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International Journal of Bioprinting In situ thermal monitoring in bioprinting

technologies related to bioprinting have emerged in the tomography but is limited to photo-curable bioinks and
state of the art, from specific 3D printers, the bioprinters, can potentially cause cell damage. 18
to specific soft biomaterials in which living cells can be The bioprinting literature is rich with articles studying
embedded, called bioinks. 4,5
the bioprinting process capabilities, the development and
Although the bioprinting literature does not typically classification of novel biomaterials, and a specific focus
use the specific terminology defined in AM standards on the tissue of interest. 5,20,21 However, few studies have
(ISO/ASTM 52900:2021), technologies similar to those been investigating solutions for in situ monitoring and
used for polymers are often employed. We can identify two inspection of bioprinted constructs, as the ones observed
main classes of bioprinting technologies: laser-assisted in the literature on more conventional AM processes. 22-27
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bioprinting and laser-free bioprinting, each of which In fact, a few papers mostly from the same research groups
includes several sub-categories. There are also hybrid have been presented inspecting the quality of the printed
technologies or those with particular processes like melt construct, detecting defects as well as tools to control and
electrowriting (MEW), magnetic 3D cell culture, acoustic correct deposition errors in situ. 28-42
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assembly, microneedle array, and single-cell printing
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among the most known. The lack of repeatability, process stability, and error
detection and monitoring in bioprinting represent key
Laser-free bioprinting offers varying resolutions technological barriers to the development of products
and speeds influenced by factors like bioprinting head of increasing complexity, especially when hard-to-print
precision, material extrusion mechanisms, nozzle biomaterials are involved and the shape fidelity layer-
types, and droplet formation methods. Extrusion-based wise can then affect the capability of printing multilayer
bioprinting (EBB) is widely studied and known for its constructs. Indeed, the industrial repeatability and
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affordability and compatibility with various biomaterials. reproducibility of the printing process is still far from
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In EBB, the impact of mechanical forces, particularly being reached, especially in EBB. 43
shear stress, on cells and bioink properties must be
properly mitigated to preserve cell viability and structural On the other side, the growing advance of non-
integrity. Inkjet bioprinting is suitable for smaller destructive sensors could bring a significant impact on
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features but requires low-viscosity biomaterials. Higher the development of new solutions for in-line inspection,
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viscoelasticity stabilizes droplets, reducing displacement, monitoring, and control in bioprinting. Among non-
while increased viscosity improves accuracy and enhances destructive and non-invasive methods for quality
cell viability and proliferation. This highlights the crucial inspection, the most used solutions are image-based
role of bioink properties in optimizing precision and methods, which are perceived as time- and cost-effective
creating tunable cell spheroids. Laser-assisted bioprinting approaches. In the context of bioprinting, image-based
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notably utilizes vat photopolymerization methods such methods could potentially streamline processes by offering
as stereolithography (SLA) and digital light processing precise visualization and automation. This might enhance
(DLP). In this sophisticated approach, light serves as the planning accuracy, reduce the likelihood of errors, and
catalyst to either initiate photopolymerization reactions or theoretically minimize material wastage, potentially
generate controlled heat and pressure, enabling the precise contributing to cost savings. Real-time monitoring might
and intricate fabrication of 3D structures. ensure quality control, allowing for quick adjustments
Stereolithography and DLP contribute significantly to and potentially minimizing trial-and-error iterations. It
the refinement and efficacy of laser-assisted bioprinting. is important to note that the actual efficiency gains would
These vat photopolymerization techniques excel in depend on various factors, including tissue complexity and
providing a higher resolution during the bioprinting the sophistication of the imaging technologies and data
process, ensuring the creation of detailed and complex mining approach. Usually, data analysis of the last layer can
structures with higher precision. The ability to precisely be done in masked time, while the process is printing the
control the initiation of photopolymerization reactions new layer. Once the data mining algorithm is also defined,
through light-based technologies enhances the accuracy different solutions to reduce the computational time can
of the deposited bioink materials. One notable advantage be considered by extracting just the relevant features and
of vat photopolymerization methods, particularly acting on them. In situ monitoring and control via image-
SLA and DLP, is their broader compatibility with a based analysis can bring advantages, which are printer
diverse range of bioinks. Two-photon polymerization capability characterization, bioink printability assessment,
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(2PP) achieves nanoscale detail using laser-induced and process optimization. 28-42 Image processing can also
photopolymerization. 16-19 Volumetric bioprinting be used for the study of reproducibility since reliable
constructs free-form structures inspired by optical production is important in the transition from research

Volume 10 Issue 3 (2024) 395 doi: 10.36922/ijb.2021