Polydimethylsiloxane Mold Research Articles

Fabrication of circular microfluidic channels by combining mechanical micromilling and soft lithography

The fabrication of microfluidic channels with complex three-dimensional (3D) geometries presents a major challenge to the field of microfluidics, because conventional lithography methods are mainly limited to rectangular cross-sections. In this paper, we demonstrate the use of mechanical micromachining to fabricate microfluidic channels with complex cross-sectional geometries. Micro-scale milling tools are first used to fabricate semi-circular patterns on planar metallic surfaces to create a master mold. The micromilled pattern is then transferred to polydimethylsiloxane (PDMS) through a two-step reverse molding process. Using these semi-circular PDMS channels, circular cross-sectioned microchannels are created by aligning and adhering two channels face-to-face. Straight and serpentine-shaped microchannels were fabricated, and the channel geometry and precision of the metallic master and PDMS molds were assessed through scanning electron microscopy and non-contact profilometry. Channel functionality was tested by perfusion of liquid through the channels. This work demonstrates that micromachining enabled soft lithography is capable of fabricating non-rectangular cross-section channels for microfluidic applications. We believe that this approach will be important for many fields from biomimetics and vascular engineering to microfabrication and microreactor technologies.

A simple and rapid formation of wet chemical etched silicon nanowire films at the air–water interface

A spontaneous assembly route to form a thin film of nanowires (NWs) was demonstrated and its feasibility was confirmed through the fabrication of a high-performance multi-Si NW field effect transistor (FET) using this route. Governed by the three mechanisms of spreading, trapping, and two-dimensional packing, the route was optimized for the concentration of Si NWs and the initial volume ratio of aqueous hydrochloride solution to isopropyl alcohol. The successfully formed Si NW thin-film was transferred on a flat polydimethylsiloxane (PDMS) mold and regulated using a repeatable conformal contact method with a new flat PDMS to prepare it for decal printing on an organic dielectric layer. Finally, after depositing the source and drain electrodes on the printed active layer, a high-performance 23-bridged Si NW FET exhibiting a μeff of 51.4 cm2 V−1s−1, an on/off drain current ratio of 105, and a Vth of −2.7 V was obtained.

Simultaneous and Dual Emissive Imaging by Micro‐Contact Printing on the Surface of Electrostatically Assembled Water‐Soluble Poly(p‐phenylene) Using FRET

AbstractPoly{[2,5‐bis(3‐sulfonatobutoxy)‐1,4‐phenylene sodium salt]‐alt‐(1,4‐phenylene)}, which is an anionically charged, water‐soluble poly(para‐phenylene) derivative with aldehyde groups at both chain ends, is prepared via the Suzuki coupling reaction in order to develop a FRET energy donor, while simultaneously dual‐fluorescence‐patterning the protein. Regardless of the end‐capping, the synthesized polymer exhibits a good solubility in water with an absorption maximum at 338 nm and a photoluminescence maximum at 417 nm, similar to those of the the end‐capped polymer. The emission spectrum of the polymer overlaps the absorption spectrum of fluorescein, and therefore, the polymer can be used as an energy donor with fluorescein as the energy acceptor in the FRET mechanism. This polymer design not only takes advantage of the introduction of biotin at both chain ends (through a reaction with the aldehyde end groups) to realize the facile interaction with streptavidin, but also brings into play the electrostatic features of the anionic sulfonate groups to fabricate an electrostatic self‐assembly with polycation for the pattern substrate. The micropattern of fluorescein‐labeled streptavidin is fabricated on the polymer‐coated substrate through micro‐contact printing using a polydimethylsiloxane mold. As a result, the polymer substrate exhibits a dual fluorescence micropattern, which results from the blue emission color from the energy donor and the FRET‐amplified green emission from the energy acceptor. The high‐resolution patterning is carried out for the application of multiplexing by simultaneously imaging the patterned green‐emitting fluorescein by FRET and the surrounding blue‐emitting polymer according to an optical detection scheme.

Simultaneous and Dual Emissive Imaging by Micro‐Contact Printing on the Surface of Electrostatically Assembled Water‐Soluble Poly(p‐phenylene) Using FRET

AbstractPoly{[2,5‐bis(3‐sulfonatobutoxy)‐1,4‐phenylene sodium salt]‐alt‐(1,4‐phenylene)}, which is an anionically charged, water‐soluble poly(para‐phenylene) derivative with aldehyde groups at both chain ends, is prepared via the Suzuki coupling reaction in order to develop a FRET energy donor, while simultaneously dual‐fluorescence‐patterning the protein. Regardless of the end‐capping, the synthesized polymer exhibits a good solubility in water with an absorption maximum at 338 nm and a photoluminescence maximum at 417 nm, similar to those of the the end‐capped polymer. The emission spectrum of the polymer overlaps the absorption spectrum of fluorescein, and therefore, the polymer can be used as an energy donor with fluorescein as the energy acceptor in the FRET mechanism. This polymer design not only takes advantage of the introduction of biotin at both chain ends (through a reaction with the aldehyde end groups) to realize the facile interaction with streptavidin, but also brings into play the electrostatic features of the anionic sulfonate groups to fabricate an electrostatic self‐assembly with polycation for the pattern substrate. The micropattern of fluorescein‐labeled streptavidin is fabricated on the polymer‐coated substrate through micro‐contact printing using a polydimethylsiloxane mold. As a result, the polymer substrate exhibits a dual fluorescence micropattern, which results from the blue emission color from the energy donor and the FRET‐amplified green emission from the energy acceptor. The high‐resolution patterning is carried out for the application of multiplexing by simultaneously imaging the patterned green‐emitting fluorescein by FRET and the surrounding blue‐emitting polymer according to an optical detection scheme.

Electrochromic Diffraction from Nanopatterned Poly(3-hexylthiophene)

Poly(3-hexylthiophene) (P3HT) films were patterned by a soft lithography technique using a nanopatterned polydimethylsiloxane (PDMS) mold to generate one-dimensional (1D) grating and two-dimensional (2D) crossed line pillar patterns. The redox currents (i(p)) were significantly increased due to the facilitated diffusion of ClO4(-) counterions associated with redox processes at the P3HT electrode as analyzing cyclic voltammetry (CV) was performed at different scan rates (ν). It was found that the diffusion coefficient (D(f), cm(2) s(-1)) for ion diffusion in the patterned electrode was much larger than that of the pristine P3HT electrode. Furthermore, the value of D(f) in the 2D electrode was three times higher than that in a pristine film. As a result of such facilitated charge transport, the electrochromic (EC) properties of the patterned P3HT electrode were greatly enhanced and dependent on the dimension of the pattern. Thus, the electrochromic efficiency (E(e)), including the coloration (E(c)) and bleaching efficiencies (E(b)), was higher as the dimension of the pattern was increased; E(e) was maximized in the 2D patterned P3HT film. In a patterned cell, electrochromic diffraction was reversibly observed with a switching efficiency (R(DE)) of 2 and 2.5 for the 1D and 2D patterned cells, respectively.

Direct fabricating patterns using stamping transfer process with PDMS mold of hydrophobic nanostructures on surface of micro-cavity

The transfer stamping process has been used to fabricate thin-film pattern in recent years. Due to the characteristics of the materials of molds and inks, residual inks on the cavities of mold and residual layers on the substrate are still a problem. To solve the problem, we present a concept for fabrication of hydrophobic nanostructures on the cavities of microstructures of the mold, which can effectively decrease the ink residing on the cavities of the mold during coating. First, the periodic nanopores are fabricated on the anodic aluminum oxide (AAO). Second, AAO membrane is employed as the template for fabricating nanostructures on the PC film by embossing. And then, by partial protrusion of the nano-structured PC film into the micro-holes of the mold, an array of protruded convex microstructures is formed. After that, polydimethylsiloxane (PDMS) mold is casted from the embossed PC film. The contact angle of nanostructures on the micro-cavities of PDMS mold is about 145°. Micro-patterns with no residual layers have been successfully transferred on the poly(ethylene terephthalate) (PET) substrate using a transfer stamping process with this PDMS mold.

Fabrication of Silicon Carbide Micropillar Arrays from Polycarbosilanes

Periodic patterns of silicon carbide (SiC) micropillars of 18 μm in length and 2 μm in diameter with a hexagonal morphology and an areal number density of micropillars of 6 × 104 mm−2 were fabricated from a mixture of two different polycarbosilane ceramic precursors. The polycarbosilanes (PCS) were molded into polydimethylsiloxane (PDMS) micromolds processed from a silicon wafer master. After cross-linking between 200° and 400°C, debonding from the elastomeric PDMS mold yielded freestanding structures of PCS micropillars. Due to the low stiffness of cross-linked PCS (E∼2.5 GPa), surface adhesion forces might result in the deformation of the micropillars into ordered domains. The PCS micropillar arrays were pyrolyzed in inert atmosphere at 900° and 1500°C to yield amorphous and nanocrystalline SiC (β-SiC) with crystallite sizes in the range of tenths of micrometers, respectively. The formation of domains due to instability of micropillars with a high aspect ratio might be of specific interest for electrical engineering and catalytic applications.

나노입자 마스크를 이용하여 제작한 초소수성 마이크로-나노 혼성구조

소수성 고분자를 사용하여 제작한 마이크로구조에 금 나노입자를 마스크로 이용하는 반응성이온식각(RIE: Reactive Ion Etching)을 적용하여 초소수성을 갖는 마이크로-나노 혼성구조를 제작하였다. 소수성 고분자로는 PFPE (perfluoropolyether bisurethane methacrylate)를 사용하였으며 마이크로 단일구조는 PDMS (polydimethylsiloxane) 몰드를 사용하는 스탬핑 방식으로 제작하였다. 다양한 형태로 제작한 PFPE 마이크로 단일구조와 마이크로-나노 혼성구조의 표면 접촉각을 측정하여 표면 미세구조에 따른 소수성의 변화를 관찰하였다. 마이크로 단일구조의 경우 접촉각은 안정적인 값을 보이지 못하였으나 단일 구조에 나노입자를 사용한 식각을 적용해 나노구조가 형성됨에 따라 <TEX>$150^{\circ}$</TEX> 이상의 접촉각을 갖는 초소수성 표면이 매우 높은 재현성으로 용이하게 형성되었다. Superhydrophobic micro-nano hybrid structures were fabricated by reactive ion etching of hydrophobic polymer micro patterns using gold nanoparticles as etch masks. Micro structures of perfluoropolyether bisurethane methacrylate (PFPE) were prepared by soft-lithographic technique using polydimethylsiloxane (PDMS) molds. Water contact angles on the surfaces of various PFPE micro structures and corresponding micro-nano hybrid structures were compared to examine the effects of micro patterning and nanostructure formation in the manifestation of superhydrophobicity. The PFPE micro-nano hybrid structures exhibited a very stable superhydrophobicity, while the micro-only structures could not reach the superhydrophobicity but only showed the unstable hydrophobicity.

Fabrication of Complex Patterns with a Wide Range of Feature Sizes from a Single Line Prepattern by Successive Application of Capillary Force Lithography

The variously shaped gold patterns can be generated from the polydimethylsiloxane (PDMS) mold using line patterns by the capillary force lithography (CFL) process, which is a kind of nanoimprint method following the two-cycle method. After fabrication of micro- or nanosized line patterns at the first cycle, the patterned substrate is used as a substrate for the second cycle of CFL. When the other stamp is placed on the first pattern, rotated by a certain angle with respect to the first stamp, only the overlapped parts remained dot-shaped after the etching process. The various shapes and sizes of patterns can be produced by controlling the CFL conditions such as polymer thickness, reactive ion etching (RIE) time, and degree of rotation angle. The key advantage of the double imprint lithography method is to get the nanosized isolated dot-shaped patterns from microsized line patterns. If we fabricate nanosized isolated dot-shaped patterns directly, we should need predesigned patterns in the form of a master, which is generally prepared by a high-cost and time-consuming process such as E-beam lithography. The successful applications of large-area periodic patterns are nanoelectronic devices, nanoelectromechanical system (NEMS), and biosensors, the template of which is the master of nanoimprint lithography (NIL) and stamp fabrication in soft lithography.

A gasbag-roller-assisted UV imprinting technique for fabrication of a microlens array on a PMMA substrate

Roller imprinting has been one of the most effective replication techniques due to its continuous nature and mass productivity. To improve the pressure uniformity and enhance the replication precision, a novel gasbag-roller-assisted UV imprinting process is designed and developed in this study. A belt-type polydimethylsiloxane (PDMS) mold with cavities for forming a microlens array is prepared. A gasbag roller is employed to exert pressure on the contact area between the PDMS mold and a PMMA substrate. Due to the isotropic pressure provided by the gasbag roller, the contact area between the mold and the substrate is increased and the uniformity of the imprinting pressure is enhanced. Precise replication and complete UV curing are thus achieved. A gasbag-roller-assisted UV-based imprinting facility is constructed, and the experiments are conducted. The results prove that a microlens array can be successfully fabricated on a 2 mm thick PMMA substrate. The shape profile and optical property of the microlens are also evaluated.

Nanoparticle-Based Surface Templating

This study is focused on making molds with micrometer-sized feature arrays, studying high solids loading nanoparticle suspension for nanoparticle-based templating, and cast forming of surface-templated materials. Polydimethylsiloxane (PDMS) and silicone molds with micrometer-sized feature arrays are made using mold cores templated by a focused ion beam microscope. Al2O3 nanoparticle suspensions are evaluated based on nanoparticle interaction energy and suspension flowability in order to obtain stable and high solids loading suspensions. Templated feature transfer ability is compared for the PDMS and silicon molds under ambient and freeze-drying conditions. This work provides a new approach for surface templating of nanoparticle-based materials.

SiC ceramic micropatterns from polycarbosilanes

Micropatterned SiC ceramics were fabricated from polycarbosilanes applying a softlithographic replication technique. A polydimethylsiloxane mould replicated from a photolithographic microstructured silicon wafer was used as master structure. The polydimethylsiloxane mould was coated with a solution containing a mixture of two different polycarbosilanes in n-octane. After treatment at 200–400 °C the cross-linked polycarbosilane films were debonded and pyrolysed at 900 °C in nitrogen and subsequently crystallised at temperatures up to 1500 °C in argon. The cross-linking and thermal degradation behaviour of the polycarbosilanes was investigated by Fourier-transform infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. X-ray diffractrometry showed the expected development of a nanocrystalline β-SiC (3 nm) as the main phase with increasing temperature. However, traces of α-SiO 2 derived from the polycarbosilane precursors were also detected by X-ray analysis. Removal of the α-SiO 2 dioxide with hydrofluoric acid in the pyrolysed samples and subsequent increased the crystallite size to 7 nm. The Young's modulus determined by nanoindentation was increased from 3 GPa after cross-linking to 110 GPa after crystallisation. Scanning electron microscopy revealed, that the initial micropatterns were fully retained in the pyrolysed and crystallised SiC ceramics. The micropatterned cross-linked and crystallised β-SiC based substrates exhibited light scattering characteristics, which qualify them as promising candidates for diffractive optical elements in microoptical applications.

Decreased lung carcinoma cell density on select polymer nanometer surface features for lung replacement therapies

Poly(lactic-co-glycolic) acid (PLGA) has been widely used as a biomaterial in regenerative medicine because of its biocompatibility and biodegradability properties. Previous studies have shown that cells (such as bladder smooth muscle cells, chondrocytes, and osteoblasts) respond differently to nanostructured PLGA surfaces compared with nanosmooth surfaces. The purpose of the present in vitro research was to prepare PLGA films with various nanometer surface features and determine whether lung cancer epithelial cells respond differently to such topographies. To create nanosurface features on PLGA, different sized (190 nm, 300 nm, 400 nm, and 530 nm diameter) polystyrene beads were used to cast polydimethylsiloxane (PDMS) molds which were used as templates to create nanofeatured PLGA films. Atomic force microscopy (AFM) images and root mean square roughness (RMS) values indicated that the intended spherical surface nanotopographies on PLGA with RMS values of 2.23, 5.03, 5.42, and 36.90 nm were formed by employing 190, 300, 400, and 530 nm beads. A solution evaporation method was also utilized to modify PLGA surface features by using 8 wt% (to obtain an AFM RMS value of 0.62 nm) and 4 wt% (to obtain an AFM RMS value of 2.23 nm) PLGA in chloroform solutions. Most importantly, lung cancer epithelial cells adhered less on the PLGA surfaces with RMS values of 0.62, 2.23, and 5.42 nm after four hours of culture compared with any other PLGA surface created here. After three days, PLGA surfaces with an RMS value of 0.62 nm had much lower cell density than any other sample. In this manner, PLGA with specific nanometer surface features may inhibit lung cancer cell density which may provide an important biomaterial for the treatment of lung cancer (from drug delivery to regenerative medicine).

Pressure dependent current-controllable devices based on organic thin film transistors by soft-contact lamination

Soft-contact lamination is a versatile technology for fabricating unconventional electronic devices, and also imparts an efficient functionality to organic thin film transistors (OTFTs). In this study, a pressure sensor based on this technique was fabricated using an elastic polydimethylsiloxane (PDMS) mold with gold (Au) electrodes. Since the effective channel length of the laminated OTFT could be controlled by external pressure on the structured PDMS mold, the changes in source–drain output currents also corresponded to the external loads without any change of the material properties. Owing to the elastic properties of the PDMS mold, the laminated device returned to its former structure after the external pressure was released. Based on this device property, reversible and time-durable operations of the device were demonstrated.

Enhancement of light extraction in GaN based LED structures using TiO2 nano-structures

TiO2 nano-patterns were formed on the indium-tin-oxide (ITO) layer of GaN based light emitting diodes (LEDs) without a residual layer using a sol-imprinting process. A polydimethylsiloxane mold replicated from a Si master was used as the imprint stamp for the sol-imprinting process. The light extraction efficiency of LEDs was enhanced by the TiO2 nano-patterns formed on the ITO layer because the TiO2 nano-patterns locally modulated the refractive index of the ITO layer and enhanced the scattering of light at the ITO layer. Consequently, directly fabricated TiO2 nano-patterns on the ITO layer can esoln hance the light extraction efficiency of LEDs without plasma-induced damage.

Fast replication of out-of-plane microlens with polydimethylsiloxane and curable polymer (NOA73)

Out-of-plane microlens, as its in-plane counterpart, is an important micro optics component that can be used in building integrated micro-optic systems for many applications. In earlier publications from our group, an ultra violet (UV) lithography based technique for out-of-plane microlens fabrication was reported. In this paper, we report a replication technology for time-efficient fabrication of out-of-plane microlens made of a curable polymer, NOA73. Microlens of cured SU-8 polymer was fabricated using a unique tilted UV lithography process, polydimethylsiloxane (PDMS) was molded using the resulting SU-8 master to form a negative mold, curable polymer NOA73 was then casted in the PDMS mold and out-of-plane microlens replica made of NOA73 was finally obtained after curing. The entire replication process took less than 5 h. Since PDMS negative mold was reusable, multiple replications of the microlens could be done with the same mold and each replication only took about 30 min. Scanning electron microscopic (SEM) images showed that NOA73 microlens replica had almost identical shape as the SU-8 master. In Comparison to the SU-8 microlens, microlens replica of UV curable polymer had slightly longer focal length and smaller numerical aperture due to the lower refractive index of NOA73. In addition, NOA73 microlens replica also had improved spectral transmission. Because of its compatibility with soft lithography technique, the reported replication process may also be used to integrate out-of-plane microlens into micro-opto-electro-mechanical-systems (MOEMS) and BioMEMS chips.

Fabrication of two-dimensional dimple and conical microlens arrays from a highly periodic toroidal-shaped liquid crystal defect array

A self-assembly fabrication method was developed for the preparation of microlens arrays (MLAs). The procedure used the focal conic structures of semi-fluorinated smectic liquid crystals (LCs), the periodic toric focal conic domains (TFCDs), which were prepared on a surface-modified substrate. This LC-based MLA system focuses light via the intrinsic molecular orientations of the TFCDs, leading to a highly efficient MLA with good optical properties. The thickness of the smectic LC film could be used to control both the microlens feature size, over the range 5–15 μm, and the microlens focal length, over the range 1–3 μm. In addition, we prepared two-dimensional hexagonally packed polymer MLAs with conical or dimple shapes by successive replica molding, using a UV-curable photopolymer (NOA63) and polydimethylsiloxane (PDMS), from a TFCD array template containing a dimple structure. The LC-based TFCD MLAs and the secondary replicated polymer MLAs, using NOA63 and PDMS molds, showed good lens performances. We anticipate that this LC self-assembly method will be applicable to the large-scale fabrication of MLAs. The method allows fabrication of dynamic MLAs that are responsive to external fields, such as electric or magnetic fields, or to thermal variations.

Sub-200nm gap electrodes by soft UV nanoimprint lithography using polydimethylsiloxane mold without external pressure

In this article, the authors present an alternative approach to electron beam lithography allowing the fabrication of gap electrodes with dimensions less than 200nm. This approach consists of the soft ultraviolet nanoimprint lithography on nonstandard sample sizes, using polydimethylsiloxane (PDMS) as a flexible mold material. By simple deposit of the PDMS mold on the substrate, they succeeded in imprinting gaps in Amonil and the transfer of patterns of polymethylmethacrylate/Amonil bilayer was achieved by etching the substrate with a suitable reactive ion etching process. The imprint pressure was ensured by only the weight of the PDMS mold.

Micromolded arrays for separation of adherent cells.

We present an efficient, yet inexpensive, approach for isolating viable single cells or colonies from a mixed population. This cell microarray platform possesses innovations in both the array manufacture and the manner of target cell release. Arrays of microwells with bases composed of detachable concave elements, termed microrafts, were fabricated by a dip-coating process using a polydimethylsiloxane mold as the template and the array substrate. This manufacturing approach enabled the use of materials other than photoresists to create the array elements. Thus microrafts possessing low autofluorescence could be fabricated for fluorescence-based identification of cells. Cells plated on the microarray settled and attached at the center of the wells due to the microrafts' concavity. Individual microrafts were readily dislodged by the action of a needle inserted through the compliant polymer substrate. The hard polymer material (polystyrene or epoxy resin) of which the microrafts were composed protected the cells from damage by the needle. For cell analysis and isolation, cells of interest were identified using a standard inverted microscope and microrafts carrying target cells were dislodged with the needle. The released cells/microrafts could be efficiently collected, cultured and clonally expanded. During the separation and collection procedures, the cells remained adherent and provided a measure of protection during manipulation, thus providing an extremely high single-cell cloning rate (>95%). Generation of a transfected cell line based on expression of a fluorescent protein demonstrated an important application for performing on-chip cell separations.

Room-temperature capillary-imprint lithography for making micro-/nanostructures in large areas

Compared to imprint techniques under the driving condition of external imprint pressure, imprint lithography under capillary force has the advantages of being suitable to produce large feature patterns, simple mold structure, very low imprint pressure, and conformal contact between the mold and the substrate. However, in existing capillary-force lithography, the necessary heating process will bring problems, such as separation between the mold and the substrate, long imprint time, shorter lifetime of the mold, etc. In this article, the authors propose a technique of room-temperature capillary-imprint lithography which is applied to making both micro- and nanostructures simultaneously in large areas under low temperature. In order to prevent the polydimethylsiloxane mold from swelling in the imprint process, a new ultraviolet curable resist is introduced in the imprint process. This resist contains no solvents and has a wide range of viscosity. Through the experiment, three-dimensional microstructures are replicated in large areas on the flat substrate as well as on curved substrate. Moreover, by utilizing a resist with low viscosity, combination features with linewidth of 1 μm and 100 nm are reproduced successfully in the resist by the imprint technique.