Non-invasive characterization of polyurethane-based tissue constructs in a rat abdominal repair model using high frequency ultrasound elasticity imaging

Structurally Diverse Gelatin-Based Electrospun Membranes for Ultrasound Imaging

Nanofiber membranes, produced through electrospinning, offer significant promise in the biomedical field due to their large surface area and strong mechanical properties. Their versatility is evident across applications such as drug delivery, wound healing, filtration, catalysis, and heritage conservation. However, the potential of electrospun membranes for advanced biomedical uses, like medical ultrasonic couplants, remains largely untapped. Current ultrasonic couplants have notable limitations, often failing to meet clinical needs and affecting patient comfort during diagnostics. To address this, gelatin/polyvinyl alcohol nanofiber membranes with various fibrous structures are developed and characterized, specifically designed for B-mode ultrasonography. Through multi-fluid electrospinning, three distinct nanofiber structures with average diameters of 325 ± 6.172, 349 ± 9.189, and 361 ± 2.117 nm are created. Electron microscopy confirmed these membranes' uniformity and smooth, hydrophilic surfaces. After crosslinking, the membranes exhibited enhanced mechanical strength and biodegradability. In ultrasound imaging trials, these nanofibers demonstrated superior clarity compared to commonly used polymer materials, revealing detailed body part structures. This study highlights the critical role of diverse nanofiber membranes in ultrasound imaging and positions them as promising alternatives to conventional couplants, with the potential to revolutionize ultrasound diagnostics and therapeutic practices.

A Quantitative Method for the Evaluation of Deep Vein Thrombosis in a Murine Model Using Three-Dimensional Ultrasound Imaging

Deep vein thrombosis (DVT) is a life-threatening condition that can lead to its sequelae pulmonary embolism (PE) or post-thrombotic syndrome (PTS). Murine models of DVT are frequently used in early-stage disease research and to assess potential therapies. This creates the need for the reliable and easy quantification of blood clots. In this paper, we present a novel high-frequency 3D ultrasound approach for the quantitative evaluation of the volume of DVT in an in vitro model and an in vivo murine model. The proposed method involves the use of a high-resolution ultrasound acquisition system and semiautomatic segmentation of the clot. The measured 3D volume of blood clots was validated to be correlated with in vitro blood clot weights with an R2 of 0.89. Additionally, the method was confirmed with an R2 of 0.91 in the in vivo mouse model with a cylindrical volume from macroscopic measurement. We anticipate that the proposed method will be useful in pharmacological or therapeutic studies in murine models of DVT.

Assessing engineered tissues and biomaterials using ultrasound imaging: In vitro and in vivo applications

Quantitative assessment of the structural, functional, and mechanical properties of engineered tissues and biomaterials is fundamental to their development for regenerative medicine applications. Ultrasound (US) imaging is a non-invasive, non-destructive, and cost-effective technique capable of longitudinal and quantitative monitoring of tissue structure and function across centimeter to sub-micron length scales. Here we present the fundamentals of US to contextualize its application for the assessment of biomaterials and engineered tissues, both in vivo and in vitro. We review key studies that demonstrate the versatility and broad capabilities of US for clinical and pre-clinical biomaterials research. Finally, we highlight emerging techniques that further extend the applications of US, including for ultrafast imaging of biomaterials and engineered tissues in vivo and functional monitoring of stem cells, organoids, and organ-on-a-chip systems in vitro.

Characterization of Tissue Scaffolds Using Synchrotron Radiation Microcomputed Tomography Imaging

Distinguishing from other traditional imaging, synchrotron radiation microcomputed tomography (SR-μCT) imaging allows for the visualization of three-dimensional objects of interest in a nondestructive and/or in situ way with better spatial resolution, deep penetration, relatively fast speed, and/or high contrast. SR-μCT has been illustrated promising for visualizing and characterizing tissue scaffolds for repairing or replacing damaged tissue or organs in tissue engineering (TE), which is of particular advance for longitudinal monitoring and tracking the success of scaffolds once implanted in animal models and/or human patients. This article presents a comprehensive review on recent studies of characterization of scaffolds based on SR-μCT and takes scaffold architectural properties, mechanical properties, degradation, swelling and wettability, and biological properties as five separate sections to introduce SR-μCT wide applications. We also discuss and highlight the unique opportunities of SR-μCT in various TE applications; conclude this article with the suggested future research directions, including the prospective applications of SR-μCT, along with its challenges and methods for improvement in the field of TE.

Design Considerations to Facilitate Clinical Radiological Evaluation of Implantable Biomedical Structures

Clinical effectiveness of implantable medical devices would be improved with in situ monitoring to ensure device positioning, determine subsequent damage, measure biodegradation, and follow healing. While standard clinical imaging protocols are appropriate for diagnosing disease and injury, these protocols have not been vetted for imaging devices. This study investigated how radiologists use clinical imaging to detect the location and integrity of implanted devices and whether embedding nanoparticle contrast agents into devices can improve assessment. To mimic the variety of devices available, phantoms from hydrophobic polymer films and hydrophilic gels were constructed, with and without computed tomography (CT)-visible TaOx and magnetic resonance imaging (MRI)-visible Fe3O4 nanoparticles. Some phantoms were purposely damaged by nick or transection. Phantoms were implanted in vitro into tissue and imaged with clinical CT, MRI, and ultrasound. In a blinded study, radiologists independently evaluated whether phantoms were present, assessed the type, and diagnosed whether phantoms were damaged or intact. Radiologists identified the location of phantoms 80% of the time. However, without incorporated nanoparticles, radiologists correctly assessed damage in only 54% of cases. With an incorporated imaging agent, the percentage jumped to 86%. The imaging technique which was most useful to radiologists varied with the properties of phantoms. With benefits and drawbacks to all three imaging modalities, future implanted devices should be engineered for visibility in the modality which best fits the treated tissue, the implanted device's physical location, and the type of required information. Imaging protocols should also be tailored to best exploit the properties of the imaging agents.

Medical imaging of tissue engineering and regenerative medicine constructs

Advancement of tissue engineering and regenerative medicine (TERM) strategies to replicate tissue structure and function has led to the need for noninvasive assessment of key outcome measures of a construct's state, biocompatibility, and function. Histology based approaches are traditionally used in pre-clinical animal experiments, but are not always feasible or practical if a TERM construct is going to be tested for human use. In order to transition these therapies from benchtop to bedside, rigorously validated imaging techniques must be utilized that are sensitive to key outcome measures that fulfill the FDA standards for TERM construct evaluation. This review discusses key outcome measures for TERM constructs and various clinical- and research-based imaging techniques that can be used to assess them. Potential applications and limitations of these techniques are discussed, as well as resources for the processing, analysis, and interpretation of biomedical images.

Radiopaque scaffolds based on electrospun iodixanol/polycaprolactone fibrous composites

Grafts based on biodegradable polymer scaffolds are increasingly used in tissue-engineering applications as they facilitate natural tissue regeneration. However, monitoring the position and integrity of these scaffolds over time is challenging due to radiolucency. In this study, we used an electrospinning method to fabricate biodegradable scaffolds based on polycaprolactone (PCL) and iodixanol, a clinical contrast agent. Scaffolds were implanted subcutaneously into C57BL/6 mice and monitored in vivo using longitudinal X-ray imaging and micro-computed tomography (CT). The addition of iodixanol altered the physicochemical properties of the PCL scaffold; notably, as the iodixanol concentration increased, the fiber diameter decreased. Radiopacity was achieved with corresponding signal enhancement as iodine concentration increased while exhibiting a steady time-dependent decrease of 0.96% per day in vivo. The electrospun scaffolds had similar performance with tissue culture-treated polystyrene in supporting the attachment, viability, and proliferation of human mesenchymal stem cells. Furthermore, implanted PCL-I scaffolds had more intense acute inflammatory infiltrate and thicker layers of maturing fibrous tissue. In conclusion, we developed radiopaque, biodegradable, biocompatible scaffolds whose position and integrity can be monitored noninvasively. The successful development of other imaging enhancers may further expand the use of biodegradable scaffolds in tissue engineering applications.

Imaging the In Vivo Degradation of Tissue Engineering Implants by Use of Supramolecular Radiopaque Biomaterials

For in situ tissue engineering (TE) applications it is important that implant degradation proceeds in concord with neo-tissue formation to avoid graft failure. It will therefore be valuable to have an imaging contrast agent (CA) available that can report on the degrading implant. For this purpose, a biodegradable radiopaque biomaterial is presented, modularly composed of a bisurea chain-extended polycaprolactone (PCL2000-U4U) elastomer and a novel iodinated bisurea-modified CA additive (I-U4U). Supramolecular hydrogen bonding interactions between the components ensure their intimate mixing. Porous implant TE-grafts are prepared by simply electrospinning a solution containing PCL2000-U4U and I-U4U. Rats receive an aortic interposition graft, either composed of only PCL2000-U4U (control) or of PCL2000-U4U and I-U4U (test). The grafts are explanted for analysis at three time points over a 1-month period. Computed tomography imaging of the test group implants prior to explantation shows a decrease in iodide volume and density over time. Explant analysis also indicates scaffold degradation. (Immuno)histochemistry shows comparable cellular contents and a similar neo-tissue formation process for test and control group, demonstrating that the CA does not have apparent adverse effects. A supramolecular approach to create solid radiopaque biomaterials can therefore be used to noninvasively monitor the biodegradation of synthetic implants.

Noninvasive high-frequency acoustic microscopy for 3D visualization of microstructure and estimation of elastic properties during hydrolytic degradation of lactide and ε-caprolactone polymers

The monitoring of degradation processes' kinetics in polymers is one of the attractive possibilities of ultrasound technique applications that provide non-destructive imaging of polymers' internal microstructure and measurements of elastic properties. In this work, biodegradable polymers and copolymers based on L,L-lactide, D,L-lactide and ε-caprolactone have been studied at different stages of hydrolysis at 37 °C by high-frequency (100 and 200 MHz) ultrasound. The acoustic microscopy technique has been developed to reveal changes in the internal microstructure and bulk sound speed in polymer samples over a hydrolysis period of 25 weeks. Ultrasound imaging provides visualization of amorphous and crystalline phases, internal imperfections, variation in packing density, and other microstructural features. Acoustic images demonstrate nucleation, growth, and the changes in internal inhomogeneities in polymers during degradation accompanied by a decrease in the polymers' molecular weight. We associate the changes in the elastic properties (the speed of a longitudinal wave) with crystallinity variations in polymers during hydrothermal aging. The results of the ultrasound investigations are supplemented by gel permeation chromatography, differential scanning calorimetry, and wide-angle X-ray spectroscopy. STATEMENT OF SIGNIFICANCE: Monitoring the kinetics of degradation processes in polymers is one of the attractive possibilities of applying ultrasound techniques that provide non-destructive imaging of the polymers' internal microstructure and measurements of elastic properties. In this work, visualization of nucleation, growth, and evolution of internal inhomogeneities in the volume of polymers and variation of values of speed of longitudinal and transverse sound waves during hydrolysis are compared with measurements of molecular weight, density, data of DSC curves, and X-ray scattering analysis. We discuss several common phenomena that occur in the volume of poly(L-lactide) and poly(D,L-lactide) over the degradation process as well as improvement of elastic properties of the poly(ε -caprolactone) and poly(L-lactide-co-caprolactone) during hydrothermal aging.

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.

Advances in bioinks and in vivo imaging of biomaterials for CNS applications

Due to increasing life expectancy incidence of neurological disorders is rapidly rising, thus adding urgency to develop effective strategies for treatment. Stem cell-based therapies were considered highly promising and while progress in this field is evident, outcomes of clinical trials are rather disappointing. Suboptimal engraftment, poor cell survival and uncontrolled differentiation may be the reasons behind dismal results. Clearly, new direction is needed and we postulate that with recent progress in biomaterials and bioprinting, regenerative approaches for neurological applications may be finally successful. The use of biomaterials aids engraftment of stem cells, protects them from harmful microenvironment and importantly, it facilitates the incorporation of cell-supporting molecules. The biomaterials used in bioprinting (the bioinks) form a scaffold for embedding the cells/biomolecules of interest, but also could be exploited as a source of endogenous contrast or supplemented with contrast agents for imaging. Additionally, bioprinting enables patient-specific customization with shape/size tailored for actual needs. In stroke or traumatic brain injury for example lesions are localized and focal, and usually progress with significant loss of tissue volume creating space that could be filled with artificial tissue using bioprinting modalities. The value of imaging for bioprinting technology is advantageous on many levels including design of custom shapes scaffolds based on anatomical 3D scans, assessment of performance and integration after scaffold implantation, or to learn about the degradation over time. In this review, we focus on bioprinting technology describing different printing techniques and properties of biomaterials in the context of requirements for neurological applications. We also discuss the need for in vivo imaging of implanted materials and tissue constructs reviewing applicable imaging modalities and type of information they can provide. STATEMENT OF SIGNIFICANCE: Current stem cell-based regenerative strategies for neurological diseases are ineffective due to inaccurate engraftment, low cell viability and suboptimal differentiation. Bioprinting and embedding stem cells within biomaterials at high precision, including building complex multi-material and multi-cell type composites may bring a breakthrough in this field. We provide here comprehensive review of bioinks, bioprinting techniques applicable to application for neurological disorders. Appreciating importance of longitudinal monitoring of implanted scaffolds, we discuss advantages of various imaging modalities available and suitable for imaging biomaterials in the central nervous system. Our goal is to inspire new experimental approaches combining imaging, biomaterials/bioinks, advanced manufacturing and tissue engineering approaches, and stimulate interest in image-guided therapies based on bioprinting.

Noninvasive ultrasound imaging for assessment of intact microstructure of extracellular matrix in tissue engineering

Development of artificial tissues or organs is one of the actual tasks in regenerative medicine that requires observation and evaluation of intact volume microstructure of tissue engineering products at all stages of their formation, from native donor tissues and decellularized scaffolds to recipient cell migration in the matrix. Unfortunately in practice, methods of vital noninvasive imaging of volume microstructure in matrixes are absent. In this work, we propose a new approach based on high-frequency acoustic microscopy for noninvasive evaluation and visualization of volume microstructure in tissue engineering products. The results present the ultrasound characterization of native rat diaphragms and lungs and their decellularized scaffolds. Verification of the method for visualization of tissue formation in the matrix volume was described in the model samples of diaphragm scaffolds with stepwise collagenization. Results demonstrate acoustic microscopic sensitivity to cell content concentration, variation in local density, and orientation of protein fibers in the volume, micron air inclusions, and other inhomogeneities of matrixes.

Biopolymeric In Situ Hydrogels for Tissue Engineering and Bioimaging Applications

BACKGROUND

Biopolymeric in situ hydrogels play a crucial role in the regenerative repair and replacement of infected or injured tissue. They possess excellent biodegradability and biocompatibility in the biological system, however only a few biopolymeric in situ hydrogels have been approved clinically. Researchers have been investigating new advancements and designs to restore tissue functions and structure, and these studies involve a composite of biometrics, cells and a combination of factors that can repair or regenerate damaged tissue.

METHODS

Injectable hydrogels, cross-linking mechanisms, bioactive materials for injectable hydrogels, clinically applied injectable biopolymeric hydrogels and the bioimaging applications of hydrogels were reviewed.

RESULTS

This article reviews the different types of biopolymeric injectable hydrogels, their gelation mechanisms, tissue engineering, clinical applications and their various in situ imaging techniques.

CONCLUSION

The applications of bioactive injectable hydrogels and their bioimaging are a promising area in tissue engineering and regenerative medicine. There is a high demand for injectable hydrogels for in situ imaging.

MRI Visualization of Injectable Ureidopyrimidinone Hydrogelators by Supramolecular Contrast Agent Labeling

Information about the in vivo location, shape, degradation, or erosion rate of injected in situ gelating hydrogels can be obtained with magnetic resonance imaging (MRI). Herein, an injectable supramolecular ureidopyrimidinone-based hydrogel (UPy-PEG) is functionalized with a modified Gadolinium(III)-DOTA complex (UPy-Gd) for contrast enhanced MRI. The contrast agent is designed to supramolecularly interact with the hydrogel network to enable high-quality imaging of this hydrogel. The applicability of the approach is demonstrated with successful visualization of the Gd-labeled UPy-PEG hydrogel after targeted intramyocardial catheter injection in a pig heart.

Imaging Biomaterial-Tissue Interactions

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.

Remote Determination of Time-Dependent Stiffness of Surface-Degrading-Polymer Scaffolds Via Synchrotron-Based Imaging

Surface-degrading polymers have been widely used to fabricate scaffolds with the mechanical properties appropriate for tissue regeneration/repair. During their surface degradation, the material properties of polymers remain approximately unchanged, but the scaffold geometry and thus mechanical properties vary with time. This paper presents a novel method to determine the time-dependent mechanical properties, particularly stiffness, of scaffolds from the geometric changes captured by synchrotron-based imaging, with the help of finite element analysis (FEA). Three-dimensional (3D) tissue scaffolds were fabricated from surface-degrading polymers, and during their degradation, the tissue scaffolds were imaged via the synchrotron-based imaging to characterize their changing geometry. On this basis, the stiffness behavior of scaffolds was estimated from the FEA, and the results obtained were compared to the direct measurements of scaffold stiffness from the load–displacement material testing. The comparison illustrates that the Young's moduli estimated from the FEA and characterized geometry are in agreement with the ones of direct measurements. The developed method of estimating the mechanical behavior was also demonstrated effective with a nondegrading scaffold that displays the nonlinear stress–strain behavior. The in vivo monitoring of Young's modulus by morphology characterization also suggests the feasibility of characterizing experimentally the difference between in vivo and in vitro surface degradation of tissue engineering constructs.

Three-dimensional imaging technologies: a priority for the advancement of tissue engineering and a challenge for the imaging community

Tissue engineering/regenerative medicine (TERM) is an interdisciplinary field that applies the principle of engineering and life sciences to restore/replace damaged tissues/organs with in vitro artificially-created ones. Research on TERM quickly moves forward. Today newest technologies and discoveries, such as 3D-/bio-printing, allow in vitro fabrication of ex-novo made tissues/organs, opening the door to wide and probably never-ending application possibilities, from organ transplant to drug discovery, high content screening and replacement of laboratory animals. Imaging techniques are fundamental tools for the characterization of tissue engineering (TE) products at any stage, from biomaterial/scaffold to construct/organ analysis. Indeed, tissue engineers need versatile imaging methods capable of monitoring not only morphological but also functional and molecular features, allowing three-dimensional (3D) and time-lapse in vivo analysis, in a non-destructive, quantitative, multidimensional analysis of TE constructs, to analyze their pre-implantation quality assessment and their fate after implantation. This review focuses on the newest developments in imaging technologies and applications in the context of requirements of the different steps of the TERM field, describing strengths and weaknesses of the current imaging approaches.

Validation of Ultrasound Elastography Imaging for Nondestructive Characterization of Stiffer Biomaterials

Ultrasound elastography (UE) has been widely used as a "digital palpation" tool to characterize tissue mechanical properties in the clinic. UE benefits from the capability of noninvasively generating 2-D elasticity encoded maps. This spatial distribution of elasticity can be especially useful in the in vivo assessment of tissue engineering scaffolds and implantable drug delivery platforms. However, the detection limitations have not been fully characterized and thus its true potential has not been completely discovered. Characterization studies have focused primarily on the range of moduli corresponding to soft tissues, 20-600 kPa. However, polymeric biomaterials used in biomedical applications such as tissue scaffolds, stents, and implantable drug delivery devices can be much stiffer. In order to explore UE's potential to assess mechanical properties of biomaterials in a broader range of applications, this work investigated the detection limit of UE strain imaging beyond soft tissue range. To determine the detection limit, measurements using standard mechanical testing and UE on the same polydimethylsiloxane samples were compared and statistically evaluated. The broadest detection range found based on the current optimized setup is between 47 kPa and 4 MPa which exceeds the modulus of normal soft tissue suggesting the possibility of using this technique for stiffer materials' mechanical characterization. The detectable difference was found to be as low as 157 kPa depending on sample stiffness and experimental setup.

Application of Elastography for the Noninvasive Assessment of Biomechanics in Engineered Biomaterials and Tissues

The elastic properties of engineered biomaterials and tissues impact their post-implantation repair potential and structural integrity, and are critical to help regulate cell fate and gene expression. The measurement of properties (e.g., stiffness or shear modulus) can be attained using elastography, which exploits noninvasive imaging modalities to provide functional information of a material indicative of the regeneration state. In this review, we outline the current leading elastography methodologies available to characterize the properties of biomaterials and tissues suitable for repair and mechanobiology research. We describe methods utilizing magnetic resonance, ultrasound, and optical coherent elastography, highlighting their potential for longitudinal monitoring of implanted materials in vivo, in addition to spatiotemporal limits of each method for probing changes in cell-laden constructs. Micro-elastography methods now allow acquisitions at length scales approaching 5-100 μm in two and three dimensions. Many of the methods introduced in this review are therefore capable of longitudinal monitoring in biomaterials and tissues approaching the cellular scale. However, critical factors such as anisotropy, heterogeneity and viscoelasity-inherent in many soft tissues-are often not fully described and therefore require further advancements and future developments.

Biocompatible, Biodegradable, and Electroactive Polyurethane-Urea Elastomers with Tunable Hydrophilicity for Skeletal Muscle Tissue Engineering

It remains a challenge to develop electroactive and elastic biomaterials to mimic the elasticity of soft tissue and to regulate the cell behavior during tissue regeneration. We designed and synthesized a series of novel electroactive and biodegradable polyurethane-urea (PUU) copolymers with elastomeric property by combining the properties of polyurethanes and conducting polymers. The electroactive PUU copolymers were synthesized from amine capped aniline trimer (ACAT), dimethylol propionic acid (DMPA), polylactide, and hexamethylene diisocyanate. The electroactivity of the PUU copolymers were studied by UV-vis spectroscopy and cyclic voltammetry. Elasticity and Young's modulus were tailored by the polylactide segment length and ACAT content. Hydrophilicity of the copolymer films was tuned by changing DMPA content and doping of the copolymer. Cytotoxicity of the PUU copolymers was evaluated by mouse C2C12 myoblast cells. The myogenic differentiation of C2C12 myoblasts on copolymer films was also studied by analyzing the morphology of myotubes and relative gene expression during myogenic differentiation. The chemical structure, thermal properties, surface morphology, and processability of the PUU copolymers were characterized by NMR, FT-IR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and solubility testing, respectively. Those biodegradable electroactive elastic PUU copolymers are promising materials for repair of soft tissues such as skeletal muscle, cardiac muscle, and nerve.

Quantitative Ultrasound for Nondestructive Characterization of Engineered Tissues and Biomaterials

Non-invasive, non-destructive technologies for imaging and quantitatively monitoring the development of artificial tissues are critical for the advancement of tissue engineering. Current standard techniques for evaluating engineered tissues, including histology, biochemical assays and mechanical testing, are destructive approaches. Ultrasound is emerging as a valuable tool for imaging and quantitatively monitoring the properties of engineered tissues and biomaterials longitudinally during fabrication and post-implantation. Ultrasound techniques are rapid, non-invasive, non-destructive and can be easily integrated into sterile environments necessary for tissue engineering. Furthermore, high-frequency quantitative ultrasound techniques can enable volumetric characterization of the structural, biological, and mechanical properties of engineered tissues during fabrication and post-implantation. This review provides an overview of ultrasound imaging, quantitative ultrasound techniques, and elastography, with representative examples of applications of these ultrasound-based techniques to the field of tissue engineering.

Non-invasive and Non-destructive Characterization of Tissue Engineered Constructs Using Ultrasound Imaging Technologies: A Review

With the rapid expansion of biomaterial development and coupled efforts to translate such advances toward the clinic, non-invasive and non-destructive imaging tools to evaluate implants in situ in a timely manner are critically needed. The required multi-level information is comprehensive, including structural, mechanical, and biological changes such as scaffold degradation, mechanical strength, cell infiltration, extracellular matrix formation and vascularization to name a few. With its inherent advantages of non-invasiveness and non-destructiveness, ultrasound imaging can be an ideal tool for both preclinical and clinical uses. In this review, currently available ultrasound imaging technologies that have been applied in vitro and in vivo for tissue engineering and regenerative medicine are discussed and some new emerging ultrasound technologies and multi-modality approaches utilizing ultrasound are introduced.

Naturally derived and synthetic scaffolds for skeletal muscle reconstruction

Skeletal muscle tissue has an inherent capacity for regeneration following injury. However, severe trauma, such as volumetric muscle loss, overwhelms these natural muscle repair mechanisms prompting the search for a tissue engineering/regenerative medicine approach to promote functional skeletal muscle restoration. A desirable approach involves a bioscaffold that simultaneously acts as an inductive microenvironment and as a cell/drug delivery vehicle to encourage muscle ingrowth. Both biologically active, naturally derived materials (such as extracellular matrix) and carefully engineered synthetic polymers have been developed to provide such a muscle regenerative environment. Next generation naturally derived/synthetic "hybrid materials" would combine the advantageous properties of these materials to create an optimal platform for cell/drug delivery and possess inherent bioactive properties. Advances in scaffolds using muscle tissue engineering are reviewed herein.

Noninvasive Quantitative Imaging of Collagen Microstructure in Three-Dimensional Hydrogels Using High-Frequency Ultrasound

Collagen I is widely used as a natural component of biomaterials for both tissue engineering and regenerative medicine applications. The physical and biological properties of fibrillar collagens are strongly tied to variations in collagen fiber microstructure. The goal of this study was to develop the use of high-frequency quantitative ultrasound to assess collagen microstructure within three-dimensional (3D) hydrogels noninvasively and nondestructively. The integrated backscatter coefficient (IBC) was employed as a quantitative ultrasound parameter to detect, image, and quantify spatial variations in collagen fiber density and diameter. Collagen fiber microstructure was varied by fabricating hydrogels with different collagen concentrations or polymerization temperatures. IBC values were computed from measurements of the backscattered radio-frequency ultrasound signals collected using a single-element transducer (38-MHz center frequency, 13-47 MHz bandwidth). The IBC increased linearly with increasing collagen concentration and decreasing polymerization temperature. Parametric 3D images of the IBC were generated to visualize and quantify regional variations in collagen microstructure throughout the volume of hydrogels fabricated in standard tissue culture plates. IBC parametric images of corresponding cell-embedded collagen gels showed cell accumulation within regions having elevated collagen IBC values. The capability of this ultrasound technique to noninvasively detect and quantify spatial differences in collagen microstructure offers a valuable tool to monitor the structural properties of collagen scaffolds during fabrication, to detect functional differences in collagen microstructure, and to guide fundamental research on the interactions of cells and collagen matrices.

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.

Ultrasound technologies for biomaterials fabrication and imaging

Ultrasound is emerging as a powerful tool for developing biomaterials for regenerative medicine. Ultrasound technologies are finding wide-ranging, innovative applications for controlling the fabrication of bioengineered scaffolds, as well as for imaging and quantitatively monitoring the properties of engineered constructs both during fabrication processes and post-implantation. This review provides an overview of the biomedical applications of ultrasound for imaging and therapy, a tutorial of the physical mechanisms through which ultrasound can interact with biomaterials, and examples of how ultrasound technologies are being developed and applied for biomaterials fabrication processes, non-invasive imaging, and quantitative characterization of bioengineered scaffolds in vitro and in vivo.

Fabrication of Bioactive Surfaces by Functionalization of Electroactive and Surface-Active Block Copolymers

Biofunctional block copolymers are becoming increasingly attractive materials as active components in biosensors and other nanoscale electronic devices. We have described two different classes of block copolymers with biofuctional properties. Biofunctionality for block copolymers is achieved through functionalization with appropriate biospecific ligands. We have synthesized block copolymers of electroactive poly(3-decylthiophene) and 2-hydroxyethyl methacrylate by atom transfer radical polymerization. The block copolymers were functionalized with the dinitrophenyl (DNP) groups, which are capable of binding to Immunoglobulin E (IgE) on cell surfaces. The block copolymers were shown to be redox active. Additionally, the triblock copolymer of α, ω-bi-biotin (poly(ethylene oxide)-b-poly (styrene)-b-poly(ethylene oxide)) was also synthesized to study their capacity to bind fluorescently tagged avidin. The surface-active property of the poly(ethylene oxide) block improved the availability of the biotin functional groups on the polymer surfaces. Fluorescence microscopy observations confirm the specific binding of biotin with avidin.

In vivo monitoring of structural and mechanical changes of tissue scaffolds by multi-modality imaging

Degradable tissue scaffolds are implanted to serve a mechanical role while healing processes occur and putatively assume the physiological load as the scaffold degrades. Mechanical failure during this period can be unpredictable as monitoring of structural degradation and mechanical strength changes at the implant site is not readily achieved in vivo, and non-invasively. To address this need, a multi-modality approach using ultrasound shear wave imaging (USWI) and photoacoustic imaging (PAI) for both mechanical and structural assessment in vivo was demonstrated with degradable poly(ester urethane)urea (PEUU) and polydioxanone (PDO) scaffolds. The fibrous scaffolds were fabricated with wet electrospinning, dyed with indocyanine green (ICG) for optical contrast in PAI, and implanted in the abdominal wall of 36 rats. The scaffolds were monitored monthly using USWI and PAI and were extracted at 0, 4, 8 and 12 wk for mechanical and histological assessment. The change in shear modulus of the constructs in vivo obtained by USWI correlated with the change in average Young's modulus of the constructs ex vivo obtained by compression measurements. The PEUU and PDO scaffolds exhibited distinctly different degradation rates and average PAI signal intensity. The distribution of PAI signal intensity also corresponded well to the remaining scaffolds as seen in explant histology. This evidence using a small animal abdominal wall repair model demonstrates that multi-modality imaging of USWI and PAI may allow tissue engineers to noninvasively evaluate concurrent mechanical stiffness and structural changes of tissue constructs in vivo for a variety of applications.

Application of ultrasound on monitoring the evolution of the collagen fiber reinforced nHAC/CS composites in vivo

To date, fiber reinforce scaffolds have been largely applied to repair hard and soft tissues. Meanwhile, monitoring the scaffolds for long periods in vivo is recognized as a crucial issue before its wide use. As a consequence, there is a growing need for noninvasive and convenient methods to analyze the implantation remolding process in situ and in real time. In this paper, diagnostic medical ultrasound was used to monitor the in vivo bone formation and degradation process of the novel mineralized collagen fiber reinforced composite which is synthesized by chitosan (CS), nanohydroxyapatite (nHA), and collagen fiber (Col). To observe the impact of cells on bone remodeling process, the scaffolds were planted into the back of the SD rats with and without rat bone mesenchymal stem cells (rBMSCs). Systematic data of scaffolds in vivo was extracted from ultrasound images. Significant consistency between the data from the ultrasound and DXA could be observed (P < 0.05). This indicated that ultrasound may serve as a feasible alternative for noninvasive monitoring the evolution of scaffolds in situ during cell growth.

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.

Engineered cell-laden human protein-based elastomer

Elastic tissue equivalence is a vital requirement of synthetic materials proposed for many resilient, soft tissue engineering applications. Here we present a bioelastomer made from tropoelastin, the human protein that naturally facilitates elasticity and cell interactions in all elastic tissues. We combined this protein's innate versatility with fast non-toxic fabrication techniques to make highly extensible, cell compatible hydrogels. These hydrogels can be produced in less than a minute through photocrosslinking of methacrylated tropoelastin (MeTro) in an aqueous solution. The fabricated MeTro gels exhibited high extensibility (up to 400%) and superior mechanical properties that outperformed other photocrosslinkable hydrogels. MeTro gels were used to encapsulate cells within a flexible 3D environment and to manufacture highly elastic 2D films for cell attachment, growth, and proliferation. In addition, the physical properties of this fabricated bioelastomer such as elasticity, stiffness, and pore characteristics were tuned through manipulation of the methacrylation degree and protein concentration. This photocrosslinkable, functional tissue mimetic gel benefits from the innate biological properties of a human elastic protein and opens new opportunities in tissue engineering.