Variation of polyelectrolyte film stiffness by photo-cross-linking: a new way to control cell adhesion

A Polymer Canvas with the Stiffness of the Bone Matrix to Study and Control Mesenchymal Stem Cell Response

Reproducing in vitro the complex multiscale physical features of human tissues creates novel biomedical opportunities and fundamental understanding of cell-environment interfaces and interactions. While stiffness has been recognized as a key driver of cell behavior, systematic studies on the role of stiffness have been limited to values in the KPa-MPa range, significantly below the stiffness of bone. Here, a platform enabling the tuning of the stiffness of a biocompatible polymeric interface up to values characteristic of human bone is reported, which are in the GPa range, by using extremely thin polymer films on glass and cross-linking the films using ultraviolet (UV) light irradiation. It is shown that a higher stiffness is related to better adhesion, proliferation, and osteogenic differentiation, and that it is possible to switch on/off cell attachment and growth by solely tuning the stiffness of the interface, without any surface chemistry or topography modification. Since the stiffness is tuned directly by UV irradiation, this platform is ideal for rapid and simple fabrication of stiffness patterns and gradients, thus representing an innovative tool for combinatorial studies of the synergistic effect of tissue environmental cues on cell behavior, and creates new opportunities for next-generation biosensors, single-cell patterning, and lab-on-a-chip devices.

Combinations of Hydrogels and Mesenchymal Stromal Cells (MSCs) for Cartilage Tissue Engineering-A Review of the Literature

Cartilage offers limited regenerative capacity. Cell-based approaches have emerged as a promising alternative in the treatment of cartilage defects and osteoarthritis. Due to their easy accessibility, abundancy, and chondrogenic potential mesenchymal stromal cells (MSCs) offer an attractive cell source. MSCs are often combined with natural or synthetic hydrogels providing tunable biocompatibility, biodegradability, and enhanced cell functionality. In this review, we focused on the different advantages and disadvantages of various natural, synthetic, and modified hydrogels. We examined the different combinations of MSC-subpopulations and hydrogels used for cartilage engineering in preclinical and clinical studies and reviewed the effects of added growth factors or gene transfer on chondrogenesis in MSC-laden hydrogels. The aim of this review is to add to the understanding of the disadvantages and advantages of various combinations of MSC-subpopulations, growth factors, gene transfers, and hydrogels in cartilage engineering.

Green and Tunable Animal Protein-Free Microcarriers for Cell Expansion

Cell culture on microcarriers emerges as an alternative of two-dimensional culture to produce large cell doses, which are required for cell-based therapies. Herein, we report a versatile and easy solvent-free greener fabrication process to prepare microcarriers based on a biosourced and compostable polymer. The preparation of the microcarrier core, which is based on poly(L-lactide) crystallization from a polymer blend, allows us to easily tune the density, porosity, and size of the microparticles. A bioadhesive coating based on biopolymers, devoid of animal protein and optimized to improve cell adhesion, is then successfully deposited on the surface of the microcarriers. The ability of these new microcarriers to expand human adipose-derived stromal cells with good yield, in semistatic and dynamic conditions, is demonstrated. Finally, bead-to-bead cell transfer is shown to increase the yield of cell production without having to stop the culture. These microcarriers are therefore a promising and efficient green alternative to currently existing systems.

Combining Top-Down and Bottom-Up with Photodegradable Layer-by-Layer Films

Layer-by-layer (LbL) self-assembly of polymer coatings is a bottom-up fabrication technique with broad applicability across a wide range of materials and applications that require control over interfacial properties. While most LbL coatings are chemically uniform in directions both tangent and perpendicular to their substrate, control over the properties of surface coatings as a function of space can enhance their function. To contribute to this rapidly advancing field, our group has focused on the top-down spatiotemporal control possible with photochemically reactive LbL coatings, harnessed through charge-shifting polyelectrolytes enabled by photocleavable ester pendants. The photolysis of the photocleavable esters degrades LbL films containing these polyelectrolytes. The chemical structures of the photocleavable groups dictate the wavelengths responsible for disrupting these coatings, ranging from ultraviolet to near-infrared in our work. In addition, spatially segregating reactive groups into "compartments" within LbL films has enabled us to fabricate reactive free-standing polymer films and multiheight photopatterned coatings. Overall, by combining bottom-up and top-down approaches, photoreactive LbL films enable precise control over the interfacial properties of polymer and composite coatings.

Surface Micro- and Nanoengineering: Applications of Layer-by-Layer Technology as a Versatile Tool to Control Cellular Behavior

Extracellular matrix (ECM) cues have been widely investigated for their impact on cellular behavior. Among mechanics, physics, chemistry, and topography, different ECM properties have been discovered as important parameters to modulate cell functions, activating mechanotransduction pathways that can influence gene expression, proliferation or even differentiation. Particularly, ECM topography has been gaining more and more interest based on the evidence that these physical cues can tailor cell behavior. Here, an overview of bottom-up and top-down approaches reported to produce materials capable of mimicking the ECM topography and being applied for biomedical purposes is provided. Moreover, the increasing motivation of using the layer-by-layer (LbL) technique to reproduce these topographical cues is highlighted. LbL assembly is a versatile methodology used to coat materials with a nanoscale fidelity to the geometry of the template or to produce multilayer thin films composed of polymers, proteins, colloids, or even cells. Different geometries, sizes, or shapes on surface topography can imply different behaviors: effects on the cell adhesion, proliferation, morphology, alignment, migration, gene expression, and even differentiation are considered. Finally, the importance of LbL assembly to produce defined topographical cues on materials is discussed, highlighting the potential of micro- and nanoengineered materials to modulate cell function and fate.

Natural hydrogels for cartilage regeneration: Modification, preparation and application

Hydrogels, consisting of hydrophilic polymers, can be used as films, scaffolds, nanoparticles and drug carriers. They are one of the hot research topics in material science and tissue engineering and are widely used in the field of biomedical and biological sciences. Researchers are seeking for a type of material that is similar to human tissues and can partially replace human tissues or organs. The hydrogel has brought possibility to solve this problem. It has good biocompatibility and biodegradability. After entering the body, it does not cause immune and toxic reactions. The degradation time can be controlled, and the degradation products are nontoxic and nonimmunogenic; the final metabolites can be excreted outside the body. Owing to the lack of blood vessels and poor migration ability of chondrocytes, the self-healing ability of damaged cartilage is limited. Tissue engineering has brought a new direction for the regeneration of cartilage. Drug carriers and scaffolds made of hydrogels are widely used in cartilage tissue engineering. The present review introduces the natural hydrogels, which are often used for cartilage tissue engineering with respect to synthesis, modification and application methods.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

This review introduces the natural hydrogels that are often used in cartilage tissue engineering with respect to synthesis, modification and application methods. Furthermore, the essential concepts and recent discoveries were demonstrated to illustrate the achievable goals and the current limitations. In addition, we propose the putative challenges and directions for the use of natural hydrogels in cartilage regeneration.

Chain conformation of poly(acrylic acid)-graft-poly(ethylene oxide)-graft-dodecyl in solution: an anomalous counter-ions condensation

A dielectric spectroscopy study on a polyelectrolyte in aqueous solutions, which contains hydrophobic groups in part of the side chains poly(acrylic acid)-graft-poly(ethylene oxide)-graft-dodecyl (PAA-g-PEO-g-dodecyl) is reported. A refined double layer polarization model was proposed to analyze the double dielectric relaxations in the dielectric spectra. Various electrical and structural parameters of the copolymers were obtained. Besides the crossover concentration, another turning point around 4 mg mL^-1 was identified through the analysis of all the dielectrical parameters including dielectric increment, relaxation time and correlation length. According to the scaling relationship between the correlation length and concentration, a necklace-like structure was predicted. In addition, 4 mg mL^-1 was proven to be the transition point between string-controlling with bead-controlling structure of the chains. In addition, the fraction of effective charges on the chains was illustrated by Ito's counter-ions fluctuation theory, as well as its linear dependence relationship with the zeta potential. Meanwhile, the counter-ions condensation behavior was consistent with the avalanche-like trend, which was predicted by theory for a hydrophobic polyelectrolyte with a necklace conformation. The results demonstrated that the electrostatic interactions were the main driving force of the necklace-like structure with pendant globules when the string-controlling structure was below 4 mg mL^-1. While hydrophobic interactions are the main driving force of the structure of bead-controlling above 4 mg mL^-1.

* Engineering Membranes for Bone Regeneration

This review is focused on the use of membranes for the specific application of bone regeneration. The first section focuses on the relevance of membranes in this context and what are the specifications that they should possess to improve the regeneration of bone. Afterward, several techniques to engineer bone membranes by using "bulk"-like methods are discussed, where different parameters to induce bone formation are disclosed in a way to have desirable structural and functional properties. Subsequently, the production of nanostructured membranes using a bottom-up approach is discussed by highlighting the main advances in the field of bone regeneration. Primordial importance is given to the promotion of osteoconductive and osteoinductive capability during the membrane design. Whenever possible, the films prepared using different techniques are compared in terms of handability, bone guiding ability, osteoinductivity, adequate mechanical properties, or biodegradability. A last chapter contemplates membranes only composed by cells, disclosing their potential to regenerate bone.

Controlled Interfacial Permeation, Nanostructure Formation, Catalytic Efficiency, Signal Enhancement Capability, and Cell Spreading by Adjusting Photochemical Cross-Linking Degrees of Layer-by-Layer Films

Interfacial properties including permeation, catalytic efficiency, Raman signal enhancement capabilities, and cell spreading efficiencies are important features that determine material functionality and applications. Here, we propose a facile method to adjust the above-mentioned properties by controlling the cross-linking degrees of multilayer using a photoactive molecule. After treating the cross-linked films in basic solutions, films with different cross-linking degrees presented varying residue thicknesses and film morphologies. As a result, these different films possessed distinct molecular loading and release characteristics. In addition, gold nanoparticles (AuNPs) of different morphological traits were generated by redox reactions coupled with diffusion within these films. The AuNP-polyelectrolyte obtained from the polyelectrolyte films of the medium cross-linking degrees displayed the highest catalytic efficiency and signal enhancement capabilities. Furthermore, cells responded to the variation of film cross-linking degrees, and on the films with the highest cross-linking degree, cells adhered with the highest speed. We expect this report to provide a general interfacial material engineering strategy for material designs.

Separation of Storage and Loss Modulus of Polyelectrolyte Multilayers on a Nanoscale: A Dynamic AFM Study

Atomic force microscopy (AFM) is used to carry out rheology measurements on the nanoscale and to determine the mechanical properties of poly(l-lysine) (PLL)/hyaluronic acid (HA) multilayer films. Storage (G') and loss modulus (G″) of the films are calculated and compared with the values obtained from quartz crystal microbalance with dissipation monitoring measurements (QCM-D). A predominant elastic behavior independently of the applied frequencies (5-100 Hz) is observed for native HA/PLL films consisting of 36 double layer. If the layers are cross-linked, the value of G' increases by 2 orders of magnitude, while the loss modulus becomes negligible, making these films a purely elastic chemical gel. The values of G' and G'' extracted from QCM-D measurements on native films are much higher, due to the different frequency regime of the applied shear stress. However, the viscoelastic ratio from the two methods is the same and proves the elastic dominated response of the multilayer in both frequency regimes.

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.

Cell Adhesion and Proliferation on the "Living" Surface of a Polyelectrolyte Multilayer

The adhesion of living eukaryotic cells to a substrate, one of the most complex problems in surface science, requires adsorption of extracellular proteins such as fibronectin. Thin films of polyelectrolyte complex made layer-by-layer (polyelectrolyte multilayers or PEMUs) offer a high degree of control of surface charge and composition-interconnected and essential variables for protein adhesion. Fibroblasts grown on multilayers of poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA, with increasing thickness exhibit good adhesion until the 12th layer of polyelectrolyte has been added, whereupon there is a sudden transition to nonadhesive behavior. This sharp change is due to the migration of excess positive charge to the surface-a previously unrecognized property of PEMUs. Precise radiotracer assays of adsorbed (125)I-albumin show how protein adsorption is related to multilayer surface charge. With more negative surface charge density from the sulfonates of PSS, more albumin adsorbs to the surface. However, a loosely held or "soft corona" exchanges with serum protein under the Vroman effect, which is correlated with poor cell adhesion. A comprehensive view of cell adhesion highlights the central role of robust protein adhesion, which is required before any secondary effects of matrix stiffness on cell fate can come into play.

Controlling cell adhesion using layer-by-layer approaches for biomedical applications

Controlling the adhesion of mammalian and bacterial cells at the interfaces between synthetic materials and biological environments is a real challenge in the biomedical fields such as tissue engineering, antibacterial coating, implantable biomaterials and biosensors. The surface properties of materials are known to profoundly influence the adhesion processes. To mediate the adhesion processes, polymeric coatings have been used to functionalize surfaces to introduce diverse physicochemical properties. The polyelectrolyte multilayer films built via the layer-by-layer (LbL) method, introduced by Moehwald, Decher, and Lvov 20years ago, has led to significant developments ranging from the fundamental understanding of cellular processes to controlling cell adhesion for biomedical applications. In this review, we focus our attention on the modification of surface physicochemical properties, using the LbL approach, to construct films which can either promote or inhibit mammalian/bacterial cell adhesion. We also discuss the emerging field of multifunctional surfaces capable of responding to specific cellular activity but being inert to the others.

Polyelectrolyte multilayer film modification for chemo-mechano-regulation of endothelial cell response

The new multilayer polyelectrolyte films (PEMs) that are able to simulate the structure and functions of the extracellular matrix have become a powerful tool for tailoring biointerfaces of “cardiovascular” implants.

Spatio-Temporal Control of LbL Films for Biomedical Applications: From 2D to 3D

Introduced in the '90s by Prof. Moehwald, Lvov, and Decher, the layer-by-layer (LbL) assembly of polyelectrolytes has become a popular technique to engineer various types of objects such as films, capsules and free standing membranes, with an unprecedented control at the nanometer and micrometer scales. The LbL technique allows to engineer biofunctional surface coatings, which may be dedicated to biomedical applications in vivo but also to fundamental studies and diagnosis in vitro. Initially mostly developed as 2D coatings and hollow capsules, the range of complex objects created by the LbL technique has greatly expanded in the past 10 years. In this Review, the aim is to highlight the recent progress in the field of LbL films for biomedical applications and to discuss the various ways to spatially and temporally control the biochemical and mechanical properties of multilayers. In particular, three major developments of LbL films are discussed: 1) the new methods and templates to engineer LbL films and control cellular processes from adhesion to differentiation, 2) the major ways to achieve temporal control by chemical, biological and physical triggers and, 3) the combinations of LbL technique, cells and scaffolds for repairing 3D tissues, including cardio-vascular devices, bone implants and neuro-prosthetic devices.

Effect of the silicon carbide nanoparticles introduction on biological properties of porous polymer coatings

The multilayer polyelectrolyte films (PEMs) seem to be a promising material to reconstruct the structure and behavior of the extracellular matrix.

Dynamic stiffness of polyelectrolyte multilayer films based on disulfide bonds for in situ control of cell adhesion

Dynamic stiffness of (poly-l-lysine/hyaluronan-SH) films was developed for in situ control of cell adhesion by using reversible disulfide linkages.

Two methods for one-point anchoring of a linear polysaccharide on a gold surface

Two strategies to achieve a one-point anchoring of a hydrolyzed pullulan (P9000) on a gold surface are compared. The first strategy consists of forming a self-assembled monolayer of a 6-amino-1-hexanethiol (AHT) and then achieving reductive amination on the surface between the aminated surface and the aldehyde of the polysaccharide reductive end sugar. The second consists of incorporating a thiol function at the extremity of the pullulan (via the same reductive amination), leading to P9000-AHT and then immobilizing it on gold by a spontaneous reaction between solid gold and thiol. The modified pullulan was characterized by NMR and size-exclusion chromatography coupled to a light-scattering detector. P9000-AHT appears to be in a disulfide dimer form in solution but recovers its unimer form with dithiothreitol (DTT) treatment. The comparison of the two strategies by contact angle and XPS revealed that the second strategy is more efficient for the pullulan one-point anchoring. P9000-AHT even in its dimer form is easily grafted onto the surface. The grafted polymer seems to be more in a coil conformation than in a rigid brush. Furthermore, QCM measurements highlighted that the second strategy leads to a grafting density of around 3.5 × 10(13) molecules·cm(-2) corresponding to a high surface coverage. The elaboration of a dense and oriented layer of polysaccharides covalently linked to a gold surface might enhance the use of such modified polysaccharides in various fields.

Layer-by-Layer Assembly of Biopolyelectrolytes onto Thermo/pH-Responsive Micro/Nano-Gels

This review deals with the layer-by-layer (LbL) assembly of polyelectrolyte multilayers of biopolymers, polypeptides (i.e., poly-l-lysine/poly-l-glutamic acid) and polysaccharides (i.e., chitosan/dextran sulphate/sodium alginate), onto thermo- and/or pH-responsive micro- and nano-gels such as those based on synthetic poly(N-isopropylacrylamide) (PNIPAM) and poly(acrylic acid) (PAA) or biodegradable hyaluronic acid (HA) and dextran-hydroxyethyl methacrylate (DEX-HEMA). The synthesis of the ensembles and their characterization by way of various techniques is described. The morphology, hydrodynamic size, surface charge density, bilayer thickness, stability over time and mechanical properties of the systems are discussed. Further, the mechanisms of interaction between biopolymers and gels are analysed. Results demonstrate that the structure and properties of biocompatible multilayer films can be finely tuned by confinement onto stimuli-responsive gels, which thus provides new perspectives for biomedical applications, particularly in the controlled release of biomolecules, bio-sensors, gene delivery, tissue engineering and storage.

Tailored freestanding multilayered membranes based on chitosan and alginate

Engineering metabolically demanding tissues requires the supply of nutrients, oxygen, and removal of metabolic byproducts, as well as adequate mechanical properties. In this work, we propose the development of chitosan (CHIT)/alginate (ALG) freestanding membranes fabricated by layer-by-layer (LbL) assembly. CHIT/ALG membranes were cross-linked with genipin at a concentration of 1 mg·mL(-1) or 5 mg·mL(-1). Mass transport properties of glucose and oxygen were evaluated on the freestanding membranes. The diffusion of glucose and oxygen decreases with increasing cross-linking concentration. Mechanical properties were also evaluated in physiological-simulated conditions. Increasing cross-linking density leads to an increase of storage modulus, Young modulus, and ultimate tensile strength, but to a decrease in the maximum hydrostatic pressure. The in vitro biological performance demonstrates that cross-linked films are more favorable for cell adhesion. This work demonstrates the versatility and feasibility of LbL assembly to generate nanostructured constructs with tunable permeability, mechanical, and biological properties.

Sprayed cells and polyelectrolyte films for biomaterial functionalization: the influence of physical PLL-PGA film treatments on dental pulp cell behavior

Further development of biomaterials is expected as advanced therapeutic products must be compliant to good manufacturing practice regulations. A spraying method for building-up polyelectrolyte films followed by the deposition of dental pulp cells by spraying is presented. Physical treatments of UV irradiation and a drying/wetting process are applied to the system. Structural changes and elasticity modifications of the obtained coatings are revealed by atomic force microscopy and by Raman spectroscopy. This procedure results in thicker, rougher and stiffer film. The initially ordered structure composed of mainly α helices is transformed into random/β-structures. The treatment enhanced dental pulp cell adhesion and proliferation, suggesting that this system is promising for medical applications.

Stiffness of cross-linked poly(dimethylsiloxane) affects bacterial adhesion and antibiotic susceptibility of attached cells

In this study, Escherichia coli RP437 and Pseudomonas aeruginosa PAO1 were used as model strains to investigate the early stage biofilm formation on poly(dimethylsiloxane) (PDMS) surfaces with varying stiffness, which were prepared by controlling the degree of cross-linking (base:curing agent ratios of 5:1, 10:1, 20:1, and 40:1 were tested). An inverse correlation between cell adhesion and substrate stiffness was observed for both species. Interestingly, it was found that the cells attached on relatively stiff substrates (5:1 PDMS) were significantly smaller than those on relatively soft substrates (40:1 PDMS). In addition to the difference in size, the cells on 5:1 PDMS substrates were also found to be less susceptible to antibiotics, such as ofloxacin, ampicillin, and tobramycin, than the cells attached on 40:1 PDMS substrates. These results reveal that surface stiffness is an important material property that influences the attachment, growth, and stress tolerance of biofilm cells.

Cytotoxicity control of SiC nanoparticles introduced into polyelectrolyte multilayer films

Nowadays, biosensor technology development is directed toward improvement of sensing devices' biocompatibility.

Rigidity-patterned polyelectrolyte films to control myoblast cell adhesion and spatial organization

In vivo, cells are sensitive to the stiffness of their micro-environment and especially to the spatial organization of the stiffness. In vitro studies of this phenomenon can help to better understand the mechanisms of the cell response to spatial variations of the matrix stiffness. In this work, we design polelyelectrolyte multilayer films made of poly(L-lysine) and a photo-reactive hyaluronan derivative. These films can be photo-crosslinked through a photomask to create spatial patterns of rigidity. Quartz substrates incorporating a chromium mask are prepared to expose selectively the film to UV light (in a physiological buffer), without any direct contact between the photomask and the soft film. We show that these micropatterns are chemically homogeneous and flat, without any preferential adsorption of adhesive proteins. Three groups of pattern geometries differing by their shape (circles or lines), size (form 2 to 100 μm) or interspacing distance between the motifs are used to study the adhesion and spatial organization of myoblast cells. On large circular micropatterns, the cells form large assemblies that are confined to the stiffest parts. Conversely, when the size of the rigidity patterns is subcellular, the cells respond by forming protrusions. Finally, on linear micropatterns of rigidity, myoblasts align and their nuclei drastically elongate in specific conditions. These results pave the way for the study of the different steps of myoblast fusion in response to matrix rigidity in well-defined geometrical conditions.

Electrochemically controlled stiffness of multilayers for manipulation of cell adhesion

Stimuli-responsive thin films attract considerable attention in different fields. Herein, an electrochemical redox multilayers with tunable stiffness is constructed through the layer-by-layer self-assembly method. The redox ferrocene modified poly(ethylenimine) play an essential role to induce multilayers' swelling/shrinking under an electrochemical stimulus, resulting reversible change of elastic modulus of the multilayers. The adhesion of fibroblast cells can be thus controlled from well spreading to round shape. Such soft multilayers with electrochemically controlled stiffness could have potentials for cell-based applications.

Liposomes as drug deposits in multilayered polymer films

The ex vivo growth of implantable hepatic or cardiac tissue remains a challenge and novel approaches are highly sought after. We report an approach to use liposomes embedded within multilayered films as drug deposits to deliver active cargo to adherent cells. We verify and characterize the assembly of poly(l-lysine) (PLL)/alginate, PLL/poly(l-glutamic acid), PLL/poly(methacrylic acid) (PMA), and PLL/cholesterol-modified PMA (PMAc) films, and assess the myoblast and hepatocyte adhesion to these coatings using different numbers of polyelectrolyte layers. The assembly of liposome-containing multilayered coatings is monitored by QCM-D, and the films are visualized using microscopy. The myoblast and hepatocyte adhesion to these films using PLL/PMAc or poly(styrenesulfonate) (PSS)/poly(allyl amine hydrochloride) (PAH) as capping layers is evaluated. Finally, the uptake of fluorescent lipids from the surface by these cells is demonstrated and compared. The activity of this liposome-containing coating is confirmed for both cell lines by trapping the small cytotoxic compound thiocoraline within the liposomes. It is shown that the biological response depends on the number of capping layers, and is different for the two cell lines when the compound is delivered from the surface, while it is similar when administered from solution. Taken together, we demonstrate the potential of liposomes as drug deposits in multilayered films for surface-mediated drug delivery.

Cell durotaxis on polyelectrolyte multilayers with photogenerated gradients of modulus

Behaviors of rat aortic smooth muscle (A7r5) and human osteosarcoma (U2OS) cells on photo-cross-linked polyelectrolyte multilayers (PEMUs) with uniform, or gradients of, moduli were investigated. The PEMUs were built layer-by-layer with the polycation poly(allylamine hydrochloride) (PAH) and a polyanion poly(acrylic acid) (PAA) that was modified with photoreactive 4-(2-hydroxyethoxy) benzophenone (PAABp). PEMUs with different uniform and gradients of modulus were generated by varying the time of uniform ultraviolet light exposure and by exposure through optical density gradient filters. Analysis of adhesion, morphology, cytoskeletal organization, and motility of the cells on the PEMUs revealed that A7r5 cells established a polarized orientation toward increasing modulus on shallow modulus gradients (approximately 4.7 MPa mm(-1)) and durotaxed toward stiffer regions on steeper gradients (approximately 55 MPa mm(-1)). In contrast, U2OS cells exhibited little orientation or durotaxis on modulus gradients. These results demonstrate the utility of photo-cross-linked PEMUs to direct vascular and osteoblast cell behavior, a potential application for PEMU coatings on biomedical implants.

Influence of polyelectrolyte film stiffness on bacterial growth

Photo-cross-linkable polyelectrolyte films, whose nanomechanical properties can be varied under UV light illumination, were prepared from poly(l-lysine) (PLL) and a hyaluronan derivative modified with photoreactive vinylbenzyl groups (HAVB). The adhesion and the growth of two model bacteria, namely Escherichia coli and Lactococcus lactis , were studied on non-cross-linked and cross-linked films to investigate how the film stiffness influences the bacterial behavior. While the Gram positive L. lactis was shown to grow slowly on both films, independently of their rigidity, the Gram negative E. coli exhibited a more rapid growth on non-cross-linked softer films compared to the stiffer ones. Experiments performed on photopatterned films showing both soft and stiff regions, confirmed a faster development of E. coli colonies on softer regions. Interestingly, this behavior is opposite to the one reported before for mammalian cells. Therefore, the photo-cross-linked (PLL/HAVB) films are interesting coatings for tissue engineering since they promote the growth of mammalian cells while limiting the bacterial colonization.

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.

Homogeneity, modulus, and viscoelasticity of polyelectrolyte multilayers by nanoindentation: refining the buildup mechanism

Atomic force microscopy, AFM, and nanoindentation of polyelectrolyte multilayers, PEMUs, made from poly(diallyldimethylammonium), PDADMA, and poly(styrene sulfonate), PSS, provided new insight into their surface morphology and growth mechanism. A strong odd/even alternation of surface modulus revealed greater extrinsic (counterion-balanced) charge compensation for fully hydrated multilayers ending in the polycation, PDADMA. These swings in modulus indicate a much more asymmetric layer-by-layer growth mechanism than previously proposed. Viscoelastic properties of the PEMU, which may contribute to cell response, were highlighted by variable indentation rates and minimized by extrapolating to zero indentation rate, at which point the surface and bulk equilibrium moduli were comparable. Variations in surface composition were probed at high resolution using force mapping, and the surface was found to be uniform, with no evidence of phase separation. AFM comparison of wet and dry films terminated with PSS and PDADMA revealed much greater swelling of the PDADMA-terminated PEMU by water, with collapse of surface roughness features in dry conditions. Dynamic and static contact angle measurements suggested less rearrangement for the glassy PSS surface.

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.

Evaluation of substrata effect on cell adhesion properties using freestanding poly(L-lactic acid) nanosheets

Investigation of the interactions between cells and material surfaces is important not only for the understanding of cell biology but also for the development of smart biomaterials. In this study, we investigated the substrate-related effects on the interaction between cell and polymeric ultrathin film (nanosheet) by modulating the mechanical properties of the nanosheet with a metal substrate or mesh. A freestanding polymeric nanosheet with tens-of-nanometers thickness composed of poly(L-lactic acid) (PLLA nanosheet) was fabricated by combination of a spin-coating technique and a water-soluble sacrificial layer. The freestanding PLLA nanosheet was collected on a stainless steel mesh (PLLA-mesh) and subsequently used for cell adhesion studies, comparing the results to the ones on a control SiO(2) substrate coated with an ultrathin layer of PLLA (PLLA-substrate). The adhesion of rat cardiomyocytes (H9c2) was evaluated on both samples after 24 h of culture. The PLLA-mesh with the tens-of-nanometers thick nanosheets induced an anisotropic adhesion of H9c2, while H9c2 on the PLLA-substrate showed an isotropic adhesion independent from the nanosheet thickness. Interestingly, an increment in the nanosheet thickness in the PLLA-mesh samples reduced the cellular anisotropy and led to a similar morphology to the PLLA-substrate. Considering the huge discrepancy of Young's modulus between PLLA nanosheet (3.5-4.2 GPa) and metal substrate (hundreds of GPa), cell adhesion was mechanically regulated by the Young's modulus of the underlying substrate when the thickness of the PLLA nanosheet was tens of nanometers. Modulation of the stiffness of the polymeric nanosheet by utilizing a rigid underlying material will allow the constitution of a unique cell culture environment.

A material's point of view on recent developments of polymeric biomaterials: control of mechanical and biochemical properties

Cells respond to a variety of stimuli, including biochemical, topographical and mechanical signals originating from their micro-environment. Cell responses to the mechanical properties of their substrates have been increasingly studied for about 14 years. To this end, several types of materials based on synthetic and natural polymers have been developed. Presentation of biochemical ligands to the cells is also important to provide additional functionalities or more selectivity in the details of cell/material interaction. In this review article, we will emphasize the development of synthetic and natural polymeric materials with well-characterized and tunable mechanical properties. We will also highlight how biochemical signals can be presented to the cells by combining them with these biomaterials. Such developments in materials science are not only important for fundamental biophysical studies on cell/material interactions but also for the design of a new generation of advanced and highly functional biomaterials.

Electrochemically addressed cross-links in polyelectrolyte multilayers: cyclic duravoltammetry

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

Osteoconductive protamine-based polyelectrolyte multilayer functionalized surfaces

The integration of orthopedic implants with host bone presents a major challenge in joint arthroplasty, spinal fusion and tumor reconstruction. The cellular microenvironment can be programmed via implant surface functionalization allowing direct modulation of osteoblast adhesion, proliferation, and differentiation at the implant--bone interface. The development of layer-by-layer assembled polyelectrolyte multilayer (PEM) architectures has greatly expanded our ability to fabricate intricate nanometer to micron scale thin film coatings that conform to complex implant geometries. The in vivo therapeutic efficacy of thin PEM implant coatings for numerous biomedical applications has previously been reported. We have fabricated protamine-based PEM thin films that support the long-term proliferation and differentiation of pre-osteoblast cells on non-cross-linked film-coated surfaces. These hydrophilic PEM functionalized surfaces with nanometer-scale roughness facilitated increased deposition of calcified matrix by osteoblasts in vitro, and thus offer the potential to enhance implant integration with host bone. The coatings can make an immediate impact in the osteogenic culture of stem cells and assessment of the osteogenic potential of new therapeutic factors.