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Gao and Zhou
parameters, including diameter of glass capillary nozzle, other authors [23,24] . This review only introduces three-
hydrostatic back pressure head in the reservoir, and dimensional (3D) printing that we are interested, and
standoff height . When other processing parameters are applications of two-dimensional (2D) printing are
[19]
maintained the same, onset jetting voltage for hole-type reviewed for comparison reasons. Most EHD printing
electrode is larger than that for plane-type electrode since applications have been focused on 2D printing, and
the less surface area of electrode in the previous type. only a few research groups have explored 3D printing
However, when the gap distance was larger than 200 µm, techniques that are restricted by several issues discussed
the onset jetting voltage was insensitive to the change in previous sections. The following sections review the
of hole diameter . The assumption that onset radius representative results of both 2D and 3D printings.
[19]
of curvature is not affected by the whole distribution of
electric field for the same nozzle seems reasonable from 3.1. Applications of EHD 2D Printing
simulation results . Thus, value of onset electric field at
[19]
the meniscus did not change with either diameter of hole 3.1.1 Electronics Applications
or gap distance . EHD inkjet printing is an attractive fabrication method for
[19]
Lee et al. used a ring-shaped gate electrode to suppress microscale electronic devices since this method can obtain
intact jet from generating satellite droplets or spraying . features with fine resolution and it is compatible with a
[20]
A maximal suppression was obtained as the inner diameter series of functional inks. The rest of this section introduces
of ring electrode is 5 times bigger than the outer diameter several applications of EHD printing in fabrication of
of nozzle, and the gap distance between nozzle tip and electronic components. As an example, Park et al. used
gate electrode is 2½ times as big as the outer diameter EHD method to print photocurable polyurethane (PU)
of nozzle . Tse developed a double layer electric field precursor as a resist layer for patterning metal electrodes
[20]
shaping print head to obtain smaller size of droplets by chemical etching . The printing results are shown in
[9]
compared to results from single layer field shaping Figure 4A and B.
print head . To maintain constant operating conditions, Metallo-organic compound solutions and inks
[21]
Barton et al. investigated a two degree of freedom control formulated from metal nanoparticle suspension are two
algorithm . They used an iterative learning control main kinds of ink for printing metallic patterns. The former
[21]
algorithm and a feedback controller to obtain feedforward ink is a true solution where the metal salt is fully dissolved
control signal. The feedforward voltage signal is applied in the solvent and this property decreases sediments
to compensate for repeatable changes, such as standoff and clogging issues arising from nanoparticles in the
height, and feedback component is employed for suspension . An uniform silver microtracks with a 35-
[25]
uncertainty in jetting operating conditions . Table 2 lists µm line width was printed by Wang et al., and its resistivity
[22]
some optional solutions for problems that usually appear was measured to be within a range of (2–4)×10 Ωm,
−8
in EHD printing process. which is 2.4 times of the theoretical value of bulk silver
−8
4. Applications of EHD Inkjet Printing (1.6 × 10 Ωm) [25,26] . As shown in Figure 4C, the edge of
micro track is more porous compared to central part, since
EHD printing is developed as a cost-effective inkjet repulsive interaction between the charged droplets leads
printing process with high resolution. With EHD to an incompletely suppressed jet breakup. Sutanto et al.
printing, functional materials can be selectively and successfully fabricated a crossover intersecting structure
precisely printed on desired substrates. Applications of using a multimaterial e-jet printing system to deposit
EHD in different industries have been summarized by conductive lines, which was made of an organic silver

Table 2. Some corresponding solutions for possible problems in EHD 3D printing
Existed problems Alternative solutions
−4
Reducing the size of droplet For large conductivity (≥10 S/m) Reducing flow rate, increasing voltage
For small conductivity (<10 S/m) Reducing flow rate, minimizing diameter of nozzle
−4
outlet
Increasing precision of placement Keeping a small distance between nozzle and substrate
Increasing yielding rate Increasing numbers of nozzle
Alleviating end effects from neighboring nozzle Inserting non-conductive/dummy nozzles; inserting metal nozzles into non-conductive
material-based multinozzle structure; increasing distance between neighboring nozzle; changing
the shape of nozzle array
Eliminating disturbance due to surface irregularity Changing configuration of electrodes, such as nozzle-ring type; applying sinusoidal AC voltage and
of the substrate, such as uneven surface, restriction print droplet whose charge alternates between positive and negative charges
of standoff distance, non-conductive material

International Journal of Bioprinting (2019)–Volume 5, Issue 1 5