Advances in Collagen-based Tissue Engineering - Natural Polymers, Biopolymers, Biomaterials, and Their Composites, Blends, and IPNs - page 207

• Poly(lactic acid) (PLA) and Poly(lactic-co-glycolic acid) (PLGA) with relative-
ly high mechanical strength, but their hydrophobic surface is not favorable for
cell seeding.
Preparation of Parallel-oriented Nano and micro-diameter Fibrils
The natural ECM has fibrillar structure, with submicron to micron sized fibers (Wess,
2008). Thus, alternative methods of scaffolds fabrication that are more homogenous
and biomimetic are needed. For the first time, there is an evidence that construction
of nano- and micro-fibrillar biomaterials/scaffolds may be possible. Implants based
on such materials are the next frontier of bioengineering. The basic building blocks of
nano- and micro-fibrillar materials are likely to be nanofibers. One of the potential ad-
vantages of this method is that it should be capable of scaling up to produce nanofibers
in quantities required to mimic the ECM. There has been significant increase, recently,
in research on electrospinning of collagen fibers for tissue engineering (Powell et al.,
2008; Yang et al., 2008).
Electrospinning and Coaxial Electrospinning
Several laboratory techniques for producing nanofibers are being developed. For ap-
plication in tissue engineering, good progress has been made with the technique of
electrospinning (Matthews et al., 2002). In the electrospinning apparatus, a pump
feeds a solution of collagen into a long narrow tube, like a syringe needle. The flow
through this tube results in formation of a longitudinally oriented solution filament,
with collagen molecules oriented in direction of flow. A high DC voltage applied to the
tube, leads to formation of an electric charge in the polymeric filament emerging from
the tube. The charge generates forces of repulsion within the filament and neutralizes
the effect of surface tension. At sufficiently high voltage, the emerging solution fila-
ment ceases to be held together by surface tension and splits into nano-fibrils. These
are collected on an earthed drying--drum and on solvent evaporation become collagen
nanofibrils. The diameter of the nanofibrils is related to the solution viscosity. A sol-
vent composition will be found experimentally, to ensure stable nano-scale homogene-
ity in the polymer solution at the required viscosity. The choice of solvents is limited
by the requirement for rapid evaporation from the nanofilaments. Since electric charge
formation is fundamental to the process, the solution will have to meet strict require-
ments with respect to the dielectric constant and resistivity. There are several variables
in the process, the effect of which will have to be investigated experimentally. It will
be of paramount importance to confirm that no traces of solvents have been left in
nanofibrils, which might be in a long term cytotoxic. A potential advantage of the
electrospinning process is that, when proven to be successful, it can be scaled-up for
larger scale production. Complex scaffold architecture is possible by using several
spinnerets in parallel, extruding fibrils at different angles. Some information on the
electrospinning methods in collagen nanofiber fabrication and its great potential for
tissue regeneration is published (Zheng et al., 2010).
Electrospinning provides an important step in producing basic construction ele-
ments for the scaffolds. It also provides the preferred way of achieving two specific
objectives: regulating the rate of biodegradation of the scaffolds and producing scaffolds