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International Journal of Bioprinting PAI for 3D bioprinted constructs

3. Applications of PAI in 3D bioprinting to study the formation of thrombosis using PAI. They
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used digital light processing (DLP)-based 3D printing to
One primary challenge in 3D bioprinting is creating create intricate patterns of internal microchannels in
complex anatomical structures that exhibit both blood vessel structures, including straight, serpentine,
physiological responses and behaviors. The application and dodecahedron patterns. To produce more complex
of PAI in 3D bioprinting depends on the intended use of structures, DLP printing relies on photoabsorbers with
the printed structure and can be broadly categorized into strong optical absorption, which limit the penetration of
in vitro and in vivo applications. In vitro imaging employs ballistic photons. Dodecahedron-patterned microchannels
PAI to assess the mechanical characteristics of bioprintable were found to be particularly complex and could not
materials without damaging them. In addition, it creates be imaged using conventional optical microscopy. In
imaging phantoms specifically designed to evaluate, comparison, PAM can access the entire volume of the
analyze, and optimize the performance of imaging devices. dodecahedron-patterned microchannels, providing a
This approach is aimed at enhancing the acquisition clear depiction of their complex volumetric structures.
of accurate medical images and improves patient care By using OR-PAM and acoustic-resolution photoacoustic
by replicating the imaging properties of living tissues. microscopy (AR-PAM) systems, the authors imaged
In contrast, in vivo imaging utilizes PAI to assess how 3D-printed blood vessels and conducted thrombosis
implanted materials affect the body in vivo, such as by studies. Blood clots exhibited low oxygenation due to
observing the degradation of biodegradable materials or the ineffective exchange of oxygen with the surrounding
monitoring angiogenesis around implanted materials.
medium during clot formation. Multi-wavelength
3.1. In vitro imaging of 3D-printed objects measurements using two wavelengths, 532 and 590
The mechanical properties and structural stability of nm, allowed the PAM to quantify the sO level of total
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bioprintable materials can be evaluated using PAI. Zhao et hemoglobin, with the images showcasing that blood clots
al. devised a technique leveraging PAI to characterize gelatin possessed much lower oxygenation levels than normal
methacryloyl (GelMA) hydrogels. This noninvasive blood. This suggests the possibility of effectively monitoring
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method is important because the porous structure of thrombosis using PAI. This study demonstrated that PAM
hydrogels affects their mechanical properties and nutrient is a practical tool for imaging the structure and function of
delivery. The proposed technique utilizes a transmission- 3D-printed blood vessels.
mode PAI system that relies on the optical contrast, deep Although the use of RBCs is a common approach in
penetration, and noninvasiveness of the PAI technology. PAI for assessing the functional properties of constructs,
The experimental results showed that mixing RBCs with a alternative mediators can be employed to monitor these
high PA contrast into the GelMA hydrogel results in certain properties. Yim et al. fabricated synthetic skin tissue that
areas exhibited a higher PA signal than the surrounding resembled human skin, and used PAI to investigate the
areas, which was attributed to the RBCs occupying the influence of skin phototype on biomedical optics. Skin
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pores in GelMA. By excluding the background signal phototype varies among individuals and depends on the
from the unmixed GelMA, the team concluded that the concentration of melanosomes, which are cellular organelles
pore distribution could be approximated by the locations that produce melanin pigments. Higher concentrations
occupied by the RBCs. During 3D biofabrication, cells of melanosomes lead to darker skin phototypes because
require a sufficient nutrient environment, which depends melanin attenuates the incident light, hindering signal
on the diffusivity of the nutrients in the hydrogel and transmission. To mitigate the effects of skin phototype
connectivity of the pores in the hydrogel. They analyzed variations, Yim et al. developed an enhanced phantom that
the interconnectivity of the pores through RBC lysis. By mimics the skin by creating synthetic melanin. It achieves
soaking the hydrogel in an RBC lysis buffer, researchers this by using polydopamine (PDA) and incorporating it into
could remove RBCs sequentially over time, and the an existing phantom lacking real melanosomes and having
reduction in the PA signal caused by RBCs was used to a bio-relevant size and shape. By using 3D bioprinting,
understand how RBCs are located and interact within the they created a customized GelMA-based phantom with a
GelMA sample. Overall, PAI is a highly effective method for thin layer of melanin-containing material that resembled
characterizing the morphology and mechanical properties the epidermal layer of the human skin (Figure 9a). They
of hydrogels, which are typically bioprintable materials. found that larger PDA clusters and higher concentrations
Using PAI, it is possible to characterize bioprinted of PDA nanoparticles produced stronger PA signals. In
constructs by measuring factors such as the microstructure addition to characterizing the material, PAI was used to
of blood vessels, blood sO , and density of biomaterials. Ma assess how well the simulated skin replicated real skin. The
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et al. created in vitro 3D-bioprinted models of blood vessels PA signal of the simulated skin was found to match that

Volume 10 Issue 4 (2024) 13 doi: 10.36922/ijb.3448