Flow-induced deformation of compliant microchannels and its effect on pressure–flow characteristics

We report theoretical and experimental investigations of flow through compliant microchannels in which one of the walls is a thin PDMS membrane. A theoretical model is derived that provides an insight into the physics of the coupled fluid–structure interaction. For a fixed channel size, flow rate and fluid viscosity, a compliance parameter \(f_{\text{p}}\) is identified, which controls the pressure–flow characteristics. The pressure and deflection profiles and pressure–flow characteristics of the compliant microchannels are predicted using the model and compared with experimental data, which show good agreement. The pressure–flow characteristics of the compliant microchannel are compared with that obtained for an identical conventional (rigid) microchannel. For a fixed channel size and flow rate, the effect of fluid viscosity and compliance parameter \(f_{\text{p}}\) on the pressure drop is predicted using the theoretical model, which successfully confront experimental data. The pressure–flow characteristics of a non-Newtonian fluid (0.1 % polyethylene oxide solution) through the compliant and conventional (rigid) microchannels are experimentally measured and compared. The results reveal that for a given change in the flow rate, the corresponding modification in the viscosity due to the shear thinning effect determines the change in the pressure drop in such microchannels.     

Capillary flow in microchannels

The surface of microchannels, especially polymer channels, often needs to be treated to acquire specific properties. This study investigated the capillary flow and the interface behavior in several glass capillaries and fabricated microchannels using a photographic technique and image analysis. The effect of air plasma treatment on the characteristics of capillary flow in three types of microfluidic chips, and the longevity of the acquired surface properties were also studied. It was observed that the dynamic contact angles in microchannels were significantly larger than those measured from a flat substrate and the angle varied with channel size. This suggests that dynamic contact angle measured in situ must be used in the theoretical calculation of capillary flow speed, especially for microfabricated microchannels since the surface properties are likely to be different from the native material. This study also revealed that plasma treatment could induce different interface patterns in the PDMS channels from those in the glass and PC channels. The PDMS channel walls could acquire different level of hydrophilicity during the plasma treatment, and the recovery to hydrophobicity is also non-homogeneous.     

Fabrication of layered polydimethylsiloxane/perfluoropolyether microfluidic devices with solvent compatibility and valve functionality

The development of multilayer soft lithography methodology has seen polydimethysiloxane (PDMS) as the preferred material for the fabrication of microfluidic devices. However, the functionality of these PDMS microfluidic chips is often limited by the poor chemical resistance of PDMS to certain solvents. Here, we propose the use of a photocurable perfluoropolyether (PFPE), specifically FOMBLIN^® MD40 PFPE, as a candidate material to provide a solvent-resistant buffer layer to make the device substantially impervious to chemically induced swelling. We first carried out a systematic study of the solvent resistance properties of FOMBLIN^® MD40 PFPE as compared with PDMS. The comparison presented here demonstrates the superiority of FOMBLIN^® MD40 PFPE over PDMS in this regard; moreover, the results permitted to categorize solvents in four different groups depending on their swelling ratio. We then present a step-by-step recipe for a novel fabrication process that uses multilayer lithography to construct a comprehensive solvent-resistant device with fluid and control channels integrated with a valve structure and also permitting easy establishment of outside connections.     

Experimental characterization of electrical current leakage in poly(dimethylsiloxane) microfluidic devices

Poly(dimethylsiloxane) (PDMS) is usually considered as a dielectric material and the PDMS microchannel wall can be treated as an electrically insulated boundary in an applied electric field. However, in certain layouts of microfluidic networks, electrical leakage through the PDMS microfluidic channel walls may not be negligible, which must be carefully considered in the microfluidic circuit design. In this paper, we report on the experimental characterization of the electrical leakage current through PDMS microfluidic channel walls of different configurations. Our numerical and experimental studies indicate that for tens of microns thick PDMS channel walls, electrical leakage through the PDMS wall could significantly alter the electrical field in the main channel. We further show that we can use the electrical leakage through the PDMS microfluidic channel wall to control the electrolyte flow inside the microfluidic channel and manipulate the particle motion inside the microfluidic channel. More specifically, we can trap individual particles at different locations inside the microfluidic channel by balancing the electroosmotic flow and the electrophoretic migration of the particle.     

一种新的纸基微流开关及其活跃方法

提出了一种新的、基于声表面波的纸基微流开关。通过软光刻技术制作内含两个微孔的聚二甲基硅氧烷(PDMS)微架,其上固定经折叠、长度可变的纸通道。PDMS微架贴附于压电基片之上,并在待连接的两微通道之下方,折叠纸通道最低端离压电基片间距为2 mm。压电基片上采用微电子工艺光刻一对叉指换能器和反射栅。当足够强度的电信号加到叉指换能器对时,激发两相向声表面波,使得压电基片上微流体输运到折叠纸通道,改变其长度,连接其上待连通的两纸基微通道,完成开关功能。对可编程微流器件提供了一种新的编程和开关控制方法。     

Bonding-less (B-less) fabrication of polymeric microsystems

A new technology of the microsystems fabrication was developed. The process is based on a so-called capillary film, which is a commercially available material. A matrix, master pattern corresponding to the microchannels structure, made of the capillary film emulsion is embedded inside a block of polydimethylsiloxane (PDMS). After polymer cross-linking, the matrix is dissolved and removed as a solution from the PDMS structure leaving network of empty spaces that can act as microchannels. In the paper, the fabrication methods of the microsystems with 2D or sandwich-like microchannels architecture were described.     

An improved bulk acoustic waves chip based on a PDMS bonding layer for high-efficient particle enrichment

In this work, we developed a feasible way to package bulk acoustic waves chip with sandwich structure by inserting a polydimethylsiloxane (PDMS) layer as the adhesive between cover glass and silicon substrate. After spin-coating and curing process, a PDMS layer was formed on one side of the cover glass and then bonded to the silicon substrate with microchannels by oxygen plasma treating. Both simulation and experiment showed that the chip was not leaking and the acoustic waves produced by the piezoelectric transducer could be propagated through the PDMS layer. Finally, a standing wave field was formed in the microchannels. Compared with traditional chip bonded by anodic bonding, simulation results showed that this packaging method did decrease the acoustic pressure in the channel, but the reduction was acceptable. After optimizing the experimental parameters, we successfully aggregated 15-μm silica spheres under a very low input power (21 dBm) at a flow velocity of 1 ml/h, and the enrichment efficiency of silica spheres was greater than 97%.     

Syringe-assisted point-of-care micropumping utilizing the gas permeability of polydimethylsiloxane

By utilizing the high gas permeability of polydimethylsiloxane (PDMS), a simple syringe-assisted pumping method was introduced. A dead-end microfluidic channel was partially surrounded by an embedded microchamber, with a thin PDMS wall isolating the dead-end channel and the embedded microchamber. A syringe was connected with the microchamber port by a short tube, and the syringe plunger was manually pulled out to generate low pressure inside the microchamber. When sample liquid was loaded in the inlet port, air trapped in the dead-end channel would diffuse into the surrounding microchamber through the PDMS wall, creating an instantaneous pumping of the liquid inside the dead-end channel. By only pulling the syringe manually, a constant low flow with a rate ranging from 0.089 to 4 nl/s was realized as functions of two key parameters: the PDMS wall thickness and the overlap area between the dead-end channel and the surrounded microchamber. This method enabled point-of-care pumping without pre-evacuating the PDMS devices in a bulky vacuum chamber.     

Novel microstructured evaporation device

Microfluidic devices have become more and more important in the field of thermal or chemical process engineering within the last years (by Schubert et al. Microscale Thermophys Eng 5:17–39, 2001). Cooling is one point of research where microchannels and other microstructured geometries provide significant advantages compared to conventional devices as they offer much higher possible surface to volume ratios and short characteristic distances. Therefore, an intense amount of heat can be transferred by these devices, which can be significantly increased by phase transition. Thus, evaporation of a fluid flow in microchannels allows very compact, fast, and powerful cooling devices. In this research study a novel microstructured evaporator geometry consisting of curved microchannels was evaluated on its evaporation properties compared to previous studies dealing with evaporation of R134a (Tetrafluoroethane) in straight microchannels (by Wibel et al. Chem Eng J 167:705–712, 2011). This novel evaporator design takes advantage of the strong centrifugal forces acting on the (evaporating) two phase flow in the curved microchannels. Due to the feasibility of very small radii of curved microchannels, strong centrifugal forces can be obtained for the fluid flow inside the microstructures. Additionally, those forces are boosted as flow velocities within the channels become higher due to the volume increase induced by evaporation. Therefore, a phase separation could take place inside the microstructure with a higher liquid fraction of evaporating coolant near one side of the curved channels during the transition to vapor. A high liquid fraction inside the evaporator is aimed by an intended removal of the evaporated gas phase from the microstructure. Experimental results of the evaporation of water and R134a as coolants demonstrate the potential of this curved geometry in comparison to evaporation in straight channels. Optical investigations of the new micro evaporator concept by high-speed videography (by Maikowske et al. Appl Therm Eng 30:1872–1876, 2010) are carried out for further improvements of the design. Various bubble formations and movements of the evaporating fluid flow were studied for various vapor fractions by using different fluids. These investigations show how the separation and extraction of the vapor fraction of the novel microstructure concept could be improved.     

Optically monitored segmented flow for controlled ultra-fast mixing and nanoparticle precipitation

Nanoparticles of drugs or colloidal carrier systems are capable of providing substantial advantages for drug bioavailability, but manufacturing nanoparticulate drugs or drug carriers remains a challenge because traditional mechanical or chemical batch mode processes might lack precise control of nanoparticle sizes. Microfluidic approaches are believed to give advantages but often do not provide chemically inert environments and lack controllable operation. Here, segmented flow devices with symmetrical design for centered organic phase injection and for nanoparticle precipitation in transparent and chemically inert glass microchannels are presented. Femtosecond laser fabrication was used to structure borosilicate glass wafers with hydrophilic microchannels of nearly circular cross section. They allow for ultra-fast mixing of solvents with aqueous fluids and subsequent precipitation of poorly water soluble drug nanoparticles or colloidal carrier particles. The best results for mixing and controlled precipitation were obtained with flow focusing and gas segmentation occurring at the same channel intersection point. In such systems, early interdiffusion of the solvent and aqueous solution before ultra-fast convective mixing in the plug is suppressed. A novel optical analysis technique revealed that the speed of mixing can be well controlled by simply adjusting the volume flow rate of the gas phase where changes in the liquid flow rate have practically no influence. In a controlled and stable Taylor flow, smallest plug volumes of 3.8 nl can be generated, which allows complete mixing in 9 ms. The production of lipid nanoparticles down to a diameter of 74 nm could already be demonstrated.     

Fabrication and characterization of hydrogel-based microvalves

Several microvalves utilizing stimuli-responsive hydrogel materials have been developed. The hydrogel components are fabricated inside microchannels using a liquid phase polymerization process. In-channel processing greatly simplifies device construction, assembly, and operation since the functional components are fabricated in situ and can perform both sensing and actuation functions. Two in situ photopolymerization techniques, "laminar stream mode" and "mask mode," have been explored. Three two-dimensional (2-D) valves were fabricated and tested (response time, pressure drop, maximum differential pressure). In addition, a hydrogel/PDMS three-dimensional (3-D) hybrid valve that physically separates the sensing and regulated streams was demonstrated. Analytical modeling was performed on the 3-D valve. Hydrogel-based microvalves have a number of advantages over conventional microvalves, including relatively simple fabrication, no external power requirement, no integrated electronics, large displacement (185 μm), and large force generation (22 mN)     

Magnetically controlled valve for flow manipulation in polymer microfluidic devices

A simple, external in-line valve for use in microfluidic devices constructed of polydimethylsiloxane (PDMS) is described. The actuation of the valve is based on the principle that flexible polymer walls of a liquid channel can be pressed together by the aid of a permanent magnet and a small metal bar. In the presence of a small NdFeB magnet lying below the channel of interest, the metal bar is pulled downward simultaneously pushing the thin layer of PDMS down thereby closing the channel stopping any flow of fluid. The operation of the valve is dependent on the thickness of the PDMS layer, the height of the channel, the gap between the chip and the magnet and the strength of the magnet. The microfluidic channels are completely closed to fluid flows ranging from 0.1 to 1.0 μL/min commonly used in microfluidic applications.     

Liquid metal microcoils for sensing and actuation in lab-on-a-chip applications

The use of metals and alloys with melting point near room temperature, called here as liquid metals, allows the integration of complex three-dimensional metallic micro structures in lab-on-chip devices. The process involves the injection molten liquid metal into microchannels and subsequent solidification at room temperature. The paper reports a technique for the fabrication of three-dimensional multilayer liquid-metal microcoils by lamination of dry adhesive films. The adhesive-based liquid metal microcoil could be used for magnetic resonance relaxometry (MRR) measurement in a lab-on-a-chip platform. Not only that the coil has a low direct-current resistance, it also has a high quality factor. In this paper, we investigate the sensing and actuating capabilities of the liquid metal microcoil. The sensing capability of the microcoil is demonstrated with the coil working as a blood hematocrit level sensor. In a MRR measurement, the transverse relaxation rate of the blood sample increases quadratically with the hematocrit level due to higher magnetic susceptibility. Furthermore, a vibrating adhesive membrane with the embedded coil was realized for electromagnetic actuation. A maximum deflection of approximately 50 μ at a low resonance frequency of 15 Hz can be achieved with a maximum driving current of 300 mA.     

Design optimization of capillary-driven micromixer with square-wave microchannel for blood plasma mixing

A numerical and experimental investigation is performed into the flow characteristics and mixing performance of three microfluidic polydimethylsiloxane blood plasma mixing devices incorporating square-wave, curved and zigzag microchannels, respectively. For each device, the plasma is introduced into the microfluidic channel under the effects of capillary action alone. Of the three devices, that with the square-wave microchannel is found to yield the best mixing performance, and is therefore selected for design optimization. Four microfluidic micromixers incorporating square-wave microchannels with different widths in the x- and y-directions are fabricated using conventional photolithography techniques. The mixing performance of the four microchannels is investigated both numerically and experimentally. The results show that given an appropriate specification of the microchannel geometry, a mixing efficiency of approximately 76 % can be obtained within 4 s. The practical feasibility of the micromixer is demonstrated by performing prothrombin time (PT) tests using a total liquid volume of 4.0 μL (2.0 μL of plasma and 2.0 μL of PT reagent). It is shown that the mean time required to complete the entire PT test (including loading, mixing and coagulation) is less than 30 s.

     

Numerical modeling of electrowetting transport processes for digital microfluidics

Electrical actuation and control of liquid droplets in Hele-Shaw cells have significant importance for microfluidics and lab-on-chip devices. Numerical modeling of complex physical phenomena like contact line dynamics, dynamic contact angles or contact angle hysteresis involved in these processes do challenge in a significant manner classical numerical approaches based on macroscopic Navier–Stokes partial differential equations. In this paper, we analyze the efficiency of a numerical lattice Boltzmann model to simulate basic transport operations of sub-millimeter liquid droplets in electrowetting actuated Hele-Shaw cells. We use a two-phase three-dimensional D3Q19 lattice Boltzmann scheme driven by a Shan–Chen-type mesoscopic potential in order to simulate the gas–liquid equilibrium state of a liquid droplet confined between two solid plates. The contact angles at the liquid–solid–gas interface are simulated by taking into consideration the interaction between fluid particles and solid nodes. The electrodes are designed as regions of tunable wettability on the bottom plate and the contact angles adjusted by changing the interaction strength of the liquid with these regions. The transport velocities obtained with this approach are compared to predictions from analytical models and very good agreement is obtained.     

Pressure-driven flow through PDMS-based flexible microchannels and their applications in microfluidics

Flexible microchannels have soft walls which undergo deformation under the influence of fluid flow. The dimensional and flexural similarity of flexible microchannels make them ideal candidates for mimicking biological structures such as blood vessels and air pathway in lungs. The analysis of fluid flow and the dynamics of interaction of cells through flexible arteries provide valuable insights about cardiovascular-related diseases. Flexible microchannels can be instrumental in the in vitro investigation of such diseases. This review discusses the recent developments in pressure-driven flow through flexible microchannels and their applications. Here we present the existing theoretical models that predict the deformation and pressure-flow characteristics of flexible microchannels and the corresponding experimental validations. We compare the models for laminar flow of Newtonian fluids through flexible microchannels with their corresponding experimental validation and enlist their limitations. We discuss in detail the various applications of flexible microchannels and their relevance in cell mechanophenotyping, micropumps, microflow stabilizers, and organ-on-chip devices. The insight into the flow dynamics provided herein will extend using flexible microchannels to develop organs-on-chip and other microfluidic applications.     

Miniaturized Vision System for Microfluidic Devices

One of the biggest obstacles for lab-on-chip (LOC) devices is the miniaturization of large-scale devices and its methodologies. Miniaturization of the current microscopic technologies combined with image processing may bring significant advantages for LOC devices in the dynamic processes of sizing, positioning, flow control and cell manipulation at different time scales. Here, we propose a vision system boarded on a polydimethylsiloxane (PDMS) polymer-based chip, which can be utilized in a complex microfluidic network for continuous monitoring of mammalian egg and donor cells of sizes in the range of 10–100 μm. The developed prototype system has sufficient resolution and is accompanied with a robust detection method for cell-based microfluidic applications. To assess its performance, an image processing algorithm was applied, and the capability of the detection method was evaluated using 11- and 26-μm particles. The results show that the proposed optical system of monitoring and illumination can be effectively incorporated into PDMS structures aiming at LOC devices.     

A Thermally Responsive PDMS Composite and Its Microfluidic Applications

This paper describes a novel composite actuator for controlled liquid actuation in microsystems which is based on a thermally responsive elastomer. The composite actuator consists of expandable microspheres incorporated in a polydimethylsiloxane (PDMS) matrix and entails the merits of both PDMS and expandable microspheres. The main characteristic of the composite actuator is to expand upon heat. The expansion is irreversible and the relative volume increase is measured up to 270% of its original volume after heating to 80 degC. The composite was used to fabricate single-use microfluidic pumps and valves. We show the displacement of liquids in the range of nanoliters even against counter pressures up to 100 kPa. Moreover, liquid flow in microchannels was entirely blocked by means of the integrated valves. The valves can withstand pressures up to 140 kPa. The devices are fabricated using low-cost materials only, and the composite actuator allows using wafer-level processing. The fluidic components based on the novel composite are highly integrable and do not require external actuators     

Long-term behavior of nonionic surfactant-added PDMS for self-driven microchips

The outstanding characteristics of polydimethylsiloxane (PDMS) owe its extensive use to the fact that it is a base material for the microfluidic devices manufacturers’. In spite of favorable physical and chemical properties, the hydrophobic surface of PDMS is a handicap when pumping aqueous solutions through microchannels using only capillary forces. There are several techniques to achieve a hydrophilic behavior of PDMS, but most of them face the problem of hydrophobic recovery after a short period of time while most commercial microdevices require long storage and distribution times. The use of surfactant-added PDMS provides a novel method to overcome hydrophobicity and to control the hydrophobic recovery over a long period of time. There are many different types of surfactants and not a deep methodology to choose one in terms of efficiency, clearance and duration of the hydrophilic behavior. This paper has compared three non-ionic surfactants with different critical micelle concentration and chemical composition: Triton X-100, Brij 35 and Tween 20. Short and long-term studies were done using 5-μL deionized water droplet on the surface of the prepared surfactant-added PDMS. The experiments demonstrated that Triton X-100 is more efficient than Brij 35 and Tween 20 since with less concentration it achieves a maximum contact angle of around 23.5°. In terms of hydrophobic recovery, the experiments showed that using surfactants and controlling humidity of samples, hydrophobic recovery of the surfactant-added PDMS was negligible after 2 months. According to these results, the use of PDMS with Triton X-100 and Brij 35 provides a good potential for building capillary driven devices without the need of tedious preprocessing techniques.