Biomedical Engineering Reference
In-Depth Information
In contrast to these glycolide polymers, polylactide (PLA) synthesized from l -
lactide or d,l-lactide monomer undergo hydrolysis at much lower rates than gly-
colide polymers. It has been observed that a part or all the parts of PLA mass still
remain without resorption when implanted up to one year. This implies that the
application of PLA to tissue engineering may be limited to the tissues which
require relatively long periods of time for regeneration.
Poly(
- caprolactone) (PCL) has often been used for scaffold studies, probably
because of its high processability (low melting point, many available organic sol-
vents, and high strength product). However, PCL scaffolds would not be applicable
to clinical tissue engineering, simply because PCL materials with high mechanical
strength is virtually non - biodegradable
in vivo
. Clearly, only biodegradable poly-
mers can be used for preclinical and clinical trials of tissue engineering.
In marked contrast to PLA and PCL, copolymers from P[LA/CL] are partially
crystalline and produce strong, elastomeric scaffolds that have biodegradation
rates ranging between those of PGA and PLA.
ε
14.5.2
Biopolymers
Biodegradable biopolymers or biomacromolecules include polysaccharides, pro-
teins (polypeptides), and nucleic acids. The most frequently used polymer for
scaffold fabrication among these biopolymers is collagen, whereas nucleic acids
have been scarcely used in tissue engineering studies. As collagen is ubiquitously
distributed in our body, it is no wonder that many researchers have chosen col-
lagen as a candidate material for scaffold fabrication, although mad cow disease
has greatly hampered the use of collagen. Porous collagen sheet can be prepared
by freeze-drying of aqueous collagen solution, followed by crosslinking with glu-
taraldehyde or dehydrothermal treatment. The porous structure can be controlled
by changing the freeze-drying temperature. Lower freezing temperature yields
scaffolds with smaller pores. Porous collagen sheets have been clinically applied
to the skin tissue engineering from the 1980s, often making composite with gly-
cosaminoglycan. Freeze-drying of aqueous solution of gelatin, denatured collagen,
also produces porous gelatin sheets, but they have been applied to tissue engineer-
ing much less frequently than collagen sheets. As collagen and gelatin are able to
serve as carrier of growth factors, the scaffolds fabricated from these polypeptides
may have features different from others.
The low mechanical strength and high rate of degradation of biopolymers can
be improved by chemical crosslinking. Fibrin glue which quickly forms upon
mixing fi brinogen with thrombin has been used as scaffold preferably by sur-
geons. This biomedical hydrogel has low mechanical strength, but is capable of
holding a large number of cells. In addition, this gel can seal another scaffold, if
it has large interstices.
Chitin and chitosan are biomaterials that have been common choices for
scaffold studies among polysaccharides. Scaffolds of high mechanical strength
can be prepared from these crystalline biopolymers, but it should be mentioned
