3D BIOPRINTING TECHNIQUES - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 61

FIGURE 3.2
Inkjet printing mechanisms. (A) Thin film resistive heater generating a vapor bubble that ejects the bio-ink material;
(B) A piezoelectric driven actuator that squeezes out a defined quantity of the bio-ink upon pulsed activation. Both
mechanisms can either work in continuous or drop-on-demand jetting mode.
The DOD approach can be further categorized into thermal (heat) or piezoelectric (mechanical com-
pression) based on the droplet actuation mechanism. A schematic of DOD ink-jet printing based on both
mechanisms is presented in Figure 3.2 . In thermal DOD, an electric current pulse applied to the heating
element (thin film resistor) rapidly vaporizes a small pocket of ink in the microfluidic chamber. The re-
sulting vapor bubble creates the pressure pulse that propels the ink droplet through the nozzle orifice and
onto the substrate. In piezoelectric DOD, a microfluidic chamber above the nozzle contains a piezoelec-
tric transducer for droplet actuation instead of a heating element. A voltage pulse applied to the transducer
causes it to expand, creating the transient pressure that results in droplet ejection. For both forms of DOD,
the rheological and surface tension properties of the ink govern their ability to be printed. The ink viscos-
ity requirements vary from system to system, but a typical threshold is around 30 mPa/s ( Reis et al., 2005;
Seerden et al., 2001; Derby, 2008 ). In addition to the ink characteristics, the orifice size, the distance be-
tween nozzle and substrate, the frequency of the current pulse and resulting temperature gradient (thermal
DOD), and the frequency of the voltage pulse and piezo-deformation characteristics of the transducer
(piezoelectric DOD) have an effect on the ejected droplet size and spatial resolution in ink-jet printing.
In addition to creating structures of nonliving biomolecules, such as DNA ( Okamoto et al., 2000 )
and proteins ( Delaney et al., 2009 ), DOD ink-jet bioprinters have been successfully used to print and
pattern live mammalian cells, opening up new and exciting avenues in the field of tissue engineering
and regenerative medicine. Ink-jet printing also offers the capability to print multiple cell types, bio-
materials, or their combinations from different printheads in a single fabrication operation, allowing
for complex multicellular patterns and constructs. The concept of tissue and organ 3D printing, which
is being widely explored today, has evolved over years starting with ink-jet bioprinting. Both thermal
and piezoelectric DOD printers have been explored for cell bioprinting, but the use of thermal ones
has been more prevalent ( Cui et al., 2012 ). In thermal ink-jet printing, while the localized temperature
around the heating element can reach between 200-300°C, it lasts for only a few microseconds, and
the ejected cells are subjected to a temperature rise of only a couple of degrees above ambient for 2 m s
( Cui et al., 2012; Roth et al., 2004; Cui et al., 2010 ).
Xu et al. (2005 ) have highlighted the initial challenges in adapting the piezoelectric mechanism for
ink-jet cell printing, which revolve around their higher ink viscosity. Primarily, the frequencies and
power employed by commercial piezoelectric printers lie within the same range of vibrating frequen-
cies (15-25 kHz) and power (10-375 W) that are known to disrupt cell membrane and cause cell lysis
during sonification ( Cui et al., 2012; Xu et al., 2005; Simons et al., 1989; Hopkins, 1991 ). Adapting
piezoelectric printers for less viscous ink to lower the frequency and power would be challenging,