Optoacoustic Neuromodulation Offers a New Modality

by James Cavuoto, editor

April 2022 issue

In recent years, neural engineering researchers have developed novel methods of neuromodulation that exploit optical or optogenetic forms of stimulation, while other have pursued ultrasound approaches. A team at Boston University is now exploring optoacoustic technology as a new neuromodulation modality in an effort to combine the best attributes of both technologies.

The optoacoustic process involves pulses of light that are converted into acoustic waves at ultrasonic frequencies. Optoacoustic technologies have been used before in imaging of biological structures—ranging from subcellular structures to organs, and even whole animals. But the BU team is making use of optoacoustics for high-precision neuromodulation to exploit the high penetration depth of ultrasound, plus the high spatial precision of photonic stimulation.

By repurposing an optoacoustic emitter originally developed for surgical guidance, the team developed the first example of optoacoustic neurostimulation. More recently, they created a new type of emitter that extends precision to the single neuron and subcellular level. In a primer published in Neurophotonics, they discuss device design considerations, potential mechanisms, and barriers to the uptake of optoacoustics as a viable neuromodulation tool. They also offer their perspective on future directions in fundamental and translational research for the field.

Optoacoustic neuromodulation could avoid problems associated with electrical stimulation and achieve better precision to reach finer nerve fibers. Some of the novel optoacoustic techniques surveyed in the primer include fiber-based neurostimulation, biocompatible films, and nanoparticle mediation. Multiplexed emitters in the form of fiber or film-based arrays may potentially be used for stimulation, for instance, to retinal ganglion cells for visual neuroprosthetic devices.

Highly miniaturized fiber-based optoacoustic emitters could also deliver drugs into cellular membranes, or be integrated into medical devices, such as catheters and needles, to provide real-time surgical guidance.

Complementary with fiber-based optoacoustic emitter devices, biocompatible optoacoustic films may serve as a new neural interface, offering multiple functions, from optoacoustic stimulation to structural support and growth guidance. As a platform for bioelectronics and basis for tissue scaffolding, biocompatible optoacoustic films have been shown to enhance regeneration effects in bone engineering. They have also proven capable of promoting neural regeneration, by increasing the secretion of brain-derived neurotrophic factors. As a light-mediated technique, the optoacoustic scaffold eliminates the requirements of wire connections and genetic modifications.

The vista is enticing, yet work remains to be done. According to senior author Ji-xin Cheng, Theodore Moustakas chair professor in photonics and optoelectronics at Boston University, “Overall, optoacoustic neuromodulation poses a number of advantages over its ultrasonic counterpart, including higher spatial temporal resolution, minimal thermal accumulation, and broad bandwidth, which make it suitable for region-specific modulation in animal models and even in human patients. On the other hand, optoacoustic neuromodulation is still at an early stage of development, and there are several challenges to be addressed by future studies.”


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