Biosensor microprobe arrays for in vivo monitoring of neurotransmitters

Real-time monitoring of neurotransmitters is of utmost importance for understanding the functioning of the brain. Especially L-glutamate and choline play a major role in chemical signalling and are highly involved in cognitive functions such as learning, memory and behaviour. In this thesis, silicon microprobe arrays were realised comprising several microelectrodes with a size of 50×150 µm2. The microtechnological fabrication process allows an application-tailored design to position the electrodes at defined target regions in the brain. Microprobes with a length up to 8 mm and a cross-section of 40×100 µm2 have been fabricated allowing minimally invasive implantation at low tissue damage to be achieved. L-glutamate and choline are detected by the amperometric detection of peroxide using the microelectrodes coated with an enzymatic membrane. An array-compatible method for spatially controlled and parallel membrane deposition by electrochemically aided adsorption and chemical co-cross-linking with glutaraldehyde was developed. The enzymatic membrane deposition is followed by electropolymerisation of m-phenylenediamine to deposit a semi-permeable membrane rejecting electroactive interferents such as ascorbic acid and dopamine that are endogenously present in the extracellular fluid. The resulting biosensors show adequate characteristics in sensitivity, detection limit as well as functional and storage lifetimes for acute monitoring of the neuro-transmitters in brain tissue. The results of an extensive in vitro assessment and in vivo functional tests are presented. In array configuration, the biosensors allow simultaneous multi-site recordings of one analyte from different brain regions or multi-analyte recordings within defined spatial brain areas. Additionally, microchannels were integrated with the biosensor arrays to perform chemical stimulation by the local delivery of neuroactive substances: SU-8 microinjector arrays comprising several microfluidic channels or the direct integration of microchannels into the silicon shaft were realised. Both allow a precise positioning of the fluidic outlet relative to the biosensors and low-volume injections. The biosensors integrated with the delivery channels enable the bilateral interaction with neuronal tissue on the neurochemical level at required temporal and spatial resolution. This thesis is a part of the European integrated project NeuroProbes aiming at realising three-dimensional arrays of multi-functional microprobes, assembled in a modular way to perform in vivo electrophysiological and biological measurements. The biosensor is one of the functionalities to be integrated in this platform. The result is a new device for neuroscientists that will allow most complete 3D mapping of neuronal circuitry and relate these complex signalling mechanisms to observable behaviour.