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Ng H Y, et al.

technique, single bioink printing is insufficient to provide field created by an in-built solenoid coil to open up
desired mechanical properties. A recent demonstrated the valve, thereby producing droplets. A recent review
using multiple bioinks to fabricate hollow channels, in article by Ng WL, et al. provided an in-depth review
which these bioinks usually have different cross-linking into the operational considerations for micro-valve
[2]
properties . The bioinks involved in creating hollow inkjet printing such as opening times of valves, printing
channels are usually UV cross-linked while bioinks pressure, nozzle size and bioinks considerations [27] . In
that are used as the sacrificial material are usually addition, they provided an interesting insight into future
thermo-responsive. Suntornnond R, et al. explained the outlook for micro-valve printing such as the need for
importance of support material in today’s bioprinting in improving cell homogeneity in bioinks, post-printing
fabricating complex structures and the need to integrate cellular damage and the prospects for hybrid bioprinting.
materials science and chemistry to understand the nature Piezo-electric inkjet printing is the most common used
[22]
of materials and reaction mechanism . method today whereby a small current is now applied
Droplet-based technique typically includes thermal, to a piezo-electric device which is typically made up of
micro-valve and piezo-electric inkjet printing methods. polycrystalline ceramic. The piezo crystal vibrates when
The bioinks, which can be living cells in culture receiving the current, thus creating an internal pressure
medium, are deposited as small droplets at any which allows for the extrusion of droplets via a nozzle .
[28]
specific pre-determined position [23] . With technological Similarly, droplet-based techniques can be classified into
advancement, we are now able to control the size of direct and indirect extrusion techniques.
each droplet with resolutions ranging from ~25 to Laser-based technique can be classified into two
300 μm. Gudapati H, et al. provided a comprehensive main categories: cell transfer technology and photo-
review on the application of droplet-based technique on polymerization technology. Cell transfer technology
various fields of tissue engineerings such as lung tissues, is mainly based on the laser-induced forward transfer
cardiac tissues, skin tissues and vascular tissues etc. effect (LIFT) which consists of a pulsed laser source,
[24] . Dai, et al. recently fabricated endothelialized fluidic a target and a base to collect the printed material. In
channels with a lumen size of approximately 1mm with brief, the target is composed of a base (such as quartz or
subsequent generation of capillary networks between glass) which is non-absorbing to the laser coated with a
channels. The fluidic channels were fabricated by thin layer of metal which is absorptive of the laser (such
dispensing gelatin using droplet-based technique. Two as titanium). After which, cells in culture medium are
parallel gelatin tubes was dispensed within a collagen deposited onto the surface of the metal coated target.
scaffold and human umbilical vein endothelial cell The laser pulse then induces vaporization of the metal
(HUVECs) with green fluorescent, fibrinogen, thrombin film, thus causing droplets to form and deposited on the
[29]
and normal human lung fibroblasts were deposited base itself . On the other hand, photo-polymerization
between the gelatin tubes. Following which, gelatin was technology harness digital micro-mirror devices to
sacrificed using thermal reversal to form fluidic channels polymerize, target and solidify the biomaterial at the
and perfused with HUVECs with red fluorescent. The irradiated region. The irradiated region and path can be
concept of angiogenic sprouting, self-assembly and pre-programmed using a simple computer-aided design
droplet-based technique were applied in this study model. This method is usually a top down bioprinting, in
to induce angiogenesis and vasculogenesis between which after completion of each layer, the platform moves
channels. Sprouting was observed from day 3 at the edge down for irradiating and bioprinting of the subsequent
of fluidic channels, and by 14 days post culture, obvious levels [30] . Kim, et al. designed a low cost, simple
capillary networks were present in collagen networks . but high resolution printing system that uses visible
[25]
Self-assembly of vascular networks is an approach in light cross-linkable bioinks [31] . In this study, the team
vascular engineering which would be further discussed created a hydrogel using polyethylene glycol diacrylate
in depth below. Thermal inkjet printing method applies (PEGDA), gelatin methacrylate (GelMA) and eosin Y
small current to a heating element, thus producing a photo-initiator with NIH 3T3 fibroblasts encapsulation.
bubble which ruptures to provide pressure pulses for A commercial projector was used as visible light
[26]
extrusion of droplets via a nozzle . Therefore, droplet projection device with a thick water filter to block
size is dependent on frequency of pressure pulses, harmful infrared radiation emitted by the projected. The
viscosity of bioink and temperature of heating element. construct was bioprinted, light treated and subsequently
Thermal inkjet is the earliest method in droplet-based assessed. It was reported that such printing methods
technique and most works involve modification of the brought about higher cellular viability as compared with
normal printers that we see at home. Micro-valve inkjet other extrusion systems, probably due to the absence of
printing is similar to thermal in which instead of using nozzle-based extrusion. As compared to UV crosslinking
pressure pulses, micro-valve makes use of a magnetic techniques, there was a slight decrease in viability. This

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