A Soft Robotic Orthosis for Wrist Rehabilitation1

Abstract

In the United States about 795,000 people suffer from a stroke each year [1]. Of those who survive the stroke, the majority experience paralysis on one side of their body, a condition known as hemiparesis. Physical therapy has been shown to restore functionality of the paretic limbs, especially when done early and often [2]. However, the effectiveness of therapy is limited by the availability of therapists and the amount of practice that patients do on their own.Robot-assisted therapy has been explored as a means of guiding patients through the time-intensive exercise regimes that most therapy techniques prescribe. Several wearable, robotic orthoses for the hand and wrist have been developed and are still being developed today [3]. However, few of these existing solutions allow for any significant range of motion, and those that do only offer one degree of freedom. The assisted degree of freedom is almost always flexion/extension, despite the fact that supination/pronation is crucial in nearly all activities of daily living. In addition, current devices are often large, heavy, and uncomfortable for the wearer, presenting significant deterrents to practice.This paper presents a soft wearable device for the wrist that provides motion-specific assistance with rehabilitation for hemiparetic stroke patients at home. Unlike conventional robot-assisted rehabilitation, this pneumatically actuated orthosis is portable, soft, and lightweight, making it more suitable for use outside of the clinic. In addition, this device supports all the degrees of freedom of the wrist, including supination and pronation, which are critical to many tasks.After a review of prior art and the current literature on biomechanics, as well as consultations with occupational therapists and stroke patients, it was determined that an ideal device should be able to assist in multiple degrees of freedom while remaining lightweight and comfortable. In addition, the device should allow for customizable levels of assistance, to provide more effective treatment for users of varying ability.The final design (Fig. 1) consists of a wearable portion tethered to an off-board power system. Crossing linear actuators on both the palmar and dorsal sides of the forearm allow the device to assist wrist movement. Any single motion can be achieved by activating a pair of actuators. For instance, flexion is achieved by activating the two actuators on the palmar side of the wrist. Supination is achieved by activating one actuator on the palmar side, and the opposite actuator on the dorsal side.After numerous potential actuator types were reviewed, McKibben actuators (a type of pneumatic artificial muscle) were selected for their ease of manufacture, low weight, overall simplicity, and capability for straightforward integration with other pneumatic orthoses of interest [4]. Preliminary testing showed that ½ in. outside diameter (OD) actuators would provide sufficient force, but concerns for user comfort necessitated investigating a more streamlined actuator configuration. More rigorous testing was done on an Instron 5544A Materials Testing System to determine the configuration of smaller diameter actuators that could output comparable levels of force (Fig. 2). While a single ¼ in. OD actuator provided insufficient force, the output from two such actuators in parallel was determined to be acceptable, considering data on wrist torques and moment arms. An operating pressure of 60 PSI was selected from additional characterization of contraction ratio as a function of pressure.The textile interface is defined as the portion of the device that transmits the motion of the actuators to the user. It consists of a glove, an elbow sleeve, and an actuator tensioning mechanism.The details of the anchoring points on both the glove and the elbow sleeve are of great importance (Fig. 3). Anchor position determines initial actuator length, which directly affects contraction length and thus range of mot