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Jang T-S, et al.
[60]
3D printing techniques. However, there are some drawbacks and good controllability .
such as limitation of material type, nozzle condition, and As mentioned above, piezoelectric or thermal force is used
natural ability of this process hinder its potential. to eject liquid drops in DOD printing systems. In piezoelectric
To date, scaffolds fabricated by nozzle based 3D printing inkjet printer, the application of external voltage to
technique show low resolution and poor mechanical piezoelectric actuator generate pressure to eject droplets
properties. Compared to laser based 3D printing and from nozzle. Thermal inkjet printer, which possesses
droplet based 3D printing technologies, the resolution of low cost, high print speed, and wide availability, uses an
nozzle based 3D printing is relatively low. Moreover, the electrical heating to generate pulses of pressure that leads to
resolution is dependent on the solid loading or particle size the vaporization of liquid. Application of air-pressure pulses
on hydrogel-based composites. As the hydrogel is extruded eject small droplets from the nozzle. Heating temperature
from the nozzle, it does not have the material strength to is usually in the range from 200 °C to 300 °C, which can
maintain the structure and result in sagging or collapse lead to denaturalization of hydrogels or biocomponents in
of unsupported parts. This phenomenon of mechanical hydrogels. However, due to the short heating time (~2 μs)
property deterioration is aggravated during the printing in the printing process, heating has shown no detrimental
[61]
process of scaffolds without the assistance of supporting effect on the stability of biocomponents in recent studies .
materials or liquid medium. When the hydrogel materials Similar to other 3D printing techniques, hydrogel
possess sensitive biocomponents such as cells or ECM or scaffolds predesigned by computer modeling are constructed
growth factor, low printing speed and external pressure layer-by-layer with deposition materials. And there are a
on materials may lead to function loss or damage of variety of material that can be used in inkjet 3D printer.
biocomponents. These can be categorized in two types by starting materials
For solving these disadvantages, other improved on platform as described in the following section.
and combined nozzle based systems have also been 2.3.1 Inkjet Based 3D Printer with Powder (I3DP-P)
reported steadily. Multi-head deposition system (MHDS),
bioExtruder, screw extrusion system (SES), combined I3DP-P system (Figure 3A) is the representative solid-
rapid freezing and plotting system, modified plotting phase rapid prototyping technology. This system can use
system and porogen-based extrusion system are some of various materials including ceramic, metals, polymers as
the novel attempts [51–56] . These techniques were investigated well as hydrogels [62–65] . The process is a 3-step process. The
to enhance manufacturing flexibility by increasing the first step in 3D printing is the spreading of powder onto a
capability of deposition in accomplishing optimum scaffold platform with a roller. Second step is the deposition liquid
requirements. binder from inkjet print head with a 2D pattern onto the
2.3 Inkjet Printer-based Hydrogel 3D Printing powder layer by bonding the adjacent powder particles
together. The final step involves lowering the layer and
Systems filling the powder on the next layer. This process is repeated
3D printing technology, also known as additive until the fabrication process is completed. The unreacted
manufacturing, originated from 2D inkjet-based printer. powder with binders can support the bonded structures,
It was first developed at the Massachusetts Institute thus no supporting material is needed. Various types of
of Technology (MIT) in 1993 by depositing liquid powder such as a single powder, surface-coated powder,
binder onto a powder bed [57] . Inkjet printer is a non- and a mixture of different powders are used in this system.
contact technology which prints droplets of ink onto a Selection of suitable biocompatible powder and binder is
material platform. It is composed of a printer head which the most important part in I3DP-P system.
possesses liquid binder cartridge and moves in the XY 2.3.2 Inkjet-based 3D Printer with Liquid (I3DP-L)
plane and a build platform that is movable along the Z
axis as shown in (Figure 2B). There are two types of I3DP-L (Figure 3B). The working
Inkjet printing process can be divided into two types; principle of the first type is similar to the I3DP-P system
continuous inkjet (CIJ) printing and drop-on-demand but the powder bed is replaced by liquid materials [66] ,
(DOD) printing [58,59] . In CIJ printing, liquid binder emerges and the second type is a direct inkjet writing system
[67]
continuously from a nozzle to form a jet which breaks up which generally uses photosensitive resins . In the case
into droplets by the Rayleigh instability, whereas individual of the former system, uncrosslinked hydrogels are filled
droplets are ejected only when electrical signals result in bed platform which moves along Z-axis and the liquid
from thermal or piezoelectric effect in DOD printing. Both crosslinker ink such as calcium chloride are printed
systems offer droplets ranging in size from 15 to several from the print head. In direct inkjet writing systems,
hundred microns. However, DOD printing is preferable for photosensitive resin ejected from inkjet printer head
fabricating biological structures of soft tissue engineering build are constructed by simultaneous curing with light.
applications due to the reduced possibility of contamination Compared with I3DP-P, fabricated scaffolds shows high

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