Microsystems – A tiny, smart and reliable key technology for the markets of tomorrow

Microsystems have crept almost unnoticed into our everyday lives. In smartphones and tablets, they serve as speakers, microphones and acceleration sensors. In cars, they monitor and manage airbags and other safety systems. The economic potential of microsystems is huge as wearables and autonomous vehicles will, in the foreseeable future, be everywhere. 

What exactly is a microsystem?

If anyone can provide a definition, it is Dr. Frank Ansorge, head of the Interconnect Systems Group at the Fraunhofer Research Institution for Microsystems and Solid State Technologies (EMFT) in Munich. Ansorge has been conducting research in this field for over 30 years. His definition, however, is deliberately vague: “Microsystems combine electronics, mechanics, pneumatics, hydraulics, optics, chemistry and biochemistry to create highly integrated intelligent solutions in a very small space. That space can cover a few millimetres or just a few micrometres.” There is indeed no precise boundary determining when a complex ‘system’ – a device, machine or plant – becomes a microsystem. And the fuzziness of the definition is reflected in the choice of terminology: Whereas MST (for microsystem technology) has become established in Europe, researchers and developers in the USA prefer the term microelectromechanical systems (MEMS). The Japanese speak of micromachines. 

 Two examples suffice to illustrate the vast spectrum of applications for microsystems – and point to the focal applications of the future. One is in low-cost medical point-of-care diagnostics, the other in intelligent and reliable microsensors for automotive systems. 

Dr. Can Dincer first came across paper sensors – known in the trade as microfluidic paper-based analytical devices, or µPADs – as a visiting researcher at London’s Imperial College in 2018. “I was immediately struck by the versatility of the analytics that is possible with these simply structured microsystems. Now, we are further developing electrochemical µPADs in Germany.” Today in the employ of the University of Freiburg, Dincer unveiled the first fruits of his research in 2019: a sensor that measures the hydrogen peroxide (H2O2) content in exhaled air. The proportion of H2O2 increases if the airways are inflamed, e.g. in the event of pneumonia, asthma, lung cancer or Covid-19. Measuring just a few centimetres square, the sensor fits snugly in a conventional face mask. By repeatedly measuring H2O2 at regular intervals, it is possible to track the progression of the disease and monitor the effectiveness of treatment. 

Conventional devices which indirectly measure elements of breath condensate have been around for some time, but they are bulky, expensive and error prone. In contrast, the paper sensor from Freiburg is inexpensive to produce and easy to use. An electrochemical measuring cell whose dimensions used to roughly resemble a coffee cup now fits on a small strip of paper. As soon as the paper is moistened by damp exhaled air, the moisture transports the H2O2 in the paper to a carbon electrode where it oxidises a dye. The subsequent back reduction supplies a current signal in proportion to the concentration of H2O2. Electrochemical µPADs are produced using commercially available equipment. To start with, the pattern of the measuring cell is applied to the paper using a wax printer. Heat causes the wax to sink into the paper and defines a reaction space. Lastly, screen printing with silver and carbon pastes produces electrodes and leads. The paper sensor is affixed to a plastic shim to enhance its stability.