Imaging Biomaterial-Tissue Interactions

Abstract

Modern biomedical imaging has revolutionized life science by providing anatomical, functional, and molecular information of biological species with high spatial resolution, deep penetration, enhanced temporal responsiveness, and improved chemical specificity. In recent years, these imaging techniques have been increasingly tailored for characterizing biomaterials and probing their interactions with biological tissues. This in turn has spurred substantial advances in engineering material properties to accommodate different imaging modalities that was previously unattainable. Here, we review advances in engineering both imaging modalities and material properties with improved contrast, providing a timely practical guide to better assess biomaterial-tissue interactions both in vitro and in vivo.

Keywords: biomaterials; contrast agent; imaging; regenerative medicine; tissue engineering.

Figure 1. Biomedical imaging of biomaterials

(a) Representative biomedical imaging modalities used in biomaterial characterization and their corresponding probing energies. US, ultrasound; MRI, magnetic resonance imaging; PAT, photoacoustic tomography; OCT, optical coherence tomography; CM, confocal microscopy; MPM, multi-photon microscopy; TEM, transmission electron microscopy; SEM, scanning electron microscopy; x-ray CT, x-ray computed tomography; PET, positron-emission tomography; SPECT, single-photon position emission tomography. (b) US elasticity imaging of a polyurethane-based tissue construct in a rat abdominal repair model at week 12. The tissue stiffness is colored from yellow (soft) to red (hard). Adapted with permission from [8]. (c) MRI image of transplanted ¹⁸F-labeled human neural stem cells (shown in color) and ECM scaffold remodeling in the stroke-damaged rat brain. Adapted with permission from [15]. (d) Confocal microscopy image of cardiomyocytes grown on a poly(glycerol sebacate) (PGS) scaffold. Adapted with permission from [20]. (e) PAT image of SWCNT-PLGA scaffold acquired at 680 nm embedded in a chicken breast tissue at the depth of 0.5 mm. SWCNT, single-walled carbon nanotube; PLGA, poly(lactic-co-glycolic acid). Adapted with permission from [7]. (f) X-ray CT image of thigh muscle pouches of a nude mouse at week 12 after implantation of zein scaffolds with rabbit mesenchymal stem cells. The white arrows indicate ectopic bone formation. Adapted with permission from [25]. (g) SEM image of a porous scaffold prepared from a composite of PLGA and hydroxyapatite (Hap), and the deposition of minerals from preosteoblasts cultured on the scaffold for 28 days. Adapted with permission from [48]. (h) PET image of transverse sections of a bioreactor chamber containing a cardiomyocyte-encapsulated fibrin scaffold under perfusion culture showing metabolic activity of the cells an hour after ¹⁸FDG labeling. ¹⁸FDG, ¹⁸F-fluordeoxyglucose. Adapted with permission from [29]. (i) Schematics showing contrast mechanisms to enhance imaging capacity.

Figure 2. Interrogating biomaterial-tissue interactions

(a) Bright-field optical image showing melanoma cells grown in a porous PLGA scaffold. Adapted with permission from [38]. (b) PAT reconstruction image showing 3D distribution of melanoma cells in a PLGA scaffold. Adapted with permission from [66]. (c) PAT images showing invasion of NIH/3T3 fibroblasts into PLGA scaffolds at days 1 and 7; the cells were stained with formazan to generate exogenous contrast. Adapted with permission from [43]. (d) Top view of the PAT/OCT images showing the ingrowth of melanoma cells from the surface into the center of the PLGA scaffolds. The melanoma cells were imaged by the PAT subsystem whereas the scaffold by the OCT subsystem, both in a label-free manner. (e) Volumetric rendering of the OCT-imaged scaffold (gray) with PAT-imaged melanoma cells (red). Adapted with permission from [38]. (f–i) Co-registered 3D reconstruction PAT images showing both the degradation of an individual PLGA scaffold and the remodeling of vasculature simultaneously in vivo. (j–m) Co-registered cross-sectional PAT images at the dotted planes as indicated in (f–i), respectively. Adapted with permission from [44].

Figure 3. Advanced technologies in tissue processing for enhanced optical imaging capability

(a–c) CLARITY. (a) Procedures for CLARITY: tissue is crosslinked with formaldehyde (red) in the presence of hydrogel monomers (blue), covalently linking tissue elements to monomers that are then polymerized into a hydrogel mesh (followed by a day-4 wash step); electric fields are subsequently applied across the sample in ionic detergent actively transport micelles into, and lipids out of, the tissue, leaving fine structure and crosslinked biomolecules in place. (b) Images of an adult mouse brains (3-month old) before and after CLARITY. (c) Confocal reconstructed image of a non-sectioned mouse brain tissue showing cortex, hippocampus, and thalamus (310 objective; stack size, 3,400 mm; step size, 2 mm). Scale bar: 400 μm. Adapted with permission from [77]. (d–i) ExM. (d, e) Schematics of (d) collapsed polyelectrolyte network, showing crosslinker (dot) and polymer chain (line), and (e) expanded network after H₂O dialysis. (f, g) Photographs of (f) a fixed mouse brain slice and (g) the sample post-ExM. Scale bars: 5 mm. Note that the size of (g) is 4.5 times that of (f) to give direct comparison of the same specimen pre-/post-expansion. (h, i) Confocal fluorescence images of a mouse brain slice stained with presynaptic (anti-Bassoon, blue) and postsynaptic (anti-Homer1, red) markers, in addition to antibody to GFP (green), (h) pre- versus (i) post-expansion. Scale bar: 5 μm. Adapted with permission from [70]. SDS, sodium dodecyl sulfate.