BackgroundPersons suffering from progressive muscular weakness, like those with Duchenne muscular dystrophy (DMD), gradually lose the ability to stand, walk and to use their arms. This hinders them from performing daily activities, social participation and being independent. Wheelchairs are used to overcome the loss of walking. However, there are currently few efficient functional substitutes to support the arms. Arm supports or robotic arms can be mounted to wheelchairs to aid in arm motion, but they are quite visible (stigmatizing), and limited in their possibilities due to their fixation to the wheelchair. The users prefer inconspicuous arm supports that are comfortable to wear and easy to control.MethodsIn this paper the design, characterization, and pilot validation of a passive arm support prototype, which is worn on the body, is presented. The A-gear runs along the body from the contact surface between seat and upper legs via torso and upper arm to the forearm. Freedom of motion is accomplished by mechanical joints, which are nearly aligned with the human joints. The system compensates for the arm weight, using elastic bands for static balance, in every position of the arm. As opposed to existing devices, the proposed kinematic structure allows trunk motion and requires fewer links and less joint space without compromising balancing precision.The functional prototype has been validated in three DMD patients, using 3D motion analysis.ResultsMeasurements have shown increased arm performance when the subjects were wearing the prototype. Upward and forward movements were easier to perform. The arm support is easy to put on and remove. Moreover, the device felt comfortable for the subjects. However, downward movements were more difficult, and the patients would prefer the device to be even more inconspicuous.ConclusionThe A-gear prototype is a step towards inconspicuousness and therefore well-received dynamic arm supports for people with muscular weakness.Electronic supplementary materialThe online version of this article (doi:10.1186/s12984-015-0072-y) contains supplementary material, which is available to authorized users.
Due to neuromuscular disorders (e.g., Duchenne Muscular Dystrophy) people often loose muscle strength and become wheelchair bound. It is important to use muscles as much as possible. To allow this, and to increase independency of patients, an arm orthosis can be used to perform activities of daily life. The orthosis compensates for the gravity force of the arm, allowing people to perform movements with smaller muscle forces. For patients, the aesthetics of the orthosis is one of the critical issues. This paper presents the state-of-the-art in passive and wearable active arm orthoses, and investigates how to proceed towards a suitable structure for a wearable passive arm orthosis, that is able to balance the arm within its natural range of motion and is inconspicuous; in the ideal case it fits underneath the clothes. Existing devices were investigated with respect to the body interface, the volume, and the workspace. According to these evaluation metrics it is investigated to what extent the devices are wearable and inconspicuous. Furthermore, the balancing principle of the devices, the architecture, force transmission through the devices, and alignment with the body joints are investigated. It appears that there is only one wearable passive orthosis presented in literature. This orthosis can perform throughout the natural workspace of the arm, but is still too bulky to be inconspicuous. The other passive orthoses were conspicuous and mounted to the wheelchair. Except one, the wearable active orthoses were all conspicuous and heavy due to a large backpack to enclose the actuators. They also could not achieve the entire natural workspace of the human arm. A future design of an inconspicuous, wearable, passive arm orthoses should stay close to the body, be comfortable to wear, and supports pronation and supination.
Due to progressive muscle weakness, the arm function in boys with Duchenne muscular dystrophy (DMD) reduces. An arm support can compensate for this loss of function. Existing arm supports are wheelchair bound, which restricts the ability to perform trunk movements. To evaluate the function of a body-bound arm support, a prototype (based on the Wilmington robotic exoskeleton (WREX) arm support) that allows trunk movements was built. In order to examine the effect of this device and to compare it with an existing wheelchair-bound device, three healthy subjects performed single joint movements (SJMs) and activities of daily living (ADL) with and without the devices. The range of motion (RoM) of the arm and the surface electromyography (sEMG) signal of five different arm muscles were measured. The range of motion increased when compared to the wheelchair-bound device, and the trunk motion was perceived as important to make specific movements easier and more natural, especially the more extreme movements like reaching for a far object and reaching to the top of the head. The sEMG signal was comparable to that of the wheelchair-bound device. This means that an arm support with trunk motion capability can increase the range of motion of the user, while the amount of support to the arm is equal.
Bistable straight-guided buckling beams are essential mechanisms for precision engineering, compliant mechanisms, and MEMS. However, a straightforward and accurate numerical modeling have not been available. When preloading effects must be included, numerical modeling becomes an even more challenging problem. This article presents a straightforward numerical model for bistable straight-guided buckling beams, which includes preloading effects as well. Adjusting the bistable force–displacement characteristic by variation of design parameters and preloading is also investigated. Both lumped compliance and distributed compliance are considered in this work. In order to validate the model, measurements have been performed. It was shown that a small precurvature of bistable straight-guided buckling beams is crucial to avoid convergence into higher order buckling modes in nonlinear analysis of ANSYS™ and to obtain reliable results. Transient analysis using ANSYS™ with subsequent preloading and motion displacements can incorporate preloading effects. Moreover, the model correction allows accurate description of the increased symmetry and energy efficiency of the bistable behavior in case of increasing (in order of effectiveness) the initial angle and preloading for the case of distributed compliance. This behavior was observed by increasing the initial angle, thickness, and length of the rigid segment for the case of lumped compliance.
Compactness is a valuable property in designs of assistive devices and exoskeletons. Current devices are large and stigmatizing in the eyes of the users. The cosmetic appearance will increase by reducing the size. The users want a device that is small enough to be worn underneath the clothes, so it becomes unnoticeable. The goals of this paper are (1) to provide an overview of the shape-changing-material-actuated large-deflection compliant rotational joints, (2) provide new introduced performance indicators that evaluate the designs on performance with respect to volume or weight and (3) design a compact active assistive elbow device as a case study. In order to reach these goals, two evolving fields of study are brought together that have great potential to reduce the size of exoskeletons: smart materials and compliant rotational joints. Smart materials have the ability to change their shape, which make them suitable as actuators. Compliant joints can be compact, since they are made out of one piece of material. An overview of shape-changing-material-actuated large-deflection compliant rotational joints is presented. Performance indicators are proposed to evaluate the existing designs and the prototype. As a case study a compact actuated rotational elbow joint is presented. An antagonistic actuator made from shape memory alloy wires is able to carry an external load and to actuate to move the arm to different positions. The compliant joint is optimized to balance the weight of the arm and to auto-align with the rotational axis of the human elbow joint. A prototype is able to generate a volume specific stall torque of 5.77 ⋅ 103 Nm/m3, produces a work density of 7.27 ⋅ 103 J/m3 based on volumes including isolation covers and the half-cycle efficiency of the device is 3.6%. The prototype is able to balance and actuate a torque of 1.1 Nm.
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