Influence of radiant exposure and repetition rate in infrared neural stimulation with near-infrared lasers

Advances in the Regulation of Neural Function by Infrared Light

In recent years, with the rapid development of optical technology, infrared light has been increasingly used in biomedical fields. Research has shown that infrared light could play roles in light stimulation and biological regulation. Infrared light has been used to regulate neural function due to its high spatial resolution, safety and neural sensitivity and has been considered a useful method to replace traditional neural regulation approaches. Infrared neuromodulation methods have been used for neural activation, central nervous system disorder treatment and cognitive enhancement. Research on the regulation of neural function by infrared light stimulation began only recently, and the underlying mechanism remains unclear. This article reviews the characteristics of infrared light, the advantages and disadvantages of infrared neuromodulation, its effects on improving individual health, and its mechanism. This article aims to provide a reference for future research on the use of infrared neural regulation to treat neuropsychological disorders.

Emerging trends in the development of flexible optrode arrays for electrophysiology

Optical-electrode (optrode) arrays use light to modulate excitable biological tissues and/or transduce bioelectrical signals into the optical domain. Light offers several advantages over electrical wiring, including the ability to encode multiple data channels within a single beam. This approach is at the forefront of innovation aimed at increasing spatial resolution and channel count in multichannel electrophysiology systems. This review presents an overview of devices and material systems that utilize light for electrophysiology recording and stimulation. The work focuses on the current and emerging methods and their applications, and provides a detailed discussion of the design and fabrication of flexible arrayed devices. Optrode arrays feature components non-existent in conventional multi-electrode arrays, such as waveguides, optical circuitry, light-emitting diodes, and optoelectronic and light-sensitive functional materials, packaged in planar, penetrating, or endoscopic forms. Often these are combined with dielectric and conductive structures and, less frequently, with multi-functional sensors. While creating flexible optrode arrays is feasible and necessary to minimize tissue-device mechanical mismatch, key factors must be considered for regulatory approval and clinical use. These include the biocompatibility of optical and photonic components. Additionally, material selection should match the operating wavelength of the specific electrophysiology application, minimizing light scattering and optical losses under physiologically induced stresses and strains. Flexible and soft variants of traditionally rigid photonic circuitry for passive optical multiplexing should be developed to advance the field. We evaluate fabrication techniques against these requirements. We foresee a future whereby established telecommunications techniques are engineered into flexible optrode arrays to enable unprecedented large-scale high-resolution electrophysiology systems.

Heat accumulation during infrared stimulation impacts the response of ex vivo rat sciatic nerve

Infrared neural stimulation (INS) is a neuromodulation technique that involves short optical pulses delivered to the neural tissue, resulting in the initiation of action potentials. In this work, we studied the compound neural action potentials (CNAP) generated by INS in five ex vivo sciatic nerves. A 1470 nm laser emitting a sequence of 0.4 ms light pulses with a peak power of 10 W was used. A single 4 mJ stimulus is not capable of eliciting a nerve response. However, repetition of the optical stimuli resulted in the induction of CNAPs. Heat accumulation induced by repetition rates as high as 10 Hz may be involved in the increase in CNAP amplitude. This sensitization effect may help to reduce the pulse energy required to evoke CNAP. In addition, these results highlight the importance of investigating the role of the slow nerve temperature dynamics in INS.

Optical neural stimulation using the thermoplasmonic effect of gold nano-hexagon

The use of nanoparticle photothermal effect as adjuvants in neuromodulation has recently received much attention, with many open questions about new nanostructures' effect on the action potential. The photothermal properties of hexagonal gold nanoparticles are investigated in this work, including the absorption peak wavelength and light-heat conversion rate, using both experimental and simulation methods. Furthermore, the ability to use these nanostructures in axonal neural stimulation and cardiac stimulation by measuring temperature changes of gold nano-hexagons under 532 nm laser irradiation is studied. In addition, their thermal effect on neural responses is investigated by modeling small-diameter unmyelinated axons and heart pacemaker cells. The results show that the increase in temperature caused by these nano-hexagons can successfully stimulate the small diameter axon and produce an action potential. Experiments have also demonstrated that the heat created by gold nano-hexagons affects toad cardiac rhythm and increases T wave amplitude. An increase in T wave amplitude on toad heart rhythm shows the thermal effect of nano hexagons heat on heart pacemaker cells and intracellular ion flows. This work demonstrates the feasibility of utilizing these nanostructures to create portable and compact medical devices, such as optical pacemakers or cardiac stimulation.

Blind Localization of Heating in Neural Tissues Induced by a Train of the Infrared Pulse Laser

Introduction: Recently, infrared lasers (wavelengths larger than 1100 nm) have been applied to stimulate neural tissues. Infrared neural stimulation (INS) has some advantages over conventional electric stimulation, including contact-free delivery, spatial precision, and lack of stimulation artifacts. In this study and based on a photothermal mechanism, we applied the heat diffusion equation to study temperature variation of a biological phantom during INS. In addition, the impact of laser parameters on spatially localized heating induced by 2 different infrared wavelengths were studied. Methods: We studied the localization of INS inside a phantom similar to cortical neural tissue. First, we analytically solved the heat diffusion equation to study the distribution of temperature inside this phantom. Then, the accuracy of analytical results was verified by heating the phantom using amplitude-modulated infrared lasers (lambda= 1450 and 1500 nm, the energy between 2 and 5 mJ and pulse duration up to 20 ms). The laser light was directed to sample by a multimode optical fiber (NA=0.22, core size= 200 microns). Finally, the impacts of laser properties on the spatial resolution of infrared heating were discerned. Results: In order to verify analytical results, we measured the maximum temperatures of the phantom during illumination of lasers and compared them with analytical results. The analytical results were in agreement with the experimental results. The effects of laser beam properties such as pulse duration, energy and repetition rate frequency on the spatial resolution were investigated. The results indicated that the spatial resolution of INS can be smaller than one millimeter. Conclusion: Here, the influences of laser properties on the localization of INS inside a biological phantom were studied. These results can be applied to improve the spatial selectivity of the peripheral nerve interface.