15 - Variable stiffness 4D printing

Abstract

Variable stiffness or compliance is a feature that is well-known in many naturally occurring materials ranging from wood and bone. For a long time, replicating this characteristic in man-made materials, without prohibitive cost, was an arduous task due to limitations imposed by the manufacturing process. However, the recent rise in additive manufacturing has facilitated the generation of functional materials that can be endowed with temporal and spatial stiffness. Four-dimensional (4D) printing offers the ability to develop variable stiffness structures by controlling their compliance using heterogeneous materials such as shape memory polymers (SMPs). The stiffness of these structures can be adjusted to introduce passive sensing to control the stiffness of the structures based on the variation in the stimulus. SMPs can be easily processed and printed while offering a high stiffness change of more than 100 times when exposed to stimuli such as temperature. This chapter will provide a brief historical account of research into variable stiffness and report the results of our investigations into the effect of fused deposition modeling fabrication parameters on the response of polymeric parts designed with variable stiffness along with both the axial and transverse directions. This chapter presents three types of variable stiffness 4D-printed structures that are realized using single and multiple materials. The single material type is achieved by changing the amount of infill percentage inside the materials, using different printing patterns of different profiles to change the directional stiffness of the materials, and hinges that are made of different printed patterns. Then, the multimaterial actuators are accomplished by using multiple materials with different stiffness values on the same structures. The bending angle values of the developed actuators vary in the range of 2.05–88.25.