4D printed hydromorph biocomposites with shape change and force generation properties: progress towards structural actuation

Purpose Plant tissues are capable of paradoxical properties of structural actuation involving shape change while maintaining load-bearing ability. Ancient civilizations used the hydroexpansion properties of wooden wedges, which released blocked forces to fracture certain rocks. Hydromorph biocomposites (HBCs) composed of fibre-reinforced polymer matrix belong to a new generation of materials that draw inspiration from natural actuators. The combination of environmental responsiveness and architecture allows for the design of programmable passive shape changes via four-dimensional (4D) printing. Design/methodology/approach This experimental study investigates structural actuation capabilities of 4D-printed HBCs. Spruce wood is used as a reference to characterize hydroexpansion properties, enabling the quantification of typical structural actuation properties, i.e. energy density. HBCs made of continuous unidirectional flax yarn (cFFs)-reinforced polylactic acid (PLA) with rectilinear shape-changing properties are 4D printed. The relationship between free and constrained hydroexpansion is examined and compared to that of smart materials, using calculated energy density. Findings Results show that hydroexpansion, blocked force and energy density of HBCs can be tuned by varying layer height, print angle orientation and interfilament distance. Energy densities can reach 37.1 ± 3.3 J.g-1.cm-3 that is comparable to different woods species during water immersion (from 9.2 ± 0.3 J.g-1.cm-3 to 477.8 ± 17.1 J.g-1.cm-3). Originality/value Although the shape-changing characteristics of hydromorph biocomposites are being increasingly studied together with their load-bearing potential, current knowledge is still limited regarding the structural actuation potential. This work aims to bring deeper insights into the original performance of HBC.