Surface acoustic wave (SAW) biosensors are a type of biosensor developed from radiofilters used in cell phones. SAW biosensors use mechanical vibrations to sense the binding events on the surface of a chip. Binding events refer to the binding of analyte or biomarker to antibody within and immunoassay and represents the quantity of a biomarker. SAW biosensors are based on the principle of piezoelectricity, which is the ability of some materials to generate electric charges when they are deformed by mechanical stress, or vice versa. By applying an alternating voltage to a pair of interdigital electrodes on a piezoelectric substrate, such as quartz or lithium niobate, SAW biosensors can generate and detect acoustic waves that propagate along the surface of the substrate1.
The acoustic waves have frequencies ranging from a few megahertz (MHz) to several gigahertz (GHz), and wavelengths in the order of micrometers. The acoustic waves are very sensitive to any change in the mass, viscosity, or conductivity of the medium in contact with the surface, such as air or liquid. Therefore, by coating the surface of the SAW biosensor with a layer of biomolecules that can selectively bind to the target molecules in the sample, the binding events can cause a change in the acoustic wave properties, such as amplitude, phase, frequency, or velocity. This change can be measured by the SAW biosensor as an electrical signal, which can be correlated to the amount of target molecules in the sample2.
There are different types of SAW biosensors, depending on the mode of propagation and polarization of the acoustic waves.
Some of the most common types are:
Shear horizontal surface acoustic wave (SH-SAW) biosensors, which use acoustic waves that have a shear horizontal polarization, meaning that the particle displacement is parallel to the surface and perpendicular to the direction of propagation. SH-SAW biosensors have low attenuation and high sensitivity in liquids, as the acoustic energy is confined near the surface and does not leak into the liquid. SH-SAW biosensors can be fabricated on various substrates, such as quartz, lithium tantalate, or gallium arsenide3.
Surface transverse wave (STW) biosensors, which use acoustic waves that have a surface transverse polarization, meaning that the particle displacement is perpendicular to both the surface and the direction of propagation. STW biosensors have higher frequencies and lower attenuation than SH-SAW biosensors, as they use a grating structure to enhance the confinement of the acoustic energy. STW biosensors can be fabricated on quartz or lithium tantalate substrates4.
Love wave biosensors, which use acoustic waves that have a shear horizontal polarization, but are guided by a thin layer of low acoustic impedance material, such as silicon oxide or polymer, on top of the piezoelectric substrate. The layer acts as a waveguide that increases the sensitivity and reduces the attenuation of the acoustic waves in liquids, as it creates a resonance effect that amplifies the interaction between the acoustic waves and the sensing layer. Love wave biosensors can be fabricated on various substrates, such as quartz, lithium niobate, or lithium tetraborate5.
Advantages of SAW biosensors
SAW biosensors have several advantages over conventional biosensors, such as:
High sensitivity and resolution: SAW biosensors can detect very small changes in mass, viscosity, or conductivity on the surface, as they operate at high frequencies and wavelengths comparable to the size of the biomolecules. SAW biosensors can achieve mass sensitivities in the range of nanograms per square centimeter, and frequency resolutions in the range of hertz2.
Label-free and non-invasive: SAW biosensors do not require any labeling, amplification, or modification of the target molecules, as they rely on the intrinsic physical properties of the molecules. SAW biosensors also do not damage or alter the biological activity of the molecules, as they use low-power acoustic waves that do not generate heat or radiation2.
Real-time and dynamic: SAW biosensors can measure the binding events in real-time, as they do not involve any scanning or optical detection. SAW biosensors can also monitor the dynamic behavior and response of the molecules to external stimuli, such as temperature, pH, or electric fields2.
Simple and compact: SAW biosensors are simple and compact devices, as they consist of only a piezoelectric chip with electrodes and a sensing layer. SAW biosensors do not require any bulky or expensive equipment, such as lasers, optics, or pumps. SAW biosensors can also be integrated into microfluidic systems, which can enable the automation and miniaturization of the sample handling and analysis2.
Applications of SAW biosensors in health care
SAW biosensors can be used to diagnose and monitor various diseases and conditions, by detecting biomarkers, such as proteins, DNA, or antibodies, in biological fluids, such as blood, urine, or saliva. For example, SAW biosensors can detect prostate-specific antigen (PSA) for prostate cancer screening, human chorionic gonadotropin (hCG) for pregnancy testing, or hepatitis B virus (HBV) for infection diagnosis2.
Biotechnology: SAW biosensors can be used to facilitate the research and development of biotechnology, by characterizing and manipulating biological molecules, such as enzymes, antibodies, or cells, in vitro or in vivo. For example, SAW biosensors can measure the activity and kinetics of enzymes for drug discovery, the affinity and specificity of antibodies for immunotherapy, or the adhesion and proliferation of cells for tissue engineering2.
Conclusion
SAW biosensors are a type of biosensor that use mechanical vibrations to sense the binding events on the surface of a chip. SAW biosensors have high sensitivity, label-free detection, real-time measurement, and simple design, which make them suitable for various applications in health care and biotechnology.
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