| The subject of this research is the fabrication and characterization of a micro flow sensor for an integrated microfluidic system. In contrast to earlier research work carried out in our center, this sensor was designed to measure liquid flow through microchannels in the μl/min range. As a part of the complex of components constituting a generic fully integrated microfluidic system for autonomous biochemical analysis, this micro flow sensor has several unique operational and physical characteristics.; The micro flow sensor depends on the forced convective heat transfer from a thermally isolated, self-heating sensing element. The Utilization of high resistive single crystalline silicon results in highly sensitive elements, which were imbedded in the microchannel walls to prevent clogging by virtue of dead volume or other interference. In essence, the sensor measures liquid flow through-the-wall, and complex electrical passivation is thus avoided.; To minimize possible thermally induced effects in the fluid and to save valuable battery lifetime, the sensor was operated in a pulsed modality. The elements were pulsed at a fixed current level (typically 3mA) for 50 to 100ms intervals at a low duty cycle. The sensing elements easily achieve full operation during the on-cycle, because of the fast time response resulting from miniature size and thermal isolation. A specific arranged pulsing scheme ignores possible thermal runaway, which can be a problem in thermally based sensors, e.g. due to thermal coupling of the sensor to its mounting and environment.; Several packaging approaches were investigated, considering boundary conditions such as biocompatibility, temperature limits and allowed assembly tolerances. A low temperature assembling and bonding scheme utilizing wax is discussed in detail.; Numerical simulations and analytical investigations of the liquid flow field and the thermal distribution were undertaken in an attempt to obtain theoretical models which can help to interpret recorded data that was gathered during the experimental analysis. |
