Flow-Independent Thermal Conductivity and Volumetric Heat Capacity Measurement of Pure Gases and Binary Gas Mixtures Using a Single Heated Wire.

Among the different techniques for monitoring the flow rate of various fluids, thermal flow sensors stand out for their straightforward measurement technique. However, the main drawback of these types of sensors is their dependency on the thermal properties of the medium, i.e., thermal conductivity (k), and volumetric heat capacity (ρcp). They require calibration whenever the fluid in the system changes. In this paper, we present a single hot wire suspended above a V-groove cavity that is used to measure k and ρcp through DC and AC excitation for both pure gases and binary gas mixtures, respectively. The unique characteristic of the proposed sensor is its independence of the flow velocity, which makes it possible to detect the medium properties while the fluid flows over the sensor chip. The measured error due to fluctuations in flow velocity is less than ±0.5% for all test gases except for He, where it is ±6% due to the limitations of the measurement setup. The working principle and measurement results are discussed.

Compact Micro-Coriolis Mass-Flow Meter with Optical Readout.

This paper presents the first nickel-plated micro-Coriolis mass-flow sensor with integrated optical readout. The sensor consists of a freely suspended tube made of electroplated nickel with a total length of 60 mm, an inner diameter of 580 µm, and a wall thickness of approximately 8 µm. The U-shaped tube is actuated by Lorentz forces. An optical readout consisting of two LEDs and two phototransistors is used to detect the tube motion. Mass-flow measurements were performed at room temperature with water and isopropyl alcohol for flows up to 200 g/h and 100 g/h, respectively. The measured resonance frequencies were 1.67 kHz and 738 Hz for water and 1.70 kHz and 752 Hz for isopropyl alcohol for the twist and swing modes, respectively. The measured phase shift between the two readout signals shows a linear response to mass flow with very similar sensitivities for water and isopropyl alcohol of 0.41mdegg/h and 0.43 mdegg/h, respectively.

Flow Ripple Reduction in Reciprocating Pumps by Multi-Phase Rectification.

Reciprocating piezoelectric micropumps enable miniaturization in microfluidics for lab-on-a-chip applications such as organs-on-chips (OoC). However, achieving a steady flow when using these micropumps is a significant challenge because of flow ripples in the displaced liquid, especially at low frequencies or low flow rates (<50 µL/min). Although dampers are widely used for reducing ripples in a flow, their efficiency depends on the driving frequency of the pump. Here, we investigated multi-phase rectification as an approach to minimize ripples at low flow rates by connecting piezoelectric micropumps in parallel. The efficiency in ripple reduction was evaluated with an increasing number (n) of pumps connected in parallel, each actuated by an alternating voltage waveform with a phase difference of 2π/n (called multi-phase rectification) at a chosen frequency. We introduce a fluidic ripple factor (RFfl.), which is the ratio of the root mean square (RMS) value of the fluctuations present in the rectified output to the average fluctuation-free value of the discharge flow, as a metric to express the quality of the flow. The fluidic ripple factor was reduced by more than 90% by using three-phase rectification when compared to one-phase rectification in the 2-60 µL/min flow rate range. Analytical equations to estimate the fluidic ripple factor for a chosen number of pumps connected in parallel are presented, and we experimentally confirmed up to four pumps. The analysis shown can be used to design a frequency-independent multi-phase fluid rectifier to the desired ripple level in a flow for reciprocating pumps.

Thermal Flow Meter with Integrated Thermal Conductivity Sensor.

This paper presents a novel gas-independent thermal flow sensor chip featuring three calorimetric flow sensors for measuring flow profile and direction within a tube, along with a single-wire flow independent thermal conductivity sensor capable of identifying the gas type through a simple DC voltage measurement. All wires have the same dimensions of 2000 µm in length, 5 µm in width, and 1.2 µm in thickness. The design theory and COMSOL simulation are discussed and compared with the measurement results. The sensor's efficacy is demonstrated with different gases, He, N2, Ar, and CO2, for thermal conductivity and thermal flow measurements. The sensor can accurately measure the thermal conductivity of various gases, including air, enabling correction of flow rate measurements based on the fluid type. The measured voltage from the thermal conductivity sensor for air corresponds to a calculated thermal conductivity of 0.02522 [W/m·K], with an error within 2.9%.

Portable and integrated microfluidic flow control system using off-the-shelf components towards organs-on-chip applications.

Organ-on-a-chip (OoC) devices require the precise control of various media. This is mostly done using several fluid control components, which are much larger than the typical OoC device and connected through fluidic tubing, i.e., the fluidic system is not integrated, which inhibits the system's portability. Here, we explore the limits of fluidic system integration using off-the-shelf fluidic control components. A flow control configuration is proposed that uses a vacuum to generate a fluctuation-free flow and minimizes the number of components used in the system. 3D printing is used to fabricate a custom-designed platform box for mounting the chosen smallest footprint components. It provides flexibility in arranging the various components to create experiment-specific systems. A demonstrator system is realized for lung-on-a-chip experiments. The 3D-printed platform box is 290 mm long, 240 mm wide and 37 mm tall. After integrating all the components, it weighs 4.8 kg. The system comprises of a switch valve, flow and pressure controllers, and a vacuum pump to control the diverse media flows. The system generates liquid flow rates ranging from 1.5 [Formula: see text]Lmin[Formula: see text] to 68 [Formula: see text]Lmin[Formula: see text] in the cell chambers, and a cyclic vacuum of 280 mbar below atmospheric pressure with 0.5 Hz frequency in the side channels to induce mechanical strain on the cells-substrate. The components are modular for easy exchange. The battery operated platform box can be mounted on either upright or inverted microscopes and fits in a standard incubator. Overall, it is shown that a compact integrated and portable fluidic system for OoC experiments can be constructed using off-the-shelf components. For further down-scaling, the fluidic control components, like the pump, switch valves, and flow controllers, require significant miniaturization while having a wide flow rate range with high resolution.

Modeling, Fabrication, and Testing of a 3D-Printed Coriolis Mass Flow Sensor.

This paper presents the modeling, fabrication, and testing of a 3D-printed Coriolis mass flow sensor. The sensor contains a free-standing tube with a circular cross-section printed using the LCD 3D-printing technique. The tube has a total length of 42 mm, an inner diameter of about 900 µm, and a wall thickness of approximately 230 µm. The outer surface of the tube is metalized using a Cu plating process, resulting in a low electrical resistance of 0.5 Ω. The tube is brought into vibration using an AC current in combination with a magnetic field from a permanent magnet. The displacement of the tube is detected using a laser Doppler vibrometer (LDV) that is part of a Polytec MSA-600 microsystem analyzer. The Coriolis mass flow sensor has been tested over a flow range of 0-150 g/h for water, 0-38 g/h for isopropyl alcohol (IPA), and 0-50 g/h for nitrogen. The maximum flow rates of water and IPA resulted in less than a 30 mbar pressure drop. The pressure drop at the maximum flow rate of nitrogen is 250 mbar.

In-line measurements of the physical and thermodynamic properties of single and multicomponent liquids.

Microfluidic devices are becoming increasingly important in various fields of pharmacy, flow chemistry and healthcare. In the embedded microchannel, the flow rates, the dynamic viscosity of the transported liquids and the fluid dynamic properties play an important role. Various functional auxiliary components of microfluidic devices such as flow restrictors, valves and flow meters need to be characterised with liquids used in several microfluidic applications. However, calibration with water does not always reflect the behaviour of the liquids used in the different applications. Therefore, several National Metrology Institutes (NMI) have developed micro-pipe viscometers for traceable inline measurement of the dynamic viscosity of liquids used in flow applications as part of the EMPIR 18HLT08 MeDDII project. These micro-pipe viscometers allow the calibration of any flow device at different flow rates and the calibration of the dynamic viscosity of the liquid or liquid mixture used under actual flow conditions. The validation of the micro-pipe viscometers has been performed either with traceable reference oils or with different liquids typically administered in hospitals, such as saline and/or glucose solutions or even glycerol-water mixtures for higher dynamic viscosities. Furthermore, measurement results of a commercially available device and a technology demonstrator for the inline measurement of dynamic viscosity and density are presented in this paper.

Calibration methods for flow rates down to 5 nL/min and validation methodology.

Improving the accuracy and enabling traceable measurements of volume, flow, and pressure in existing drug delivery devices and in-line sensors operating at very low flow rates is essential in several fields of activities and specially in medical applications. This can only be achieved through the development of new calibrationmethods and by expanding the existing metrological infrastructure to perform micro-flow and nano-flow measurements. In this paper, we will investigate new traceable techniques for measuring flow rate, from 5 nL/min to 1,500 nL/min and present the results of an inter-comparison between nine laboratories for the calibration of two different flow meters and a syringe pump.

Method for Keyhole-Free High-Aspect-Ratio Trench Refill by LPCVD.

In micro-machined micro-electromechanical systems (MEMS), refilled high-aspect-ratio trench structures are used for different applications. However, these trenches often show keyholes, which have an impact on the performance of the devices. In this paper, explanations are given on keyhole formation, and a method is presented for etching positively-tapered high-aspect ratio trenches with an optimised trench entrance to prevent keyhole formation. The trench etch is performed by a two-step Bosch-based process, in which the cycle time, platen power, and process pressure during the etch step of the Bosch cycle are studied to adjust the dimensions of the scallops and their location in the trench sidewall, which control the taper of the trench sidewall. It is demonstrated that the amount of chemical flux, being adjusted by the cycle time of the etch step in the Bosch cycle, relates the scallop height to the sidewall profile angle. The required positive tapering of 88° to 89° for a keyhole-free structure after a trench refill by low-pressure chemical vapour deposition is achieved by lowering the time of the etch step.

Toward a modular, integrated, miniaturized, and portable microfluidic flow control architecture for organs-on-chips applications.

Microfluidic organs-on-chips (OoCs) technology has emerged as the trend for in vitro functional modeling of organs in recent years. Simplifying the complexities of the human organs under controlled perfusion of required fluids paves the way for accurate prediction of human organ functionalities and their response to interventions like exposure to drugs. However, in the state-of-the-art OoC, the existing methods to control fluids use external bulky peripheral components and systems much larger than the chips used in experiments. A new generation of compact microfluidic flow control systems is needed to overcome this challenge. This study first presents a structured classification of OoC devices according to their types and microfluidic complexities. Next, we suggest three fundamental fluid flow control mechanisms and define component configurations for different levels of OoC complexity for each respective mechanism. Finally, we propose an architecture integrating modular microfluidic flow control components and OoC devices on a single platform. We emphasize the need for miniaturization of flow control components to achieve portability, minimize sample usage, minimize dead volume, improve the flowing time of fluids to the OoC cell chamber, and enable long-duration experiments.

Air Damping Analysis of a Micro-Coriolis Mass Flow Sensor.

A micro-Coriolis mass flow sensor is a resonating device that measures small mass flows of fluid. A large vibration amplitude is desired as the Coriolis forces due to mass flow and, accordingly, the signal-to-noise ratio, are directly proportional to the vibration amplitude. Therefore, it is important to maximize the quality factor Q so that a large vibration amplitude can be achieved without requiring high actuation voltages and high power consumption. This paper presents an investigation of the Q factor of different devices in different resonant modes. Q factors were measured both at atmospheric pressure and in vacuum. The measurement results are compared with theoretical predictions. In the atmospheric environment, the Q factor increases when the resonance frequency increases. When reducing the pressure from 1 bar to 0.1 bar, the Q factor almost doubles. At even lower pressures, the Q factor is inversely proportional to the pressure until intrinsic effects start to dominate, resulting in a maximum Q factor of approximately 7200.

Heavily-Doped Bulk Silicon Sidewall Electrodes Embedded between Free-Hanging Microfluidic Channels by Modified Surface Channel Technology.

Surface Channel Technology is known as the fabrication platform to make free-hanging microchannels for various microfluidic sensors and actuators. In this technology, thin film metal electrodes, such as platinum or gold, are often used for electrical sensing and actuation purposes. As a result that they are located at the top surface of the microfluidic channels, only topside sensing and actuation is possible. Moreover, in microreactor applications, high temperature degradation of thin film metal layers limits their performance as robust microheaters. In this paper, we report on an innovative idea to make microfluidic devices with integrated silicon sidewall electrodes, and we demonstrate their use as microheaters. This is achieved by modifying the original Surface Channel Technology with optimized mask designs. The modified technology allows to embed heavily-doped bulk silicon electrodes in between the sidewalls of two adjacent free-hanging microfluidic channels. The bulk silicon electrodes have the same electrical properties as the extrinsic silicon substrate. Their cross-sectional geometry and overall dimensions can be designed by optimizing the mask design, hence the resulting resistance of each silicon electrode can be customized. Furthermore, each silicon electrode can be electrically insulated from the silicon substrate. They can be designed with large cross-sectional areas and allow for high power dissipation when used as microheater. A demonstrator device is presented which reached 119 . 4 ∘ C at a power of 206 . 9 m W , limited by thermal conduction through the surrounding air. Other potential applications are sensors using the silicon sidewall electrodes as resistive or capacitive readout.

A Flow-Through Microfluidic Relative Permittivity Sensor.

In this paper, we present the design, simulation, fabrication and characterization of a microfluidic relative permittivity sensor in which the fluid flows through an interdigitated electrode structure. Sensor fabrication is based on an silicon on insulator (SOI) wafer where the fluidic inlet and outlet are etched through the handle layer and the interdigitated electrodes are made in the device layer. An impedance analyzer was used to measure the impedance between the interdigitated electrodes for various non-conducting fluids with a relative permittivity ranging from 1 to 41. The sensor shows good linearity over this range of relative permittivity and can be integrated with other microfluidic sensors in a multiparameter chip.

Disposable DNA Amplification Chips with Integrated Low-Cost Heaters.

Fast point-of-use detection of, for example, early-stage zoonoses, e.g., Q-fever, bovine tuberculosis, or the Covid-19 coronavirus, is beneficial for both humans and animal husbandry as it can save lives and livestock. The latter prevents farmers from going bankrupt after a zoonoses outbreak. This paper describes the development of a fabrication process and the proof-of-principle of a disposable DNA amplification chip with an integrated heater. Based on the analysis of the milling process, metal adhesion studies, and COMSOL MultiPhysics heat transfer simulations, the first batch of chips has been fabricated and successful multiple displacement amplification reactions are performed inside these chips. This research is the first step towards the development of an early-stage zoonoses detection device. Tests with real zoonoses and DNA specific amplification reactions still need to be done.

µ-Coriolis Mass Flow Sensor with Resistive Readout.

This paper presents a µ -Coriolis mass flow sensor with resistive readout. Instead of measuring a net displacement such as in a capacitive readout, a resistive readout detects the deformation of the suspended micro-fluidic channel. It allows for actuation at much higher amplitudes than for a capacitive readout, resulting in correspondingly larger Coriolis forces in response to fluid flow. A resistive readout can be operated in two actuation vibrational modes. A capacitive readout can only be operated in one of these two modes, which is more sensitive to external disturbances. Three types of devices have been realized. We present measurement results for all three devices. One device clearly outperforms the other two, with a flow sensitivity of 2.22 ∘ / ( g / h ) and a zero-flow stability of 0.02 g / h over 30 min. Optimization of the metal strain gauges and/or implementation of poly-Silicon strain gauges could further improve performance.

Primary standards for measuring flow rates from 100 nl/min to 1 ml/min - gravimetric principle.

Microflow and nanoflow rate calibrations are important in several applications such as liquid chromatography, (scaled-down) process technology, and special health-care applications. However, traceability in the microflow and nanoflow range does not go below 16 µl/min in Europe. Furthermore, the European metrology organization EURAMET did not yet validate this traceability by means of an intercomparison between different National Metrology Institutes (NMIs). The NMIs METAS, Centre Technique des Industries Aérauliques et Thermiques, IPQ, Danish Technological Institute, and VSL have therefore developed and validated primary standards to cover the flow rate range from 0.1 µl/min to at least 1 ml/min. In this article, we describe the different designs and methods of the primary standards of the gravimetric principle and the results obtained at the intercomparison for the upper flow rate range for the various NMIs and Bronkhorst High-Tech, the manufacturer of the transfer standards used.

Primary standard for liquid flow rates between 30 and 1500 nl/min based on volume expansion.

An increasing number of microfluidic systems operate at flow rates below 1 µl/min. Applications include (implanted) micropumps for drug delivery, liquid chromatography, and microreactors. For the applications where the absolute accuracy is important, a proper calibration is required. However, with standard calibration facilities, flow rate calibrations below ~1 µl/min are not feasible because of a too large calibration uncertainty. In the current research, a traceable flow rate using a certain temperature increase rate is proposed. When the fluid properties, starting mass, and temperature increase rate are known, this principle yields a direct link to SI units, which makes it a primary standard. In this article, it will be shown that this principle enables flow rate uncertainties in the order of 2-3% for flow rates from 30 to 1500 nl/min.

Standing balance evaluation using a triaxial accelerometer.

This paper presents a new inherently triaxial accelerometer-based system for determining the ability to maintain balance while standing. In this study, the accelerometer was placed at the back of the subject at the approximate height of the centre of mass. The data were processed to obtain five performance parameters. Paired t-tests indicated that the accelerometer measurements were able to distinguish between the different test conditions as well as or better than simultaneous AMTI force platform measurements (P < or = 0.05). The accelerometer system is fully portable, independent of inclination in space, low-cost and allows long term measurements of standing balance.