SAW devices are widely used to filter and condition electronic signals in cellular phones and other consumer electronics, with over 4 billion devices produced worldwide for communications applications alone. Prices of many of these chips to OEM phone manufacturers, in high volume, are less than $0.15 each.
A SAW device consists of a polished piezoelectric substrate, on which interdigitated metal electrode structures have been produced using standard photolithographic processes. The interdigitated electrodes are connected in sets to bus bars, forming transducer(s). When the SAW is used as a wireless device, an antenna connected to the bus bars allows a radio frequency (RF) signal to be applied to the transducer, producing voltages that alternate with time on groups of electrodes. The time-varying difference in voltage between adjacent electrodes produces alternating strains in the underlying piezoelectric, transducing the electromagnetic RF signal into a mechanical disturbance on the substrate, known as a surface acoustic wave. This wave propagates outward on the surface of the substrate, and in general is either reflected back to the launching transducer or is received by another transducer. In either configuration, the resulting surface wave is transduced back into another RF signal, which is sent through an antenna back to a receiver that can read the sensor response. Since any environmental property that interacts with the wave can affect wave propagation characteristics that can be measured, the device is useful as a sensor.
While SAW sensor devices are somewhat more complex than simple SAW devices, and hence are somewhat more expensive to produce, the manufacturing infrastructure exists to drive down costs with increasing volumes, provided the devices are developed to utilize standard manufacturing processes. This makes SAW sensors a cost-effective enabling solution for distributed wireless sensing for a wide range of applications.
SAW (surface acoustic wave) sensors utilize the same basic low-cost device technology that is widely used for signal processing applications such as radio frequency (RF) and intermediate frequency (IF) filtering in cell phones and consumer electronics, adapted to measure chemical, biological, or physical properties of the device environment.
SAW sensors have been used in both wired and wireless configurations to measure physical properties such as temperature, pressure, strain, and torque. Recently, wireless SAW sensors for physical parameters have been combined with RFID coding, to produce sets of individually identifiable wireless sensors.
SAW devices coated with chemically selective films have been used as chemical vapor sensors for more than 20 years. Work in the 1980’s by Hank Wohltjen and colleagues at the U. S. Naval Research Labs established the high sensitivity of these devices to modifications of materials at the surface of the device, and recognized the potential to monitor film changes including transitions and vapor absorption.
Piezoelectric devices can be coated with biologically specific coatings such as antibodies, DNA, RNA, aptamers, and other molecules to produce biosensors for target analytes, also known as piezoelectric bioaffinity sensors. Piezoelectric biosensors have been widely used for laboratory based bioanalytical assays, including detection of viruses, bacteria, proteins, nucleic acids, and other molecules, and for the direct real-time monitoring of affinity interactions, including determination of the kinetic rate constants for the interactions . Most piezoelectric biosensors utilize mass changes induced by formation of biocomplexes, although biosensors using changes in fluid viscosity and sensors responding to changes in electrical conductivity have also been investigated.
In addition to being useful directly as sensors, individually coded SAW devices can be used to provide uniquely identifiable wireless links to external impedance varying or voltage producing sensors. This approach is advantageous in that it would allow the elimination of wiring harnesses to existing sensors on airframes or in test facilities, allowing a small group of SAW interface devices to be qualified for use with a large number of existing qualified sensors.