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.
The stimuli‐responsive properties of films deposited in a layer‐by‐layer manner are reviewed herein in relation to their physicochemical properties. The ability of these films to grow either linearly or exponentially with the number of deposition steps as well as their charge compensation mode allows the fine‐tuning of their dynamic response to external stimuli, such as pH changes, changes in temperature, or mechanical stresses.
There exist two types of polyelectrolyte multilayers: those whose thickness increases linearly with the number of deposition steps, which are nicely structured, and those whose thickness increases ...exponentially, which resembles hydrogels. This simple picture has recently slightly evolved with the finding that some exponentially growing films enter into a linear growth phase after a certain number of deposition steps. In this study, we investigate the buildup process of hyaluronic acid/poly(l-lysine) (HA/PLL) multilayers that constitute one of the best known exponentially growing systems. The films are built by using two deposition methods: the well-known dipping method and the more recent spraying method where the polyelectrolyte solutions are sprayed alternately onto a vertical substrate. The goal of this study is twofold. First, we investigate the influence of the main parameters (i.e., spraying rate and spraying time) of the spraying method on the film growth process. We find that, as for the dipping method, the film thickness first evolves exponentially with the number of deposition steps, and after a given number of deposition steps, it follows a linear evolution. We find that similar behavior is observed with the dipping method. Second, because the spraying method allows the very fine variation of the different parameters of the buildup, we use this method to investigate the exponential-to-linear transition. We find that this transition always takes place after about 12 deposition steps whatever the values of the parameters controlling the deposition process. We discuss our results in light of a model proposed by Hübsch et al. (Hübsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980−1985) and later by Salomäki et al. (Salomäki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232−11240) in which it is assumed that the exponential-to-linear transition is due to a film restructuring that progressively forbids the diffusion of one of the polyelectrolytes constituting the film over part of the film. This “forbidden” zone then grows with the number of deposition steps so that the outer zone of the film that is still concerned with diffusion keeps a constant thickness and moves upward as the total film thickness increases.
Abstract
Layer-by-layer (LbL) deposition method of polyelectrolytes is a versatile way of developing functional nanoscale coatings. Even though the mechanisms of LbL film development are ...well-established, currently there are no predictive models that can link film components with their final properties. The current health crisis has shown the importance of accelerated development of biomedical solutions such as antiviral coatings, and the implementation of machine learning methodologies for coating development can enable achieving this. In this work, using literature data and newly generated experimental results, we first analyzed the relative impact of 23 coating parameters on the coating thickness. Next, a predictive model has been developed using aforementioned parameters and molecular descriptors of polymers from the DeepChem library. Model performance was limited because of insufficient number of data points in the training set, due to the scarce availability of data in the literature. Despite this limitation, we demonstrate, for the first time, utilization of machine learning for prediction of LbL coating properties. It can decrease the time necessary to obtain functional coating with desired properties, as well as decrease experimental costs and enable the fast first response to crisis situations (such as pandemics) where coatings can positively contribute. Besides coating thickness, which was selected as an output value in this study, machine learning approach can be potentially used to predict functional properties of multilayer coatings, e.g. biocompatibility, cell adhesive, antibacterial, antiviral or anti-inflammatory properties.
The design and control of molecular systems that self‐assemble spontaneously and exclusively at or near an interface represents a real scientific challenge. We present here a new concept, an active ...seed layer that allows to overcome this challenge. It is based on enzyme‐assisted self‐assembly. An enzyme, alkaline phosphatase, which transforms an original peptide, Fmoc‐FFY(PO42−), into an efficient gelation agent by dephosphorylation, is embedded in a polyelectrolyte multilayer and constitutes the “reaction motor”. A seed layer composed of a polyelectrolyte covalently modified by anchoring hydrogelator peptides constitutes the top of the multilayer. This layer is the nucleation site for the Fmoc‐FFY peptide self‐assembly. When such a film is brought in contact with a Fmoc‐FFY(PO42−) solution, a nanofiber network starts to form almost instantaneously which extents up to several micrometers into the solution after several hours. We demonstrate that the active seed layer allows convenient control over the self‐assembly kinetics and the geometric features of the fiber network simply by changing its peptide density.
Peptidic hydrogel assembly line: The growth of peptide‐based hydrogel exclusively from the surface can be realized and controlled by using a bioactive seed layer. Confined into the seed layer, an enzyme ensures the gradient production of self‐assembling peptides from the surface in a continuous and autonomous way (see scheme).
The aim of the present work was to assemble extracellular matrix components into polyelectrolyte multilayers using the layer-by-layer deposition method. The films are constructed with type-I collagen ...and hyaluronic acid. The construction exhibits the general features observed during polyelectrolyte multilayer buildup: alternate positive and negative values of the zeta potential of the film during its construction and regular increase of the film thickness with the number,
n, of deposition step. This increase is shown to be linear with
n. As expected for a linearly growing film, the confocal microscopy shows that when the film is brought in contact with a collagen solution, collagen does not diffuse into the film but interacts only with its outer layer. However, the films are not constituted of homogeneously distributed polyanion/polycation complexes as it is usually observed, but they are formed of fibers as imaged by AFM. The typical width of these fibers increases with the number of deposition steps. Finally, it is found that chondrosarcoma cells spread well and synthesize extracellular matrix components only on the collagen ending films, whereas no cellular matrix was found for HA ending ones. Such architectures may be further functionalized by inclusion of active drugs, peptides, proteins…, and could be used as tunable biomaterial interfaces.
Two types of polyelectrolyte multilayer films have been reported in the literature. These are (i) films whose mass and thickness increase linearly as the number of deposited bilayers increases and ...(ii) films that grow exponentially. We present a model for the buildup of exponentially growing films that allows a discussion of the behavior of them in a unified manner. This model is based on the diffusion both in and out the whole film of part of the chains of at least one of the polyelectrolytes constituting the multilayer. In short, the film is brought into contact with the solution of polyelectrolytes that are able to diffuse into the film. Inside of the film, chains of this polyelectrolyte constitute the “free” chains. At the subsequent rinsing step, some of them diffuse outward from the film. The remaining chains leave the film as it is brought into contact further with the polyelectrolyte solution of opposite charge. As the “free” chains reach the film/solution interface, they are complexed by the polyelectrolytes of opposite charge. These complexes, which are composed of both types of polyelectrolytes, contribute to the formation of the additional mass of the multilayer. The model relies on the evaluation of the electrostatic potential in the film within the framework of the Debye−Hückel approximation and takes into consideration the Donnan effect, which is due to noncompensated fixed charges in the film. It also includes the situation where none of the polyelectrolytes diffuse within the multilayer, in which case the film grows linearly. The model predicts the existence of a free-energy barrier that prevents total diffusion of any “free” polyelectrolyte outward from the film during a rinsing step, following contact with a polyelectrolyte solution. It also predicts that usually only one of the two polyelectrolytes that comprise the film diffuses readily into it. Both polyelectrolytes that comprise the film can diffuse “into” and “out of” the multilayer only when the concentration of noncompensated fixed charges within the film is very small. Several predictions of the model are discussed in the light of experimental results that have already been published or are new.
The design of stimuli-responsive polymer assemblies for the controlled release of bioactive molecules has raised considerable interest these two last decades. Herein, we report the design of ...mechanically responsive drug-releasing films made of polyelectrolyte multilayers. A layer-by-layer (LbL) reservoir containing biodegradable polyelectrolytes is capped with a mechanosensitive LbL barrier and responds to stretching by a total enzymatic degradation of the film. This strategy is successfully applied for the release in solution of an anticancer drug initially loaded within the architecture.
For many years it has been a major challenge to regenerate damaged tissues using synthetic or natural materials. To favor the healing processes after tendon, cornea, muscle, or brain injuries, ...aligned collagen-based architectures are of utmost interest. In this study, we define a novel aligned coating based on a collagen/alginate (COL/ALG) multilayer film. The coating exhibiting a nanofibrillar structure is cross-linked with genipin for stability in physiological conditions. By stretching COL/ALG-coated polydimethylsiloxane substrates, we developed a versatile method to align the collagen fibrils of the polymeric coating. Assays on cell morphology and alignment were performed to investigate the properties of these films. Microscopic assessments revealed that cells align with the stretched collagen fibrils of the coating. The degree of alignment is tuned by the stretching rate (i.e., the strain) of the COL/ALG-coated elastic substrate. Such coatings are of great interest for strategies that require aligned nanofibrillar biological material as a substrate for tissue engineering.
Actin cytoskeleton forms a physical connection between the extracellular matrix, adhesion complexes and nuclear architecture. Because tissue stiffness plays key roles in adhesion and cytoskeletal ...organization, an important open question concerns the influence of substrate elasticity on replication and transcription. To answer this major question, polyelectrolyte multilayer films were used as substrate models with apparent elastic moduli ranging from 0 to 500 kPa. The sequential relationship between Rac1, vinculin adhesion assembly, and replication becomes efficient at above 200 kPa because activation of Rac1 leads to vinculin assembly, actin fiber formation and, subsequently, to initiation of replication. An optimal window of elasticity (200 kPa) is required for activation of focal adhesion kinase through auto-phosphorylation of tyrosine 397. Transcription, including nuclear recruitment of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1), occurred above 50 kPa. Actin fiber and focal adhesion signaling are not required for transcription. Above 50 kPa, transcription was correlated with αv-integrin engagement together with histone H3 hyperacetylation and chromatin decondensation, allowing little cell spreading. By contrast, soft substrate (below 50 kPa) promoted morphological changes characteristic of apoptosis, including cell rounding, nucleus condensation, loss of focal adhesions and exposure of phosphatidylserine at the outer cell surface. On the basis of our data, we propose a selective and uncoupled contribution from the substrate elasticity to the regulation of replication and transcription activities for an epithelial cell model.
Polymers that “click”: A polymer film is obtained by the CuI‐catalyzed Sharpless click reaction between two polymers, bearing either azide or alkyne groups, both present simultaneously in a CuII ...solution (see picture). The CuI morphogen is generated at an electrode by applying an adequate potential. This concept can be extended to supramolecular films formed by coordination complexes.
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