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3D-printing and microfluidics
printers are able to concurrently print multiple materials. and high-resolution fabrication of microfluidic devices
A rigid microfluidic device with flexible membranes as is highly coveted. The multiscale 3D-printing must be
the pneumatic valve has been demonstrated using this able to adjust the printing resolution and printing speed
approach . Nevertheless, the number of materials that according to the required specifications.
[36]
can be printed concurrently is small, and they can only
be printed using the same process. The ultimate goal is to 3. Microfluidics for 3D-printing
be able to print complex microfluidic devices with many The relationship between 3D-printing and microfluidics
types of materials, such as a rigid plastic microfluidic could go the other way around. Microfluidics could
chip with flexible membranes, metal electrodes, also serve as the enabler of 3D-printing technologies.
hydrogel matrix, nanoparticle-packed beds, and magnetic Extrusion-based 3D-printing is one of the most popular
composite actuators, all in one go. technologies, especially in bioprinting. As the scope of
To accomplish multi-material printing, multiple bioprinting expands, the type of materials to be printed
printing processes must be integrated to cope with becomes more and more intricate. New applications
different bonding mechanisms. While plastic materials often require printing multiphase and multicomponent
can be printed by extruding molten plastic or crosslinking materials that cannot be handled by the conventional
photopolymer resins with a low-energy light source, extrusion printhead. Microfluidics, with its exceptional
metal powders require a high-energy laser or electron ability to manipulate a small amount of fluids, has been
beam to bond together. Furthermore, the material incorporated into the printhead to add a layer of fluidic
feeding mechanisms are also drastically different for control for sophisticated bioprinting.
different 3D-printing processes. In FDM, material is fed
to the extruder in the form of filaments; in SLA, liquid- 3.1. Current Development in Microfluidics-
resin is kept in a reservoir and reflows after each layer enabled 3D-printing
is printed; in inkjet printers, liquid resins are feed to
the printhead through a tubing; and in SLS and SLM, As a matter of fact, microfluidic components are employed
precursor materials in the form powders are loaded into in inkjet 3D printer, such as MJF, to dispense liquid in the
a powder bed and spread by a roller after each layer is form of droplets through microfabricated nozzles. More
printed. These material bonding and feeding mechanisms complex microfluidics-enabled 3D-printing arises from
are incompatible. To realize multiprocess printing, the the need to print hydrogel fibers with controlled gelation
partially printed parts need to be transferred between and composition. Early solutions employ coaxial flow to
platforms, and the printing processes must have the ability extrude hydrogel microfibers with cells encapsulated in
to resume from the breakpoint. A technique known as the the fiber core. Ozawa et al. created a coaxial flow system
[53]
print-pause-print (PPP) is able to suspend the printing by cascading tapered capillary tubing . The coaxial flow
process for users to add prefabricated components focused the cell suspension in the first capillary into the
(e.g., electrodes) to the partially printed parts and resume core of the fiber. The gel matrix precursor was injected
the printing from the breakpoint to embed these added from the second capillary to encapsulate the core flow.
parts within the 3D-printed microfluidic device . This The gelling agent was introduced as the sheath flow
[49]
technique points out a possible direction for multiprocess from the third capillary, crosslinking the gel matrix and
3D-printing. However, it does not address the challenges forming a coaxial fiber. Pancreatic β cells encapsulated in
associated with the cross-platform transfer of the partially these microfibers maintained their viability and functions.
printed parts. Similar approaches were demonstrated by several other
The quality of 3D-printed microfluidic devices can groups. Instead of cascaded capillary tubing, a manifold
be significantly improved using ultrahigh-resolution with two orthogonal inlets and a nozzle outlet as the
3D-printing technologies such as 2PP. However, it would printhead was used to couple the gel matrix precursor
be impractical to print the entire microfluidic device and the gelling agent into a coaxial flow. The printing of
solely using 2PP due to the extremely slow printing alginate hydrogel microfibers was demonstrated using
speed. In many microfluidic devices, a large portion of this setup in which the cell-laden alginate was focused
the device body plays a structural rather than functional by the sheath flow containing Ca 2+[54-56] . The manifold
role, which means a big part of the device body can be could be replaced by a microfluidic chip with the same
printed with a fast and low-resolution process. Only the configuration and function [57,58] . Capillary tubing or
parts that form the microfluidic architectures need to be needles were inserted into the microfluidic channel to
printed with a high-resolution process. These parts contain generate the coaxial flow.
microscale structures that directly interact with the fluids; Microfluidics has since moved beyond the simple
hence, their surface quality is more critical. Therefore, coaxial flow. In addition to microchannels, various
multiscale 3D-printing that provides both high-speed types of microfluidic components have been included in

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