Tracking Sulfonated Polystyrene Diffusion in a Chitosan/Carboxymethyl Cellulose Layer-by-Layer Film: Exploring the Internal Architecture of Nanocoatings.

Since chitosan presents the ability to interact with a wide range of molecules, it has been one of the most popular natural polymers for the construction of layer-by-layer thin films. In this study, depth-profiling X-ray photoelectron spectroscopy (XPS) was employed to track the diffusion of sulfonated polystyrene (SPS) in carboxymethyl cellulose/chitosan (CMC/Chi) multilayers. Our findings suggest that the CMC/Chi film does not constitute an electrostatic barrier sufficient to block diffusion of SPS, and that diffusion can be controlled by adjusting the diffusion time and the molecular weight of the polymers that compose the CMC/Chi system. In addition to monitoring the diffusion, it was also possible to observe a process of preferential interaction between Chi and SPS. Thus, the nitrogen N 1s peak, due to functional groups found exclusively in chitosan chains, was the key factor to identifying the molecular interactions involving chitosan and the different polyanions. Accordingly, the presence of a strong polyanion such as SPS shifts the N 1s peak to a higher level of binding energy. Such results highlight that understanding the fundamentals of polymer interactions is a major step to fine-tuning the internal architecture of LbL structures for specific applications (e.g., drug release).

Investigation of the Internal Chemical Composition of Chitosan-Based LbL Films by Depth-Profiling X-ray Photoelectron Spectroscopy (XPS) Analysis.

Chitosan-based thin films were assembled using the layer-by-layer technique, and the axial composition was accessed using X-ray photoelectron spectroscopy with depth profiling. Chitosan (CHI) samples possessing different degrees of acetylation ([Formula: see text]) and molecular weight ([Formula: see text]) produced via the ultrasound-assisted deacetylation reaction were used in this study along with two different polyanions, namely, sodium polystyrenesulfonate (PSS) and carboxymethylcellulose (CMC). When chitosan, a positively charged polymer in aqueous acid medium, was combined with a strong polyanion (PSS), the total positive charge of chitosan, directly related to its [Formula: see text], was the key factor affecting the film formation. However, for CMC/CHI films, the pH of the medium and [Formula: see text] of chitosan strongly affected the film structure and composition. Consequently, the structure and the axial composition of chitosan-based films can be finely adjusted by choosing the polyanion and defining the chitosan to be used according to its DA and [Formula: see text] for the desired application, as demonstrated by the antibacterial tests.

Depth-profiling X-ray photoelectron spectroscopy (XPS) analysis of interlayer diffusion in polyelectrolyte multilayers.

Functional organic thin films often demand precise control over the nanometer-level structure. Interlayer diffusion of materials may destroy this precise structure; therefore, a better understanding of when interlayer diffusion occurs and how to control it is needed. X-ray photoelectron spectroscopy paired with C60(+) cluster ion sputtering enables high-resolution analysis of the atomic composition and chemical state of organic thin films with depth. Using this technique, we explore issues common to the polyelectrolyte multilayer field, such as the competition between hydrogen bonding and electrostatic interactions in multilayers, blocking interlayer diffusion of polymers, the exchange of film components with a surrounding solution, and the extent and kinetics of interlayer diffusion. The diffusion coefficient of chitosan (M = ∼100 kDa) in swollen hydrogen-bonded poly(ethylene oxide)/poly(acrylic acid) multilayer films was examined and determined to be 1.4*10(-12) cm(2)/s. Using the high-resolution data, we show that upon chitosan diffusion into the hydrogen-bonded region, poly(ethylene oxide) is displaced from the film. Under the conditions tested, a single layer of poly(allylamine hydrochloride) completely stops chitosan diffusion. We expect our results to enhance the understanding of how to control polyelectrolyte multilayer structure, what chemical compositional changes occur with diffusion, and under what conditions polymers in the film exchange with the solution.

Optimization of amine-rich multilayer thin films for the capture and quantification of prostate-specific antigen.

It is demonstrated that poly(allylamine hydrochloride)/poly(styrenesulfonate) (PAH/SPS) multilayer films can be successfully tailored for the capture and detection of small biomolecules in dilute concentrations. Based on in vitro results, these films could be potentially applied for rapid and high-throughput diagnosis of dilute biomarkers in serum or tissue. PAH presents functional amino groups that can be further reacted with desired chemistries in order to create customizable and specific surfaces for biomolecule capture. A variety of film assembly characteristics were tested (pH, molecular weight of PAH, and ionic strength) to tune the biotinylation and swelling behavior of these films to maximize detection capabilities. The resultant optimized biotinylated PAH/SPS 9.3/9.3 system was utilized in conjunction with quantum dots (Qdots) to capture and detect a dilute biomarker for prostate cancer, prostate-specific antigen (PSA). Compared to previous work, our system presents a good sensitivity for PSA detection within the clinically relevant range of 0.4-100 ng/mL.

Tuning the properties of mucin via layer-by-layer assembly.

Multilayer films consisting of bovine submaxillary mucin (BSM) and poly(allylamine hydrochloride) (PAH) were prepared on various substrates using layer-by-layer assembly. The effects of both the assembly pH and ionic strength on multilayer characteristics were investigated by assessing film thicknesses (10-80 nm), surface wetting characteristics, and cell repulsion. Also, the dynamic assembly behavior was monitored using quartz crystal microbalance with dissipation monitoring (QCM-D) to further understand the effect of assembly pH on film characteristics. Assembly studies revealed that substantial amounts of BSM adhere to the outermost surface only at low pH conditions. The resulting multilayer films assembled at low pH conditions were found to exhibit hydrophilic and cell repellent behavior. In addition, it was found that batch-to-batch variations of the biopolymer BSM could dramatically alter properties.

Sugar-mediated disassembly of mucin/lectin multilayers and their use as pH-Tolerant, on-demand sacrificial layers.

The layer-by-layer (LbL) assembly of thin films on surfaces has proven to be an extremely useful technology for uses ranging from optics to biomedical applications. Releasing these films from the substrate to generate so-called free-standing multilayer films opens a new set of applications. Current approaches to generating such materials are limited because they can be cytotoxic, difficult to scale up, or have undesirable side reactions on the material. In this work, a new sacrificial thin film system capable of chemically triggered dissolution at physiological pH of 7.4 is described. The film was created through LbL assembly of bovine submaxillary mucin (BSM) and the lectin jacalin (JAC) for a (BSM/JAC) multilayer system, which remains stable over a wide pH range (pH 3-9) and at high ionic strength (up to 5 M NaCl). This stability allows for subsequent LbL assembly of additional films in a variety of conditions, which could be released from the substrate by incubation in the presence of a competitive inhibitor sugar, melibiose, which selectively disassembles the (BSM/JAC) section of the film. This novel multilayer system was then applied to generate free-standing, 7 µm diameter, circular ultrathin films, which can be attached to a cell surface as a "backpack". A critical thickness of about 100 nm for the (BSM/JAC) film was required to release the backpacks from the glass substrate, after incubation in melibiose solution at 37 °C for 1 h. Upon their release, backpacks were subsequently attached to murine monocytes without cytotoxicity, thereby demonstrating the compatibility of this mucin-based release system with living cells.

Templated nanopores for robust functional surface porosity in poly(methyl methacrylate).

A novel "sink and etch" technique is used to generate stable surface nanoporosity in poly(methyl methacrylate). Layer-by-layer assembly is first used to conformally coat PMMA substrates with a uniform layer of silica nanoparticles. Thermal annealing is then applied to cause sinking and engulfment of the silica nanoparticles into the thermoplastic PMMA surface. By selectively etching away the layer of embedded silica nanoparticles, a conformal porous layer of inversely templated structure can be obtained in the PMMA surface. Characterization with atomic force microscopy shows that a variety of nanoporous surface morphologies can be achieved simply by controlling the duration and temperature of thermal annealing. The nanoporous surfaces consisting of either as assembled silica nanoparticles or templated inverse porosity in PMMA were compared in terms of their antireflective (AR) properties. Measuring AR properties provided a quantitative means to compare the stability of these porous AR surfaces before and after several cleaning cycles. Our results show that while both types of surface porosity can provide excellent AR properties (optimized for 300-400 nm), the porous layer generated by the "sink and etch" technique showed superior mechanical stability.

Spray-layer-by-layer assembly can more rapidly produce optical-quality multistack heterostructures.

Automated spray-layer-by-layer (LbL) assembly was used to create highly reflective structurally colored thin films with high reflectance at near-UV light wavelengths. Reflectance peaks were tuned by fabricating alternating stacks of high (TiO(2) nanoparticles) and low (SiO(2) nanoparticles) refractive index materials using a non-quarter-wave design. Spray-assembled multilayer heterostructures fabricated with up to 840 individual polymer or nanoparticle deposition steps presented similar roughness and refractive index values compared to Bragg stacks obtained via immersion LbL assembly. Such complex multilayer heterostructures, however, could be fabricated in significantly shorter times; the time required to deposit a complete bilayer was only about 90 s, compared to 36 min for the immersion assembly of the same system. Optimization of the experimental parameters was performed to achieve uniform coatings and relatively smooth interfaces and surfaces. We observed that the spraying times of the nanoparticle and polymer solutions are the main parameters that determine the thickness, optical properties, and uniformity of the assembled films. Ellipsometry, atomic force microscopy (AFM), UV-vis spectroscopy, and transmission electron microscopy (TEM) were used to characterize the samples. The nanoparticle thin films were iridescent and presented relatively narrow peaks of high reflectance (∼90%) at visible and near-UV wavelengths of light.

Durable antifog films from layer-by-layer molecularly blended hydrophilic polysaccharides.

Mechanically durable, long-lasting antifog coatings based on polysaccharides were developed using a layer-by-layer (LBL) assembly process. The unique properties of these coatings are a result of a molecular-level blending of the polysaccharides, with multilayers containing chitosan and carboxymethyl cellulose providing the best overall properties. The antifog properties resulted from a strong interaction between the polar and H-bonding elements of the assembled polymers and water molecules and the concomitant formation of thin films of water. Environmental scanning electron microscopy (ESEM) studies confirmed that fogging coatings are decorated with light scattering, micrometer-sized droplets of water whereas antifogging coatings remain droplet free. To improve the mechanical durability of the multilayer films on substrates, the surface was modified via self-assembly of epoxy-functionalized silane molecules. Cross-linking chemistry was then applied to improve the mechanical robustness of the LBL films on various surfaces. These films were characterized using several techniques: optical profilometery (PL), spectroscopic ellipsometry (EL), contact angle goniometry (CA), and atomic force microscopy (AFM). The antifog properties of the films were evaluated by several tests under different environmental conditions. This work demonstrates that the unique water-adsorbing properties of polysaccharides can be exploited to create permanent antifog properties, which may be useful for various applications.

Layer-by-layer deposited chitosan/silk fibroin thin films with anisotropic nanofiber alignment.

Chitosan/silk fibroin multilayer thin films were assembled using layer-by-layer deposition. The resultant multilayer films contained nanofibers aligned parallel to the dipping direction. Fiber deposition and orientation was enabled uniquely by a judicious choice of solvent and drying conditions and layer-by-layer assembly with chitosan. The deposition of oriented nanofibers was found to be the result of a unique combination of layer-by-layer and Langmuir-Blodgett type processing. Fiber orientation was confirmed by fast Fourier transform (FFT) analysis of optical micrographs and atomic force microscopy (AFM). Bidirectional fiber alignment was realized by rotating the substrate between multilayer deposition steps. Infrared spectroscopy revealed that the silk fibroin adopted the silk II secondary structure in the deposited films. We anticipate that these anisotropic films are able to combine the biocompatibility of a natural polymer system with the mechanical strength of SF, two properties useful in many biological applications including scaffolds suitable for guiding cell attachment and spreading.

Bioactive polyelectrolyte multilayers: hyaluronic acid mediated B lymphocyte adhesion.

A strategy was developed to produce thin, biopolymer-based polyelectrolyte multilayer films, based on hyaluronic acid and chitosan, that are able to effectively bind B lymphocytes. These films explore CD44-hyaluronate interactions and provide a method to make surface-bound B cell arrays without the need for nonselective covalent chemistry. The rational design of these films using solution deposition variables, such as ionic strength and pH, allows one to maximize and fine tune this binding efficiency ex vivo. This work suggests two important conditions for successfully attaching B cells to hyaluronate-containing polyelectrolyte multilayer films: (1) hyaluronic acid is required for the proposed CD44-mediated binding mechanism, and (2) hyaluronic acid deposition conditions that favor loops and tails, such as low pH and with added salt, result in more available CD44 binding ligands and higher cell binding efficiency. Chitosan-terminated films prepared without NaCl in the deposition solutions and hyaluronic acid-terminated films prepared with salt, both under pH 3.0 assembly conditions, presented a similar high lymphocyte binding efficiency. In the former case, however, the binding strength was weaker due to a significant electrostatic contribution to the binding. Bioactive polyelectrolyte multilayers for selective binding of lymphocytes hold great promise in fields ranging from cell-based biosensors to immune system engineering.

Freely suspended cellular "backpacks" lead to cell aggregate self-assembly.

Cellular "backpacks" are a new type of anisotropic, nanoscale thickness microparticle that may be attached to the surface of living cells creating a "bio-hybrid" material. Previous work has shown that these backpacks do not impair cell viability or native functions such as migration in a B and T cell line, respectively. In the current work, we show that backpacks, when added to a cell suspension, assemble cells into aggregates of reproducible size. We investigate the efficiency of backpack-cell binding using flow cytometry and laser diffraction, examine the influence of backpack diameter on aggregate size, and show that even when cell-backpack complexes are forced through small pores, backpacks are not removed from the surfaces of cells.

pH-responsive reversibly swellable nanotube arrays.

We demonstrate a technique for synthesizing substrate-bound arrays of submicrometer-sized reversibly swellable tubes by using porous templates. The sacrificial template approach allows straightforward control over the tube length, diameter, and lateral arrangement of the resultant surface-bound nanotubes. We also explored methods for varying the tube opening structure by altering the pore shape at the surface of the template. A specific PEM system composed of poly(allylamine hydrochloride) and poly(acrylic acid) was chosen as the building block for the nanotube arrays because of its ability to undergo pH-triggered swelling-deswelling transitions. The activation of this transition results in dramatic changes in the length and diameter of the nanotubes as characterized in situ via confocal laser scanning microscopy (CLSM). The pH-driven reversible swelling-deswelling and nanoporosity behavior observed with planar films and nanotubes of this PEM system is a direct consequence of the breaking and reforming of ionic cross-links.

Reversible Self-Healing for Preserving Optical Transparency and Repairing Mechanical Damage in Composites.

This research concentrates on the healing of optical properties, roughness, contact angle hysteresis, and shallow scratches in polymer/nanoparticle composites. A series of ternary composite blends [epoxy/halloysite nanotubes (HNTs)/cellulose acetate butyrate (CAB)] with various CAB concentrations were fabricated and subjected to a series of mechanical damages. The optimized concentration of a nanoparticle is 1.0 vol %, and the CAB concentration is 3.0 vol % based on the mechanical reinforcement and wear resistance. Nanoscale scratching, microlevel falling-sand test, and macrolevel Taber abrasions were utilized to damage the surfaces. The induced damage (roughness and surface scratch up to hundreds of nanometers in depth) healed upon heating. At any temperatures above the softening transition of the semi-interpenetrating network structure of the polymer composites, CAB migrates into the microcracks, and the essential mechanical parameters (modulus, strength, strain to failure) are recovered; in our particular epoxy/HNTs/CAB system, optical transparency is also recovered efficiently. CAB also moves to the macroscopic air/specimen interface and favorably modifies the surface properties, reducing the roll-off angles of water droplets from ∼90° to ∼20°. Through an appropriate choice of CAB additives with different molecular weights, the healing temperature can be tailored.

Substrata mechanical stiffness can regulate adhesion of viable bacteria.

The competing mechanisms that regulate adhesion of bacteria to surfaces and subsequent biofilm formation remain unclear, though nearly all studies have focused on the role of physical and chemical properties of the material surface. Given the large monetary and health costs of medical-device colonization and hospital-acquired infections due to bacteria, there is considerable interest in better understanding of material properties that can limit bacterial adhesion and viability. Here we employ weak polyelectrolyte multilayer (PEM) thin films comprised of poly(allylamine) hydrochloride (PAH) and poly(acrylic acid) (PAA), assembled over a range of conditions, to explore the physicochemical and mechanical characteristics of material surfaces controlling adhesion of Staphylococcus epidermidis bacteria and subsequent colony growth. Although it is increasingly appreciated that eukaryotic cells possess subcellular structures and biomolecular pathways to sense and respond to local chemomechanical environments, much less is known about mechanoselective adhesion of prokaryotes such as these bacteria. We find that adhesion of viable S. epidermidis correlates positively with the stiffness of these polymeric substrata, independently of the roughness, interaction energy, and charge density of these materials. Quantitatively similar trends observed for wild-type and actin analogue mutant Escherichia coli suggest that these results are not confined to only specific bacterial strains, shapes, or cell envelope types. These results indicate the plausibility of mechanoselective adhesion mechanisms in prokaryotes and suggest that mechanical stiffness of substrata materials represents an additional parameter that can regulate adhesion of and subsequent colonization by viable bacteria.

Structural color in porous, superhydrophilic, and self-cleaning SiO2/TiO2 Bragg stacks.

Thin-film Bragg stacks exhibiting structural color have been fabricated by a layer-by-layer (LbL) deposition process involving the sequential adsorption of nanoparticles and polymers. High- and low-refractive-index regions of quarter-wave stacks were generated by calcining LbL-assembled multilayers containing TiO(2) and SiO(2) nanoparticles, respectively. The physical attributes of each region were characterized by a recently developed ellipsometric method. The structural color characteristics of the resultant nanoporous Bragg stacks could be precisely tuned in the visible region by varying the number of stacks and the thickness of the high- and low-refractive-index stacks. These Bragg stacks also exhibited potentially useful superhydrophilicity and self-cleaning properties.

Superoleophilic Titania Nanoparticle Coatings with Fast Fingerprint Decomposition and High Transparency.

Low surface tension sebaceous liquids such as human fingerprint oils are readily deposited on high energy surfaces such as clean glass, leaving smudges that significantly lower transparency. There have been several attempts to prevent formation of these dactylograms on glass by employing oil-repellent textured surfaces. However, nanotextured superoleophobic coatings typically scatter visible light, and the intrinsic thermodynamic metastability of the composite superoleophobic state can result in failure of the oil repellency under moderate contact pressure. We develop titania-based porous nanoparticle coatings that are superoleophilic and highly transparent and which exhibit short time scales for decomposition of fingerprint oils under ultraviolet light. The mechanism by which a typical dactylogram is consumed combines wicking of the sebum into the nanoporous titania structure followed by photocatalytic degradation. We envision a wide range of applications because these TiO2 nanostructured surfaces remain photocatalytically active against fingerprint oils in natural sunlight and are also compatible with flexible glass substrates.

Macrophages with cellular backpacks for targeted drug delivery to the brain.

Most potent therapeutics are unable to cross the blood-brain barrier following systemic administration, which necessitates the development of unconventional, clinically applicable drug delivery systems. With the given challenges, biologically active vehicles are crucial to accomplishing this task. We now report a new method for drug delivery that utilizes living cells as vehicles for drug carriage across the blood brain barrier. Cellular backpacks, 7-10 µm diameter polymer patches of a few hundred nanometers in thickness, are a potentially interesting approach, because they can act as drug depots that travel with the cell-carrier, without being phagocytized. Backpacks loaded with a potent antioxidant, catalase, were attached to autologous macrophages and systemically administered into mice with brain inflammation. Using inflammatory response cells enabled targeted drug transport to the inflamed brain. Furthermore, catalase-loaded backpacks demonstrated potent therapeutic effects deactivating free radicals released by activated microglia in vitro. This approach for drug carriage and release can accelerate the development of new drug formulations for all the neurodegenerative disorders.

Spray-Coated Halloysite-Epoxy Composites: A Means To Create Mechanically Robust, Vertically Aligned Nanotube Composites.

Halloysite nanotube-filled epoxy composites were fabricated using spray-coating methods. The halloysite nanotubes (HNTs) were aligned by the hydrodynamic flow conditions at the spray nozzle, and the polymer viscosity helped to preserve this preferential orientation in the final coatings on the target substrates. Electron microscopy demonstrated a consistent trend of higher orientation degree in the nanocomposite coatings as viscosity increased. The nanoindentation mechanical performances of these coatings were studied using a Hysitron TriboIndenter device. Composites showed improvements up to ∼50% in modulus and ∼100% in hardness as compared to pure epoxy, and the largest improvements in mechanical performance correlated with higher alignment of HNTs along the plane-normal direction. Achieving this nanotube alignment using a simple spray-coating method suggests potential for large-scale production of multifunctional anisotropic nanocomposite coatings on a variety of rigid and deformable substrates.

Enhanced Wear Resistance of Transparent Epoxy Composite Coatings with Vertically Aligned Halloysite Nanotubes.

The influence of nanoparticle orientation on wear resistance of transparent composite coatings has been studied. Using a nozzle spray coating method, halloysite nanotubes (HNTs) were aligned in the in-plane and out-of-plane directions and in various randomly oriented states. Nanoscratching, falling sand, and Taber Abrasion tests were used to characterize the wear resistance at different length scales. Composites consistently displayed better wear resistance than pure epoxy. Samples with out-of-plane particle orientations exhibited better wear-resistant behavior than those with in-plane particle distributions. In nanoscratching tests, the out-of-plane orientation decreases the normalized scratch volume by as much as 60% compared to pure epoxy. In the falling sand and Taber Abrasion tests, out-of-plane aligned halloysite particles resulted in surfaces with smaller roughness based on stylus profilometry and SEM observations. The decrease in roughness values after these wear tests can be as large as 67% from pure epoxy to composites. Composites with higher out-of-plane particle orientation factors exhibited better light transmittance after sand impingements and other wear tests. This study suggests a useful strategy for producing material systems with enhanced mechanical durability and more durable optical properties.