Identifying optimal parameters for infrared neural stimulation in the peripheral nervous system

Infrared neuroglial modulation of spinal locomotor networks

Infrared neural stimulation (INS) emerges as a promising tool for stimulating the nervous system by its high spatial precision and absence of the use of exogenous agents into the tissue, which led to the first successful proof of concept in human brain. While neural networks have been the focal point of INS research, this technique is also non cell type specific as it triggers activity in non electrically excitable cells. Despite increasing interest, there remains to demonstrate well defined simultaneous astrocytic and neuronal signals in response to INS. Using calcium imaging, we show that INS has the capacity to initiate calcium signaling in both astrocytes and neurons simultaneously from the rostral lumbar spinal cord, each exhibiting distinct temporal and amplitude characteristics. Importantly, the mechanism underlying infrared-induced neuronal and astrocytic calcium signaling differ, with neuronal activity relying on sodium channels, whereas induced astrocytic signaling is predominantly influenced by extracellular calcium and TRPV4 channels. Furthermore, our findings demonstrate the frequency shift of neuronal calcium oscillations through infrared stimulation. By deepening our understanding in INS fundamentals, this technique holds great promise for advancing neuroscience, deepening our understanding of pathologies, and potentially paving the way for future clinical applications.

Quantifying tissue temperature changes induced by infrared neural stimulation: numerical simulation and MR thermometry

Infrared neural stimulation (INS) delivered via short pulse trains is an innovative tool that has potential for us use for studying brain function and circuitry, brain machine interface, and clinical use. The prevailing mechanism for INS involves the conversion of light energy into thermal transients, leading to neuronal membrane depolarization. Due to the potential risks of thermal damage, it is crucial to ensure that the resulting local temperature increases are within non-damaging limits for brain tissues. Previous studies have estimated damage thresholds using histological methods and have modeled thermal effects based on peripheral nerves. However, additional quantitative measurements and modeling studies are needed for the central nervous system. Here, we performed 7 T MRI thermometry on ex vivo rat brains following the delivery of infrared pulse trains at five different intensities from 0.1-1.0 J/cm2 (each pulse train 1,875 nm, 25 us/pulse, 200 Hz, 0.5 s duration, delivered through 200 µm fiber). Additionally, we utilized the General BioHeat Transfer Model (GBHTM) to simulate local temperature changes in perfused brain tissues while delivering these laser energies to tissue (with optical parameters of human skin) via three different sizes of optical fibers at five energy intensities. The simulation results clearly demonstrate that a 0.5 second INS pulse train induces an increase followed by an immediate drop in temperature at stimulation offset. The delivery of multiple pulse trains with 2.5 s interstimulus interval (ISI) leads to rising temperatures that plateau. Both thermometry and modeling results show that, using parameters that are commonly used in biological applications (200 µm diameter fiber, 0.1-1.0 J/cm2), the final temperature increase at the end of the 60 sec stimuli duration does not exceed 1°C with stimulation values of 0.1-0.5 J/cm2 and does not exceed 2°C with stimulation values of up to 1.0 J/cm2. Thus, the maximum temperature rise is consistent with the thermal damage threshold reported in previous studies. This study provides a quantitative evaluation of the temperature changes induced by INS, suggesting that existing practices pose minimal major safety concerns for biological tissues.

Infrared neuromodulation-a review

Infrared (IR) neuromodulation (INM) is an emerging light-based neuromodulation approach that can reversibly control neuronal and muscular activities through the transient and localized deposition of pulsed IR light without requiring any chemical or genetic pre-treatment of the target cells. Though the efficacy and short-term safety of INM have been widely demonstrated in both peripheral and central nervous systems, the investigations of the detailed cellular and biological processes and the underlying biophysical mechanisms are still ongoing. In this review, we discuss the current research progress in the INM field with a focus on the more recently discovered IR nerve inhibition. Major biophysical mechanisms associated with IR nerve stimulation are summarized. As the INM effects are primarily attributed to the spatiotemporal thermal transients induced by water and tissue absorption of pulsed IR light, temperature monitoring techniques and simulation models adopted in INM studies are discussed. Potential translational applications, current limitations, and challenges of the field are elucidated to provide guidance for future INM research and advancement.

Flexible, Transparent, and Cytocompatible Nanostructured Indium Tin Oxide Thin Films for Bio-optoelectronic Applications

Electrical stimulation has been used successfully for several decades for the treatment of neurodegenerative disorders, including motor disorders, pain, and psychiatric disorders. These technologies typically rely on the modulation of neural activity through the focused delivery of electrical pulses. Recent research, however, has shown that electrically triggered neuromodulation can be further enhanced when coupled with optical stimulation, an approach that can benefit from the development of novel electrode materials that combine transparency with excellent electrochemical and biological performance. In this study, we describe an electrochemically modified, nanostructured indium tin oxide/poly(ethylene terephthalate) (ITO/PET) surface as a flexible, transparent, and cytocompatible electrode material. Electrochemical oxidation and reduction of ITO/PET electrodes in the presence of an ionic liquid based on d-glucopyranoside and bistriflamide units were performed, and the electrochemical behavior, conductivity, capacitance, charge transport processes, surface morphology, optical properties, and cytocompatibility were assessed in vitro. It has been shown that under selected conditions, electrochemically modified ITO/PET films remained transparent and highly conductive and were able to enhance neural cell survival and neurite outgrowth. Consequently, electrochemical modification of ITO/PET electrodes in the presence of an ionic liquid is introduced as an effective approach for tailoring the properties of ITO for advanced bio-optoelectronic applications.

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.

Label-free intraoperative nerve detection and visualization using ratiometric diffuse reflectance spectroscopy

Iatrogenic nerve injuries contribute significantly to postoperative morbidity across various surgical disciplines and occur in approximately 500,000 cases annually in the US alone. Currently, there are no clinically adopted means to intraoperatively visualize nerves beyond the surgeon's visual assessment. Here, we report a label-free method for nerve detection using diffuse reflectance spectroscopy (DRS). Starting with an in vivo rat model, fiber- and imaging-based DRS independently identified similar wavelengths that provided optimal contrast for nerve identification with an accuracy of 92%. Optical property measurements of rat and human cadaver tissues verify that the source of contrast between nerve and surrounding tissues is largely due to higher scattering in nerve and differences in oxygenated hemoglobin content. Clinical feasibility was demonstrated in patients undergoing thyroidectomies using both probe-based and imaging-based approaches where the nerve were identified with 91% accuracy. Based on our preliminary results, DRS has the potential to both provide surgeons with a label-free, intraoperative means of nerve visualization and reduce the incidence of iatrogenic nerve injuries along with its detrimental complications.

Targeted Optical Neural Stimulation: A New Era for Personalized Medicine

Targeted optical neural stimulation comprises infrared neural stimulation and optogenetics, which affect the nervous system through induced thermal transients and activation of light-sensitive proteins, respectively. The main advantage of this pair of optical tools is high functional selectivity, which conventional electrical stimulation lacks. Over the past 15 years, the mechanism, safety, and feasibility of optical stimulation techniques have undergone continuous investigation and development. When combined with other methods like optical imaging and high-field functional magnetic resonance imaging, the translation of optical stimulation to clinical practice adds high value. We review the theoretical foundations and current state of optical stimulation, with a particular focus on infrared neural stimulation as a potential bridge linking optical stimulation to personalized medicine.

Infrared neural stimulation markedly enhances nerve functionality assessment during nerve monitoring

In surgical procedures where the risk of accidental nerve damage is prevalent, surgeons commonly use electrical stimulation (ES) during intraoperative nerve monitoring (IONM) to assess a nerve's functional integrity. ES, however, is subject to off-target stimulation and stimulation artifacts disguising the true functionality of the specific target and complicating interpretation. Lacking a stimulation artifact and having a higher degree of spatial specificity, infrared neural stimulation (INS) has the potential to improve upon clinical ES for IONM. Here, we present a direct comparison between clinical ES and INS for IONM performance in an in vivo rat model. The sensitivity of INS surpasses that of ES in detecting partial forms of damage while maintaining a comparable specificity and sensitivity to more complete forms. Without loss in performance, INS is readily compatible with existing clinical nerve monitoring systems. These findings underscore the clinical potential of INS to improve IONM and surgical outcomes.

Differential immunostaining patterns of transient receptor potential (TRP) ion channels in the rat nodose ganglion

Vagal afferents regulate numerous physiological functions including arterial blood pressure, heart rate, breathing, and nociception. Cell bodies of vagal afferents reside in the inferior vagal (nodose) ganglia and their stimulation by various means is being considered as a way to regulate cardiorespiratory responses and control pain sensations. Stimulation of the nodose by exposure to infrared light is recently being considered as a precise way to elicit responses. These responses would likely involve the activity of temperature-sensitive membrane-bound channels. While papers have been published to track the expression of these transient receptor potential ion channels (TRPs), further studies are warranted to determine the in situ expression of the endogenous TRP proteins in the nodose ganglia to fully understand their pattern of expression, subcellular locations, and functions in this animal model. TRP ion channels are a superfamily of Na+ /Ca2+ -channels whose members are temperature- and/or mechano-sensitive and therefore represent a potential set of proteins that will be activated directly or indirectly by infrared light. Here, we report the spatial localization of six TRP channels, TRPV1, TRPV4, TRPM3, TRPM8, TRPA1, and TRPC1, from nodose ganglia taken from juvenile male Sprague-Dawley rats. The channels were detected using immunohistology with fluorescent tags on cryosections and imaged using confocal microscopy. All six TRP channels were detected with different levels of intensity in neuronal cell bodies and some were also detected in axonal fibers and blood vessels. The TRP receptors differed in their prevalence, in their patterns of expression, and in subcellular expression/localization. More specifically, TRPV1, TRPV4, TRPA1, TRPM8, TRPC1, and TRPM3 were found in vagal afferent cell bodies with a wide range of immunostaining intensity from neuron to neuron. Immunostaining for TRPV1, TRPV4, and TRPA1 appeared as fine particles scattered throughout the cytoplasm of the cell body. Intense TRPV1 immunostaining was also evident in a subset of axonal fibers. TRPM8 and TRPC1 were expressed in courser particles suggesting different subcellular compartments than for TRPV1. The localization of TRPM3 differed markedly from the other TRP channels with an immunostaining pattern that was localized to the periphery of a subset of cell bodies, whereas a scattering or no immunostaining was detected within the bulk of the cytoplasm. TRPV4 and TRPC1 were also expressed on the walls of blood vessels. The finding that all six TRP channels (representing four subfamilies) were present in the nodose ganglia provides the basis for studies designed to understand the roles of these channels in sensory transmission within vagal afferent fibers and in the responses elicited by exposure of nodose ganglia to infrared light and other stimuli. Depending on the location and functionality of the TRP channels, they may regulate the flux of Na+ /Ca2+ -across the membranes of cell bodies and axons of sensory afferents, efferent (motor) fibers coursing through the ganglia, and in vascular smooth muscle.

High-precision neural stimulation through optoacoustic emitters

Neuromodulation poses an invaluable role in deciphering neural circuits and exploring clinical treatment of neurological diseases. Optoacoustic neuromodulation is an emerging modality benefiting from the merits of ultrasound with high penetration depth as well as the merits of photons with high spatial precision. We summarize recent development in a variety of optoacoustic platforms for neural modulation, including fiber, film, and nanotransducer-based devices, highlighting the key advantages of each platform. The possible mechanisms and main barriers for optoacoustics as a viable neuromodulation tool are discussed. Future directions in fundamental and translational research are proposed.

Visualizing Lipid Dynamics Role in Infrared Neural Stimulation using Stimulated Raman Scattering

Infrared neural stimulation, or INS, uses pulsed infrared light to yield label-free neural stimulation with broad experimental and translational utility. Despite its robust demonstration, INS's mechanistic and biophysical underpinnings have been the subject of debate for more than a decade. The role of lipid membrane thermodynamics appears to play an important role in how fast IR-mediated heating nonspecifically drives action potential generation. Direct observation of lipid membrane dynamics during INS remains to be shown in a live neural model system. We used hyperspectral stimulated Raman scattering (hsSRS) microscopy to study biochemical signatures of high-speed vibrational dynamics underlying INS in a live neural cell culture model. Findings suggest that lipid bilayer structural changes are occurring during INS in vitro in NG108-15 neuroglioma cells. Lipid-specific signatures of cell SRS spectra varied with stimulation energy and radiant exposure. Spectroscopic observations agree with high-speed ratiometric fluorescence imaging of a conventional lipophilic membrane structure reporter, di-4-ANNEPS. Overall, the presented findings support the hypothesis that INS causes changes in the lipid membrane of neural cells by changing lipid membrane packing order. Furthermore, this work highlights the potential of hsSRS as a method to study biophysical and biochemical dynamics safely in live cells.

In Vivo Quantification of the Effectiveness of Topical Low-Dose Photodynamic Therapy in Wound Healing Using Two-Photon Microscopy

The effect of low-dose photodynamic therapy on in vivo wound healing with topical application of 5-aminolevulinic acid and methylene blue was investigated using an animal model for two laser radiation doses (1 and 4 J/cm2). A second-harmonic-generation-to-auto-fluorescence aging index of the dermis (SAAID) was analyzed by two-photon microscopy. SAAID measured at 60-80 μm depths was shown to be a suitable quantitative parameter to monitor wound healing. A comparison of SAAID in healthy and wound tissues during phototherapy showed that both light doses were effective for wound healing; however, healing was better at a dose of 4 J/cm2.

Vagus nerve bundle stimulation using 1505-nm laser irradiation in an in-vivo rat model

Laser nerve stimulation using near-infrared laser irradiation has recently been studied in the peripheral nervous system as an alternative method to conventional electrical nerve stimulation. Bringing this method to the vagus nerve model could leverage this emerging stimulation approach to be tested in broader preclinical applications. Here, we report the capability of the laser nerve stimulation method on the rat vagus nerve bundle with a 1505-nm diode laser operated in continuous-wave mode. Studies of the stimulation threshold and laser-induced acute thermal injury to the nerve bundle were also performed to determine a temperature window for safe, reliable and reproducible laser stimulation of the rat vagus nerve bundle. The results show that laser stimulation of the vagus nerve bundle provides reliable and reproducible nerve stimulation in a rat model. These results also confirm a threshold temperature of >42°C with acute nerve damage observed above 46°C. A strong correlation was obtained between the laser time required to raise the nerve temperature above the stimulation threshold and the mean arterial pressure response. Advantages of the method such as non-contact delivery of external stimulus signals at mm scaled distance in air, enhanced spatial selectivity and electrical artefact-free measurements may indicate its potential to counteract the side effects of conventional electrical vagus nerve stimulation.