B. Ahmed, C.F. Reiche, F. Solzbacher, J. Körner
Leibniz University Hannover,
Keywords: smart hydrogel, biomedical sensor, power transfer
Summary:Smart, i.e. stimulus-responsive, hydrogels are a promising choice for sensing elements in biomedical contexts due to their versatility in material composition and properties. They can be tailored to selectively undergo a volume phase transition in response to a target analyte or other physical/chemical stimulus. Within a certain range, the material’s (de)swelling is proportional to the stimulus strength and can be used for quantification thereof if properly calibrated . Despite these promising properties, only very few sensors based on smart hydrogels are available yet. The main challenge is the development of suitable transduction concepts for the swelling state as these have to fulfil a comprehensive set of requirements, including: high sensitivity (for small volume changes), robustness with respect to cross-sensitivities, unrestricted and unaltered stimulus interaction with the hydrogel, potential for miniaturization and, specifically for biomedical applications, biocompatibility of the complete sensor setup. Based on our previous work of a mechanical bending sensor concept [2,3] we have developed a much more stable, robust and reproducible approach based on modulated power transfer which we are presenting here. The sensor consists of two polyimide sensor sheets with embedded metal strip lines. They are placed in parallel with an aluminum shield in between, except for the tip region. In this part, a smart hydrogel is sandwiched between the sheets. One sensor sheet is powered with a sinusoidal voltage at the resonance frequency of the embedded metal trace. At the second sensor sheet, the received voltage signal is measured as the output signal. Since the sensor sheets are electromagnetically insulated and shielded from each other except at the tip region where the hydrogel is sitting, a volume change of the hydrogel alters the distance between the sensor sheets and hence, the amount of transferred electromagnetic energy. Two effects occur in this setup: (i) the hydrogel bends the sensor sheets and therefore causes a shift of their respective impedances and (ii) the distance between the sheets is altered, changing the amount of capacitive and inductive coupling and therefore the transferred energy. These effects form a complex interplay, resulting in a very sensitive yet reproducible and robust sensor signal. The measured output voltage can be directly linked to the amount of the hydrogel’s volume change and consequently, the stimulus concentration. We have conducted a series of experiments with a glucose sensitive hydrogel that included repeated cycling, and step tests for medically relevant glucose concentrations (5 mMol to 20 mMol) in PBS buffer. All tested sensors exhibited very stable and reproducible results for many cycles and repetitions. Our current target application is the integration of the sensor concept into a catheter and future work will include further miniaturization and the development of a wireless readout.  F. Pinelli et al., Mater. Today Chem. 17:100317, 2020  J. Koerner et al., TechConnect Briefs 3:206, 2018  B. Ahmed et al., Sens. Actuator A Phys. 347:113954, 2022 Florian Solzbacher declares financial intrest in Sentiomed, Inc. and Blackrock Neurotech managed by the University of Utah’s conflict of interest management.