Micro total analysis systems. Recent developments.

Microfabrication has greatly matured and proliferated in use amongst many disciplines. There has been great interest in micromachined flow sensors due to the benefits of miniaturization: low cost, small device footprint, low power consumption, greater sensitivity, integration with on-chip circuitry, etc. This paper reviews the theory of thermal flow sensing and the different configurations and operation modes available. Material properties relevant to micromachined thermal flow sensing and selection criteria are also presented. Finally, recent applications of micromachined thermal flow sensors are presented. Detailed tables of the reviewed devices are included.

/pdf/micromachined-thermal-flow-sensors-a-review-1fahvwga27.pdf

Micromachined Thermal Flow Sensors—A Review

A module may be provided with at least one opening, the opening being an endpoint of a microfluidic channel that passes through at least part of the module. A set of multiple such modules may be arranged into an arrangement of modules, which may be coupled together using one or more coupling mechanisms included on each module. The arrangement of modules may fit within a regular polyhedral grid, and each module within the arrangement of modules may have a form suitable for arrangement of the modules within the regular polyhedral grid. Fluid may then flow through at least a subset of the arrangement of modules via the microfluidic channel of each module of the subset of the arrangement of modules. Some modules may include sensors, actuators, or inner microfluidic channel surface coatings. The arrangement of modules may form a microfluidic circuit that can perform a microfluidic circuit function.

https://www.pnas.org/content/pnas/111/42/15013.full.pdf

Discrete elements for 3d microfluidics

There is an indispensable need for fluid flow rate and direction sensors in various medical, industrial and environmental applications. Besides the critical demands on sensing range of flow parameters (such as rate, velocity, direction and temperature), the properties of different target gases or liquids to be sensed pose challenges to the development of reliable, inexpensive and low powered sensors. This paper presents an overview of the work done on design and development of Microelectromechanical system (MEMS)-based flow sensors in recent years. In spite of using some similar principles, diverse production methods, analysis strategies, and different sensing materials, MEMS flow sensors can be broadly categorized into three main types, namely thermal sensors, piezoresistive sensors and piezoelectric sensors. Additionally, some key challenges and future prospects for the use of the MEMS flow sensors are discussed briefly.

/pdf/design-and-applications-of-mems-flow-sensors-a-review-515rj4x2ar.pdf

Design and applications of MEMS flow sensors: A review

A new flow transducer for measuring the flow rate of a conducting fluid in a microchannel is reported. In this paper, the measure of flow of such fluid under laminar flow conditions based on the change of electrical admittance is established with the aid of a pair of electrodes parallel to the line of flow in a glass–PDMS microfluidic device. This flow sensor is simple in design and can be integrated to most of the microfluidic platforms. The effect of flow rate of the electrolyte, the frequency of the applied ac voltage, the voltage applied across the detector electrodes, and the conductivity of the electrolyte are varied to optimize for high sensitivity. The optimized values are then used to demonstrate the measurements of very low flow rates (<1 nL s−1). This flow sensor can be extended towards the measurement of chemical and biochemical buffers and reagents.

http://biomint.eng.uci.edu/pdf/John%20Abe%20Lab%20on%20a%20Chip.pdf

Microfluidic flow transducer based on the measurement of electrical admittance.

This paper introduces a micro-sensor compatible with Bio-MEMS applications for wide-range thermal flow rate measurements in liquids. Based on the applied materials and geometry, the sensor allows an outstanding resolution at minimum thermal cross-talk, enabling flow rate measurements down to 100 μg/h in water, i.e. 100 nl/h. Active and passive measuring principles, their respective applications, and the dynamic flow range coverage are presented.

High resolution flow characterization in Bio-MEMS

Using thin-film technology with its characteristic low masses and high aspect ratios a new range of possibilities is made available for the use of dynamic thermal measuring principles. Based on this, a micromachined sensor was developed for the measurement of transient thermal signal responses leading to the thermal characterization of fluids at low sample volumes. The achieved resolution allowed the measurement of thermal parameters of the investigated fluids, i.e., thermal conductivity and specific heat, inside microfluidic systems at a high sensitivity, enabling the detection of inter-fluid boundaries, e.g., as found in micromixers and -reactors, making the sensor a useful tool for micro fluidic system characterization. This is achieved via the measurement of the frequency dependent thermal signal response.

Dynamic thermal sensor - principles in MEMS for fluid characterization

https://asmedigitalcollection.asme.org/IMECE/proceedings-pdf/doi/10.1115/IMECE2000-1143/6787512/537_1_imece2000-1143.pdf

Flow Monitoring in Microdialysis Systems for Continuous Sampling

A novel method is presented for the fast dynamic analysis of thermal material parameters. For this purpose a miniaturized sensor system is used applying thermal pulses and monitoring the resulting transient thermal responses at a temporal resolution of few microseconds and an absolute detectable temperature gradient of below 1mK. The involved masses and absolute capacities are reduced to a minimum requiring a fluid volume of only few nanoliters. The detectable resolution for thermal parameters is in the range of μW/Km, respectively mJ/kgK. A reduced need for long term stability of the ambient as well as the robustness of the presented sensor make it a superior competitor to existing macro- and micro-approaches.

Herbert Ernst

Papers

High resolution flow characterization in Bio-MEMS

Dynamic thermal sensor - principles in MEMS for fluid characterization

Flow Monitoring in Microdialysis Systems for Continuous Sampling

Fast thermal characterization of nano-liter fluid volumes