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 [Formula: see text] mm−1 when saturated. Moisture diffusion follows a two-stage process ([Formula: see text] m²/s; [Formula: see text] 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 ([Formula: see text]C; 0.3[Formula: see text]C), surpassing typical building conditions (< 60°C). A module of six flaps highlights constructability. This work lays a scientific groundwork for circular hygromorphic building applications and delivers a persuasive proof-of-concept for biomaterial-based kinetic architectural systems.