Ultrafast, High-Strain, and Strong Uniaxial Hydrogel Actuators from Recyclable Nanofibril Networks.

Synthetic polymeric hydrogels mimic biological tissues and are suitable for future lifelike machines. However, in polymer hydrogel actuators, actuation is isotropic, so they must be crosslinked or placed in a turgor membrane to achieve high actuation pressures, severely impeding their performance. Here, we show that organizing cellulose nanofibrils in anisotropic fibrillar networks leads to hydrogel sheets with a mechanical in-plane reinforcement that generates a uniaxial, out-of-plane strain with performance far surpassing polymer hydrogels. These fibrillar hydrogel actuators expand uniaxially by 250 times with an initial rate of 100-150% s -1 , compared to <10 times and <1% s -1 in directional strain rate for isotropic hydrogels, respectively. The blocking pressure was up to 0.9 MPa, similar to turgor actuators, while the time to reach 90% of the max pressure was 1-2 minutes, compared to 10 minutes to hours for crosslinked polymers or turgor actuators. We showcase uniaxial actuators that lift objects 120 000 times their weight and soft grippers that grasp objects. In addition, the hydrogels could be recycled without a loss in performance. The uniaxial swelling allowed adding channels through the gel for local solvent delivery, further increasing the actuation rate and cyclability. Thus, fibrillar networks can overcome the major drawbacks of hydrogel actuators and is a significant advancement toward hydrogel-based lifelike machines. This article is protected by copyright. All rights reserved.