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International Journal of Bioprinting PAI for 3D bioprinted constructs

wound healing primarily manifests as alterations in the provided by MiROM was confirmed by prior validation
vascular network morphology, emphasizing shifts in using standard FTIR spectroscopy. PA images were
vessel tortuosity and direction rather than changes in the captured at wavenumbers of 2850 and 1550 cm , targeting
−1
individual vessel length and diameter. the CH bond of lipids and the amide II band of proteins.
2
Liu et al. introduced functional imaging to quantify The lateral resolution, measured at approximately 5.3 μm
−1
the blood flow speed within blood vessels, in addition to at 2850 cm , proved adequate for discerning individual
sO and vascular morphology analysis, using wide-field adipocytes and their respective lipid droplets (LDs)
2
PAM. They simultaneously acquired PA images at three Moreover, MiROM offered an intrinsic molecular contrast
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wavelengths (532, 545, and 558 nm) out of single laser in thick, freshly excised tissue, distinctly outlining the
pulse, achieved through a temporal and spectral separation abundant lipid clusters within the pancreatic acinar glands
using stimulated Raman scattering shifters. To estimate the embedded in proteins. By adding excitation between 1085
−1
blood flow speed, dual-pulse PA flowmetry was performed and 1000 cm , which targeted C–O stretching and C–O–H
using two isosbestic wavelengths (532 and 545 nm) by deformation, carbohydrates became detectable, in addition
measuring the PA signal decay in submicroseconds. Using to lipids and proteins. Collectively, the localized contrast
the three-wavelength optical-resolution photoacoustic in living 3T3-L1 adipocytes highlights the accumulation of
microscopy (OR-PAM) system, the mouse ear was imaged glucose preceding lipogenesis.
in vivo simultaneously at each wavelength, allowing for Compared with UV-, VIS-, or NIR-PAM, MIR-PAM
the quantification of parameters such as the hemoglobin often faces challenges in resolving fine details within
concentration, blood flow speed, and sO . Notably, they microscopic tissue structures due to the inverse relationship
2
implemented additional compensation to calibrate the between the spatial resolution and the wavelength of the
systematic errors in the sO calculation arising from light source. Moreover, the prominent absorption of MIR
2
incomplete thermal relaxation between the short time gap by water can result in an unclear contrast in the MIR-PAM
between the two laser pulses. With this compensation, the images of fresh biological samples. To overcome these
sO values were measured to be 96% in the artery and 65% limitations, Shi et al. presented UV-localized MIR PAM
2
in the vein, consistent with well-established physiological (ULM-PAM), a novel approach to indirectly measure
norms. Likewise, the arterial blood flow speed decreased the MIR molecular contrast using a localized UV laser,
from approximately 7 mm/s at the root to approximately leveraging the Grüneisen relaxation effect. Briefly, it
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3.3 mm/s at the ear tip, while the venous flow maintained measures the difference in the UV PA signal under a
stable at approximately 2 mm/s along the ear. tightly focused UV beam before and after rapidly heating
the sample with a burst of MIR excitation. The change
2.4. MIR-PAI: Complex in the PA amplitude was directly proportional to the
intra-/inter-cellular architecture local temperature change, which was proportional to the
Lipids, proteins, and carbohydrates possess distinctive absorption coefficient in the MIR wavelength range. This
fingerprint infrared (IR) spectra due to their rich covalent strategy provides the advantage of preserving the spatial
bonds. Specifically, lipid molecules exhibit specific resolution at the UV beam size while circumventing the
absorption bands associated with alkyl chain (C–H bond) strong background signal from water through indirect
stretching, while proteins display characteristic absorption measurements using UV excitation. The excellent spatial
bands related to amide I and amide II vibrations. Conversely, resolution of the ULM-PAM allowed subcellular molecular
carbohydrates present distinct absorption bands for C–O imaging, enabling the mapping of intracellular lipids,
and C–O–H stretching. These attributes are demonstrated proteins, and even cell nuclei with UV excitation in mouse
in previous Fourier-transform infrared spectroscopy fibroblast cells.
(FTIR) studies. Therefore, pairing MIR laser sources with
PAM has enabled metabolic imaging of the protoplasm Collagen is a major component of the ECM, a network
within biological tissues, including fat-rich adipocytes that provides structural support and biochemical cues to
in connective tissue, collagen in the extracellular matrix cells. The mechanical strength of collagen is influenced
(ECM), and fibroblasts. by its molecular arrangement, with collagen fibers
typically exhibiting a unidirectional alignment in highly
Pleitez et al. pioneered MIR optoacoustic microscopy stressed tissues, such as tendons. Park et al. introduced
(MiROM) to conduct label-free metabolic imaging of dichroism-sensitive PAM (DS-PAM), which modulates
live cells, selectively targeting specific molecular bonds the polarization angle of the incident 1540 nm excitation
in carbohydrates, lipids, and proteins, allowing for the beam to determine the orientation of fibers.79 Depending
real-time observation of their distribution and dynamics on the polarization angle of the incident beam, dichroic
within cells and tissues. The biomolecular contrast materials exhibit anisotropic absorption coefficients that
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Volume 10 Issue 4 (2024) 10 doi: 10.36922/ijb.3448