Programmable and reconfigurable biobased composite materials and metasurfaces with hygromorph behaviour

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Humidity responsive materials are attractive to the research community because of their adaptive characteristics to the surrounding environment, without need of using external devices to provide the stimuli. Natural fibre hygromorph composites are promising because of their significant mechanical performance, low cost and environmental friendly characteristics. However, current hygromorph composites have limitations in terms of adaptation to the surrounding relative humidity, because of their one-to-one relationship between the morphed shapes and the internal moisture they contain to trigger the actuation. This feature limits the application scenarios, because the required motions will only occur at specific humidity conditions. In other words, designed hygromorph composites will only work as actuators when the humidity is at a certain value, which may not be available during practical operational conditions.

To overcome the one-to-one relationship limitation, a shape memory polymer has been here selected as the matrix of hygromorph composites reinforced with flax fibres. The combined materials (called HyTemCs) are responsive to both moisture and thermal stimuli. At room temperature, the HyTemC is sensitive to moisture like the existing hygromorph composites. HyTemC do not show morphing over the glass transition temperature of the shape memory polymer matrix, although they absorb the same amount of moisture as in the room temperature case. In this thesis, the materials and mechanisms related to HyTemCs are introduced. Their moisture diffusion, hrgroscopic expansion, hygro-elastic properties are also modelled and tested, and the actuation performance of these biobased composites evaluated. In this work, two prototypes using these novel hygromorph biobased composites have been manufactured: one is a gripper with five fingers made using HyTemCs, and the other is an active morphing electroadhesive metasurface actuated by HyTemC. The prototypes show that the bending motion from flat to curvy state can be realised in both water and air environments; this is something not provided by current state-of-the-art hygromorph biobased composites. The HyTemCs can be also cut into small elements and distributed into an actuating pattern, hence their potential use as platforms for programmable electroadhesive metasurfaces. These metasurfaces change shapes from flat into several complex ones, like concave, convex, sine wave, dual concave. The objects handled by these biobased metasurface can be fragile, because of the high conformal matching between the contacting surfaces and the absence of compressive adhesion.

Hygromorph composites absorb moisture, produce hygroscopic expansion and morph because of the moisture-induced internal stresses. In order to understand the mechanism, a new 3D moisture diffusion method has been also proposed in this thesis. Existing models to design hygromorph composites are based on one-dimensional diffusion theories along the thickness direction, rather than along the three spatial directions of the laminate. The new method predicts the diffusion effects in different flax fibre laminates designs and architectures, together with the internal stresses of the composites induced by the three-dimensional hygroscopic expansion. This model, together with the actuation and adaptive performance of the shape memory-based polymer hygromorphs developed in this PhD work, can be useful to design the next generation of biobased and sustainable smart actuators and materials.
Date of Award3 Oct 2023
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorFabrizio Scarpa (Supervisor), Giuliano Allegri (Supervisor) & Antoine Le Duigou (Supervisor)

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