PDMS Replica Research Articles

A Pulse Width Modulated Microfluidic Diluter

The preparation of solutions of different concentrations is often an essential part of chemical or biological assays. We have developed a pulsed microfluidic dilution concept, suitably for flexible programming of an accurate output solution concentration. Only minute amounts of chemicals are needed and dilution series with high resolution can be generated. The concept of our dilution chip is similar to digital analog conversion in electronics, using pulse width modulation (PWM).By means of PDMS replica molding, a multi-stages PWM diluter has been constructed. Fluorescence imaging protocols with microscope/SLR camera as well as electrochemical probing with microelectrodes were employed for characterization and calibration, showing that the design allows for accurate dilution over 2 orders of magnitudes with high controllability, and at the same time minimal external components. This device concept can be applied in stand-alone diluter circuits or as a component in more sophisticated fully integrated analytical devices.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Preparation of material surface structure similar to hydrophobic structure of lotus leaf

Nano/micro replication, a technique widely applied in the microelectronics field, was introduced to prepare the hydrophobic bionics microstructure on material surface. Poly(vinyl alcohol) (PVA) and polystyrene (PS) moulds of the mastoid microstructure on lotus leaf surface were prepared respectively by the nano/micro replication technology. And poly(dimethylsiloxane) (PDMS) replicas with the mastoid-like microstructure were prepared from these two kinds of polymer moulds. Scanning electronic microscope (SEM) was employed to investigate the morphology and microstructures on moulds and replicas. Both the static and dynamic contact angles between water droplet and PDMS replicas’ surface were also measured. As a result, similar microstructure can be observed clearly on the surface of PDMS replicas and the static contact angle on PDMS replicas was enhanced dramatically by the existence of these microstructures.

Micropatterning of proteins on the surface of three-dimensional poly(ethylene glycol) hydrogel microstructures

This paper describes micropatterning of proteins on the surface of three-dimensional hydrogel microstructures. Poly(ethylene glycol) (PEG)-based hydrogel microstructures were fabricated on a glass substrate using a poly(dimethylsiloxane) (PDMS) replica as a molding insert and photolithography. The lateral dimension and height of the hydrogel microstructures were easily controlled by the feature size of the photomask and depth of the PDMS replica, respectively. Bovine serum albumin (BSA), a model protein, was covalently immobilized to the surface of the hydrogel microstructure via a 5-azidonitrobenzoyloxy N-hydroxysuccinimide bifunctional linker at a surface density of 1.48 mg cm −2. The immobilization of BSA on the PEG hydrogel surface was demonstrated with XPS by confirming the formation of a new nitrogen peak, and the selective immobilization of fluorescent-labeled BSA on the outer region of the three-dimensional hydrogel micropattern was demonstrated by fluorescence. A hydrogel microstructure could immobilize two different enzymes separately, and sequential bienzymatic reaction was demonstrated by reacting glucose and Amplex Red with a hydrogel microstructure where glucose oxidase was immobilized on the surface and peroxidase was encapsulated. Activity of immobilized glucose oxidase was 16.5 U mg −1, and different glucose concentration ranged from 0.1 to 20 mM could be successfully detected.

Fabrication of complex multilevel microchannels in PDMS by using three-dimensional photoresist masters

This paper demonstrates a new method of implementing complex microchannels in PDMS, which is simply constructed using three-dimensional photoresist structures as a master mold for the PDMS replica process. The process utilizes UV-insensitive LOR resist as a sacrificial layer to levitate the structural photoresist. In addition, the thickness of photoresist structures can be controlled by multi-step UV exposure. By using these techniques, various three-dimensional photoresist structures were successfully implemented, including the recessed cantilevers, suspended bridges, and the complex plates with micro-pits or micro-villi. We demonstrate that the three-dimensional photoresist structures are applicable to implementing complex multiple microchannels in PDMS by using the PDMS replica method.

Use of hydrogel microstructures as templates for protein immobilization

AbstractIn this study, protein micropatterns were created on the surface of three-dimensional hydrogel microstructures. Poly(ethylene glycol)(PEG)-based hydrogel microstructures were fabricated on a glass substrate using a poly(dimethylsiloxane) (PDMS) replica as a molding insert and photolithography. The lateral dimension and height of the hydrogel microstructures were easily controlled by the feature size of the photomask and depth of the PDMS replica, respectively. Bovine serum albumin (BSA), a model protein, was covalently immobilized to the surface of the hydrogel microstructure via a 5-azidonitrobenzoyloxy N-hydroxysuccinimide bifunctional linker, which has a phenyl azide group and a protein-binding N-hydroxysuccinimide group on either end. The immobilization of BSA on the PEG hydrogel surface was demonstrated with XPS by confirming the formation of a new nitrogen peak, and the selective immobilization of fluorescent-labeled BSA on the outer region of the three-dimensional hydrogel micropattern was demonstrated by fluorescence. A hydrogel microstructure could immobilize two different enzymes separately, and sequential bienzymatic reaction was demonstrated by reacting glucose and Amplex Red with a hydrogel microstructure where glucose oxidase was immobilized on the surface and peroxidase was encapsulated.

Generation of Various Complex Patterned Structures From a Single Ellipsoidal Dot Prepattern by Capillary Force Lithography

Creating many different complex structures from a single prepattern is achieved from either a master mold or stamping onto a solid substrate by patterning with a PDMS replica mold. Various metal patterns including ellipses, dots, rings, chains of dots (or rings), ribbons and wavy lines with various feature sizes can be generated from a single pattern of isolated ellipsoidal dots by simply varying the residual polymer film thickness and the reactive ion etching (RIE) time.

Parallel Fabrication of RNA Microarrays by Mechanical Transfer from a DNA Master

A new method for fabrication of RNA microarrays is described. The approach involves cohybridization of a short, biotinylated DNA oligonucleotide and an RNA probe sequence to DNA templates spotted onto a master array. Next, the short DNA sequence and the RNA probe are linked using a T4 DNA ligase. Finally, a poly(dimethylsiloxane) (PDMS) monolith modified on the surface with streptavidin is brought into conformal contact with the master array. This results in binding of the biotinylated DNA/RNA oligonucleotides to the PDMS surface. When the two substrates are mechanically separated, the DNA/RNA oligonucleotides transfer to the PDMS replica, and the DNA oligonucleotides remaining on the master array are ready to template another RNA replica array. This sequence can be repeated for at least 18 cycles using a single master array. RNA arrays consisting of up to three different oligonucleotide sequences and consisting of up to 2500 individual approximately 70 microm spots have been prepared.

Rapid Fabrication of Micro‐ and Nanoscale Patterns by Replica Molding from Diatom Biosilica

AbstractDiatoms are single‐celled micro‐algae that possess an exoskeleton (called frustule) comprised of diverse and highly ordered 3D porous silica structures and that hold considerable promise for biological or biomimetic fabrication of nanostructured materials and devices. We have used, for the first time, a soft lithographic approach of replica molding to replicate porous diatom structures into polymers. Two centric diatom species, Coscinodiscus sp., Thalassiosira eccentrica cultured in our laboratory were used as masters for replication. In the first step, replica molding onto soft and elastic polymer using poly(dimethylsiloxane) PDMS produced a negative replica of the diatom frustule. These PDMS replicas were then used as a mold to fabricate the positive polymer replicas of diatoms using a mercaptol ester type UV curable polymer (NOA 60). Fabricated polymer replicas were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). In all cases, diatom micro‐ and nanoscale porous structures were successfully transferred with high precision into polymer replicas. Such an accomplishment effectively demonstrates the potential for using diatoms as blueprints for rapid and simple fabrication of polymer nanostructures. The prepared replicas were used as diffraction gratings and as nanowells to hold polymeric nanoparticles effectively demonstrating the functional properties of these biomimetic structures.

Polyimide liquid crystal alignment layers prepared by soft-lithography

Elastomeric replicas of surface relief structures were prepared by molding poly(dimethylsiloxane) (PDMS) precursors against photo-inscribed surface-relief-gratings on azo polymer films. The PI solutions were micro-contact printed with the elastomeric PDMS replica on quartz slides. Good surface-relief-grating structures were formed on the quartz slides. The quartz slides covered with surface-relief-grating polymide (PI) films were then assembled into liquid crystal (LC) cells. The transmittance passing through the cell between crossed polarizers changed periodically with a regular 90° separation of the rotational angle. The pretilt angle of the cell was found to be 2.8°. Results showed that the PI films with surface-relief-grating structures by this micro-contact printing process have good liquid crystal alignment ability. This preparation method of alignment layers can be considered a potentially useful technique in the LC display (LCD) industry in the future.

Fabrication of UV epoxy resin masters for the replication of PDMS-based microchips

This paper presents two methods for the fabrication of UV epoxy resin masters for the replication of PDMS-based microfluidic chips. In the first method, the epoxy resin master is fabricated from a negative glass template manufactured using conventional lithography and wet etching techniques. However, in the second method, the master is produced simply by exposing a layer of UV epoxy resin coated on a glass substrate. Although the first method enables the replication of multiple PDMS structures from a single master, the latter method avoids the requirement for a wet chemical etching process and enables the epoxy master to be produced in 40 min or less. The experimental results show that the epoxy resin masters enable the mass production of PDMS replicas with highly precise geometrical tolerances. A series of electrokinetic focusing experiments are performed using PDMS microchips replicated from the current epoxy resin masters. The experimental results obtained for the width of the electrokinetically-focused sample stream under different focusing ratios are found to be in good agreement with the theoretical predictions. The sample handling characteristics of the microfluidic chips are also investigated. It is shown that the sample flow can be electrokinetically pre-focused into a narrow stream and then guided to the desired outlet port by applying a simple voltage control model. Finally, it is demonstrated that through an appropriate alignment of the sample flow and the conductivity gradient, the electrokinetic instability phenomenon can be induced at a relatively low electrical field strength of 0.35 kV/cm.

Microfabrication of PDMS microchannels using SU-8/PMMA moldings and their sealing topolystyrene substrates

In this paper we show increased interfacial bonding of a photoresist (SU-8) depositedonto highly polished poly(methyl methacrylate) (PMMA) substrates containing aUV absorber. We also demonstrate the facile sealing/covalent bonding of100 µm × 20 µm PDMS replica microchannels, made from these PMMA/SU-8 moldings, onto thermallypolymerized styrene. Sealing was achieved through radio-frequency plasma discharge(RFPD) oxygen plasma treatment.

Biomimetic materials replicating Schwann cell topography enhance neuronal adhesion and neurite alignment in vitro

It is well established that Schwann cells (SCs) promote and enhance axon guidance and nerve regeneration by providing multiple cues, including extracellular matrix, cell surface molecules, neurotrophic factors and cellular topography. Which of the elements of the complex environment associated with SCs provides the essential information for directed nerve growth is unclear, because, until now, it has been impossible to investigate their contributions individually. Our development of biomimetic materials that replicate the micro- and nanoscale topography of SCs has allowed us to investigate for the first time the role of cellular topography in directing nerve growth. Dorsal root ganglion (DRG) neurons were cultured on flat poly(dimethyl siloxane) (PDMS) and on PDMS replicas with protruding SC topography. Image analysis showed that more neurons adhered to the replicas than to the flat substrates, and that neurite growth on the replicas followed the underlying SC pattern. Neuronal alignment was dependent on cell density. Live SCs derived from the DRG also grew along the replica SC pattern. These results suggest that the combination of micro- and nanoscale topographical cues provided by SCs can influence nerve growth and point toward design parameters for future nerve guidance channels.

Fabrication of PDMS/glass microchips by twofold replication of PDMS and its application in genetic analysis

In this paper, we describe a simple method for fabrication of high quality poly(dimethylsiloxane) (PDMS)/glass microchip by twofold replica molding of PDMS. This technique first served to transfer the negative microchannels from the glass template to the PDMS substrate as a master, and then this PDMS master with positive microchannels was used to replicate the PDMS replica with negative microchannels. Finally, the PDMS replica was bound to a glass sheet by UV radiation. The fabricated microchips were successfully applied for the detection of C677T mutation from the human methylenetetrahydrofolate reductase gene.

바이오 멤스 및 마이크로 시스템 적용을 위한 3차원 마이크로 유로 제작

This paper reports a new technology to implement complex PDMS microchannels, which are simply constructed using three-dimensional photoresist structures as mold for PDMS replica process. The process utilizes LOR resist as a sacrificial layer to levitate the structural photoresist and multi-step exposure to control the thicknesses of photoresist structures. Various shapes of photoresist structures were successfully fabricated. Using the PDMS replica method, the three-dimensional photoresist structures are demonstrated to be applicable for implementing complex microchannels in PDMS. In addition, more complex multilevel microchannels are constructed by bonding two PDMS layers with just single PDMS alignment.

Crystallization on Surfaces of Well-Defined Topography

Single crystals of calcite with regular patterned surfaces comprising close-packed arrays of hemispherical cavities or domes were produced by crystallization on colloidal monolayers or PDMS replicas of these monolayers, respectively. Perfect replication of the substrate topography was achieved for all colloidal particles, irrespective of their size and surface chemistry when the substrate geometry permitted unrestricted ion flow to the growing crystal. This work demonstrates that crystallization within a mould provides a very general route to producing single crystals with curved surfaces and unusual morphologies and that such patterning can be applied from the micro- to the nanoscale.

실리콘 결정면을 이용한 LCD-BLU용 도광판의 미세산란구조 형성

LCD-BLU(liquid crystal device-back light unit)에 사용되는 도광판의 미세 산란패턴을 만드는 새로운 방법으로서, 실리콘 웨이퍼의 비등방 식각에 의해 자연적으로 형성되는 3차원 결정면 구조를 이용하는 방법을 제안하였다. 실리콘 3차원 결정면을 갖는 도광판과 프리즘 시트의 원판을 설계 및 제작하였고, casting 공정을 통해 PDMS 재질로 복제된 도광판을 제작하여 특성을 분석하였다. 측정 결과, 기존 인쇄형 도광판에 비해 실리콘피라미드 패턴의 도광판이 <TEX>$10\%$</TEX> 증가된 정면 휘도 효율을 가질 수 있음을 실험적으로 검증하였다. Luminous efficiency and uniformity in a LCD-BLU are mainly determined by fine scattering patterns formed on the light guide panel. We propose a novel fabrication method of 3-dimensional scattered patterns based on anisotropic etching of silicon wafers. Micro-pyramid patterns with 70.5 degree apex-angle and micro-prism patterns with 109.4 degree apex-angle can be self-constructed by the wet, anisotropic etching of (100) and (110) silicon wafers, respectively, and those patterns are easily duplicated by the PDMS replica process. Experimental results on spatial and angular distributions of irradiation from the light guide panel with the micro-pyramid patterns were very consistent with the calculation results. Surface roughness of the silicon-based micro-patterns is free from any artificial defects since the micro-patterns are inherently formed with silicon crystal surfaces. Therefore, we expect that the silicon based micro-patterning process makes it possible to fabricate perfect 3-dimensional micro-structures with crystal surface and apex angles, which may guarantee mass-reproduction of the light guide panels in LCD-BLU.

Fabrication of Nano‐Precision PDMS Replica Using Two‐Photon Photopolymerization and Vacuum Pressure Difference Technique.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Fabrication of Nano-precision PDMS Replica Using Two-photon Photopolymerization and Vacuum Pressure Difference Technique

It is undoubtingly believed that a revolution in miniaturization, in the field of microelectromechanical systems (MEMS), nano- and biotechnology etc., is under way. Multidimensional micro structures are fabricated by mainly two methods, such as laser rapid prototyping and lithography. Since it is a process that builds features layer by layer, rapid prototyping is a very slow process and is difficult to fabricate smaller objects. 1 On the other hand, the conventional lithographic technique requires an etching process that limits to build complicated figures. Recently, two-photon photopolymerization has attracted increasing attention as an innovative tool for micro- and nanofabrication due to its intrinsic multi-dimensional fabrication capability and subdiffraction limited spatial resolution. 2,3 Two-photon absorption probability strongly depends quadratically on excitation beam intensity confining the process at the vicinity of the focus. Thus, by using two-photon excitation one can activate chemical and physical processes only for a tightly focused spot in the bulk medium with high spatial resolution. This substantial increase in structural complexity was only possible by using chromophores with high two-photon absorption cross-sections (σ ). It is apparent that the synthesis of chromophores with even larger two-photon activity will unleash improvements in the capability and performance of a variety of two-photon absorption application including multi-dimensional nano- and microfabrication. In this paper, we describe the synthesis of newly designed highly efficient two-photon chromophore and fabrication of three-dimensional nano-precision PDMS replica using vacuum pressure difference technique (VPDT) and twophoton photopolymerization. The two-photon absorbing chromophore 3 with long alkyl chain for improved solubility was synthesized using palladium catalyzed coupling reaction designed by Heck 4 from compound 1 and 2. The compound 1 was obtained from Vilsmeier-Haack fomylation of triphenylamine followed by Wittig raction, and the 2,7dibromofluorene was reacted with excess amount of ethylhexylbromide to give compound 2. The resulting twophoton chromophore 3 was extensively purified before use as a light sensitizer/photoinitiator in two-photon photopolymerization. 5 The optical transparency of a chromophore at the wavelength of operation is crucial for successful 3D microfabrication with two-photonpolymerzation. As shown in steady-state absorption and emission spectra (Figure 1), the two-photon chromophore 3 has no absorption beyond 475 nm making itself ideal for two-photon application at the wavelength of 780 nm which is wavelength of our interest. Two-photon absorption cross-section of chromophore 3 at 780 nm was found to be σ = 4.7 × 10 −48 cm 4 s/photon with

Prototyping of microfluidic devices in poly(dimethylsiloxane) using solid-object printing.

A solid-object printer was used to produce masters for the fabrication of microfluidic devices in poly(dimethylsiloxane) (PDMS). The printer provides an alternative to photolithography for applications where features of > 250 microm are needed. Solid-object printing is capable of delivering objects that have dimensions as large as 250 x 190 x 200 mm (x, y, z) with feature sizes that can range from 10 cm to 250 microm. The user designs a device in 3-D in a CAD program, and the CAD file is used by the printer to fabricate a master directly without the need for a mask. The printer can produce complex structures, including multilevel features, in one unattended printing. The masters are robust and inexpensive and can be fabricated rapidly. Once a master was obtained, a PDMS replica was fabricated by molding against it and used to fabricate a microfluidic device. The capabilities of this method are demonstrated by fabricating devices that contain multilevel and tall features, devices that cover a large area (approximately 150 cm2), and devices that contain nonintersecting, crossing channels.

Photolithography with transparent reflective photomasks

A new type of photomask was fabricated by casting a prepolymer of a transparent, elastomeric polymer (polydimethylsiloxane, PDMS) against a Si(100) master whose surface has been patterned with V-shaped trenches or pyramidal pits using anisotropic etching. The PDMS replica, when placed in contact with a film of photoresist and illuminated, acts as a photomask. The sidewalls of the trenches and pits in the silicon master meet with the plateaus in dihedral angles of 54°; as a result, the PDMS replica selectively blocks the incident light in regions where it has sloping features by total internal reflection, and acts as a reflective contact mask for photolithography. The feasibility of this new type of photomask has been demonstrated by the fabrication of micropatterns in photoresist (and in an underlying silicon substrate) with smaller feature sizes and higher complexities than those present on the original chrome mask used in patterning the silicon master. The patterns produced using these elastomeric photomasks can be changed by varying the pressure applied in contacting them.