Microfabricated Multimode Bio-interface with High Density Microelectrode Array and Photonic Detectors

Presently, the cells' electrical activity is measured either by extracellular microelectrode array (MEA) or by microscopic fluorescence imaging methods. The MEA is a non-invasive in vitro technique which allows long-term recording at multicellular level while fluorescence imaging technique enables to record the cells' action potential from subcellular to the organ level either in vivo or in vitro. These techniques have a number of limitations due to the fact that they require complex electro-optical recording equipment. As a matter of fact, because these systems are expensive, require skilled operators, suffer from problems of keeping the preparations under physiological conditions and permit to conduct only one experiment at a time, they are not suited for long-term and massively parallel measurements. In order to overcome these limitations, it would be highly desirable to obtain a hybrid sensor which combines simultaneous electrical and optical recording capabilities with sufficient spatiotemporal resolution to resolve distributed electrical activity and associated changes in intracellular ion concentrations, both at the single cell level and at the level of entire networks of excitable cells. The objective of this thesis is to design and to fabricate a novel type of hybrid sensor array which allows measuring simultaneously dynamic changes in the extracellular electrical potential and intracellular ion concentrations at a high spatiotemporal resolution. This is realised by a novel approach using a microfabricated glass interface, hereafter called bio-interface, between the electronic readout circuit and the cell network. This bio-interface features an array of 4096 implemented sensing elements, i.e., electrodes and photodiodes, on an area of 3.7 mm2. It is flip-chip bonded to an integrated circuit (IC) by means of high-density indium bumps forming a multi-mode sensor array (MMSA). The IC provides a selectable local amplification of the electrical signals sensed by the microelectrodes, photodetectors, and the processing capabilities to read out all pixels sequentially. The final goal of this work consists of the implementation of a fully operational and validated MMSA measurement platform, which can be used as core instrument for biological research projects. Since the fabricated MMSA essentially represents a microscope-less fluorescence imager with microscopic resolution and embedded electrical signal recording capabilities, it has the potential to turn into a new key measurement technique for both basic and applied biological research in fields as diverse as developmental biology, cardiac electrophysiology, neurophysiology, drug screening, and toxicological studies. This thesis demonstrates the design and fabrication process of the biointerface and the post-processing in connection with the IC and packaging issues. The bio-interface was successfully fabricated and flip-chip bonded to the readout electronics by means of high density indium bumps. These bumps feature a diameter of 15 µm at a 30 µm pitch. Tests show a bump yield of 100 %. The photodiode array was optically characterized and the device biocompatibility was validated by culturing a monolayer of cardiomyocytes on top of the bio-interface.