Unraveling the Effect of the Hydration Level on the Molecular Mobility of Nanolayered Polymeric Systems

Marine-origin polysaccharides-based free-standing multilayered membranes as sustainable nanoreservoirs for controlled drug delivery

The layer-by-layer (LbL) assembly technology has been widely used to functionalise surfaces and precisely engineer robust multilayered bioarchitectures with tunable structures, compositions, properties, and functions at the nanoscale by resorting to a myriad of building blocks exhibiting complementary interactions. Among them, marine-origin polysaccharides are a sustainable renewable resource for the fabrication of nanostructured biomaterials for biomedical applications owing to their wide bioavailability, biocompatibility, biodegradability, non-cytotoxicity, and non-immunogenic properties. Chitosan (CHT) and alginate (ALG) have been widely employed as LbL ingredients to shape a wide repertoire of size- and shape-tunable electrostatic-driven multilayered assemblies by exploring their opposite charge nature. However, the insolubility of CHT in physiological conditions intrinsically limits the range of bioapplications of the as-developed CHT-based LbL structures. Herein, we report the preparation of free-standing (FS) multilayered membranes made of water-soluble quaternised CHT and ALG biopolymers for controlled release of model drug molecules. The influence of the film structure in the drug release rate is studied by assembling two distinct set-ups of FS membranes, having the model hydrophilic drug fluorescein isothiocyanate-labelled bovine serum albumin (FITC-BSA) either as an intrinsic building block or added as an outer layer after the LbL assembly process. Both FS membranes are characterised for their thickness, morphology, in vitro cytocompatibility, and release profile, with those having FITC-BSA as an intrinsic LbL ingredient denoting a more sustained release rate. This work opens up new avenues for the design and development of a wide array of CHT-based devices for biomedical applications, overcoming the limitations associated with the insolubility of native CHT under physiological conditions.

Chitosan-Based Biomaterials: Insights into Chemistry, Properties, Devices, and Their Biomedical Applications

Chitosan is a marine-origin polysaccharide obtained from the deacetylation of chitin, the main component of crustaceans’ exoskeleton, and the second most abundant in nature. Although this biopolymer has received limited attention for several decades right after its discovery, since the new millennium chitosan has emerged owing to its physicochemical, structural and biological properties, multifunctionalities and applications in several sectors. This review aims at providing an overview of chitosan properties, chemical functionalization, and the innovative biomaterials obtained thereof. Firstly, the chemical functionalization of chitosan backbone in the amino and hydroxyl groups will be addressed. Then, the review will focus on the bottom-up strategies to process a wide array of chitosan-based biomaterials. In particular, the preparation of chitosan-based hydrogels, organic–inorganic hybrids, layer-by-layer assemblies, (bio)inks and their use in the biomedical field will be covered aiming to elucidate and inspire the community to keep on exploring the unique features and properties imparted by chitosan to develop advanced biomedical devices. Given the wide body of literature that has appeared in past years, this review is far from being exhaustive. Selected works in the last 10 years will be considered.

Green approaches for extraction, chemical modification and processing of marine polysaccharides for biomedical applications

Over the past few decades, natural-origin polysaccharides have received increasing attention across different fields of application, including biomedicine and biotechnology, because of their specific physicochemical and biological properties that have afforded the fabrication of a plethora of multifunctional devices for healthcare applications. More recently, marine raw materials from fisheries and aquaculture have emerged as a highly sustainable approach to convert marine biomass into added-value polysaccharides for human benefit. Nowadays, significant efforts have been made to combine such circular bio-based approach with cost-effective and environmentally-friendly technologies that enable the isolation of marine-origin polysaccharides up to the final construction of a biomedical device, thus developing an entirely sustainable pipeline. In this regard, the present review intends to provide an up-to-date outlook on the current green extraction methodologies of marine-origin polysaccharides and their molecular engineering toolbox for designing a multitude of biomaterial platforms for healthcare. Furthermore, we discuss how to foster circular bio-based approaches to pursue the further development of added-value biomedical devices, while preserving the marine ecosystem.

Effect of Different Crosslinking Strategies on Physical Properties and Biocompatibility of Freestanding Multilayer Films Made of Alginate and Chitosan

Freestanding multilayer films prepared by layer-by-layer technique have attracted interest as promising materials for wound dressings. The goal is to fabricate freestanding films using chitosan (CHI) and alginate (ALG) including subsequent crosslinking to improve the mechanical properties of films while maintaining their biocompatibility. Three crosslinking strategies are investigated, namely use of calcium ions for crosslinking ALG, 1-ethyl-3-(-3-dimethylaminopropyl) carbodiimide combined with N-hydroxysuccinimide for crosslinking ALG with CHI, and Genipin for crosslinking chitosan inside the films. Different characteristics, such as surface morphology, wettability, swelling, roughness, and mechanical properties are investigated showing that films became thinner, exhibited rougher surfaces, had lower water uptake, and increased mechanical strength after crosslinking. Changes of wettability are moderate and dependent on the crosslinking method. In vitro cytotoxicity and cell attachment studies with human dermal fibroblasts show that freestanding CHI-ALG films represent a poorly adhesive substratum for fibroblasts, while studies using incubation of plastic-adherent fibroblast beneath floating films show no signs of cytotoxicity in a time frame of 7 days. Results from cell experiments combined with film characteristics after crosslinking, indicate that crosslinked freestanding films made of ALG and CHI may be interesting candidates for wound dressings.

* 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.

Multilayered membranes with tuned well arrays to be used as regenerative patches

Membranes have been explored as patches in tissue repair and regeneration, most of them presenting a flat geometry or a patterned texture at the nano/micrometer scale. Herein, a new concept of a flexible membrane featuring well arrays forming pore-like environments to accommodate cell culture is proposed. The processing of such membranes using polysaccharides is based on the production of multilayers using the layer-by-layer methodology over a patterned PDMS substrate. The detached multilayered membrane exhibits a layer of open pores at one side and a total thickness of 38±2.2µm. The photolithography technology used to produce the molds allows obtaining wells on the final membranes with a tuned shape and micro-scale precision. The influence of post-processing procedures over chitosan/alginate films with 100 double layers, including crosslinking with genipin or fibronectin immobilization, on the adhesion and proliferation of human osteoblast-like cells is also investigated. The results suggest that the presence of patterned wells affects positively cell adhesion, morphology and proliferation. In particular, it is seen that cells colonized preferentially the well regions. The geometrical features with micro to sub-millimeter patterned wells, together with the nano-scale organization of the polymeric components along the thickness of the film will allow to engineer highly versatile multilayered membranes exhibiting a pore-like microstructure in just one of the sides, that could be adaptable in the regeneration of multiple tissues.

STATEMENT OF SIGNIFICANCE

Flexible multilayered membranes containing multiple micro-reservoirs are found as potential regenerative patches. Layer-by-layer (LbL) methodology over a featured PDMS substrate is used to produce patterned membranes, composed only by natural-based polymers, that can be easily detached from the PDMS substrate. The combination of nano-scale control of the polymeric organization along the thickness of the chitosan/alginate (CHT/ALG) membranes, provided by LbL, together with the geometrical micro-scale features of the patterned membranes offers a uniqueness system that allows cells to colonize 3-dimensionally. This study provides a promising strategy to control cellular spatial organization that can face the region of the tissue to regenerate.

Multilayered Hollow Tubes as Blood Vessel Substitutes

The available therapies for cardiovascular pathologies often require the replacement of diseased vascular grafts. However, the current blood vessel substitutes are unsuitable for small-diameter blood vessel replacements. Herein, we propose the creation of multilayered hollow tubes as blood vessel substitutes. Hollow tubes were obtained by building-up multilayers of marine-derived polysaccharides (i.e., chitosan and alginate) on sacrificial tubular templates using layer-by-layer technology and template leaching. A cross-linking degree of ≈59% was achieved using genipin, which is reflected in an increase of the mechanical properties and a decrease of the water uptake. To further improve the cell adhesive properties of the multilayers, fibronectin (FN) was immobilized on the surface of the hollow tubes. The in vitro biological performance of human umbilical vein endothelial cells (HUVECs) and human aortic smooth muscle cells (HASMCs) was assessed. In addition, to perform the culture of HUVECs on the inner side and the HASMCs on the outer side of the tubes, an in-house developed apparatus was created that allowed us to feed cells with their respective culture medium. The developed hollow tubes were shown to be a suitable structure to promote cell adhesion, spreading, and proliferation. It is our belief that the creation of these functional structures will open a new research field in order to develop innovative multilayered tubular structures for cardiovascular TE applications.

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.

Elastic chitosan/chondroitin sulfate multilayer membranes

Freestanding multilayered films were obtained using layer-by-layer (LbL) technology from the assembly of natural polyelectrolytes, namely chitosan (CHT) and chondroitin sulfate (CS). The morphology and the transparency of the membranes were evaluated. The influence of genipin (1 and 2 mg ml(-1)), a naturally-derived crosslinker agent, was also investigated in the control of the mechanical properties of the CHT/CS membranes. The water uptake ability can be tailored by changing the crosslinker concentration that also controls the Young's modulus and ultimate tensile strength. The maximum extension tends to decrease upon crosslinking with the highest genipin concentration, compromising the elastic properties of CHT/CS membranes: nevertheless, when using a lower genipin concentration, the ultimate tensile stress is similar to the non-crosslinked one, but exhibits a significantly higher modulus. Moreover, the crosslinked multilayer membranes exhibited shape memory properties, through a simple hydration action. The in vitro biological assays showed better L929 cell adhesion and proliferation when using the crosslinked membranes and confirmed the non-cytotoxicity of the developed CHT/CS membranes. Within this research work, we were able to construct freestanding biomimetic multilayer structures with tailored swelling, mechanical and biological properties that could find applicability in a variety of biomedical applications.

Polysaccharide-based freestanding multilayered membranes exhibiting reversible switchable properties

The design of self-standing multilayered structures based on biopolymers has been attracting increasing interest due to their potential in the biomedical field. However, their use has been limited due to their gel-like properties. Herein, we report the combination of covalent and ionic cross-linking, using natural and non-cytotoxic cross-linkers, such as genipin and calcium chloride (CaCl2). Combining both cross-linking types the mechanical properties of the multilayers increased and the water uptake ability decreased. The ionic cross-linking of multilayered chitosan (CHI)-alginate (ALG) films led to freestanding membranes with multiple interesting properties, such as: improved mechanical strength, calcium-induced adhesion and shape memory ability. The use of CaCl2 also offered the possibility of reversibly switching all of these properties by simple immersion in a chelate solution. We attribute the switch-ability of the mechanical properties, shape memory ability and the propensity for induced-adhesion to the ionic cross-linking of the multilayers. These findings suggested the potential of the developed polysaccharide freestanding membranes in a plethora of research fields, including in biomedical and biotechnological fields.

pH Responsiveness of Multilayered Films and Membranes Made of Polysaccharides

We investigated the pH-dependent properties of multilayered films made of chitosan (CHI) and alginate (ALG) and focused on their postassembly response to different pH environments using a quartz crystal microbalance with dissipation monitoring (QCM-D), swelling studies, ζ potential measurements, and dynamic mechanical analysis (DMA). In an acidic environment, the multilayers presented lower dissipation values and, consequently, higher moduli when compared with the values obtained for the pH used during the assembly (5.5). When the multilayers were exposed to alkaline environments, the opposite behavior occurred. These results were further corroborated by the ability of this multilayered system to exhibit a reversible swelling-deswelling behavior within the pH range from 3 to 9. The changes in the physicochemical properties of the multilayer system were gradual and different from those of individual solubilized polyelectrolytes. This behavior is related to electrostatic interactions between the ionizable groups combined with hydrogen bonding and hydrophobic interactions. Beyond the pH range of 3-9, the multilayers were stabilized by genipin cross-linking. The multilayered films also became more rigid while the pH responsiveness conferred by the ionizable moieties of the polyelectrolytes was preserved. This work demonstrates the versatility and feasibility of LbL methodology to generate inherently pH stimulus-responsive nanostructured films. Surface functionalization using pH responsiveness endows several biomedical applications with abilities such as drug delivery, diagnostics, microfluidics, biosensing, and biomimetic implantable membranes.