Shape memory polymer hygromorph inspired by the pinecone in dry and wet conditions. Image credit: Qinyu Li.
Shape memory polymer hygromorph inspired by the pinecone in dry and wet conditions. Image credit: Qinyu Li.
The design of the grasping structure consists of a stiff base and five connected strips of the hygromorph bio-based laminate. The evolution of the shape in water and air (RH = 50%) is displayed. The maximum lifting weight of the ball is around 2.3 g in air, while the five strips only weigh 2.8 g. Three strips can hold six pens of around 40 g weight in air with ease.
The design of the grasping structure consists of a stiff base and five connected strips of the hygromorph bio-based laminate. The evolution of the shape in water and air (RH = 50%) is displayed. The maximum lifting weight of the ball is around 2.3 g in air, while the five strips only weigh 2.8 g. Three strips can hold six pens of around 40 g weight in air with ease.

Smart materials transform in shape in response to external triggers such as heat, light, electricity or a magnetic field, bending, twisting and folding. Existing moisture-responsive materials, however, have proved difficult to apply in practice usefully because humidity tends to be uncontrolled in real operating environments. Now researchers from the University of Bristol, Imperial College London, Université Bretagne Sud, and Harbin Institute of Technology have created a new class of natural fiber-reinforced shape memory polymer that autonomously morphs in a controlled fashion into different shapes in response to humidity [Li et al., Applied Materials Today 27 (2022) 101414, https://doi.org/10.1016/j.apmt.2022.101414].

“When these composites are laminated according to special stacking sequences, they become hygromorphs – changing their shape and deforming in curved configurations, like pinecones scales,” explains Fabrizio Scarpa of the University of Bristol, who led the work with Qinyu Li and Jinsong Leng.

Inspired by the way in which natural fibers absorb moisture and swell asymmetrically, the team created the hygro-thermo responsive shape-morphing composite HyTemC from natural flax fibers embedded in a shape memory polymer matrix. By preheating the composite in controlled humidity conditions, the architecture of the flax fibers and their distribution within the matrix can be programmed. Once cooled, the composite will respond autonomously to specific humidity/temperature levels. Simultaneously, the flax fibers provide reinforcement to the composite, creating a much stronger material.

“These programmable features in a bio-based smart material can be used to develop actuators and sensors based on eco-friendly and high-performance materials for a whole range of possible applications from robotics, programmable shape changing/morphing structures to environmental monitoring,” says Scarpa.

Two proof-of-concept devices demonstrate the capabilities of the material: a bio-robotic five-fingered ‘hand’ and an actuator for a smart frame. The robotic hand can pick up a ball or cylindrical objects many times heavier than its own weight, both in water and humid conditions. The smart frame, meanwhile, provides concave or convex surfaces in response to changes in humidity.

“This new class of shape memory polymer hygromorph broadens the design space for smart actuators by using temperature as a stimulus to store shapes,” points out Scarpa. “Moreover, these new hygromorphs are remarkably stiff [so] can also be used for secondary load bearing applications.”

This is, the researchers believe, the first example of how sustainable plant fiber-based materials can be programmed for use in robotics. They are now working to reduce the actuation time by increasing the fiber volume fraction and using 4D printing/additive manufacturing to produce thinner laminates.