Micro-contact printing of polydiacetylene liposomes using hydrophilic stamps

Micron-sized polydiacetylene (PDA) liposome patterns have been fabricated on titanium (Ti) substrates using a micro-contact printing (micro-CP) technique. Two types of stamps (PDMS and agarose) and inking methods ("soaking" and "dropping") are used for micro-CP, and we compare their effect on the morphology of the PDA patterns. The size and morphology of the patterned PDA liposomes are analysized by optical and fluorescence microscopies and atomic force microscopy (AFM). When the agarose stamp is inked by the "dropping" method, PDA patterns are most efficiently transferred to the Ti substrate. However, the thickness of the transferred PDA patterns is not homogeneous, with the edge of the transferred pattern being thicker than its center. In contrast, when the PDMS stamp is used for micro-CP, the center of the pattern is thicker than the edge. Red fluorescence patterns are readily obtained by heat treatment of the PDA-immobilized solid substrate. The intensity of the fluorescence of the samples is consistent with the results of optical microscopy and AFM experiments.     

Carbon nanowires fabrications via top down approach

Carbon nanowires are fabricated by the Langmuir Blodgett (LB) method via the top down approach on amorphous carbon. Thick a-C films (500 nm to 1 microm) have been successfully deposited after the treatment on silicon. The anisotropic etching of carbon using reactive ion etching (RIE) has been verified giving near vertical sidewalls. The LB method for depositing monolayer requires a hydrophilic surface. Plasma treatment is being performed on the silicon oxide hard mask to reduce the surface energy thereby making the surface from hydrophobic to hydrophilic. PS balls which are being deposited by LB method have one disadvantage which is the low adhesion of the PS ball to the silicon oxide surface. This adhesion is being improved by subjecting the PS ball to annealing which changes the shape and increase the contact area between the PS balls and the silicon oxide surface. As carbon and PS ball is vulnerable to oxygen plasma, a modified recipe of CF4:Ar was being used to etch the silicon oxide hard mask. There is almost little chemical reaction of the CF4 on carbon and PS ball. Carbon nanowires were successfully fabricated using polystyrene (PS) balls of diameter 450 nm. Through a series of steps, carbon nanowire of 500 nm in length and diameter approximately 250 nm can be produced.     

Controllable and Facile Fabrication of Gold Nanostructures for Selective Metal‐Assisted Etching of Silicon

A method with the combination of organic‐vapor‐assisted polymer swelling and nanotransfer printing (nTP) is used to manufacture desirable patterns consisting of gold nano‐clusters on silicon wafers for Au‐assisted etching of silicon. This method remarkably benefits to the size control and regional selection of the deposited Au. By tuning the thickness of the Au films deposited on the polydimethylsiloxane (PDMS) stamps, along with the swelling of PDMS stamps in acetone atmosphere, the Au films are cracked into diverse nanostructures. These nanostructures are covalently transferred onto silicon substrates in a large scale and enable to accelerate the chemical etching of silicon. The etched areas are composed of porous structures which can be readily distinguished from the surroundings on optical microscope. PDMS stamps and the Au clusters provide the control over the feature of the etched areas and the porous silicon, respectively. The silicon surfaces with patterned porous features offer a platform for exploiting new functional templates, for example, they present a diversity of antireflective and fluorescent performance.     

Stable Non‐Covalent Large Area Patterning of Inert Teflon‐AF Surface: A New Approach to Multiscale Cell Guidance

Micro‐ and nano‐patterning of cell adhesion proteins is demonstrated to direct the growth of neural cells, viz. human neuroblastoma SHSY5Y, at precise positions on a strongly antifouling substrate of technolological interest. We adopt a soft‐lithographic approach with oxygen plasma modified PDMS stamps to pattern human laminin on Teflon‐AF films. These patterns are based on the interplay of capillary forces within the stamp and non‐covalent intermolecular and surface interactions. Remarkably, they remain stable for several days upon cell culture conditions. The fabrication of substrates with adjacent antifouling and adhesion‐promoting regions allows us to reach absolute spatial control in the positioning of neuroblastoma cells on the Teflon‐AF films. This patterning approach of a technologically‐relevant substrate can be of interest in tissue engineering and biosensing.     

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.     

Easy Fabrication of Electrically Insulating Nanogaps by Transfer Printing

An easy and cost‐effective method to reproducibly fabricate nanogaps over a large area is introduced. Gold is evaporated on low‐aspect‐ratio polydimethylsiloxane (PDMS) stamps at an angle of 60°. Afterwards, the stamp is brought into contact with a silicon/silicon dioxide substrate and subsequently peeled at rates varying from 1 to 3 mm s^?1, resulting in the fabrication of nanogaps between two gold electrodes. The fabrication of insulating nanogaps with a width down to 50 nm is demonstrated.     

Shear assay measurements of cell adhesion on biomaterials surfaces

The paper examines the adhesion of human osterosarcoma (HOS) cells to selected biomaterials surfaces that are relevant to implantable biomedical systems and bio-micro-electro-mechanical systems (BioMEMS). The four biomaterials that were explored include: silicon, silicon coated with a nanoscale layer of titanium, Ti–6Al–4V, and poly-di-methy-siloxane (PDMS). The interfacial strengths between the HOS cells and the biomaterials surfaces were determined using a shear assay technique. The adhesion forces were determined using a combination of confocal microscopy images of the three-dimensional cell structure, and computational fluid dynamics (CFD) simulations that coupled actual cell morphologies and non-Newtonian fluid properties in the computation of the adhesion forces. After cell detachment by the shear assay, immunofluorescence staining of the biomedical surfaces was used to reveal the proteins associated with cell detachment. These revealed that the nano-scale Ti coating increases the cell/surface adhesion strength. Silicon with Ti coating has the strongest adhesion strength, while the other surfaces had similar adhesion strength. The measured strengths are shown to be largely associated with the detachment of focal adhesion proteins from extra-cellular matrix (ECM) proteins.     

Formation and properties of surface-anchored amine-terminated poly(dimethylsiloxane) assemblies with tunable physico-chemical characteristics

Surface-anchored amine-terminated poly(dimethysiloxane) (PDMS) assemblies with tunable physico-chemical characteristics were fabricated with a simple two-step procedure. Firstly, 3-glycidoxypropylmethyldimethoxysilane (GPDMS) molecules were self-assembled on silicon surface, and then coupled to PDMS through a surface ring-opening reaction. The structure and morphology of the amine-terminated PDMS assemblies were characterized with various techniques such as ellipsometry, contact angle goniometer, grazing angle attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The GPDMS monolayers were truly monomolecular films with a virtually normal molecular orientation of densely packed molecules, which were firmly tethered to the hydroxylated silicon substrate. Self-assembly of PDMS molecules resulted in the formation of homogeneous films ~ 6.3 nm thick with the surface roughness ~ 0.898 nm. The calculation of grafting parameters from experimental measurements indicated that the presence of homogeneous and densely grafted PDMS films allowed us to predict a “brushlike” regime for the polymer chains in good solvents.     

Mechanical Characterization of Thin Films by Use of Atomic Force Acoustic Microscopy

The atomic force acoustic microscopy (AFAM) technique has been used to determine elastic properties of films with thicknesses decreasing from several hundreds of nanometers to several nanometers. It has been shown that metal films as thin as 50 nm can be characterized directly without the need to consider the influence of the substrate. For films with thicknesses ranging from about 30 to 50 nm, measurement parameters can be chosen such as to allow characterization of the elastic properties of either the film or the film–substrate interface. This attribute has been combined with the ability of the method to obtain qualitative stiffness images to show variations in the film–substrate adhesion. The AFAM technique has been also used to determine the indentation modulus of thin films of silicon oxide with thicknesses ranging from 7 to 28 nm. In this case, elastic properties of the substrate had to be considered. The examples of the applications of the AFAM method reported here for characterization of elastic properties of very thin films have shown that this technique has the lateral and depth resolution required to characterize the very thin films used nowadays in microelectronics industry.     

Electrospinning of polystyrene/dicyanopyrazine-linked porphyrin nanofibers

We have fabricated fluorescing polystyrene/dicyanopyrazine-linked porphyrin (PS/4-TDCPP) nanofibers using the electrospinning technique. UV-vis spectroscopy shows a strong Soret band and two relatively weak Q bands from the PS/4-TDCPP films and fibers, and reveals that the 4-TDCPP molecules are homogeneously dispersed in the films and fibers. Scanning electron microscopy (SEM) reveals the effect of solvent and collecting distance on the morphology of the electro-spun PS/4-TDCPP fibers. Fibers spun from a 50% dimethlyformamide (DMF), 50% methylethylketone (MEK) solution have ultra-fine structures with an average diameter of 300 nm. In the case of fibers from pure DMF and DMF:MEK (1:3) solutions, beads are formed along the length of the fibers. Variation of the collecting distance from 20 to 30 cm does not induce significant differences in the morphology of the electro-spun PS/4-TDCPP fibers. However, at a collecting distance of 15 cm, many beads are formed along the fibers. Acid-sensing capability of the PS/4-TDCPP fibers is demonstrated by fluorescence microscopy.     

Optical waveguide self-assembled from organic dye molecules in solution

Fiber-shaped H-aggregates with lengths of up to 300 microm are synthesized by self-assembly of thiacyanine (TC) dye molecules in solution. Photoluminescence (PL) images and spatially resolved PL spectra of the fibers that are transferred onto a glass substrate reveal that the fibers act as single-mode optical waveguides that propagate PL in the range of 520 to 560 nm over 250 microm without any loss.     

Directed collagen patterning on gold-coated silicon substrates via micro-contact printing

The ability to create biologically functional systems from non-biological materials has importance in the arena of tissue engineering and medical device implantation. Directing the immobilization of proteins to specified regions on a substrate has attracted a lot of attention as one potential approach. Functionalization of the surface of gold-coated silicon wafers was accomplished by micro-contact printing a hydrophilic (or hydrophobic) self-assembled monolayer (SAM) atop the gold coating using poly(dimethylsiloxane) (PDMS) stamps. Afterwards, the substrate was soaked in a solution of hydrophobic (or hydrophilic) surfactant molecules which filled in the un-stamped area. The intention was to use carbodiimide coupling to attach fluorescently labeled collagen to COOH-terminated (hydrophilic) regions of the substrate. However, even in the presence of the reagents for this reaction, the collagen preferred to assemble on the hydrophobic regions. The results suggest that micro-contact printing may provide a simple mechanism for patterning collagen onto surfaces simply using selective adsorption. This might be useful for examining directed cell interactions, or to enhance the biocompatibility of inorganic materials used as substrates in tissue engineering or devices that are to be implanted into the body.     

Site-specific stamping of graphene micro-patterns over large areas using flexible stamps

Site-specific stamping has the potential of becoming a low-cost, high-throughput method for depositing specific-shaped graphene micro-patterns over large areas on a wide variety of substrates. The use of an approach involving flexible stamps presented here represents an important advance towards reaching that potential. This approach entails lithographic creation (dry etching) of high-quality micro-pillar arrays of highly oriented pyrolytic graphite (HOPG) over large areas. This is followed by embedding the micro-pillar arrays in polydimethylsiloxane (PDMS), and detaching them from the HOPG base. This results in flexible stamps containing embedded HOPG micro-pillar arrays with freshly cleaved stamping surfaces. The flexible HOPG/PDMS stamps are then brought into contact with substrate surfaces to site-specifically stamp graphene or few-layer graphene (FLG) arrays over large areas. The freshly cleaved nature of the micro-pillar surfaces in the flexible stamps, the low elastic modulus of the flexible stamps and the elimination of sidewall deposits on the micro-pillars allow for more uniform stamping, relative to the use of stiff HOPG stamps from earlier studies. This approach has the potential to expand the substrate choice for graphene or FLG stamping to include curved and/or flexible substrates that could have an impact on the burgeoning field of flexible/stretchable electronics.     

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.     

Direct transfer patterning of gold films with minimal processing steps

We were able to reduce the processing steps of transfer printing of thin gold films through prolonged evaporation times. We suspect the reduced evaporation rate to cause diffusion of small chain molecules (oligomers) in the PDMS (poly(dimethylsiloxane)) stamp to facilitate the transfer. Typical wrinkling of the PDMS surface was avoided by fabricating thin stamps of approximately 50 μm with polymer backing. The transferred films with a thickness of 20 nm showed enhanced edge resolution and a roughness of 1.31 nm root mean square. We were further able to fabricate 3D structures, indicating stability of the transferred films. Adhesion problems remain a limitation for contacting purposes.     

Shaping Metallic Nanolattices: Design by Microcontact Printing from Wrinkled Stamps

A method for the fabrication of well‐defined metallic nanostructures is presented here in a simple and straightforward fashion. As an alternative to lithographic techniques, this routine employs microcontact printing utilizing wrinkled stamps, which are prepared from polydimethylsiloxane (PDMS), and includes the formation of hydrophobic stripe patterns on a substrate via the transfer of oligomeric PDMS. Subsequent backfilling of the interspaces between these stripes with a hydroxyl‐functional poly(2‐vinyl pyridine) then provides the basic pattern for the deposition of citrate‐stabilized gold nanoparticles promoted by electrostatic interaction. The resulting metallic nanostripes can be further customized by peeling off particles in a second microcontact printing step, which employs poly(ethylene imine) surface‐decorated wrinkled stamps, to form nanolattices. Due to the independent adjustability of the period dimensions of the wrinkled stamps and stamp orientation with respect to the substrate, particle arrays on the (sub)micro‐scale with various kinds of geometries are accessible in a straightforward fashion. This work provides an alternative, cost‐effective, and scalable surface‐patterning technique to fabricate nanolattice structures applicable to multiple types of functional nanoparticles. Being a top‐down method, this process could be readily implemented into, e.g., the fabrication of optical and sensing devices on a large scale.     

R.F. magnetron sputtering of multilayered c-BN films on cemented carbide tool

A c-BN thin film was deposited using a B4C target in a r.f. magnetron sputtering system. The c-BN layer was coated with a TiAIN adhesion layer (approximately 2 microm), boron carbide (approximately 1 microm) and BCN (10 approximately 15 nm) nano-gradient layer system. The c-BN layers with thicknesses of more than 0.5 microm were successfully deposited onto cemented carbide substrates. The high resolution XPS spectra analysis of B1s and N1s revealed that the c-BN film was mainly composed of sp3 BN bonds.     

Revealing the dependence of cell spreading kinetics on its spreading morphology using microcontact printed fibronectin patterns

Since the dawn of in vitro cell cultures, how cells interact and proliferate within a given external environment has always been an important issue in the study of cell biology. It is now well known that mammalian cells typically exhibit a three-phase sigmoid spreading on encountering a substrate. To further this understanding, we examined the influence of cell shape towards the second rapid expansion phase of spreading. Specifically, 3T3 fibroblasts were seeded onto silicon elastomer films made from polydimethylsiloxane (PDMS), and micro-contact printed with fibronectin stripes of various dimensions. PDMS is adopted in our study for its biocompatibility, its ease in producing very smooth surfaces, and in the fabrication of micro-contact printing stamps. The substrate patterns are compared with respect to their influence on cell spreading over time. Our studies reveal, during the early rapid expansion phase, 3T3 fibroblasts are found to spread radially following a law; meanwhile, they proliferated in a lengthwise fashion on the striped patterns, following a law. We account for the observed differences in kinetics through a simple geometric analysis which predicted similar trends. In particular, a t² law for radial spreading cells, and a t¹ law for lengthwise spreading cells.     

Fabrication of nano- and micro-scale UV imprint stamp using diamond-like carbon coating technology

Two-dimensional (2-D) and three-dimensional (3-D) diamond-like carbon (DLC) stamps for ultraviolet nanoimprint lithography were fabricated with two methods: namely, a DLC coating process, followed by focused ion beam lithography; and two-photon polymerization patterning, followed by nanoscale-thick DLC coating. We used focused ion beam lithography to fabricate 70 nm deep lines with a width of 100 nm, as well as 70 nm deep lines with a width of 150 nm, on 100 nm thick DLC layers coated on quartz substrates. We also used two-photon polymerization patterning and a DLC coating process to successfully fabricate 200 nm wide lines, as well as 3-D rings with a diameter of 1.35 microm and a height of 1.97 microm, and a 3-D cone with a bottom diameter of 2.88 microm and a height of 1.97 microm. The wafers were successfully printed on an UV-NIL using the DLC stamps without an anti-adhesive layer. The correlation between the dimensions of the stamp's features and the corresponding imprinted features was excellent.     

Zinc oxide nanorod growth on gold islands prepared by microsphere lithography on silicon and quartz

Gold islands, vapor deposited on silicon and quartz by microsphere lithography patterning, are used to nucleate arrays of ZnO nanorods. ZnO is grown on approximately 0.32 microm2 Au islands by carbothermal reduction in a tube furnace. Scanning electron microscopy (SEM) and energy dispersive atomic X-ray spectroscopy (EDS) confirm that the gold effectively controls the sites of nucleation of ZnO. Atomic force microscopy (AFM) shows that approximately 30 nm diameter nanorods grow horizontally, along the surface. Alloy droplets that are characteristic of the vapor-liquid-solid (VLS) mechanism are observed at the tips of the nanorods. The spatial growth direction of VLS catalyzed ZnO nanorods is along the substrate when they nucleate from gold islands on silicon and quartz. The energy of adhesion of the VLS droplet to the surface can account for the horizontal growth.