Unparalleled nanofibril hydrogel actuators by mimicking nature's design.

Soft actuators aim to bridge the gap between rigid machines and soft matter by mimicking the flexibility and compliance of natural muscles and tissues. Stimuli-responsive hydrogels can fit this purpose through their softness and ability for large reversible shape morphing. However, the performance of hydrogel actuators is restricted by their diffusion-limited water transport, which leads to slow responses, and uniform volumetric changes that generate limited actuation forces and strains. Thus, there is a need to develop faster hydrogel actuators that can efficiently convert swelling into actuation force and strain. Anisotropic cellulose nanofibril (CNF) hydrogels offer a route to overcome these limitations by assembling charged fibrils into dense, layered sheets that are reinforced in-plane while remaining compliant in the thickness direction. This architecture redirects water uptake into uniaxial expansion or, under confinement, into high blocking pressure, thereby combining large strain, high force, and rapid response. Here, we establish how the structure and processing of charged CNF networks govern swelling-driven actuation. Specifically, we examine how fibril properties, including aspect ratio and charge density, together with sheet fabrication parameters, including drying conditions and actuator area, control the translation of water uptake into directional strain and force. Optimization of these parameters results in CNF networks that expand uniaxially by 220 times within an hour with initial strain rates of 190-300% s-1, reaching blocking pressures up to 4.9 MPa in less than a minute. These hydrogels are a great step towards hydrogel-based artificial muscles, which have been prevented by the slow response of previously reported stimuli-responsive hydrogels. Further development of fibrillar hydrogel actuators can lead to truly lifelike artificial muscles.