SCIENTIFIC HIGHLIGHTS

Remote control of neural cell behavior using induced dipoles in electroactive conducting implanted materials

Electrostimulation of neurons is possible through the induced dipoles on implanted transparent conductive materials, with distinct effects depending on the material and its intercalation properties

Electrostimulation of the Central nervous system to alleviate neurological symptoms or trying to repair damaged tissue is possible through implanted electrodes in direct electrical contact to a power source. When using intercalation materials like Iridium Oxide and its hybrids with nanocarbons as electrodes, the charge capacity of the final electrode is enlarged several orders of magnitude, with a simultaneous decrease of impedance. In those cases DC fields render neural repair in short periods of time. Nano-structuring induced by the coupling of iridium oxide around carbon nanotubes or graphene yields stable coatings with large cyclability not present for graphite combinations or even for IrOx alone. This work shows a significant discovery that opens a new possibility: response of neural cells is also possible using electrodes without direct contact to the power source. Immersed conducting materials in the biological media, with an external field applied by remote external electrodes, render a local dipole between the borders of the material that, in turn, may induce chemical reactions for certain potentials.

This effect is known as bipolar electrochemistry, and the final induced dipole potential depends on the applied external field, the material and the geometry of the electrochemical cell. Within the electrochemical safe window of water, intercalation of ions and ionic gradients along the material occur, yielding gradients that modulate the cell growth. The observed neural effects are quite distinct depending on the material. Thus, Iridium oxide materials favor speed of dendrite growth, while PEDOT conducting polymers favor growth turning towards a specific direction. In contrast noble metals show smaller or no effects.  Electroactive materials are believed to act differently thanks to the ionic gradients created within the material depending on the specific intercalation properties. Thus, iridium oxide is truly a hydrated oxohydroxide that allows H+, Na+, K+ and OH- intercalation/deintercalation processes at negative and positive poles. PEDOT-PSS on the other hand, has a large PSS anion that cannot move out of the structure and that results in cation intercalation only. Diffusion of ions is also different for both types of materials, and result in a different impedance behavior.

The significance of the observation is related to the design of new implants in biological systems, when electrostimulation is planned, and allow to explore a new remote control procedure, even in cases where electrodes need to be transparent (retina).

Authors:
Ann M. Rajnicek,2 Zhiqiang Zhao,2 Javier Moral-Vico,1 Ana M. Cruz,2 Colin D. McCaig,2 and Nieves Casañ-Pastor1 

Affiliation:
1Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
2School of Medical Sciences-University of Aberdeen, UK

Publication:
Controlling Nerve Growth with an Electric Field Induced Indirectly in Transparent Conductive Substrate Material
Advanced Healthcare Materials 7, 1800473 (2018)
DOI: 10.1002/adhm.201800473

Figure:
Remote electrostimulation of neural cells through induced dipoles on A-C) metals and transparent nanostructured conducting materials. D) Cell geometry for remote control and E) pH changes observed at the poles of gold/titanium  coatings for small and large voltages.

Acknowledgments

This work has been possible thanks to the financing of EU, Fundació MaratoTV3 and MINEICO grants, and to the deep collaboration among University of Aberdeen School of Medical Sciences and ICMAB with planned electric field protocols designed commonly by bio and materials scientists

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