Surface-chemical and -morphological gradients

Continuous stationary phase gradient preparation on planar chromatographic media using vapor phase deposition of silane

A new, versatile, and straightforward vapor phase deposition (VPD) approach was used to prepare continuous stationary phase gradients (cSPGs) on silica thin-layer chromatography (TLC) plates using phenyldimethylchlorosilane (PDCS) as a precursor. A mixture of paraffin oil and PDCS was placed at the bottom of an open-ended rectangular chamber, allowing the reactive silanes to evaporate and freely diffuse under a controlled atmosphere. As the volatile silane diffused across the length of the TLC plate, it reacted with the surface silanol groups thus functionalizing the surface in a gradient fashion. Characterization of the gradient TLC plates was done through UV visualization and diffuse reflectance spectroscopy (DRS). Visualizing the fluorescent gradient plates under UV radiation shows the clear presence of a gradient with the side closest to the vapor source undergoing the most modification. More quantitative characterization of the shape of the gradient was provided by DRS. The DRS showed that the degree of modification and shape of the gradient was dependent on the concentration of silane, VPD time, and relative humidity. To evaluate the chromatographic performance, a mixture of three aromatic compounds (acetaminophen (A), aspirin (As), and 3-hydroxy-2-naphthoic acid (3H)) was spotted on the high (GHP) and low phenyl (GLP) ends of the gradient TLC plates and the results compared to the separations carried out on unmodified and uniformly modified plates. The GHP TLC plates showed retention factors (Rf) of 0.060 ± 0.006, 0.391 ± 0.006, and 0.544 ± 0.006, whereas the unmodified plate displayed Rf values of 0.059 ± 0.006, 0.092 ± 0.003, and 0.037 ± 0.002 for the analytes A, As, and 3H, respectively. From the Rf values, it was observed that each modified plate exhibited different selectivity for the analytes. The GHP TLC plates exhibited better separation performance, and improved resolution compared to the GLP, unmodified, and uniformly modified plates. Overall, VPD is a new, cost-effective method for creating a gradient on the stationary phase which has the potential to advance chromatographic separation capabilities.

An imprint-based approach to replicate nano- to microscale roughness on gelatin hydrogel scaffolds: surface characterization and effect on endothelialization

Biologization of biomaterials with endothelial cells (ECs) is an important step in vascular tissue engineering, aiming at improving hemocompatibility and diminishing the thrombo-inflammatory response of implants. Since subcellular topography in the scale of nano to micrometers can influence cellular adhesion, proliferation, and differentiation, we here investigate the effect of surface roughness on the endothelialization of gelatin hydrogel scaffolds. Considering the micron and sub-micron features of the different native tissues underlying the endothelium in the body, we carried out a biomimetic approach to replicate the surface roughness of tissues and analyzed how this impacted the adhesion and proliferation of human umbilical endothelial cells (HUVECs). Using an imprinting technique, nano and micro-roughness ranging from Sa= 402 nm to Sa= 8 μm were replicated on the surface of gelatin hydrogels. Fluorescent imaging of HUVECs on consecutive days after seeding revealed that microscale topographies negatively affect cell spreading and proliferation. By contrast, nanoscale roughnesses of Sa= 402 and Sa= 538 nm promoted endothelialization as evidenced by the formation of confluent cell monolayers with prominent VE-cadherin surface expression. Collectively, we present an affordable and flexible imprinting method to replicate surface characteristics of tissues on hydrogels and demonstrate how nanoscale roughness positively supports their endothelialization.

Surface-Initiated Polymerization with an Initiator Gradient: A Monte Carlo Simulation

Due to the difficulty of accurately characterizing properties such as the molecular weight (Mn) and grafting density (σ) of gradient brushes (GBs), these properties are traditionally assumed to be uniform in space to simplify analysis. Applying a stochastic reaction model (SRM) developed for heterogeneous polymerizations, we explored surface-initiated polymerizations (SIPs) with initiator gradients in lattice Monte Carlo simulations to examine this assumption. An initial exploration of SIPs with 'homogeneously' distributed initiators revealed that increasing σ slows down the polymerization process, resulting in polymers with lower molecular weight and larger dispersity (Đ) for a given reaction time. In SIPs with an initiator gradient, we observed that the properties of the polymers are position-dependent, with lower Mn and larger Đ in regions of higher σ, indicating the non-uniform properties of polymers in GBs. The results reveal a significant deviation in the scaling behavior of brush height with σ compared to experimental data and theoretical predictions, and this deviation is attributed to the non-uniform Mn and Đ.

Thickness Gradient in Polymer Coating by Reactive Layer-by-Layer Assembly on Solid Substrate

The study describes a simple yet robust methodology for forming gradients in polymer coatings with nanometer-thickness precision. The thickness gradients of 0-20 nm in the coating are obtained by a reactive layer-by-layer assembly of polyester and polyethylenimine on gold substrates. Three parameters are important in forming thickness gradients: (i) the incubation time, (ii) the incubation concentration of the polymer solutions, and (iii) the tilt angle of the gold substrate during the dipping process. After examining these parameters, the characterization of the anisotropic surface obtained under the best conditions is presented in the manuscript. The thickness profile and nanomechanical characterization of the polymer gradients are characterized by atomic force microscopy. The roughness analysis has demonstrated that the coating exhibited decreasing roughness with increasing thickness. On the other hand, Young's moduli of the thin and thick coatings are 0.50 and 1.4 MPa, respectively, which assured an increase in mechanical stability with increasing coating thickness. Angle-dependent infrared spectroscopy reveals that the C-O-C ester groups of the polyesters exhibit a perpendicular orientation to the surface, while the C≡C groups are parallel to the surface. The surface properties of the polymer gradients are explored by fluorescence microscopy, proving that the dye's fluorescence intensity increases as the coating thickness increases. The significant benefit of the suggested methodology is that it promises thickness control of gradients in the coating as a consequence of the fast reaction kinetics between layers and the reaction time.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Magnetic-field-controlled counterion migration within polyionic liquid micropores enables nano-energy harvest

Efficient separation of positive and negative charges is essential for developing high-performance nanogenerators. In this article, we describe a method that was not previously demonstrated to separate charges which enables us to fabricate a magnetic energy harvesting device. The magnetic field induces the migration of the mobile magnetic counterions (Dy(NO₃)₄^-) which establishes anion gradients within a layer of polyionic liquid micropores (PLM). The PLM is covalently cross-linked on which the positive charges are fixed on the matrix, that is, immobile. In a device with a structure of Au/dielectric//mag-PLM//dielectric/Au, the charge gradient is subsequently transformed into the output voltage through electrostatic induction. Removing the magnetic field leads to the backflow of magnetic anions which produces a voltage with a similar magnitude but reversed polarity. The parameters in fabricating the magnetic PLM such as photoinitiator concentration, UV irradiation time, water treatment time, and temperature are found to dramatically influence the size of micropores and the effective concentration of magnetic anions. Under optimized conditions, an output voltage with an amplitude of approximately 4 V is finally achieved. We expect this new method could find practical applications in further improving the output performance.

Recent Growth of Wettability Gradient Surfaces: A Review

This review reports the recent progress and future prospects of wettability gradient surfaces (WGSs), particularly focusing on the governing principles, fabrication methods, classification, characterization, and applications. While transforming the inherent wettability into artificial wettability via bioinspiration, topographic micro/nanostructures are produced with changed surface energy, resulting in new droplet wetting regimes and droplet dynamic regimes. WGSs have been mainly classified in dry and wet surfaces, depending on the apparent surface states. Wettability gradient has long been documented as a surface phenomenon inducing the droplet mobility in the direction of decreasing wettability. However, it is herein critically emphasized that the wettability gradient does not always result in droplet mobility. Indeed, the sticky and slippery dynamic regimes exist in WGSs, prohibiting or allowing the droplet mobility, respectively. Lastly, the stringent bottlenecks encountered by WGSs are highlighted along with solution-oriented recommendations, and furthermore, phase change materials are strongly anticipated as a new class in WGSs. In all, WGSs intend to open up new technological insights for applications, encompassing water harvesting, droplet and bubble manipulation, controllable microfluidic systems, and condensation heat transfer, among others.

Reaction-Free Concentration Gradient Generation in Spatially Nonuniform AC Electric Fields

The ability to generate stable, spatiotemporally controllable concentration gradients is critical for both electrokinetic and biological applications such as directional wetting and chemotaxis. Electrochemical techniques for generating solution and surface gradients display benefits such as simplicity, controllability, and compatibility with automation. Here, we present an exploratory study for generating microscale spatiotemporally controllable gradients using a reaction-free electrokinetic technique in a microfluidic environment. Methanol solutions with ionic fluorescein isothiocyanate (FITC) molecules were used as an illustrative electrolyte. Spatially nonuniform alternating current (AC) electric fields were applied using hafnium dioxide (HfO₂)-coated Ti/Au electrode pairs. Results from spatial and temporal analyses along with control experiments suggest that the FITC ion concentration gradient in bulk fluid (over 50 μm from the electrode) was established due to spatial variation of electric field density, and was independent of electrochemical reactions at the electrode surface. The established ion concentration gradients depended on both amplitudes and frequencies of the oscillating AC electric field. Overall, this work reports a novel approach for generating stable and spatiotemporally tunable gradients in a microfluidic chamber using a reaction-free electrochemical methodology.

Influence of Hyaluronan Density on the Behavior of Breast Cancer Cells with Different CD44 Expression

Molecular gradients are common in biosystems and play an essential role in physiological and pathological processes. During carcinogenesis, for example, hyaluronan (HA) homeostasis is dysregulated by cancer cells and the altered synthesis and degradation processes result in the formation of HA gradients within the tumor microenvironment. Herein, a platform is developed to study the biological role of HA gradient in breast cancer cells. Cells with different aggressiveness and expression of CD44-the main HA receptor usually overexpressed in breast cancers, are selected for this study. The developed platform is compatible with several imaging modalities and allows assessment of cell density, morphology, CD44 expression, and cell motility in a function of HA density. Using high-throughput analysis, it is shown that cells that do not express CD44 do not change along the gradient, while CD44 positive cells respond differently to the HA gradient depending on the level of CD44 expression and HA density. This different response is associated with the activation of different signaling pathways by the CD44-HA interactions.

Single Molecule Spectroscopy Studies of Acid-Base Chemical Gradients Using Nile Red as a Probe of Local Surface Acidity

Single molecule spectroscopy studies of local acidity along bifunctional acid-base gradients are reported. Gradients are prepared by directional vapor phase diffusion and subsequent reaction of 3-aminopropyl-trimethoxysilane with a uniform silica film. Gradient formation is confirmed by spectroscopic ellipsometry and by static water contact angle measurements. X-ray photoelectron spectroscopy is used to characterize the nitrogen content and degree of nitrogen protonation along the gradient. Nile Red is employed as the probe dye in single molecule spectroscopy studies of these gradients. While Nile Red is well-known for its solvent sensitivity, it is used here, for the first time, to sense the acid/base properties of the film in two-color wide-field fluorescence imaging experiments. The data reveal broad bimodal distributions of Nile Red emission spectra that vary along the gradient direction. The single molecule results are consistent with solution phase ensemble acid/base studies of the dye. The former reveal a gradual transition from a surface dominated by basic aminosilane sites at the high-amine end of the gradient to one dominated by acidic silanol sites at the low-amine end. The sub-diffraction-limited spatial resolution afforded by superlocalization of the single molecules reveals spatial correlations in the acid/base properties of the gradient over ∼200 nm distances. These studies provide data relevant to the use of aminosilane-modified silica in bifunctional, cooperative chemical catalysis.

Initial-position-driven opposite directional transport of a water droplet on a wedge-shaped groove

The transport direction of water droplets on a functionalized surface is of great significance due to its wide applications in microfluidics technology. The prevailing view is that a water droplet on a wedge-shaped groove always moves towards the wider end. In this paper, however, molecular dynamics simulations show that a water droplet can move towards the narrower end if placed at specific positions. It is found that the direction of water droplet transport on a grooved surface is related to its initial position. The water droplet moves towards the wider end only when it is placed near the wider end initially. If the water droplet is placed near the narrower end, it will move in the opposite direction. The novel phenomenon is attributed to the opposite interactions of the groove substrate and the groove upper layers with water droplets. Two effective models are proposed to exploit the physical origin of different transport directions of water droplets on a wedge-shaped groove surface. The study provides an insight into the design of nanostructured surfaces to effectively control the droplet motion.

High-Throughput and Combinatorial Approaches for the Development of Multifunctional Polymers

High-throughput (HT) development of new multifunctional polymers is accomplished by the combination of different HT tools established in polymer sciences in the last decade. Important advances are robotic/HT synthesis of polymer libraries, the HT characterization of polymers, and the application of spatially resolved polymer library formats, explicitly microarray and gradient libraries. HT polymer synthesis enables the generation of material libraries with combinatorial design motifs. Polymer composition, molecular weight, macromolecular architecture, etc. may be varied in a systematic, fine-graded manner to obtain libraries with high chemical diversity and sufficient compositional resolution as model systems for the screening of these materials for the functions aimed. HT characterization allows a fast assessment of complementary properties, which are employed to decipher quantitative structure-properties relationships. Moreover, these methods facilitate the HT determination of important surface parameters by spatially resolved characterization methods, including time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. Here current methods for the high-throughput robotic synthesis of multifunctional polymers as well as their characterization are presented and advantages as well as present limitations are discussed.

Dynamic manipulation of droplets using mechanically tunable microtextured chemical gradients

Materials and strategies applicable to the dynamic transport of microdroplets are relevant to surface fluidics, self-cleaning materials, thermal management systems, and analytical devices. Techniques based on electrowetting, topographic micropatterns, and thermal/chemical gradients have advanced considerably, but dynamic microdroplet transport remains a challenge. This manuscript reports the fabrication of mechano-tunable, microtextured chemical gradients on elastomer films and their use in controlled microdroplet transport. Specifically, discreet mechanical deformations of these films enabled dynamic tuning of the microtextures and thus transport along surface-chemical gradients. The interplay between the driving force of the chemical gradient and the microtopography was characterized, facilitating accurate prediction of the conditions (droplet radius and roughness) which supported transport. In this work, the use of microtextured surface chemical gradients in mechano-adaptive materials with microdroplet manipulation functionality was highlighted.

Directed self-propulsion of droplets on surfaces absent of gradients for cargo transport

HYPOTHESIS

Manipulating droplet transportation without inputting work is desired and important in microfluidic systems. Although the creation of wettability gradient on surfaces has been employed to achieve this goal, the transport distance is very limited, hindering its applications in long-term operations.

EXPERIMENTS

Here, we show that programming long-ranged transport of droplets on surfaces can be achieved by the addition of trisiloxane surfactants and the creation of deep grooves. The former provides Marangoni stress to actuate the droplet motion and also reduces the inherent contact line pinning. The latter acts as a railing to guide the motion of surfactant-laden droplets to follow various layouts with geometric features of roads.

FINDINGS

It is found that the droplets with microliters can move over 20 cm. This work-free method is applicable to a variety of substrate materials and liquids. By using self-running shuttles, a convenient platform for liquid cargos transport is developed and demonstrated. Moreover, the coalescence of cargos carried by different shuttles is accomplished in a three-branch layout, revealing new droplet microreactors.

Gradient wettability induced by deterministically patterned nanostructures

We report a large-scale surface with continuously varying wettability induced by ordered gradient nanostructures. The gradient pattern is generated from nonuniform interference lithography by utilizing the Gaussian-shaped intensity distribution of two coherent laser beams. We also develop a facile fabrication method to directly transfer a photoresist pattern into an ultraviolet (UV)-cured high-strength replication molding material, which eliminates the need for high-cost reactive ion etching and e-beam evaporation during the mold fabrication process. This facile mold is then used for the reproducible production of surfaces with gradient wettability using thermal-nanoimprint lithography (NIL). In addition, the wetting behavior of water droplets on the surface with the gradient nanostructures and therefore gradient wettability is investigated. A hybrid wetting model is proposed and theoretically captures the contact angle measurement results, shedding light on the wetting behavior of a liquid on structures patterned at the nanoscale.

Amine/phenyl gradient derived base layer as a comprehensive extractive phase for headspace cooled in-tube microextraction of volatile organic compounds in saliva

A gradient derived base layer extractive phase was synthesized and applied for the determination of volatile organic compounds (VOCs) in saliva samples using the headspace cooled in-tube microextraction (HS-CITME) method. The base layers from three different sols of phenyltriethoxysilane (PTES), octyltrimethoxysilane (OTMS) and methyltrimethoxysilane (MTMS) as nonpolar precursors were individually dip coated on the stainless steel wires (SSW). Then, the hydrolyzed polar precursor aminopropyltriethoxysilane (APTES) reacted with the silanol groups already formed on the surface of SSWs via controlled rate infusion (CRI) method. The presence of polar and non-polar functional groups on the surface of substrate was evaluated by Fourier-transform infrared spectroscopy (FTIR) while the morphology and thickness of the most suitable gradient coating (amine/phenyl) were also investigated by scanning electron microscopy (SEM). Assessment of the gradient extractive phase efficiency was carried out determining a group of VOCs with different polarities coupled with gas chromatography-mass spectrometry (GCMS) and the improved performance of the synthesized base layer coatings was observed. Furthermore, a cooling device was designed and implemented to the extracting system to improve the efficiency by influencing the exothermic nature of process. The data were analyzed by principal component analysis (PCA), and hierarchical cluster analysis (HCA) and the results were interpreted by polarities of analytes. Finally, under the optimized conditions, the limits of detection (LOD) and limits of quantification (LOQ) were 0.15 and 0.50 ng L^-1, respectively. The intra-day and inter-day relative standard deviations (RSDs) at 5 and 50 ng L^-1 (n = 3) using a single extractive phase were 2-6 and 10-17, respectively. The data associated with RSDs% for three extractive phases were between 16 and 19 %. Eventually, the method was conveniently applied to the extraction of VOCs from saliva samples of smokers and satisfactory relative recoveries (RR%) (95-108 %) were achieved and low quantities of VOCs were detected.

Anisotropy-induced directional self-transportation of low surface tension liquids: a review

Inspired by natural surfaces such as butterfly wings, cactus leaves, or the Nepenthes alata plant, synthetic materials may be engineered to directionally transport liquids on their surface without external energy input. This advantageous feature has been adopted for various mechanical and chemical processes, e.g. fog harvesting, lubrication, lossless chemical reactions, etc. Many studies have focused on the manipulation and transport of water or aqueous droplets, but significantly fewer have extended their work to low surface tension (LST) liquids, although these fluids are involved in numerous industrial and everyday processes. LST liquids completely wet most surfaces which makes spontaneous transportation an active challenge. This review focuses on recently developed strategies for passively and directionally transporting LST liquids.

On the Importance of Silane Infusion Order on the Microscopic and Macroscopic Properties of Multifunctional Charge Gradients

Four multicomponent charge gradients containing acidic and basic functionalities were prepared via sol-gel processes and the controlled-rate infusion (CRI) method to more clearly understand how preparation conditions influence macroscopic properties. CRI is used to form gradients by infusing reactive alkoxysilanes into a glass vial housing a vertically oriented modified silicon wafer. The concentration and time of infusion of the silane solutions were kept constant. Only the sequence of infusion of the silane solutions was changed. The first set of samples was prepared by initially infusing a solution containing 3-aminopropyltriethoxysilane (APTES) followed by a mercaptopropyltrimethoxysilane (MPTMS) solution. The individual gradients were formed either in an aligned or opposed fashion with respect to the initial gradient. The second set of samples was prepared by infusing the MPTMS solution first followed by the APTES solution, again in either an aligned or opposed fashion. To create charge gradients (NH3 +, SO3 -), the samples were immersed into H2O2. The extent of modification, the degree of protonation of the amine, and the thicknesses of the individual layers were examined by X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. The wettability of the individual gradients was assessed via static contact angle measurements. The results demonstrate the importance of infusion order and how it influences the macroscopic and microscopic properties of gradient surfaces including the surface concentration, packing density, degree of protonation, and ultimately wettability. When the gradient materials are prepared via infusion of the APTES sol first, it results in increased deposition of both the amine and thiol groups as evidenced by XPS. Interestingly, the total thickness evaluated from ellipsometry was independent of the infusion order for the aligned gradients, indicative of significant differences in the film density. For the opposed gradients, however, the infusion of APTES first leads to a significantly thicker composite film. Furthermore, it also leads to a more pronounced gradient in the protonation of the amine, which introduces a very different surface wettability. The use of aminosilanes provides a viable approach to create gradient surfaces with different functional group distributions. These studies demonstrate that the controlled placement of functional groups on a surface can provide a new route to prepare gradient materials with improved performance.

Nano-micrometer surface roughness gradients reveal topographical influences on differentiating responses of vascular cells on biodegradable magnesium

Distinctively directing endothelial cells (ECs) and smooth muscle cells (SMCs), potentially by surface topography cue, is of central importance for enhancing bioefficacy of vascular implants. For the first time, surface gradients with a broad range of nano-micrometer roughness are developed on Mg, a promising next-generation biodegradable metal, to carry out a systematic study on the response of ECs and SMCs. Cell adhesion, spreading, and proliferation are quantified along gradients by high-throughput imaging, illustrating drastic divergence between ECs and SMCs, especially in highly rough regions. The profound role of surface topography overcoming the biochemical cue of released Mg²⁺ is unraveled at different roughness ranges for ECs and SMCs. Further insights into the underlying regulatory mechanism are gained at subcellular and gene levels. Our work enables high-efficient exploration of optimized surface morphology for modulating favored cell selectivity of promoting ECs and suppressing SMCs, providing a potential strategy to achieve rapid endothelialization for Mg.

•

Surface topography stimuli was engineered on Mg with varying roughness gradients.

•

Ridge/valley network feature on Mg overperforms the influence of Mg²⁺.

•

Optimized roughness facilitates proliferation of ECs while suppressing SMCs.

Shape controlled assembly of carboxylic acids: formation of a binary monolayer by intercalation into molecular nanotunnels

Binary self-assembled monolayers (SAMs) combining a Y-shaped aromatic carboxylic acid (1,3,5-benzenetribenzoic acid, H3BTB) and a cage-type alicyclic carboxylic acid (adamantane carboxylic acid, AdCA) were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The SAMs, prepared by molecular adsorption from solution on Au substrates modified by underpotential deposition of Ag, exhibit a pronounced dependence of their structure on the assembly protocol. Exposing an H3BTB SAM to AdCA, the highly regular row structure of the native H3BTB layer persists and STM imaging does not show signs of AdCA adsorption. This is in striking contrast to the disordered arrangements of H3BTB and the presence of AdCA employing the inverted adsorption sequence or coadsorption of the two molecules. However, spectroscopic analysis of the H3BTB SAM exposed to AdCA reveals the presence also of the latter, suggesting that the AdCA molecules are hidden in the nanotunnels of the H3BTB monolayer. Direct evidence for the intercalation of AdCA is obtained by STM manipulation experiments which lay bare areas of AdCA molecules upon local removal of H3BTB. Surprisingly, these are densely packed and arranged into a highly ordered monolayer. Formation of such a compact AdCA layer is explained by expulsion of AdCA from the H3BTB nanotunnels of the surrounding intact mixed SAM, driven by release of stress in the nanotunnels built up when AdCA is intercalated.

Charge Density Gradients of Polymer Thin Film by Gaseous Phase Quaternization

We report on the rapid formation of charge density gradients in polymer films by exposing poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) films resting on flat silica substrates to methyl iodide (i.e., MI, also known as iodomethane) vapors. We adjust the charge gradient by varying the MI concentration in solution and the process time. The thickness of the parent PDMAEMA film does not affect the diffusion of MI through and the reaction kinetics in the films. Instead, the diffusion of MI through the gaseous phase constitutes the limiting step in the overall process.

Extending the Range of Controlling Protein Adsorption via Subsurface Architecture

Recently, it has been shown that water, confined in a plasma polymer subsurface chemical gradient, nanometers below the surface, significantly reduced the amount of adsorbed protein bovine serum albumin (BSA). Relating to this effect, we proposed the hypothesis that oriented water molecules within the subsurface gradient generate a long-range dipolar field, which interacts with dipolar proteins such as BSA near the surface region. This study extends the above used in situ multistep plasma deposition process to introduce plasma oxidation modifications of the subsurface architecture with the aim to further control the effect on protein adsorption. Neutron reflectivity measurements reveal that the oxidation time increases the amount of matrix-confined water. There is, however, an optimal oxidation time to obtain minimal protein adsorption, which suggests that a minimal distance between confined water molecules plays an important role. Altogether we can extend the range of controlling the adsorbed protein mass by the introduction of this additional plasma oxidation step.

Designing biomimetic liquid diodes

Just as the innovation of electronic diodes that allow the current to flow in one direction provides a foundation for the development of digital technologies, the engineering of surfaces or devices that allow the directional and spontaneous transport of fluids, termed liquid diodes, is highly desired in a wide spectrum of applications ranging from medical microfluidics, advanced printing, heat management and water collection to oil-water separation. Recent advances in manufacturing, visualization techniques, and biomimetics have led to exciting progress in the design of various liquid diodes. In spite of exciting progress, formulating a general framework broad enough to guide the design, optimization and fabrication of engineered liquid diodes remains a challenging task to date. In this review, we first present an overview of the development of biological and engineered liquid diodes to elucidate how to control the surface chemistry and topography to regulate the transport of liquids without the need for external energy. Then the latest design strategies allowing for the creation of longitudinal and transverse liquid diodes are discussed and compared. We also define some figures of merit such as the rectification coefficient and the transport velocity and distance to quantify the performance of liquid diodes. Finally, we highlight perspectives on the development of engineered liquid diodes that transcend nature and adapt to various practical applications.

Graded Protein/PEG Nanopattern Arrays: Well-Defined Gradient Biomaterials to Induce Basic Cellular Behaviors

Gradient biomaterials have shown enormous potential in high-throughput screening of biomaterials and material-induced cell migration. To make the screening process more rapid and precise, improving the regularity of morphological structure and chemical modification on gradient biomaterials have attracted much attention. In this paper, we present a novel fabrication strategy to introduce ordered nanopattern arrays into gradient biomaterials, through combining surface-initiated atom transfer radical polymerization and inclined reactive-ion etching based on colloidal lithography. Graded protein/poly(ethylene glycol) (PEG) nanopattern arrays on a quartz substrate were fabricated and applied to affect the behaviors of cells. Owing to the continuously changed ratio of two different components, the corresponding cell adhesion density along the substrate showed obvious graded distribution after culturing for 24 h. Meanwhile, the cytoskeleton showed obvious polarization after culturing for 7 days, which is parallel with the direction of gradient. Additionally, oriented migration was generated when mouse MC3T3-E1 cells were cultured on the graded protein/PEG nanopattern arrays. On the basis of the ordered and well-defined nanopatterns, the correlation between the extracellular matrix and corresponding expressions generated by different stimuli can be investigated.

Directed motion of water droplets on multi-gradient photopolymer surfaces

Rapid and directional movement of water droplets across a photopolymer surface with inscribed wettability and Laplace pressure gradient is demonstrated.

Gradient extractive phase prepared by controlled rate infusion method: An applicable approach in solid phase microextraction for non-targeted analysis

The aim of this study is to introduce an extractive phase based on gradient concept by continuous changing in chemical functional groups for non-targeted analysis. For this purpose, three different two-component coatings containing (3-aminopropyl)trimethoxysilane (APTES) as polar and either phenyltriethoxysilane (PTES), octyl-trimethoxysilane (OTMS) or methyltrimethoxysilane (MTMS) as nonpolar precursors were formed on the modified stainless steel wires using controlled rate infusion (CRI) method. The presence of polar and/or non-polar functional groups on the surface of substrate was evaluated by Fourier-transform infrared spectroscopy (FTIR) together with contact angles determined alongside the gradient surface. The morphology and thickness of the prepared fibers were also investigated by scanning electron microscopy (SEM). Furthermore, uniform single-component fibers from polar (APTES) and nonpolar (PTES) coatings were fabricated in order to be compared with the gradient sorbent. The gradient phase was implemented as a fiber coating in headspace- or immersed-solid phase microextraction of various compounds including chlorobenzenes, polycyclic aromatic hydrocarbons, chlorophenols and volatile organic compounds (Log K_ow range: -0.77 to 4.64). Under the optimized condition, the limits of detection and quantification were obtained in the range of 0.01-0.5 μg L^-1 and 0.05-1.5 μg L^-1, respectively. The intra-day and inter-day relative standard deviations of 2-10% and 11-17% were achieved, respectively. The method was successfully applied to the extraction of VOCs from real water sample and relative recoveries were between 89 and 105%. The capability and efficiency of the gradient coating appears to be quite appropriate for non-targeted analysis.

Vapor-Phase Plotting of Organosilane Chemical Gradients

Vapor-phase plotting of organosilane-based self-assembled monolayer (SAM) gradients is demonstrated for the first time. Patterned SAMs are formed by delivering gas-phase organotrichlorosilane precursors to a reactive silica surface using a heated glass capillary. The capillary is attached via a short flexible tube to a reservoir containing the precursor dissolved in toluene. The proximal end of the capillary is positioned at an experimentally optimized distance of 30 μm above the substrate during film deposition. The capillary is mounted to a stepper-motor-driven X, Y plotter for raster scanning above the surface. Two different organotrichlorosilane precursors are employed in this initial demonstration: n-octyltrichlorosilane and 3-cyanopropyltrichlorosilane. The dependence of SAM deposition on ambient relative humidity, capillary-substrate separation, raster-scanning speed, and solvent viscosity and volatility is explored and optimum deposition conditions are identified. The optimized procedures are used to plot uniformly modified square "pads" and gradients of the silanes. Film formation is verified and the gradient profiles are obtained by sessile drop water contact angle measurements, spectroscopic ellipsometry measurements of film thickness, and X-ray photoelectron spectroscopy mapping. The resolution of the plotting process is currently in the millimeter range and depends on capillary diameter and distance from the substrate surface. Vapor-phase plotting affords a unique direct-write method for producing patterned and chemically graded SAMS that may find applications in microfluidic devices, planar chromatography, and optical and electronic devices.

Spontaneous directional transportations of water droplets on surfaces driven by gradient structures

Spontaneous directional transportation of droplets on solid surfaces driven by structure gradients has attracted much attention due to its large-scale applications, such as heat transfer, microfluidic devices, water collection, and separation. It also provides new insight for theoretical research into the interactions between droplets and solid surfaces. This review article summarizes recent progress in the spontaneous directional transportation of droplets on surfaces with structure gradients. Currently, surfaces with structure gradients can be divided into three types: wedge corners with a gradient opening angle, wedge-shaped surfaces, and conical substrates. This review focuses on their basic theory, detailed transport processes, fabrication methods, influence factors and application development. Finally, a perspective of this mode of transportation for future development is proposed.

Design and Fabrication of Wettability Gradients with Tunable Profiles through Degrafting Organosilane Layers from Silica Surfaces by Tetrabutylammonium Fluoride

Surface-bound wettability gradients allow for a high-throughput approach to evaluate surface interactions for many biological and chemical processes. Here we describe the fabrication of surface wettability gradients on flat surfaces by a simple, two-step procedure that permits precise tuning of the gradient profile. This process involves the deposition of homogeneous silane SAMs followed by the formation of a surface coverage gradient through the selective removal of silanes from the substrate. Removal of silanes from the surface is achieved by using tetrabutylammonium fluoride which selectively cleaves the Si-O bonds at the headgroup of the silane. The kinetics of degrafting has been modeled by using a series of first order rate equations, based on the number of attachment points broken to remove a silane from the surface. Degrafting of monofunctional silanes exhibits a single exponential decay in surface coverage; however, there is a delay in degrafting of trifunctional silanes due to the presence of multiple attachment points. The effects of degrafting temperature and time are examined in detail and demonstrate the ability to reliably and precisely control the gradient profile on the surface. We observe a relatively homogeneous coverage of silane (i.e., without the presence of islands or holes) throughout the degrafting process, providing a much more uniform surface when compared to additive approaches of gradient formation. Linear gradients were formed on the substrates to demonstrate the reproducibility and tuneability of this subtractive approach.

Organosilane Chemical Gradients: Progress, Properties, and Promise

Chemical gradients play an important role in nature, driving many different phenomena critical to life, including the transport of chemical species across membranes and the transport, attachment, and assembly of cells. Taking a cue from these natural processes, scientists and engineers are now working to develop synthetic chemical gradients for use in a broad range of applications, such as in high-throughput investigations of surface properties, as means to guide the motions and/or assembly of liquid droplets, vesicles, nanoparticles, and cells and as new media for stationary-phase-gradient chemical separations. Our groups have been working to develop new methods for preparing chemical gradients from organoalkoxysilane and organochlorosilane precursors and to obtain a better understanding of their properties on macroscopic to microscopic length scales. This review highlights our recent work on the development of controlled-rate infusion and infusion-withdrawal dip-coating methods for the preparation of gradients on planar glass and silicon substrates, on thin-layer chromatography plates, and in capillaries and monoliths for liquid chromatography. We also cover the new knowledge gained from the characterization of our gradients using sessile drop and Wilhelmy plate dynamic water contact angle measurements, X-ray photoelectron spectroscopy mapping, and single-molecule tracking and spectroscopy. Our studies reveal important evidence of phase separation and cooperative interactions occurring along multicomponent gradients. Emerging concepts and new directions in the preparation and characterization of organosilane-based chemical gradients are also discussed.

Single Molecule Catch and Release: Potential-Dependent Plasmid DNA Adsorption along Chemically Graded Electrode Surfaces

Single molecule detection methods were employed to study the potential dependent adsorption and desorption of dye labeled plasmid DNA along chemical gradients prepared on indium tin oxide (ITO) electrodes. Gradients were formed over silica-base-layer-coated ITO surfaces by exposing them in a directional fashion to aminopropyltrimethoxysilane from the vapor phase. Sessile drop water contact angle measurements, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy were used to verify that a gradient was formed and to characterize its wettability, thickness, and composition as a function of position. The gradient-coated ITO electrode served as both the working electrode and a window into the electrochemical cell used to manipulate DNA adsorption. For single molecule studies, the electrochemical cell was filled with buffer solution containing YOYO-1-labeled plasmid DNA. Fluorescence videos acquired along the gradients depicted clear position-, potential-, and pH-dependent variations in DNA adsorption and desorption. The results demonstrate that DNA adsorption was largely independent of applied potential and irreversible at high amine coverage (i.e., multilayers), under pH ∼ 6 buffer. DNA adsorption became more reversible as the amine coverage decreased and the solution pH increased. Potential dependent control over DNA adsorption and desorption was best achieved at monolayer-to-submonolayer aminosilane coverage under pH ∼ 8 buffer. The knowledge gained in these studies will aid in the development of electrochemical methods for the capture and release of DNA and other polyelectrolytes at electrode surfaces.

Modulation of Wetting Gradients by Tuning the Interplay between Surface Structuration and Anisotropic Molecular Layers with Bipolar Electrochemistry

A new simple and versatile method for the preparation of surface-wetting gradients is proposed. It is based on the combination of electrode surface structuration introduced by a sacrificial template approach and the formation of a tunable molecular gradient by bipolar electrochemistry. The gradient involves the formation of a self-assembled monolayer on a gold surface by selecting an appropriate thiol molecule and subsequent reductive desorption by means of bipolar electrochemistry. Under these conditions, completion of the reductive desorption process evolves along the bipolar surface with a maximum strength localized at the cathodic edge and a decreasing driving force towards the middle of the surface. The remaining quantity of surface-immobilized thiol, therefore, varies as a function of the axial position, resulting in the formation of a molecular gradient. The surface of the bipolar electrode is characterized at each step of the modification by recording heterogeneous electron transfer. Also, the evolution of static contact angles measured with a water droplet deposited on the surface directly reveals the presence of the wetting gradient, which can be modulated by changing the properties of the thiol. This is exemplified with a long, hydrophobic alkane-thiol and a short, hydrophilic mercaptan.

Graded nanowell arrays: a fine plasmonic "library" with an adjustable spectral range

We present an effective approach for fabricating graded plasmonic arrays based on ordered micro-/nanostructures with a geometric gradient. Ag nanowell arrays with graded geometric parameters were fabricated and systematically investigated. The order of the graded plasmonic arrays is generated by colloidal lithography, while the geometric gradient is the result of inclined reactive ion etching. The surface plasmon resonance (SPR) peaks were measured at different positions, which move gradually along the Ag nanowell arrays with a geometric gradient. Such micro-/nanostructure arrays with graded and integrated SPR peaks can work as a fine plasmonic "library" (FPL), and the spectral range can be controlled using a "coarse adjustment knob" (lattice constant) and a "fine adjustment knob" (pore diameter). Additionally, the spectral resolution of the FPL is high, which benefits from the high value of the full height/full width at half-maximum and the small step size of the wavelength shift (0.5 nm). Meanwhile, the FPL could be effectively applied as a well-defined model to verify the plasmonic enhancement in surface enhanced Raman scattering. As the FPL is an integrated optical material with graded individual SPR peaks, it can not only be a theoretical model for fundamental research, but also has great potential in high-throughput screening of optical materials, multiplex sensors, etc.

Nanotribological properties of nanostructured poly(cysteine methacrylate) brushes

The nanomechanical properties of zwitterionic poly(cysteine methacrylate) (PCysMA) brushes grown from planar surfaces by atom transfer radical polymerisation have been characterised by friction force microscopy (FFM). FFM provides quantitative insights into polymer structure-property relationships and in particular illuminates the dependence of brush swelling on chain packing in nanostructured materials. In ethanol, which is a poor solvent for PCysMA, a linear friction-load relationship is observed, indicating that energy dissipation occurs primarily through ploughing. In contrast, in a good solvent for PCysMA such as water, a non-linear friction-load relationship is observed that can be fitted by Derjaguin-Muller-Toporov (DMT) mechanics, suggesting that the relatively small modulus of the swollen polymer leads to a large contact area and consequently a significant shear contribution to energy dissipation. The brush grafting density was varied by using UV photolysis of C-Br bonds at 244 nm to dehalogenate the surface in a controlled fashion. The surface shear strength increases initially as the brush grafting density is reduced, but then decreases for UV doses greater than 0.5 J cm^-2, reaching a limiting value when the brush thickness is ca. 50% that of a brush monolayer. Below this critical grafting density, a collapsed brush layer is obtained. For nm-scale gradient brush structures formed via interferometric lithography, the mean width increases as the period is increased, and the lateral mobility of brushes in these regions is reduced, leading to an increase in brush height as the grafted chains become progressively more extended. For a width of 260 nm, the mean brush height in water and ethanol is close to the thickness of a dense unpatterned brush monolayer synthesised under identical conditions. Both the surface shear stress measured for PCysMA brushes under water and the coefficient of friction measured in ethanol are closely correlated to the feature height, and hence to the chain conformation.

Detailed Study of BSA Adsorption on Micro- and Nanocrystalline Diamond/β-SiC Composite Gradient Films by Time-Resolved Fluorescence Microscopy

The adsorption of bovine serum albumin (BSA) on micro- and nanocrystalline diamond/β-SiC composite films synthesized using the hot filament chemical vapor deposition (HFCVD) technique has been investigated by confocal fluorescence lifetime imaging microscopy. BSA labeled with fluorescein isothiocyanate (FITC) was employed as a probe. The BSA^FITC conjugate was found to preferentially adsorb on both O-/OH-terminated microcrystalline and nanocrystalline diamond compared to the OH-terminated β-SiC, resulting in an increasing amount of BSA adsorbed to the gradient surfaces with an increasing diamond/β-SiC ratio. The different strength of adsorption (>30 times for diamond with a grain size of 570 nm) coincides with different surface energy parameters and differing conformational changes upon adsorption. Fluorescence data of the adsorbed BSA^FITC on the gradient film with different diamond coverage show a four-exponential decay with decay times of 3.71, 2.54, 0.66, and 0.13 ns for a grain size of 570 nm. The different decay times are attributed to the fluorescence of thiourea fluorescein residuals of linked FITC distributed in BSA with different dye-dye and dye-surface distances. The longest decay time was found to correlate linearly with the diamond grain size. The fluorescence of BSA^FITC undergoes external dynamic fluorescence quenching on the diamond surface by H- and/or sp²-defects and/or by amorphous carbon or graphite phases. An acceleration of the internal fluorescence concentration quenching in BSA^FITC because of structural changes of albumin due to adsorption, is concluded to be a secondary contributor. These results suggest that the micro- and nanocrystalline diamond/β-SiC composite gradient films can be utilized to spatially control protein adsorption and diamond crystallite size, which facilitates systematic studies at these interesting (bio)interfaces.

Macroporous materials: microfluidic fabrication, functionalization and applications

This article provides an up-to-date highly comprehensive overview (594 references) on the state of the art of the synthesis and design of macroporous materials using microfluidics and their applications in different fields.

Ordered Micro/Nanostructures with Geometric Gradient: From Integrated Wettability "Library" to Anisotropic Wetting Surface

Geometric gradients within ordered micro/nanostructures exhibit unique wetting properties. Well-defined and ordered microsphere arrays with geometric gradient (OMAGG) are successfully fabricated through combining colloidal lithography and inclined reactive ion etching (RIE). During the inclined RIE, the graded etching rates in vertical direction of etcher chamber are the key to generating a geometric gradient. The OMAGG can be used as an effective mask for the preparation of micro/nanostructure arrays with geometric gradient by selective RIE. Through this strategy, a well-defined wettability "library" with graded silicon cone arrays is fabricated, and the possibility of screening one desired "book" from the designated wettability "library" is demonstrated. Meanwhile, the silicon cone arrays with geometric gradient (SCAGG) can be applied to control the wetting behavior of water after being modified by hydrophilic or hydrophobic chemical groups. Based on this result, a temperature-responsive wetting substrate is fabricated by modifying poly n-isopropyl acrylamide (PNIPAM) on the SCAGG. These wettability gradients have great potential in tissue engineering, microfluidic devices, and integrated sensors.

Directional Fluid Transport in Thin Porous Materials and its Functional Applications

Directional fluid motion driven by the surface property of solid substrate is highly desirable for manipulating microfluidic liquid and collecting water from humid air. Studies on such liquid motion have been confined to dense material surfaces such as flat panels and single filaments. Recently, directional fluid transport through the thickness of thin porous materials has been reported by several research groups. Their studies not only attract fundamental, experimental and theoretical interest but also open novel application opportunities. This review article summarizes research progress in directional fluid transport across thin porous materials. It focuses on the materials preparation, basic properties associated with directional fluid transport in thin porous media, and their application development. The porous substrates, type of transporting fluids, structure-property attributes, and possible directional fluid transport mechanism are discussed. A perspective for future development in this field is proposed.

Cell Growth on ("Janus") Density Gradients of Bifunctional Zeolite L Crystals

Nanoparticle density gradients on surfaces have attracted interest as two-dimensional material surfaces that can mimic the complex nano-/microstructure of the native extracellular matrix, including its chemical and physical gradients, and can therefore be used to systematically study cell-material interactions. In this respect, we report the preparation of density gradients made of bifunctional zeolite L crystals on glass surfaces and the effects of the density gradient and biopolymer functionalization of zeolite L crystals on cell adhesion. We also describe how we created "Janus" density gradient surfaces by gradually depositing two different types of zeolite L crystals that were functionalized and loaded with different chemical groups and guest molecules onto the two distinct sides of the same glass substrate. Our results show that more cells adhered on the density gradient of biopolymer-coated zeolites than on uncoated ones. The number of adhered cells increased up to a certain surface coverage of the glass by the zeolite L crystals, but then it decreased beyond the zeolite density at which a higher surface coverage decreased fibroblast cell adhesion and spreading. Additionally, cell experiments showed that cells gradually internalized the guest-molecule-loaded zeolite L crystals from the underlying density gradient containing bifunctional zeolite L crystals.

Molecular Combing of λ-DNA using Self-Propelled Water Droplets on Wettability Gradient Surfaces

Surface wettability gradients were used to elongate and align double stranded λ-DNA. Gradients were prepared by vapor phase deposition of octyltrichlorosilane (C8-silane) and fluorinated octyltrichlorosilane (F-silane) precursors. Gradient formation was confirmed by water contact angle and ellipsometric film thickness measurements. Placement of a droplet of aqueous DNA solution on the hydrophobic end of each gradient led to spontaneous motion of the droplet toward the hydrophilic end and deposition of the DNA. Fluorescence imaging of surface-adsorbed YOYO-1 labeled DNA molecules revealed that they are elongated and aligned perpendicular to the droplet-surface contact line at all positions along the gradient, consistent with a dominant role played by surface tension forces in elongating the DNA. The density of adsorbed DNA was found to be greatest on the C8-silane gradient at its hydrophobic end. DNA density decreased toward the hydrophilic end, while the length of the elongated DNA was less dependent on position. The elongation of DNA molecules by spontaneous droplet motion on chemical gradient surfaces has possible applications in DNA barcoding and studies of DNA-protein interactions.

Surface-Anchored Poly(N-isopropylacrylamide) Orthogonal Gradient Networks

We present a versatile synthetic route leading toward generating surface-attached polyacrylamide gels, in which the cross-link density varies continuously and gradually across the substrate in two orthogonal directions. We employ free radical polymerization to synthesize random copolymers comprising ~5% of photoactive methacrylyloxybenzophenone (MABP), ~5% of thermally active styrene sulfonyl azide (SSAz), and ~90% of N-isopropylacrylamide (NIPAAm) units. The presence of MABP and SSAz in the copolymer facilitates control over the cross-link density of the gel in an orthogonal manner using photoactivated and thermally activated cross-linking chemistries, respectively. Spectroscopic ellipsometry is employed to determine the degree of swelling of the gel in water and methanol as a function of position on the substrate. Network swelling varies continuously and gradually across the substrate and is high in regions of low gel fractions and low in regions of high gel fractions.

Roughness gradients on zirconia for rapid screening of cell-surface interactions: Fabrication, characterization and application

Roughness is one of the key parameters for successful osseointegration of dental implants. The understanding of how roughness affects cell response is thus crucial to improve implant performance. Surface gradients, which allow rapid and systematic investigations of cell-surface interactions, have the potential to facilitate this task. In this study, a novel method aiming to produce roughness gradients at the surface of zirconia using hydrofluoric acid etching was implemented. The topography was exhaustively characterized at the microscale and nanoscale by white light interferometry and atomic force microscopy, including the analysis of amplitude, spatial, hybrid, functional, and fractal parameters. A rapid screening of the influence of roughness on human mesenchymal stem cell morphology was conducted and potential correlations between roughness parameters and cell morphology were investigated. The roughness gradient induced significant changes in cell area (p < 0.001), aspect ratio (p = 0.01), and solidity (p = 0.026). Nanoroughness parameters were linearly correlated to cell solidity (p < 0.005), while microroughness parameters appeared nonlinearly correlated to cell area, highlighting the importance of multiscale optimization of implant topography to induce the desired cell response. The gradient method proposed here drastically reduces the efforts and resources necessary to study cell-surface interactions and provides results directly transferable to industry. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 2502-2514, 2016.