Dual-Stimuli-Responsive Polymer Composite with Ultrawide Tunable Stiffness Range Triggered by Water and Temperature

Variable stiffness materials are an important class of materials that are useful in soft robotics, reconfigurable devices, deployable vehicles, and artificial muscles. Most variable stiffness polymers respond to one environmental cue, such as temperature, light, moisture, or magnetic field, which results in a limited tunable stiffness range. Combining multiple stimulation mechanisms can result in a wider stiffness range and expanded application areas. We have developed a variable stiffness composite with an ultrawide tunable stiffness range using bacterial cellulose and a bistable electroactive polymer (BSEP) matrix that responds to water and temperature, respectively. The composite utilizes phase- changing BSEP, with a sharp transition temperature tuned to around human body temperature, as the matrix material. Bacterial cellulose (BC) nanofibers form a percolative network within the matrix, resulting in a significant mechanical reinforcement from 300 MPa for a neat BSEP matrix, to a storage modulus as high as 1 GPa for the composite material. With the combined stimulation mechanisms, the BC-BSEP composite exhibits a large stiffness change of 24 000-fold from around 1 GPa to 40 kPa. A modulus variation of this magnitude has not been achieved by cellulose composites. The composite combines the reinforcement effect from the BC nanofibers for a high modulus at room temperature and a large modulus differential in BSEP when heated (300 MPa–40 kPa), resulting in a material with an ultrawide modulus change. The composite also shows good biocompatibility for possible applications in the biomedical area.