A Low-Profile Soft Robotic Sixth-Finger for Grasp Compensation in Hand-Impaired Patients1

摘要

Impairment of hand function is commonly observed in patients suffering from neurological disorders, such as stroke [1]. The patients will lose the ability to control the hand and perform activities of daily living (ADLs). In order to restore hand function, rehabilitation programs comprising repetitive task practice are required [2]. Various robotic devices with the ability to assist with repetitive hand movements have been proposed [3]. One class of robotic devices is exoskeletons, which couple to the user through multiple limb segments and provide movement assistance. Several research groups have developed wearable soft exoskeletons that are suitable for use as an at-home assistive device that can provide assistance for ADLs and task-specific training [4,5].Recently, alternative approach such as designing robotic extra finger has been explored in order to achieve compensation of grasping function in hand-impaired patients [6]. The grasping function is achieved by the opposition between the extra finger and the paretic hand. This approach is particularly useful for patients with severe spasticity when wearable exoskeletons fail to provide enough force to guide the hand movements. However, current devices (with robotic extra finger) are bulky as they consist of components, such as servo motors. These rigid components will affect the portability and wearability of the device.The aim of this work is to design a soft robotic sixth-finger that provides grasp compensation for hand-impaired patients. Unlike current devices, the device presented in this work utilizes underactuated soft actuator that is flexible, making it more portable and lightweight.The device consists of a soft actuator and a wearable wrist brace (weight < 100 g). The actuator is integrated with the wrist brace through a rotating knob (Fig. 1(a)). Due to the flexibility of the actuator, it can be wrapped around and attached to the wrist brace through Velcro strap when the device is not in use (Fig. 1(b)). The actuator can be rotated to its working position with the rotating knob (Fig. 1(c)). The wearer can use his/her healthy hand to switch between the rest and the working position.A two-part 3D-printed mold is required to fabricate the actuators. The lower-part mold (channel mold) is used to create a pneumatic channel inside the actuators, which will inflate upon pressurization, while the upper-part mold (outer layer mold) is used to impose the corrugated outer layer at the top of the actuators (Fig. 2(b)). Upon pressurization, the top surface of the actuators expands due to the inflation of the embedded pneumatic channel. The strain-limiting fabric restricts the elongation at the bottom surface. This results in the bending of the actuators upon actuation.The actuator is able to generate a distributed force along its length when it is actuated. The distributed force exerted by the actuator was measured over increasing pressures using a customized force measurement setup (Fig. 3(a)). The system consisted of a compression load cell (FC22, Measurement Specialties, Inc., Hampton, VA) and a mounting platform. A constraining platform was positioned on top of the actuator. During pressurization, the actuator flexed and started to contact with the constraining platform, which constrained the height and the curvature of the actuator. The initial force generated along the actuator was transmitted to the distal end, where it could be measured by the load cell (Fig. 3(a)).The actuator force increased with increased pressure (Fig. 3(b)). In the experiment, the maximum initial force and maximum actuation pressure tested for the actuator were 9.17 N and 120 kPa, respectively. According to a previous study which estimated that a minimum force of 8 N was required to achieve a palmar grasp and manipulate most objects of daily living [4], the force generated by our soft actuator was thus considered to be sufficient.A healthy participant was recruited to evaluate the feasibility of t