Seeing Through the Surface: Non-invasive Characterization of Biomaterial–Tissue Interactions Using Photoacoustic Microscopy

Abstract

At the intersection of life sciences, materials science, engineering, and medicine, regenerative medicine stands out as a rapidly progressing field that aims at retaining, restoring, or augmenting tissue/organ functions to promote the human welfare. While the field has witnessed tremendous advancements over the past few decades, it still faces many challenges. For example, it has been difficult to visualize, monitor, and assess the functions of the engineered tissue/organ constructs, particularly when three-dimensional (3D) scaffolds are involved. Conventional approaches based on histology are invasive and therefore only convey end-point assays. The development of volumetric imaging techniques such as confocal and ultrasonic imaging has enabled direct observation of intact constructs without the need of sectioning. However, the capability of these techniques is often limited in terms of penetration depth and contrast. In comparison, the recently developed photoacoustic microscopy (PAM) has allowed us to address these issues by integrating optical and ultrasonic imaging to greatly reduce the effect of tissue scattering of photons with one-way ultrasound detection while retaining the high optical absorption contrast. PAM has been successfully applied to a number of studies, such as observation of cell distribution, monitoring of vascularization, and interrogation of biomaterial degradation. In this review article, we highlight recent progress in non-invasive and volumetric characterization of biomaterial-tissue interactions using PAM. We also discuss challenges ahead and envision future directions.

Inverse Opal Scaffolds and Their Biomedical Applications

“…Imaging biomaterial-tissue interactions has been historically challenging largely due to the poor imaging contrast between the biomaterials and/or the cells. [128,129] It becomes even more challenging in the in vivo setting because of the increase in imaging depth and complexity of the tissue microenvironment, as well as the interferences from the biological system. [128-131] To address this issue, collaborative efforts from imaging science and materials engineering have been formed over the past decade.…”

“…The unique integration of optical and ultrasound mechanisms allows PAM/PAT to achieve imaging at both high resolution and deep penetration without compromising the contrast, due to the reduced scattering of ultrasound by biological tissues relative to that of light. [128,129,133,134] Taking this advantage, we and our collaborators have used PAM/PAT to non-invasively characterize the interactions between the cells and inverse opal scaffolds in vitro and in real time. For example, by seeding B16 melanoma cells that possess intrinsic absorption contrast onto a PLGA inverse opal scaffold, the distribution of these cells inside the scaffold over a distance of about 1.5 mm could be easily resolved using acoustic-resolution PAM (AR-PAM; Figure 22A,B).…”

“…[135,136] Furthermore, by integrating PAM with other imaging modalities, it became possible to simultaneously visualize the cells and inverse opal scaffolds, revealing the valuable information about cell responses to their microenvironment (Figure 22C,D). [129] …”

Third harmonic generation microscopy of cells and tissue organization

“…Photoacoustic imaging uses excitation by laser light, which is absorbed by tissue and partially converted into heat; this leads to ultrasonic emission, which is detected for image reconstruction (Beard, 2011). Thus, the high contrast of photo-absorption is combined with the low scattering properties of ultrasound (Zhang et al, 2015). Optical absorption properties of tissue components vary with physiological conditions, such as hemoglobin concentration and oxygen saturation, and this can be exploited, for instance, for quantitative measurements of blood oxygen saturation.…”

“…Optical absorption properties of tissue components vary with physiological conditions, such as hemoglobin concentration and oxygen saturation, and this can be exploited, for instance, for quantitative measurements of blood oxygen saturation. The current resolution of photoacoustic imaging is in the range of approximately 100 µm, but can be improved to cell-level resolution by combining photoacoustic imaging with microscopy (PAM) (Zhang et al, 2015). Compared to photoacoustic imaging, the penetration depth of PAM is reduced from several millimeters to less than 1 mm.…”