Biomedical Engineering Reference
In-Depth Information
show that two different impacts occur during the process under the specified conditions. First impact
happens at the computation start time, and second impact happens at approximately 2.2
m
s. During the
process, the peripheral particles 19139 (top) and 19150 (bottom) are subject to a higher stress level than
the inner particle 19144, which indicates that the cell membrane has a higher impact-induced mechanical
stress during laser bioprinting. Also, the bottom particle 19150 undergoes a higher stress than the top par-
ticle 19139.
Figure 4.9
also shows that the second impact has a negligible effect on the particles 19139
(top) and 19144 (inner). However, the bottom particle 19150 experiences higher stresses during the
second impact compared to the first impact (1.33 MPa vs. 0.96 MPa), which means that it is important
to study the stress information of the bottom particles during both impacts. In this simulation, the bot-
tom particle 19150 experiences the first impact-induced stress peak at 0.2
m
s and the second peak at ap-
proximately 2.6
m
is The probability of experiencing a second impact is highest for the bottom peripheral
particles, lowest for the inner particles with the top peripheral particles falling somewhere in between.
Through modeling studies (
Wang et al., 2007, 2008
), it has been found that cell peripheral regions,
especially the bottom peripheral region, usually experience a higher stress level than the inner regions.
This indicates that the cell membrane can be adversely affected by the impact-induced mechanical
injury during laser bioprinting. Additionally, the cell mechanical loading profile and the post-transfer
cell viability depend on the cell droplet initial velocity and the substrate coating thickness. Generally, a
larger initial velocity poses a higher probability of cell injury, and substrate coating can significantly re-
lieve the cell mechanical injury severity. Furthermore, two important impact processes occur during the
cell droplet landing process: first impact between the cell droplet and the substrate coating, and second
impact between the cell and the substrate. It is assumed that impact-induced cell injury depends on the
magnitudes of stress, acceleration, and/or shear strain, and also the cell loading history. In fact, over
the entire impact duration, the collective cell momentum change, rather than peak stress, acceleration and/
or strain, appears to be critical in determining the cell viability during laser bioprinting and deposition.
4.4
CELL VIABILITY
Post-deposition cell viability has been demonstrated for laser-assisted transfer techniques in terms of
cell survival, adhesion, mobility, proliferation, and differentiation. Damage to the cells can be due to
mechanical, thermal, or irradiative sources. Modeling of mechanical forces is detailed earlier. Due
to the properties of the sacrificial hydrogel layer, thermal and irradiative injury is considered negligible.
For example, in MAPLE-DW, for a print ribbon coating of 100
m
m, approximately 5
m
m depth would
be affected by UV laser irradiation. In addition, thermal cell injury does not occur due to disparate time
scales. During a typical 8-12 ns laser pulse, cell ejection from the print ribbon will occur within a few
m
s (as previously shown) and heat conduction does not permeate through the first biopolymer layer to
cause cell damage. As explained earlier in the discussion of ancillary materials, the choice of medium
for both ribbon and substrate coating are crucial to cell viability (
Schiele et al., 2010
;
Lin et al., 2010
;
Riggs et al., 2011
).
Similarly, in a study on the transfer of “vulnerable cell types” via AFA-LIFT,
Hopp et al. (2005)
demonstrated successful transfer of rat Schwann and astroglial cells as well as pig lens epithelial cells.
Two weeks after transfer, the cells survived, proliferated, differentiated, and regained their original
phenotypes. The three cell types were each isolated and then cultured in HEPES-buffered Dulbecco's
modified Eagle's medium (HDMEM). Cells were then harvested from culture and prepared for
Search WWH ::
Custom Search