Gradient chemical micro patterns: a reference substrate for surface nanometrology

Fabrication of Polysulfobetaine Gradient Coating via Oxidation Polymerization of Pyrogallol To Modulate Biointerfaces

A surface with a gradient physical or chemical feature, such as roughness, hardness, wettability, and chemistry, serves as a powerful platform for high-throughput investigation of cell responses to a biointerface. In this work, we developed a continuous antifouling gradient surface using pyrogallol (PG) chemistry. A copolymer of a zwitterionic monomer, sulfobetaine methacrylate, and an amino monomer, aminoethyl methacrylate, were synthesized (pSBAE) and deposited on glass slides via the deposition of self-polymerized PG. A gradient of pSBAE was fabricated on glass slides in 7 min in the presence of an oxidant, ammonium persulfate, by withdrawing the reaction solution. The modified glass slide showed a wettability gradient, determined by measuring the water contact angle. Cell adhesion and protein adsorption were well correlated with surface wettability. We expect that this simple and faster method for the fabrication of a continuous chemical gradient is applicable for high-throughput screening of surface properties to modulate biointerfaces.

Transition from Spin Dewetting to continuous film in spin coating of Liquid Crystal 5CB

Spin dewetting refers to spontaneous rupture of the dispensed solution layer during spin coating, resulting in isolated but periodic, regular sized domains of the solute and is pre-dominant when the solute concentration (C_n) is very low. In this article we report how the morphology of liquid crystal (LC) 5CB thin films coated on flat and patterned PMMA substrate transform from spin dewetted droplets to continuous films with increase in C_n. We further show that within the spin dewetted regime, with gradual increase in the solute concentration, periodicity of the isotropic droplets (λ_D) as well as their mean diameter (d_D), gradually decreases, till the film becomes continuous at a critical concentration (C_n*). Interestingly, the trend that λ_D reduces with increase in C_n is exact opposite to what is observed in thermal/solvent vapor induced dewetting of a thin film. The spin dewetted droplets exhibit transient Radial texture, in contrast to Schlieren texture observed in elongated threads and continuous films of 5CB, which remains in the Nematic phase at room temperature. Finally we show that by casting the film on a grating patterned substrate it becomes possible to align the spin dewetted droplets along the contours substrate patterns.

Thermally tailored gradient topography surface on elastomeric thin films

We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (L), resulting in a surface with a gradient topography with progressively varying feature heights (hF). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting.

Shape-controlled fabrication of micron-scale surface chemical gradients via electrochemically activated copper(i) "click" chemistry

We report an electrochemical method for the shape-controlled fabrication of micron-scale surface-bound chemical gradients. The approach is based on employing platinum microelectrode arrays on glass for the establishment of a Cu(i) solution gradient via local electrochemical reduction of Cu(ii) (cathodic reaction), and oxidation of the generated Cu(i) back to Cu(ii) (anodic reaction), under ambient conditions. The Cu(i) solution gradient, in the presence of an alkyne in solution and an azide monolayer on the glass surface in between the platinum electrodes, is exploited for the surface-confined gradient fabrication via the Huisgen 1,3-dipolar cycloaddition (CuAAC). Owing to the high sensitivity of the CuAAC on the Cu(i) concentration, we demonstrate here the control of the shape of the micron-scale surface gradient, in terms of steepness and surface density, as a function of the reaction conditions. The surface gradients were assessed by fluorescence microscopy and time-of-flight secondary ion mass spectrometry (Tof-SIMS). Moreover, bi-component and biomolecular gradients have been fabricated and a method for the electrochemically mediated patterning of surface chemical gradients on external azide-functionalized substrates has been developed for the implementation of bi-directional 2D surface gradients.

Lithographic Tuning of Polymeric Thin Film Surfaces by Stress Relaxation

We report a facile soft lithography (SL) technique that allows fabrication of patterned polymeric surfaces with feature height varying between 0 and h₀, using a single stamp. The method relies on the partial relaxation of the applied stress in a viscoelastic polymer thin film imprinted under a transient external load using a flexible stamp. The applicability of the technique is demonstrated for thermosetting (rubbery) as well as thermoplastic (glassy) polymers over a wide range of lateral dimensions. The lateral dimension and the periodicity of imprinted patterns remain identical to that of the original stamp. The method has potential applications in creating templates for performing combinatorial experiments related to wetting and dewetting studies, adhesion, nanotribology, microfluidics, etc.

Spatially resolved solid-state 1H NMR for evaluation of gradient-composition polymeric libraries

Polyurethane libraries consisting of films with composition gradients of aliphatic polyisocyanate and hydroxy-terminated polyacrylate resin were characterized using methods of (1)H NMR microimaging (i.e., magnetic resonance imaging, (MRI)) and solid-state NMR. Molecular mobilities and underlying structural information were extracted as a function of the relative content of each of the two components. Routine NMR microimaging using the spin-echo sequence only allows investigations of transverse relaxation of magnetization at echo times >2 ms. A single-exponential decay was found, which is likely due to free, noncross-linked polymer chains. The mobility of these chains decreases with increasing content of the aliphatic polyisocyanate. The concept of a 1D NMR profiler is introduced as a novel modality for library screening, which allows the convenient measurement of static solid-state NMR spectra as a function of spatial location along a library sample that is repositioned in the rf coil between experiments. With this setup the complete transverse relaxation function was measured using Bloch decays and spin echoes. For all positions within the gradient-composition film, relaxation data consisted of at least three components that were attributed to a rigid highly cross-linked resin, an intermediate cross-linked but mobile constituent, and the highly mobile free polymer chains (the latter is also detectable by MRI). Analysis of this overall relaxation function measured via Bloch decays and spin echoes revealed only minor changes in the mobilities of the individual fractions. Findings with respect to the most mobile components are consistent with the results obtained by NMR microimaging. The major effect is the significant increase in the rigid-component fraction with the addition of the hydroxy-terminated polyacrylate resin.

Programmable chemical gradient patterns by soft grayscale lithography

A method for fabricating chemical gradients on planar and nonplanar substrates using grayscale lithography is reported. Compliant grayscale amplitude masks are fabricated using a vacuum-assisted microfluidic filling protocol that employs dilutions of a carbon-black-containing polydimethylsiloxane emulsion (bPDMS) within traditional clear PDMS (cPDMS) to create planar, fully self-supporting mask elements. The mask is then placed over a surface functionalized with a hydrophobic coumarin-based photocleavable monolayer, which exposes a polar group upon irradiation. The mask serves to modulate the intensity of incident UV light, thereby controlling the density of molecules cleaved. The resulting molecular-level grayscale patterns are characterized by condensation microscopy and imaging mode time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Due to the inherent flexibility of this technique, the photofuse as well as the gradient patterns can be designed for a wide range of applications; in this paper two proof-of-concept demonstrations are shown. The first utilizes the ability to control the resulting contact angle of the surface for the fabrication of a passive pressure-sensitive microfluidic gating system. The second is a model surface modification process that utilizes the functional groups deprotected during the photocleavage to pattern the deposition of moieties with complementary chemistry. The spatial layout, resolution, and concentration of these covalently linked molecules follow the gradient pattern created by the grayscale mask during exposure. The programmable chemical gradient fabrication scheme presented in this work allows explicit engineering of both surface properties that dictate nonspecific interactions (surface energy, charge, etc.) and functional chemistry necessary for covalent bonding.

Effect of Relative Humidity on Chemical Heterogeneity Imaging with Atomic Force Microscopy

In this study, a well-controlled humidity system is used to enhance the sensitivity of AFM for characterizing surface chemical heterogeneity of patterned self-assembled monolayers (SAMs) and hydrophilic-hydrophobic polymeric brush specimens. Dependence of the AFM friction contrasts on the surface energy differences between the hydrophilic regions and hydrophobic regions of the chemically heterogeneous samples has been investigated as a function of relative humidity (RH). Effects of RH and surface chemistry on tip-sample adhesion are also investigated. Both AFM image contrast and tip-sample adhesion forces between the hydrophilic and hydrophobic regions significantly depend on RH and follow the similar trend as a function of RH. The results clearly demonstrate that, by using proper RH at the tip-sample environment, chemically heterogeneous regions can be distinguished with the AFM.

Water-collecting capability of radial-wettability gradient surfaces generated by controlled surface reactions

In this work, we developed a controlled oxidation reaction of vinyl-terminated self-assembled monolayers (SAMs) to carboxylic acid-terminated ones to generate radially inward wettability gradient surfaces. The hydrophobicity was introduced on a silicon wafer by SAMs of 10-undecenyltrichlorosilane, and after the initial drop in oxidation, followed by the dilution-by-dropping method, radial-wettability gradient surfaces having hydrophilic centers and hydrophobic exteriors were generated. This direct drop reaction on the SAMs did not require an elastomeric stamp to be fabricated, which allowed for facile tuning of the gradients in terms of sizes and shapes. The fabricated wettability gradient surfaces possessed a water-collecting capability toward the hydrophilic center, which was inactive on previous linear wettability gradient surfaces.

Generation of monolayer gradients in surface energy and surface chemistry for block copolymer thin film studies

We utilize a vapor deposition setup and cross-diffusion of functionalized chlorosilanes under dynamic vacuum to generate a nearly linear gradient in surface energy and composition on a silicon substrate. The gradient can be tuned by manipulating chlorosilane reservoir sizes and positions, and the gradient profile is independent of time as long as maximum coverage of the substrate is achieved. Our method is readily amenable to the creation of gradients on other substrate surfaces, due to the use of vapor deposition, and with other functionalities, due to our use of functionalized chlorosilanes. Our gradients were characterized using contact angle measurements and X-ray photoelectron spectroscopy. From these measurements, we were able to correlate composition, contact angle, and surface energy. We generated a nearly linear gradient with a range in mole fraction of one component from 0.15 to 0.85 (34 to 40 mJ/m(2) in surface energy) to demonstrate its utility in a block copolymer thin film morphology study. Examination of the copolymer thin film surface morphology with optical and atomic force microscopy revealed the expected morphological transitions across the gradient.

Effects of humidity and sample surface free energy on AFM probe-sample interactions and lateral force microscopy image contrast

Contrast between hydrophilic and hydrophobic domains and probe-sample adhesion forces as a function of relative humidity (RH) and sample surface free energy have been investigated using hydrophilic and hydrophobic atomic force microscopy (AFM) probes. For hydrophobic probes, the adhesion force is low, and the AFM image contrast between hydrophilic and hydrophobic domains is poor over the 0-93% RH. For hydrophilic probes, the image contrast between the hydrophilic and hydrophobic domains is poor at low RH but improved at high RH. This image contrast change is related to adhesion force differences between the two domains. In turn, the enhanced adhesion and image contrasts at elevated RH are attributed to capillary forces, which are large over the hydrophilic domains but greatly diminished over the hydrophobic domains. The adhesion force increases slightly with sample surface free energy at low RH, but increases rapidly with increasing sample surface free energy at high RH. The results indicate that for AFM in air, tailoring the RH of the probe-sample environment and utilizing a hydrophilic probe can enhance imaging of materials chemical heterogeneity with nanoscale spatial resolution.

Molecular gradients of bioinertness reveal a mechanistic difference between mammalian cell adhesion and bacterial biofilm formation

Chemical gradients play an important role in guiding the activities of both eukaryotic and prokaryotic cells. Here, we used molecularly well-defined chemical gradients formed by self-assembled monolayers (SAMs) on gold films to reveal that mammalian cell adhesion and bacterial biofilm formation respond differently to a gradient of surface chemistry that resists cell attachment. Gradient self-assembled monolayers (SAMs) consisting of two mixed alkanethiols were fabricated by differential exposure of the gold film to one alkanethiol, followed by soaking in another alkanethiol solution. A gradient in bioinertness that resisted cell attachment was created on SAMs from a gradient in the surface density of HS(CH2)11(OCH2CH2)3OH, backfilled with either HS(CH2)11OH or HS(CH2)11CH3. Measurements of the amounts of mammalian cells and bacterial biofilms on these gradient surfaces reveal that, for mammalian cells, a critical density of adhesion ligands from absorbed proteins on surfaces exists for supporting maximum adhesion and proliferation, whereas for the bacterium Escherichia coli , the amount of biofilm formed on surfaces increased linearly with the surface density of adhesive groups (methyl or hydroxyl groups) in different media. These results are consistent with mammalian cell adhesion requiring an anchorage via specific molecular recognitions and suggest that biofilms can form by immobilization of bacteria via nonspecific interaction between bacteria and surfaces.

Strategies and applications for incorporating physical and chemical signal gradients in tissue engineering

From embryonic development to wound repair, concentration gradients of bioactive signaling molecules guide tissue formation and regeneration. Moreover, gradients in cellular and extracellular architecture as well as in mechanical properties are readily apparent in native tissues. Perhaps tissue engineers can take a cue from nature in attempting to regenerate tissues by incorporating gradients into engineering design strategies. Indeed, gradient-based approaches are an emerging trend in tissue engineering, standing in contrast to traditional approaches of homogeneous delivery of cells and/or growth factors using isotropic scaffolds. Gradients in tissue engineering lie at the intersection of three major paradigms in the field-biomimetic, interfacial, and functional tissue engineering-by combining physical (via biomaterial design) and chemical (with growth/differentiation factors and cell adhesion molecules) signal delivery to achieve a continuous transition in both structure and function. This review consolidates several key methodologies to generate gradients, some of which have never been employed in a tissue engineering application, and discusses strategies for incorporating these methods into tissue engineering and implant design. A key finding of this review was that two-dimensional physicochemical gradient substrates, which serve as excellent high-throughput screening tools for optimizing desired biomaterial properties, can be enhanced in the future by transitioning from two dimensions to three dimensions, which would enable studies of cell-protein-biomaterial interactions in a more native tissue-like environment. In addition, biomimetic tissue regeneration via combined delivery of graded physical and chemical signals appears to be a promising strategy for the regeneration of heterogeneous tissues and tissue interfaces. In the future, in vivo applications will shed more light on the performance of gradient-based mechanical integrity and signal delivery strategies compared to traditional tissue engineering approaches.

Atomic force microscopy characterization of the chemical contrast of nanoscale patterns fabricated by electron beam lithography on polyethylene glycol oxide thin films

The present paper shows that atomic force microscopy (AFM) imaging of friction force and phase lag in ambient air can be used to characterize the chemical contrast induced by electron beam (EB) irradiation on polyethylene glycol oxide (PEO) surface. Time-of-flight secondary emission mass spectroscopy measurements showed that the EB irradiation generates chemical contrast on PEO surface by decreasing the ether bond density. The AFM measurements showed smaller phase lag and lower friction and adhesive forces on the EB irradiated PEO surface, as compared to the non-irradiated PEO surface. While the chemical contrast in friction force had a linear dependence on the EB irradiation dose, the dependence of the chemical contrast in the phase lag was strongly non-linear. As the friction and adhesive forces depended on the AFM probe hydrophilicity and air humidity, the contrast in friction and adhesive forces is ascribed to different capillary condensation of ambient water vapour at the AFM tip contact with the EB irradiated and non-irradiated PEO surfaces, respectively.

Surface-bound soft matter gradients

This feature article describes the progress realized over the past half century in the field of surface-bound gradient structures created on or from soft materials (oligomers and/or polymers), or those enabling the study of the behavior of soft materials. By highlighting our work in the field and accounting for the contribution of other groups, we emphasize the exceptional versatility of gradient assemblies in facilitating fast screening of physicochemical phenomena, acting as "recording media" for monitoring a process, and playing a key role in the design and fabrication of surface-bound molecular and macromolecular motors capable of directing a transport phenomenon.

Anisotropic wetting on tunable micro-wrinkled surfaces

We examine the wettability of rough surfaces through a measurement approach that harnesses a wrinkling instability to produce model substrate topographies. Specifically, we probe the wetting of liquids on anisotropic micro-wrinkled features that exhibit well-defined aspect ratios (amplitude wavelength of the wrinkles) that can be actively tuned. Our study provides new insight into the wetting behavior on rough surfaces and into the interpretation of related liquid contact-angle measurements. In particular, we find that droplet wetting anisotropy is governed primarily by the roughness aspect ratio. In addition, comparison of our measurements to theoretical models demonstrates that droplet distortions and observed contact angles on surfaces with a strongly anisotropic texture can be quantitatively attributed to the difference in the energetic barriers to wetting along and perpendicular to substrate features.

Versatile platform for creating gradient combinatorial libraries via modulated light exposure

This article details the design, construction, and operation of flexible system that modulates light exposure for the purpose of fabricating continuous and discrete gradient combinatorial libraries. Designed for versatility, the device combines "off the shelf" components, modular accessories, and flexible computer control, so that it can be used for a variety of combinatorial research applications. Salient aspects and capabilities of the instrument are illustrated through two practical examples. The first case demonstrates how user defined exposure functions can be used to create continuous surface energy gradient libraries with a linear profile. The second example illustrates the creation of continuous and discrete libraries for mapping exposure-property functions in a photocurable polymer system.

Pattern-directed to isotropic dewetting transition in polymer films on micropatterned surfaces with differential surface energy contrast

Surface chemical patterns can both cause and direct dewetting in overlying thin polymer films. In this paper we focus on a key factor in this phenomenon, the magnitude of the surface energy difference between surface pattern domains (Δ). To probe the influence of Δ on film dewetting, we utilize novel combinatorial test patterns exhibiting a gradient in Δ. Specifically, our test patterns consist of a series of micron-scale striped regions that continuously change in their surface energy () relative to background striped regions having a fixed and calibrated . Using polystyrene (PS) films as a demonstration case, we employ these test patterns to quantify the morphology and kinetics of dewetting as Δ diminishes. Our study indicates a transition from pattern-directed to isotropic PS dewetting at critical Δ values. For Δ > 14 mJ m, ordered droplet arrays are formed, while for Δ < 7 mJ m, the dewetting is isotropic. A competition between these limiting behaviors is found for a "crossover regime", 7 mJ m < Δ < 14 mJ m. These combinatorial test patterns provide a powerful approach for investigating the large number of parameters that govern the stability of ultrathin polymer films, and the physical factors that influence the dewetted film morphology.

Substrate surface energy dependent morphology and dewetting in an ABC triblock copolymer film

A gradient combinatorial approach was used to examine the effect of substrate surface energy on the morphology and stability of films of a poly(isoprene-b-styrene-b-ethylene oxide) triblock copolymer that exhibits an alternating gyroid morphology in the bulk. Atomic force microscopy data across our surface energy (water contact angle) library suggest a transformation to predominantly surface parallel lamellae with an antisymmetric ordering. For substrate water contact angles below 70 degrees the film exhibited autophobic dewetting from an adsorbed half-period triblock copolymer monolayer at longer annealing times. X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure analysis along gradient specimens indicated that the substrate surface energy governed the composition profile of the monolayer, and this variation in chemical expression was key to whether the film was stable or autophobically dewet. These observations demonstrate that enthalpic interactions, in addition to entropic considerations, can play a major role in autophobic dewetting of block copolymer films.

UVO-tunable superhydrophobic to superhydrophilic wetting transition on biomimetic nanostructured surfaces

A novel strategy for a tunable sigmoidal wetting transition from superhydrophobicity to superhydrophilicity on a continuous nanostructured hybrid film via gradient UV-ozone (UVO) exposure is presented. Along a single wetting gradient surface (40 mm), we could visualize the superhydrophobic (thetaH2O > 165 degrees and low contact angle hysteresis) transition (165 degrees > thetaH2O > 10 degrees ) and superhydrophilic (thetaH2O < 10 degrees within 1 s) regions simply through the optical images of water droplets on the surface. The film is prepared through layer-by-layer assembly of negatively charged silica nanoparticles (11 nm) and positively charged poly(allylamine hydrochloride) with an initial deposition in a fractal manner. The extraordinary wetting transition on chemically modified nanoparticle layered surfaces with submicrometer- to micrometer-scale pores represents a competition between the chemical wettability and hierarchical roughness of surfaces as often occurs in nature (e.g., lotus leaves, insect wings, etc).