Andres Canales

Multifunctional fibers for simultaneous optical, electrical and chemical interrogation of neural circuits in vivo

Brain function depends on simultaneous electrical, chemical and mechanical signaling at the cellular level. This multiplicity has confounded efforts to simultaneously measure or modulate these diverse signals in vivo. Here we present fiber probes that allow for simultaneous optical stimulation, neural recording and drug delivery in behaving mice with high resolution. These fibers are fabricated from polymers by means of a thermal drawing process that allows for the integration of multiple materials and interrogation modalities into neural probes. Mechanical, electrical, optical and microfluidic measurements revealed high flexibility and functionality of the probes under bending deformation. Long-term in vivo recordings, optogenetic stimulation, drug perturbation and analysis of tissue response confirmed that our probes can form stable brain-machine interfaces for at least 2 months. We expect that our multifunctional fibers will permit more detailed manipulation and analysis of neural circuits deep in the brain of behaving animals than achievable before.

Optogenetic entrainment of neural oscillations with hybrid fiber probes

OBJECTIVE Optogenetic modulation of neural activity is a ubiquitous tool for basic investigation of brain circuits. While the majority of optogenetic paradigms rely on short light pulses to evoke synchronized activity of optically sensitized cells, many neurobiological processes are associated with slow local field potential (LFP) oscillations. Therefore, we developed a hybrid fiber probe capable of simultaneous electrophysiological recording and optical stimulation and used it to investigate the utility of sinusoidal light stimulation for evoking oscillatory neural activity in vivo across a broad frequency range. APPROACH We fabricated hybrid fiber probes comprising a hollow cylindrical array of 9 electrodes and a flexible optical waveguide integrated within the core. We implanted these probes in the hippocampus of transgenic Thy1-ChR2-YFP mice that broadly express the blue-light sensitive cation channel channelrhodopsin 2 (ChR2) in excitatory neurons across the brain. The effects of the sinusoidal light stimulation were characterized and contrasted with those corresponding to pulsed stimulation in the frequency range of physiological LFP rhythms (3-128 Hz). MAIN RESULTS Within hybrid probes, metal electrode surfaces were vertically aligned with the waveguide tip, which minimized optical stimulation artifacts in neurophysiological recordings. Sinusoidal stimulation resulted in reliable and coherent entrainment of LFP oscillations up to 70 Hz, the cutoff frequency of ChR2, with response amplitudes inversely scaling with the stimulation frequencies. Effectiveness of the stimulation was maintained for two months following implantation. SIGNIFICANCE Alternative stimulation patterns complementing existing pulsed protocols, in particular sinusoidal light stimulation, are a prerequisite for investigating the physiological mechanisms underlying brain rhythms. So far, studies applying sinusoidal stimulation in vivo were limited to single stimulation frequencies. We show the feasibility of sinusoidal stimulation in vivo to induce coherent LFP oscillations across the entire frequency spectrum supported by the gating dynamics of ChR2 and introduce a hybrid fiber probe tailored to continuous light stimulation.

Multifunctional Fibers as Tools for Neuroscience and Neuroengineering

Multifunctional devices for modulation and probing of neuronal activity during free behavior facilitate studies of functions and pathologies of the nervous system. Probes composed of stiff materials, such as metals and semiconductors, exhibit elastic and chemical mismatch with the neural tissue, which is hypothesized to contribute to sustained tissue damage and gliosis. Dense glial scars have been found to encapsulate implanted devices, corrode their surfaces, and often yield poor recording quality in long-term experiments. Motivated by the hypothesis that reducing the mechanical stiffness of implanted probes may improve their long-term reliability, a variety of probes based on soft materials have been developed. In addition to enabling electrical neural recording, these probes have been engineered to take advantage of genetic tools for optical neuromodulation. With the emergence of optogenetics, it became possible to optically excite or inhibit genetically identifiable cell types via expression of light-sensitive opsins. Optogenetics experiments often demand implantable multifunctional devices to optically stimulate, deliver viral vectors and drugs, and simultaneously record electrophysiological signals from the specified cells within the nervous system. Recent advances in microcontact printing and microfabrication techniques have equipped flexible probes with microscale light-emitting diodes (μLEDs), waveguides, and microfluidic channels. Complementary to these approaches, fiber drawing has emerged as a scalable route to integration of multiple functional features within miniature and flexible neural probes. The thermal drawing process relies on the fabrication of macroscale models containing the materials of interest, which are then drawn into microstructured fibers with predefined cross-sectional geometries. We have recently applied this approach to produce fibers integrating conductive electrodes for extracellular recording of single- and multineuron potentials, low-loss optical waveguides for optogenetic neuromodulation, and microfluidic channels for drug and viral vector delivery. These devices allowed dynamic investigation of the time course of opsin expression across multiple brain regions and enabled pairing of optical stimulation with local pharmacological intervention in behaving animals. Neural probes designed to interface with the spinal cord, a viscoelastic tissue undergoing repeated strain during normal movement, rely on the integration of soft and flexible materials to avoid injury and device failure. Employing soft substrates, such as parylene C and poly-(dimethylsiloxane), for electrode and μLED arrays permitted stimulation and recording of neural activity on the surface of the spinal cord. Similarly, thermally drawn flexible and stretchable optoelectronic fibers that resemble the fibrous structure of the spinal cord were implanted without any significant inflammatory reaction in the vicinity of the probes. These fibers enabled simultaneous recording and optogenetic stimulation of neural activity in the spinal cord. In this Account, we review the applications of multifunctional fibers and other integrated devices for optoelectronic probing of neural circuits and discuss engineering directions that may facilitate future studies of nerve repair and accelerate the development of bioelectronic medical devices.

Electronic, optical, and chemical interrogation of neural circuits with multifunctional fibers (Conference Presentation)

Despite recent advances in microfabrication and nanofabrication, integrating multiple modes of communication with the brain into a single biocompatible neural probe remains a challenge. These multifunctional neural probes may further our understanding of normal and disrupted functions of neural circuits manifested in neurological conditions, such as Parkinson’s disease. Here, we present a novel family of probes fabricated using a thermal drawing process. In this process, a macroscopic template (preform) containing the desired features is drawn by applying heat and tension into a fiber that conserves the original geometry of the preform but at a much smaller scale. Being composed of soft materials, such as polymers, conductive composites, and low melting temperature metals, fiber based neural probes minimize the damage to the surrounding tissue when implanted. Furthermore, fiber drawing enables straightforward integration features allowing for simultaneous electrical, optical and chemical interrogation of the brain. We demonstrate the utility of these probes for one-step optogenetics, in which a viral vector carrying opsin genes is injected through the same device then used to optically stimulate neurons and record their response as electrical activity. With these probes, we also show, for the first time, recordings of electrical activity in the spinal cord of freely moving mice.