OPE Journal

RESEARCH 38 No 35 | May 2021 | OPE journal Stretchable, multi-modal sensors for vital data monitoring and healthcare applications An article written by Sina Martin, Sebastian Reitelshöfer and Jörg Franke from FAU Erlangen-Nürnberg (Erlangen, Germany) The growing interest in flexible electron - ics also represents a trend in printed organic electronics, as flexible circuits are typically small and lightweight and allow better integra- tion into a variety of applications. One of the most promising areas is wearables, lifestyle and healthcare products, where sensors and circuits must adapt their shape to the curved and con- stantly changing shape of the human body. For optimal signal acquisition, it is critical that the sensor is worn close to the body and that deformations accompany body movement. Such flexible sensor arrays are also the focus of various research projects such as “Smart- NIV” at TU Braunschweig, which monitors the breathing of new-borns.Another example is dehydration monitoring, which is being inves- tigated as part of the “SeLe” project at FAU Erlangen. This new generation of sensor cir- cuits is designed to be flexible and even allows slight stretching by morphological adaptation of the conductive elements, but is not yet as flexible and stretchable as human skin. More than flexible Dielectric elastomers (DE) overcome this prob- lem because the active area is fully stretchable by more than 100% of its original length while maintaining electrical conductivity. In addition, they offer high reliability and can withstand a high number of load cycles while returning to their original state. These sensors enable the detection of various mechanical loads, such as stress, pressure and shear forces, and can be used for non-contact proximity measurement. The elastomeric material composition allows them to conform to any shape and provides optimal adaptation to human skin. Together with the thin layer geometry, this allows easy integration into various applications such as textiles. Working principle of dielectric elastomers (DE) DE belong to the group of smart materials that can adjust their geometry according to an applied electric field. Since their structure corre - sponds to that of parallel plate capacitors, their dimension is proportionally linked to the meas- urable capacitance. Because of this behaviour, they can be used to measure different forces in the deformed state. In addition to capacitive measurement, resistive approaches can also be used to measure electrode resistance, which increases with greater deformation. The system is based on elastomers such as polyurethanes or silicones, which act as an insulating dielectric between two flexible, conductive layers. Suitable materials include carbon and silver particles, which have differ- ent properties in terms of flexibility and con - ductivity. These conductive particles can be processed with solvents or encapsulated in a polymer matrix to increase flexibility and make the system more resistant to delamination. Manufacturing techniques for DE Various processes are used for the additive production of alternating layers of DE. On the one hand, screen printing or roll-to-roll tech- niques enable the production of large and thin layers. On the other hand, digital printing techniques such as inkjet, aerosol jet or direct ink writing allow easy shape customisation for individual products and a non-contact manufacturing method for printing on curved surfaces of different materials. At the Institute for Factory Automation and Production Systems (FAPS) located at the FAU Erlangen, we modified an aerosol jet printing process for automated stacking of thin, alternating layers of conductive and insulating materials. The materials, namely the two separate components of RTV-2 silicone and graphene ink, are processed separately. Through a collision nebuliser, the material is converted into a gas droplet mixture by pneumatic aerosol generation. Additionally, the particular ink is homogenised during the process by ultrasound that also creates ink droplets, resulting in a hybrid aerosol genera- tion principle. The aerosol then passes through the virtual impactor, where a negative gas flow reduces the amount of gas and smaller droplets with lower kinetic energy, resulting in higher density and a more homogeneous aerosol. In the final process step, the silicone components are combined, which is the start- ing point of polymerisation. Under moderate heat, the materials are deposited on the surface of the substrate and cure immediately. Fig 1: Schematic drawing of a dielectric elastomer sensor that can, among others, detect stretch and pressure loads