Electrical neural interfaces are unlocking new possibilities for studying, restoring, and augmenting brain functions.
These devices provide direct communication pathways to connect the nervous system with the external world.
Recent innovations in neural electrode design, electronics, wireless transmission, integration of electrical and optical modalities, and neural data processing are rapidly advancing the capabilities of electrical neural interfaces.
Key Facts:
- Neural electrodes with smaller dimensions induce less immune response and enable more stable, long-term recordings.
- Wireless transmission of data and power to implants eliminates cumbersome wires and enables freely-moving experiments.
- Application-specific integrated circuits allow highly efficient neural recording and stimulation in compact form factors.
- Combining electrical and optical modalities provides complementary spatiotemporal resolution and cell-type specificity.
- Novel spike sorting algorithms automate the detection and classification of neuronal firing from large-scale, high-density electrode arrays.
Source: Neuron 2020
Optimized Electrode Design Enables More Precise Interaction with Neurons
A key component of electrical neural interfaces is the implanted electrode array, which must be optimized to interact with neurons and neural tissue.
Conventional rigid, large electrodes tend to cause sustained immune reactions that degrade recording quality over time.
Recent advances have focused on using smaller, flexible materials to improve biocompatibility.
Ultra-small carbon fiber electrodes (7 μm diameter) show reduced neuronal cell death and glial scarring compared to traditional silicon probes in rodent studies.
Flexible neural electrodes made from soft polymers, like parylene and polyimide (1 μm thickness), instead of silicon and metals, better match the mechanical properties of brain tissue.
This results in less compression damage and seamless integration with the brain.
Images of neurons and blood vessels near ultraflexible probes implanted in rodents reveal stable, long-term recordings for months with minimal immune response.
High-density, large-scale electrode arrays are needed to record broad neural populations.
Novel fabrication techniques now allow flexible polymer electrodes with up to 1000 channels, enabling recording of distributed circuits across brain regions in rodents.
Small dimensions are key; minimizing the probe footprint reduces tissue displacement during implantation.
This new generation of electrodes – smaller, flexible, and scalable – provides unmatched spatiotemporal precision to chronically monitor and modulate neural activity.
Wireless Systems Enable New Neuroscience Experiments
Eliminating cumbersome wires is critical for many neuroscience and clinical applications.
Wireless power and data transmission to implanted devices is now possible through electromagnetic, magnetic or acoustic modalities.
Radio-frequency and near-field inductive coupling use electromagnetic fields and have the advantage of commercially available components.
However, RF absorption in tissue constrains device size and implant depth unless wired to a superficial receiver.
Inductive coupling requires relatively large (> 1 cm) pickup coils, limiting implantation in small animals.
Newer acoustic and magnetic methods show promise for wirelessly powering mm-scale devices.
Ultrasound delivers power by pressure waves to a piezoelectric receiver, enabling bi-directional data communication.
Similarly, magneto-electric coupling uses magnetic fields and miniature resonators to deliver power.
These techniques efficiently transfer data and mW-levels of power to miniaturized stimulators and recorders implanted centimeters deep, as demonstrated in freely moving rats.
Wireless systems greatly expand experimental possibilities, from home cage behavior to neuroprosthetics, while minimizing invasiveness.
Further co-development of electrodes, electronics and wireless techniques will enable even smaller, more capable devices.
Specialized Integrated Circuits Enable Efficient Neural Interfacing
Custom integrated circuits are essential to connect implanted electrodes to external equipment for stimulation, recording and signal processing.
Smaller, lower power electronics allow more electrodes per device and battery-powered operation.
Application-specific integrated circuits (ASICs) contain the circuitry to amplify and filter weak neural signals, convert them to digital streams, and program electrical stimulation patterns based on wireless commands.
State-of-the-art ASICs enable acquisition of up to a thousand neural recording channels at only ~50 μW per channel, comparable to benchtop neurophysiology systems.
Small wireless implants utilize ASICs to rectify incoming power waveforms and decode digital data for stimulation.
The latest mm-scale stimulators and recorders transduce power and data magnetically or acoustically to achieve autonomous functionality using only microwatts.
Embedding sensors and signal processors directly on miniaturized ASICs can further enhance performance and capabilities.
Continued optimization of neural interface electronics will enable ever more powerful chronic brain-machine interfaces.
Lighter, lower power systems will improve quality of life for neuroprosthetic patients.
Combining Optical & Electrical Modalities Provides Complementary Perspectives
Optical imaging offers wide field-of-view, cellular specificity and high spatial resolution but slower timescales.
Electrical recording detects precise spike timing of surrounding neurons.
Together these modalities achieve concurrent, multiscale monitoring of neural activity.
Recent advances have enabled combined imaging and electrical interfacing.
Novel transparent, flexible electrodes allow unobstructed optical access.
Advanced spike sorting algorithms can isolate neurons near stimulation sites.
Micro-LEDs integrated onto probes permit targeted optogenetic control alongside electrical recording in behaving animals.
This merging of modalities leverages the strengths of both techniques.
The future integration of electrical, optical and chemical sensing will provide an unprecedented multifaceted view of brain function.
Smarter AI Algorithms Extract Meaning from Expanding Datasets
Improved electrodes and electronics expand the scale of neural data acquisition.
Extracting meaningful information from these massive datasets requires new computational tools.
Recent algorithmic advances enable automated spike detection and sorting from hundreds of electrodes and localization of signal sources in field potential recordings.
Classical manual spike sorting does not scale to the thousands of channels provided by high-density probes.
New density-based clustering algorithms like Kilosort and Mountainsort enable automated, near real-time sorting of large-scale recordings.
Continued development of drift correction and resolution of coincident spikes will further enhance performance.
Understanding the origin of field potential signals measured at multiple sites is challenging. Current source density analysis can now localize underlying synaptic dynamics.
Information theoretic tools like transfer entropy assess directional connections between brain regions.
Dimensionality reduction techniques extract lower-dimensional signal features for advanced analyses.
Innovative neural data processing methods empower interrogation of vastly more complex circuits, unlocking new vistas for neuroscience research.
More sophisticated algorithms are essential to elucidating the brain’s dizzying intricacies.
The Path Forward: An Exciting Future for Brain-Machine Interfaces
Recent advances in electrical neural interface technology are rapidly expanding experimental possibilities, while clinical brain-computer interfaces are already improving lives.
As electrodes continue minimzing tissue disruption, electronics become more efficient, and algorithms get smarter, revolutionary new applications await.
Drastically enlarging the web of neurons that can be simultaneously tracked will reveal new principles of neural coding and computation.
Seamless human-computer interaction may one day enable fluid augmentation of natural cognition.
Electrical interfacing with individual cells could allow precision control of sensory experience, memory formation, and motor actions.
The future is bright for this vibrant field.
Upcoming innovations in neural interface technology will unravel more of the brain’s mysteries and harness its extraordinary capabilities.
This foundational work brings us steps closer to truly revolutionary breakthroughs in neuroscience and medicine.
References
- Study: Recent advances in electrical neural interface engineering: minimal invasiveness, longevity and scalability
- Authors: Lan Luan et al. (2020)
