**1. Introduction**

Over the past three decades, the emergence of small portable lab-on-a-chip biosensors has developed into an important area in biology and medicine [1]. Lab-on-a-chip devices promise to perform all the functions of a traditional laboratory on a miniature microfluidic platform [2]. These small devices allow full automation of analysis, reduce the sample volumes, and reduce the time of analysis as well [2] SAW devices are ideally suited to provide the sensing element in the Lab-on-a-chip platform [3]. One limitation that has plagued lab-on-a-chip devices has been the lack of a miniaturized sensor that facilitates the development of high-sensitivity assays. Several technologies have been explored including Mass spectrometry field [4, 5], electrochemical

sensors field [6], optical detection field [7, 8], surface plasmon resonance spectroscopy field [9], and interferometry [10, 11] but each has significant limitations in complex biological matrices. The SAW biosensor technology is based on Surface acoustic waves (SAW), a type of acoustic wave that propagates along a surface of a solid material [12–14]. Propagating SAW are impacted by mass loading and changes in viscoelastic properties of the media on the surface of the sensor and compared to a reference, as seen in **Figure 1a** and **b** [15]. These waves were first described by John William Strutt, 3rd (Lord Rayleigh), in an article on the propagation of acoustic waves in a piezoelectric material [16]. White and Voltmer introduced the concept of using interdigital transducers (IDTs) as a more efficient method of generating surface acoustic waves [17–19]. This method is still used today to generate surface acoustic waves on a piezoelectric material [20, 21]. The piezoelectric materials that are typically used for fabricating SAW devices are listed in **Table 1** [17].

To date, the primary use of SAW devices has been in the telecommunications industry, specifically as filters in cellular telephones and other smart devices [17]. SAW devices operate in the following manner: a metal IDT deposited on a piezoelectric surface is driven by a sinusoidal power source with a period specific to the IDT design [22]. This causes the electrode to vibrate, generating an acoustic wave that is perpendicular to the direction of the IDTs [23]. The penetration depth of the acoustic wave is relatively shallow. This is because the wave produces an evanescent field that cannot penetrate more than a few nanometers into the substrate from which the sensors are fabricated. If a guiding layer is used (a top layer), the acoustic wave propagation will be confined to the substrate-sample interface [24]. The confinement of the wave via a guiding layer maximizes the energy density of the acoustic wave at the substratesample interface.

The velocity of an acoustic wave in a piezoelectric material is an intrinsic property and varies slightly as a function of temperature [25–27] In the case of lithium tantalate, one of the more popular substrates used for the fabrication of SAW filters and devices, the wave velocity is 4200 m s−1 [28]. Values for the acoustic properties of commonly used piezoelectric substrates are listed in **Table 1** below. The acoustic wave velocity is approximately 104 –105 times smaller than the velocity of electromagnetic waves [29]. Surface acoustic waves can be generated and detected by spatially periodic, interdigital electrodes that are deposited on the planar surface of a piezoelectric plate. Excitation

#### **Figure 1.**

*Acoustic wave measurement using (a) a delay line SAW device with measurement performed via a two-port vector network analyzer (VNA). Connections are made using a customized printed circuit board with connectors to the VNA. In (b) there is a plot of the change in phase of the reference, the change in phase of the sample and the difference between the two channels.*

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


#### **Table 1.**

*Properties of common piezoelectric substrates used in the fabrication of surface acoustic wave devices. Courtesy of The Roditi International Corporation Ltd, UK.*

of the interdigitated electrode with a radio frequency source generates a periodic electric field, thus permitting piezoelectric coupling to a traveling surface wave. The center frequency of the acoustic wave generated by the sensor, fc, is governed by the Rayleigh wave velocity (VR). VR depends on the piezoelectric substrate and the electrode width (a) of a single finger, according to the equation fc = VR/4a [30]. The velocity for the SAW generated by the device depends on the properties of the piezoelectric substrate (crystal) that is used in the fabrication of the sensor and its crystallographic orientation. Computer models have allowed the careful sorting of numerous crystallographic orientations to enable the discovery of different types of acoustic waves. Where vs. is the SAW velocity and fc is the center frequency of the device. The SAW velocity is an important parameter determining the center frequency. The mass sensitivity is given by Sauerbrey's equation, where:

$$
\Delta \mathbf{f} / \mathbf{f}\_{\mathbf{o}} \approx \Delta \mathbf{V} / \mathbf{V}\_{\mathbf{o}} \tag{1}
$$

and

$$\mathbf{f}\_0 = \nu \wr \mathcal{X} \tag{2}$$

Propagation loss is one of the major factors that determine the insertion loss of a device and is caused by wave scattering at crystalline defects and surface irregularities. Materials that show high electromechanical coupling factors combined with small temperature coefficients of delay are generally preferred. The free surface velocity, Vf, of the material is a function of the cut angle and propagation direction. The TCD is an indication of the frequency shift expected for a transducer due to a temperature change and is also a function of the cut angle and propagation direction. The substrate is chosen based on the device design specifications, which include operating temperature, fractional bandwidth, and insertion loss.

Indeed, the fabrication of SAW devices requires a few critical components including the physical deposition of a metal on the surface of the piezoelectric substrate, etching of that metal deposited on the surface, optional deposition of guiding, and/or sensing layer. SAW devices mainly have two kinds of structures. The first is a design that features two sets of IDTs where a sinusoidal radio frequency (*RF*) is applied to one side of the IDT structure while the other is connected to the ground. The *RF* is applied to the first set of IDTs or the input IDTs, which generates an acoustic wave, as is seen in **Figure 1a**. The wave is transmitted through a delay line and is received by the second set of output IDTs. The signal is then captured and analyzed. The second type of SAW termed a resonator, features one set of IDTs with grating reflectors that can trap the surface wave. The signal from the IDT can be amplified and then fed back to the input IDT.

There are several factors that affect the transmission of the acoustic wave in a SAW device and have to be closely monitored or controlled in order to generate repeatable results. These include temperature, pressure, humidity, and mass loading. Indeed, SAW devices can be operated as sensors for temperature, pressure, humidity and mass loading. Temperature effects are the most challenging when trying to perform sensitive acoustic-wave sensing in liquid media. This is controlled in part by including a reference delay line. The reference is functionalized with a passivating agent to minimize nonspecific binding (e.g., PEG). For biological or medical applications, the active sensing line is functionalized with capturing agents that interact specifically with the analyte being targeted. An Analyte can be quantitatively detected by monitoring changes in gas pressure, liquid pressure or from the increase mass due to binding of a biological molecule to a targeting molecule or the increase in the fluid density or mass increases due to absorption of the target. The sensitivity of SAW devices increases as the square of the frequency; therefore, higher frequencies lead to smaller, more sensitive instruments. However, the frequency also determines the depth within the sample that the device can probe; for example, for a solution placed atop a sensor, the higher frequencies will examine a shallower depth than lower frequencies. Thus, the operating frequency of the SAW must be considered when selecting targets, probes, and conjugation schemes to functionalize the active sensing region of a SAW device.

Aside from the layer of targeting molecules that is typically used to decorate a SAW device, an additional guiding layer is often deposited to enhance the sensitivity of the device. The guiding layer traps the energy of the acoustic wave near the surface of the device to increase sensitivity to surface perturbations. The SAW sensors are inherently capable of detecting analytes in solution concentrations on the order of parts-per-billion (ppb) by mass, through the use of higher frequencies >300 MHz Ideally, the guiding layer needs to have a lower density and lower acoustic velocity than the piezoelectric substrate. Materials that have been utilized in past include polymers such as poly-methyl-methacrylate and Novolac and oxides including silicon dioxide and silicon monoxide. Changes in the mass loading of the surface, if all else is kept constant, affect the acoustic wave velocity as it travels across the delay line from the input IDT to the output IDT. The time delay is a result of the interactions between any adsorbed mass, i.e. the analyte, and results in a phase shift between the applied and the detected sinusoidal wave. In the case where there is only a single IDT, the round-trip time is measured by the applied signal. In recent years, there has been an increasing demand for portable, disposable and inexpensive sensors for biological and medical applications. Due to these increasing demands for miniature sensors, SAW devices have received renewed interest for use as sensors in biochemical assays and as detectors *Development of Simple and Portable Surface Acoustic Wave Biosensors for Applications… DOI: http://dx.doi.org/10.5772/intechopen.106630*

in microfluidic biosensors, particularly since it is a label-free technique. In this review, we will outline the use of SAW biosensors in biology and medicine.

## **2. Principles for SAW biosensors**

Surface acoustic waves are generated in a SAW device with the application of a sinusoidal *RF* to one side of the IDT patterned on the piezoelectric material, while the other side is connected to the ground. An image of a typical measurement scheme is shown in **Figure 1a** with a custom-built printed circuit board and connections with a vector network analyzer (VNA). The wavelength of the acoustic wave generated will be a function of the material properties, the shape, and layout of the IDTs, and the material deposited as the guiding layer. The main parameter utilized in data analysis is the velocity of the acoustic wave. Each piezoelectric material will allow an acoustic wave to propagate at different velocities for a given wavelength. The larger the wave velocity, the smaller time needed to have the acoustic wave travel from the input IDT to the output IDT. Another important parameter for SAW devices is the frequency that it operates. Only when the wavelength of the applied R*F* is equal to the intrinsic wavelength of the SAW, i.e. the period of the IDT, can the SAW be stimulated to produce an acoustic wave described by Eq. (2). Measurements of acoustic wave velocity is often reported as a phase shift between a reference channel and a sample channel. An image of a typical measurement is seen in **Figure 1a** while the plot of phase shift is seen in **Figure 1b**.

The IDTs can be fabricated as periodic bars with uniform lengths, widths, and gap spacing. They can be designed to give bi-directional or unidirectional acoustic wave propagation. Small IDT gap spacing and widths and thus smaller wavelengths often result in a higher frequency. In some cases, different IDT width/gap ratios can generate higher harmonic waves. The velocity of the wave that is generated can be influenced by several factors such as temperature and the analyte's concentration change. The temperature has a significant effect on the wave velocity. This effect can be described as the temperature coefficient of the frequency (TCF). TCF is described as a relative change in frequency with the temperature. Other factors that influence the device's sensitivity include humidity and pressure. Any slight fluctuations in the pressure will impact the mass loading of the device. Any change in the humidity results in changes to the electric field. The electric field strength is also impacted by charged particles that are suspended in the solution with comparatively high dipole moments (**Figure 2**).

#### **Figure 2.**

*Image of a surface acoustic wave biosensor where (a) is an image of new unused sensors while (b) shows an image of used sensors and (c) is a high magnification image of one of the sample channels and the reference channels from (b).*
