Modeling the Growth Processes of Polyelectrolyte Multilayers Using a Quartz Crystal Resonator

Design of Tailor-Made Biopolymer-Based Capsules for Biological Application by Combining Porous Particles and Polysaccharide Assembly

Various approaches have been described in the literature to demonstrate the possibility of designing biopolymer particles with well-defined characteristics, such as size, chemical composition or mechanical properties. From a biological point of view, the properties of particle have been related to their biodistribution and bioavailability. Among the reported core-shell nanoparticles, biopolymer-based capsules can be used as a versatile platform for drug delivery purposes. Among the known biopolymers, the present review focuses on polysaccharide-based capsules. We only report on biopolyelectrolyte capsules fabricated by combining porous particles as a template and using the layer-by-layer technique. The review focuses on the major steps of the capsule design, i.e., the fabrication and subsequent use of the sacrificial porous template, multilayer coating with polysaccharides, the removal of the porous template to obtain the capsules, capsule characterisation and the application of capsules in the biomedical field. In the last part, selected examples are presented to evidence the major benefits of using polysaccharide-based capsules for biological purposes.

Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)

The response of the quartz crystal microbalance (QCM, also: QCM-D for "QCM with Dissipation monitoring") to loading with a diverse set of samples is reviewed in a consistent frame. After a brief introduction to the advanced QCMs, the governing equation (the small-load approximation) is derived. Planar films and adsorbates are modeled based on the acoustic multilayer formalism. In liquid environments, viscoelastic spectroscopy and high-frequency rheology are possible, even on layers with a thickness in the monolayer range. For particulate samples, the contact stiffness can be derived. Because the stress at the contact is large, the force is not always proportional to the displacement. Nonlinear effects are observed, leading to a dependence of the resonance frequency and the resonance bandwidth on the amplitude of oscillation. Partial slip, in particular, can be studied in detail. Advanced topics include structured samples and the extension of the small-load approximation to its tensorial version.

Topology Challenge for the Assessment of Living Cell Deposits with Shear Bulk Acoustic Biosensor

Shear bulk acoustic type of resonant biosensors, such as the quartz crystal microbalance (QCM), give access to label-free in-liquid analysis of surface interactions. The general understanding of the sensing principles was inherited from past developments in biofilms measurements and applied to cells while keeping the same basic assumptions. Thus, the biosensor readouts are still quite often described using 'mass' related terminology. This contribution aims to show that assessment of cell deposits with acoustic biosensors requires a deep understanding of the sensor transduction mechanism. More specifically, the cell deposits should be considered as a structured viscoelastic load and the sensor response depends on both material and topological parameters of the deposits. This shifts the paradigm of acoustic biosensor away from the classical mass loading perspective. As a proof of the concept, we recorded QCM frequency shifts caused by blood platelet deposits on a collagen surface under different rheological conditions and observed the final deposit shape with atomic force microscopy (AFM). The results vividly demonstrate that the frequency shift is highly impacted by the platelet topology on the bio-interface. We support our findings with numerical simulations of viscoelastic unstructured and structured loads in liquid. Both experimental and theoretical studies underline the complexity behind the frequency shift interpretation when acoustic biosensing is used with cell deposits.

Highly selective sensor for the detection of Hg2+ ions using homocysteine functionalised quartz crystal microbalance with cross-linked pyridinedicarboxylic acid

This study reports an insightful portable vector network analyser (VNA)-based measurement technique for quick and selective detection of Hg2+ ions in nanomolar (nM) range using homocysteine (HCys)-functionalised quartz-crystal-microbalance (QCM) with cross-linked-pyridinedicarboxylic acid (PDCA). The excessive exposure to mercury can cause damage to many human organs, such as the brain, lungs, stomach, and kidneys, etc. Hence, the authors have proposed a portable experimental platform capable of achieving the detection in 20-30 min with a limit of detection (LOD) 0.1 ppb (0.498 nM) and a better dynamic range (0.498 nM-6.74 mM), which perfectly describes its excellent performance over other reported techniques. The detection time for various laboratory-based techniques is generally 12-24 h. The proposed method used the benefits of thin-film, nanoparticles (NPs), and QCM-based technology to overcome the limitation of NPs-based technique and have LOD of 0.1 ppb (0.1 μg/l) for selective Hg2+ ions detection which is many times less than the World Health Organization limit of 6 μg/l. The main advantage of the proposed QCM-based platform is its portability, excellent repeatability, millilitre sample volume requirement, and easy process flow, which makes it suitable as an early warning system for selective detection of mercury ions without any costly measuring instruments.

Investigations of the high-frequency dynamic properties of polymeric systems with quartz crystal resonators

Opportunities arising from the use of the rheometric quartz crystal microbalance (RheoQCM) as a fixed frequency rheometer operating at 15 MHz are discussed. The technique requires the use of films in a specified thickness range that depends on the mechanical properties of the material of interest. A regime map quantifying the appropriate thicknesses is developed, based on the properties of a highly crosslinked epoxy sample that is representative of a broad class of polymeric materials. Relative errors in the measured film properties are typically in the range of several percent or less and are minimized by using a power law model to relate the rheological properties at two different resonant harmonics of the quartz crystal. Application of the RheoQCM technique is illustrated by measuring the temperature- and molecular weight-dependent properties of polystyrene and poly(methyl methacrylate) in the vicinity of the glass transition.

Oxidative Layer-By-Layer Multilayers Based on Metal Coordination: Influence of Intervening Graphene Oxide Layers

Layer-by-layer (LbL) fabricated oxidative multilayers consisting of successive layers of inorganic polyphosphate (PP) and Ce(IV) can electrolessly form thin conducting polymer films on their surface. We describe the effect of substituting every second PP layer in the (PP/Ce) multilayers for graphene oxide (GO) as a means of modifying the structure and mechanical properties of these (GO/Ce/PP/Ce) films and enhancing their growth. Both types of LbL films are based on reversible coordinative bonding between the metal ions and the oxygen-bearing groups in PP and GO, instead of purely electrostatic interactions. The GO incorporation leads to the doubling of the areal mass density and to a dry film thickness close to 300 nm after 4 (GO/Ce/PP/Ce) tetralayers. The film roughness increases significantly with thickness. The (PP/Ce) films are soft materials with approximately equal shear storage and loss moduli, but the incorporation of GO doubles the storage modulus. PP displays a marked terminating layer effect and practically eliminates mechanical losses, making the (GO/Ce/PP/Ce) films almost pure soft elastomers. The smoothness of the (PP/Ce) films and the PP-termination effects are attributed to the reversible coordinative bonding. The (GO/Ce/PP/Ce) films oxidize pyrrole and 3,4-ethylenedioxythiophene (EDOT) and form polypyrrole and PEDOT films on their surfaces. These polymer films are considerably thicker than those formed using the (PP/Ce) multilayers with the same nominal amount of cerium layers. The GO sheets interfere with the polymerization reaction and make its kinetics biphasic. The (GO/Ce) multilayers without PP are brittle and thin.

pH-Controlled Electrochemical Deposition of Polyelectrolyte Complex Films

Polyelectrolyte complex (PEC) films made from oppositely charged polymer chains have applications as drug-delivery vehicles, separation membranes, and biocompatible coatings. Conventional layer-by-layer techniques for polyelectrolyte coatings are low-throughput and multistep processes that are quite slow for building films on the order of micrometers. In this work, PEC films are electrochemically deposited using a rapid one-pot method, yielding thick (1 μm) films within short experimental time scales (5 min). This rapid electrodeposition is achieved by exploiting the reduction of hydrogen peroxide at mild electrode potentials that avoid water electrolysis yet trigger the pH-responsive self-assembly of a PEC film composed of poly(acrylic) acid and poly(allylamine) HCl. In situ rheology using an electrochemical quartz crystal microbalance quantified the shear modulus-density product of the deposited layer to be on the order of 10⁷ Pa g/cm³ at a frequency of 15 MHz, with a viscoelastic phase angle of approximately 50°. This electrodeposition scheme furthers the development of PEC coatings for more high-throughput applications, where a fast and efficient single-step approach would be desirable for obtaining coatings.

3D solid supported inter-polyelectrolyte complexes obtained by the alternate deposition of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate)

This work addresses the formation and the internal morphology of polyelectrolyte layers obtained by the layer-by-layer method. A multimodal characterization showed the absence of stratification of the films formed by the alternate deposition of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate). Indeed the final organization might be regarded as three-dimensional solid-supported inter-polyelectrolyte films. The growth mechanism of the multilayers, followed using a quartz crystal microbalance, evidences two different growth trends, which show a dependency on the ionic strength due to its influence onto the polymer conformation. The hydration state does not modify the multilayer growth, but it contributes to the total adsorbed mass of the film. The water associated with the polyelectrolyte films leads to their swelling and plastification. The use of X-ray photoelectron spectroscopy has allowed for deeper insights on the internal structure and composition of the polyelectrolyte multilayers.

Layer-by-layer assembled oxidative films as general platform for electrodeless formation of conducting polymers

Fabrication of precisely tailored layers of conductive polymers in thin film assemblies is an attractive extension of the layer-by-layer technique. This approach provides tools for fabricating thin films with customized optical and electrical properties. In this paper, we study inorganic layer-by-layer assembled films prepared using polyphosphate and cerium(IV). It is shown that these multilayers can oxidize certain monomers from the adjacent aqueous solution to produce conducting polymer layers. We studied the thermodynamic factors that allow the aforementioned autopolymerization. A five bilayer polyphosphate/cerium(IV) film was shown to possess high oxidative power in acidic solutions. It was found that the polymerization of pyrrole, aniline and 3,4-ethylenedioxythiophene in contact with the redox active multilayer is thermodynamically favored. The rate of polymer formation and the thickness of the conducting film could be controlled by the concentration of the monomer in solution and the number of cerium/polyphosphate bilayers in the oxidative film. The oxidative polymerization of pyrrole was unambiguously recognized on UV-vis spectra with characteristic reduction and oxidation bands. The film formation was not restricted by charge diffusion and the reaction formally followed first-order kinetics. The results suggest that the reaction takes place effectively within the whole pre-existing polypyrrole film and it continues until all oxidant in the film was used. The spectral changes that are characteristic for conducting polypyrrole are shown on spectroelectrochemical analysis of the films indicating that cationic (polaron) and dicationic (bipolaron) species are involved in the redox processes of the film. The functional polymer films formed are found to be electroactive and conducting. Therefore, they fully resemble of conducting polymer films prepared using traditional electropolymerization.

pH-induced changes in the fabrication of multilayers of poly(acrylic acid) and chitosan: fabrication, properties, and tests as a drug storage and delivery system

Multilayers of poly(acrylic acid), PAA, and chitosan, CHI, have been built by the layer-by-layer (LbL) method from aqueous solutions at different pH values and analyzed by the dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results showed that under all of the assembly conditions considered the growth of the films is nonlinear. The thickness of the PAA layers increases as the pH of the assembling solutions decreases, whereas the adsorption of CHI is almost unaffected by the pH conditions. The comparison of the thickness obtained by D-QCM and by ellipsometry has allowed us to calculate the water content of the films, showing that the multilayers are highly hydrated, with an average water content higher than 20%. The analysis of D-QCM data has provided high-frequency values of the complex shear modulus that are in the megapascal range and shows a transition from mainly viscous to mainly elastic behavior for the added PAA layers, depending on the pH. The monomer surface density in each layer (obtained from the combination of ellipsometry and differential refractive index measurements) indicated that the monomer density depends on the assembly conditions. It was found that the adsorption kinetics is a bimodal process, with characteristic times that depend on the number and nature of the layers. Finally, the possibility of using of these multilayers as a drug storage and delivery system has been evaluated.

Nanofilm biomaterials: localized cross-linking to optimize mechanical rigidity and bioactivity

Nanofilm biomaterials, formed by the layer-by-layer assembly of charged macromolecules, are important systems for a variety of cell-contacting biomedical and biotechnological applications. Mechanical rigidity and bioactivity are two key film properties influencing the behavior of contacting cells. Increased rigidity tends to improve cells attachment, and films may be rendered bioactive through the incorporation of proteins, peptides, or drugs. A key challenge is to realize films that are simultaneously rigid and bioactive. Chemical cross-linking of the polymer framework--the standard means of increasing a film's rigidity--can diminish bioactivity through deactivation or isolation of embedded biomolecules or inhibition of film biodegradation. We present here a strategy to decouple mechanical rigidity and bioactivity, potentially enabling nanofilm biomaterials that are both mechanically rigid and bioactive. Our idea is to selectively cross-link the outer region of the film, resulting in a rigid outer skin to promote cell attachment, while leaving the film interior (with any embedded bioactive species) unaffected. We propose an approach whereby an N-hydroxysulfosuccinimide (sulfo-NHS) activated poly(L-glutamic acid) is added as the terminal layer of a multilayer film and forms (covalent) amide bonds with amino groups of poly(L-lysine) placed previously within the film. We characterize film assembly and cross-linking extent via quartz crystal microbalance with dissipation monitoring (QCMD), Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), and laser scanning confocal microscopy (LSCM) and measure the attachment and metabolic activity of preosteoblastic MC3T3-E1 cells. We show cross-linking to occur primarily at the film surface and the subsequent cell attachment and metabolic activity to be enhanced compared to native films. Our method appears promising as a means to realize films that are simultaneously mechanically rigid and bioactive.

Multiple functionalities of polyelectrolyte multilayer films: new biomedical applications

The design of advanced functional materials with nanometer- and micrometer-scale control over their properties is of considerable interest for both fundamental and applied studies because of the many potential applications for these materials in the fields of biomedical materials, tissue engineering, and regenerative medicine. The layer-by-layer deposition technique introduced in the early 1990s by Decher, Moehwald, and Lvov is a versatile technique, which has attracted an increasing number of researchers in recent years due to its wide range of advantages for biomedical applications: ease of preparation under "mild" conditions compatible with physiological media, capability of incorporating bioactive molecules, extra-cellular matrix components and biopolymers in the films, tunable mechanical properties, and spatio-temporal control over film organization. The last few years have seen a significant increase in reports exploring the possibilities offered by diffusing molecules into films to control their internal structures or design "reservoirs," as well as control their mechanical properties. Such properties, associated with the chemical properties of films, are particularly important for designing biomedical devices that contain bioactive molecules. In this review, we highlight recent work on designing and controlling film properties at the nanometer and micrometer scales with a view to developing new biomaterial coatings, tissue engineered constructs that could mimic in vivo cellular microenvironments, and stem cell "niches."

Investigation of layer-by-layer assembled heparin and chitosan multilayer films via electrochemical spectroscopy

The layer-by-layer (LbL) assembly as a simple and effective method has been extensively used to prepare polyelectrolyte films but the buildup mechanism is expected to be further clarified. In this work, the structure and formation mechanism of LbL-assembled heparin/chitosan multilayer composite films were characterized by electrochemical system, scanning electron microscope and atom force microscope. The results revealed that the film grew linearly in the first 10 bilayers based on measured linear increase of film resistance with number of layers, while the film grew exponentially in the later 10 bilayers based on measured nonlinear increase of film resistance. The charge-transfer resistance increased in an oscillatory way or a linear way at different growing stages, which was discussed with their formation mechanism and the interfacial structure on electrode. A buildup mode of the LbL film was suggested based on the structural and electrochemical characters.

Construction of Viscoelastic Biocompatible Films via the Layer-by-Layer Assembly of Hyaluronan and Phosphorylcholine-Modified Chitosan

Films of hyaluronan (HA) and a phosphorylcholine-modified chitosan (PC-CH) were constructed by the polyelectrolyte multilayer (PEM) deposition technique and their buildup in 0.15 M NaCl was followed by atomic force microscopy, surface plasmon resonance spectroscopy (SPR), and dissipative quartz crystal microbalance (QCM). The HA/PC-CH films were stable over a wide pH range (3.0-12.0), exhibiting a stronger resistance against alkaline conditions as compared to HA/CH films. The loss and storage moduli, G' and G", of the films throughout the growth of eight bilayer assemblies were derived from an impedance analysis of the QCM data recorded in situ. Both G' and G" values were one order of magnitude lower than the moduli of HA/CH films. The fluid gel-like characteristics of HA/PC-CH multilayers were attributed to their high water content (50 wt %), which was estimated by comparing the surface coverage values derived from SPR and QCM measurements. Given the versatility of the PEM methodology, HA/PC-CH films are attractive tools for developing biocompatible surface coatings of controlled mechanical properties.