On-chip Valves Research Articles

Simple Method for On-Demand Droplet Trapping in a Microfluidic Device Based on the Concept of Hydrodynamic Resistance.

We demonstrate an innovative method to catch the desired droplets from a train of droplets and immobilize them in traps located in an integrated microfluidic device. To this end, water-in-oil droplets are generated in a flow-focusing junction and then guided to a channel connected to chambers designated for on-demand droplet trapping. Each chamber is connected to a side channel through a batch of microposts. The side channels are also connected to the flexible poly(vinyl chloride) tubes, which can be closed by attaching binder clips. The hydrodynamic resistance of the routes in the device can be changed by opening and closing the binder clips. In this way, droplets are easily guided into individual traps based on the user's demand. A set of numerical simulations was also conducted to investigate the authenticity of the employed idea and to find the optimal geometry for implementing our strategy. This simple method can be easily employed for on-demand droplet trapping without using on-chip valves or complex off-chip actuators proposed in previous studies.

Generalizable Electrochemical Biosensors Using DNA-Analyte Conjugates Integrated with Pneumatic Oscillators for Microfluidic Workflow Automation

Sensors based on electrochemical (EC) readout offer low cost, miniaturization, and adaptability to the point-of-care (POC). Nonetheless, most EC sensors are specialized to a particular target, and there remains a need for a robust EC biosensor platform for the multitude of biomarkers that are not EC-active, do not undergo enzymatic conversion, or are not suited for potentiometry [1,2]. While aptamer-based EC sensors have been proven for sensing in living animals with temporal resolution of a few seconds [3], most method development has been target-focused, lacking generalizability [4]. Presently, the clinical EC toolbox is a conglomerate of targeted methods, and there is a pressing need to develop a single EC platform amenable to rapid, generalizable, quantitative readout of multiple classes of clinically relevant targets. A direct, generalized EC sensing approach with minimal added reagents or amplification steps is preferred [5,6]. Our group has been working to address this need and expand to more analyte classes for several years, and in 2019 we designed a versatile DNA-nanostructure architecture attached to gold electrode surfaces [6]. In this work, we will discuss our efforts to expand the generalizability of our sensor platform, chiefly through custom synthesis of varied DNA-analyte bioconjugates to incorporate within the DNA-nanostructure, specifically DNA-peptide and DNA-small molecule conjugates. Using the same DNA nano-architecture, sensors have been validated in 98% human serum for a variety of targets, several encompassing the human clinical range—a peptide drug (exendin-4) [7], a larger protein (creatine kinase), and smaller molecules or steroid hormones (testosterone, estradiol, progesterone, cortisol). Overall, this new DNA nanostructure platform provides a generalizable sensor with minimal workflow, direct-readout, and the capability to expand EC sensing to a wide variety of clinically important analytes. These sensors can measure antibodies directly without any reagent addition, and the small-molecule or peptide sensors require just a two-step workflow. While the workflow is minimal, the EC measurements benefit from well-controlled solution pH, ionic strength, etc. As such, automated fluidic handling can enhance the user-friendliness of the sensors while maintaining measurement quality. We will also discuss several advancements in automating microfluidic flow control, leveraging digital circuit analogies to develop on-chip pneumatic circuits [8,9]. Using devices fabricated by inexpensive, resin-based 3D printers, we have developed robust pneumatic oscillators that can control on-chip valves for pumping solutions to the EC sensors. The oscillators are automated and tunable from 0.5 to 100 Hz and can pump multiple solutions on microdevices with only a single vacuum input line. Antibody reagents and samples can be mixed automatically then introduced to the electrode, and buffer washes can be included to improve measurement consistency. Ultimately, by integrating this automated microfluidic control with our generalizable EC sensors in the future, many different analytes should be measurable consistently and in a user-friendly manner.

Biocompatible High-Resolution 3D-Printed Microfluidic Devices: Integrated Cell Chemotaxis Demonstration.

We demonstrate a method to effectively 3D print microfluidic devices with high-resolution features using a biocompatible resin based on avobenzone as the UV absorber. Our method relies on spectrally shaping the 3D printer source spectrum so that it is fully overlapped by avobenzone's absorption spectrum. Complete overlap is essential to effectively limit the optical penetration depth, which is required to achieve high out-of-plane resolution. We demonstrate the high resolution in practice by 3D printing 15 μm square pillars in a microfluidic chamber, where the pillars are separated by 7.7 μm and are printed with 5 μm layers. Furthermore, we show reliable membrane valves and pumps using the biocompatible resin. Valves are tested to 1,000,000 actuations with no observable degradation in performance. Finally, we create a concentration gradient generation (CG) component and utilize it in two device designs for cell chemotaxis studies. The first design relies on an external dual syringe pump to generate source and sink flows to supply the CG channel, while the second is a complete integrated device incorporating on-chip pumps, valves, and reservoirs. Both device types are seeded with adherent cells that are subjected to a chemoattractant CG, and both show clear evidence of chemotactic cellular migration. Moreover, the integrated device demonstrates cellular migration comparable to the external syringe pump device. This demonstration illustrates the effectiveness of our integrated chemotactic assay approach and high-resolution biocompatible resin 3D printing fabrication process. In addition, our 3D printing process has been tuned for rapid fabrication, as printing times for the two device designs are, respectively, 8 and 15 min.

Reliable cell retention of mammalian suspension cells in microfluidic cultivation chambers

Microfluidic cultivation, with its high level of environmental control and spatio-temporal resolution of cellular behavior, is a well-established tool in today’s microfluidics. Yet, reliable retention of (randomly) motile cells inside designated cultivation compartments still represents a limitation, which prohibits systematic single-cell growth studies. To overcome this obstacle, current approaches rely on complex multilayer chips or on-chip valves, which makes their application for a broad community of users infeasible. Here, we present an easy-to-implement cell retention concept to withhold cells inside microfluidic cultivation chambers. By introducing a blocking structure into a cultivation chamber’s entrance and nearly closing it, cells can be manually pushed into the chamber during loading procedures but are unable to leave it autonomously in subsequent long-term cultivation. CFD simulations as well as trace substance experiments confirm sufficient nutrient supply within the chamber. Through preventing recurring cell loss, growth data obtained from Chinese hamster ovary cultivation on colony level perfectly match data determined from single-cell data, which eventually allows reliable high throughput studies of single-cell growth. Due to its transferability to other chamber-based approaches, we strongly believe that our concept is also applicable for a broad range of cellular taxis studies or analyses of directed migration in basic or biomedical research.

Microfluidic on-chip valve and pump for applications in immunoassays.

On-chip valves can simplify a microfluidic chip and make it easy to operate. However, most on-chip valves already reported still need complicated manufacture and sophisticated supporting devices. In this work, we present a straightforward on-chip valve, which can be serially connected, to form an on-chip pump. The liquid can horizontally flow one way by the regular deformations of flexure strips in the two valves at both sides of the chamber under pressure changes in microchannels generated by repeated vertical movements of linear actuators. The volume of this system including the chip and the supporting device is 0.65 cubic decimeters, which is much smaller than that of reported systems with a volume of at least 12 cubic decimeters, and the weight of this system is only 0.56 kg, making it possible for point-of-care testing. We carry out an immunoassay of folic acid on chip, and the results show satisfactory reproducibility with acceptable coefficients of variation. We determine 163 clinical human serum samples for folic acid. Furthermore, we detect transferrin, cobalamin and folic acid simultaneously on one chip with both sandwich and competitive binding immunoassay methods. We anticipate that this on-chip valve and pump can be applied in immunoassays and other biosensing applications.

Application Mapping and Control-system Design for Microfluidic Biochips with Distributed Channel Storage

Continuous-flow microfluidic biochips have emerged as a potential low-cost and fast-responsive lab-on-chip platform. They have attracted much attention due to their capability of performing various biochemical applications concurrently and automatically within a coin-sized chip area. To improve execution efficiency and reduce fabrication cost, a distributed channel-storage architecture can be implemented in which the same channels can be switched between the roles of transportation and storage. Accordingly, fluid transportation, caching, and fetch can be performed simultaneously through different flow paths. Such a flow-path planning needs to be considered carefully in the mapping procedure from a biochemical application to a given biochip architecture. Moreover, all the on-chip valves should be actuated correctly and promptly to temporally block the fluid transportation in unwanted directions and seal the fluids in caching channels. Such an exact control of the valves needs to be considered systematically in control-system design to support the mapping scheme for bioassay execution. In this article, we formulate the practical mapping-control co-design problem for microfluidic biochips with distributed channel storage, considering application mapping, valve synchronization, and control-system design simultaneously, and present an efficient synthesis flow to solve this problem systematically. Given the protocol of a biochemical application and the corresponding chip layout in the flow layer, our goal is to map the biochemical application onto the chip with short execution time. Meanwhile, a practical control system considering the real valve-switching requirements can be constructed efficiently with low fabrication cost. Experimental results on multiple real-life bioassays and synthetic benchmarks demonstrate the effectiveness of the proposed design flow.

Air microfluidics-enabled soft robotic transtibial prosthesis socket liner toward dynamic management of residual limb contact pressure and volume fluctuation.

Residual limb volume fluctuation and the resulting contact pressures are some of the key factors leading to skin ulcerations, suboptimal prosthetic functioning, pain, and diminishing quality of life of transtibial amputees. Self-management of socket fit is complicated by peripheral neuropathy, reducing the perception of pressure and pain in the residual limb. We introduce a novel proof-of-concept for a transtibial prosthetic socket liner with the potential to dynamically adjust the fit between the limb and socket. The core of the technology is a small air microfluidic chip (10 cm3 and 10 g) with 10 on-chip valves that enable sequential pressurizing of 10 actuators in custom sizes to match the pressures required by the residual limb's unique anatomy. The microfluidic chip largely reduced the number of electromechanical solenoid valves needed for sequential control of 10 actuators (2 instead of 10 valves), resulting in the reduction of the required power, size, mass, and cost of the control box toward an affordable and wearable prosthetic socket. Proof-of-concept testing demonstrated that the applied pressures can be varied in the desired sequence and to redistribute pressure. Future work will focus on integrating the system with biofidelic prosthetic sockets and residual limb models to investigate the ability to redistribute pressure away from pressure-sensitive regions (e.g., fibular head) to pressure tolerant areas. Overall, the dynamic prosthesis socket liner is very encouraging for creating a dynamic socket fit system that can be seamlessly integrated with existing socket fabrication methods for managing residual limb volume fluctuations and contact pressure.

Multiple interactions between microfluidic droplets and on-chip pneumatic valves

Multiple interactions between microfluidic droplets and on-chip pneumatic valves

Fluidic handling system for PCR-based sample-to-answer detection of viral nucleic acids

Sample-to-answer detection of a viral infectious disease is becoming increasingly important to prevent pandemic outbreaks. However, only a few microfluidic chips that use nucleic acid isolation and detection have implemented all-in-one assays in a simple and effective platform. Here, we present a microfluidic platform that isolates and detects viral RNA of influenza A (H1N1) virus from clinical swab samples in a fully automated, effective, and highly sensitive manner. The device with a vertically movable top layer and a rotationally mobile bottom layer exploits surface tension in a solution. In this way, the device effectively orchestrates multiple functions, including transporting, mixing, and washing of reagents and magnetic beads without the need for on-chip valves, pumps, and microfluidic networks that increase operational and structural complexity. With integrated heating and detection functions, the device provides fully automated sample-to-answer detection with real-time polymerase chain reaction in 60 min for positive results with high sensitivity with the limit of detection of 26 copies/reaction. Our platform is useful for accurate diagnosis of infectious diseases in point-of-care settings.

Synterface

Flow-based microfluidic biochips can be used to perform bioassays by manipulating a large number of on-chip valves. These biochips are increasingly used today for biomolecular recognition, single-cell screening, and point-of-care disease diagnostics, and design-automation solutions for flow-based microfluidics enable the mapping and optimization of bimolecular protocols and software-based valve control. However, a key problem that has not received adequate attention is chip-to-world interfacing, which requires the use of off-chip control equipment to provide control signals for the on-chip valves. This problem is exacerbated by the increase in the number of valves as chips get more complex. To address the interfacing problem, we present an efficient pin-count minimization (synthesis) problem, referred to as Synterface, which uses on-chip microfluidic logic gates and optimization based on concepts from linear algebra. We present results to show that Synterface significantly reduces pin-count and simplifies the external interface for flow-based microfluidics.

An automated microfluidic device for time-lapse imaging of mouse embryonic stem cells.

Long-term, time-lapse imaging studies of embryonic stem cells (ESCs) require a controlled and stable culturing environment for high-resolution imaging. Microfluidics is well-suited for such studies, especially when the media composition needs to be rapidly and accurately altered without disrupting the imaging. Current studies in plates, which can only add molecules at the start of an experiment without any information on the levels of endogenous signaling before the exposure, are incompatible with continuous high-resolution imaging and cell-tracking. Here, we present a custom designed, fully automated microfluidic chip to overcome these challenges. A unique feature of our chip includes three-dimensional ports that can connect completely sealed on-chip valves for fluid control to individually addressable cell culture chambers with thin glass bottoms for high-resolution imaging. We developed a robust protocol for on-chip culturing of mouse ESCs for minimum of 3 days, to carry out experiments reliably and repeatedly. The on-chip ESC growth rate was similar to that on standard culture plates with same initial cell density. We tested the chips for high-resolution, time-lapse imaging of a sensitive reporter of ESC lineage priming, Nanog-GFP, and HHex-Venus with an H2B-mCherry nuclear marker for cell-tracking. Two color imaging of cells was possible over a 24-hr period while maintaining cell viability. Importantly, changing the media did not affect our ability to track individual cells. This system now enables long-term fluorescence imaging studies in a reliable and automated manner in a fully controlled microenvironment.

Generation of 3D Microparticles in Microchannels with Non-rectangular Cross Sections

Flow lithography in a conventional rectangular microchannel is limited to fabrication of particles for which only the shapes of top perimeters are controlled. We present a flow lithography technique for fabrication of microparticles of diverse 3D shapes and multiple layers using non-rectangular microchannels with designed cross sections that allow the creation of complex shapes and diverse cross-sectional shapes. Variations in cross-sectional shape allow high-throughput, on-demand production of microparticles in unconventional shapes such as tetrahedrons and pyramids. Multilayered 3D particles were generated in an enlarging triangular channel combined with on-chip PDMS valves, which allow particle alignment and fluid exchange. These 3D microparticles are expected to further expand the wide variety of applications of microparticles, especially in drug delivery and tissue engineering fields.

Assessing reusability of microfluidic devices: Urinary protein uptake by PDMS-based channels after long-term cyclic use

In the search for transformative technologies for person-centered health monitoring, reusability of microfluidic chips would be a critical design consideration in many biomedical applications. With this unmet need in mind, in this study, we develop and validate a novel microfluidic platform for automated sample pumping with an integrated channel-cleaning procedure. The proposed system leverages micropumps and on-chip solenoid valves to dynamically control fluid flow. We provide a thorough characterization of the custom-designed chip, including quantitative measures of the protein uptake by the chip, as well as cross-contamination using both simulated samples and human urine samples. The effectiveness of the cleaning procedure is assessed by testing the samples collected from the cleaning chip with commercially available urine dipstick protein tests. The results of the longitudinal protein level measurement of the urine samples after the cleaning cycles show high accuracy of protein measurement and negligible protein cross-contamination. Additionally, the cleaning procedure after pumping each sample results in a very low protein uptake (150 ng/cm2). We have also demonstrated that the efficiency of the automated cleaning microfluidic device can be further improved by an anti-fouling coating via PLL-g-PEG pretreatment and a posttreatment via Trypsin. The developed platform could potentially be further miniaturized and integrated into point-of-care devices to provide an effective cleaning process and to enable the reusability of microfluidic devices.

A microfluidic device for automated, high-speed microinjection of Caenorhabditis elegans.

The nematode worm Caenorhabditis elegans has been widely used as a model organism in biological studies because of its short and prolific life cycle, relatively simple body structure, significant genetic overlap with human, and facile/inexpensive cultivation. Microinjection, as an established and versatile tool for delivering liquid substances into cellular/organismal objects, plays an important role in C. elegans research. However, the conventional manual procedure of C. elegans microinjection is labor-intensive and time-consuming and thus hinders large-scale C. elegans studies involving microinjection of a large number of C. elegans on a daily basis. In this paper, we report a novel microfluidic device that enables, for the first time, fully automated, high-speed microinjection of C. elegans. The device is automatically regulated by on-chip pneumatic valves and allows rapid loading, immobilization, injection, and downstream sorting of single C. elegans. For demonstration, we performed microinjection experiments on 200 C. elegans worms and demonstrated an average injection speed of 6.6 worm/min (average worm handling time: 9.45 s/worm) and a success rate of 77.5% (post-sorting success rate: 100%), both much higher than the performance of manual operation (speed: 1 worm/4 min and success rate: 30%). We conducted typical viability tests on the injected C. elegans and confirmed that the automated injection system does not impose significant adverse effect on the physiological condition of the injected C. elegans. We believe that the developed microfluidic device holds great potential to become a useful tool for facilitating high-throughput, large-scale worm biology research.

Microfluidic valve array control system integrating a fluid demultiplexer circuit

This paper proposes an efficient control method for the large-scale integration of microvalves in microfluidic systems. The proposed method can control 2n individual microvalves with 2n + 2 control lines (where n is an integer). The on-chip valves are closed by applying pressure to a control line, similar to conventional pneumatic microvalves. Another control line closes gate valves between the control line to the on-chip valves and the on-chip valves themselves, to preserve the state of the on-chip valves. The remaining control lines select an activated gate valve. While the addressed gate valve is selected by the other control lines, the corresponding on-chip valve is actuated by applying input pressure to the control line to the on-chip valves. Using this method would substantially reduce the number of world-to-chip connectors and off-chip valve controllers. Experiments conducted using a fabricated 28 microvalve array device, comprising 256 individual on-chip valves controlled with 18 (2 × 8 + 2) control lines, yielded switching speeds for the selected on-chip valve under 90 ms.

Automation Highlights from the Literature

Automation Highlights from the Literature

Simple bilayer on-chip valves using reversible sealability of PDMS

Simple bilayer on-chip valves exploiting the reversible sealability of PDMS were realized by patterning the non-covalent area between two parallel microchannels.

A hybrid paper and microfluidic chip with electrowetting valves and colorimetric detection

Sequential fluid delivery with minimized external equipment is vital towards a point-of-care diagnostic device. In this work, we have further developed the On-chip Electrowetting Valves concept for the sequential delivery of the reagents to the reaction site in a miniaturized capillary-driven microfluidic chip. Specifically, a disposable polymeric microfluidic device was developed containing capillary force driven microchannels. The device was fabricated using laser ablation and inkjet printing and required no external pumping equipment. The assay was conducted on the microchip containing microfluidic channels with embedded electrowetting valves and a porous membrane patterned with capture molecules and colloidal gold labels. To conduct the assay, the microchip was connected with a low voltage supply which was capable of sequentially opening the valves, delivering the sample and the rinsing reagent to generate visual results. Using T7 bacteriophage as a model, we have demonstrated the development of the device, operation of the valves and execution of the automated assay.

Stand-alone microfluidic system using partly disposable PDMS microwell array for high throughput cell analysis

This paper presents a stand-alone microfluidic system composed of a reusable valve control part and a disposable microwell-array-embedded fluidic chip for high throughput cell analysis. The valve control part and the fluidic chip are fabricated separately and combined together in experiments. Since valve control of the device and fluid injection are driven by solenoid actuators and electro-osmosis (EO) micropumps, respectively, access-tube-free system is achieved. Utilization of addressable valve control method makes possible to drive 25 on-chip hydraulic valves by 10 solenoid actuators. The prototype fluidic chip with 16 microwell array and 4 reagent injection ports on top of palm-top size substrate (30×40mm) is fabricated and integrated to make a replacement of multi-kind reagents inside the system. With this system, other functional fluidic networks can be replaced easily. After testing on/off switching performances of on-chip valves, multi-reagent exchange trial is carried out, revealing precise reagent injection of ten-nanoliter order to each microwell. The proposed system also shows high performance of power consumption, realizing introduction of two reagents to 16 microwells with 11.9Wh.

Systematic characterization of feature dimensions and closing pressures for microfluidic valves produced via photoresist reflow

Multilayer soft lithography (MSL) provides a convenient and low-cost method for fabricating poly(dimethyl siloxane) (PDMS) microfluidic devices with on-chip valves for automated and precise control of fluid flow. MSL casting molds for flow channels typically incorporate small patches of rounded positive photoresist at valve locations to achieve the rounded cross-sectional profile required for these valves to function properly. Despite the importance of these rounded features for device performance, a comprehensive characterization of how the rounding process affects feature dimensions and closing pressures has been lacking. Here, we measure valve dimensions both before and after rounding and closing pressures for 120 different valve widths and lengths at post-rounding heights between 15 and 84 μm, for a total of 1200 different geometries spanning a wide range of useful sizes. We find that valve height and width after rounding depend strongly on valve aspect ratios, with these effects becoming more pronounced for taller and narrower features. Based on the measured data, we provide a simple fitted model and an online tool for estimating the pre-rounding dimensions needed to achieve desired post-rounding dimensions. We also find that valve closing pressures are well explained by modelling valve membranes in a manner analogous to a suspension bridge, shedding new light on device physics and providing a practical model for estimating closing pressures during device design.