Focal neurostimulation of propagating calcium waves in human neural cells using megahertz-range single-pulse focused ultrasound

Focused ultrasound (FUS) offers a promising neurostimulation technique that addresses the trade-off between invasiveness and spatial selectivity found in conventional electromagnetic-based methods. Evidence supporting its theoretical selectivity and comprehensive descriptions of the underlying biophysical mechanisms remain insufficient to ensure adequate control and safety. Based on observations of causal, focal, and propagating calcium waves in human neural cells in vitro evoked using single-pulse megahertz-range FUS, a transdural FUS approach relying on an intracranial implant is proposed to overcome frequency limitations associated with transcranial ultrasound. Using an extended parameter space (0.7 -8 MHz), calcium waves were shown to be selectively elicited by cavitation or acoustic radiation force. Cavitation, predominant at frequencies under 5 MHz, induced dispersed and unpredictable responses that could lead to cell damage. FUS radiation force however, predominant at frequencies over 5 MHz, elicited focal and predictable responses, without affecting cell viability. FUS-evoked intercellular calcium waves were shown to propagate across the surrounding neural network via intracellular and extracellular pathways driven by calcium amplification mechanisms. Collectively, these findings lay the foundation for developing wearable neurostimulation approaches to manage chronic neurological conditions.