Neurophotonics: a comprehensive review, current challenges and future trends

Exploring the interface between quantum biology, microwave technology, and neuroscience

Microwave technology, essential in quantum computing, is now being explored for clinical diagnostics. Passive microwave radiometry (MWR) detects endogenous microwave emissions from biological tissues, offering non-invasive insights that are distinct from conventional thermodynamic methods. This review highlights the potential of MWR to monitor physiological changes in conditions such as stroke, inflammation, brain injury, and degenerative diseases. Emerging evidence suggests that variations in microwave emissions reflect quantum-level biological processes, including protein folding and enzymatic activity. These findings open new avenues for real-time diagnostics and personalised treatment strategies. By integrating quantum biology, microwave sensing, and neuroscience, MWR could become a powerful tool in clinical practice. Further research is needed to validate these technologies and translate them into routine medical use.

Small form factor implantable neural probe with efficient flip chip µLED for in vivo optogenetics

Optogenetics is a widely used tool to dissect neural circuits with optical stimulation, but it requires that light is delivered to photosensitive neurons inside the brain. Implantable neural probes with microscale LEDs (µLEDs) are an emerging approach to delivering light to the brain with superior light output control. However, approaches to integrate µLEDs in neural probes depend on complex fabrication processes. Here, we developed an implantable small form factor neural probe that integrates highly efficient commercial flip chip µLEDs using only standard lithography processes in silicon and a custom automated LED mounting approach with custom 3D-printed tools on a pick-and-place machine. The probe has a cross-sectional area under 0.013 mm2 but can output up to 2.5 mW of optical power with an irradiance of 175 mW/mm2. Due to the high plug efficiency of the LED, the neural probe can perform stimulation protocols up to 20 Hz and 80% duty cycles without surpassing estimated hotspot temperature elevations above 1 ºC. The neural probes were validated in vivo, with brain activity in the motor cortex of transgenic mice being reliably modulated by pulsed light emitted from the probe.

Imaging the enteric nervous system

The enteric nervous system (ENS) has garnered increasing scientific interest due to its pivotal role in digestive processes and its involvement in various gastrointestinal and central nervous system (CNS) disorders, including Crohn's disease, Parkinson's disease, and autism. Despite its significance, the ENS remains relatively underexplored by neurobiologists, primarily because its structure and function are less understood compared to the CNS. This review examines both pioneering methodologies that initially revealed the intricate layered structure of the ENS and recent advancements in studying its three-dimensional (3-D) organization, both in fixed samples and at a functional level, ex-vivo or in-vivo. Traditionally, imaging the ENS relied on histological techniques involving sequential tissue sectioning, staining, and microscopic imaging of single sections. However, this method has limitations representing the full complexity of the ENS's 3-D meshwork, which led to the development of more intact preparations, such as whole-mount preparation, as well as the use of volume imaging techniques. Advancements in 3-D imaging, particularly methods like spinning-disk confocal, 2-photon, and light-sheet microscopies, combined with tissue-clearing techniques, have revolutionized our understanding of the ENS's fine structure. These approaches offer detailed views of its cellular architecture, including interactions among various cell types, blood vessels, and lymphatic vessels. They have also enhanced our comprehension of ENS-related pathologies, such as inflammatory bowel disease, Hirschsprung's disease (HSCR), and the ENS's involvement in neurodegenerative disorders like Parkinson's (PD) and Alzheimer's diseases (AD). More recently, 2-photon or confocal in-vivo imaging, combined with transgenic approaches for calcium imaging, or confocal laser endomicroscopy, have opened new avenues for functional studies of the ENS. These methods enable real-time observation of enteric neuronal and glial activity and their interactions. While routinely used in CNS studies, their application to understanding local circuits and signals in the ENS is relatively recent and presents unique challenges, such as accommodating peristaltic movements. Advancements in 3-D in-vivo functional imaging are expected to significantly deepen our understanding of the ENS and its roles in gastrointestinal and neurological diseases, potentially leading to improved diagnostic and therapeutic strategies.

Electromagnetic radiation and biophoton emission in neuronal communication and neurodegenerative diseases

The intersection of electromagnetic radiation and neuronal communication, focusing on the potential role of biophoton emission in brain function and neurodegenerative diseases is an emerging research area. Traditionally, it is believed that neurons encode and communicate information via electrochemical impulses, generating electromagnetic fields detectable by EEG and MEG. Recent discoveries indicate that neurons may also emit biophotons, suggesting an additional communication channel alongside the regular synaptic interactions. This dual signaling system is analyzed for its potential in synchronizing neuronal activity and improving information transfer, with implications for brain-like computing systems. The clinical relevance is explored through the lens of neurodegenerative diseases and intrinsically disordered proteins, where oxidative stress may alter biophoton emission, offering clues for pathological conditions, such as Alzheimer's and Parkinson's diseases. The potential therapeutic use of Low-Level Laser Therapy (LLLT) is also examined for its ability to modulate biophoton activity and mitigate oxidative stress, presenting new opportunities for treatment. Here, we invite further exploration into the intricate roles the electromagnetic phenomena play in brain function, potentially leading to breakthroughs in computational neuroscience and medical therapies for neurodegenerative diseases.

Low-NA two-photon lithography patterning of metal/dielectric tapered optical fibers for depth-selective, volumetric optical neural interfaces

Optical neural implants allow neuroscientists to access deep brain regions, enabling to decipher complex patterns of neural activity. In this field, the use of optical fibers is rapidly increasing, and the ability to generate high-quality metal patterns on their non-planar surface would further extend their application. Here, we propose to use alternating metal shielding and dielectric confinement to engineer the mode-division properties of tapered optical fiber neural implants. This is accomplished through an unconventional application of two-photon lithography (TPL), which employs a low-numerical aperture objective to pattern extensive waveguide sections at both low and high curvature radii. The low-NA TPL is used to polymerize a mask of photoresist, while the rest of the taper undergoes wet metal etching. This implies no direct destructive interaction between the laser beam and the metal to be removed, preserving the optical properties of the dielectric waveguide and of the metal coating. The advantages provided by the presented fabrication method, combined with the intrinsic modal properties of the dielectric waveguide, enable the engineering of the light guiding mechanisms, achieving depth-selective light delivery with a high extinction ratio. The device's light emission and collection properties were investigated in quasi-transparent media and highly scattering brain slices, finding that our proposed method facilitates 360° symmetric light collection around the dielectric-confined section with depth resolution. This opens a perspective for the realization of optical neural implants that can interface the implant axis all-around, with low-NA TPL that can also be applied on other types of non-planar surfaces.

Microfabrication Technologies for Nanoinvasive and High-Resolution Magnetic Neuromodulation

The increasing demand for precise neuromodulation necessitates advancements in techniques to achieve higher spatial resolution. Magnetic stimulation, offering low signal attenuation and minimal tissue damage, plays a significant role in neuromodulation. Conventional transcranial magnetic stimulation (TMS), though noninvasive, lacks the spatial resolution and neuron selectivity required for spatially precise neuromodulation. To address these limitations, the next generation of magnetic neurostimulation technologies aims to achieve submillimeter-resolution and selective neuromodulation with high temporal resolution. Invasive and nanoinvasive magnetic neurostimulation are two next-generation approaches: invasive methods use implantable microcoils, while nanoinvasive methods use magnetic nanoparticles (MNPs) to achieve high spatial and temporal resolution of magnetic neuromodulation. This review will introduce the working principles, technical details, coil designs, and potential future developments of these approaches from an engineering perspective. Furthermore, the review will discuss state-of-the-art microfabrication in depth due to its irreplaceable role in realizing next-generation magnetic neuromodulation. In addition to reviewing magnetic neuromodulation, this review will cover through-silicon vias (TSV), surface micromachining, photolithography, direct writing, and other fabrication technologies, supported by case studies, providing a framework for the integration of magnetic neuromodulation and microelectronics technologies.

Brain-implantable needle-type CMOS imaging device enables multi-layer dissection of seizure calcium dynamics in the hippocampus

Objective: Current neuronal imaging methods mostly use bulky lenses that either impede animal behavior or prohibit multi-depth imaging. To overcome these limitations, we developed a lightweight lensless biophotonic system for neuronal imaging, enabling compact and simultaneous visualization of multiple brain layers.Approach: Our developed 'CIS-NAIST' device integrates a micro-CMOS image sensor, thin-film fluorescence filter, micro-LEDs, and a needle-shaped flexible printed circuit. With this device, we monitored neuronal calcium dynamics during seizures across the different layers of the hippocampus and employed machine learning techniques for seizure classification and prediction.Main results: The CIS-NAIST device revealed distinct calcium activity patterns across the CA1, molecular interlayer, and dentate gyrus. Our findings indicated an elevated calcium amplitude activity specifically in the dentate gyrus compared to other layers. Then, leveraging the multi-layer data obtained from the device, we successfully classified seizure calcium activity and predicted seizure behavior using Long Short-Term Memory and Hidden Markov models.Significance: Taken together, our 'CIS-NAIST' device offers an effective and minimally invasive method of seizure monitoring that can help elucidate the mechanisms of temporal lobe epilepsy.