Biomedical Engineering Reference
In-Depth Information
[
49
] does not provide a natural microenvironment for chondrocytes, at least for the
cell layer near to the carrier. Teixeira et al. [
39
] could demonstrate that endo-
chondral ossification of a growth plate cartilage to bone took place on biphasic
calcium phosphate scaffolds because of their mechanical and chemical analogy to
the mineral phase of hard tissue. Bone formation was also observed after
implantation of calcium phosphate ceramics in ectopic tissues such as muscles
[
50
], leading to the hypothesis that calcium phosphates as hydroxyapatite support
hard tissue synthesis.
During discussion of the reduced cartilage quality when cultured on top of a
carrier compared to unsupported tissue, mass transfer limitations due to the carrier
have to be kept in mind. Tissue generation on top of a carrier is affected by a
insufficient nutrient supply from the bottom. Despite porosity of more than 50% of
the Sponceram HA
carrier, diffusion of nutrients and gases is hindered by the
carrier itself and additionally by the impermeable clamping device used to fix the
carrier in the liquid. Therefore, waste products such as lactate and CO
2
can only be
slowly removed from the matrix, which may lead to a decrease in pH. Low pH
may result in a reduction of the osmotic pressure which induces cell reactions
[
51
]. Mass transport limitations of the cartilage tissue can be improved by using
bioreactor systems such as a flow chamber or loading reactors [
35
,
52
,
53
].
4 Time Course of Cartilage Formation
Although cartilage-carrier constructs and cartilage without a carrier are cultivated
under the same conditions, cartilage grown without a carrier attained a signifi-
cantly higher quality of the matrix. Knowledge of the progress of cartilage may
help to identify differences in cartilage development and further optimize culti-
vation protocols by defining time windows for biochemical and biomechanical
stimulations. Additionally, in consideration of the clinical applications of in vitro
generated tissue, it is necessary to reduce the cultivation time before implantation.
The development of cartilage was thus studied over a long time period. Constructs
cultivated with and without carriers were harvested after 1-5 weeks of cartilage
high-density cell culture (step d) as shown in Figs.
5
and
6
.
Only slight changes in cartilage quality could be observed during cultivation of
cartilage-carrier constructs. In contrast, biochemical and biomechanical properties
varied over time during cultivation without carriers. By increasing cultivation
time, the masses of cartilage increased significantly (a \ 0.05). Young's modulus
and GAG content showed a maximum after 3 weeks of cultivation. Significant
differences in the Young's modulus were detected between the first to the
third week and second to fifth week (p \ 0.001), and there was also a significant
difference in the GAG content between the third to the first and fifth weeks
(p = 0.002). Changes in Young's modulus may be correlated with the GAG
content in tissue engineered cartilage, as zonal organization and orientation of the
collagen network could not be observed in unsupported cartilage using the
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