N. Farhoudi, H.-Y. Leu, J. Magda, F. Solzbacher, C.F. Reiche
University of Utah,
Keywords: implantable, sensor, ultrasound, hydrogel
Summary:Hydrogels consist of a cross-linked network of hydrophilic polymers and they can contain absorb water more than 90% of their dry weight . Through the addition of specific functional groups into the hydrogel structure, the hydrogel becomes “smart” . In other words, it can undergo a reversible volume change in response to an environmental change in concentration of a specific biomedical analyte  Over the past decades, smart hydrogels have been used as a transduction element in numerous sensing schemes . Nonetheless, these sensors often need active electronic elements in their sensing component or wire/optical fiber connections, which hinders their use as implantable sensors. Here we introduce a sensing approach  that employs the resonance absorption of ultrasound waves in an array of micro-fabricated hydrogel resonator pillars. If the frequency of the ultrasound matches a mechanical resonant frequency of the hydrogel resonators array, the ultrasound waves are absorbed by the structure. Otherwise, if there is no frequency match, most of the waves pass through the hydrogel structure with minimum attenuation. The resonance frequency of the pillars depends on the swelling state of the hydrogel. Therefore, tracking the resonance frequency can give information about the changes in the environmental concentration of the biomedical analyte the hydrogel is sensitive to. The proposed sensor doesn’t require the implantation of an active component inside the tissue and is free from wire connections, which makes it a good candidate for in vivo sensing. Additionally, the readout is based on ultrasound which is harmless to living tissue and has the potential to be integrated with widely available medical ultrasound imaging equipment. Furthermore, the sensing principle is applicable to any smart hydrogel which makes it extremely versatile. The presented method for fabricating these sensors is also simple and cost-effective. These advantages together with the promising results obtained from finite element simulations (figure 1) and proof-of-principle experiments (figure 2) indicate a great potential of corresponding devices for implantable applications. Florian Solzbacher declares financial interest in Blackrock LCC and Sentiomed, Inc. Jules Magda declares financial interest in Applied Biosensors LLC. Funded by the Joe W. and Dorothy Dorsett Brown and the Olive Tupper Foundations.