Advances in Collagen-based Tissue Engineering - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 219

diffusion and proliferation. The ECM is virtually absent from the spinal cord. The ma-
jor part of the tissue of the spinal cord consists of large neurons which are responsible
for the receptive, integrative and motor functions of the nerve system. The second
component is neuroglial cells, smaller but more numerous than neurons. They are re-
sponsible for supporting and protecting neurons. The scaffold for implanting in spinal
cord will have to be biologically and physically compatible with the environment of
the spinal cord. It's structure will need to have directional orientation for both guid-
ing the ends of the severed nerves and for guiding stem cells and neurotrophic factors
to the location of the nerve repair process. A supply of stem cells from outside to the
repair area is essential, because mature neuron cells do not divide and once the adult
complement of neurons has been generated, no stem cells persist to generate more, or
to repair damage. Two main forms of elongated nerve cells will need accommodation
in the scaffold. The first is neuron, the nerve cell which may be 5-150 μm in diam-
eter. The second is axon, a thin, about 15 μm in diameter, extension growing out of
neuron; its function is to conduct signals towards distant target, which may be up to
1 m away. Axons are more frequently severed than neuronsand research on animals
has shown that they are much easier to re-grow and regenerate. A newly generated or
repaired axon will tend to grow along a directionally oriented fibril or pore channel
of the scaffold. Structural support of axonal regeneration will be combined with tar-
geted delivery systems for stem cells and also for therapeutic drugs and neurotrophic
factors to regionalize growth of specific nerve cells. In relation to blood, the cerebral
and spinal fluids are low in cellular nutrients. Scaffold permeability to a wide range
of molecular sizes will be crucial for access of oxygen and nutrients and for removal
of metabolic waste.
The author of this review has credible means, supported by experimental evidence,
of adjusting the values of the known parameters of the scaffold. She has developed
experimentally techniques for adjusting either or both of these properties by co-po-
lymerization/grafting of collagen with minor proportions of other biopolymers. These
include the internal surface area-to-volume ratio, the ranges of dimensions of inter-
connecting pores, diameters and orientation of fibrils and pore channels, as well as
the equilibrium water content. Two additional properties of the scaffold may require
adjustment: the rate of biodegradation after implanting and the mechanical strength.
The results of these experiments will be accessible in another original publication.
Regeneration of peripheral nerve is also important topic in regenerative medicine.
Ideally, a nerve guide material, first of all, should provide guidance and support for
regeneration axons, exhibit biodegradability properties and have a shelf-life appro-
priate to the nerve trauma (Alluin et al., 2009; Parenteau-Bareil et al., 2010; Wang
et al., 2009). Among the known nerves guides collagen-based composite materials
such as CultiGuide ® -composite poly-caprolactone and porous collagen-based beads,
RevoNerv ® -bioresorbable porcine collagen Type I+III nerve conduit and bovine col-
lagen type I tube from Integra Life Sciences Cor. are developed. Although surgery
techniques improved over the years, the clinical results of peripheral nerve repair re-
main unsatisfactory.