Design and experimental testing of a force-augmenting exoskeleton for the human hand

 You'll have to ask your doctor what they are doing to get this tested for stroke patients. Not doing anything should be grounds for firing.

Design and experimental testing of a force-augmenting exoskeleton for the human hand

• Emily R. Triolo &
• Brett F. BuSha 

Journal of NeuroEngineering and Rehabilitation volume 19, Article number: 23 (2022) Cite this article

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Abstract

Background

Many older Americans suffer from long-term upper limb dysfunction, decreased grip strength, and/or a reduced ability to hold objects due to injuries and a variety of age-related illnesses. The objective of this study was to design and build a five-fingered powered assistive exoskeleton for the human hand, and to validate its ability to augment the gripping and pinching efforts of the wearer and assist in performing ADLs.

Methods

The exoskeleton device was designed using CAD software and 3-D printed in ABS. Each finger’s movement efforts were individually monitored by a force sensing resistor at each fingertip, and proportionally augmented via the microcontroller-based control scheme, linear actuators, and rigid exoskeleton structure. The force production of the device and the force augmenting capability were assessed on ten healthy individuals with one 5-digit grasping test, three pinching tests, and two functional tests.

Results

Use of the device significantly decreased the forearm muscle activity necessary to maintain a grasping effort (67%, p < 0.001), the larger of two pinching efforts (30%, p < 0.05), and the palmer pinching effort (67%, p < 0.001); however, no benefit by wearing the device was identified while maintaining a minimal pinching effort or attempting one of the functional tests.

Conclusion

The exoskeleton device allowed subjects to maintain independent control of each digit, and while wearing the exoskeleton, in both the unpowered and powered states, subjects were able to grasp, hold, and move objects such as a water bottle, bag, smartphone, or dry-erase marker.

Introduction

In general, grip strength has been seen to gradually decline between 60 to 75 years of age- this decline more drastically noted among men [1]. Additionally, approximately 795,000 Americans suffer from a stroke, the leading cause of serious long-term disability, per year, reducing mobility, including upper limb dysfunction, in over half of stroke victims age 65 and older [2]. Upper limb dysfunction, including decreased grip strength and/or diminished ability to hold objects is also prevalent in populations with carpal tunnel syndrome [3].

Robotic exoskeleton devices can be primarily designed to augment user strength in order to assist with activities of daily living (ADLs), or as rehabilitative devices that are used under the guidance of a physical therapist to help patients regain greater functionality of damaged joints and/or muscles [4]. Assistive exoskeletons for the hand can be grouped according to how the augmenting forces enhance the concentric movement of the digits. Devices have been designed to apply the augmenting forces to the dorsal aspect of the fingers via mechanical linkages [5,6,7,8] or fabric-based pneumatic bladders [9]. A ventral approach has also been used, where pseudo-tendons applied tension that is transmitted to the digits through soft [10] or hard exoskeleton structures [11]. Heo, et al. [12] and Bos, et al. [13] have both published comprehensive listings and reviews of exoskeleton devices for the hand.

Regardless of the technique used to apply the augmenting force, for an assistive device to function, finger movement or another indication of the user’s intent to move must be sensed and transformed into a signal that controls the application of the assistive forces. Ideally, there needs to be a consistent coordination between the device and the user that results in a coupling of the human hand and the augmenting system, allowing the robotic device to consistently provide assistance as needed through the detection and amplification of the user’s effort. Some grip-assistive devices, however, have pre-programed algorithms with which users do not initiate by intent to move. These types of devices, such as the HERO Grip Glove [14] move the user’s hand through gripping and/or pinching patterns that allow for a set force production, which is then augmented by a user’s own strength. Devices by Yap et al. and Polygerinos et al. operate in a similar fashion, where the user shows intent to move, and the device then moves through a pre-determined motion without any subsequent input from a user [15, 16]. Such devices can both be used for hand motion training with the guidance of a physical therapist, as well as assist in ADLs.

Hand exoskeleton designs vary in overall weight, complexity, and cost. In attempts to provide the full range of motion of the human hand to the user, most of these devices have become both bulky and complex, and due to this are restricted to a single functional activity- either hand-opening or pinching. These exoskeleton devices often use a single motor or driving feature to assist multiple fingers [14], such as with Yoo et al.’s design which used one motor to drive three fingers [17] and Gasser et al.’s design which uses two motors to control four fingers [18]. Alternatively, some devices actively assist fewer than all five fingers [19], for example, Pu et al.’s, Nycz et al.’s, and Gasser et al.’s designs exclude the thumb [6, 18, 20]. Devices that allow for more degrees of freedom and independently assist all five digits become exceedingly cost prohibitive as more joints, motors, and custom electrical components become necessary [5, 21]. These additional motors and therefore batteries will also make the device heavier and potentially tethered to a power source dependent on the current draw [9, 22].

A previous design from our laboratory used machined aluminum segments to construct exoskeleton digits with a desktop computer-based control system tethered to the device [23, 24]. In order to reduce both the manufacturing cost and time of these previous prototypes as well as the weight, the most recent exoskeletons were designed to be constructed with 3-D printed thermoplastics. Furthermore, a minicomputer-based control system replaced the desktop computer and associated data acquisition hardware, which provided a further reduction in cost, weight, and complexity, and also allowed for greater freedom of movement [11, 25]. Even though exoskeletons with 1 or 2 fingers are simpler to implement, most ADLs require at least 3 fingers to be assisted by an exoskeleton [26]. Exoskeletons with 3 or 4 fingers could assist with most ADLs, the realism for the user would decrease as the number of fingers decrease [26]. Additionally, having fewer fingers limits the grasping positions the user can make with their hand, as well as limit the objects the user can lift. For example, a 4 or 3-fingered exoskeleton could assist a user in picking up objects of uniform shapes (e.g., cup, reusable water bottle) but not objects that are oddly shapes or have varying thicknesses (e.g., cell phone, wine glass). Increasing the number of independently controlled exoskeleton digits would allow for the control to lift objects such as this and increase the mobility to the point where it feels natural to the user. For training and rehabilitation devices specifically, being able to independently control each finger is imperative for re-developing muscle and flexibility in every finger. All these reasons listed are why most modern powered exoskeletons for the human hand use a five-fingered design despite the added complexity and weight [15, 16, 22, 27,28,29], and why we have decided to move from a three-fingered design [11, 25, 30] to a five-fingered design [31].

The main objective of this study was to design and produce a wearable powered exoskeleton for the human hand to improve structural stability of the fingers while also augmenting pinching and grasping efforts, and to validate that the device augments both the user’s pinching and grasping efforts and ability to perform ADLs by evaluating healthy human subjects. The exoskeleton device should be user friendly, allow for individual finger movement, and be cost optimized. This device aims to not compromise cost and weight for individual, independent movement of all five fingers. To be user friendly, the device must be able to incorporate a range of sizes that users may experience on a daily basis, as well as have a minimal user interface, and be easily donned and doffed. Additionally, the device must be portable and easily carried, and the batteries should last multiple hours. For cost optimization, electronic components must be commercially available, and the device should be modular such that broken parts are able to be replaced as necessary. Additionally, the modularity of the design must be such that different sized pieces are able be added and removed for users of differing size in the future. The exoskeleton structure was designed using CAD (computer aided design) software to enclose all five fingers of the right hand and was 3-D printed in ABS plastic. Each finger’s movement efforts were individually monitored and proportionally augmented via the microcontroller-based control scheme, linear actuators, and rigid exoskeleton structure.

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