Basically, there are three levels of biocommunications where electronics and biology could interface: molecular, cellular and skeletal. For any implanted bionic material it is the initial interactions at the biomolecular level that will determine longer term performance. While bionics is often associated with skeletal level enhancements, electronic communication with living cells is of interest with a view to improving the results of tissue engineering or the performance of implants such as bionic ears or eyes.
Pioneer researchers, such as Peter Fromhertz from the Max-Plank-Institute of Biochemistry in Germany, have worked for more than 20 years on interfacing neurons and silicon devices. They are experimenting with individual neurons from different parts of the brain by cultivating them and trying to establish ex vivo neural networks. The goal is to stimulate neurons with electric signals and observe how the live network reacts and modifies itself.
These studies could result in valuable findings that improve our understanding of how a neural network modifies its structure during the learning phase and the rules that govern the way synapses and neurites grow. Analysis of the electro-physiological activity of neurons could one day enable scientists to develop artificial prostheses for bypassing injured zones and restore brain functionality, or to realize neuro-diagnostic tools for monitoring the reaction of biological neurons to selected chemical species or newly developed drugs.
Making another step in this direction, researchers in Europe have now demonstrated the possibility of integrating living neural cells and organic semiconductor thin-films made of a few monolayers of pentacene. These results are promising for the development of electronic transducers based on organic field-effect transistors with ultra-thin-films, which may be used for real-time monitoring of biological activities at the level of interconnected living cells.
"Monitoring electrical and chemical signaling within neural networks is a fundamental issue in neuroscience" Dr. Fabio Biscarini explains to Nanowerk. "Extracellular metal electrodes can record network activity, but the resolution is too poor to record individual cell responses or single chemical events. The widely used patch clamp approach provides a highly sensitive method for the detection of single cell responses or channel reactions both in vivo and in vitro, but it only allows real-time monitoring for one or very few cells and it is difficult to upscale in number as well as downscale in size."
"A less invasive approach consists of coupling neurons to inorganic semiconductor devices, such as field effect transistors," continues Biscarini, a research scientist at the Institute for the Study of Nanostructured Materials (CNR) in Bologna, Italy. "Although important observations, e.g., the 'firing' in small neural networks, were accomplished using this approach, it struggles with sensitivity due to low capacitive coupling and faces difficulties in chemically tailoring the surface for the attachment of neurons. More recently, it was proposed that silicon nanowires may be used to monitor the action potential on axon membranes. Thus, low dimensional charge transport devices seem to be a promising route for overcoming limitations due to low sensitivity. Attempts to adhere and grow neurons on functionalized carbon nanotubes, are headed in the same direction and have already been successful."
Biscarini, whose CNR team collaborated with scientists from the Laboratory of Cellular and Developmental Neurobiology at the Hungarian Academy of Sciences, and the Center for NanoScience (CeNS) at the University of Munich in Germany, has just published a paper in Advanced Functional Materials that shows the possibility to couple organic semiconductors and living cells ("Neural Networks Grown on Organic Semiconductors"). This is just the first paper of a series that will appear in the coming months, so stay tuned.
a) AFM image of a differentiated NE-4C-derived neuron (arrows). b) The magnified image shows that the morphology of the pentacene thin-film is neither damaged nor modified by the interactions with the cell. c) Same image as (b), with a different height scale to highlight the axon morphology. Inset shows the unaltered pentacene film from (b). The change in height scale highlights the axons. (Reprinted with permission from Wiley VCH-Verlag)
The researchers explain that it was not so obvious that cells would live on molecular semiconductor ultra-thin films (just a few molecular layer thick).
"π-conjugated molecules are often mutagenic or toxic, and, in addition, the forces exerted by the cells adhering on a 'soft' surface can disrupt it," says Biscarini. "This was not the case in our experiments and we showed that stem cells can adhere, grow and remain viable on pentacene – one of the most relevant organic semiconductors – for many days, and eventually be differentiated into neurons and glia cells, the former interconnected into dense networks. It is an indication that organic electronics materials can be interfaced directly to living cells. We then see no major problems in interfacing organic electronics devices to cells. There are many organic semiconductors available and this may indeed be important for the biological compatibility issue in the long term."
Despite the similarity, organic transistors are very different from silicon based ones. In an organic transistor, carriers move within a few monolayers of the organic semiconductor layer. By making the organic semiconductor layer very thin (3-5 nm – that's why they are called 'organic ultra-thin film transistors'), the charge carriers can interact with the outer environment, for instance to a solution containing biomolecules or cells in their culture medium.
Biscarini points out that this proximity effect enhances the changes in the transistor parameters due to this coupling; and without the need to have a passivation layer. "It was not obvious that organic semiconductors and especially organic electronics devices would sustain the presence of water, containing electrolytes. Our evidence points to the possibility to do so, and this can provide an opportunity to investigate biological systems in a gentle, non invasive way. We believe it is worth exploring this possibility."
In the short term, the European team aims to monitor slow dynamics of phenomena like aggregation of proteins, denaturation, or simply motion of large biomolecules. Later, they plan to transduce the electrical and chemical signals from living cells, interconnected to form neural networks, into electrical signals of the organic field effect transistor.
"It would be fantastic to have not only transduction, but also specific recognition and then develop a new label free sensor," says Biscarini. "But this is a much more complicated goal at this stage."
Applications arising from this work could be biosensors, which are biocompatible, non-invasive, and easy to fabricate. A first goal could be to monitor cell cultures in vitro. Organic electronics would also offer an easy way to adapt to curved geometries, which may be useful in many parts of a living organism. And these are just some indications of where nanotechnology can be used in the future.
The CRN and CeNS teams are partners in the European BIODOT project. The vision of BIODOT is a hybrid bio-organic technology for transduction of dynamical phenomena of biosystems in vitro. One objective of the project is to develop a new sensor for neurodegenerative diseases. Such a sensor could then be used for controlling drug dispensing in loco-regional therapies and may become extremely useful for early monitoring and therapy.
"We still have to solve many technical problems, which are not trivial at all, for which there is no ready-to-use solution, and to understand many phenomena ongoing at the interface between biological systems and organic semiconductors," says Biscarini. "But in BIODOT we have a vast group with diverse competences which makes me feeling optimistic about our prospects. It would be a success if by the end of the project, in October 2009, we would have reproducible signals transduced from an organic transistor, even though we may not be able to interpret them. Interpretation of the signal and device stability may be two challenges we might have to face subsequently. And at some point it will be necessary to address ethical implications, as the device may also serve to influence or change the biological system it is used in."