Page 373 - IJB-10-3

International Journal of Bioprinting Multi-physical field control inkjet bioprinting

printhead. Therefore, considering the rheological properties 3. Results and discussion
of GelMA and the temperature range suitable for cell 3.1. Control of pressure field to adjust microdroplet
survival, the printhead system temperature was set at 37°C.
diameter and velocity
The temperature-controlled chamber is responsible The key to MFCPIB was the precise control of the temperature
for microdroplet shaping after injection. An observation of the microdroplets, and the diameters and velocity of the
window made it possible to monitor print status, and the microdroplets profoundly influenced printing. Because the
thermal insulation cover of resin reduced heat exchange amount of heat needed to cool a microdroplet depended
between the chamber and the outside environment. on its diameter, the velocity of a microdroplet determined
Usually, only the baseplate of the temperature-controlled the time available for heat exchange with the temperature
chamber could be used for temperature control. Since field. Therefore, it was necessary to use the pressure field to
microdroplets were deposited on the floor for a long time, control the microdroplet diameter and velocity precisely.
it was difficult for cells to survive for a long time at 4°C or In this section, we studied the law and mechanism of the
lower. Therefore, it was challenging for the temperature of pressure field regulating droplet velocity and diameter.
only the cooling floor chamber to meet the requirements.
We developed a temperature-controlled chamber with 3.1.1. Pressure field control and optimization
upper and lower covers to solve this problem. The upper The deformation of the piezoelectric ceramic was
cover and baseplate inside the chamber were refrigerated controlled by adjusting the driving wave. The deformation
of the piezoelectric ceramic changed the pressure field
using the Peltier effect for cooling. The baseplate has six inside the printhead. The GelMA in the printhead is
semiconductor chips (TEC2-19008, Yandong, China), and maintained at a temperature of 37°C. Rheological testing
the top cover has four semiconductor chips. The air in the results displayed in Figure 6E indicate that GelMA has very
temperature-controlled chamber transferred heat between low viscoelasticity at this temperature, with the storage
the top cover and the baseplate, keeping the internal gas modulus and loss modulus measuring approximately
temperature uniform and low enough to shape the GelMA 0.01 Pa, which is almost negligible. It is worth noting that
microdroplets. After 15 min of system operation, a uniform GelMA has been observed to exhibit a Newtonian fluid
temperature field can be formed in the temperature- behavior under 37°C. This means that it can be described
controlled chamber. by the Navier–Stokes equation, which is commonly used to

2.6.3. Printing assist system calculate the behavior of fluids that follow Newton’s laws of
The third part of the printing system is the printing assist viscosity. Specifically, Equation II can be used to predict the
system, which includes motion control and monitoring flow of the fluid under various conditions and to calculate
equipment to ensure high-quality printing. A 3D printer various properties such as speed and pressure: 36
with motion resolution and positioning accuracy of 10 Dv 2 (II)
p
μm in the X, Y, and Z directions made in this laboratory ρ Dt = ρ f −∇+ µ∇ v
(SIA Inkjet Bioprinter) was utilized for driving the
displacement of the printing platform. The monitoring where v, t, ρ, f, p, and are the fluid speed, time, density,
equipment includes a high-speed camera (DimaxHS4, body force, pressure, and dynamic viscosity, respectively.
Pco, Germany), thermal imager (A615, FLIR, USA), and The left side of this equation represents the rate of change
a strobe light emitting diode (LED). The monitoring in momentum per unit volume of fluid. On the right
equipment is separately fixed with tripods. The high- side, the first term represents the volumetric force acting
speed camera photographed falling microdroplets to on the fluid, the second represents the pressure acting
measure their diameters and velocities. The diameter and on the fluid, and the third represents the viscous force
velocity of the microdroplet were measured by adopting acting on the fluid. When piezoelectric ceramics vibrate,
37
the Image-Pro Plus (version 6.0, MEDIA CYBERNETICS, the boundary conditions of the control equation change,
USA). The image’s resolution acquired in this work is and the pressure on the fluid microelements in contact
1920 × 1080 pixels, and one pixel corresponds to an area with the piezoelectric ceramics changes. This change also
of 22.32 × 37.74 μm . In the image processing software, simultaneously occurs at other microelements, forming
2
the microdroplet diameter and velocity were calculated a pressure wave that changes the pressure field inside the
according to the number of marked pixels and the time printhead. The pressure field ultimately determined the
per frame. A thermal imager was used to measure the air formation of the microdroplets. The two most essential
temperature field in the temperature-controlled chamber parameters in the driving wave were the voltage amplitude
and the temperature of the printed structure. We used the and pulse width, which determined the size and duration
printing system to conduct printing experiments. of the deformation of the piezoelectric ceramic. Thus, these

Volume 10 Issue 3 (2024) 365 doi: 10.36922/ijb.2120