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International Journal of Bioprinting In situ thermal monitoring in bioprinting

to industrial application, and more precisely to clinical propose the use of infrared (IR) thermal imaging for in situ
studies. inspection and monitoring of multi-layered 3D-bioprinted
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In the case of in situ process monitoring, even fewer constructs. This solution is specifically suitable for
studies have used image analysis to obtain layer-wise transparent bioinks, where images in the visible range
information in grid-like printing considering non- make edge detection hard to be implemented to detect
industrial devices. 28,30,35-42 Most of the approaches used top- local defects and severe deviation from the nominal shape.
view imaging, while some others considered also the lateral In this case, thermal imaging allows a clear identification
perspective to oversee the height of the printed scaffold. 36,37 of the printed geometry, which is usually deposited at
Additional work was done to emphasize the role of in situ a different temperature with respect to the underneath
monitoring to investigate the influence of the rheological layer. This thermal signature is exploited to identify and
properties and printing parameters on the shape of the reconstruct the printed geometry.
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3D-printed lines. A different approach was used by Wang As a second main advantage, the thermal-based in situ
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et al. and Yang et al. with the use of optical coherence monitoring can be used to identify defects arising on the
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tomography (OCT) to acquire high-resolution images of last printed layer only. In fact, by exploiting the thermal
hydrogel scaffolds with the aim of accurately quantifying differences between layers, the last printed layer geometry
relevant morphological parameters (pore size, pore shape, can be easily distinguished from the shapes printed on
surface area, porosity, and interconnectivity) for non- the underlying layers. Again, this capability can be hardly
destructive geometric assessment and characterization of obtained with top-view imaging in the visible range, which
printed scaffolds. Jin et al. obtained pictures of separate layers allows one to obtain at each location the cumulative effect
to observe rough surfaces, fractured lines, and irregular of the materials printed on different layers.
abnormalities. Eventually, Armstrong et al. 40-42 contributed Our novel solution opens new opportunities for
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significantly to the literature showing how laser scanners detecting different possible defects, such as uneven
can be used for monitoring the nozzle trajectory and the depositions and shape deviations with respect to the
filament width in extrusion-based 3D printing, fostering nominal pattern, which may arise during the bioprinting
some possible solutions for in situ in-line feedback control. process. The proposed monitoring system based on
Also in our previous work, we used a camera operating in thermal imaging would fit in the context of advanced
the visible to acquire images from above a construct for the manufacturing solutions, improving the digitization of
identification of drift processes affecting EBB. 28
processes and systems, the management of “Big Data,”
Many of those presented works, however, suffer from and the fusion/integration of information from multiple
the typical limitations of approaches operating in the sensors. It would also open the opportunity to develop a
field of visible light, namely the difficulty in discerning process control system, to modify control inputs to correct
between target and background when they present very errors in subsequent layers. Such an approach could be used
similar features or when, as in the case of bioinks, they are not only to study geometry as has been done in this work
completely transparent, a feature that is very common and but also to study the thermal history of thermoresponsive
also essential to enable biological readouts and microscopic materials 46-48 widely used in this field. This would be a key
inspections of bioprinted constructs. contribution to defining a new method to quantitatively
As far as we know, Moncal et al. are the only ones evaluate the accuracy of printed constructs and improve
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in the literature proposing to use a thermal camera in their quality.
bioprinting. However, this study has a quite different The paper is structured as follows: In section 2,
perspective compared with the one proposed in our study. the biomaterials and the monitoring system used are
In fact, Moncal et al. propose the use of thermal imaging described. In section 3, the preliminary results of such an
to thermally control the extrusion process and ensure the approach are herein presented. In section 4, critical issues
appropriate temperatures of the printbed and printhead and advantages of this innovative approach are discussed.
for promoting crosslinking for thermoresponsive Finally, the main conclusions are drawn in section 5.
hydrogels. In other words, thermal imaging is used in
this study to measure the local temperature during the 2. Materials and methods
extrusion process.
2.1. Biomaterials
The aim of this paper is to propose a novel solution Two different test campaigns were conducted. In the first
that can overcome the previously mentioned limits of in one, the capability of inspecting the extrusion process
situ monitoring and in-line defect detection in bioprinting with thermal imaging was firstly tested without cells and
when visible-range imaging is assumed. In fact, we considering a commercial bioink, namely CELLINK Bioink

Volume 10 Issue 3 (2024) 396 doi: 10.36922/ijb.2021