Variable Stiffness Structure for Limb Attachment Maxime Bureau, Thierry Keller, Rosemarie Velik, Joel Perry, Jan Veneman Quality of Life Unit – Biorobotics Department Tecnalia Research and Innovation Paseo Mikeletegi 1, 20009 Donostia-San Sebastián, Spain thierry.keller / jan.veneman @tecnalia.com Abstract—In robotic rehabilitation, the way of attachment of the robotic device to the users limb constitutes a crucial element of product quality, particularly for assuring good fitting, comfort, accuracy, usability, and safety. In this article, we present a new technological concept – ‘Variable Stiffness Structure’ – allowing for an improvement of these aspects in the ‘robotic device to limb’- connection by offering a compound of materials that are together able to switch from a flexible textile-like state to a more rigid state by applying negative pressure. Keywords-limb connection; robotic rehabilitation; body fitting element I. INTRODUCTION Physical therapy has the aim to treat disorders of the musculoskeletal or neuromuscular system in order to restore maximal functional independence of individual patients. In the last decades, a number of robotic devices have been introduced to facilitate such therapy and release the therapist from part of his tasks being time- and resource consuming and/or needing a large physical effort in manual therapy. A specific and essential component of robotic devices for physical therapy is the connective part that attaches/fixes the robot’s end effectors, its exoskeleton parts, or the device in general to the human limb(s). Two examples are given in figure 1, one is device for arm training, the ArmAssist [1] in development at Tecnalia, and the other the Lokomat, the well-known gait rehabilitation system from Hocoma [2]. In these examples the connective parts, that are the subject of this paper, are clearly visible Although the aspect of limb attachment and fixation in rehabilitation robotics design does often not receive major attention in research, neither in the design process nor in the publications in the field, it is a crucial component that determines the eventual properties of the robot towards the user, i.e. the patient that receives training. Leaving out this part becomes indispensable as soon as reaching towards the commercialization of a device. Comfort, safety, accuracy, and speed of the don/doff procedure are crucial factors defining the product quality, and all of them are partially determined and/or affected by the design of the connective parts. Particularly in stroke rehabilitation, which is characterized by intensive repetitive training, there is increased need for optimization of this aspect as the involved robotics may have to be used for long durations of training, with a relative high level of (assistive) force interaction. In general, the design of the connective parts is most critical when the transmitted forces are highest. This is typically the case in gait and balance rehabilitation, where the full body weight comes into play [3]. Nevertheless, its importance is also not to be neglected in upper extremity devices. Today, the most common approach to solution for device attachment are constructions of stiff and flexible parts, similar like orthoses, using, e.g., fiber reinforced polypropylene, carbon fiber reinforced thermoplastic composite material, Velcro straps, textile belts, and similar. In many cases, to be applicable to full range of patient measures, different elements have to be replaced and attached to the device. From the functional viewpoint, an ideal solution would provide a (1) comfortable connection that (2) is sufficiently rigidly connected to assure safety and precise control of the robot in the rehabilitation exercises, (3) is easy and fast to attach and detach to/from the limb, (4) needs only one component to automatically fit to any patient, and (5) is easy to clean. Figure 1. ArmAssist [1] (left) and the Lokomat [2] (right). The limb attachment or connective parts that are encircled in these pictures are the topic of this paper. II. PROPOSED SOLUTION AND ITS IMPLEMENTATION In order to approach the abovementioned ideal characteristics of the connective part of a robotic device, we propose a new technological concept: ‘Variable Stiffness’.. This technology recently patented by Tecnalia provides a ‘material’ that is able to switch from a flexible textile-like state to a more rigid state (similar to HDPE) by applying negative pressure (vacuum). This ‘material’, referred to as