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A Novel Microsensor for Measurement of Liquid Flow and Diffusivity

Deliverables

A microscale sensor for fluid velocity measurements has been developed and its properties investigated. The sensor can be made with tip diameters below 10 microns and the fluid velocity detection limit can be down to 5 microns/second. The sensor consists of a slim transducer (e.g. a gas microsensor) surrounded by a tracer gas reservoir in the form of a tiny glass tube. The tip of the tracer reservoir is closed by a silicon membrane that is penetrated by the transducer tip. The tracer gas diffuses out through the tip membrane and the concentration build-up at the tip is measured by the transducer. As the tracer build-up is a function of the fluid velocity at the sensor tip, the transducer signal becomes an inverse measure of fluid velocity. The tracer gas can be selected from a range of substances depending on the suitability in the relevant situation. For instance, in systems where the oxygen concentration is high or variable, acetylene can be used as a tracer along with an acetylene transducer, whereas in systems where long-term stability is important and where the oxygen concentration is not variable, oxygen can be used as a tracer, as the oxygen transducer is very long-term stable. The sensor tip is sharpened such that it can penetrate tissue without tearing it. This means that the sensor tip can be inserted into small biological channels, e.g. blood vessels and kidney tubules, without damage to the channel wall, making it a unique tool for investigating fluid velocities at very low velocities. There are alternative methods for measuring flow velocity (e.g. Laser-Doppler techniques and particle imaging systems), but these are inferior to the technique presented here with respect to velocity sensitivity (Laser-Doppler) and spatial resolution (imaging). The sensor signal is acquired with picoammeter technology and the positioned with positioning equipment, both of which are readily available.
A calibration set-up for calibration of flow velocity sensors has been developed. The set-up consists of a sensor holder connected to a precision motor via a spindle. Via specialised software, the sensor can be moved at well-defined speeds between 10 microns/second and 60 mm/second relative to a water phase in a narrow linear groove, in which the sensor tip is immersed. The relatively high resistance in the narrow groove ensures that the water phase is practically stagnant. By moving the sensor in a range of speeds and logging the signal, a calibration curve for the flow velocity sensor can be produced.
A circular calibration set up for calibration of flow velocity sensors has been developed. The set up consists of a sensor holder connected to a precision motor. Via specialised software, the sensor can be moved in at well-defined speeds between 10 microns/second and 60 mm/second relative to a water phase in a narrow circular groove, in which the sensor tip is immersed. The relatively high resistance in the narrow groove ensures that the water phase is practically stagnant. By moving the sensor in a range of speeds and logging the signal, a calibration curve for the flow velocity sensor can be produced.
A flow velocity sensor is constructed according to the principle of a transducer surrounded by a concentric reservoir closed by a membrane, which is penetrated by the transducer. The reservoir contains a tracer that diffuses through the membrane and into the surrounding medium. The transducer can measure the concentration build-up in the medium of the tracer and transducer signal thus becomes is a function of the fluid velocity. The fluid velocity sensor is fixed with its tip inside a tube or channel. By letting fluid pass through the channel at well-defined flows, a calibration can be produced. This calibration can then be used to calculate the flow from signals produced during unknown flows. As the mentioned sensor principle is very sensitive to low fluid velocities, the flow measurement setup can measure very low flow rates.
A microscale sensor for fluid velocity measurements has been developed and its properties investigated. The sensor can be made with tip diameters below 10 microns and the fluid velocity detection limit can be down to 5microns/second. The sensor consists of a slim transducer (e.g. a gas microsensor) surrounded by a tracer gas reservoir in the form of a tiny glass tube. The tip of the tracer reservoir is closed by a silicon membrane, which is penetrated by the transducer tip. The tracer gas diffuses out through the tip membrane and the concentration build-up at the tip is measured by the transducer. As the tracer build-up is a function of the fluid velocity at the sensor tip, the transducer signal becomes an inverse measure of fluid velocity. In most cases, hydrogen will be the preferred tracer as it is possible to make a reliable and fast transducer for hydrogen and as hydrogen is not present in detectable concentrations in most environments. Also oxygen and acetylene have been used as tracers along with suitable transducers. Thus the sensor is a unique tool for investigating fluid velocities at very low velocities and at very high spatial resolution in a variety of systems within biology, medicine and hydrology. There are alternative methods for measuring flow velocity (e.g. Laser-Doppler techniques and particle imaging systems), but these are inferior to the technique presented here with respect to velocity detection limit (Laser-Doppler) and spatial resolution (imaging). The sensor signal is acquired with picoammeter technology and the sensor is positioned with positioning equipment, both of which are readily available.
The exploitable result is a microscale sensor for perfusion. The spatial resolution can be in the order of a few tens of microns, depending on the sensor size and perfusion rate. The sensor has not yet been applied in physiological studies, but has shown promising performance in other natural and artificial model systems and thus holds promise to be a useful tool in medical research dealing with different aspects of tissue perfusion.
The exploitable result is a microscale sensor for apparent diffusivity. The spatial resolution depends on the grain size of the actual system; in soft sediments and biofilms it can be below 100 microns. Different versions differing with respect to used tracer gas have been developed to suit different measuring environments. The sensor has proved useful in biogeochemical research in sediments and biofilms.