Shift of resonant frequency in biological piezoelectric sensors

Here, we examine the principles of how biological piezoelectric sensors and impedance analyzers can be used to detect changes in resonant frequency, including some practical applications and key aspects of using the technique.

Impedance analyzers have been used on a commercial basis and increasingly for biological applications since the 1980s. The techniques are relatively easy to learn and use, while equipment has also become increasingly portable and cost-effective due to technological advances over recent decades.

Impedance analyzers are used to measure static resistance and dynamic (or complex) impedance, whether the measurement is between connections of an electrical device under test (DUT) or between two or more points on a material that is under analysis.

In biological applications, electrical impedance depends on the level of opposition to the flow of electrical currents through the body or sample tissue(s) being measured. In humans, impedance may be measured from the wrist to opposite ankle, using up to four electrode sensors and a very low current of between 1-10 microamperes. The results can then be interpreted to gauge various medical criteria such as total body water (TBW) or changes in cellular structure, particularly through cellular surface characteristics.

Biological sensors are different from standard electrical sensors in that the resonant frequency changes are caused by cells reacting with and adhering to a differently coated surface. This adherence will increase the biomass, which will lead to decreased resonance and, therefore, a change in the resonant frequency. This principle is used in quartz crystal microbalance (QCM) systems.

A QCM measures changes in density or film thickness (such as those caused by cellular surface deposits) within a certain area. A quartz crystal resonator generates various frequencies and then the changes in the response received from the tissue or sample are recorded. In addition to the resonator, the system also includes a quartz crystal sensor within a temperature-controlled mounting, as well as an electronic controller that monitors the frequency response over time.

Modern techniques allow QCMs to be used with liquids, giving precise results and therefore very effective for determining the affinity of proteins and other molecules to surfaces with defined characteristics. This methodology means that polymers, viruses and other biomolecules including bacteria and pathogens can be detected and investigated.

Apart from measuring the resonant frequency, another characteristic known as the dissipation factor can also be determined as part of the analysis process. Dissipation factor is related to frequency response bandwidth and gives an indication of the viscoelasticity of the sample being examined. Over recent years, improvements in crystal design, transducer engineering and the measurement of very short time intervals have combined to improve the accuracy of this technique.

For analyzing the resonant frequency and any changes over time, an impedance analyzer is necessary. The standard analyzer software measures the conductance and its peak (i.e. resonant frequency) over the frequency range in question, using the software fitting tool and a single measurement sweep.

This standard measurement can be extended so that the resonant frequency value is tracked continuously and displayed over time. This is known as a frequency shift monitor and an example can be seen in the example picture on this page as well as in the video section area. This tracker solution or a similar configuration can be implemented for your project through a separate customized systems project. Of course a multi-channel solution (using eight channels, for instance) is also possible if a parallel frequency shift tracking system is needed, such as the shown example picture.

Customized Systems

At SinePhase, impedance analysis technology is not limited to a standard configuration; our products can be customized and are available with bespoke implementations and options to suit your requirements. Our experienced specialists will work with you to specify, design and manufacture customized hardware solutions that are based on our successful platform.

Extra functionality enables additional analysis with just a few keystrokes and gives quick indications of significant information including Q factor and half-value-bandwidth. Additionally, the extra correlation tool calculates and plots graphs of frequency shifts to illustrate current and earlier measurements and highlight any differences.

With our customized software, your individual analyzer system can be best adapted to your application environment, with performance parameters optimized through:

• parallel or multiplexed multi-channel inputs.
• tailored frequency range(s).
• customized adapters and connections.
• tailored measurement accuracy for specific impedance magnitudes.
• seamless integration of SinePhase hardware and software into multifunctional test configurations and rigs.

If you wish to discuss your requirements or require further information about customized biological measurement solutions, please contact us – we will be pleased to help.