Single-step replicable microfluidic check valve for rectifying and sensing low Reynolds number flow

This paper presents a new microfluidic check valve well suited for low Reynolds number flow rate sensing, micropump flow rectification, and flow control in lab-on-a-chip devices. The valve uses coupling between fluid movement in a channel and an elastomeric column (flap) suspended in the fluid path to generate a strong anisotropic flow resistance. Soft lithography-based molding techniques were used to fabricate the valve, allowing for a low-cost, single-step fabrication process. Three valves—having heights of 25, 50, and 75 μm, respectively—were fabricated and experimentally evaluated; the best of them demonstrated a maximum fluidic diodicity of 4.6 at a Reynolds number of 12.6 and a significant diodicity of 1.6 at the low Reynolds number of 0.7. The valve’s notable low Reynolds number response was realized by adopting a design methodology that balances the stiffness of the elastomer flap and adhesion forces between the flap and its seat. A pair of elastomer check valves integrated with a miniature membrane actuator demonstrated a flow rectification efficiency of 29.8%. The valve’s other notable features include a wide bandwidth response, the ability to admit particles without becoming jammed, and flow rate sensing capability based on optical flap displacement measurements.     

Two-stage polymer embossing of co-planar microfluidic features for microfluidic devices

A two-stage embossing technique for fabricating microchannels for microfluidic devices is presented. A micromachined aluminum mold is used to emboss a polyetherimide (PEI) substrate with a relatively high glass transition temperature (T_g). The embossed PEI is then used as a mold for embossing an amorphous polyethylene terephthalate (APET) substrate with a lower T_g. The resulting APET substrate has the same features as those of the aluminum mold. Successful transfer of features from the aluminum mold to the APET substrate was verified by profilometry, and an application of this method in production of a microfluidic device is presented.     

A microfluidic capacitance sensor for fluid discrimination and characterization

A microfluidic device with embedded capacitive sensing is proposed. The purpose of the device is fluid discrimination and characterization in a microchannel on the basis of the dielectric permittivity. The device is fabricated in a hybrid cost-effective technology which innovatively combines PDMS (PolyDiMethylSiloxane) soft photolithography and screen printing techniques. A microchannel, realized in a PDMS layer, is placed in the field of a sensing capacitor formed by electrodes screen-printed on a glass substrate. Fluids inside the microchannel affect the capacitance, that is in the order of femtofarads, which is measured by a tailored electronic interface system. The electronic system features a sensitivity of 100 V/pF and a resolution threshold of 0.06 fF. Experimental results obtained for different fluids injected in the microchannel demonstrate the ability of the system to discriminate the fluids and to estimate their dielectric permittivity both as pure samples and as mixtures at varying solute fractions. This makes the device a promising building block for fluid mixing monitoring in microfluidic systems.     

高聚物微流控芯片上集成化气动微阀的研制

报道了一种新型的聚甲基丙烯酸甲酯(PMMA)/聚二甲基硅氧烷(PDMS)复合芯片。该芯片采用PMMA-PDMS…PDMS-PMMA的四层构型,以在芯片上集成气动微阀。具有液路和控制通道网路的PMMA基片与PDMS弹性膜间采用不可逆封接,分别形成液路半芯片和控制半芯片,而2个半芯片则依靠PDMS膜间的粘性实现可逆封接,组成带有微阀的全芯片。这种制备方法解决了制备PMMA-PDMS-PMMA三层结构芯片的封接难题,封接过程简单可靠。其控制部分和液路部分可以单独更换,可进一步降低使用成本,尤其适合一次性应用场合。初步实验表明:该微阀具有良好的开关性能和耐用性。     

Surface molecular property modifications for poly(dimethylsiloxane) (PDMS) based microfluidic devices

Fast advancements of microfabrication processes in past two decades have reached to a fairly matured stage that we can manufacture a wide range of microfluidic devices. At present, the main challenge is the control of nanoscale properties on the surface of lab-on-a-chip to satisfy the need for biomedical applications. For example, poly(dimethylsiloxane) (PDMS) is a commonly used material for microfluidic circuitry, yet the hydrophobic nature of PDMS surface suffers serious nonspecific protein adsorption. Thus the current major efforts are focused on surface molecular property treatments for satisfying specific needs in handling macro functional molecules. Reviewing surface modifications of all types of materials used in microfluidics will be too broad. This review will only summarize recent advances in nonbiofouling PDMS surface modification strategies applicable to microfluidic technology and classify them into two main categories: (1) physical approach including physisorption of charged or amphiphilic polymers and copolymers, as well as (2) chemical approach including self assembled monolayer and thick polymer coating. Pros and cons of a collection of available yet fully exploited surface modification methods are briefly compared among subcategories.     

Development of microfluidic devices incorporating non-spherical hydrogel microparticles for protein-based bioassay

This paper describes the fabrication of a microfluidic device for use in protein-based bioassays that effectively incorporates poly(ethylene glycol) (PEG) hydrogel microparticles within a defined region. The microfluidic device is composed of a polymerization chamber and reaction chamber that are serially connected through the microchannel. Various shapes and sizes of hydrogel microparticles were fabricated in the polymerization chamber by photopatterning and moved to the reaction chamber by pressure-driven flow. All of the hydrogel microparticles were retained within the reaction chamber due to an in-chamber integrated microfilter with smaller mesh size than hydrogel microparticles. Hydrogel microparticles were able to encapsulate enzymes without losing their activity, and different concentrations of glucose were detected by sequential bienzymatic reaction of hydrogel-entrapped glucose oxidase (GOX) and peroxidase (POD) inside the microfluidic device using fluorescence method. Importantly, there was a linear correspondence between fluorescence intensity and the glucose concentration over the physiologically important range of 1.00–10.00 mM. D. Choi and E. Jang contributed equally to this work.     

Fluorescence detection in a micro flow cytometer without on-chip fibers

This paper describes a novel concept of integrated on-chip fiber free laser-induced fluorescence detection system. The poly-dimethylsiloxane (PDMS) chip was fabricated using soft lithography and was bonded with a glass substrate of 150 μm thickness that reduced the distance of channel-to-sidewall to less than 180 μm. The cells and particles detection was conducted by an external single fiber close to the glass substrate that transmitted laser light for simultaneous excitation and receipt of the emission light signals. The performance of the proposed device was demonstrated using fluorescence beads, stained white blood cells, and yeast cells. The experimental results showed the simplicity and flexibility of the proposed device configuration which can provide convenient on-chip integration interface for fast, high throughput, and low-cost laser-induced fluorescence detection micro flow cytometer.     

聚二甲基硅氧烷微流体芯片的制作技术

基于MEMS技术的微流体芯片在分析化学和生物医学领域显示了巨大的应用潜力。作为构建微流体芯片的基底材料———聚二甲基硅氧烷(PDMS)已经表现出了许多的优点:良好的电绝缘性、较高的热稳定性、优良的光学特性以及简单的加工工艺等。采用浇注法制作了PDMS电泳微芯片,对PDMS微流体芯片的加工工艺、封装方法和结构特征进行了探讨,并提出了相应的解决方案。     

A novel technique for fabrication of micro- and nanofluidic device with embedded single carbon nanotube

This paper describes a novel technique for fabrication of micro- and nanofluidic device that consists of a carbon nanotube (CNT) and a polydimethylsiloxane (PDMS) microchannel. Single CNT was placed at desired locations using dielectrophoresis (DEP) and PDMS microchannel was constructed on the aligned CNT via photolithography and soft lithography techniques. This technique enables a CNT to be seamlessly embedded in a PDMS microchannel. Moreover, controlling the PDMS curing condition enables the construction of the device with or without a CNT (the device without CNT has a trace nanochannel in PDMS). Preliminary flow tests such as capillary effect and pressure-driven flow were performed with the fabricated devices. In the capillary effect tests, the flow stopped at the nanochannel in both devices. In the pressure-driven flow lower flow resistance was observed in the device with a CNT.     

Correlations of droplet formation in T-junction microfluidic devices: from squeezing to dripping

In this work, we have systematically analyzed the scaling law of droplet formation by cross-flow shear method in T-junction microfluidic devices. The droplet formation mechanisms can be distinguished by the capillary number for the continuous phase (Ca_c), which are the squeezing regime (Ca_c < 0.002), dripping regime (0.01 < Ca_c < 0.3), and the transient regime (0.002 < Ca_c< 0.01). Three corresponding correlations have been suggested in the different range of Ca_c. In the dripping regime, we developed a modified capillary number for the continuous phase (Ca_c′) by considering the influence of growing droplet size on the continuous phase flow rate. And the modified model could predict droplet diameter more accurately. In the squeezing regime, the final plug length was contributed by the growth and ‘squeeze’ stages based on the observation of dynamic break-up process. In the transient regime, we firstly suggested a mathematical model by considering the influences of the above two mechanisms. The correlations should be very useful for the application of controlling droplet size in T-junction microfluidic devices.     

Development of a biochip with serially connected pneumatic balloons for cell-stretching culture

The stretch stimulus is one of the most important mechanical stimuli sensed by cells. In the present study, a simple microdevice was developed to study the effects of the application of different strain magnitudes to cells. The pressure drop effect in a microchannel was utilized to generate a wide range of strain magnitudes in a single device. The microdevice consisted of 2 layers of polydimethylsiloxane (PDMS), and no alignment process was needed to fabricate it. Eight serially connected balloon structures were included in the device. Cells cultured on the surface of the balloons were stretched by inflating the balloons pneumatically. Finite element analysis (FEA) revealed that 8 different balloons in a single device could generate strains of 14.3-7.7% when air pressure was applied at 50 kPa to the air inlet. Cell culture experiments confirmed the useful application of a wide range of strain magnitudes to cells cultured on balloons inflated to different degrees. The new microdevice utilizing the pressure drop effect is a convenient instrument for the study of cellular mechanotransduction.     

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.     

复合式微流控脱水芯片的研制

报道了一种复合式微流控脱水芯片。采用玻璃、聚二甲氧基硅氧烷(PDMS)和聚碳酸酯(PC)三种材质,采用不可逆封接方法分别制得玻璃—PDMS液路半芯片、PC—PDMS气路半芯片,中间夹一层聚四氟乙烯(PTFE)多孔滤膜,将两个半芯片可逆封接形成玻璃—PDMS…PDMS—PC结构的全芯片。该制备方法简单可靠,其液路半芯片和气路半芯片可以单独更换,使得使用成本降低。实验表明:该芯片脱水性能良好,可用于有机合成步骤中含水试剂的高效除水。     

GMI detection of magnetic-particle concentration in continuous flow

We describe a microfluidic device for the determination of the concentration of magnetic beads under continuous flow of the carrier fluid by means of the Giant Magneto-Impedance effect (GMI). The microfluidic chip is composed of a 10 μl chamber situated on top of the GMI sensing material, which is inserted in a microstrip transmission line for the impedance measurements. Two different GMI materials have been used for the experiments: an amorphous ribbon and a permalloy based multilayer, sputtered onto the same polymeric material of the microfluidic chamber. Detection tests in continuous flow have been performed using solutions containing two types of beads: one made of ferromagnetic microparticles and the other made of superparamagnetic nanoparticles. A basic calculation of the magnitude of the fringe field created by the beads assures a detectable signal on the sensors, but the experimental difficulties severely condition the measurements. The results show a promising sensitivity for both types of particles but also reveal an important number of detection problems that must be overcame before the prototype become functional.     

A clip-on electroosmotic pump for oscillating flow in microfluidic cell culture devices

Recent advances in microfluidic devices put a high demand on small, robust and reliable pumps suitable for high-throughput applications. Here we demonstrate a compact, low-cost, directly attachable (clip-on) electroosmotic pump that couples with standard Luer connectors on a microfluidic device. The pump is easy to make and consists of a porous polycarbonate membrane and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) electrodes. The soft electrode and membrane materials make it possible to incorporate the pump into a standard syringe filter holder, which in turn can be attached to commercial chips. The pump is less than half the size of the microscope slide used for many commercial lab-on-a-chip devices, meaning that these pumps can be used to control fluid flow in individual reactors in highly parallelized chemistry and biology experiments. Flow rates at various electric current and device dimensions are reported. We demonstrate the feasibility and safety of the pump for biological experiments by exposing endothelial cells to oscillating shear stress (up to 5 dyn/cm²) and by controlling the movement of both micro- and macroparticles, generating steady or oscillatory flow rates up to ± 400 μL/min.     

一种基于MEMS技术的压电微泵的研究

介绍了一种基于MEMS技术的压电微泵。该微泵利用聚二甲基硅氧烷(PDMS)作为泵膜,使用了一个主动阀和一个被动阀,并利用压电双晶片作为驱动部件。压电双晶片和PDMS泵膜的组合可以产生较大的泵腔体积改变和压缩比,显著降低了加工成本,并提高了成品率。对压电微泵的输出流量进行了测试,结果显示:电压、频率以及背压对流量均有显著影响。在100 V,25Hz的方波驱动下,该压电微泵的最大输出流量为458μL/m in,最大输出压力为6 kPa。     

Design and evaluation of orifice arrangement for particle-excitation flow control valve

In this article, we report a new particle-excitation flow control valve. The purpose of this study is the development of a particle-excitation flow control valve that can precisely control pneumatic cylinders. We have reported this flow control valve principle. The valve, driven by a PZT vibrator, has a simple lightweight structure with large flow rate. We report the relationship between the orifice arrangement and flow rate characteristics of the valve. We have designed a new prototype for the purpose of high controllability. We have measured flow-rate characteristics and confirmed the conditions necessary for continuous adjustment of flow quantity. The control valve works successfully to realize a change in flow rate.     

Quantum dot light emitting devices for photomedical applications

While OLEDs have struggled to find a niche lighting application that can fully take advantage of their unique form factors as thin, flexible, lightweight and uniformly large‐area luminaire, photomedical researchers have been in search of low‐cost, effective illumination devices with such form factors that could facilitate widespread clinical applications of photodynamic therapy (PDT) or photobiomodulation (PBM). Although existing OLEDs with either fluorescent or phosphorescent emitters cannot achieve the required high power density at the right wavelength windows for photomedicine, the recently developed ultrabright and efficient deep red quantum dot light emitting devices (QLEDs) can nicely fit into this niche. Here, we report for the first time the in‐vitro study to demonstrate that this QLED‐based photomedical approach could increase cell metabolism over control systems for PBM and kill cancerous cells efficiently for PDT. The perspective of developing wavelength‐specific, flexible QLEDs for two critical photomedical fields (wound repair and cancer treatment) will be presented with their potential impacts summarized. The work promises to generate flexible QLED‐based light sources that could enable the widespread use and clinical acceptance of photomedical strategies including PDT and PBM.     

Bright red-to-yellow organic light-emitting devices based on polarization-induced spectral shifts and broadening

We demonstrated red and yellow organic light-emitting devices (OLEDs) with the structure of ITO/NPB/AlQ:DCJTB/AlQ/LiF/Al, where the NPB, AlQ and DCJTB are 4, 4′-bis[N-(1-naphthyl)-N-henylamino] biphenyl, tris(8-quinolinolato)aluminum and 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran, respectively. Electroluminescent (EL) behaviors of these devices have been examined with different concentrations of DCJTB doped into AlQ matrix. The emission color of the devices depends on the doping concentrations of DCJTB. For red and yellow OLEDs, a maximum luminescence of 2750 cd/m² and 21,700 cd/m² was obtained, respectively. The peak emission wavelength shift of DCJTB was found to be due to the polarization effects. It is of particular interest that the EL spectrum of DCJTB got broadening with the doping concentrations and current densities of the devices in our experiments.     

Interference in thin radiating layers and its application to thin film electroluminescent devices

Results are reviewed of theoretical and experimental studies on interference of light radiated by a thin luminescence layer placed between two reflecting planes which form a Fabry-Perot cavity. This paper considers first the interference effect on the spectral and angular dependences of the emission from the cavity, and then the influence of the method and the intensity of luminescence excitation upon the interference effects. The possibility of producing ‘colour’ thin film electroluminescent devices based on interference of the emission is discussed. Some parameters of such ‘green’ and ‘red’ devices on the basis of the ZnS:Mn films known at present are given.