Polyelectrolyte multilayers and degradable polymer layers as multicompartment films

Advancing bone tissue engineering one layer at a time: a layer-by-layer assembly approach to 3D bone scaffold materials

The layer-by-layer (LbL) assembly technique has shown excellent potential in tissue engineering applications. The technique is mainly based on electrostatic attraction and involves the sequential adsorption of oppositely charged electrolyte complexes onto a substrate, resulting in uniform single layers that can be rapidly deposited to form nanolayer films. LbL has attracted significant attention as a coating technique due to it being a convenient and affordable fabrication method capable of achieving a wide range of biomaterial coatings while keeping the main biofunctionality of the substrate materials. One promising application is the use of nanolayer films fabricated by LbL assembly in the development of 3-dimensional (3D) bone scaffolds for bone repair and regeneration. Due to their versatility, nanoscale films offer an exciting opportunity for tailoring surface and bulk property modification of implants for osseous defect therapies. This review article discusses the state of the art of the LbL assembly technique, and the properties and functions of LbL-assembled films for engineered bone scaffold application, combination of multilayers for multifunctional coatings and recent advancements in the application of LbL assembly in bone tissue engineering. The recent decade has seen tremendous advances in the promising developments of LbL film systems and their impact on cell interaction and tissue repair. A deep understanding of the cell behaviour and biomaterial interaction for the further development of new generations of LbL films for tissue engineering are the most important targets for biomaterial research in the field. While there is still much to learn about the biological and physicochemical interactions at the interface of nano-surface coated scaffolds and biological systems, we provide a conceptual review to further progress in the LbL approach to 3D bone scaffold materials and inform the future of LbL development in bone tissue engineering.

Endogenous FGF-2 levels impact FGF-2/BMP-2 growth factor delivery dosing in aged murine calvarial bone defects

Bone repair in elderly mice has been shown to be improved or negatively impacted by supplementing the highly osteogenic bone morphogenetic protein-2 (BMP-2) with fibroblast growth factor-2 (FGF-2). To better predict the outcome of FGF-2 supplementation, we investigated whether endogenous levels of FGF-2 play a role in optimal dosing of FGF-2 for augmenting BMP-2 activity in elderly mice. In vivo calvarial bone defect studies in Fgf2 knockout mice with wildtype controls were conducted with the growth factors delivered in a highly localized manner from a biomimetic calcium phosphate/polyelectrolyte multilayer coating applied to a bone graft substitute. Endogenous FGF-2 levels were measured in old mice versus young and found to decrease with age. Optimal dosing for improving bone defect repair correlated with levels of endogenous FGF-2, with a larger dose of FGF-2 required to have a positive effect on bone healing in the Fgf2 knockout mice. The same dose in wildtype old mice, with higher levels of FGF-2, promoted chondrogenesis and increased osteoclast activity. The results suggest a personalized medicine approach, based on a knowledge of endogenous levels of FGF-2, should guide FGF-2 supplementation in order to avoid provoking excessive bone resorption and cartilage formation, both of which inhibited calvarial bone repair.

Growth characteristics of human bone marrow mesenchymal stromal cells at cultivation on synthetic polyelectrolyte nanofilms in vitro

This study examines the adhesive properties and cytotoxicity of polyelectrolyte nanofilms from polyethyleneimine (PEI), polyallylamine hydrochloride (PAH) and sodium polystyrene sulfonate (PSS) on human bone marrow mesenchymal stromal cells (h-MSCs) and mouse adipose tissue (m-MSC) in vitro. Films are formed on 24- and 96-well culture plates in the combinations: PEI, PAH, PEI-PSS, PEI-PSS-PAH, PEI-PSS-PEI. An analysis of the culture results show that direct contact of h-MSCs with the PEI surface promotes adhesion (93–95% of adhesive cells versus 40% in the control). On the PEI surface, h-MSCs are evenly distributed, form colonies and 80% monolayer after 72 h of culture, as in the control on culture plastic. On nanofilms from PAH and PEI-PSS-PAH, cells grow in the form of rosette-like colonies with long and thin processes similar to neurites. The cytotoxic properties of PSS were revealed in direct contact with h-MSCs (more than 40% of nonviable cells with damaged plasma membranes). On the PSS surface, cells lost their adhesiveness. To culture and stably grow the cell mass of h-MSCs, it is better to use monolayer nanofilms made of highly adhesive and non-toxic PEI polyelectrolyte, which can bind the growth factors of blood serum and platelet lysate, ensuring the growth of h-MSCs under in vitro deprivation conditions.

Antibacterial Layer-by-Layer Coatings for Medical Implants

The widespread occurrence of nosocomial infections and the emergence of new bacterial strands calls for the development of antibacterial coatings with localized antibacterial action that are capable of facing the challenges posed by increasing bacterial resistance to antibiotics. The Layer-by-Layer (LbL) technique, based on the alternating assembly of oppositely charged polyelectrolytes, can be applied for the non-covalent modification of multiple substrates, including medical implants. Polyelectrolyte multilayers fabricated by the LbL technique have been extensively researched for the development of antibacterial coatings as they can be loaded with antibiotics, antibacterial peptides, nanoparticles with bactericide action, in addition to being capable of restricting adhesion of bacteria to surfaces. In this review, the different approaches that apply LbL for antibacterial coatings, emphasizing those that can be applied for implant modification are presented.

Recent Advances in Hybrid Biomimetic Polymer-Based Films: from Assembly to Applications

Biological membranes, in addition to being a cell boundary, can host a variety of proteins that are involved in different biological functions, including selective nutrient transport, signal transduction, inter- and intra-cellular communication, and cell-cell recognition. Due to their extreme complexity, there has been an increasing interest in developing model membrane systems of controlled properties based on combinations of polymers and different biomacromolecules, i.e., polymer-based hybrid films. In this review, we have highlighted recent advances in the development and applications of hybrid biomimetic planar systems based on different polymeric species. We have focused in particular on hybrid films based on (i) polyelectrolytes, (ii) polymer brushes, as well as (iii) tethers and cushions formed from synthetic polymers, and (iv) block copolymers and their combinations with biomacromolecules, such as lipids, proteins, enzymes, biopolymers, and chosen nanoparticles. In this respect, multiple approaches to the synthesis, characterization, and processing of such hybrid films have been presented. The review has further exemplified their bioengineering, biomedical, and environmental applications, in dependence on the composition and properties of the respective hybrids. We believed that this comprehensive review would be of interest to both the specialists in the field of biomimicry as well as persons entering the field.

Sustained delivery of growth factors with high loading efficiency in a layer by layer assembly

Layer by layer (LBL) assembly has garnered considerable interest due to its ability to generate multifunctional films with high tunability and versatility in terms of substrates and polyelectrolytes, allowing the option to use complex devices and drugs. Polyelectrolytes, such as growth factors (GFs), are essential, but costly, delicate, biological molecules that have been used in various tissue regeneration applications. For this reason, the controlled drug delivery of efficiently loaded GFs via LBL assembly (GF-LBL) can contribute to the establishment of cost-effective biologically triggered biomedical applications. We have developed an LBL method to load GFs (specifically, transforming growth factor beta 1, platelet-derived growth factor ββ, and insulin growth factor 1), with up to 90% efficiency approximately, by gas plasma surface activation and tuning the pH to increase the ionic strength of polyelectrolytes. Poly(styrenesulfonate) (PSS) and poly(ethyleneimine) (PEI) have been used to provide the initial necessary charge for multilayer build-up. Heparin and dextran sulphate have been investigated as counter polyelectrolytes to enhance the activity of GFs by protecting their ligands, where heparin resulted in the highest achievable loading efficiency for all GFs. Oxygen gas plasma and acidic pH levels also resulted in a significant increase in GF loading efficiency. The three GFs were released by diffusion and erosion in a controlled manner over lengthy time scales and the bioactivity was maintained for up to 14 days. When tested as implants in vitro, GF-LBL constructs increased fibroblast proliferation, influenced cell morphology and migration, and enhanced myofibroblast differentiation, indicating that the biological functionalities of the GFs were preserved. In conclusion, this developed LBL assembly method can provide a simple drug delivery system, which may yield more effective applications for tissue regeneration as well as biomedical sciences at large.

pH-Induced Changes in Polypeptide Conformation: Force-Field Comparison with Experimental Validation

Microsecond-long all-atom molecular dynamics (MD) simulations, circular dichroism, laser Doppler velocimetry, and dynamic light-scattering techniques have been used to investigate pH-induced changes in the secondary structure, charge, and conformation of poly l-lysine (PLL) and poly l-glutamic acid (PGA). The employed combination of the experimental methods reveals for both PLL and PGA a narrow pH range at which they are charged enough to form stable colloidal suspensions, maintaining their α-helix content above 60%; an elevated charge state of the peptides required for colloidal stability promotes the peptide solvation as a random coil. To obtain a more microscopic view on the conformations and to verify the modeling performance, peptide secondary structure and conformations rising in MD simulations are also examined using three different force fields, i.e., OPLS-AA, CHARMM27, and AMBER99SB*-ILDNP. Ramachandran plots reveal that in the examined setup the α-helix content is systematically overestimated in CHARMM27, while OPLS-AA overestimates the β-sheet fraction at lower ionization degrees. At high ionization degrees, the OPLS-AA force-field-predicted secondary structure fractions match the experimentally measured distribution most closely. However, the pH-induced changes in PLL and PGA secondary structure are reasonably captured only by the AMBER99SB*-ILDNP force field, with the exception of the fully charged PGA in which the α-helix content is overestimated. The comparison to simulations results shows that the examined force fields involve significant deviations in their predictions for charged homopolypeptides. The detailed mapping of secondary structure dependency on pH for the polypeptides, especially finding the stable colloidal α-helical regime for both examined peptides, has significant potential for practical applications of the charged homopolypeptides. The findings raise attention especially to the pH fine tuning as an underappreciated control factor in surface modification and self-assembly.

Biomimetic Extracellular Environment Based on Natural Origin Polyelectrolyte Multilayers

Surface modification of biomaterials is a well-known approach to enable an adequate biointerface between the implant and the surrounding tissue, dictating the initial acceptance or rejection of the implantable device. Since its discovery in early 1990s layer-by-layer (LbL) approaches have become a popular and attractive technique to functionalize the biomaterials surface and also engineering various types of objects such as capsules, hollow tubes, and freestanding membranes in a controllable and versatile manner. Such versatility enables the incorporation of different nanostructured building blocks, including natural biopolymers, which appear as promising biomimetic multilayered systems due to their similarity to human tissues. In this review, the potential of natural origin polymer-based multilayers is highlighted in hopes of a better understanding of the mechanisms behind its use as building blocks of LbL assembly. A deep overview on the recent progresses achieved in the design, fabrication, and applications of natural origin multilayered films is provided. Such films may lead to novel biomimetic approaches for various biomedical applications, such as tissue engineering, regenerative medicine, implantable devices, cell-based biosensors, diagnostic systems, and basic cell biology.

Coating Strategies Using Layer-by-layer Deposition for Cell Encapsulation

The layer-by-layer (LbL) deposition technique is widely used to develop multilayered films based on the directed assembly of complementary materials. In the last decade, thin multilayers prepared by LbL deposition have been applied in biological fields, namely, for cellular encapsulation, due to their versatile processing and tunable properties. Their use was suggested as an alternative approach to overcome the drawbacks of bulk hydrogels, for endocrine cells transplantation or tissue engineering approaches, as effective cytoprotective agents, or as a way to control cell division. Nanostructured multilayered materials are currently used in the nanomodification of the surfaces of single cells and cell aggregates, and are also suitable as coatings for cell-laden hydrogels or other biomaterials, which may later be transformed to highly permeable hollow capsules. In this Focus Review, we discuss the applications of LbL cell encapsulation in distinct fields, including cell therapy, regenerative medicine, and biotechnological applications. Insights regarding practical aspects required to employ LbL for cell encapsulation are also provided.

Ordered and kinetically discrete sequential protein release from biodegradable thin films

Multidrug regimens can sometimes treat recalcitrant diseases when single-drug therapies fail. Recapitulating complex multidrug administration from controlled release films for localized delivery remains challenging because their release kinetics are frequently intertwined, and an initial burst release of each drug is usually uncontrollable. Kinetic control over protein release is demonstrated by cross-linking layer-by-layer films during the assembly process. We used biodegradable and naturally derived components and relied on copper-free click chemistry for bioorthogonal covalent cross-links throughout the film that entrap but do not modify the embedded protein. We found that this strategy restricted the interdiffusion of protein while maintaining its activity. By depositing a barrier layer and a second protein-containing layer atop this construct, we generated well-defined sequential protein release with minimal overlap that follows their spatial distribution within the film.

Strong resistance of (tridecafluoro-1,1,2,2-tetrahydrooctyl)triethoxysilane (TTS) nanofilm to protein adsorption

In this report, the properties of fluorocarbon-containing (tridecafluoro-1,1,2,2-tetrahydrooctyl)triethoxysilane (TTS) (C14 H19 F13 O3 Si) nanofilm coated on silicon surface and its potential to resist protein adsorption were examined. Thickness and wettability of the silicon surface before and after TTS nanofilm coating were examined by ellipsometry and contact angle goniometry, respectively. The same techniques were used to examine protein layer on nonmodified and TTS-coated silicon surface. In addition, bright-field optical microscopy and fluorescence spectrophotometry were used to provide visual, qualitative description of adsorbed proteins and the specific signal of fluorescence-labeled bovine serum albumin (BSA), respectively, on bare and TTS-coated silicon surface. Single-component protein solution of four model proteins, namely BSA, human fibrinogen, bovine serum immunoglobulin G, and fibronectin, was prepared, and the adsorption responses of these four proteins on TTS nanofilm were examined, using nonmodified silicon surface as comparison. TTS substantially reduces the adsorption of all four proteins tested. Our results indicate that fluorocarbon-containing TTS, once coated on surfaces, is an effective molecule for resisting protein adsorption. This will open up potential applications, particularly for silicon-containing implant devices such as glass.

Micropatterned polyelectrolyte nanofilms promote alignment and myogenic differentiation of C2C12 cells in standard growth media

Alignment of skeletal myoblasts is considered a critical step during myotube formation. The C2C12 cell line is frequently used as a model of skeletal muscle differentiation that can be induced by lowering the serum concentration in standard culture flasks. In order to mimic the striated architectures of skeletal muscles in vitro, micro-patterning techniques and surface engineering have been proven as useful approaches for promoting elongation and alignment of C2C12 myoblasts, thereby enhancing the outgrowth of multi-nucleated myotubes upon switching from growth media (GM) to differentiative media (DM). Herein, a layer-by-layer (LbL) polyelectrolyte multilayer deposition was combined with a micro-molding in capillaries (MIMIC) method to simultaneously provide biochemical and geometrical instructive cues that induced the formation of tightly apposed and parallel arrays of differentiating myotubes from C2C12 cells maintained in GM media for 15 days. This study focuses on two different types of patterned/self-assembled nanofilms based on alternated layers of poly (allylamine hydrochloride) (PAH)/poly(sodium 4-styrene-sulfonate) (PSS) as biocompatible but not biodegradable polymeric structures, or poly-L-arginine sulfate salt (pARG)/dextran sulfate sodium salt (DXS) as both biocompatible and biodegradable surfaces. The influence of these microstructures as well as of the nanofilm composition on C2C12 skeletal muscle cells' differentiation and viability was evaluated and quantified, pointing to give a reference for skeletal muscle regenerative potential in culture conditions that do not promote it. At this regard, our results validate PEM microstructured devices, to a greater extent for (PAH/PSS)₅-coated microgrooves, as biocompatible and innovative tools for tissue engineering applications and molecular dissection of events controlling C2C12 skeletal muscle regeneration without switching to their optimal differentiative culture media in vitro.

Polyelectrolyte Multilayer Assemblies on Materials Surfaces: From Cell Adhesion to Tissue Engineering

Controlling the bulk and surface properties of materials is a real challenge for bioengineers working in the fields of biomaterials, tissue engineering and biophysics. The layer-by-layer (LbL) deposition method, introduced 20 years ago, consists in the alternate adsorption of polyelectrolytes that self-organize on the material's surface, leading to the formation of polyelectrolyte multilayer (PEM) films.1 Because of its simplicity and versatility, the procedure has led to considerable developments of biological applications within the past 5 years. In this review, we focus our attention on the design of PEM films as surface coatings for applications in the field of physical properties that have emerged as being key points in relation to biological processes. The numerous possibilities for adjusting the chemical, physical, and mechanical properties of PEM films have fostered studies on the influence of these parameters on cellular behaviors. Importantly, PEM have emerged as a powerful tool for the immobilization of biomolecules with preserved bioactivity.

Polyelectrolyte multilayers in tissue engineering

The layer-by-layer assembly of sequentially adsorbed, alternating polyelectrolytes has become increasingly important over the past two decades. The ease and versatility in assembling polyelectrolyte multilayers (PEMs) has resulted in numerous wide ranging applications of these materials. More recently, PEMs are being used in biological applications ranging from biomaterials, tissue engineering, regenerative medicine, and drug delivery. The ability to manipulate the chemical, physical, surface, and topographical properties of these multilayer architectures by simply changing the pH, ionic strength, thickness, and postassembly modifications render them highly suitable to probe the effects of external stimuli on cellular responsiveness. In the field of regenerative medicine, the ability to sequester growth factors and to tether peptides to PEMs has been exploited to direct the lineage of progenitor cells and to subsequently maintain a desired phenotype. Additional novel applications include the use of PEMs in the assembly of three-dimensional layered architectures and as coatings for individual cells to deliver tunable payloads of drugs or bioactive molecules. This review focuses on literature related to the modulation of chemical and physical properties of PEMs for tissue engineering applications and recent research efforts in maintaining and directing cellular phenotype in stem cell differentiation.

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.

Modular assembly of layer-by-layer capsules with tailored degradation profiles

Herein we report the preparation of layer-by-layer (LbL) assembled, biodegradable, covalently stabilized capsules with tunable degradation properties. Poly(L-glutamic acid) modified with alkyne moieties (PGA(Alk)) was alternately assembled with poly(N-vinyl pyrrolidone) (PVPON) on silica particles via hydrogen-bonding. The films were cross-linked with a bis-azide linker, followed by removal of the sacrificial template and PVPON at physiological pH through hydrogen bond disruption, yielding one-component PGA(Alk) capsules. To control the kinetics and location of capsule degradation, a number of approaches were investigated. First, a degradable bis-azide cross-linker was incorporated into the inherently enzymatically degradable capsules. Second, we assembled low-fouling capsules composed of nondegradable poly(N-vinyl pyrrolidone-ran-propargyl acrylate) (PVPON(Alk)) via hydrogen bonding with poly(methacrylic acid) (PMA) and combined this with the aforementioned system (PGA(Alk)/PVPON) to produce stratified hybrid capsules. The degradation profiles of these stratified capsules can be closely controlled by the number as well as the position of nondegradable barrier layers in the systems. The facile tailoring of the degradation kinetics makes this stratified LbL approach promising for the design of tailored drug-delivery vehicles.

Emerging trend in nano-engineered polyelectrolyte-based surrogate carriers for delivery of bioactives

IMPORTANCE OF THE FIELD

In recent decades a new colloidal drug delivery system based on layer-by-layer (LbL) technology has emerged, which offers promising means of delivering bioactive agents, specifically biological macromolecules including peptides and DNA. Nano-engineered capsules specifically fabricated from biocompatible and biodegradable polyelectrolytes (PEs) can provide a better option for encapsulation of cells thereby protecting cells from immunological molecules in the body, and their selective permeability can ensure the survival of encapsulated cells.

AREAS COVERED IN THIS REVIEW

This review encompasses a strategic approach to fabricate nano-engineered microcapsules through meticulous selection of polyelectrolytes and core materials based on LbL technology. The content of the article provides evidence for its wide array of applications in medical therapeutics, as indicated by the quantity of research and patents in this area. Recent developments and approaches for tuning drug release, biocompatibility and cellular interaction are discussed thoroughly.

WHAT THE READER WILL GAIN

This review aims to provide an overview on the development of LbL capsules with specific orientation towards drug and macromolecular delivery and its integration with other drug delivery systems, such as liposomes.

TAKE HOME MESSAGE

Selection of PEs for the fabrication of LbL microcapsules has a profound effect on stability, drug release, biocompatibility and encapsulation efficacy. The release can be easily modulated by varying different physicochemical as well as physiological conditions. Scale-up approaches for the fabrication of LbL microcapsules by means of automation must be considered to improve the possibility of application of LbL microcapsules on a large scale.

Low-fouling, biofunctionalized, and biodegradable click capsules

We report the synthesis of covalently stabilized hollow capsules from biodegradable materials using a combination of click chemistry and layer-by-layer (LbL) assembly. The biodegradable polymers poly(L-lysine) (PLL) and poly(L-glutamic acid) (PGA) were modified with alkyne and azide moieties. Linear film buildup was observed for both materials on planar surfaces and colloidal silica templates. A variation of the assembly conditions, such as an increase in the salt concentration and variations in pH, was shown to increase the individual layer thickness by almost 200%. The biodegradable click capsules were analyzed with optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Capsules were uniform in size and had a regular, spherical shape. They were found to be stable between pH 2 and 11 and showed reversible, pH-responsive shrinking/swelling behavior. We also show that covalently stabilized PLL films can be postfunctionalized by depositing a monolayer of heterobifunctional poly(ethylene glycol) (PEG), which provides low-fouling properties and simultaneously enhances specific protein binding. The responsive, biodegradable click films reported herein are promising for a range of applications in the biomedical field.

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."

Poly(L-lysine) nanostructured particles for gene delivery and hormone stimulation

In this work, we designed replica particles based on poly (L-lysine) (PLL) polymers crosslinked via a homobifunctional linker to support coadsorption of a plasmid DNA and a peptide hormone for concurrent transfection and induction of a cellular function. PLL replica particles (PLL(RP)) were prepared by infiltrating polymer into mesoporous silica (MS) particles, crosslinking the adsorbed chains by using a homobifunctional crosslinker and finally removing the template particles. Moreover, we verified their cytotoxicity. Furthermore, based on this PLL(RP) gene delivery system, we simultaneously evaluated the melanin stimulation and gene expression in these cells by fluorescence microscopy. To further understand the bi-functionality, we labeled the SPT7pTL and PGA-alpha-MSH with YOYO-1 and Rhodamine, respectively, to follow its intracellular pathway by confocal microscopy. Our data suggests that the PLL(RP) is a promising vector for gene therapy and hormone stimulation.

Nanovehicular intracellular delivery systems

This article provides an overview of principles and barriers relevant to intracellular drug and gene transport, accumulation and retention (collectively called as drug delivery) by means of nanovehicles (NV). The aim is to deliver a cargo to a particular intracellular site, if possible, to exert a local action. Some of the principles discussed in this article apply to noncolloidal drugs that are not permeable to the plasma membrane or to the blood-brain barrier. NV are defined as a wide range of nanosized particles leading to colloidal objects which are capable of entering cells and tissues and delivering a cargo intracelullarly. Different localization and targeting means are discussed. Limited discussion on pharmacokinetics and pharmacodynamics is also presented. NVs are contrasted to micro-delivery and current nanotechnologies which are already in commercial use. Newer developments in NV technologies are outlined and future applications are stressed. We also briefly review the existing modeling tools and approaches to quantitatively describe the behavior of targeted NV within the vascular and tumor compartments, an area of particular importance. While we list "elementary" phenomena related to different level of complexity of delivery to cancer, we also stress importance of multi-scale modeling and bottom-up systems biology approach.

Transfection ability and intracellular DNA pathway of nanostructured gene-delivery systems

Considerable efforts have been devoted to the design of structured materials with functional properties. Polyelectrolyte multilayer films are now a well-established nanostructured concept with numerous potential applications, in particular as biomaterial coatings. This technique allows the preparation of nanostructured architectures exhibiting specific properties for cell-activation control and local drug delivery. In this study, we used a multilayered system made of poly-(l-lysine)/hyaluronic acid (PLL/HA) as a reservoir for active DNA complexes with nonviral gene-delivery vectors, PLL, beta-cyclodextrin (CD), and PLL-CD. When embedded into the multilayered films, the transfection efficiencies of the DNA complexes and the cell viability were improved. The highest transfection efficiency was obtained with the PLL-CD/plasmid DNA (pDNA) complexes. We found that this high transfection efficiency was related to an efficient internalization of the complexes in the cell cytoplasm and selected nuclei domains through a nonendocytotic pathway. For the first time, we report the intracellular pathway of the pDNA in complexes incorporated into the multilayered system.

Micro-stratified architectures based on successive stacking of alginate gel layers and poly(l-lysine)-hyaluronic acid multilayer films aimed at tissue engineering

A micro-stratified 3D scaffold was designed by successive stacking of alginate gel layers (AGLs) and poly(l-lysine)-hyaluronic acid (PLL-HA) multilayer films. AGLs are obtained by complexation of alginate by Ca2+ ions. Alginate solutions are first sprayed onto a solid substrate inclined such that the excess of solution be removed by natural drainage. A CaCl2 solution is then either sprayed onto the substrate or the alginate covered substrate is dipped into a CaCl2 solution. The spraying of the CaCl2 solution leads to micro-porous AGLs, whereas the dipping in a CaCl2 aqueous solution leads to a more homogeneous gel layer without porosity. The second process also allows the formation of AGLs with a controlled thickness. With the goal of stacking different AGLs and PLL-HA films, the influence of a PLL-HA precursor film on the formation of AGLs is firstly investigated. It is found that when an alginate solution is sprayed on a PLL-HA multilayer built in the presence of CaCl2, the multilayer plays the role of reservoir of Ca2+ ions and of PLL chains, which both diffuse out of the multilayer film and complex alginate chains. This leads to the formation of a "pre-alginate gel". When this film is further dipped in the CaCl2 solution, an additional AGL forms, which is, however, free of PLL chains. Finally after the build-up of a PLL-HA film on the top of AGL, we succeeded in designing micro-stratified 3D scaffolds constituted by alternating strata of AGLs and PLL-HA films. This micro-stratified gel provides a new scaffold design with a perfectly controlled build-up: AGL aims to be a 3D scaffold for cell culture, and the PLL-HA multilayers should act as reservoirs for biologically active molecules.

Exponential growth of LBL films with incorporated inorganic sheets

The fastest growth pattern of layer-by-layer (LBL) assembled films is exponential LBL (e-LBL), which has both fundamental and practical importance. It is associated with "in-and-out" diffusion of flexible polymers and thus was considered to be impossible for films containing clay sheets with strong barrier function, preventing diffusion. Here, we demonstrate that e-LBL for inorganic sheets is possible in a complex tricomponent film of poly(ethyleneimine) (PEI), poly(acrylic acid) (PAA), and Na(+)-montmorillonite (MTM). This system displayed clear e-LBL patterns in terms of both initial accumulation of materials and unusually thick individual bilayers later in the deposition process with film thicknesses reaching 200 microm for films composed of 200 pairs of layers. Successful incorporation of MTM layers was observed by scanning electron microscopy and thermo-gravimetric analysis. Surprisingly, the growth rate was found to be nearly identical in films with and without clay layers, which suggests fast permeation/reptation of polyelectrolytes between the nanosheets during the "in-and-out" diffusion of polymer. In considering these findings, e-LBL growth property is expected for a wide array of available inorganic nanoscale components and have a potential to greatly expand the e-LBL field and LBL field altogether. The large thickness and rapid growth of the films affords fast preparation of nanostructured materials which is essential for multiple practical applications ranging from optical devices to ultrastrong composites.

Controlled delivery of anti-sense oligodeoxynucleotide from multilayered biocompatible phosphorylcholine polymer films

Fabrication of polymeric multilayered films based on the electrostatic self-assembly of polycations and polyanions is a promising approach for controlled loading and release in gene delivery. In this study, we have fabricated a series of multilayered films based on alternate deposition between positively-charged cationic phosphorylcholine copolymer (PC copolymer) and negatively-charged c-myc anti-sense oligodeoxynucleotide (AS-ODN). The growth of film thickness and increase of ODN loading capacity were monitored by spectroscopic ellipsometry (SE) and confocal laser scanning microscopy (CLSM). After elution into PBS buffer under physiological conditions, the elution profile was monitored by UV spectrometry and gel electrophoresis. Employing a secondary transgenic vector, the cellular uptake of the eluted AS-ODN into HeLa cells was evaluated by fluorescent microscopy and FACS analysis. The biological effect of eluted AS-ODN was evaluated by cell growth inhibition. The results showed that AS-ODN loading capacity increased almost linearly with the number of PC polymer/ODN bilayers and was also strongly dependent upon the cationic charge density. Through swelling, a non-degradable release mechanism, the AS-ODN release was characterized by two distinguishable release regimes: a fast release regime during the first 6 hour period and a slow release regime from 6 hour to the 8th day, both of which were characterized by zero-order kinetics. Gel electrophoresis showed excellent DNA integrity and strong transfection was observed when the eluted ODN was transfected into HeLa cells. Cell growth was significantly inhibited by eluted AS-ODN, indicating its full bioactivity. These results demonstrate that PC multilayered polymer films are capable of delivering a prescribed amount of anti-sense ODN with a controllable kinetic profile and that the multilayer process is more efficient and reliable than most other existing coating approaches largely based on single-layer fabrication.

Characterization of polyelectrolyte multilayer films on polyethylene terephtalate vascular prostheses under mechanical stretching

Layer-by-layer (LBL) polyelectrolyte films offer extensive potentials to enhance surface properties of vascular biomaterials. From the time of implantation, PET prostheses are continuously subjected to multiple mechanical stresses such as important distorsions and blood pressure. In this study, three LBL films, namely (1) poly(sodium 4-styrenesulfonate)/poly(allylamine hydrochloride), (2) poly(L-lysine)/hyaluronan, and (3) poly(L-lysine)/poly(L-glutamic acid) were built on to isolated PET filaments, thread, and vascular prostheses. The three LBL films uniformly covered the surface of the PET samples with rough, totally smooth, and "wrinkled" appearances respectively for (PAH/PSS)(24), (PLL/HA)(24), and (PLL/PGA)(24) systems. We then assessed the behavior of these LBL films, in an aqueous environment [by environmental scanning electronic microscopy (ESEM)], when subjected to unidirectional longitudinal stretches. We found that stretching induces ruptures in the multilayer films on isolated filaments for longitudinal stretches of 14% for (PSS/PAH)(24), 13% for (PLL/PGA)(24), and 30% for (PLL/HA)(24) films. On threads, the rupture limit is enhanced to be respectively 26, 20, and 28%. Most interestingly, we found that on vascular prosthesis no rupture is visible in any of the three multilayers types, even for elongations of 200% (200% undergone by the PET prostheses is representative of those encountered during graft deployment) which by far exceeds elongations observed under physiological conditions (10-20%, blood pressure). In term of mechanical behaviors, these preliminary data constitute a first step toward the possible use of LBL film to coat and functionalize vascular prosthesis.

Reversible loading and unloading of nanoparticles in "exponentially" growing polyelectrolyte LBL films

The exponentially growing layer-by-layer (LBL) films made from poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) were used to load and unload the CdTe nanoparticles (NPs). The reversible loading of NPs were investigated through UV-vis studies and further confirmed by confocal microscopy. In addition the LBL films were also compared for the release kinetics for pH 9 and 7 and films capped with (PDDA-PSS)10 layers. The amount of released particles at pH 9 was found to be at least 2 orders of magnitude higher than those at pH 7 and with (PDDA-PSS)10 capped layers after 25 h. This variation in film response for CdTe-particle release presents a route for studies in which highly swollen exponentially growing LBL films can be loaded with functionalized NPs for biological applications and explored as carriers to hold the NPs inside the films for self-assembly.

Response of fibroblast activity and polyelectrolyte multilayer films coating titanium

OBJECTIVES

The study of surface properties is a recent and crucial issue in the biomaterial fields applied to Odontology. The reference biomaterial in dental implantology is titanium. The principal objective is a perfect bio-integration in the oral ecosystem, both in terms of mucosal and bone tissues. The aim of this work was to optimize the tissue-titanium interface by applying polyelectrolyte multilayer films on the surface of titanium.

METHODS

The experimental study was undertaken on pure titanium samples. Two types of film ending with polycations or polyanions were selected. Both film types were built with a first poly(ethyleneimine) (PEI) base layer and composed either of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) or of hyaluronic acid (HA) and poly(l-lysine) (PLL) layers. Final architectures were as follows: PEI-(PSS/PAH)(10), or PEI-(PSS/PAH)(10)-PSS, or chemically cross-linked PEI-(HA/PLL)(10) or PEI-(HA/PLL)(10)-HA. An analysis of the physicochemical characteristics of the surfaces was carried out by tensiometry measurements (dynamic contact angle, wettability, contact angle hysteresis) and atomic force microscopy. A biological study with human fibroblasts was followed over a 7-day culture period at days 0, 2, 4 and 7 to observe the cellular response in terms of morphology (scanning electron microscopy) and viability (Mosmann's test).

RESULTS

The results showed that polyelectrolyte multilayer films could be successfully deposited onto titanium as previously described for glass or composite. Fibroblast adhesion and proliferation was strongly dependent on film type. SEM observations of cells on the different films agreed with the viability cell test. Furthermore, films containing PSS/PAH generated a better cellular response than films containing cross-linked HA/PLL.

CONCLUSION

PSS/PAH polyelectrolyte films coating titanium could represent a new approach for oral bio-integration with great potential for clinical application in the fields of dental implantology. More particularly, the specific biofunctionalization of PSS/PAH films coating titanium could be envisioned by introducing layers of molecules that encourage the bio-integration process between the films.

Development of an artificial neuronal network with post-mitotic rat fetal hippocampal cells by polyethylenimine

The selection of appropriate surface materials that promote cellular adhesion and growth is an important consideration when designing a simplified neuronal network in vitro. In the past, extracellular matrix proteins such as laminin (LN) or positively charged substances such as poly-l-lysine (PLL) have been used. In this study, we examined the ability of another positively charged polymer, polyethyleneimine (PEI), to promote neuronal adhesion, growth and the formation of a functional neuronal network in vitro. PEI, PLL and LN were used to produce grid-shape patterns on glass coverslips by micro-contact printing. Post-mitotic neurons from the rat fetal hippocampus were cultured on the different polymers and the viability and morphology of these neurons under serum-free culture conditions were observed using fluorescent microscopy and atomic force microscopy (AFM). We show that neurons cultured on the PEI- and PLL-coated surfaces adhered to and extended neurites along the grid-shape patterns, whereas neurons cultured on the LN-coated coverslips clustered into clumps of cells. In addition, we found that the neurons on the PEI and PLL-coated grids survived for more than 2 weeks in serum-free conditions, whereas most neurons cultured on the LN-coated grids died after 1 week. Using AFM, we observed some neurosynapse-like structures near the neuronal soma on PEI-coated coverslips. These findings indicate that PEI is a suitable surface for establishing a functional neuronal network in vitro.

Multilayered films fabricated from plasmid DNA and a side-chain functionalized poly(beta-amino ester): surface-type erosion and sequential release of multiple plasmid constructs from surfaces

Hydrolytically degradable polyamines can be used to fabricate multilayered polyelectrolyte films that erode and release DNA in aqueous environments. Past studies have investigated films fabricated from poly(beta-amino ester) 1 and the influence of polymer backbone structure on film erosion and the release of anionic polyelectrolytes. This investigation sought to characterize the influence of polymer side-chain structure on the stability of multilayered films in physiologically relevant media. Here, we report on the fabrication and characterization of multilayered films approximately 150 nm thick assembled from plasmid DNA and side-chain functionalized polymer 2. We observed large differences in the behavior of films fabricated from polymer 2 as compared to films fabricated from polymer 1. Whereas films fabricated from polymer 1 erode and release DNA over approximately 2 days when incubated in phosphate-buffered saline, films fabricated from polymer 2 erode and release DNA over approximately 2 weeks. In addition, whereas films fabricated from polymer 1 undergo complex nanometer-scale physical transformations in aqueous media, characterization of the surfaces of films fabricated from polymer 2 by atomic force microscopy (AFM) demonstrates that the surfaces of these materials remain smooth and uniform during erosion. The apparent surface-type erosion of these materials permits the fabrication of ultrathin films with architectures that provide control over the timing and the order in which two different DNA constructs are released from surfaces. For example, the order in which two different DNA constructs are released from films and expressed by cells can be controlled to measurable extents by the relative order in which they are deposited during fabrication. These results suggest approaches to the localized and sequential release of multiple different DNA constructs to cells or tissues from the surfaces of tissue engineering scaffolds or implantable devices coated with multilayered films.

Cell apoptosis control using BMP4 and noggin embedded in a polyelectrolyte multilayer film

Programmed cell death (apoptosis) is a genetically regulated process of cell elimination essential during development. During development, programmed cell death is involved in the specific shaping of organs, in the elimination of cells having achieved their program, and in regulating the number of cells to differentiate. Tooth development includes these three aspects and was used here as a model to study the control of apoptosis. Bone morphogenetic proteins (BMPs) are currently considered as playing a major role in signaling apoptosis. This apoptosis could be stopped by treatments with a BMP antagonist ("Noggin"). We selected a model system made by a layer-by-layer approach using poly-L-glutamic acid (PlGA) and poly-L-lysine (PlL) films into which BMP4 and/or Noggin have been embedded. Our results indicate that in situ control of apoptosis during tooth differentiation mediated by both BMP4 and Noggin embedded in a polyelectrolyte multilayer film is possible. We show here for the first time that in the presence of BMP4 and Noggin embedded in a multilayered film, we can induce or inhibit cell death in tooth differentiation, and conserve their biological effects.

Phospholipid bilayers as biomembrane-like barriers in layer-by-layer polyelectrolyte films

Dipalmitoylphosphatidylcholine (DPPC) bilayer was created on the surface of an exponentially growing poly(glutamic acid)/poly(lysine) (PGA/PLL) layer-by-layer polyelectrolyte film. The lipid bilayer decreased the surface roughness of the polyelectrolyte film. The layer-by-layer construction of the polyelectrolyte film could be continued on the top of the DPPC layer. The lipid bilayer, however, formed a barrier in the interior of the polyelectrolyte film, which blocked the diffusion (a prerequisite for exponential growth) of the polyelectrolytes. Thus, a new growth regime started in the upper part of the polyelectrolyte film, which was added to embed the DPPC bilayer. The structure and the dynamics of the DPPC bilayer on the polyelectrolyte film surface remained similar to that of its hydrated multibilayers, except that the phase transition became wider. In the case of embedded DPPC bilayers, in addition, the phase-transition temperature also decreased. This is the result of interactions with the nonconcerted movements of the barrier-separated lower and higher parts of the polyelectrolyte film. Gramicidin A (GRA) as a model of lipid-soluble peptides and proteins was successfully incorporated into such DPPC films. The DPPC films, either with or without GRA, were remarkably stable; as many heating-cooling cycles to measure phase transition could be carried out without visible alterations as wanted.

Multilayered films fabricated from an oligoarginine-conjugated protein promote efficient surface-mediated protein transduction

The conjugation of cationic protein transduction domains to proteins results in an increase in the extent to which proteins are internalized by cells. This investigation sought to determine whether the conjugation of a protein transduction domain to a functional protein could be used to facilitate the incorporation of the protein into multilayered polyelectrolyte films and, subsequently, whether these films could be used to promote surface-mediated protein transduction. We demonstrate that it is possible to fabricate multilayered assemblies 80 nm thick using sodium polystyrene sulfonate (SPS) and bovine pancreatic ribonuclease (RNase A) conjugated to the cationic protein transduction domain nonaarginine (R(9)) using an entirely aqueous layer-by-layer process. We demonstrate further that the conjugation of R(9) to RNase A permits the assembly of multilayered films under conditions that do not allow for the incorporation of the unmodified protein. This result suggests that R(9) functions as a cationic anchor and serves to increase the strength of electrostatic interactions with SPS and facilitate layer-by-layer assembly. We also demonstrate that RNase A-R(9)/SPS films dissolve rapidly in physiologically relevant media and that macroscopic objects coated with these materials can be used to mediate high levels of protein transduction in mammalian cells. These results suggest the basis of general methods that could contribute to the design of materials that permit spatial and temporal control over the delivery of therapeutic proteins to cells and tissues.

Cytotoxicity of polyethyleneimine (PEI), precursor base layer of polyelectrolyte multilayer films

Polyethyleneimine (PEI) is a synthetic polymer commonly used as precursor base layer in polyelectrolyte multilayer films. However, the biological properties of this cationic macromolecule are poorly understood. The aim of this experimental investigation was to evaluate in vitro the biocompatibility of PEI towards two different human cell lines. The experimental investigation was undertaken on pure titanium (Ti) and nickel-titanium (NiTi) alloy samples with an average surface roughness of Ra=0.3microm. A biological study was undertaken at day 0 (2h after seeding), day 2, day 4 and day 7 to observe the cellular response of fibroblasts and osteoblasts cell lines in terms of morphology, adhesion (as observed by scanning electron microscopy), and viability (Mosmann's test). The results showed that PEI can be successfully deposited onto Ti or NiTi alloy, but generates a detrimental cellular response on both substrates as illustrated by a decrease of both fibroblast and osteoblast adhesion and proliferation over a 7-day culture period. These results suggest that PEI is potentially cytotoxic and may not be biocompatible enough in clinical applications using high molecular weight. As a consequence, polyelectrolyte multilayer films, which are promising in prosthesis and implantology fields, could not be coated with PEI at a high molecular weight. A lower molecular weight should be considered or a more biocompatible molecular base as precursor layer of polyelectrolyte multilayer films would be better to use for a good human bio-integration.

Control of drug accessibility on functional polyelectrolyte multilayer films

A surface coating based on polylysine/hyaluronic acid multilayers was designed and acted as a reservoir for an antiproliferative agent, paclitaxel (Taxol). Absolutely no chemical modification of polyelectrolytes or of the drug was needed and the final architecture was obtained in an extremely simple way using the layer-by-layer method. The paclitaxel dose available for human colonic adenocarcinoma cells HT29 seeded on the films could be finely tuned. Moreover, the accessibility of the drugs was controlled by adding on the top of the drug reservoir a capping made of synthetic polyelectrolyte multilayers. This capping was also required to allow adhesion of HT29 cells. Paclitaxel activity was maintained after embedding in the polyelectrolyte multilayers and cellular viability could be reduced by about 80% 96 h after seeding. The strategy described in this paper could be valuable for various other drug/cell systems.