Fabrication of topologically complex three-dimensional microfluidic systems in PDMS by rapid prototyping

This paper describes a procedure for making topologically complex three-dimensional microfluidic channel systems in poly(dimethylsiloxane) (PDMS). This procedure is called the "membrane sandwich" method to suggest the structure of the final system: a thin membrane having channel structures molded on each face (and with connections between the faces) sandwiched between two thicker, flat slabs that provide structural support. Two "masters" are fabricated by rapid prototyping using two-level photolithography and replica molding. They are aligned face to face, under pressure, with PDMS prepolymer between them. The PDMS is cured thermally. The masters have complementary alignment tracks, so registration is straightforward. The resulting, thin PDMS membrane can be transferred and sealed to another membrane or slab of PDMS by a sequence of steps in which the two masters are removed one at a time; these steps take place without distortion of the features. This method can fabricate a membrane containing a channel that crosses over and under itself, but does not intersect itself and, therefore, can be fabricated in the form of any knot. It follows that this method can generate topologically complex microfluidic systems; this capability is demonstrated by the fabrication of a "basketweave" structure. By filling the channels and removing the membrane, complex microstructures can be made. Stacking and sealing more than one membrane allows even more complicated geometries than are possible in one membrane. A square coiled channel that surrounds, but does not connect to, a straight channel illustrates this type of complexity.     

Highly elastic conductive polymeric MEMS

Polymeric structures with integrated, functional microelectrical mechanical systems (MEMS) elements are increasingly important in various applications such as biomedical systems or wearable smart devices. These applications require highly flexible and elastic polymers with good conductivity, which can be embedded into a matrix that undergoes large deformations. Conductive polydimethylsiloxane (PDMS) is a suitable candidate but is still challenging to fabricate. Conductivity is achieved by filling a nonconductive PDMS matrix with conductive particles. In this work, we present an approach that uses new mixing techniques to fabricate conductive PDMS with different fillers such as carbon black, silver particles, and multiwalled carbon nanotubes. Additionally, the electrical properties of all three composites are examined under continuous mechanical stress. Furthermore, we present a novel, low-cost, simple three-step molding process that transfers a micro patterned silicon master into a polystyrene (PS) polytetrafluoroethylene (PTFE) replica with improved release features. This PS/PTFE mold is used for subsequent structuring of conductive PDMS with high accuracy. The non sticking characteristics enable the fabrication of delicate structures using a very soft PDMS, which is usually hard to release from conventional molds. Moreover, the process can also be applied to polyurethanes and various other material combinations.     

Highly elastic conductive polymeric MEMS

Abstract

Polymeric structures with integrated, functional microelectrical mechanical systems (MEMS) elements are increasingly important in various applications such as biomedical systems or wearable smart devices. These applications require highly flexible and elastic polymers with good conductivity, which can be embedded into a matrix that undergoes large deformations. Conductive polydimethylsiloxane (PDMS) is a suitable candidate but is still challenging to fabricate. Conductivity is achieved by filling a nonconductive PDMS matrix with conductive particles. In this work, we present an approach that uses new mixing techniques to fabricate conductive PDMS with different fillers such as carbon black, silver particles, and multiwalled carbon nanotubes. Additionally, the electrical properties of all three composites are examined under continuous mechanical stress. Furthermore, we present a novel, low-cost, simple three-step molding process that transfers a micro patterned silicon master into a polystyrene (PS) polytetrafluoroethylene (PTFE) replica with improved release features. This PS/PTFE mold is used for subsequent structuring of conductive PDMS with high accuracy. The non sticking characteristics enable the fabrication of delicate structures using a very soft PDMS, which is usually hard to release from conventional molds. Moreover, the process can also be applied to polyurethanes and various other material combinations.     

Use of a corona discharge to selectively pattern a hydrophilic/hydrophobic interface for integrating segmented flow with microchip electrophoresis and electrochemical detection

Segmented flow in microfluidic devices involves the use of droplets that are generated either on- or off-chip. When used with off-chip sampling methods, segmented flow has been shown to prevent analyte dispersion and improve temporal resolution by periodically surrounding an aqueous flow stream with an immiscible carrier phase as it is transferred to the microchip. To analyze the droplets by methods such as electrochemistry or electrophoresis, a method to "desegment" the flow into separate aqueous and immiscible carrier phase streams is needed. In this paper, a simple and straightforward approach for this desegmentation process was developed by first creating an air/water junction in natively hydrophobic and perpendicular PDMS channels. The air-filled channel was treated with a corona discharge electrode to create a hydrophilic/hydrophobic interface. When a segmented flow stream encounters this interface, only the aqueous sample phase enters the hydrophilic channel, where it can be subsequently analyzed by electrochemistry or microchip-based electrophoresis with electrochemical detection. It is shown that the desegmentation process does not significantly degrade the temporal resolution of the system, with rise times as low as 12 s reported after droplets are recombined into a continuous flow stream. This approach demonstrates significant advantages over previous studies in that the treatment process takes only a few minutes, fabrication is relatively simple, and reversible sealing of the microchip is possible. This work should enable future studies in which off-chip processes such as microdialysis can be integrated with segmented flow and electrochemical-based detection.     

Torque-actuated valves for microfluidics

This paper describes torque-actuated valves for controlling the flow of fluids in microfluidic channels. The valves consist of small machine screws (> or =500 microm) embedded in a layer of polyurethane cast above microfluidic channels fabricated in poly(dimethylsiloxane) (PDMS). The polyurethane is cured photochemically with the screws in place; on curing, it bonds to the surrounding layer of PDMS and forms a stiff layer that retains an impression of the threads of the screws. The valves were separated from the ceiling of microfluidic channels by a layer of PDMS and were integrated into channels using a simple procedure compatible with soft lithography and rapid prototyping. Turning the screws actuated the valves by collapsing the PDMS layer between the valve and channel, controlling the flow of fluids in the underlying channels. These valves have the useful characteristic that they do not require power to retain their setting (on/off). They also allow settings between "on" and "off" and can be integrated into portable, disposable microfluidic devices for carrying out sandwich immunoassays.     

Fabrication of microfluidic devices containing patterned microwell arrays

A rapid fabrication and prototyping technique to incorporate microwell arrays with sub-10 μm features within a single layer of microfluidic circuitry is presented. Typically, the construction of devices that incorporate very small architecture within larger components has required the assembly of multiple elements to form a working device. Rapid, facile production of a working device using only a single layer of molded polydimethylsiloxane (PDMS) and a glass support substrate is achieved with the reported fabrication technique. A combination of conventional wet-chemical etching for larger (≥20 μm) microchannel features and focused ion beam (FIB) milling for smaller (≤10 μm) microwell features was used to fabricate a monolithic glass master mold. PDMS/glass hybrid chips were then produced using simple molding and oxygen plasma bonding methods. Microwell structures were loaded with 3 μm antibody-functionalized dye-encoded polystyrene spheres, and a sandwich immunoassay for common cytokines was performed to demonstrate proof-of-principle. Potential applications for this device include highly parallel multiplexed sandwich immunoassays, DNA/RNA hybridization analyses, and enzyme linked immunosorbent assay (ELISA). The fabrication technique described can be used for rapid prototyping of devices wherever submicrometer- to micrometer-sized features are incorporated into a microfluidic device.     

Ultrathin planar graphene supercapacitors

With the advent of atomically thin and flat layers of conducting materials such as graphene, new designs for thin film energy storage devices with good performance have become possible. Here, we report an "in-plane" fabrication approach for ultrathin supercapacitors based on electrodes comprised of pristine graphene and multilayer reduced graphene oxide. The in-plane design is straightforward to implement and exploits efficiently the surface of each graphene layer for energy storage. The open architecture and the effect of graphene edges enable even the thinnest of devices, made from as grown 1-2 graphene layers, to reach specific capacities up to 80 μFcm(-2), while much higher (394 μFcm(-2)) specific capacities are observed multilayer reduced graphene oxide electrodes. The performances of devices with pristine as well as thicker graphene-based structures are examined using a combination of experiments and model calculations. The demonstrated all solid-state supercapacitors provide a prototype for a broad range of thin-film based energy storage devices.     

Controlling electroosmotic flow in poly(dimethylsiloxane) separation channels by means of prepolymer additives

The electroosmotic flow (EOF) in a poly(dimethylsiloxane) (PDMS) separation channel can be altered and controlled by adding a carboxylic acid to the prepolymer prior to curing. When the prepolymer is doped with 0.5 wt % undecylenic acid (UDA), the electroosmotic mobility in a modified PDMS channel rises to (7.6 +/- 0.2) x 10(-4) cm(2) V(-1) s(-1) (in HEPES buffer at pH 8.5), which is nearly twice that in the native PDMS channel. Because this modification does not significantly change the hydrophobicity of the PDMS surface, it is possible to combine the modified PDMS with a dynamic coating of n-dodecyl beta-d-maltoside (DDM), which prevents protein sticking (see Huang, B.; Wu, H. K.; Kim, S.; Zare, R. N. Lab Chip 2005, 5, 1005-1007). The modified PDMS channel with a dynamic coating of DDM generates an electroosmotic mobility of (5.01 +/- 0.09) x 10(-4) cm(2) V(-1) s(-1), which shows excellent reproducibility both in successive runs and during storage in water. Combining this surface modification and the dynamic coating of DDM is an effective means for both providing stable EOF in the PDMS channels and preventing protein adsorption on the channel walls. To demonstrate these effects, we show that the electrophoretic separation of immunocomplexes in free solution can be readily accomplished in a microfluidic chip made of UDA-doped (0.5 wt %) PDMS with a dynamic coating of DDM.     

Characterization and resolution of evaporation-mediated osmolality shifts that constrain microfluidic cell culture in poly(dimethylsiloxane) devices

Evaporation is a critical problem when handling submicroliter volumes of fluids. This paper characterizes this problem as it applies to microfluidic cell culture in poly(dimethylsiloxane) (PDMS) devices and provides a practical solution. Evaporation-mediated osmolality shifts through PDMS membranes with varying thicknesses (10, 1, 0.2, or 0.1 mm) were measured over 96 h. Even in humidified cell culture incubators, evaporation through PDMS and associated shifts in the osmolality of culture media was significant and prevented mouse embryo and human endothelial cell growth and development. A simple diffusion model, where the measured diffusion coefficient for PDMS matches reported values of approximately 10-9 m2/s, accounts for these evaporation and osmolality shifts. To overcome this problem, a PDMS-parylene-PDMS hybrid membrane was developed that greatly suppresses evaporation and osmolality shifts, yet possesses thinness and the flexibility necessary to interface with deformation-based microfluidic actuation systems, maintains the clarity for optical microscopy, and enables the successful development of single-cell mouse embryos into blastocysts under static conditions and culture of human endothelial cells under dynamic recirculation of submicroliter volumes of media. These insights and methods demonstrated specifically for embryo and endothelial cell studies will be generally useful for understanding and overcoming evaporation-associated effects in microfluidic cell cultures.     

Chemical-assisted bonding of thermoplastics/elastomer for fabricating microfluidic valves

Thermoplastics such as cyclic olefin copolymer (COC) and polymethylmethacrylate (PMMA) have been increasingly used in fabricating microfluidic devices. However, the state-of-the-art microvalve technology is a polydimethylsiloxane (PDMS)-based three-layer structure. In order to integrate such a valve with a thermoplastics-based microfluidic device, a bonding method for thermoplastics/PDMS must be developed. We report here a method to bond COC with PDMS through surface activation by corona discharge, surface modification using 3-(trimethoxysilyl)propyl methacrylate (TMSPMA), and thermal annealing. The method is also applicable to PMMA. The bonding strength between thermoplastics and PDMS was represented by the peeling force, which was measured using a method established by the International Organization for Standardization (ISO). The bonding strength measurement offered an objective and quantitative indicator for protocol optimization, as well as comparison with other PDMS-associated bonding methods. Using optimized bonding conditions, two valve arrays were fabricated in a COC/PDMS/COC device and cyclic operations of valve closing/opening were successfully demonstrated. The valve-containing devices withstood 100 psi (～689 KPa) without delamination. Further, we integrated such valve arrays in a device for protein separation and demonstrated isoelectric focusing in the presence of valves.     

Synthesis and characterisation of enhanced barrier polyurethane for encapsulation of implantable medical devices

Polymeric membranes have been used as interfaces between implantable devices and biological tissues to operate as a protective barrier from water exchanging and to enhance biocompatibility. Polyurethanes have been used as biocompatible membranes for decades. In this study, copolymers of polyether urethane (PEU) with polydimethylsiloxane (PDMS) were synthesised with the goal of creating materials with low water permeability and high elasticity. PDMS was incorporated into polymer backbone as a part of the soft segment during polyurethane synthesis and physical properties as well as water permeability of resulting copolymer were studied in regard to PDMS content. Increase in PDMS content led to increase of microphase separation of the copolymer and corresponding increase in elastic modulus. Surface energy of the polymer was decreased by incorporating PDMS compared to unmodified PEU. PDMS in copolymer formed a hydrophobic surface which caused reduction in water permeability and water uptake of the membranes. Thus, PDMS containing polyurethane with its potent water resistant properties demonstrated a great promise for use as an implantable encapsulation material.     

Surface characterization of plasma-treated and PEG-grafted PDMS for micro fluidic applications

The contact angle measurements have shown that polydimethyl siloxane (PDMS) surfaces treated by air plasma can recover up to about 40% of its hydrophobic nature in less than 20 min of air exposure. Therefore, poly(ethylene glycol) (PEG) silane was grafted after plasma treatment to permanently change the PDMS surface as hydrophilic in nature for micro fluidic application. The surface chemistry of plasma-treated and PEG-grafted PDMS substrate has been studied using X-ray photoelectron spectroscopy (XPS). The proportion of carbon atoms as C-Si and hydrocarbon decreased for both plasma-treated as well as PEG-grafted PDMS surfaces. The plasma treatment had increased the proportion of carbon atoms as CO and C(O)OX in C1s, whereas grafting of PEG silane decreased the proportion of C(O)OX and an increase in C-OX and CO functionalities. This is due to the interaction of OCH₃ on Si (in PEG silane) with C-OX and C(O)OX on plasma-treated PDMS by covalent bonding. Therefore, an increase in CO and C-OX functionalities and relative decrease in C(O)OX is expected. The plasma treatment of micro channels had increased the fluid velocity by a factor or four and similar measurements were observed in PEG grafted micro channel in PDMS chip. This indicates that the fluid velocity depends on the hydrophilic nature of substrate. The effect of nature of fluids on the fluid velocity in PDMS-based micro channel was also studied. It was observed that the fluid velocity was decreased with decreasing the pH values of the fluid.     

支撑层对PDMS复合膜渗透汽化性能的影响

以相转化法制备的聚偏氟乙烯(PVDF)、聚丙烯腈(PAN)、聚砜(PSF)三种多孔膜作为支撑层,制备聚二甲基硅氧烷(PDMS)复合膜用于渗透汽化乙醇/水混合物的分离。采用能量色散X射线光谱仪(EDX)定量表征了PDMS在支撑层表面的厚度(L0)和支撑层内的渗入深度(Li),研究发现,PDMS在各支撑层表面的厚度、支撑层内渗入的厚度有显著差异,PDMS复合膜的渗透通量与(L0+Li)间存在近似的线性关系,表明PDMS在支撑层中渗入深度不同是造成不同底膜支撑的PDMS复合膜渗透汽化性能差异的根本原因。文中提出选择层总厚度(Lt=L0+Li)概念,通过线性拟合得到PDMS复合膜渗透通量与Lt之间的定量关系,可以用来估算PDMS复合膜的渗透通量,并预测复合膜渗透通量极大值。     

双沟道层对氮掺杂非晶氧化铟锌薄膜晶体管的影响

采用射频磁控溅射法, 在热氧化p型硅基片上制备了双沟道层非晶氧化铟锌(a-IZO)和氮掺杂氧化铟锌(a-IZON)薄膜晶体管(TFTs), 并研究了双沟道层对器件电学性能和温度稳定性的影响。研究发现, a-IZO/IZON双沟道层TFTs具有较高的场效应迁移率, 为23.26 cm²/(V•s), 并且其阈值电压相较于单层a-IZO-TFTs正向偏移。这是由于氮掺杂可以减少沟道层中的氧空位, 抑制载流子浓度, 使器件具有更好的阈值电压。而a-IZO层避免了由于氮掺杂导致的场效应迁移率和开态电流的下降, 提升了器件的电流开关比。从298 K至423 K的器件转移特性曲线中发现, 双沟道层器件相较于单沟道层器件的温度稳定性更佳, 这可归因于a-IZON层的保护作用。氮掺杂可以减少氧在背沟道层表面的吸收/解吸反应, 改善器件的稳定性。     

Transformation of poly(dimethylsiloxane) into thin surface films of SiO x by UV/ozone treatment. Part II: segregation and modification of doped polymer blends

UV/ozone treatment of organic polymers having silicone additives to produce oxidized layers was achieved by doping a host polymer or prepolymer with a silicone additive, poly(dimethylsiloxane) (PDMS). The concentration of PDMS in the host polymer was low, typically in the range of 0.1–2.0% by weight. Host polymers were polyethylene, polyimide, and polyurethane. After film formation, the presence of PDMS was detected on the surface using X-ray photoelectron spectroscopy (XPS), consistent with wetting angle measurements that revealed a hydrophobic surface. The doped blend was then subjected to exposure in a UV/ozone environment such that a thin, stable barrier of SiO _x was formed at the surface of the film. Rate of film modification was monitored by XPS and measurement of advancing contact angle using deionized water. XPS measurements also showed some evidence of modified fragments of the host polymer near the surface. Significant segregation of PDMS and subsequent transformation to silicon oxides has been demonstrated to occur in these doped systems. The stability of the modified glassy surface formed by UV/ozone treatment of a commercially available epoxy formulation containing a silicone additive was shown to be superior to that obtained by other treatment techniques, e.g., oxygen plasma modification.     

Flexographically printed fluidic structures in paper

This Technical Note demonstrates a simple method based on flexographic printing of polystyrene to form liquid guiding boundaries and layers on paper substrates. The method allows formation of hydrophobic barrier structures that partially or completely penetrate through the substrate. This unique property enables one to form very thin fluidic channels on paper, leading to reduced sample volumes required in point-of-care diagnostic devices. The described method is compatible with roll-to-roll flexography units found in many printing houses, making it an ideal method for large-scale production of paper-based fluidic structures.     

The preparation of laminated ceramic composites by sedimentation and sieving

Two simple methods have been developed to fabricate layered microcomposites, with alternative Al₂O₃ layers as thin as 20 m in Ce-TZP matrix. One is sieving different dry powder into a die alternatively and pressed into greenbody, the other is realized by rapid particle sedimentation occurred in aqueous suspending systems of Al₂O₃ and ZrO₂ powder. These methods avoid the thermolamination step associated with tape casting and adoption of organic assistant. Finally the possibility of processing thin layers by xerox has been explored.     

Stretchable magnetoelectronics

We fabricated [Co/Cu] multilayers revealing a giant magnetoresistance (GMR) effect on free-standing elastic poly(dimethylsiloxane) (PDMS) membranes. The GMR performance of [Co/Cu] multilayers on rigid silicon and on free-standing PDMS is similar and does not change with tensile deformations up to 4.5%. Mechanical deformations imposed on the sensor are totally reversible, due to the elasticity of the PDMS membranes. This remarkable performance upon stretching relies on a wrinkling of GMR layers on top of the PDMS membrane.     

Polymer-based carbon-nanotube film with reversible signal tracking capabilities

This study presents a polymer-based carbon-nanotube (CNT) sensing polymer with reversible signal tracking capabilities. The sensing polymer was prepared by dispersing multi-walled CNTs (MWCNTs) and silver nanoparticles into the polydimethylsiloxane (PDMS) polymer matrix. Before curing the PDMS prepolymer, MWCNTs were aligned in the prepolymer, using the dielectrophoresis (DEP) technique. Under an external force, the polymer increased and retained resistivity, which could be recovered to its original value by repeating DEP. Similar resistivity behaviors induced by temperature elevation and DEP were also observed. This study also presents the measured performance and repeatability. The potential applications of the sensing polymer include reusable inertia switches, footstep tracking carpets, and temperature switches.     

离子硫化层与热喷涂硫化层的摩擦学性能比较

分别采用低温离子渗硫和等离子喷涂的方法在45^#钢表面制备了硫化层。在摩擦磨损试验机上对比研究了这两种硫化层在油润滑条件下的摩擦学性能。利用XRD分析了硫化层的相结构，用SEM观察了硫化层的表面及磨面形貌并进行了能谱分析。结果表明，各硫化层的摩擦学性能明显优于原始基体表面，其中离子硫化层的减摩性和耐磨性更好，耐热喷涂硫化层的抗擦伤性更佳。造成这种差别的主要原因在于两种硫化层的成膜机理不同。