Biomedical Engineering Reference
In-Depth Information
O
CO
O
O
C
O
N
H
CH 2
NN
N
CH 2
N
H
O
CH 2
CH 2
O
C
C
N
H
x
H
H
H
n
FIGURE 21.11
Schematic representation of TPE structure.
Due to the possibility of tailoring their structure and properties by careful selection of nontoxic
monomers (or by using substrates leading to nontoxic by-products), designed biodegradable poly-
urethane systems are becoming interesting potential materials for cartilage reconstruction [50].
21.4 TISSUE ENGINEERING APPROACH
Current trends in AC regeneration involve the potential use of chondrocytes or chondroprogenitor
cells (i.e., mesenchymal stem cells) as a source for TE. Combinations of cells with biodegradable
scaffolds and growth factors are widely tested in the context of cartilage regeneration. One of the
main issues in the cartilage TE is to design suitable scaffolds. In natural tissues, the extracellular
matrix that surrounds cells in the body not only physically supports cells but also regulates their pro-
liferation, differentiation, and morphogenesis. A scaffold, therefore, needs to provide the necessary
support for cells to proliferate and maintain their differentiated functions. Ideally, a scaffold should
have the following characteristics: (i) 3D and highly porous structure with an interconnected pore
network for cell growth and transport of nutrients and metabolic wastes, (ii) biocompatible and bio-
resorbable with a controllable degradation and resorption rate to match cell or tissue growth in vitro
and in vivo , (iii) suitable surface chemistry for cell attachment, proliferation, and differentiation,
and (iv) mechanical properties to match those of the tissue at the site of implantation.
Three-dimensional scaffolds must support the loading of an appropriate cell source to allow
successful infi ltration and attachment in conjunction with appropriate bioactive molecules in order
to promote cellular differentiation and maturation. Scaffolds could be modifi ed with cell adhesion
peptides, which show some promise of cell specifi city. To make scaffolds more cell-friendly, the
hydroxyl, carboxyl, and amine groups could be used on their surface to facilitate the attachment of
peptides or biological molecules. Additionally, the presence of protein growth factors in the scaffold
might stimulate differentiation and metabolism of the cells. Common growth factors are TGF-b and
bone morphogenetic protein (BMP).
Two main types of polymeric materials used for scaffolds in AC regeneration—natural and
synthetic biomaterials.
Collagen, fi brin, hyaluronan, alginate, starch, and chitosan-based matrices are among the most
popular natural scaffold materials as they offer a substrate that would normally be found in the
structure of native AC. Collagen (Figure 21.12) is a main component of the extracellular matri-
ces of many tissues including skin, cartilage, bone, and ligaments. This fi brous polypeptide is
mainly composed of glycine, proline, and hydroxyproline aminoacids. Fibrin can be produced from
patient's own blood [51]. Alginate and chitosan are polysaccharides. Alginate can be obtained from
brown algae while chitosan (Figure 21.13) from chitin by a deacetylation process. Chitin, however,
is derived from the exoskeletons of crustaceans (shrimp, crab, and other shellfi sh).
With natural materials one may face problems of immunogenic compatibility, batch inconsis-
tency, and low mechanical strength. Specifi cally engineered collagen-based scaffolds combined
with unique peptide technology are currently offered to treat cartilage defects. However, the result-
ing materials are far away from the ideal cartilage.
Most of the mentioned natural polymers could be used in gel form. This is a signifi cant advan-
tage because the cell can be added to the scaffold made of gels and injected into the body before the
material gelation. Then, the gel can be hardened by altering the pH of the solution [52] or by light
agitation [53], allowing the scaffold to fi ll out the defect in the tissue. This TE approach does not
require large incision to delivery cell with solid scaffolds into the body.
 
Search WWH ::




Custom Search