Diffusive Mass Transport Studies Using Biosensors.
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Diffusive mass transport is fundamental for many scientific research areas including physics, chemistry, biology, pharmacy, medicine and geography. In tissue engineering and regenerative medicine, the diffusive mass transport property of artificial and natural biological materials is a key parameter for understanding 3D scaffolds towards designing vascular networks capable of mimicking natural tissues. The aim was to understand diffusion coefficient differences for biomedical materials of different geometrical shapes and matrix properties, including collagen gels and polymeric membranes. Theoretical work involved producing analytical expressions for diffusion, variously in a planar sheet, a cylinder and a sphere for different initial and boundary conditions. Dynamic amperometric current responses at recessed, membrane covered planar and hanging mercury drop electrodes were also studied. Experimentally, glucose and lactate needle enzyme electrodes were fabricated and an experimental rig was designed to measure analyte concentrations within gels. The analyte diffusion coefficient in a collagen gel was obtained by fitting the simulated to the experimental concentration profiles. Also, a membrane covered planar electrode system was developed to measure the diffusion coefficient of electrochemically active solute through various polymeric barriers. Here, a fit of the simulated to the experimental amperometric current transients was made. Conventionally, a drug release curve is used to characterise drug release, which depends on drug concentration and substrate geometric size and shape. A more intrinsic property, the effective diffusion coefficient, independent of drug concentration or substrate, was determined by fitting calculated drug release to experimental curves. Finally, solute diffusion across dual laminar flows in a microfluidic system was analysed and used to determine ammonia diffusion coefficient in aqueous solution. The key novelty of this work was the construction of a series of accurate but simple expressions for mass transport in various geometric matrices which enabled the determination of diffusion coefficients by a specific analytical expression obtained from Fick’s Laws and the best fit, avoiding extensive numerical computation such as finite element methods. For all the above, corresponding one point equations were also derived to give initial rapid estimates of diffusion coefficients.
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