Experimental Characterisation of Hydraulic Fiber-Reinforced Soft Actuators for Snake-Like Robots Matheus S. Xavier 1 , Andrew Fleming 2 and Yuen Yong 3 Abstract— This article describes the design and fabrication of fiber-reinforced soft actuators for a snake-like robot designed to operate inside constrained tubes. The actuators include bending, extension and torsion. These actuators were exper- imentally characterised using water as the driving fluid with the aid of a water pressure sensor connected to Arduino and video recordings. It is shown that fiber wrapping, geometry of cross-section and elastomer selection are the main parameters affecting the levels of extension, bending and torsion of these actuators. Then, multi-material soft actuators are developed and used to present a soft robot capable of crawling a pipe, a mechanism that could be explored in steerable catheters, endoscopes and pipe inspection devices. I. INTRODUCTION Conventional robot manipulators (“vertebrate robots” or discrete robots) [1] are constructed from rigid links con- nected through joints with a single degree of freedom (DOF) and have been employed in many industrial applications with excellent speed and accuracy. These robots are very efficient for open environments but might not reach desired end- effector positions as constraints are added [2]. By increasing the number of discrete joints and making the rigid links very short, serpentine robots are achieved. Serpentine robots produce smooth curves similar to a snake [3]. Continuum robots (“invertebrate robots”) [2]–[5], on the other hand, do not contain rigid links and can be defined as infinite degrees of freedom robots with elastic structures [5]. These robots can bend, extend/contract and sometimes twist at any point along their structure and produce motion through the generation of smooth curves [3], [4]. Due to their conformal deformation, they can adapt to delicate objects, lowering the number of parts required for a given task and increasing safety and dexterity [6]. Furthermore, continuum robots can be lightweight and employed within constrained environments with restricted access [2], [3]. Continuum style robots composed of highly deformable and compliant materials, such as silicone rubber, are de- nominated soft robots [7]–[9]. These flexible (elastomeric) materials typically exhibit large strains and low elastic mod- uli (Young’s modulus ∼ 10 2 − 10 6 Pa) [10]. They show high dexterity but with low accuracy and difficult (motion) control and sensing [8], [9]. The majority of soft robots is composed of pressure-driven actuators, the so-called soft fluidic actuators (or elastic inflatable actuators) [11], [12]. All authors are with the Precision Mechatronics Lab at the School of Electrical Engineering and Computer Science, The Uni- versity of Newcastle, 2308 Callaghan, New South Wales, Australia (e-mails: matheus.xavier@uon.edu.au, andrew.fleming@newcastle.edu.au, yuenkuan.yong@newcastle.edu.au). These actuators are driven by the deformation of a chamber or membrane with pneumatics or hydraulics, can output high force densities (high power-to-weight ratio) and are relatively inexpensive [7], [10], [13]. Soft fluidic actuators can be classified according to their motion into four categories: extending, contracting, bending and twisting. These actuators have specifically engineered anisotropic flexible structures to achieve each of these four motions [7], [14]. The simplest design for soft fluidic ac- tuators consists of a single chamber. However, unless fibers are wrapped around the single chamber actuator, the actuator behaves as a balloon [13] and high radial expansion occurs [15]. The most investigated design in the literature is the multi-chambered actuator. A popular example is the Pne- uNets [16]–[18], which consist of an extensible top layer and an inextensible but flexible bottom layer. Differently from slow PneuNets [16], [19] wherein a block of silicone rubber has embedded air chambers, the fast PneuNets developed by Mosadegh et al. [17] contains gaps between the inside walls of each chamber. Other designs for multi-chambered actu- ators (fast PneuNets) using trapezoidal [15] and triangular [20] bellows have also been proposed. In this article, fiber-reinforced actuators [10], [21], [22] are analysed. For these actuators, fibers (Kevlar fiber, for example) can be arranged along their length to achieve different motions (Fig. 1): (a) double helical wrapping and strain limiting layer for bending; (b) double helical wrapping for extending; (c) single helical wrapping and strain limiting layer for twisting and bending; and (d) single helical wrap- ping for twisting and extending. Moreover, an actuator can be fabricated with different motion types in different segments, thus combining multiple behaviours. Soft actuators are commonly manufactured with the aid of additive manufacturing (AM). In most cases, molds are 3D- printed into which silicones are cast and consolidated [23]. The use of 3D printing (a subset of AM) allows the design of complex features (free form capability) and high precision molds with a lower number of manufacturing steps [24], [25]. After curing the rubber (possibly in multiple stages), tubes are inserted for actuation. The fabrication of some actuators also include bonding fabric [19] and/or fiber reinforcements to the actuator [10]. The majority of soft actuators proposed in the literature are characterised using air [15], [17], [19]. However, the employment of hydraulic actuators represent a safer alter- native for biomedical applications (steerable catheters) since physiological saline can be used to drive the actuators [26], [27]. Furthermore, the different types of actuators are scat-