Energy-autonomous wireless piezoelectric MEMS sensors and actuators for medical technology and industry 4.0 - E-PISA

Microelectromechanical systems (MEMS) have become elementary components of our everyday life: As sensors in consumer electronics and medical technology, and as sensors and actuators in industry. Their influence ranges from the detection of acoustic and mechanical vibrations to the detection of motion, temperature and humidity measurement, from gas analytics to micro-optical, medical technology products. Industry roadmaps predict exponential growth to as many as one trillion sensors per year by 2021, driven by IoT (Internet-of-Things) development. Especially for applications in medical diagnostics, "smart systems" will be needed in the future that can be manufactured in an energy-autonomous, wireless and highly miniaturized manner. Examples are catheter-based pressure measurements or endoscope-based imaging techniques for direct (in-vivo) application in the human body. The aim of the work of this research group is to develop solutions for these tasks and to verify them on the basis of two demonstrators which, compared to the state of the art, enable highly miniaturized, non-invasive or minimally invasive diagnostics and thus greatly reduce the burden on the patient.
The in-vivo pressure transducer is a demonstrator for a purely optically driven MEMS pressure sensor. Using a membrane with AlN actuator, it is demonstrated that it is possible to implement zero-point control with optical power supply and with optical position detection. In perspective, such a system can become smaller than 500 µm and thus allow access to the finest blood vessels. In contrast to previous technologies, the electrical connections are omitted for the catheter line), which means that a strong miniaturization of the sensor and MRI suitability can be realized.
An optical MEMS (MOEMS) has emerged that enables endoscopic optical coherent tomography. The method is suitable to analyze tissue layers in their depth non-invasively and to partially replace diagnostics based on endoscopic biopsies. The highly miniaturized system requires constant control of the imaging mechanical-motion elements. To implement this control with low noise, it is necessary to integrate a highly sensitive position detection directly with the MEMS.
There is also a great need for energy-autonomous sensors for industrial applications, for example in the condition monitoring of equipment, but also new functional materials such as fiber composites. The task here is to detect movements, vibrations and damage, e.g. due to cracks in the material, combined with the generation of vibration signals (so-called acoustic emission (AE)) without constantly consuming energy. A third demonstrator was created for this purpose:
Industrial Monitoring Inertial Sensor for Shock and Vibration Detection. This generates an intrinsic charge when a mechanical event occurs and can thus be used in an extremely energy-efficient manner for stand-alone sensor nodes. Unlike existing systems, the piezoelectric inertial sensor requires energy only when a measurement is made. The quiescent current of the system is minimized. The battery life and thus the maintenance intervals of a self-sufficient sensor node can thus be maximized.
All three demonstrators are based on the piezoelectric transducer principle. This allows both sensory and actuator functionalities to be generated. Compared to the established capacitive MEMS, they are characterized by a more compact size and the possibility of intrinsic charge generation of the sensory component. The use of AlN as the piezoelectrically active transducer material provides a CMOS-compatible, environmentally friendly alternative in contrast to the use of established materials such as lead zirconate titanate (PZT) and polyvinyl fluoride (PVDF). An additional increase in the sensitivity of the sensing component results from the combination (monolithic integration) of carbon nanotube (CNT)-based transistors as amplifier elements in close proximity (very short signal paths) to the electromechanical transducer elements.Abschlussbericht des Projektes E-PISA (PDF)

Fig 1: Actuated 2D MOEMS on wafer level

Fig 2: Impression of a MOEMS next to a ladybug

Fig 3: Photography of a diced 2D MOEMS with integrated sensor elements

Fig 4: Waferlevel photography of round sensors with diameter of 300 µm to 1000 µm for catheter based pressure sensing