76 research outputs found
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Engineering Poly(ethylene glycol) hydrogel microenvironment for osteogenic differentiation of human mesenchymal stem cells
Poly(ethylene glycol) (PEG)-based hydrogels have emerged as important class of biomaterials for use as cell and drug delivery vehicles for tissue engineering and also as two- and three-dimensional cell culture substrates. PEG is commonly used due to its hydrophilic and bio-inert properties. The versatility of PEG chemistry allows for independent tailoring of biophysical properties and biochemical functionalization of the microenvironment experienced by cells allowing researchers to conduct systematic studies, in vitro, to study and understand the role of physiological cues experienced by cells in a biologically relevant fashion. The main goal of this thesis is to design appropriate hydrogel substrates to design biomaterials platforms to study and understand the role of extracellular matrix (ECM) cues on osteogenic differentiation of human mesenchymal stem cells. First, the role of phosphate functional groups, found abundantly in the mineral phase of bone, on osteogenic differentiation of hMSCs in the absence of osteogenic supplements was studied. Role of sequestered serum proteins in mediating the adhesion and interaction of hMSCs with the phosphate functional groups was characterized. The role of focal adhesion kinase (FAK) mediated integrin signaling on inducing osteogenic differentiation in hMSCs cultured on phosphate functionalized hydrogels was assessed. Second, thiol-ene photopolymerization chemistry was exploited to synthesize α5 integrin priming hydrogels and the role of substrate elasticity on osteogenic differentiation of hMSCs induced by α5β1 signaling was studied. cyl(RRETAWA) peptide that specifically binds to α5 integrin was synthesized and the effect on soluble delivery to hMSCs was assessed by measuring ALP activity. Hydrogel formulations were identified to independently control their peptide functionalization and elasticity. The interplay of substrate elasticity and peptide functionalization on hMSC adhesion and focal adhesions formation was quantified. Towards understanding the role of substrate elasticity on osteogenic differentiation of hMSCs induced by α5β1 integrin signaling, the ALP activity of the cultures as a function of substrate elasticity and peptide concentration was measured. Last, towards development of chemical strategies that allow dynamic tunablity of biochemical environment experienced by cells, we have designed and synthesized addition-fragmentation-chain transfer capable allyl sulfide functionalized PEG hydrogel networks. Biochemical patterning via photo-initiated thiol-ene reactions on ally sulfide was studied and characterized with fibronectin based CRGDS motif as a model thiol containing bioactive compound. The ability to reversibly exchange any thiol containing biochemical cues in these hydrogels was demonstrated and the kinetics of exchange reactions was characterized
Microstructural Engineering of Porous Cathodes for SOFC Applications
LSCF [(La0.6Sr0.4)0.98 (Co0.2 Fe0.8) O3-delta], a solid oxide fuel cell (SOFC) cathode material was fabricated and foamed through a polymeric in situ foaming process to build an optimum porous architecture. The changes in the porous cathode microstructure with changes in the in situ foaming parameters were qualitatively investigated through back-scattered scanning electron microscope (SEM) imaging. Later, a quantitative analysis of the pore size, shape, area and distribution was completed on the same samples through a computational image analysis program called Image J (National Institute of Health, NIH). Electrochemical testing of the foamed cathode under different processing conditions including the baseline (un-foamed) cathode was performed through electrochemical impedance spectroscopy (EIS) of cathode symmetrical electrolyte-supported cells.;The porous cathode architecture formed through in situ foaming with 70% solids loading and a polymer precursor composition of 8:4:1 volume ratio (isocyanate: PEG: surfactant) within an terpineol/cellulose printing vehicle yielded the optimum microstructure displaying a substantial decrease in the electrode polarization resistance. It displayed a broad pore size distribution, higher mean pore area and more elongated pore channels with ∼40% and ∼50% less polarization than the baseline cell at 750°C and 800°C, respectively. These measurements were completed at open circuit voltage (OCV), 100 mA and 300 mA loading. Electrochemical Impedance Spectroscopy (EIS) testing for this cathode displayed ∼0.08 O cm2 -- polarization at 800°C (at OCV) and ∼50% increase in maximum power density with the foamed cathode over the baseline. Further improvements in the foamed cathode performance were obtained through the nano-catalyst incorporation into this microstructure. Platinum (Pt) nano-catalyst was impregnated into the microstructure using water based precursor (H2PtCl6.6H2O) solution; the interconnected porosity permitted the efficient infiltration of the solution throughout the bulk of the microstructure using a lower number of processing steps than the baseline (unfoamed) microstructure (per infiltration cycle). Also, a homogeneous dispersion of the nano-catalyst across the foamed cathode led to higher power densities, which is further reported in this study
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Direct foamed and nano-catalyst impregnated solid-oxide fuel cell (SOFC) cathodes
A binder system containing polyurethane precursors was used to in situ foam (direct foam) a (La{sub 0.6}Sr{sub 0.4}){sub 0.98} (Co{sub 0.2} Fe{sub 0.8})O{sub 3-{delta}} (LSCF) cathode composition upon a yttrium-stabilized zirconia (YSZ) electrolyte coated with a porous #3;10 mm thick cathode active layer. The YSZ electrolyte was #3;110 mm in thickness, and a full cell was created by application of a Ni/(Ce{sub 0.9}Gd{sub 0.1})O{sub 2} cermet as the baseline anode. Cells possessing the foamed LSCF cathode were compared to cells constructed via standard methods in terms of resultant microstructure, electrochemical performance, and introceptive character. The foamed cathode tended to possess a high level of tortuous porosity which was ellipsoidal and interconnected in character. Both the standard and foamed cathode structures were subjected to an infiltration process, and the resultant microstructure was examined. The impregnation efficiency of the foamed cathode was at least #3;10% greater per deposition than that of an unfoamed porous LSCF cathode. The SOFC with the Pt nano-catalyst impregnated foamed cathode demonstrated a maximum power density of 593 mW/cm{sup 2} utilizing wet H{sub 2} fuel, which is 52% higher than a SOFC with the baseline Pt-impregnated LSCF cathode (#3;390 mW/cm{sup 2}) at 800 {degrees}C. The cathode compositional and microstructural alterations obtainable by foaming led to the elevated power performance, which was shown to be quite high relative to standard SOFCs with a thick YSZ electrolyte
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