Biomedical Engineering Reference
In-Depth Information
applications, gelatin is currently used both as a sacrificial and transfer material, as well as a receiving
substrate (
Schiele et al., 2011
). As previously described, a laser pulse vaporizes the sacrificial gelatin,
ejecting a droplet of cells. What is truly powerful about this approach is its exploitation of the thermally
reversible properties of gelatin. On the print ribbon, exploiting gelatin's thermal properties allows for
partial encapsulation of the biopayload, giving the opportunity for a wide variety of cells and other pay-
load to be transferred. Further, this technique does not require cells to attach to the matrix on the ribbon
(as with Matrigel
®
), and thus allows cells to be printed in their trypsinized state. This is less traumatic to
the cells because it does not disrupt the focal adhesions, thereby allowing even nonadherent cells to be
printed. In addition to gelatin, it is possible to use other materials with temperature-dependent gela-
tion or viscosity, such as glycerol (
Guillemot et al., 2010
), to encapsulate the biopayload for transfer.
Gelatin's thermal properties also offer unique benefits when used on the receiving substrate. Following
transfer, after a brief incubation period, the gelatin on the receiving substrate is liquefied, leaving only
the LDW-patterned cells. This further takes advantage of gelatin's thermal reversibility to minimize the
influence of potentially unwanted matrix factors in simple cell studies.
5.2.4
DYNAMIC RELEASE LAYERS
A dynamic release layer on the ribbon is often used to control the material interacting directly with
the laser, decoupling the laser from the transfer material. Dynamic release layers are thin, sacrificial
layers of material that interact with the laser at its operational wavelength, and can amplify energy
from the laser (
Figure 5.3
). Some example materials for this layer are triazene or a metal/metal oxide
(
Schiele et al., 2009; Ringeisen et al., 2008
). With LDW, the size of the transferred material droplet is
directly related to laser fluence. However, inconsistencies of the transfer material on the ribbon, such
as uneven coating, could produce unwanted spot-to-spot variation in the pattern. The use of a dynamic
release layer has been shown to help minimize the energy threshold required for transfer, and decrease
the thermal impact on the transfer material (
Banks et al., 2008
). The other benefit of using such a
dynamic release layer is that it provides a consistent material at the ribbon interface, thereby granting
a more predictable laser-material interaction when utilizing a variety of transfer materials. Without a
dynamic release layer, each transferred material will exhibit different transfer dynamics due to specific
laser-material interactions. The use of a dynamic release layer will result in a consistent vapor pocket,
independent of the transfer material. However, utilizing a dynamic release layer may impair or preclude
ribbon and substrate visualization.
5.2.5
ADDITIONAL HYDROGELS AND HYDROGEL PROCESSING USED IN LDW
An extensive amount of hydrogels and hydrogel blends have been used for LDW applications. The
use of a dynamic release layer has made it simple to change between different hydrogel materials on
the ribbon as a transfer material for cells or other biological contents. To date, the most commonly
used hydrogels as a transfer material are gelatin, Matrigel
®
, and alginate (
Barron et al., 2004a; Koch
et al., 2010; Koch et al., 2012
). The same hydrogels are also commonly used to coat the receiving
substrate during printing, to cushion the impact of printed cells, and to immobilize the printed spots
to increase pattern fidelity. The feature that these hydrogels have in common is that they have rapid
and controllable gelation properties that benefit immobilization of the biopayload. There has also been
use of fibrinogen/hyaluronic acid and collagen both as transfer material and for coating the receiving
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