Integration of Surface Acoustic Wave and Microfluidic Technologies for Liquid-Phase Sensing Applications

Doktorsavhandling, 2019

Acoustic wave microdevices are based on piezoelectric materials, and have been in commercial use for more than 60 years. The largest user of these devices is the telecommunications industry, where mobile phones and base stations use acoustic wave technology for radio frequency (RF) filtering.  The relationship between acoustic waves and mechanical motion of piezoelectric substrates is nowadays also used in industrial sensing, which includes automotive (e.g. torque and tire pressure sensors), and medical (gas-chemical and biochemical sensors) applications. Acoustic wave sensors are cheap, robust, and very sensitive. The transfer of sensing data is uncomplicated, and can often be achieved passively without external power source, and wirelessly. An acoustic wave sensor detects changes in mass deposited on its surface, and responds to these changes by a shift in resonance frequency. Such devices are essentially highly miniaturized balances with direct electrical readout.
 
For use in medical diagnostics and treatment, sensors capable of in-liquid operation are important. This thesis describes the development and testing of a new generation of acoustic wave in-liquid sensors, and the integration with a microfluidic sample delivery system.  Microfluidics is a modern means of manipulating tiny volumes of sample solutions in a small device, often for the purpose of processing, and delivering them to the analytical detector. Such integration provides practical advantages, most importantly the ability to mass-produce, parallelize and easily introduce additional functionality within a compact analytical platform.
 
The new technology described in this thesis is a resonant surface acoustic wave sensor, embedded in a polydimethylsiloxane (PDMS) microfluidic sample delivery system. In this sensor device, mass deposition is not directly measured on the acoustic wave-generating resonator, but indirectly on associated reflector structures, reducing loss of signal, and simplifying signal readout. New sensor layouts were designed, simulated, fabricated, assembled and tested repeatedly, until optimal performance was achieved.

The new acoustic wave technology provides opportunities for the development of very advanced hand-held instruments for high-performance chemical analysis and medical diagnostics, for example to detect pathogenic microorganisms. Thanks to the simplicity and small size of the sensor, such instruments  can be built at low cost, consume very little energy, and can be wirelessly  attached to common devices, such as cellular phones, for data processing.