Fabrication of microfluidic systems in poly(dimethylsiloxane)

Nanostructured Amorphous Silicas Hydrophobized by Various Pathways

Various nanostructured amorphous silicas [fumed silicas such as crude (A-300), hydro-compacted (cA-300, TS 100), and precipitated silica Syloid 244] were modified by different polydimethylsiloxanes such as PDMS5, PDMS100, PDMS200, PDMS1000, and PDMS12500 (the label numbers show the viscosity (η) values) using dimethyl carbonate (DMC) as a siloxane-bond-breaking reagent. In addition, hexamethyldisilazane was used to modify fumed silica cA-300. The nanocomposites were characterized using microscopy, infrared spectroscopy, thermodesorption, nitrogen adsorption-desorption, solid-state NMR spectroscopy, small-angle X-ray scattering, and zeta-potential methods. It was found that the morphological, textural, and structural characteristics of silicas grafted with PDMS depend strongly not only on the type and content of the polymers used but also on the organization of nonporous nanoparticles (NPNP) in secondary structures (aggregates of NPNP and agglomerated aggregates, ANPNP), as well on the reaction temperature (T _r). Specifically, we determined that ANPNP with a macro/mesoporous character are favorable for the effective modification of the silicas studied with short polymers and no DMC addition but at higher temperatures or for a longer silicone polymer with the presence of DMC and at lower temperatures. In particular, the PDMS/DMC-modified silicas are of great interest from a practical point of view because they remain in a dispersed state with no strong compaction of the secondary structures after modification, and this corresponds to a better distribution of the modified nanoparticles in polymeric or other matrices.

Fabrication of a Polydimethylsiloxane Fluidic Chip Using a Sacrificial Template Made by Fused Deposition Modeling 3D Printing and Application for Flow-injection Analysis

Fluidic chip fabrication technologies using three-dimensional (3D) printing have received broad attention recently. Herein, we describe a new method for fabricating polydimethylsiloxane (PDMS) fluidic chips using a 3D-printed polyvinyl alcohol (PVA) or acrylonitrile butadiene styrene (ABS) template and polymer coating. In this method, polyethylene glycol (PEG) was coated on the 3D-printed template. This coated template was immersed in liquid PDMS, and subsequently the PDMS was cured. Space can be created between the template and PDMS by removing this liquid PEG from the channel. This space renders template removal easier. A flow path is formed by dissolving the template with a solvent. These PDMS chips are used for flow injection measurement.

Oblique Colloidal Lithography for the Fabrication of Nonconcentric Features

Herein, we describe the development of oblique colloidal lithography (OCL) and establish a systematic patterning strategy for creating libraries of nanosized nonconcentric plasmonic structures. This strategy combines OCL, capillary force lithography, and several wet and ion etching steps. Hexagonal arrays of nonconcentric gold features were created on glass substrates with highly controllable geometric parameters. The size, geometry, and eccentricity of the gold features could be independently tuned by controlling the experimental conditions. Gaps within surface elements could be shrunk to as small as 30 nm, while the total patterned area was about l cm². The goal was to devise a method that offers a high degree of control over the resolution and morphology of asymmetric structures without the need to resort to electron beam lithography. This technique also enabled the development of numerous surface patterns through the stepwise fabrication of separate elements. Complex features, including dots-surrounded nonconcentric targets, nonconcentric hexagram-disks, and nonconcentric annular aperture arrays, were demonstrated, and their optical properties were characterized. Indeed, spectroscopic studies and FDTD simulations demonstrated that Fano resonances could readily be generated by the nonconcentric gold features. Consequently, our patterning strategy should enable the high-throughput investigation of plasmonic coupling and Fano resonances as a function of the physical parameters of the elements within the nanopattern array.

Microfluidic Surface Titrations of Electroactive Thin Films

We report the use of microfluidic surface titrations (MSTs) for studying electroactive self-assembled monolayers (eSAMs) and other thin films. The technique of MST utilizes a microfluidic generation-collection dual channel electrode (DCE) configuration to quantify the charge associated with electroactive thin films that might or might not be in direct contact with an electrode surface. This technique allows for quantitative measurement of surface coverages, Γ, as low as 30 pmol cm^-2 for electrodeposited Cu thin films. Additionally, we show that it is possible to quantify Γ for ferrocene (Fc)-terminated alkylthiols in mixed-monolayer eSAMs. Interestingly, MSTs sometimes reveal a two-fold higher eSAM concentration compared to direct electrochemical measurements. This finding suggests that in these instances not all the constituent Fc-moieties of the eSAM are in sufficiently close proximity to the surface to be addressable via direct electrochemistry.

Dual Imprinted Polymer Thin Films via Pattern Directed Self-Organization

Synthetic topographically patterned films and coatings are typically contoured on one side, yet many of nature's surfaces have distinct textures on different surfaces of the same object. Common examples are the top and bottom sides of the butterfly wing or lotus leaf, onion shells, and the inside versus outside of the stem of a flower. Inspired by nature, we create dual (top and bottom) channel patterned polymer films. To this end, we first develop a novel fabrication method to create ceramic line channel relief structures by converting the oligomeric residue of stamped poly(dimethylsiloxane) (PDMS) nanopatterns on silicon substrates to glass (SiOx, silica) by ultraviolet-ozone (UVO) exposure. These silica patterned substrates are flow coated with polystyrene (PS) films and confined within an identically patterned top confining soft PDMS elastomer film. Annealing of the sandwich structures drives the PS to rapidly mold fill the top PDMS pattern in conjunction with a dewetting tendency of the PS on the silica pattern. Varying the film thickness h, from less than to greater than the pattern height, and varying the relative angle between the top-down and bottom-up patterned confinement surfaces create interesting uniform and nonuniform digitized defects in PS channel patterns, as also a defect-free channel regime. Our dual patterned polymer channels provide a novel fabrication route to topographically imprinted Moiré patterns (whose applications range from security encrypting holograms to sensitive strain gauges), and their basic laser light diffractions properties are illustrated and compared to graphical simulations and 2D-FFT of real-space AFM channel patterns. While traditional "geometrical" and "fringe" Moiré patterns function by superposition of two misaligned optical patterned transmittance gratings, our topographic pattern gratings are quite distinct and may allow for more unique holographic optical characteristics with further development.

One-Step Fabrication of a Microfluidic Device with an Integrated Membrane and Embedded Reagents by Multimaterial 3D Printing

One of the largest impediments in the development of microfluidic-based smart sensing systems is the manufacturability of integrated, complex devices. Here we propose multimaterial 3D printing for the fabrication of such devices in a single step. A microfluidic device containing an integrated porous membrane and embedded liquid reagents was made by 3D printing and applied for the analysis of nitrate in soil. The manufacture of the integrated, sealed device was realized as a single print within 30 min. The body of the device was printed in transparent acrylonitrile butadiene styrene (ABS) and contained a 400 μm wide structure printed from a commercially available composite filament. The composite filament can be turned into a porous material through dissolution of a water-soluble material. Liquid reagents were integrated by briefly pausing the printing before resuming for sealing the device. The devices were evaluated by the determination of nitrate in a soil slurry containing zinc particles for the reduction of nitrate to nitrite using the Griess reagent. Using a consumer digital camera, the linear range of the detector response ranged from 0 to 60 ppm, covering the normal range of nitrate in soil. To ensure that the sealing of the reagent chamber is maintained, aqueous reagents should be avoided. When using the nonaqueous reagent, the multimaterial device containing the Griess reagent could be stored for over 4 days but increased the detection range to 100-500 ppm. Multimaterial 3D printing is a potentially new approach for the manufacture of microfluidic devices with multiple integrated functional components.

Droplet Microfluidic Flow Cytometer For Sorting On Transient Cellular Responses Of Genetically-Encoded Sensors

Fluorescent biosensors are important measurement tools for in vivo quantification of pH, concentrations of metal ions and other analytes, and physical parameters such as membrane potential. Both the development of these sensors and their implementation in examining cellular heterogeneity requires technology for measuring and sorting cells based on the fluorescence levels before and after chemical or physical perturbations. We developed a droplet microfluidic platform for the screening and separation of cell populations on the basis of the in vivo response of expressed fluorescence-based biosensors after addition of an exogenous analyte. We demonstrate the capability to resolve the responses of two genetically encoded Zn2+ sensors at a range of time points spanning several seconds and subsequently sort a mixed-cell population of varying ratios with high accuracy.

Wearable Potentiometric Chloride Sweat Sensor: The Critical Role of the Salt Bridge

The components of sweat provide an array of potential biomarkers for health and disease. Sweat chloride is of interest as a biomarker for cystic fibrosis, electrolyte metabolism disorders, electrolyte balance, and electrolyte loss during exercise. Developing wearable sensors for biomarkers in sweat is a major technological challenge. Potentiometric sensors provide a relatively simple technology for on-body sweat chloride measurement, however, equilibration between reference and test solutions has limited the time over which accurate measurements can be made. Here, we report on a wearable potentiometric chloride sweat sensor. We performed parametric studies to show how the salt bridge geometry determines equilibration between the reference and test solutions. From these results, we show a sweat chloride sensor can be designed to provide accurate measurements over extended times. We then performed on-body tests on healthy subjects while exercising to establish the feasibility of using this technology as a wearable device.

A One Donor-Two Acceptor Lipid Bilayer FRET Assay Based on Asymmetrically Labeled Liposomes

The fusion of two opposing membranes is essential in biological functions such as fertilization, viral entry, membrane trafficking and synaptic transmission. Before the membrane bilayers are fully connected, at some stage a hemifusion intermediate-when the outer leaflets are merged but not the inner leaflets-is formed. However, the position of hemifusion in the energy landscape and the duration of it vary and have not been fully mapped out. To date, there has not been a way to differentiate lipid mixing of the two leaflets directly in a single experiment. Herein we demonstrate labeling of the outer and inner leaflets with different fluorophores, which can be distinguished by their fluorescence lifetimes. As a proof of concept, the asymmetrically labeled liposomes were used as acceptor liposomes in a novel one donor-two acceptor Förster resonance energy transfer (FRET) assay to monitor membrane fusion reactions mediated by the synaptic proteins soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) in microfluidic devices. Initial hemifusion was clearly indicated by the acceptor fluorescence lifetime originating solely from FRET acceptors on the outer leaflet (Oregon Green 488; τ_Fl ∼ 4.8 ns). Progression to full fusion was then indicated by the significantly increasing lifetime contribution from acceptors on the inner leaflet (nitrobenzoxadiazole; τ_Fl ∼ 6.7 ns). The new labeling strategy creates many possibilities in the design of bulk and single-molecule experiments.

Core-Shell Microgels with Switchable Elasticity at Constant Interfacial Interaction

Hydrogels based on poly(N-isopropylacrylamide) (pNIPAAm) exhibit a thermo-reversible volume phase transition from swollen to deswollen states. This change of the hydrogel volume is accompanied by changes of the hydrogel elastic and Young's moduli and of the hydrogel interfacial interactions. To decouple these parameters from one another, we present a class of submillimeter sized hydrogel particles that consist of a thermosensitive pNIPAAm core wrapped by a nonthermosensitive polyacrylamide (pAAm) shell, each templated by droplet-based microfluidics. When the microgel core deswells upon increase of the temperature to above 34 °C, the shell is stretched and dragged to follow this deswelling into the microgel interior, resulting in an increase of the microgel surficial Young's modulus. However, as the surface interactions of the pAAm shell are independent of temperature at around 34 °C, they do not considerably change during the pNIPAAm-core volume phase transition. This feature makes these core-shell microgels a promising platform to be used as building blocks to assemble soft materials with rationally and independently tunable mechanics.

Post-Formation Shrinkage and Stabilization of Microfluidic Bubbles in Lipid Solution

Medical ultrasound imaging often employs ultrasound contrast agents (UCAs), injectable microbubbles stabilized by shells or membranes. In tissue, the compressible gas cores can strongly scatter acoustic signals, resonate, and emit harmonics. However, bubbles generated by conventional methods have nonuniform sizes, reducing the fraction that resonates with a given transducer. Microfluidic flow-focusing is an alternative production method which generates highly monodisperse bubbles with uniform constituents, enabling more-efficient contrast enhancement than current UCAs. Production size is tunable by adjusting gas pressure and solution flow rate, but solution effects on downstream stable size and lifetime have not been closely examined. This study therefore investigated several solution parameters, including the DSPC/DSPE-PEG2000 lipid ratio, concentration, viscosity, and preparation temperature to determine their effects on stabilization. It was found that bubble lifetime roughly correlated with stable size, which in turn was strongly influenced by primary-lipid-to-emulsifier ratio, analogous to its effects on conventional bubble yield and Langmuir-trough compressibility in existing studies. Raising DSPE-PEG2000 fraction in solution reduced bubble surface area in proportion to its reduction of lipid packing density at low compression in literature. In addition, the surface area was found to increase proportionately with lipid concentration above 2.1 mM. However, viscosities above or below 2.3-3.3 mPa·s seemed to reduce bubble size. Finally, lipid preparation at room temperature led to smaller bubbles compared to preparation near or above the primary lipid's phase transition point. Understanding these effects will further improve on postformation control over microfluidic bubble production, and facilitate size-tuning for optimal contrast enhancement.

A Diffusion-Based and Dynamic 3D-Printed Device That Enables Parallel in Vitro Pharmacokinetic Profiling of Molecules

The process of bringing a drug to market involves many steps, including the preclinical stage, where various properties of the drug candidate molecule are determined. These properties, which include drug absorption, distribution, metabolism, and excretion, are often displayed in a pharmacokinetic (PK) profile. While PK profiles are determined in animal models, in vitro systems that model in vivo processes are available, although each possesses shortcomings. Here, we present a 3D-printed, diffusion-based, and dynamic in vitro PK device. The device contains six flow channels, each with integrated porous membrane-based insert wells. The pores of these membranes enable drugs to freely diffuse back and forth between the flow channels and the inserts, thus enabling both loading and clearance portions of a standard PK curve to be generated. The device is designed to work with 96-well plate technology and consumes single-digit milliliter volumes to generate multiple PK profiles, simultaneously. Generation of PK profiles by use of the device was initially performed with fluorescein as a test molecule. Effects of such parameters as flow rate, loading time, volume in the insert well, and initial concentration of the test molecule were investigated. A prediction model was generated from this data, enabling the user to predict the concentration of the test molecule at any point along the PK profile within a coefficient of variation of ∼ 5%. Depletion of the analyte from the well was characterized and was determined to follow first-order rate kinetics, indicated by statistically equivalent (p > 0.05) depletion half-lives that were independent of the starting concentration. A PK curve for an approved antibiotic, levofloxacin, was generated to show utility beyond the fluorescein test molecule.

Integrated Microfluidic System for Size-Based Selection and Trapping of Giant Vesicles

Vesicles composed of phospholipids (liposomes) have attracted interest as artificial cell models and have been widely studied to explore lipid-lipid and lipid-protein interactions. However, the size dispersity of liposomes prepared by conventional methods was a major problem that inhibited their use in high-throughput analyses based on monodisperse liposomes. In this study, we developed an integrative microfluidic device that enables both the size-based selection and trapping of liposomes. This device consists of hydrodynamic selection and trapping channels in series, which made it possible to successfully produce an array of more than 60 monodisperse liposomes from a polydisperse liposome suspension with a narrow size distribution (the coefficient of variation was less than 12%). We successfully observed a size-dependent response of the liposomes to sequential osmotic stimuli, which had not clarified so far, by using this device. Our device will be a powerful tool to facilitate the statistical analysis of liposome dynamics.

Highly Stretchable and Transparent Microfluidic Strain Sensors for Monitoring Human Body Motions

We report a new class of simple microfluidic strain sensors with high stretchability, transparency, sensitivity, and long-term stability with no considerable hysteresis and a fast response to various deformations by combining the merits of microfluidic techniques and ionic liquids. The high optical transparency of the strain sensors was achieved by introducing refractive-index matched ionic liquids into microfluidic networks or channels embedded in an elastomeric matrix. The microfluidic strain sensors offer the outstanding sensor performance under a variety of deformations induced by stretching, bending, pressing, and twisting of the microfluidic strain sensors. The principle of our microfluidic strain sensor is explained by a theoretical model based on the elastic channel deformation. In order to demonstrate its capability of practical usage, the simple-structured microfluidic strain sensors were performed onto a finger, wrist, and arm. The highly stretchable and transparent microfluidic strain sensors were successfully applied as potential platforms for distinctively monitoring a wide range of human body motions in real time. Our novel microfluidic strain sensors show great promise for making future stretchable electronic devices.

Enhanced In Vitro Biocompatibility of Chemically Modified Poly(dimethylsiloxane) Surfaces for Stable Adhesion and Long-term Investigation of Brain Cerebral Cortex Cells

Studies on the mammalian brain cerebral cortex have gained increasing importance due to the relevance of the region in controlling critical higher brain functions. Interactions between the cortical cells and surface extracellular matrix (ECM) proteins play a pivotal role in promoting stable cell adhesion, growth, and function. Poly(dimethylsiloxane) (PDMS) based platforms have been increasingly used for on-chip in vitro cellular system analysis. However, the inherent hydrophobicity of the PDMS surface has been unfavorable for any long-term cell system investigations due to transitory physical adsorption of ECM proteins on PDMS surfaces followed by eventual cell dislodgement due to poor anchorage and viability. To address this critical issue, we employed the (3-aminopropyl)triethoxysilane (APTES) based cross-linking strategy to stabilize ECM protein immobilization on PDMS. The efficiency of surface modification in supporting adhesion and long-term viability of neuronal and glial cells was analyzed. The chemically modified surfaces showed a relatively higher cell survival with an increased neurite length and neurite branching. These changes were understood in terms of an increase in surface hydrophilicity, protein stability, and cell-ECM protein interactions. The modification strategy could be successfully applied for stable cortical cell culture on the PDMS microchip for up to 3 weeks in vitro.

Microfluidic Assay To Study the Combinatorial Impact of Substrate Properties on Mesenchymal Stem Cell Migration

As an alternative to complex and costly in vivo models, microfluidic in vitro models are being widely used to study various physiological phenomena. It is of particular interest to study cell migration in a controlled microenvironment because of its vital role in a large number of physiological processes, such as wound healing, disease progression, and tissue regeneration. Cell migration has been shown to be affected by variations in the biochemical and physical properties of the extracellular matrix (ECM). To study the combinatorial impact of the ECM physical properties on cell migration, we have developed a microfluidic assay to induce migration of human bone marrow derived mesenchymal stem cells (hBMSCs) on polydimethylsiloxane (PDMS) substrates with varying combinatorial properties (hydrophobicity, stiffness, and roughness). The results show that although the initial cell adhesion and viability appear similar on all PDMS samples, the cell spreading and migration are enhanced on PDMS samples exhibiting intermediate levels of hydrophobicity, stiffness, and roughness. This study suggests that there is a particular range of substrate properties for optimal cell spreading and migration. The influence of substrate properties on hBMSC migration can help understand the physical cues that affect cell migration, which may facilitate the development of optimized engineered scaffolds with desired properties for tissue regeneration applications.

Protein immobilization techniques for microfluidic assays

Microfluidic systems have shown unequivocal performance improvements over conventional bench-top assays across a range of performance metrics. For example, specific advances have been made in reagent consumption, throughput, integration of multiple assay steps, assay automation, and multiplexing capability. For heterogeneous systems, controlled immobilization of reactants is essential for reliable, sensitive detection of analytes. In most cases, protein immobilization densities are maximized, while native activity and conformation are maintained. Immobilization methods and chemistries vary significantly depending on immobilization surface, protein properties, and specific assay goals. In this review, we present trade-offs considerations for common immobilization surface materials. We overview immobilization methods and chemistries, and discuss studies exemplar of key approaches-here with a specific emphasis on immunoassays and enzymatic reactors. Recent "smart immobilization" methods including the use of light, electrochemical, thermal, and chemical stimuli to attach and detach proteins on demand with precise spatial control are highlighted. Spatially encoded protein immobilization using DNA hybridization for multiplexed assays and reversible protein immobilization surfaces for repeatable assay are introduced as immobilization methods. We also describe multifunctional surface coatings that can perform tasks that were, until recently, relegated to multiple functional coatings. We consider the microfluidics literature from 1997 to present and close with a perspective on future approaches to protein immobilization.

Polydimethylsiloxane-based conducting composites and their applications in microfluidic chip fabrication

This paper reviews the design and fabrication of polydimethylsiloxane (PDMS)-based conducting composites and their applications in microfluidic chip fabrication. Owing to their good electrical conductivity and rubberlike elastic characteristics, these composites can be used variously in soft-touch electronic packaging, planar and three-dimensional electronic circuits, and in-chip electrodes. Several microfluidic components fabricated with PDMS-based composites have been introduced, including a microfluidic mixer, a microheater, a micropump, a microdroplet controller, as well as an all-in-one microfluidic chip.

Effect of oxidation on the wettability of poly(dimethylsiloxane) surfaces

The wetting of amorphous poly(dimethylsiloxane) (PDMS) surfaces by water has been studied using molecular dynamics simulations. PDMS surfaces were generated by compressing a long PDMS chain between two elastic boundaries at atmospheric pressure. Oxidation of the PDMS surface, achieved in real systems by exposure to air plasma or corona discharge, was modeled by replacing methyl groups on the PDMS chain with hydroxyl groups. Three surfaces of varying degrees of oxidation were characterized by measuring the water contact angle and the roughness. The dependence of the microscopic contact angle on drop size was measured from time averaged density profiles. The macroscopic contact angle was measured directly using a cylindrical drop of infinite length with zero contact line curvature. The measured macroscopic contact angle ranged from approximately 125 degrees on the untreated surface to 75 degrees on the most oxidized surface studied. The line tension was found to increase with increasing degree of oxidation, from a negligible value on the untreated surface to approximately 5x10(-11) J m(-1) on the most heavily oxidized surface.

BIOMIMETIC NANOSCALE REACTORS AND NETWORKS

Methods based on self-assembly, self-organization, and forced shape transformations to form synthetic or semisynthetic enclosed lipid bilayer structures with several properties similar to biological nanocompartments are reviewed. The procedures offer unconventional micro- and nanofabrication routes to yield complex soft-matter devices for a variety of applications for example, in physical chemistry and nanotechnology. In particular, we describe novel micromanipulation methods for producing fluid-state lipid bilayer networks of nanotubes and surface-immobilized vesicles with controlled geometry, topology, membrane composition, and interior contents. Mass transport in nanotubes and materials exchange, for example, between conjugated containers, can be controlled by creating a surface tension gradient that gives rise to a moving boundary or by induced shape transformations. The network devices can operate with extremely small volume elements and low mass, to the limit of single molecules and particles at a length scale where a continuum mechanics approximation may break down. Thus, we also describe some concepts of anomalous fluctuation-dominated kinetics and anomalous diffusive behaviours, including hindered transport, as they might become important in studying chemistry and transport phenomena in these confined systems. The networks are suitable for initiating and controlling chemical reactions in confined biomimetic compartments for rationalizing, for example, enzyme behaviors, as well as for applications in nanofluidics, bioanalytical devices, and to construct computational and complex sensor systems with operations building on chemical kinetics, coupled reactions and controlled mass transport.

Geometrically mediated breakup of drops in microfluidic devices

Microfluidic technology offers capabilities for the precise handling of small fluid volumes dispersed as droplets. To fully exploit this potential requires simultaneous generation of multiple size droplets. We demonstrate two methods for passively breaking larger drops into precisely controlled daughter drops using pressure-driven flow in simple microfluidic configurations: (i) a T junction and (ii) flow past isolated obstacles. We quantify conditions for breakup at a T junction and illustrate sequential breakup at T junctions for making small drops at high dispersed phase volume fractions.

Physics and applications of microfluidics in biology

Fluid flow at the microscale exhibits unique phenomena that can be leveraged to fabricate devices and components capable of performing functions useful for biological studies. The physics of importance to microfluidics are reviewed. Common methods of fabricating microfluidic devices and systems are described. Components, including valves, mixers, and pumps, capable of controlling fluid flow by utilizing the physics of the microscale are presented. Techniques for sensing flow characteristics are described and examples of devices and systems that perform bioanalysis are presented. The focus of this review is microscale phenomena and the use of the physics of the scale to create devices and systems that provide functionality useful to the life sciences.

Soft lithography in biology and biochemistry

Soft lithography, a set of techniques for microfabrication, is based on printing and molding using elastomeric stamps with the patterns of interest in basrelief. As a technique for fabricating microstructures for biological applications, soft lithography overcomes many of the shortcomings of photolithography. In particular, soft lithography offers the ability to control the molecular structure of surfaces and to pattern the complex molecules relevant to biology, to fabricate channel structures appropriate for microfluidics, and to pattern and manipulate cells. For the relatively large feature sizes used in biology (> or = 50 microns), production of prototype patterns and structures is convenient, inexpensive, and rapid. Self-assembled monolayers of alkanethiolates on gold are particularly easy to pattern by soft lithography, and they provide exquisite control over surface biochemistry.