Magnetically Inserted Neural Electrodes: Tissue Response and Functional Lifetime

Abstract

Neural recording and stimulation have great clinical potential. Long-term neural recording remains a challenge, however, as implantable electrodes eventually fail due to the adverse effects of the host tissue response to the indwelling implant. Astrocytes and microglia attempt to engulf the electrode, increasing the electrical impedance between the electrode and neurons, and possibly pushing neurons away from the recording site. Faster insertion speed, finer tip geometry, smaller size, and lower material stiffness all seem to decrease damage caused by insertion and reduce the intensity of the tissue response. However, electrodes that are too small result in buckling, making insertion impossible. In this paper, we assess the viability of high-speed (27.8 m/s) deployment of 25 μm, ferromagnetic microelectrodes into rat brain. To characterize functionality of magnetically inserted electrodes, 4 Long-Evans rats were implanted for 31 days with impedance measurements and neural recordings taken daily. Performance was compared to 150 μm diameter PlasticsOne electrodes since 25 μm electrodes buckled during "slow speed" insertion. Platinum-iron magnetically inserted electrodes resolved single unit activity throughout the duration of the study in one rat, and saw no significant change (p=0.970) in impedance (4.54% increase) from day 0 (Z0 ≈ 144 kΩ,Z31 ≈ 150 kΩ). These findings provide a proof-of-concept for magnetic insertion as a viable insertion method that enables nonbuckling implantation of small (25 μm) microelectrodes, with potential for neural recording applications.

An Injectable Neural Stimulation Electrode Made from an In‐Body Curing Polymer/Metal Composite

“…A number or previous studies have taken advantage of silicone (PDMS) composites doped with conductive filler particles in order to create a conductive material, including for use the development of neural electrodes . Additionally, other in‐body curing PDMS composite materials have been proposed in the literature and are commonly used in small animal studies due to PDMS being physiological inert and favorable curing properties; PDMS cures in an aqueous environment without the formation of toxic products or exothermic heat . These advancements in conductive and in‐body curing polymers have not been applied to create an injectable electrode, but they provide a basis for the use of silver particles as a conductive filler within a stretchable silicone substrate.…”

Materials and technologies for soft implantable neuroprostheses

“…Several strategies have been proposed to facilitate the insertion of soft implants. These strategies include the use of insertion aides 86 , dissolvable, rigid carriers 87 , stiffening of the implant backbone 88 and larger designs (>1-mm 2 cross-section). An interesting alternative is the implementation of mechanically adaptable materials: that is, engineered polymers that soften once in vivo.…”

Implantable Neural Interfaces and Wearable Tactile Systems for Bidirectional Neuroprosthetics Systems

“…For recording, small active sites are preferred since they are more selective for single units whereas voltage rapidly decays with the distance from the neuronal source. However when the size of the active site is decreased, the electrical impedance and the thermal noise increase thus inevitably attenuating the signal‐to‐noise ratio . Differently, when using larger sites for neural recording, there is an averaging of the signal among strong responses from close neurons and weak responses from distant neurons since the larger site can be modeled as a set of small sites in parallel.…”

“…Comparison among different strategies of stimulation of PNS to evoke tactile percepts. Parameters that have been considered are: i) number of subjects (S) and implant duration; ii) type of implanted electrode, number of channels, nerve target; iii) impedance measurement; iv) type of stimulation pattern; v) threshold charge/current over time; vi) …”