Showing papers by "George M. Whitesides published in 2012"

Camouflage and Display for Soft Machines

Abstract: Synthetic systems cannot easily mimic the color-changing abilities of animals such as cephalopods. Soft machines--machines fabricated from soft polymers and flexible reinforcing sheets--are rapidly increasing in functionality. This manuscript describes simple microfluidic networks that can change the color, contrast, pattern, apparent shape, luminescence, and surface temperature of soft machines for camouflage and display. The color of these microfluidic networks can be changed simultaneously in the visible and infrared--a capability that organisms do not have. These strategies begin to imitate the functions, although not the anatomies, of color-changing animals.

Elastomeric Origami: Programmable Paper-Elastomer Composites as Pneumatic Actuators

Abstract: The development of soft pneumatic actuators based on composites consisting of elastomers with embedded sheet or fiber structures (e.g., paper or fabric) that are flexible but not extensible is described. On pneumatic inflation, these actuators move anisotropically, based on the motions accessible by their composite structures. They are inexpensive, simple to fabricate, light in weight, and easy to actuate. This class of structure is versatile: the same principles of design lead to actuators that respond to pressurization with a wide range of motions (bending, extension, contraction, twisting, and others). Paper, when used to introduce anisotropy into elastomers, can be readily folded into 3D structures following the principles of origami; these folded structures increase the stiffness and anisotropy of the elastomeric actuators, while being light in weight. These soft actuators can manipulate objects with moderate performance; for example, they can lift loads up to 120 times their weight. They can also be combined with other components, for example, electrical components, to increase their functionality.

Measuring Markers of Liver Function Using a Micropatterned Paper Device Designed for Blood from a Fingerstick

Abstract: This paper describes a paper-based microfluidic device that measures two enzymatic markers of liver function (alkaline phosphatase, ALP, and aspartate aminotransferase, AST) and total serum protein. A device consists of four components: (i) a top plastic sheet, (ii) a filter membrane, (iii) a patterned paper chip containing the reagents necessary for analysis, and (iv) a bottom plastic sheet. The device performs both the sample preparation (separating blood plasma from erythrocytes) and the assays; it also enables both qualitative and quantitative analysis of data. The data obtained from the paper-microfluidic devices show standard deviations in calibration runs and “spiked” standards that are acceptable for routine clinical use. This device illustrates a type of test useable for a range of assays in resource-poor settings.

A Paper-Based Multiplexed Transaminase Test for Low-Cost, Point-of-Care Liver Function Testing

Abstract: In developed nations, monitoring for drug-induced liver injury via serial measurements of serum transaminases (aspartate aminotransferase (AST) and alanine aminotransferase (ALT)) in at-risk individuals is the standard of care. Despite the need, monitoring for drug-related hepatotoxicity in resource-limited settings is often limited by expense and logistics, even for patients at highest risk. This manuscript describes the development and clinical testing of a paper-based, multiplexed microfluidic assay designed for rapid, semi-quantitative measurement of AST and ALT in a fingerstick specimen. Using 223 clinical specimens obtained by venipuncture and 10 fingerstick specimens from healthy volunteers, we have shown that our assay can, in 15 minutes, provide visual measurements of AST and ALT in whole blood or serum which allow the user to place those values into one of three readout “bins” ( 5x ULN, corresponding to tuberculosis/HIV treatment guidelines) with >90% accuracy. These data suggest that the ultimate point-of-care fingerstick device will have high impact on patient care in low-resource settings.

Separation of Nanoparticles in Aqueous Multiphase Systems through Centrifugation

Abstract: This paper demonstrates the use of aqueous multiphase systems (MuPSs) as media for rate-zonal centrifugation to separate nanoparticles of different shapes and sizes. The properties of MuPSs do not change with time or during centrifugation; this stability facilitates sample collection after separation. A three-phase system demonstrates the separation of the reaction products (nanorods, nanospheres, and large particles) of a synthesis of gold nanorods, and enriches the nanorods from 48 to 99% in less than ten minutes using a benchtop centrifuge.

Paper‐Based, Capacitive Touch Pads

Abstract: This paper describes low-cost, thin, and pliable touch pads constructed from a commercially available, metallized paper commonly used as packaging material for beverages and book covers. The associated electronics with the individual keys in the touch pads detect changes in capacitance or contact with fi ngers by using the effective capacitance of the human body and the electrical impedance across the tip of a fi nger. To create the individual keys, a laser cutter ablates lines through the fi lm of evaporated aluminum on the metallized paper to pattern distinct, conductive regions. This work includes the experimental characterization of two types of capacitive buttons and illustrates their use with applications in a keypad with 10 individually addressable keys , a keypad that conforms to a cube, and a keypad on an alarmed cardboard box. With their easily arrayed keys, environmentally benign material, and low cost, the touch pads have the potential to contribute to future developments in disposable, fl exible electronics, active, “smart” packaging, user interfaces for biomedical instrumentation, biomedical devices for the developing world, applications for monitoring animal and plant health, food and water quality, and disposable games (e.g., providers of content for consumer products). There is no simple method of integrating buttons with structures on single-use or throw-away devices. Current commercial buttons are not thin enough, inexpensive enough, or easy enough to array seamlessly with paper-based products for disposable applications. The touch pads in this work are thin ( ∼ 60 μ m in some cases), simple to array, fabricated by etching patterns into metallized paper, low-cost ( < $0.25 m − 2 for the thin grade of metallized paper we use in this work), and lightweight

Electrical Resistance of Ag-TS-S(CH2)(n-1)CH3//Ga2O3/EGaln Tunneling Junctions

Abstract: Tunneling junctions having the structure AgTS–S(CH2)n−1CH3//Ga2O3/EGaIn allow physical–organic studies of charge transport across self-assembled monolayers (SAMs). In ambient conditions, the surface of the liquid metal electrode (EGaIn, 75.5 wt % Ga, 24.5 wt % In, mp 15.7 °C) oxidizes and adsorbs―like other high-energy surfaces―adventitious contaminants. The interface between the EGaIn and the SAM thus includes a film of metal oxide, and probably also organic material adsorbed on this film; this interface will influence the properties and operation of the junctions. A combination of structural, chemical, and electrical characterizations leads to four conclusions about AgTS–S(CH2)n−1CH3//Ga2O3/EGaIn junctions. (i) The oxide is ∼0.7 nm thick on average, is composed mostly of Ga2O3, and appears to be self-limiting in its growth. (ii) The structure and composition (but not necessarily the contact area) of the junctions are conserved from junction to junction. (iii) The transport of charge through the junction...

Paper-Based Analytical Device for Electrochemical Flow-Injection Analysis of Glucose in Urine

Abstract: This article describes a new design for a paper-based electrochemical system for flow-injection analysis. Capillary wicking facilitates a gravity-driven flow of buffer solution continuously through paper and nitrocellulose, from a buffer reservoir at one end of the device to a sink at the other. A difference in height between the reservoir and the sink leads to a continuous and constant flow. The nitrocellulose lies horizontally on a working electrode, which consists of a thin platinum layer deposited on a solid support. The counter and reference electrodes are strategically positioned upstream in the buffer reservoir. A simple pipetting device was developed for reliable application of (sub)microliter volumes of sample without the need of commercial micropipets; this device did not damage the nitrocellulose membrane. Demonstration of the system for the determination of the concentration of glucose in urine resulted in a noninvasive, quantitative assay that could be used for diagnosis and monitoring of dia...

The SAM, Not the Electrodes, Dominates Charge Transport in Metal-Monolayer//Ga2O3/Gallium–Indium Eutectic Junctions

Abstract: The liquidmetal eutectic of gallium and indium (EGaIn) is a useful electrode for making soft electrical contacts to self-assembled monolayers (SAMs). This electrode has, however, one feature whose effect on charge transport has been incompletely understood: a thin (approximately 0.7 nm) film;consisting primarily of Ga2O3;that covers its surface when in contact with air. SAMs that rectify current have been measured using this electrode in Ag TS -SAM//Ga2O3/EGaIn (where Ag TS = template-stripped Ag surface) junctions. This paper organizes evidence, both published and unpublished, showing that the molecular structure of the SAM (specifically, the presence of an accessiblemolecular orbitalasymmetricallylocated withintheSAM), not thedifferencebetween the electrodes or the characteristics of the Ga2O3 film, causes the observed rectification. By examining and ruling out potential mechanisms of rectification that rely either on the Ga2O3 film or on the asymmetry of the electrodes, this paper demonstrates that the structure of the SAM dominates charge transport through Ag TS -SAM//Ga2O3/EGaIn junctions, and that the electrical characteristics of the Ga2O3 film have a negligible effect on these measurements.

A microfluidic device for whole-animal drug screening using electrophysiological measures in the nematode C. elegans

Abstract: This paper describes the fabrication and use of a microfluidic device for performing whole-animal chemical screens using non-invasive electrophysiological readouts of neuromuscular function in the nematode worm, C. elegans. The device consists of an array of microchannels to which electrodes are attached to form recording modules capable of detecting the electrical activity of the pharynx, a heart-like neuromuscular organ involved in feeding. The array is coupled to a tree-like arrangement of distribution channels that automatically delivers one nematode to each recording module. The same channels are then used to perfuse the recording modules with test solutions while recording the electropharyngeogram (EPG) from each worm with sufficient sensitivity to detect each pharyngeal contraction. The device accurately reported the acute effects of known anthelmintics (anti-nematode drugs) and also correctly distinguished a specific drug-resistant mutant strain of C. elegans from wild type. The approach described here is readily adaptable to parasitic species for the identification of novel anthelmintics. It is also applicable in toxicology and drug discovery programs for human metabolic and degenerative diseases for which C. elegans is used as a model.

The rate of charge tunneling through self-assembled monolayers is insensitive to many functional group substitutions.

Abstract: At its conception, the field of molecular electronics promised to provide the ability to engineer the rate of charge transport, by design of the molecular structure of electronic junctions.[1] The hypothesis was that the electronic and geometrical structure of molecules in a junction would have a significant and predictable effect on the rate and mechanism of charge transport through their influence on the energetic topography of the tunneling barrier. Here we show the preparation and electrical characterization of junctions (Figure 1) of the structure AgTS/S(CH2)4CONH(CH2)2R//Ga2O3/EGaIn (AgTS = template-stripped silver surface[2]; R = tail group; EGaIn = eutectic gallium and indium alloy; Ga2O3 = a passivating metal oxide film on the surface of the EGaIn[3–5]) including a range of common aliphatic, aromatic, and heteroaromatic organic tail groups. We demonstrate that the rate of charge transport across these self-assembled monolayers (SAMs) is surprisingly insensitive to changes in the structure of the organic molecules of which they are composed. This study is based on a physical-organic design: that is, the information it provides comes from comparisons of rates of tunneling across related structures, rather than from the interpretation of the absolute values of single measurements. Figure 1 A) Schematic description of tunneling junction consisting of a template-stripped Ag bottom-electrode supporting a SAM, and contacted by a Ga2O3/EGaIn top-electrode. B) A schematic of one junction. C) The numbering system based on non-hydrogen atoms in ... Targets for shaping the tunneling barriers of molecular junctions have included electron–donor–bridge–acceptor molecules,[1a,6] molecular quantum dot systems,[7] aromatic molecules,[8] and complex organic molecules with multiple functional groups.[9] Many of these studies ostensibly shaping the tunneling barriers of molecular junctions have, however, been difficult to interpret because, when they were carried out, there were no experimental systems that generated well-characterized, statistically validated data. This paper characterizes the rates of charge transport by tunneling across a series of molecules—arrayed in SAMs—containing a common head group and body (HS(CH2)4CONH(CH2)2-) and structurally varied tail groups (-R); these molecules are assembled in junctions of the structure AgTS/SAM//Ga2O3/EGaIn. Over a range of common aliphatic, aromatic, and heteroaromatic organic tail groups, changing the structure of R does not significantly influence the rate of tunneling. In making these measurements, we utilize C12 and C18 alkanethiols as calibration standards to allow comparison with results from other types of junctions. Limited studies[4,5,10–15] of charge transport using a range of junctions have described the relation between molecular structure and the rate of tunneling. For example, Venkataraman et al.[14] reported that the rate of charge transport through a series of diaminobenzenes depends on the alignment of the metal Fermi level to the closest molecular orbital. Chiechi and Solomon et al.[15] compared the rate of charge transport through three different anthracene derivatives of approximately the same thickness, and concluded that conjugation influences the rate of charge transport. Studies exploring the correlation between molecular structure and charge transport based on systematic physical–organic measurements of the rate of charge transport over a wide range of structures are sparse. This paper describes tunneling rates through SAMs of molecules with a variety of molecular structures including aromatic, heterocyclic, and aliphatic moieties. We have previously examined ferrocene-terminated SAMs[4] and SAMs comprising odd-and even-numbered n-alkanethiolates.[5]

Aqueous multiphase systems of polymers and surfactants provide self-assembling step-gradients in density.

Abstract: This Communication demonstrates the generation of over 300 phase-separated systems—ranging from two to six phases—from mixtures of aqueous solutions of polymers and surfactants. These aqueous multiphase systems (MuPSs) form self-assembling, thermodynamically stable step-gradients in density using a common solvent, water. The steps in density between phases of a MuPS can be very small (Δρ ≈ 0.001 g/cm3), do not change over time, and can be tuned by the addition of co-solutes. We use two sets of similar objects, glass beads and pellets of different formulations of Nylon, to demonstrate the ability of MuPSs to separate mixtures of objects by differences in density. The stable interfaces between phases facilitate the convenient collection of species after separation. These results suggest that the stable, sharp step-gradients in density provided by MuPSs can enable new classes of fractionations and separations based on density.

Statistical Tools for Analyzing Measurements of Charge Transport

Abstract: This paper applies statistical methods to analyze the large, noisy data sets produced in measurements of tunneling current density (J) through self-assembled monolayers (SAMs) in large-area junctions. It describes and compares the accuracy and precision of procedures for summarizing data for individual SAMs, for comparing two or more SAMs, and for determining the parameters of the Simmons model (β and J0). For data that contain significant numbers of outliers (i.e., most measurements of charge transport), commonly used statistical techniquese.g., summarizing data with arithmetic mean and standard deviation and fitting data using a linear, least-squares algorithmare prone to large errors. The paper recommends statistical methods that distinguish between real data and artifacts, subject to the assumption that real data (J) are independent and log-normally distributed. Selecting a precise and accurate (conditional on these assumptions) method yields updated values of β and J0 for charge transport across both odd and even n-alkanethiols (with 99% confidence intervals) and explains that the so-called odd−even effect (for n-alkanethiols on Ag) is largely due to a difference in J0 between odd and even n-alkanethiols. This conclusion is provisional, in that it depends to some extent on the statistical model assumed, and these assumptions must be tested by future experiments.

Dependence of avidity on linker length for a bivalent ligand-bivalent receptor model system.

Abstract: This paper describes a synthetic dimer of carbonic anhydrase, and a series of bivalent sulfonamide ligands with different lengths (25 to 69 A between the ends of the fully extended ligands), as a m...

The Core Apoptotic Executioner Proteins CED-3 and CED-4 Promote Initiation of Neuronal Regeneration in Caenorhabditis elegans

Abstract: A critical accomplishment in the rapidly developing field of regenerative medicine will be the ability to foster repair of neurons severed by injury, disease, or microsurgery. In C. elegans, individual visualized axons can be laser-cut in vivo and neuronal responses to damage can be monitored to decipher genetic requirements for regeneration. With an initial interest in how local environments manage cellular debris, we performed femtosecond laser axotomies in genetic backgrounds lacking cell death gene activities. Unexpectedly, we found that the CED-3 caspase, well known as the core apoptotic cell death executioner, acts in early responses to neuronal injury to promote rapid regeneration of dissociated axons. In ced-3 mutants, initial regenerative outgrowth dynamics are impaired and axon repair through reconnection of the two dissociated ends is delayed. The CED-3 activator, CED-4/Apaf-1, similarly promotes regeneration, but the upstream regulators of apoptosis CED-9/Bcl2 and BH3-domain proteins EGL-1 and CED-13 are not essential. Thus, a novel regulatory mechanism must be utilized to activate core apoptotic proteins for neuronal repair. Since calcium plays a conserved modulatory role in regeneration, we hypothesized calcium might play a critical regulatory role in the CED-3/CED-4 repair pathway. We used the calcium reporter cameleon to track in vivo calcium fluxes in the axotomized neuron. We show that when the endoplasmic reticulum calcium-storing chaperone calreticulin, CRT-1, is deleted, both calcium dynamics and initial regenerative outgrowth are impaired. Genetic data suggest that CED-3, CED-4, and CRT-1 act in the same pathway to promote early events in regeneration and that CED-3 might act downstream of CRT-1, but upstream of the conserved DLK-1 kinase implicated in regeneration across species. This study documents reconstructive roles for proteins known to orchestrate apoptotic death and links previously unconnected observations in the vertebrate literature to suggest a similar pathway may be conserved in higher organisms.

Comparison of SAM-Based Junctions with Ga2O3/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions

Abstract: This paper compares the J(V) characteristics obtained for self-assembled monolayer (SAM)-based tunneling junctions with top electrodes of the liquid eutectic of gallium and indium (EGaIn) fabricated using two different procedures: (i) stabilizing the EGaIn electrode in PDMS microchannels and (ii) suspending the EGaIn electrode from the tip of a syringe. These two geometries of the EGaIn electrode (with, at least when in contact with air, its solid Ga2O3 surface film) produce indistinguishable data. The junctions incorporated SAMs of SCn–1CH3 (with n = 12, 14, 16, or 18) supported on ultraflat, template-stripped silver electrodes. Both methods generated high yields of junctions (70–85%) that were stable enough to conduct measurements of J(V) with statistically large numbers of data (N = 400–1000). The devices with the top electrode stabilized in microchannels also made it possible to conduct measurements of J(V) as a function of temperature, almost down to liquid nitrogen temperatures (T = 110–293 K). The ...

Contact de-electrification of electrostatically charged polymers.

Abstract: The contact electrification of insulating organic polymers is still incompletely understood, in part because multiple fundamental mechanisms may contribute to the movement of charge. This study describes a mechanism previously unreported in the context of contact electrification: that is, “contact de-electrification”, a process in which polymers charged to the same polarity discharge on contact. Both positively charged polymeric beads, e.g., polyamide 6/6 (Nylon) and polyoxymethylene (Delrin), and negatively charged polymeric beads, e.g., polytetrafluoroethylene (Teflon) and polyamide-imide (Torlon), discharge when the like-charged beads are brought into contact. The beads (both with charges of ∼±20 μC/m2, or ∼100 charges/μm2) discharge on contact regardless of whether they are made of the same material, or of different materials. Discharge is rapid: discharge of flat slabs of like-charged Nylon and Teflon pieces is completed on a single contact (∼3 s). The charge lost from the polymers during contact de-...

Structural Transformation by Electrodeposition on Patterned Substrates (STEPS): A New Versatile Nanofabrication Method

Abstract: Arrays of high-aspect-ratio (HAR) nano- and microstructuresareofgreatinterestfordesigningsurfacesfor applications in optics, bionano interfaces, microelectrome- chanical systems, and microfluidics, but the difficulty of systematically and conveniently varying the geometries of these structures significantly limits their design and optimi- zationfor a specific function. This paperdemonstrates a low- cost, high-throughput benchtop method that enables a HAR array to be reshaped with nanoscale precision by electro- deposition of conductive polymers. The method—named STEPS (structural transformation by electrodeposition on patterned substrates)—makes it possible to create patterns with proportionally increasing size of original features, to convert isolated HAR features into a closed-cell substrate with a continuous HAR wall, and to transform a simple parent two- dimensional HAR array into new three-dimensional patterned structures with tapered, tilted, anisotropic, or overhanging geometries by controlling the deposition conditions. We demonstrate the fabrication of substrates with continuous or discrete gradients of nanostructure features, as well as libraries of various patterns, starting from a single master structure. By providing exemplary applications in plasmonics, bacterial patterning, and formation of mechanically reinforced structures, we show that STEPS enables a wide range of studies of the effect of substrate topography on surface properties leading to optimization of the structures for a specific application. This research identifies solution-based deposition of conductive polymers as a new tool in nanofabrication and allows access to 3D architectures that were previously difficult to fabricate.

Replacing −CH2CH2– with −CONH– Does Not Significantly Change Rates of Charge Transport through AgTS-SAM//Ga2O3/EGaIn Junctions

Abstract: This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a −CH2CH2– group with a −CONH– group changes the dipole moment and polarizability of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure AgTS-S(CH2)mX(CH2)nH//Ga2O3/EGaIn (TS = template stripped, X = −CH2CH2– or −CONH–, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS∼CONH∼R) using an amide-b...

Soft Lithographic Approaches to Nanofabrication

Abstract: This chapter reviews soft lithographic approaches to nanofabrication: that is, the generation and replication of patterns using a polymeric stamp bearing relief features. Soft lithographic methods are playing roles in science and technology where conventional methods of fabrication – for example, those developed for semiconductor manufacturing – are not applicable. This chapter begins by reviewing recent developments in preparing topographic masters and materials for stamps, and then describes methods of using these masters to make patterns by soft lithography. The chapter is organized into four sections, according to classes of methods: (1) printing, in which the stamp transfers an ‘ink’ to a substrate; (2) molding, in which a stamp templates the formation of thermally or ultraviolet (UV)-curable polymeric replicas; (3) phase-shifting edge lithography, using a conformable, transparent phase mask bearing relief features, whose edges form patterns in a film of photoresist upon exposure; and (4) nanoskiving (thin sectioning with an ultramicrotome), in which a topographically patterned epoxy structure supporting a metallic or polymeric film is embedded and sectioned to produce a pattern of nanostructures whose linewidths correspond to the thicknesses of the thin films. This chapter emphasizes materials and structures that are amenable to soft lithography, and the applications enabled by them.

Measuring Binding of Protein to Gel-Bound Ligands Using Magnetic Levitation

Abstract: This paper describes the use of magnetic levitation (MagLev) to measure the association of proteins and ligands. The method starts with diamagnetic gel beads that are functionalized covalently with small molecules (putative ligands). Binding of protein to the ligands within the bead causes a change in the density of the bead. When these beads are suspended in a paramagnetic aqueous buffer and placed between the poles of two NbFeB magnets with like poles facing, the changes in the density of the bead on binding of protein result in changes in the levitation height of the bead that can be used to quantify the amount of protein bound. This paper uses a reaction-diffusion model to examine the physical principles that determine the values of rate and equilibrium constants measured by this system, using the well-defined model system of carbonic anhydrase and aryl sulfonamides. By tuning the experimental protocol, the method is capable of quantifying either the concentration of protein in a solution, or the binding affinities of a protein to several resin-bound small molecules simultaneously. Since this method requires no electricity and only a single piece of inexpensive equipment, it may find use in situations where portability and low cost are important, such as in bioanalysis in resource-limited settings, point-of-care diagnosis, veterinary medicine, and plant pathology. It still has several practical disadvantages. Most notably, the method requires relatively long assay times and cannot be applied to large proteins (>70 kDa), including antibodies. The design and synthesis of beads with improved characteristics (e.g., larger pore size) has the potential to resolve these problems.

ac electric fields drive steady flows in flames.

Abstract: We show that time-oscillating electric fields applied to plasmas present in flames create steady flows of gas. Ions generated within the flame move in the field and migrate a distance δ before recombining; the net flow of ions away from the flame creates a time-averaged force that drives the steady flows observed experimentally. A quantitative model describes the response of the flame and reveals how δ decreases as the frequency of the applied field increases. Interestingly, above a critical frequency, ac fields can be used to manipulate flames at a distance without the need for proximal electrodes.

A Simple Two-Dimensional Model System to Study Electrostatic-Self-Assembly

Abstract: This paper surveys the variables controlling the lattice structure and charge in macroscopic Coulombic crystals made from electrically charged, millimeter-sized polymer objects (spheres, cubes, and cylinders). Mechanical agitation of these objects inside planar, bounded containers caused them to charge electrically through contact electrification, and to self-assemble. The processes of electrification and self-assembly, and the characteristics of the assemblies, depended on the type of motion used for agitation, on the type of materials used for the objects and the dish, on the size and shape of the objects and the dish, and on the number of objects. Each of the three different materials in the system (of the dish and of the two types of spheres) influenced the electrification. Three classes of structures formed by self-assembly, depending on the experimental conditions: two-dimensional lattices, one-dimensional chains, and zero-dimensional ‘rosettes’. The lattices were characterized by their structure (disordered, square, rhombic, or hexagonal) and by the electrical charges of individual objects; the whole lattices were approximately electrically neutral. The lattices observed in this study were qualitatively different from ionic crystals; the charge of objects had practically continuous values which changed during agitation and self-assembly, and depended on experimental conditions which included the lattice structure itself. The relationship between charge and structure led to the coexistence of regions with different lattice structures within the same assembly, and to transformations between different lattice structures during agitation.

Magnetic Levitation as a Platform for Competitive Protein−Ligand Binding Assays

Abstract: This paper describes a method based on magnetic levitation (MagLev) that is capable of indirectly measuring the binding of unlabeled ligands to unlabeled protein. We demonstrate this method by measuring the affinity of unlabeled bovine carbonic anhydrase (BCA) for a variety of ligands (most of which are benzene sulfonamide derivatives). This method utilizes porous gel beads that are functionalized with a common aryl sulfonamide ligand. The beads are incubated with BCA and allowed to reach an equilibrium state in which the majority of the immobilized ligands are bound to BCA. Since the beads are less dense than the protein, protein binding to the bead increases the overall density of the bead. This change in density can be monitored using MagLev. Transferring the beads to a solution containing no protein creates a situation where net protein efflux from the bead is thermodynamically favorable. The rate at which protein leaves the bead for the solution can be calculated from the rate at which the levitation height of the bead changes. If another small molecule ligand of BCA is dissolved in the solution, the rate of protein efflux is accelerated significantly. This paper develops a reaction-diffusion (RD) model to explain both this observation, and the physical-organic chemistry that underlies it. Using this model, we calculate the dissociation constants of several unlabeled ligands from BCA, using plots of levitation height versus time. Notably, although this method requires no electricity, and only a single piece of inexpensive equipment, it can measure accurately the binding of unlabeled proteins to small molecules over a wide range of dissociation constants (K(d) values within the range from ~10 nM to 100 μM are measured easily). Assays performed using this method generally can be completed within a relatively short time period (20 min-2 h). A deficiency of this system is that it is not, in its present form, applicable to proteins with molecular weight greater than approximately 65 kDa.

Capacitive, paper-based accelerometers and touch sensors

Abstract: Accelerometers and capacitive touch sensors fabricated from inexpensive, lightweight, disposable substrate materials, such as paper, are provided. These can be fabricated using simple technologies, such as laser cutting and screen printing. In one embodiment, a touch sensor includes a parallel plate capacitor having a fixed plate formed of a substrate material having a conductive layer and a deflectable plate formed of a paper substrate material having a conductive layer. In a second embodiment, a touch sensor includes a parallel plate capacitor formed of an exterior conductive layer deposited on a paper substrate material and an interior conductive layer deposited on a substrate material. In a third embodiment, a touch sensor includes an active electrode and a grounded electrode patterned on the surface of a paper substrate material.. In another embodiment, an accelerometer includes a parallel plate capacitor containing a fixed plate and a free plate containing a paper substrate. Upon an applied acceleration, the distance between the plate of the parallel plate capacitor in an accelerometer changes, eliciting a change in the capacitance of the sensor. Measurement of capacitance can be correlated to the acceleration or deceleration applied to the accelerometer.

MEMS force sensors fabricated using paper substrates

Abstract: MEMS devices fabricated using inexpensive substrate materials such as paper or fabric, are provided. Using paper as a substrate, low cost, simple to prepare, lightweight, disposable piezoresistive sensors, including accelerometers are prepared. Signal-processing circuitry can also be patterned on the substrate material. The sensors can be utilized as two-dimensional sensors, or the paper substrate material can be folded to arrange the sensors in a three dimensional conformation. For example, three sensors can be patterned on a paper substrate and folded into a cube such that the three sensors are orthogonally positioned on the faces of a cube, permitting simultaneous measurement of accelerations along three orthogonal directions (x-y-z). These paper-based sensors can be mass produced by incorporating highly developed technologies for automatic paper cutting, folding, and screen-printing. Also provided are methods of modifying paper for use as a substrate material in MEMS devices.

Effect of Surfactant Hydrophobicity on the Pathway for Unfolding of Ubiquitin

Abstract: This paper describes the interaction between ubiquitin (UBI) and three sodium n-alkyl sulfates (SCnS) that have the same charge (Z = −1) but different hydrophobicity ( n= 10, 12, or 14). Increasing the hydrophobicity of the n-alkyl sulfate resulted in (i) an increase in the number of distinct intermediates (that is, complexes of UBI and surfactant) that form along the pathway of unfolding, (ii) a decrease in the minimum concentrations of surfactant at which intermediates begin to form (i.e., a more negative ΔGbinding of surfactant for UBI), and (iii) an increase in the number of surfactant molecules bound to UBI in each intermediate or complex. These results demonstrate that small changes in the hydrophobicity of a surfactant can significantly alter the binding interactions with a folded or unfolded cytosolic protein.