ENGINEERING 2D AND 3D CELLULAR MICROENVIRONMENTS USING LASER DIRECT WRITE - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 110

substrate ( Koch et al., 2012; Gruene et al., 2011a ). Current research in LDW has not yet focused on
customizing receiving substrate properties, which can be achieved using a combination of natural and
synthetic hydrogels.
Hydrogels as substrate coating materials allow for the manipulation of both the mechanical
and biochemical properties of the environment. The biochemical properties of the surface can be
engineered either by using protein-based gels or grafting functional moieties into the polymer that
makes up the gel. Additionally, the mechanical properties of the hydrogel are tunable by manipulat-
ing the amount of cross-linking and the relative polymer concentration. LDW applications have not
yet explored this parameter space. However, nonpatterning applications have begun customizing
both of these properties in hydrogels for a wide range of applications. One example of manipulation
of hydrogel biochemical and mechanical properties is within alginate gels, where the elastic modu-
lus and RGD grafting density were independently manipulated to optimize the environment for
stem cell differentiation ( Huebsch et al., 2010 ). Printed material can also be manipulated, through
in situ cross-linking on the receiving substrate. Example of this include the printing of cells sus-
pended in fibrinogen to substrates of thrombin, which after a period of incubation, forms fibrin
( Gruene et al., 2011a ), and the printing of alginate into a substrate of calcium, to fabricate and
localize microbeads ( Kingsley et al., 2013 ).
5.2.6 CUSTOMIZABLE TOPOGRAPHY OF NONHYDROGEL RECEIVING SUBSTRATES
Another means to control the receiving substrate surface properties is through the use of engi-
neered nonhydrogel materials or scaffolds. One such example of this is electrospun nanofibrous
structures. As a surface coating material, electrospun fibers can be fabricated with a variety of
natural and synthetic polymers, where fabrication parameters allow for the tuning of stiffness,
topography, degradation, and fiber size ( Pham et al., 2006 ). Fibers can also be chemically treated
after fabrication with desired functional units, similar to protein grafting in hydrogels. Addition-
ally, electrospun fibers can be fabricated in such a way that they are aligned, giving cells direc-
tional cues ( Schaub et al., 2013 ). Utilizing electrospun fiber substrates with cell printing is another
way to achieve idealized microenvironments, with the addition of directing cell growth through
properties of the fibers and their structural alignment. A 1-day time course of fibroblasts printed on
electrospun fibers ( Figure 5.4 ) indicates that cells maintain registry to the printed pattern. Aligned
fibers appear to direct the cell elongation and migration after LDW. Printing onto substrates with
topographical features enables the fabrication of unique constructs or cell studies that are difficult
to perform using other techniques.
5.3 LDW APPLICATIONS IN 2D
LDW was first used in 2D for electronics applications ( Chrisey et al., 2000 ). Once it was shown that
LDW could be adapted for use in soft materials transfer and deposition of biologics, 2D printing of
nucleic acids ( Colina et al., 2005; Fernández-Pradas et al., 2004 ), proteins ( Dinca et al., 2008 ), and
even live cells ( Wu et al., 2001 ) was demonstrated. These bioprinting approaches hold many prom-
ising applications, ranging from biosensors fabrication, to the creation of small grafts or biological
constructs, to building spatially precise cultures for in vitro diagnostics and cellular signaling studies.