Browsing by Subject "tissue engineering"
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Item Biomedical Applications of Emulsion Templated Scaffolds(2014-03-28) Moglia, Robert ScottEach year, millions of patients undergo reconstructive surgery to treat injuries caused by trauma, deformities, or tumor resection. Repair of these defects often requires the use of tissue grafts to promote healing. Current treatment options rely on a limited supply of donor tissue or synthetic materials that lack bioactivity and have a high rate of revision treatments. Tissue engineered grafts provide a temporary matrix that supports tissue regeneration and circumvents complications associated with traditional treatment options. In addition to biodegradable and biocompatible requirements, an injectable scaffold would offer the advantage of space-filling irregular defects without the need for expensive fabrication to shape or custom-build devices. To this end, we have utilized emulsion templating to create injectable polyHIPE scaffolds that are biodegradable, highly porous, polymerize at body temperature, and possess appropriate and tunable mechanical properties for tissue regeneration. PolyHIPE grafts developed for this purpose exhibited tunable pore sizes (5 ?m to 1 mm) and a wide range of mechanical properties (modulus = 50 kPa-50 MPa). The biodegradable macromers used in these polyHIPEs were designed to polymerize at body temperature and have a low viscosity prior to cure, eliminating the use of toxic solvents common in fabricating biodegradable polyHIPEs. New methodology was developed to permit the rational selection of macromers based on prediction of molecular hydrophobicity and structural analysis of surfactant chemical structure in contrast to the traditional trial-and-error approach. Redox initiation was also studied as a means to decrease polyHIPE cure times from hours to minutes, comparable to current bone cements used currently in the clinic. This new initiation method also improved mechanical properties and had minimal effects on pore structure. The use of a double-barrel syringe also allowed emulsions to be stored for up to 6 months prior to cure with no negative effects on pore structure. Finally, these polyHIPEs were used to make porous microspheres, via a double-emulsion technique, to improve scaffold bioactivity. These microspheres successfully incorporated rhBMP-2 growth factor, a potent osteoinductive agent used in many bone graft procedures. Current rhBMP-2 delivery methods are expensive and pose safety risks due to the excessive amounts of growth factor used. These microspheres offer a means to gradually deliver site-specific dosages of rhBMP-2 directly in the polyHIPE scaffolds, potentially improving tissue regeneration. In summary, we have developed a library of injectable porous materials that can be used to improve tissue regeneration. Furthermore, the emulsion structure-property relationships explored here can be used in designing future polyHIPEs for tissue engineering or other applications.Item Characterization of Engineered Tissue by Multimodal Optical Imaging and Biaxial Mechanical Testing(2014-04-28) Bai, YuqiangTo better understand the relationships between mechanical stimuli and cellular responses, we developed a 3D tissue bioreactor coupling to both a biaxial mechanical testing platform and a stage for multimodal microscopy. Fibroblast seeded cruciform fibrin gels were investigated. A multimodal nonlinear optical microscopy-optical coherence microscopy (NLOM-OCM) system was developed to delineate relative spatial distributions of original fibrin, deposited collagen and fibroblasts non-invasively. Serial in-culture mechanical testing platform was also applied to track the evolution of bulk mechanical properties under sterile conditions. Wall stress depends on sample thickness and our multimodal imaging system measured evolving construct thickness as a function of mechanical stretch during biaxial tests. Through one month culture, cell and deposited collagen randomly distributed in non-stretched constructs. While under stretched condition, cell and deposited collagen fibers, which aligned with cell bodies, appeared preferentially parallel with principal stretch. Surprisingly both non-stretched and stretched constructs showed isotropic mechanical properties with increasing stiffness with culture time. In summary, our biaxial bioreactor system integrating both NLOM-OCM and mechanical testing provided complementary microstructural information and mechanical properties and thus may broaden fundamental understanding of soft tissue mechanics and mechanobiology.Item Development of Osteoinductive, High Porosity PolyHIPEs as Injectable Bone Grafts(2014-08-25) Robinson, Jennifer LindseyInjectable bone grafts are space-filling materials that integrate with native bone to repair large defects from congenital deformities, trauma, and tumor resection. However, current injectable bone grafts limit healing due to lack of biodegradability, high temperatures during hardening, reduced porosity, and brittle compressive properties. In this work, polymerized high internal phase emulsions (polyHIPEs) were developed as high porosity scaffolds for injectable bone grafting applications. Methods to modulate polyHIPE pore architecture, compressive properties, and degradation rates were established. Injectable polyHIPEs with pore sizes ranging from 1- 200 ?m, compressive properties comparable to human cancellous bone, and degradation profiles spanning days to months were fabricated by altering compositional parameters (i.e. organic phase composition including macromers and surfactant, addition of an electrolyte in aqueous phase, and the phase in which initiator is soluble) and processing parameters (i.e. mixing speed, and cure and storage conditions). Cytocompatibility of all HIPE components was first verified and human mesenchymal stem cell (hMSC) adhesion and morphology was then assessed on polyHIPE sections. In order to confer an osteoinductive character to the bone grafts, calcium phosphate nanoparticles and demineralized bone matrix were incorporated into the polyHIPEs. PolyHIPE pore size and compressive properties were negligibly altered with the incorporation of particles. Alkaline phosphatase activity, a marker of osteoblast differentiation, was elevated from hMSCs on polyHIPE sections with osteoinductive particles. Finally, hMSC viability post encapsulation in the polyHIPE was investigated to demonstrate the potential use as a cell carrier and viable cells were observed at 3 hours post encapsulation. Overall, these studies highlight the potential of injectable polyHIPEs as improved bone grafts that can deliver and retain autologous hMSCs at the defect site while also inducing osteogenic differentiation for enhanced bone regeneration. Elucidation of key structure-property relationships in emulsion templated scaffolds can be used in future studies to further modulate cell-material interactions.Item Study of Cell Material Interactions for Vascular Tissue Engineering Application(2012-07-16) Qu, XinIn the US alone, more than 500,000 coronary artery bypass procedures are performed annually. Tissue engineering shows the potential to construct functional grafts to overcome the limited availability of autologous saphenous veins, relatively poor elasticity and low compliance of synthetic materials (mainly Dacron and polytetrafluoroethylene). In order to meet the low modulus associate with myocyte differentiation, the high suture retention and an ultimate tensile strength (UTS) sufficient to withstand implantation and peak physiological stresses, we designed and characterized a multi-component scaffold comprised of polyurethane electrospun mesh layers bonded together by a fibrin hydrogel matrix. We have demonstrated this composite construct retains the high tensile strength and suture retention strength but displays a "J-shaped" mechanical response similar to that of native coronary artery. To improve our design, poly(ethylene glycol) diacrylate based hydrogel system was utilized as a blank slate to study the phenotypic regulation by cell material interactions. Fibrinogen, fibronectin, laminin and collagen type IV were incorporated into the hydrogel to mimic the stimuli from extracellular matrix (ECM) proteins. Surprisingly, no significant effect was detected on induction of smooth muscle cell (SMC) differentiation marker expression, activation of mitogen-activated protein (MAP) kinases pathway, or alteration of surface integrin expression profile. However, fibronectin showed repression of undesired phenotypes in SMC differentiation. In contrast to ECM proteins, glycosaminoglycans (GAGs) showed more influence on regulating SMC phenotype. By using a scaffold environment intended to be mimetic of early atherosclerosis, the impact of GAG identity on SMC foam cell formation was explored. We focused on chondroitin sulfate C (CSC), dermatan sulfate (DS), and an intermediate molecular weight hyaluronan (HA_IMW, ~400 kDa), the levels and/or distribution of which are significantly altered in atherosclerosis. CSC and DS hydrogels were associated with greater SMC phagocytosis of apolipoprotein B than HA_IMW gels. However, only SMCs in DS constructs maintained increased expression of adipocyte marker A-FABP relative to HA_IMW gels over 35 days of culture. Combined, our results suggested interesting roles for fibronectin and HA_IMW in repression of undesired phenotypes in SMC differentiation, which could give insights into rational design of novel biomaterials for vascular tissue engineering applications.Item Tissue Engineering Approaches for the Treatment of Knee Joint Damage(2012-07-16) McMahon, Rebecca ErinThere are more than 150,000 anterior cruciate ligament reconstructions each year with the goal of recovering the balance between knee stability and mobility. As many as 25 percent of these procedures will end in joint instability that can cause further damage. The risk of developing degenerative joint disease (DJD) increases in patients with previous knee injury, resulting in a higher instance of total knee arthroplasty (TKA). There are more than 400,000 TKA procedures each year, but the waiting lists for this surgery shows that many more patients are hoping to undergo this procedure. TKA provides improved knee function and pain relief for patients suffering from DJD. Although this procedure is considered successful, as younger patients undergo this treatment, the long-term performance must be improved. Major mechanisms of failure include component loosening from stress-shielding, poor integration of the implant with native tissue, and ion release from the implant. TiNb alloys are more biocompatible than currently used alloys, such as NiTi, and have mechanical properties closer to bone, so they would reduce the instance of stress shielding. TiNb can be made porous for better integration with the native bone and has superior corrosion resistance than NiTi. Engineered ligaments have generally failed to achieve mechanical properties sufficiently similar to their native counterparts, but also lack the osteochondral interface critical to the transfer of load between ligament and bone. The osteochondral interface could be incorporated through a gradient of inorganic content toward the bony insertion ends of the ligament graft, as we showed that in increase of inorganic content resulted in the transdifferentiation of osteoblasts toward chondrocyte-like cells (bone to cartilage-like). A composite scaffold composed of an electrospun mesh with either a hydrogel component or extracellular matrix (ECM) produced by the cells may be a suitable tissue engineered ligament graft. The non-linear stress-strain behavior seen in native ligament is exhibited by both of these systems, and the ECM produced by these systems is consistent with ligament tissue. The ECM-electrospun mesh composite exhibited higher elastic modulus than the fibrin-electrospun mesh composite, but required extensive pre culture while the fibrin-electrospun mesh composite could be fabricated in situ.