Shear Horizontal Surface Acoustic Wave Sensor Research Articles

Sensitivity measurements for a 250 MHz quartz shear-horizontal surface acoustic wave biosensor under liquid viscous loading

Surface acoustic wave (SAW) devices have been used in biochemical assays due to their high sensitivity. The device sensitivity is a function of changes in the density and viscosity of the liquid. Here, we studied the effect of fluid viscosity using a 250 MHz quartz shear-horizontal (SH)-SAW biosensor by monitoring different concentrations of binary aqueous/glycerol solutions. In this study, the sensitivity of the biosensor was determined by fitting the data to models derived from perturbation theory. Measurements in water were used as the reference. For a 0% to 50% glycerol solution, an 87°–204° separation in the phase shift was observed. The slope of the plot of the phase shift vs (ηρ)0.5 was used to indicate the sensor’s sensitivity. The sensitivity for our 250 MHz quartz SH-SAW sensors was calculated to be 3.7×10−3m2sKg. The corresponding mass sensitivity was determined to be 9.25 × 105m2Kg. The limit of detection was calculated to be 36 picograms (pg), while the limit of quantification or LOQ was calculated to be 109 pg. Traditionally, liquid phase measurements have been challenging for SAW devices because liquids dampen the vibrating sensors severely. This problem has been largely solved using a transverse (shear) wave instead of the more popular longitudinal or Rayleigh waves. Liquid measurements are now possible using transverse waves, also known as shear waves, because transverse waves are only minimally attenuated by liquids. Shear-horizontal SAW sensors (SH-SAW) show great promise as label-free biosensors because of their ability to handle liquid samples. However, the viscosity of the liquid still induces loading effects and can be measured when the liquid is loaded onto the SH-SAW propagating surface (delay line). When the liquid above the delay line is perturbed by physical or chemical changes, such as binding to a receptor, it alters the propagating acoustic wave. The SH-SAW device can measure these changes in liquid properties as a change in the wave’s phase compared to the original wave. The device’s phase shift was recorded as a function of the changes in the density and viscosity of the binary glycerol solution and used to determine the sensitivity in the linear dynamic range of responses.

Application of Shear Horizontal Surface Acoustic Wave (SH-SAW) Immunosensor in Point-of-Care Diagnosis.

Point-of-care testing (POCT), also known as on-site or near-patient testing, has been exploding in the last 20 years. A favorable POCT device requires minimal sample handling (e.g., finger-prick samples, but plasma for analysis), minimal sample volume (e.g., one drop of blood), and very fast results. Shear horizontal surface acoustic wave (SH-SAW) biosensors have attracted a lot of attention as one of the effective solutions to complete whole blood measurements in less than 3 min, while providing a low-cost and small-sized device. This review provides an overview of the SH-SAW biosensor system that has been successfully commercialized for medical use. Three unique features of the system are a disposable test cartridge with an SH-SAW sensor chip, a mass-produced bio-coating, and a palm-sized reader. This paper first discusses the characteristics and performance of the SH-SAW sensor system. Subsequently, the method of cross-linking biomaterials and the analysis of SH-SAW real-time signals are investigated, and the detection range and detection limit are presented.

Real-Time Viscoelasticity Monitoring of Cardiomyocytes Based on Love Wave Biosensor

Introduction In addition to electrophysiological signals [1-2], mechanical properties such as contraction and relaxation of cardiomyocytes are also important properties. They are essential for the evaluation of drug efficacy and toxicity, and they can change the viscoelasticity of cells, thus providing the possibility for quantitative detection. The acoustic wave sensors are able to detect this slight change in viscoelasticity, which may not be easily noticed under the microscope in the early stages. At present, it has been reported that QCM-D can be used to detect viscoelastic properties of pre-osteoblast cells [3]. Besides, shear-horizontal surface acoustic wave sensor has been applied for the monitoring of cell spreading and attachment [4]. However, no study has realized the label free, real-time and quantitative detection of cardiac viscoelasticity for drug evaluation based on acoustic wave sensors.Recently, the surface acoustic wave (SAW) sensor has been greatly developed as a well-performed mass and viscoelasticity sensing device for label-free immunoassay. In particular, the Love Wave sensor has shown its advantages in biosensor applications due to its high sensitivity and stability in the liquid phase [5]. Hence, it is a promising device capable of monitoring viscoelastic properties of a population of cells in a label free and real-time way.As shown in Fig.1a, a self-designed Love Wave biosensor was used to monitor the viscoelastic properties of HL-1 cardiomyocytes in this study. Our previous studies have shown that insertion loss (IL) of Love Wave biosensor is a more suitable parameter for the quantitative detection of viscoelasticity than phase. Then, two different drugs were added to observe the effects on the viscoelastic properties of HL-1 cells. Method In this study, the Love Wave biosensor is constructed based on a piezoelectric quartz substrate. Interdigitated transducers (IDTs) with Ti/Au (20/200 nm) are deposited to generate acoustic waves. The input and output IDTs electrodes consist of 50 split-finger pairs with a preset wavelength λ=28 μm, which determines the center frequency of the sensor to be about 152 MHz. The distance between IDTs centers is 200λ and the acoustic aperture is 75λ. Afterwards, the IDTs patterned substrate is guided with a 3 μm SiO2 film deposited by plasma enhanced chemical vapor deposition (PECVD). Au layer (200 nm) is deposited on top of guiding layer to improve the cell attachment on the sensor surface. A polydimethylsiloxane (PDMS) chip (Fig.1b) was designed and used as the cell culturing chamber. The height and volume of this cavity are about 10 mm and 110 μL, respectively. A portable multi-channel detection system was developed to collect both the IL and phase signals of Love Wave biosensors. Then the HL-1 cardiac cells with the density of 50,000 cells/well were seeded on the surface of Love Wave biosensor (Fig.1c). To test the function of the detection system, isoprenaline (ISO) and verapamil (VRP) with different concentrations were used to induce changes in the viscoelastic properties of HL-1 cells. Results and Conclusions In Fig.1c, HL-1 cells attach and grow well on the pre-coated Love Wave biosensor, which demonstrates the biocompatibility of the sensor. The insertion loss signals of sensor after treated by ISO and VRP are shown in Fig.1d. It can be found that the insertion loss of the control group decreases slightly, which may be due to cell proliferation. VRP is a calcium channel antagonist that blocks the L-type calcium channel and prevents calcium influx into the cell, thereby decreasing the cardiomyocyte contraction force. With the addition of VRP solutions, the insertion loss of Love Wave biosensors increases with the increase of VRP concentration within the dosage range tested. In contrast to VRP, ISO is positive inotropic drug. After ISO treatment, HL-1 cells tend to be contractive due to the traction forces. Therefore, the higher the ISO concentration, the tighter the contact of the cells with the sensor substrate. Thus, insertion loss decreases after ISO treatment which changed positively correlated with concentration. Moreover, the optical microscope photos also proved that the cells after ISO treatment seems to be smaller than control group, while the cells after VRP treatment seemed to be larger. According to our preliminary experimental results, the affection on cell viscoelasticity of different drugs can be monitored by recording the insertion loss value of the SAW biosensor. Our results demonstrated that the proposed cell-based SAW biosensor is a potential, convenient and quantitative platform for in vitro cardiac viscoelasticity evaluation, especially in the early stages. It has promising applications in cell monitoring, drug evaluation and many other fields.

Study of frequency dependence of shear horizontal surface acoustic wave sensor for engine oil measurements

A shear horizontal surface acoustic wave (SH-SAW) sensor can be applied for measuring liquid properties. Measurements of engine oils are one of the SH-SAW sensor applications. In our previous study, the possibility of the SH-SAW sensor for monitoring degradation of the engine oil was discussed. In this study, the SH-SAW sensors with three frequencies were applied to discuss the frequency dependence of the sensor sensitivity. The two primary subjects discussed in this paper are: first, a comparison between used engine oil extracted from a scooter traveling 6000 km and new engine oil, and second, heating effect of the new engine oil. In the comparison between used and new engine oils, although the sensor responses related to liquid viscosity can be explained by the decrease in viscosity, those related to liquid electrical characteristics cannot be explained using the electrical perturbation theory. An effective single layer and an apparent relative permittivity were introduced to explain the obtained results. The results indicated that the higher frequency SH-SAW sensor is suitable for engine oil monitoring. Moreover, an amplification of the SH-SAW was observed when the heated engine oil was measured. This phenomenon could only be explained using a negative conductivity.

Development of measurement system using online software for shear-horizontal surface acoustic wave sensor

In the medical field, point-of-care testing (POCT) is garnering attention. An online inspection system can be realized by combining POCT and the Internet. Herein, we report the performances of a measurement system developed using online software for a shear horizontal surface acoustic wave (SH-SAW) sensor. The developed SH-SAW measurement system does not require a phase detection circuit. The developed system can be used when the system is connected to the Internet. This enables an online measurement system that can be used “by anyone, at anytime, and anywhere.” In this study, we used samples of glycerol and water mixtures and then measured the amplitude ratio and phase difference of the SH-SAW sensor. The validity of the developed system is confirmed by comparing the measured results with theoretical values.

Multifunctional and Sensitivity Enhancement of Hybrid Acoustoplasmonic Sensors Fabricated on 36XY-LiTaO3 with Gold Nanoparticles for the Detection of Permittivity, Conductivity, and the Refractive Index.

Integration of high-sensitivity sensors with multiple sensing performance for the environmental detection of permittivity (εr), conductivity (σ), and the refractive index (n) is required to support Societies 5.0. However, there are still many sensors with low sensitivity that stand alone. A shear-horizontal surface acoustic wave (SH-SAW) sensor is usually used because of its high-sensitivity performance in detecting electrical properties. Moreover, localized surface plasmon resonance (LSPR) sensors show remarkable optical side capability. Here, we have successfully combined these advantages with an additional benefit of sensitivity enhancement. We propose a hybrid acoustoplasmonic sensor generated by integrating SH-SAW and LSPR devices to simultaneously detect εr, σ, and n. The SH-SAW sensor was fabricated on a 36XY-LiTaO3 substrate using a developed interdigital transducer. Then, the LSPR sensor was implemented by the deposition of gold nanoparticles (AuNPs) on the propagation surfaces of the SH-SAW sensor. Fascinatingly, the AuNPs not only generate the LSPR effect but also enhance the SH-SAW sensor sensitivity. Comprehensive investigations were performed with atomic force microscopy imaging, CST software used for plasmonic E-field simulation, and hybrid sensing evaluation. Moreover, the SH-SAW sensitivity enhancement achieved using AuNPs was verified by frequency-domain and time-domain measurements. Thus, the SH-SAW sensor with AuNPs has a wide εr detection range (25-85), sensing capabilities for ultrasmall σ (0.00528-0.02504 S/m), and high sensitivity for n detection (45.5-201.9 nm/RIU). The cross-sectional effects were also evaluated. The effect of the LSPR device on the SH-SAW device was examined by turning the light OFF or ON (hereafter OFF/ON). The impact of the SH-SAW device on the LSPR device was investigated by turning the sine signal OFF/ON. We found that the SH-SAW sensor was not impacted by light. Interestingly, the presence of the SH-SAW sensor affects the positions of the AuNPs, which consistently generates a small blueshift in the LSPR effect. However, insignificant variation was noted in independent performances. In general, the SH-SAW sensor with AuNPs shows multifunctional independent characteristics and high-sensitivity performance, making it suitable for a chemical environment, with the possibility of integration with a wireless network.

Detection of Transferrin Receptor CD71 on a Shear Horizontal Surface Acoustic Wave Biosensor

A semi-empirical model was applied to estimate the frequency shift of a shear-horizontal surface-acoustic-wave (SH-SAW) biosensor for detecting a disease-related biomarker antigen transferrin receptor (CD71) in the sample. In the simulation to investigate its sensitivity, a shift of the SH-SAW resonant frequency occurred by applying an incremental surface mass density change on the surface. The semi-empirical model was proposed and developed by using the experimental and numerical results to relate the concentration of the biomarker to the frequency shift. Results indicated that the thickness of the SiO2 guiding layer affects the sensitivity of SH-SAW sensing, and the dependence is non-monotonically. The SH-SAW sensor was used for specific detection of biotin at a varied concentration. With the concentration of the targeted antigen in the range 0.4 ∼4.2 μg/mL, a typical exponential relation was found between the quantitative target and the frequency shift. Measurement results showed that the mass-loading effect of the antibody-antigen has a reliable response with a sensitivity of 0.94 kHz/(μg/mL). Effects of the sample flow rate on the antigen- antibody interaction and thus the frequency shift of the SH-SAW sensor were also evaluated. It is demonstrated that the proposed model provides a useful approach to analyze effectively the frequency shift dependence on the concentration and the flow rates of sensed molecules in a flow-type SH-SAW sensor.

Shear-horizontal surface acoustic wave sensor for non-invasive monitoring of dynamic cell spreading and attachment in wound healing assays

A Love-wave based biosensor is introduced for analyzing a standardized wound healing assay by observing cell growth and quantifying cell detachment processes. Utilizing the piezoelectric material LiTaO3 36° XY-cut with a thin SiO2-cover layer, shear horizontal surface acoustic waves (SAW) are excited and detected by a set of Interdigital Transducers. Epithelial cells, being cultivated on the substrate and invading the sensors delay line cause a phase shift in the transmitted SAW signal. This phase shift correlates exactly with the surface coverage of the invading cells. After wound healing, emerging fluctuations in the phase shift signal provide information about the cell growth in a confluent cell layer. Additionally, the signal slope allows to quantify the cell detachment process induced by apoptosis, necrosis or cell lysis substances, respectively. Furthermore, culture conditions like temperature or osmolality can be simultaneously monitored by SAW. Based on a theoretical approach and using FEM simulations, we identified the acoustoelectric interaction as the main reason for the phase shift in various frequency- and time-dependent studies. Our model is validated by experimental data and allows predicting the phase change caused by variations in the cell-substrate distance or the volume ratio of the nucleus and the complete cell.

Finite element simulation for sensitivity measurement of a shear horizontal surface acoustic wave micro pressure sensor with a groove structure

Shear horizontal surface acoustic wave (SH-SAW) sensors have great application potential due to their advantages of easy integration, miniaturization and suitability in liquid environments. In this paper, the finite element method is used to analyse a new SH-SAW micro pressure sensor, in which there are many groove structures along the direction of wave propagation on the delay path. We use the transient response simulation method to calculate and analyse the output voltage signal at the output interdigital transducer and surface average stress at the delay path of this new SH-SAW sensor, and its pressure sensitivity is analysed by uniformly applying an appropriate surface pressure on the resonant beam formed after grooving. The simulation results show that the surface average stress can be enhanced in a certain range of groove depth during the vibration of the groove structure. When the groove depth and width are set to 0.7 μm and 0.5 μm, respectively, the sensitivity of the SH-SAW sensor with a groove structure is four times higher than that of the traditional SH-SAW sensor. The increase of pressure sensitivity is the result of the increase of average stress caused by the groove structure. The new groove structure SH-SAW sensor provides a new basis for research on high-sensitivity micro-pressure sensors and lays a foundation for subsequent device design and manufacture.

Frequency Shift of a SH-SAW Biosensor with Glutaraldehyde and 3-Aminopropyltriethoxysilane Functionalized Films for Detection of Epidermal Growth Factor.

The frequency shift of a shear-horizontal surface-acoustic-wave (SH-SAW) biosensor in which the concentration of biomolecule is determined by the amount of its adsorption on the sensing film was studied. Simulation results were compared with experimental results to investigate its sensitivity and to develop a model to estimate the concentration of a cancer-related biomarker antigen epidermal growth factor (EGF) in the sample, with two types of sensing films, 3-aminopropyltriethoxysilane (APTES) and glutaraldehyde. With the concentration of the targeted biomarker varying from 0.2 to 5 ng/mL, a typical exponential relationship was found between the concentration and the frequency shift of the SH-SAW sensor. Measurement results showed a clear response of this immunosensor to the mass-loading effects of the antibody–antigen. The sensitivity of the glutaraldehyde film is greater than that of the APTES film owing to the chemisorption of the antibody. In the simulation, a shift of the SH-SAW resonant frequency due to added mass occurred on applying an incremental surface mass density on the sensing film, while in real applications, the concentration of the targeted biomarker to be absorbed in the sensing film is demanded. An empirical model was proposed to calculate the frequency shift in the simulation of the SH-SAW biosensor, corresponding to the concentration of specific biomolecules absorbed on a specific film. From the semi-empirical model, the sensitivity level is found to be 0.641 and 1.709 kHz/(ng/mL) for APTES and glutaraldehyde sensing films, respectively, at a biomarker concentration of less than 1 ng/mL. The developed method is useful for quickly estimating the frequency shift with respect to the concentration of the target molecules in the simulation for SH-SAW sensors.

Feasibility Study on Shear Horizontal Surface Acoustic Wave Sensors for Engine Oil Evaluation.

On site monitoring of engine oil is required. The features of a shear horizontal surface acoustic wave (SH-SAW) sensor include simultaneous detection of mechanical and electrical properties of liquids (such as viscosity, relative permittivity, and conductivity) and loaded mass on the sensor surface. In this paper, the used engine oil extracted from a motorbike was measured using the SH-SAW sensor. The degradation factors of the used engine oil were experimentally discussed. Especially, the influences of the particles in the engine oil, heating effect, and water contained in the engine oil were considered by comparing the differences between new and used engine oils. The results indicate that the influence of the water contained in the engine oil is the primary cause of the degradation of the used engine oil.

Properties of engine oil measured using a surface acoustic wave sensor

Shear horizontal surface acoustic wave (SH-SAW) devices are used as sensors for liquid measurements. Liquid properties, such as viscosity and conductivity, can be measured using SH-SAW sensors. Measurements of the deterioration of engine oil are required. In this study, an SH-SAW sensor was applied to measure the properties of three types of oil. First, the properties of olive oils with iron particles were measured to simulate the deterioration of oil. Secondly, two types of engine oil were heated to evaluate the thermal changes in their properties. The properties of the engine oil used by a motorbike for about one year were measured and compared with those of the new one. The obtained results in this study strongly suggest that the SH-SAW sensors for detecting the mechanical and electrical properties of liquids were effective in measuring the properties of oil.

Real-time monitoring of methanol concentration using a shear horizontal surface acoustic wave sensor for direct methanol fuel cell without reference liquid measurement

In recent years, there has been an increasing demand for sensors that continuously measure liquid concentrations and detect abnormalities in liquid environments. In this study, a shear horizontal surface acoustic wave (SH-SAW) sensor is applied for the continuous monitoring of liquid concentrations. As the SH-SAW sensor functions using the relative measurement method, it normally needs a reference at each measurement. However, if the sensor is installed in a liquid flow cell, it is difficult to measure a reference liquid. Therefore, it is important to establish an estimation method for liquid concentrations using the SH-SAW sensor without requiring a reference measurement. In this study, the SH-SAW sensor is installed in a direct methanol fuel cell to monitor the methanol concentration. The estimated concentration is compared with a conventional density meter. Moreover, the effect of formic acid is examined. When the fuel temperature is higher than 70 °C, it is necessary to consider the influence of liquid conductivity. Here, an estimation method for these cases is also proposed.

Estimation of liquid properties by inverse problem analysis based on shear horizontal surface acoustic wave sensor responses

A shear horizontal surface acoustic wave (SH-SAW) sensor can detect liquid properties, such as viscosity, density, permittivity, and conductivity. The advantage of using the SH-SAW sensors is the simultaneous detection of the mechanical and electrical properties of liquids. In this paper, we proposed a method of estimating the density and viscosity of liquids based on the inverse problem analysis. Glycerol or ethanol aqueous solutions were measured. The estimated and literature values were compared. For glycerol aqueous solutions, when the concentration is low, those values agree well. However, when the concentration is high, those values did not agree because the bulk modulus of glycerin solutions cannot be assumed as constant. On the other hand, as the bulk modulus of ethanol aqueous solutions can be assumed to be the same as that of water, the deviations between those values were small. Therefore, the proposed method is effective when the bulk modulus is assumed as constant.

Analysis of binary mixtures of aqueous aromatic hydrocarbons with low-phase-noise shear-horizontal surface acoustic wave sensors using multielectrode transducer designs.

The present work investigates a compact sensor system that provides rapid, real-time, in situ measurements of the identities and concentrations of aromatic hydrocarbons at parts-per-billion concentrations in water through the combined use of kinetic and thermodynamic response parameters. The system uses shear-horizontal surface acoustic wave (SH-SAW) sensors operating directly in the liquid phase. The 103 MHz SAW sensors are coated with thin sorbent polymer films to provide the appropriate limits of detection as well as partial selectivity for the analytes of interest, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), which are common indicators of fuel and oil accidental releases in groundwater. Particular emphasis is placed on benzene, a known carcinogen and the most challenging BTEX analyte with regard to both regulated levels and its solubility properties. To demonstrate the identification and quantification of individual compounds in multicomponent aqueous samples, responses to binary mixtures of benzene with toluene as well as ethylbenzene were characterized at concentrations below 1 ppm (1 mg/L). The use of both thermodynamic and kinetic (i.e., steady-state and transient) responses from a single polymer-coated SH-SAW sensor enabled identification and quantification of the two BTEX compounds in binary mixtures in aqueous solution. The signal-to-noise ratio was improved, resulting in lower limits of detection and improved identification at low concentrations, by designing and implementing a type of multielectrode transducer pattern, not previously reported for chemical sensor applications. The design significantly reduces signal distortion and root-mean-square (RMS) phase noise by minimizing acoustic wave reflections from electrode edges, thus enabling limits of detection for BTEX analytes of 9-83 ppb (calculated from RMS noise); concentrations of benzene in water as low as ~100 ppb were measured directly. Reliable quantification of BTEX analytes in binary mixtures is demonstrated in the sub-parts-per-million concentration range.

Label-free Detection of Protein Released during Platelet Activation by CNT-Enhanced Love Mode SAW Sensors.

Platelet-rich plasma (PRP) has been applied in a series of clinical treatments. PRP contains high-concentrated platelets, which, when activated, could secret a variety of growth factors and cytokines, to promote and/or enhance healing of injured tissues. Non-activated platelets suspension could be prepared by an isolation method of centrifugation and washing currently. However, it is not clear whether platelets, if any, are already activated during this process and there is no simple method to monitor their activation accordingly. Shear-Horizontal Surface Acoustic Wave sensors (SH-SAW, Love Mode) are promising in fundamental biology as well as biomedical engineering, detecting cell behaviors in liquid in a non-invasive, simple and quantitative manner. In this study, Love mode sensors are adopted for the label-free detection of protein secreted by platelets. Carbon nanotube (CNT) is reported as an advisable platform of both non-specific protein adsorption and specific protein binding. For further improvement of Love mode sensor performance, novel CNT -coated parylene-C film is prepared on its surface as both the acoustic-wave-guiding layer and bio-interface layer. The S21 loss curves of Love mode sensors were recorded and the corresponding resonance frequencies were extracted. The results showed that the CNT-enhanced sensor possessed an increased resonance frequency shift when compared to normal sensor with single parylene-C film under identical collagen concentrations. Then, the modified sensor is used for label-free detection of protein released by various concentrations of platelets. The results revealed high sensitivity and consistency, indicating the potential of CNT-enhanced Love mode sensors in cell-based applications.

Identification and Quantification of Aqueous Aromatic Hydrocarbons Using SH-Surface Acoustic Wave Sensors

A need exists for compact sensor systems capable of in situ monitoring of groundwater for accidental releases of fuel and oil. The work reported here addresses this need, using shear horizontal surface acoustic wave (SH-SAW) sensors, which function effectively in liquid environments. To achieve enhanced sensitivity and partial selectivity for hydrocarbons, the devices are coated with thin chemically sensitive polymer films. Various polymer materials are investigated with the goal of identifying a set of coatings suitable for a sensor array. The system is tested with compounds indicative of fuel and oil releases, in particular, the BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), in the low milligrams/liters to high micrograms/liters concentration range. Particular emphasis is placed on detection of benzene, a known carcinogen. It was observed that within the above concentration range, responses to multiple analytes in a mixture are additive, and there is a characteristic response time for each coating/analyte pair, which is largely independent of concentration. With the use of both the steady-state and transient-response information of SH-SAW sensor devices coated with three different polymer materials, poly(ethyl acrylate), poly(epichlorohydrin), and poly(isobutylene), a response pattern was obtained for benzene that is easily distinguishable from those of the other BTEX compounds. The time courses of the responses to binary analyte mixtures were modeled accurately using dual-exponential fits, yielding a characteristic concentration-independent time constant for each analyte/coating pair. Benzene concentration was quantified in the aqueous phase in the presence of the other BTEX compounds.

Measurement of Plasma Clotting Using Shear Horizontal Surface Acoustic Wave Sensor

The monitoring of blood coagulation is important during operation. In this study, a shear horizontal surface acoustic wave (SH-SAW) sensor is applied to monitor plasma clotting. An SH-SAW sensor with a metallized surface for mechanical perturbation detection can detect plasma clotting. As plasma clotting is a gel formation reaction, the SH-SAW sensor detects viscoelastic property changes. On the other hand, an SH-SAW sensor with a free surface for electrical perturbation detection detects only the liquid mixing effect. No electrical property changes due to plasma clotting are obtained using this sensor. A planar electrochemical sensor is also used to monitor plasma clotting. In impedance spectral analysis, plasma clotting is measured. However, in the measurement of time responses, no differences between clotting and nonclotting are obtained. Therefore, the SH-SAW sensor is useful for monitoring plasma clotting.

Signal change of surface acoustic wave (SAW) caused by H2O2 damage to SV40-T2 cells cultivated on SH-SAW sensor

We developed a cellular biosensing system based on the direct assembly of rat- or human-cultured cells onto a shear horizontal-surface acoustic wave (SH-SAW) device. This novel design enables the sensitive monitoring of changes in physical cell-characteristics, such as viscosity or density inside the cells. In our system, the change of phase of SAW and insertion loss was observed to be time-dependent when the rat lung epithelial cells were treated with 1.0mM H2O2. Immunofluorescence microscopy showed the disappearance of the zonula occludens protein from the cell–cell contact region. Furthermore, the transepithelial electrical resistance of the cells gradually decreased owing to this treatment. These results suggest the correlation between the change of SH-SAW phase, the insertion loss of the SH-SAW parameter, and the destruction of the tight junction of the cells on SH-SAW devices.

Methanol Sensor Using Shear Horizontal Surface Acoustic Wave Device for Direct Methanol Fuel Cell

A liquid-phase sensor is realized that uses a shear horizontal surface acoustic wave (SH-SAW) device. The advantage of using an SH-SAW sensor is the simultaneous detection of liquid properties, such as density, viscosity, permittivity, and conductivity. In this study, the SH-SAW sensor is applied as a methanol (MeOH) sensor for a direct methanol fuel cell (DMFC). As the generation efficiency of the DMFC depends on the methanol concentration, the monitoring of MeOH concentration is needed. The SH-SAW sensor is applied to the MeOH sensor to measure the electrical properties. The efficiency of the DMFC is also influenced by materials contained in the fluid. If a component of the SH-SAW sensor is eluted into the fuel, the SH-SAW sensor cannot be used in the DMFC. The SH-SAW sensor was dipped into the fuel at 80 °C and it was found that the concentration of eluted components from the SH-SAW sensor was low and that the efficiency was not influenced by such components. Then, the temperature characteristics of the SH-SAW sensor with various MeOH solutions were measured. The maximum operating temperature of the DMFC is 80 °C and the minimum temperature depends on the place of installation. In this work, the minimum temperature is assumed to be -5 °C. Thus, the SH-SAW sensor must measure the MeOH concentration from -5 to 80 °C without malfunction. The results indicate that the MeOH concentration can be measured when the temperature is lower than 60 °C. If the temperature is higher than 60 °C, bubbles are generated on the sensor surface and the sensor is influenced by them. A liquid flow system was demonstrated to reduce the influence of bubbles.