###### Abstract  
This paper presents the design of a legged robot with gecko-mimicking mechanism and mushroom-shaped adhesive microstructure (MSAMS) that can climb surfaces under reduced gravity. The design principle, adhesion performance and roles of different toes of footpad are explored and discussed in this paper. The effect of the preload velocity, peeling velocity and thickness of backing layering on the reliability of the robot are investigated. Results show that pull-force is independent of preload velocity, while the peeling force is relying on peeling velocity, and the peel strength increased with the increasing thickness of the backing layer. The climbing experiments show that the robot can climb under mimic zero gravity by using multiple toes facilitating adhesion. The robot with new type of footpads also provides a good platform for testing different adhesive materials for the future space applications.  
legged robot; mushroom-shaped adhesive microstructure; adhesion performance; reduced gravity; climbing 2018 8 114 2018  
each step. Several groups have developed legged robots that use dry adhesives and capable of such a contact formation/breakage control [14; 15; 16]. One of the most impressive robots is the Stickybot [17], which utilizes directional polymer stalks (DPS) to effectively climb vertical surfaces, such as glass, plastic, and polished wood panels. Although the climbing abilities of the robot on ground and wall have been reported, its ability to perform locomotion under zero gravity, when contact formation by DPS is not supported by the gravity force, is still unknown.  
Legged climbing robot involves the controlled application of forces during the leg-substrate contact and release, in order to propel the body forwards. The achievement of strong attachment and easy removal of the adhesive is the critical issues, when we apply the adhesive to the robot's footpad. Previous studies have considered the effect of the peel angle, the length and the shape of the peel region of the adhesive itself on the adhesion performance. However, the influences of the shape and stiffness of the footpad itself are rarely studied.  
IBSS_Gecko is a legged mechanism for a gecko-inspired robot, which is first introduced by [PERSON] and [PERSON] (Figure 1a) [18]. Using a single toe which is composed of a spring-loaded runner sheet coated with pressure-sensitive adhesive at each foot, IBSS_Gecko_6 was able to climb on smooth vertical surface [19]. The improvements presented in this work are built upon the basic design principles of the robot. Inspired by the geckos, we introduce here a footpad with multiple toes (Figure 1b) and biomimetic MSAMS (Figure 1c-e) [10], to increase adaptability and stability of the climbing robot. The legged robot with only one toe is difficult to cope with the different environments. And the novel fibrillar adhesives may enable the robot to climb on surfaces regardless of the gravity.  
This paper is organized in the following manner. Section 2 briefly reviews the biomechanics of a gecko climbing on a surface. Based on the biological principle in micro and macro scales, we develop a gecko-inspired footpad. The manner of footpad with strong attachment and easy-removal properties is designed and tested in Section 3. Furthermore, experiments on the IBSS_Gecko on the challenged condition-reduced gravity is designed, analyzed and discussed in Section 4. Finally, conclusions and future work are reported in Section 5.