Reaction–Diffusion Process for Hydrogels with a Tailored Layer Structure

Abstract

The architecture of hydrogel composites results in not only synergistic property enhancement but also superior functionality. The reaction–diffusion (RD) process is acommon phenomenon throughout nature which induced ordered structure on a length scale from microscopic to macroscopic. Different from commonly used inorganic salts or oppositely charged nanoparticles for the RD process, a modified RD process was used for layered chitosan hydrogel (L-CH) and layered magnetic chitosan hydrogel (L-MCH). During the modified RD process reported in this paper, the protonated chitosan (CS-NH3+) with iron ions (Fe3+ and Fe2+) was used as an inner-reactant and hydroxide ion (-OH−) was used as an out-reactant. The protonated chitosan (CS-NH3+) not only played the role of an inner-reactant but also the reaction medium which controlled the diffusion behavior of the out-reactant (-OH−). A series of ordered layers were constructed and the ordered layers were parallel with the longitudinal axis. The layer width of L-CH and L-MCH can be tailored by varying interval time T. The mean layer width of L-MCH increased from 50 ± 5.8 μm to 90 ± 6.4 μm when the interval time T increased from 2 min to 5 min. The tailored layer structure of L-CH and L-MCH obeyed the time law and spacing law, which declared that the L-CH and L-MCH were constructed via the reaction–diffusion process. We also show that the tailored layer structure endows hydrogel with enhanced mechanical properties, especially toughness. The yield strength of magnetic chitosan hydrogel was improved significantly (from 95.1 ± 7.6 kPa to 401.7 ± 12.1 kPa, improved by about 4 times) when 10 wt. % magnetite nanoparticles were involved. The enhancement of the mechanical propertieswas due to a physical crosslinking effect of magnetite nanoparticles on chitosan. For L-MCH, the probe displacement reached 28.93 ± 2.6 mm when the rupture occurred, which was as high as 284.7% compared with that of the non-layered hydrogel. The tailor-made hydrogels might be possible for application as a tough implantable scaffold.