Strong injectable chitosan thermosensitive hydrogels can be created, without chemical modification, by combining sodium hydrogen carbonate (SHC) with another weak base, namely beta-glycerophosphate (BGP) or phosphate buffer. Here we studied the influence of gelation agent concentration on the mechanical properties, gelation kinetics, osmolality, swelling and compatibility for cell encapulation, in order to find This is the peer reviewed version of the article published in final form at http://doi.org/10.1002/mabi.201600435. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.2 the most optimal formulations and demonstrate their potential for cell therapy and tissue engineering.The new formulations present up to a 50 fold increase of the Young's modulus after gelation compared with conventional chitosan-BGP hydrogels, while reducing the ionic strength to the level of iso-osmolality. Increasing PB concentration accelerates gelation but reduces the mechanical properties. Increasing BGP also has this effect, but to a lesser extent. Cells can be easily encapsulated by mixing the cell suspension within the hydrogel solution at room temperature, prior to rapid gelation at body temperature.After encapsulation, L929 mouse fibroblasts were homogeneously distributed within scaffolds and presented a strongly increased viability and growth compared with conventional chitosan-BGP hydrogels. Two particularly promising formulations were evaluated with human mesenchymal stem cells. Their viability and metabolic activity were maintained over 7 days in vitro.

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Hydrogels are extensively used for tissue engineering, cell therapy or controlled release of bioactive factors. Nondestructive techniques that can follow their viscoelastic properties during polymerization, remodeling, and degradation are needed, since these properties are determinant for their in vivo efficiency. In this work, we proposed the viscoelastic testing of bilayered materials (VeTBiM) as a new method for nondestructive and contact‐less mechanical characterization of soft materials. The VeTBiM method measures the dynamic displacement response of a material, to a low amplitude vibration in order to characterize its viscoelastic properties. We validated VeTBiM by comparing data obtained on various agar and chitosan hydrogels with data from rotational rheometry, and compression tests. We then investigated its potential to follow the mechanical properties of chitosan hydrogels during gelation and in the presence of papain and lysozyme that induce fast or slow enzymatic degradation. Due to this nondestructive and contactless approach, samples can be removed from the instrument and stored in different conditions between measurements. VeTBiM is well adapted to follow biomaterials alone or with cells, over long periods of time. This new method will help in the fine tuning of the mechanical properties of biomaterials used for cell therapy and tissue engineering. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2565–2573, 2017.

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