Niall McAlinden

Cortical implants are a proven clinical neurotechnology with the potential to transform our understanding of cognitive processes. These processes rely on complex neuronal networks that are difficult to selectively probe or stimulate. Optogenetics offers cell-type specificity, but achieving the density and coverage required for chronic, high-resolution modulation remains a challenge. We fabricated 100-element μ LED arrays (200 μ m pixel pitch, 2 × 2 mm ² footprint) coupled into chronically implantable systems for optogenetic stimulation of the mouse cortex. The μ LEDs remain stable for over 300 hours continuous operation time in vivo, allowing for months-long chronic experiments. Simultaneous electrophysiology recordings confirmed robust neuronal responses at low μ LED drive currents (< 5 mA), minimising thermal effects. Here we show that our device can be chronically implanted in freely-behaving mice and drive behavioural outcomes from spatiotemporal, patterned, optogenetic stimulation of auditory cortical circuits.

SignificanceCurrent preclinical evaluation of Alzheimer’s disease pathology in mouse models relies on post-mortem analyses, which hinders the development and optimization of therapeutic approaches. Although in vivo methods exist, monitoring amyloid plaque signals across multiple brain regions in freely behaving animals remains a significant challenge.AimWe aim to develop an optical approach to address this challenge.ApproachWe used flat and tapered optical fibers in an Alzheimer’s mouse model.ResultsWe first confirmed that conventional flat fiber-based photometry can detect amyloid plaque signals across multiple brain regions under anesthesia after injecting a blood-brain-barrier-permeable tracer, Methoxy-X04. The depth profile of in vivo fluorescent signals is correlated with histological signals. A machine learning approach could distinguish between in vivo fluorescent signals of mice with and without amyloid plaques. Next, after validating the feasibility of depth-resolved fiber photometry ex vivo, we chronically implanted a tapered fiber to monitor amyloid plaque signals in freely behaving mice. After injecting Methoxy-X04, fluorescent signals increased in a depth-specific manner in Alzheimer’s mice, but not in their wild-type littermates.ConclusionsOur approach expands the capabilities of fiber photometry to monitor molecular pathologies, such as amyloid plaques, even in a freely behaving condition.

Cortical implants are a proven clinical neurotechnology with the potential to transform our understanding of cognitive processes. These processes rely on complex neuronal networks that are difficult to selectively probe or stimulate. Optogenetics offers cell-type specificity, but achieving the density and coverage required for chronic, high-resolution modulation remains a challenge. Here we present a 100-element µLED array (200 µm pixel pitch, 2 × 2 mm2 footprint) coupled into a miniaturised, flexible system suitable for chronic implantation and optogenetic stimulation of the surface of the mouse cortex. The µLEDs can remain stable for over 300 hours continuous operation time in-vivo , allowing for months-long chronic experiments. Simultaneous electrophysiology recordings confirmed robust neuronal responses corresponding to low µLED drive currents (<5 mA), minimising thermal effects and supporting future wireless operation. The spatial resolution of neuronal responses was consistent with a simulated model of light scattering in the cortical layers, enabling device optimisation. Behavioural experiments with chronically implanted mice demonstrated robust learning during discrimination tasks using spatially distinct optogenetic stimulation patterns.

Objective. Optogenetics allows the manipulation of neural circuits in vivo with high spatial and temporal precision. However, combining this precision with control over a significant portion of the brain is technologically challenging (especially in larger animal models). Approach. Here, we have developed, optimised, and tested in vivo, the Utah Optrode Array (UOA), an electrically addressable array of optical needles and interstitial sites illuminated by 181 μLEDs and used to optogenetically stimulate the brain. The device is specifically designed for non-human primate studies. Main results. Thinning the combined μLED and needle backplane of the device from 300 μm to 230 μm improved the efficiency of light delivery to tissue by 80%, allowing lower μLED drive currents, which improved power management and thermal performance. The spatial selectivity of each site was also improved by integrating an optical interposer to reduce stray light emission. These improvements were achieved using an innovative fabrication method to create an anodically bonded glass/silicon substrate with through-silicon vias etched, forming an optical interposer. Optical modelling was used to demonstrate that the tip structure of the device had a major influence on the illumination pattern. The thermal performance was evaluated through a combination of modelling and experiment, in order to ensure that cortical tissue temperatures did not rise by more than 1 °C. The device was tested in vivo in the visual cortex of macaque expressing ChR2-tdTomato in cortical neurons. Significance. It was shown that the UOA produced the strongest optogenetic response in the region surrounding the needle tips, and that the extent of the optogenetic response matched the predicted illumination profile based on optical modelling—demonstrating the improved spatial selectivity resulting from the optical interposer approach. Furthermore, different needle illumination sites generated different patterns of low-frequency potential activity.

Optogenetics allows manipulation of neural circuits in vivo with high spatial and temporal precision. However, combining this precision with control over a significant portion of the brain is technologically challenging (especially in larger animal models). Here, we have developed, optimised, and tested in vivo, the Utah Optrode Array (UOA), an electrically addressable array of optical needles and interstitial sites illuminated by 181 µLEDs and used to optogenetically stimulate the brain. The device is specifically designed for non-human primate studies. Thinning the combined µLED and needle backplane of the device from 300 µm to 230 µm improved the efficiency of light delivery to tissue by 80%, allowing lower µLED drive currents, which improved power management and thermal performance. The spatial selectivity of each site was also improved by integrating an optical interposer to reduce stray light emission. These improvements were achieved using an innovative fabrication method to create an anodically bonded glass/silicon substrate with through-silicon vias etched, forming an optical interposer. Optical modelling was used to demonstrate that the tip structure of the device had a major influence on the illumination pattern. The thermal performance was evaluated through a combination of modelling and experiment, in order to ensure that cortical tissue temperatures did not rise by more than 1°C. The device was tested in vivo in the visual cortex of macaque expressing ChR2-tdTomato in cortical neurons. It was shown that the strongest optogenetic response occurred in the region surrounding the needle tips, and that the extent of the optogenetic response matched the predicted illumination profile based on optical modelling – demonstrating the improved spatial selectivity resulting from the optical interposer approach. Furthermore, different needle illumination sites generated different patterns of low-frequency potential (LFP) activity.

There is current demand for pattern programmable devices that can deliver light to deep brain structures for use in optogenetic experiments. Here we introduce a microLED optrode device with 181 individually addressable sites, each with the capability to optogenetically excite thousands of neurons in vivo. The device consists of a 10 x 10 glass microneedle array directly integrated with a custom fabricated microLED device. 100 microLEDs couple to the needle tips for intra-cortical light delivery and 81 microLEDs sit at interstitial points for surface illumination. Light delivery and thermal properties are comprehensively evaluated.

Optogenetics is a powerful tool for neural control that supports perturbation of specific cell types. The Utah Optrode Array, paired with a µLED array, is an optical stimulation device for precise multi-site light delivery in deep cortical layers. We describe recent developments in the evolution of this device, including methods for improved light coupling and device encapsulation.

In vivo electrophysiology is the gold standard technique used to investigate sub-second neural dynamics in freely behaving animals. However, monitoring cell-type-specific population activity is not a trivial task. Over the last decade, fiber photometry based on genetically encoded calcium indicators has been widely adopted as a versatile tool to monitor cell-type-specific population activity in vivo . However, this approach suffers from low temporal resolution. Here, we combine these two approaches to monitor both sub-second field potentials and cell-type-specific population activity in freely behaving mice. By developing an economical custom-made system, and constructing a hybrid implant of an electrode and a fiber optic cannula, we simultaneously monitor artifact-free pontine field potentials and calcium transients in cholinergic neurons across the sleep-wake cycle. We find that pontine cholinergic activity co-occurs with sub-second pontine waves, called P-waves, during rapid eye movement sleep. Given the simplicity of our approach, simultaneous electrophysiological recording and cell-type-specific imaging provides a novel and valuable tool for interrogating state-dependent neural circuit dynamics in vivo .

We present an electrically addressable optrode array capable of delivering light to 181 sites in the brain, each providing sufficient light to optogenetically excite hundreds of neurons in vivo , developed with the aim to allow behavioural studies in large mammals. The device is a glass microneedle array directly integrated with a custom fabricated microLED device, which delivers light to 100 needle tips and 81 interstitial surface sites, giving 2-level optogenetic excitation of neurons in vivo . Light delivery and thermal properties are evaluated, with the device capable of peak irradiances > 80 mW/mm ² per needle at 50 ms pulse widths with tissue temperature increase less than 1 ^° C. Future designs are explored through optical and thermal modelling and benchmarked against the current device.