A micro PDMS flow sensor based on time-of-flight measurement for conductive liquid

Transdermal extraction of interstitial fluid (ISF) offers an attractive method for non-invasive blood glucose monitoring. In order to calculate blood glucose concentration accurately, precise volume measurement of transdermally extracted ISF is required due to human skin’s varying permeability. In this paper, we presented a novel flow sensor fabricated from polydimethylsiloxane (PDMS), designed to measure the volume of conductive liquid. The flow sensor consists of two pairs of metal electrodes, which are fabricated in the PDMS channel. The volume of liquid is measured utilizing the time-of-flight of the two electrode pairs’ resistance while the liquid is flowing through the flow sensor. 1–14 μL normal saline solution was measured, the flow sensor measured volumes correlate very well (R ² = 0.9996 and R ² = 0.9975 for vacuum pump and syringe pump situations respectively) with the actual volumes. And the coefficient of variation for 10 times 10 μL normal saline solution measurement is 0.0077 (vacuum pump) and 0.0381 (syringe pump), respectively. The demonstrated flow sensor provides excellent functionality for conductive liquid.     

A two-stage electrophoretic microfluidic device for nucleic acid collection and enrichment

Sample purification and enrichment is an important and usually time-consuming step for on-chip nucleic acid detection and analysis. This paper presents an electrophoretic DNA focusing method in microfluidic devices to enrich nucleic acid concentration by around 2700-fold. The electrical waveforms applied to five individual electrodes are such designed that DNAs move successively to the collection electrodes at high speed, while the interferences from bubbles due to electrohydrolysis are minimized. In a spiral channel with a total length of 48 cm, 1 ml DNA sample is purified and enriched by 57 times at a flow rate of 30 μl/min at first. The captured DNAs are then released and transported to the second microfluidic chamber where DNAs are collected and concentrated by 49 times. Thus, in about 40 min, the two-stage device can extract DNAs from 1 ml sample volume and enrich its concentration by 2790-fold. A trade-off exists between the process throughput and the DNA collection efficiency. A DNA capture efficiency of 99.7 % is reached when the flow rate is 1 μl/min, and the maximum DNA capture throughput is achieved at a flow rate of 30 μl/min. As a platform technology, the device can be integrated into bio-sensing and genetic analysis assays for DNA extraction and pre-concentration.     

Liquid film thickness measurement underneath a gas slug with miniaturized sensor matrix in a microchannel

Measurement of liquid film thickness is essential for understanding the dynamics of two-phase flow in microchannels. In this work, a miniaturized sensor matrix with impedance measurement and MEMS technology to measure the thin liquid film underneath a bubble in the air–water flow in a horizontal microchannel has been developed. This miniaturized sensor matrix consists of 5 × 5 sensors where each sensor is comprised of a transmitter and a receiver electrode concentrically. The dimension and performance of the sensor electrodes were optimized with simulation results. The maximum diameter of the sensor ring is 310 µm, allowing a measurable range of liquid film thickness up to 83 µm. These sensors were distributed on the surface of a wafer with photolithography technology, covering a total length of 8 mm and a width of 2 mm. A spatial resolution of 0.5 × 2.0 mm² and a temporal resolution of 5 kHz were achieved for this sensor matrix with a measurement accuracy of 0.5 µm. A series of microchannels with different heights were used in the calibration in order to achieve the signal-to-thickness characteristics of each sensor. This delicate sensor matrix can provide detailed information on the variation of film thickness underneath gas–water slug directly, accurately and dynamically.     

Component design and testing for a miniaturised autonomous sensor based on a nanowire materials platform

We present the design considerations of an autonomous wireless sensor and discuss the fabrication and testing of the various components including the energy harvester, the active sensing devices and the power management and sensor interface circuits. A common materials platform, namely, nanowires, enables us to fabricate state-of-the-art components at reduced volume and show chemical sensing within the available energy budget. We demonstrate a photovoltaic mini-module made of silicon nanowire solar cells, each of 0.5 mm² area, which delivers a power of 260 μW and an open circuit voltage of 2 V at one sun illumination. Using nanowire platforms two sensing applications are presented. Combining functionalised suspended Si nanowires with a novel microfluidic fluid delivery system, fully integrated microfluidic–sensor devices are examined as sensors for streptavidin and pH, whereas, using a microchip modified with Pd nanowires provides a power efficient and fast early hydrogen gas detection method. Finally, an ultra-low power, efficient solar energy harvesting and sensing microsystem augmented with a 6 mAh rechargeable battery allows for less than 20 μW power consumption and 425 h sensor operation even without energy harvesting.     

Differential C<Superscript>4</Superscript>D sensor for conductive and non-conductive fluidic channel

This paper presents a novel design of a differential C⁴D (DC⁴D) sensor based on three electrodes for both conductive and non-conductive fluidic channel. This structure consists of two single C⁴D with an applied carrier sinusoidal signal to the center electrode as the excitation electrode. The electrodes are directly bonded on the PCB with built-in differential amplifier and signal processing circuit in order to reduce the parasitic component and common noise. In the non-conductive fluidic channel, the output voltage and capacitance changes 214.39 mV and 14 fF, respectively when a 3.83 μl tin particle crosses an oil channel. In conductive fluidic channel, the output voltage and admittance change up to 300 mV and 0.07 μS for the movement of a 4.88 μl plastic particle through channel. Moreover, the voltage change of this sensor is linear relation with the volume of investigated particle. This sensor also allows measuring velocity of particle inside fluidic channel and resistivity of the conductive fluidic.     

A low power micro fluxgate sensor with improved magnetic core

The influence of an improved magnetic core on the micro fluxgate sensor about sensitivity and power consumption is investigated and discussed in this paper. We have fabricated the micro solenoid fluxgate sensors based on the MEMS technologies, with the electroplating permalloy cores, which are easy to process and used in common; and the amorphous soft magnetic ribbon cores, which have better soft magnetic performances but be hard to be integrated, respectively. Four magnetic core structures are designed, including rectangular structure, unequal width rectangular structure, multi rectangular ring structure and spiral structure. Spiral structure can improve the performances of the fluxgate sensor significantly, both sensitivity and power consumption. The micro fluxgate sensors with the amorphous soft magnetic ribbon cores are promoted in all aspects than those with the electroplating permalloy cores, including ultra low power consumption of 2.4 mW with unequal width rectangular structure, and high sensitivity of 118 V/T with rectangular structure in wide linear range of 0–800 μT.     

Flexible time–temperature indicator: a versatile platform for laminated paper-based analytical devices

Time–temperature indicators are devices employed for monitoring thermal history of perishable food products from storage to consumption. As the food industry has opted for intelligent packaging, which also involves thin foldable wrappings, a requirement arises for a flexible device capable of attaching and sensing on a curved product surface. In this study, we have fabricated a distance-based flexible time–temperature indicator (FTTI) by employing electronic cutting machine and through press lamination of thermoplastic films; we have incorporated the FTTI with a unique starting actuation switch made of thin (150 μm) cover glass. Flow profile in 0.45 μm nitrocellulose membrane is measured for oleic, octanoic, and decanoic acids representing test temperatures of 5, 15 and 30 °C, respectively. Results demonstrate the sensor to be robust and flexible with precise responsiveness to oscillating temperature conditions. Further improvement in time-scale is achieved by employing a series of fan-shaped cut channels. This two-dimensional flow increases the device time by 170% in comparison with straight channel and improves scale readability by achieving a linear distance-time relation in the porous membrane. The advantages of low cost, simple design, freedom from equipment, robustness, and flexibility render the FTTI a versatile platform for distance-based diagnostics, food quality control, and environmental monitoring devices.     

Analytical study on cantilever resonance type magnet-integrated sensor device for micro-magnetic field detection

A beam-shaped cantilever resonance type magnetic sensor device has been proposed with a micro magnet. Two structural designs, named as design 1 and design 2, have been comparatively analyzed using ANSYS in order to obtain larger frequency shifts (higher magnetism sensitivity) due to the applied exterior magnetic field. The analytical results show that, in the range of 0–10 mT, the frequency shifts are small, while under 100 mT, a relatively larger frequency shift of about 30 Hz can be theoretically obtained. The power consumption of the proposed devices has been further theoretically studied for preliminary understanding. Using the well-known displacement equations, the estimated power consumption is around 0.21 μW, which is very lower than that of the reported magnetic field sensors. This implies that it is possible to fabricate higher sensitive magnetic field sensor with lower power consumption.     

Fabrication of micro sloping structures of SU-8 by substrate penetration lithography

In this paper, three-dimensional (3D) micro sloping structures were fabricated by ordinary mask pattern and diffraction phenomenon. Especially, we fabricated the structures with SU-8 negative photoresist and substrate penetration lithography. In this method, exposure is performed arranging in order of a mask, a substrate and the SU-8 resist. There is a gap that is equal to the thickness of the substrate between resist and mask. In narrow slit of mask, resist is less exposed than usual because of Fraunhofer diffraction. The amount of exposure depends on slit width so that the height of SU-8 resist can be controlled. A 173 μm height of structure was obtained in the case of 27 μm width slit and 24.2 μm height of structure was obtained in the case of 7.4 μm width slit. By using this method, high aspect ratio 3D SU-8 structures with smooth sloping were fabricated in the length of 100–300 μm and in the height of 50–200 μm with rectangular triangle mask pattern. In the same way, there is influence of Fresnel diffraction on edge of aperture so that micro taper structures were fabricated. A lot of taper structures were fabricated by the method to make the surface repellency. The contact angle was achieved more than 160° in this study.     

A printed circuit board capacitive sensor for air bubble inside fluidic flow detection

This paper presents a three-electrode capacitive fluidic sensor for detecting an air bubble inside a fluidic channel such as blood vessels, oil or medical liquid channels. The capacitor is designed and fabricated based on a printed circuit board (PCB). The electrodes are fabricated by using copper via structure through top to bottom surface of the PCB. A plastic pipe is layout through the capacitive sensor and perpendicular to the PCB surface. Capacitance of sensor changes when an air bubble inside fluidic flow cross the sensor. The capacitance change can be monitored by using a differential capacitive amplifier, a lock-in amplifier, filter and an NI acquisition card. Signal is processed and calculated on a computer. Air bubble inside the liquid flow are detected by monitor the unbalance signal between the three electrode potential voltages. Output voltage depends on the volume of the air bubble due to dielectric change between capacitor’s electrodes. Output voltage is up to 53 mV when an 2.28 mm³ air bubble crosses the sensing channel. Air bubble velocity can be estimated based on the output pulse signal. This proposed fluidic sensor can be used for void fraction detection in medical devices and systems; fluidic characterization; and water–gas, oil–water and oil–water–gas multiphase flows in petroleum technology. That structure also can apply to the micro-size for detecting in microfluidic to monitor and control changes in microfluidic channels.     

Design and fabrication of microtunnel and Si-diaphragm for ZnO based MEMS acoustic sensor for high SPL and low frequency application

The purpose of the paper is to design and fabricate a ZnO-based MEMS acoustic sensor for higher sound pressure level (SPL) measurement in the range of 120–200 dB and low frequency infrasonic wave detection. The thickness of silicon diaphragm was optimized for higher SPL using MEMS-CAD-Tool COVENTORWARE. The microtunnel which relates the cavity to the atmosphere was designed and simulated analytically for low cut-off frequency of the sensor in infrasonic band. The resonance frequency of the sensor was obtained using modal analysis. The sensitivity of the sensor was also estimated using COVENTORWARE. The optimized Si-diaphragm thickness for the intended SPL range was determined and found to be 50 μm. The lower cut-off frequency of the sensor for a 10 μm-deep microtunnel was found to be 0.094 Hz. The resonance frequency of the sensor was obtained using modal analysis and found to be 78.9 kHz. Based on simulation results, the MEMS acoustic sensor with 10 μm-deep microtunnel was fabricated. The optimum sensitivity of sensor was calculated using simulated results and found to be 116.4 μVolt/Pa. The lower cut-off frequency of the sensor can be utilized to detect low frequency sounds. The high SPL sensing capability of the device up to 200 dB facilitates detection of high sound pressure level in launch vehicles, rocket motors and weapons’ discharge applications.     

Numerical simulation of gas flow in rough microchannels: hybrid kinetic–continuum approach versus Navier–Stokes

The flow field in a rough microchannel is numerically analyzed using a hybrid solver, dynamically coupling kinetic and Navier–Stokes solutions computed in rarefied and continuum subareas of the flow field, respectively, and a full Navier–Stokes solver. The rough surface is configured with triangular roughness elements, with a maximum relative roughness of 5 % of the channel height. The effects of Mach number, Knudsen number (or Reynolds number) and roughness height are investigated and discussed in terms of Poiseuille number and mass flow rate. Discrepancies between full Navier–Stokes and hybrid solutions are analyzed, assessing the range of validity of Navier–Stokes equations provided with first-order slip boundary conditions for modeling gas flow along a rough surface. Results indicate that the roughness increases Poiseuille number and decreases mass flux in comparison with those for the smooth microchannel. Increasing rarefaction results in further enhancement of roughness effect. At the same time, the compressibility effect is more noticeable than the roughness one, although the compressibility effect is alleviated by increase in the rarefaction. It was found that, although the Navier–Stokes solution of the flow in a smooth channel is accurate up to Kn = 0.1, when relative roughness height is higher than 1.25 % significant errors already appear at Kn = 0.02.     

An experimental study on an electro-osmotic flow-based silicon heat spreader

An experimental study has been carried out to estimate the heat transfer improvement offered by a novel electro-osmotic (EO) heat spreader for microprocessor cooling. The proposed design of the elliptical silicon heat spreader can be fabricated at the back surface of the microprocessor as an integral part. Thus, no extra space may be required. The EO heat spreader developed is 0.6 mm³ in volume and it contains a pair of thin gold film electrodes of approximately 1 μm thickness for applying an external electric field that induces electro-osmotic flow. The inner channel surfaces of the heat spreader are electrically insulated with a thin SiO₂ layer to minimize current leakage into the wafer. The EO heat spreader constructed is able to generate a flow rate of 0.2028 μl/min at 2 V/mm. With this heat spreader, the temperature of a heat source may be reduced by up to 4°C without the aid of any external mechanical devices. The heat spreader has the potential to make the temperature uniform, if the heat source is non-uniform in nature.     

High aspect ratio metal micro and nano pillars for minimal footprint MEMS suspension

Vertical nano and micro pillars perpendicularly rising from a substrate offer two lateral translatory–rotatory degrees of freedom. Electroforming allows their production as small footprint integrated suspension elements of micro to nano scale. This paper demonstrates the design of a novel inertial sensor concept with acceleration sensor and gyroscope function using only one inertial mass. Experimental results using UV Direct LIGA with AZ 125 nXT show the feasibility of a technology demonstrator with a copper micro pillar of 400 μm length and 40 μm diameter. Further work using x-ray Direct LIGA is scheduled for the production of the pillar with a length of 100 μm and a diameter of 3–6 μm. Fabrication concepts and pilot tests show promising possibilities for miniaturization towards nano scale pillars for minimal footprint suspension in MEMS.     

Design,fabrication and characterization of micro flow sensor inspired by biological hair cell

Design, fabrication and characterization of micro flow sensor were investigated based on the inspiration of biological hair cell in a nature. The micro scale artificial hair cell sensor was designed as considering two parts; first the high aspect ratio cilium structure which works as a hair cell of fish and second the mechanoreceptor structure where the drag force by flow are actually measured. Parameters of cilium structure were designed based on static modelling as follow: 300 μm diameter and 2 mm height. The high aspect ratio cilium structure was precisely fabricated using a hot embossing process with the developed separated micro mold system prepared by LIGA (from the German Lithographi, Galvanoformung, Abformung) process. The mechanoreceptor was formulated with a force sensitive resistor with four symmetric electrodes to analyze the direction and the magnitude of target flow. Performance of assembled sensor was characterized using the prepared water channel. Flow velocity was sensed with the magnitude of signal and the direction of flow was distinguished by analyzing the signals from four mechanoreceptors.     

Assessing the sun glint effect on the data bias of CrIS shortwave surface channels near 3.7 μm

The Suomi National Polar-orbiting Partnership (NPP) satellite was successfully launched on 28 October 2011. The on-board Cross-track Infrared Sounder (CrIS) provides the hyperspectral infrared radiance covering a spectral range of 3.92–15.4 μm, inheriting the task to improve numerical weather prediction (NWP) from previous hyperspectral sounders. The so-called sun glint effect results in large biases in CrIS shortwave surface channels near 3.7 μm and therefore impedes the usage of those channels in the operational data assimilation, because the data biases are required to be evaluated appropriately by any data assimilation system. This work assesses the sun glint effect on bias characteristics of those shortwave surface channels near 3.7 μm, with the help of a sun glint model developed in the community radiative transfer model (CRTM). It is demonstrated that the daytime biases of those shortwave surface channels are decreased markedly after applying sun glint correction with values close to 0 K. The dependence of daytime biases on sensor zenith angles is also eliminated by using the sun glint model. It is seen that the differences between daytime and night-time biases can reach 0.6 K near mid-latitudes in the southern hemisphere after including the sun glint effect, which implies that the sun glint model needs further enhancement. Overall, the direct assimilation of CrIS shortwave surface channels near 3.7 μm is possibly accomplished by utilizing the sun glint model implemented in CRTM during both daytime and night-time.     

Design and fabrication of an electrochemically actuated microvalve

A microfluidic valve based on electrochemical (ECM) actuation was designed, fabricated using UV-LIGA microfabrication technologies. The valve consists of an ECM actuator, polydimethylsiloxane (PDMS) membrane and a micro chamber. The flow channels and chamber are made of cured SU-8 polymer. The hydrogen gas bubbles were generated in the valve microchamber with Pt black electrodes (coated with platinum nanoparticles) and filled with 1 M of NaCl solution. The nano particles coated on the working electrode helps to boost the surface-to-volume ratio of the electrode for faster reversible electrolysis and faster valve operation. To test the functionality of the microvalve, a simple micropump based on ECM principle was also integrated in the system to deliver a microscopic volume of fluid through the valve. The experimental results have showed that an approximately 300 μm deflection of valve membrane was achieved by applying a bias voltage of ?1.5 V across the electrodes. The pressure in the valve chamber was estimated to be about 200 KPa. Experimental results proved that the valve can be easily operated by controlling the electrical signals supplied to the ECM actuators.     

Development of a micro biochip integrated traveling wave micropumps and surface plasmon resonance imaging sensors

Since most of miniaturized surface plasmon resonance (SPR) sensing systems need commercially available peristaltic or syringe pumps, it is difficult to reduce the system size, biosample volume, and the production cost. In this paper, a compact biochip for clinical diagnosis is presented. The proposed biochip is integrated traveling wave micropumps and SPR imaging sensors on one chip. The micropump is composed of flexible microchannel and piezoelectric bimorph actuator array, and achieved the maximum flow rate 336 μl/min. The SPR imaging biosensor can quantitatively measure biosamples with multi microchannels, i.e. one biosample and two reference flows to obtain an analytical curve. The SPR imaging measurements with bovine serum albumin solutions were carried out using the prototype of the proposed diagnostic system composed of a pair of the micropump and the sensor. Since the clear SPR signal curve was observed, it was confirmed that the proposed system can be applicable to the clinical diagnosis.     

Improving particle detection sensitivity of a microfluidic resistive pulse sensor by a novel electrokinetic flow focusing method

A novel and simple method of improving the particle detection sensitivity of a microfluidic resistive pulse sensor was presented in this paper. This novel electrokinetic flow focusing method utilizes a focusing solution (with high resistivity) flowing from the upstream focusing channel to the downstream focusing channel. The focusing solution in the sensing gate works like a virtual insulation wall that greatly narrows the gate and thus improves the detection sensitivity. An equation was derived to relate the magnitude of the output signal to the resistivity and the width of the focusing solution. The width of the focused particle solution under different voltages was numerically predicted. The results show that the magnitude of output signal increases with the decrease in the width of the focused particle stream. More importantly, the detection sensitivity can be improved by decreasing the space occupied by the focusing solution in the upstream and downstream channels as much as possible. Detection of 1 μm particle with a sensing gate of 30 × 40 × 10 μm (width × length × height) was successfully achieved. The proposed method is simple and advantageous in detecting smaller particles without fabricating a small sensing gate.     

High throughput micro droplet generator array controlled by two-dimensional dynamic virtual walls

A chamber-free two-dimensional-array micro droplet generator has been realized by precise time-delayed control of micro bubble arrays as virtual chamber walls. Droplets can be ejected out by the bubbles around the ejection site in specific configuration of excitation, thus replacing physical chamber walls for pressure preservation. The micro droplet generator array was fabricated by heater lithography and direct nozzle formation on a laminated SU-8 dry film without any solid chamber wall among heaters. The nozzle density of this compact droplet generator can be five to ten times higher than that of commercial inkjet printheads in one-dimensional formats. The volume and initial speed of the generated droplets was 3.6–5.7 pL and 14–15 m/s, respectively, meeting the standard of commercial printheads. The micro droplet generator is free of satellite droplets due to the precise meniscus control. The analyzed data shows the meniscus undergoes a “push–pull–push” progress which effectively cuts the liquid column short. The refilling time of the innovative micro droplet generator was estimated to be 0.296 μs from the simplified chamber model, and it was one-tenth of the commercial printheads. In addition, the frequency response was estimated to be higher than 20 kHz by observing the meniscus fluctuation condition. Finally, a 3 × 5 heater array was used to generate two droplets simultaneously, which shows that the crosstalk problem can be eliminated by precise time-delayed control. An interlacing operation was also proposed to address the large array control algorithm. To summarize, a 330-dpi monolithic micro droplet generator prototype has been proposed for high speed and large 2D format printing.