Thermal Behavior of Polyelectrolyte Multilayer Microcapsules. 1. The Effect of Odd and Even Layer Number

Dependence of Sub-Micron Vaterite Container Release Properties on pH and Ionic Strength of the Surrounding Solution

We report on the synthesis and characterization of porous monodisperse vaterite containers with controllable average sizes from 400 nm to 10 μm. Possible release strategies of enclosed substances via recrystallization or by pH-change are presented. As a model experiment, a fluorescent marker was encapsulated and imaged by two-photon microscopy to monitor the dye release. The release process was found to be controllable via the immersion medium’s properties. Release times can be further tuned by covering the containers with additional polymer layers, creating a flexible system with promising perspectives for pharmaceutical applications.

Thermal transitions in dry and hydrated layer-by-layer assemblies exhibiting linear and exponential growth

Layer-by-layer (LbL) assemblies are remarkable materials, known for their tunable mechanical, optical, and surface properties in nanoscale films. However, questions related to their thermal properties still remain unclear. Here, the thermal properties of a model LbL assembly of strong polyelectrolytes, poly(diallyldimethylammonium chloride)/poly(styrene sulfonate) (PDAC/PSS), assembled from solutions of varying ionic strength (0-1.25 M NaCl) are investigated using quartz crystal microbalance with dissipation (QCM-D) and modulated differential scanning calorimetry. Hydrated exponentially growing films (assembled from 0.25 to 1.25 M NaCl) exhibited distinct thermal transitions akin to a glass transition at 49-56 °C; linearly growing films (assembled without added salt) did not exhibit a transition in the temperature range investigated and were glassy. Results support the idea that exponentially growing films have greater segmental mobility than that of linearly growing films. On the other hand, all dry LbL assemblies investigated were glassy at room temperature and did not exhibit a T(g) up to 250 °C, independent of ionic strength. For the first time, thermal transitions such as T(g) values can be measured for LbL assemblies using QCM-D by monitoring fluctuations in changes in dissipation, allowing us to probe the film's internal structure as a function of film depth.

Probing aggregation and fibril formation of insulin in polyelectrolyte multilayers

Ultrathin films are useful for coating materials and controlling drug delivery processes. Here, we explore the use of polyelectrolyte multilayers as templates for the formation of two-dimensional protein networks, which represent biocompatible and biodegradable ultrathin films. In a first step, we have studied the lateral aggregation and amyloid fibril formation of bovine insulin that is adsorbed at and confined within planar polyelectrolyte multilayers, assembled with poly(diallyldimethylammonium chloride) (PDDA), poly(styrenesulfonic acid) (PSS), and hyaluronic acid (HA). Si-PDDA-PSS-(insulin-PSS)(x) and Si-PDDA-PSS-(insulin-HA)(x) multilayers (x=1-4) have been prepared and characterized in the fully hydrated state by using X-ray reflectometry, attenuated total reflection-Fourier transform infrared spectroscopy and confocal fluorescence microscopy. The obtained data demonstrate a successful build-up of the insulin-polyelectrolyte multilayers on silicon wafers that grow strongly in thickness upon insulin adsorption on PSS and HA layers. The secondary structure analysis of insulin, based on the vibrational amide I'-band, indicates an enhanced intermolecular β-sheet formation within the multilayers at 70°C and pD=2, i.e. at conditions that promote insulin amyloid fibrils rich in β-sheet contents. However, insulin that is confined between two polyelectrolyte layers rather forms amorphous aggregates as can be inferred from confocal fluorescence images. Remarkably, when insulin is deposited as the top-layer, a partial conversion into a two-dimensional fibrillar network can be induced by adding amyloid seeds to the solution. Thus, the results of this study illustrate the capability of polyelectrolyte multilayers as templates for the growth of protein networks.

Effect of thickness on the thermal properties of hydrogen-bonded LbL assemblies

Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 μm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.

Polymeric multilayer capsules delivering biotherapeutics

Polymeric multilayer capsules have emerged as a novel drug delivery platform. These capsules are fabricated through layer-by-layer sequential deposition of polymers onto a sacrificial core template followed by the decomposition of this core yielding hollow capsules. The resulting nanometer thin membrane is permselective, allowing diffusion of water and ions but excluding larger molecules. Moreover, the sequential fabrication procedure allows a precise fine-tuning of the capsules' physicochemical and biological properties. These properties have put polymeric multilayer capsules under major attention in the field of drug delivery. In this review we focus on polymeric multilayer capsule mediated delivery of biotechnological macromolecular drugs such as peptides, proteins and nucleic acids.

Stimuli-responsive LbL capsules and nanoshells for drug delivery

Review of basic principles and recent developments in the area of stimuli responsive polymeric capsules and nanoshells formed via layer-by-layer (LbL) is presented. The most essential attributes of the LbL approach are multifunctionality and responsiveness to a multitude of stimuli. The stimuli can be logically divided into three categories: physical (light, electric, magnetic, ultrasound, mechanical, and temperature), chemical (pH, ionic strength, solvent, and electrochemical) and biological (enzymes and receptors). Using these stimuli, numerous functionalities of nanoshells have been demonstrated: encapsulation, release including that inside living cells or in tissue, sensors, enzymatic reactions, enhancement of mechanical properties, and fusion. This review describes mechanisms and basic principles of stimuli effects, describes progress in the area, and gives an outlook on emerging trends such as theranostics and nanomedicine.

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.

Smart nanocontainers as depot media for feedback active coatings

Among the grand challenges at present are ways to develop systems with low consumption of raw materials and with little load on the environment. In view of this it is of utmost importance to avoid or to delay processes causing material destruction. This is especially urgent, since many protective substances have associated health hazards, and new routes to improve the situation are a main concern of this contribution. Nanocapsules (nanocontainers) with controlled release properties of the shell can be used to fabricate a new family of active coatings, with quick response to changes of the coating environment or coating integrity. The release of active materials encapsulated into nanocapsules is triggered by various external and internal factors, thus preventing spontaneous leakage of the active component. The coating can have several active functionalities when several types of nanocapsules loaded with corresponding active agent are incorporated simultaneously into a coating matrix. We highlight recent achievements in development and application of filled responsive containers in biomedical and self-healing protective coatings.

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.

Polymeric microcapsules with light responsive properties for encapsulation and release

This review is dedicated to recent developments on the topic of light sensitive polymer-based microcapsules. The microcapsules discussed are constructed using the layer-by-layer self-assembly method, which consists in absorbing oppositely charged polyelectrolytes onto charged sacrificial particles. Microcapsules display a broad spectrum of qualities over other existing microdelivery systems such as high stability, longevity, versatile construction and a variety of methods to encapsulate and release substances. Release and encapsulation of materials by light is a particularly interesting topic. Microcapsules can be made sensitive to light by incorporation of light sensitive polymers, functional dyes and metal nanoparticles. Optically active substances can be inserted into the shell during their assembly as a polymer complex or following the shell preparation. Ultraviolet-addressable microcapsules were shown to allow for remote encapsulation and release of materials. Visible- and infrared- addressable microcapsules offer a large array of release strategies for capsules, from destructive to highly sensitive reversible approaches. Besides the Introduction and Conclusions, this review contains in four sections reviewing the effects of light 1) on polymer-based microcapsules, 2) microcapsules containing metal nanoparticles and 3) functional dyes, as well as a fourth section that revisits the implications of light addressable polymeric microcapsules as a microdelivery system for biological applications.

Vesicle-templated layer-by-layer assembly for the production of nanocapsules

Hollow structures on the submicrometer scale (nm) are obtained with the assembly of polyelectrolytes according to the layer-by-layer (LbL) technique. Following the LbL procedure, polymers alginate and chitosan were alternatively adsorbed on a vesicular template made of didodecyldimethylammonium bromide (DDAB). Evidence for the removal of the vesicular template entrapped in the alginate/chitosan film is presented. The removal of the vesicular template was achieved through interactions between a nonionic surfactant (Triton X100) and the double-chained surfactant forming the vesicles. The application of this approach allowed the production of hollow nanospheres with a mild procedure, avoiding the use of strong acids or other extreme working conditions that can modify the shell integrity. The obtained nanostructures were characterized by means of dynamic light scattering (DLS), the zeta potential, and scanning electron microscopy (SEM). The SEM analysis demonstrated the presence, after the core removal process, of nanocapsules indistinguishable in size and shape from the parent core-shell system. The analysis of the surface charge of the hollow nanocapsules, after the core dissolution, by zeta potential measurements, indicates good aggregate stability. DLS measurements showed that the size of the nanocapsules is on the order of hundreds of nanometers. Moreover, the size of both the core-shell and the hollow particles did not appear to be perturbed by variations in temperature or ionic strength.

Biocompatible protein nanocontainers for controlled drugs release

We designed a biocompatible carrier for controlled release of hydrophobic drugs. The designed carrier was prepared by sonicating oil in a protein aqueous solution forming a protein nanocontainer composed of an inner gel core and an outer protein shell. Two model drugs were loaded into the designed nanocontainers by dissolving drugs in the oil phase before sonication. The loading capacity was up to 0.9 mg/mL for the amphiphilic drug rifampicin, while it reached to 19 mg/mL for the hydrophobic drug indomethacin. The encapsulated drugs were released at different temperatures. At 37 degrees C, only less than 20% of the drug was released due to the protection by the gel core. Increasing temperature to 40 degrees C led to a completely release of the remaining drug. The drug release showed drastic temperature dependence. The biocompatibility of the protein nanocontainers was evaluated by incubating the nanocontainers in the 3T3 cell and B-LCL cell lines. Both experiments indicated an excellent biocompatibility of the designed nanocontainers.

Neutron reflectometry study of swelling of polyelectrolyte multilayers in water vapors: influence of charge density of the polycation

We studied the swelling of polyelectrolyte (PE) multilayers (PEM) in water (H2O) vapors. The PEM were made from polyanion poly(styrene sulfonate) (PSS) and polycation poly(diallyldimethylammonium chloride)-N-methyl-N-vinylacetamide (pDADMAC-NMVA). While PSS is a fully charged polyanion, pDADMAC-NMVA is a random copolymer made of charged pDADMAC and uncharged NMVA monomer units. Variation of the relative amount of these two units allows for controlling the charge density of pDADMAC-NMVA. The degree of swelling was studied as a function of the relative humidity in the experimental chamber (respectively water concentration in the gas phase) for PEM prepared from PSS and pDADMAC-NMVA with their different charge densities--100%, 89% and 75%. The films were prepared by means of spraying technique and consisted of six PE couples-PSS/pDADMAC-NMVA. Neutron reflectometry was applied as main tool to observe the swelling process. The technique allows to obtain in a single experiment information about film thickness and amount of water in the film. The experiments were complemented with AFM measurements to obtain the thickness of the films. It was found that the film thickness increases when the charge density of the polycation decreases. The swelling of the PEM increases with the relative humidity and it depends on the charge density of pDADMAC-NMVA. The swelling behavior is 2-fold, splitting up in a charge dependent mode with relatively little volume increase, and a second mode with high volume expansion, which is independent from charge density of PEM. The "swelling transition" occurs for all samples at a relative humidity about 60% and a volume increase of ca. 20%. The results were interpreted according to the Flory-Huggins theory which assumes a phase separation in PEM network at higher water contents.

Ordering of Fe(3)O(4) nanoparticles in polyelectrolyte multilayer films

In our work we have focused on the incorporation of magnetite nanoparticles (NPs) into poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) polyelectrolyte multilayers (PEMs). The main goal of presented studies was to control the two-dimentional ordering of NPs within polyelectrolyte films. The ordering of NPs depended on the treatment of the underlying polyelectrolyte films. The NPs were uniformly distributed in freshly prepared samples leading to an interfacial mixture of polyelectrolytes and particles, while a highly concentrated layer of NP was formed only when the PEMs were exposed to elevated temperature after their preparation. The observed effect was correlated to glass-melt phase transitions of the PEMs. Such ordering of functionalized species in a polymer matrix may enhance the response from the studied nanocomposites.

Variation of ion-exchange capacity, zeta potential, and ion-transport selectivities with the number of layers in a multilayer polyelectrolyte film

The properties of poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films vary dramatically with the number of polyelectrolyte layers deposited. Attenuated total reflectance infrared spectroscopy of (PDADMAC/PSS)n films deposited on a Ge crystal shows that coatings with fewer than 7 PDADMAC/PSS bilayers do not absorb significant amounts of SCN- or Ni(CN)4(2-) but coatings with more than 7 bilayers exhibit an ion-exchange capacity of about 0.5 mol/L of film. Consistent with ion-exchange, Ni(CN)4(2-) is the anion that is predominantly absorbed from equimolar mixtures of SCN- and Ni(CN)4(2-), even though SCN- initially exchanges into the film more rapidly than Ni(CN)4(2-). For silicon-supported PSS/PDADMAC films terminated with PSS, zeta potentials change from negative to positive as the number of adsorbed bilayers increases, presumably because of a high number of anion-exchange sites inside the film. These changes in film properties dramatically affect ion transport through (PSS/PDADMAC)nPSS-coated alumina membranes. The Cl-/SO4(2-) selectivities of these membranes are >30 with (PSS/PDADMAC)4PSS films but only 3 with (PSS/PDADMAC)6PSS films. Trends in zeta potentials and selectivities with increasing numbers of bilayers are consistent with the exponential growth mechanism, where a polycation absorbs throughout the film to create large numbers of anion-exchange sites, and during polyanion deposition, some of the polycation diffuses to the surface of the film to complex with polyanions from solution. Apparently, not all of the charge on the polycation is compensated by the polyanion; therefore, anion-exchange sites remain in the film, and the presence of this positive charge yields decreased Cl-/SO4(2-) selectivity.

Interaction of IAPP and insulin with model interfaces studied using neutron reflectometry

The islet amyloid polypeptide (IAPP) and insulin are coproduced by the beta-cells of the pancreatic islets of Langerhans. Both peptides can interact with negatively charged lipid membranes. The positively charged islet amyloid polypeptide partially inserts into these membranes and subsequently forms amyloid fibrils. The amyloid fibril formation of insulin is also accelerated by the presence of negatively charged lipids, although insulin has a negative net charge at neutral pH-values. We used water-polymer model interfaces to differentiate between the hydrophobic and electrostatic interactions that can drive these peptides to adsorb at an interface. By applying neutron reflectometry, the scattering-length density profiles of IAPP and insulin, as adsorbed at three different water-polymer interfaces, were determined. The islet amyloid polypeptide most strongly adsorbed at a hydrophobic poly-(styrene) surface, whereas at a hydrophilic, negatively charged poly-(styrene sulfonate) interface, the degree of adsorption was reduced by 50%. Almost no IAPP adsorption was evident at this negatively charged interface when we added 100 mM NaCl. On the other hand, negatively charged insulin was most strongly attracted to a hydrophilic, negatively charged interface. Our results suggest that IAPP is strongly attracted to a hydrophobic surface, whereas the few positive charges of IAPP cannot warrant a permanent immobilization of IAPP at a hydrophilic, negatively charged surface at an ionic strength of 100 mM. Furthermore, the interfacial accumulation of insulin at a hydrophilic, negatively charged surface may represent a favorable precondition for nucleus formation and fibril formation.

Adhesive interaction between polyelectrolyte multilayers of polyallylamine hydrochloride and polyacrylic acid studied using atomic force microscopy and surface force apparatus

In the present work, the adhesion between substrates treated with identical polyelectrolyte multilayers (PEM) from polyallylamine hydrochloride (PAH) and poly(acrylic acid) (PAA) was studied using atomic force microscopy (AFM) and the surface force apparatus (SFA). The AFM measurements, conducted under wet conditions for PEMs formed at pH 7.5, showed a higher adhesion (pull-off force) when PAH was adsorbed in the outermost layers. There was also a difference depending on the molecular mass of the polymers, demonstrating a greater adhesion for the low molecular mass combination of polyelectrolytes. Furthermore, the time in contact showed to be of importance, with increasing pull-off forces with contact time at maximum load. The SFA measurements were conducted under dry conditions, at 100% RH, and under wet conditions for PEMs adsorbed at pH 7.5/3.5. The SFA adhesion measurements showed that under dry conditions, the adhesive forces between two high energetic mica substrates were lowered when they were covered by PEMs before the measurements. The thickness of the adsorbed layers was also measured using SFA. This showed that there was a significant swelling when the dry layers were exposed to 100% RH or to wet conditions. The swelling was higher, indicating a less rigid layer, when PAH was adsorbed in the outermost layer than when the PEM was capped with PAA.

Toward self-assembly of nanoparticles on polymeric microshells: near-IR release and permeability

We present a novel approach to construct hollow polymeric microcontainers that can be remotely addressed using a low-power near-infrared laser to release encapsulated materials. Microshells possessing walls with aggregates of gold nanoparticles are found to release encapsulated materials upon near-IR irradiation, while shells containing the same amount of nonaggregated gold nanoparticles did not release their contents. The permeability of thermally shrunk microcapsules to dextran molecules is the lowest for shells containing nonaggregated nanoparticles and the highest for microcapsules with no nanoparticles. The wall thickness, roughness, influence of concentration of encapsulated materials, and general shrinking behavior of the microcapsules are studied. Aggregation of nanoparticles increases the absorption coefficient in the near-infrared part of electromagnetic spectrum. The temperature increase upon near-infrared laser illumination for different gold nanoparticle distributions is simulated. Important implications of this approach are expected in development of drug delivery systems as well as in temperature- and light-sensitive materials and membranes.

Dry tin dioxide hollow microshells and extreme ultraviolet radiation induced by CO2 laser illumination

Low-density tin dioxide (SnO2) is required for radiating monochromatic extreme ultraviolet (EUV) light with low debris and high conversion efficiency from a laser. In this paper, tin dioxide nanoparticle hollow microcapsules were successfully fabricated by a layer-by-layer template technique. The obtained capsules have a rougher surface (30 nm in rms) compared to the freshly prepared polyelectrolyte capsules. Based on the X-ray diffraction (XRD) results, the tin dioxide nanoparticles well maintained their size after they were assembled on the capsules' surfaces. In order to remove the polymer template, a heat treatment was introduced, and after the heat treatment the capsule sizes shrank about 71% (the average size was from 4.9 to 3.5 mum), and the obtained capsules maintained their round shape after water evaporation. The narrowest bandwidth at the 13.5 nm emission in the EUV region was observed when the capsules were irradiated by a CO2 laser with an intensity of 2.9 x 10(10) W/cm (2).

Hydrothermal-induced structure transformation of polyelectrolyte multilayers: from nanotubes to capsules

The assembled polyelectrolyte nanotubes composed of poly(styrenesulfonate) and poly(allylamine hydrochloride) multilayers by using the layer-by-layer assembly combined with the porous template method can be transformed into capsules by a high-temperature treatment. Scanning electron microscopy and confocal laser scanning microscopy images revealed the whole transition process. The structure transformation of polyelectrolyte multilayers after annealing can be initiated by the input of thermal energy which leads to a breakage of ion pairs between oppositely charged polyelectrolyte groups. The transition process from tubes to capsules is supposed to be driven by the Raleigh instability and leads to the generated polyelectrolyte capsules with different sizes.