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3D-printing and microfluidics
silicon-based microfluidic devices is simpler compared to components offer excellent fluidic control of 3D-printing
MEMS devices, it is still a daunting task for biomedical inks, simplifying multi-material, and high-throughput
researchers to take on. This issue is not resolved until the parallel printing. The laminar flow profile of microfluidics
polydimethylsiloxane (PDMS)-based soft lithography, allows concurrent printing of multiple inks through a single
which is a simple molding-based fabrication technique, nozzle and time-controlled crosslinking of hydrogel inks
is developed . Although traditional micromachining using hydrodynamic focusing. Furthermore, additional
[10]
process is still involved in PDMS-based fabrication, it functional components, such as surface acoustic waves,
is limited to the making of molds. With the ready-made can be incorporated to modulate the distribution of
mold, the chip fabrication workflow is reduced to pouring chemical constituents in multiphase inks. These works
PDMS, punching access ports and bonding PMDS to point out a new direction in which 3D-printing and
glass. Compared to the silicon-based microfluidic devices, microfluidics could work synergistically to accomplish
PMDS-based devices find a bigger audience among previously unattainable tasks.
biomedical researchers. The PDMS-based device is made In this perspective article, we evaluate the up-to-
more popular by the invention of PDMS-based multilayer date development of 3D-printed microfluidics and
pneumatic valves and pumps , which enables system- microfluidics-enabled 3D-printing with a strong emphasis
[11]
level integration of multifaced devices for intricate tasks on their limitations. We would express our opinions on the
such as single-cell analysis [3,12] . future innovations required to overcome these limitations
The PDMS-based microfluidics has its pros and cons. and to develop new high-value applications. We hope
On the one hand, PDMS is able to precisely replicate to answer whether 3D-printing is more well-suited for
the lithographically defined patterns with nanometer microfluidics or it is the other way around, but we will
resolution. In addition, PDMS is biocompatible and leave the discussion open.
well-suited for cell studies [13-15] . It also has favorable
optical properties such as great transparency and low 2. 3D-printing for Microfluidics
autofluorescence, which is compatible with various 3D-printing is an umbrella term encompassing a number
optical sensing modalities. The low cost of PDMS and the of additive manufacturing technologies, but not all of
reusability of the mold make PDMS-based microfluidic them are applicable to printing microfluidic devices.
devices reasonably affordable. On the other hand, PDMS Based on their suitability for microfluidics, we loosely
is water vapor permeable. Samples in PDMS chips are categorize 3D-printing into extrusion-based technology
susceptible to evaporation and bubbles in the event of (e.g., fused deposition modeling [FDM]), liquid resin-
a heating or prolonged incubation. PDMS is also prone based technology (e.g., stereolithography [SLA],
to protein fouling, which would affect the accuracy of digital light processing, and two-photon polymerization
biosensing. Furthermore, the fabrication of a PDMS- [2PP]) which also includes inkjet-based 3D-printing
based microfluidic device still heavily relies on manual (e.g., material jetting) due to the similar curing
assembly. mechanism, powder-based technology (e.g., Multi Jet
Nowadays, as microfluidic devices are designed to Fusion [MJF], selective laser sintering [SLS], selective
tackle more intricate tasks, the architecture of microfluidic laser melting [SLM], and electron beam melting),
devices becomes more complex, and more sophisticated and other less common 3D-printing technologies. The
fabrication techniques are in demand. Therefore, it technical aspects of these 3D-printing technologies have
is sensible to fabricate microfluidic devices by three- been discussed extensively in many reviews [16-21] ; hence,
dimensional (3D)-printing, which is well-recognized for we will skip it in this article. Majority of microfluidic
its unique ability to monolithically fabricate complex devices are fabricated with extrusion-based technology
structures using a near-net-shape additive manufacturing or liquid resin-based technology.
process. As a matter of fact, a great number of 3D-printed The fabrication of microfluidic devices by 3D-printing
microfluidic devices have been reported in the past few can be either direct or indirect. Direct 3D-printing
years followed by several review papers that provide a constructs the microfluidic chip by enclosing the
fairly comprehensive evaluation of these devices and an microchannels and other microfluidic components with
optimistic future outlook on 3D-printed microfluidics [16-20] . the ink materials. Indirect 3D-printing produces a mold
One of the reviews even touts 3D-printing as the upcoming using the ink materials, and the chip is fabricated by
revolution in microfluidics . casting PMDS against the mold. The final microfluidic
[21]
While majority studies employ 3D-printing for chip does not consist of any ink materials. In this
microfluidic device fabrication, a number of studies perspective article, we will mainly focus on the direct
go the other way around and incorporate microfluidic printing approach except in a few cases in which a
components in 3D-printers for added functions and sacrificial mold is required for complex 3D microfluidic
improved printing performance. These microfluidic networks.

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