Fiber reinforced elastomeric enclosures (FREEs) are soft pneumatic actuators that can contract and generate forces upon pressurization. Typical engineering applications utilize FREEs in their straight cylindrical configuration and derive actuation displacement and forces from their ends. However, there are several instances in nature, such as an elephant trunk, snakes and grapevine tendrils, where a spiral configuration of muscle systems is used for gripping, thereby establishing a mechanical connection with uniform force distribution. Inspired by these examples, this paper investigates the constricting behavior of a contracting FREE actuator deployed in a spiral or coiled configuration around a cylindrical object. Force balance is used to model the blocked force of the FREE, which is then related to the constriction force using a string model. The modeling and experimental findings reveal an attenuation in the blocked force, and thus the constriction force caused by the coupling of peripheral contact forces acting in the spiral configuration. The usefulness of the coiled FREE configuration is demonstrated in a soft arm orthosis for crutch users that provides a constriction force around the forearm. This design minimizes injury risk by reducing wrist load and improving wrist posture.

Patients that use crutches for ambulation experience forces as high as 50% of body weight and extreme extension angles at the wrist, which increases the risk of joint injury such as carpal tunnel syndrome. We have designed and fabricated a soft pneumatic sleeve to reduce the wrist loading by transferring part of the load to the forearm. The sleeve uses a Fiber Reinforced Elastomeric Enclosure (FREE). FREEs are soft pneumatic actuators that can generate force and moment upon inflation. We have used a contracting FREE, which is wrapped in a helical shape around the forearm as part of the sleeve. Upon actuation, it contracts in length and reduces in diameter, thereby generating a constricting force around the forearm. In this paper, we describe the modeling of the constricting force generated by the helical FREE. We can model the FREE as a string due to its negligible bending stiffness. The constriction force can be expressed in terms of the axial tensile force generated in the FREE upon actuation and the geometry of the helix. To obtain the axial force, we have used a model previously reported in literature that uses a constrained volume maximization formulation. We validate the string model by comparing with experimental results.

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Lofstrand, or forearm, crutch users experience a concerning amount of force in their wrists that can approach 30% of body weight during swing-through gait, along with hyperextension of the wrist [1]. Such repetitive large loads and poor wrist posture could cause joint pain, injury and carpal tunnel syndrome in long-term crutch users [2]. To address these issues in Lofstrand crutch users, we previously reported two different designs. The first involved attaching a passive orthosis to the grip handle and shaft that directly supported the wrist [3]. The second proposed a soft pneumatic sleeve attached to the forearm cuff of the crutch along with a concept to harvest pneumatic energy during walking via a piston pump at the crutch tip, i.e., the Pneumatic Ergonomic Crutch (PEC) [4]. This paper first describes the full system design of the PEC, which contains an improved pneumatic sleeve orthosis, an energy-harvesting shock-absorbing crutch tip, and a control system with three modes of operation and safety protection. We also present pilot data from one healthy control subject performing a proposed biomechanical evaluation of the pneumatic sleeve orthosis. More tests will be conducted with a group of able-bodied subjects to evaluate the overall effectiveness of the PEC.

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