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.
Examples of applications in medical technology and mechanical/plant engineering
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.
Example 1: Rapid point-of-care medical diagnostics
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.
Electrochemical µPADs versus conventional devices
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.
Fantastically flexible: The sensor chip design can be tailored to all kinds of point-of-care measurements
Dincer explains: “Our membranes can also serve as substrates for biomolecules such as nucleic acids, antibodies and enzymes. As a result, multi-level reaction chains can be triggered on the chip, each of which delivers electrochemically detectable signals, as in the case of H2O2.” For example, a chip prepared with the enzyme glucose oxidase produces H2O2 if the exhaled air contains glucose. This provides a bloodless and non-invasive way to measure blood sugar concentrations. The Dincer team is currently working on a different, polymer-based measuring system which detects microRNAs in blood serum. MicroRNAs are highly specific signal molecules for a wide range of conditions, such as certain types of cancer. To selectively capture only a single one of the many microRNAs, the Freiburg-based team uses a key mechanism known colloquially as “gene scissors” – or, more technically, as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR technology). “Our microsensor works without RNA replication and is five to ten times more sensitive than optical methods that use CRISPR/Cas. Better still, our method does not need any complicated laboratory equipment. In a few years’ time, it could be deployed in every doctor’s surgery as a rapid test for all kinds of diseases,” says Dincer, who is also currently working flat out to develop a microsensor that identifies Covid-19.
Example 2: Intelligent plugs for mechanical, plant and automotive engineering that predict their own failures
The mechanical, plant and automotive engineering industries need large quantities of electrical connectors such as plugs, terminals and cable lugs. The Fraunhofer EMFT Group around Dr. Frank Ansorge upgrades these vital components with microsensors and microelectronics. “The integrated microsystem technology doesn’t make the connectors larger or heavier, but it does make them intelligent,” Ansorge explains. “They can transmit energy or data and also handle diagnostic functions.” One such plug, for example, constantly checks the temperature around the contact point, as this can rise due to mechanical stresses and material fatigue. The built-in analytical electronics wirelessly report any such increase to a device that collects information about the current states of many connectors. Since degradation processes usually follow a typical temperature curve, a connector’s loss of quality can be spotted at an early stage. It can then be replaced before it fails completely.
The intelligent diagnostic plug monitors not only its own reliability but also that of connected devices. “We now fit the plugs with microsensors which also measure the flow of current, the contact force and other physical parameters,” Ansorge says. “That gives us information about the current status of motors, pumps, headlamps and other consumers.” Microsystems thus control the macrosystem. Since the effects being measured are small and occur in unadulterated form only in the immediate vicinity of the contact, using heavily miniaturized sensors, mounting technologies and connection technologies is of paramount importance.
Integration methods: Harnessing additive manufacturing processes to make complex diagnostic plugs
The methods used to integrate housings, contacts and microsystems differ as widely as the mountings and functions of the plugs themselves. Complex three-dimensional structures made of plastic, metal and semiconductors can be produced using 3D MID technology (MID stands for “moulded interconnect devices”), where injection moulding techniques successively form several layers of plastic, some of them metallised. Additive technologies such as 3D printing and selective laser melting are gaining in importance and can be used to build complex plugs in thin layers, step by step. “We have shown that plugs can do much more than merely establish contact,” Ansorge says. “Together with partners in industry, we now need to develop low-cost and sustainable methods of mass production.” His vision is what he calls the ‘bionic plug’. What that might look like, however, he is not yet telling.