The world’s smallest impedance spectroscopy system in pill form

May 16, 2023

(Nanowerk News) The larger the industrial machine, the more difficult it is to troubleshoot a malfunction by detecting unwanted oil pressure deviations or even outside line leaks. It often takes specialist staff ages to find what seems to be the equivalent of a needle in a haystack. This can lead to production losses and high costs.

The situation is similar to the identification of the causes of disease in humans. If a patient complains of abdominal pain, there is usually no escape from a complex gastroscopy or colonoscopy. Electrochemical impedance spectroscopy can be of great help in such cases.

In this process, radio waves are sent through a medium from one electrode to a second electrode to obtain the frequency spectrum (i.e., the specific fingerprint) of the medium. If any change in the properties of a substance or liquid is identified, this may indicate rapidly developing component corrosion or the presence of certain clinical features.

Previously, impedance analyzers were not small and portable enough to be used for this purpose. With these applications in mind, researchers at the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin, with support from MST and Sensry, worked to develop a compact and modular IoT (Internet of Things) sensor that can measure impedance and transmit wirelessly. As a result, the sensor is not only waterproof but also biomedically compatible. Impedance measurement in hard-to-reach areas: Super mini IoT sensor. (Image: Fraunhofer IZM)

The sensor consists of a biocompatible polymer, and despite its small surface area of ​​only 11 × 16 square millimeters, it accommodates the necessary two electrodes, as well as various components for environmental property analysis, including six sensors for measuring a wide range of data parameters. In addition to ambient temperature, pressure, humidity and sound, this small but highly versatile device can also track its own acceleration behavior, as well as rotation and ambient noise. Light and color properties can be detected through the integrated light sensor.

In a more concrete scenario, in the event of an engine failure, a sensor could be inserted into the oil line, for example, so that it flows throughout the system. Accurate data on environmental properties are wirelessly transmitted in real time to custom-built software with a web interface for PCs and smartphones. If the sensor reaches a point where the pressure or liquid spectrum deviates from requirements, it is an indication that the cause of the problem has been successfully localized. To make it easier for users to analyze the collected data, frequency spectra of certain liquids, such as oil and water, are included in the software.

The main challenge faced during the manufacture of sensors is the miniaturization of components. In particular, reducing the coil diameter for wireless charging to 10 millimeters proved to be a significant constraint. However, the sophisticated system design makes it possible to overcome these challenges. At the start of the project, Sensry GmbH provided circuit diagrams and Kalisto firmware as a basis for sensor development.

To ensure that a total of more than 70 passive and active components can be mounted on a flexible and biocompatible printed circuit board, the circuit board was designed using liquid crystal polymers and manufactured in four layers by DYCONEX, an MST company. Despite its multi-layered design, the thickness of the circuit board is only 175 micrometers, which makes it as thick as a human hair.

The system-in-a-package is built on a six-layer interposer and is the core of the sensor, as this is where the IoT system is integrated. Thanks to the built-in induction coil, the device can be charged wirelessly using Qi technology without having to open the capsule. However, classic DC charging is also possible via the docking station which is used to calibrate and program the sensors.

To prevent components from overheating during operation, the sensor is filled with epoxy resin which isolates the components from each other and dissipates heat to the outside. On the underside, the sensor has a 0.5-millimeter-thin four-layer ceramic board manufactured by Micro Systems Engineering GmbH, an MST company, which houses the electrodes for impedance spectroscopy as well as the pressure sensor. As a trade show demonstration unit, IoT sensors demonstrate how intelligent system design and semiconductor packaging can be used to greatly downsize electronics without sacrificing any functionality.

Research on electrochemical impedance spectroscopy is well under way, and the possibilities of medical technology are far from exhausted.

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