Sensors and Actuators A 316 (2020) 112398 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal h om epage: www.elsevier.com/locate/sna Multi-material 3D printing of caterpillar-inspired soft crawling robots with the pneumatically bellow-type body and anisotropic friction feet Xinjun Sheng a,b,1 , Haipeng Xu a,1 , Ningbin Zhang a , Ningyuan Ding a , Xiangyang Zhu a,b , Guoying Gu a,b,∗ a Robotics Institute, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China b State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai, 200240, China a r t i c l e i n f o Article history: Received 12 September 2020 Received in revised form 20 October 2020 Accepted 21 October 2020 Available online 31 October 2020 Keywords: Soft crawling robot Pneumatically bellow-type body Anisotropic friction feet Passive synergy locomotion model Hybrid multi-material 3D printing a b s t r a c t Due to large shape-changing ability and high adaptability, soft crawling robots become a promising can- didate in applications with unpredictable terrain and complex environments. However, designing and fabricating of soft crawling robots with hybrid soft and rigid components are still elusive. Here, we present a novel caterpillar inspired pneumatically-driven soft crawling robot, which can be directly 3D printed with multiple materials and without complex assembling process. To mimic the biological structure and morphological locomotion of caterpillars, we design the soft crawling robot with a pneumatically driven bellow-type body, 12 anisotropic frictional feet, and two end caps, and introduce a passive synergy loco- motion model between the crawling robot’s body and feet. By selecting different cross-section shape of the feet, we characterize the moving performance of soft crawling robots. Finally, we integrate a pneu- matic closed-loop control system to drive the soft crawling robots with a periodic gait and demonstrate their motion capability in a curve plastic tube. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Animals can take advantages of their flexible bodies and anisotropic frictional feet or skin to efficiently move in various environments [1–5]. For example, caterpillars utilize the antero- grade wave of dorsal deformation and the controllable prolegs and thoracic legs to propel themselves moving unidirectional [6,7]. Mimicking biological structures has recently inspired rich develop- ment of soft crawling robots [8–11]. Among the reported developments, we can find that the combi- nation of a stretchable body and anisotropic frictional feet or skin plays a significant role in the locomotion of crawling robots [10–15]. Therefore, one of the key issues in soft crawling robots is to design and control the synergy between the deformation of the body and directional friction of feet to achieve desired behaviors and func- tionalities (e.g., locomotion), which can be roughly classified into two categories. The first category is to active control the body and feet independently. Through a synergy control strategy, the robot can achieve a directional locomotion such as presented in [16]. The ∗ Corresponding author. E-mail address: guguoying@sjtu.edu.cn (G. Gu). 1 These authors contribute equally to the current work. other category is to design a passive coupled mechanism of body deformation and directional friction force of feet or skin. In this case, only one actuation source without complex control strategy can enable the robot to crawl efficiently as reported in [11,15]. On the other hand, it is still a challenge to fabricate the soft crawling robots with multiple components effectively and sim- ply. Casting techniques are the most common way to fabricate the soft robots by replica molding process [17–19]. However, this approach may be laboriously intensive and limited by manual fabrication of hybrid soft and rigid materials. Alternatively, addi- tive manufacturing (also termed as 3D printing) has drawn much attention for rapid fabrication of soft robotic systems, including combustion-powered jumpers, multilegged robots and stiffness- tunable actuators [20–25]. Recently, shape-memory alloy and motor tendons actuated 3D printed soft crawling robots have been developed and show a significant advantage in the simple and fast fabrication [8,14,15,26]. However, the pneumatically-driven 3D printed soft crawling robots has not been demonstrated. In this paper, we develop a novel caterpillar inspired pneumatically-driven soft crawling robot, which can be directly 3D printed without complex assemble process. The soft crawl- ing robot is composed of a pneumatic bellow-type body, 12 anisotropic frictional feet, and two end caps. To further mimic the morphological locomotion of caterpillars, we introduce a pas- https://doi.org/10.1016/j.sna.2020.112398 0924-4247/© 2020 Elsevier B.V. All rights reserved.