Bioelectronic interfaces: “The shape of the electrode plays a big and important role“

Interview with Prof. Andreas Offenhäusser, Institute of Complex Systems and Peter Grünberg Institute, Bioelectronics, Forschungszentrum Jülich

Knowing what holds the world together at its core – in some ways, medicine also pursues this dream when it seeks to measure ever more precisely what happens in our bodies. In the future, bioelectronic interfaces, whose electrodes are able to communicate with individual cells, could play a big role in this.


Photo: Prof. Andreas Offenhäusser

Prof. Andreas Offenhäusser; ©Ralf-Uwe Limbach/ Forschungszentrum Jülich

At, Prof. Andreas Offenhäusser talks about electrodes with a very specific shape: as nanometer-sized mushrooms, they are “swallowed up” by cells and are thus able to measure electrical and electrochemical cell signals. This could be the basis for new, more precise implants.

Prof. Offenhäusser, you study nanometer-sized electrodes that are meant to measure cell signals. Where could you use these electrodes?

Andreas Offenhäusser: There are different application areas. One possible application is with cochlear implants, retinal implants or with deep brain stimulation where you want to exchange information between electronic circuit and brain. You could make an implant for example that conveys better signals than any implants on the market today – if they are actually already on the market.

Secondly, we are also building a platform to develop in vitro test systems. We could use these to simulate and understand the pathogenesis of cells. Thirdly, you could study neural signal processing and use this knowledge to develop chips in the future.

How can we imagine a bioelectronic interface?

Offenhäusser: On the one hand, we have biology, the nerve cell, which is generally in close contact with the electrode. Currently, it is made up of silicon, gold, platinum or other metals that are unproblematic for cells. We must not use any materials here that lead to a contamination of the cell or cause an immune response. This is why any future system we develop that is based on this technology also needs to have a surface coating that prevents negative cell responses.

The space between nerve cell and electrode is filled with an electrolyte solution, since the cell is situated in this medium. Biopolymers, which the cell discards and surrounds itself with, are also likely to be between the cell membrane and the electrode surface.
Foto: Scanning of an electrode

This scanning electron microscope picture shows an electrode that has been swallowed by a cell; ©Forschungszentrem Jülich

You have also looked at the shape of the electrode in your research. How does it affect cell behavior?

Offenhäusser: The shape of the electrode plays a big and important role. The simplest shape we are dealing with is a planar electrode found on a chip. Here cells assume a relatively flat shape. Signals between cell and electrode are exchanged either in an electrochemical or electrical manner.

We are now working on giving these electrodes a three-dimensional structure, so the cells interact better with them. A kind of mushroom shape has turned out to be the ideal shape: a stem with a wide umbrella-like cap the cell wants to absorb and literally swallow up. However, it is not able to swallow the stem, which is permanently attached to the surface; it can only enclose it. In doing so, it establishes good contact with the electrode.

Why does this particular shape have this kind of effect?

Offenhäusser: It basically addresses a natural reaction of cells to foreign bodies, the phagocytosis process. This means that cells absorb small or nanoparticles for example when they come in contact with them.

In earlier tests, we already experimented with other three-dimensional structures, but chose the wrong dimensions and misjudged the cell. The cell was not able to enclose structures that are too small or too narrow. Just recently, we were able to determine the best shape and show that the mushroom shape enables an almost ideal configuration between cell and electrode. We examined cell cross-sections on the structures with the scanning electron microscope and were able to see exactly how tightly the cells adhere to the electrodes.

Graphic: Theoretical models of possible elctrode shapes

The Jülich researchers tested different shapes of 3D nano electrodes and how closely they are swallowed by cells in theoretical models; ©Forschungszentrum Jülich

What cell signals can you measure with this?

Offenhäusser: We measure electrical signals of cells. When a cell generates an action potential, ion channels open up and currents flow across the cell membrane, which we can measure. We then see a change in the surface potential of the electrodes. Currents can also develop when neurotransmitters of the cell respond to the electrode surface. We can detect them if they are electrochemically active and when we apply voltage to the electrode.

What would a concrete interface look like, also in terms of proportions?

Offenhäusser: To measure signals from a cell, the electrode should be a little smaller than the nerve cell body. It could then easily cover the entire electrode. You need a much tinier structure to be able to measure signals in the axons, the long threadlike part of a nerve cell. Here we are reaching our limit with metal electrodes and have to change over to nanoelectronic components.

We produce those types of dimensions, which are also being used for building transistors for computers or Smartphones, in traditional clean rooms. We only need to adapt our chips so they survive in a saline solution. This is the challenge for our development. Apart from that, we are building on classic semiconductor technology.

In your assessment, what significance do bioelectronic interfaces hold today and in the future?

Offenhäusser: They are already very important in the implant field today. The cochlear implant is a successful model and I think eventually, retinal implants will become a successful system as well. Bioelectronic interfaces will also become more and more significant in deep brain stimulation, since we live in an aging society.

Photo: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.