Growing nerve cells on chips

a) Biohybrid chip with functionalized waveguides and overgrown neurons (black). Individual cells are optically interconnected for stimulation and readout. b) Zoom into hybrid synaptic attachment region, showing array of waveguide crossings (grey) and protein-decorated surfaces (green).
© Jürgen Klingauf, Wolfram Pernice

Project title: Biohybrid neurosynaptic chips interfaced with nanostructured, integrated optics
Principal investigators: Jürgen Klingauf, Wolfram Pernice
Project time: 11/2017 - 12/2018
Project code: FF-2017-10

Nerve cells in the brain are linked with each other through a large number of connections. They use these connections, known as synapses, to transmit signals from cell to cell and communicate with one another. During this process of synaptic transmission, the emitting nerve cell, electrically stimulated, releases signalling molecules which, in turn, electrically stimulate receiving cells. These are stored in vesicles in the cell process – the axon – of the emitting nerve cell and are released when the vesicles merge with the cell membrane.

In this project, biophysicists and nanophysicists are aiming to jointly develop a new model which will enable them to analyse the variability of synapses in the signal emitting nerve cells. Not much is currently known about this. The researchers want to know whether the synapses change their structure and activity immediately after they have become active. Can nerve cells activate their individual synapses on a process to different extents, depending on the activity a receiving nerve cell requires? In order to eventually find answers to these questions, the researchers are developing a chip onto which they place both nerve cells and, at certain spots, proteins which induce the formation of synapses. The aim is for artificial synapses to grow between nerve cells and chip structures enabling the communication between nerve cells and chip. In the process, optical waveguides on the chip stimulate the cells, receive optical signals from the artificial synapses and pass these on to other cells and synapses.

Detailed project description:

Prof. Jürgen Klingauf, a biophysicist, and his team have already developed special nerve cell cultures for the experiment which produce synapses with a substrate at defined spots. The cells come from mice and were cultivated in a culture medium outside the organism, on glass coverslips or chips. They form a fluorescent protein which makes it possible to make those synaptic vesicles – which merge with the membrane and transmit signalling molecules – glow.

The aim is to place these nerve cells on the optical chip. A team of nanophysicists led by Prof. Wolfram Pernice are engaged on the technical development of the chip. This process is called nano-production because the structures on the chip are on a nanometer-scale. They are tiny components which are about a thousand times smaller than the diameter of a human hair. Using electron beam lithography, the researchers produce a grid on the chip. On this grid are optical waveguides, which are fibres that can transmit light. Onto defined spots on these optical waveguides the researchers place special proteins. These proteins play a part in the signal receiving nerve cells during synapse formation. In this way, the nerve cells distributed over the chip begin to form artificial synapses with the protein nano-structures.

Ultimately, this is how the model works: a nerve cell on the chip is stimulated by a light pulse, whereupon the cell begins to become active. The synaptic vesicles with the fluorescent proteins merge with the cell membrane of the artificial synapses on the optical waveguides. The chip recognizes these optical signals, amplifies them, and passes them on to their destinations – other synapses or nerve cells. This model involves a so-called biohybrid system as it combines biological and technical elements with one another – the nerve cell and chip are communicating with each other. The researchers use high-resolution light microscopy to visualize the processes.

There is, however, one technical difficulty in developing the chips – placing the proteins precisely on the optical waveguides and thus exactly guiding the synapses being formed. To solve this problem, the researchers are collaborating with a team led by Dr. Michael Hirtz at the Karlsruhe Institute of Technology. Using Dip-Pen Nanolithography, which basically works just like an inkjet printer, the researchers in Karlsruhe can “paint” the docking stations and spray the proteins on, exactly where they need to be.

The researchers’ aim is to produce an integrated circuit and thus optically wire up artificial synapses at different positions. First, there are questions to be answered, such as: Are the synapses positioned correctly on the chip? Or: What are the optical responses from individual synapses when the cells are stimulated? With this new model, the researchers ultimately want not only to be able to trigger in exactly the synapses, but also to visualize their plasticity, i.e. their dynamics and the changes in their activities depending to the stimulation patterns. In the long term, their aim is to develop a system enabling them to better analyse how nerve cells communicate with one another.