A softening laminar electrode for recording single unit activity from the rat hippocampus

A Zátonyi,D Meszéna,Z Fekete,R Modi,G Orbán,I Ulbert,A Joshi-Imre,W E Voit,M Ecker,G Márton,A Pongrácz

doi:10.1038/s41598-019-39835-6

Abstract

Softening neural implants that change their elastic modulus under physiological conditions are promising candidates to mitigate neuroinflammatory response due to the reduced mechanical mismatch between the artificial interface and the brain tissue. Intracortical neural probes have been used to demonstrate the viability of this material engineering approach. In our paper, we present a robust technology of softening neural microelectrode and demonstrate its recording performance in the hippocampus of rat subjects. The 5 mm long, single shank, multi-channel probes are composed of a custom thiol-ene/acrylate thermoset polymer substrate, and were micromachined by standard MEMS processes. A special packaging technique is also developed, which guarantees the stable functionality and longevity of the device, which were tested under in vitro conditions prior to animal studies. The 60 micron thick device was successfully implanted to 4.5 mm deep in the hippocampus without the aid of any insertion shuttle. Spike amplitudes of 84 µV peak-to-peak and signal-to-noise ratio of 6.24 were achieved in acute experiments. Our study demonstrates that softening neural probes may be used to investigate deep layers of the rat brain.

Highlights

Implantable microelectrodes interface with neuronal populations by means of electrical, optical and chemical transduction[1]
The target temperature is reached in approximately 1300 seconds, while the storage modulus stabilizes at 300 MPa approximately 500 seconds later
Considering that the relationship between intracortical probe stiffness alone and the foreign body response that follows the traumatic injury of implantation is not well understood, we argue that the technology presented here provides a valuable tool for research

Summary

Introduction

Implantable microelectrodes interface with neuronal populations by means of electrical, optical and chemical transduction[1]. The same study demonstrated that the soft implants at 12.7 MPa Young’s modulus, tethered in rat brain, have fully recovered from neuron loss in the immediate vicinity of the probes by sixteen weeks post-implantation This suggests that tissue compliant probes can be fabricated from materials presenting elasticity in the order of 10 MPa and perhaps above. Several newly-engineered substrate materials have been proposed as mechanically adaptive components for soft neuroprostheses These polymeric materials undergo chemical or temperature based activation resulting in a lower Young’s modulus after placed in the living tissue, but they maintain a Young’s modulus of several GPa and provide easy handling before and during implantation. Fabrication details, electrochemical testing in saline solution and acute in vivo performance of the constructed probes are presented

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