Combining Hydromorphic and Shape-Shifting Effects for Programmable Passive Actuation in 4D-Printed Biocomposites

Abstract

4D printing involves the fabrication of structures that respond dynamically to external stimuli. While various actuation mechanisms have been extensively studied, the integration of multiple actuation effects remains relatively unexplored. This study examines the combined influence of hydromorphic and shape-shifting effects in a biocomposite containing 40 wt% cellulose fibres. The biocomposite was combined with polylactide (PLA) and processed via 3D printing to produce bi-material strips, which were then comprehensively analysed for their material properties and shape-changing behaviour. Upon immersion in water, the biocomposite expanded by 3.2 % perpendicular to the printing direction and exhibited a marked decrease in Young’s modulus with increasing water content. Bi-material strips 1 mm thick displayed pronounced hydromorphic behaviour, achieving maximum curvatures of 0.036 - 0.052 mm-1 depending on the passive-to-active layer thickness ratio. Thicker strips (4 mm) reached 0.012 mm-1 under the same conditions. When both heat and water were applied as external stimuli, the biocomposite and neat PLA layers became simultaneously active, and curvatures of up to 0.057 mm⁻¹ were obtained for 4 mm strips. This curvature surpassed the hydromorphic-only and shape-shifting-only responses by factors of 4.77 and 1.82, respectively. The results demonstrate that integrating water- and heat-induced stimuli increases achievable curvature and accelerates the response, enabling high-curvature deformations even in relatively thick structures. The approach therefore holds promise for applications that demand significant shape transformations without compromising structural integrity.