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Laser-assisted bioprinting at different wavelengths and pulse durations with a metal dynamic release layer: A parametric study

The effect of laser parameters on the printing pro- In this study, a metal layer is used, since it can be
cess depends on the applied laser-absorbing material. evaporated with all laser wavelengths, while polymer
Different materials have individual advantages and layers usually require UV wavelength below 400 nm.
disadvantages. If no DRL is applied, the process is As DRL, we apply 60-nm thick layers of gold sputter-
(strongly) dependent on the optical properties of the coated onto glass slides.
matrix material at the applied wavelength; furthermore,
cells near to the substrate might be harmed by laser 2. Material and Methods
radiation. Metals are good laser-absorbing materials 2.1 The Printing Process and Setup
and offer good wettability, enabling a homogeneous
distribution of hydrogel layers. However, during laser The laser printing device, in general, consists of the
printing, some metal debris are also transferred into pulsed laser source and two horizontal co-planar glass
the printed construct which is undesired for printed slides (the upper one hereinafter is called the “donor
tissue, even when gold and titanium are inert and bio- slide” and the lower one is called the “collector slide”).
compatible materials. The distance between the two slides is usually set to
Polymers as DRLs are hypothesized to be trans- about 1 millimeter. The donor slide is coated under-
formed completely into gaseous phase by laser- neath with a thin layer of laser-absorbing material and
induced photo-chemical reactions; however, actually a thicker layer of biomaterial to be printed, which
also some polymer debris become part of the printed can be a hydrogel with embedded cells. Laser pulses
structure. Even if they are nontoxic, they may have an are focused through the donor slide on the inter-
impact on the cell behavior. Additionally, due to our face between the donor slide and absorption layer.
experience, the distribution of hydrogel layers on This layer is evaporated in the focal spot, generating a
some polymers is not as homogeneous and reproduci- vapor bubble, which rapidly expands. After a few
ble as on metals such as gold. microseconds, the bubble reaches its peak volume
[2]
Alternatively, Schiele et al. used gelatin as a DRL expansion and starts re-collapsing. Due to inertia, the
for LaBP at 193-nm laser wavelength. Gelatin is free accelerated hydrogel continues its motion and flows as
of growth factors and matrix components that may a jet towards the collector slide (Figure 1).
The laser focal spot can be moved in x/y direction
influence cell behavior. However, the gelatin melted in the interfacial area between the donor slide and ab-
within one hour after printing at 37 °C. This is the sorption layer. Furthermore, the glass slides can be
typical temperature in human bodies and cell culture. moved relatively to each other. Thus, any pre-defined
Thus, it is not quite clear if this technique is suitable two-dimensional pattern and also three-dimensional
for 3-D printing, since cell-containing 3-D printed patterns can be generated layer-by-layer. A more de-
objects would possibly melt before cells could estab- tailed description of the printing process and setup
lish intercellular connections to maintain the 3-D cell was given by Gruene et al. [14]
pattern.
Other groups also applied an absorption layer sys- 2.2 The Applied Lasers
tem that is not completely evaporated, but experience
a bulging effect. Lin et al. [13] used cyanoacrylate to Two pulsed lasers have been applied, a Ytterbium:
glue brass foil on a quartz substrate; then they spread YAG fiber laser (YLPM-1-A4-20-20, IPG Photonics
cells in medium on the foil. Laser pulses evaporated the Corp., Oxford, MA, USA) and a Neodym:YAG diode-
cyanoacrylate and the generated bubble rapidly bulged pumped solid-state laser (Pulselas P-355-100-HP, Al-
the foil. The cell-medium compound was accelerated phalasGmbh, Göttingen, Germany). The Yb:YAG fi-
[8]
thereby and formed a jet. Brown et al. used a 7-µm ber laser offers 7 different pulse durations in the nano-
thick layer of polyimide, which was only partially second range (8, 14, 20, 30, 50, 100, 200 ns) at a laser
evaporated near the quartz substrate as a confined wavelength of 1064 nanometer. The repetition rate
pocket of gas. The vapor pressure forced the remaining can be chosen in the range of 1.6 to 1000 kHz, and the
polyimide layer away from the glass as a rapidly ex- maximum power is 20 watt. The maximum pulse en-
panding blister. Thereby, the polyimide surface re- ergy depends on the pulse duration and repetition rate.
mains intact. This bulging or blister effect avoids the The Nd:YAG laser offers three different wavelengths
contamination of the printed structure with debris. So (1064 nm, 532 nm, 355 nm; fundamental wavelength,
far, only low-viscosity liquids have been printed and it second, and third harmonics) at repetition rates in the
is not quite clear if also small droplets of hydrogels range of 0.4 to 1 kHz. Laser parameters are listed in
with higher viscosities can be printed by this way. Table 1.

44 International Journal of Bioprinting (2017)–Volume 3, Issue 1