High-precision neural stimulation by a highly efficient candle soot fiber optoacoustic emitter

Abstract

Highly precise neuromodulation with a high efficacy poses great importance in neuroscience. Here we developed a candle soot fiber optoacoustic emitter (CSFOE), capable of generating a high pressure of over 10 MPa with a central frequency of 12.8 MHz, enabling highly efficient neuromodulation in vitro. The design of the fiber optoacoustic emitter, including the choice of the material and the thickness of the layered structure, was optimized in both simulations and experiments. The optoacoustic conversion efficiency of the optimized CSFOE was found to be 10 times higher than the other carbon-based fiber optoacoustic emitters. Driven by a single laser, the CSFOE can perform dual-site optoacoustic activation of neurons, confirmed by calcium (Ca²⁺) imaging. Our work opens potential avenues for more complex and programmed control in neural circuits using a simple design for multisite neuromodulation in vivo.

Keywords: fiber; neural modulation; neural stimulation; optoacoustic; optoacoustic conversion efficiency; optoacoustic stimulation; photoacoustic.

FIGURE 1

COMSOL simulation of CSFOE performance. (A) Schematic of CSFOE. (B) Illustration of the CSFOE model used in simulation. Not to scale. (C) Representative ultrasound waveform, simulated at t = 400 ns under an input of a 3 ns pulsed laser. (D) Acoustic waveforms simulated at different thicknesses of the CS layer. (E) Peak-to-peak acoustic pressure plotted as a function of candle soot layer thickness.

FIGURE 2

Fabrication and characterization of CSFOE. (A) Key steps of CSFOE fabrication. (Left) Candle soot deposition on an optical fiber tip. (Middle) PDMS coating on the surface of the CS layer using a nanoinjector. (Right) Images of samples after CS deposition and PDMS coating, respectively. Scale bars: 200 μm. (B) Thickness of the CS layer obtained as a function of deposition duration. Inset: representative image of a fiber coated with candle soot. (C) Transmission ratio plotted as a function of the thickness of CS layer. (D) Schematic of experimental configuration of photoacoustic signal measurement using a 40 μm needle hydrophone. Created with BioRender.com. (E) Acoustic signal of CSFOE as a function of the candle soot layer thickness detected by the hydrophone. Laser condition: 1,030 nm, 1.7 kHz repetition rate, 46 μJ per pulse. (F) Peak to peak pressure plotted as a function of the thickness of CS under the same laser condition as (E). (G) Representative photoacoustic waveform (Black) detected by the hydrophone and its FFT frequency spectrum (Red). laser condition: 1,030 nm, 56 μJ pulse energy. (H,I) Acoustic signal and peak-to-peak pressure generated by CSFOE detected at different distances from the CSFOE tip. Each data point was an average of three trials. laser condition: 1,030 nm, 56 μJ pulse energy, distance between hydrophone and CSFOE: 70 μm (J) Photoacoustic signal propagation in the medium detected by a linear transducer array. Fiber tip (Yellow), PA waveform (red). (K,L) Photoacoustic waveforms and peak-to-peak pressures measured at different laser pulse input. Each data point was an average of three trials, distance between hydrophone and CSFOE: 70 μm.

FIGURE 3

Activation of GCaMP6f-expressing cortical neurons by CSFOE stimulation. (A,B) Representative fluorescence of neurons from three batches stimulated by CSFOE before stimulation (A) and after stimulation (B). (C) Map of the maximum fluorescence change ΔF/F₀ induced by the CSFOE stimulation. Laser condition: 3 ms duration, pulse energy 65μJ. Scale bar: 200 μm. (D) Calcium trace shows repeatable activation of the same neuron. Laser condition: 3 ms duration, pulse energy 56 μJ. (E–G) Colormaps of fluorescence change in neurons stimulated by CSFOE with a laser pulse energy of 65 μJ (E) (N = 20), 56 μJ (F) (N = 10), and 46 μJ (G) (N = 10). (H) Average calcium traces of neurons obtained from (E,F,G) with the pulse energy of 65μJ (Red) (N = 20), 56 μJ(Blue)(N = 10) and 46 μJ (black) (N = 10), respectively. The shaded region corresponds to one standard deviation. Laser turns on at t = 5 s (red dashed lines). The duration of each stimulation was fixed at 3 ms. (I) Average of maximum fluorescence intensity changes shown in (E–G). Error bars represent standard deviation. ***p < 0.001, one way AVOVA test. (J) Average calcium traces of neurons of CSFOE stimulation, Laser only control and TTX control. Each group was repeated three times on three different dishes of neurons (N = 10 for each condition). (K) Colormaps of fluorescence change in neurons of a laser only control group. (L) Average calcium traces with (Red) (N = 30) and without (Blue) synaptic blocker (N = 30). Laser is on at t = 5 s. laser condition: 1,030 nm, 1.7 kHz repetition rate, 65 μJ pulse energy (M) Time constant of the decay of the fluorescence trace with and without synaptic blockers. ***p < 0.001, two sample t-test. (N) Spatial distribution of maximum neuronal calcium response induced by CSFOE stimulation. Scale bar: 200 μm. Dashed circle: indication of the CSFOE position. (O) Maximum ΔF/F₀ changes over the distance away from the CSFOE (N = 6 for 0–50 μm group, N = 4 for 50–100 μm group, N = 5 for 100–150 μm group, N = 7 for > 150 μm group).

FIGURE 4

Comparison of CSFOE and FOC. (A) Schematics of the CSFOE and FOC. (B) Photoacoustic signal of CSFOE and FOC, measured by a 5 MHz transducer under the same laser condition: 1,030 nm, 3 ns, 1.7 kHz, 48 mW. (C) Temperature rise measured by a thermal probe placed at the surface of CSFOE and FOC, respectively. (D) Representative calcium traces of GCaMP6f transfected neurons stimulated by CSFOE (Blue) and FOC (Red) under the same laser energy input of 56 μJ. (E) Temperature rise measured by the thermal probe placed at ∼10 μm away from CSFOE under the laser energy used in neuromodulation experiments. Red vertical bars indicate the laser on. Laser condition: 1,030 nm, 1.7 kHz repetition rate, 3 ms duration for each burst. Laser pulse energy is shown in the figure. (F) Measured maximum temperature increases at corresponding pulse energy inputs (N = 5 for each energy).

FIGURE 5

Dual site neuron stimulation by CSFOE. (A) Schematic of dual site stimulation using two CSFOEs with a fiber splitter. Created with BioRender.com. (B) Map of the max △F/F₀ image of two sites of neurons stimulated by two CSFOE. (C) Colormaps of fluorescence changes in neurons at two sites stimulated by CSFOE. (D) Representative calcium traces from traces shown in (C) from neurons at site 1 (Red) and site 2 (Black).