Improving water-responsive actuation of microfibrillated cellulose films by aluminum cross-linking

This study focuses on developing and characterizing monolayer films with the purpose of designing a water-responsive bio-based bilayer design, incorporating microfibrillated cellulose (MFC) and carboxymethylcellulose (CMC). MFC, with their high aspect ratio and surface area, provide a network of highly hydrogen-bonded microfibers exhibiting superior mechanical properties. To achieve a highly water-sensitive material, CMC with a high substitution degree of the carboxylic group was incorporated into the MFC matrix. Additionally, to maintain film integrity after water uptake, the films were cross-linked with aluminum ions (Al3+), which improved the film resistance without compromising the water uptake. MFC, MFC/CMC (1:1 mass ratio), and Al3+-cross-linked MFC/CMC films were prepared by vacuum filtration, and their response to water was monitored through water uptake and changes in mechanical properties. Water-responsive actuators were designed and prepared by combining tape with MFC or MFC/CMC layers, with and without Al3+ cross-linking. Bending was observed after incubation at controlled relative humidity or by immersion in water. Al3+ cross-linking clearly improved mechanical resistance, which provided stable films that bent in response to water uptake. The work presented here can be used to control actuation in cellulose-based films and gives insight into the parameters involved in the film bending.