**3. Biological and medical application of SAW devices**

Changes in the surface mass can attenuate the wave velocity. These minute changes are measured by either measuring the changes in the resonance frequency of the piezoelectric material or by measuring the time the SAW travels from the input IDT to the output IDT. In the case when there is only a single IDT, then we measure the round-trip time.

The temperature dependence of the SAW measurements is a property that can be used in temperature sensors. Borrero et al. report a SAW resonator-based temperature sensor that was fabricated from 128o Y-X LiNbO3 crystal. The device operated at a frequency of 65 MHz and was also capable of measuring pressure and impedance. The one-port SAW resonator had an IDT width of 15 μm, 20 IDT finger pairs, and an acoustic aperture of 15λ. There were also 100 electrodes on each side of the IDT pairs. A linear dynamic range was established between 50 and 200°C, while the frequency had a linear response with temperature.

Wireless temperature sensing is possible on SAW devices. This technique takes advantage of the round-trip flight time of travel. Wireless SAWs are made not to require any power supply so they can be used for remote sensing. Reindl et al. designed a delay line wireless SAW temperature sensor. The device operates by sending a VHF/UHF band RF burst delivered by a radar transceiver. The SAW then performed a measurement. Since the temperature affects the changes affect the wave velocity, measurement of the response pattern can be used to determine the temperature. The resolution of this system was ±0.2°C. This type of system could be used for biological applications in extremely isolated locations or for patients who have been isolated from the general population due to a high communicable infection.

SAW sensors can be fabricated and implanted in the body to monitor corebody temperatures in real-time. Martin and colleagues packaged a single-port resonator (single IDT) SAW temperature sensor, in a ceramic and connected it to a small antenna. In vivo tests in dogs demonstrated the capacity to perform wireless interrogation of samples. SAW devices fabricated from materials having large TCF are well suited for fabricating temperature sensors but are not well suited to fabricating other types of sensors. To perform other types of sensing, methods should be developed to compensate for the effects of TCF. One solution is to introduce a guiding layer of a material that reduces the TCF such as SiO2. SiO2 is often used to compensate for the negative TCF for most piezoelectric materials used to fabricate SAW devices. Zhang et al., published a report that experimentally verified that a SiO2 layer of a thickness of 0.3λ gives a TCF of zero for LiNbO3. A large electro-mechanical coupling coefficient of 7.92% was also observed when the thick SiO2 guiding layer was used. Another method for performing temperature compensation would be to add a second SAW device and a mixer cell [1, 31]. In such a configuration, one SAW acts as a reference while the other SAW acts as a sensing unit [1, 31]. If the two SAW sensors are placed in such a manner that both devices experience the same temperature with only one sensor actually sensing changes, then any interference occurring to both systems would be canceled out after the mixer [1, 31]. The SAW velocity is strongly affected by the pressure applied to the piezoelectric material. Therefore, a SAW pressure sensor would be a device that exploits this pressurefrequency relationship [32]. To enhance the sensitivity, often a method similar

*Development of Simple and Portable Surface Acoustic Wave Biosensors for Applications… DOI: http://dx.doi.org/10.5772/intechopen.106630*

to that of Grousset et al. is followed where the area below the sensing area is etched [8]. In that report, they used an AT-cut quartz film, operated at 430 MHz that was etched by Deep Reactive Ion Etching (DRIE) to expose the sensing area. The resonance frequency showed a linear relationship with the applied pressure and had a sensitivity of 25.8 kHz/bar from 0 to 4.8 bar.

SAW pressure sensors can be implanted in the human body. Liang et al. reported a blood pressure sensor where they amplified the signal from a SAW device by using a Colpitts oscillator. A static test showed a 1.75 kHz/mmHg sensitivity with a standard deviation less than 1 mmHg. Another wireless *in vivo* SAW device was reported by Murphy et al. that was designed to monitor blood pressure remotely from inside the left ventricle of the heart of a living porcine subject. A prototype of the device was able to monitor changes in blood pressure around the clock and then compared the results with a commercial catheter-tip transducer. The primary challenge to building *in vivo* SAW devices is to have a well-designed antenna to deliver the *RF* signal and to receive the data with minimal signal loss.

### **4. Molecular biosensors**

Due to their small sizes, ability to monitor label-free, and high accuracy, SAW devices are ideally suited to function as biosensors. They can be operated remotely, they can be used as implantable devices and they facilitate real-time measurements for patients remotely. The high accuracy of the SAW supports it to use as a bacterial cell monitor, viral particle monitoring, and a DNA detection system. Cai et al. used a SAW device to detect DNA sequences and cells. As is frequently done for SAW devices, gold was deposited on the guiding layer to form a sensing layer for DNA detection. The hybridization of the target single-stranded DNA target molecules (ssDNA) with the DNA probe resulted in a frequency shift of the SAW resonator and could be recorded and measured. DNA detection using this device achieved a sensitivity of 6.7 x 10−16 g/cm2 per Hz. The device was also capable of detecting a single EMT6 and 3 T3 cancer cell. Biochemical assays need to be able to monitor in liquid environments to known concentrations, however, the immersion of a traditional Rayleigh SAW tends to radiate the acoustic energy into the liquid because the displacement component is perpendicular to the surface. A type of SAW called the Love mode SAW is capable of performing analysis in liquid environments. Love-mode SAW devices guided acoustic modes which propagate in a thin layer deposited on a substrate. The acoustic energy is focused in the guiding layer where the displacement component propagates parallel to the surface. When using traditional piezoelectric substrates, SiO2 and PMMA are frequently used as the waveguiding layer. The soft polymer poly (dimethylsiloxane) (PDMS) is often used to fabricate the channels in the device. Zhang et al. has reported a prostate-specific antigen (PSA) biosensor based on a love mode device. The sensor used LiTaO3 with aluminum IDTs which were coated with a SiO2 guiding layer and then gold forming the sensing layer. A PDMS microfluidic channel was subsequently added to the device to ensure that liquid can flow between the IDTs. The detection limit of this system was 10 mg/ml. The images and mass loading effects of a similar Love wave SAW device was used to measure the mass loading resulting from the covalent attachment of streptavidin-coated one-micron magnetic beads are shown in **Figure 3**.

#### **Figure 3.**

*Confirmation of the mass-loading correlation between magnetic nanoparticles. (a) Shows covalently bonded to the surface of the SAW sensor and (b) shows the resulting phase shifts. The semi-log plot illustrates the expected direct proportionality between mass and phase shift.*
