Rice husk 4D printing in architecture: hygromorphic bi-directional self-shaping building skin

This study presents rice husk (RH) 4D printing to create compliantly lightweight, humidity-responsive shading skins from biowaste for buildings. It combines agro-waste upcycling, pellet-based 3D printing, and kinetic architectural design to produce a novel hygromorphic RH– polylactic acid (PLA) biocomposite. The design features a triangular bilayer flap (150 × 150 × 150 mm) with an active layer of 20 wt% RH–PLA (0.3 mm thick) laminated between two passive PLA sheets (0.3 mm each, ≤ 50% infill) oriented perpendicularly. This enables fast, reversible hygromorphic actuation, with full recovery at low humidity (∼20%) and curvatures up to 2.5×10−3 mm−1 when saturated. Moisture diffusion follows a two-stage process (D1=7.49×10−11 m²/s; D2=3.66×10−12 m²/s), achieving peak uptake of 2.373% due to layer-specific hygroexpansion. Accelerated aging tests of 100 humidity cycles confirm long-term durability, with consistent curvatures (0.0045 ± 0.0001 mm−1 absorption; 0.0031 ± 0.0002 mm−1 desorption) and reduced stress to 4.61 ± 0.61 MPa. Thermogravimetric analysis shows strong thermal stability (T50%;=364–373∘C; 0.3T5%;=96.9–101.7∘C), surpassing typical building conditions (