5 - MEMS devices

The sensor cell is a small cavity filled with a solution of a glucose sensitive enzyme and contacted with metal electrodes. This cell can exchange substances trough an array of small holes that are etched trough the top. The sensor sensitivity can be adjusted by varying the number and size of these holes. Since the exchange between the sensor cell and the analyzed liquid is based on molecular diffusion through micropores, the actual material exchange is minimized, leading to a long lifetime and stability of the sensor cell. The surface of the porous sensor top can be coated additionally with a thin polymer film as an additional diffusion barrier that favors diffusion of small molecules, like glucose, over diffusion of large molecules, like proteins, into the sensor cell.

Living cell based biosensor uses a light-addressable potentiometric sensor (LAPS) for real-time pH measuring. The living cells (bacteria, yeast) immobilized on a glass cover change the pH of the content in flow channel during their metabolism. The pH change can be detected as modulation of measured current function when the flow is paused.

DNA-sensors are the most sophisticated transducers with molecular receptors. DNA molecules are specific to their counterparts and to proteins built-up of amino-acid sequences according to the DNA "encoding". The basis for the nucleic-acid hybridization is the DNA base pairing namely the strong interaction between two complementary nucleic acid strands. Once fabricated, the DNA chip probe cantilevers are ready for hybridization. The nucleic acid to be analyzed - the target - is incubated with the array using the fluidics station. After the hybridization reaction is complete, the array is inserted into the scanner, where patterns of hybridization are detected. The hybridization data are collected as light reflected from the cantilever surface. Probes that perfectly match the target increase the mass on the sensor area, this produces a small warp in the cantilever so, that it can deflect the laser beam. Since the sequence and position of each probe on the cantilevers are known, by complementarity, the identity of the target nucleic acid applied to the probe array can be determined.

In surface acoustic wave (SAW) devices, the acoustic wave energy is constrained to the surface; the wave propagates along the surface of a solid medium. Waves can be generated quite easily in piezoelectric substrates using an interdigital transducer (IDT) electrode. The first interdigital transducer excites an SAW whose frequency is mainly determined by the elastic constants of the piezoelectric material and the geometrical sizes of the generator IDT. The receiver IDT will receive this wave after traveling along the surface of the substrate. The propagation path is the selective receptor layer. All changes in the boundary conditions for SAW propagation lead to a variation of the SAW received by the second IDT. The SAW resonator is a delay line with an amplified feedback. Standing waves are formed by reflection from gratings. Any change in the environment leads to a change in the resonance frequency. Detection limit is 0.05 pg/mm².

The basic principle of the OWLS method is the following: linearly polarized light (laser) is coupled by a diffraction grating into the waveguide layer, provided that the incoupling condition is fulfilled. The incoupling is a resonance phenomenon that occurs at a precise angle of incidence, which depends on the refractive indices of the medium covering the surface of the waveguide. The light is guided by total internal reflection to the ends of the waveguide layer where it is detected by photodiodes. By varying the angle of incidence of the light the mode spectrum can be obtained from which the effective refractive indices and hence the adsorbed mass per unit area can be calculated.