Biomedical Engineering Reference
In-Depth Information
solution jet under the electrostatic forces in the solution as the solvent evaporates.
In the basic electrospinning set-up, there is no special orientation of the fibers
dropping down to the collector. However, by incorporating a rotating device, like
rotating drum, rod or plate, electrospinning has been shown to be able to form
aligned fiber meshes. In particular, electrospinning is able to fabricate various
nanofiber assemblies in situ. This gives electrospinning an important edge over
other larger-scale nanofiber production methods [103, 105, 106].
To fabricate 3D nanostructured porous polymer scaffolds, solvent-casting/
particulate-leaching techniques have also been widely used for nano-engineering
tissue [107]. Briefly, this technique involves producing a suspension of polymer
composites in a solvent. Salt particles are ground and sieved into small particles
and those of the desired size (mostly 100±200 m range paticles) are transferred
into a mold. A polymer suspension is then cast into the salt-filled mold. The
solvent is then removed by evaporation in air and/or in vacuum. After evapora-
tion of the solvent, the salt crystals are leached by immersion in water to form a
porous structure. Using this technique, the pore size can be controlled by the size
of the porogen particles and the porosity can be controlled by the salt/polymer
composite ratio. However, this method usually involves organic solvents, which
leads to many concerns about the toxicity of the solvent and associated cleaning
procedures.
Chemical etching can also be used to fabricate nanostructured polymer
scaffolds. For example, sodium hydroxide has been used to etch PLGA three
dimensional scaffolds to have nanoscale features within 10 minutes. In vitro
studies have demonstrated that such NaOH-treated PLGA three-dimensional
scaffolds enhanced chondrocyte functions by being more hydrophilic with a
greater degree of nano-roughness than non-treated scaffolds [108].
One of the common shortcomings of the fabrication technologies just discussed
is the lack of precise control of the three-dimensional porous architecture of the
scaffolds. 3D printing has successfully overcome such gaps. This process
generates components by ink-jet printing a binder on sequential powder layers.
Operation parameters such as the speed, flow rate and drop position can be
computer controlled to produce complex 3D polymer scaffolds. The disadvan-
tages of 3D printing techniques are limited material selection and inadequate
resolution. The materials must be injectable and the resolution is determined by
the size of binder drops, the jet and powder particles, which makes it difficult to
design and fabricate scaffolds with fine biological inspired nanostructures.
Besides the techniques mentioned above, many other useful methods are
available to fabricate nanostructured scaffolds, such as gas foaming/particulate-
leaching, phase separation and emulsion freeze drying, fiber meshes/bonding,
etc. In short, nanotechniques have greatly enhanced material fabrication methods,
improved the hierarchical structure of tissue scaffolds to mimic those of natural
tissue components (from micron to nano level) and provided the possibility to
make bio-mimic nanostructured scaffolds to improve cartilage growth.
