Researchers at the University of Stuttgart have been experimenting with hygromorphic smart structures as passive systems to generate movement in various applications. Hygromorphic means that they change shape when the structures come into contact with water, or in this case, when the humidity in the environment changes.
Read on for more information.
In general, material limitations hamper the reliability and repeatability of moisture sensitive actuation. The authors of the article proposed a co-design method for 4D printing hygromorphic structures using biobased cellulose-filled filaments with variable stiffness and hygroresponsiveness, and designed mesoscale structuring into printed elements.
The research team successfully designed, fabricated, and tested 4D-printed prototypes that can morph in response to relative humidity, with rapid, reversible, and repeatable motion over numerous cycles.
The structures were able to alter their shape in a matter of minutes, in conditions of 35% to 90% relative humidity (RH), corresponding to natural changes in RH in daily and seasonal weather cycles. This means that the structures can act when the weather changes, which can be beneficial in providing shade or shelter for the building’s inhabitants when the weather changes.
You can see one of the structures printed in the image below. The graph shows the petals of the structure opening as the RH increases over time.
For the 4D structures, the research team produced biocomposite filament materials by combining native cellulose powder and two partially biobased thermoplastic matrix polymers.
The three-step process involved drying the materials, producing pellets by compounding, and extruding them into filaments with a constant diameter of 1.75mm. The dimensional accuracy of the produced filaments was ensured by continuous measurement with a laser micrometer and wiremaster.
You can see a close up of one of the printed components, its composition and the driven part in the image below.
How does it work
The structure works thanks to a functional bilayer scheme, consisting of a hygroscopic active layer printed with cellulose-filled filaments and a moisture-stable restrictive layer printed with pure polymers. Many readers will know that plastics can be a bit hygroscopic, and this normally negative aspect was used as a feature in this design work. The plastic matrix in these experiments consisted of PK and TPU polymers.
An optional tie layer can also be added to maintain the restrictive layer between two compatible material layers, preventing delamination of the overall layer. The trajectory design parameters were adjusted to adjust the mesostructural anisotropy and porosity to physically program the shape change in the bilayer constructions.
The bending curvature was controlled by variations in the thickness of the layers and the fill ratio of the restraining layer.
The researchers conclude that, given the rapid response time of the actuation, as well as the relative humidity range in which the actuation occurs, this method can be used to produce intelligent 4D printed structures in adaptive architectural building envelopes. outdoor. For example, structures made using this method can be opened in high RH conditions (usually during the fall/winter or at night) and closed to provide shade in low RH conditions (usually in the hot summer).
You can read the full paper, titled “Co-design of biobased cellulose-filled filaments and mesostructures for 4D printing of moisture-sensitive smart structures” at this link.
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