# Detachable Soft Actuators with Tunable Stiffness Based on Wire Jamming
## 1 Introduction
Based on the characteristics of fiber jamming, researchers presented three arrangement options and applied fiber jamming transition to a manipulator [35; 36]. In addition, hybrid jamming structures have been reported, consisting of two or more different jamming media. Therefore, these hybrid structures combine the advantages of different jamming schemes [37; 38; 39]. At present, the research areas of fiber jamming and hybrid jamming are still in their infancy and many factors have not been studied. During the design of soft actuators and stiffness-tunable structures, 3D printing technology provides an excellent, systematic tool to directly fabricate structures with elastomeric materials [11; 26; 40]. [PERSON] et al. [41] developed soft robots that mimicked cactus stem geometries and used 3D printing technology to fabricate thin elastomeric layers. In order to solve the problem of fabricating soft actuators with complex inner geometry, a method of 3D printing with fused deposition modelling (FDM) is proposed [42]. [PERSON] et al. [43] presented a magnetic multi-material printing technique and fabricated soft programmable composites with tunable Poisson's ratio and shiftable mechanical behaviors. However, current 3D printing technology still has some limitations, such as speed, material compatibility and resolution [40]. Therefore, further research is needed for 3D printing materials and printing devices applied to soft robots. To further study the properties of the stiffness tunability of wire/fiber jamming structures and explore their applications (Figure 1a,b), we propose a paradigm to design and fabricate a type of tunable-stiffness soft actuator based on wire/fiber jamming. The stiffness-tunable soft actuator mainly consists of a soft pneumatic actuator and a wire/fiber jamming mechanism. Through rigorous analysis of various operating conditions, a new wire jamming structure with flexible 3D bending capability and a wide range of stiffness variation is developed to meet the needs of different applications. Bending and torsion experiments are carried out to investigate the performance of our wire jamming mechanisms, and the results show a remarkable rise in bending stiffness under vacuum pressure. In this study, a fiber-reinforced pneumatic actuator is used as a driving unit because it not only has a flat surface but also has a smaller expansion coefficient under air pressure (Figure 1c) [44; 45; 46]. To understand the bending performance of the fiber-reinforced actuator, we develop a theoretical model and determined the geometric parameters of the soft actuator (Section S1 and Figures S1 and S2 Supplementary Materials) [45]. In addition, uniaxial tensile tests are conducted to analyze the tensile properties of silicone rubber (see Section S2 and Figure S3 in the Supplementary Materials for details). The bending state of the actuator can be obtained from FE simulation results (Figure 1d). Since both the inner chamber structure and the outer structure are quite simple, the fiber-reinforced soft actuators are fabricated by 3D-printed molds. In order to improve the flexibility and universal adaptability, we use a modular approach by integrating Velcro into the design of variable stiffness actuators, where the Velcro acts as an inextensible layer as well as a standardized connector for the actuator and wire jamming mechanism. Because of the advantages of rapidity, convenience and reliability, this paper applies Velcro to wearable devices, such as detachable soft gloves. At present, scholars have developed a variety of functional gloves. For example, [PERSON] et al. [47; 48] developed multifunctional materials with self-healing, conductive and stretchable properties, and applied them to wearable sensors and bionic gloves. [PERSON] et al. [49] designed a waterproof and flexible nanocomposite membrane and attached it to a commercial glove to detect joint movements.