Showing papers by "George M. Whitesides published in 2003"

Solvent compatibility of poly(dimethylsiloxane)-based microfluidic devices.

Abstract: This paper describes the compatibility of poly(dimethylsiloxane) (PDMS) with organic solvents; this compatibility is important in considering the potential of PDMS-based microfluidic devices in a number of applications, including that of microreactors for organic reactions. We considered three aspects of compatibility: the swelling of PDMS in a solvent, the partitioning of solutes between a solvent and PDMS, and the dissolution of PDMS oligomers in a solvent. Of these three parameters that determine the compatibility of PDMS with a solvent, the swelling of PDMS had the greatest influence. Experimental measurements of swelling were correlated with the solubility parameter, δ (cal1/2 cm-3/2), which is based on the cohesive energy densities, c (cal/cm3), of the materials. Solvents that swelled PDMS the least included water, nitromethane, dimethyl sulfoxide, ethylene glycol, perfluorotributylamine, perfluorodecalin, acetonitrile, and propylene carbonate; solvents that swelled PDMS the most were diisopropylam...

Microfluidic devices fabricated in Poly(dimethylsiloxane) for biological studies

Abstract: This review describes microfluidic systems in poly(dimethylsiloxane) (PDMS) for biological studies. Properties of PDMS that make it a suitable platform for miniaturized biological studies, techniques for fabricating PDMS microstructures, and methods for controlling fluid flow in microchannels are discussed. Biological procedures that have been miniaturized into PDMS-based microdevices include immunoassays, separation of proteins and DNA, sorting and manipulation of cells, studies of cells in microchannels exposed to laminar flows of fluids, and large-scale, combinatorial screening. The review emphasizes the advantages of miniaturization for biological analysis, such as efficiency of the device and special insights into cell biology.

The 'right' size in nanobiotechnology.

Abstract: The biological and physical sciences share a common interest in small structures (the definition of 'small' depends on the application, but can range from 1 nm to 1 mm). A vigorous trade across the borders of these areas of science is developing around new materials and tools (largely from the physical sciences) and new phenomena (largely from the biological sciences). The physical sciences offer tools for synthesis and fabrication of devices for measuring the characteristics of cells and sub-cellular components, and of materials useful in cell and molecular biology; biology offers a window into the most sophisticated collection of functional nanostructures that exists.

Surface tension-powered self-assembly of microstructures - the state-of-the-art

Abstract: Because of the low dimensional power of its force scaling law, surface tension is appropriate for carrying out reshaping and assembly in the microstructure size domain. This paper reviews work on surface tension powered self-assembly of microstructures. The existing theoretical approaches for rotational assembly are unified. The demonstrated fabrication processes are compared. Mechanisms for accurately determining the assembled shape are discussed, and the limits on accuracy and structural distortion are considered. Applications in optics, electronics and mechanics are described. More complex operations (including the combination of self-assembly and self-organization) are also reviewed.

Kosmotropes Form the Basis of Protein-Resistant Surfaces

Abstract: This paper presents a hypothesis relating the exclusion of a molecule (solute) from the surface of a protein in aqueous solution with the ability of that molecule to render surfaces “protein-resistant”, that is, resistant to the adsorption of proteins from aqueous buffer. While few current data test this hypothesis, it does suggest that surfaces presenting groups derived from certain osmolytesmolecules synthesized by cells to relieve osmotic stresswill be protein-resistant. These predictions were tested by constructing protein-resistant, self-assembled monolayers (SAMs) based on the osmolytes betaine and taurine. Examination of data from the literature also revealed that most of the known protein-resistant surfaces are based on displays of kosmotropesmolecules that stabilize the native structure of proteins. The connection between protein resistance, kosmotropicity, and biological function as an osmolyte may illuminate all three properties.

Formation and structure of self-assembled monolayers of alkanethiolates on palladium.

Abstract: The adsorption of n-alkanethiols onto polycrystalline thin films of palladium containing a strong (111) texture produces well-organized, self-assembled monolayers. The organization of the alkane chains in the monolayer and the nature of the bonding between the palladium and the thiol were studied by contact angle measurements, optical ellipsometry, reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). The XPS data reveals that a compound palladium-sulfide interphase is present at the surface of the palladium film. The RAIR spectra, ellipsometry data, and wetting properties show that the palladium-sulfide phase is terminated with an organized, methyl-terminated monolayer of alkanethiolates. The local molecular environment of the alkane chains transitions from a conformationally disordered, liquidlike state to a mostly all-trans, crystalline-like structure with increasing chain length (n = 8-26). The intensities and dichroism of the methylene and methyl stretching modes support a model for the average orientation of an ensemble of all-trans-conformer chains with a tilt angle of approximately 14-18 degrees with respect to the surface normal and a twist angle of the CCC plane relative to the tilt plane of approximately 45 degrees. The SAMs are stable in air, although the sulfur present at the surface oxidizes in air over a period of 2-5 days at room temperature. The differences in chain organization between SAMs formed by microcontact printing and by solution deposition are also examined by RAIRS and XPS.

Fabrication of Complex Three-Dimensional Microchannel Systems in PDMS

Abstract: This paper describes a method for fabricating three-dimensional (3D) microfluidic channel systems in poly(dimethylsiloxane) (PDMS) with complex topologies and geometries that include a knot, a spiral channel, a “basketweave” of channels, a chaotic advective mixer, a system with “braided” channels, and a 3D grid of channels. Pseudo-3D channels, which are topologically equivalent to planar channels, are generated by bending corresponding planar channels in PDMS out of the plane into 3D shapes. True 3D channel systems are formed on the basis of the strategy of decomposing these complex networks into substructures that are planar or pseudo-3D. A methodology is developed that connects these planar and/or pseudo-3D structures to generate PDMS channel systems with the original 3D geometry. This technique of joining separate channel structures can also be used to create channel systems in PDMS over large areas by connecting features on different substrates. The channels can be used as templates to form 3D structu...

Method and apparatus for fluid dispersion

Abstract: A microfluidic method and device for focusing and/or forming discontinuous sections of similar or dissimilar size in a fluid is provided. The device can be fabricated simply from readily-available, inexpensive material using simple techniques.

Electrochemical desorption of self-assembled monolayers noninvasively releases patterned cells from geometrical confinements.

Abstract: This report describes a method to pattern mammalian cells using self-assembled monolayers (SAMs), and then to use electrochemical desorption of these monolayers to release cells from their patterns. This method uses an oligo(ethyleneglycol)-terminated SAM to prevent,and a methyl-terminated SAM to allowadsorption of proteins and attachment of bovine capillary endothelial cells. Electrochemical removal of the oligo(ethyleneglycol)-terminated SAM allowed proteins to adsorb onto areas that had been previously inert and enabled cells to migrate into these areas. This straightforward technique is useful in bioassays for drug screening and for fundamental studies in cell biology.

Geometric determinants of directional cell motility revealed using microcontact printing

Abstract: Mammalian cells redirect their movement in response to changes in the physical properties of their extracellular matrix (ECM) adhesive scaffolds, including changes in available substrate area, shap...

Adsorption of Proteins to Hydrophobic Sites on Mixed Self-Assembled Monolayers†

Abstract: This paper describes a technique that uses mixed self-assembled monolayers of two alkanethiolates (-S(CH2)11(OCH2CH2)6OR, R = a hydrophobic group, and -S(CH2)11(OCH2CH2)nOH, n = 3, 6, EGnOH), in combination with surface plasmon resonance spectroscopy, to study the influence of the size and shape of R, and its density at the surface, on the hydrophobic adsorption of proteins at solid−liquid interfaces. Detailed results were obtained for β-galactosidase, carbonic anhydrase, lysozyme, and RNase A using R = C(C6H5)3, CH(C6H5)2, and CH2(C6H5). A hard-sphere model is used to rationalize the adsorption; this model, although very approximate, helps to interpret qualitative trends in the data. Using this model, the extent to which adsorbed proteins undergo conformational rearrangements appears to depend on the density of the hydrophobic groups at the surface and on the concentration of protein in solution. This paper describes the first step toward the development of a system that will allow the study of hydrophob...

Electrostatic self-assembly of macroscopic crystals using contact electrification

Abstract: 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.

Using self-assembly for the fabrication of nano-scale electronic and photonic devices

Abstract: Challenges facing the scaling of microelectronics to sub-50 nm dimensions and the demanding material and structural requirements of integrated photonic and microelectromechanical systems suggest that alternative fabrication technologies are needed to produce nano-scale devices. Inspired by complex, functional, self-assembled structures and systems found in Nature we suggest that self-assembly can be employed as an effective tool for nanofabrication. We define a self-assembling system as one in which the elements of the system interact in pre-defined ways to spontaneously generate a higher order structure. Self-assembly is a parallel fabrication process that, at the molecular level, can generate three-dimensional structures with sub-nanometer precision. Guiding the process of self-assembly by external forces and geometrical constraints can reconfigure a system dynamically on demand. We survey some of the recent applications of self-assembly for nanofabrication of electronic and photonic devices. Five self-assembling systems are discussed: 1) self-assembled molecular monolayers; 2) self-assembly in supramolecular chemistry; 3) self-assembly of nanocrystals and nanowires; 4) self-assembly of phase-separated block copolymers; 5) colloidal self-assembly. These techniques can generate features ranging in size from a few angstroms to a few microns. We conclude with a discussion of the limitations and challenges facing self-assembly and some potential directions along which the development of self-assembly as a nanofabrication technology may proceed.

Three-Dimensional Self-Assembly of Metallic Rods with Submicron Diameters Using Magnetic Interactions

Abstract: Metallic rods with submicron diameters that contain disklike ferromagnetic sections self-assemble into highly stable, hexagonally close-packed arrays of rods. The rods were fabricated by electrodeposition in porous alumina membranes and comprised alternating sections of gold and nickel. The thicknesses of the ferromagnetic nickel sections were approximately one-half the diameter of the rods (400 nm); this geometry orients the “easy” axis of magnetization perpendicular to the long axis of the rod. After magnetization of the rods with a rare-earth magnet, followed by sonication of the suspension, the rods spontaneously assembled into three-dimensional (3D) bundles that, on average, contained 15−30 rods. A macroscopic model of the rods suggests that the most stable orientation of the magnetic dipoles for rods in a defect-free, hexagonally close-packed arrangement is in concentric rings with the dipoles oriented head-to-tail. This configuration minimizes the energy of the bundle and does not generate a net di...

Asymmetric Dimers Can Be Formed by Dewetting Half-Shells of Gold Deposited on the Surfaces of Spherical Oxide Colloids

Abstract: Asymmetric dimers consisting of gold microcrystals and spherical silica colloids have been fabricated by depositing thin films of gold onto the spherical colloids to form half-shells, followed by annealing at elevated temperatures. The capability and feasibility of this procedure have been demonstrated with silica and titania beads of 0.2−2 μm in diameter and γ-Fe2O3/polystyrene@SiO2 core−shell particles 0.5 μm in size. The dimensions of gold microcrystals could be conveniently varied in the range of 100−650 nm by controlling the thickness of gold films and/or the diameter of the spherical colloids. This method provides another route to asymmetric dimers made of colloidal particles that could be different in size, chemical composition, surface functionality, density or sign of surface charge, bulk property, or a combination of these properties.

A miniaturized, parallel, serially diluted immunoassay for analyzing multiple antigens

Abstract: This paper describes a microfluidic immunoassay that is applicable to the parallel determination of multiple analytes and that requires only a few microliters of sample. This assay relies on a microchannel network that achieves serial dilution of analytes; this network replaces manual dilutions employed in traditional immunoassays and enables the analysis of multiple analytes simultaneously. The immunoassay was demonstrated by an analysis of concentrations of antibodies against the HIV viral proteins gp120 and gp41 in human serum.

Controlling flows in microchannels with patterned surface charge and topography.

Abstract: This Account reviews two procedures for controlling the flow of fluids in microchannels. The first procedure involves patterning the density of charge on the inner surfaces of a channel. These patterns generate recirculating electroosmotic flows in the presence of a steady electric field. The second procedure involves patterning topography on an inner surface of a channel. These patterns generate recirculation in the cross-section of steady, pressure-driven flows. This Account summarizes applications of these flow to mixing and to controlling dispersion (band broadening).

Millimeter-scale self-assembly and its applications

Abstract: Self-assembly is a concept familiar to chemists. In the molecular and nanoscale regimes, it is often used as a strategy in fabricating regular 3D structures—that is, crystals. Self-assembly of components with sizes in the µm-to-mm range is less familiar to chemists; this type of self-assembly may, however, become technologically important in the future. In this size range, self-assembly offers methods to form regular 3D structures from components too small or too numerous to be manipulated by other means, and methods to incorporate function into these structures; it also offers simplicity and economy. This paper focuses on the use of self-assembly to build functional systems of compo- nents with sizes in the range from microns to millimeters. It compares the principles of self- assembly at the molecular and millimeter scales, reviews the possible applications of meso- scale, self-assembled systems, and outlines some of the most important issues in the use of self-assembly to build functional systems.

Patterning Spherical Surfaces at the Two-Hundred-Nanometer Scale Using Soft Lithography†

Abstract: Two soft lithographic techniques—topographically directed photolithography (TOP) and near-field contact-mode photolithography—have been used to pattern spherical surfaces with features as small as 175 nm. Each technique has the ability to pattern more than a 60° arc of a spherical surface, albeit with distortions at the edge. Use as an optical polarizer demonstrates an application of these types of patterned surface.

Replication of vertical features smaller than 2 nm by soft lithography.

Abstract: This communication demonstrates a simple, soft lithographic approach to the replication and metrology of nanoscale vertical displacements. We patterned test structures with regular patterns that minimize artifacts in measurements by atomic force microscopy. A composite stamp of poly(dimethylsiloxane) (PDMS) molded against the original test structure served as a template to generate polyurethane replicas. We replicated vertical displacements down to approximately 1.5 nm. This replication demonstrates the capability of soft lithography to reproduce features with dimensions similar to those of large molecules.

A tool for studying contact electrification in systems comprising metals and insulating polymers.

Abstract: We describe an analytical system for in situ measurement of the charge that develops by contact electrification when a ferromagnetic sphere rolls on the surface of a polymer. This system makes it possible to survey the ability of polymeric surfaces to charge by contact electrification. Because the measurement of charge using this tool does not require physical contact of the charged sphere with the measuring electrode, it also enables the kinetics of charging to be examined. The research has focused on the contact charging of spheres having a core-and-shell geometry (a common core of ferromagnetic steel, and a variable shell of thin films of metals, or metals with surface oxides) rolling on the surface of polymeric slabs; it has generated an internally consistent set of data that include the polarity and magnitude of charging for a homologous series of polymers that differ chemically in the pendant group on a polyethylene backbone.

Using bifunctional polymers presenting vancomycin and fluorescein groups to direct anti-fluorescein antibodies to self-assembled monolayers presenting d-alanine-d-alanine groups.

Abstract: This paper describes the synthesis of bifunctional polyacrylamides containing pendant vancomycin (Van) and fluorescein groups, and the use of these polymers to direct antibodies against fluorescein to self-assembled monolayers (SAMs) presenting d-alanine-d-alanine (dAdA) groups. These polymers bind biospecifically to these SAMs via interactions between the dAdA and Van groups and serve as a molecular bridge between the anti-fluorescein antibodies and the SAM. The binding events were characterized using surface plasmon resonance spectroscopy and fluorescence microscopy. The paper demonstrates that polyvalent, biospecific, noncovalent interactions between a polymer and a surface can be used to tailor the properties of the surface in molecular recognition. It also represents a first step toward the design of polymers that direct arbitrarily chosen antibodies to the surfaces of cells.

Solving Mazes Using Microfluidic Networks

Abstract: This work demonstrates that pressure-driven flow in a microfluidic network can solve mazelike problems by exploring all possible solutions in a parallel fashion. Microfluidic networks can be fabricated easily by soft lithography and rapid prototyping. To find the best path between the inlet and the outlet of these networks, the channels are filled with a fluid, and the path of a second, dyed fluid moving under pressure-driven flow is traced from the inlet to the outlet. Varying the viscosities of these fluids allows the behavior of the system to be tailored. For example, filling the channels with immiscible fluids of different viscosities enhances the resolution of paths of different fluidic resistances.

Synthesis of Free-Standing Quasi-Two-Dimensional Polymers

Abstract: This paper describes a synthesis of free-standing, 10−15-nm-thick polymer films of well-defined lateral size and shape. The three key elements of this procedure are (1) formation by microcontact printing (μCP) of a patterned, self-assembled monolayer (SAM) with hydrophobic regions (alkane-terminated) and adsorption-resistant regions (oligo(ethylene oxide)-terminated); (2) initiation of spatially selective growth of films of poly(electrolyte) multilayers by adsorption on the hydrophobic regions of the patterned SAM; and (3) dry transfer of these films to a water-soluble sacrificial backing, from which the films can be released into solution. This technique exploits the hydrophobic effect as an interaction that can be switched off when it is not needed: during the growth of the films in aqueous buffer, the hydrophobic effect anchors the polymers to the surface; once these films have been dried after synthesis, they are bound to the substrate only by van der Waals interactions and can be transferred nondest...

Two-dimensional colloid crystals obtained by coupling of flow and confinement

Abstract: This Letter describes the generation of 2D colloidal lattices in microchannels by coupling the laminar flow of dispersions of spherical colloids and geometrical confinement. We describe a non-equilibrium, convective, mechanism leading to formation of ordered 2D structures of both closed-packed hexagonal and non-closed-packed rhombic symmetries. The number and types of possible lattices is determined by the ratio of the width of the channel to the diameter of the particle. The structures tend to return to a regular lattice after a defect is introduced; that is, for example, they tend to self-repair disorder induced by particle polydispersity, contaminants, and flow instabilities. The stability of different lattices is analyzed numerically for particles with different polydispersity.

Customization of Poly(dimethylsiloxane) Stamps by Micromachining Using a Femtosecond‐Pulsed Laser

Abstract: This work describes the use of focused, high-intensity light from a Ti:sapphire laser that generates femtosecond pulses to create topographical structure in a flat surface of poly(di-methylsiloxane) (PDMS), and the use of the PDMS surfaces patterned using this procedure for a range of purposes. PDMS patterned in surface bas-relief is the material most widely used for printing and stamping in soft lithography, [1,2] and a material widely used in microfluidic systems. [3] The bas-relief patterns required in these applications are usually fabricated by casting PDMS against a complementary bas-relief pattern in photoresist, fabricated in turn by photolithography. This process works well, but is not applicable to the preparation of PDMS stamps required for all types of problems; printing on spherical surfaces is an example. The technique described here permits the non-photolitho-graphic generation of surface topography for use in soft lithography and microfluidics; this technique has three useful characteristics: i) It generates features smaller than those generated by standard rapid-prototyping techniques based on printed transparency masks. [1,2] ii) It is applicable to fabrication on non-planar surfaces. iii) It can be used in fabrication of large-area patterns. The rough, recessed features generated by this technique are useful for applications where large surface area to volume ratios are desired, e.g., in catalysis, [7] for super-hydrophobic surfaces, [8±10] in surface-enhanced Raman scattering, [11,12] and as magnetic field concentrators. [13±15] It has the disadvantages that it is a serial process, and that the laser and positioning equipment are relatively specialized. The only requirement for the material is that it must be transparent to 800 nm light: PDMS works well, but other transparent elastomeric polymers should also work. Laser ablation is a common technique for direct writing of patterns into the surfaces of metals, semiconductors, and polymers. [16,17] Most of the techniques that have been described for laser writing work through an absorption mechanism for light that is linear in light intensity. This sort of photochemistry typically requires doping of the material with sensitizers [18±20] and/or the use of UV lasers. [21,22] We have previously described the production of stamps for microcontact printing (lCP) by ablation of a doped polymer with a diode-pumped Nd:YVO 4 laser. [20] Line widths of the features produced through this technique are ~ 5 lm. This technique, as described, is substantially less useful than rapid prototyping. Another process, developed by Hull and co-workers, used a focused ion beam (FIB) to write …

Sub-100 nm confinement of magnetic nanoparticles using localized magnetic field gradients

Abstract: Ferromagnetic rods containing thin sections of diamagnetic metal create intense magnetic field gradients that attract and confine magnetic nanoparticles to regions of space as small as 20 nm. The rods (80 nm diameter) comprised alternating sections of CoNi ( approximately 350 nm) and Au (20-160 nm) formed by electrodeposition into porous polycarbonate membranes. Upon magnetizing the rods, large magnetic gradients (106-107 T/m) form at the boundaries between ferromagnetic and diamagnetic sections. These gradients attract and confine magnetic nanoparticles to attoliter volumes of space surrounding the rod. This method provides a new tool for generating intense, highly localized magnetic field gradients, by design, and confining magnetic nanoparticles in these gradients.