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.
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.