Photoacoustic Device Modulates Single Neurons with High Spatiotemporal Resolution

An instrument developed at Boston University (BU) could advance fundamental knowledge in the field of neuroscience and lead to treatments for neurological diseases. The tapered fiber optoacoustic emitter (TFOE) uses the optoacoustic effect to enable neuromodulation with single-cell precision. It provides high spatial resolution for ultrasound stimulation, generating an ultrasound field with a spatial precision of 39.6 µm. It also enables optoacoustic activation of single neurons and subcellular structures.

To advance spatial resolution and optoacoustic conversion efficiency, the research team, led by professors Chen Yang and Ji-Xin Cheng, used a new approach to fiber engineering and a new deposition method to build TFOE. The BU team previously developed a fiber-based optoacoustic converter that realized neural stimulation at submillimeter spatial resolution. However, the level of resolution was insufficient for targeting subtypes of neurons at single-cell level or subcellular structures. In addition, the device did not allow stable integration with patch clamp on the same cell being stimulated.

To build a device with greater precision and to integrate single-cell neuromodulation with single-cell electrophysiology recording, the researchers tapered fibers to a tip with a diameter as small as 20 μm, using a reproducible and controlled tapering strategy developed by the team. The researchers then developed a deposition method that produced a uniform and controllable coating thickness of about 10 µm on the 20-µm fiber tip.

Schematic of TFOE enabling single neuron stimulation. A 3-ns pulsed laser is delivered to TFOE to generate an acoustic signal via the photoacoustic effect (a). Fabrication of TFOE. The coating material is casted on a metal mesh followed by a punch-through method to coat the tapered fiber tip. Optical images show that TFOE has a tip with an overall diameter of 20 µm (b). Characterization of the pressure generated by TFOE and successful stimulation shown by calcium imaging (c). Acoustic pressure generated as a function of the distance from TFOE tip (left). TFOE-induced stimulation of GCaMP6f-expressing single neuron (middle left). TFOE selectively stimulates axon (red) and dendrites (yellow and green) of a multipolar neuron (middle right). TFOE integrated with whole-cell patch-clamp. Excitatory and inhibitory neurons are genetically coded in dark and red (right). Courtesy of Linli Shi, Ying Jiang, Fernando R. Fernandez, Guo Chen, Lu Lan, Heng-ye Man, John A. White, Ji-Xin Cheng, Chen Yang.

The researchers made TFOE’s optoacoustic conversion layer with carbon nanotubes with improved solubility, and embedded them in a thermo-expansive polydimethylsiloxane matrix. This made for highly efficient optoacoustic signal generation from the tapered fiber tip, increasing light-to-sound conversion efficiency. It also prevented light from leaking out of the thin coating.

The BU team demonstrated single-cell stimulations and subcellular stimulation of axons and dendrites using TFOE. The researchers also demonstrated the device’s high temporal resolution, showing that a single acoustic pulse with a submicrosecond duration was able to activate neurons.

TFOE generated a near-field acoustic wave that allowed optoacoustic stimulation to be integrated with patch-clamp recording on single neurons — the “gold standard” for high-fidelity analysis of the biophysical mechanisms of neuromodulation, and a challenge for traditional ultrasound to achieve. The researchers further found that coupling TFOE with ex vivo brain slice electrophysiology revealed cell-type-specific responses to acoustic stimulation.

The nongenetic neural stimulation technique could serve as a platform technology for high-precision stimulation of the neural system and investigations into the mechanisms of ultrasound neural stimulation. It could potentially be used as a surgical tool for precise stimulation of a single nerve. Its use on humans is not limited, like optogenetics’ use, by the need for viral transfection. The device’s demonstrated spatial resolution of 39.6 µm has no precedent, to the best of the researchers’ knowledge. Spatial resolution can be tuned by changing the diameter of the fiber.

Successful TFOE stimulation was achieved with a single laser pulse of 3 ns, generating a submicrosecond acoustic pulse that the researchers believe to be the shortest duration of acoustic stimuli for successful neuromodulation on record.

TFOE, which has no metal components, is immune to electromagnetic interference and is compatible with functional magnetic resonance imaging (fMRI). Given the increasing popularity of ultrasound neuromodulation, the research team forecasts that TFOE, which is compact and cost-effective, could help increase the number of opportunities to apply the optoacoustic effect in the field of neuroscience.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-021-00580-z).