Multi-material level set-based topology optimization of compliant mechanisms using moving wide spline curves with constrained ends

Compliant mechanisms are devices that achieve desired deformation through structural flexibility in response to applied loads, without relying on conventional joints or lubrication. This makes them highly suitable for applications in aerospace, biomedical engineering, and microelectromechanical systems (MEMS). Various topology optimization methods have been proposed for designing compliant mechanisms. However, these approaches frequently generate optimal structures with hinge-like regions, which limit their engineering usefulness. To address this issue, a topology optimization method based on geometrical representation using spline curves has been introduced. While this approach effectively avoids the formation of locally thin regions, it tends to produce smaller output displacements compared to conventional methods. Meanwhile, multi-material topology optimization has emerged as a promising strategy to enhance performance by strategically distributing materials with different properties, enabling designs that are unattainable with single-material configurations. This study proposes a novel multi-material topology optimization method for hinge-free compliant mechanisms, based on a geometrical representation using moving wide spline curves with constrained ends. The proposed framework consists of a two-stage optimization strategy. In the first stage, an optimal hinge-free configuration is generated via spline-based geometry control for realizing deformation in a desired direction while maintaining sufficient stiffness. In the second stage, a multi-material topology optimization problem within the obtained external geometry in the first stage is solved to enhance the deformation behavior. The effectiveness and efficiency of the proposed method are demonstrated through numerical examples involving both 2D and 3D compliant mechanisms.