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

Advances in natural and synthetic macromolecules with stem cells and extracellular vesicles for orthopedic disease treatment

Serious orthopedic disorders resulting from myriad diseases and impairments continue to pose a considerable challenge to contemporary clinical care. Owing to its limited regenerative capacity, achieving complete bone tissue regeneration and complete functional restoration has proven challenging with existing treatments. By virtue of cellular regenerative and paracrine pathways, stem cells are extensively utilized in the restoration and regeneration of bone tissue; however, low survival and retention after transplantation severely limit their therapeutic effect. Meanwhile, biomolecule materials provide a delivery platform that improves stem cell survival, increases retention, and enhances therapeutic efficacy. In this review, we present the basic concepts of stem cells and extracellular vesicles from different sources, emphasizing the importance of using appropriate expansion methods and modification strategies. We then review different types of biomolecule materials, focusing on their design strategies. Moreover, we summarize several forms of biomaterial preparation and application strategies as well as current research on biomacromolecule materials loaded with stem cells and extracellular vesicles. Finally, we present the challenges currently impeding their clinical application for the treatment of orthopedic diseases. The article aims to provide researchers with new insights for subsequent investigations.

There is still plenty of room for layer-by-layer assembly for constructing nanoarchitectonics-based materials and devices

Nanoarchitectonics approaches can produce functional materials from tiny units through combination of various processes including atom/molecular manipulation, chemical conversion, self-assembly/self-organization, microfabrication, and bio-inspired procedures. Existing fabrication approaches can be regarded as fitting into the same concept. In particular, the so-called layer-by-layer (LbL) assembly method has huge potential for preparing applicable materials with a great variety of assembling mechanisms. LbL assembly is a multistep process where different components can be organized in planned sequences while simple alignment options provide access to superstructures, for example helical structures, and anisotropies which are important aspects of nanoarchitectonics. In this article, newly-featured examples are extracted from the literature on LbL assembly discussing trends for composite functional materials according to (i) principles and techniques, (ii) composite materials, and (iii) applications. We present our opinion on the present trends, and the prospects of LbL assembly. While this method has already reached a certain maturity, there is still plenty of room for expanding its usefulness for the fabrication of nanoarchitectonics-based materials and devices.

Dual-functional antibacterial and osteogenic nisin-based layer-by-layer coatings

Implanted biomaterials can be regarded in a cornerstone in the domain of bone surgery. Their surfaces are expected to fulfil two particular requirements: preventing the settlement and the development of bacteria, and stimulating bone cells in view to foster osseointegration. Therefore, a modern approach consists in the design of dual functional coatings with both antibacterial and osteogenic features. To this end, we developed ultrathin Layer-by-Layer (LbL) coatings composed of biocompatible polyelectrolytes, namely chondroitin sulfate A (CSA) and poly-l-lysine (PLL). The coatings were crosslinked with genipin (GnP), a natural and biocompatible crosslinking agent, to increase their resistance against environmental changes, and to confer them adequate mechanical properties with regards to bone cell behaviors. Antibacterial activity was obtained with nisin Z, an antimicrobial peptide (AMP), which is active against gram-positive bacteria. The coatings had a significant bactericidal impact upon Staphylococcus aureus, with fully maintained bone cell adhesion, proliferation and osteogenic differentiation.

Polyelectrolyte Multilayers: An Overview on Fabrication, Properties, and Biomedical and Environmental Applications

Polyelectrolyte multilayers are versatile materials that are used in a large number of domains, including biomedical and environmental applications. The fabrication of polyelectrolyte multilayers using the layer-by-layer technique is one of the simplest methods to obtain composite functional materials. The properties of the final material can be easily tuned by changing the deposition conditions and the used building blocks. This review presents the main characteristics of polyelectrolyte multilayers, the fabrication methods currently used, and the factors influencing the layer-by-layer assembly of polyelectrolytes. The last section of this paper presents some of the most important applications of polyelectrolyte multilayers, with a special focus on biomedical and environmental applications.

Anti-biofouling assembly strategies for protein & cell repellent surfaces: a mini-review

The protein/cell interactions with the surface at the blood-biomaterial interface generally control the efficiency of biomedical devices. A wide range of active processes and slow kinetics occur simultaneously with many biomaterials in healthcare applications, leading to multiple biological reactions and reduced clinical functions. In this work, we present a brief review of studies as the interface between proteins and biomaterials. These include mechanisms of resistance to proteins, protein-rejecting polyelectrolyte multilayers, and coatings of hydrophilic, polysaccharide and phospholipid nature. The mechanisms required to attain surfaces that resist adhesion include steric exclusion, water-related effects, and volume effects. Also, approaches in the use of hydrophilic, highly hydrated, and electrically neutral coatings have demonstrated a good ability to decrease cell adhesion. Moreover, amongst the available methods, the approach of layer-by-layer deposition has been known as an interesting process to manipulate protein and cell adhesion behavior.

Multifunctional Scaffolds and Synergistic Strategies in Tissue Engineering and Regenerative Medicine

The increasing demand for organ replacements in a growing world with an aging population as well as the loss of tissues and organs due to congenital defects, trauma and diseases has resulted in rapidly evolving new approaches for tissue engineering and regenerative medicine (TERM). The extracellular matrix (ECM) is a crucial component in tissues and organs that surrounds and acts as a physical environment for cells. Thus, ECM has become a model guide for the design and fabrication of scaffolds and biomaterials in TERM. However, the fabrication of a tissue/organ replacement or its regeneration is a very complex process and often requires the combination of several strategies such as the development of scaffolds with multiple functionalities and the simultaneous delivery of growth factors, biochemical signals, cells, genes, immunomodulatory agents, and external stimuli. Although the development of multifunctional scaffolds and biomaterials is one of the most studied approaches for TERM, all these strategies can be combined among them to develop novel synergistic approaches for tissue regeneration. In this review we discuss recent advances in which multifunctional scaffolds alone or combined with other strategies have been employed for TERM purposes.

Effect of crosslinking processing on the chemical structure and biocompatibility of a chitosan-based hydrogel

Chitosan (CS)-based hydrogels with different structures were prepared to ensure the bioavailability of bioactive components. With the electrostatic interaction between CS and anionic crosslinkers, the structure of the CS-based hydrogel changed and influenced the swelling ability, which was beneficial for maintaining bioactive ingredients in the hydrogel. Compared with sodium hexametaphosphate, hydrogels crosslinked by sodium tripolyphosphate (STPP) had a higher swelling capacity and more stable release profile (no more than 10% BSA in the upper gastrointestinal tract), which could deliver bioactive ingredients to the colon. Moreover, due to electrostatic interactions, the surface of the CS-based hydrogel became hydrophilic, which helped Caco2 cells to grow on it. 118.86%-147.22% cell viability was found on the CS-based hydrogel. Furthermore, with different crosslinkers and concentrations in the crosslinking process, the release properties and safety of the hydrogels were varied, but the STPP-crosslinked CS hydrogel presented good cell adhesivity for bioactive components to the colon.

The role of carboxylic groups in heparin-mimicking polymer-functionalized surfaces for blood compatibility: Enhanced vascular cell selectivity

Blood compatibility is an eternal topic of biomedical materials. The effect of heparin-mimicking polymers (HMPs) on blood compatibility has been well studied, especially the synergistic effect of sugar unit and sulfonate/sulfate unit. However, carboxylic groups also play an important role in HMPs. In this work, copolymers of sodium 4-vinyl-benzenesulfonate (SS) and 2-methacrylamido glucopyranose (MAG) (poly(SS-co-MAG)) and poly(acrylate acid) (PAA) were self-assembled on Au surfaces with different feed ratios. When self-assembly of poly(SS-co-MAG) alone, the optimized feed ratio of SS and MAG for vascular cell selectivity was 1:1 (PS1M1); at this ratio the Au-PS1M1 surface showed the highest human umbilical vein endothelial cells (HUVECs) density and the lowest human umbilical vein smooth muscle cells (HUVSMCs) density. When self-assembly of PAA alone (surface designated as Au-PAA), the proliferation of both HUVECs and HUVSMCs was inhibited. Compared with either PS1M1 or PAA alone, the surfaces modified with both PAA and PS1M1 at the feed ratio of 1:1 (material designated as Au-PSM/PAA-2) showed enhanced promoting effect on HUVECs as well as enhanced inhibiting effect on HUVSMCs, indicating stronger vascular cell selectivity of carboxylic groups in the presence of sugar and sulfonate units.

Enhancement of Intracellular Calcium Ion Mobilization by Moderately but Not Highly Positive Material Surface Charges

Electrostatic forces at the cell interface affect the nature of cell adhesion and function; but there is still limited knowledge about the impact of positive or negative surface charges on cell-material interactions in regenerative medicine. Titanium surfaces with a variety of zeta potentials between −90 mV and +50 mV were generated by functionalizing them with amino polymers, extracellular matrix proteins/peptide motifs and polyelectrolyte multilayers. A significant enhancement of intracellular calcium mobilization was achieved on surfaces with a moderately positive (+1 to +10 mV) compared with a negative zeta potential (−90 to −3 mV). Dramatic losses of cell activity (membrane integrity, viability, proliferation, calcium mobilization) were observed on surfaces with a highly positive zeta potential (+50 mV). This systematic study indicates that cells do not prefer positive charges in general, merely moderately positive ones. The cell behavior of MG-63s could be correlated with the materials’ zeta potential; but not with water contact angle or surface free energy. Our findings present new insights and provide an essential knowledge for future applications in dental and orthopedic surgery.

Vascular cell responses to silicone surfaces grafted with heparin-like polymers: surface chemical composition vs. topographic patterning

Heparin-like polymers are promising synthetic materials with biological functionalities, such as anticoagulant ability, growth factor binding to regulate cellular functions, and inflammation mediation, similar to heparin. The biocompatibility of heparin-like polymers with well-defined chemical structures has inspired many researchers to design heparin-like surfaces to explore their biological applications. The concept of the recombination of functional heparin structural units (sulfonate- and glyco-containing units) was proven to be successful in designing heparin-mimicking surfaces. However, besides surface structural units, topographic patterning is also an important contributor to the biological activity of the surfaces modified with heparin-like polymers. In this work, both surface structural units and topographic patterning were taken into account to investigate the vascular cell behaviors on the silicone surfaces. A facile method for the production of patterned bromine-containing polydimethylsiloxane surface (PDMS-Br) was developed from a one-step multicomponent thermocuring procedure and replica molding using a nanohole-arrayed silicon template. Different structural units of heparin-like polymers, i.e. homopolymer of sulfonate-containing sodium 4-vinylbenzenesulfonate (pSS), homopolymer of glyco-containing 2-(methacrylamido)glucopyranose (pMAG), and copolymers of MAG and SS (pSG), were then introduced on the flat and patterned PDMS-Br surface using visible light-induced graft polymerization. For the flat surfaces, compared with the PDMS-Br surface, pSS-grafted and pSG-grafted surfaces significantly increased cell densities of both human umbilical vein endothelial cells (HUVECs) and human umbilical vein smooth muscle cells (HUVSMCs), indicating that they are "vascular cell-friendly". In contrast, the pMAG-grafted surface showed decreased cell attachment of both HUVECs and HUVSMCs, indicating that the pMAG-grafted surface is "vascular cell-resistant". Moreover, surface topographic patterning enhanced the cell responses to the corresponding flat surfaces. That is to say, surface patterning can make the "vascular cell-friendly" surface still friendly, and the "vascular cell-resistant" surface much more resistant. The combination of surface structural units and topographic patterning shows promise in the preparation of new heparin-like surfaces with improved cell compatibility that is suitable for blood-compatible biomaterials.

A closer physico-chemical look to the Layer-by-Layer electrostatic self-assembly of polyelectrolyte multilayers

The fabrication of polyelectrolyte multilayer films (PEMs) using the Layer-by-Layer (LbL) method is one of the most versatile approaches for manufacturing functional surfaces. This is the result of the possibility to control the assembly process of the LbL films almost at will, by changing the nature of the assembled materials (building blocks), the assembly conditions (pH, ionic strength, temperature, etc.) or even by changing some other operational parameters which may impact in the structure and physico-chemical properties of the obtained multi-layered films. Therefore, the understanding of the impact of the above mentioned parameters on the assembly process of LbL materials plays a critical role in the potential use of the LbL method for the fabrication of new functional materials with technological interest. This review tries to provide a broad physico-chemical perspective to the study of the fabrication process of PEMs by the LbL method, which allows one to take advantage of the many possibilities offered for this approach on the fabrication of new functional nanomaterials.

Engineering Thiolated Surfaces with Polyelectrolyte Multilayers

Interfaces bearing firmly attached thiol groups are useful for many applications requiring the versatile and facile chemistry of the -SH functionality. In this work, rugged ultrathin films were prepared on substrates using layer-by-layer assembly. The surface of these smooth films was capped with a co-polymer containing benzyl mercaptan units. The utility of this coating was illustrated by three applications. First, thiol-ene "click" chemistry was used to introduce the Arg-Gly-Asp (RGD) adhesive peptide sequence on a surface that otherwise resisted good adhesion of fibroblasts. This treatment promoted cell adhesion and spreading. Similar Michael addition chemistry was employed to attach poly(ethylene glycol) to the surface, which reduced fouling by (adhesion of) serum albumin. Finally, the affinity of gold for -SH was exploited by depositing a layer of gold nanoparticles on the thiolated surface or by evaporating a tenacious film of gold without using the classical chromium "primer" layer.

Proliferation of Cells with Severe Nuclear Deformation on a Micropillar Array

Cellular responses on a topographic surface are fundamental topics about interfaces and biology. Herein, a poly(lactide- co-glycolide) (PLGA) micropillar array was prepared and found to trigger significant self-deformation of cell nuclei. The time-dependent cell viability and thus cell proliferation was investigated. Despite significant nuclear deformation, all of the examined cell types (Hela, HepG2, MC3T3-E1, and NIH3T3) could survive and proliferate on the micropillar array yet exhibited different proliferation abilities. Compared to the corresponding groups on the smooth surface, the cell proliferation abilities on the micropillar array were decreased for Hela and MC3T3-E1 cells and did not change significantly for HepG2 and NIH3T3 cells. We also found that whether the proliferation ability changed was related to whether the nuclear sizes decreased in the micropillar array, and thus the size deformation of cell nuclei should, besides shape deformation, be taken into consideration in studies of cells on topological surfaces.

Surface sulfonates lock serum albumin into a “hard” corona

Albumin is more easily displaced on a surface bearing carboxylate compared to sulfonate functionality, which controls the composition of the eventual protein corona.

Self-assembled phosphate-polyamine networks as biocompatible supramolecular platforms to modulate cell adhesion

The modulation of cell adhesion via biologically inspired materials plays a key role in the development of realistic platforms to envisage not only mechanistic descriptions of many physiological and pathological processes but also new biointerfacial designs compatible with the requirements of biomedical devices. In this work, we show that the cell adhesion and proliferation of three different cell lines can be easily manipulated by using a novel biologically inspired supramolecular coating generated via dip coating of the working substrates in an aqueous solution of polyallylamine in the presence of phosphate anions-a simple one-step modification procedure. Our results reveal that selective cell adhesion can be controlled by varying the deposition time of the coating. Cell proliferation experiments showed a cell type-dependent quasi-exponential growth demonstrating the nontoxic properties of the supramolecular platform. After reaching a certain surface coverage, the supramolecular films based on phosphate-polyamine networks displayed antiadhesive activity towards cells, irrespective of the cell type. However and most interestingly, these antiadherent substrates developed strong adhesive properties after thermal annealing at 37 °C for 3 days. These results were interpreted based on the changes in the coating hydrophilicity, topography and stiffness, with the latter being assessed by atomic force microscopy imaging and indentation experiments. The reported approach is simple, robust and flexible, and would offer opportunities for the development of tunable, biocompatible interfacial architectures to control cell attachment for various biomedical applications.

Stressful Surfaces: Cell Metabolism on a Poorly Adhesive Substrate

The adhesion and proliferation of cells are exquisitely sensitive to the nature of the surface to which they attach. Aside from cell counting, cell "health" on surfaces is typically established by measuring the metabolic rate with dyes that participate in the metabolic pathway or using "live/dead" assays with combinations of membrane permeable/impermeable dyes. The binary information gleaned from these tests-whether cells are attached or not, and whether they are living or dead-provides an incomplete picture of cell health. In the present work, proliferation rates and net metabolism of 3T3 fibroblasts seeded on "biocompatible" ultrathin polyelectrolyte multilayer films and on control tissue culture plastic were compared. Cells adhered to, and proliferated on, both surfaces, which were shown to be nontoxic according to live/dead assays. However, adhesion was poorer on the multilayer surface, illustrated by diffuse organization of the actin cytoskeleton and less-developed focal adhesions. Proliferation was also slower on the multilayer. When normalized for the total number of cells, it was shown that cells on multilayers experienced a five-day burst of metabolic stress, after which the metabolic rate approached that of the control surface. This initial state of high stress has not been reported or appreciated in studies of cell growth on multilayers, although the observation period for this system is usually a few days.

Polyelectrolyte multilayers for bio-applications: recent advancements

The synergistic relationship between structure and the bulk properties of polyelectrolyte multilayer (PEM) films has generated tremendous interest in their application for loading and release of bioactive species. Layer-by-layer assembly is the simplest, cost effective process for fabrication of such PEMs films, leading to one of the most widely accepted platforms for incorporating biological molecules with nanometre precision. The bulk reservoir properties of PEM films render them a potential candidate for applications such as biosensing, drug delivery and tissue engineering. Various biomolecules such as proteins, DNA, RNA or other desired molecules can be incorporated into the PEM stack via electrostatic interactions and various other secondary interactions such as hydrophobic interactions. The location and availability of the biological molecules within the PEM stack mediates its applicability in various fields of biomedical engineering such as programmed drug delivery. The development of advanced technologies for biomedical applications using PEM films has seen rapid progress recently. This review briefly summarises the recent successes of PEM being utilised for diverse bio-applications.

Macroion adsorption-electrokinetic and optical methods

Recent studies on macroion adsorption at solid/liquid interfaces evaluated by electrokinetic and optical methods are reviewed. In the first section a description of electrokinetic phenomena at a solid surface is briefly outlined. Various methods for determining both static and dynamic properties of the electrical double layer, such as the appropriate location of the slip plane, are presented. Theoretical approaches are discussed concerning quantitative interpretation of streaming potential/current measurements of homogeneous macroscopic interfaces. Experimental results are presented, involving electrokinetic characteristics of bare surfaces, such as mica, silicon, glass etc. obtained from various types of electrokinetic cells. The surface conductivity effect on zeta potential is underlined. In the next section, various theoretical approaches, proposed to determine a distribution of electrostatic potential and flow distribution within macroion layers, are presented. Accordingly, the influence of the uniform as well as non-uniform distribution of charges within macroion layer, the dissociation degree, and the surface conductance on electrokinetic parameters are discussed. The principles, the advantages and limits of optical techniques as well as AFM are briefly outlined in Section 4. The last section is devoted to the discussion of experimental data obtained by streaming potential/current measurements and optical methods, such as reflectometry, ellipsometry, surface plasmon resonance (SPR), optical waveguide lightmode spectroscopy (OWLS), colloid enhancement, and fluorescence technique, for mono- and multilayers of macroions. Results of polycations (PEI, PAMAM dendrimers, PAH, PDADMAC) and polyanions (PAA, PSS) adsorption on mica, silicon, gold, and PTFE are quantitatively interpreted in terms of theoretical approaches postulating the three dimensional charge distribution or the random sequential adsorption model (RSA). Macroion bilayer formation, experimentally examined by streaming current measurements, and theoretically interpreted in terms of the comprehensive formalism is also reviewed. The utility of electrokinetic measurements, combined with optical methods, for a precise, in situ characteristics of macroion mono- and multilayer formation at solid/liquid interfaces is pointed out.

Layer-by-Layer polyelectrolyte assemblies for encapsulation and release of active compounds

Soft assemblies obtained following the Layer-by-Layer (LbL) approach are accounted among the most interesting systems for designing biomaterials and drug delivery platforms. This is due to the extraordinary versatility and flexibility offered by the LbL method, allowing for the fabrication of supramolecular multifunctional materials using a wide range of building blocks through different types of interactions (electrostatic, hydrogen bonds, acid-base or coordination interactions, or even covalent bonds). This provides the bases for the building of materials with different sizes, shapes, compositions and morphologies, gathering important possibilities for tuning and controlling the physico-chemical properties of the assembled materials with precision in the nanometer scale, and consequently creating important perspective for the application of these multifunctional materials as cargo systems in many areas of technological interest. This review studies different physico - chemical aspects associated with the assembly of supramolecular materials by the LbL method, paying special attention to the description of these aspects playing a central role in the application of these materials as cargo platforms for encapsulation and release of active compounds.