Silicone-Based Adhesives with Highly Tunable Adhesion Force for Skin-Contact Applications.

A fundamental approach to fabricating silicone-based adhesives with highly tunable adhesion force for the skin-contact applications is presented. Liquid blends consisting of vinyl-multifunctional polydimethylsiloxane (V-PDMS), hydride-terminated PDMS (H-PDMS), and a tackifier composed of a silanol-terminated PDMS/MQ resin mixture and the MQ resin are used as the adhesive materials. The peel adhesion force of addition-cured adhesives on the skin is increased by increasing the H-PDMS molecular weights and the tackifier content, and decreasing the H-PDMS/V-PDMS ratio. There is an inverse relationship between the adhesion force and the Young's modulus. The low-modulus adhesives with a low H-PDMS/V-PDMS ratio exhibit enhanced adhesion properties. The low-modulus adhesives with the high MQ resin content show significantly enhanced adhesion properties. These adhesives exhibit a wide range of modulus (2-499 kPa), and their adhesion force (0.04-5.38 N) is superior to commercially available soft silicone adhesives (0.82-2.79 N). The strong adhesives (>≈2 N) provide sufficient adhesion for fixing the flexible electrocardiogram (ECG) device to the skin in most daily activity. The human ECG signals are successfully recorded in real time. These results suggest that the silicone-based adhesives should be useful as an atraumatic adhesive for the skin-contact applications.

Attogram mass sensing based on silicon microbeam resonators.

Using doubly-clamped silicon (Si) microbeam resonators, we demonstrate sub-attogram per Hertz (ag/Hz) mass sensitivity, which is extremely high sensitivity achieved by micro-scale MEMS mass sensors. We also characterize unusual buckling phenomena of the resonators. The thin-film based resonator is composed of a Si microbeam surrounded by silicon nitride (SiN) anchors, which significantly improve performance by providing fixation on the microbeam and stabilizing oscillating motion. Here, we introduce two fabrication techniques to further improve the mass sensitivity. First, we minimize surface stress by depositing a sacrificial SiN layer, which prevents damage on the Si microbeam. Second, we modify anchor structure to find optimal design that allows the microbeam to oscillate in quasi-one dimensional mode while achieving high quality factor. Mass loading is conducted by depositing Au/Ti thin films on the local area of the microbeam surface. Using sequential mass loading, we test effects of changing beam dimensions, position of mass loading, and distribution of a metal film on the mass sensitivity. In addition, we demonstrate that microbeams suffer local micro-buckling and global buckling by excessive mass loading, which are induced by two different mechanisms. We also find that the critical buckling length is increased by additional support from the anchors.

Integrated Flexible Electronic Devices Based on Passive Alignment for Physiological Measurement.

This study proposes a simple method of fabricating flexible electronic devices using a metal template for passive alignment between chip components and an interconnect layer, which enabled efficient alignment with high accuracy. An electrocardiogram (ECG) sensor was fabricated using 20 µm thick polyimide (PI) film as a flexible substrate to demonstrate the feasibility of the proposed method. The interconnect layer was fabricated by a two-step photolithography process and evaporation. After applying solder paste, the metal template was placed on top of the interconnect layer. The metal template had rectangular holes at the same position as the chip components on the interconnect layer. Rectangular hole sizes were designed to account for alignment tolerance of the chips. Passive alignment was performed by simply inserting the components in the holes of the template, which resulted in accurate alignment with positional tolerance of less than 10 µm based on the structural design, suggesting that our method can efficiently perform chip mounting with precision. Furthermore, a fabricated flexible ECG sensor was easily attachable to the curved skin surface and able to measure ECG signals from a human subject. These results suggest that the proposed method can be used to fabricate epidermal sensors, which are mounted on the skin to measure various physiological signals.

Flexible Nonstick Replica Mold for Transfer Printing of Ag Ink.

We report the fabrication of flexible replica molds for transfer printing of Ag ink on a rigid glass substrate. As mold precursors, acrylic mixtures were prepared from silsesquioxane-based materials, silicone acrylate, poly(propylene glycol) diacrylate, 3,3,4,4,5,5,6,6,7,7,8,8, 9,9,10,10,10-heptadecafluorodecyl methacrylate, and photoinitiator. By using these materials, the replica molds were fabricated from a silicon master onto a flexible substrate by means of UV-assisted molding process at room temperature. The wettability of Ag ink decreased with increase in the water contact angle of replica molds. On the other hand, the transfer rate of Ag ink onto adhesive-modified substrates increased with increase in the water contact angle of replica molds. Transferred patterns were found to be thermally stable on the photocurable adhesive layer, whereas Ag-ink patterns transferred on non-photocurable adhesives were distorted by thermal treatment. We believe that these characteristics of replica molds and adhesives offer a new strategy for the development of the transfer printing of solution-based ink materials.

Fabrication of Large-Area Hierarchical Structure Array Using Siliconized-Silsesquioxane as a Nanoscale Etching Barrier.

A material approach to fabricate a large-area hierarchical structure array is presented. The replica molding and oxygen (O2) plasma etching processes were combined to fabricate a large-area hierarchical structure array. Liquid blends consisting of siliconized silsesquioxane acrylate (Si-SSQA), ethylene glycol dimethacrylate (EGDMA), and photoinitiator are developed as a roughness amplifying material during O2 plasma etching. Microstructures composed of the Si-SSQA/EGDMA mixtures are fabricated by replica molding. Nanoscale roughness on molded microstructures is realized by O2 etching. The nanoscale roughness on microstructures is efficiently controlled by varying the etching time and the weight ratio of Si-SSQA to EGDMA. The hierarchical structures fabricated by combining replica molding and O2 plasma etching showed superhydrophilicity with long-term stability, resulting in the formation of hydroxyl-terminated silicon oxide layer with the reorientation limit. On the other hand, the hierarchical structures modified with a perfluorinated monolayer showed superhydrophobicity. The increment of water contact angles is consistent with increment of the nano/microroughness of hierarchical structures and decrement of the top contact area of water/hierarchical structures.

Application of thermal initiator for characteristic improvements of polymeric replica mold for UV nanoimprint lithography.

We report the improvement of the hardness and modulus properties in a silsesquioxane-based soft replica mold by adding thermal initiator, without deteriorating the UV transmittance at the wavelength of 365 nm. It is found that thermal initiator used for this work contributes to improving the hardness and modulus values up to 0.175 and 3.585 GPa, while the UV transmittance value is still above 75%. The optimized soft replica mold built on a flexible plastic substrate allows submicron-scale patterns to be transferred onto a rigid Si substrate by means of UV-NIL process. Consequently, we demonstrate that the developed soft replica mold can be a suitable replacement for typical hard molds, promising further use in mold-based nanolithography for the fabrication of high-resolution nanopatterns over large areas.

Direct nanopatterning of silsesquioxane/poly(ethylene glycol) blends with high stability and nonfouling properties.

A free-radical-polymerizable SSQ/PEG blend with direct patternability has been proposed as an ideal nonfouling material for nanostructure-based biomedical applications. Cured SSQ/PEG networks show an UV transparency of >90% at 365 nm, high resistance to organic/aqueous solutions, hydrophilicity and Young's moduli of 1.898-2.815 GPa. SSQ/PEG patterns with 25-nm linewidths, 25-nm spacing, and an aspect ratio of 4:1 were directly fabricated on transparent substrates by UV embossing, and cured SSQ/PEG networks with long-term stability under chemical, thermal, and biological stress showed strong resistance to the nonspecific adsorption of biomolecules. These characteristics may offer a new strategy for the development of a number of medical nanodevice applications such as labs-on-a-chip.

Photocurable silsesquioxane-based formulations as versatile resins for nanoimprint lithography.

Methacrylate octafunctionalized silsesquioxane (SSQMA) was shown to be an ideal material with high performance for ultraviolet (UV)-based nanoimprint lithography (NIL). The total viscosity of SSQMA-based formulations was adjusted to between 0.8 and 50 cP by incorporating low-viscosity acrylic additives, making the formulations suitable for UV-based NIL. The cured SSQMA-based formulations showed numerous desirable characteristics, including low volumetric shrinkage (4%), high Young's modulus (2.445-4.272 GPa), high resistance to oxygen plasma, high transparency to UV light, and high resistance to organic/aqueous media, as a functional imprint material for UV-based NIL and step-and-flash imprint lithography (SFIL). Using both techniques, the SSQMA-based formulations were easily transferred to relief structures with excellent imprint fidelity and minimal residual thickness. Formulations containing 50% SSQMA (wt %) were able to reproduce high-aspect-ratio nanostructures with aspect ratios as high as 4.5 using bilayer SFIL. Transparent rigiflex molds and hard replica molds with sub-50-nm size features were reproducibly duplicated by using UV-NIL with the SSQMA-based resin. Nanostructures with feature sizes down to 50 nm were successfully reproduced using these molds in both UV- and thermal-NIL processes. After repeating 20 imprinting cycles at relatively high temperature and pressure, no detectable collapse or contamination on the replica surface was observed. These properties of the SSQMA-based resins make them suitable as inexpensive and convenient components in all NIL processes that are based on physical contact.

Direct fabrication of integrated 3D Au nanobox arrays by sidewall deposition with controllable heights and thicknesses.

This paper provides a unique strategy for controlling integrated hollow nanostructure arrays such as boxes or pillars at the nanometer scale. The key merit of this technique is that it can overcome resolution limits by sidewall deposition and deposit various materials using a sputtering method. The sputtering method can be replaced by other dry deposition techniques such as pulsed laser deposition (PLD) for complex functional materials. Furthermore, it can produce low-cost large-area fabrication and high reproducibility using the NIL (nanoimprint lithograph) process. The fabrication method consists of a sequence of bilayer spin-coating, UV-NIL, RIE (reactive ion etching), sputtering, ion milling and piranha cleaning processes. By changing the deposition time and molds, various thicknesses and shapes can be fabricated, respectively. Furthermore, the fabricated Au box nanostructure has a bending zone of the top layer and a approximately 17 nm undercut of the bottom layer as observed by SEM (scanning electron microscope). The sidewall thickness was changed from 12 to 61 nm by controlling the deposition time, and was investigated to understand the relationship with blanket thicknesses and geometric effects. The calculated sidewall thickness matched well with experimental results. Using smaller hole-patterned molds, integrated nanobox arrays, with inner squares measuring approximately 160 nm, and nanopillar arrays, with inside pores measuring approximately 65 nm, were fabricated under the same conditions.

Replica mold for nanoimprint lithography from a novel hybrid resin.

The use of durable replica molds with high feature resolution has been proposed as an inexpensive and convenient route for manufacturing nanostructured materials. A simple and fast duplication method, involving the use of a master mold to create durable polymer replicas as imprinting molds, has been demonstrated using both UV- and thermal nanoimprinting lithography (NIL). To obtain a high-durability replicating material, a dual UV/thermal-curable, organic-inorganic hybrid resin was synthesized using a sol-gel-based combinatorial method. The cross-linked hybrid resin exhibited high transparency to UV light and resistance to organic solvents. Molds made of this material showed good mechanical properties (Young's modulus=1.76 GPa) and gas permeability. The low viscosity of the hybrid resin (approximately 29 cP) allowed it to be easily transferred to relief nanostructures on transparent glass substrates using UV-NIL at room temperature and low pressure (0.2 MPa) over a relatively short time (80 s). A low surface energy release agent was successfully coated onto the hybrid mold surface without destroying the imprinted nanostructures, even after O2 plasma treatment. Nanostructures with feature sizes down to 80 nm were successfully reproduced using these molds in both UV- and thermal-NIL processes. After repeating 10 imprinting cycles at relatively high temperature and pressure, no detectable collapse or contamination of the replica surface was observed. These results indicate that the hybrid molds could tolerate repeated UV- and thermal-NIL processes.

Nanoarrays of tethered lipid bilayer rafts on poly(vinyl alcohol) hydrogels.

Lipid rafts are cholesterol- and sphingolipid-rich domains that function as platforms for signal transduction and other cellular processes. Tethered lipid bilayers have been proposed as a promising model to describe the structure and function of cell membranes. We report a nano(submicro) array of tethered lipid bilayer raft membranes (tLBRMs) comprising a biosensing platform. Poly(vinyl alcohol) (PVA) hydrogel was directly patterned onto a solid substrate, using ultraviolet-nanoimprint lithography (UV-NIL), as an inert barrier to prevent biofouling. The robust structures of the nanopatterned PVA hydrogel were stable for up to three weeks in phosphate-buffered saline solution despite significant swelling (100% in height) by hydration. The PVA hydrogel strongly restricted the adhesion of vesicles, resulting in an array of highly selective hydrogel nanowells. tLBRMs were not formed by direct vesicle fusion, although raft vesicles containing poly(ethylene glycol) lipopolymer were selectively immobilized on gold substrates patterned with PVA hydrogel. The deposition of tLBRM nano(submicro) arrays was accomplished by a mixed, self-assembled monolayer-assisted vesicle fusion method. The monolayer was composed of a mixture of 2-mercaptoethanol and poly(ethylene glycol) lipopolymer, which promoted vesicle rupture. These results suggest that the fabrication of inert nanostructures and the site-selective modification of solid surfaces to induce vesicle rupture may be essential in the construction of tLBRM nano(submicro) arrays using stepwise self-assembly.

Self-organized functional lipid vesicle array for sensitive immunoassay chip.

We report the self-assembly immobilization of functional lipid vesicles (FLVs) by electrostatic interaction onto N-inscription-nanosized geometrics. The well-organized three-dimensional physical structures of liposome were observed by AFM. Generally, two involved forces for the binding to surfaces and the repulsion between individual liposome are necessary to array lipid vesicles individually similar to the physical configuration in solution. The immobilized FLVs demonstrated clearly defined redox activity in electrochemical measurements. We observed a notable current decrease, indicating the binding of the capture antibody with the target human serum albumin (HSA) antigen. We believe these findings can be related to various vesicles applications such as drug delivery system, nanobiosensors and nano-scale membrane function studies.

Facile and rapid direct gold surface immobilization with controlled orientation for carbohydrates.

Effective surface immobilization is a prerequisite for numerous carbohydrate-related studies including carbohydrate-biomolecule interactions. In the present work, we report a simple and rapid modification technique for diverse carbohydrate types in which direct oriented immobilization onto a gold surface is accomplished by coupling the amine group of a thiol group-bearing aminophenyl disulfide as a new coupling reagent with an aldehyde group of the terminal reducing sugar in the carbohydrate. To demonstrate the generality of this proposed reductive amination method, we examined its use for three types of carbohydrates: glucose (monosaccharide), lactose (disaccharide), and GM1 pentasaccharide. Through successful mass identifications of the modified carbohydrates, direct binding assays on gold surface using surface plasmon resonance and electrochemical methods, and a terminal galactose-binding lectin assay using atomic force microscopy, we confirmed several advantages including direct and rapid one-step immobilization onto a gold surface and exposure of functional carbohydrate moieties through oriented modification of the terminal reducing sugar. Therefore, this facile modification and immobilization method can be successfully used for diverse biomimetic studies of carbohydrates, including carbohydrate-biomolecule interactions and carbohydrate sensor or array development for diagnosis and screening.

A novel route for immobilization of oligonucleotides onto modified silica nanoparticles.

A novel approach for immobilization of probe oligonucleotides that uses zirconium phosphate modified silica nanoparticles is proposed. The surface modification of nanoparticles was carried out in two stages. Initially binding of Zr4+ to the surface of silica nanoparticles and later treated with phosphoric acid for terminal phosphate groups. Oligonucleotide probes modified with amine group at 5'-end were strongly binds to the phosphate terminated silica nanoparticles with imidazole in presence of 0.1 mol L(-1) EDC [N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide], as phosphate groups are more reactive towards amine group. Various studies, i.e., synthesis of silica nanoparticles, their surface modification, probe immobilization, measurement of hybridization and effect of bovine serum albumin (BSA) were carried out during optimization of reaction conditions. The significant reduction in the background signal was observed by treating the probe modified silica nanoparticles with bovine serum albumin prior to hybridization. The probe modified silica nanoparticles were retained their properties and the hybridization was induced by exposure of single-stranded DNA (ssDNA) containing silica nanoparticles to the complementary DNA in solution. The decrease in the fluorescence signal for one mismatch and three mismatch was observed upon hybridization of probe with target DNAs, while there was no response for the random target ssDNA under the same experimental conditions. The intensity of fluorescence signal was linear to the concentration of target DNA ranging from 3.9 x 10(-9) to 3.0 x 10(-6)mol L(-1). A detection limit of 1.22 x 10(-9) mol L(-1) of oligonucleotides can be estimated. The proposed hybridization assay is simple and possesses good analytical characteristics and it can provide an effective and efficient route in the development of DNA biosensors and biochips.