Highly stretchable hydrogels from complex coacervation of natural polyelectrolytes.

The controlled complex coacervation of oppositely charged hyaluronic acid (Mw ≈ 800-1000 kg mol-1) and chitosan (Mw ≈ 160 kg mol-1, degree of acetylation = 15%) led to hydrogels with controllable properties in terms of elasticity and strength. In this work, we performed desalting by dialysis of high ionic strength solutions of mixed polyelectrolytes and showed that the control of the pH during the polyelectrolyte assembly greatly impacts the mechanical properties of the hydrogel. First, for pHs from 5.5 to 7.5, a slight coacervation was observed due to low chitosan protonation and poor polyelectrolyte associations. Then, for pHs from 3.0 to 5.5, coacervation and syneresis led to free-standing and easy to handle hydrogels. Finally, for pHs from 2.0 to 3.0 (close to the pKa of the hyaluronic acid), we observed the unusual stretchability of these hydrogels that could arise from the pre-folding of hyaluronic acid chains while physical crosslinking was achieved by hyaluronic acid/chitosan polyelectrolyte complexation.

In Vitro Mechanical Property Evaluation of Chitosan-Based Hydrogels Intended for Vascular Graft Development.

Vascular grafts made of synthetic polymers perform poorly in cardiac and peripheral bypass applications. In these applications, chitosan-based materials can be produced and shaped to provide a novel scaffold for vascular tissue engineering. The goal of this study was to evaluate in vitro the mechanical properties of a novel chitosan formulation to assess its potential for this scaffold. Two chitosan-based hydrogel tubes were produced by modulating chitosan concentration. Based on the standard ISO 7198:1998, the hydrogel tubes were characterized in vitro in terms of suture retention strength, tensile strength, compliance, and burst pressure. By increasing chitosan concentration, suture retention value increased to reach 1.1 N; average burst strength and elastic moduli also increased significantly. The compliance seemed to exhibit a low value for chitosan tubes of high concentration. By modulating chitosan concentration, we produced scaffolds with suitable mechanical properties to be implanted in vivo and withstand physiological blood pressures.

Liposome-loaded chitosan physical hydrogel: toward a promising delayed-release biosystem.

This work deals with the elaboration of an original biosystem in view of its application as drug delayed-release device in biomedical area. This innovative "hybrid" system is composed of phosphatidylcholine liposomes entrapped within a chitosan physical hydrogel (only constituted of polymer and water). To this end, pre-formed liposomes were suspended into chitosan solutions, and the polymer gelation process was subsequently carried out following particular experimental conditions. This liposome incorporation did absolutely not prevent the gel formation as shown by rheological properties of the resulting tridimensional matrix. The presence of liposomes within the hydrogel was confirmed by fluorescence and cryo-scanning electron microscopies. Then, the expected concept of delayed-release of this "hybrid" system was proved using a model water soluble molecule (carboxyfluorescein, CF) encapsulated in liposomes, themselves incorporated into the chitosan hydrogel. The CF release was assayed after repeated and intensive washings of hydrogels, and was found to be higher in the CF-in-hydrogel systems in comparison with the CF-in-liposomes-in-hydrogel ones, demonstrating a CF delayed-release thanks to lipid vesicles.

Bioresorption mechanisms of chitosan physical hydrogels: a scanning electron microscopy study.

Tissue-engineered biodegradable medical devices are widely studied and systems must present suitable balance between versatility and elaboration simplicity. In this work, we aim at illustrating that such equilibrium can be found by processing chitosan physical hydrogels without external cross-linker. Chitosan concentration, degree of acetylation, solvent composition, and neutralization route were modulated in order to obtain hydrogels exhibiting different physico-chemical properties. The resulting in vivo biological response was investigated by scanning electron microscopy. "Soft" hydrogels were obtained from chitosan of high degree of acetylation (35%) and by the neutralization with gaseous ammonia of a chitosan acetate aqueous solutions presenting low polymer concentration (Cp=1.6% w/w). "Harder" hydrogels were obtained from chitosan with lower degree of acetylation (5%) and after neutralization in sodium hydroxide bath (1M) of hydro-alcoholic chitosan solutions (50/50 w/w water/1,2-propanediol) with a polymer concentration of 2.5% w/w. Soft and hard hydrogels exhibited bioresorption times from below 10 days to higher than 60 days, respectively. We also evidenced that cell colonization and neo-vascularization mechanisms depend on the hydrogel-aggregated structure that is controlled by elaboration conditions and possibly in relation with mechanical properties. Specific processing conditions induced micron-range capillary formation, which can be assimilated to colonization channels, also acting on the resorption scenario.

Physicochemical modulation of chitosan-based hydrogels induces different biological responses: interest for tissue engineering.

Polysaccharide-based hydrogels are remarkable materials for the development of tissue engineering strategies as they meet several critical requirements for such applications and they may partly mimic the extracellular matrix. Chitosan is widely envisioned as hydrogel in biomedical fields for its bioresorbability, biocompatibility, and fungistatic and bacteriostatic properties. In this study, we report that the modulation of the polymer concentration, the degree of acetylation, the gelation processes [or neutralization routes (NR)] in the preparation of different chitosan-based hydrogels lead to substantially and significantly different biological responses. We show that it is possible to tune the physicochemical characteristics, mechanical properties, and biological responses of such matrices. Physical hydrogels prepared from highly acetylated chitosan were softer, degraded quickly in vivo, and were not suitable for in vitro culture of human mesenchymal stem and progenitor derived endothelial cells. In contrast, for a same chitosan concentration and obtained by the same processing route, a low degree of acetylation chitosan hydrogel provided a more elastic material, better cell adhesion on its surface and tissue regeneration, and restored tissue neo-vascularization as well. This work offers promising and innovative perspectives for the design of hydrogel materials with tunable properties for tissue engineering and regenerative medicine.