Measuring temperature effects on the dielectric properties of biological liquids by an experimental microwave system

In recent years, microwave biosensing based on resonant elements led to an ever-growing interest in biomedical research for monitoring, therapeutic and diagnostic applications due to its minimal invasiveness and high detection sensitivity. The dielectric properties of the biological medium under test (MUT) determine how these resonant structures perform. In particular, the temperature is crucial for inducing dielectric intrinsic changes in MUT. Furthermore, at the cellular level, environmental temperature changes strongly affect the dielectric properties of cell cultures. On this basis, this paper proposes to study the effects of temperature on the dielectric properties of a biological solution in the range of 25°C -37°C, using a measurement method based on microwave biosensing. We used an experimental system consisting of a resonant circular biosensor for temperature measurements of one of the most used biological solutions, the phosphate-buffered saline (PBS) solution, in a thermostatic chamber. The circular probe provided the measurement response to temperature variations in terms of frequency and signal attenuation. In experimental tests, 6.0 ml of PBS were cooled stepwise from 37 °C to 25°C at 3 °C steps into a standard Petri dish. The experimental results demonstrated the ability of the measurement system to detect significant temperature changes of the PBS solution in both frequency and attenuation of the reflection coefficient| S11|, also pointing out the need for accurate temperature control during the measurement of the dielectric properties of biologic fluids.