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Abstract— Almost 1 million Americans suffer from debilitative
disorders or injuries to the hand, which result in decreased grip
strength and/or impaired ability to hold objects. The objective
of this study was to design and test the functioning of a five-
digit exoskeleton for the human hand that augments pinching
and grasping efforts. The exoskeleton digits and the wrist and
forearm structure was computer designed and 3-D printed
using ABS plastic, while the housing for the control system,
motors, and batteries was constructed from laser-cut acrylic.
The user’s finger movement efforts were monitored with force
sensing resistors (FSR) located within the fingertips of the
exoskeleton. A microcomputer-based control system monitored
the FSRs and commanded linear actuators that augmented the
wearer’s force production. The exoskeleton device was tested
on six healthy individuals. Using the device for grasping efforts
significantly decreased the muscle activity necessary to
maintain a constant force (p < 0.001); however, no significant
benefit was identified during pinching efforts. In conclusion, a
novel 5-digit exoskeleton was designed, and functional testing
identified a significant benefit of using the device during
grasping efforts.
I. INTRODUCTION
Over 795,000 Americans suffer from debilitative
disorders such as arthritis, or injuries to the hand that result in
decreased grip or pinching strength and/or diminished ability
to hold objects [1]. Use of powered assistive device augments
muscle strength and mechanical support of the hand and
fingers would improve the quality of life of such individuals.
Robotic exoskeleton devices provide assistance by
detecting and amplifying a user’s hand movement efforts via
active and/or passive mechanisms [2]. This type of assistance
can increase functionality of the hand as well as promote
therapeutic treatment compliance, increasing the
effectiveness of rehabilitation [3]. Additionally, robot-
assisted rehabilitation can improve hand function after stroke
and other trauma [4]. Although many different functional
devices are currently in development [5, 6], most were
designed for rehabilitation, and no intended to assist with
everyday tasks.
Previously, in this laboratory, hand exoskeletons were
designed with machined aluminum pieces and a personal
computer-based control system [7, 8]. Recent designs were
constructed using 3-D printed plastics and replaced the
*Research supported by the School of Engineering, The College of New
Jersey.
E. R. Triolo and B. F. BuSha are with the Department of Biomedical
Engineering, The College of New Jersey, Ewing, NJ, 08628 USA (e-mail:
busha@tcnj.edu).
M. H. Stella is with the School of Engineering, The College of New
Jersey, Ewing, NJ 08628 USA
personal computer control system with a microcontroller to
reduce manufacturing time, cost, and weight [9, 10]. The
most recent design from this laboratory was a three-fingered
exoskeleton 3-D printed from thermoplastic controlled by an
Arduino-based system [11].
The objective of this study was to design and test the
functioning of a five-digit exoskeleton for the human hand
that augments pinching and grasping efforts. The exoskeleton
structure, which enclosed all 5 digits of the hand, was
computer modeled and 3-D printed in ABS plastic. A
microcomputer-based control system monitored each finger’s
movement efforts and applied an augmenting force
proportional to the efforts through an exoskeleton structure.
II. METHODS
A. Mechanical Structure
The exoskeleton structure was designed to surround and
support all five fingers of the hand, prevent any movement of
the wrist joint, and to direct electric motor-generated gripping
and pinching forces to the exoskeleton’s digits. The
exoskeleton structure was designed in Solidworks and 3-D
printed with ABS plastic (Dimension SST 1200es). The wrist
and forearm structure (WFS) was designed to be lightweight,
and easily attached to and removed from the user’s wrist
using a velcro strap, as illustrated in Fig. 1. The electric
motors, control system, batteries were housed in a plastic
container fixed to the dorsal aspect of the WFS. The
exoskeleton digits (EXDs) were attached using a pin and cap
A force augmenting exoskeleton for the human hand designed for
pinching and grasping
E. R. Triolo, Student Member, IEEE, M.H Stella, B. F. BuSha, Senior Member, IEEE
Figure 1. Schematic of the exoskeleton’s wrist and forearm
structure, which allowed for mounting of the motors, batteries, and
control system, and for attachment points for the 5 exoskeleton
digits. The structure was 3-D printed as one piece.
978-1-5386-3646-6/18/$31.00 ©2018 IEEE 1875
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design, as shown in Fig. 2, so as to allow for replacement and
repair without the need to re-print the WFS.
Each EXD, apart from the thumb, was composed of three
pieces, consisting of a proximal, middle, and distal ring
structure, each connected by flexible joints that restricted
finger movement to an appropriate range of motion at the
distal, proximal, and metacarpophangeal joints. Each
component of the index and pinky digits were 3-D printed
independently and assembled by connecting the flexible
joints. The ring and middle digits were designed similarly to
the pinky and index ones, but also include a flexible knuckle
design, and were therefore 3-D printed in two parts each. The
first assembly included the knuckle joint, proximal, and
middle components of the digit, while the second assembly
consisted of the distal component. The thumb was directly
attached to the wrist, with the distal component of the thumb
having the ability to be attached after printing. To limit range
of motion, mechanical stops were integrated into the top
portions of each digit to prevent hyperextension of the finger
joints.
Finger movement efforts were quantified with force
sensing resistors (FSR) located at the inner ventral aspect of
the tip of each of the 5 exoskeleton digits, the point of contact
for the fingertips of the wearer during grasping or pinching
maneuvers. Holes in the dorsal aspect of each of the
exoskeleton digits provided a tunnel for the FSR wires, while
holes in the bottom of the exoskeleton digits allowed for
polymer cables to connect the tips of the exoskeleton digits to
the linear actuators.
B. Electrical Components and Control System
FSRs provided input to a control system, implemented
with a microcontroller (Arduino micro), which commanded
force-augmenting linear actuators (Actuonix). A 6V
rechargeable battery powered the motors, and a 9V
rechargeable battery powered the microcontroller. FSRs and
the motors were attached using crimped connections, which
allowed for easy replacement of faulty components.
When powered, the device automatically initiated a ten-
second calibration sequence, indicated with an illuminated
LED. During the calibration, the user made several grasping
efforts that allowed for independent calibration of each digit.
No force sensed by an FSR mapped to the actuator being
IfThP IfG MfG RfG HandG
0
10
20
Force (N)
Exoskeleton Force Production
Figure 4. Average force production of the orthotic device during
an index finger and thumb pinch (IfThP), index finger grasp (IfG),
middle finger grasp (MfG), ring finger grasp (RfG), and a full hand
grasp (HandG).
Figure 3. From top to bottom, a dorsal, lateral, and ventral image of the
device with a user’s hand in place.
Figure 2. The index digit exoskeleton was comprised of 3 concentric
ring structures with bilateral joints. The most distal section encased the
fingertip, and the section proximal to the wrist and forearm structure
attached with a pin and securing cap. The topmost image shows the
dorsal aspect of the digit with the tunnels for the FSRs and wires, and
the bottom image shows the tunnels for the polymer cables. Individual
parts were 3-D printed separately using a thermoplastic.
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fully extended (fully extended finger), while the maximum
fingertip force mapped to the actuator being fully contracted
(fully contracted finger). This allowed for independent
control of each of the five digits.
The electrical components were housed in a box that was
laser-cut from a 3mm thick acrylic sheet, assembled using
comb joints. Openings were placed in the side of the box
facing the exoskeleton digits to accommodate the linear
actuators and the wires connected to the FSRs. The opposite
side of the box housed two switches that controlled power to
the device, and an LED.
III. EXOSKELETON FORCE PRODUCTION
To assess the device’s maximum grasping and pinching
force production, the unworn device was commanded to
produce: pinching forces with the index and thumb digits;
grasping forces with the index, middle, and ring digits
independently; and grasping forces with all digits. The
exoskeleton was secured in a custom stand, and the digits
were placed around a hand force dynamometer (BioPac
Systems Inc.). A software routine commanded the device to
produce three 10-second contractions for each maneuver,
with an uncontracted pause of 10-seconds between
contractions.
IV. EXPERIMENTAL PROTOCOL
Six healthy test subjects, aged 18 to 23, participated in the
study. The subjects were informed of the experimental
protocol prior to participation, and each provided written
consent. This study was approved by the Institutional Review
Board of The College of New Jersey.
An initial fit test was conducted to ensure that subjects’
hand fit within the device. Subjects whose hands did not fit
were excluded from participation. Subjects also needed to be
able to comfortably move all fingers as well as make full
grasping and pinching efforts, while wearing the device.
Grasping and pinching forces were recorded using a hand
dynamometer (BioPac Systems Inc.) while surface
electromyography (EMG) of the forearm muscles was
measured with three surface EMG electrodes (Heart Trace,
Cardiology Shop) Grasp force (Newtons) and EMG activity
(mV) were simultaneously recorded by a data acquisition
system (Biopac Systems, Inc.).
Prior to starting the experimental procedure, the force
production of the exoskeleton device was calibrated to each
subject’s maximum clenching force. Following the
calibration, each subject produced a minimum of four
grasping efforts of 25N while bare handed, with the
unpowered exoskeleton on, and with the powered
exoskeleton on. The same sets of tests were repeated while
the subject produced 15N pinching efforts with his/her thumb
and forefinger. To assist in the maintenance of the target
clenching and pinching forces, visual feedback of the
subject’s efforts was provided on a computer monitor. The
recorded forces were processed with a low pass filter of
66.5Hz. The EMG was recorded with a bandpass filter of 5-
1000Hz, and moving time averaged.
A locally designed algorithm implemented in MATLAB
was used to identify the peak pinching and grasping force per
test and to extract 1 second of force and EMG data with the
peak as the midpoint. Baseline values of the force and EMG
data immediately prior to or following the force effort were
subtracted from the test values to account for any baseline
drift.
The average force for each extracted 1-second interval
was divided by the concomitant average EMG in order to
normalize the measurements for any variations in force
production. At a constant force while wearing the device, any
decrease in the force/EMG relationship would indicate that
the forearm muscles produced less electrical activity for a
given grasping or pinching effort [11, 12]. The force/EMG
measurements for each type of effort by each subject are
presented as the average ± sample standard deviation.
Results of the bare handed, unpowered exoskeleton on,
and powered exoskeleton on tests of full-handed grasping and
pinching were compared using a one-way repeated measures
ANOVA, and multiple comparisons were assessed with the
Fisher’s L.S.D. Statistical analyses were performed using
Bare UnPowered Powered
0
2500
5000
Force/EMG
(a.u.)
Grasping Manuever
***
**
Figure 6. Average grasping force/forearm muscle EMG relationship
across efforts of 4 efforts in 6 subjects, expressed in arbitrary units
(a.u.). Error bars indicate standard deviation. There was a significant
difference across all 3 states (*, p = 0.01), and between the barehanded
and powered device states (**, p < 0.001), and between the unpowered
and the powered device states (**, p < 0.001).
Bare UnPowered Powered
0
600
1200
Force/EMG
(a.u.)
Pinching Manuever
Figure 5. Average index-thumb pinching force/forearm muscle EMG
relationship across 5 efforts in 6 subjects, expressed in arbitrary units
(a.u.). Error bars indicate standard deviation. There was no statistical
difference between the average Force/EMG relationship when the
subjects were barehanded, wearing the unpowered device, or while
wearing a fully powered and functioning device.
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OriginPro 2015 (OriginLab) with a statistically significant
difference identified with a p < 0.05.
V. RESULTS
The unworn exoskeleton device produced a maximum
grasping force of 17.2 Newtons, and a maximum pinching
force of 5.0 Newtons. Independently, the middle exoskeleton
digit generated a grasp of 10.3 Newtons, while the ring and
index digits generated forces of 9.2 and 7.0 Newtons
respectively, as illustrated in Fig. 4.
Although there was a trend towards a benefit of wearing
the device, no statistically significant difference was
observed in the average pinching force/EMG relationship
between the barehanded, unpowered device and powered
device trials (p > 0.05), as illustrated in Fig. 5
During full-hand grasping efforts, there was no significant
difference in the forces produced across the three testing
states (p> 0.05), however a significant difference in the
force/EMG relationship was identified across testing states (p
= 0.01). Although no significant difference was identified
between bare handed and wearing the unpowered device (p >
0.05), there was a significant increase in the force/EMG
relationship between the barehanded and the powered device
states (p < 0.001), and between the unpowered and powered
device states (p < 0.001), as illustrated in Fig. 6. By using the
exoskeleton, the subjects were able to maintain a steady force
while their forearm muscles produced significantly less EMG
activity than that produced while bare handed.
VI. CONCLUSION
In this study, a five-fingered, 3-D printed, battery-
powered, and force augmenting orthotic exoskeleton for the
human hand was developed and tested. A microcomputer-
based control system proportionally contracted corresponding
assistive linear actuators based on input of FSRs located on
the ventral aspect of the EXD tips. The five-digit exoskeleton
design allowed subjects to maintain independent control over
all fingers. While wearing the unpowered and powered
exoskeleton, subjects were able to maintain independent
finger movement, pick up a common object, such as a water
bottle, as well as smaller and more delicate object, such as a
smartphone.
Independently 3-D printing the components of the device
was cost effective and allowed for rapid replacement of
broken parts. The modular design of the control system,
including crimped connections to the FSR and motors, also
allowed for fast and easy replacement of the FSRs, batteries,
LEDs, and/or linear actuators.
In a sample of six subjects, the device provided a
significant reduction in forearm muscle activation during a
constant force grasping efforts, as compared to bare-handed
efforts. Although during a pinching task there was no
statistically significant improvement in the force/EMG
relationship, the average pinching force/EMG was reduced
during efforts with the unpowered device, and increased in
the powered device trials, as compared to the barehanded
trials. This difference could be attributed to the fact that
during the pinching task, the device was only providing
augmented support for two digits, as opposed to the grasping
trials in where all 5 digits were assisted by the motors. In
future studies, a further optimization of the control algorithm
may enhance the force augmentation provided by the device,
thus further improving the benefit during pinching
maneuvers.
ACKNOWLEDGMENT
The authors would like to acknowledge the technical
support of Mr. Joe Zanetti and Mr. Michael Steeil.
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