Characterization of smart hydrogel-based ultrasound resonators for implantable sensing applications

Many biomedical sensing concepts for continuous monitoring of analytes rely on implanting electronic components inside the body to operate, which raises issues about long-term biocompatibility. In recent reports, an implantable sensing modality was reported in which ultrasound absorption in smart hydrogel resonators at a particular probing frequency is used to track the changes in ionic strength and glucose concentration of an analyte solution. This sensing concept allows the implanted component to be free from electronics, with corresponding possible advantages with respect to biocompatibility and lifetime of the device. However, an unsuitable probing frequency can undermine the received signal's quality from the implants or even entirely cause a signal loss. Here we present our work on creating an ultrasound characterization system and using it to determine optimum probing frequencies for the hydrogel resonator structures within a given frequency window. Furthermore, we demonstrate that the signal amplitude depends on the probing frequency's location relative to the frequency response peaks at a fixed dynamic range for swelling of smart hydrogels.