Introduction

Optimization of methodologies toward engineering the life sciences and healthcare remains a grand challenge. In the field of neurotechnology, tremendous progress has been made in a fundamental understanding of the nervous system and in building technology to diagnose and treat some neurological diseases. However, our understanding of nervous system function and technological approaches to measuring and manipulating neuronal circuits needs to be improved.

Neural prosthetic devices are artificial extensions of body parts which allow a disabled individual to restore the body functions. Here, a neuroelectronic device which interfaces neuronal tissue with electronics is the key to restore the disabled body functions. Also, in vivo monitoring of the electrical signals from multiple cells during nerve excitation and cell-to-cell communication are important for design and development of novel materials and methods for laboratory analysis. In vitro biological applications such as drug screening and cell separation also require cell-based biosensors. Nowadays, the best approach to study the electrophysiological activity of neurons and cardiac cells in vitro and in vivo is based on planar microelectrode arrays or field-effect transistors which can be integrated with microfluidic devices. These methods allow the simultaneous monitoring and stimulation of large populations of excitable cells over many days and weeks and enable insights into long-term effects such as adaptivity in neuronal networks.

Basics

Silicon-based microstructures are gaining more and more importance in fundamental neuroscience and biomedical research. Precise and long-lasting neuroelectronic hybrid systems are in the center of research and development in this field. The interaction of a neuronal cell with an electronic device is schematically depicted in Fig.1a. Sufficient electrical coupling between the cell and the (gate) electrode for extracellular signal recording is achieved only when a cell or a part of a cell is located directly on top of the (gate) electrode. Electrical signals recorded by these devices show lower signals and a higher noise level (owing to a weaker coupling to the (gate) electrode) compared to intracellular electrodes or patch pipettes (see Fig. 1b).

Neurons, Coupling, Fig. 1
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(a) Schematic of a neuron on an extracellular electrode: intracellular (upper orange electrode) and extracellular (lower yellow electrode) signals can be recorded (b) Action potential of neuron (approx. 100 mV) recorded by an intracellular electrode (upper trace) and an extracellular electrode (lower trace)

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For extracellular signal recordings from electrically active cells in culture, two main concepts have been developed in the past: microelectrode arrays (MEAs) (see Fig. 2a) with metalized contacts on silicon or glass substrates have been used to monitor cardiac impulse propagation from dissociated embryonic myocytes [1–3], dissociated invertebrate neurons [4, 5] and mammalian neurons [6] spinal cord [7], and mouse dorsal root ganglia [8]. Alternatively, arrays of field-effect transistors (FETs) (see Fig. 2b) are used for extracellular recordings having either non-metalized transistor gates with cells growing directly on the gate oxide [9–11] or metalized gates. The latter were in direct contact with the electrolyte [12] or they were electrically insulated, so-called floating gates [13–15]. With these noninvasive methods, the electrical activity of single cells and networks of neurons can be observed over an extended period of time. Meanwhile both concepts are growing together by designing MEAs inside a CMOS process with on-chip amplification and filtering [16, 17].

Neurons, Coupling, Fig. 2
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(a) Substrate-embedded microelectrode: the metal electrode (red) is exposed to the electrolyte while the feed lines are covered with an isolation layer (green-blue) (b) Open-gate field-effect transistor for the recording of extracellular signals

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Neuron-Electrode Coupling

For a quantitative understanding of the extracellular signals recorded by electronic devices, it is necessary to explain the experimental situation in detail. A schematic picture of a typical experimental situation is depicted in Fig. 3. Here, the neuroelectronic hybrid is formed by the neuron, the cleft between neuron and the sensor surface, and the electronic device. Outside the neuron and inside the cleft, there is extracellular electrolyte solution. By electrical excitation, the ion channels in the cell’s membrane open and ions can flow from across the cell membrane. While in the upper part of the cell (free membrane), these ions just enter the surrounding electrolyte bath directly; it is different at the attached membrane. Here, the ions have to pass the cleft before entering/leaving the bath. The cleft acts as a resistance typically called seal resistance R J [10, 18]. The magnitude of R J is typically in the order of several 100 kW up to MW corresponding to a typical cleft thickness of 40–150 nm [19, 20]. The voltage V J , which determines the voltage at the (gate) electrode, is mainly determined by the seal resistance R J , and the current the flows across it.

Neurons, Coupling, Fig. 3
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Schematics of the neuroelectronic hybrid. The cell membrane is divided into free (FM) and attached membrane (AM) with the respective values of membrane area (AFM, A JM) and membrane capacitance (C FM, CJM) and resistance (R FM, R JM). C G and R G are the capacitance and the resistance of the (gate) electrode, respectively. The seal resistor R J represents the electrical properties of the cleft between the membrane and the sensor surface. In case of patch-clamp experiments, the intracellular voltage V M can be determined

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Future Directions

Although this noninvasive method of extracellular recordings allows monitoring the electrical activity of single cells and networks of neurons over an extended period of time with good time resolution, it does not allow detecting subthreshold signals of neuronal cells. In the last years, a number of research groups began to study the combination of planar (2D) electrodes with intracellular recordings. This includes the use of gold mushroom-shaped protrusions, nanopillar electrodes, and nanorods. These developments may improve our understanding of neuroscience in the future [21–23].

Cross-References

Neural Stimulation Electrodes and Sensors