Page 104 - IJB-9-2

International Journal of Bioprinting Holistic charge-based MEW scaffold model

inaccuracies and limitations, this model, as shown later, existence of intersection points. Both of them promote the
proves useful to understand many phenomena during the charge dissipation process, either by way of inverse corona
MEW process. discharge or contact discharge. In this way, the net polarity
of the scaffold is negative (β < 1). The characteristic surface
3.3. Interplay between wall morphology and is shown in Figure 5A and the lateral characteristic curves
residual charge are shown in Figure 5D-F (β = 0.1 or 0.5). Since the lateral
It is known that for a conservative force field, the characteristic curve changes insignificantly from the
→
relationship between the force F and the corresponding center to the periphery of the scaffold (Figure 5D-F), the
→
potential energy E can be expressed as F =−∇E . In lateral deviation caused by the global effect, if present, is
p
p
other words, the force is pointed into the direction, in insignificant. Moreover, in this case, the lateral deviation
which the corresponding potential energy decreases can be corrected due to the local and polarization effect
fastest, and the magnitude of the force depends on how fast (considering their effects on the energy surface at the
the potential energy decreases in that direction. As a prescribed locations discussed in section 3.2.1). Therefore,
particular 1D case, the strength of global, local, and the overall jet deposition trajectory is expected to follow
∂E p the red curve in Figure 5A. As N increases, the net polarity
polarization effect is evaluated by . In this way, the of the scaffold transitions from negative to positive values
∂x (β > 1). If the amount of positive residual charges is not too
energy surfaces and their lateral characteristic curves or large (for example β = 1.5), the global and local effects are
characteristic surfaces can represent how the jet is exposed insignificant, as shown in Figure 5D-F (β = 1.5). Therefore,
to the force due to the residual charge retained in the the initial lateral deviation caused by the global and local
scaffold, which, in turn, determines the fiber morphology effect can still be corrected to the prescribed locations,
by affecting the jet deposition trajectory. The characteristic as shown in the red curve in Figure 5B. On higher layers
surfaces help to understand the dynamic jet deposition (i.e., larger N values), the residual charge amount becomes
process, while the lateral characteristic curves are more even larger. In this case, the lateral deviation initiated by
useful for comparing the effects of different parameters on the global effect and strengthened by the local effect is
the jet deposition process. In the following sections, either significant (for example, Figure 5D-F, β = 3) enough that
of these two characteristic representations will be invoked when the polarization effect arises, the jet has deviated
for different analytical aims. beyond the region dominated by the polarization effect.
It has been shown that at a low collector temperature, Therefore, the jet deposition trajectory will be shown as
due to the increase of residual charge amount and decrease the red curve in Figure 5C. In this case, the initial lateral
of polarization extent, a “spindle-like” structure can be deviation cannot be corrected as it was in the previous two
observed between two adjacent intersection points . The cases. On subsequent layers, due to the repulsion from
[33]
formation of this structure has been preliminarily explained preexisting deviating fibers and the local effect, the new
based on the energy and dynamical analyses. However, in fibers are likely to deviate to both sides of the fiber wall
the previous studies, the electric force on the incoming jet and form a complete “spindle.” Once the “spindle-like”
segment is assumed to be caused by a single preexisting structure has been initiated, the following fibers will be
fiber that resides in the same vertical plane, in which discretely deposited, and the process of contact discharge
the jet is deposited . In contrast, for the present model is even further adversely affected, resulting in a larger
[33]
development, all of the fibers on the topmost two layers β. A larger β means a stronger global and local effect, as
have been considered. Moreover, the amount of residual shown in Figure 5D and 5E, which, further, promote the
charge is significantly affected by the fiber morphologies formation of the “spindle-like” structure. In other words,
in the scaffold. With other conditions (layer number, fiber there exists a positive feedback control loop between the
diameter, printing time, etc.) held constant, the residual jet deviation caused by the charge effects, as shown in
charge amount retained in a discretely deposited wall is Figure 5I. Therefore, the earlier the lateral deviation is
larger than that in a compactly deposited wall . Based initiated, the more apparent the “spindle-like” structure will
[30]
on these observations, it is now possible to explain the be. Based on these analyses, the overall wall morphology is
formation of a “spindle-like” structure from a panoramic expected to be as shown in Figure 5G.
perspective. It has been shown how the lateral deviation is initiated
When the layer number N is small, the distance in the formation of a “spindle-like” structure, yet it still
between the deposited fiber and the collector is small. needs explanation that both ends of the “spindle” are
Moreover, the conditions for “fiber sagging” are favored still pinned on the corresponding intersection points. As
due to the lack of geometrical barriers caused by the shown in Figure 5H, the incoming jet segment around

Volume 9 Issue 2 (2022) 96 https://doi.org/10.18063/ijb.v9i2.656