Acoustic and electromagnetic waves have an exquisite property in that they pass right through materials such as proteins, cellular membranes, nucleic acids, polymers, carbon nanotubes and many others, bringing with them critical information about samples electrical, mechanical and conformational states for better decisions in healthcare, bioprocessing and aerospace. Acoustic biosensors offer a simple, cost-effective sensing platform that responds to a variety of interfacial phenomena, such as DNA hybridisation, antigen-antibody binding and ligand-protein interactions. We have been developing a number of approaches to using acoustic principles for sensing applications. Early work involved investigating the sensing properties of surface skimming bulk waves (SSBW). However, it became clear that whilst these sophisticated devices were useful for monitoring biological interactions, they were too expensive to compete with conventional medical diagnostics technologies. The ability to reduce the costs of fabrication of acoustic sensor technology, whilst retaining the substantive advantages of general applicability and ease of use became a major research target. One way to achieve this with acoustic devices was to use less expensive, non-piezoelectric, substrates and eliminate all microfabrication steps, by using remote excitation. This thinking led to the exploitation of magnetic direct generation for the non-contact excitation of acoustic waves in glass plates and an approach for substantially improving the transduction efficiency that utilises continuous wave resonances and thin film generation. These devices use glass plates coated with thin films of aluminium to create free-standing acoustic resonance cavities with high transduction efficiencies and Q factors.
The enhanced magnetic direct generation concept was evolved into a new strategy for chemical sensing based on frequency tunable acoustic devices, the so-called magnetic acoustic resonator sensor (MARS). The device comprises a circular 0.5mm-thick resonant plate fabricated from a variety of non-piezoelectric materials and coated on the underside with a 2.5μm-thick aluminium film. Harmonic radial shear waves over several orders of magnitude frequency range can be induced in the resonant plate by enhanced magnetic direct generation using a non-contacting RF coil and an NdFeB magnet. In a further refinement, a planar spiral coil has been used to induce hypersonic evanescent waves in a quartz substrate with the unique ability to focus the acoustic wave down onto the biorecognition layer. These special sensing conditions were achieved by applying an RF current to a coaxial waveguide and spiral coil, such that wideband repeating electrical resonance conditions could be established over the MHz-GHz frequency range. At an operating frequency of 1.09 GHz, the evanescent wave depth of a quartz crystal hypersonic resonance is reduced to 17 nm, approximately the same as the thickness of a single IgG monolayer, and minimising unwanted coupling to the bulk fluid. The technique has been exploited to generate a novel bioanalytical technique based on multifrequency acoustic devices, the so-called acoustic spectrophonometer, which develops the concept of the acoustic “fingerprint”. In addition, the remote acoustic spectroscopy (RAS) system aims to enhance the characteristics of MARS by reducing the size of the sensing element and making it accessible to electromagnetic interrogation over greater distances (several centimetres) such that it can operate as a truly remote sensing element that is the unique in requiring no antenna, metallization or circuitry, whilst providing MHz-GHz spectroscopic measurements. This simple format lends itself well to biochemical measurements in immersed or subcutaneous samples.
The acoustic sensor has been further developed into an acoustic dielectric and mechanical spectrometer and as a potentially implantable glucose sensor. In vivo glucose monitoring is required for tighter glycaemic control and we have constructed a miniature implantable device based on a magnetic acoustic resonance sensor (MARS). A ~600–800 nm thick glucose-responsive poly(acrylamide-co-3-acrylamidophenylboronic acid) (poly(acrylamide-co-3-APB)) film was polymerised on the quartz disc (12 mm in diameter and 0.25 mm thick) of the MARS. The swelling/shrinking of the polymer film induced by the glucose binding to the phenylboronate caused changes in the resonance amplitude of the quartz disc in the MARS. A linear relationship between the response of the MARS and the glucose concentration in the range 0–15 mM was observed, with the optimum response of the MARS sensor being obtained when the polymer films contained ~20 mol% 3-APB. The MARS glucose sensor also functioned under flow conditions with a response almost identical to the sensor under static or non-flow conditions. The results suggest that the MARS could offer a promising strategy for developing a small subcutaneously implanted continuous glucose monitor.
List of Projects and Members:
Electromagnetic Techniques for Non-destructive Testing – Dr Adrian Stevenson
Polymeric Acoustic Holography – Miss Nan Li