Optical spectroscopy and imaging for the noninvasive evaluation of engineered tissues

Nonlinear optical response of heme solutions

Heme is the prosthetic group for cytochrome that exists in nearly all living organisms and serves as a vital component of human red blood cells (RBCs). Tunable optical nonlinearity in suspensions of RBCs has been demonstrated previously, however, the nonlinear optical response of a pure heme (without membrane structure) solution has not been studied to our knowledge. In this work, we show optical nonlinearity in two common kinds of heme (i.e., hemin and hematin) solutions by a series of experiments and numerical simulations. We find that the mechanism of nonlinearity in heme solutions is distinct from that observed in the RBC suspensions where the nonlinearity can be easily tuned through optical power, concentration, and the solution properties. In particular, we observe an unusual phenomenon wherein the heme solution exhibits negative optical nonlinearity and render self-collimation of a focused beam at specific optical powers, enabling shape-preserving propagation of light to long distances. Our results may have potential applications in optical imaging and medical diagnosis through blood.

Fundamentals and Applications of Raman-Based Techniques for the Design and Development of Active Biomedical Materials

Raman spectroscopy is an analytical method based on light-matter interactions that can interrogate the vibrational modes of matter and provide representative molecular fingerprints. Mediated by its label-free, non-invasive nature, and high molecular specificity, Raman-based techniques have become ubiquitous tools for in situ characterization of materials. This review comprehensively describes the theoretical and practical background of Raman spectroscopy and its advanced variants. The numerous facets of material characterization that Raman scattering can reveal, including biomolecular identification, solid-to-solid phase transitions, and spatial mapping of biomolecular species in bioactive materials, are highlighted. The review illustrates the potential of these techniques in the context of active biomedical material design and development by highlighting representative studies from the literature. These studies cover the use of Raman spectroscopy for the characterization of both natural and synthetic biomaterials, including engineered tissue constructs, biopolymer systems, ceramics, and nanoparticle formulations, among others. To increase the accessibility and adoption of these techniques, the present review also provides the reader with practical recommendations on the integration of Raman techniques into the experimental laboratory toolbox. Finally, perspectives on how recent developments in plasmon- and coherently-enhanced Raman spectroscopy can propel Raman from underutilized to critical for biomaterial development are provided.

Generalization of the Ratiometric Method to Extend pH Range Measurements of the BCECF Probe

There is a variety of fluorescent probes for pH measurements and which are mainly used for biological systems. In general, they can be classified into two groups. The first group includes fluorescent pH probes which exhibit a single fluorescence emission peak. For these probes, the fluorescence excitation profile is pH-dependent and the shape of the emission spectra remains almost constant. Hence, the ratiometric pH measurement–which makes pH determination independent of the probe concentration-is implemented when the excitation is performed at two excitation wavelengths and the fluorescence emission is measured at one wavelength. The second group exhibits a dual fluorescence emission peak. Here, each protonated or deprotonated form exhibits characteristics emission and/or absorption spectra. Shifts between spectra obtained for protonated and deprotonated species can be exploited in order to perform a ratiometric measurement. In this study we present a methodology that evaluates the precision of the ratiometric measurements based on multiple wavelengths excitation to determine the optimum wavelengths combination for pH determination in biological samples. This methodology using the BCECF probe is applied to measure the pH drift in cell culture medium. It exhibits a high precision and significantly extends the range of validity for pH measurements spanning from very acidic to basic.

Conservation of orbital angular momentum and polarization through biological waveguides

A major roadblock to the development of photonic sensors is the scattering associated with many biological systems. We show the conservation of photonic states through optically self-arranged biological waveguides, for the first time, which can be implemented to transmit light through scattering media. The conservation of optical properties of light through biological waveguides allows for the transmission of high bandwidth information with low loss through scattering media. Here, we experimentally demonstrate the conservation of polarization state and orbital angular momentum of light through a self-arranged biological waveguide, several centimeters long, in a sheep red blood cell suspension. We utilize nonlinear optical effects to self-trap cells, which form waveguides at 532 nm and 780 nm wavelengths. Moreover, we use the formed waveguide channels to couple and guide probe beams without altering the information. The formed biological waveguides are in a sub-diffusive scattering regime, so the photons’ information degrades insignificantly over several centimeters of propagation through the scattering media. Our results show the potential of biological waveguides as a methodology for the development of novel photonic biosensors, biomedical devices that require optical wireless communication, and the development of new approaches to noninvasive biomedical imaging.

The vascularization paradox of non-union formation

Despite major research efforts to elucidate mechanisms of non-union formation, failed fracture healing remains a common complication in orthopedic surgery. Adequate vascularization has been recognized as a crucial factor for successful bone regeneration, as newly formed microvessels guarantee the supply of the callus tissue with vital oxygen, nutrients, and growth factors. Accordingly, a vast number of preclinical studies have focused on the development of vascularization strategies to stimulate fracture repair. However, recent evidence suggests that stimulation of blood vessel formation is an oversimplified approach to support bone regeneration. This review discusses the role of vascularization during bone regeneration and delineates a phenomenon, for which we coin the term “the vascularization paradox of non-union-formation”. This view is based on the results of a variety of experimental studies that suggest that the callus tissue of non-unions is indeed densely vascularized and that pro-angiogenic mediators, such as vascular endothelial growth factor, are sufficiently expressed at the facture site. By gaining further insights into the molecular and cellular basis of non-union vascularization, it may be possible to develop more optimized treatment approaches or even prevent the non-union formation in the future.

Raman spectroscopy in skeletal tissue disorders and tissue engineering: present and prospective

Musculoskeletal disorders are the most common reason of chronic pain and disability representing worldwide an enormous socio-economic burden. In this review, new biomedical application fields for Raman spectroscopy (RS) technique related to skeletal tissues are discussed showing that it can provide a comprehensive profile of tissue composition in situ, in a rapid, label-free, and non-destructive manner. RS can be used as a tool to study tissue alterations associated to aging, pathologies, and disease treatments. The main advantage with respect to currently applied methods in clinics is its ability to provide specific information on molecular composition, which goes beyond other diagnostic tools. Being compatible with water, RS can be performed without pre-treatment on unfixed, hydrated tissue samples, without any labelling and chemical fixation used in histochemical methods. This review provides first the description of basic principles of RS as a biotechnology tool and introduces into the field of currently available RS based techniques, developed to enhance Raman signal. The main spectral processing statistical tools, fingerprint identification and available databases are mentioned. The recent literature has been analysed for such applications of RS as tendon and ligaments, cartilage, bone, and tissue engineered constructs for regenerative medicine. Several cases of proof-of-concept preclinical studies have been described. Finally, advantages, limitations, future perspectives, and challenges for translation of RS into clinical practice have been also discussed.

3-D Cell Culture Systems in Bone Marrow Tissue and Organoid Engineering, and BM Phantoms as In Vitro Models of Hematological Cancer Therapeutics-A Review

We review the state-of-the-art in bone and marrow tissue engineering (BMTE) and hematological cancer tissue engineering (HCTE) in light of the recent interest in bone marrow environment and pathophysiology of hematological cancers. This review focuses on engineered BM tissue and organoids as in vitro models of hematological cancer therapeutics, along with identification of BM components and their integration as synthetically engineered BM mimetic scaffolds. In addition, the review details interaction dynamics of various BM and hematologic cancer (HC) cell types in co-culture systems of engineered BM tissues/phantoms as well as their relation to drug resistance and cytotoxicity. Interaction between hematological cancer cells and their niche, and the difference with respect to the healthy niche microenvironment narrated. Future perspectives of BMTE for in vitro disease models, BM regeneration and large scale ex vivo expansion of hematopoietic and mesenchymal stem cells for transplantation and therapy are explained. We conclude by overviewing the clinical application of biomaterials in BM and HC pathophysiology and its challenges and opportunities.

Photoacoustic Imaging in Tissue Engineering and Regenerative Medicine

Several imaging modalities are available for investigation of the morphological, functional, and molecular features of engineered tissues in small animal models. While research in tissue engineering and regenerative medicine (TERM) would benefit from a comprehensive longitudinal analysis of new strategies, researchers have not always applied the most advanced methods. Photoacoustic imaging (PAI) is a rapidly emerging modality that has received significant attention due to its ability to exploit the strong endogenous contrast of optical methods with the high spatial resolution of ultrasound methods. Exogenous contrast agents can also be used in PAI for targeted imaging. Applications of PAI relevant to TERM include stem cell tracking, longitudinal monitoring of scaffolds in vivo, and evaluation of vascularization. In addition, the emerging capabilities of PAI applied to the detection and monitoring of cancer and other inflammatory diseases could be exploited by tissue engineers. This article provides an overview of the operating principles of PAI and its broad potential for application in TERM. Impact statement Photoacoustic imaging, a new hybrid imaging technique, has demonstrated high potential in the clinical diagnostic applications. The optical and acoustic aspect of the photoacoustic imaging system works in harmony to provide better resolution at greater tissue depth. Label-free imaging of vasculature with this imaging can be used to track and monitor disease, as well as the therapeutic progression of treatment. Photoacoustic imaging has been utilized in tissue engineering to some extent; however, the full benefit of this technique is yet to be explored. The increasing availability of commercial photoacoustic systems will make application as an imaging tool for tissue engineering application more feasible. This review first provides a brief description of photoacoustic imaging and summarizes its current and potential application in tissue engineering.

Phase-contrast 3D tomography of HeLa cells grown in PLLA polymer electrospun scaffolds using synchrotron X-rays

Advanced imaging is useful for understanding the three-dimensional (3D) growth of cells. X-ray tomography serves as a powerful noninvasive, nondestructive technique that can fulfill these purposes by providing information about cell growth within 3D platforms. There are a limited number of studies taking advantage of synchrotron X-rays, which provides a large field of view and suitable resolution to image cells within specific biomaterials. In this study, X-ray synchrotron radiation microtomography at Diamond Light Source and advanced image processing were used to investigate cellular infiltration of HeLa cells within poly L-lactide (PLLA) scaffolds. This study demonstrates that synchrotron X-rays using phase contrast is a useful method to understand the 3D growth of cells in PLLA electrospun scaffolds. Two different fiber diameter (2 and 4 µm) scaffolds with different pore sizes, grown over 2, 5 and 8 days in vitro, were examined for infiltration and cell connectivity. After performing visualization by segmentation of the cells from the fibers, the results clearly show deeper cell growth and higher cellular interconnectivity in the 4 µm fiber diameter scaffold. This indicates the potential for using such 3D technology to study cell-scaffold interactions for future medical use.

Quantitative ultrasound imaging of cell-laden hydrogels and printed constructs

In the present work we have revisited the application of quantitative ultrasound imaging (QUI) to cellular hydrogels, by using the reference phantom method (RPM) in combination with a local attenuation compensation algorithm. The investigated biological samples consisted of cell-laden collagen hydrogels with PC12 neural cells. These cell-laden hydrogels were used to calibrate the integrated backscattering coefficient (IBC) as a function of cell density, which was then used to generate parametric images of local cell density. The image resolution used for QUI and its impact on the relative IBC error was also investigated. Another important contribution of our work was the monitoring of PC12 cell proliferation. The cell number estimates obtained via the calibrated IBC compared well with data obtained using a conventional quantitative method, the MTS assay. Evaluation of spectral changes as a function of culture time also provided additional information on the cell cluster size, which was found to be in close agreement with that observed by microscopy. Last but not least, we also applied QUI on a 3D printed cellular construct in order to illustrate its capabilities for the evaluation of bioprinted structures. STATEMENT OF SIGNIFICANCE: While there is intensive research in the areas of polymer science, biology, and 3D bio-printing, there exists a gap in available characterisation tools for the non-destructive inspection of biological constructs in the three-dimensional domain, on the macroscopic scale, and with fast data acquisition times. Quantitative ultrasound imaging is a suitable characterization technique for providing essential information on the development of tissue engineered constructs. These results provide a detailed and comprehensive guide on the capabilities and limitations of the technique.

Evaluating Cell Metabolism Through Autofluorescence Imaging of NAD(P)H and FAD

SIGNIFICANCE

Optical imaging using the endogenous fluorescence of metabolic cofactors has enabled nondestructive examination of dynamic changes in cell and tissue function both in vitro and in vivo. Quantifying NAD(P)H and FAD fluorescence through an optical redox ratio and fluorescence lifetime imaging (FLIM) provides sensitivity to the relative balance between oxidative phosphorylation and glucose catabolism. Since its introduction decades ago, the use of NAD(P)H imaging has expanded to include applications involving almost every major tissue type and a variety of pathologies. Recent Advances: This review focuses on the use of two-photon excited fluorescence and NAD(P)H fluorescence lifetime techniques in cancer, neuroscience, tissue engineering, and other biomedical applications over the last 5 years. In a variety of cancer models, NAD(P)H fluorescence intensity and lifetime measurements demonstrate a sensitivity to the Warburg effect, suggesting potential for early detection or high-throughput drug screening. The sensitivity to the biosynthetic demands of stem cell differentiation and tissue repair processes indicates the range of applications for this imaging technology may be broad.

CRITICAL ISSUES

As the number of applications for these fluorescence imaging techniques expand, identifying and characterizing additional intrinsic fluorophores and chromophores present in vivo will be vital to accurately measure and interpret metabolic outcomes. Understanding the full capabilities and limitations of FLIM will also be key to future advances.

FUTURE DIRECTIONS

Future work is needed to evaluate whether a combination of different biochemical and structural outcomes using these imaging techniques can provide complementary information regarding the utilization of specific metabolic pathways.

Applications of SNAP-tag technology in skin cancer therapy

BACKGROUND

Cancer treatment in the 21st century has seen immense advances in optical imaging and immunotherapy. Significant progress has been made in the bioengineering and production of immunoconjugates to achieve the goal of specifically targeting tumors.

DISCUSSION

In the 21st century, antibody drug conjugates (ADCs) have been the focus of immunotherapeutic strategies in cancer. ADCs combine the unique targeting of monoclonal antibodies (mAbs) with the cancer killing ability of cytotoxic drugs. However, due to random conjugation methods of drug to antibody, ADCs are associated with poor antigen specificity and low cytotoxicity, resulting in a drug to antibody ratio (DAR) >1. This means that the cytotoxic drugs in ADCs are conjugated randomly to antibodies, by cysteine or lysine residues. This generates heterogeneous ADC populations with 0 to 8 drugs per an antibody, each with distinct pharmacokinetic, efficacy, and toxicity properties. Additionally, heterogeneity is created not only by different antibody to ligand ratios but also by different sites of conjugation. Hence, much effort has been made to find and establish antibody conjugation strategies that enable us to better control stoichiometry and site-specificity. This includes utilizing protein self-labeling tags as fusion partners to the original protein. Site-specific conjugation is a significant characteristic of these engineered proteins. SNAP-tag is one such engineered self-labeling protein tag shown to have promising potential in cancer treatment. The SNAP-tag is fused to an antibody of choice and covalently reacts specifically in a 1:1 ratio with benzylguanine (BG) substrates, eg, fluorophores or photosensitizers, to target skin cancer. This makes SNAP-tag a versatile technique in optical imaging and photoimmunotherapy of skin cancer.

CONCLUSION

SNAP-tag technology has the potential to contribute greatly to a broad range of molecular oncological applications because it combines efficacious tumor targeting, minimized local and systemic toxicity, and noninvasive assessment of diagnostic/prognostic molecular biomarkers of cancer.

Optical force-induced nonlinearity and self-guiding of light in human red blood cell suspensions

Osmotic conditions play an important role in the cell properties of human red blood cells (RBCs), which are crucial for the pathological analysis of some blood diseases such as malaria. Over the past decades, numerous efforts have mainly focused on the study of the RBC biomechanical properties that arise from the unique deformability of erythrocytes. Here, we demonstrate nonlinear optical effects from human RBCs suspended in different osmotic solutions. Specifically, we observe self-trapping and scattering-resistant nonlinear propagation of a laser beam through RBC suspensions under all three osmotic conditions, where the strength of the optical nonlinearity increases with osmotic pressure on the cells. This tunable nonlinearity is attributed to optical forces, particularly the forward-scattering and gradient forces. Interestingly, in aged blood samples (with lysed cells), a notably different nonlinear behavior is observed due to the presence of free hemoglobin. We use a theoretical model with an optical force-mediated nonlocal nonlinearity to explain the experimental observations. Our work on light self-guiding through scattering bio-soft-matter may introduce new photonic tools for noninvasive biomedical imaging and medical diagnosis.

Noninvasive Optical Assessment of Implanted Engineered Tissues Correlates with Cytokine Secretion

Fluorescence lifetime sensing has been shown to noninvasively characterize the preimplantation health and viability of engineered tissue constructs. However, current practices to monitor postimplantation construct integration are either qualitative (visual assessment) or destructive (tissue histology). We employed label-free fluorescence lifetime spectroscopy for quantitative, noninvasive optical assessment of engineered tissue constructs that were implanted into a murine model. The portable system was designed to be suitable for intravital measurements and included a handheld probe to precisely and rapidly acquire data at multiple sites per construct. Our model tissue constructs were manufactured from primary human cells to simulate patient variability based on a standard protocol, and half of the manufactured constructs were stressed to create a range of health states. Secreted amounts of three cytokines that relate to cellular viability were measured in vitro to assess preimplantation construct health: interleukin-8 (IL-8), human β-defensin 1 (hBD-1), and vascular endothelial growth factor (VEGF). Preimplantation cytokine secretion ranged from 1.5 to 33.5 pg/mL for IL-8, from 3.4 to 195.0 pg/mL for hBD-1, and from 0.1 to 154.3 pg/mL for VEGF. In vivo optical sensing assessed constructs at 1 and 3 weeks postimplantation. We found that at 1 week postimplantation, in vivo optical parameters correlated with in vitro preimplantation secretion levels of all three cytokines (p < 0.05). This correlation was not observed in optical measurements at 3 weeks postimplantation when histology showed that the constructs had re-epithelialized, independent of preimplantation health state, supporting the lack of a correlation. These results suggest that clinical optical diagnostic tools based on label-free fluorescence lifetime sensing of endogenous tissue fluorophores could noninvasively monitor postimplantation integration of engineered tissues.

Raman Spectroscopy as a Tool for Quality and Sterility Analysis for Tissue Engineering Applications like Cartilage Transplants

At present, the production of tissue engineered cartilage requires the concurrent production of two identical transplants. One transplant is used for destructive quality control and the second one is implanted into the patient. A non-invasive characterization of such tissue engineering samples would be a promising tool to achieve a production process of just one transplant that is both implanted and tested. Raman spectroscopy is a method that satisfies this requirement by analyzing cells without lysis, fixation or the use of any chemicals. This pure optical technique is based on inelastic scattering of laser photons by molecular vibrations of biopolymers. Characteristic peaks in Raman spectra of cells could be assigned to typical biochemical molecules present in biological samples. For the analysis of chondrocytes present in cartilage transplants, the determination of the cell vitality as well as the discrimination of another cell type have been studied by Raman spectroscopy. Another bottleneck in such biological processes under GMP conditions is sterility control, as most of the commonly used methods require long cultivation times. Raman spectroscopy provides a good alternative to conventional methods in terms of time saving. In this study, the potential of Raman spectroscopy as a quality and sterility control tool for tissue engineering applications was studied by analyzing and comparing the spectra of cell and bacteria cultures.

Near-Infrared Spectroscopy Predicts Compositional and Mechanical Properties of Hyaluronic Acid-Based Engineered Cartilage Constructs

Hyaluronic acid (HA) has been widely used for cartilage tissue engineering applications. However, the optimal time point to harvest HA-based engineered constructs for cartilage repair is still under investigation. In this study, we investigated the ability of a nondestructive modality, near-infrared spectroscopic (NIR) analysis, to predict compositional and mechanical properties of HA-based engineered cartilage constructs. NIR spectral data were collected from control, unseeded constructs, and twice per week by fiber optic from constructs seeded with chondrocytes during their development over an 8-week period. Constructs were harvested at 2, 4, 6, and 8 weeks, collagen and sulfated glycosaminoglycan content measured using biochemical assays, and the mechanical properties of the constructs evaluated using unconfined compression tests. NIR absorbances associated with the scaffold material, water, and engineered cartilage matrix, were identified. The NIR-determined matrix absorbance plateaued after 4 weeks of culture, which was in agreement with the biochemical assay results. Similarly, the mechanical properties of the constructs also plateaued at 4 weeks. A multivariate partial least square model based on NIR spectral input was developed to predict the moduli of the constructs, which resulted in a prediction error of 10% and R value of 0.88 for predicted versus actual values of dynamic modulus. Furthermore, the maximum increase in moduli was calculated from the first derivative of the curve fit of NIR-predicted and actual moduli values over time, and both occurred at ∼2 weeks. Collectively, these data suggest that NIR spectral data analysis could be an alternative to destructive biochemical and mechanical methods for evaluation of HA-based engineered cartilage construct properties.

Vibrational spectroscopy and imaging: applications for tissue engineering

Tissue engineering (TE) approaches strive to regenerate or replace an organ or tissue. The successful development and subsequent integration of a TE construct is contingent on a series of in vitro and in vivo events that result in an optimal construct for implantation. Current widely used methods for evaluation of constructs are incapable of providing an accurate compositional assessment without destruction of the construct. In this review, we discuss the contributions of vibrational spectroscopic assessment for evaluation of tissue engineered construct composition, both during development and post-implantation. Fourier transform infrared (FTIR) spectroscopy in the mid and near-infrared range, as well as Raman spectroscopy, are intrinsically label free, can be non-destructive, and provide specific information on the chemical composition of tissues. Overall, we examine the contribution that vibrational spectroscopy via fiber optics and imaging have to tissue engineering approaches.

Quantitative, Label-Free Evaluation of Tissue-Engineered Skeletal Muscle Through Multiphoton Microscopy

The lack of tools for assessing engineered tissue viability and function in a noninvasive manner is a major regulatory and translational challenge facing tissue engineers. Label-free, nonlinear optical molecular imaging (OMI) has utilized endogenous nicotinamide adenine dinucleotide and flavin adenine dinucleotide fluorescence to indicate metabolic activity. Similarly, second harmonic generation (SHG) signals from myosin and collagen can measure overall muscle structural integrity and function. The purpose of this study was to demonstrate these OMI techniques for the first time in engineered skeletal muscle and to develop a novel method for evaluating our engineered skeletal muscle units (SMUs) before implantation. Three experimental groups were studied: Control, Steroid Supplemented, and Metabolically Stressed SMUs. After imaging and analysis in ImageJ, a redox ratio (RR) metric was calculated to indicate metabolic activity, and a structure ratio metric was calculated to reflect structural composition. In addition, function was evaluated as tetanic force production in response to electrical stimulation. In living tissues, the RRs successfully distinguished control and metabolically stressed SMUs in both monolayer and 3D form. OMI of myosin and collagen SHG similarly differentiated control SMUs from the steroid supplemented group. With respect to function, steroid supplementation significantly increased active force generation. When comparing functional and OMI measures, a significant correlation was present between overall myosin density and active force generation. This work demonstrates the potential for using label-free OMI to evaluate tissue-engineered skeletal muscle constructs. The positive correlation between structural OMI measures and force production suggests that OMI could potentially serve as an accurate predictor of functional behaviors, such as integration and tissue regeneration, after implantation. This noninvasive OMI methodology, demonstrated for the first time in engineered skeletal muscle, could prove invaluable for assessing our tissue engineering technology and confirming release criteria for validation.

Nonlinear Self-Action of Light through Biological Suspensions

It is commonly thought that biological media cannot exhibit an appreciable nonlinear optical response. We demonstrate, for the first time to our knowledge, a tunable optical nonlinearity in suspensions of cyanobacteria that leads to robust propagation and strong self-action of a light beam. By deliberately altering the host environment of the marine bacteria, we show experimentally that nonlinear interaction can result in either deep penetration or enhanced scattering of light through the bacterial suspension, while the viability of the cells remains intact. A theoretical model is developed to show that a nonlocal nonlinearity mediated by optical forces (including both gradient and forward-scattering forces) acting on the bacteria explains our experimental observations.

Cell-matrix mechanical interaction in electrospun polymeric scaffolds for tissue engineering: Implications for scaffold design and performance

Engineered scaffolds produced by electrospinning of biodegradable polymers offer a 3D, nanofibrous environment with controllable structural, chemical, and mechanical properties that mimic the extracellular matrix of native tissues and have shown promise for a number of tissue engineering applications. The microscale mechanical interactions between cells and electrospun matrices drive cell behaviors including migration and differentiation that are critical to promote tissue regeneration. Recent developments in understanding these mechanical interactions in electrospun environments are reviewed, with emphasis on how fiber geometry and polymer structure impact on the local mechanical properties of scaffolds, how altering the micromechanics cues cell behaviors, and how, in turn, cellular and extrinsic forces exerted on the matrix mechanically remodel an electrospun scaffold throughout tissue development. Techniques used to measure and visualize these mechanical interactions are described. We provide a critical outlook on technological gaps that must be overcome to advance the ability to design, assess, and manipulate the mechanical environment in electrospun scaffolds toward constructs that may be successfully applied in tissue engineering and regenerative medicine.

STATEMENT OF SIGNIFICANCE

Tissue engineering requires design of scaffolds that interact with cells to promote tissue development. Electrospinning is a promising technique for fabricating fibrous, biomimetic scaffolds. Effects of electrospun matrix microstructure and biochemical properties on cell behavior have been extensively reviewed previously; here, we consider cell-matrix interaction from a mechanical perspective. Micromechanical properties as a driver of cell behavior has been well established in planar substrates, but more recently, many studies have provided new insights into mechanical interaction in fibrillar, electrospun environments. This review provides readers with an overview of how electrospun scaffold mechanics and cell behavior work in a dynamic feedback loop to drive tissue development, and discusses opportunities for improved design of mechanical environments that are conducive to tissue development.

Two-photon FLIM of NAD(P)H and FAD in mesenchymal stem cells undergoing either osteogenic or chondrogenic differentiation

Background

Metabolic plasticity and the versatility of different lineages of stem cells as they satisfy their energy demands are not completely understood. In this study we investigated the metabolic changes in mesenchymal stem cells (MSCs) undergoing differentiation in two directions, osteogenic and chondrogenic, using two-photon fluorescence microscopy combined with FLIM.

Methods

Differentiation was induced by incubating the human bone marrow MSCs in osteogenic or chondrogenic mediums. Cellular metabolism was examined on the basis of the fluorescence of the metabolic cofactors NAD(P)H and FAD. The optical redox ratio (FAD/NAD(P)H) and the fluorescence lifetimes of NAD(P)H and FAD were traced using two-photon fluorescence microscopy combined with FLIM. The cells were imaged before the induction of differentiation (day 0) and on days 7, 14, and 21 of osteogenic and chondrogenic differentiation.

Results

Based on the data for the FAD/NAD(P)H redox ratio and on the fluorescence lifetimes of protein-bound NAD(P)H, we registered a metabolic shift toward a more glycolytic status in the process of MSC differentiation. The difference was that, in osteogenic differentiation, an increase in oxidative phosphorylation preceded the shift to the glycolytic status in the process of such MSC differentiation. The fluorescence lifetime characteristics of FAD indicated the stimulation of an unknown metabolic pathway, where protein-bound FAD participates.

Conclusions

In this study, probing of the metabolic status of MSCs during osteogenic and chondrogenic differentiation was implemented for the first time with the use of optical metabolic imaging of the two cofactors - NAD(P)H and FAD. Our data suggest that biosynthetic processes, associated, presumably, with the synthesis of collagen, drive energy metabolism in differentiating cells, and promote a metabolic shift from a more oxidative to a more glycolytic state.

Label-Free Imaging of Umbilical Cord Tissue Morphology and Explant-Derived Cells

In situ detection of MSCs remains difficult and warrants additional methods to aid with their characterization in vivo. Two-photon confocal laser scanning microscopy (TPM) and second harmonic generation (SHG) could fill this gap. Both techniques enable the detection of cells and extracellular structures, based on intrinsic properties of the specific tissue and intracellular molecules under optical irradiation. TPM imaging and SHG imaging have been used for label-free monitoring of stem cells differentiation, assessment of their behavior in biocompatible scaffolds, and even cell tracking in vivo. In this study, we show that TPM and SHG can accurately depict the umbilical cord architecture and visualize individual cells both in situ and during culture initiation, without the use of exogenously applied labels. In combination with nuclear DNA staining, we observed a variance in fluorescent intensity in the vessel walls. In addition, antibody staining showed differences in Oct4, αSMA, vimentin, and ALDH1A1 expression in situ, indicating functional differences among the umbilical cord cell populations. In future research, marker-free imaging can be of great added value to the current antigen-based staining methods for describing tissue structures and for the identification of progenitor cells in their tissue of origin.

Methods of Monitoring Cell Fate and Tissue Growth in Three-Dimensional Scaffold-Based Strategies for In Vitro Tissue Engineering

In the field of tissue engineering, there is a need for methods that allow assessing the performance of tissue-engineered constructs noninvasively in vitro and in vivo. To date, histological analysis is the golden standard to retrieve information on tissue growth, cellular distribution, and cell fate on tissue-engineered constructs after in vitro cell culture or on explanted specimens after in vivo applications. Yet, many advances have been made to optimize imaging techniques for monitoring tissue-engineered constructs with a sub-mm or μm resolution. Many imaging modalities have first been developed for clinical applications, in which a high penetration depth has been often more important than lateral resolution. In this study, we have reviewed the current state of the art in several imaging approaches that have shown to be promising in monitoring cell fate and tissue growth upon in vitro culture. Depending on the aimed tissue type and scaffold properties, some imaging methods are more applicable than others. Optical methods are mostly suited for transparent materials such as hydrogels, whereas magnetic resonance-based methods are mostly applied to obtain contrast between hard and soft tissues regardless of their transparency. Overall, this review shows that the field of imaging in scaffold-based tissue engineering is developing at a fast pace and has the potential to overcome the limitations of destructive endpoint analysis.

Probing metabolic states of differentiating stem cells using two-photon FLIM

The ability of stem cells to differentiate into specialized cell types presents a number of opportunities for regenerative medicine, stem cell therapy and developmental biology. Because traditional assessments of stem cells are destructive, time consuming, and logistically intensive, the use of a non-invasive, label-free approach to study of cell differentiation provides a powerful tool for rapid, high-content characterization of cell and tissue cultures. Here, we elucidate the metabolic changes in MSCs during adipogenic differentiation, based on the fluorescence of the metabolic co-factors NADH, NADPH, and FAD using the methods of two-photon fluorescence microscopy combined with FLIM. To estimate the contribution of energy metabolism and lipogenesis in the observed changes of the metabolic profile, a separate analysis of NADH and NADPH is required. In our study we demonstrated, for the first time, an increased contribution of protein-bound NADPH in adipocytes that is associated with lipogenesis. The optical redox ratio FAD/NAD(P)H decreased during adipogenic differentiation, and that this was likely to be explained by the intensive biosynthesis of lipids and the enhanced NADPH production associated with this. Based on the data on the fluorescence lifetime contribution of protein-bound NAD(P)H, we registered a metabolic switch from glycolysis to oxidative phosphorylation in adipocytes.

Nondestructive Assessment of Engineered Cartilage Composition by Near Infrared Spectroscopy

Tissue engineering presents a strategy to overcome the limitations of current tissue healing methods. Scaffolds, cells, external growth factors and mechanical input are combined in an effort to obtain constructs with properties that mimic native tissues. However, engineered constructs developed using similar culture environments can have very different matrix composition and biomechanical properties. Accordingly, a nondestructive technique to assess constructs during development such that appropriate compositional endpoints can be defined is desirable. Near infrared spectroscopy (NIRS) analysis is a modality being investigated to address the challenges associated with current evaluation techniques, which includes nondestructive compositional assessment. In the present study, cartilage tissue constructs were grown using chondrocytes seeded onto polyglycolic acid (PGA) scaffolds in similar environments in three separate tissue culture experiments and monitored using NIRS. Multivariate partial least squares (PLS) analysis models of NIR spectra were calculated and used to predict tissue composition, with biochemical assay information used as the reference data. Results showed that for combined data from all tissue culture experiments, PLS models were able to assess composition with significant correlations to reference values, including engineered cartilage water (at 5200 cm(-1), R = 0.68, p = 0.03), proteoglycan (at 4310 cm(-1), R = 0.82, p = 0.007), and collagen (at 4610 cm(-1), R = 0.84, p = 0.005). In addition, degradation of PGA was monitored using specific NIRS frequencies. These results demonstrate that NIR spectroscopy combined with multivariate analysis provides a nondestructive modality to assess engineered cartilage, which could provide information to determine the optimal time for tissue harvest for clinical applications.

Combining 3D human in vitro methods for a 3Rs evaluation of novel titanium surfaces in orthopaedic applications

In this study, we report on a group of complementary human osteoblast in vitro test methods for the preclinical evaluation of 3D porous titanium surfaces. The surfaces were prepared by additive manufacturing (electron beam melting [EBM]) and plasma spraying, allowing the creation of complex lattice surface geometries. Physical properties of the surfaces were characterized by SEM and profilometry and 3D in vitro cell culture using human osteoblasts. Primary human osteoblast cells were found to elicit greater differences between titanium sample surfaces than an MG63 osteoblast-like cell line, particularly in terms of cell survival. Surface morphology was associated with higher osteoblast metabolic activity and mineralization on rougher titanium plasma spray coated surfaces than smoother surfaces. Differences in osteoblast survival and metabolic activity on titanium lattice structures were also found, despite analogous surface morphology at the cellular level. 3D confocal microscopy identified osteoblast organization within complex titanium surface geometries, adhesion, spreading, and alignment to the biomaterial strut geometries. Mineralized nodule formation throughout the lattice structures was also observed, and indicative of early markers of bone in-growth on such materials. Testing methods such as those presented are not traditionally considered by medical device manufacturers, but we suggest have value as an increasingly vital tool in efficiently translating pre-clinical studies, especially in balance with current regulatory practice, commercial demands, the 3Rs, and the relative merits of in vitro and in vivo studies. Biotechnol. Bioeng. 2016;113: 1586-1599. © 2015 The Authors. Biotechnology and Bioengineering Published by Wiley Periodicals, Inc.

Mesoscopic Fluorescence Molecular Tomography for Evaluating Engineered Tissues

Optimization of regenerative medicine strategies includes the design of biomaterials, development of cell-seeding methods, and control of cell-biomaterial interactions within the engineered tissues. Among these steps, one paramount challenge is to non-destructively image the engineered tissues in their entirety to assess structure, function, and molecular expression. It is especially important to be able to enable cell phenotyping and monitor the distribution and migration of cells throughout the bulk scaffold. Advanced fluorescence microscopic techniques are commonly employed to perform such tasks; however, they are limited to superficial examination of tissue constructs. Therefore, the field of tissue engineering and regenerative medicine would greatly benefit from the development of molecular imaging techniques which are capable of non-destructive imaging of three-dimensional cellular distribution and maturation within a tissue-engineered scaffold beyond the limited depth of current microscopic techniques. In this review, we focus on an emerging depth-resolved optical mesoscopic imaging technique, termed laminar optical tomography (LOT) or mesoscopic fluorescence molecular tomography (MFMT), which enables longitudinal imaging of cellular distribution in thick tissue engineering constructs at depths of a few millimeters and with relatively high resolution. The physical principle, image formation, and instrumentation of LOT/MFMT systems are introduced. Representative applications in tissue engineering include imaging the distribution of human mesenchymal stem cells embedded in hydrogels, imaging of bio-printed tissues, and in vivo applications.

Generation and Assessment of Functional Biomaterial Scaffolds for Applications in Cardiovascular Tissue Engineering and Regenerative Medicine

Current clinically applicable tissue and organ replacement therapies are limited in the field of cardiovascular regenerative medicine. The available options do not regenerate damaged tissues and organs, and, in the majority of the cases, show insufficient restoration of tissue function. To date, anticoagulant drug-free heart valve replacements or growing valves for pediatric patients, hemocompatible and thrombus-free vascular substitutes that are smaller than 6 mm, and stem cell-recruiting delivery systems that induce myocardial regeneration are still only visions of researchers and medical professionals worldwide and far from being the standard of clinical treatment. The design of functional off-the-shelf biomaterials as well as automatable and up-scalable biomaterial processing methods are the focus of current research endeavors and of great interest for fields of tissue engineering and regenerative medicine. Here, various approaches that aim to overcome the current limitations are reviewed, focusing on biomaterials design and generation methods for myocardium, heart valves, and blood vessels. Furthermore, novel contact- and marker-free biomaterial and extracellular matrix assessment methods are highlighted.

Non-invasive Assessments of Adipose Tissue Metabolism In Vitro

Adipose tissue engineering is a diverse area of research where the developed tissues can be used to study normal adipose tissue functions, create disease models in vitro, and replace soft tissue defects in vivo. Increasing attention has been focused on the highly specialized metabolic pathways that regulate energy storage and release in adipose tissues which affect local and systemic outcomes. Non-invasive, dynamic measurement systems are useful to track these metabolic pathways in the same tissue model over time to evaluate long term cell growth, differentiation, and development within tissue engineering constructs. This approach reduces costs and time in comparison to more traditional destructive methods such as biochemical and immunochemistry assays and proteomics assessments. Towards this goal, this review will focus on important metabolic functions of adipose tissues and strategies to evaluate them with non-invasive in vitro methods. Current non-invasive methods, such as measuring key metabolic markers and endogenous contrast imaging will be explored.

A computer-designed scaffold for bone regeneration within cranial defect using human dental pulp stem cells

A computer-designed, solvent-free scaffold offer several potential advantages such as ease of customized manufacture and in vivo safety. In this work, we firstly used a computer-designed, solvent-free scaffold and human dental pulp stem cells (hDPSCs) to regenerate neo-bone within cranial bone defects. The hDPSCs expressed mesenchymal stem cell markers and served as an abundant source of stem cells with a high proliferation rate. In addition, hDPSCs showed a phenotype of differentiated osteoblasts in the presence of osteogenic factors (OF). We used solid freeform fabrication (SFF) with biodegradable polyesters (MPEG-(PLLA-co-PGA-co-PCL) (PLGC)) to fabricate a computer-designed scaffold. The SFF technology gave quick and reproducible results. To assess bone tissue engineering in vivo, the computer-designed, circular PLGC scaffold was implanted into a full-thickness cranial bone defect and monitored by micro-computed tomography (CT) and histology of the in vivo tissue-engineered bone. Neo-bone formation of more than 50% in both micro-CT and histology tests was observed at only PLGC scaffold with hDPSCs/OF. Furthermore, the PLGC scaffold gradually degraded, as evidenced by the fluorescent-labeled PLGC scaffold, which provides information to tract biodegradation of implanted PLGC scaffold. In conclusion, we confirmed neo-bone formation within a cranial bone defect using hDPSCs and a computer-designed PLGC scaffold.

Fluorescent hydrogels for embryoid body formation and osteogenic differentiation of embryonic stem cells

Substrate mechanics (e.g., stiffness and topography of the microenvironment) are likely critical for driving normal morphogenesis and tissue development. As such, substrate mechanics imposed by hydrogels have been exploited to guide the lineage differentiation of stem cells and to drive stemness. In this work, we chemically modified gelatin hydrogels through glyceraldehyde cross-linking to render them suitable for cell culture. The modified hydrogels proved to be ideal for embryonic stem cell osteogenesis, initially providing a soft nonadhesive surface for the formation of embryoid bodies. They subsequently degraded in culture to afford a harder surface during osteoblast differentiation. The gels synthesized are highly fluorescent, relatively easy to prepare, and can potentially aid in overcoming the challenge of imaging changes to the microenvironments of cells during three-dimensional cell culture. Exploiting these materials could lead to the development of tissue-engineered products of increased complexity and rational treatment strategies.

Current concepts and future of noninvasive procedures for diagnosing oral squamous cell carcinoma--a systematic review

Background

Oral squamous cell carcinoma (OSCC) has a remarkably high incidence worldwide, and a fairly serious prognosis, encouraging further research into advanced technologies for noninvasive methods of making early diagnoses, ideally in primary care settings.

Objectives

Our purpose was to examine the validity of using advanced noninvasive technologies in diagnosis of OSCC by identifying and evaluating relevant published reports.

Data source

MEDLINE, EMBASE, and CINAHL were searched to identify clinical trials and other information published between 1990 and 10 June 2014; the searches of MEDLINE and EMBASE were updated to November 2014. Study selection: Studies of noninvasive methods of diagnosing OSCC, including oral brush biopsy, optical biopsy, saliva-based oral cancer diagnosis, and others were included.

Data extraction

Data were abstracted and evaluated in duplicate for possible relevance on two occasions at an interval of 2 months before being included or excluded.

Data synthesis

This study identified 163 studies of noninvasive methods for diagnosing OSCC that met the inclusion criteria. These included six studies of oral brush biopsy, 42 of saliva-based oral diagnosis, and 115 of optical biopsy. Sixty nine of these studies were assessed by the modified version of the QUADAS instrument. Saliva-based oral cancer diagnosis and optical biopsy were found to be promising noninvasive methods for diagnosing OSCC.

Limitation

The strength of evidence was rated low for accuracy outcomes because the studies did not report important details required to assess the risk for bias.

Conclusions

It is clear that screening for and early detection of cancer and pre-cancerous lesions have the potential to reduce the morbidity and mortality of this disease. Advances in technologies for saliva-based oral diagnosis and optical biopsy are promising pathways for the future development of more effective noninvasive methods for diagnosing OSCC that are easy to perform clinically in primary care settings.

Imaging strategies for tissue engineering applications

Tissue engineering has evolved with multifaceted research being conducted using advanced technologies, and it is progressing toward clinical applications. As tissue engineering technology significantly advances, it proceeds toward increasing sophistication, including nanoscale strategies for material construction and synergetic methods for combining with cells, growth factors, or other macromolecules. Therefore, to assess advanced tissue-engineered constructs, tissue engineers need versatile imaging methods capable of monitoring not only morphological but also functional and molecular information. However, there is no single imaging modality that is suitable for all tissue-engineered constructs. Each imaging method has its own range of applications and provides information based on the specific properties of the imaging technique. Therefore, according to the requirements of the tissue engineering studies, the most appropriate tool should be selected among a variety of imaging modalities. The goal of this review article is to describe available biomedical imaging methods to assess tissue engineering applications and to provide tissue engineers with criteria and insights for determining the best imaging strategies. Commonly used biomedical imaging modalities, including X-ray and computed tomography, positron emission tomography and single photon emission computed tomography, magnetic resonance imaging, ultrasound imaging, optical imaging, and emerging techniques and multimodal imaging, will be discussed, focusing on the latest trends of their applications in recent tissue engineering studies.

The potential of label-free nonlinear optical molecular microscopy to non-invasively characterize the viability of engineered human tissue constructs

Nonlinear optical molecular imaging and quantitative analytic methods were developed to non-invasively assess the viability of tissue-engineered constructs manufactured from primary human cells. Label-free optical measures of local tissue structure and biochemistry characterized morphologic and functional differences between controls and stressed constructs. Rigorous statistical analysis accounted for variability between human patients. Fluorescence intensity-based spatial assessment and metabolic sensing differentiated controls from thermally-stressed and from metabolically-stressed constructs. Fluorescence lifetime-based sensing differentiated controls from thermally-stressed constructs. Unlike traditional histological (found to be generally reliable, but destructive) and biochemical (non-invasive, but found to be unreliable) tissue analyses, label-free optical assessments had the advantages of being both non-invasive and reliable. Thus, such optical measures could serve as reliable manufacturing release criteria for cell-based tissue-engineered constructs prior to human implantation, thereby addressing a critical regulatory need in regenerative medicine.

Multimodal ultrasound-photoacoustic imaging of tissue engineering scaffolds and blood oxygen saturation in and around the scaffolds

Preclinical, noninvasive imaging of tissue engineering polymeric scaffold structure and/or the physiological processes such as blood oxygenation remains a challenge. In vitro or ex vivo, the widely used scaffold characterization modalities such as porosimetry, electron or optical microscopy, and X-ray microcomputed tomography have limitations or disadvantages-some are invasive or destructive, others have limited tissue penetration (few hundred micrometers) and/or show poor contrast under physiological conditions. Postmortem histological analysis, the most robust technique for the evaluation of neovascularization is obviously not appropriate for acquiring physiological or longitudinal data. In this study, we have explored the potential of ultrasound (US)-coregistered photoacoustic (PA) imaging as a noninvasive multimodal imaging modality to overcome some of the above challenges and/or provide complementary information. US-PA imaging was employed to characterize poly(lactic-co-glycolic acid) (PLGA) polymer scaffolds or single-walled carbon nanotube (SWCNT)-incorporated PLGA (SWCNT-PLGA) polymer scaffolds as well as blood oxygen saturation within and around the scaffolds. Ex vivo, PLGA and SWCNT-PLGA scaffolds were placed at 0.5, 2, and 6 mm depths in chicken breast tissues. PLGA scaffolds could be localized with US imaging, but generate no PA signal (excitation wavelengths 680 and 780 nm). SWCNT-PLGA scaffolds generated strong PA signals at both wavelengths due to the presence of the SWCNTs and could be localized with both US and PA imaging depths between 0.5-6 mm (lateral resolution = 90 μm, axial resolution = 40 μm). In vivo, PLGA and SWCNT-PLGA scaffolds were implanted in subcutaneous pockets at 2 mm depth in rats, and imaged at 7 and 14 days postsurgery. The anatomical position of both the scaffolds could be determined from the US images. Only SWCNT-PLGA scaffolds could be easily detected in the US-PA images. SWCNT-PLGA scaffolds had significant four times higher PA signal intensity compared with the surrounding tissue and PLGA scaffolds. In vivo blood oxygen saturation maps around and within the PLGA scaffolds could be obtained by PA imaging. There was no significant difference in oxygen saturation for the PLGA scaffolds at the two time points. The blood oxygen saturation maps complemented the histological analysis of neovascularization of the PLGA scaffolds.

Fluorescent imaging of endothelial cells in bioengineered blood vessels: the impact of crosslinking of the scaffold

Fluorescent imaging is a useful tool to monitor and evaluate bioengineered tissues and organs. However, autofluorescence emitted from the scaffold can be comparable or even overwhelm signals generated by fluorescently labelled cells and biomarkers. Using standard fluorescent microscopy techniques, a simple and easy-to-measure signal to noise ratio metric was developed, which can facilitate the selection of fluorescent biomarkers and the respective biomaterials for tissue engineering studies. Endothelial cells (MS1) expressing green-fluorescent protein and red fluorescent protein (mKate) were seeded on poly(epsilon-caprolactone)-collagen hybrid scaffolds that were prepared by crosslinking with glutaraldehyde, genipin and ethyl(dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide. All scaffolds had comparable mechanical properties, which could meet the requirements for vascular graft applications. ethyl(dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide crosslinked scaffolds had a high signal to noise ratio value because of its low autofluorescence in green and red channels. Genipin crosslinked scaffolds had a high signal to noise ratio only in the green channel, while glutaraldehyde crosslinked scaffolds had a low signal to noise ratio in both green and red channels. The signal to noise ratio was independent of the exposure time. The data show that although similar in their mechanical properties and ability to support cell growth, scaffolds crosslinked with different agents have significant differences in causing autofluorescence of the scaffolds. This result indicates that scaffold's preparation method may have a significant impact on direct imaging of fluorescently labelled cells on scaffolds used for tissue engineering. Copyright © 2014 John Wiley & Sons, Ltd.

Three-dimensional characterization of tissue-engineered constructs by contrast-enhanced nanofocus computed tomography

To successfully implement tissue-engineered (TE) constructs as part of a clinical therapy, it is necessary to develop quality control tools that will ensure accurate and consistent TE construct release specifications. Hence, advanced methods to monitor TE construct properties need to be further developed. In this study, we showed proof of concept for contrast-enhanced nanofocus computed tomography (CE-nano-CT) as a whole-construct imaging technique with a noninvasive potential that enables three-dimensional (3D) visualization and quantification of in vitro engineered extracellular matrix (ECM) in TE constructs. In particular, we performed a 3D qualitative and quantitative structural and spatial assessment of the in vitro engineered ECM, formed during static and perfusion bioreactor cell culture in 3D TE scaffolds, using two contrast agents, namely, Hexabrix® and phosphotungstic acid (PTA). To evaluate the potential of CE-nano-CT, a comparison was made to standardly used techniques such as Live/Dead viability/cytotoxicity, Picrosirius Red staining, and to net dry weight measurements of the TE constructs. When using Hexabrix as the contrast agent, the ECM volume fitted linearly with the net dry ECM weight independent from the flow rate used, thus suggesting that it stains most of the ECM. When using PTA as the contrast agent, comparing to net weight measurements showed that PTA only stains a part of the ECM. This was attributed to the binding specificity of this contrast agent. In addition, the PTA-stained CE-nano-CT data showed pronounced distinction between flow conditions when compared to Hexabrix, indicating culture-specific structural ECM differences. This novel type of information can contribute to optimize bioreactor culture conditions and potentially critical quality characteristics of TE constructs such as ECM quantity and homogeneity, facilitating the gradual transformation of TE constructs in well-characterized TE products.

Tissue discrimination by uncorrected autofluorescence spectra: a proof-of-principle study for tissue-specific laser surgery

Laser surgery provides a number of advantages over conventional surgery. However, it implies large risks for sensitive tissue structures due to its characteristic non-tissue-specific ablation. The present study investigates the discrimination of nine different ex vivo tissue types by using uncorrected (raw) autofluorescence spectra for the development of a remote feedback control system for tissue-selective laser surgery. Autofluorescence spectra (excitation wavelength 377 ± 50 nm) were measured from nine different ex vivo tissue types, obtained from 15 domestic pig cadavers. For data analysis, a wavelength range between 450 nm and 650 nm was investigated. Principal Component Analysis (PCA) and Quadratic Discriminant Analysis (QDA) were used to discriminate the tissue types. ROC analysis showed that PCA, followed by QDA, could differentiate all investigated tissue types with AUC results between 1.00 and 0.97. Sensitivity reached values between 93% and 100% and specificity values between 94% and 100%. This ex vivo study shows a high differentiation potential for physiological tissue types when performing autofluorescence spectroscopy followed by PCA and QDA. The uncorrected autofluorescence spectra are suitable for reliable tissue discrimination and have a high potential to meet the challenges necessary for an optical feedback system for tissue-specific laser surgery.

Imaging challenges in biomaterials and tissue engineering

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.

Non-invasive monitoring of cell metabolism and lipid production in 3D engineered human adipose tissues using label-free multiphoton microscopy

Non-linear optical microscopy methods can characterize over time multiple functional properties of engineered tissues during development. Here, we demonstrate how the combined use of third-harmonic generation (THG) and two-photon excited fluorescence (2PEF) imaging can provide direct quantitative biomarkers of adipogenic stem cell differentiation and metabolic state, respectively. Specifically, we imaged over nine weeks silk scaffolds embedded with human mesenchymal stem cells and exposed to either propagation (PM) or adipogenic differentiation media (AM). THG was employed to visualize the formation of lipid droplets. 2PEF was used to assess the metabolic state of the cells through the redox ratio defined based on the endogenous FAD and NADH fluorescence. The redox ratio of cells in the AM scaffold was significantly lower than that in the PM scaffold during week 5 and 9, and correlated with significant increases in lipid-to-cell volume ratio, and number and size of lipid droplets in the AM scaffold. These findings indicate that the decrease in redox ratio during adipogenic differentiation is associated with fatty acid synthesis and lipid accumulation. Our methods therefore enabled us to identify and measure dynamic correlations between lipid droplet formation and cell metabolic state, while providing insight on the spatial heterogeneity of the observed signals.

Taking a deep look: modern microscopy technologies to optimize the design and functionality of biocompatible scaffolds for tissue engineering in regenerative medicine

This review focuses on modern nonlinear optical microscopy (NLOM) methods that are increasingly being used in the field of tissue engineering (TE) to image tissue non-invasively and without labelling in depths unreached by conventional microscopy techniques. With NLOM techniques, biomaterial matrices, cultured cells and their produced extracellular matrix may be visualized with high resolution. After introducing classical imaging methodologies such as µCT, MRI, optical coherence tomography, electron microscopy and conventional microscopy two-photon fluorescence (2-PF) and second harmonic generation (SHG) imaging are described in detail (principle, power, limitations) together with their most widely used TE applications. Besides our own cell encapsulation, cell printing and collagen scaffolding systems and their NLOM imaging the most current research articles will be reviewed. These cover imaging of autofluorescence and fluorescence-labelled tissue and biomaterial structures, SHG-based quantitative morphometry of collagen I and other proteins, imaging of vascularization and online monitoring techniques in TE. Finally, some insight is given into state-of-the-art three-photon-based imaging methods (e.g. coherent anti-Stokes Raman scattering, third harmonic generation). This review provides an overview of the powerful and constantly evolving field of multiphoton microscopy, which is a powerful and indispensable tool for the development of artificial tissues in regenerative medicine and which is likely to gain importance also as a means for general diagnostic medical imaging.

Dynamic, nondestructive imaging of a bioengineered vascular graft endothelium

Bioengineering of vascular grafts holds great potential to address the shortcomings associated with autologous and conventional synthetic vascular grafts used for small diameter grafting procedures. Lumen endothelialization of bioengineered vascular grafts is essential to provide an antithrombogenic graft surface to ensure long-term patency after implantation. Conventional methods used to assess endothelialization in vitro typically involve periodic harvesting of the graft for histological sectioning and staining of the lumen. Endpoint testing methods such as these are effective but do not provide real-time information of endothelial cells in their intact microenvironment, rather only a single time point measurement of endothelium development. Therefore, nondestructive methods are needed to provide dynamic information of graft endothelialization and endothelium maturation in vitro. To address this need, we have developed a nondestructive fiber optic based (FOB) imaging method that is capable of dynamic assessment of graft endothelialization without disturbing the graft housed in a bioreactor. In this study we demonstrate the capability of the FOB imaging method to quantify electrospun vascular graft endothelialization, EC detachment, and apoptosis in a nondestructive manner. The electrospun scaffold fiber diameter of the graft lumen was systematically varied and the FOB imaging system was used to noninvasively quantify the affect of topography on graft endothelialization over a 7-day period. Additionally, results demonstrated that the FOB imaging method had a greater imaging penetration depth than that of two-photon microscopy. This imaging method is a powerful tool to optimize vascular grafts and bioreactor conditions in vitro, and can be further adapted to monitor endothelium maturation and response to fluid flow bioreactor preconditioning.

Noninvasive metabolic imaging of engineered 3D human adipose tissue in a perfusion bioreactor

The efficacy and economy of most in vitro human models used in research is limited by the lack of a physiologically-relevant three-dimensional perfused environment and the inability to noninvasively quantify the structural and biochemical characteristics of the tissue. The goal of this project was to develop a perfusion bioreactor system compatible with two-photon imaging to noninvasively assess tissue engineered human adipose tissue structure and function in vitro. Three-dimensional (3D) vascularized human adipose tissues were engineered in vitro, before being introduced to a perfusion environment and tracked over time by automated quantification of endogenous markers of metabolism using two-photon excited fluorescence (TPEF). Depth-resolved image stacks were analyzed for redox ratio metabolic profiling and compared to prior analyses performed on 3D engineered adipose tissue in static culture. Traditional assessments with H&E staining were used to qualitatively measure extracellular matrix generation and cell density with respect to location within the tissue. The distribution of cells within the tissue and average cellular redox ratios were different between static and perfusion cultures, while the trends of decreased redox ratio and increased cellular proliferation with time in both static and perfusion cultures were similar. These results establish a basis for noninvasive optical tracking of tissue structure and function in vitro, which can be applied to future studies to assess tissue development or drug toxicity screening and disease progression.

Raman spectroscopy in biomedicine - non-invasive in vitro analysis of cells and extracellular matrix components in tissues

Raman spectroscopy is an established laser-based technology for the quality assurance of pharmaceutical products. Over the past few years, Raman spectroscopy has become a powerful diagnostic tool in the life sciences. Raman spectra allow assessment of the overall molecular constitution of biological samples, based on specific signals from proteins, nucleic acids, lipids, carbohydrates, and inorganic crystals. Measurements are non-invasive and do not require sample processing, making Raman spectroscopy a reliable and robust method with numerous applications in biomedicine. Moreover, Raman spectroscopy allows the highly sensitive discrimination of bacteria. Rama spectra retain information on continuous metabolic processes and kinetics such as lipid storage and recombinant protein production. Raman spectra are specific for each cell type and provide additional information on cell viability, differentiation status, and tumorigenicity. In tissues, Raman spectroscopy can detect major extracellular matrix components and their secondary structures. Furthermore, the non-invasive characterization of healthy and pathological tissues as well as quality control and process monitoring of in vitro-engineered matrix is possible. This review provides comprehensive insight to the current progress in expanding the applicability of Raman spectroscopy for the characterization of living cells and tissues, and serves as a good reference point for those starting in the field.

Zinc phthalocyanine labelled polyethylene glycol: preparation, characterization, interaction with bovine serum albumin and near infrared fluorescence imaging in vivo

Zinc phthalocyanine labelled polyethylene glycol was prepared to track and monitor the in vivo fate of polyethylene glycol. The chemical structures were characterized by nuclear magnetic resonance and infrared spectroscopy. Their light stability and fluorescence quantum yield were evaluated by UV-Visible and fluorescence spectroscopy methods. The interaction of zinc phthalocyanine labelled polyethylene glycol with bovine serum albumin was evaluated by fluorescence titration and isothermal titration calorimetry methods. Optical imaging in vivo, organ aggregation as well as distribution of fluorescence experiments for tracking polyethylene glycol were performed with zinc phthalocyanine labelled polyethylene glycol as fluorescent agent. Results show that zinc phthalocyanine labelled polyethylene glycol has good optical stability and high emission ability in the near infrared region. Imaging results demonstrate that zinc phthalocyanine labelled polyethylene glycol can track and monitor the in vivo process by near infrared fluorescence imaging, which implies its potential in biomaterials evaluation in vivo by a real-time noninvasive method.