Showing papers by "George M. Whitesides published in 1997"

Geometric control of cell life and death.

Abstract: Human and bovine capillary endothelial cells were switched from growth to apoptosis by using micropatterned substrates that contained extracellular matrix-coated adhesive islands of decreasing size to progressively restrict cell extension. Cell spreading also was varied while maintaining the total cell-matrix contact area constant by changing the spacing between multiple focal adhesion-sized islands. Cell shape was found to govern whether individual cells grow or die, regardless of the type of matrix protein or antibody to integrin used to mediate adhesion. Local geometric control of cell growth and viability may therefore represent a fundamental mechanism for developmental regulation within the tissue microenvironment.

Self-Assembly of Mesoscale Objects into Ordered Two-Dimensional Arrays

Abstract: Regular arrays of topologically complex, millimeter-scale objects were prepared by self-assembly, with the shapes of the assembling objects and the wettability of their surfaces determining the structure of the arrays. The system was composed of solid objects floating at the interface between perfluorodecalin and water and interacting by lateral capillary forces; patterning of the wettability of the surfaces of the objects directs these forces. Self-assembly results from minimization of the interfacial free energy of the liquid-liquid interface. Calculations suggest that this strategy for self-assembly can be applied to objects on a micrometer scale.

Soft lithographic methods for nano-fabrication

Abstract: Soft lithography is a low-cost, non-photolithographic strategy for carrying out micro- and nano-fabrication This unconventional approach consists of techniques based on self-assembly and replica molding of organic molecules and polymeric materials Four such techniques, microcontact printing (µCP), replica molding, micromolding in capillaries (MIMIC), and microtransfer molding (µTM), have been demonstrated for the fabrication of patterns and structures of a variety of materials with dimension ≥30 nm This review describes these techniques and their applications in fabrication and manufacturing at the sub-100 nm scale

Microscopic patterning of orientated mesoscopic silica through guided growth

Abstract: The supramolecular assembly of surfactant molecules at a solid–liquid interface can produce tubular structures with diameters of around 10 nm (refs 1,2,3,4), which can be used for the templated polymerization of mesoporous silica thin films3,4,5. The orientation of the tubules depends primarily on the nature of the substrate–surfactant interaction. These nanostructured films hold much promise for applications such as their use as orientated nanowires6, sensor/actuator arrays7,8,9 and optoelectronic devices10. But a method of patterning the tubules and orientating them into designed arrangements is required for many of these possibilities to be realized. Here we describe a method that allows the direction of growth of these tubules to be guided by infiltrating a reaction fluid into the microcapillaries of a mould in contact with a substrate11. An electric field applied tangentially to the surface within the capillaries induces electro-osmotic flow, and also enhances the rates of silica polymerization around the tubules by localized Joule heating. After removal of the mould, patterned bundles of orientated nanotubules remain on the surface. This method permits the formation of orientated mesoporous channels on a non-conducting substrate with an arbitrary microscopic pattern.

Microfabrication through Electrostatic Self-Assembly

Abstract: We use electrostatic interactions to direct the patterning of gold disks having ∼10-μm diameters on functionalized surfaces. Planar and curved substrates with patterned surface charge were generated either by microcontact printing or by photolithography. Small charged gold disks were generated by electroplating gold into photoresist molds and derivatizing these disks with charged self-assembled monolayers. When agitated as a suspension in contact with the patterned surfaces, the charged gold disks deposited specifically but as disordered aggregates on regions presenting the opposite charge. Positively-charged disks deposited on phosphonate-, carboxylate-, and SiOH-terminated surfaces but not on trimethylammonium- and dimethylammonium-terminated surfaces, and vice versa for negatively-charged disks. Methyl- and CF3-terminated surfaces resisted deposition of disks of either charge. Selective and dense assembly was achieved in methanol, ethanol, 2-propanol, and dioxane; in water, deposition was nonspecific. ...

Using an elastomeric phase mask for sub-100 nm photolithography in the optical near field

Abstract: Bringing an elastomeric phase mask into conformal contact with a layer of photoresist makes it possible to perform photolithography in the near field of the mask. This technique provides an especially simple method for forming features with sizes of 90–100 nm in photoresist: straight lines, curved lines, and posts, on both curved and planar surfaces. It combines experimental convenience, new optical characteristics, and applicability to nonplanar substrates into a new approach to fabrication. Nanowire polarizers for visible light illustrate one application for this technique.

Solvent‐assisted microcontact molding: A convenient method for fabricating three‐dimensional structures on surfaces of polymers

Abstract: Registered names. trademarks. etc. used in this journal. even without specific, indications thereof, are not to be considered unprotected by law. Printed in the Federal Republic of Germany

Three-dimensional self-assembly of millimetre-scale components

Abstract: The spontaneous association of molecules, termed molecular self-assembly, is a successful strategy for the generation of large, structured molecular aggregates1. The most important source of inspiration for this strategy is the biological world, in which many processes involve interfacial interactions and shape selectivity that guide the formation of complex, multicomponent three-dimensional structures. The success of molecular self-assembly notwithstanding, many objectives in science and technology require the assembly of components that are much larger than molecules: examples include microelectronic and microelectro-mechanical systems, sensors and microanalytical and micro-synthetic devices2. Photolithography, the principal technique used to make such microstructures, has certain limitations: it cannot easily form non-planar or three-dimensional structures; it generates structures that are metastable; and it can be used only for a limited set of materials3. Here we describe an approach for the self-assembly of millimetre-scale components that uses two concepts to direct the assembly process: shape recognition and the minimization of liquid–liquid interfacial free energies4. These play a role in other spontaneous self-assembly phenomena, such as the formation of bubble rafts5,6, the patterned dewetting of surfaces7,8, and the coalescence of liquid drops9. We apply self-assembled monolayer molecular films10 to the surfaces of shaped macroscopic objects to render them hydrophilic or hydrophobic, depending on the terminal groups of the bound molecules. In aqueous solution, hydrophobic surfaces bearing a thin film of a hydrophobic, lubricating liquid adhere to similar surfaces with complementary shapes, while being able to adjust their relative alignment to ensure a good fit. In this way, the components assemble into well defined aggregates, which can be bound permanently when the hydrophobic liquid films consist of a polymerizable adhesive.

Selective Deposition of Conducting Polymers on Hydroxyl-Terminated Surfaces with Printed Monolayers of Alkylsiloxanes as Templates

Abstract: This paper describes the use of patterned self-assembled monolayers (SAMs) in area-selective deposition of conducting polymers (polypyrrole and polyaniline) on insulating, hydroxyl-terminated surfaces such as Si/SiO2 and glass. Patterned SAMs of octadecylsiloxane were generated on the hydroxyl-terminated surfaces using microcontact printing; they defined and directed the deposition of conducting polymers. The rate of deposition on the hydrophobic surface is higher than that on the hydrophilic surface: immersion of a substrate patterned with a methyl-terminated SAM in an appropriate aqueous polymerization bath produced a “positive” pattern of the conducting polymer on the surface. The conducting polymer deposited on the hydrophobic region of a surface completely covered by the polymer could be readily removed by transferring it to adhesive tape to form a “positive” pattern on the tape, leaving a “negative” pattern on the surface of the substrate. The conducting polymer deposited on the hydrophobic surface...

Extending Microcontact Printing as a Microlithographic Technique

Abstract: This paper describes a number of approaches that have been employed to reduce the size of features of self-assembled monolayers (SAMs) generated using microcontact printing (μCP). In μCP, an elastomeric stamp is used to print patterned SAMs of alkanethiolates on the surfaces of coinage metals and SAMs of alkylsiloxanes on Si/SiO2. It is a convenient technique for generating patterned microstructures with feature sizes ≥ 500 nm. The capability of this technique could be extended to produce features smaller than 500 nm using the following approaches: (1) μCP with mechanical deformation of the elastomeric stampthat is, with lateral compression or uniaxial stretching in the plane of the stamp and with pressure perpendicular to the plane of the stamp; (2) μCP with physical alternation of the elastomeric stampthat is, with a stamp that has been swelled with a solvent or a stamp whose dimensions have been reduced by extraction of an inert filler; (3) μCP with reduction in the size of features resulting from pro...

Noncovalent Polycationic Coatings for Capillaries in Capillary Electrophoresis of Proteins

Abstract: The adsorption of proteins with net positive charges (pI > pH) on the walls of fused-silica capillaries is a common problem in the analysis of proteins by capillary electrophoresis. This paper explores the use of polycationic polymers as noncovalent coatings to limit this problem. The behavior of three sets of proteins was compared using uncoated and coated capillaries: (i) a protein charge ladder obtained by acetylation of lysozyme (EC 3.2.1.17); (ii) a protein charge ladder obtained by acetylation of carbonic anhydrase II (EC 4.2.1.1); (iii) a test panel of proteins with a range of values of molecular weight and pI. Four polycationic polymers were examined: polyethylenimine (PEI; MWav = 15 000), Polybrene (MWav = 25 000), poly(methoxyethoxyethyl)ethylenimine (MWav = 64 000), and poly(diallyldimethylammonium chloride) (MWav = 10 000). Detection of proteins with high pI was readily achieved using the first three of these polycationic polymer coatings but not with the poly(diallyldimethylammonium chlorid...

Generation and in Situ Evaluation of Libraries of Poly(acrylic acid) Presenting Sialosides as Side Chains as Polyvalent Inhibitors of Influenza-Mediated Hemagglutination

Abstract: This paper describes a simple, microscale method for generating and evaluating libraries of derivatives of poly(acrylic acid) (pAA) that present mixtures of side chains that influence their biologi...

Using Surface Plasmon Resonance Spectroscopy To Measure the Association of Detergents with Self-Assembled Monolayers of Hexadecanethiolate on Gold

Abstract: This paper describes the use of surface plasmon resonance (SPR) spectroscopy to measure the rates and extents of association of four detergentssodium dodecyl sulfate (SDS), β-octyl glucoside, Triton X-100, and Tween 20to self-assembled monolayers (SAMs) of alkanethiolates on gold. SAMs presenting hexaethylene glycol groups resisted the adsorption of all four detergents. These same detergents associated with hydrophobic SAMs presenting methyl groups; the concentration of detergent molecules on the surface was 120−280 pmol/cm2. The associations of the detergents with the hydrophobic SAM were described well by the Langmuir adsorption isotherm. The dissociation constants Kd (M) for the desorption of the detergents from the surface correlated with the critical micelle concentration (cmc, M) of the detergents in solution, and followed the relationship cmc ≈ 7 (±2)Kd. The efficacy of SDS in removing the protein fibrinogen adsorbed on a hydrophobic SAM depended strongly on the concentration of detergent. SDS at a...

Formation and Reaction of Interchain Carboxylic Anhydride Groups on Self-Assembled Monolayers on Gold

Abstract: This paper demonstrates a new method for the synthesis of mixed self-assembled monolayers (mixed SAMs) by reaction of a reactive intermediatean interchain carboxylic anhydridewith alkylamines. The interchain anhydride was prepared in high yield from a SAM of 16-mercaptohexadecanoic acid (HS(CH2)15COOH) on gold by treatment with trifluoroacetic anhydride. Alkylamines reacted cleanly with the interchain anhydride to generate a mixed SAM, which comprised a mixture of acids and amides in approximately 1:1 ratio on the surface. The SAMs of the interchain anhydride and the mixed SAM were characterized using polarized infrared external reflectance spectroscopy, X-ray photoelectron spectroscopy, contact angle, and ellipsometry. Control of the wettability of the SAMs was demonstrated by allowing the interchain anhydride to react with alkylamines having different alkyl groups; this model system gave wetting data consistent with earlier studies of mixed monolayers and verified the ability of this method to manipulat...

Using Self-Assembled Monolayers That Present Oligo(ethylene glycol) Groups to Control the Interactions of Proteins with Surfaces

Abstract: This chapter reviews the use of self-assembled monolayers of alkanethiolates on gold to control the interactions of proteins and cells with man-made materials. The work is based on the ability of monolayers that present oligo(ethylene glycol) groups to resist the non-specific adsorption of protein. The chapter describes the use of functionalized monolayers for the bioand chemo-specific adsorption of proteins. The chapter concludes with a discussion of techniques that can pattern the formation of monolayers and that can prepare tailored substrates for the control of cell attachment.

Fabrication and Characterization of Glassy Carbon MEMS

Abstract: This paper describes the fabrication of free-standing high-carbon microstructures by soft-lithographic techniques; these structures ranged in complexity from simple beams to complex, suspended defl...

Imbibition and Flow of Wetting Liquids in Noncircular Capillaries

Abstract: Micromolding in capillaries (MIMIC) has been used to study the dynamics of imbibition of liquid prepolymers in micrometer-scale, rectangular capillaries formed between an elastomer containing relief structures and a self-assembled monolayer (SAM) of alkanethiolates on gold supported on Si/SiO2. Self-assembled monolayers provide a wide range of solid/vapor interfacial free energies (γSV) and a control over wetting and spreading of liquids in the capillaries. The dynamic shapes of the liquids in the capillaries are obtained as solid polymers cross-linked photochemically and examined using scanning electron and atomic force microscopies. The shape of the imbibing liquid on a surface with high γSV shows that precursor structures precede macroscopic flow; on low γSV, these precursor structures are absent. Rates of capillary imbibition are linearly correlated to the cosine of static advancing contact angles (θa) of a liquid prepolymer on different SAMs. Different regimes of spreading are observed on a surface h...

On-Line Detection of Nonspecific Protein Adsorption at Artificial Surfaces

Abstract: A detailed understanding of the interaction of proteins with artificial surfaces is essential for many applications in medicine and biochemistry. The affinity of surfaces toward proteins may, for instance, remove pharmacological proteins from media or control the adherence of pathogenic bacteria to protheses. Only a few analytical techniques now exist that can be used to study the binding process in real time, using unlabeled proteins. By investigating the adsorption kinetics of fibrinogen at differently terminated self-assembled monolayers (SAMs) of alkanethiols on thin gold films, it is demonstrated that acoustic plate-mode sensors are a promising analytical tool for studying the adsorption of proteins. In agreement with previous studies for fibrinogen, it is shown in situ that hexa(ethylene glycol)-terminated SAMs (HS(CH2)11(OCH2CH2)6OH) exhibit very low protein adsorption and that methyl-terminated SAMs (HS(CH2)11CH3) tend to adsorb large amounts of protein nonspecifically. The observed adsorption kin...

Hydrogen-Bonded Tapes Based on Symmetrically Substituted Diketopiperazines: A Robust Structural Motif for the Engineering of Molecular Solids

Abstract: A series of eight symmetrically substituted diketopiperazines (DKPs) derived from 1-amino-1-carboxycycloalkanes (n = 3−7; 3,3,5,5-tetramethylcyclohexane; 4,4-dimethylcyclohexane; 2-indan) were synthesized and their crystal structures determined. In the solid state, all eight compounds form two pairs of hydrogen bonds with two adjacent molecules to form a one-dimensional structure that we refer to as "tapes". These molecules represent a range of volumes and shapes that contain a common molecular fragment (DKP ring). We examined this series of compounds with three objectives in mind: (i) to establish the ability of the hydrogen-bonded "tape" motif to persist through these differences in volume and shape; (ii) to provide a series of structurally related compounds to use to test computational methods of predicting crystal structure from molecular structure; (iii) to search for qualitative correlations between molecular structure and crystal packing. All compounds form tapes and with one exception, all tapes pack with their long axes parallel. When viewed down their long axis, two types of tapes emerge: planar and nonplanar. The type of tape that forms reflects the conformation adapted by the DKP ringplanar or boat. Planar tapes form when the angle (α) between the two planes defined by the cis-amides in the DKP ring is 180°; nonplanar tapes form when α < 180°. Five of the eight compounds studied form planar tapes, the remaining three compounds form nonplanar tapes. Despite the variability in volume and shape represented by this series of molecules, the persistence of the tape motif in their crystalline solids suggests that the hydrogen-bonding interactions between DKPs dominate the packing arrangement of these molecules. Void space in the crystalline solid is minimized by parallel alignment of tapes that pack in a manner that permits the interdigitation of substituents on adjacent tapes.

Using microcontact printing to fabricate microcoils on capillaries for high resolution proton nuclear magnetic resonance on nanoliter volumes

Abstract: This letter describes a method for producing conducting microcoils for high resolution proton nuclear magnetic resonance (1H-NMR) spectroscopy on nanoliter volumes. This technique uses microcontact printing and electroplating to form coils on microcapillaries. Nuclear magnetic resonance spectra collected using these microcoils, have linewidths less than 1 Hz for model compounds and a limit of detection (signal-to-noise ratio=3) for ethylbenzene of 2.6 nmol in 13 min.

Elastomeric light valves

Abstract: Electromagnetic radiation valves are provided. In one system, a transparent, elastomeric article is provided having a contoured surface including protrusions and intervening indentations. The contoured surface is placed adjacent a transparent plate and a void pattern defined by indentations of the contoured surface is filled with an opaque fluid. The protrusions decrease in cross-sectional area in the direction in which they extend away from the bulk of the elastomeric article, thus compression of the elastomeric protrusions against the transparent plate results in an increase in surface area in elastomeric protrusions against the transparent plate due to deformation of the protrusions, and the opaque fluid is displaced from the voids. Increased transmission of the electromagnetic radiation results. In another system, a similar elastomeric article includes an array of corner cubes that are totally internally reflective of the electromagnetic radiation. Increased deformation of the protrusions of the transparent plate results in decreased total internal reflection of light, due to the progressive destruction of the corner-cube geometry, and the passage of light is increased with compression.

Self-assembly of mesoscale objects

Abstract: Self-assembling systems include component articles that can be pinned at a fluid/fluid interface, or provided in a fluid, or provided in proximity of a surface, and caused to self-assemble optionally via agitation. A self-assembling electrical circuit is provided.

Carbonic Anhydrase-Inhibitor Binding: From Solution to the Gas Phase

Abstract: In this report, we compare the kinetic stabilities of noncovalent complexes between bovine carbonic anhydrase II(BCAII, EC 4.2.1.1) and para-substituted benzenesulfonamide inhibitors in the gas phase and in solution. These BCAII-inhibitor systems are attractive model systems due to the stability of carbonic anhydrase (CA) and its well characterized structure and ligand complexes, providing a basis for inferences regarding the protein structure in the gas phase and its ligand interactions. CA is a roughly spherical Zn(II) metalloenzyme having a conical binding pocket which catalyzes the hydration of CO{sub 2} to bicarbonate. A large body of data correlate structures of sulfonamide ligands with their binding constants to CA. A set of eight inhibitors was selected for this study, covering a wide range of binding affinities and varying in the length of their tails and aromatic content. The results demonstrate that relative stabilities of BCAII-inhibitor complexes differ substantially between the gas and liquid phases and also show the dominant role of polar surface interactions in the gas phase. 12 refs., 1 fig., 1 tab.

Using soft lithography to fabricate GaAs/AlGaAs heterostructure field effect transistors

Abstract: This letter describes the fabrication of functional GaAs/AlGaAs field effect transistors using micromolding in capillaries—a representative soft lithographic technique. The fabrication process involved three soft lithographic steps and two registration steps. Room temperature characteristics of these transistors resemble those of field effect transistors fabricated by photolithography. The fabrication of functional microelectronic devices using multilayer soft lithography establishes the compatibility of these techniques with the processing methods used in device fabrication, and opens the door for their development as a technique in this area.

Tight Binding of a Dimeric Derivative of Vancomycin with Dimeric L-Lys-D-Ala-D-Ala

Abstract: The ligand/receptor pair consisting of a synthetic dimeric derivative of vancomycin (V), linked at the C terminus by p-xylylenediamine (V-CONHCH2C6H4CH2NHCO-V), and a dimeric derivative of l-Lys-d-Ala-d-Ala, [CH2CONeH(Nα-Ac)-l-Lys-d-Ala-d-Ala-CO2-]2, provides a new system with which to study the influence of divalency on the strength of binding. A competitive assay using affinity capillary electrophoresis (ACE) has been developed and used to estimate the dissociation constant of the divalent complex (≈ 1.1 nM) and the enhancement in binding (∼103) relative to the corresponding monomeric interaction between unmodified monomeric vancomycin and diacetyl-l-Lys-d-Ala-d-Ala.

Nanometer scale patterning and pattern transfer on amorphous Si, crystalline Si, and SiO2 surfaces using self-assembled monolayers

Abstract: Microcontact printing was used to form nanometer scale patterns of self-assembled monolayers (SAMs) on amorphous Si, crystalline Si, and SiO2 using octadecyltrichlorosilane (OTS) as the ink and an elastomer as the stamp. The patterns were subsequently transferred into crystalline Si substrates or amorphous Si films using the SAM of OTS as the resist film. Atomic force microscopy was used to characterize the quality of the SAM and the resulting patterns. Using a Si pillar structure as the master, “pancakes” of less than 80 nm in size were formed by over etching of the patterned OTS film on amorphous Si using KOH. The size of the resulting amorphous Si pancakes can be controlled by the etching time.

Fabrication of glassy carbon microstructures by pyrolysis of microfabricated polymeric precursors

Abstract: Communications formed with this method is limited by the resolution of the microcontact printing (-0.2 pm)J2] by the ability to set the angle and position of the stamp relative to the object to be patterned (-0.01\", and-2 Fm for our apparatus))'] and by uncertainties in the dimensions of the curved support. The composition of these parts is limited to materials that can be electrodeposited. We believe that our method offers a simple means for fabrication of complex three-dimensional microstructures that will be a valuable complement to other means for microfabrication. Many applications in biotechnology and MEMS should be found. This paper describes a method of fabricating microstruc-tures of glassy carbon and other high-carbon solids. This method uses micromolding of polymers to form precursors to these structures, and pyrolysis of these polymeric micro-structures to form the carbon solids. These microstructures are stiff and can be either electrically insulating or conducting , depending on their thermal history. In addition, carbon structures can be very stable thermally, and the surface of carbon is readily functionalized.[',21 We are developing carbon microstructures as components in microelectromecha-nical systems (MEMS), microreactors, and other systems in which thermal stability, chemical inertness, engineered surface properties, and electrical conductivity are useful characteristics. The mechanical, electrical, and chemical properties of high-carbon solids can be controlled over wide ranges by the temperature at which these solids are prepared.'1231 The range of electrical properties that can be achieved is especially interesting: the conductivity of a phenol-formalde-hyde resin can be controlled over a range of-10L9!x' cm-' by pyrolysis. Insulating, semiconducting, and semimetallic behaviors have been observed for pyrolyzed novolac photoresists, depending on the pyrolysis conditions.[42s1 Glassy carbon is often produced by carbonization of organic polymeric precursors; resins of furfuryl alcohol, phenol -formaldehyde, acetone-furfural, or furfuryl alcohol-phenol copolymers are among those commonly used for this purpose.'*] In this work, copolymers of phenol-formal-dehyde or furfuryl alcohol-phenol were formed in patterns on appropriate substrates (silica, Si/Si02, glassy carbon) using micro-molding in capillaries (MIMIC)[6-s1 and micro-transfer molding (pTM).I9' The resulting polymeric structures were carbonized at high temperatures in vacuum (600-1100 \"C, 10-6-10-7 torr) or in an inert atmosphere (ar-gon), either on the substrate or after being lifted from the surface, to yield high-carbon structures having micron-scale dimensions. Supported structures of a phenol-formaldehyde resin and of a furfuryl alcohol-modified phenolic resin were prepared by MIMIC, using a patterned poly(dimethylsi1oxane)

Fabrication of carbon microstructures

Abstract: A technique for forming high-carbon structural articles, such as glassy carbon articles, is provided that involves molding fluid precursors of the article into a predetermined shape, followed by carbonization of the articles An elastomeric mold can be used to mold the precursors, and the resultant, free-standing articles have features on the micron or nanometer scale A variety of useful articles including sensors, actuators, microelectromechanical systems, transmission electron microscopy grids, and the like are provided