Seeing the Sound—A Non-Invasive Acoustic Neural Interface Facilitated by Mechanoluminescent Nanomaterials

Understanding the complex neural circuitry and its correlation to specific behaviors requires spatially and temporally precise modulation of neuron subtypes in certain brain regions. For decades, neural stimulation has predominantly been achieved by traditional electrical stimulation electrodes. More recently, optogenetics, which utilizes visible light to achieve neural modulation by genetically expressing microbial opsins in mammalian cells, has gained great popularity due to its rapid and precise control of neural activities. However, owing to the limited tissue penetration of visible light, invasive craniotomy and intracranial implantation of tethered optical fibers are usually required for <i>in vivo</i> optogenetic modulation.<br/>Here, we report a new method termed ‘sono-optogenetics’, which provides non-invasive optogenetic neuromodulation in the brain without any scalp incision, craniotomy, or brain implant. Sono-optogenetics delivers nanoscopic light sources—mechanoluminescent nanoparticles—via the endogenous blood circulation, and provides millisecond-timescale switching of 470 nm light emission for optogenetic neuromodulation via brain-penetrant focused ultrasound. Furthermore, the mechanoluminescent nanoparticles could be recharged by an external photoexcitation light in superficial blood vessels during circulation, enabling repetitive through-scalp optogenetic stimulation and inducing behavioral responses in live mice. Unlike the conventional ‘outside-in’ approaches of optogenetics with fiber implantation, our method, which combines non-invasive ultrasound excitation and intravenous delivery of mechanoluminescent nanoparticles, provides an ‘inside-out’ approach to deliver nanoscopic light emitters via the intrinsic circulatory system and switch them on and off at any time and location of interest in the brain. <b>This work was published in PNAS (Wu, X. et al. Sono-optogenetics facilitated by a circulation-delivered rechargeable light source for minimally invasive optogenetics. <i>Proc. Natl Acad. Sci. USA</i> 116, 26332–26342 (2019)) and was awarded the Science and PINS Prize for Neuromodulation in 2020.</b><br/>To further optimize this acoustic neural interface and extend its utility, we developed a biomineral-inspired suppressed dissolution approach to synthesize multicolor mechanoluminescent nanoparticles with diameters down to 20 nm and bright emissions that cover the entire visible spectrum, including Sr₂MgSi₂O₇:Eu, Dy (470 nm), ZnS:Cu, Al (534 nm), ZnS:Mn (578 nm), and CaTiO₃:Pr (610 nm). We were able to, for the first time, visualize the ultrasound-mediated localized light emission produced by the nanoscopic light source in the vascular systems of multiple mouse organs, including kidney, liver, and brain. Specifically, upon non-invasive transcranial focused ultrasound stimulation, the mechanolumiescence generated inside the brain was sufficient to activate channelrhodopsin-2 (ChR2)-expressing neurons in live mice. <b>This work was recently published in JACS (Yang, F. and Wu, X. et al. Palette of rechargeable mechanoluminescent fluids produced by a biomineral-inspired suppressed dissolution approach. <i>J. Am. Chem. Soc.</i> 144, 18406–18418 (2022)).</b><br/>In summary, sono-optogenetics not only represents a non-invasive acoustic neural interface for neuromodulation, but also provides a unique tool of generating localized visible light inside highly scattering biological tissues for many optical biotechniques, such as fluorescence imaging, light-controlled gene-editing, and photodynamic therapy.