A 2-D micro-magnetic neuro-stimulation platform

Abstract

The aim of this thesis is the development of a novel in-vitro neuro-stimulation tool based on the micro-scale implementation of the transcranial magnetic stimulation (TMS) principle. The project involves the design, fabrication and testing of single coil geometries and a two dimensional array of micro-coils for establishing spatio-temporal magnetic flux profiles. The proposed device can induce a localised electric field in the vicinity of the coils that can instigate the stimulation of single or multiple neurons in vitro. The first steps of this project covered the investigation of all the parameters that affect the efficiency of a micro-coil structure, in an attempt to achieve an induced electric field above the stimulation threshold of a neuron (e.g. spatial derivative electric field intensity: ∂Ex/∂x>11kV/m2 [1]). The investigation is based on a parametric study with COMSOL Multiphysics simulation software, while for the design of the structure further experimental limitations were taken into account. The fabrication steps for the development of the micro-coils include two photolithographic steps while the further increase of micro-coils’ thickness was achieved with electroplating. The packaging, the bio-compatible encapsulation with Parylene-C and the functionalization of the material, in terms of hydrophilicity, are also presented and complete the platform prototyping. The micro-coils are characterized electrically with an impedance frequency sweep while a further monitoring of their electromagnetic behaviour was performed with magnetic nanoparticles trapping and inductive measurements between different coils in the same array. Their ability to stimulate magnetically neural cells was evaluated firstly with a phantom gel with electric properties (electrical permittivity and conductivity) similar to neural tissue, with the use of bio-oriented simulations with NEURON software + COMSOL and with biological validation in vitro. Finally, the main challenge of this method is to define the limits of safe operation of the micro-inductors prior to their failure due to Joule heating and electromigration phenomena. In this direction, an electrothermal study was performed to define the maximum current capacity that could safely hold.

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Identifiers

ORCID for Themis Prodromakis: orcid.org/0000-0002-6267-6909

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