George M. Whitesides

Bio: George M. Whitesides is an academic researcher from Harvard University. The author has contributed to research in topics: Microcontact printing & Self-assembled monolayer. The author has an hindex of 240, co-authored 1739 publications receiving 269833 citations. Previous affiliations of George M. Whitesides include University of California, Davis & University of Texas at Austin.

Fabrication of Micrometer‐Scale, Patterned Polyhedra by Self‐Assembly

Abstract: We recently proposed and demonstrated a strategy for fabricating self-assembling, three-dimensional (3D) electrical networks. In this demonstration, we used millimeter scale building blocks (polyhedra) whose faces were patterned with copper connectors and devices (light-emitting diodes). One significant hurdle to implementing self-assembly in practical systems is that of miniaturizing the assemblies. To do so would require us to construct building blocks similar to those of ~1 mm scale, but on the micrometer scale. The building blocks must have the following characteristics: a) polyhedral structures, b) faces patterned with arbitrary patterns that would serve as connectors, and c) microelectronic devices attached to the faces of the polyhedron. It is difficult to fabricate micrometer scale polyhedral structures. Structures with these dimensions are usually fabricated by projection lithography, and this technique is inherently planar. Most methods of fabrication in 3D utilize processes such as surface micromachining that are precise and versatile, but also expensive and limited in the range of materials that can be used and the types of structures that can be generated. It is also difficult to generate arbitrarily patterned structures in 3D or on curved surfaces. Techniques for patterning have been limited to microcontact printing, projection lithography on spherical substrates using elaborate optics, and shell plating onto die-cast mandrills. Fabricating devices on 3D objects is extremely difficult; this is because processes (e.g., ion implantation) used to build silicon-based microdevices are inherently planar techniques. This paper describes the fabrication of patterned polyhedra, having 100±300 lm sides, by the spontaneous folding of two-dimensional (2D) structures under the influence of the surface tension of liquid solder. Our examination of this approach was stimulated by the early work of Pister and Shimoyama on micromachined hinges and by the extensive research of Syms and others on the use of capillary forces in liquid solder and similar methods for directly shrinking polymer joints for the assembly of non-planar microstructures. The structures we describe can be patterned and processed in 2D using conventional techniquesÐphotolithography, evaporation, electrodeposition, etchingÐthat have been extensively developed by the semiconductor industry. In the past, auto-folding has been used primarily to actuate micrometer scale components in microelectromechanical systems (MEMS) devices. In our work, we demonstrate that the self-assembling process of auto-folding can be used as a strategy for fabricating patterned 3D components from 2D precursors. We have also demonstrated that it is possible to build 3D polyhedra whose faces contain single crystal silicon chipsÐthe most primitive electronic device, i.e., a resistor. The approach we demonstrate has four steps: 1) The desired structures are designed in planar form as a series of unconnected but adjacent faces. 2) The faces are fabricated in 2D on a sacrificial layer using a combination of photolithography, evaporation, etching, and electrodeposition. 3) The ensemble of faces is covered with a thin film of liquid solder by dip coating. 4) The structure is released from the substrate by dissolving the sacrificial layer, and allowed to fold under the influence of the surface tension of the molten solder. This strategy is sketched in Figure 1. We experimented with many different materials, structures, and processes. Figure 2 shows scanning electron microscopy (SEM) images of folded metallic polyhedra and the 2D precursors of these structures. The metallic faces of the polyhedra contained either holes (the trigonal pyramid in Fig. 2) or solid faces (as seen in the tetragonal pyramid, cube, and hexagonal prism). The faces ranged in size between 100±300 lm (on a side). The 2D precursors contained faces that were not hinged; the faces were aligned as close to each other as possible (given the mask and photolithographic capabilities). For 200±300 lm faces, spacings between 8 and 15 lm worked well; for 100 lm faces, a spacing of 8 to 10 lm was required. When the 2D structures were dipped in solder, the solder bridged the faces and formed a continuous layer. The 2D precursors were released from the wafer by dissolving a sacrificial layer on which they were built. The precursors were heated above the melting point of the solder. The liquid solder tried to minimize its surface area (capillarity); this process drew the faces together to form a compact 3D polyhedron. The equilibrated 3D polyhedron was, at this point, filled with solder; the folding thus worked best when the volume of the solder present was equal to the volume of the polyhedron. Since the volume of solder present was equal to that deposited on the 2D precursor, the critical step controlling the yield of the process was the deposition of solder. We controlled the amount of solder deposited by changing the surface tension of the liquid solder, as well as by changing the solder±copper interfacial energy. The surface tension of liquids decreases approximately linearly with increasing temperatures; as a result, when the solder dip-coating was carried out at elevated temperatures (100 C for a solder with melting point, m.p., 47 C), a smaller volume of solder was deposited. The solder± copper interfacial energy was also controlled using fluxes and acids that aid in cleaning organic contaminants and dissolving oxide layers at the solder and copper surfaces. When the con-

Fabrication of magnetic microfiltration systems using soft lithography

Abstract: Arrays of nickel posts were used as magnetic elements in a microfiltration device that is compatible with microfluidic systems. The combination of microtransfer molding—a soft lithography technique—and electrodeposition generated nickel posts ∼7 μm in height and ∼15 μm in diameter inside a microfluidic channel. Once magnetized by a magnetic field from an external, permanent, neodymium–iron–boron magnet, these nickel posts generated strong magnetic field gradients and efficiently trapped superparamagnetic beads moving past them in a flowing stream of water. These nickel post arrays were also used to separate magnetic beads from nonmagnetic beads.

Self-assembly of 10-microm-sized objects into ordered three-dimensional arrays.

Abstract: This paper describes the self-assembly of small objectspolyhedral metal plates with largest dimensions of 10 to 30 μminto highly ordered, three-dimensional arrays. The plates were fabricated using photolithography and electrodeposition techniques, and the faces of the plates were functionalized to be hydrophobic or hydrophilic using self-assembled monolayers (SAMs). Self-assembly occurs in water through capillary interactions between thin films of a hydrophobic liquid (a liquid prepolymer adhesive) coated onto the hydrophobic faces of the plates; coalescence of the adhesive films reduces the interfacial free energy of the system and drives self-assembly. By altering the size and surface-patterning of the plates, the external morphologies of the aggregates were varied. Curing the adhesive furnished mechanically stable aggregates that were characterized by scanning electron microscopy (SEM). For assemblies formed by plates partially composed of a sacrificial material, a subsequent etching step furnished ful...

Equilibrium constants for thiol-disulfide interchange reactions: a coherent, corrected set

Abstract: Equilibrium constants (K eq ) for the thiol-disulfide interchange reactions between dithiothreitol (DTT) and lipoic acid (14.2±0.7), lipoic acid (Lip) and mercaptoethanol (13.3 M±1.0 M), and mercaptoethanol (ME) and glutathione (GSH or GSSG) (1.20±0.10) were measured in D 2 O at pD 7.0 by 1 H NMR spectroscopy. Two of these equilibrium constants [DTT and Lip (21..±0.9), and Lip and ME (8.6±0.7)] were also measured in D 2 O/CD 3 OD

Measuring Densities of Solids and Liquids Using Magnetic Levitation: Fundamentals

Abstract: This paper describes an analytical system that uses magnetic levitation to measure densities of solids and water-immiscible organic liquids with accuracies ranging from ±0.0002 to ±0.02 g/cm3, depe...

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

The Theory of Island Biogeography

Abstract: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

Large-scale pattern growth of graphene films for stretchable transparent electrodes

Abstract: Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites

Abstract: Multilayer films of organic compounds on solid surfaces have been studied for more than 60 years because they allow fabrication of multicomposite molecular assemblies of tailored architecture. However, both the Langmuir-Blodgett technique and chemisorption from solution can be used only with certain classes of molecules. An alternative approach—fabrication of multilayers by consecutive adsorption of polyanions and polycations—is far more general and has been extended to other materials such as proteins or colloids. Because polymers are typically flexible molecules, the resulting superlattice architectures are somewhat fuzzy structures, but the absence of crystallinity in these films is expected to be beneficial for many potential applications.