Page 376 - IJB-10-3

International Journal of Bioprinting Multi-physical field control inkjet bioprinting

injection was stable when the voltage amplitude ranged voltage amplitude and velocity diameter. Therefore, if the
from 132 V to 288 V. We conducted three sets of tests at the material is changed, only the equation coefficients need to
same voltage amplitude to determine the average diameter be determined through experiments.
and velocity of microdroplets. Both the microdroplet
diameter and velocity increased with increasing voltage 3.1.3. Effect of pulse width on diameter and velocity
amplitude, and the relationship was approximately Notably, the pulse width of the piezoelectric ceramic
proportional. The microdroplet diameter ranged from 502 directly affected the ejection process because the pulse
μm to 668 μm when the applied voltage varied from 132 V width determined the duration of the positive pressure
to 288 V (Figure 4A). field. We controlled the frequency and duty ratio in the
actuation waveform to adjust the pulse width. When the
The experimental results were consistent with the pulse width applied to the piezoceramic tube changed,
simulation results of the pressure field and voltage the time the pressure field stayed at its maximum positive
amplitude. We utilized linear equations to establish a value changed, and the time the microdroplets were pulled
correlation between pressure, microdroplet velocity, apart changed. As shown in Figure 1B, when the duration
and diameter. This approach is based on the fact that as of the high pulse width level increased, the duration
the driving voltage increases, the piezoelectric ceramic of the positive pressure field increased. In contrast, the
deforms more, resulting in a more substantial pressure piezoceramic expanded earlier when the pulse width
field. As a consequence, the liquid column experiences a decreased, which increased the negative pressure in the
greater acceleration, leading to the formation of larger- tube and caused the liquid column to be pulled apart sooner
diameter microdroplets with faster velocities. We used to form microdroplets. Therefore, the pulse width affected
this linear relationship to control microdroplet formation, the moment when the liquid column snapped, which
diameter, and velocity. Equations IV and V are suitable for changed the diameters and velocities of the microdroplets.
piezoelectric inkjet printing. Experimental observation Then, we experimentally studied how the pulse width
can solve the formula coefficient when there is a change affected the diameter and velocity of the microdroplet. In
in the printing material or printhead structure. The the experiment, we kept the voltage amplitude at 160 V and
relationship between the microdroplet diameter and the changed the duty ratio or frequency to change the pulse
voltage amplitude was linear, as defined by Equation IV: width. We used a high-speed camera to film a microdroplet
(IV) falling in the above experiments.
Da Ub= 1 + 1
As shown in Figure 5, the microdroplet diameter and
velocity exponentially increased at first and remained
where D and U are the diameter and voltage amplitude,
and a and b are 1.09 and 372.44. Figure 4B shows that constant as the pulse width increased. When the pulse
width was too small, GelMA could not be ejected from the
1
1
when the voltage amplitude was 132 V, the minimum speed nozzle, and it only floated periodically at the nozzle orifice
was approximately 0.2 m/s. When the voltage amplitude because the piezoelectric ceramic shrank too quickly
was 288 V, the maximum speed reached approximately to allow a microdroplet to form. When the pulse width
1.84 m/s. The GelMA microdroplets took about 0.1 to 0.5 increased to 0.25 ms, microdroplets were ejected from the
s to gel when exposed to cold air. The fitted equation of the nozzle orifice. According to the functional relationship
microdroplet velocity and voltage amplitude is shown in between pressure and pulse width obtained by simulation,
Equation V: in the range from 0.25 ms to 0.8 ms, there were exponential
Va Ub= − (V) relationships between pulse width and microdroplet
2
2
diameter and velocity. The microdroplet diameter ranged
from 480 μm to 658 μm while the pulse width varied
where V and U are velocity and voltage amplitude, and from 0.25 ms to 0.8 ms (Figure 5A). Equation VI was
a and b are 0.01 and 1.02. The experiment utilized high- calculated by double-integrating the pressure and pulse
2
2
speed camera footage to illustrate changes in microdroplet width function (Equation III) and fitting the test data (the
diameter and velocity, as depicted in Figure 4C. The formula derivation process shown in the Supplementary
resulting graph confirms a linear relationship between File). Equations VI and VII are suitable for piezoelectric
voltage amplitude and the adjustments in microdroplet inkjet printing. Experimental observation can solve the
diameter and velocity. Multiple voltage amplitudes were formula coefficient when there is a change in the printing
tested, and their respective diameter and velocity were material or printhead structure. The relationship between
displayed. It is worth noting that altering the printed microdroplet diameter and the pulse width is defined by
material will not affect the linear relationship between Equation VI:

Volume 10 Issue 3 (2024) 368 doi: 10.36922/ijb.2120