Self-assembly of components larger than molecules into ordered arrays is an efficient way of preparing microstructured materials with interesting mechanical and optical properties. Although crystallization of identical particles or particles of different sizes or shapes can be readily achieved, the repertoire of methods to assemble binary lattices of particles of the same sizes but with different properties is very limited. This paper describes electrostatic self-assembly of two types of macroscopic components of identical dimensions using interactions that are generated by contact electrification. The systems we have examined comprise two kinds of objects (usually spheres) made of different polymeric materials that charge with opposite electrical polarities when agitated on flat, metallic surfaces. The interplay of repulsive interactions between like-charged objects and attractive interactions between unlike-charged ones results in the self-assembly of these objects into highly ordered, closed arrays. Remarkably, some of the assemblies that form are not electroneutral-that is, they possess a net charge. We suggest that the stability of these unusual structures can be explained by accounting for the interactions between electric dipoles that the particles in the aggregates induce in their neighbours.

This manuscript describes the fabrication and use of a three-dimensional magnetic trap for diamagnetic objects in an aqueous solution of paramagnetic ions; this trap uses permanent magnets. It demonstrates trapping of polystyrene spheres, and of various types of living cells: mouse fibroblast ͑NIH-3T3͒, yeast (Saccharomyces cerevisiae), and algae (Chlamydomonas reinhardtii). For a 40 mM solution of gadolinium (III) diethylenetriaminepentaacetic acid ͑Gd· DTPA͒ in aqueous buffer, the smallest cell (particle) that could be trapped had a radius of ϳ2.5 m. The trapped particle and location of the magnetic trap can be translated in three dimensions by independent manipulation of the permanent magnets. This letter a1so characterizes the biocompatibility of the trapping solution.The ability to position cells, on surfaces and in suspension, is broadly useful in cellular biology. Microcontact printing of self-assembled monolayers 1-3 and other techniques of surface engineering 4,5 are used for confining and controlling the mobility of cells on surfaces. Optical traps can confine and manipulate cells and microspheres in suspension, and have been used to determine the elasticity of the cell membrane, 6-8 to observe cell division, 9 to measure inhibition of cell adhesion, 10 and to strain cells to induce activity in signaling pathways. 11 While optical traps have enabled many experiments in biophysics, they also have limitations. The laser power required to trap a micron-sized particle (for example, a cell) is proportional to the ratio of the refractive index of the particle to that of the medium. 12 Since the ratio of the refractive indices for most biological materials to biocompatible fluid media is near unity, trapping requires lasers having powers up to ϳ100 mW. This high laser power can raise the local temperature in the liquid by several degrees, 13 and this heating can damage or kill a trapped cell. In addition, traditional optical tweezers cannot trap objects with ratios of refractive indices of the object to the environment of less than unity (e.g., a gas-filled glass sphere in water 14 or a water drop in liquid parafilm 15 ) or greater than 1.5 (e.g., diamond particles in water). 16 Optical tweezers are restrictive in the size of the particle that can be trapped; particles must be ഛ10 m in diameter. The trapping force of optical tweezers is minimal outside the focus of the laser beam, requiring objects to enter the focus of the laser before they can be trapped. The small capture volume for many particles (only a few cubic microns) results in significant waiting periods before objects are trapped in dilute samples. The short working distance of the objective lens used to focus the light restricts the trap to regions near ͑Ͻ200 m͒ the surface. The high-powered lasers and infinity-corrected, high numerical aperture objectives used to construct an optical trap are expensive.Although the use of magnetism to manipulate objects was described by Thales ͑c.500 B.C.͒, the trapping of objects in a stable magnetic equilibrium...

This paper describes the fabrication of a fluidic device for detecting and separating diamagnetic materials that differ in density. The basis for the separation is the balance of the magnetic and gravitational forces on diamagnetic materials suspended in a paramagnetic medium. The paper demonstrates two applications of separations involving particles suspended in static fluids for detecting the following: (i) the binding of streptavidin to solid-supported biotin and (ii) the binding of citrate-capped gold nanoparticles to amine-modified polystyrene spheres. The paper also demonstrates a microfluidic device in which polystyrene particles that differ in their content of CH2Cl groups are continuously separated and collected in a flowing stream of an aqueous solution of GdCl3. The procedures for separation and detection described in this paper require only gadolinium salts, two NdFeB magnets, and simple microfluidic devices fabricated from poly(dimethylsiloxane). This device requires no power, has no moving parts, and may be suitable for use in resource-poor environments.

This report describes the spontaneous folding of flat elastomeric sheets, patterned with magnetic dipoles, into free-standing, 3D objects that are the topological equivalents of spherical shells. The path of the self-assembly is determined by a competition between mechanical and magnetic interactions. The potential of this strategy for the fabrication of 3D electronic devices is demonstrated by generating a simple electrical circuit surrounding a spherical cavity.folding ͉ microfabrication ͉ 3D structure ͉ soft lithography ͉ soft electronics T he strategies used to form 3D micro-and nanostructures in cells and by humans differ. Proteins, RNAs, and their aggregates, the most complex, 3D molecular structures in nature, form by the spontaneous folding of linear precursors (1). The ubiquity of this strategy reflects the efficiency with which the cell synthesizes linear precursors by sequential formation of covalent bonds. Microelectronic devices, the most complex 3D structures generated by humans, are fabricated by stacking and connecting planar layers (2). This strategy is dictated by the availability of highly developed methods for parallel microfabrication in 2D and the absence of effective, general methods for fabrication in 3D (3).Folding of connected, 2D plates [using robotics (4) or spontaneous folding (5-9)] can yield 3D microelectromechanical systems (MEMS) and microelectronic devices. We (10, 11) and others (4, 12) have explored a number of routes to small 3D shapes based on self-assembly. These strategies are still early in their development.Here, we explore a new strategy for formation of 3D objects that combines the advantages of planar microfabrication with those of 3D self-assembly. Our approach comprises four steps (Fig. 1a): (i) cutting the 3D surface of interest into connected sections that ''almost'' unfold into a plane (unpeeling a sphere as one unpeels an orange is an example); (ii) flattening this surface and projecting it onto a plane; (iii) fabricating the planar projection in the form of an elastomeric membrane patterned with magnetic dipoles; and (iv) allowing this patterned membrane to fold into an ''almost-correct'' 3D shape by self-assembly. This strategy offers the potential to transform easily patterned, functionalized planar sheets into 3D structures and devices. It also raises the problem of designing and generating stable 3D structures by decomposing and projecting these structures into 2D shapes and then balancing the shapes of 2D cuts, the placement of magnetic dipoles, and the mechanical characteristics of the membrane.Converting sheets into 3D objects by folding and creasing is a very old, remarkably interesting, and still incompletely resolved problem in applied mathematics (13-16). The inverse problemmapping the surface of a 3D shape (specifically, the surface of the Earth) onto a flat sheet-has been at the core of cartography since the times of Frisius (1508Frisius ( -1555 and Mercator (1512Mercator ( -1594. Although it is known that a flat, inextensible surface cannot fold i...