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Soft pneumatic actuators for pushing fingers into extension
Journal of NeuroEngineering and Rehabilitation volume 21, Article number: 146 (2024)
Abstract
Background
Compliant pneumatic actuators possess many characteristics that are desirable for wearable robotic systems. These actuators can be lightweight, integrated with clothing, and accommodate uncontrolled degrees of freedom. These attributes are especially desirable for hand exoskeletons, where the soft actuator can conform to the highly variable digit shape. In particular, locating the pneumatic actuator on the palmar side of the digit may have benefits for assisting finger extension and resisting unwanted finger flexion, but this configuration requires suppleness to allow digit flexion while retaining sufficient stiffness to assist extension.
Methods
To meet these needs, we designed an actuator consisting of a hollow chamber long enough to span the joints of each digit while sufficiently narrow not to inhibit finger adduction. We explored the geometrical design parameter space for this chamber in terms of shape, dimensions, and wall thickness. After fabricating an elastomer-based prototype for each actuator design, we measured active extension force and passive resistance to bending for each chamber using a mechanical jig. We also created a finite element model for each chamber to enable estimation of the impact of chamber deformation, caused by joint rotation, on airflow through the chamber. Finally, we created a prototype hand exoskeleton with the chamber parameters yielding the best outcomes.
Results
A rectangular cross-sectional area was preferable to a semi-obround shape for the chamber; wall thickness also impacted performance. Extension joint torque reached 0.33 N-m at a low chamber pressure of 48.3 kPa. The finite element model confirmed that airflow for the rectangular chamber remained high despite deformation resulting from joint rotation. The hand exoskeleton created with the rectangular chambers enabled rapid movement, with a cycle time of 1.1 s for voluntary flexion followed by actuated extension.
Conclusions
The developed soft actuators are feasible for use in promoting finger extension from the palmar side of the hand. This placement utilizes pushing rather than pulling for digit extension, which is more comfortable and safer. The small chamber volumes allow rapid filling and evacuation to facilitate relatively high frequency finger movements.
Background
Hand impairment is a common occurrence following injury to the central nervous system. Substantial hand motor deficits are likely to occur after stroke [1], the most common cause of major long-term disability in the U.S. and a primary cause of disability throughout the world [2, 3]. Hand deficits are also associated with cerebral palsy (CP) [4, 5], the most common movement disorder in children [6, 7]. Reduced motor control of the hand has ramifications for self-care, employment, and social interactions.
In these clinical populations, common therapeutic practice for upper extremity rehabilitation involves repetitive practice of movement [8, 9] (e.g., constraint-induced movement therapy [10,11,12] and HABIT [13, 14]). Exoskeletons can facilitate this practice by providing assistance of desired movement [15, 16] and resistance of undesired movement. These devices typically employ rigid actuators, however, that may introduce considerable mass and inertia, potentially disturbing control and movement of the hand. Soft actuators have advantages in terms of weight, comfort, and conformation to different shapes [17,18,19,20]. These actuators may be especially well suited to the hand, where space is limited, additional mass is costly, and there are many degrees of freedom.
Many individuals with hand impairment especially have difficulty independently moving their digits. Therapeutic practice of finger individuation is needed and could be promoted by soft hand exoskeletons. Current soft actuator designs for the hand, however, are typically focused on pushing the digits into flexion from the dorsal side using a bellows-type approach [18, 21,22,23]. For stroke survivors or individuals with CP, finger extension is typically affected to a greater degree than digit flexion [24]. Involuntary coactivation of finger flexor muscles and muscle compartments leads to involuntary flexion of multiple fingers when trying to move only one digit [25]. Thus, active assistance of desired extension and resistance of unwanted flexion may be preferable to assistance of flexion for facilitating task practice. For rehabilitation therapy, the degree of assistance/resistance would ideally be variable and customized to each digit. Additionally, directly driving the finger without the need for external transmission, such as linkages or cables required with some solutions such as McKibben actuators [26], would be beneficial in order to reduce bulk and the number of required components, while increasing comfort. Furthermore, to facilitate therapeutic practice, the provided assistance should allow rapid, independent movement of the digits.
Given these target design criteria, we focused on palmar placement of the actuators, which would directly push (rather than pull) the digits into extension. Pushing reduces compressive joint forces relative to pulling while avoiding rubbing over the joints as the finger flexes. Rigid finger actuators have been positioned on the palm in the past to provide finger extension, but their presence limits finger flexion and precludes grasping of objects [27]. Similarly, stiffer pneumatic actuators such as PneuNets [28] could impose substantial resistance to desired flexion. Formerly, we developed polyurethane-based actuators that could assist digit extension from the palmar surface of the hand [29, 30]. When deflated, the actuators provided little added bulk or flexion resistance. The polyurethane actuators, however, are difficult to fabricate and are susceptible to kinking when bent, reducing airflow and the assistance provided.
The goal of this work was to design and test elastomer-based pneumatic chambers that could directly aid finger extension and resist unwanted flexion for each digit independently from the palmar side of the hand. To explore the design space, we evaluated a set of chambers with varying geometric characteristics: shape, size, and wall thickness. Each chamber was tested over a range of pressures and bending angles. Finite element models (FEMs) were created to estimate airflow through the chamber, as the airflow, and thus assistance provided to the finger, can become compromised as the chamber is distorted during finger flexion. These actuators were then incorporated into a soft glove designed to facilitate therapeutic practice of hand movements, including object grasp-and-release and rapid individuated movements of the digits. We hypothesized that a rectangular cross-sectional shape would yield higher extension force, higher flow rate, and lower passive bending resistance than a semi-obround shape, and that the extension force produced would increase with increased pressure and bending angle. A preliminary analysis of initial experimental results was presented in a conference paper [31].
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