Fiber based electrode for intercortical recordings

Background

Neurons form the fundamental information processing units of human brain. They communicate amongst themselves through an action potential resulting from electro-chemical activities. Essentially, axons of transmitting neurons move closer to the dendrites of receiving neurons for a shot of ion release. This action creates an electric potential of around +40mV that last for around 1 ms. For any information process, large number of neurons work and communicate in parallel, and any neuron can be excitatory or inhibitory depending upon the characteristics of synapses it generate. Brain Machine Interfacing (BMI) primarily need to read these signals that are generated from different neurons for any specific activity of the living being.

Signals from neurons present in intercortical region of brain are essentially weak, as is evident from the action potential. Earlier studies included non-invasive (EEG) and semi-invasive methods for signal recording. As an alternative, a safe invasive technique to collect signals through direct physical contact with neurons has become a challenging research task. One of the methods involves inserting a fine metallic electrode into brain and subsequently recording the signals. Despite the partial success of this method there exist several practical issues including but not limited to minimum possible diameter of electrode and biocompatibility.

Properties of a suitable electrode include optimized bio-compatibility, conductivity, diameter, surface-morphology, tensile and bending characteristics. Studies have shown that polymers exhibit more compatibility than their metallic counterparts in several biomedical applications; sutures and substrates are common examples. At the Neural Instrumentation Lab in Temple University, we are in the process of developing a nanoscale fiber-based electrode that can be used to record neuron signals.

Synopsis

First phase of the development addresses two prime requirements, minimal diameter and conductivity. It is well documented that neurons, axons and synaptic cleft are of the order of few micrometer to few hundred nanometer. This poses a stringent constraint on the maximum allowable diameter of the electrode; for a precise interaction with individual neurons or synaptic cleft, it needs to be in nanoscale. Nano-sized fibers with appropriate conductivity can be obtained using several methods, and electrospinning is one of the most widely used techniques; Meltblown and self-assembled nano structures are other methods to develop a nanoscale fiber electrode.

In the process of electrospinning, a small droplet of polymer solution pumped out of a syringe is electrostatically charged and an exposure to high potential difference will attenuate the polymer into thin strands of fibers. Fibers can be collected on a grounded plate or roller. Another variation is Melt-Electrospinning – it uses a polymer melt instead of polymer solution. In either case, the principle of attenuation remains same except the processing parameters. For a core-sheath configuration of electro-spun fibers, coaxial electrospinning which uses a sheath polymer extruded coaxially along with the core polymer is employed. Schematic of electrospinning is shown in Figure-1.

Figure-1 Schematic of Electrospinning

Conductivity can be incorporated into a fiber by choosing a polymer that is conductive or by doping conductive additives. Polymers of the Polyacetylene, Poly (p-Phenylene), Polypyrrole and Polyaniline families possess conductive characteristics ranging from 10⁵ to 10¹ S cm^-1. Thus, with a suitable solvent these polymers can be spun into a conductive fiber. In the method of doping, carbon nanotubes are added to make the polymer conductive. Carbon nanotubes (Single walled and Multiwalled) are chemical structures that are purely (up to 99%) carbon and that makes them conductive. Though they are conductive, production mechanism for a long length nanotubes is still under research. But, a suitable quantity of these nanotubes can be doped under controlled distribution in the polymer at the time of extrusion to make it conductive.

Link: A glimpse on Nanofiber Electrode