Hollow microneedles provide an innovative and minimally invasive method for delivering functional cells into the skin. Microneedle cell delivery represents a potential new treatment option for cell therapy approaches including skin repigmentation, wound repair, scar and burn remodelling, immune therapies and cancer vaccines.
SUMMARYThis paper presents a comprehensive finite-element modelling approach to electro-osmotic flows on unstructured meshes. The non-linear equation governing the electric potential is solved using an iterative algorithm. The employed algorithm is based on a preconditioned GMRES scheme. The linear Laplace equation governing the external electric potential is solved using a standard pre-conditioned conjugate gradient solver. The coupled fluid dynamics equations are solved using a fractional step-based, fully explicit, artificial compressibility scheme. This combination of an implicit approach to the electric potential equations and an explicit discretization to the Navier-Stokes equations is one of the best ways of solving the coupled equations in a memory-efficient manner. The local time-stepping approach used in the solution of the fluid flow equations accelerates the solution to a steady state faster than by using a global time-stepping approach. The fully explicit form and the fractional stages of the fluid dynamics equations make the system memory efficient and free of pressure instability. In addition to these advantages, the proposed method is suitable for use on both structured and unstructured meshes with a highly non-uniform distribution of element sizes. The accuracy of the proposed procedure is demonstrated by solving a basic micro-channel flow problem and comparing the results against an analytical solution. The comparisons show excellent agreement between the numerical and analytical data. In addition to the benchmark solution, we have also presented results for flow through a fully three-dimensional rectangular channel to further demonstrate the application of the presented method.
Using an SPTS Technologies Ltd. Pegasus deep reactive-ion etching (DRIE) system, an advanced two-step etching process has been developed for hollow microneedles in applications of transdermal blood sampling and drug delivery. Because of the different etching requirements of both narrow deep hollow and large open cavity, hollow etch and cavity etch steps have been achieved separately. This novel two-step etching process is assisted with a bi-layer etching mask. Results show that the etch rate of silicon during this hollow etch step was about 7.5 microm/min and the etch rate of silicon during this cavity etch step was about 8-10 microm/min, using the coil plasma etching power between 2.0 and 2.8 kW. Especially for the microneedle bores etch, the deeper it etched, the slower the etch rate was. The microneedle bores have successfully been obtained 75-150 microm in inner diametre and 700-1000 microm long with high aspect ratio DRIE, meanwhile, the vertical sidewall structures have been achieved with the high etch load exposed area over 70% for the cavity etch step.
In the current study, the modified Navier—Stokes equations together with the Poisson—Boltzmann and Laplace equations have been used to numerically model electro-osmotic flow (EOF) in straight microchannels. Flow experiments have been carried out using microchannels etched into silicon wafer surfaces. The numerical results from the present study have been compared against experimental data and an analytical solution. The results indicate that the numerical simulations are an accurate representation of EOF and that this model could be used as a tool in the design and analysis of complex EOF driven systems.
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