Page 97 - IJB-9-2

International Journal of Bioprinting Holistic charge-based MEW scaffold model

synergistic or antagonistic . Moreover, the printing with PCL pellets was heated by a double-walled heating
[30]
sequence may also affect the printing accuracy due to its vessel made from alumina ceramics. Between the outer
effect on the residual charge . and inner walls of the heating vessels are a heating pad
[32]
The relationship between the fiber morphologies and (New Era Pump Systems, Farmingdale, NY, USA) set
the residual charge is not only represented by the effect at 95°C. The syringe was connected to an air dispenser
of fiber morphologies on the residual charge but also the (Intellispense, Agoura Hills, CA, USA), which can
effect of residual charge on the fiber morphologies. Efforts regulate its output air pressure. The needle of the syringe
have been made in a previous study to clarify this problem, was connected to the positive terminal of a high voltage
in which the collector temperature was tuned to change supply (Stanford Research Systems, Sunnyvale, CA, USA),
the charge polarization extent [25,26] as well as the amount of while the grounded negative terminal was connected to
residual charge. In this way, different fiber morphologies, the collector plate. The collector plate (203 mm × 203 mm
including the “spindle-like” structures and “wall-with-gap” × 3.3 mm, shown in Figure 1), was mounted on an XY
structures, were observed at different collector temperature moving platform consisting of two slides (Velmex, USA)
values. A model based on charge mechanism was put driven by step motors. The ambient temperature is kept at
forward to explain the formation of these structures, which 22.5 ± 1°C. The key parameters enabling the MEW process
revealed how residual charge amount as well as charge are shown in Table 1.
polarization extent affect the wall morphologies. However,
the proposed model assumes that the jet is only exposed to 2.3. Imaging and data measurement
the electric force from the deposited fiber right under the A 1080P, 60FPS, and industry microscope camera with
jet, which is a valid approximation only when the interfiber 0.5× – 4.5× objective (Lapsun, CN) were used to take
distance is very large. To overcome this limitation, a more high-resolution images of samples. The mass of samples
realistic model needs to be developed to characterize the was measured by a high-precision electronic balance (HFS
case when interfiber distance is not large. FA204, CA, USA). All the temperatures were measured by
In this study, as a development of the previous work, the thermometer (Traceable 4039, USA). Error bars in the
an analytical model based on electric potential energy figures represent the standard deviation of samples.
calculation was constructed [33,34] . Instead of simply
considering a pair of charge points in the deposited fiber
right under the jet, this model comprehensively considers
all the charges in all fibers within the topmost two layers
of the scaffold, so it is a more “holistic” approximation of
the actual process. Based on the analyses on the energy
variation with different parameters, the interplay between
the residual charge and fiber morphologies is clarified. This
will provide a guideline for tuning different parameters to
decrease the amount of residual charges and improve the
jet deposition accuracy.

2. Materials and methods
2.1. Materials
Figure 1. Schematic of melt electrowriting system.
Poly (ε-caprolactone) (PCL) is in the form of pellet with
an average molecular weight (Mw) of 45,600 g/mol and Table 1. The parameters and their values or range in the
polydispersity of 1.219 (Capa 6500, Perstop Ltd., UK). MEW process.

2.2. MEW system configurations Parameters Values
A 5-mL pneumatic syringe (Intellispence, Agoura Hills, Polymer temperature 95°C
CA, USA) was loaded with PCL pellets and placed in the Voltage (U) 16 kV
heating oven (90°C) overnight with its needle-end upside. Pressure (P) 70 kPa
After each experiment, the syringe was kept at room Tip-to-collector distance 15 mm
temperature to avoid unnecessary thermal degradation
during storage. The MEW system used in this study is Interfiber distance 2 mm
schematized in Figure 1. The pneumatic syringe loaded Stage speed (v) 15 or 16 mm/s

Volume 9 Issue 2 (2022) 89 https://doi.org/10.18063/ijb.v9i2.656