NANOTECHNOLOGY: A TOOLKIT FOR CELL BEHAVIOR - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 10

and vitronectin) are nanometer-scale in dimension. Therefore, novel nanostructured ceramics, which
mimic the nanostructure of natural bone, have become quite popular. These nanobiomaterials exhibit
increased surface area, and contribute improved surface roughness, surface wettability, and cytocom-
patibility ( Zhang et al . , 2008 ) to tissue-engineered bone constructs. For example, nHA is commonly
used in bone tissue engineering, due to its natural abundance in native bone tissue. In one study, nHA
and bone marrow aspirate (BMA) were combined to form a paste for use in posterolateral fusion
surgeries across 46 patients. When evaluated 12 months later, there was no difference between bone
formation rates of iliac crest autografts (the gold standard in this surgery) and the nHA/BMA mix-
ture ( Robbins et al . , 2014 ). nHA has been shown to not only improve osteoblast behavior ( Webster
et al . , 2000 , Webster, 2001), but also improve bone-marrow-derived MSCs behavior in vitro ( Castro
et al . , 2014 ) and improve new bone formation in vivo ( Huber et al . , 2006 , Chang et al . , 2001 ) Another
highly studied ceramic material is zirconium dioxide or zirconia. It enhances fracture toughness in
other ceramics. Kong et al . studied HA-added zirconia-alumina nanocomposites in load-bearing or-
thopedic applications ( Kong et al . , 2005 ). The HA-added zirconia-alumina nanocomposites contained
biphasic calcium phosphates of HA/TCP and had higher flexural strength than conventionally mixed
HA-added zirconia-alumina composites. The in vitro tests showed that the proliferation and differen-
tiation of osteoblasts on this nanocomposite gradually increased as the amount of added HA increased.
Although nHA and other ceramics are powerful materials when used alone, they can be made more
versatile in combination with polymers to create a nanocomposite tissue-engineered scaffold. This ap-
proach can effectively allow for the fabrication of biomimetic physical and chemical gradients in bone,
cartilage, and osteochondral tissue. Using osteochondral tissue as an example, osteochondral defects,
caused by osteoarthritis and trauma, present a common and serious clinical problem. They are notoriously
difficult to regenerate due to complex inherent, stratified soft/hard tissue structure, poor cell mobility
within the matrix, lack of vascular network, and limited local progenitor cells. Continuous gradients of
proteins and collagen fibers are found throughout the cartilage zones and are essential for load transfer
and directed cell behavior ( Zhang et al . , 2009b ; Dormer et al . , 2012 ; Dormer et al . , 2010 ). The subchon-
dral bone and calcified zone also contain a gradient of calcified ECM (nHA) ( Zhang et al . , 2012 ), which
provides osteochondral structural integrity ( Zhang et al . , 2009b ). Considering the unique graded structure
of osteochondral tissue, ceramic nanoparticles can be embedded within a polymer or hydrogel scaffold
to form a ceramic gradient originating in the rigid subchondral region and terminating with zero ceramic
component in the articulating cartilage region of a scaffold ( Khanarian et al . , 2012 ). The stiffness of the
native microenvironment provides an essential stimulus that helps to shepherd the phenotypic differen-
tiation of pluripotent and multipotent stem cells, such as MSCs, in conjunction with chemical and other
stimuli. Therefore, implantable multiphasic scaffolds that leverage spatially controlled stiffness gradients,
morphogenetic factors, and configurable geometries are being developed in an effort to direct the differen-
tiation and phenotypic expression of bone-marrow-derived MSCs towards osteogenic and chondrogenic
cell types in one construct ( Wang et al . , 2009 ; Wang et al . , 2010a ; Chen et al . , 2011 ). Several researchers
also utilize biomimetic spatially controlled gradients ( Erisken et al . , 2008 ; Zhang et al . , 2005 ) with graded
PCL/nHA composite fiber meshes. The fibrous meshes were subsequently seeded with mouse preosteo-
blast cells and after a 4-week culture period, a deposited ECM was observed exhibiting gradations of
collagen type I and calcium that were similar to the gradients that exist in the osteochondral site.
Similar to osteochondral tissue, the ligament is another highly integrated tissue, which must perform
in a complex manner under a high stress environment. In order to best simulate regeneration and repair
of this environment, techniques employing two or more disparate biomaterials to create composite