Transduction of smart hydrogel swelling state by advanced flexible CNT-based MEMS capacitors on a membrane platform (HydroMEMS)

• Applicants: Dr.-Ing. Sascha Hermann; Professorin Dr.-Ing. Julia Körner
• Subject Area: Measurement Systems ; Solid State and Surface Chemistry, Material Synthesis ; Microsystems ; Polymer Materials
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 548242085

Smart hydrogels are a specific class of three-dimensional polymer networks prepared by physical or chemical cross-linking of hydrophilic molecules, which are capable of responding to an external stimulus with a volume-phase-transition. The responsiveness can be tailored for a wide variety of physical and chemical stimuli, e.g. temperature, humidity, organic analytes and biochemical compounds. This, in combination with a broad variety of possible compositions and an easily achievable biocompatibility makes smart hydrogels very interesting candidates for sensing elements, specifically in the biomedical context. However, so far the smartness is rarely harnessed in a sensor context as there is a lack of suitable transduction concepts for reliable and robust detection of the hydrogel’s swelling state and changes thereof. Key challenges include the potential sensitivity and measuring range, biocompatibility and potential of encapsulation of transducer for use in harsh environments and mitigation of cross-sensitivities and other disturbances associated with the sensing material. This project aims at exploring a novel miniaturizable and energy efficient smart sensor concept based on an electromechanical transduction principle for swelling state detection. It relies on self-forming flexible microstructures created by vertically aligned carbon nanotube (VACNT) pillar electrodes whose resistive/capacitive properties are altered by deformation induced swelling of the smart hydrogel. Furthermore, sensing element (hydrogel) and transducer (CNT electrodes) are physically separated by a biocompatible passivation layer. Within the project, fundamental studies on the underlying physics will be carried out through simulations as well as comprehensive experiments, to gain insights into the interplay between hydrogel swelling, resulting force distribution and transducer output signal. We will investigate the potential of this sensor approach by studying different transducer / hydrogel configurations for linear as well as switch-type behavior, the latter being achieved by integration of defined air gaps within the CNT structures which goes beyond the current technological state of the art for VACNTs. Our target demonstrator application for evaluation of sensor properties and concept feasibility is volatile organic compound detection in a gaseous environment. Furthermore, we will study degradation effects and evaluate long-term performance as well as develop robust operating regimes of the closed loop sensor concept.

• DFG Programme: Research Grants