Effect of textile pre-stretch and printed geometry on the curvature of PLA-Lycra 4D textiles

4D textiles are a specific class of 4D printed materials obtained by printing flat patterns on elastically pre-tensioned textiles and being able to switch from planar systems to complex 3D objects after the textile pre-stretch is released. The mechanical balance between textile recovering strain and printed structure stiffness determines the final shape. This study is carried out by coupling pre-stretched Lycra to PLA and explores ways to control 4D textile shape transformations by varying pre-stretch (10% ÷ 60%), printed structure geometry (bar-shaped and star-shaped elements; star-shaped patterns), printed element thickness (0.3 ÷ 3 mm) and mutual distance (2 ÷ 15 mm). By adjusting these parameters, a wide set of out-of-plane curvatures are obtained, ranging from flat, to dome-like and highly curved, wrapped or coiled shapes. Digital optical methods, including digital image analysis, 3D scanning, and digital image correlation, are used to evaluate the complexity of the final shape and strain state evolution during shape transformation. The geometry variation is measured in terms of height increase (maximum 45 mm for a star-shaped system, 30 mm for a multiple star pattern) and of area decrease (maximum 80% for a star-shaped system, 60% for a multiple star pattern). While most shape transformations occur immediately after printing (‘direct 4D printing’), further shape evolutions may be triggered by heating above the PLA glass transition, allowing for the creation of dynamic structures whose shape changes upon external stimuli. The adhesion between the 3D printed element and the stretched textile is also examined, with a focus on determining the role of interfacial strength and the conditions that could enhance it. This study provides an overview of the primary design variables and valuable maps of their impacts on shape transformations in this broad scenario of influencing parameters.