In-vitro Modeling of Electrode-tissue Parameters

Maran Ma and Timothy E. Kennedy

Department of Neurology and Neurosurgery, Montreal Neurological Institute, McGill University, Montréal, Canada

1 OBJECTIVES

The long-term causes underlying the failure of neural

recording electrodes is an active question in the

community of neural implant users and developers. It

is known that a variety of phenomena contribute to

signal degradation, but to engineer better devices, the

impact of each factor must be quantified and

prioritized. Excluding modes of general implant

failure such as connectors and meningeal responses

(Barrese, et al., 2013), the causes of gradual loss of

signal fidelity on individual electrodes include: 1)

insulation breakdown (Prasad, et al., 2014), 2)

biofouling (Malaga, et al., 2015), 3) glial

ensheathment (Polikov, et al., 2005), and 4) neuronal

death (Biran, et al., 2005). It is difficult to identify the

dominating problem based on existing literature,

which offer different conclusions (Prasad, et al.,

2014; Malaga, et al., 2015).

One challenge to teasing these factors apart is that

they occur simultaneously in-vivo. Another is that

impedance, the primary method of monitoring

electrode status, is affected by multiple factors. Not

surprisingly the correlation between impedance and

signal quality has been reported to be weak in

longterm studies of intracortical arrays, and changes

in strength and direction during different time periods

post implantation (Barrese, et al., 2013).

The purpose of this project is to build a simple and

cost effective in-vitro setup to model each

phenomenon individually, and quantify its impact on

both impedance spectra and electrophysiological

recording quality.

2 METHODS

2.1 Hardware Components

A custom adapter board was built to interface with

commercially available multi-electrode array (MEA)

plates (single well plate, Axion Biosystems).

Electrical contact is made via Z-axis elastomer, and

secured in place with a 3D printed housing. The

adapter board was laid out such that electrode

positions in the MEA mapped to a corresponding

field of headers.

Figure 1: A. Adapter board for interfacing with MEA plate.

B. MEA plate seated in adapter board, two electrode sites

connected to impedance converter board.

A single-chip impedance converter / network

analyser (AD5933, Analog Devices) is used to

perform electrochemical impedance spectroscopy.

The device was modified for optimal measurement of

1Khz -10Khz range by adding a 4Mhz oscillator.

A generic data acquisition module with built-in

instrumentation amplifier (DAQPad-6259, National

Instruments) and custom LabVIEW programs were

setup for electrophysiological recordings.

2.2 Biological Components

Culture of rat embryonic (E18) cortical neurons and

postnatal (P1) astrocytes and microglia will be used

to model neural and glial-scar tissue in the MEA

plate. Inflammation inducing factors (such as TGFβ1)

will be added to convert glial cells to a state of

reactive gliosis. The HEK293 cell-line is used to

generate reference data on effects of cell density.

2.3 Test Conditions

The four conditions will be modelled as follows:

• Biofouling: incubate MEA plate with 100%

fetal bovine serum (FBS).

Ma M. and Kennedy T.

In-vitro Modeling of Electrode-tissue Parameters.

In NEUROTECHNIX 2017 - Extended Abstracts (NEUROTECHNIX 2017), pages 12-13

• Glial ensheathment: seed glial cells at

various densities on top of a neural culture.

• Neuronal death: increase distance between

MEA surface and neural culture with

hydrogel coating or a dense glial layer.

• Insulation breakdown: natural degradation

of the MEA plate (insulation layer begins to

detach with repeated usage/sterilization).

3 RESULTS

Preliminary testing of the in-vitro setup showed

sufficient sensitivity and consistency in impedance

measurements to proceed to electrophysiological

experiments.

Differences in spectra were observed after

modifications to the MEA surface: before and after

PDL coating (Figure 2A), after one day of FBS

incubation (Figure 2B), and at HEK293 cell densities

of ~50% and ~80% confluence (Figure 2C).

Figure 2: A. Before and after PDL coating, measured in

DMEM solution with 10% FBS (typical for HEK293 cell

culturing). B. After one day of incubation in serum (100%

FBS). C. HEK293 cell culture at different densities.

4 DISCUSSION

This system for in-vitro impedance testing and

electrophysiology combines relevant aspects of a

neural electrode to tissue interface in a simple setup.

Commercial disposable MEA dishes are affordable,

well fabricated to work with in-vitro methods, and

transparent for convenient imaging. The custom

adapter board enables flexible access to MEA sites

for connecting to any instrumentation. The single-

chip impedance converter and general purpose data

acquisition box provide low-cost and programmable

options for interrogating the MEA plate.

Preliminary impedance data show promise that

different electrode-tissue conditions yield different

spectra. This may offer a means to reconcile

observations in the literature regarding impedance

and signal quality trends. A potential further

application of this system is screening of electrode

coating materials for enhancing biocompatibility.

REFERENCES

Barrese, J. C. et al., 2013. Failure mode analysis of silicon-

based intracortical microelectrode arrays in non-human

primates. Journal of Neural Engineering, 10(6).

Biran, R., Martin, D. C. & Tresco, P. A., 2005. Neuronal

cell loss accompanies the brain tissue response to

chronically implanted silicon microelectrode arrays.

Experimental Neurology, 195(1).

Malaga, K. A. et al., 2015. Data-driven model comparing

the effects of glial scarring and interface interactions on

chronic neural recordings in non-human primates.

Journal of Neural Engineering, 13(1).

Polikov, V. S., Tresco, P. A. & Reichert, W. M., 2005.

Response of brain tissue to chronically implanted

neural electrodes. Journal of Neuroscience Methods,

148(1).

Prasad, A. et al., 2014. Abiotic-biotic characterization of

Pt/Ir microelectrode arrays in chronic implants.

Frontiers in Neuroengineering, 7(2).

~50% confluence

~80% confluence