Biomedical Engineering Reference
In-Depth Information
17.5
In-vitro
degradation
Since tissue engineering aims at the regeneration of new tissues, biomaterials
are expected to be degradable and absorbable with a proper rate to match
the speed of new tissue formation. The degradation behavior has a crucial
impact on the long-term performance of a tissue-engineered cell/polymer
construct. The degradation kinetics may affect a range of processes such as
cell growth, tissue regeneration and host response. The mechanism of
aliphatic polyester biodegradation is the bio-erosion of the material mainly
determined by the surface hydrolysis of the polymer. Scaffolds can lead to
heterogeneous degradation, due to neutralization of carboxylic end groups
located at the surface by the external buffer solution (in-vitro or in-vivo).
These phenomena reduce the acidity at the surface whereas, in the bulk, the
degradation rate is enhanced by autocatalysis due to carboxylic end groups
of aliphatic polyesters. In general, the amount of absorbed water depends on
the diffusion coefficients of chain fragments within the polymer matrix,
temperature, buffering capacity, pH, ionic strength, additions in the matrix
and the medium and the processing history. Different polyesters can exhibit
quite distinct degradation kinetics in aqueous solutions. For example, PGA
is a stronger acid and is more hydrophilic than PLA, which is hydrophobic
due to its methyl groups.
Of particular significance for application in tissue engineering are debris
and crystalline by-products, as well as the acidic degradation products of
PLA, PGA, PCL and their copolymers (Niiranen et al., 2004). Several
groups have incorporated basic compounds to stabilize the pH of the
environment surrounding the polymer and to control its degradation.
Bioglass
â
and calcium phosphates have been introduced (Rich et al., 2002).
Naocomposites showed a strongly enhanced polymer degradation rate when
compared to the neat polymer (Dunn et al., 2001). As mentioned with
regard to PLLA/ND nanocomposites (Fig. 17.3), improvement of
osteoconductivity of PLLA nanocomposites (i.e. the deposition of the HA
crystal on the surface) was observed. Fast degradation and superior
bioactivity make these nanocomposites a promising material for orthopedic
applications (Dunn et al., 2001).
In contrast, the degradation rates of biopolyester elastomer/MWNT
nanocomposites tended to decrease with the increase of MWNT loadings
(above 1 wt%) in SBF solution (Liu et al., 2009). Investigation of the
degradation behavior of biopolymer-based nanocomposites when nano-
structures are incorporated into the matrices is to be continued.
Allen et al. (2008) reported on the biodegradation of SWNTs through
natural, enzymatic catalysis. By incubating SWNTs with a natural horse-
radish peroxidase (HRP) and a low concentration of H
2
O
2
(
40
μ
M) at 4
8
C
~
over 12 weeks under
static conditions,
the degradation of SWNTs
