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LIFT hydrogel printing: A defined route for highly controlled process

Figure 5. Time resolved images of the hyaluronic acid sodium salt (2%) transferring process for various
laser fluences. Scale bar: 400 µ.

formation, as well as at the final moment during The whole process, ending with droplet
the impingement of a gel microdroplet with an transfer, begins with the absorption of the laser
acceptor plate (or landing) . Hence, according pulse and rapid heating to high temperatures of
[50]
to the calculations, at a jet speed of 50 m/s, the the area of the absorbing layer and nearby thin
accelerations can reach 7 × 10 m/s at the initial layers of the donor glass plate and gel. In the
8
2
moment and 6 × 10 m/s during the landing case of short (nanosecond) pulses, when it can be
9
2
process. Caused by dramatic accelerations and assumed that heat spreads only in the direction of
fluid dynamics, mechanical stress can have a the optical axis, the increase in temperature under
negative effect on transferred objects, for example, laser irradiation with fluence F will be:
it can lead to severe deformation of cells, rupture 0
a F /(ρ
of their membranes and subsequent cell deaths . ∆T ≈⋅ 0 Eff ⋅ C PEff h ⋅ Eff (1)
[50]
)
As shown in Wang and Chrisey , the acceleration
[50]
at the final stage can be reduced by almost an order where a is the absorption coefficient of laser
of magnitude to 7 × 10 m/s when the acceptor radiation by a metal energy absorbing layer; ρ ,
2
8
Eff
plate is coated with a 40-μm thick acceptor gel С РEff , and h are the effective density, heat capacity,
Eff
layer. Such a coating during landing significantly and thickness of a thin heated gel/metal / glass
reduces dynamic stress, which leads to a decrease layer. The h value is determined by the thermal
Eff
in mechanical damage of the cells and increases diffusion length of the donor plate materials (fused
post-transfer cell viability up to 95% . It is worth silica) and gel: where τ is the time, at which the
[51]
noting that with an increase in the thickness of temperature reaches its maximum, which in most
the gel on the acceptor plate, dynamic stress will cases are equal to the laser pulse duration, D – is
decrease. However, with a large thickness of the the thermal diffusivity . Taking into account
[53]
gel, negative effects associated with immersion of D silica = 5.8 × 10 m /s and D water = 1.4 × 10 m /s,
−7
2
2
−7

cells at a great depth can be observed, which are at τ = 8 ns we get: h = 68 nm + 34 nm = 102 nm.
Eff
explained by a decrease in cell metabolism due to Thus, all the main processes of heat transfer are
diffusion limitations . associated with the heating of this thin layer.
[52]
84 International Journal of Bioprinting (2020)–Volume 6, Issue 3