Controlled Electrodissolution of Polyelectrolyte Multilayers: A Platform Technology Towards the Surface‐Initiated Delivery of Drugs

A novel method for the electrochemical dissolution of polyelectrolyte multilayers from the surface of an electrode for applications in controlled drug delivery is reported. Biodegradable and biocompatible multilayer films based on poly(L ‐lysine) and heparin have been selected as a model system, and have been built on an indium tin oxide semiconductor substrate. The build‐up and dissolution processes of the multilayers is followed by electrochemical optical waveguide light mode spectroscopy. The formation and stability of the polyelectrolyte multilayers have been found to depend on the applied potential and the ionic strength of the buffer. The application of potentials above a threshold of 1.8 V induces dissolution, which follows single‐exponential kinetics, of the polyelectrolyte multilayer film. The rate of this process can be varied by an on–off profile of the potential, leading to the controlled release of heparin into the bulk. Atomic force microscopy investigations show that the electrodissolution of the polyelectrolyte multilayers is a local phenomenon that leads to the formation of nanoporous films.     

Short‐Time Tuning of the Biological Activity of Functionalized Polyelectrolyte Multilayers

This article demonstrates the tuning of the biological activity of a surface functionalized by a polyelectrolyte multilayer. The interaction of protein A with macrophages is used as the model system. The film consists of two polypeptides, poly(lysine) and poly(glutamic acid); each “build‐up” solution is a mixture of the respective D ‐ and L ‐enantiomers (d and l enantiomers). Cells are deposited on top of the film, and they produce tumor necrosis factor alpha (TNF‐α) as they come into contact with the protein. Depending upon the d/l‐enantiomer ratio of the polyelectrolyte solutions used for the film build‐up, and the embedding depth of the protein, the production of TNF‐α commences after a varying induction time and displays a transition from no‐production to full‐production, which takes place over a period of time that depends on the film's composition and embedding depth. Thus, it is shown that by changing these two parameters the timing of the protein's activity can be accurately tuned.     

Ag‐Functionalized Carbon Molecular‐Sieve Membranes Based on Polyelectrolyte/Polyimide Blend Precursors

We prepared dense flat‐sheet Ag‐functionalized carbon molecular‐sieve (CMS) membranes from blends of P84 co‐polyimide and a sulfonated poly(ether ether ketone) with a Ag⁺ counterion (AgSPEEK). These blends offer the possibility of producing new functionalized precursor structures, which were previously not possible, such as integrally skinned asymmetric hollow fibers. Membranes prepared at a pyrolysis end temperature of 800 °C showed a maximum permeability for all tested gases at a Ag content of approximately 2.5 wt.‐% (He permeability P_He = 465 Barrer (1 Barrer = 7.5 × 10^–18 m² s^–1 Pa^–1), Pequation/tex2gif-inf-2.gif = 366 Barrer, Pequation/tex2gif-inf-4.gif = 91.8 Barrer, Pequation/tex2gif-inf-6.gif = 10.3 Barrer). The maximum achieved selectivity for O₂ over N₂ with CMS membranes based on these blends was αequation/tex2gif-inf-10.gif = 13.5 (Ag content: 4.5 wt.‐%, Pequation/tex2gif-inf-14.gif = 52.7 Barrer). The CO₂ over N₂ selectivity reached a value of 48.9 (Ag content: 4.5 wt.‐%, Pequation/tex2gif-inf-18.gif = 191 Barrer). These observations are explained by the formation of selective bypasses around Ag nanoclusters in the CMS matrix.     

Biofunctional Polyelectrolyte Multilayers and Microcapsules: Control of Non‐Specific and Bio‐Specific Protein Adsorption

Layers of the polyelectrolytes poly(allylamine hydrochloride) (PAH, polycationic) and poly(styrene sulfonate) (PSS, polyanionic) are consecutively adsorbed on flat silicon oxide surfaces, forming stable, ultrathin multilayer films. Subsequently, a final monolayer of the polycationic copolymer poly(L ‐lysine)‐graft‐poly(ethylene glycol) (PLL‐g‐PEG) is adsorbed onto the PSS‐terminated multilayer in order to impart protein resistance to the surface. The growth of each of the polyelectrolyte layers and the protein resistance of the resulting [PAH/PPS]_n(PLL‐g‐PEG) multilayer (n = 1–4) are followed quantitatively ex situ using X‐ray photoelectron spectroscopy and in situ using real‐time optical‐waveguide lightmode spectroscopy. In a second approach, the same type of [PAH/PSS]_n(PLL‐g‐PEG) multilayer coatings are successfully formed on the surface of colloidal particles in order to produce surface‐functionalized, hollow microcapsules after dissolution of the core materials (melamine formaldehyde (MF) and poly(lactic acid) (PLA; colloid diameters: 1.2–20 μm). Microelectrophoresis and confocal laser scanning microscopy are used to study multilayer formation on the colloids and protein resistance of the final capsule. The quality of the PLL‐g‐PEG layer on the microcapsules depends on both the type of core material and the dissolution protocols used. The greatest protein resistance is achieved using PLA cores and coating the polyelectrolyte microcapsules with PLL‐g‐PEG after dissolution of the cores. Protein adsorption from full serum on [PAH/PPS]_n(PLL‐g‐PEG) multilayers (on both flat substrates and microcapsules) decreases by three orders of magnitude in comparison to the standard [PAH/PPS]_n layer. Finally, biofunctional capsules of the type [PAH/PPS]_n(PLL‐g‐PEG/PEG‐biotin) (top copolymer layer with a fraction of the PEG chains end‐functionalized with biotin) are produced which allow for specific recognition and immobilization of controlled amounts of streptavidin at the surface of the capsules. Biofunctional multilayer films and capsules are believed to have a potential for future applications as novel platforms for biotechnological applications such as biosensors and carriers for targeted drug delivery.     

Re‐endothelialization of Human Umbilical Arteries Treated with Polyelectrolyte Multilayers: A Tool for Damaged Vessel Replacement

The use of cryopreserved arteries for vascular tissue engineering provides a promising way for vessel replacement. Unfortunately cryopreservation induces structural changes that strongly modify the mechanical properties and alter the thrombogenicity of the vessel after implantation. We present here a new procedure to treat the inner coating of cryopreserved arteries with poly(sodium‐4‐styrene sulfonate)/poly(allylamine hydrochloride) polyelectrolyte multilayers. We show that this treatment improves the mechanical properties of the cryopreserved vessel. It also allows the adhesion and spreading of endothelial cells so that the internal structure of the vessel closely resembles that of fresh arteries. Finally, we verify by PECAM‐1 and von‐Willebrand‐factor (vWF) expression that this treatment preserves the phenotype of the endothelial cells. This study should open new routes towards the development of future, new biocompatible tissue substitutes allowing long‐term functionality after implantation.     

Antibody‐Functionalized Hybrid Superparamagnetic Nanoparticles

Superparamagnetic hybrid nanoparticles (ca. 80 nm) are obtained. They consist of an inner iron oxide core coated by a silica shell, which is in turn functionalized with amine or carboxyl groups and covalently coupled to a monoclonal antibody (anti‐hCG; hCG = human chorionic gonadotropin). The prepared nanoparticles show a specific magnetic moment (per gram of iron) that is comparable to that measured for commercial superparamagnetic iron oxide preparations. The bioactivity of the antibody‐conjugated magnetic nanoparticles is verified by a standard bioassay. These results indicate the potential of the hybrid nanoparticles prepared for use as enhanced contrast agents in magnetic resonance imaging applications.     

Primary Neuron/Astrocyte Co‐Culture on Polyelectrolyte Multilayer Films: A Template for Studying Astrocyte‐Mediated Oxidative Stress in Neurons

We engineered patterned co‐cultures of primary neurons and astrocytes on polyelectrolyte multilayer (PEM) films without the aid of adhesive proteins/ligands to study the oxidative stress mediated by astrocytes on neuronal cells. A number of studies have explored engineering co‐culture of neurons and astrocytes predominantly using cell lines rather than primary cells owing to the difficulties involved in attaching primary cells onto synthetic surfaces. To our knowledge this is the first demonstration of patterned co‐culture of primary neurons and astrocytes for studying neuronal metabolism. In our study, we used synthetic polymers, namely poly(diallyldimethylammoniumchloride) (PDAC) and sulfonated poly(styrene) (SPS) as the polycation and polyanion, respectively, to build the multilayers. Primary neurons attached and spread preferentially on SPS surfaces, while primary astrocytes attached to both SPS and PDAC surfaces. SPS patterns were formed on PEM surfaces, either by microcontact printing SPS onto PDAC surfaces or vice‐versa, to obtain patterns of primary neurons and patterned co‐cultures of primary neurons and astrocytes. We further used the patterned co‐culture system to study the neuronal response to elevated levels of free fatty acids as compared to the response in separated monoculture by measuring the level of reactive oxygen species (ROS; a widely accepted marker of oxidative stress). The elevation in the ROS levels was observed to occur earlier in the patterned co‐culture system than in the separated monoculture system. The results suggest that this technique may provide a useful tool for engineering neuronal co‐culture systems, that may more accurately capture neuronal function and metabolism, and thus could be used to obtain valuable insights into neuronal cell function and perhaps even the pathogenesis of neurodegenerative diseases.     

Self‐Healing Actuating Adhesive Based on Polyelectrolyte Multilayers

Creating actuators capable of mechanical motion in response to external stimuli is a key for design and preparation of smart materials. The lifetime of such materials is limited by their eventual wear. Here, self‐healable and adhesive actuating materials are demonstrated by taking advantage of the solvent responsive of weak polyelectrolyte multilayers consisting of branched poly(ethylenimine)/poly(acrylic acid) (BPEI/PAA). BPEI/PAA multilayers are dehydrated and contract upon contact with organic solvent and become sticky when wetted with water. By constructing an asymmetric heterostructure consisting of a responsive BPEI/PAA multilayer block and a nonresponsive component through either layer‐by‐layer assembly or the paste‐to‐curl process, smart films that actuate upon exposure to alcohol are realized. The curl degree, defined as degrees from horizontal that the actuated material reaches, can be as high as ≈228.9°. With evaporation of the ethanol, the curled film returns to its initial state, and water triggers fast self‐healing extends the actuator's lifetime. Meanwhile, the adhesive nature of the wet material allows it to be attached to various substrates for possible combination with hydrophobic functional surfaces and/or applications in biological environments. This self‐healable adhesive for controlled fast actuation represents a considerable advance in polyelectrolyte multilayers for design and fabrication of robust smart advanced materials.     

Carbon Nanotube Coatings on Bioglass‐Based Tissue Engineering Scaffolds

The coating of highly porous Bioglass^® based 3D scaffolds with multi‐walled carbon nanotubes (CNT) was investigated. Foam like Bioglass^® scaffolds were fabricated by the replica technique and electrophoretic deposition was used to deposit homogeneous layers of CNT throughout the scaffold pore structure. The optimal experimental conditions were determined to be: applied voltage 15 V and deposition time 20 minutes, utilizing a concentrated aqueous suspension of CNT with addition of a surfactant and iodine. The scaffold pore structure remained invariant after the CNT coating, as assessed by SEM. The incorporation of CNTs induced a nanostructured internal surface of the pores which is thought to be beneficial for osteoblast cell attachment and proliferation. Bioactivity of the scaffolds was assessed by immersion studies in simulated body fluid (SBF) for periods of up to 2 weeks and the subsequent determination of hydroxyapatite (HA) formation. The presence of CNTs can enhance the bioactive behaviour of the scaffolds since CNTs can serve as template for the ordered formation of a nanostructured HA layers, which does not occur on uncoated Bioglass^® surfaces.     

Poly(ϵ‐caprolactone)‐Functionalized Carbon Nanotubes and Their Biodegradation Properties

Biodegradable poly(?‐caprolactone) (PCL) has been covalently grafted onto the surfaces of multiwalled carbon nanotubes (MWNTs) by the “grafting from” approach based on in‐situ ring‐opening polymerization of ?‐caprolactone. The grafted PCL content can be controlled easily by adjusting the feed ratio of monomer to MWNT‐supported macroinitiators (MWNT‐OH). The resulting products have been characterized with Fourier‐transform IR (FTIR), NMR, and Raman spectroscopies, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). After PCL was coated onto MWNT surfaces, core/shell structures with nanotubes as the “hard” core and the hairy polymer layer as the “soft” shell are formed, especially for MWNTs coated with a high density of polymer chains. Such a polymer shell promises good solubility/dispersibility of the MWNT–PCL nanohybrids in low‐boiling‐point organic solvents such as chloroform and tetrahydrofuran. Biodegradation experiments have shown that the PCL grafted onto MWNTs can be completely enzymatically degraded within 4 days in a phosphate buffer solution in the presence of pseudomonas (PS) lipase, and the carbon nanotubes retain their tubelike morphologies, as observed by SEM and TEM. The results present possible applications for these biocompatible PCL‐functionalized CNTs in bionanomaterials, biomedicine, and artificial bones.     

Porous Nanostructured Optical Filters Rendered Insensitive to Humidity by Vapor‐Phase Functionalization

Water adsorption in many porous optical coatings can cause a detrimental red‐shift in the optical properties. In this study, a porous nanostructured rugate filter is fabricated using the glancing‐angle deposition (GLAD) technique, and rendered insensitive to large changes in ambient humidity by post‐deposition vapor‐phase functionalization. A central defect mode is added to the stop band of the filter by integrating a phase shift into the sinusoidal refractive‐index profile of the film. By monitoring the wavelength of the defect mode under variable humidity conditions, a six‐times reduction in sensitivity to water vapor is observed upon functionalization with 3,3,3‐trifluoropropyltrichlorosilane. Electrical characterization and advancing aqueous contact‐angle measurements are used to verify the hydrophobic properties of the functionalized thin films.     

Tailor‐Made Polymer Multilayers

The fabrication of multilayer assemblies from polymeric compounds is an important tool for precise control of film architecture on the nanoscale. In this report, a general, novel approach for the preparation of well‐defined polymeric multilayers is described. To achieve this, sulfonyl‐azide group containing polymers are first generated and deposited as thin films onto solid (organic) substrates. Upon thermal activation, the system crosslinks and binds to the substrate via C–H bond insertion. Through step‐and‐repeat procedures, multilayer assemblies are then generated where all the individual layers are linked to each other. As the assembly process does not require any specific molecular interactions, the described process represents a general strategy to generate tailor‐made multilayer surface coatings with wide range of film thickness and composition.     

Percolation‐Controlled Metal/Polyelectrolyte Complexed Films for All‐Solution‐Processable Electrical Conductors

The use of solution‐processable electrically conducting films is imperative for realizing next‐generation flexible and wearable devices in a large‐scale and economically viable way. However, the conventional approach of simply complexing metallic nanoparticles with a polymeric medium leads to a tradeoff between electrical conductivity and material properties. To address this issue, in this study, a novel strategy is presented for fabricating all‐solution‐processable conducting films by means of metal/polyelectrolyte complexation to achieve controlled electrical percolation; this simultaneously imparts superior electrical conductivity and good mechanical properties. A polymeric matrix comprised of polyelectrolyte multilayers is first formed using layer‐by‐layer assembly, and then Ag nanoparticles are gradually synthesized and gradationally distributed inside the polymeric matrix by means of a subsequent procedure of repeated cationic exchange and reduction. During this process, electrical percolation between Ag nanoparticles and networking of electrical pathways is facilitated in the surface region of the complexed film, providing outstanding electrical conductivity only one order of magnitude less than that of metallic Ag. At the same time, the polymer‐rich underlying region imparts strong, yet compliant, binding characteristics to the upper Ag‐containing conducting region while allowing highly flexible mechanical deformations of bending and folding, which consequently makes the system outperform existing materials.     

Sulfur‐Functionalized Mesoporous Carbon

A simple synthesis route to mesoporous carbons that contain heteroaromatic functionality is described. The sulfur‐functionalized mesoporous carbon (S‐FMC) materials that have been prepared show excellent thermal stability, as well as excellent hydrothermal stability, and stability over a wide range of pH values. These materials also show excellent mercury sorption performance over a broad range of pH, much broader than is possible with thiol‐based functionality or most silica‐based sorbents. The superior performance of these mesoporous heterocarbons as heavy‐metal sorbent material is demonstrated. These materials are shown to be stable at elevated temperatures and extreme pHs, making them ideally suited as a new class of absorbent material.     

Multivalent‐Ion‐Mediated Stabilization of Hydrogen‐Bonded Multilayers

Hydrogen‐bonding interactions are an important alternative to electrostatic interactions for assembling multilayer thin films of uncharged components. Herein, a new method is reported for rendering such films stable at pH values close to physiological conditions. Multilayer films based on hydrogen bonding are assembled by the alternate deposition of poly[(styrene sulfonic acid)‐co‐(maleic acid)] (PSSMA) and poly(N‐isopropylacrylamide) (PNiPAAm) at pH 2.5. The use of PSSMA results in multilayers that contain free styrene sulfonate groups, as these moieties do not interact with the PNiPAAm functional groups. Subsequent infiltration of a multivalent ion (Ce⁴⁺ or Fe³⁺) leads to an increase in the total film mass, with little impact on the film morphology, as determined by using atomic force microscopy. To examine the film stability, the resulting films have been exposed to elevated pH (7.1). While there is substantial swelling of the multilayers (25 % and 55 % for Ce⁴⁺‐ and Fe³⁺‐stabilized films, respectively), film loss is negligible. This provides a stark contrast with non‐stabilized films, which disassemble almost immediately upon exposure to pH 7.1. This method represents a simple and effective strategy for stabilizing hydrogen‐bonded structures non‐covalently. Further, the multivalent ions also render the films responsive to changes in the local redox environment, as demonstrated by film disassembly after exposure of Fe³⁺‐treated films to iodide solutions.     

Probing Individual Layers in Functional Oxide Multilayers by Wavelength‐Dependent Raman Scattering

The integration of functional oxides on silicon requires the use of complex heterostructures involving oxides of which the structure and properties strongly depend on the strain state and strain‐mediated interface coupling. The experimental observation of strain‐related effects of the individual components remains challenging. Here, a Raman scattering investigation of complex multilayer BaTiO₃/LaNiO₃/CeO₂/YSZ thin‐film structures on silicon is reported. It is shown that the Raman signature of the multilayers differs significantly for three different laser wavelengths (633, 442, and 325 nm). The results demonstrate that Raman scattering at various wavelengths allows both the identification of the individual layers of functional oxide multilayers and monitoring of their strain state. It is shown that all of the layers are strained with respect to the bulk reference samples, and that strain induces a new crystal structure in the embedded LaNiO₃. Based on this, it is demonstrated that Raman scattering at various wavelengths offers a well‐adapted, non‐destructive probe for the investigation of strain and structure changes, even in complex thin‐film heterostructures.     

Spin‐Structured Multilayers: A New Class of Materials for Precision Spintronics

Magnetoelectronic multilayer devices are widely used in today's information and sensor technology. Their functionality, however, is limited by the inherent properties of magnetic exchange or dipolar coupling which constrain possible spin configurations to collinear or perpendicular alignments of adjacent layers. Here, a deposition procedure is introduced that allows for a new class of layered materials in which complex spin structures can be accurately designed to result in a multitude of new and precisely adjustable spintronic and magnetoresistive properties. The magnetization direction and coercivity of each individual layer are determined by the deposition process in oblique incidence geometry and can be completely decoupled from neighboring layers. This applies for layers of any ferromagnetic material down to layer thicknesses of a few nm and lateral dimensions of a few 100 nm, enabling the design of efficient and compact magnetoelectronic devices, encompassing precision magnetoresistive sensors as well as layer systems with multiple addressable remanent states for magnetic memory applications.     

Dual‐Function Scattering Layer of Submicrometer‐Sized Mesoporous TiO2 Beads for High‐Efficiency Dye‐Sensitized Solar Cells

Submicrometer‐sized (830 ± 40 nm) mesoporous TiO₂ beads are used to form a scattering layer on top of a transparent, 6‐µm‐thick, nanocrystalline TiO₂ film. According to the Mie theory, the large beads scatter light in the region of 600–800 nm. In addition, the mesoporous structure offers a high surface area, 89.1 m² g^?1, which allows high dye loading. The dual functions of light scattering and electrode participation make the mesoporous TiO₂ beads superior candidates for the scattering layer in dye‐sensitized solar cells. A high efficiency of 8.84% was achieved with the mesoporous beads as a scattering layer, compared with an efficiency of 7.87% for the electrode with the scattering layer of 400‐nm TiO₂ of similar thickness.     

High‐Performance Polyaniline Prepared via Polymerization in a Self‐Stabilized Dispersion

We report a new synthetic method for the preparation of high‐performance polyaniline, implementing a new concept of self‐stabilized dispersion polymerization. In contrast with conventional homogeneous or dispersion polymerization methods that use an aqueous medium containing aniline, acid, and oxidant, our new polymerization is performed in a heterogeneous biphasic system of organic and aqueous medium without any stabilizers. Here, the monomers and growing polymer chains act as stabilizers, resulting in excellent dispersion of the organic phase inside the aqueous reaction medium. Polymerization based on this concept has produced high‐quality samples with few structural defects; the doped polyanilines exhibit conductivities (σ_DC ～ 600–800 S cm^–1) almost three times that of the samples prepared using conventional methods. Moreover, the new polyaniline system shows a honeycomb‐like morphology with great porosity, leading to a system with excellent processibility.     

Effects of Crystallinity in Spin‐On Pure‐Silica‐Zeolite MFI Low‐Dielectric‐Constant Films

Low‐dielectric‐constant (low‐κ) materials are a critical requirement for future generations of computer microprocessors. As a unique class of porous silicas, pure silica zeolites (PSZs) have been shown to be a promising low‐κ material with excellent mechanical strength (e.g., elastic modulus of 16–18 GPa) due to their crystalline nature. In the present study, we show for the first time that higher crystallinity of spin‐on PSZ MFI films leads to lower κ values and less moisture sensitivity—two critical properties of a porous low‐κ material. We have also advanced the two‐stage synthesis method to produce zeolite nanoparticles with high yield (77 %) and a small diameter (< 80 nm). A κ value of 1.6 is obtained from the silylated highly crystalline PSZ MFI film and the κ value only increases by 12.5 % after exposure to ambient conditions for a period of 24 h.