Modelling and analysis of wirelessly interrogated SAW based micropumps for drug delivery applications.

Abstract

Many types of life threatening global health problems such as cardiovascular deceases, cancer, and diabetes have placed human life at high risk. These critical health problems may be eliminated and/or controlled with effective early diagnostic and/or targeted treatment methodologies. Conventional drug delivery methods such as oral tablets or injections consist of various limitations. Among them, the problem with variable absorption profiles and need of frequent dosing are yet to be successfully addressed. Therefore conventional methods are not effective for delivering the drug within their therapeutic range. The implementation of targeted micro drug delivery methods is recognised as a critical solution space for twenty first century healthcare. Micro Electromechanical Systems (MEMS) based typical micropump is a fundamental part of a drug delivery system which provides the actuation source to effectively transfer an accurate amount of fluid/drug to a targeted location. However, the lack of availability of accurate and easy to use, implantable and low-powered micropumps has been identified as a significant problem. Furthermore, the ease of control of implantable biological devices would be greatly improved by incorporation of wireless and secure actuator mechanism with no battery attached with the device. Therefore, in this thesis, several significant contributions to address the above highlighted issues are presented and discussed. In this thesis, various types of actuation and micropump mechanisms were reviewed; in addition to investigating how Surface Acoustic Wave (SAW) devices can be used for secure, wireless and batteryless actuation. Consequently, SAW based novel transcutaneous interrogation mechanism was proposed for low–powered electrostatic actuations, without the need for active electronics to meet the biocompatibility requirements. A SAW correlator was used for the secure interrogation, where the device responds only to a uniquely coded RF signal, which has to be matched with the code implanted in the SAW correlator. The proposed micro actuation mechanism was demonstrated by utilising a Finite Element Model (FEM). This allowed the investigation of this device performance using a sophisticated computational numerical method. A new theoretical analysis was also developed to derive both electric potential and electrostatic force equations for SAW based microactuators. Then the Rayleigh-Ritz method based theoretical model was developed to validate the FEM results. Based on these results the SAW based low-powered actuator is able to achieve displacements up to 3 μm at low operating voltages. Once the proposed mechanism was verified both analytically and using FEM, the modelling was then extended to analyse the performance of SAW based microdiaphragms, as a critical performance dictator for the diaphragm part of the micropumps. Several new methods were developed and modelled to overcome the existing drawbacks in flat microdiaphragms, such as the incorporation of highly effective corrugated profiles, and effective use of flexible materials. As a result a number of these corrugation profiles were examined using FEA. As a result it is demonstrated that the proposed design approaches have substantially enhanced microdiaphragm performance, compared to a flat diaphragm. As much as the effectiveness of microdiaphragms, the flow rectification mechanism also dictates a critical role in micropumps. In this research, the proposed micropump was designed to be valveless for simplicity and ease of fabrication, and used diffuser elements for flow rectification. However, most of the existing computational analyses of diffusers are mainly based on 2D or simplified 3D models. Hence, the relationship between diffuser parameters, Reynolds number, and the diffuser performance at microscale, are not well established. Therefore, FEM based Computational Fluid Dynamic (CFD) was successfully utilised to analyse flat–walled diffuser elements. These analyses provide a qualitative and quantitative relationship between the diffuser efficiency and Reynolds numbers for laminar flow. Building on the developed actuation mechanism and various corrugated micro diaphragms, and diffuser models, an integrated device analysis are presented. This includes a full 3D model of the SAW based electrostatically actuated, diffuser micropump, and complex microfluidic behaviour of the micropump was analysed. A strong emphasis was given in utilising CFD to analyse the Fluid–Solid Interaction (FSI) phenomena of the micropump and the overall pumping effect was successfully demonstrated. The knowledge and new contributions made in this thesis in modelling, simulation, and analysis of implantable drug delivery micropumps, will be able to effectively utilise in a range of fields such as advanced computational numerical modelling of Bio–MEMS, secure transcutaneous communication, and implantable drug delivery systems and other biomedical aplications.