Polyelectrolyte Multilayers with Reversible Thermal Responsivity

Sustained growth factor delivery from bioactive PNIPAM-grafted-chitosan/heparin multilayers as a tool to promote growth and migration of cells

Delivery of growth factors (GFs) is challenging for regulation of cell proliferation and differentiation due to their rapid inactivation under physiological conditions. Here, a bioactive polyelectrolyte multilayer (PEM) is engineered by the combination of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) and glycosaminoglycans to be used as reservoir for GF storage. PNIPAM-grafted-chitosan (PChi) with two degrees of substitution (DS) are synthesized, namely LMW* (DS 0.14) and HMW (DS 0.03), by grafting low (2 kDa) and high (10 kDa) molecular weight of PNIPAM on the backbone of chitosan (Chi) to be employed as polycations to form PEM with the polyanion heparin (Hep) at pH 4. Subsequently, PEMs are chemically crosslinked to improve their stability at physiological pH 7.4. Resulting surface and mechanical properties indicate that PEM containing HMW is responsive to temperature at 20 °C and 37 °C, while LMW is not. More importantly, Hep as terminal layer combined with HMW allows not only a better retention of the adhesive protein vitronectin but also a sustained release of FGF-2 at 37 °C. With the synergistic effect of vitronectin and matrix-bound FGF-2, significant promotion on adhesion, proliferation, and migration of 3T3 mouse embryonic fibroblasts is achieved on HMW-containing PEM compared to Chi-containing PEM and exogenously added FGF-2. Thus, PEM containing PNIPAM in combination with bioactive glycosaminoglycans like Hep represents a versatile approach to fabricate a GF delivery system for efficient cell culture, which can be potentially served as cell culture substrate for production of (stem) cells and bioactive wound dressing for tissue regeneration.

Engineering of Stable Cross-Linked Multilayers Based on Thermo-Responsive PNIPAM-Grafted-Chitosan/Heparin to Tailor Their Physiochemical Properties and Biocompatibility

The thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) is ubiquitously applied in controlled drug release and tissue engineering. However, the lack of bioactivity of PNIPAM restricts its use in cell-containing systems being a thermo-responsive adhesive substratum with no regulating effect on cell growth and differentiation. In this study, integrating PNIPAM with chitosan into PNIPAM-grafted-chitosan (PNIPAM-Chi) allows a layer-by-layer assembly with bioactive heparin to fabricate PNIPAM-modified polyelectrolyte multilayers (PNIPAM-PEMs). Grafting PNIPAM chains of either 2 (LMW) or 10 kDa (HMW) on the chitosan backbone influences the cloud point (CP) temperature in the range from 31 to 33 °C. PNIPAM-Chi with either a higher molecular weight or a higher degree of substitution of PNIPAM chains exhibiting a significant increase in diameter above CP as ensured by dynamic light scattering is selected to fabricate PEM with heparin as a polyanion at pH 4. Little difference of layer growth is detected between the chosen PNIPAM-Chi used as polycations by surface plasmon resonance, while multilayers formed with HMW-0.02 are more hydrated and show striking swelling-and-shrinking abilities when studied with quartz crystal microbalance with dissipation monitoring. Subsequently, the multilayers are covalently cross-linked using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide to strengthen the stability of the systems under physiological conditions. Ellipsometry results confirm the layer integrity after exposure to the physiological buffer at pH 7.4 compared to those without cross-linking. Moreover, significantly higher adhesion and more spreading of C3H10T1/2 multipotent embryonic mouse fibroblasts on cross-linked PEMs, particularly with heparin terminal layers, are observed owing to the bioactivity of heparin. The slightly more hydrophobic surfaces of cross-linked PNIPAM-PEMs at 37 °C also increase cell attachment and growth. Thus, layer-by-layer constructed PNIPAM-PEM with cross-linking represents an interesting cell culture system that can be potentially employed for thermally uploading and controlled release of growth factors that further promotes tissue regeneration.

Towards standard electrolytes for sodium-ion batteries: physical properties, ion solvation and ion-pairing in alkyl carbonate solvents

The currently emerging sodium-ion battery technology is in need of an optimized standard organic solvent electrolyte based on solid and directly comparable data. With this aim we have made a systematic study of "simple" electrolyte systems consisting of two sodium salts (NaTFSI and NaPF6) dissolved in three different alkyl carbonate solvents (EC, PC, DMC) within a wide range of salt concentrations and investigated: (i) their more macroscopic physico-chemical properties such as ionic conductivity, viscosity, thermal stability, and (ii) the molecular level properties such as ion-pairing and solvation. From this all electrolytes were found to have useful thermal operational windows and electrochemical stability windows, allowing for large scale energy storage technologies focused on load levelling or (to a less extent) electric vehicles, and ionic conductivities on par with analogous lithium-ion battery electrolytes, giving promise to also be power performant. Furthermore, at the molecular level the NaPF6-based electrolytes are more dissociated than the NaTFSI-based ones because of the higher ionic association strength of TFSI compared to PF6- while two different conformers of DMC participate in the Na+ first solvation shells - a Na+ affected conformational equilibrium and induced polarity of DMC. The non-negligible presence of DMC in the Na+ first solvation shells increases as a function of salt concentration. Overall, these results should both have a general impact on the design of more performant Na-conducting electrolytes and provide useful insight on the very details of the importance of DMC conformers in any cation solvation studies.

Thermoresponsive polymers and their biomedical application in tissue engineering - a review

Thermoresponsive polymers hold great potential in the biomedical field, since they enable the fabrication of cell sheets, in situ drug delivery and 3D-printing under physiological conditions. In this review we provide an overview of several thermoresponsive polymers and their application, with focus on poly(N-isopropylacrylamide)-surfaces for cell sheet engineering. Basic knowledge of important processes like protein adsorption on surfaces and cell adhesion is provided. For different thermoresponsive polymers, namely PNIPAm, Pluronics, elastin-like polypeptides (ELP) and poly(N-vinylcaprolactam) (PNVCL), synthesis and basic chemical and physical properties have been described and the mechanism of their thermoresponsive behavior highlighted. Fabrication methods of thermoresponsive surfaces have been discussed, focusing on PNIPAm, and describing several methods in detail. The latter part of this review is dedicated to the application of the thermoresponsive polymers and with regard to cell sheet engineering, the process of temperature-dependent cell sheet detachment is explained. We provide insight into several applications of PNIPAm surfaces in cell sheet engineering. For Pluronics, ELP and PNVCL we show their application in the field of drug delivery and tissue engineering. We conclude, that research of thermoresponsive polymers has made big progress in recent years, especially for PNIPAm since the 1990s. However, manifold research possibilities, e.g. in surface fabrication and 3D-printing and further translational applications are conceivable in near future.

Thermally-Induced Actuations of Stimuli-Responsive, Bicompartmental Nanofibers for Decoupled Drug Release

Stimuli-responsive anisotropic microstructures and nanostructures with different physicochemical properties in discrete compartments, have been developed as advanced materials for drug delivery systems, tissue engineering, regenerative medicine, and biosensing applications. Moreover, their stimuli-triggered actuations would be of great interest for the introduction of the functionality of drug delivery reservoirs and tissue engineering scaffolds. In this study, stimuli-responsive bicompartmental nanofibers (BCNFs), with completely different polymer compositions, were prepared through electrohydrodynamic co-jetting with side-by-side needle geometry. One compartment with thermo-responsiveness was composed of methacrylated poly(N-isopropylacrylamide-co-allylamine hydrochloride) (poly(NIPAM-co-AAh)), while the counter compartment was made of poly(ethylene glycol) dimethacrylates (PEGDMA). Both methacrylated poly(NIPAM-co-AAh) and PEGDMA in distinct compartments were chemically crosslinked in a solid phase by UV irradiation and swelled under aqueous conditions, because of the hydrophilicity of both poly(NIPAM-co-AAh) and PEGDMA. As the temperature increased, BCNFs maintained a clear interface between compartments and showed thermally-induced actuation at the nanoscale due to the collapsed poly(NIPAM-co-AAh) compartment under the PEGDMA compartment of identical dimensions. Different model drugs, bovine serum albumin, and dexamethasone phosphate were alternately loaded into each compartment and released at different rates depending on the temperature and molecular weight of the drugs. These BCNFs, as intelligent nanomaterials, have great potential as tissue engineering scaffolds and multi-modal drug delivery reservoirs with stimuli-triggered actuation and decoupled drug release.

Cargo shuttling by electrochemical switching of core-shell microgels obtained by a facile one-shot polymerization

Controlling and understanding the electrochemical properties of electroactive polymeric colloids is a highly topical but still a rather unexplored field of research. This is especially true when considering more complex particle architectures like stimuli-responsive microgels, which would entail different kinetic constraints for charge transport within one particle. We synthesize and electrochemically address dual stimuli responsive core-shell microgels, where the temperature-responsiveness modulates not only the internal structure, but also the microgel electroactivity both on an internal and on a global scale. In detail, a facile one-step precipitation polymerization results in architecturally advanced poly(N-isopropylacrylamide-co-vinylferrocene) P(NIPAM-co-VFc) microgels with a ferrocene (Fc)-enriched (collapsed/hard) core and a NIPAM-rich shell. While the remaining Fc units in the shell are electrochemically accessible, the electrochemical activity of Fc in the core is limited due to the restricted mobility of redox active sites and therefore restricted electron transfer in the compact core domain. Still, prolonged electrochemical action and/or chemical oxidation enable a reversible adjustment of the internal microgel structure from core-shell microgels with a dense core to completely oxidized microgels with a highly swollen core and a denser corona. The combination of thermo-sensitive and redox-responsive units being part of the network allows for efficient amplification of the redox response on the overall microgel dimension, which is mainly governed by the shell. Further, it allows for an electrochemical switching of polarity (hydrophilicity/hydrophobicity) of the microgel, enabling an electrochemically triggered uptake and release of active guest molecules. Hence, bactericidal drugs can be released to effectively kill bacteria. In addition, good biocompatibility of the microgels in cell tests suggests suitability of the new microgel system for future biomedical applications.

PPEGMEMA-based cationic copolymers designed for layer-by-layer assembly

We have synthesized three PPEGMEMA-based cationic copolymers with similar amine contents but with systematic variation in the average length of the PEG side chains. The positively charged copolymers were paired with alginate to fabricate layer-by-layer assembled multilayered films. It was demonstrated that the polymeric design, in terms of the systematic variation in the average length of the PEG units, affects the polyelectrolyte multilayer growth mechanism and can be used to tune the structural properties and the water content of the layers. In addition, by partial cross-linking of the amine groups present in the copolymer backbone, disintegration of the film induced by pH changes was prevented. Finally, it was demonstrated how the cross-linked multilayered film can exhibit cationic, zwitterionic and anionic properties depending on the pH value and how these changes are associated with swelling, layer contraction and changes in water content.

PPEGMEMA-based cationic copolymers were synthesized and utilized in a layer-by-layer assembly process to prepare a multilayered film. Film properties were tuned with systematic variation in the average length of the PEG side chains.

Molecular Origin of the Glass Transition in Polyelectrolyte Assemblies

Water plays a central role in the assembly and the dynamics of charged systems such as proteins, enzymes, DNA, and surfactants. Yet it remains a challenge to resolve how water affects relaxation at a molecular level, particularly for assemblies of oppositely charged macromolecules. Here, the molecular origin of water's influence on the glass transition is quantified for several charged macromolecular systems. It is revealed that the glass transition temperature (Tg) is controlled by the number of water molecules surrounding an oppositely charged polyelectrolyte-polyelectrolyte intrinsic ion pair as 1/Tg ∼ ln(nH2O/nintrinsic ion pair). This relationship is found to be "general", as it holds for two completely different types of charged systems (pH- and salt-sensitive) and for both polyelectrolyte complexes and polyelectrolyte multilayers, which are made by different paths. This suggests that water facilitates the relaxation of charged assemblies by reducing attractions between oppositely charged intrinsic ion pairs. This finding impacts current interpretations of relaxation dynamics in charged assemblies and points to water's important contribution at the molecular level.

Layer-by-Layer Assembled Microgels Can Combine Conflicting Properties: Switchable Stiffness and Wettability without Affecting Permeability

Responsive interfacial architectures of practical interest commonly require the combination of conflicting properties in terms of their demand upon material structure. Switchable stiffness, wettability, and permeability, key features for tissue engineering applications, are in fact known to be exclusively interdependent. Here, we present a nanoarchitectonic approach that decouples these divergent properties by the use of thermoresponsive microgels as building blocks for the construction of three-dimensional arrays of interconnected pores. Layer-by-layer assembled poly( N-isopropylacrylamide- co-methacrylic acid) microgel films were found to exhibit an increase in hydrophobicity, stiffness, and adhesion properties upon switching the temperature from below to above the lower critical solution temperature, whereas the permeability of redox probes through the film remained unchanged. Our findings indicate that the switch in hydrophilicity and nanomechanical properties undergone by the microgels does not compromise the porosity of the film, thus allowing the free diffusion of redox probes through the polymer-free volume of the submicrometer pores. This novel approach for decoupling conflicting properties provides a strategic route for creating tailorable scaffolds with unforeseen functionalities.

Temperature-responsive nanogel multilayers of poly(N-vinylcaprolactam) for topical drug delivery

We report nanothin temperature-responsive hydrogel films of poly(N-vinylcaprolactam) nanoparticles (νPVCL) with remarkably high loading capacity for topical drug delivery. Highly swollen (νPVCL)_n multilayer hydrogels, where n denotes the number of nanoparticle layers, are produced by layer-by-layer hydrogen-bonded assembly of core-shell PVCL-co-acrylic acid nanoparticles with linear PVPON followed by cross-linking of the acrylic acid shell with either ethylene diamine (EDA) or adipic acid dihydrazide (AAD). We demonstrate that a (νPVCL)₅ film undergoes dramatic and reversible swelling up to 9 times its dry thickness at pH = 7.5, indicating 89v/v % of water inside the network. These hydrogels exhibit highly reversible ∼3-fold thickness changes with temperature variations from 25 to 50°C at pH = 5, the average pH of human skin. We also show that a (νPVCL)₃₀ hydrogel loaded with ∼120µgcm^-2 sodium diclofenac, a non-steroidal anti-inflammatory drug used for osteoarthritis pain management, provides sustained permeation of this drug through an artificial skin membrane for up to 24h at 32°C (the average human skin surface temperature). The cumulative amount of diclofenac transported at 32°C from the (νPVCL)₃₀ hydrogel after 24h is 12 times higher than that from the (νPVCL)₃₀ hydrogel at 22°C. Finally, we demonstrate that the (νPVCL) hydrogels can be used for multiple drug delivery by inclusion of Nile red, fluorescein and DAPI dyes within the νPVCL nanoparticles prior to hydrogel assembly. Using confocal microscopy we observed the presence of separate dye-loaded νPVCL compartments within the hydrogel matrix with all three dyes confined to the nanogel particles without intermixing between the dyes. Our study provides opportunity for development of temperature-responsive multilayer hydrogel coatings made via the assembly of core-shell nanogel particles which can be used for skin-sensitive materials for topical drug delivery.

Non-additive impacts of covalent cross-linking on the viscoelastic nanomechanics of ionic polyelectrolyte complexes

This study elucidates the influences of adding covalent cross-linking on the nanomechanical viscoelasticity of ionically cross-linked polyelectrolyte networks.

Response of Swelling Behavior of Weak Branched Poly(ethylene imine)/Poly(acrylic acid) Polyelectrolyte Multilayers to Thermal Treatment

Weak polyelectrolyte multilayers (PEMs) prepared by the layer-by-layer technique have attracted a great deal of attention as smart responsive materials for biological and other applications in aqueous medium, but their dynamic behavior as a function of exposure to a wide temperature range is still not well understood. In this work, the thermally dependent swelling behavior of PEMs consisting of branched poly(ethylenimine) and poly(acrylic acid) is studied by temperature controlled in situ spectroscopic ellipsometry. Because of diffusion and interpenetration of polyelectrolytes during film deposition, the PEMs densify with increasing bilayer number, which further affects their water uptake behavior. Upon heating to temperatures below 60 °C, the worsened solvent quality of the PEM in water causes deswelling of the PEMs. However, once heated above this critical temperature, the hydrogen bonds within the PEMs are weakened, which allows for chain rearrangement within the film upon cooling, resulting in enhanced water uptake and increased film thickness. The current work provides fundamental insight into the unique dynamic behavior of weak polyelectrolyte multilayers in water at elevated temperatures.

Temperature-induced structure switch in thermo-responsive micellar interpolyelectrolyte complexes: toward core-shell-corona and worm-like morphologies

The spontaneous formation and thermo-responsiveness of a colloidally-stable interpolyelectrolyte complex (IPEC) based on a linear temperature-sensitive diblock copolymer poly(vinyl sulfonate)31-b-poly(N-isopropyl acrylamide)27 (PVS31-b-PNIPAM27) and a star-shaped quaternized miktoarm polymer poly(ethylene oxide)114-(poly(2-(dimethylamino)ethyl methacrylate)17)4 (PEO114-(qPDMAEMA17)4) was investigated in aqueous media at 0.3 M NaCl by means of dynamic light scattering (DLS), small angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). The micellar macromolecular co-assemblies show a temperature-dependent size and morphology, which result from the lower critical solution temperature (LCST) behavior of the PNIPAM-blocks. Hence, the micellar co-assemblies grow upon heating. At 60 °C, spherical core-shell-corona co-assemblies are proposed with a hydrophobic PNIPAM core, a water-insoluble IPEC shell, and a hydrophilic PEO corona. These constructs develop into a rod-like structure upon extended equilibration. In turn, PEO-arms and PNIPAM-blocks within a hydrophilic mixed two-component corona surround the water-insoluble IPEC domain at 20 °C, thereby forming spherical core-corona co-assemblies. Reversibility of the structural changes is suggested by the scattering data. This contribution addresses the use of a combination of oppositely charged thermo-responsive and bis-hydrophilic star-shaped polymeric components toward IPECs of diverse morphological types.

Building polyzwitterion-based multilayers for responsive membranes

We systematically investigate the assembly of multilayers based on a polyzwitterion (PSBMA) and a polycation (PDADMAC) for the development of ionic strength responsive membranes. Although the polyzwitterion is essentially charge neutral, we show that specific electrostatic interactions with the PDADMAC allow for the formation of stable multilayers. The growth of this LbL system is monitored on model surfaces (silica) via optical reflectometry for different pH values and ionic strengths. While no effect of pH on the layer growth is observed, we did observe a strong dependence on the ionic strength. Upon increasing the ionic strength during deposition from 0.005 to 0.5 M NaCl, the adsorbed amount is significantly decreased, a behavior that is opposite to classical LbL systems. Similar results to those obtained on silica are also observed on top of classical LbL systems and on polymeric membranes. This demonstrates that the growth of the polyzwitterion multilayers is independent of the substrate. Coating these polyzwitterion multilayers on hollow fiber membranes via dip-coating yields membranes that are stimuli responsive toward the ionic strength of the filtration solution, with an increase in permeability of up to 108% from 0 to 1.5 M NaCl. We show that the fabrication of the polyzwitterion multilayers is an easy and controlled way to provide surfaces, such as membranes, with the specific functionalities of polyzwitterions.

Thermosensitive multilayer hydrogels of poly(N-vinylcaprolactam) as nanothin films and shaped capsules

We report on nanothin multilayer hydrogels of cross-linked poly(N-vinylcaprolactam) (PVCL) that exhibit distinctive and reversible thermoresponsive behavior. The single-component PVCL hydrogels were produced by selective cross-linking of PVCL in layer-by-layer films of PVCL-NH(2) copolymers assembled with poly(methacrylic acid) (PMAA) via hydrogen bonding. The degree of the PVCL hydrogel film shrinkage, defined as the ratio of wet thicknesses at 25°C to 50°C, was demonstrated to be 1.9±0.1 and 1.3±0.1 for the films made from PVCL-NH(2)-7 and PVCL-NH(2)-14 copolymers, respectively. No temperature-responsive behavior was observed for non-cross-linked two-component films due to the presence of PMAA. We also demonstrated that temperature-sensitive PVCL capsules of cubical and spherical shapes could be fabricated as hollow hydrogel replicas of inorganic templates. The cubical (PVCL)(7) capsules retained their cubical shape when temperature was elevated from 25°C to 50°C exhibiting 21±1% decrease in the capsule size. Spherical hydrogel capsules demonstrated similar shrinkage of 23±1%. The temperature-triggered capsule size changes were completely reversible. Our work opens new prospects for developing biocompatible and nanothin hydrogel-based coatings and containers for temperate-regulating drug delivery, cellular uptake, sensing, and transport behavior in microfluidic devices.

Preparation and characterisation of multilayer films and microcapsules containing poly(N-isopropylacrylamide-co-sodium vinylsulphonate) and applicability on controlled release

The copolymers of N-isopropylacrylamide and sodium vinylsulphonate were synthesised by free radical polymerisation. The layer-by-layer self-assembly of the copolymers with poly(allylamine) hydrochloride was performed through assembling onto silicon wafer to form multilayer films and onto CaCO₃ microparticles doped with poly(styrene sulphonate) as well as deltamethrin microcrystals to form microcapsules. The multilayer films and microcapsules were characterised by atomic force microscopy, transmission electron microscopy and scanning electron microscopy. The release behaviour of deltamethrin in the microcapsules under different conditions was also investigated by high performance liquid chromatography. Results show that these deltamethrin microcapsules have good thermo-sensitive properties and deltamethrin release can be controlled via changing temperature or self-assembly layers.

Light responsive polymer membranes: a review

In recent years, stimuli responsive materials have gained significant attention in membrane separation processes due to their ability to change specific properties in response to small external stimuli, such as light, pH, temperature, ionic strength, pressure, magnetic field, antigen, chemical composition, and so on. In this review, we briefly report recent progresses in light-driven materials and membranes. Photo-switching mechanisms, valved-membrane fabrication and light-driven properties are examined. Advances and perspectives of light responsive polymer membranes in biotechnology, chemistry and biology areas are discussed.

Layer-by-layer assembly of two temperature-responsive homopolymers at neutral pH and the temperature-dependent solubility of the multilayer film

We fabricated a layer-by-layer (LbL) film of temperature-responsive homopolymers at neutral pH and studied its temperature-dependent solubility. We first measured the cloud point of mixed solutions of temperature-responsive polymers. The significant decrease of cloud point suggested that the intermolecular interaction between two polymer chains of different kinds was stronger than that between two polymer chains of the same kind. Strong intermolecular interaction between two polymer chains of different kinds is a prerequisite for LbL assembly. On the basis of the decrease of cloud point of mixed solutions of temperature-responsive homopolymers, we selected poly(N-vinylcaprolactam) (PVCL) and poly(2-hydroxypropyl acrylate) (PHPA) for LbL assembly. LbL films of the two polymers were fabricated at neutral pH at a constant temperature. When the film was immersed in purified water at a temperature lower than the assembly temperature, it can be partially dissolved with a diffusion-limited dissolution process. The temperature-responsive solubility of the LbL film is closely connected to the phase behavior of mixed solutions of the two polymers. Additionally, as compared to multilayer films of neutral polymers and poly(carboxylic acid)s, the PVCL/PHPA multilayer film is relatively stable when it was immersed in buffer solutions near physiological pH at the assembly temperature. Such LbL films with temperature-responsive solubility might be used as a dissolvable film or a smart capsule.

Polymer assemblies for controlled delivery of bioactive molecules from surfaces

Localized delivery of bioactive compounds from surfaces of biomedical devices affords significant therapeutic benefits, and often relies on the capability of surface coatings to provide spatial and temporal control over release rate. The layer-by-layer technique presents a unique means to construct surface coatings that can conform to a variety of biomaterial surfaces and serve as matrices enabling controlled delivery of bioactive molecules from surfaces. The versatility of layer-by-layer assembly enables construction of surface coatings of diverse chemistry and internal architecture with controlled release properties. This review focuses on recent developments in constructing such layered matrices using linear polymers, polymer nanoparticles and block copolymer micelles, including micelles with stimuli-responsive cores, as film building blocks and in controlling release rate of therapeutics from these matrices via degradation, application of pH, ionic strength, temperature, light, electric field and chemical or biological stimuli. Challenges and opportunities associated with fabrication of stratified multilayer films capable of multi-stage delivery of multiple drugs are also discussed.

Electrochemically addressed cross-links in polyelectrolyte multilayers: cyclic duravoltammetry

In situ nanoindentation was performed on a multilayer of poly(acrylic acid) and a high molecular weight, pendant chain polyviologen under controlled electrochemical potential. The modulus of the thin film of polyelectrolyte complex was reversibly modulated, by about an order of magnitude, upon changing the state of charge within the material using the electrochemically active and addressable viologen repeat units. The applied potential, under aqueous conditions, is believed to control the extent of cross-link formation. Simultaneous quartz crystal microbalance measurements revealed the flux of ions into or out of the multilayer during redox cycling. Apparent film modulus also depends on the identity of the last layer.

Dual responsive nanostructured surfaces for biomedical applications

In this paper, we describe the construction and characteristics of thermoresponsive, thin nanostructured films prepared by layer-by-layer sequential assembly of chitosan-graft-NIPAAm and alginate. FTIR and (1)H NMR spectra have confirmed the introduction of NIPAAm moieties onto the chitosan backbone. The LCST of the synthesized copolymer was found to be around 31-33 °C. The formation of the polyelectrolyte multilayers containing the copolymer and alginate was followed in situ by quartz crystal microbalance with dissipation monitoring technique and ex situ by UV-vis measurements. Our results revealed the linear increase of the multilayer film growth and the influence of the presence of salt. Moreover, AFM analysis has confirmed that PNIPAAm is able to reconform upon temperature swaps even when combined with other layers in a polyelectrolyte multilayer, demonstrating that the nanoassemblies are thermoresponsive. Preliminary results showed that, upon reducing culture temperature below PNIPAAm LCST, a gradual detachment of cell sheets from these PNIPAAm-based coatings has occurred.

Ion diffusion coefficients through polyelectrolyte multilayers: temperature and charge dependence

The diffusion coefficient is a fundamental parameter for devices exploiting the ion transport properties of polyelectrolyte multilayers (PEMUs) and complexes. Here, the transport of ferricyanide through a multilayer made from poly(diallyldimethylammonium chloride) (PDADMA) and polystyrene sulfonate (PSS) was studied as a function of temperature or salt concentration. Accurate and precise measurements of ion diffusion coefficients were obtained using steady-state electrochemistry to determine the flux and Fourier transform infrared (FTIR) spectroscopy to measure the PEMU concentration. It was found that the concentration of ferricyanide inside the film decreased with temperature. Membrane transport is strongly thermally activated with activation energy 98 kJ mol(-1). A potential shift with decreasing salt concentration in cyclic voltammograms was translated into a differential flux caused by significantly higher diffusion coefficients for ferricyanide as compared to ferrocyanide.

N-isopropylacrylamide-based thermoresponsive polyelectrolyte multilayer films for human mesenchymal stem cell expansion

Human mesenchymal stem cells (hMSCs) are colony-forming unit fibroblasts (CFU-F) derived from adult bone marrow and have significant potential for many cell-based tissue-engineering applications. Their therapeutic potential, however, is restricted by their diminishing plasticity as they are expanded in culture. In this study, we used N-isopropylacrylamide (NIPAM)-based thermoresponsive polyelectrolyte multilayer (N-PEMU) films as culture substrates to support hMSC expansion and evaluated their effects on cell properties. The N-PEMU films were made via layer-by-layer adsorption of thermoresponsive monomers copolymerized with charged monomers, positively charged allylamine hydrochloride (PAH), or negatively charged styrene sulfonic acid (PSS) and compared to fetal bovine serum (FBS) coated surfaces. Surface charges were shown to alter the extracellular matrix (ECM) structure and subsequently regulate hMSC responses including adhesion, proliferation, integrin expression, detachment, and colony forming ability. The positively charged thermal responsive surfaces improved cell adhesion and growth in a range comparable to control surfaces while maintaining significantly higher CFU-F forming ability. Immunostaining and Western blot results indicate that the improved cell adhesion and growth on the positively charged surfaces resulted from the elevated adhesion of ECM proteins such as fibronectin on the positively charge surfaces. These results demonstrate that the layer-by-layer approach is an efficient way to form PNIPAM-based thermal responsive surfaces for hMSC growth and removal without enzymatic treatment. The results also show that surface charge regulates ECM adhesion, which in turn influences not only cell adhesion but also CFU-forming ability and their multi-lineage differentiation potential.

Dynamic aspects of films prepared by a sequential deposition of species: perspectives for smart and responsive materials

The deposition of surface coatings using a step-by-step approach from mutually interacting species allows the fabrication of so called "multilayered films". These coatings are very versatile and easy to produce in environmentally friendly conditions, mostly from aqueous solution. They find more and more applications in many hot topic areas, such as in biomaterials and nanoelectronics but also in stimuli-responsive films. We aim to review the most recent developments in such stimuli-responsive coatings based on layer-by-layer (LBL) depositions in relationship to the properties of these coatings. The most investigated stimuli are based on changes in ionic strength, temperature, exposure to light, and mechanical forces. The possibility to induce a transition from linear to exponential growth in thickness and to change the charge compensation from "intrinsic" to "extrinsic" by controlling parameters such as temperature, pH, and ionic strength are the ways to confer their responsiveness to the films. Chemical post-modifications also allow to significantly modify the film properties.

Polyelectrolyte multilayers of diblock copolymer micelles with temperature-responsive cores

We report on assembly and stimuli-response behavior of layer-by-layer (LbL) films of pH- and temperature-responsive cationic diblock copolymer micelles (BCMs) of poly(2-(dimethylamino)ethyl methacrylate)-block-poly(N-isopropylacrylamide) (PDMA-b-PNIPAM) and a linear polyanion polystyrene sulfonate (PSS). As a function of solution pH at temperatures above lower critical solution temperature (LCST) of PNIPAM, PDMA-b-PNIPAM micelles have been demonstrated earlier to exhibit an abrupt change in micellar aggregation number and hydrodynamic size between larger and smaller BCMs (LBCMs and SBCMs, respectively). Here, LBCMs or SBCMs were included within LbL films through self-assembly with a polyanion, and film pH and temperature responses were studied using ellipsometry and atomic force microscopy (AFM). Both types of micelle preserved their micellar morphology when adsorbed at the surface of oxidized silicon wafers coated with PSS-terminated precursor layer at a constant pH. Response of adsorbed BCMs to temperature and pH variations was strongly dependent on whether or not BCMs were coated with the PSS layer. While monolayers of LBCMs lost their original dry morphology in response to pH or temperature variations, depositing a PSS layer atop LBCMs inhibited such irreversible restructuring. As a result of wrapping around and strong binding of PSS chains with LBCM micelles, BCM/PSS assemblies preserved their original dry state morphology despite the application of pH and temperature triggers. However, the wet-state film response to pH and temperature stimuli was drastically different. Swelling of BCM/PSS multilayers was strongly affected by temperature but was almost independent of pH due to neutralization of BCM PDMA's coronal charge with PSS. Cycling the temperature below and above PNIPAM's LCST caused PNIPAM chains within BCM cores to swell or collapse, resulting in reversible swelling transitions in the entire BCM/PSS assemblies. Temperature-controlled switching between the hydrophobic and hydrophilic state of assembled micellar cores was also used to regulate the release of a micelle-loaded hydrophobic pyrene dye, whose release rate increased at temperatures below PNIPAM's LCST.

Reversible "closing" of an electrode interface functionalized with a polymer brush by an electrochemical signal

The poly(4-vinyl pyridine) (P4VP)-brush-modified indium tin oxide (ITO) electrode was used to switch reversibly the interfacial activity by the electrochemical signal. The application of an external potential (-0.85 V vs Ag|AgCl|KCl, 3M) that electrochemically reduced O(2) resulted in the concomitant consumption of hydrogen ions at the electrode interface, thus yielding a higher pH value and triggering the restructuring of the P4VP brush on the electrode surface. The initial swollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)(6)](4-) redox species, but the electrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The produced hydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting its redox process at the electrode surface. The interface's return to the electrochemically active state was achieved by disconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of the electrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggered inhibition ("closing") of the electrode interface. The application of this approach to different interfacial systems will allow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlled activity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating as electroswitchable drug releasers).

Hydrogen-bonded layer-by-layer temperature-triggered release films

A hydrogen-bonded layer-by-layer (LbL) technique was used to build multilayers of neutral, temperature-responsive polymers such as poly(N-isopropylacrylamide) (PNIPAM), poly(N-vinylcaprolactam) (PVCL), poly(vinyl methyl ether) (PVME), or poly(acrylamide) (PAAm) with a polycarboxylic acid such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), or poly(ethacrylic acid) (PEAA). For all multilayers involving temperature-responsive polymers, the temperature used during or after self-assembly had a significant effect on film stability with pH changes. The proximity of the self-assembly or post-self-assembly temperature to the critical temperature of phase separation of a neutral polymer from solution resulted in a higher pH stability of multilayers. However, for polymers with a lower critical solution temperature (LCST) such as PNIPAM, PVCL, or PVME within PNIPAM/PMAA, PVCL/PMAA, or PVME/PMAA multilayers, the critical pH of film disintegration (pH(crit)) increased in the temperature range from 10 to 37 degrees C, whereas for polymer films with an upper critical solution temperature (UCST), such as PAAm within PAAm/PMAA, the film showed the opposite trend. Using a hydrogen-bonded polyvinylpyrrolidone (PVPON)/PMAA system, which is not responsive to temperature changes, we constructed hybrid films with lower [PNIPAM/PMAA](n) and higher [PVPON/PMAA](m) strata and obtained free-floating [PVPON/PMAA](m) films by temperature-triggered dissolution of the PNIPAM/PMAA layers at a constant pH value. The kinetics of [PVPON/PMAA](m) film release was strongly dependent on the number of bilayers within the PNIPAM/PMAA stratum, indicating significant interpenetration between PNIPAM/PMAA and PVPON/PMAA bilayers. Importantly, the use of PEAA instead of PAA or PMAA in film assembly enabled the construction of hydrogen-bonded LbL films that can be released by applying temperature as a trigger at near-physiological pH values. This feature makes such release layers attractive candidates for future tissue engineering applications.

Temperature-induced swelling and small molecule release with hydrogen-bonded multilayers of block copolymer micelles

We report on reversible temperature-triggered swelling transitions in hydrogen-bonded multilayer films of a polycarboxylic acid and stimuli-responsive block copolymer micelles (BCMs). A neutral hydrogen-bonding temperature-responsive diblock copolymer, poly(N-vinylpyrrolidone)-b-poly(N-isopropylacrylamide) (PVPON-b-PNIPAM), was synthesized by macromolecular design via the interchange of xanthates (MADIX). The block copolymer exhibited reversible micellization, forming PNIPAM-core micelles with PVPON coronae in 0.01 M buffer solutions at temperatures higher than 34 degrees C, or in solutions with high salt concentrations (C(NaCl) > 0.4 M) at 20 degrees C. The PVPON-b-PNIPAM BCMs were then assembled with poly(methacrylic acid) (PMAA) at acidic pH and higher temperature using the layer-by-layer (LbL) technique. Within the hydrogen-bonded multilayer, BCMs were stabilized through hydrogen bonding between PVPON and PMAA units and, unlike in solution, did not dissociate into unimers in low-salt solution at T < 34 degrees C. Instead, PVPON-b-PNIPAM BCMs reversibly swelled within film in response to temperature- or salt-concentration variations, reflecting collapse and dissolution of the BCM PNIPAM cores. The capacity of BCM/PMAA films to retain hydrophobic molecules was also dramatically dependent on temperature and/or ionic strength. The characteristic release time of pyrene from a [BCM/PMAA](10) film decreased from 80 to 10 min upon a decrease in temperature from 37 to 20 degrees C. In addition, at 20 degrees C, ionic strength was also capable of controlling the collapse of PNIPAM micellar cores and the subsequent film swelling and pyrene release rate. Incorporation of stimuli-responsive BCM micelles within LbL films opens new opportunities in designing nanoscale films capable of controlling molecular swelling, transport, and diffusion in response to environmental stimuli.

Switchable selectivity for gating ion transport with mixed polyelectrolyte brushes: approaching 'smart' drug delivery systems

A pH-responsive mixed polyelectrolyte brush from tethered polyacrylic acid (PAA) and poly(2-vinylpyridine) (P2VP) (PAA:P2VP = 69:31 by weight) was prepared and used for selective gating transport of anions and cations across the thin film. An ITO glass electrode was modified with the polymer brush and used to study the switchable permeability of the mixed brush triggered by changes in pH of the aqueous environment in the presence of two soluble redox probes: [Fe(CN)(6)](4-) and [Ru(NH(3))(6)](3+). The responsive behavior of the brush was also investigated using the in situ ellipsometric measurements of the brush swelling, examination of the brush morphology with atomic force microscopy (AFM), and contact angle measurements of the brush samples extracted from aqueous solutions at different pH values. The mixed brush demonstrated a bipolar permselective behavior. At pH<3 the positively charged P2VP chains enabled the electrochemical process for the negatively charged redox probe, [Fe(CN)(6)](4-), while the redox process for the positively charged redox probe was effectively inhibited. On the contrary, at pH>6 a reversible redox process for the positively charged redox probe, [Ru(NH(3))(6)](3+), was observed, while the redox process for the negatively charged redox species, [Fe(CN)(6)](4-), was fully inhibited. Stepwise changing the pH value and recording cyclic voltammograms for the intermediate states of the polymer brush allowed electrochemical observation of the brush transition from the positively charged state, permeable for the negatively charged species, to the negatively charged state, permeable for the positively charged species. The data of ellipsometric, AFM and contact angle measurements are in accord with the electrochemical study. The discovered properties of the brush could be used for the development of 'smart' sensors and drug delivery systems, for example, a smart drug delivery capsule which could release negatively charged molecules of drugs in acidic conditions, while positively charged molecules of drugs will be released in neutral conditions.