A recent study published in Advanced Materials introduced a 4D printing method to create tubular moisture-responsive actuators called meta biocomposites. These structures use continuous flax fiber (cFF)-reinforced materials that change shape in response to humidity. The study explored how to design and program these materials for controlled structural actuation.
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Background
4D printing builds on 3D printing by using materials that respond to environmental changes over time. Hygromorphic biocomposites (HBCs) are a type of 4D-printed material that reacts to humidity. They are made from renewable resources like natural fibers.
Natural fibers act as both sensors and actuators and help scale the mechanical performance of composite materials. However, because of their complex polysaccharide composition, natural fibers are moisture-sensitive, and upon moisture intake, they exhibit architecture-controlled large hygroexpansion.
This study developed a 4D-printed tubular biocomposite designed for precise moisture-triggered shape changes. The goal was to create sustainable, programmable actuators for applications like solar tracking and adaptive structures.
Material Preparation and Experimental Setup
The cFF-reinforced polybutylene adipate terephthalate (PBAT) filament used for 4D printing was produced via co-extrusion. A customized 3D printer was then used to fabricate the samples, maintaining a nozzle temperature of 145 °C and a printing speed of 6 mm/s. The interfilament distance was set at 0.6 mm, with a layer height of 0.3 mm, to ensure precise structural formation.
To analyze the internal structure, the specimens’ transverse cross-sections were examined using a scanning electron microscope (SEM). Before exposure to humidity, the samples were dried in a vacuum oven at 40 °C for at least 48 hours. They were then placed in a controlled chamber at 90 % relative humidity (RH) and 25 °C to induce moisture-driven shape changes.
A prototype of the hygro-sensitive solar tracker was tested in a hermetically sealed enclosure with transparent walls, allowing observation of shape changes under approximately 90 % RH. This high humidity level was maintained using a saturated potassium sulfate (K2SO4). Gravimetric measurements were conducted throughout conditioning to track the moisture content absorbed by the samples.
The actuation behavior of the 4D-printed tubular biocomposites was assessed under two different conditions. In one setup, the samples were free to rotate, while in the second, their movement was restricted to measure the torque generated. Together, these tests provided data to calculate the energy density of the 4D-printed tubular meta biocomposites and evaluate their mechanical performance.
Key Findings
Researchers successfully developed bioinspired, moisture-sensitive rotational actuators using a combination of biomimicry and 4D printing. The tubular meta biocomposites were designed based on the architecture of plant fiber cell walls, allowing them to respond dynamically to humidity changes.
A customized 3D printer enabled the fabrication of tubular biocomposites reinforced with cFF. These materials exhibited high sensitivity to RH due to the hygrophilic nature of flax fiber and its significant transverse hygroscopic expansion.
The actuators demonstrated rotational movement and torque when exposed to 90 % RH. Their performance was influenced by structural parameters such as fiber inclination angle, tube length, inner diameter, and thickness.
Experimental data confirmed a specific torque of up to 29 N m/kg, highlighting their enhanced applicability compared to natural fibers. Additionally, the design flexibility provided by 4D printing allowed exploring various structural configurations and achieving a wide range of actuation in the form of responsiveness (torque and rotation) and reactivity.
The study also confirmed the potential of these biocomposites for solar tracking applications. A specimen programmed with specific structural parameters exhibited heliotropic behavior when exposed to daily RH variations.
Conclusion
This study introduced a novel 4D-printed tubular meta biocomposite using sustainable materials for moisture-driven structural actuation. The cFF/PBAT biocomposites leveraged their high moisture-induced shape-changing ability to achieve programmable mechanical responses.
Instead of replicating the entire bilayer structure of natural fibers, the researchers simplified the design by focusing on the S2 layer of plant cell walls. This approach improved the programmability of the hygro-mechanical coupling between hygroscopic expansion and twisting behavior.
Experimental, analytical, and finite element modeling confirmed the influence of mesostructural and geometric parameters on actuation performance. The findings demonstrated a balance between torque and rotation, with energy density comparable to that of natural fiber cell walls. These results highlight the potential of 4D-printed biocomposites for applications requiring environmentally responsive materials.
Journal Reference
Josselin, M., Castro, M., Cesare, ND., Scarpa, F., Duigou, AL. (2025). Sustainable 4D Printed Meta Biocomposite Materials for Programmable Structural Shape Changing. Advanced Materials. DOI: 10.1002/adma.202418656, https://advanced.onlinelibrary.wiley.com/doi/10.1002/adma.202418656
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