The requirements for dielectric measurements on polar liquids lie largely in two areas. First there is scientific interest in revealing the structure of and interactions between the molecules - this can be studied through dielectric spectroscopy. Secondly, polar liquids are widely used as dielectric reference and tissue equivalent materials for biomedical studies and for mobile telecommunications, health and safety related measurements. This review discusses these roles for polar liquids and surveys the techniques available for the measurement of their complex permittivity at RF and Microwave frequencies. One aim of the review is to guide researchers and metrologists in the choice of measurement methods and in their optimization. Particular emphasis is placed on the importance of traceability in these measurements to international standards

A review of RF and microwave techniques for dielectric measurements on polar liquids

The advantages and limitations of using partial differential analysis to assess the methodological uncertainty associated with the measurement of the dielectric properties of a material are discussed and an alternative pragmatic approach is proposed. It relies on repeat measurements of the dielectric properties of reference liquids and an analysis to estimate random and systematic uncertainties. Examples of measurement uncertainty are provided for well-defined monomolecular materials and for less homogeneous materials at microwave frequencies. All examples relate to measurement with an open-ended coaxial probe but the methodology is not specific to this technique. Examination of the components of uncertainty in the dielectric properties of biological tissue shows that, where the system is free of methodological bias, random fluctuations originating from sampling and natural inhomogeneity dominate the uncertainty budget. In such cases, the mean value of the measured parameter and the standard error of the mean can be taken as a good measure of the true value and its associated uncertainty.

https://iopscience.iop.org/article/10.1088/0031-9155/51/23/006/pdf

Dielectric measurement: error analysis and assessment of uncertainty

Electromagnetic (EM) medical technologies are rapidly expanding worldwide for both diagnostics and therapeutics. As these technologies are low-cost and minimally invasive, they have been the focus of significant research efforts in recent years. Such technologies are often based on the assumption that there is a contrast in the dielectric properties of different tissue types or that the properties of particular tissues fall within a defined range. Thus, accurate knowledge of the dielectric properties of biological tissues is fundamental to EM medical technologies. Over the past decades, numerous studies were conducted to expand the dielectric repository of biological tissues. However, dielectric data is not yet available for every tissue type and at every temperature and frequency. For this reason, dielectric measurements may be performed by researchers who are not specialists in the acquisition of tissue dielectric properties. To this end, this paper reviews the tissue dielectric measurement process performed with an open-ended coaxial probe. Given the high number of factors, including equipment- and tissue-related confounders, that can increase the measurement uncertainty or introduce errors into the tissue dielectric data, this work discusses each step of the coaxial probe measurement procedure, highlighting common practices, challenges, and techniques for controlling and compensating for confounders.

https://www.mdpi.com/2075-4418/8/2/40/pdf

Open-ended coaxial probe technique for dielectric measurement of biological tissues: challenges and common practices

In this paper, a complementary split-ring resonator (CSRR)-loaded patch is proposed as a microfluidic ethanol chemical sensor The primary objective of this chemical sensor is to detect ethanol's concentration First, two tightly coupled concentric CSRRs loaded on a patch are realized on a Rogers RT/Duroid 5870 substrate, and then a microfluidic channel engraved on polydimethylsiloxane (PDMS) is integrated for ethanol chemical sensor applications The resonant frequency of the structure before loading the microfluidic channel is 472 GHz After loading the microfluidic channel, the 550 MHz shift in the resonant frequency is ascribed to the dielectric perturbation phenomenon when the ethanol concentration is varied from 0% to 100% In order to assess the sensitivity range of our proposed sensor, various concentrations of ethanol are tested and analyzed Our proposed sensor exhibits repeatability and successfully detects 10% ethanol as verified by the measurement set-up It has created headway to a miniaturized, non-contact, low-cost, reliable, reusable, and easily fabricated design using extremely small liquid volumes

https://www.mdpi.com/1424-8220/16/11/1802/pdf

Complementary Split-Ring Resonator-Loaded Microfluidic Ethanol Chemical Sensor.

Metamaterial elements/arrays exhibit a sensitive response to fluids yet with a small footprint, therefore, they have been an attractive choice to realize various sensing devices when integrated with microfluidic technology. Micro-channels made from inexpensive biocompatible materials avoid any contamination from environment and require only microliter–nanoliter sample for sensing. Simple design, easy fabrication process, light weight prototype, and instant measurements are advantages as compared to conventional (optical, electrochemical and biological) sensing systems. Inkjet-printed flexible sensors find their utilization in rapidly growing wearable electronics and health-monitoring flexible devices. Adequate sensitivity and repeatability of these low profile microfluidic sensors make them a potential candidate for point-of-care testing which novice patients can use reliably. Aside from degraded sensitivity and lack of selectivity in all practical microwave chemical sensors, they require an instrument, such as vector network analyzer for measurements and not readily available as a self-sustained portable sensor. This review article presents state-of-the-art metamaterial inspired microfluidic bio/chemical sensors (passive devices ranging from gigahertz to terahertz range) with an emphasis on metamaterial sensing circuit and microfluidic detection. We also highlight challenges and strategies to cope these issues which set future directions.

Review of Recent Metamaterial Microfluidic Sensors

This report summarises the results of a recent programme of dielectric reference liquid measurements at National Physical Laboratory (NPL). It is intended that full details of the methods of measurement will be published in the scientific literature in the near future. Comprehensive tables of dielectric relaxation parameters for high purity methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, dimethyl sulphoxide and ethanediol are presented over a range of temperatures (in most cases at 5o intervals in the range 10 - 50 oC). These were derived from measurements at NPL between the years 1997 and 2000 at frequencies extending up to 5 GHz. The relaxation parameters are generally those of the single or double Debye models, which are found to offer a good fit to the measured data in nearly all cases. In addition, static permittivity data are presented for pure water, acetone, silicone oil and cyclohexane. All of the data presented is traceable to UK National Standards. Advisory notes on the use of the tables are provided.

R N Clarke

Papers

Tables of the complex permittivity of dielectric reference liquids at frequencies up to 5 GHz.