and in non-buffered (Li 2006 ).
On the contrary, the absence of ionic strength in distilled water supported osmotic
exchanges and enhanced water absorption and promoted the surface/interior dif-
ferentiation in the early stages (Li et al. 1990 ). It was also emphasized that the
pH = 7.4 phosphate buffer improved solubilization of degradation byproducts in
comparison to distilled water or acidic media.

2.5 Degradation of PLLA and PHB Polymers

The degradation of PLLA and the copolymers generally involves random
hydrolysis of their ester bonds. Lactic acid is the degradation product of PLA,
which can enter the tricarboxylic acid cycle and can be excreted as water and
carbon dioxide. In general, PLA is more hydrophilic than PGA, and is more
resistant to hydrolytic attack than PGA. Table 2.3 shows the comparative weight
loss for different biodegradable polymers given as time for 10 % weight loss at
37 °C and pH 7.4.