Microfluidic Flow Sensor Research Articles

Comprehensive overview of microfluidic flow sensors: principles, materials, and fabrication

Comprehensive overview of microfluidic flow sensors: principles, materials, and fabrication

Microfluidic flow sensor based on chronoamperometric measurements in a microchannel

Microfluidics explores the fluid behavior at micron scale and is therefore suited to applications that need precise manipulation of fluids at small length or volume scales. In a span of over two decades, microfluidics has become an essential tool for modern genome sequencing platforms, single-cell analysis and common diagnostic tests. Despite many successful applications, the field has various challenges: Fabrication processes are yet to transition to commercial materials, microfluidic systems are reliant on skilled operators, and supporting equipment. Technological innovations in automation are needed to enable easy to use, completely on-chip microfluidic systems. Sensors are a critical component of any automated system. Fluid and flow properties such as flow rate, pressure, temperature, viscosity are critical to applications in drug development, and material/chemical synthesis. The flow rate (μL/min or nL/min) predictions based on external (off-chip) flow measurements can lead to erroneous calculations. The present work explores a technique based on chronoamperometry to measure flow rates inside a microchannel. Chronoamperometry based redox cycling of aqueous NaCl was performed by using interdigitated electrodes (IDE). The sensor measures current transients (which are modulated by the flow rate of the solution) effected by redox reactions on surface of the electrodes. The sensor demonstrated a sensitivity of 0.016 (μL/min)−1 change in charge transfer per unit change in flow rate and 3σ resolution of 9.3 μL/min. The calibration curve is linear in the range 0 – 200 μL/min, with a limit of detection (LoD) of 5 μL/min, making it suitable for various microfluidics applications.

On-chip resistive microfluidic flow sensor with reduced analysis time using transient analysis

On-chip resistive microfluidic flow sensor with reduced analysis time using transient analysis

A Microfluidic Flow Sensor Based on CuO Nws Enhanced Solid-Liquid Contact Electrification for Trapping Platelets on Collagen and Its Quantification

Platelet number plays an important role in several kinds of diseases such as leukemia, hepatitis C, diabetes, COVID-19, microbial infections, etc. These diseases can result in thrombocytopenia, a condition where patients suffer from reduced platelets, thus requiring the need to frequently monitor the platelet count. Currently existing techniques are complex, costly, and time consuming. The target diseases continue to evolve urging the need to develop rapid, facile point-of-care devices with high precision. In this regard, the current study is focused on the development of a platelet quantification technique based on trapping of platelets in a microfluidic channel adhered with collagen layer, and CuO NWs grown on Cu tube integrated on both the sides of the channel. This trapping of platelets results in a change in the triboelectric output performance from solid-liquid contact electrification due to change in flow rate of platelet content in the plasma. These changes in triboelectric output are enhanced by CuO NWs due to its high surface-area-to-volume ratio. The study of flow sensing by integrating the microfluidic device with solid-liquid contact electrification has been reported for the first time. The flow sensing signal was optimized by variation in viscosity of the flowing liquid, channel geometry, flow rate, and triboelectric voltage measuring methods. The platelet rich plasma (PRP) and platelet poor plasma (PPP) were prepared and mixed in different ratios to prepare the solutions with different concentration of platelets (0% - 100%). The calibration curve for platelet quantification has shown a variation of 9 V for change in platelet concentration from 0% to 100%. The proposed device exhibited a linear response with the variation in platelet counts. The obtained results prove its potential and feasibility as a point-of-care device.

Microfluidic thermal flow sensor based on phase-change material with ultra-high thermal sensitivity

A microfluidic thermal mass flow sensor with ultra-high thermal sensitivity, based on planar micro-machining technology and a phase-change material is developed.

Integrated membrane-free thermal flow sensor for silicon-on-glass microfluidics.

Lab-on-a-chip (LOC) forms the basis of new-generation portable analytical systems. LOC allows the manipulation of ultralow flows of liquid reagents and multistep reactions on a microfluidic chip, which requires a robust and precise instrument to control the flow of liquids on a chip. However, commercially available flow meters appear to be a standalone option adding a significant dead volume of tubes for connection to the chip. Furthermore, most of them cannot be fabricated within the same technological cycle as microfluidic channels. Here, we report on a membrane-free microfluidic thermal flow sensor (MTFS) that can be integrated into a silicon-glass microfluidic chip with a microchannel topology. We propose a membrane-free design with thin-film thermo-resistive sensitive elements isolated from microfluidic channels and a 4'' wafer silicon-glass fabrication route. It ensures MTFS compatibility with corrosive liquids, which is critically important for biological applications. MTFS design rules for the best sensitivity and measurement range are proposed. A method for automated thermo-resistive sensitive element calibration is described. The device parameters are experimentally tested for hundreds of hours with a reference Coriolis flow sensor demonstrating a relative flow error of less than 5% within the range of 2-30 μL min-1 along with a sub-second time response.

A suspended polymeric microfluidic sensor for liquid flow rate measurement in microchannels

In this study, a microfluidic cantilever flow sensor was designed and manufactured to monitor liquid flow rate within the range of 100–1000 µl/min. System simulation was also performed to determine the influential optimal parameters and compare the results with experimental data. A flowmeter was constructed as a curved cantilever with dimensions of 6.9 × 0.5 × 0.6 mm3 and a microchannel carved with a CO2 laser inside the cantilever beam. The fabrication substance was Polydimethylsiloxane. Different flow rates were injected using a syringe pump to test the performance of the flowmeter. Vertical displacement of the cantilever was measured in each flowrate using a digital microscope. According to the results, the full-scale overall device accuracy was up to ± 1.39%, and the response time of the sensor was measured to be 6.3 s. The microchip sensitivity was 0.126 µm/(µl/min) in the range of measured flow rates. The sensor could also be utilized multiple times with an acceptable error value. The experimental data obtained by the constructed microchip had a linear trend (R2 = 0.995) and were of good consistency with simulation results. Furthermore, according to the experimental and the simulation data, the initially curved cantilever structure had a higher bending and sensitivity level than a perfectly straight cantilever construction.

A disposable smart microfluidic platform integrated with on-chip flow sensors

Microfluidic devices are powerful tools for biological, biomedical, chemical, and pharmaceutical applications, but their commercialization is still hindered by the lack of methods to automatically control fluid flow in a low-cost, simple, accurate, and safe manner. This study introduces a disposable smart microfluidic platform (DIS-μChip), which can be fully automated and utilized for a wide range of applications. On-chip microfluidic flow sensors are integrated with the platform and placed at all inlet and outlet channels, thereby allowing the DIS-μChip to be fully automated with a pressure control system. Furthermore, these confer a self-diagnosis function through monitoring of all the input and output flow rates. The DIS-μChip consists of a disposable polymeric microchannel superstrate and a permanent multifunctional substrate, which could be assembled and disassembled using only vacuum pressure. The superstrate was fabricated by combining a polydimethylsiloxane microchannel structure with a polyethylene terephthalate (PET) thin film. The substrate contains sense electrodes for the on-chip-integrated flow sensors and functional components for creating an energy field, which can penetrate the PET thin film and manipulate the fluid in the microchannels of the superstrate. Owing to the film-chip technique, the superstrate was disposable and could prevent biological cross-contamination, which cannot be realized with conventional flow sensors. The usefulness of the DIS-μChip was demonstrated by using it to isolate circulating tumor cells from the blood of patients with pancreatic cancer and to obtain cancer-specific genetic information from them with droplet digital PCR.

Proof-of-Concept Experiment of Microfluidic Flow Sensor Based on Microcavity- Secondary-Flow Observation

We propose a new type of microfluidic flow sensor, where the velocity measurements are not done on the fluid sample itself but on a second, immiscible fluid in contact with the sample. This allows us to use a low-cost, straight-forward, and well-developed particle tracking velocimetry on samples otherwise unsuitable for tracer particles. Our proof-of-concept experimental setup consists of a microfluidic device with the main channel filled with the sample and rectangular side-cavities, adjacent and opened to the main channel, filled with immiscible fluid with tracer particles. The flow in the main channel induces a circular secondary flow of the cavity liquid. We observed and analyzed the side-cavity particle movement and found that its characteristic parameters show a good linear correlation with flow velocity in the main channel over the whole observed range, from approx. $600~\mu \text{m}$ /s (volumetric flow rate $\Phi _{v} \cong 200$ nl/min, Reynolds number $\textit {Re} \cong 10^{-3}$ ) to approx. $3500 ~\mu \text{m/s}\left ({\Phi _{v} \cong 1200~ \text{nl/min}, ~\text{Re} \cong 10^{-2}}\right)$ .

A passive, biocompatible microfluidic flow sensor to assess flows in a cerebral spinal fluid shunt

Abstracts Hydrocephalus is a disease in which a buildup of cerebral spinal fluid in the ventricles of the brain can cause brain trauma and death if untreated. The current standard of treatment is to insert a shunt to drain the fluid from the ventricles in the brain to the abdomen. A shortcoming of this approach is that the shunt may get clogged, and this failure is not easily detected. We have developed a novel microfluidic sensor for application in a cerebrospinal fluid shunt. The sensor system is totally passive and no implanted electronics are required. The sensor itself is a bending based cantilever made from biocompatible polydimethylsiloxane (PDMS) and is capable of measuring flow rates from 20 ml/hr to 120 ml/hr, which is the indicated range for the flow rate of cerebral spinal fluid. The sensor is paired with an optical detection system that uses a small light spot to read the changes in sensor position due to flow. The light is input from outside the brain into the implanted optical system and the output light is measured by a camera external to the body. The sensor stability was verified by running cerebral spinal fluid over the cantilever continuously for two weeks. The ability of the sensor to measure pulsed flow and linearly increasing flow rates was also verified. All of the materials used for the device are biocompatible materials and are amenable for manufacturing and animal tests and clinical studies.

A disposable microfluidic flow sensor with a reusable sensing substrate

In many biological, medical and pharmaceutical applications, disposable flow sensors are highly desired to prevent biological contamination. However, due to their high cost, conventional microfluidic flow sensors are difficult to be used as disposable. This paper introduces a disposable microfluidic flow sensor (disposable-μFSensor) consisting of a disposable microchannel superstrate and reusable sensing substrate, which can be assembled and disassembled simply by vacuum pressure. Because the disposable microchannel superstrate is fabricated easily and cheaply using a silicone-coated release polymer thin film, it can be replaced after a single use, while the sensing substrate can be reused continuously. Experimental results show that the sensitivity, reproducibility, response time, long-term stability, resolution, and accuracy of the disposable-μFSensor are comparable to those of commercial microfluidic flow sensors. One of the key advantages of the disposable-μFSensor is that the measurable flow rate range can be adjusted very broadly, from 10 to 650 μL/min, by simply replacing only the disposable microchannel that has different bypass channel design, without having to change the sensing substrate. To demonstrate the usefulness of the disposable-μFSensor for biological, medical and pharmaceutical applications, the developed sensor was used to control the flow rates of a droplet generator, with the generated droplets having variations in diameter and volume of less than 1.25% and 4%, respectively. In summary, we report that the disposable-μFSensor is reproducible and accurate for various applications at low cost as well as resistant to biological contamination, in contrast to conventional flow sensors.

Fluid–Structure Interaction of High Aspect-Ratio Hair-Like Micro-Structures Through Dimensional Transformation Using Lattice Boltzmann Method

3D printed hair-like micro-structures have been previously demonstrated in a novel micro-fluidic flow sensor aimed at sensing air flows down to rates of a few milliliters per second. However, there is a lack of in-depth understanding of the structural response of these ‘micro-hairs' under a fluid flow field. This paper demonstrates the use of lattice Boltzmann methods (LBM) to understand this structural response towards a better optimization of the micro-hair flow sensors designed to suit the end applications' needs. The LBM approach was chosen as an efficient alternative to simulate Navier–Stokes equations for modeling fluid flow around complex geometries primarily for improved accuracy and simplicity with lesser computational costs. As the spatial dimensions of the sensor's flow channel are much larger in comparison to the actual micro-hairs (the sensing element), a multidimensional approach of combining two-dimensional (D2Q9) and three-dimensional (D3Q19) lattice configurations were implemented for improved computational speeds and efficiency. The drag force on the micro-hairs was estimated using the momentum-exchange method in the D3Q19 configuration and this drag force is transferred to the structural analysis model which determines the micro-hair deformation using Euler–Bernoulli beam theory. The entirety of the LBM Fluid–Structure Interaction (FSI) model was implemented within MATLAB and the obtained results are compared against the numerical model implemented on a commercially available software package.

Highly sensitive microfluidic flow sensor based on aligned piezoelectric poly(vinylidene fluoride-trifluoroethylene) nanofibers

A microfluidic flow sensor based on aligned piezoelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers has been developed. The flow sensor is able to linearly measure low flow rates ranging from 13 μl/h to 301 μl/h with a sensitivity of 0.36 mV per 1 μl/h, and the highest voltage difference of 120 mV at a flow rate of 451 μl/h. Moreover, the viscosity of the ethylene glycol aqueous solution ranging from 1 mPa·s to 16.1 mPa·s at 25 °C can be detected in dynamic flow with a stable output. These findings highlight the potential of piezoelectric P(VDF-TrFE) nanofibers in multiferroic applications.

Modelling and evaluation of a thermal microfluidic sensor fabricated on plastic substrate

The simulation and the experimental evaluation of a thermal microfluidic flow sensor are presented in this work. The sensor was evaluated in calorimetric mode with water flow. The simulation was performed with COMSOL package. Various combinations of microchannel geometries were tested in different input flows. The optimum pair of upstream–downstream sensing elements were extracted theoretically and verified experimentally. The operational parameters of the device were simulated by 3D models and the optimum conditions were determined for various flow ranges.

A multi-range PCB-MEMS microfluidic flow sensor with adjustable sensitivity

The complete characterization of a polymer microfluidic flow sensor is presented in this paper. The device fabrication is based on a combination of PCB and MEMS techniques, enabling effective thermal isolation and direct communication to the macroworld. The sensor performance can be tuned based on the precise definition of structural and operational parameters, these being the microchannel crossection, the sensing element positioning, and the sensor operating principle. By combining the aforementioned parameters, an extended measurement range and high sensitivity in specified flow rate regions can be achieved.

Modelling and evaluation of a thermal microfluidic sensor fabricated on plastic substrate

Both the simulation and the experimental evaluation of a thermal microfluidic flow sensor are presented in this work. The sensor was evaluated in the calorimetric principle of operation with water as the test liquid. The simulation of the sensor operation was performed into the framework of Comsol® software package. Various combinations of microchannel geometries were tested with different input flow rate values. The operational parameters of the device were simulated by 3D models. The dominant operation parameter was determined to be the distance of the upstream-downstream sensing elements to the heater, the effect of which was theoretically extracted and experimentally verified.

Novel microfluidic flow sensor based on a microchannel capped by porous silicon

AbstractThis work concerns the fabrication, modeling and characterization of a novel microfluidic flow sensor based on a microchannel capped with a porous silicon membrane, on top of which the sensor active elements are integrated. The microchannel is formed through a two‐step anodization process. In the first step a mesoporous silicon layer is formed on a lithographically defined area, while in the second step a channel is formed underneath the porous layer by electropolishing. The channel is buried into bulk silicon and capped with a free‐standing porous silicon layer, which is co‐planar with the Si substrate. The overall developed process for the formation of the device will be described and simulation and device characterization results will be presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microfluidic flow rate detection based on integrated optical fiber cantilever

We demonstrate an integrated microfluidic flow sensor with ultra-wide dynamic range, suitable for high throughput applications such as flow cytometry and particle sorting/counting. A fiber-tip cantilever transduces flow rates to optical signal readout, and we demonstrate a dynamic range from 0 to 1500 microL min(-1) for operation in water. Fiber-optic sensor alignment is guided by preformed microfluidic channels, and the dynamic range can be adjusted in a one-step chemical etch. An overall non-linear response is attributed to the far-field angular distribution of single-mode fiber output.

Planar cmos compatible process for the fabrication of buried microchannels in silicon, using porous-silicon technology

This work presents a new method for the fabrication of buried microchannels, covered with porous silicon (PS). The specific method is a two-step electrochemical process, which combines PS formation and electropolishing. In a first step a PS layer with a specific depth is created at a predefined area and in the following step a cavity underneath is formed, by electropolishing of silicon. The shape of the microchannel is semi-cylindrical due to isotropic formation. The method allows accurate control of the dimensions of both PS and the cavity. The formation conditions of the PS layer and the cavity were optimized so as to obtain smooth microchannel walls. In order to obtain stable structures the area underneath the PS masking layer was transformed into n-type by implantation, taking advantage of the selectivity of PS formation between n- and p-type silicon. With this technique, a monocrystalline support for the PS layer is formed on top of the cavity. Various microchannel diameters with different thickness of capping PS layer were obtained. The process is CMOS compatible and it uses only one lithographic step and leaves the surface of the wafer unaffected for further processing. A microfluidic thermal flow sensor was fabricated using this technology, the experimental evaluation of which is in progress.

Simulation and optimization of a microfluidic flow sensor

With microfluidic systems becoming more prominent, modeling of microfluidic components within microfluidic systems is becoming important. This paper describes a technique for modeling microfluidic flow rate microsensors using a standard circuit simulator, SPICE3f5. The model includes a liquid Poiseuille flow through an integrated microfluidic microchannel. The model provides insight and understanding of many physical and dynamic properties of flow sensors located within microfluidic environments. This method is cost-effective as electrical, thermal, and fluidic simulations can be performed in the same design environment. This method also offers both steady-state and dynamic operation simulation of the flow sensor. Results of several simulations are presented that verify the usefulness of our methods for both modeling and optimization of microfluidic flow sensors. Lastly, a sensor is fabricated using the optimized geometry and experimental results are compared to the simulation results.