Imaging challenges in biomaterials and tissue engineering

Abstract

Biomaterials are employed in the fields of tissue engineering and regenerative medicine (TERM) in order to enhance the regeneration or replacement of tissue function and/or structure. The unique environments resulting from the presence of biomaterials, cells, and tissues result in distinct challenges in regards to monitoring and assessing the results of these interventions. Imaging technologies for three-dimensional (3D) analysis have been identified as a strategic priority in TERM research. Traditionally, histological and immunohistochemical techniques have been used to evaluate engineered tissues. However, these methods do not allow for an accurate volume assessment, are invasive, and do not provide information on functional status. Imaging techniques are needed that enable non-destructive, longitudinal, quantitative, and three-dimensional analysis of TERM strategies. This review focuses on evaluating the application of available imaging modalities for assessment of biomaterials and tissue in TERM applications. Included is a discussion of limitations of these techniques and identification of areas for further development.

Fig. 1

Schematic of TERM strategies along with identified imaging needs associated with each.

Fig. 2

(A) Range of energies/frequencies on the electromagnetic spectrum used by 3D imaging modalities. (B) Approximate ranges of spatial resolution and imaging depth achievable by imaging modalities. US = ultrasound, PAM = photoacoustic microscopy, MRI = magnetic resonance imaging, MFM = multiphoton fluorescence microscopy, OCT = optical coherence tomography.

Fig. 3

Examples of ultrasound images produced for TERM applications. (A) Color image of flow in a tissue engineered vascular graft. Figure reproduced with permission, from Tillman et al. [6]. (B) Strain maps overlaid on a B-scan before (top two) and after (bottom two) degradation showing changes in mechanical properties in vitro. Figure reproduced with permission, from Kim et al. [21].

Fig. 4

Images of biomaterials, cells, and tissue structure generated using optical imaging techniques. (A) Fluorescent image of porous synthetic hydrogel that exhibits auto-fluorescence (green) loaded with fluorescently tagged fibrin (red). (B) OCT images of a biomaterial scaffolds seeded with MG63 bone cells. Figure reproduced with permission from IOP publishing from Yang et al. http://dx.doi.org/10.1088/0031-9155/51/7/001 [46]. (C) Combined luminescence and μCT demonstrating co-localization of bacteria (orange) and a subcutaneous tumor (green) within the animal. Lower images show a magnification the tumor from mouse: showing regions of tumor (FLuc green/blue), vasculature (contrast agent – red) and bacterial (orange/yellow) signals. Figure reproduced with permission, from Cronin et al. [229]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Examples of photoacoustic and PAM/US images from TERM applications. (A) PAM images revealing neovasculature in a porous scaffold at 1 (top) and 6 (bottom) weeks post-implantation. Images produced using the same methods described in Cai et al. [82]. (B) Dual ultrasound (left) and photoacoustic (right) imaging of adipose-derived stem cells in a fibrin gel at days 1 (top) and 16 (bottom). Images produced using the same methods described in Chung et al. [86].

Fig. 6

Examples of MRI images from TERM applications. (A) Four different MRI contrast mechanism images of a 38-week phalange model. Figure reproduced with permission, from Potter et al. [94]. (B) In vitro MR imaging of collagen hydrogels loaded with mesenchymal stem cells. Differences in signal are observed between scaffolds with labeled cells (bottom) versus unlabeled cells (top). Figure reproduced with permission, from Heymer et al. [109]. (C) Depiction of cartilage in a patient 6 months after chondrocyte transplantation. Arrows mark the area of cartilage repair. Figure reproduced with permission, from Welsch et al. [120]. (D) Fiber trajectories obtained by DTI representing collagen fibers in an artery: red (x-direction), green (y-direction) and blue (z-direction). Figure reproduced with permission, from Ghazanfari et al. [133]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

Example of X-ray images from TERM applications. (A) μCT images showing bone regeneration in cranial defects in response to growth factor treatment over time. Figure reproduced with permission from IOP publishing from Umoh et al. http://dx.doi.org/10.1088/0031-9155/54/7/020 [169]. (B) EPIC-μCT in an in vitro cartilage degradation model. Representative 3D EPIC-μCT images of control and IL-1-stimulated explants demonstrating progressive increases in attenuation in treated samples. Figure reproduced with permission, from Palmer et al. [184]. (C) Cell structures imaged in the soft tissue region of articular cartilage. Figure reproduced with permission, from Zehbe et al. [196]. (D) X-ray PC CT of explanted hydrogels implanted adjacent to skeletal muscle. The fibrovascular tissue, muscle and hydrogel can be identified.

Fig. 8

Examples of nuclear imaging in TERM applications: (A) SPECT 12 weeks postoperative showing activity in scaffolds seeded with cells (C and D) versus defects with no cells (A and B). Figure reproduced with permission, from Zhou et al. [222]. (B) Transverse and coronal fusion of computed tomography (CT) and micro-positron computed tomography (PET) images showing the distribution of ¹⁸FDG labeled cells injected in the myocardium. Figure reproduced with permission, from Terrovitis et al. [225].