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Designs and Applications of EHD 3D Printing

A B The control of layer-by-layer scaffold structure
mentioned above was poor, and thus, researchers
explored a precision controlled deposition method to
fabricate 3D structures with controlled fiber diameter,
orientation, and pore size. Particularly, Wei et al.
applied a hybrid hierarchical approach to fabricate 3D
scaffolds using polycaprolactone (PCL), a phase-change,
biocompatible, and biodegradable printing material.
C
Melting extrusion was employed under pneumatic
pressure to produce a filament of hundreds of microns
in diameter, and EHD jetting was used to deposit small
feature with single micron resolution. Both 2D and 3D
patterns were fabricated as shown in Figure 7C-E [44,45] .
Cai et al. used a similar EHD direct writing technique to
build a 3D PCL scaffold with controlled fiber diameter
Figure 6. (A) Five stripes and each stripe built by single-layered and orientation [Figure 7F and G], and collagen was
printing. (B) Each stripe built by three-layered printing (adapted grafted to scaffold to improve regeneration of cartilage .
[46]
from Wang and Stark ). (C) Field emission scanning electron Collagen-grafted scaffold helps chondrocytes to maintain
[40]
microscope cross-sectional image of electrohydrodynamic jet- healthy phenotypes, and it also increases production of
printed Ag mesh on the surface (adapted from Seong et al. ). cartilage-like extracellular matrices, when compared with
[33]
conventional PCL scaffolds .
[46]
3.2.1 Electronics Applications Ahn et al. used the EHD printing method to fabricate a
In Figure 6A, Wang et al. used silver ink to deposit 3D porous structure, and the result showed that biological
five overlapping drops of 88 µm size by a single-layered capabilities of scaffolds benefit from roughened surface
[47]
printing . Figure 6B indicates that droplets were mainly and enhanced water absorptivity . Instead of traditional
[40]
accumulated vertically rather than spreading laterally substrates, a polyethylene oxide (PEO) solution bath was
by three-layered printing and a denser morphology was used as a ground target to provide an elastically cushion to
obtained compared to porous structures built by single- prevent plotted struts crumbling, as shown in Figure 7H
layered printing . Previously, EHD jet printing has and i. In addition, a grounded copper plate was immersed
[40]
been applied to the fabrication of devices on a 2D flat in the PEO bath, and previously deposited layers of struts
substrate. Seong et al. succeeded in printing lines with can be dissolved in the remnant solvents to eliminate
[47]
a width of 7 µm and height of 1.43 µm on a 3D-curved residue charges .
surface with curvature, diameter, and height of 5.5 m ,
−1
60 mm, and 10 mm, respectively, as shown in Figure 6C. 4. Conclusion
The resulting sheet resistance is below 1.5 Ω . EHD phenomenon has been known for more than a
[41]
century, while EHD technology was just applied as a
3.2.2 Biotechnology Applications
high-resolution additive manufacturing method a decade
Sullivan et al. presented three ground electrode ago. EHD inkjet printing utilizes a unique cone-jet
geometries and they were the ring-shaped electrode, a phenomenon to obtain a thin liquid jet without nozzle
plate shaped electrode and a point-shaped electrode. The blockage as commonly experienced by conventional
ring-shaped and a plate shaped electrode produced a cone- inkjet printing. It is necessary to keep cone-jet in stable
shaped spray diverging droplets, while the point-shaped operating regime to achieve the goal of high resolution.
electrode focus the spray on a point. They employed the Many functional materials have been directly deposited
point-shaped electrode to create a self-supporting arch on a variety of substrates by EHD deposition in different
between two adjacent vertical wall or pillar structures . industrial areas, such as rapid prototyping, electronics,
[42]
In addition, they were able to build a three-dimensional and biotechnology. However, low production rate of EHD
structure by using the same electrode geometry and this inkjet printing is a main drawback that impedes widespread
structure is biocompatible . applications. This is, especially, a concern when small
[42]
Ahmad et al. utilized a controlled 3D writing method diameter jets are required for high-resolution printing.
by decreasing deposition distance to build ordered and Although a multinozzle printing head is considered as a
complex structures . In Figure 7A and b, an overlapped remedy to this problem, unstable jets may appear in this
[43]
and orientated 3D grid pattern was printed using a pure PU circumstance because of an asymmetric electric field
polymer solution which is widely used as a biomaterial arising from neighboring nozzles. Researchers developed
for blood contact applications . different methods to alleviate the interaction of adjacent
[43]
8 International Journal of Bioprinting (2019)–Volume 5, Issue 1