Additive manufacturing of hygromorphic structures using regenerated cellulose/PLA biocomposites

Three-dimensional printing technologies are at the forefront of the current industrial revolution, enabling the production of complex shapes using a wide range of materials without relying on large production facilities. Evolving from these, 4D printing is a new concept in which printed objects respond to environmental stimuli and change shape or other characteristics with time in a controlled and predicted manner. Bioderived and/or biodegradable thermoplastics reinforced with bioderived fibres have compelling attributes for 3D printing applications; they can be recycled, used in smart materials (4D printing) and if necessary, can biodegrade at the end of their life cycle. In this work, we demonstrate the potential use of biocomposites based on poly(lactic acid) (PLA) with a high content of regenerated cellulose fibres (lyocell) in hygromorphic structures, 3D printed using fused deposition modelling (FDM). The shape-change potential was assessed by 3D printing bi-material strips with passive and active layers composed of neat PLA and the composite, respectively and by measuring and modelling the change in curvature with the increase in water absorption. The main mechanism involved in this behaviour is related to the anisotropic swelling of the cellulose fibres that are preferentially aligned in the composite during printing. Using this mechanism and a bi-material layout, prototypes of different sizes with 2D to 3D transformation triggered by moisture were 3D printed