The larger an industrial machine, the more difficult it is to detect an unwanted deviation in oil pressure or even a leak in a line from the outside in the event of a malfunction. It often takes a long time for specialist personnel to find the needle in the haystack. This results in production downtime and high costs. The situation is similar when it comes to detecting the causes of illness in humans. If patients complain of pain in the abdomen, there is usually no way around a complex gastroscopy or colonoscopy. In such cases, electrochemical impedance spectroscopy can provide a remedy.

In this procedure, a frequency spectrum is sent from one electrode through a medium to a second electrode: A spectrum can be derived from this, i.e. a specific fingerprint of this medium. If changes in material or fluid properties become apparent, this can be an indication of both the progressive corrosion of a component or the presence of a specific disease.

Until now, impedance analyzers have not been small and mobile enough to be used for these purposes. Researchers at the Fraunhofer Institute for Reliability and Microintegration IZM in Berlin, with the support of MST and Sensry, have therefore developed a compact and modular IoT sensor for these applications that can measure impedances and transmit them wirelessly. As a result, it is not only waterproof, but also biomedically compatible.

The sensor is made of a biocompatible polymer and combines the two electrodes required with numerous components for analyzing environmental properties, including six sensors for measuring a wide range of data, on an 11 mm x 16 mm surface. In addition to temperature, pressure, humidity and sound in the environment, the sensor can also record its own acceleration behavior, rotation or ambient noise. Light and colors can be determined by an integrated light sensor.

In the event of a machine malfunction, for example, the sensor is inserted into an oil line, which then flows through the entire system. Precise data on the properties of the environment is transmitted wirelessly in real time to specially developed software with a web interface for PCs and smartphones. If a point is reached at which the pressure or the fluid spectrum deviates from the norm, this is an indication that the cause of the problem has been successfully localized. The spectra of some liquids, such as oil or water, are already stored in the software to make it easier to classify the collected data.

The miniaturization of the components posed a major challenge in the production of the sensor. In particular, reducing the coil diameter to 10 mm for wireless charging was a hurdle. However, a sophisticated system design made it possible to overcome this challenge. At the start of the project, Sensry provided its circuit diagrams and Kalisto firmware as the basis for the development of the sensor.

The core of the spectroscopy capsule contains the system-in-package, a flexible circuit board and a ceramic plate.

© Fraunhofer IZM

To accommodate a total of over 70 passive and active components on a flexible and biocompatible circuit board, it was designed from a liquid crystal polymer and manufactured in four layers by Dyconex, an MST company. Nevertheless, it is 175 µm thin and therefore barely thicker than a human hair. A system-in-package was produced on a six-layer interposer and forms the core of the sensor, as this is where the IoT system is combined. Thanks to a built-in induction coil, the capsule does not need to be opened for wireless charging and can be charged wirelessly via Qi technology. However, a classic DC charging process is also possible via a docking station for calibrating and programming the sensor. To prevent the very small components from getting too hot during operation, the sensor is also filled with an epoxy resin that insulates the components from each other and dissipates the heat to the outside. At the lower end, it is sealed with a 0.5 mm thin four-layer ceramic plate, which was manufactured by Micro Systems Engineering, an MST company, and on which the electrodes for impedance spectroscopy are mounted alongside the pressure sensor.