Biomedical Engineering Reference
In-Depth Information
While LDW offers capabilities for generating spatially precise 2D cellular microenvironments, it has
even more power beyond micropatterning approaches in fabricating 3D microenvironments. Similar to
ink-jet printing, LDW has demonstrated the ability to print cells and biomaterials in a layer-by-layer
fashion. While the throughput of LDW is lower than that of ink-jet, it offers higher resolution, potentially
allowing controlled studies of 3D cell-cell interactions in complex geometries. For 3D bioprinting ap-
proaches, in order to get sufficient height to the structure, fairly robust materials such as hydrogels are
often used to ensure both cell viability within the printed construct and 3D structural integrity. Although
this restriction on material configuration can help maintain construct geometry, it is also a limitation from
a cellular perspective; in order for cells to proliferate and migrate, they must break down the nanoporous
hydrogel, and replace it with ECM. 3D constructs have been realized using layer-by-layer approaches,
but they take weeks, or even months to become fully cellularized because of the geometric restriction of
nanoporous hydrogels. Despite this potential limitation, future applications could include complex
in vitro
tumor models, vessels, lamina, and other structures where the small attainable size is not a restriction.
Another recent method for creating 3D microenvironments using LDW is the fabrication of 3D mi-
crobeads of controlled size and placement (
Kingsley et al., 2013
). In contrast to traditional methods for
fabricating microbeads, LDW allows fabrication and placement in a single step, via
in situ
crosslinking of
a hydrogel. This enables cells encapsulated in the microbead to be precisely placed in 3D microenviron-
ments. What follows is that this method allows the study of cells within a 3D microenvironment, but on a
2D substrate, which, in turn, permits high-quality imaging and analysis. This feature also makes microbead
printing compatible with planar (2D) LDW, so hybrid 2D/microbead constructs can be fabricated (
Kingsley
et al., 2013
). Hybrid constructs allow 2D cellular studies based on point sources of material or factors
delivered by beads. Encapsulated cells within beads can potentially deliver factors continuously, or beads
themselves could be used for delivery. Beads can serve also as nodes at precise spatial locations to direct
2D spatial migration. The true power of this technique is realized when additional processing of microbeads
with a cationic polymer allows them to be shelled, and the hydrogel liquefied, leaving a macroporous cap-
sule that allows cellular migration and proliferation within the boundary of the capsule. The macroporous
structure afforded by microbead printing and capsule formation may also allow a highly cellular structure
to be realized much more rapidly because cells do not have to break down matrix in order to proliferate.
While the LDW field seems to be moving in the direction of 3D patterning, the potential of 2D LDW
has not yet been fully realized. As discussed, micropatterning, ink-jet printing, and LDW are complimen-
tary cell printing techniques that offer unique advantages for particular applications. LDW is particularly
well suited for applications that require spatial precision on homogeneous substrate and/or evolution of
the printed structure are desired. 3D layer-by-layer and microbead printing approaches both hold promise
for studying 3D cellular microenvironments, and allow a wide range of applications based on the same
technology. The coming decade holds great promise for the advancement of LDW and cellular studies
for tissue engineering and regenerative medicine based on 2D and 3D control of the microenvironment.
ACKNOWLEDGMENTS
We would like to thank Nick Schaub and Dr. Ryan Gilbert (RPI) for providing electrospun fiber substrates,
Dr. Yubing Xie (SUNY CNSE) for providing human breast cancer cells, and Dr. Guohao Dai (RPI) for providing
normal human lung fibroblasts. This work was also supported, in part, by NIH R56-DK088217 (DTC) and DoD,
Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellow-
ship, 32 CFR 168a (ADD).
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