Hybrid soft-rigid foot with dry adhesive material designed for a gecko-inspired climbing robot

Geckos are the largest animals capable of unassisted vertical climbing. They can effectively exploit the adhesive area (approximately 0.16 cm²) of each foot to support relatively heavy weight (approximately 0.06 kg) with at least two feet during locomotion on different slopes and walls. Over the last few decades, researchers have developed gecko-inspired dry adhesive materials based on the van der Waals force and used them as the feet for climbing robots. However, to date, these robots have not fully exploited the performance of the material. Therefore, the dry adhesive material areas on each foot need to be large (3 - 16 cm²) to support their light weight (up to approximately 0.17 - 0.45 kg) with at least two feet during locomotion. To improve the utilization of the adhesive material for heavier payloads, we propose a hybrid soft-rigid foot with a sandwich structure. This compact foot consists of three main parts or layers (rigid, soft, and dry adhesive material parts). The rigid part is used to generate sufficient pressure to achieve an omni-directional adhesive force. The soft part allows the foot to gain the maximum advantage from the adhesive material by maximizing adhesive area. The material part essentially provides strong adhesion. Aside from the foot design, we also propose gecko-inspired attaching and detaching strategies for the foot to achieve the largest adhesive force possible while requiring a low peeling force. Experimental results demonstrate that a foot with an adhesive area of approximately 12.56 cm² and proper softness can support a weight of approximately 1.6 kg (or approximately 3.2 kg with at least two feet). In conclusion, this study provides a solution to enhance payload capacity and a guideline for further foot design with hybrid soft-rigid layers of climbing robots.