Polyelectrolyte Multilayer Formation on Neutral Hydrophobic Surfaces

Pyrene-labeled poly(allylamine hydrochloride) (PAH-Py) has been used to trace the conformational and depletion behaviors of polyelectrolyte chains on neutral hydrophobic Teflon-AF, octadecyltrichlorosilane, or poly(dimethylsiloxane) during assembling by the layer-by-layer deposition technique. The results are used to qualitatively describe how the final quality of multilayer thin films relies on salt stabilization. PAH-Py chains adsorb onto the surfaces in a stretched conformation and form a uniform and dense layer, confirming Dobrynin and Rubinstein's theory. Without an added electrolyte in the polyelectrolyte solutions used for building up multilayers, the PAH-Py layer was seriously depleted, and the extended chains on the surface recoiled during the assembly process, forming coagulated structures of polyelectrolyte complex. In comparison, uniform and flat multilayer thin films were achieved on the surfaces with an added electrolyte, presenting a significant role of salt stabilization on the multilayer

Catechol-Modified Polyions in Layer-by-Layer Assembly to Enhance Stability and Sustain Release of Biomolecules: A Bioinspired Approach

Although layer–by–layer (LbL) assembly technique has been successfully used in various areas of nanobiotechnology, some LbL-assembled nanostructures have suffered from a lack of stability when they are exposed to certain changes in aqueous environments. In addition, the interlayer diffusion of polyelectrolytes throughout the film during assembly generally limits the control of film architecture and release characteristics. To overcome these limitations, we have utilized a strategy to conjugate catechol groups, largely present in mussel adhesive proteins, to branched poly­(ethyleneimine) (BPEI) and poly­(acrylic acid) (PAA). Only a fraction of amine or acid groups are modified with catechol groups, thereby preserving their charged nature for use in LbL assembly, while integrating the beneficial adhesive features of catechol groups into LbL films. The structure, physico–chemical properties, and stability of LbL films composing BPEI and PAA without and with catechol modifications were compared. The incorporation of catechol groups led to a doubling of the average film thickness and linear film growth. Upon exposure to PBS pH 7.4, the catechol-containing LbL films underwent far fewer changes in the degree of ionization and film thickness and exhibited stronger mechanical properties, indicative of their enhanced film stability. Finally, when LbL films with catechol modifications were used as physical barrier layers between radiolabeled 14C–dextran sulfate (14C–DS) and 3H–heparin sulfate (3H–HS), we observed two different release rates composed of an abrupt release from the surface of 3H–HS, together with a sustained release from the underlying 14C–DS. Overall, these films provide a bioinspired multifunctional platform for the systematic incorporation and assembly of biological therapeutics into controlled release films at physiological conditions for biomedical applications

Efficient Transport Networks in a Dual Electron/Lithium-Conducting Polymeric Composite for Electrochemical Applications

In this work, an all-functional polymer material composed of the electrically conductive poly­(3,4-ethylenedioxythiophene):poly­(4-styrenesulfonic acid) (PEDOT:PSS) and lithium-conducting poly­(ethylene oxide) (PEO) was developed to form a dual conductor for three-dimensional electrodes in electrochemical applications. The composite exhibits enhanced ionic conductivity (∼10^–4 S cm^–1) and, counterintuitively, electronic conductivity (∼45 S cm^–1) with increasing PEO proportion, optimal at a monomer ratio of 20:1 PEO:PEDOT. Microscopy reveals a unique morphology, where PSS interacts favorably with PEO, destabilizing PEDOT to associate into highly branched, interconnected networks that allow for more efficient electronic transport despite relatively low concentrations. Thermal and X-ray techniques affirm that the PSS–PEO domain suppresses crystallinity, explaining the high ionic conductivity. Electrochemical experiments in lithium cell environments indicate stability as a function of cycling and improved overpotential due to dual transport characteristics despite known issues with both individual components

Engineering Ionic and Electronic Conductivity in Polymer Catalytic Electrodes Using the Layer-By-Layer Technique

The platinum loading, electronic and ionic conductivity, tuned porosity, and electrode potential of layer-by-layer (LBL) conducting polymer films for thin film catalytic electrodes are presented. Films of polyaniline (PANi)/poly(acrylic acid) (PAA) or PANi/poly(acrylic acid)-co-polyacrylamide (PAA-co-PAAm) of 3.0-μm thickness were pH-tuned to induce porosity as they were assembled. Three different techniques were used to dose the LBL PANi films with platinum. The first method used reductive precipitation of platinum and ruthenium salts adsorbed within LBL films of PANi/PAA-co-PAAm. The second method, termed polyelectrolyte colloidal platinum stabilization, was applied to load platinum nanoclusters into LBL films of either PANi/PAA or PANi/poly(styrene sulfonate) films. The third method used a PANi/platinum powder dispersion to load platinum crystals into LBL films of PANi/PAA-co-PAAm or poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAA-co-PAMPS). The first method yielded the best metal loadings with maximum platinum loadings of 0.3 mg cm-2, and the resulting Pt-containing PANi/PAA-co-PAAm films were further examined for their electrochemical characteristics. The electrode potential and chronopotentiometric current control in the resulting electrodes were examined for the best-performing LBL PANi film assembled in this study. The catalyzed PANi/PAA-co-PAAm electrodes exhibited an electrode potential similar to that of pure platinum, a relatively high and stable electrical conductivity of 2.3 S cm-1, and an ionic conductivity of up to 10-5 S cm-1

Tuning the Glass Transition of and Ion Transport within Hydrogen-Bonded Layer-by-Layer Assemblies

The influence of pH and ionic strength on the structure and properties of hydrogen-bonded layer-by-layer (LbL) assemblies of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) is explored. The degree of inter- and intramolecular hydrogen bonding is estimated from Fourier-transform infrared spectroscopy, the glass transition temperature is measured using differential scanning calorimetry of bulk free-standing films, and ionic conductivity is studied using electrochemical impedance spectroscopy. Results indicate that (PEO/PAA) LbL films assembled without added salt are sensitive to pH, with a Tg decrease (59−26 °C) and intermolecular hydrogen bonding increase (27 to 51% COOH groups bonding with PEO) with increasing assembly pH (2 to 3). Films assembled in the presence of 0.1 M lithium triflate exhibit properties independent of assembly pH (Tg ∼ 48 °C and 12% COOH groups bonding with PEO), presumably due to the “screening” of hydrogen bonds. Ionic conductivity is found to range from 10-6 to 10-10 S cm-1, depending on humidity, plasticization, and salt content

Factors Influencing the Interdiffusion of Weak Polycations in Multilayers

One promising aspect of the electrostatic multilayer assembly techniques is the ability to consistently and predictably create controlled heterostructures that may be of interest for active devices, designed biomaterials, membranes, or other composite thin film structures. This promise is mitigated by the challenge of controlling diffusion and exchange processes that can take place in certain layer-by-layer assembled film systems and cause unanticipated or unwanted materials distributions and in extreme cases completely disrupt the assembly. To further understanding toward prediction and control of these processes, we investigate a series of polyamines and their interdiffusion and exchange within preassembled multilayer films to explore the role of polyion degree of ionization, hydrophobicity/hydrophilicity of the backbone, basicity of amine groups, and polyion topology in a polyelectrolyte interdiffusion and exchange process. Interdiffusion of these polyamines will be examined within a polyhexylviologen (PXV)/poly(acrylic acid) multilayer model system in which conditions favor exchange of the polyamine with PXV, a strong polycation containing quarternary ammonium groups along the backbone. Four amine-containing polycations were examined:  linear and branched polyethylene imine (LPEI and BPEI), polyamidoamine (PAMAM) dendrimer, and poly(allylamine hydrochloride) (PAH). It was found that fully charged polycations in dilute aqueous solution are unable to diffuse through the multilayer film whereas partially charged polycations have the necessary mobility. Remarkably, despite strong differences in the nature of the polycation, as in the case of PAH, LPEI, and BPEI, for every polyamine there existed the same critical degree of ionization in solution below which interdiffusion was possible, which was seen to be near 70% in these exchange experiments with PXV. Only for the highly branched PAMAM dendrimer was this value different; the critical degree of ionization for PAMAM was observed to be 55%. Kinetics of the interdiffusion were significantly impacted by the polyion degree of ionization and molecular weight

Hydrogen-Bonding Layer-by-Layer-Assembled Biodegradable Polymeric Micelles as Drug Delivery Vehicles from Surfaces

We present the integration of amphiphilic block copolymer micelles as nanometer-sized vehicles for hydrophobic drugs within layer-by-layer (LbL) films using alternating hydrogen bond interactions as the driving force for assembly for the first time, thus enabling the incorporation of drugs and pH-sensitive release. The film was constructed based on the hydrogen bonding between poly(acrylic acid) (PAA) as an H-bond donor and biodegradable poly(ethylene oxide)-block-poly(ϵ-caprolactone) (PEO-b-PCL) micelles as the H-bond acceptor when assembled under acidic conditions. By taking advantage of the weak interactions of the hydrogen-bonded film on hydrophobic surfaces, it is possible to generate flexible free-standing films of these materials. A free-standing micelle LbL film of (PEO-b-PCL/PAA)60 with a thickness of 3.1 µm was isolated, allowing further characterization of the bulk film properties, including morphology and phase transitions, using transmission electron microscopy and differential scanning calorimetry. Because of the sensitive nature of the hydrogen bonding employed to build the multilayers, the film can be rapidly deconstructed to release micelles upon exposure to physiological conditions. However, we could also successfully control the rate of film deconstruction by cross-linking carboxylic acid groups in PAA through thermally induced anhydride linkages, which retard the drug release to the surrounding medium to enable sustained release over multiple days. To demonstrate efficacy in delivering active therapeutics, in vitro Kirby−Bauer assays against Staphylococcus aureus were used to illustrate that the drug-loaded micelle LbL film can release significant amounts of an active antibacterial drug, triclosan, to inhibit the growth of bacteria. Because the micellar encapsulation of hydrophobic therapeutics does not require specific chemical interactions, we believe this noncovalent approach provides a new route to integrating active small, uncharged, and hydrophobic therapeutics into LbL thin films for biological and biomedical coatings

Nanolayered siRNA Dressing for Sustained Localized Knockdown

The success of RNA interference (RNAi) in medicine relies on the development of technology capable of successfully delivering it to tissues of interest. Significant research has focused on the difficult task of systemic delivery of RNAi; however its local delivery could be a more easily realized approach. Localized delivery is of particular interest for many medical applications, including the treatment of localized diseases, the modulation of cellular response to implants or tissue engineering constructs, and the management of wound healing and regenerative medicine. In this work we present an ultrathin electrostatically assembled coating for localized and sustained delivery of short interfering RNA (siRNA). This film was applied to a commercially available woven nylon dressing commonly used for surgical applications and was demonstrated to sustain significant knockdown of protein expression in multiple cell types for more than one week <i>in vitro</i>. Significantly, this coating can be easily applied to a medically relevant device and requires no externally delivered transfection agents for effective delivery of siRNA. These results present promising opportunities for the localized administration of RNAi

Nano- and Microporous Layer-by-Layer Assemblies Containing Linear Poly(ethylenimine) and Poly(acrylic acid)

The structure and morphology as well as the mechanism of formation of porous polyelectrolyte multilayers consisting of linear poly(ethylenimine) (LPEI) and poly(acrylic acid) (PAA) have been systematically investigated as a function of pH. The structures obtained exhibit dramatic differences with small changes in the pH of multilayer assembly and pH of postassembly treatment, yielding an observed range of pore sizes from tens of nanometers to micrometers and pore volume fractions from 0 to 77%. The porous phase transition is quite rapid (−6 and 10−9 S cm−1 were observed. The asymmetric membrane LbL structure, first reported here, holds many potential applications in terms of filtration, catalysis, drug delivery, etc

Tuning the Glass Transition of and Ion Transport within Hydrogen-Bonded Layer-by-Layer Assemblies

The influence of pH and ionic strength on the structure and properties of hydrogen-bonded layer-by-layer (LbL) assemblies of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) is explored. The degree of inter- and intramolecular hydrogen bonding is estimated from Fourier-transform infrared spectroscopy, the glass transition temperature is measured using differential scanning calorimetry of bulk free-standing films, and ionic conductivity is studied using electrochemical impedance spectroscopy. Results indicate that (PEO/PAA) LbL films assembled without added salt are sensitive to pH, with a Tg decrease (59−26 °C) and intermolecular hydrogen bonding increase (27 to 51% COOH groups bonding with PEO) with increasing assembly pH (2 to 3). Films assembled in the presence of 0.1 M lithium triflate exhibit properties independent of assembly pH (Tg ∼ 48 °C and 12% COOH groups bonding with PEO), presumably due to the “screening” of hydrogen bonds. Ionic conductivity is found to range from 10-6 to 10-10 S cm-1, depending on humidity, plasticization, and salt content