This review presents an overview of 20 years of worldwide development in the field of biosensors based on special types of surface acoustic wave (SAW) devices that permit the highly sensitive detection of biorelevant molecules in liquid media (such as water or aqueous buffer solutions). 1987 saw the first approaches, which used either horizontally polarized shear waves (HPSW) in a delay line configuration on lithium tantalate (LiTaO3) substrates or SAW resonator structures on quartz or LiTaO3 with periodic mass gratings. The latter are termed “surface transverse waves” (STW), and they have comparatively low attenuation values when operated in liquids. Later Love wave devices were developed, which used a film resonance effect to significantly reduce attenuation. All of these sensor approaches were accompanied by the development of appropriate sensing films. First attempts used simple layers of adsorbed antibodies. Later approaches used various types of covalently bound layers, for example those utilizing intermediate hydrogel layers. Recent approaches involve SAW biosensor devices inserted into compact systems with integrated fluidics for sample handling. To achieve this, the SAW biosensors can be embedded into micromachined polymer housings. Combining these two features will extend the system to create versatile biosensor arrays for generic lab use or for diagnostic purposes.

Surface acoustic wave biosensors: a review

A review of miniaturised humidity sensors is presented. Recent achievements in capacitive-, hygrometric-, gravimetric-, optical- and integrated sensors are discussed. Attention is paid to a general perspective of miniaturised humidity sensors, emphasising on integration issues and technological challenges. A table summarising most of the reviewed devices is included.

Recent achievements in miniaturised humidity sensors—a review of transduction techniques

https://www.repository.cam.ac.uk/bitstream/1810/264501/1/Fu_et_al-2017-Progress_in_Material_Science-VoR.pdf

Advances in piezoelectric thin films for acoustic biosensors, acoustofluidics and lab-on-chip applications

Studies on humidity sensors fabricated with organic polymers for the last 10 years are reviewed. Several useful methods for improving the characteristics of humidity sensors based on polymers are proposed. In the case of a resistive-type sensor, cross-linking of hydrophilic polymers or formation of interpenetrated polymer networks with a hydrophobic polymer makes the hydrophilic polymers durable at high humidities. Graft polymerization is another method of preparing water-resistive humidity sensors. In the case of capacitive-type sensors, cross-linking is also useful to modify the hydrophobic polymer to produce a sensor with small hysteresis, high selectivity and high sensitivity.

Humidity sensors based on polymer thin films

Abstract Surface acoustic wave (SAW) devices offer many attractive features for application as vapour phase chemical microsensors. This paper describes the characteristics of SAW devices and techniques by which they can be employed as vapour sensors. The perturbation of SAW velocity by polymeric coating films is investigated both theoretically and experimentally. Highest sensitivity can be achieved when the device is used as the resonating element in a delay line oscillator circuit. A simple equation has been developed from theoretical considerations which offers reasonably accurate quantitative predictions of SAW device frequency shifts when subjected to a given mass loading. In this mode the SAW device behaves very like conventional bulk-wave quartz crystal microbalances except that the sensitivity can be several orders of magnitude higher and the device size can be several orders of magnitude smaller. Detection of mass changes of a few femtograms by a SAW device having a surface area of 10−4 cm2 is theoretically possible.

Mechanism of operation and design considerations for surface acoustic wave device vapor sensors

The performance of low-cost commercially available polymeric humidity sensors has been compared to that of prototype thick- and thin-film polymer-based sensors that will be optimized for improved performance at low relative humidity (RH). The commercially available humidity sensors examined include polymeric capacitive and resistive types. The prototype sensors utilize both thick- and thin-film interdigitated electrodes, on quartz and ceramic substrates, with polyimide sensing films. Both capacitive and surface acoustic wave (SAW) sensing mechanisms have been studied for the thin-film structure. The commercial capacitive microsensors have a linear response in the range 5–95% RH. The resistive sensors are non linear, but the signal-conditioning circuit can be modified for improved sensitivity in a particular RH range. The prototype thick-film sensor has a non-linear response, while the response of the thin-film prototype sensor is comparable to that of the commercial capacitive microsensors. The SAW sensor has the potential for higher sensitivity than the commercial capacitive microsensors, but is less linear.

A study of low-cost sensors for measuring low relative humidity

Surface acoustic wave (SAW) devices have been studied for the last twenty years as highly sensitive yet relatively inexpensive microsensors for applications ranging from temperature and stress to gas and biological sensing. This wide range of applications is due to the SAW microsensors' high sensitivity to several physical parameters including mass, temperature, stress, and conductivity. Their low cost results from the use of standard batch microelectronic fabrication techniques for their manufacture. In this paper several chemical sensing applications for SAW devices are described. These include: gas detection; thin-film polymer characterization; dew-point measurements; surface energy measurements; and as a method to measure surface cleanliness. Experimental results are presented along with comparisons to other measurement techniques.

http://iopscience.iop.org/article/10.1088/0964-1726/6/6/002/pdf

Surface acoustic wave microsensors and applications

Optical dew point hygrometers have several drawbacks including high cost, frequent mirror contamination, instability under continuous use, and inability to detect the frost-point transition. In this work a novel hybrid sensor, and a surface acoustic wave (SAW) oscillator were used to compare optical and SAW dew point measurement techniques in the areas of condensation sensitivity, surface contamination effects, frost point transition behavior and dew point measurement resolution. The SAW technique was found to better control condensation density than the optical technique, allowing for a resolution of 0.03°C versus 0.2°C for a commercial optical dew point hygrometer. The SAW technique was also found to be much less susceptible than the optical technique to surface contamination. The frost point transition was found to cause significant measurement instability when controlling condensation density with SAW velocity or optical reflection voltage while SAW amplitude provided stable measurements. A Teflon Al coating provided accurate SAW dew point measurements at extremely low condensation densities, which may allow for improved measurements at very low relative humidities.

Improved dew point measurements based on a SAW sensor

Surface acoustic wave (SAW) devices have been studied for the last twenty years as highly sensitive yet relatively inexpensive microsensors for applications ranging from gas and biological sensing to thin film and surface characterization. This wide range of applications is due to SAW microsensors high sensitivity to several physical parameters including mass, conductivity, permittivity, stress, temperature and electric fields. Their low cost results from the use of standard batch microelectronic fabrication techniques for their manufacture. In this work several SAW sensing applications are described. These include: gas detection; thin film polymer characterization; dew-point measurements; surface energy measurements; and as a method to measure surface cleanliness. Experimental results are presented along with comparisons to other measurement techniques.

A surface acoustic wave (SAW) sensor has been used to study water-vapor condensation and to measure the dew point. SAW sensor measurements of condensation density are compared with visual observations of condensation density. SAW theory predictions of surface density are in agreement with the experimental results. The minimum detectable surface density of the SAW sensor is shown to be more than two orders of magnitude smaller than that which can be detected by optical techniques. The high sensitivity of the SAW sensor suggests that it may be able to improve dew-point measurement accuracy by compensating for surface contamination. SAW sensor dew-point measurements are particularly sensitive to the surface density of the dew that is maintained on the sensor surface, the cleanliness of the sensor surface, and gas flowrates. Preliminary studies of hydrophilic and hydrophobic chemical treatments of the sensor surface also indicate an effect on dew-point measurements.

Patrick R. Story

Papers

A study of low-cost sensors for measuring low relative humidity

Surface acoustic wave microsensors and applications

Improved dew point measurements based on a SAW sensor

Surface acoustic wave microsensors and applications

A study of condensation and dew point using a SAW sensor