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Zhang Y
breed of its own, which we will discuss separately in later between layers. In bulk parts, the external surface can
sections.) Although many 3D-printing systems claim a be easily polished. In 3D-printed microfluidic devices,
printing resolution in the true microscale (<100 µm), the it is almost impossible to polish the internal surface of
actual size of 3D-printed microfluidic features mostly the microfluidic components. Besides the welding lines,
fall in the millimeter to the submillimeter range, because the surface of 3D-printed microfluidic components is
each feature is constructed by several 3D-printed lines. often speckled with particulates and microcavities .
[48]
In a rough estimate, the minimal size of a microfluidic The desire to achieve a smooth surface in microfluidics
feature is one order of magnitude larger than the printer is beyond esthetics but rather a practical concern. The
resolution . Many current high-value applications of imperfections may substantially alter the flow behavior
[45]
microfluidics require a high degree of parallelization and the way the surface interacts with biomolecules,
and the ability to handle micro-objects such as single leading to unexpected analytical outcomes. To improve
cells, both of which demand microfluidic features with the surface quality of 3D-printed microfluidic features,
a size ranging from several microns to tens of microns. one could either improve the printer resolution or develop
For example, the microfluidic feature in the microdroplet micro polishing techniques to smoothen the surface post
generator chip, which is used to create microscale droplets printing.
to encapsulate single cells, usually contains features in Several other issues of 3D-printed microfluidic devices,
the range of tens of microns [3,46] . To achieve a microfluidic such as biocompatibility and optical transparency, have
feature in this size range, the printer resolution has to reach also been noted [16,18,21] . However, these problems have
single-digital microns or even lower. Although some of been mitigated with the development of new materials.
the latest liquid resin-based 3D printers have a submicron Nevertheless, a more systematic investigation of these
vertical resolution, their lateral resolution is still not high materials would greatly benefit the community.
enough . So far, the only 3D-printing technology that is
[47]
capable of true microscale fabrication is 2PP. 2PP pushes 2.3. Future of 3D-printed Microfluidics
the printer resolution to the diffraction limit of the optics, To answer the question set in the previous section, I
reaching a submicron printer resolution in both lateral and do not believe 3D-printed microfluidics is quite there.
vertical directions. Many amazing 3D microstructures The upcoming 3D-printing revolution in microfluidics
have been fabricated using 2PP. However, the problem might be on its way, but definitely not in sight yet. Thus
of 2PP is its slow printing speed. In addition to the far, 3D-printing has only been used as an alternative
microscale features, the microfluidic devices also consist fabrication technique for microfluidics, hence does not
of other macroscale features. These large features would add much new value to the field. The functions of most
take too long for 2PP to print, which defeats the whole reported 3D-printed microfluidic devices can be easily
purpose of rapid prototyping using 3D-printing. realized by conventional microfluidic devices fabricated
The quality of the 3D-printed microfluidic devices
is another big concern. First, the dimension fidelity of using 2D lithography techniques. Compared to the
flattened 2D microfluidic network, the 3D microfluidic
3D-printing is poor at the microscale. In one study, the network does not show significant advantages.
measured dimension of millimeter and submillimeter 3D-printing adds value by offering monolithic near-
features is <6% off nominal . However, when attempting net-shape fabrication of complex structures. To truly
[29]
to print true microscale features (<100 µm), the measured revolutionize microfluidics, in addition to improving
dimension is more than 60% off-nominal (Table 1). In
addition, the side wall of the 3D-printed microfluidic the printer resolution, 3D-printing must also improve its
channel may be leaning, resulting in an undesired capability of multi-material, multiprocess, and multiscale
trapezoid cross section. Second, the surface of 3D-printed printing to monolithically create highly integrated and
parts is known to be rough with evident welding lines multifunctional microfluidic devices for high-value
biomedical applications.
Besides the microfluidic architecture, a fully functional
Table 1. Dimension fidelity of microfluidic channels printed with bioanalytical microfluidic platform also includes sensing
SLA. Reproduced from Ref. Monaghan et al. with permission elements, solid substrate for molecule and cell adsorption,
[29]
from the Royal Chemical Society. active actuators, and other components. In traditional
Channel Distance to the next channel Measured width lithography-based microfabrication, these components
width (μm) in CAD model (μm) on test piece (μm) are usually fabricated separately and assembled manually
500 1000 949±2.64 at the chip bonding stage. Solid substrate and surface
500 500 494±2.10 modification are usually introduced post-fabrication
500 250 258±1.94
500 125 130±4.49 by packing additional materials (e.g., particles or gel
500 62.5 104±0.84 matrix) in the microfluidic network or through in situ
SLA: Stereolithography; CAD: Computer-aided design chemical reactions. At present, several inkjet-based 3D
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