B.H. Timmer

A closed-loop controlled electrochemically actuated micro-dosing system

In this paper a closed-loop controlled micromachined dosing system is presented, for the accurate manipulation of liquids in microsystems down to the nanoliter range. The applied driving force to dispense liquids originates from the electrochemical generation of gas bubbles by the electrolysis of water. The proposed dosing system comprises a micromachined channel/reservoir structure in silicon, capped with a Pyrex® cover on which a set of platinum electrodes is patterned. By adopting an interdigitated electrode geometry, the electrodes can be used for electrochemical gas generation as well as for the simultaneous determination of the total gas bubble volume, via an impedance measurement of the gas/liquid mixture in the reservoir. As this measured gas bubble volume equals the dosed liquid volume, active control of dosed volumes can be obtained. It will be shown that the cell impedance can be applied to accurately determine the generated gas volume and that by using this parameter in a closed-loop control system, dosed volumes can be controlled in the nanoliter range.

Micro-evaporation electrolyte concentrator

The sensitivity of miniaturized chemical analysis systems depends most of the time on the obtainable detection limit. Concentrating the analyte prior to the detection system can enhance the detection limit. In this writing an analyte concentrator is presented that makes use of evaporation to increase the ion concentration of an electrolyte. The evaporation rate can be enhanced using forced convection. In order to control the evaporation rate a nitrogen flow is fed over a liquid channel covered with a hydrophobic vapor permeable membrane. Water vapor can pass through this membrane in contrast to water itself because of the hydrophobic nature of the membrane surface. An electrolyte conductivity detector is used to measure directly the concentration effect as a function of the nitrogen flow velocity. The influence of the convective nitrogen flow and the residence time of the analyte inside the concentrator are investigated in this paper. It is shown that the evaporation rate is enlarged with an increase in convective flow. The concentration effect is also enhanced when the residence time of the analyte inside the concentrator is increased. The higher concentration enhancement due to the longer residence time, however, results in an increase in water vapor present in the nitrogen flow. This results in a lower normalized evaporation rate when the available evaporation time is enlarged.

Selective low concentration ammonia sensing in a microfluidic lab-on-a-chip

In the medical community, there is a considerable interest in a diagnostic breath analyzer for ammonia that is selectively enough to measure in exhaled air and small enough for the small volumes available in such an application. An indirect measurement system for low gaseous ammonia concentrations has been miniaturized and integrated on a chip in order to reach this goal. The detection limit of the system was calculated to be 1.1 parts per billion (ppb). The response time was determined to be 1.6 min with a gas How of 50 ml/min. The required gas volume for one measurement is therefore sufficiently small, although sampling assistance is required for breath analysis. The selectivity of the system is sufficient to measure ammonia concentrations in the low-ppb range. The system is even sufficiently selective to be used in environments that contain elevated carbon dioxide levels, like exhaled air. The lower ammonia concentration expected in diagnostic breath analysis applications, 50 ppb, was demonstrated to be detectable

Fluorocarbon coated micromachined gas sampling device

A thin micromachined alternative for commercially available hydrophobic microporous membranes used in diffusion gas samplers has been realized. The thin membrane is made of Si3N4 and is etched porous. Underneath the membrane a channel is created. The membrane is rendered hydrophobic with a fluorocarbon coating. Bubble point and contact angle measurements have been performed on the membrane.

A Closed Loop Controlled Electrochemically Actuated Micro Dosing System for in Situ Sensor Calibration

In this contribution a closed loop controlled electrochemically actuated micro dosing system is presented, which accurately doses calibration liquids in microsystems for the in-situ calibration of chemical sensors. The system comprises an electrolyte filled reservoir with platinum electrodes, connected to a meander channel filled with the appropriate calibrant. By the electrochemical production of gas bubbles in the electrolyte filled reservoir, this calibration liquid can be driven out into a carrier channel that leads to the sensor(s) to be calibrated. Closed loop control of actual dosed volumes is obtained by determining the total generated gas bubble volume via an impedance measurement in the electrolyte reservoir. In this work, a conductivity sensor integrated in the carrier channel of the dosing system is calibrated as an example.