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Designs and Applications of EHD 3D Printing
to employ a highly viscous liquid . It is used in the droplets is slow on the insulating substrates, the residual
[1]
field of micro- or nano-manufacturing for patterning charge of droplets deposited onto a substrate will change
of a large class of materials on a variety of substrates the electrostatic field distribution and may interrupt the
without adversely affecting the chemical properties of the subsequent printing behavior . Park et al. used a sinusoidal
[8]
[8]
deposited materials. This paper is organized as follows. ac voltage to switch the charge polarity of droplets and
We describe different types of EHD printing machines charges in the subsequent droplet to neutralize charges
designed on the basis of fundamental EHD mechanisms. in the previous droplet. A visualization system includes a
The ensuing section discusses about the application of high-speed charge-coupled device camera, a microscopic
EHD inkjet printing in different fields of research. zoom lens system, and an illumination source. Since
the timescale for the most relevant EHD phenomena
2. EHD Inkjet Printing Machines and Control is very short (less than a microsecond), a conventional
continuous-illumination source will cause blurring
An EHD inkjet printing system usually consists of a images by overexposure of jet motion. Thus, a high-speed
fluid supply system, linear/rotary motorized or manual shutter system or a short duration flash with an open
positioners, a high voltage power supply, and visualizing shutter is suitable for high-speed events. The external
and imaging devices . The fluid supply system delivers disturbances, such as vibration coming from the building,
[2]
liquid ink through a pipeline to the tip of the nozzle at the may be damped by vibrational isolation platform. These
desired flow rate. The supply system can be a syringe or attributes often lead to demands for careful control and
vacuum pump or a liquid reservoir with a constant liquid optimization for each ink material. Table 1 summarizes
[3]
height [4,5] . The dispensing nozzle (usually made from the optional devices for each subsystem of EHD print.
stainless steel) is compatible with interchangeable syringe Park demonstrated that a nozzle with a diameter of
and can be purchased from commercial companies. The 50 µm can create dot diameter not <20 µm in graphic arts.
tiny size of nozzle (smaller than 50 µm) can be fabricated In addition, they developed an EHD inkjet printing system
by pulling glass pipettes or borosilicate capillaries and the as shown in Figure 1a and a droplet with submicrometer size
tip of nozzle is coated by a thin layer of metal. To prevent was able to be produced by a microcapillary with a diameter
wetting of the ink and obtain a large contacting angle at from 0.3 to 30 µm . The resulting high resolution is partially
[9]
the nozzle outlet, metal nozzles are usually processed attributed to a fine nozzle with sharp tips. Another reason is
by chemical agents. The nozzle is grounded while a that small distance between nozzle and substrate shown in
high negative potential is applied to the substrate and Figure 1b can minimize lateral variations in the placement
this prevents charge accumulation and potential shock of droplets. Furthermore, breakup occurring in electrospray
hazards during the operation of the pumping system . can be avoided at such short distance , and a sharp corner is
[6]
[5]
The gap between the nozzle tip and the substrate, known also benefited from a small gap. Fluid is propelled through a
as standoff height, is critical for stable printing. Thus, it glass capillary by a syringe pump. A high-speed camera with
is necessary to use a high precision motorized stage to 66,000 frames per second was used to observe the dynamics
adjust standoff height. A high DC voltage power source of EHD jetting, and both pulsating and stable jet modes are
is employed to produce a strong electric field to deform captured by imaging devices. Park et al. used a solution of
the liquid meniscus, and a function or pulse generator is a conducting polymer ink to electrohydrodynamically print
used to facilitate drop-on-demand printing. To supply high-resolution patterns with 10 µm droplet diameter , as
[9]
voltage and ink material to multiple nozzles separately, a shown in Figure 1c. Figure 1d shows EHD printing results
voltage distributor and a multichannel syringe pump are of silicon nanoparticles and single crystal Si rods dispersed
necessary . Since the decay rate of residual charges on in 1-octanol .
[7]
[9]
Table 1. Functions of each working device in different EHD printing platforms
Names of EHD printing Optional devices Function
subsystems
Fluid supply Syringe pump/vacuum pump/liquid Delivering ink to the tip of printing nozzle
reservoir, pipeline and connector
Positioner Linear/rotary motorized positioner/ Transmitting nozzle to printing position accurately; adjusting distance
manual positioner between two electrodes
Power supply High voltage DC/AC power supply Providing strong electric field to produce stable cone-jet
(up to 10 kV)
Visualizing and imaging device High-speed CCD camera, Microscopic Observing deformation of cone and high-speed emission of droplet
zoom lens, LED light
Vibration isolation platform Mechanical vibration isolation platform/ Preventing vibration from effecting printing results
optical tables

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