Passive micropumping in microfluidics for point-of-care testing

Gravity-driven flow control in a fully integrated microfluidic cartridge for molecular point-of-care testing

Molecular point-of-care testing (POCT) system is crucial for the timely prevention and control of infectious diseases. We recently proposed a gravity-driven microfluidic cartridge for molecular POCT detection, without the need for external sources or actuators, demonstrating the advantages in terms of the reduced cartridge size and low development costs. How to achieve precise control of liquid flow behavior is challenging for the gravity-driven cartridge. In this work, we explored the underlying mechanism of flow control in the cartridge and offered optimized solutions for our cartridge design to achieve precise control of dynamic flow rates and enhance pumping efficiency significantly. Through the computational fluid dynamics simulations, we demonstrated that adopting an asymptotic contraction chamber geometry design and a closed-loop air flow channel design with the cartridge inlet can facilitate stable laminar flow of the liquid in our microfluidic cartridge, enabling precise control of flow velocity. We further optimized the microchannel diameter and the contact angle of the liquid with the microchannel wall. The effectiveness of the optimized cartridge for POCT detection was well validated by the accurate detection of the human papillomavirus type 16 virus in the 120 clinical swab samples.

Integrated high-throughput drug screening microfluidic system for comprehensive ocular toxicity assessment

Traditional experimental methodologies suffer from a few limitations in the toxicological evaluation of the preservatives added to eye drops. In this study, we overcame these limitations by using a microfluidic device. We developed a microfluidic system featuring a gradient concentration generator for preservative dosage control with microvalves and micropumps, automatically regulated by a programmable Arduino board. This system facilitated the simultaneous toxicological evaluation of human corneal epithelial cells against eight different concentrations of preservatives, allowing for quadruplicate experiments in a single run. In our study, the IC50 values for healthy eyes and those affected with dry eyes syndrome showed an approximately twofold difference. This variation is likely attributable to the duration for which the preservative remained in contact with corneal cells before being washed off by the medium, suggesting the significance of exposure time in the cytotoxic effect of preservatives. Our microfluidic system, automated by Arduino, simulated healthy and dry eye environments to study benzalkonium chloride toxicity and revealed significant differences in cell viability, with IC50 values of 0.0033% for healthy eyes and 0.0017% for dry eyes. In summary, we implemented the pinch-to-zoom feature of an electronic tablet in our microfluidic system, offering innovative alternatives for eye research.

Optimization of Newtonian fluid pressure in microcantilever integrated flexible microfluidic channel for healthcare application

The demand for microfluidic pressure sensors is ever-increasing in various industries due to their crucial role in controlling fluid pressure within microchannels. While syringe pump setups have been traditionally used to regulate fluid pressure in microfluidic devices, they often result in larger setups that increase the cost of the device. To address this challenge and miniaturize the syringe pump setup, the researcher introduced integrated T-microcantilever-based microfluidic devices. In these devices, microcantilevers are incorporated, and their deflections correlate with the microchannel's pressure. When the relative pressure of fluid (plasma) changes, the T-microcantilever deflects, and the extent of this deflection provides information on fluid pressure within the microchannel. In this work, finite element method (FEM) based simulation was carried out to investigate the role of material, and geometric parameters of the cantilever, and the fluid viscosity on the pressure sensing capability of the T-microcantilever integrated microfluidic channel. The T-microcantilever achieves a maximum deflection of 127μm at a 5000μm/s velocity for Young's modulus(E) of 360 kPa of PDMS by employing a hinged structure. On the other hand, a minimum deflection of 4.05 × 10-5μm was attained at 5000μm/s for Young's modulus of 1 TPa for silicon. The maximum deflected angle of the T-cantilever is 20.46° for a 360 kPa Young's modulus while the minimum deflection angle of the T-cantilever is measured at 13.77° for 900 KPa at a fluid velocity of 5000μm s-1. The T-cantilever functions as a built-in microchannel that gauges the fluid pressure within the microchannel. The peak pressure, set at 8.86 Pa on the surface of the cantilever leads to a maximum deflection of 0.096μm (approximately 1μm) in the T-cantilever at a 1:1 velocity ratio. An optimized microfluidic device embedded with microchannels can optimize fluid pressure in a microchannel support cell separation.

Interlinking spatial dimensions and kinetic processes in dissipative materials to create synthetic systems with lifelike functionality

Biological systems spontaneously convert energy input into the actions necessary to survive. Motivated by the efficacy of these processes, researchers aim to forge materials systems that exhibit the self-sustained and autonomous functionality found in nature. Success in this effort will require synthetic analogues of the following: a metabolism to generate energy, a vasculature to transport energy and materials, a nervous system to transmit 'commands', a musculoskeletal system to translate commands into physical action, regulatory networks to monitor the entire enterprise, and a mechanism to convert 'nutrients' into growing materials. Design rules must interconnect the material's structural and kinetic properties over ranges of length (that can vary from the nano- to mesoscale) and timescales to enable local energy dissipations to power global functionality. Moreover, by harnessing dynamic interactions intrinsic to the material, the system itself can perform the work needed for its own functionality. Here, we assess the advances and challenges in dissipative materials design and at the same time aim to spur developments in next-generation functional, 'living' materials.

Self-Powered Microfluidics for Point-of-Care Solutions: From Sampling to Detection of Proteins and Nucleic Acids

Self-powered microfluidics presents a revolutionary approach to address the challenges of healthcare in decentralized and point-of-care settings where limited access to resources and infrastructure prevails or rapid clinical decision-making is critical. These microfluidic systems exploit physical and chemical phenomena, such as capillary forces and surface tension, to manipulate tiny volumes of fluids without the need for external power sources, making them cost-effective and highly portable. Recent technological advancements have demonstrated the ability to preprogram complex multistep liquid operations within the microfluidic circuit of these standalone systems, which enabled the integration of sensitive detection and readout principles. This chapter first addresses how the accessibility to in vitro diagnostics can be improved by shifting toward decentralized approaches like remote microsampling and point-of-care testing. Next, the crucial role of self-powered microfluidic technologies to enable this patient-centric healthcare transition is emphasized using various state-of-the-art examples, with a primary focus on applications related to biofluid collection and the detection of either proteins or nucleic acids. This chapter concludes with a summary of the main findings and our vision of the future perspectives in the field of self-powered microfluidic technologies and their use for in vitro diagnostics applications.

Fully integrated sample-in-answer-out platform for viral detection using digital reverse transcription recombinase polymerase amplification (dRT-RPA)

Recombinase polymerase amplification (RPA) is one of the most promising diagnostic methods for pathogen detection, owing to the simplified isothermal amplification technique. Using one-step digital reverse transcription RPA (dRT-RPA) to detect viral RNA provides a fast diagnosis and absolute quantification. Here, we present a chip that purifies, digitalizes, and detects viral RNA of SARS-CoV-2 in a fully automated and sensitive manner. The chip purifies the RNA using the surface charge concept of magnet bead-RNA binding, then mixes the RNA with the amplification reagents, digitalizes the amplification mixture, and performs dRT-RPA. RNA-bead complex is transported among purification buffers that are separated by an oil phase. For reagent manipulation and mixing, a magnetic valve system is integrated on the chip, where an external magnet controls the reagent direction and time of addition. Besides, a novel vacuum system is suggested to drive and regulate the reagents into two fluid systems simultaneously in ∼2 min. We also developed a cost-effective way to perform fluorescent detection for dRT-RPA on chip by using EvaGreen® dye. With integrated heating and optical detection system, the on-chip dRT-RPA presents a sample-to-answer detection platform for absolute viral RNA quantitation in 37 min and a sensitivity as low as 10 RNA copies/μL. Hence, this platform is expected to be a useful tool for accurate and automated diagnosis of infectious diseases.

Microfluidic pumps for cell sorting

Microfluidic cell sorting has shown promising advantages over traditional bulky cell sorting equipment and has demonstrated wide-reaching applications in biological research and medical diagnostics. The most important characteristics of a microfluidic cell sorter are its throughput, ease of use, and integration of peripheral equipment onto the chip itself. In this review, we discuss the six most common methods for pumping fluid samples in microfluidic cell sorting devices, present their advantages and drawbacks, and discuss notable examples of their use. Syringe pumps are the most commonly used method for fluid actuation in microfluidic devices because they are easily accessible but they are typically too bulky for portable applications, and they may produce unfavorable flow characteristics. Peristaltic pumps, both on- and off-chip, can produce reversible flow but they suffer from pulsatile flow characteristics, which may not be preferable in many scenarios. Gravity-driven pumping, and similarly hydrostatic pumping, require no energy input but generally produce low throughputs. Centrifugal flow is used to sort cells on the basis of size or density but requires a large external rotor to produce centrifugal force. Electroosmotic pumping is appealing because of its compact size but the high voltages required for fluid flow may be incompatible with live cells. Emerging methods with potential for applications in cell sorting are also discussed. In the future, microfluidic cell sorting methods will trend toward highly integrated systems with high throughputs and low sample volume requirements.

An integrated digital polymerase chain reaction chip for multiplexed meat adulteration detection

Meat adulteration detection is a common concern of consumers. Here, we proposed a multiplex digital polymerase chain reaction method and a low-cost device for meat adulteration detection. Using a polydimethylsiloxane microfluidic device, polymerase chain reaction reagents could be pump-free loaded into microchambers (40 × 40 chambers) automatically. Due to the independence of multiplex fluorescence channels, deoxyribonucleic acid templates extracted from different animal species could be distinguished by one test. In this paper, we designed primers and probes for four types of meat (beef, chicken, pork, and duck) and labeled each of the four fluorescent markers (hexachlorocyclohexane [HEX], 6-carboxyfluorescein [FAM], X-rhodamine [ROX], and cyanine dyes 5 [CY5]) on the probes. Specific detection and mixed detection experiments were performed on four types of meat, realizing a limit of detection of 3 copies/µL. A mixture of four different species can be detected by four independent fluorescence channels. The quantitative capability of this method is found to meet the requirements of meat adulteration detections. This method has great potential for point-of-care testing together with portable microscopy equipment.

Finger-Actuated Micropump of Constant Flow Rate without Backflow

This paper presents a finger-actuated micropump with a consistent flow rate and no backflow. The fluid dynamics in interstitial fluid (ISF) extraction microfluidics are studied through analytical, simulation, and experimental methods. Head losses, pressure drop, diodocity, hydrogel swelling, criteria for hydrogel absorption, and consistency flow rate are examined in order to access microfluidic performance. In terms of consistency, the experimental result revealed that after 20 s of duty cycles with full deformation on the flexible diaphragm, the output pressure became uniform and the flow rate remained at nearly constant levels of 2.2 μL/min. The flow rate discrepancy between the experimental and predicted flow rates is around 22%. In terms of diodicity, when the serpentine microchannel and hydrogel-assisted reservoir are added to the microfluidic system integration, the diodicity increases by 2% (Di = 1.48) and 34% (Di = 1.96), respectively, compared to when the Tesla integration (Di = 1.45) is used alone. A visual and experimentally weighted analysis finds no signs of backflow. These significant flow characteristics demonstrate their potential usage in many low-cost and portable microfluidic applications.

Open-channel microfluidic chip based on shape memory polymer for controllable liquid transport

Open microfluidics has attracted increasing attention over the last decade because of its flexibility and simplicity with respect to cell culture and clinical diagnosis. However, traditional valves and pumps are difficult to integrate on open-channel microfluidic chips, in which a liquid is usually driven by capillary forces. Poor fluid control performance is a common drawback of open microfluidics. Herein, we proposed a method for controlling the liquid flow in open channels by controlling the continuous Laplace pressure induced by the deformation of the shape memory microstructures. The uniformly arranged cuboidal microcolumns in the open channels have magnetic/light dual responses, and the bending angle of the microcolumns can be controlled by adjusting Laplace pressure using near-infrared laser irradiation in a magnetic field. Laplace pressure and capillary force drove the liquid flow together, and the controllable fluid transport was realized by adjusting the hydrophilicity of the channel surface and the bending angle of the microcolumns. We demonstrated the controllability of the flow rate and the directional transport of water along a preset path. In addition, the start and stop of water transport were realized via local hydrophobic modification. The proposed strategy improves poor fluid control in traditional open systems and makes fluid flow highly controllable. We tried to extract and detect rhodamine B in tiny droplets on the open microfluidic chip, demonstrating the advantages of the proposed strategy in the separation and analysis of tiny samples.

Microfluidic solutions for biofluids handling in on-skin wearable systems

On-skin wearable systems for biofluid sampling and biomarker sensing can revolutionize the current practices in healthcare monitoring and personalized medicine. However, there is still a long path toward complete market adoption and acceptance of this fascinating technology. Accordingly, microfluidic science and technology can provide excellent solutions for bridging the gap between basic research and clinical research. The research gap has led to the emerging field of epidermal microfluidics. Moreover, recent advances in the fabrication of highly flexible and stretchable microfluidic systems have revived the concept of micro elastofluidics, which can provide viable solutions for on-skin wearable biofluid handling. In this context, this review highlights the current state-of-the-art platforms in this field and discusses the potential technologies that can be used for on-skin wearable devices. Toward this aim, we first compare various microfluidic platforms that could be used for on-skin wearable devices. These platforms include semiconductor-based, polymer-based, liquid metal-based, paper-based, and textile-based microfluidics. Next, we discuss how these platforms can enhance the stretchability of on-skin wearable biosensors at the device level. Next, potential microfluidic solutions for collecting, transporting, and controlling the biofluids are discussed. The application of finger-powered micropumps as a viable solution for precise and on-demand biofluid pumping is highlighted. Finally, we present the future directions of this field by emphasizing the applications of droplet-based microfluidics, stretchable continuous-flow micro elastofluidics, stretchable superhydrophobic surfaces, liquid beads as a form of digital micro elastofluidics, and topological liquid diodes that received less attention but have enormous potential to be integrated into on-skin wearable devices.

3D printed microfluidics: advances in strategies, integration, and applications

The ability to construct multiplexed micro-systems for fluid regulation could substantially impact multiple fields, including chemistry, biology, biomedicine, tissue engineering, and soft robotics, among others. 3D printing is gaining traction as a compelling approach to fabricating microfluidic devices by providing unique capabilities, such as 1) rapid design iteration and prototyping, 2) the potential for automated manufacturing and alignment, 3) the incorporation of numerous classes of materials within a single platform, and 4) the integration of 3D microstructures with prefabricated devices, sensing arrays, and nonplanar substrates. However, to widely deploy 3D printed microfluidics at research and commercial scales, critical issues related to printing factors, device integration strategies, and incorporation of multiple functionalities require further development and optimization. In this review, we summarize important figures of merit of 3D printed microfluidics and inspect recent progress in the field, including ink properties, structural resolutions, and hierarchical levels of integration with functional platforms. Particularly, we highlight advances in microfluidic devices printed with thermosetting elastomers, printing methodologies with enhanced degrees of automation and resolution, and the direct printing of microfluidics on various 3D surfaces. The substantial progress in the performance and multifunctionality of 3D printed microfluidics suggests a rapidly approaching era in which these versatile devices could be untethered from microfabrication facilities and created on demand by users in arbitrary settings with minimal prior training.

Closed-loop Control Systems for Pumps used in Portable Analytical Systems

The demand for accurate control of the flowrate/pressure in chemical analytical systems has given rise to the adoption of mechatronic approaches in analytical instruments. A mechatronic device is a synergistic system which combines mechanical, electronic, computer and control components. In the development of portable analytical devices, considering the instrument as a mechatronic system can be useful to mitigate compromises made to decrease space, weight, or power consumption. Fluid handling is important for reliability, however, commonly utilized platforms such as syringe and peristaltic pumps are typically characterized by flow/pressure fluctuations and slow responses. Closed loop control systems have been used effectively to decrease the difference between desired and realized fluidic output. This review discusses the way control systems have been implemented for enhanced fluidic control, categorized by pump type. Advanced control strategies used to enhance the transient and the steady state responses are discussed, along with examples of their implementation in portable analytical systems. The review is concluded with the outlook that the challenge in adequately expressing the complexity and dynamics of the fluidic network as a mathematical model has yielded a trend towards the adoption of experimentally informed models and machine learning approaches.

Lab at home: a promising prospect for on-site chemical and biological analysis

The continuing pursuit for a healthy life has led to the urgent need for on-site analysis. In response to the urgent needs of on-site analysis, we propose a novel concept, called lab at home (LAH), for building automated and integrated total analysis systems to perform chemical and biological testing at home. It represents an emerging research area with broad prospects that has not yet attracted sufficient attention. In this paper, we discuss the urgent need, challenges, and future prospects of this area, and the possible roadmap for achieving the goal of LAH has also been proposed.

A passive and programmable 3D paper-based microfluidic pump for variable flow microfluidic applications

Paper has attracted significant attention recently as a microfluidic component and platform, especially in passive pumping devices due to its porous and uniform absorbing nature. Many investigations on 1D and 2D fluid flows were carried out. However, no experimental work has been reported on the three-dimensional effect in porous geometry to improve pumping characteristics in microchannels. Therefore, in this study, the fluid flow in 3D paper-based passive pumps was investigated in microchannels using cylindrical pumps. The effect of pump diameter, porosity, and programmability was investigated to achieve desired flow variations. The results indicated that the flow rate of water increased with an increase in the diameter and porosity of paper pumps. Maximum flow rates achieved for 14 mm diameter pumps of 0.5 and 0.7 porosities were 5.29 mm3/s (317.4 μl/min) and 6.97 mm3/s (418.2 μl/min), respectively. The total volume of fluid imbibition ranged between 266 and 567 μl for 8 and 14 mm diameter pumps, respectively. Moreover, 3D passive pumps can transport larger volumes of liquid with an improved flow rate, programmability, and control, in addition to being inexpensive and simple to design and fabricate. Most importantly, a single 3D paper pump showed an increasing, decreasing, and constant flow rate all in a single microchannel. With these benefits, the passive pumps can further improve the pumping characteristics of microfluidic platforms enabling a cost effective and programmable point-of-care diagnostic device.

Microfluidic chip and isothermal amplification technologies for the detection of pathogenic nucleic acid

The frequency of outbreaks of newly emerging infectious diseases has increased in recent years. The coronavirus disease 2019 (COVID-19) outbreak in late 2019 has caused a global pandemic, seriously endangering human health and social stability. Rapid detection of infectious disease pathogens is a key prerequisite for the early screening of cases and the reduction in transmission risk. Fluorescence quantitative polymerase chain reaction (qPCR) is currently the most commonly used pathogen detection method, but this method has high requirements in terms of operating staff, instrumentation, venues, and so forth. As a result, its application in the settings such as poorly conditioned communities and grassroots has been limited, and the detection needs of the first-line field cannot be met. The development of point-of-care testing (POCT) technology is of great practical significance for preventing and controlling infectious diseases. Isothermal amplification technology has advantages such as mild reaction conditions and low instrument dependence. It has a promising prospect in the development of POCT, combined with the advantages of high integration and portability of microfluidic chip technology. This study summarized the principles of several representative isothermal amplification techniques, as well as their advantages and disadvantages. Particularly, it reviewed the research progress on microfluidic chip-based recombinase polymerase isothermal amplification technology and highlighted future prospects.

Novel Pumping Methods for Microfluidic Devices: A Comprehensive Review

This review is an account of methods that use various strategies to control microfluidic flow control with high accuracy. The reviewed systems are divided into two large groups based on the way they create flow: passive systems (non-mechanical systems) and active (mechanical) systems. Each group is presented by a number of device fabrications. We try to explain the main principles of operation, and we list advantages and disadvantages of the presented systems. Mechanical systems are considered in more detail, as they are currently an area of increased interest due to their unique precision flow control and "multitasking". These systems are often applied as mini-laboratories, working autonomously without any additional operations, provided by humans, which is very important under complicated conditions. We also reviewed the integration of autonomous microfluidic systems with a smartphone or single-board computer when all data are retrieved and processed without using a personal computer. In addition, we discuss future trends and possible solutions for further development of this area of technology.

Visible-light and near-infrared fluorescence and surface-enhanced Raman scattering point-of-care sensing and bio-imaging: a review

This review article deals with the concepts, principles and applications of visible-light and near-infrared (NIR) fluorescence and surface-enhanced Raman scattering (SERS) in in vitro point-of-care testing (POCT) and in vivo bio-imaging. It has discussed how to utilize the biological transparency windows to improve the penetration depth and signal-to-noise ratio, and how to use surface plasmon resonance (SPR) to amplify fluorescence and SERS signals. This article has highlighted some plasmonic fluorescence and SERS probes. It has also reviewed the design strategies of fluorescent and SERS sensors in the detection of metal ions, small molecules, proteins and nucleic acids. Particularly, it has provided perspectives on the integration of fluorescent and SERS sensors into microfluidic chips as lab-on-chips to realize point-of-care testing. It has also discussed the design of active microfluidic devices and non-paper- or paper-based lateral flow assays for in vitro diagnostics. In addition, this article has discussed the strategies to design in vivo NIR fluorescence and SERS bio-imaging platforms for monitoring physiological processes and disease progression in live cells and tissues. Moreover, it has highlighted the applications of POCT and bio-imaging in testing toxins, heavy metals, illicit drugs, cancers, traumatic brain injuries, and infectious diseases such as COVID-19, influenza, HIV and sepsis.

A frugal microfluidic pump

Manipulation of fluid flow is paramount for microfluidic device operation. Conventional microfluidic pumps are often expensive, bulky, complicated, and not amenable in limited resource settings. Here, we introduce a Fully self-sufficient, RobUst, Gravity-Assisted, Low-cost (FRUGAL) microfluidic pump. The pump consists of a syringe, a syringe holder and loading masses. The system is easy to assemble, inexpensive, portable, and electrical power-free. Inside the syringe, the fluid is driven by the pressure from the weight of the loading masses. During operation, the exerted pressure is dynamically controllable and stable for hours. These features are useful for optimization of microfluidics assays and dynamic temporal studies. We demonstrate the application of this system to control the formation of water-in-oil droplet emulsion. Benefitting from its simplicity and versatility, the frugal microfluidic pump will enable global adoption of microfluidic technology in chemistry and biomedical applications, especially in limited resource environments.

Passively driven microfluidic device with simple operation in the development of nanolitre droplet assay in nucleic acid detection

Since nucleic acid amplification technology has become a vital tool for disease diagnosis, the development of precise applied nucleic acid detection technologies in point-of care testing (POCT) has become more significant. The microfluidic-based nucleic acid detection platform offers a great opportunity for on-site diagnosis efficiency, and the system is aimed at user-friendly access. Herein, we demonstrate a microfluidic system with simple operation that provides reliable nucleic acid results from 18 uniform droplets via LAMP detection. By using only micropipette regulation, users are able to control the nanoliter scale of the droplets in this valve-free and pump-free microfluidic (MF) chip. Based on the oil enclosure method and impermeable fabrication, we successfully preserved the reagent inside the microfluidic system, which significantly reduced the fluid loss and condensation. The relative standard deviation (RSD) of the fluorescence intensity between the droplets and during the heating process was < 5% and 2.0%, respectively. Additionally, for different nucleic acid detection methods, the MF-LAMP chip in this study showed good applicability to both genome detection and gene expression analysis.

A passive Stokes flow rectifier for Newtonian fluids

Non-linear effects of the Navier–Stokes equations disappear under the Stokes regime of Newtonian fluid flows disallowing a flow rectification behavior. Here we show that passive flow rectification of Newtonian fluids is obtainable under the Stokes regime of both compressible and incompressible flows by introducing nonlinearity into the otherwise linear Stokes equations. Asymmetric flow resistances arise in shallow nozzle/diffuser microchannels with deformable ceiling, in which the fluid flow is governed by a non-linear coupled fluid–solid mechanics equation. The proposed model captures the unequal deflection profile of the deformable ceiling depending on the flow direction under the identical applied pressure, permitting a larger flow rate in the nozzle configuration. Ultra-low aspect ratio microchannels sealed by a flexible membrane have been fabricated to demonstrate passive flow rectification for low-Reynolds-number flows (0.001 < Re < 10) of common Newtonian fluids such as water, methanol, and isopropyl alcohol. The proposed rectification mechanism is also extended to compressible flows, leading to the first demonstration of rectifying equilibrium gas flows under the Stokes flow regime. While the maximum rectification ratio experimentally obtained in this work is limited to 1.41, a higher value up to 1.76 can be achieved by optimizing the width profile of the asymmetric microchannels.

An Integrated Centrifugal Degassed PDMS-Based Microfluidic Device for Serial Dilution

We propose an integrated serial dilution generator utilizing centrifugal force with a degassed polydimethylsiloxane (PDMS) microfluidic device. Using gas-soluble PDMS as a centrifugal microfluidic device material, the sample can be dragged in any arbitrary direction using vacuum-driven force, as opposed to in a single direction, without adding further actuation components. The vacuum-driven force allows the device to avoid the formation of air bubbles and exhibit high tolerance in the surface condition. The device was then used for sample metering and sample transferring. In addition, centrifugal force was used for sample loading and sample mixing. In this study, a series of ten-fold serial dilutions ranging from 100 to 10-4 with about 8 μL in each chamber was achieved, while the serial dilution ratio and chamber volume could easily be altered by changing the geometrical designs of the device. As a proof of concept of our hybrid approach with the centrifugal and vacuum-driven forces, ten-fold serial dilutions of a cDNA (complementary DNA) sample were prepared using the device. Then, the diluted samples were collected by fine needles and subject to a quantitative polymerase chain reaction (qPCR), and the results were found to be in good agreement with those for samples prepared by manual pipetting.

Biomarker Analysis on a Power-free Microfluidic Chip Driven by Degassed Poly(dimethylsiloxane)

Point-of-care testing (POCT) of biomarkers, such as proteins and nucleic acids, is a hot topic in modern medical engineering toward the early diagnosis of various diseases including cancer. Although microfluidic chips show great promise as a new platform for POCT, external pumps and valves for driving those chips have hindered the realization of POCT on the chips. To eliminate the need for pumps and valves, a power-free microfluidic pumping method utilizing degassed poly(dimethylsiloxane) (PDMS) was invented in 2004. In this article, the working principle of the degas-driven power-free microfluidic chip is first described, and then applications of those chips to biomarker analysis are reviewed. The biomarker analysis on the chip was typically achieved with a small sample volume of ∼1 μL and a short analysis time of ∼20 min. For protein analysis, the sandwich immunoassay format was adopted. The limit of detection (LOD) was improved by three orders of magnitude by using laminar flow-assisted dendritic amplification (LFDA), which was a newly devised amplification method specialized for microfluidic chips. For analysis of nucleic acids such as DNA and microRNA, the sandwich hybridization format was adopted, and the LFDA was also effective to reduce the LOD. With the LFDA, typical LOD values for proteins and nucleic acids were both around 1 pM.

Predicting Dimensions in Microfluidic Paper Based Analytical Devices

The main problem for the expansion of the use of microfluidic paper-based analytical devices and, thus, their mass production is their inherent lack of fluid flow control due to its uncontrolled fabrication protocols. To address this issue, the first step is the generation of uniform and reliable microfluidic channels. The most common paper microfluidic fabrication method is wax printing, which consists of two parts, printing and heating, where heating is a critical step for the fabrication of reproducible device dimensions. In order to bring paper-based devices to success, it is essential to optimize the fabrication process in order to always get a reproducible device. Therefore, the optimization of the heating process and the analysis of the parameters that could affect the final dimensions of the device, such as its shape, the width of the wax barrier and the internal area of the device, were performed. Moreover, we present a method to predict reproducible devices with controlled working areas in a simple manner.

Electrochemical Detection and Point-of-Care Testing for Circulating Tumor Cells: Current Techniques and Future Potentials

Circulating tumor cells (CTCs) are tumor cells that escaped from the primary tumor or the metastasis into the blood and they play a major role in the initiation of metastasis and tumor recurrence. Thus, it is widely accepted that CTC is the main target of liquid biopsy. In the past few decades, the separation of CTC based on the electrochemical method has attracted widespread attention due to its convenience, rapidness, low cost, high sensitivity, and no need for complex instruments and equipment. At present, CTC detection is not widely used in the clinic due to various reasons. Point-of-care CTC detection provides us with a possibility, which is sensitive, fast, cheap, and easy to operate. More importantly, the testing instrument is small and portable, and the testing does not require specialized laboratories and specialized clinical examiners. In this review, we summarized the latest developments in the electrochemical-based CTC detection and point-of-care CTC detection, and discussed the challenges and possible trends.

Capillary-Driven Flow Microfluidics Combined with Smartphone Detection: An Emerging Tool for Point-of-Care Diagnostics

Point-of-care (POC) or near-patient testing allows clinicians to accurately achieve real-time diagnostic results performed at or near to the patient site. The outlook of POC devices is to provide quicker analyses that can lead to well-informed clinical decisions and hence improve the health of patients at the point-of-need. Microfluidics plays an important role in the development of POC devices. However, requirements of handling expertise, pumping systems and complex fluidic controls make the technology unaffordable to the current healthcare systems in the world. In recent years, capillary-driven flow microfluidics has emerged as an attractive microfluidic-based technology to overcome these limitations by offering robust, cost-effective and simple-to-operate devices. The internal wall of the microchannels can be pre-coated with reagents, and by merely dipping the device into the patient sample, the sample can be loaded into the microchannel driven by capillary forces and can be detected via handheld or smartphone-based detectors. The capabilities of capillary-driven flow devices have not been fully exploited in developing POC diagnostics, especially for antimicrobial resistance studies in clinical settings. The purpose of this review is to open up this field of microfluidics to the ever-expanding microfluidic-based scientific community.