Optically-generated focused ultrasound for noninvasive brain stimulation with ultrahigh precision

Understanding brain function and dysfunction necessitates neuromodulation techniques with ultrahigh precision. Current methods, such as electrical stimulation and optogenetics, have limitations in spatial precision. Electrical stimulation suffers from current leakage, limiting precise targeting, while optogenetics, though offering sub-cellular resolution, relies on viral transfection and has limited penetration depth and efficiency for transcranial applications. Transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS) provide only centimeter-level resolution. Transcranial focused ultrasound (tFUS) offers millimeter-level precision but is limited by its relatively low frequency, resulting in lower spatial resolution. High-frequency ultrasound shows promise for improved resolution but has not been demonstrated non-invasively at the desired 0.1 mm level. This study addresses this unmet need by developing optically-generated focused ultrasound (OFUS) for non-invasive neuromodulation with ultrahigh precision.

The literature extensively covers various brain stimulation techniques, highlighting their strengths and limitations in spatial resolution and invasiveness. Electrical stimulation, including deep brain stimulation, while clinically effective, suffers from spatial limitations due to current leakage. Optogenetics, though providing excellent cellular specificity, is invasive due to viral transfection requirements and suffers from limited light penetration depth and efficiency. Transcranial techniques like tDCS and TMS provide limited spatial resolution. tFUS offers better resolution than tDCS and TMS but is limited by the frequency-resolution trade-off. High-frequency ultrasound demonstrates promise but lacks non-invasive, ultrahigh-precision application. The optoacoustic effect, converting light into ultrasound, is explored as an alternative, with research focusing on materials like PDMS and various carbon-based absorbers to optimize light-to-sound conversion efficiency. Previous studies using fiber-based optoacoustic emitters achieve submillimeter resolution but require surgical implantation.

This study developed optically-generated focused ultrasound (OFUS) using a curved soft optoacoustic pad (SOAP) for non-invasive neuromodulation. SOAP was fabricated by embedding candle soot nanoparticles in a curved polydimethylsiloxane (PDMS) film. The curved design and choice of nanoparticles were optimized to achieve high numerical aperture (NA) for tighter spatial focusing and maximize focal pressure. Numerical simulations using the k-wave toolbox in MATLAB were used to optimize the SOAP geometry (radius and transverse diameter) to achieve desired lateral and axial resolution. Four different optoacoustic materials (heat shrink membrane (HSM), carbon nanotube-PDMS (CNT-PDMS), carbon nanoparticles-PDMS (C-PDMS), and carbon black-PDMS (CS-PDMS)) were tested to optimize optoacoustic conversion efficiency. The CS-PDMS combination proved most efficient. Scanning electron microscopy (SEM), stimulated Raman scattering (SRS), and photothermal imaging were used to characterize the CS-PDMS composite material and confirm the distribution of CS nanoparticles within the PDMS. A needle hydrophone was used to measure the generated ultrasound waveforms, validating the superior performance of CS-PDMS. In vitro and in vivo experiments using calcium imaging and electrophysiological recordings demonstrated the effectiveness and precision of OFUS neuromodulation in mouse motor cortex.

The study successfully demonstrated optically-generated focused ultrasound (OFUS) for non-invasive brain stimulation with an ultrahigh lateral resolution of 83 µm, a two-order-of-magnitude improvement over conventional transcranial focused ultrasound (tFUS). The optimized soft optoacoustic pad (SOAP) using candle soot nanoparticles in PDMS exhibited significantly improved optoacoustic conversion efficiency, generating ~48 MPa at the ultrasound focus with a 0.62 mJ/cm² laser input. In vitro experiments showed effective neuromodulation with a single ultrasound cycle. In vivo experiments demonstrated submillimeter transcranial stimulation of the mouse motor cortex. The required acoustic energy for OFUS neuromodulation was four orders of magnitude lower than that of tFUS. Calcium imaging and electrophysiological recordings confirmed the reliability and safety of OFUS stimulation. The developed OFUS technique significantly advances non-invasive neuromodulation capabilities.

The results demonstrate the successful development of OFUS, a non-invasive neuromodulation technique with unprecedented spatial precision. The ultrahigh resolution achieved by OFUS overcomes limitations of existing techniques, enabling targeted stimulation of small neuronal populations. The significantly reduced energy requirements compared to tFUS enhance the safety and efficacy of the method. The successful in vivo demonstration of submillimeter transcranial stimulation in the mouse motor cortex validates the translational potential of OFUS for neuroscience research and clinical applications. Future studies should explore the application of OFUS in various neurological disorders and investigate the potential for further enhancing spatial resolution and penetration depth.

This study introduces optically-generated focused ultrasound (OFUS) as a groundbreaking non-invasive neuromodulation technique with ultrahigh precision. The achieved submillimeter resolution, significantly lower energy requirements, and successful in vivo demonstration showcase its potential for advancing neuroscience research and treating neurological diseases. Future research should focus on exploring the therapeutic potential of OFUS in diverse neurological conditions and optimizing the technique for improved penetration depth and targeting capabilities.

The current study primarily focused on mouse models. Further research is needed to validate the effectiveness and safety of OFUS in larger animal models and ultimately in humans. The penetration depth achieved might be limited, requiring further optimization for deep brain stimulation applications. The long-term effects of OFUS stimulation also need to be thoroughly investigated.