Evaluation of a stable non-aqueous coupling medium for transcranial ultrasonic stimulation and sonogenetics

BACKGROUND: Transcranial ultrasonic stimulation (TUS) and sonogenetics enable non-invasive neuromodulation with high spatial precision. Their efficacy depends critically on the stability and acoustic performance of the coupling medium placed between the ultrasound transducer and the skull. Degassed water, although acoustically ideal, requires labor-intensive preparation and lacks long-term stability, limiting its practicality for routine use. NEW METHOD: We developed a stable non-aqueous coupling medium consisting of 45% polyurethane (PU) and 55% tetraethylene glycol dimethyl ether (TEGDME). Acoustic transmission rates were theoretically calculated and experimentally measured, and the applicability of the medium was evaluated in vivo using sonogenetics in mice, expressing the mechanosensitive ion channel MscL-M22S. RESULTS: The PU+TEGDME medium demonstrated practically usable transmission efficiency, along with excellent stability and resistance to bubble formation. In vivo experiments showed reliable ultrasound-evoked electromyogram responses in forelimb muscles and robust c-Fos expression in the targeted cortical region, confirming effective neuronal activation. COMPARISON WITH EXISTING METHODS: Degassed water showed the highest theoretical transmission; however, its preparation burden may limit uptake. In contrast, the PU+TEGDME medium provided robust transmission performance, while offering improved handling and reproducibility. CONCLUSIONS: The PU+TEGDME coupling medium provides a practical and effective alternative for TUS and sonogenetics, combining practical acoustic transmittance with operational stability. Its ease of preparation and reproducibility make it suitable for routine use in ultrasonic neuromodulation experiments.