Biosensor microprobes with integrated microfluidic channels for bi-directional neurochemical interaction.

This paper reports on silicon-based microprobes, 8 mm long and 250 µm × 250 µm cross-section, comprising four recessed biosensor microelectrodes (50 µm × 150 µm) per probe shank coated with an enzymatic layer for the selective detection of choline at multiple sites in brain tissue. Integrated in the same probe shank are up to two microfluidic channels for controlled local liquid delivery at a defined distance from the biosensor microelectrodes. State-of-the-art silicon micromachining processing was applied for reproducible fabrication of these experiment-tailored multi-functional probe arrays. Reliable electric and fluidic interconnections to the microprobes are guaranteed by a custom-made holder. The reversible packaging method implemented in this holder significantly reduces cost and assembly time and simplifies storage of the biosensor probes between consecutive experiments. The functionalization of the electrodes is carried out using electrochemically aided adsorption. This spatially controlled deposition technique enables a parallel deposition of membranes and is especially useful when working with microelectrode arrays. The achieved biosensors show adequate characteristics to detect choline in physiologically relevant concentrations at sufficient temporal and spatial resolution for brain research. Sensitivity to choline better than 10 pA µm(-1), detection limit below 1 µM and response time of 2 s were obtained. The proposed combination of biosensors and microfluidic injectors on the same microprobe allows simultaneous chemical stimulation and recording as demonstrated in an agarose gel-based brain phantom.

Enzyme-based choline and L-glutamate biosensor electrodes on silicon microprobe arrays.

Brain-implantable microprobe arrays, 6.5 mm shaft-length, incorporating several recessed Pt microelectrodes (50 µm×150 µm) and an integrated Ag/AgCl reference electrode fabricated by silicon micromachining dry etching techniques (DRIE) are described. The microelectrodes are coated by an enzyme membrane and a semi-permeable m-phenylenediamine layer for the selective detection of the neurotransmitters choline and L-glutamate at physiologically relevant concentrations. The functionalisation is based on electrochemically aided adsorption (EAA) combined with chemical co-cross-linking using glutaraldehyde and electrochemical polymerisation, respectively. These deposition methods are fully compatible with the fabricated microprobe arrays for the simultaneous detection of several analytes in different brain target areas. They are spatially controlled and allow fabricating biosensors on several microelectrodes in parallel or providing a cross-talk-free coating of closely spaced microelectrodes with different enzyme membranes. A sensitivity of 132±20 µA mM(-1) cm(-2) for choline and 95±20 µA mM(-1) cm(-2) for L-glutamate with limits of detections below 0.5 µM was obtained. The results of in vitro and in vivo experiments confirm the functional viability of the choline and l-glutamate biosensors.

High-density electrode array for imaging in vitro electrophysiological activity.

The development of a high-density active microelectrode array for in vitro electrophysiology is reported. Based on the Active Pixel Sensor (APS) concept, the array integrates 4096 gold microelectrodes (electrode separation 20 microm) on a surface of 2.5 mmx2.5 mm as well as a high-speed random addressing logic allowing the sequential selection of the measuring pixels. Following the electrical characterization in a phosphate solution, the functional evaluation has been carried out by recording the spontaneous electrical activity of neonatal rat cardiomyocytes. Signals with amplitudes from 130 microVp-p to 300 microVp-p could be recorded from different pixels. The results demonstrate the suitability of the APS concept for developing a new generation of high-resolution extracellular recording devices for in vitro electrophysiology.

Host-guest interactions in thin membranes: selective ion transport and transduction into electronic signals.

Synthetic receptor molecules that selectively complex with charged guest molecules can be used to transport salts through liquid membranes and to transduce chemical information into electronic signals. In both cases the receptor molecules are present in thin membranes in contact with aqueous solutions. Extreme lipophilicity of the receptor molecules is therefore required: calix crown ethers and calixspherands meet these requirements. Their synthesis and complexation properties will be discussed. In order to mimic the large rates of transport through biomembranes, thin supported liquid membranes (less than 100 microns), in which the receptor molecules are present, were investigated. The selective ion transport has been studied as a function of the experimental parameters and interpreted via computer simulations of the transport processes. The transduction of complexation into electronic signals can be achieved via the 'immobilization' of receptor molecules on the gate surface of an ISFET chip. Parameters that govern the signal transduction in multilayer systems have been studied and stimulated.