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International Journal of Bioprinting Bottom-up and top-down VAT photopolimerization

concentration: C2C12 and MSCs were mixed with (GelMA the printing volume was determined to be around 3 ×
3% w/v and PEGDA 15% w/v), while HUVECs were mixed 3 × 3 cm. In the case of microfluidics bioprinting, the
with (GelMA 5% w/v and PEGDA 15% w/v). microfluidics working chamber has a diameter of 8 mm.

As illustrated in Figure 4, the fluorescence image The lateral resolution (XY) of the DLP is in the order of 10
revealed the capability of the system to print the spatially microns, and the Z resolution of 20 nm can be achieved
distributed cell-laden bioinks, laying down the basis by the Thorlabs Z platform. To avoid cross-contamination
for future fabrication of functional multi-material issues during the bioprinting process, we added a washing
musculoskeletal tissues. The addition of more PEGDA bath in between each material selection (the washing
could result in more mechanical stability. It will make it process is illustrated on Figure 1f). The process can easily
easier to handle the tissue microenvironment fabrication. be adapted for work with more than two materials by
Still, PEGDA led to more encapsulated cell toxicity; flowing different biomaterials into the chip or by using
indeed, 30%–40% v/v concentrations of PEGDA might more biomaterials containers in the bottom-up and top-
be inappropriate for biological components. The GelMA/ down approaches. However, it should be noted here that
PEGDA bioink used in the microfluidics fabrication careful selection of the final mechanical properties of the
processes of the musculoskeletal environment, the soft support is necessary, in order to avoid flow-induced
proposed PI concentration, and the UV exposure were delamination that could result in accelerated degradation
found to be safe for the proposed cell bioink application. as well as channel clogging. Examples of multi-material
printing were also presented showing structures made
4. Discussion with two discrete hard–soft parts, respectively (Figure
2c(i) and (iii)). Furthermore, a construct consisting of six
Herein, we presented a novel bioprinting system, which soft and six hard alternating regions was fabricated (Figure
facilitates the fabrication of multi-material constructs laden 2c(ii)). Those soft–hard material combinations could be
with three different cell types, as a proof of concept for a used in order to model bone-to-soft tissue interfaces [41,42] .
musculoskeletal tissue model on-a-chip. The incorporation Especially by combining more regions in one construct
of a rotating mirror to the printing system allowed the fast and controlling the mechanical properties of each region,
switch between the bottom-up and top-down printing tissue models consisting of more than two materials can be
modes and the combination of both approaches on a single fabricated, allowing the study of complex systems such as
musculoskeletal junction model. As a future upgrade of the the intervertebral disc regeneration .
[43]
dual printer, a fully automated system could be developed, The prepared bioinks were made from GelMA and
in order to couple the movement of the stage with the mirror PEGDA, due to their wide use in biofabrication applications
rotation, thus further optimizing the fabrication time and as well as their light responsiveness . By combining
[44]
limiting the manual steps of the process. This modification the two materials in different concentrations while also
could facilitate the implementation of the dual printer in changing the concentration of the photoinitiator, we
the mass production of a wide range of organs-on-chips could get a compression modulus range from ~22 kPa to
using the bottom-up, top-down, or combined processes.
~870 kPa. The measured moduli showed an increase with
The wide range of constructs that can be fabricated both the GelMA and PEGDA concentrations when using
using the dual-printing approach was highlighted 0.05% LAP. Interestingly, this trend was not the same when
by the different geometries (Figure 2). Scaffolds with the concentration of the photoinitiator was increased.
interconnected pores and intricate patterns can be More precisely, PEGDA 15% mixed with 3% of GelMA
generated using the bottom-up approach, providing showed an increased modulus when compared to the
mechanical support as well as a suitable environment for PEGDA 15%/GelMA 0% formulation. However, when the
cells to grow . The top-down approach was used for the concentration of GelMA was increased to 5%, the modulus
[40]
fabrication of complex microvasculature patterns from showed a decrease of approximately 32.5%. Furthermore,
GelMA and PEGDA. On their own, these constructs can the concentration increases of GelMA from 0% to 3% and
be used as a soft support for vascularized tissue models, subsequently to 5% resulted in a 27.6% and a further 8%
and the generated channels can be used for the perfusion modulus decrease when mixing with 35% PEGDA.
of medium or other fluids. The maximum printing volume,
which depends on the biomaterial container used, was Most musculoskeletal diseases do not have a curative
determined for each bioprinting configuration. The DLP treatment yet. Some pathologies, such as amyotrophic
can project over an area of 7 × 12 cm. Nevertheless, for lateral sclerosis, Duchenne’s disease, or Lou Gehrig’s
[45,46]
the case of bottom-up and top-down approaches and disease, still do not have real treatment . Some
for the proposed biomaterial container configuration, treatments for more common chronic diseases, such as
cholesterolemia and diabetes, also have unwanted muscle

Volume 10 Issue 2 (2023) 539 doi: 10.36922/ijb.1017