Development of Novel Soft Materials using 3D-Printed Lattice Structures and Application for Soft Modular Robots

In recent years, soft robots made of soft, flexible materials have been attracting attention as an alternative to conventional robots made of hard rigid bodies such as metal [1]. Soft robots are expected to play an active role in fields where there are many opportunities to interact with humans, such as collaborative work, nursing care, and welfare. These soft robots are often made of polymers such as rubber, gel, and plastic [2]. By controlling the properties of flexible materials, such as large elastic deformation, elastic modulus, and viscoelasticity, it is possible to design and develop functionalities that take advantage of the softness, which has not been possible with conventional robot technology [3]. In this study, we investigated how to control the physical properties of soft matter by compositing the 3D printed lattice structure of soft matter with silicone rubber. In addition, to develop a soft robot using this technology, we attempted to create a pneumatically driven modular robots by incorporating the soft lattice structure into a hollow 3D silicone structure. These modular robots are made of voxel units, which can be rearranged to change the model and deformation behavior easily. 3D model data of the lattice, called “Igeta structure” [4] (beam and girder structure) was designed using OpenSCAD, script-based 3D CAD software. Five 3D models of lattice structures with different crossing angles were created and they were 3D printed with an FDM 3D printer using Thermoplastic polyurethane (TPU) material. The printed lattice models were placed in a 20mm cubic mold, and silicone rubber (Ecoflex00-30, Smooth-on) was poured into the mold and cured. Compression tests in three orthogonal axes (X, Y, and Z) were performed on these five models using an ORIENTEC STA-1150 universal testing machine. As a result, it was found that the elastic anisotropy in the X- and Y-axis directions increased as the intersection angle moved away from orthogonal. In the next step, we fabricated a pneumatically driven soft modular robot incorporating an anisotropic elastic structure. One module consists of a voxel-shaped casing made of silicone (Ecoflex00-30) and an internal structure housed in the casing. We designed the module so that the deformation behavior of the module is defined by the internal structure housed in the casing. The casing was made from a hollow three-dimensional silicone structure with one side open using a mold, and silicone was poured and cured. The module was then fabricated by pouring liquid silicone on a flat surface, placing the open side of the silicone cube containing the 3D printed internal structure on top of it, and curing it. Each voxel could be joined with 3D printed connectors to create several modular robots. We investigated the behavior of the voxels under reduced pressure. The results showed that the soft modular robot could achieve uniaxial bending and shear deformation behaviors by considering the combination of modules. In conclusion, our pneumatic module, which can control elastic anisotropy and various deformations using only soft materials, is expected to dramatically expand the range of future soft robot designs. Figure 1 shows the Young’s modulus of Igeta structure and silicone composite models calculated from compression tests. Anisotropy of Young’s modulus varied depending on the crossing angles of the lattice structure. In particular, the Young's modulus in the x-axis is remarkably changed. Figure 2 shows the fabricated pneumatic modular robot. Each voxel was deformed according to its anisotropy also the whole thing was deformed along with it.These results confirm that it is possible to change the anisotropy of the elastic material depending on the shape of the lattice structure. Creating modular blocks that can easily recombine these structures will be useful for designing soft robots and confirming their functions.