Intrinsic Surface‐Drying Properties of Bioadhesive Proteins

Sessile marine mussels must “dry” underwater surfaces before adhering to them. Synthetic adhesives have yet to overcome this fundamental challenge. Previous studies of bioinspired adhesion have largely been performed under applied compressive forces, but such studies are poor predictors of the ability of an adhesive to spontaneously penetrate surface hydration layers. In a force‐free approach to measuring molecular‐level interaction through surface‐water diffusivity, different mussel foot proteins were found to have different abilities to evict hydration layers from surfaces—a necessary step for adsorption and adhesion. It was anticipated that DOPA would mediate dehydration owing to its efficacy in bioinspired wet adhesion. Instead, hydrophobic side chains were found to be a critical component for protein–surface intimacy. This direct measurement of interfacial water dynamics during force‐free adsorptive interactions at solid surfaces offers guidance for the engineering of wet adhesives and coatings.     

Atmospheric plasma treatment of porous polymer constructs for tissue engineering applications

Porous 3D polymer scaffolds prepared by TIPS from PLGA (53:47) and PS are intrinsically hydrophobic which prohibits the wetting of such porous media by water. This limits the application of these materials for the fabrication of scaffolds as supports for cell adhesion/spreading. Here we demonstrate that the interior surfaces of polymer scaffolds can be effectively modified using atmospheric air plasma (AP). Polymer films (2D) were also modified as control. The surface properties of wet 2D and 3D scaffolds were characterised using zeta-potential and wettability measurements. These techniques were used as the primary screening methods to assess surface chemistry and the wettability of wet polymer constructs prior and after the surface treatment. The surfaces of the original polymers are rather hydrophobic as highlighted but contain acidic functional groups. Increased exposure to AP improved the water wetting of the treated surfaces because of the formation of a variety of oxygen and nitrogen containing functions. The morphology and pore structure was assessed using SEM and a liquid displacement test. The PLGA and PS foam samples have central regions which are open porous interconnected networks with maximum pore diameters of 49 microm for PLGA and 73 microm for PS foams.     

Adhesion of Sol-Gel-Derived Organic-Inorganic Hybrid Coatings on Polyester

Inorganic coatings, including metal-oxide coatings, provide polymer surfaces with excellent abrasion and wear resistance, and protection against environmental degradation. However, one drawback associated with the incorporation of such ceramic coatings to polymeric materials is the adhesion characteristic at the ceramic-polymer interface. In this paper, two strategies for adhesion enhancement of ceramic coatings on polymer substrates were proposed: (1) formation of chemical bonds through surface condensation reactions, and (2) development of interlocked ceramic and polymeric networks through diffusion of alkoxide precursors. The current research has focused on the adhesion of sol-gel-derived organic-inorganic hybrid coatings on polyester by forming chemical bonds between the polymer substrate and the hybrid coatings, as well as developing interlocked polymeric and inorganic networks at the interface. Contact angle, wettability tests, and chemicalanalysis were done to verify the effectiveness of the adhesion of organic-inorganic hybrid coatings on polyester substrates. In addition, dry and wet thermal cycling tests were done to analyze the adhesion behavior of the hybrid coatings on polyester, followed by microscopy examination. It was found that although both approaches resulted in excellent adhesion of hybrid coatings on polyester, adhesion with interlocked ceramic and polymeric networks was far better than that with chemical bonds in the presence of water at elevated temperatures.     

Surface modification of a liquid‐crystalline polymer for copper metallization

The effects of different surface modifications on the adhesion of copper to a liquid‐crystalline polymer (LCP) were investigated with X‐ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, contact‐angle measurements, and pull tests. High pull‐strength values were achieved when copper was sputter‐deposited onto plasma and reactive‐ion‐etching (RIE)‐pretreated LCP surfaces. The values were comparable to the reference pull strengths obtained with laminated copper on the LCP. The adhesion was relatively insensitive to the employed feed gas in the pretreatments. The surface characterizations revealed that for RIE and plasma treatments, the enhanced adhesion was attributable to the synergistic effects of the increased surface roughness and polar component of the surface free energy of the polymer. However, if the electroless copper deposition was performed on RIE‐ or plasma‐treated surfaces, very poor adhesion was measured. Good adhesion between the LCP substrate and electrolessly deposited copper was achieved only in the case of wet‐chemical surface roughening as a result of the creation of a sufficient number of mechanical interlocking sites, together with a significant loss of oxygen functionalities, on the surface. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 623–636, 2003     

Understanding and tuning of polymer surfaces for dialysis applications

Tailoring membrane properties for biomedical applications, e.g., hemodialysis, have been a challenge which material scientists have been addressing for last few decades. The fundamental challenge lies in identifying and controlling the parameters which are responsible for yielding cytocompatibility and hemocompatibility to the material. The present article is an attempt to understand the physical parameters which are responsible for the biological manifestations of a polymer membrane. Two types of dialysis membranes, viz., high performance membrane and high cutoff, have been synthesized. Membrane surfaces were modified via dry and wet annealing, and conditions of annealing were optimized. Subsequently, physical and surface properties of the membranes after annealing were investigated. In‐depth investigation of biological and blood response has been undertaken on the basis of fundamental parameters like polarizability and surface rigidity. Cell adhesion, proliferation, protein adsorption, hemolysis, platelet adhesion, thrombus formation, and complement activation tests were performed on the membranes. It was observed that dry heating increases surface smoothness but in the process develops cracks on membrane surface as well as increases work of adhesion for blood contact. On the other hand, wet heating of membrane surface not only improves biological performance but it is also easy to retrofit with existing spinning technologies for spinning dialysis membranes. In‐house spinning technology was used to synthesize hemodialysis membranes which were annealed at the optimized conditions, and their surfaces were compared with commercial fibers to ascertain the rationale of annealing as a facile method to lend desired surface properties to membranes. Copyright © 2016 John Wiley & Sons, Ltd.     

Bacterial adhesion on hybrid cationic nanoparticle-polymer brush surfaces: ionic strength tunes capture from monovalent to multivalent binding

This paper describes the creation of hybrid surfaces containing cationic nanoparticles and biocompatible PEG (polyethylene glycol) brushes that manipulate bacterial adhesion for potential diagnostic and implant applications. Here, ～10 nm cationically functionalized gold nanoparticles are immobilized randomly on negative silica surfaces at tightly controlled surface loadings, and the remaining areas are functionalized with a hydrated PEG brush, using a graft copolymer of poly-l-lysine and PEG (PLL-PEG), containing 2000 molecular weight PEG chains and roughly 30% functionalization of the PLL. The cationic nanoparticles attract the negative surfaces of suspended Staphylococcus aureus bacteria while the PEG brush exerts a steric repulsion. With the nanoparticle and PEG brush heights on the same lengthscale, variations in ionic strength are demonstrated to profoundly influence the capture of S. aureus on these surfaces. For bacteria captured from gentle flow, a crossover from multivalent to univalent binding is demonstrated as the Debye length is increased from 1 to 4 nm. In the univalent regime, 1 um diameter spherical bacteria are captured and held by single nanoparticles. In the multivalent regime, there is an adhesion threshold in the surface density of nanoparticles needed for bacterial capture. The paper also documents an interesting effect concerning the relaxations in the PLL-PEG brush itself. For brushy surfaces containing no nanoparticles, bacterial adhesion persists on newly formed brushes, but is nearly eliminated after these brushes relax, at constant mass in buffer for 12h. Thus brushy relaxations increase biocompatibility.     

Standard Practice for Bulk Sample Collection and Swab Sample Collection of Visible Powders Suspected of Being Biological Agents from Nonporous Surfaces: collaborative study

The draft ASTM Standard, "Standard Practice for Bulk Sample Collection and Swab Sample Collection of Visible Powders Suspected of Being Biological Agents from Nonporous Surfaces," was validated in a collaborative study consisting of 6 teams comprised of Civil Support personnel and First Responders, 2 levels of Bacillus anthracis Sterne and Bacillus thuringiensis Kurstaki spores, and 7 nonporous surfaces. The sample collection standard includes collection of the bulk sample (Method A) using a dry swab to push the sample onto a collection card and collection of residual sample (Method B) using an onsite test kit followed by a wet swab intended for additional onsite testing. Method A is to be performed prior to Method B in order to preserve unadulterated sample as potential criminal evidence. While statistical differences were observed between surfaces, between teams, and the interaction of surfaces and teams for the various sample types collected, these differences are due to the very low variability of the data and a much more narrow distribution than an ideal normal distribution, rather than to any practical differences. The data demonstrate that from both the 1.0 and 0.01 g powder samples, high levels of spores (mean >10(6) CFU) are recovered from the 7 surfaces by both the dry swab used in bulk sample collection (Method A) and the wet swab (Method B) sampling of the residual powder after bulk sample collection. Thus, after bulk sample collection, there is a high level of residual spores remaining for onsite biological testing and both Methods A and B should be performed in the field.     

聚(甲基丙烯酸-2-羟乙酯)改性金表面用于构建生物检测基材

采用电子活化再生原子转移自由基聚合(AGET ATRP)的方法将聚(甲基丙烯酸-2-羟乙酯)(PHEMA)接枝在金表面,对经修饰的金表面的生物惰性做了系统的研究,并利用PHEMA的羟基末端固定生物素(biotin)分子,以biotin对抗生物素蛋白(avidin)的识别为模型,研究了不同厚度的PHEMA对结合avidin的影响,以及该表面作为生物检测基材的可行性.生物惰性研究表明,PHEMA修饰的金表面不但能够有效的排斥纤维蛋白原(Fg)、人血清白蛋白(HSA)和溶菌酶(Lys)的非特异性吸附,还能够抑制3种细胞(L02、L929和EC)的黏附,是一种良好的抗污表面.通过控制聚合时间制备了不同厚度的PHEMA-biotin修饰的表面,同位素125I标记HSA吸附结果表明这几种表面均能够有效排斥非特异性蛋白质吸附,特异性FITC-avidin吸附结果表明,厚度较小时(16 nm)由于荧光淬灭而难以检测到荧光信号,厚度在16 nm和49 nm之间,荧光信号随厚度增加而增强,通过比较信噪比,认为厚度在49 nm以上时比较理想.该表面在应用于QCM与荧光检测中均表现出良好的检测性能.     

Different functionalization of the internal and external surfaces in mesoporous materials for biosensing applications using "click" chemistry

We report the use of copper(I)-catalyzed alkyne-azide cycloaddition reaction (CuAAC) to selectively functionalize the internal and external surfaces of mesoporous materials. Porous silicon rugate filters with narrow line width reflectivity peaks were employed to demonstrate this selective surface functionalization approach. Hydrosilylation of a dialkyne species, 1,8-nonadiyne, was performed to stabilize the freshly fabricated porous silicon rugate filters against oxidation and to allow for further chemical derivatization via "click" CuAAC reactions. The external surface was modified through CuAAC reactions performed in the absence of nitrogen-based Cu(I)-stabilizing species (i.e., ligand-free reactions). To subsequently modify the interior pore surface, stabilization of the Cu(I) catalyst was required. Optical reflectivity measurements, water contact angle measurements, Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to demonstrate the ability of the derivatization approach to selectively modify mesoporous materials with different surface chemistry on the exterior and interior surfaces. Furthermore, porous silicon rugate filters modified externally with the cell-adhesive peptide Gly-Arg-Gly-Asp-Ser (GRGDS) allowed for cell adhesion via formation of focal adhesion points. Results presented here demonstrate a general approach to selectively modify mesoporous silicon samples with potential applications for cell-based biosensing.     

A new Phosphorylated Polysaccharide for Biomedical Applications: Generation of 3D Scaffolds with Osteogenic Activity and Coating of Titanium Oxide Surfaces

Summary: A new phosphorylated derivative of carboxymethylcellulose and amidic carboxymethylcellulose containing one phosphate group for each disaccharide unit was synthesized using sodium trimetaphospahte (STMP) as the phosphating agent. The new polysaccharide was characterized by infrared spectroscopy (FT-IR) and the amount of phosphate groups was determined by elemental analysis. These modified polysaccharides were used both to prepare 3D scaffolds and functionalize titanium oxide surfaces with the aim to improve the osseointegration with the host tissue. The presence of phosphate groups modify the physical-chemical properties of the hydrogels with respect to the native ones. The evaluation of the bioactivity of the phosphorylated carboxymethylcellulose hydrogels towards osteoblast-like cells showed a significant increase in the osteocalcin production. The modified surfaces were chemically characterized by means of X-ray photoelectron spectroscopy (XPS) and FT-IR, whereas the surface topography was analysed by Atomic Force Measurements (AFM) measurements before and after the polysaccharide coating. In vitro biological tests using osteoblast-like cells demonstrated that phosphorylated carboxymethylcellulose functionalized TiO₂ surfaces promoted better cell adhesion and significantly enhanced their proliferation. These findings suggest that the phosphate polysaccharide both as a 3D scaffold and as a surface coating promotes osteoblast growth potentially improving the biomaterial osseointegration rate.     

Chemical Surface Modifications of Titanium Implants

Summary: In the present work, various surface modifications have been applied to titanium surface. The aim of the modifications was to improve cell adhesion and to determine their influence on the properties of titanium surface. The unmodified and modified surfaces were observed using SEM. Subtle changes in modified surface layer of titanium samples were examined using the Auger Electron Microanalysis and Photoelectron Spectroscopy. The properties of surfaces were evaluated by contact angle and roughness measurements. The results revealed large differences in morphology of Ti modified with different procedures whereas only minor differences in the chemistry of the surfaces were detected. Preliminary quantitative measurements (cell number, viability and differentiation) of the MG 63 osteoblast-like cells in the direct contact with the surface of the investigated materials show that both the not pre-treated titanium surface and the surfaces modified by the methods used in the this work are all well tolerated by the living cells. Within the experimental scatter all the surfaces provided good substrate for proliferation and growth of the cells.     

Specific and nonspecific interaction forces between Escherichia coli and silicon nitride, determined by poisson statistical analysis

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.     

Evaluation of the adhesion properties of inorganic materials with high surface energies

With the aim of checking the validity of methods for characterizing the adhesion between inorganic materials with high surface energies, the properties of the adhesion between an inorganic material (indium tin oxide (ITO)) and model surfaces with various surface energies (Cl-, NH2-, CH(3)-, and CF3-functionalized surfaces) were evaluated using atomic force microscopy (AFM) and the Johnson-Kendall-Roberts (JKR) apparatus. For this purpose, the AFM tip and the JKR lens were modified with ITO using radio frequency (rf) magnetron sputtering. The work of adhesion between the ITO coating and each model surface was estimated using AFM and the JKR apparatus and compared with the result obtained from contact angle measurements. The adhesion forces determined from the force-displacement curves (AFM) were found to agree with the predictions of the Derjaguin-Muller-Toporov (DMT) theory. The JKR equation used in the interpretation of the JKR experiments was modified by taking into account the differences between the surface and bulk moduli of the ITO-coated poly(dimethylsiloxane) (PDMS) lens. The ratio of the surface modulus to the bulk modulus we used in this modified JKR equation was obtained by determining the slope of the attracting part of the force-displacement curve. The values of the work of adhesion calculated using the modified JKR equation were also found to agree with the values obtained from contact angle measurements. We conclude that the two methods using AFM and the JKR apparatus can be used in the evaluation of the work of adhesion between inorganic materials with high surface energies such as metal and metal oxide surfaces.     

Colloid probe AFM investigation of interactions between fibrinogen and PEG-like plasma polymer surfaces

Interaction forces between surfaces designed to be protein resistant and fibrinogen (Fg) were investigated in phosphate-buffered saline with colloid probe atomic force microscopy. The surfaces of the silica probes were coated with a layer of fibrinogen molecules by adsorption from the buffer. The technique of low-power, pulsed AC plasma polymerization was used to make poly(ethylene glycol) (PEG)-like coatings on poly(ethylene teraphthalate) by using diethylene glycol vinyl ether as the monomer gas. The degree of PEG-like nature of the films was controlled by use of a different effective plasma power in the chamber for each coating, ranging from 0.6 to 3.6 W. This produced a series of thin films with a different number of ether carbons, as assessed by X-ray photoelectron spectroscopy. The interaction force measurements are discussed in relation to trends observed in the reduction of fibrinogen adsorption, as determined quantitatively by (125)I radio-labeling. The plasma polymer coatings with the greatest protein-repelling properties were the most PEG-like in nature and showed the strongest repulsion in interaction force measurements with the fibrinogen-coated probe. Once forced into contact, all the surfaces showed increased adhesion with the protein layer on the probe, and the strength and extension length of adhesion was dependent on both the applied load and the plasma polymer surface chemistry. When the medium was changed from buffer to water, the adhesion after contact was eliminated and only appeared at much higher loads. This indicates that the structure of the fibrinogen molecules on the probe is changed from an extended conformation in buffer to a flat conformation in water, with the former state allowing for stronger interaction with the polymer chains on the surface. These experiments underline the utility of aqueous surface force measurements toward understanding protein-surface interactions, and developing nonfouling surfaces that confer a steric barrier against protein adsorption.     

Forces between Polymer Surfaces and Self-Assembled Monolayers

Gold-coated atomic force microscope (AFM) tips functionalized with amine-, hydroxyl-, carboxylic acid-, and methyl-terminated alkanethiol molecules were used to probe the adhesive forces of polystyrene and poly(acrylic acid) films in dry air (relative humidity < 0.5%). X-ray photoelectron spectroscopy (XPS) and contact angle measurements confirmed the quality and uniformity of similarly treated gold surfaces and the polymer films. XPS indicated that the amine-functionalized thiol films were protonated and comprised of multilayers. Contact angle data were used to calculate surface free energies, and DMT theory yielded the works of adhesion and interfacial free energies for the tip-substrate combinations. In the case of polystyrene, the work of adhesion followed the order methyl > carboxylic acid > hydroxyl > amine. For poly(acrylic acid), the observed order was hydroxyl > amine > carboxylic acid > methyl.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Evaluating protein attraction and adhesion to biomaterials with the atomic force microscope

Failure of implanted biomaterials is commonly due to nonspecific protein adsorption, which in turn causes adverse reactions such as the formation of fibrous capsules, blood clots, or bacterial biofilm infections. Current research efforts have focused on modifying the biomaterial interface to control protein reactions. Designing biomaterial interfaces at the molecular level, however, requires an experimental technique that provides detailed, dynamic information on the forces involved in protein adhesion. The goal of this study was to develop an atomic force microscope (AFM)-based technique to evaluate protein adhesion on biomaterial surfaces. In this study, the AFM was used to evaluate (i) protein-protein, (ii) protein-substrate, and (iii) protein-dextran interactions. The AFM was first used to measure the pull-off forces between bovine serum albumin (BSA) tips/BSA surfaces and BSA tips/anti-BSA surfaces. Results from these protein-protein studies were consistent with the literature. More importantly, the successful measurement of antibody-antigen binding interactions demonstrates that both the BSA and anti-BSA proteins retain their folded conformation and remain functional following our immobilization protocol. The AFM was also used to quantify the physiochemical interactions of proteins during adhesion to various self-assembled monolayers (SAMs) and dextran-coated substrates representative of potential biomaterial interface modifications. Dextran, which renders surfaces very hydrophilic, was the only surface coating that BSA protein did not adhere to. Hydrophobic interactions were not found to play a significant role in BSA adhesion. Therefore, the dextran molecules may resist protein adhesion by repulsive steric effects or hydration pressure. Moreover, the AFM-based methodology provides dynamic, quantitative information about protein adhesion at the nanoscale level.     

Teflon hierarchical nanopillars with dry and wet adhesive properties

The superior material properties of β‐keratin along with the hierarchical high‐aspect‐ratio structure of geckos' foot pad have enabled geckos to stick readily and rapidly to almost any surfaces in both dry and wet conditions. In this research, nonsticky fluoropolymer (Teflon AF) resembling β‐keratin rigidity and having an extremely low surface energy and dielectric constant was applied to fabricate a novel dry adhesive consisting of high‐aspect‐ratio nanopillars terminated with a “fluffy” top layer. Both the nanopillars and the terminating layer are fabricated concurrently by replica molding using a nanoporous anodic aluminum oxide membrane as the mold. These Teflon AF hierarchical nanostructures are shown to have an exceptional capacity to generate strong adhesion in both dry conditions and under water because of combined actions of van der Waals forces, electrostatic attractions, and hydrophobic effects. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Liquid transport in a multi-layer froth

When a gas is sparged into an aqueous solution of surfactant in a column, a froth layer is formed above the layer of liquid. If water is injected into the rising column of froth at a particular position, two distinct layers are formed in the froth: a wet layer below the injection plane, and a dry layer above it. In this study, a phase coexistence analysis was performed on the three-layer system. Predictions from the governing drift-flux equation were compared, using two different representations of the drainage terms, which had one and two adjustable parameters, respectively. The analysis was used to predict liquid fractions and superficial liquid velocities for the wet and dry froth zones. These were compared to experimental measurements performed with a laboratory froth column. Both versions of the analysis were in good agreement with experiment. That with the single-parameter drainage term (derived from the Richardson-Zaki equation) was marginally superior. Theoretical predictions of the liquid fraction and overflow rate from the dry froth layer are supported by the data. Considerable convective motions were observed in the wet froth layer, driven by differences in density between the entering wash liquid and the surrounding froth.     

Choose sides: differential polymer adhesion

AFM-based single molecule desorption measurements were performed on surface end-grafted poly(acrylic acid) monolayers as a function of the pH of the aqueous buffer to study the adhesion properties of polymers that bridge two surfaces. These properties were found to depend on the adhesion forces of both surfaces in a differential manner, which is explained with a simple model in analogy to the Bell-Evans formalism used in dynamic force spectroscopy. The measured interaction forces between the poly(acrylic acid) chains and silicon nitride AFM tips depend on the grafting density of the polymer monolayers as well as on the contour length of the polymer chains. This study demonstrates that the stability of polymer bridges is determined by the adhesion strengths on both surfaces, which can be tuned by using pH-dependent polyelectrolyte monolayers.