Spinodal Metamaterials as Pneumatic Actuators for Complex Shape Morphing

Active 3D printed structures capable of shape morphing when subject to an external stimulus are highly useful for a vast amount of applications, ranging from soft robotics to biomedical devices. To actuate such structures, pneumatics offer a readily available and reliable solution and enjoy increasing popularity. However, when used to actuate shape morphing structures, pneumatic actuation usually takes place via monolithic linear actuators. Therefore, a mechanism is inherently needed to translate the linear actuation to a shape change of the overall structure. Even though this allows for simple and standardizable actuators, the involved mechanisms increase the manufacturing and assembly cost and complex target shapes are difficult to achieve. Other pneumatically actuated shape-morphing structures employ more complex designs, but are often manually designed for a certain use case, leaving the design process tedious and the full potential of additive manufacturing remains unused. <br/>The concept proposed in this study takes the conversion of using pneumatic pressure for complex shape morphing to the structural level and avoids any translating mechanisms. To do so, we employ spinodal metamaterials, which have a microstructure consisting of two phases with a complex labyrinthine geometry. Our approach is to 3D print these metamaterials, using an elastomer for one phase while leaving the other phase void and to actuate the metamaterial by applying pneumatic pressure to the void phase. In this study, we investigate the relations between the design of the spinodal microstructure and the material deformation when pneumatically actuated. The focus lies on the possibility of introducing anisotropy into the microstructure, leading to a highly anisotropic deformation response of the material. This anisotropy can be varied continuously within the metamaterial, allowing for designing for a desired deformation when actuated. <br/>Our characterization of these metamaterials starts by generating a set of microstructures with varying degrees, types and directions of anisotropy and different volume fractions of the material phase. The pneumatic actuation is then simulated using nonlinear Finite Element Analysis, considering the nonlinear (hyperelastic) behavior of the elastomer, as well as the option of applying a negative pneumatic pressure (i.e. pull vacuum). The active metamaterial concept is also verified experimentally on a selected set of designs and the experimental and numerical results are compared. Using the gathered knowledge, a more complex structure is designed in which the anisotropy is varied, such that when actuated, a complex shape change is obtained. <br/>The results of this study demonstrate the validity and potential of the concept by its ability to translate pneumatic actuation into a significant deformation. Considering tunability, varying the microstructure, especially its anisotropy, provides a powerful tool to control the material response to pneumatic actuation. Varying the volume fraction of the material phase can increase the resulting deformation while lowering the stiffness or vice versa. Overall, the presented concept for an active metamaterial represents a promising approach for shape morphing, as it allows for complex 3D-to-3D shape changes.