Bioadhesive Hydrogel-Coupled and Miniaturized Ultrasound Transducer System for Long-Term, Wearable Neuromodulation

Transcranial focused ultrasound has become a promising non-invasive approach for neuromodulation applications, particularly for neurodegenerative diseases and psychiatric illnesses. However, its implementation in wearable neuromodulation has thus far been limited due to the devices’ large size, which needs external supporting systems for the neuromodulation process. Furthermore, the need for ultrasound gel for acoustic coupling between the device and skin limits the viability for long-term use, due to its inherent susceptibility to dehydration and lack of adhesiveness to form a stable interface. Here, we report a wearable miniaturized ultrasound device with size comparable to standard EEG/ECG electrodes integrated with bioadhesive hydrogel to achieve efficient acoustic intensity upon ultrasound stimulation for long-term, wearable primary somatosensory cortical stimulation. Specifically, air-cavity Fresnel lens (ACFAL) based self-focusing acoustic transducer (SFAT) was fabricated using a lithography-free microfabrication process. Our transducer was able to achieve an acoustic intensity of up to 30.7 W/cm ² (1.92 MPa) in free-field with a focal depth of 10 mm. Bioadhesive hydrogel was developed to address the need for long-term stability of acoustic couplant for ultrasound application. The hydrogel demonstrated less than 13% attenuation in acoustic intensity and stable adhesion force of 0.961 N/cm over 35 days. Leveraging our bioadhesive hydrogel-integrated wearable ultrasound transducer, we were able to suppress somatosensory evoked potentials elicited by median nerve stimulation via functional electrical stimulation over 28 days, demonstrating the efficacy of our transducer for long-term, wearable neuromodulation in the brain.