Kent Leach
University of California, Davis, Biomedical Engineering, Faculty Member
Raw data to determine the osteogenic potential of an osteoconductive composite hydrogel formed of RGD-alginate/RGD-hyaluronate supplemented with biomineralized microparticles and tested in an ovine iliac crest bone defect.
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To realize the promise of 3D bioprinting, it is imperative to develop bioinks that possess the biological and rheological characteristics needed for the printing of cell-laden tissue grafts. Alginate is widely used as a bioink because its... more
To realize the promise of 3D bioprinting, it is imperative to develop bioinks that possess the biological and rheological characteristics needed for the printing of cell-laden tissue grafts. Alginate is widely used as a bioink because its rheological properties can be modified through pre-crosslinking or the addition of thickening agents to increase printing resolution. However, modification of alginate’s physicochemical characteristics using common crosslinking agents can affect its cytocompatibility. Therefore, we evaluated the printability, physicochemical properties, and osteogenic potential of four common alginate bioinks: alginate-CaCl2 (alg-CaCl2), alginate-CaSO4 (alg-CaSO4), alginate-gelatin (alg-gel) and alginate-nanocellulose (alg-ncel) for the 3D bioprinting of patient-specific osteogenic grafts. While all bioinks possessed similar viscosity, printing fidelity was lower in the pre-crosslinked bioinks. When used to print geometrically defined constructs, alg-CaSO4 and alg-...
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Epithelial barriers are the body's natural defense system to regulating passage from one domain to another. In our efforts to understand what can and cannot cross these barriers, models have emerged as a reductionist approach to... more
Epithelial barriers are the body's natural defense system to regulating passage from one domain to another. In our efforts to understand what can and cannot cross these barriers, models have emerged as a reductionist approach to rigorously study and investigate this question. In particular, in vitro tissue models have become prominent as there is an increased exploration of understanding biological molecular transport. Herein, we introduce the pertinent physiology, then discuss recent studies and approaches for building models of five epithelial tissues: skin, the gastrointestinal tract, the lungs, the blood-brain barrier, and the placenta. In particular, we evaluated literature from the past 5 years utilizing a tissue model to evaluate molecular transport. We then compare physiology of these tissues and discuss similarities in approaches, across tissues, to validate these models. We conclude with a summary of the approaches of growing interest across multiple tissues and an out...
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Current approaches in tissue engineering and regenerative medicine have focused on controlling the presentation of various factors that influence cellular behavior and tissue formation. Numerous biomaterials have been utilized as sites... more
Current approaches in tissue engineering and regenerative medicine have focused on controlling the presentation of various factors that influence cellular behavior and tissue formation. Numerous biomaterials have been utilized as sites for new tissue growth by migrating or transplanted cells, nanoscale control of cellular behavior through the presentation of specific peptide sequences, and depots for growth factor release. More recently, the development of bioresponsive materials has emerged as a promising approach to cede control of temporal macromolecule presentation and material degradation to invading cell populations. Biomaterials now have the potential of possessing multiple functions in the process of tissue regeneration. This review summarizes some of the recent advances in the use of multifunctional biomaterials in the arena of tissue engineering. Specifically, the potential of various materials is described as it pertains to the control of cellular behavior, integration of...
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Fluorescence Lifetime Imaging (FLIm) was used to study the zonal and depth-dependent changes in the structure, and biochemistry of bovine articular cartilage to evaluate its potential for nondestructively measuring cartilage health.
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There is a substantial need to prolong cell persistence and enhance functionality in situ to enhance cell-based tissue repair. Bone morphogenetic protein-2 (BMP-2) is often used at high concentrations for osteogenic differentiation of... more
There is a substantial need to prolong cell persistence and enhance functionality in situ to enhance cell-based tissue repair. Bone morphogenetic protein-2 (BMP-2) is often used at high concentrations for osteogenic differentiation of mesenchymal stem cells (MSCs) but can induce apoptosis. Biomaterials facilitate the delivery of lower doses of BMP-2, reducing side effects and localizing materials at target sites. Photocrosslinked alginate hydrogels (PAHs) can deliver osteogenic materials to irregular-sized bone defects, providing improved control over material degradation compared to ionically cross-linked hydrogels. It is hypothesized that the delivery of MSCs and BMP-2 from a PAH increases cell persistence by reducing apoptosis, while promoting osteogenic differentiation and enhancing bone formation compared to MSCs in PAHs without BMP-2. BMP-2 significantly decreases apoptosis and enhances survival of photoencapsulated MSCs, while simultaneously promoting osteogenic differentiati...
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Research Interests: Bioengineering, Materials Science, Biomedical Engineering, Tissue Engineering, Stem Cells, and 14 moreMedicine, Linear models, DNA, Adipose tissue, Cartilage, Collagen, Animals, Ultrasonics, Horses, Glycosaminoglycans, Time Factors, Elastic Modulus, Biochemistry and cell biology, and Tissue Scaffolds
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Fibrin gels are a promising material for use in promoting bone repair and regeneration due to their ease of implant formation, tailorability, biocompatibility, and degradation by natural processes. However, these materials lack necessary... more
Fibrin gels are a promising material for use in promoting bone repair and regeneration due to their ease of implant formation, tailorability, biocompatibility, and degradation by natural processes. However, these materials lack necessary osteoconductivity to nucleate calcium, integrate with surrounding bone, and promote bone formation. Polymeric substrata formed from poly(lactide-co-glycolide) (PLG) are widely used in bone tissue engineering. A carbonated apatite layer of bone-like mineral can be successfully grown on the surface of PLG microspheres after a multiday incubation process in modified simulated body fluid. Such coatings improve the osteoconductivity of the polymer, provide nucleation sites for cell-secreted calcium, and enhance the potential osseointegration with host tissue. We examined the capacity of mineralized polymeric microspheres suspended within fibrin hydrogels to enhance the osteoconductivity of fibrin gels and increase the osteogenic potential of these materials. The inclusion of microparticles, both nonmineralized and mineralized, reduced the capacity of mesenchymal stem cells (MSCs) to contract the gel. When cultured in osteogenic media, we detected a near linear increase in both calcium and phosphate incorporation in gels containing mineralized microspheres and entrapped MSCs. The osteoconductivity of acellular fibrin gels with mineralized and nonmineralized microspheres was assessed in a rodent calvarial bone defect over 12 weeks. Compared to untreated rodent calvarial bone defects, we detected significant increases in early vascularization when treated with fibrin gels, with greater vascularization, on average, occurring with gels containing microspheres. We detected a trend for increased bone mineral density in gels containing mineralized microspheres after 12 weeks. These findings demonstrate that the osteoconductivity of fibrin gels can be increased by inclusion of mineralized microspheres, but additional signals may be required to rapidly accelerate bone repair.
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Tissue engineering is a multidisciplinary field that aims to regulate tissue structure and function in vivo, and provide more physiologically relevant model systems for in vitro studies. Three common strategies for tissue engineering... more
Tissue engineering is a multidisciplinary field that aims to regulate tissue structure and function in vivo, and provide more physiologically relevant model systems for in vitro studies. Three common strategies for tissue engineering include (1) the delivery of isolated cells or cell substitutes ...
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Non-viral gene vectors are commonly used for gene therapy owing to safety concerns with viral vectors. However, non-viral vectors are plagued by low levels of gene transfection and cellular expression. Current efforts to improve the... more
Non-viral gene vectors are commonly used for gene therapy owing to safety concerns with viral vectors. However, non-viral vectors are plagued by low levels of gene transfection and cellular expression. Current efforts to improve the efficiency of non-viral gene delivery are focused on manipulations of the delivery vector, whereas the influence of the cellular environment in DNA uptake is often ignored. The mechanical properties (for example, rigidity) of the substrate to which a cell adheres have been found to mediate many aspects of cell function including proliferation, migration and differentiation, and this suggests that the mechanics of the adhesion substrate may regulate a cell's ability to uptake exogeneous signalling molecules. In this report, we present a critical role for the rigidity of the cell adhesion substrate on the level of gene transfer and expression. The mechanism relates to material control over cell proliferation, and was investigated using a fluorescent resonance energy transfer (FRET) technique. This study provides a new material-based control point for non-viral gene therapy.
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Mesenchymal stem cells (MSCs) seeded in composite implants formed of hydroxyapatite (HA) and poly(lactide-co-glycolide) (PLG) exhibit increased osteogenesis and enhanced angiogenic potential. Endothelial colony forming cells (ECFCs) can... more
Mesenchymal stem cells (MSCs) seeded in composite implants formed of hydroxyapatite (HA) and poly(lactide-co-glycolide) (PLG) exhibit increased osteogenesis and enhanced angiogenic potential. Endothelial colony forming cells (ECFCs) can participate in de novo vessel formation when implanted in vivo. The aim of this study was to determine the capacity of HA-PLG composites to co-transplant MSCs and ECFCs, with the goal of accelerating vascularization and resultant bone formation. The incorporation of HA into PLG scaffolds improved the efficiency of cell seeding and ECFC survival in vitro. We observed increases in mRNA expression and secretion of potent angiogenic factors by MSCs when cultured on HA-PLG scaffolds compared to PLG controls. Upon implantation into an orthotopic critical-sized calvarial defect, ECFC survival on composite scaffolds was not increased in the presence of MSCs, nor did the addition of ECFCs enhance vascularization beyond increases observed with MSCs alone. Micr...
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Research Interests: Materials Engineering, Mechanical Engineering, Biomedical Engineering, Tissue Engineering, Bone Regeneration, and 13 moreImmersion, Collagen, Compressive Strength, Animals, Horses, Mineralization, Matrix, Bone Marrow Transplantation, Solutions, DBM, Tensile Strength, X Ray Microtomography, and Bone Transplantation
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Decellularization techniques have been developed in an attempt to reduce the antigenicity of xenogeneic biomaterials, a critical barrier in their use as tissue engineering scaffolds. However, numerous studies have demonstrated inadequate... more
Decellularization techniques have been developed in an attempt to reduce the antigenicity of xenogeneic biomaterials, a critical barrier in their use as tissue engineering scaffolds. However, numerous studies have demonstrated inadequate removal and subsequent persistence of antigens in the biomaterial following decellularization, resulting in an immune response upon implantation. Thus, methods to enhance antigen removal (AR) are critical for the use of xenogeneic biomaterials in tissue engineering and regenerative medicine. In the present study, AR methods incorporating protein solubilization principles were investigated for their ability to reduce antigenicity of bovine pericardium (BP) for heart valve tissue engineering. Bovine pericardium following AR (BP-AR) was assessed for residual antigenicity, tensile properties, and extracellular matrix composition. Increasing protein solubility during AR significantly decreased the residual antigenicity of BP-AR-by an additional 80% compared to hypotonic solution or 60% compared to 0.1% (w/v) SDS decellularization methods. Moreover, solubilizing agents have a dominant effect on reducing the level of residual antigenicity of BP-AR beyond that achieved by AR additives alone. Tested AR methods did not compromise the tensile properties of BP-AR compared to native BP. Furthermore, residual cell nuclei did not correlate to residual antigenicity, demonstrating that residual nuclei counts may not be an appropriate indicator of successful AR. In conclusion, AR strategies promoting protein solubilization significantly reduced residual antigens compared to decellularization methods without compromising biomaterial functional properties. This study demonstrates the importance of solubilizing protein antigens for their removal in the generation of xenogeneic scaffolds.