Suck Won Hong

Stretchable, Transparent Graphene Interconnects for Arrays of Microscale Inorganic Light Emitting Diodes on Rubber Substrates

This paper describes the fabrication and design principles for using transparent graphene interconnects in stretchable arrays of microscale inorganic light emitting diodes (LEDs) on rubber substrates. We demonstrate several appealing properties of graphene for this purpose, including its ability to spontaneously conform to significant surface topography, in a manner that yields effective contacts even to deep, recessed device regions. Mechanics modeling reveals the fundamental aspects of this process, as well as the use of the same layers of graphene for interconnects designed to accommodate strains of 100% or more, in a completely reversible fashion. These attributes are compatible with conventional thin film processing and can yield high-performance devices in transparent layouts. Graphene interconnects possess attractive features for both existing and emerging applications of LEDs in information display, biomedical systems, and other environments.

solderable and electroplatable flexible electronic circuit on a porous stretchable elastomer

A variety of flexible and stretchable electronics have been reported for use in flexible electronic devices or biomedical applications. The practical and wider application of such flexible electronics has been limited because commercial electronic components are difficult to be directly integrated into flexible stretchable electronics and electroplating is still challenging. Here, we propose a novel method for fabricating flexible and stretchable electronic devices using a porous elastomeric substrate. Pressurized steam was applied to an uncured polydimethylsiloxane layer for the simple and cost-effective production of porous structure. An electroplated nickel anchor had a key role in bonding commercial electronic components on elastomers by soldering techniques, and metals could be stably patterned and electroplated for practical uses. The proposed technology was applied to develop a plaster electrocardiogram dry electrode and multi-channel microelectrodes that could be used as a long-term wearable biosignal monitor and for brain signal monitoring, respectively.

Improved Density in Aligned Arrays of Single‐Walled Carbon Nanotubes by Sequential Chemical Vapor Deposition on Quartz

[*] Prof. J. A. Rogers Department of Materials Science and Engineering, Chemistry Mechanical Science and Engineering, Electrical and Computer Engineering Beckman Institute for Advanced Science and Technology Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois 61801 (USA) E-mail: jrogers@uiuc.edu Dr. S. W. Hong Department of Materials Science and Engineering Frederick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois 61801 (USA)

Robust Self‐Assembly of Highly Ordered Complex Structures by Controlled Evaporation of Confined Microfluids

The evaporative self-assembly of nonvolatile solutes such as polymers, nanocrystals, and carbon nanotubes has been widely recognized as a nonlithographic means of producing a diverse range of intriguing complex structures. The spatial variation of evaporative flux and possible convection mean, however, that these non-equilibrium dissipative structures (e.g., coffee rings, fingering patterns, and polygonal network structures) are often irregular and stochastically organized. Yet for many applications in microelectronics, data storage devices, and biotechnology, it is highly desirable to achieve surface patterns that have a well-controlled spatial arrangement. To date, only a few elegant studies have centered on the precise control of the evaporation process to produce ordered structures. When compared with conventional lithographic techniques, surface patterning by controlled solvent evaporation is simple, cost-effective, and offers a lithography and external-field-free means of organizing nonvolatile materials into ordered microscopic structures over large surface areas. For example, it has been recently demonstrated that constraining a drop of solution in a restricted geometry formed by placing a sphere against a flat substrate results in controlled evaporation. Consequently, the repetitive pinning and depinning of the solution s contact line produces a lateral surface pattern that consists of hundreds of concentric, highly ordered “coffee rings”, the gradients of which vary in width and height. 13, 17] The ability to engineer an evaporative self-assembly process that yields a wide range of complex, self-organizing structures over large areas other than strictly concentric rings 13, 17] offers tremendous potential for applications in electronics, optoelectronics, and sensors. The formation of periodic assemblies of polymeric squares, triangular contour lines, and ellipses would be effectively mediated by controlled solvent evaporation that is precisely guided by the shape of the curved upper surface of a confined curve-on-flat geometry. Herein, we demonstrate a facile, robust, and one-step method of evaporating polymer solutions in curve-on-flat geometries to create versatile, highly regular microstructures in a precisely controlled environment, as well as offering a comprehensive study of the influence of different upper surfaces on complex structure formation by controlled evaporation. Our method further enhances current fabrication approaches to creating highly ordered structures in a simple and cost-effective manner, with the potential to be tailored for use in photonics, electronics, 20] optoelectronics, microfluidic devices, nanotechnology, and biotechnology. A linear conjugated polymer, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, molecular weight = 50–300 kg mol ) was used as the nonvolatile solute. The choice of system was motivated by its numerous potential applications in the areas of light-emitting diodes, solar cells, and biosensors. A solution of MEH-PPV in toluene was prepared at a concentration of 0.05 mg mL . The key to our approach is the use of a simple confined geometry consisting of a curved upper surface on a flat lower substrate (curve-on-flat geometry) that forms a microscopic gap in which the MEH-PPV toluene solution is loaded; this results in a capillary-held microfluid (a liquid capillary bridge, see Figure 1a, Figure 3a, and the Experimental Section). The three curved surfaces used in the study were a square pyramid (the area of the sides = 1.0 1.0 cm and Hpyramid (pyramid height) = 100 mm (Figure 1a)), a triangular-slice sphere (R, (the radius of curvature) = 1.65 cm; D, the diameter = 1.5 cm; the arm width at A, C, and D = 800 mm (Figure 3a)), and a chisel lens (D = 1.0 cm, Hchisel (the chisel height) = 100 mm (Figure S2 a in the Supporting Information). The surfaces were made of aluminum, stainless steel, and fused silica, respectively. Unlike previous work in which evaporation occurred over the entire droplet surface, 8] evaporation of the solvent in this case was restricted to the edge of the capillary within the curve-on-flat geometry (Figure 1a and Figure 3a). Figure 1a, b, illustrates the route to concentric square stripes formed by evaporation in the square-pyramid-on-flat geometry. The loss of toluene at the capillary edge by evaporation triggered pinning of the contact line (the “stick”), thereby forming the outermost MEH-PPV square. During deposition of the MEH-PPV, the evaporation of toluene caused the initial contact angle of the capillary edge to gradually decrease to a critical angle, at which the capillary force (the depinning force) became greater than the pinning force. This led the contact line to jump, or “slip,” to a new position, thus developing a new square (Figure 1b). 25, 26] It is worth noting that the pyramid (the upper surface) provided a unique environment to guide the “stick–slip” motions of the contact line of the evaporating MEH-PPV microfluid, thereby forcing MEH-PPV to deposit in a manner that conformed to the square-shaped sides of the pyramid [*] S. W. Hong, M. Byun, Prof. Z. Lin Department of Materials Science and Engineering Iowa State University Ames, IA 50011 (USA) Fax: (+ 1)515-294-7202 E-mail: zqlin@iastate.edu

Stimulated myoblast differentiation on graphene oxide-impregnated PLGA-collagen hybrid fibre matrices

Background Electrospinning is a simple and effective method for fabricating micro- and nanofiber matrices. Electrospun fibre matrices have numerous advantages for use as tissue engineering scaffolds, such as high surface area-to-volume ratio, mass production capability and structural similarity to the natural extracellular matrix (ECM). Therefore, electrospun matrices, which are composed of biocompatible polymers and various biomaterials, have been developed as biomimetic scaffolds for the tissue engineering applications. In particular, graphene oxide (GO) has recently been considered as a novel biomaterial for skeletal muscle regeneration because it can promote the growth and differentiation of myoblasts. Therefore, the aim of the present study was to fabricate the hybrid fibre matrices that stimulate myoblasts differentiation for skeletal muscle regeneration.ResultsHybrid fibre matrices composed of poly(lactic-co-glycolic acid, PLGA) and collagen (Col) impregnated with GO (GO-PLGA-Col) were successfully fabricated using an electrospinning process. Our results indicated that the GO-PLGA-Col hybrid matrices were comprised of randomly-oriented continuous fibres with a three-dimensional non-woven porous structure. Compositional analysis showed that GO was dispersed uniformly throughout the GO-PLGA-Col matrices. In addition, the hydrophilicity of the fabricated matrices was significantly increased by blending with a small amount of Col and GO. The attachment and proliferation of the C2C12 skeletal myoblasts were significantly enhanced on the GO-PLGA-Col hybrid matrices. Furthermore, the GO-PLGA-Col matrices stimulated the myogenic differentiation of C2C12 skeletal myoblasts, which was enhanced further under the culture conditions of the differentiation media.ConclusionsTaking our findings into consideration, it is suggested that the GO-PLGA-Col hybrid fibre matrices can be exploited as potential biomimetic scaffolds for skeletal tissue engineering and regeneration because these GO-impregnated hybrid matrices have potent effects on the induction of spontaneous myogenesis and exhibit superior bioactivity and biocompatibility.

Evolution of Ordered Block Copolymer Serpentines into a Macroscopic, Hierarchically Ordered Web

In the process of drying, a sessile drop containing nonvolatile solutes such as polymers, nanocrystals, and carbon nanotubes readily self-assembles into a number of concentric “coffee rings” through the repetitive “stick–slip” motion of a threephase contact line. However, owing possibly to convection and a lack of control over the evaporation process of the drop, the rings are often irregular and stochastically organized. The challenge therefore remains to use evaporative selfassembly rationally to prepare such rings and other dissipative structures (e.g., fingering patterns and polygonal network structures ) of high regularity and fidelity for use in microelectronics, data storage devices, and biotechnology applications. To date, only a few studies have focused on precise control of the evaporation process to produce such highly ordered structures. Surface patterning by controlled solvent evaporation offers a lithographyand externalfield-free means to organize nonvolatile materials into ordered microscopic structures over large surface areas in a simple and cost-effective manner. Control over the spatial arrangement of components (i.e., formation of hierarchically ordered structures) is highly desirable for many applications. To date, numerous studies have focused on creating hierarchically ordered structures using lithographic techniques. However, lithographic methods involve high processing and maintenance costs and require an iterative, multistep procedure that makes the structure formation process more complex and less reliable. Herein, we demonstrate a robust method to create hierarchically ordered structures consisting of diblock copolymers using two consecutive self-assembly processes at different length scales. First, the controlled evaporative self-assembly of a diblock copolymer solution, together with formation of fingering instabilities arising from the unfavorable interfacial interaction between one block and the substrate, occurs in a restricted geometry comprising a spherical lens on a flat substrate that yields concentric serpentines of diblock copolymer at the microscopic scale. Subsequently, upon solvent vapor annealing, these serpentines self-organize into a macroscopic web; at the same time, nanoscopic constituents of the diblock copolymer self-assemble into domains oriented vertically to the web surface. The resulting highly ordered structures exhibit two independent characteristic dimensions: global web-like macrostructures with local regular microporous mesh arrays by a top-down mechanism; and, by a bottom-up approach, vertical nanoscopic domains of selfassembled diblock copolymer that span the entire web. In short, hierarchical structures are formed with significant potential for applications in optical and optoelectronic materials and devices. Diblock copolymers composed of two chemically distinct chains covalently linked at one end are thermodynamically driven to self-assemble into a range of well-ordered nanoscopic domains (or nanodomains, historically called microdomains), for example, spheres, cylinders, and lamellae, depending on the volume fractions of their components. The size of the domains is dictated by the molecular weight of the polymer, typically in a range of 10 to 100 nm, which renders a density of 10 nanostructures per square inch, an attractive alternative to fabricating nanometer-scale structures. Thus, an asymmetric diblock copolymer, polystyrene-bpoly(methyl methacrylate) (PS-b-PMMA) with a cylindrical morphology was used as a nonvolatile solute and dissolved in toluene. The hydrophobic PS block forms nanocylinders within the hydrophobic PMMA matrix in PS-b-PMMA. A drop of PS-b-PMMA toluene solution was loaded in a restricted geometry composed of a spherical lens on a silicon substrate (i.e., “sphere-on-Si”, schematically illustrated in Figure 1a), leading to a capillary-held polymer solution, the evaporation rate of which is highest at its extremity (see the Experimental Section). In marked contrast to the copious past work in which a sessile drop was allowed to evaporate over the entire surface, 2] evaporation in the sphere-on-Si geometry was restricted to the edge of the drop (Figure 1a). Upon pinning of the drop, fingering instabilities set in owing to unfavorable interfacial interaction between the PS block and the Si substrate (PS possesses a positive value of the Hamaker constant A), 20] thus resulting in the formation of a “coffee ring” with a serpentine appearance in the outer region of X (i.e., X1, where X is the distance from the sphere/Si contact center, Figure 1b). The drop then depinned and formed a new ring with fingers, contracting the drop interface inward (Figure 2a, taken in the intermediate region X2). Finally, the drop reached the sphere/Si contact center (the inner region X3) and the solution evaporated, locking in the patterns. Consequently, the repetitive pinning and depinning (i.e., stick–slip motion) of the contact line, together with the concurrent fingering instabilities of the rings, produced a [*] S. W. Hong, J. Wang, Prof. Z. Lin Department of Materials Science and Engineering Iowa State University, Ames, IA 50011 (USA) Fax: (+ 1)515-294-7202 E-mail: zqlin@iastate.edu

Enhanced Osteogenesis by Reduced Graphene Oxide/Hydroxyapatite Nanocomposites

Recently, graphene-based nanomaterials, in the form of two dimensional substrates or three dimensional foams, have attracted considerable attention as bioactive scaffolds to promote the differentiation of various stem cells towards specific lineages. On the other hand, the potential advantages of using graphene-based hybrid composites directly as factors inducing cellular differentiation as well as tissue regeneration are unclear. This study examined whether nanocomposites of reduced graphene oxide (rGO) and hydroxyapatite (HAp) (rGO/HAp NCs) could enhance the osteogenesis of MC3T3-E1 preosteoblasts and promote new bone formation. When combined with HAp, rGO synergistically promoted the spontaneous osteodifferentiation of MC3T3-E1 cells without hindering their proliferation. This enhanced osteogenesis was corroborated from determination of alkaline phosphatase activity as early stage markers of osteodifferentiation and mineralization of calcium and phosphate as late stage markers. Immunoblot analysis showed that rGO/HAp NCs increase the expression levels of osteopontin and osteocalcin significantly. Furthermore, rGO/HAp grafts were found to significantly enhance new bone formation in full-thickness calvarial defects without inflammatory responses. These results suggest that rGO/HAp NCs can be exploited to craft a range of strategies for the development of novel dental and orthopedic bone grafts to accelerate bone regeneration because these graphene-based composite materials have potentials to stimulate osteogenesis.

Bioinspired piezoelectric nanogenerators based on vertically aligned phage nanopillars

Bioinspired nanogenerators based on vertically aligned phage nanopillars are inceptively demonstrated. Vertically aligned phage nanopillars enable not only a high piezoelectric response but also a tuneable piezoelectricity. Piezoelectricity is also modulated by tuning of the proteins dipoles in each phage. The sufficient electrical power from phage nanopillars thus holds promise for the development of self-powered implantable and wearable electronics.

Enhanced Neural Cell Adhesion and Neurite Outgrowth on Graphene-Based Biomimetic Substrates

Neural cell adhesion and neurite outgrowth were examined on graphene-based biomimetic substrates. The biocompatibility of carbon nanomaterials such as graphene and carbon nanotubes (CNTs), that is, single-walled and multiwalled CNTs, against pheochromocytoma-derived PC-12 neural cells was also evaluated by quantifying metabolic activity (with WST-8 assay), intracellular oxidative stress (with ROS assay), and membrane integrity (with LDH assay). Graphene films were grown by using chemical vapor deposition and were then coated onto glass coverslips by using the scooping method. Graphene sheets were patterned on SiO2/Si substrates by using photolithography and were then covered with serum for a neural cell culture. Both types of CNTs induced significant dose-dependent decreases in the viability of PC-12 cells, whereas graphene exerted adverse effects on the neural cells just at over 62.5 ppm. This result implies that graphene and CNTs, even though they were the same carbon-based nanomaterials, show differential influences on neural cells. Furthermore, graphene-coated or graphene-patterned substrates were shown to substantially enhance the adhesion and neurite outgrowth of PC-12 cells. These results suggest that graphene-based substrates as biomimetic cues have good biocompatibility as well as a unique surface property that can enhance the neural cells, which would open up enormous opportunities in neural regeneration and nanomedicine.

Monolithic integration of arrays of single-walled carbon nanotubes and sheets of graphene.

Sheets of graphene and arrays of single-walled carbon nanotubes (SWNTs) are formed separately using chemical vapor deposition techniques onto different optimized growth substrates. Techniques of transfer printing provide a route to integration, yielding two terminal devices and transistors in which patterned structures of graphene form the electrodes and the SWNTs arrays serve as the semiconducting channels.