Page 106 - IJB-9-2

International Journal of Bioprinting Holistic charge-based MEW scaffold model

point P in Figure 5H is far-field from the preexisting In the second category, some parameters can factor
fibers at previous instants; thus, the lateral deviation into in the morphology-charge interplay by affecting
for the new fiber is insignificant. When the jet segment fiber morphologies directly. Specifically, small fiber
encounters the intersecting point (whose elevation is diameter, small layer number, low stage speed, and large
higher than elsewhere), its distance to the preexisting interfiber distance facilitate the formation of a fibrous
fiber is thought to be instantaneously decreased to an wall of compactly deposited fibers [30] . In this way, the
infinitesimal value (which amounts to the jet directly incoming fiber can contact the preexisting fiber, and the
“jumping” from P to R on the toolpath in Figure 5C), charge dissipation process by contact discharge can be
and the incoming jet segment is still within the region enhanced.
dominated by the polarization effect (encircled in purple In the third category, some parameters play their roles by
rectangle in Figure 5F). Therefore, the lateral deviation, directly affecting the global, local, and polarization effects.
if present, is minimal, and the jet is redirected to the It is evident that an increase in the voltage and collector
prescribed locations due to the polarization effect at temperature will strengthen the polarization effect, which
the intersection points. This effect is represented by the serves to mitigate the lateral deviation of fibers and decrease
pinning of the “spindle-like” structures at the intersection the amount of residual charge. The effects of lateral location
points.
and grid number require additional scrutiny. The effect of
3.4. Effects of different parameters on morphology- lateral location as well as grid number K, which are the two
charge interplay examples of this case, was experimentally studied herein to
validate the proposed model.
Now, it is clear that fiber morphologies and residual
charge amount are mutually affected. The proposed model, The effect of lateral location on jet deposition can be
including the lateral characteristic curve or surface as well understood from Figure 6A and 6B. Based on Figure 6A,
as the global, local, and polarization effects, provides useful it is clear that from the center to the periphery of the
representations to qualitatively analyze their interplay scaffold, the global effect is increasingly significant, which
and the effect of different parameters on this interplay makes the initiation of lateral deviation occur earlier (i.e.,
(Figure 5I). In this section, these parametric effects will smaller layer number N). Moreover, by comparing the
be discussed in detail. These parameters can be classified slopes of segments ①②③ in Figure 6B, this trend for the
into three separate categories based on whether they lateral deviation will be further strengthened by the local
directly affect (i) the residual charge amount, (ii) the fiber effect. As stated earlier, the earlier the lateral deviation
morphologies, and (iii) the charge effects (global, local, arises, the more apparent the “spindle-like” structure
and polarization). will be. Therefore, the “spindle-like” structure becomes
increasingly significant from the center to the periphery of
In the first category, many parameters can factor into
this morphology-charge interplay by affecting the residual the scaffold, as shown in Figure 6C.
charge amount directly. Based on a previous study, Next, the effect of grid number K on jet deposition is
increase of substrate conductivity and deposition time shown in Figure 7. Similar to the analysis for the effect of
helps to decrease the residual charge amount considering lateral location, it can be easily concluded that an increase
that the residual charge is mainly dissipated to the in the grid number K facilitates weakening of both the
collector and the amount of residual charge decays with global effect (Figure 7A) and local effect (Figure 7B), thus
time . In addition, temperatures of material, collector, making the “spindle-like” structure less significant (shown
[30]
and ambience can affect the charge carrier mobility, in Figure 7C-E).
whose increase results in a decrease in the residual charge
amount. Moreover, the layer number N affects the distance 3.5. Explanation on “bridging” phenomenon for
between the new fiber and the collector. Therefore, an parallel walls
increase in N inhibits the charge dissipation process. A recent study showed that the “bridging” phenomenon
Furthermore, voltage plays its role mainly by affecting (Figure 8A) was observed when fibers were printed into
the electric field strength, whose increase facilitates parallel walls instead of scaffolds with interweaving
[26]
the charge dissipation process as well as increases the structures . It is found that the minimum achievable S / f
polarization extent. Finally, fiber diameter affects the d increases with layer number N and decreases with fiber
f
residual charge amount by affecting the surface-area-to- diameter d, which can be readily explained after slight
f
volume ratio, whose increase promotes the retention of modifications of the proposed holistic model. First, fibers
volumetric charge, thus facilitating the accumulation of are printed in the same direction. Second, the number of
residual charge. layers taken into consideration is as large as ten. Since the

Volume 9 Issue 2 (2022) 98 https://doi.org/10.18063/ijb.v9i2.656