Biomedical Engineering Reference
In-Depth Information
blocks are guanine (G) and cytosine (C) DNA base-pairs [93]. Their units
undergo a hierarchical assembly to form a six-membered supermacrocycle by
the formation of 18 hydrogen bonds in physiological conditions, then the
rosettes form a stable stack with an inner channel 11
Ê
in diameter based on
electrostatic forces, base stacking interactions and hydrophobic effects. It was
reported that such nanotubes have a similar size and morphology with collagen,
enhance select protein adsorption and consequently a variety of cell functions,
and can be functionalized with peptides or drugs tailored for specific applica-
tions [94±98]. With the self-assembly of such nanomaterials, they can also
increase in viscosity from a solution at room temperature to a gel at body
temperature to possess a strong affinity to bond to other components such as
nano-HA and hydrogels [98, 99].
A second class of self-assembled orthopedic nanostructured biomaterials has
also been inspired from proteins and peptides. These nanostructured bio-
materials are genetically selected/designed peptides with specific binding to
functional solids, tailoring their binding and self-assembly characteristics,
developing bifunctional peptide/protein genetic constructs with both material
binding and biological activity, and using these as molecular synthesizers,
erectors and assemblers [100]. For example, the PuraMatrix
TM
hydrogel peptide
is a new synthetic 16-amino-acid peptide developed by 3DM (Cambridge, MA).
When it is exposed to physiological salt conditions, the peptide forms a hydrogel
because of ionic bonds and hydrophobic interactions. It was stated that the
nanofiber structure resembled natural collagen with fiber diameters of 5±10 nm
and promoted cell (such as osteoblasts and fibroblasts) proliferation greater than
any other synthetic scaffold, such as PGA.
9.3.3 Nanotechniques for biomaterial fabrication
As mentioned, in articular cartilage, tissues are organized into three-dimensional
functional structures. As shown in Fig. 9.13, cartilage at this level can be divided
into four zones: (a) the superficial tangential zone, which comprises 10±20% of
cartilage thickness with collagen (mostly type II) fibers oriented parallel to the
joint surface to resist shear stress; (b) the middle zone, which comprises 60% of
the cartilage thickness, (c) the deep zone, which comprises 30% of the cartilage
thickness with collagen (mostly type II) fibers vertically oriented to enhance
compression stress, and (d) the calcified cartilage zone where cartilage inter-
faces with the bone. To engineer such functional extracellular matrices success-
fully, the scaffolds have to be designed with micron and nanostructures, aligned
fiber orientation, controlling material composition, facilitating cell distribution
and guiding tissue regeneration in three dimensions.
Because electrospinning can fabricate a variety of organic and inorganic
materials, generating different sizes of fibers and controlling fiber orientation in
scaffolds, many have studied electrospinning as a very promising technique for
