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International Journal of Bioprinting Microfluidic-assisted 3D bioprinting

2.2.3. Light-assisted technologies the creation of a spinneret with a diameter of 12 μm for
Light-assisted 3D printing comprehends different spinning microfibers. 71
approaches based on the use of an intense light source,
typically in the UV spectrum, to polymerize photosensitive 3. MST for the production of biocompatible
resins composed of functional monomers that join to form fibers: practical and theoretical aspects
polymers after absorbing a sufficient amount of energy. 57
The progress in the fabrication of microfluidic tools
Stereolithography apparatus (SLA) and digital light remarkably contributed to the creation and advancement of
processing (DLP) use light to cure the photoresin in a novel biofabrication approaches, fostering the development
layer-by-layer fashion. While SLA-based approaches of increasingly reliable in vitro models of human tissues.
exploit a moving mirror galvanometer to deflect the entire Thanks to MST technology, microfluidic devices are used not
light beam in a single spot, a spatial light modulating only to confine and culture cells in dedicated 3D platforms
element (i.e., a digital micromirror device (DMD) or (i.e., organ-on-a-chip) but also to fabricate biocompatible
a liquid-crystal display (LCD)) in DLP technology fibers homing cells. After extrusion, spun fibers can be either
produces a dynamic mask to illuminate each layer collected in a coagulation bath, 72-75 coiled around a rotating
entirely, making DLP faster than SLA. Even though mandrel, 63,76-78 or deposited on a substrate in a predefined
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optical resolution down to 2 μm is theoretically achieved, shape through a 3D printer (see section 4).
the minimum cross-section of an enclosed channel
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reported is 75 μm. Moreover, the objects manufactured 3.1. Approaches and available platforms for
with SLA-DLP technologies exhibit an unparalleled level fiber formation
of surface smoothness compared to FDM and MJM MST falls under the umbrella of wet-spinning methods,
(average roughness, R =0.35 μm ). a wide class of fibers spinning techniques that involves
38
a
The superior fabrication characteristics of SLA-DLP the use of materials in the liquid state to be transformed
make this technology the best candidate for the fabrication into solid or gel form after passing through a coagulation
bath or a crosslinking solution. In MST, such liquids are
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of entire microfluidic devices or patterned molds. The first confined and manipulated in microchannels.
attempts to employ laser-based approaches in this field
date back between the late 1990s and the first 2000s. Various microfluidic solutions have been engineered
61
60
Since then, SLA-based technologies have begun to be over the past few decades. Coaxial spinning systems are
crucial for the realization of 3D-printed microfluidic chips the most common platforms and are able to produce a
for a number of applications in the biofabrication field, coaxial flow of the material precursor and the crosslinking
including fiber spinning. 62-64 solution, which come into contact at the tip of the
spinneret, enabling quick gelation of the gel precursor.
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Different from SLA-DLP technologies, direct laser
writing (DLW) enables to cure femtoliter volumes of Coaxial spinning systems can be realized by arranging
the resin (i.e., 3D voxels) by exploiting the multi-photon two concentric glass capillaries or metallic needles. Also,
81,82
polymerization (MPP) process. In this case, a high- they can be composed by single or multiple capillaries
75,83,84
intensity and extra-fast pulsed laser (in the order of assembled in series to obtain multi-layered flow 72,73 as
femtoseconds) is employed. The non-linear absorption of well as coaxial needles combined within a glass tube.
two or more photons causes photopolymerization to occur In the majority of wet-spinning systems, the
only in the focus of the laser beam as it is insufficient to phenomenon of hydrodynamic focusing (HF) is exploited
polymerize the surrounding regions. This results in the to produce coaxial flow and, in turn, fibers. Due to the
ability to realize feature size down to 100 nm (beyond laminar regime dominating in microchannels, fluids can
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the diffraction limit) with free-form manufacturing ability be manipulated via hydrodynamic focusing by forcing
and high reproducibility. The high cost of MPP equipment, a central fluid stream (core)) through a boundary fluid
however, is a significant barrier to the widespread use of (sheath), forming a coaxial flow. Crosslinking agents are
this technology. 37,66 Recently, DLW is attracting numerous included in one of the two solutions, often the sheath fluid,
85
microfluidic communities to realize chips for the most to promote crosslinking and produce compact fibers.
67
diverse applications including particle handling and Additionally, hydrodynamic focusing not only enables
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molecular detection. The extreme accuracy in creating fluid focusing but also acts as a lubricant, enabling the
sub-micrometric structures is also exploited for realizing extrusion of the solid fiber. However, to ensure the stability
complex micro- and nanoarchitectures to mimic intricate of the whole process, the maintenance of a laminar regime
biological environments. 66,69,70 Moreover, DLW-based imposes to not have a large difference in core and sheath
systems have also been harnessed to directly write fluid viscosities. The sheath fluid can be combined with
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nanostructures on-chip (in-chip fabrication), enabling the central one from an angle of 90° or at 45° as it minimally
Volume 10 Issue 1 (2024) 51 https://doi.org/10.36922/ijb.1404