THE POWER OF CAD/CAM LASER BIOPRINTING AT THE SINGLE-CELL LEVEL: EVOLUTION OF PRINTING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 82

the ribbon-biopolymer interface and cell layer. MAPLE-DW uses low-powered lasers in UV or near-
UV range, typical pulse duration is 8 ns. Single pulses focused at the quartz ribbon-absorption layer
interface cause volatile bubble formation that enables transfer. Bubble formation and the beginning
of material transfer are schematically represented in Figure 4.2 . Shallow penetration depth associated
with low-power UV lasers prevents direct effects and interaction between the cells and laser, preventing
adverse effects on the cells ( Riggs et al., 2011 ).
4.2.4 ANCILLARY MATERIALS
Materials chosen for the print ribbon and receiving substrate can maintain cell viability before print-
ing and promote cell proliferation, movement, and differentiation after printing. Poor choices for these
mediums can lead to cell damage and death. In general, hydrogel biopolymers are used for print ribbons
and substrates, but the amounts and biopolymer materials used should be carefully considered.
Cells are embedded in biopolymer hydrogel precursors on the print ribbon. The print ribbon re-
quires a biopolymer that allows cell adherence and admits cell ejection with a single laser pulse. For
nonfilm assisted transfer methods, such as MAPLE-DW, this layer acts as a volatile sacrificial me-
dium. Shear thinning and controllable viscosity are in new biopolymers for print applications, because
these qualities permit adjustments to compensate for altered printing parameters and while maintaining
single pulse ejection. Less viscous biopolymers (e.g. hydrogels) reduce the propagation and magnitude
of the stress wave generated, and thus subsequent cell damage, during printing ( Wang et al., 2009 ).
Gelatin has been an effective biopolymer, used for both film-assisted and non-film-assisted methods
( Hopp et al., 2005 ; Ringeisen, Kim, et al., 2004 ). Gelatin is amenable to thermal manipulation and can
be partially cross-linked using heat, which reduces pressure on the embedded cells during printing.
Thickness of this coating affects cell viability and varies based on the system and cells of interest.
Ringeisen, Kim, et al . (2004) found that cell viability increased from 50% to more than 95% when their
Matrigel ® biopolymer coating increased from 20 m m to 40 m m for pluripotent embryonal carcinoma
cells transferred using the MAPLE-DW platform.
Biopolymers with various growth factors and/or extracellular matrix (ECM) keep cells moist
and promote cell adherence. However, growth factors and ECM can introduce additional variability and
complications. ECM causes cells to bind firmly to the coated print ribbon, and thus requires more power
to achieve cell ejection. This in turn causes cells to be exposed to more irradiation and suffer greater
eventual impact damage. Matrigel ® , one such biopolymer, is often a good choice of biopolymer and has
been extensively utilized by laser DW researchers, even though it has been shown to have unintended
effects on cells, such as unintended stem cell differentiation ( Riggs et al., 2011 ; Vukicevic et al., 1992 ).
The receiving substrate serves to cushion impact-induced stress, maintain moisture, and provide the
appropriate growth environment for post-transfer cells. Gelatin and Matrigel ® are both used as receiv-
ing substrate biopolymers. Impact-induced stress and modeling is discussed in the mechanistics section
of this chapter.
4.3 MAPLE-DW MECHANICS
The cell transfer process in MAPLE-DW occurs in three sequential events: cellular droplet forma-
tion, cellular droplet travel, and cellular droplet landing. Analytical modeling of the two main
events—cellular droplet formation and landing—and their effect on postdeposition cell viability are
discussed in detail next.