Thermoplasmonic Optical Fiber for Localized Neural Stimulation

Highly Branched Au Superparticles as Efficient Photothermal Transducers for Optical Neuromodulation

Precise neuromodulation is critical for interrogating cellular communication and treating neurological diseases. Nanoscale transducers have emerged as effective interfaces to exert photothermal effects and modulate neural activities with a high spatiotemporal resolution. Ideal materials for this application should possess strong light absorption, high photothermal conversion efficiency, and great biocompatibility for clinical translation. Here, we show that the structurally designed 3D Au superparticles with a highly branched morphology can be promising candidates for nongenetic and remote neuromodulation. The structure-induced blackbody-like absorption endows Au superparticles with photothermal conversion efficiency over 90%, much higher than that of conventional Au nanorods. With the biocompatible polydopamine ligands, Au superparticles can be readily interfaced with primary mouse hippocampal neurons and other cells and can photostimulate or inhibit their activities in both cell networks or with a single-cell resolution. These findings highlight the importance of structural designs as powerful tools to promote the performance of plasmonic materials in neuromodulation and related research of neuroscience and neuroengineering.

Gold Nanorod-Embedded PDMS Micro-Pillar Array for Localized Photothermal Stimulation

Gold nanorods (GNRs) are one of the most promising biomaterial choices for the photothermal activation of neurons due to their relative biocompatibility, unique photothermal properties, and broad optical tunability through their synthetic shape control. While photothermal stimulation using randomly accumulated GNRs successfully demonstrates the potential treatment of functional neural disorders by modulating the neuronal activities using localized heating, there are limited demonstrations to translate this new concept into large-arrayed neural stimulations. In this paper, we report an arrayed PDMS micropillar platform in which GNRs are embedded as pixel-like, arrayed photothermal stimulators at the tips of the pillars. The proposed platform will be able to localize GNRs at predetermined pillar positions and create thermal stimulations using near-infrared (NIR) light. This will address the limitations of randomly distributed GNR-based approaches. Furthermore, a flexible PDMS pillar structure will create intimate interfaces on target cells. By characterizing the spatiotemporal temperature change in the platform with rhodamine B dye, we have shown that the localized temperature can be optically modulated within 4°C, which is in the range of temperature variation required for neuromodulation using NIR light. We envision that our proposed platform has the potential to be applied as a photothermal, neuronal stimulation interface with high spatiotemporal resolution.

Sandwich Layer-Modified Ω-Shaped Fiber-Optic LSPR Enables the Development of an Aptasensor for a Cytosensing-Photothermal Therapy Circuit

The metastasis of cancer cells is a principal cause of morbidity and mortality in cancer. The combination of a cytosensor and photothermal therapy (PTT) cannot completely eliminate cancer cells at one time. Hence, this study aimed to design a localized surface plasmonic resonance (LSPR)-based aptasensor for a circuit of cytosensing-PTT (COCP). This was achieved by coating a novel sandwich layer of polydopamine/gold nanoparticles/polydopamine (PDA/AuNPs/PDA) around the Ω-shaped fiber-optic (Ω-FO). The short-wavelength peak of the sandwich layer with strong resonance exhibited a high refractive index sensitivity (RIS). The modification with the T-shaped aptamer endowed FO-LSPR with unique characteristics of time-dependent sensitivity enhancement behavior for a sensitive cytosensor with the lowest limit of detection (LOD) of 13 cells/mL. The long-wavelength resonance peak in the sandwich layer appears in the near-infrared region. Hence, the rate of increased localized temperature of FO-LSPR was 160 and 30-fold higher than that of the bare and PDA-coated FO, indicating strong photothermal conversion efficiency. After considering the localized temperature distribution around the FO under the flow environment, the FO-LSPR-enabled aptasensor killed 77.6% of cancer cells in simulated blood circulation after five cycles of COCP. The FO-LSPR-enabled aptasensor improved the efficiency of the cytosensor and PTT to effectively kill cancer cells, showing significant potential for application in inhibiting cancer metastasis.

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.

Biological Applications of Thermoplasmonics

Thermoplasmonics has emerged as an extraordinarily versatile tool with profound applications across various biological domains ranging from medical science to cell biology and biophysics. The key feature of nanoscale plasmonic heating involves remote activation of heating by applying laser irradiation to plasmonic nanostructures that are designed to optimally convert light into heat. This unique capability paves the way for a diverse array of applications, facilitating the exploration of critical biological processes such as cell differentiation, repair, signaling, and protein functionality, and the advancement of biosensing techniques. Of particular significance is the rapid heat cycling that can be achieved through thermoplasmonics, which has ushered in remarkable technical innovations such as accelerated amplification of DNA through quantitative reverse transcription polymerase chain reaction. Finally, medical applications of photothermal therapy have recently completed clinical trials with remarkable results in prostate cancer, which will inevitably lead to the implementation of photothermal therapy for a number of diseases in the future. Within this review, we offer a survey of the latest advancements in the burgeoning field of thermoplasmonics, with a keen emphasis on its transformative applications within the realm of biosciences.

Implantable Flexible Sensors for Health Monitoring

Flexible sensors, as a significant component of flexible electronics, have attracted great interest the realms of human-computer interaction and health monitoring due to their high conformability, adjustable sensitivity, and excellent durability. In comparison to wearable sensor-based in vitro health monitoring, the use of implantable flexible sensors (IFSs) for in vivo health monitoring offers more accurate and reliable vital sign information due to their ability to adapt and directly integrate with human tissue. IFSs show tremendous promise in the field of health monitoring, with unique advantages such as robust signal reading capabilities, lightweight design, flexibility, and biocompatibility. Herein, a review of IFSs for vital signs monitoring is detailly provided, highlighting the essential conditions for in vivo applications. As the prerequisites of IFSs, the stretchability and wireless self-powered properties of the sensor are discussed, with a special attention paid to the sensing materials which can maintain prominent biosafety (i.e., biocompatibility, biodegradability, bioresorbability). Furthermore, the applications of IFSs monitoring various parts of the body are described in detail, with a summary in brain monitoring, eye monitoring, and blood monitoring. Finally, the challenges as well as opportunities in the development of next-generation IFSs are presented.

Functional material-mediated wireless physical stimulation for neuro-modulation and regeneration

Nerve injuries and neurological diseases remain intractable clinical challenges. Despite the advantages of stem cell therapy in treating neurological disorders, uncontrollable cell fates and loss of cell function in vivo are still challenging. Recently, increasing attention has been given to the roles of external physical signals, such as electricity and ultrasound, in regulating stem cell fate as well as activating or inhibiting neuronal activity, which provides new insights for the treatment of neurological disorders. However, direct physical stimulations in vivo are short in accuracy and safety. Functional materials that can absorb energy from a specific physical field exerted in a wireless way and then release another localized physical signal hold great advantages in mediating noninvasive or minimally invasive accurate indirect physical stimulations to promote the therapeutic effect on neurological disorders. In this review, the mechanism by which various physical signals regulate stem cell fate and neuronal activity is summarized. Based on these concepts, the approaches of using functional materials to mediate indirect wireless physical stimulation for neuro-modulation and regeneration are systematically reviewed. We expect that this review will contribute to developing wireless platforms for neural stimulation as an assistance for the treatment of neurological diseases and injuries.

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.

Plasmonic Nanostructure Engineering with Shadow Growth

Physical shadow growth is a vacuum deposition technique that permits a wide variety of 3D-shaped nanoparticles and structures to be fabricated from a large library of materials. Recent advances in the control of the shadow effect at the nanoscale expand the scope of nanomaterials from spherical nanoparticles to complex 3D shaped hybrid nanoparticles and structures. In particular, plasmonically active nanomaterials can be engineered in their shape and material composition so that they exhibit unique physical and chemical properties. Here, the recent progress in the development of shadow growth techniques to realize hybrid plasmonic nanomaterials is discussed. The review describes how fabrication permits the material response to be engineered and highlights novel functions. Potential fields of application with a focus on photonic devices, biomedical, and chiral spectroscopic applications are discussed.

Dual Behavior Regulation: Tether-Free Deep-Brain Stimulation by Photothermal and Upconversion Hybrid Nanoparticles

Optogenetics has been plagued by invasive brain implants and thermal effects during photo-modulation. Here, two upconversion hybrid nanoparticles modified with photothermal agents, named PT-UCNP-B/G, which can modulate neuronal activities via photostimulation and thermo-stimulation under near-infrared laser irradiation at 980 nm and 808 nm, respectively, are demonstrated. PT-UCNP-B/G emits visible light (410-500 nm or 500-570 nm) through the upconversion process at 980 nm, while they exhibit efficient photothermal effect at 808 nm with no visible emission and tissue damage. Intriguingly, PT-UCNP-B significantly activates extracellular sodium currents in neuro2a cells expressing light-gated channelrhodopsin-2 (ChR2) ion channels under 980-nm irradiation, and inhibits potassium currents in human embryonic kidney 293 cells expressing the voltage-gated potassium channels (KCNQ1) under 808-nm irradiation in vitro. Furthermore, deep-brain bidirectional modulation of feeding behavior is achieved under tether-free 980 or 808-nm illumination (0.8 W cm^-2 ) in mice stereotactically injected with PT-UCNP-B in the ChR2-expressing lateral hypothalamus region. Thus, PT-UCNP-B/G creates new possibility of utilizing both light and heat to modulate neural activities and provides a viable strategy to overcome the limits of optogenetics.

Engineering optical tools for remotely controlled brain stimulation and regeneration

Neurological disorders are one of the world's leading medical and societal challenges due to the lack of efficacy of the first line treatment. Although pharmacological and non-pharmacological interventions have been employed with the aim of regulating neuronal activity and survival, they have failed to avoid symptom relapse and disease progression in the vast majority of patients. In the last 5 years, advanced drug delivery systems delivering bioactive molecules and neuromodulation strategies have been developed to promote tissue regeneration and remodel neuronal circuitry. However, both approaches still have limited spatial and temporal precision over the desired target regions. While external stimuli such as electromagnetic fields and ultrasound have been employed in the clinic for non-invasive neuromodulation, they do not have the capability of offering single-cell spatial resolution as light stimulation. Herein, we review the latest progress in this area of study and discuss the prospects of using light-responsive nanomaterials to achieve on-demand delivery of drugs and neuromodulation, with the aim of achieving brain stimulation and regeneration.

Macromolecular rate theory explains the temperature dependence of membrane conductance kinetics

The factor Q10 is used in neuroscience to adjust reaction rates of voltage-activated membrane conductances to different temperatures and is widely assumed to be constant. By performing an analysis of published data of the reaction rates of sodium, potassium, and calcium membrane conductances, we demonstrate that 1) Q10 is temperature dependent, 2) this relationship is similar across conductances, and 3) there is a strong effect at low temperatures (<15°C). We show that macromolecular rate theory (MMRT) explains this temperature dependency. MMRT predicts the existence of optimal temperatures at which reaction rates decrease as temperature increases, a phenomenon that we also found in the published data sets. We tested the consequences of using MMRT-adjusted reaction rates in the Hodgkin-Huxley model of the squid's giant axon. The MMRT-adjusted model reproduces the temperature dependence of the rising and falling times of the action potential. Furthermore, the model also reproduces these properties for different squid species that live in different climates. In a second example, we compare spiking patterns of biophysical models based on human pyramidal neurons from the Allen Cell Types database at room and physiological temperatures. The original models, calibrated at 34°C, failed to generate realistic spikes at room temperature in more than half of the tested models, while the MMRT produces realistic spiking in all conditions. In another example, we show that using the MMRT correction in hippocampal pyramidal cell models results in 100% differences in voltage responses. Finally, we show that the shape of the Q10 function results in systematic errors in predicting reaction rates. We propose that the optimal temperature could be a thermodynamical barrier to avoid over excitation in neurons. While this study is centered on membrane conductances, our results have important consequences for all biochemical reactions involved in cell signaling.

High temporal resolution transparent thermoelectric temperature sensors for photothermal effect sensing

We propose inkjet-printed high-speed and transparent temperature sensors based on the thermoelectric effect for direct monitoring of the photothermal effect. They consist of highly transparent organic thermoelectric materials that allow excellent biocompatibility and sub-ms temporal resolution, simultaneously. Our transparent thermoelectric temperature sensors can be used to advance various photothermal biomedical applications.

A Micron-Range Displacement Sensor Based on Thermo-Optically Tuned Whispering Gallery Modes in a Microcapillary Resonator

A novel micron-range displacement sensor based on a whispering-gallery mode (WGM) microcapillary resonator filled with a nematic liquid crystal (LC) and a magnetic nanoparticle- coated fiber half-taper is proposed and experimentally demonstrated. In the proposed device, the tip of a fiber half-taper coated with a thin layer of magnetic nanoparticles (MNPs) moves inside the LC-filled microcapillary resonator along its axis. The input end of the fiber half-taper is connected to a pump laser source and due to the thermo-optic effect within the MNPs, the fiber tip acts as point heat source increasing the temperature of the LC material in its vicinity. An increase in the LC temperature leads to a decrease in its effective refractive index, which in turn causes spectral shift of the WGM resonances monitored in the transmission spectrum of the coupling fiber. The spectral shift of the WGMs is proportional to the displacement of the MNP-coated tip with respect to the microcapillary's light coupling point. The sensor's operation is simulated considering heat transfer in the microcapillary filled with a LC material having a negative thermo-optic coefficient. The simulations are in a good agreement with the WGMs spectral shift observed experimentally. A sensitivity to displacement of 15.44 pm/µm and a response time of 260 ms were demonstrated for the proposed sensor. The device also shows good reversibility and repeatability of response. The proposed micro-displacement sensor has potential applications in micro-manufacturing, precision measurement and medical instruments.

Thermoplasmonic Regulation of the Mitochondrial Metabolic State for Promoting Directed Differentiation of Dental Pulp Stem Cells

Regulating stem cell differentiation in a controllable way is significant for regeneration of tissues. Herein, we report a simple and highly efficient method for accelerating the stem cell differentiation of dental pulp stem cells (DPSCs) based on the synergy of the electromagnetic field and the photothermal (thermoplasmonic) effect of plasmonic nanoparticles. By simple laser irradiation at 50 mW/cm² (10 min per day, totally for 5 days), the thermoplasmonic effect of Au nanoparticles (AuNPs) can effectively regulate mitochondrial metabolism to induce the increase of mitochondrial membrane potential and further drive energy increase during the DPSC differentiation process. The proposed method can specifically regulate DPSCs' cell differentiation toward odontoblasts, with the differentiation time reduced to only 5 days. Simultaneously, the molecular profiling change of mitochondria within DPSCs during the cell differentiation process is revealed by in situ surface-enhanced Raman spectroscopy. It clearly demonstrates that the expression of hydroxyproline and glutamate gradually increases with prolonging of the differentiation days. The developed method is simple, robust, and rapid for stem cell differentiation of DPSCs, which would be beneficial to tissue engineering and regenerative medicine.

Enhancement of Thermoplasmonic Neural Modulation Using a Gold Nanorod-Immobilized Polydopamine Film

Photothermal neural activity inhibition has emerged as a minimally invasive neuromodulation technology with submillimeter precision. One of the techniques involves the utilization of plasmonic gold nanoparticles (AuNPs) to modulate neural activity by photothermal effects ("thermoplasmonics"). A surface modification technique is often required to integrate AuNPs onto the neural interface. Here, polydopamine (pDA), a multifunctional adhesive polymer with a wide light absorption spectrum, is introduced both as a primer layer for the immobilization of gold nanorods (GNRs) on the neural interface and as an additional photothermal agent by absorbing near-infrared red (NIR) lights for more efficient photothermal effects. First, the optical and photothermal properties of pDA as well as the characteristics of GNRs attached onto the pDA film are investigated for the optimized photothermal neural interface. Due to the covalent bonding between GNR surfaces and pDA, GNRs immobilized on pDA showed strong attachment onto the surface, yielding a more stable photothermal platform. Lastly, when photothermal neural stimulation was applied to the primary rat hippocampal neurons, the substrate with GNRs immobilized on the pDA film allowed more laser power-efficient photothermal neuromodulation as well as photothermal cell death. This study suggests the feasibility of using pDA as a surface modification material for developing a photothermal platform for the inhibition of neural activities.

Holographic Manipulation of Nanostructured Fiber Optics Enables Spatially-Resolved, Reconfigurable Optical Control of Plasmonic Local Field Enhancement and SERS

Integration of plasmonic structures on step-index optical fibers is attracting interest for both applications and fundamental studies. However, the possibility to dynamically control the coupling between the guided light fields and the plasmonic resonances is hindered by the turbidity of light propagation in multimode fibers (MMFs). This pivotal point strongly limits the range of studies that can benefit from nanostructured fiber optics. Fortunately, harnessing the interaction between plasmonic modes on the fiber tip and the full set of guided modes can bring this technology to a next generation progress. Here, the intrinsic wealth of information of guided modes is exploited to spatiotemporally control the plasmonic resonances of the coupled system. This concept is shown by employing dynamic phase modulation to structure both the response of plasmonic MMFs on the plasmonic facet and their response in the corresponding Fourier plane, achieving spatial selective field enhancement and direct control of the probe's work point in the dispersion diagram. Such a conceptual leap would transform the biomedical applications of holographic endoscopic imaging by integrating new sensing and manipulation capabilities.

Optoplasmonic Modulation of Cell Metabolic State Promotes Rapid Cell Differentiation

Cell differentiation plays a vital role in mediating organ formation and tissue repair and regeneration. Although rapid and effective methods to stimulate cell differentiation for clinical purposes are highly desired, it remains a great challenge in the medical fields. Herein, a highly effective and conceptual optical method was developed based on a plasmonic chip platform (made of 2D AuNPs nanomembranes). through effective light-augmented plasmonic regulation of cellular bioenergetics (CBE) and an entropy effect at bionano interfaces, to promote rapid cell differentiation. Compared with traditional methods, the developed optoplasmonic method greatly shortens cell differentiation time from usually more than 10 days to only about 3 days. Upon the optoplasmonic treatment of cells, the conformational and vibration entropy changes of cell membranes were clearly revealed through theoretical simulation and fingerprint spectra of cell membranes. Meanwhile, during the treatment process, bioenergetics levels of cells were elevated with increasing mitochondrial membrane potential (Δψ_m), which accelerates cell differentiation and proliferation. The developed optoplasmonic method is highly efficient and easy to implement, provides a new perspective and avenue for cell differentiation and proliferation, and has potential application prospects in accelerating tissue repair and regeneration.

Metal-Nanostructure-Decorated Spider Silk for Highly Sensitive Refractive Index Sensing

Highly sensitive detection of refractive index (RI) is essential for the analysis of the bio-microenvironment and basic cellular reactions. To achieve this, optic-fiber RI sensors based on localized surface plasmon resonance (LSPR) have been widely used for their flexibility and high sensitivity. However, the current optic-fiber RI sensors are mainly fabricated using glass, which makes them face the challenges in biocompatibility and biosafety. In this work, a RI sensor with high sensitivity is fabricated using metal-nanostructure-decorated spider silk. The spider silk, which is directly dragged from Araneus ventricosus, is natural protein-based biopolymer with low attenuation, good biocompatibility and biodegradability, large RI, great flexibility, and easy functionalization. Hence, the spider silk can be an ideal alternative to glass for sensing in biological environments with a wide RI range. Different kinds of metal nanostructures, such as gold nanorods (GNRs), gold nanobipyramids (GNBP), and Ag@GNRs, are decorated on the surface of the spider silk utilizing the surface viscidity of the silk. By directing a beam of white light into the spider silk, the LSPR of the metal nanostructures was excited and a highly sensitive RI sensing (the highest sensitivity of 1746 nm per refractive index was achieved on the GNBP-decorated spider silk) was obtained. This work may pave a new way to precise and sensitive biosensing and bioanalysis.

Gold nanostructures: synthesis, properties, and neurological applications

Recent advances in technology are expected to increase our current understanding of neuroscience. Nanotechnology and nanomaterials can alter and control neural functionality in both in vitro and in vivo experimental setups. The intersection between neuroscience and nanoscience may generate long-term neural interfaces adapted at the molecular level. Owing to their intrinsic physicochemical characteristics, gold nanostructures (GNSs) have received much attention in neuroscience, especially for combined diagnostic and therapeutic (theragnostic) purposes. GNSs have been successfully employed to stimulate and monitor neurophysiological signals. Hence, GNSs could provide a promising solution for the regeneration and recovery of neural tissue, novel neuroprotective strategies, and integrated implantable materials. This review covers the broad range of neurological applications of GNS-based materials to improve clinical diagnosis and therapy. Sub-topics include neurotoxicity, targeted delivery of therapeutics to the central nervous system (CNS), neurochemical sensing, neuromodulation, neuroimaging, neurotherapy, tissue engineering, and neural regeneration. It focuses on core concepts of GNSs in neurology, to circumvent the limitations and significant obstacles of innovative approaches in neurobiology and neurochemistry, including theragnostics. We will discuss recent advances in the use of GNSs to overcome current bottlenecks and tackle technical and conceptual challenges.

Hybridized nanolayer modified Ω-shaped fiber-optic synergistically enhances localized surface plasma resonance for ultrasensitive cytosensor and efficient photothermal therapy

Inadequate sensitivity and side-effect are the main challenges to develop cytosensors combining with therapeutic potential simultaneously for cancer diagnosis and treatment. Herein, localized surface plasma resonance (LSPR) based on hybridized nanolayer modified Ω-shaped fiber-optic (HN/Ω-FO) was developed to integrate cytosensor and plasmonic photothermal treatment (PPT). On one hand, hybridized nanolayers improve the coverage of nanoparticles and refractive index sensitivity (RIS). Moreover, the hybridized nanoploymers of gold nanorods/gold nanoparticles (AuNRs/AuNPs) also result in intense enhancement in electronic field intensity (I). On the other hand, Ω-shaped fiber-optic (Ω-FO) led to strong bending loss in its bending part. To be specific, a majority of light escaped from fiber will interact with HN. Thus, HN/Ω-FO synergistically enhances the plasmonic, which achieved the goal of ultrasensitive cytosensor and highly-efficient plasmonic photothermal treatment (PPT). The proposed cytosensor exhibits ultrasensitivity for detection of cancer cells with a low limit of detection down to 2.6 cells/mL was realized just in 30 min. HN/Ω-FO-based LSPR exhibits unique characteristics of highly efficient, localized, and geometry-dependent heat distribution, which makes it suitable for PPT to only kill the cancer cells specifically on the surface or surrounding fiber-optic (FO) surface. Thus, HN/Ω-FO provides a new approach to couple cytosensor with PPT, indicating its great potential in clinical diagnosis and treatment.

Closed-loop control of neural spike rate of cultured neurons using a thermoplasmonics-based photothermal neural stimulation

Objective.Photothermal neural stimulation has been developed in a variety of interfaces as an alternative technology that can perturb neural activity. The demonstrations of these techniques have heavily relied on open-loop stimulation or complete suppression of neural activity. To extend the controllability of photothermal neural stimulation, combining it with a closed-loop system is required. In this work, we investigated whether photothermal suppression mechanism can be used in a closed-loop system to reliably modulate neural spike rate to non-zero setpoints.Approach. To incorporate the photothermal inhibition mechanism into the neural feedback system, we combined a thermoplasmonic stimulation platform based on gold nanorods (GNRs) and near-infrared illuminations (808 nm, spot size: 2 mm or 200μm in diameter) with a proportional-integral (PI) controller. The closed-loop feedback control system was implemented to track predetermined target spike rates of hippocampal neuronal networks cultured on GNR-coated microelectrode arrays.Main results. The closed-loop system for neural spike rate control was successfully implemented using a PI controller and the thermoplasmonic neural suppression platform. Compared to the open-loop control, the target-channel spike rates were precisely modulated to remain constant or change in a sinusoidal form in the range below baseline spike rates. The spike rate response behaviors were affected by the choice of the controller gain. We also demonstrated that the functional connectivity of a synchronized bursting network could be altered by controlling the spike rate of one of the participating channels.Significance.The thermoplasmonic feedback controller proved that it can precisely modulate neural spike rate of neural activityin vitro. This technology can be used for studying neuronal network dynamics and might provide insights in developing new neuromodulation techniques in clinical applications.

Laser Power Determination Using Light-to-Heat Conversion Rate of Nanoplasmonic Substrates for Neural Stimulation

Since neurons have temperature sensitive properties, gold nanorod (GNR)-mediated photothermal stimulation has been developed as a neuromodulation application. As an in vitro photothermal platform, GNR-layer was integrated with substrates to effectively apply heat stimulation to the cultured neurons. However, identifying optimal laser power for a targeted temperature on the substrate requires the consideration of thermal properties of the GNR-coated substrates. In this report, we suggest a simple numerical method to determine incident laser power on the substrates for a targeted temperature.

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.