Microfiber-directed boundary flow in press-fit microdevices fabricated from self-adhesive hydrophobic surfaces

We report a rapid microfluidic device construction technique which does not employ lithography or stamping methods. Device assembly physically combines a silicon wafer, an elastomer (poly(dimethylsiloxane) (PDMS)), and microfibers to form patterns of hydrophobic channels, wells, elbows, or orifices that direct fluid flow into controlled boundary layers. Tweezers are used to place glass microfibers in a defined pattern onto an elastomeric (PDMS) hydrophobic film. The film is then manually pressed onto a hydrophobic silicon wafer, causing it to adhere to the silicon wafer and form a liquid-tight seal around the fibers. Completed in 15 min, the technique results in an operable microdevice with micrometer-scale features of nanoliter volume. Microfiber-directed boundary flow is achieved by use of the surface wetting properties of the hydrophilic glass fiber and the hydrophobicity of surrounding surfaces. The simplicity of this technique allows quick prototyping of microfluidic components, as well as complete biosensor systems, such as we describe for the detection of pathogenic bacteria.     

Photo‐Reconfigurable Azopolymer Etch Mask: Photofluidization‐Driven Reconfiguration and Edge Rectangularization

Directional photofluidization of azobenzene materials has provided unprecedented opportunities for the structural reconfiguration of circular holes, line gaps, ellipsoidal holes, and nanofunnel‐shaped micro/nanoarchitectures. However, all the reconfigured structures have a parabolic or round wall due to the tendency of the photofluidized azobenezene materials to minimize the surface area, which limits their use as a reconfigurable etch‐mask for the lithography process. In this work, a simple method is presented that can change the round walls of azopolymer architectures into rectangular walls, which is named rectangularization. By irradiating far‐field light on reconfigured azopolymer in a conformal contact with a flat polydimethylsiloxane (PDMS) film, the round wall transforms to a rectangular one because the azopolymer adheres along the PDMS surface while being photofluidized. As a result, the rectangularization process creates a variety of structural features and sizes ranging from a few micrometers to 150 nm having a rectangular wall. By exploiting the rectangularization process, the concept of a photo‐reconfigurable etch mask is achieved, which transfers the mask patterns to a silicon pattern with a high structural fidelity and imparts a considerable flexibility to the lithography process.     

Prototyping of masks, masters, and stamps/molds for soft lithography using an office printer and photographic reduction

This paper describes a practical method for the fabrication of photomasks, masters, and stamps/molds used in soft lithography that minimizes the need for specialized equipment. In this method, CAD files are first printed onto paper using an office printer with resolution of 600 dots/in. Photographic reduction of these printed patterns transfers the images onto 35-mm film or microfiche. These photographic films can be used, after development, as photomasks in 1:1 contact photolithography. With the resulting photoresist masters, it is straightforward to fabricate poly(dimethylsiloxane) (PDMS) stamps/molds for soft lithography. This process can generate microstructures as small as 15 microm; the overall time to go from CAD file to PDMS stamp is 4-24 h. Although access to equipment-spin coater and ultraviolet exposure tool-normally found in the clean room is still required, the cost of the photomask itself is small, and the time required to go from concept to device is short. A comparison between this method and all other methods that generate film-type photomasks has been performed using test patterns of lines, squares, and circles. Three microstructures have also been fabricated to demonstrate the utility of this method in practical applications.     

Facile patterning of reduced graphene oxide film into microelectrode array for highly sensitive sensing

In this study, we develop a new technique to fabricate a reduced graphene oxide (rGO)-based microelectrode array (MEA) with low-cost soft lithography. To prepare patterned rGO, a polydimethylsiloxane (PDMS) mold with an array of microwells on its surface is fabricated using soft lithography, and GO is assembled on an indium tin oxide (ITO) electrode with a layer-by-layer method. The rGO pattern is formed by closely contacting the assembled GO film onto the ITO electrode with the PDMS mold filled with hydrazine solution in the microwells to selectively reduce the localized GO into the rGO. The MEA with patterned rGO as the microelectrode is characterized with Kelvin probe force microscopy (KFM), atomic force microscopy (AFM), and cyclic voltammetry (CV) with ferricyanide in aqueous solution as the redox probe. The KFM and AFM results demonstrate that each rGO pattern prepared under the present conditions is 3 μm in diameter, which is close to that of the PDMS mold we use. The CV results show that the rGO patterned onto the ITO exhibits a sigmoid-shaped voltammogram up to 200 mVs(-1) with a microampere level current response, suggesting that the rGO-based electrode fabricated with soft lithography behalves like a MEA. To demonstrate the potential electroanalytical application of the rGO-based MEA, prussian blue (PB) is electrodeposited onto the rGO-based MEA to form the PB/rGO-based MEA. Electrochemical studies on the formed PB/rGO-based MEA reveal that MEA shows a lower detection limit and a larger current density for the detection of H(2)O(2), as compared with the macroscopic rGO electrode. The method demonstrated here provides a simple and low-cost strategy for the fabrication of graphene-based MEA that are useful for electroanalytical applications.     

不可逆封合微流控芯片中高活性蛋白质阵列的制备

保持生物分子的高活性是在不可逆封合微流控芯片中构筑微阵列芯片的关键问题.首先,利用MEMS技术和表面修饰方法制作了一种聚二甲基硅氧烷(PDMS)/玻璃芯片.应用光刻技术制作了PDMS盖片上的通道,同时用光刻剥离技术制作了玻璃基片上的金膜图案.进而,使用双官能团修饰剂3-氨丙基三甲氧基硅氧烷(APTMS)在玻璃基体和金膜图案上进行选择性表面修饰以吸附形成蛋白质阵列,并在其上覆盖一层水溶性聚乙烯醇(PVA)来保护蛋白质,既可避免其在加热处理过程中的高温伤害,又能防止在PDMS盖片与玻璃基片进行不可逆封合过程中的氧等离子体轰击造成的活性伤害.然后,通入水溶液冲洗除去PVA膜.使用荧光显微镜和原子力显微镜(AFM)考察蛋白质阵列质量,并结合免疫反应实验和细胞捕获固定实验评估蛋白质阵列的活性.结果表明,使用该方法可在不可逆封合的微流控芯片制作中构筑具有直径为200μm的高分辨率蛋白质阵列图案,蛋白质保持高的免疫活性,且可用于固定Hela细胞.     

Self-organized meso-patterning of soft solids by controlled adhesion: elastic contact lithography

The surface of a soft elastic film becomes unstable and deforms when a rigid flat plate is brought into its contact proximity, without application of any external pressure. These isotropic undulations have a characteristic wavelength, lambda approximately 3H, where H is the film thickness. The wavelength is independent of the adhesive interactions and the mechanical properties of the film. We present here a mini-review of our recent work on techniques of aligning, modulating, and ordering the instability structures by the use of simple 1-D patterned stamps, by changing the stamp-surface separation, by slow shearing of a flat stamp and by confining the instability in soft narrow channels. The generality of the technique for different soft materials is illustrated by patterning cross-linked polydimethylsiloxane (PDMS), aluminum coated PDMS and hydrogels films. Use of a flexible stamp such as a metal foil provides enhanced conformal contact by adhesive forces, which aids large area patterning without critically maintaining a parallel configuration and uniform pressure between the stamp and the film. The technique has the potential to develop into a new soft lithography tool--"Elastic Contact Lithography" suitable for rapid, large area micron and sub-micron self-organized patterning of a variety of soft materials without any special equipments.     

基于光学光刻技术的PDMS微纳敏感结构薄膜加工工艺研究

利用光学光刻技术进行多种形状规格聚二甲基硅氧烷(PDMS)微纳敏感结构的并行加工。研究了基于光学光刻技术的PDMS微纳敏感结构薄膜加工工艺,并加工出不同形状、、不同大小的PDMS微纳敏感结构。此外,对加工后的PDMS微纳敏感结构薄膜进行表征,观察了不同形状、不同大小的PDMS微纳敏感结构的形貌特征。结果表明:基于光学光刻技术的PDMS微纳敏感结构薄膜加工工艺具有加工效率高,加工过程中不需要刻蚀、激光加工等复杂工艺和昂贵设备的特点,并且实现了大批量PDMS微纳敏感结构的并行加工。同时,掩模板上的微纳图案较好地转移到PDMS薄膜上,具有较好的图案曝光成像和图形转移效果。此外,制备的PDMS微纳敏感结构形状较规则、排列整齐,并且不同形状,不同大小的PDMS微纳敏感结构都具有高度一致性较好,侧面轮廓分明,侧壁角接近90°的特征。     

Replication of a thin polydimethylsiloxane stamp and its application to dual-nanoimprint lithography for 3D hybrid nano/micropatterns

This paper presents the fabrication of a thin and flexible polydimethylsiloxane (PDMS) stamp with a thickness of a few tens of um and its application to nanoimprint lithography (NIL). The PDMS material generally has a low elastic modulus and high adhesive characteristics. Therefore, after being treated, the thin PDMS stamp is easily deformed and torn, adhering to itself and other materials. This paper introduces the use of a metal ring around the flange of a thin PDMS stamp to assist with the handling of this material. A PDMS stamp with a motheye pattern in nanometer scale was inserted between a substrate and a microstamp with concave patterns in micrometer scale. Subsequently, three-dimensional (3D) hybrid nano/micropatterns were fabricated by pressing these two stamps and curing the resist. The fabricated hybrid patterns were measured and verified in both the microscale and nanoscale. The process, termed "dual NIL," can be applied to the fabrication of optical components or bio-sensors that require repetitive nanopatterns on micropatterns.     

Polyelectrolyte multilayers coating for organic solvent resistant microfluidic chips

The layer-by-layer (LbL) deposition technique was used to coat and protect poly(methyl methacrylate) (PMMA) and poly(dimethylsiloxane) (PDMS) substrate from organic solvent. PMMA and PDMS substrates were protected by polyelectrolyte multilayers (PEM) thin films of either poly(diallydimethiyl ammonium chloride) and Poly(styrene sulfonate) PDADMAC/PSS or chitosan/alginate. The PEM depositited on the PMMA and PDMS substrates improved the organic solvent resistance with the best results obtained from the chitosan/alginate over the PDADMAC/PSS pair. The more hydrophilic character of the chitosan/alginate and the PDADMAC/PSS film caused a significant decreasing rate of organic solvent pentration into the PMMA substrate which retain transparent optical properties for up to 30 dipping in acetonitrile. A 20 layers chitosan alginate film also decreased PDMS substrate swelling when exposed to chloroform vapor. The PEMs coating could protect the PMMA and PDMS sample against organic solvent and vapor which could make them useful in microfluidic systems even in agressive environment.     

纳米光子结构软膜紫外光固化纳米压印

紫外光固化纳米压印是实现纳米结构批量复制的一种新技术．其特点是低成本和高分辨，而且可以达到极高的套刻精度．为了得到大面积图案的均匀复制，可用聚二甲基硅氧烷（PDMS）制备透光的压印软模板．其母版图案可由高分辨率电子柬曝光和反应离子刻蚀的方法在硅片基底上获得，然后用浇注的方法将图案转移到PDMS上．本实验特别发展了紫外光固化纳米压印适用于软膜压印的双层膜图型转移技术．该双层膜由廉价的光胶和聚甲基丙烯酸甲脂（PMMA）构成．对光胶层的压印可用普通的光学曝光仪实现．然后再将图案用反应离子刻蚀的方法转移到PMMA层中．为了证明方案的可行性，在两种不同材料的半导体基片上压印了三角晶格的光子晶体和准晶结构的图案，并用剥离的方法将它们转移到金属薄膜上，最后成功地进行了硅片刻蚀实验．相信这一纳米制做方法对大面积纳米光子结构和光学集成芯片的制造是普遍适用的．     

Controlling orientation of V(2)O(5) nanowires within micropatterns via microcontact printing combined with the gluing Langmuir-Blodgett technique

Previously, we suggested a facile method to transfer dioctadecyldimethylammonium bromide (DODAB)/V(2)O(5) nanowire hybrid patterns onto both hydrophobic and hydrophilic substrates via microcontact printing combined with the Langmuir-Blodgett (LB) technique (Park et al 2007 Nanotechnology 18 405301). Herein, we report on the delicate control of the orientation of V(2)O(5) nanowires within the micropatterns transferred via the gluing LB technique using a patterned polydimethylsilicate (PDMS) stamp. According to the orientation of the PDMS line patterns relative to the air-water interface, the aligned orientation of the nanowires, either parallel or perpendicular to the patterns, could be obtained and attributed to the moving direction of the water menisci formed between the PDMS stamp and water. In particular, addition of a small amount of ethanol in the subphase enhanced the dispersion of the DODAB at the air-water interface as well as the aggregation of V(2)O(5) nanowires, resulting in alignment of the V(2)O(5) nanowires via compression of the hybrid LB film by a barrier. Directional alignment of nanowires has potentially broad applications in the fabrication of aligned nanowire devices.     

Formation of nanometer-scale structures using conventional optical lithography

We investigated a new method to form the line-and-space patterns with a nanometer-scale using the conventional optical lithography technique and the metal deposition/liftoff process. The ashing of the negative photo resist defined by the conventional optical lithography technique results in the proper profiles including a high aspect ratio (i.e., height/width value of the patterns) for the formation of nanometer-scale structures. We demonstrated that the metal electrodes with the nanometer-scale gap of 20 nm or less can be easily obtained by the newly proposed method without employing the highly sophisticated lithography tools including an electron beam lithography.     

聚乙烯/硅橡胶共混膜的制备及其透气性能

为了提高聚乙烯薄膜的透气性能,采用挤出吹塑的方法制备了硅橡胶（聚二甲基硅氧烷,PDMS）含量为1%~5%的聚乙烯（PE）/硅橡胶共混薄膜,用扫描电镜研究了硅橡胶在聚乙烯中的分布,测试了共混膜的透气性能。结果表明,由于吹膜过程中的拉伸作用,硅橡胶颗粒呈细长型分布于聚乙烯基体中,硅橡胶的加入并不能明显改善聚乙烯的透气性能。...     

Improvement of transmittance by fabricating broadband subwavelength anti-reflection structures for polycarbonate

We report on how to increase transmittance of a 0.2 mm thick polycarbonate (PC) film by periodic subwavelength anti-reflection structures in the visible spectral range. Subwavelength anti-reflection structures like moth-eyes are fabricated into the polycarbonate substrate itself by thermal nano-imprinting lithography (TH-NIL), which uses silicon stamps that have periodic structures such as gratings (lines and spaces) and pillared dots, and are fabricated by laser interference lithography (LIL) and transformer coupled plasma etching. To increase transmittance of a polycarbonate film, we control the periods and shapes of patterns, the number of patterned surfaces, and the overlapping direction of patterns that are fabricated into its surfaces. As a result of this, we show that average transmittance improves as the pattern period gets shorter and as both surfaces of the film are patterned. We also show that the spectrum range gets larger as the pattern period gets shorter and is determined by the longer pattern period in the case of designing a film to have different pattern period on its surfaces. The maximum average transmittance of a polycarbonate film increases up to approximately 6% compared to a bare sample in the 470-800 nm spectral range.     

Enhanced adhesion of bioinspired nanopatterned elastomers via colloidal surface assembly

We describe a scalable method to fabricate nanopatterned bioinspired dry adhesives using colloidal lithography. Close-packed monolayers of polystyrene particles were formed at the air/water interface, on which polydimethylsiloxane (PDMS) was applied. The order of the colloidal monolayer and the immersion depth of the particles were tuned by altering the pH and ionic strength of the water. Initially, PDMS completely wetted the air/water interface outside the monolayer, thereby compressing the monolayer as in a Langmuir trough; further application of PDMS subsequently covered the colloidal monolayers. PDMS curing and particle extraction resulted in elastomers patterned with nanodimples. Adhesion and friction of these nanopatterned surfaces with varying dimple depth were studied using a spherical probe as a counter-surface. Compared with smooth surfaces, adhesion of nanopatterned surfaces was enhanced, which is attributed to an energy-dissipating mechanism during pull-off. All nanopatterned surfaces showed a significant decrease in friction compared with smooth surfaces.     

疏水可拉伸碳纳米管/聚二甲基硅氧烷复合薄膜的性能

基于化学气相沉积制备三维多孔多壁碳纳米管(MWNTs)海绵,在其内均匀填充聚二甲基硅氧烷(PDMS)制备出碳纳米管/聚二甲基硅氧烷复合薄膜。复合PDMS的碳纳米管海绵保持着自身的三维结构,成为导电网络和力学骨架;均匀填充的PDMS使复合薄膜具有较高的拉伸性能。碳纳米管与聚二甲基硅氧烷之间的协同作用,使MWNTs/PDMS复合薄膜具有良好的力学强度(3.7 MPa)、拉伸性(207%)和弹性。MWNTs/PDMS复合薄膜对应变有稳定可靠的响应,应变为10%、20%、50%、80%和100%时电阻变化率(△R/R₀)分别为0.9%、1.4%、2.3%、3.5%和4.6%,灵敏因子(GF)为别为0.09、0.07、0.046、0.044和0.046。MWNTs/PDMS复合薄膜的性能具有良好的稳定性,不受拉伸速度和循环次数影响。同时,MWNTs/PDMS复合薄膜还保持了碳纳米管和PDMS的疏水能力。     

基于干共混法控制柔性模板上金属薄膜微裂纹形貌

以500 nm的SiO₂纳米粒子为填料,聚二甲基硅氧烷(PDMS)为包覆纳米粒子的聚合物,通过一种新颖的干共混法制备了SiO₂纳米粒子含量高达83.8wt%的SiO₂/PDMS柔性模板。所制备柔性模板的弹性模量为16.58 MPa,热膨胀系数为96×10?6/℃,较SiO₂直接掺入PDMS中湿共混制备的纯PDMS柔性模板,弹性模量提高了91.56%,热膨胀系数降低了69.23%,且该柔性模板具有良好的透明度。最后,在柔性模板表面采用磁控溅射法沉积银薄膜,利用SEM和AFM对银薄膜表面形貌进行表征。结果显示,银薄膜表面光滑,粗糙度很小,且具有良好稳定性。干共混法制备的柔性模板有效抑制了金属薄膜微裂纹的产生,为制造导电性良好的电极及大面积的金属薄膜提供了新方案。      

Growth of high-aspect ratio horizontally-aligned ZnO nanowire arrays

A method of fabricating horizontally-aligned zinc-oxide (ZnO) nanowire (NW) arrays with full control over the width and length is demonstrated. SEM images reveal the hexagonal structure typical of zinc oxide NWs. Arrays of high-aspect ratio horizontal ZnO NWs are fabricated by making use of the lateral overgrowth from dot patterns created by electron beam lithography (EBL). An array of patterned wires are lifted off and transferred to a flexible PDMS substrate with possible applications in several key nanotechnology areas.     

Creating self-organized submicrometer contact instability patterns in soft elastic bilayers with a topographically patterned stamp

The surface of a thin elastic bilayer becomes spontaneously unstable when it is brought in proximity to another rigid contactor. The instability patterns, which are random and isotropic, exhibit a dominant lateral length scale of instability λ, which linearly scales with the bilayer thickness (h) as: λ = R(F)h. It is known that for an elastic bilayer, R(F) exhibits a nonlinear dependence on the ratios of individual film thicknesses (H) and shear moduli (M) of the two constituent layers, and can have values as low as 0.5 under specific conditions. This is in contrast to a near constant value of R(F) ≈ 3 for a single layer elastic film. (1) These isotropic contact instability patterns in a bilayer can be ordered, aligned and modulated using a topographically patterned stamp. The precise morphology of the aligned structures depends on commensuration between λ and the stamp periodicity (λ(P)), and on the intersurface separation distance. A variety of patterns, like an array of circular holes, double periodic channels, etc., in addition to a positive and a negative replica of the stamp pattern, can be engineered with a simple stamp having 1D grating structure. A lower value of R(F) in a bilayer allows generating patterns with sub 500 nm lateral resolution, which is impossible to create by elastic contact lithography (ECL) of a single layer film due to strong surface tension effects in ultrathin films. Thus, control of elastic instability in a bilayer with a patterned stamp represents a flexible soft lithography tool allowing modulation of length scales, morphology, and order.     

Combined laser interference and photolithography patterning of a hybrid mask mold for nanoimprint lithography

A lithography technique that combines laser interference lithography (LIL) and photolithography, which can be a valuable technique for the low cost production of microscale and nanoscale hybrid mask molds, is proposed. LIL is a maskless process which allows the production of periodic nanoscale structures quickly, uniformly, and over large areas. A 257 nm wavelength Ar-Ion laser is utilized for the LIL process incorporating a Lloyd's mirror one beam inteferometer. By combining LIL with photolithography, the non-selective patterning limitation of LIL are explored and the design and development of a hybrid mask mold for nanoimprint lithography process, with uniform two-dimensional nanoscale patterns are presented. Polydimethylsiloxane is applied on the mold to fabricate a replica of the stamp. Through nanoimprint lithography using the manufactured replica, successful transfer of the patterns is achieved, and selective nanoscale patterning is confirmed with pattern sizes of around 180 nm and pattern aspect ratio of around 1.44:1.