Design, characterisation and optimisation of a SAW correlator driven, wireless, passive microvalve for biomedical applications.

Abstract

The culmination of rapid advances made in the areas of microelectromechanical systems (MEMS), nonregenerative power sources, nanotechnology, and biomedical engineering have resulted in the expansion of their horizons in modern medicine for the deployment of a wide array of implantable devices. However, the lifetime and remote interrogability of implants, specifically used for drug delivery applications, has been an issue of contention, as their deployment period is limited by the battery life and the device size. Furthermore, not much research effort is directed towards remotely controlled flow manipulation using passive components. These shortcomings are addressed in this thesis by employing surface acoustic wave (SAW) technology to design a novel RF powered, secure coded, active microvalve with fully passive components. By combining the complex signal processing capabilities of the acoustic wave correlator with the electrostatic actuation of the microchannel, the advantages of both the mechanisms are incorporated into a novel microvalve design. Fluid pumping can be achieved at ultrasonic frequencies by electrostatically actuating the edge clamped microchannel, placed in between the compressor interdigital transducer’s (IDT’s) of two identical SAW correlators. The ability to wirelessly administer doses of drug accurately, for an extended period of time, at an inaccessible target location, through an implanted microvalve has the potential to revolutionise health care for long-term, controlled drug release applications. Three specific and diverse areas within MEMS, the new device builds on, are investigated by taking a comprehensive design, modelling, optimisation and experimental validation approach for majority of the research endeavors in the thesis. The first area corresponds to SAW technology followed by microfluidics, and body-centric communications; driven by the ultimate goal to demonstrate the operational feasibility of a human implanted, wirelessly controlled microvalve. The proposed specialised design necessitated a thorough understanding of the multiple coupled physics phenomena at the process level, before fabrication, for the critical investigation and refinement of the individual microvalve components. A comprehensive finite element modelling technique, where the complete set of partial differential equations are solved, was used to design these microvalve components with low level of abstraction to enable an automatic inclusion of the majority of the second order effects. As a starting point for the FEM modelling of SAW devices, an infinite periodic grating was modelled to analyse the freely propagating eigenmodes and eigenvalues with modal analysis; and electrically active waves and electrical admittance with harmonic analysis. A curve fitting technique was employed to extract the COM/P-matrix model parameters from these FEM results. Furthermore, an experimental validation of the parameters extracted using this novel combination of FEM and fitting techniques was carried out by fabricating a number of delaylines and comparing the physical structure response with the formulated P-matrix model. On the other hand, the modelling of a 2 and 3-dimensional, 5×2-bit Barker sequence encoded acoustic wave correlator was demonstrated using FEM. The correlator’s response was quantified in terms of harmonic analysis, to obtain the electrical admittance and output voltage profile, and transient analysis, to study the acoustic wave propagating characteristics and correlation pulses. The validation of these simulation results was carried out by fabricating the SAW correlators using optical lithographic techniques. A good agreement between the numerical and experimental results highlighted the feasibility and potential of using FEM for application specific modelling of SAW correlators. The complexity involved in combining the electroacoustic correlation and electrostatic actuation mechanisms, necessitated a systematic design and optimization of the novel microvalve which is best possible with FEM. In this thesis, the emphasiswas on the design and optimisation of a novel microfluidic structure through the deflection analysis, both, to verify the functionality of the concept and to investigate the working range of the structure. Secure interrogability of the microvalve was demonstrated by utilising finite element modelling of the complete structure and the quantitative deduction of the code dependent, harmonic and dynamic transient microchannel actuation. A numerical and experimental analysis of the biotelemetry link for the microvalve was undertaken in the vicinity of numerical and physical human body phantoms, respectively. To accurately account for the path losses and to address the design optimisation, the receiver coil/antenna was solved simultaneouslywith the transmitter coil/antenna in the presence of a human body simulant using 3-dimensional, high frequency electromagnetic, FEM modelling. The received relative signal strength was numerically and experimentally derived for a miniature (6×6×0.5 mm), square spiral antenna/coil when interrogated by a hand-held 8×5×0.2 cm square spiral antenna/coil in the near field. Finally, the experimental results confirmed well with the FEM analysis predictions and hence ascertained the applicability of the developed system for secure interrogation and remote powering of the newly proposed microvalve.