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International Journal of Bioprinting Simulation-based comparative analysis of nozzles for bioprinting

1. Introduction main advantages are the possibility to use high-viscosity
bioinks with high cell density as well as their simple process
Bioprinting is one of the most studied applications for microextrusion; the accurate deposition of a small
of additive manufacturing. The combination of novel number of cells in a fast process with high cell viability
manufacturing processes with standard tissue engineering (80%–90%) for inkjet; or the high printing resolution for
protocols is leading to a revolution in the medical vat polymerization. Additionally, the main disadvantages
field. Bioprinting is formally defined as “the process of of the previous bioprinting techniques are the relatively low
producing tissues or organs similar to natural body parts printing speed with moderate cell viability (40%–80%) for
[1]
and containing living cells, using 3D printing” . One of the microextrusion and the limited bioink and cell density, as
main reasons this technology is widely studied is because well as the complexity of the system for inkjet, and harmful
it minimizes the rejection risk when cells from the own wavelengths for photopolymerization, complex relationships
[2]
patient are used in the process . Researchers are using between printing parameters and reduced number of
bioprinting to study the generation of vascular, neural, nontoxic materials available for vat polymerization. Despite
bone, cardiac, skin, or muscle tissues [3,4] . Since each tissue its low printing speed, microextrusion is the most versatile
has very specific properties and functionality depending technique and allows the use of highly viscous materials
on the role that plays within the body, the procedure in the with high cell density [2,7,8] .
bioprinting process as well as the main material must be a
perfect match with the cells of the target tissue. Regarding bioprinting materials, the interaction

Techniques used in bioprinting can be classified between the material and cell viability [14,15] , printability [16-18] ,
according to ASTM standards, as extrusion-based crosslinking [19-21] , or shape fidelity [22,23] have been analyzed
(microextrusion), jetting-based (inkjet, laser-assisted), in many studies. Hydrogels are the most common material
and vat polymerization stereolithography (SLA) , among in microextrusion bioprinting. Despite they are mainly
[5]
which microextrusion bioprinting is the most used composed of water, their usual behavior is closer to a shear-
technique. Microextrusion can be considered a combination thinning non-Newtonian material. In this type of materials,
of a fluid-dispensing system and an automated robotic the viscosity plays a major role in how the material flows.
system. This technique can be performed pneumatically or In general, the higher the viscosity is, the higher the inner
mechanically (piston or screw-driven) . While the former pressure and the shear stress are, meaning that higher force
[6]
facilitates the configuration of bioprinting parameters such is needed to obtain a proper flow. Previous studies showed
as the dispensing pressure, the latter provides a more stable that inner pressure and shear stress can provoke cell lysis,
volumetric flow [2,7,8] . i.e., cells die due to the break of their cellular membrane [24-26] .
As for viscosity, high values are usually required to achieve
Jetting-based bioprinting consists of the noncontact the best shape fidelity . Viscosity is also sensitive to
[27]
deposition of defined sized droplet into a substrate. temperature [28-31] and it is widely studied together with the
There are two main technologies related to jetting-based concentration of components to assure printability [17,18,32] .
bioprinting, inkjet and laser-assisted. While the former uses So as viscosity affects the nozzle inner flow and cellular
mechanical methods (thermal, piezoelectric, electrostatic, viability, it is a key factor to observe [33,34] . Therefore, the
or electrohydrodynamic) to generate controlled size actual behavior of the bioink flowing inside the nozzle is
droplets, the latter uses a laser to heat a biomaterial layer, an important aspect to determine but difficult to control
causing a thermal expansion of a tiny portion of the in experimental tests. With the small nozzle size, the
material, forming a droplet that falls into a substrate [9,10] . nozzle inner geometry greatly influences the material flow.
Additionally, the smaller is the nozzle, the more difficult it
Finally, vat polymerization, also known as SLA, refers
to the photopolymerization of a photocurable liquid, a bio- is to sensorize it and to experimentally measure the flow
without disturbing it or using any scaling technique. There
resin, by a specific light source. Depending on the light are many studies about bioprinting hydrogels but most of
source and its movement, vat polymerization can be classified them are experimental and focused on bioprinting results
as (1) stereolithography (SLA) when the light source is a while the study of crucial inner parameters such as pressure
movable laser beam that directly irradiates the resin; (2) or shear stress is often neglected or overlooked [18,35] . For
digital light processing (DLP) when a digital micro-mirror this reason, computational simulations were proposed as
device projects a silhouette of the layer; or (3) two-photon a helpful tool to obtain hard-to-measure, bioprinting inner
polymerization (2PP) when a femtosecond laser emits two parameters [36,37] . Previous studies showed that cell viability
photons that excite the resin, causing the polymerization .
[5]
was highly impacted by the nozzle inner pressure, and even
Some authors [11-13] have reviewed the most commonly more by the shear stress . In this sense, Blaeser et al.
[24]
[38]
used bioprinting techniques. All of them agree that the classified the shear stress ranges that affect cellular viability
Volume 9 Issue 4 (2023) 210 https://doi.org/10.18063/ijb.730