Ajay Tikka

Radio frequency controlled microvalve for biomedical applications

In this paper we propose the use of a RF controlled microvalve for implementation on a PZT substrate for biomedical applications. Such device has a huge range of applications such as parallel mixing of photo-lithographically defined nanolitre volumes, flow control in pneumatically driven microfluidic systems and lab-on-chip applications. The microvalve makes use of direct actuation mechanisms at the microscale level to allow its use in vivo applications. A number of acoustic propagation modes are investigated and their suitability for biomedical applications, in terms of the required displacement, device size and operation frequency. A theoretical model of the Surface Acoustic Wave (SAW) device is presented and its use in micro-valve application was evaluated using ANSYS tools. Furthermore, the wireless aspect of the device is considered through combining the RF antenna with the microvalve simulation by assuming a high carrier frequency with a small peak-to-peak signal. A new microvalve structure which uses a parallel type piezoelectric bimorph actuator was designed and simulated using ANSYS tools. Then, further optimization of the device was carried out to achieve a better coupling between electrical signal and mechanical actuation within the SAW device.

Secure wireless actuation of an implanted microvalve for drug delivery applications

The capability to wirelessly control fluid flow through a microvalve can emerge as an attractive technology enabling various biomedical applications such as remote drug delivery and in vitro diagnostics. Contactless powering of such a microvalve is best addressed by near-field inductive coupling due to its close proximity to the external interrogator. In this paper, we propose the use of the same technique for secure remote interrogation and powering of a human implantable, surface acoustic wave (SAW) correlation-based, passive microvalve. This is carried out by interrogating the microvalve with a Barker sequence-encoded BPSK signal. 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 optimization, the receiver coil/antenna was solved simultaneously with the transmitter coil/antenna in the presence of a human body simulant using three-dimensional, high frequency electromagnetic FEM modelling. The received relative signal strength was numerically and experimentally derived for a miniature (6 mm × 6 mm × 0.5 mm), square spiral antenna/coil when interrogated by a handheld 8 cm × 5 cm × 0.2 cm square spiral antenna/coil in the near-field. Finally, the experimental results agreed 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.

Improving the security and actuation of wireless controlled microvalve

A wireless microvalve would have a wide range of applications, including biomedical applications such as fertility control and nano-litre drug delivery. Arguably the most important aspect for such a device is a secure method to actuate the valve, such that it is not actuated through the spectrum of electromagnetic radiation already present in the surrounding environment. Additionally, many of the possible applications are sensitive to electromagnetic (EM) radiation so the device should be designed to only require the minimum amount of EM input to actuate the valve. To overcome this problem, we propose the use of a coded interdigital transducer (IDT) to respond only to a coded signal. For the wireless microvalve to be useful in biomedical applications, the IDTs response to a specifically coded RF signal must be much greater than its response to another coded RF signal, even if the two codes are very similar, i.e. improve the signal ratio of the device. In this research we demonstrate a number of code sequences that have a correlation function such that the peak response is unique and can be used to provide a high signal-to-noise ratio (SNR) surface acoustic wave. That results in a unique activation of the device when the interrogating RF signal code sequence matches the stored code sequence in the device. Also we will investigate the trade-off between the needed code length to ensure secure operation and the area constrain of the device within the context of biomedical application. For this purpose, the IDT is modelled as a pulse compression filter, which correlates the input signal with a stored replica.

A remotely interrogatable passive microactuator using SAW correlation

In this paper, a novel remotely interrogatable ultrasonic microactuator is introduced utilising the complex signal processing functionality of an acoustic wave correlator. Fluid pumping can be achieved at ultrasonic frequencies by electrostatically actuating an edge clamped conducting diaphragm placed above the compressor IDT of the correlator. Secure interrogability of the wireless actuator is demonstrated by encoding the SAW correlator with a 5times2-bit Barker sequence and quantitatively deducing the code dependent diaphragm motion using finite element analysis. Moreover, the displacement optimization of the microactuator is addressed by the analyses of the device response for various acoustic mode excitations.

Finite Element Modelling of SAW Correlator

Numerical simulations of SAW correlators so far are limited to delta function and equivalent circuit models. These models are not accurate as they do not replicate the actual behaviour of the device. Manufacturing a correlator to specifically realise a different configuration is both expensive and time consuming. With the continuous improvement in computing capacity, switching to finite element modelling would be more appropriate. In this paper a novel way of modelling a SAW correlator using finite element analysis is presented. This modelling approach allows the consideration of different code implementation and device structures. This is demonstrated through simulation results for a 5×2-bit Barker sequence encoded SAW correlator. These results show the effect of both bulk and leaky modes on the device performance at various operating frequencies. Moreover, the ways in which the gain of the correlator can be optimised though variation of design parameters will also be outlined.

Secure wireless powering and interrogation of an implantable microvalve

As the power is at a premium in a wireless powered microvalve, it necessitated the design and development of a contactless powering system specific to implant functionality. We present a custom design and development of the biotelemetry system for the implanted microvalve, using FEM modelling and experimental validation in the presence of numerical and physical human body phantoms, respectively. A comprehensive 3-dimensional FEM modelling and physical validation of an inductively coupled link comprising a 6×6×0.5 mm conformal spiral implanted antenna and a 8×5×0.2 cm spiral transmitter antenna is demonstrated. In addition to the investigation of the quality factor and inductance of individual coils, an analysis of the received relative signal response of the implanted antenna is presented.

Finite element analysis of a 3-dimensional acoustic wave correlator response for variable acoustic modes

Complex signal processing functions can be performed by acoustic wave correlators, with simple structures, through the variation of electrode patterns. Numerical simulations of Surface Acoustic Wave (SAW) correlators, previously limited to analytical techniques like delta function and equivalent circuit models, require simplification of second order effects such as backscattering, charge distribution, diffraction, and mechanical loading. With the continual improvement in computing capacity, the adaptation of finite element modelling (FEM) is more eficient for full scale simulation of electromechanical phenomena without model oversimplification. This is achieved by resolving the complete set of partial differential equations. In this paper a novel way of modelling a 3-dimensional acoustic wave correlator using finite element analysis is presented. This modelling approach allows the consideration of different code implementation and device structures. This is demonstrated through the simulation results for a Barker sequence encoded acoustic wave correlator. The device response for various surface, bulk, and leaky modes, determined by the excitation frequency, are presented. Moreover, the ways in which the gain of the correlator can be optimised though variation of design parameters is also outlined.

Comparative analysis of switching performance of transistors in SOS process for RF applications

Silicon-on-sapphire (SOS) technology is gaining rapid ground in RF applications due to its inherent low parasitic capacitance and the availability of high Q passive components. In this paper, performance of different transistors in SOS technology for switching applications has been verified. Quality factor, OFF performance, and harmonic distortion of all N-types transistors have been simulated and a comparative analysis is provided. Based on this analysis it can be concluded that for the same W/L ratio, NL and IN transistors in the FC process give higher quality factor and also higher Con over Coff, while in the GC process, IN devices perform best.

Modelling a surface acoustic wave based remotely actuated microvalve

We present a normally closed, remotely actuated, secure coded, electrostatically driven active microvalve using passive components. This is carried out by utilizing the complex signal processing capabilities of two identical 5 ? 2-bit Barker sequence encoded acoustic wave correlators. An electrostatically driven microchannel, comprising two conducting diaphragms as the top and bottom walls, is placed in between the compressor interdigital transducers of the two correlators. Secure interrogability of the microvalve is demonstrated by finite element modelling of the complete structure and the quantitative deduction of the code dependent microchannel actuation. Furthermore, the influence of the excited acoustic modes of the correlator on the microchannel deflection is investigated to optimize the microvalve design.

Loading Analysis of a Remotely Interrogatable Passive Microvalve

We present the dynamic loading analysis of a normally closed, remotely actuated, secure coded, electrostatically driven, active microvalve using passive components. The design employs a synergetic approach to incorporates the advantages of both electroacoustic correlation and electrostatic actuation into the microvalve structure. This is carried out by utilising the complex signal processing capabilities of two identical, 5×2-bit Barker sequence encoded, acoustic wave correlators. An electrostatically driven microchannel, comprising of two conducting diaphragms as the top and bottom walls, is placed in between the compressor IDT’s of the two correlators. Secure interrogability of the microvalve is demonstrated by the 3-D finite element modelling of the complete structure and the quantitative deduction of the harmonic code dependent microchannel actuation. Furthermore, the dynamic transient analysis is employed to investigation the nonlinear time response of the microvalve and other performance criteria of the structure such as microchannel opening dynamics and the microvalve loading time.