  Compliant mechanisms actuated by pneumatic loads are receiving increasing
attention due to their direct applicability as soft robots that perform tasks
using their flexible bodies. Using multiple materials to build them can further
improve their performance and efficiency. Due to developments in additive
manufacturing, the fabrication of multi-material soft robots is becoming a real
possibility. To exploit this opportunity, there is a need for a dedicated
design approach. This paper offers a systematic approach to developing such
mechanisms using topology optimization. The extended SIMP scheme is employed
for multi-material modeling. The design-dependent nature of the pressure load
is modeled using the Darcy law with a volumetric drainage term. Flow
coefficient of each element is interpolated using a smoothed Heaviside
function. The obtained pressure field is converted to consistent nodal loads.
The adjoint-variable approach is employed to determine the sensitivities. A
robust formulation is employed, wherein a min-max optimization problem is
formulated using the output displacements of the eroded and blueprint designs.
Volume constraints are applied to the blueprint design, whereas the strain
energy constraint is formulated with respect to the eroded design. The efficacy
and success of the approach are demonstrated by designing pneumatically
actuated multi-material gripper and contractor mechanisms. A numerical study
confirms that multiple-material mechanisms perform relatively better than their
single-material counterparts.
