The SMART(R) Wrist-Hand Orthosis (WHO) for Quadriplegic Patients
John B. Makaran, MESc, P. Eng.
Douglas K. Dittmer, MD, FRCPC
Ralph O. Buchal, PhD
Dawn B. MacArthur, CO(c)
ABSTRACT
The SMART? Wrist-Hand Orthosis
(WHO) is a new method of providing
grasping function for quadriplegic patients. The use of shape memory alloy
(SMA) actuators in rehabilitation technology (SMART) allows for a new type
of actuator in addition to traditional actuators such as Bowden cables, pneumatic cylinders, electric motors and linear ratchets. It provides excellent function and ease of use.
Key Words: SMART?, Shape Memory Alloy (SMA), Wrist-Hand Orthosis (WHO), quadriplegic
Introduction
Attendant care for the long-term management of quadriplegic patients is expensive, reflecting the lack of independence of activities of daily living
(ADL) these people experience. Assistive devices, including orthoses, can
enhance their independence. A prototype orthosis, employing a new technology, is introduced here.
The flexor-hinge hand orthosis was
created based on the principle of a
modified three-jaw chuck described by
Nickel, Perry and Garrett (1). Immobilizing the thumb in opposition and semi-flexing the interphalangeal joints of
the index and middle fingers allow the
index and middle fingers to move simultaneously toward the thumb. An object
can be grasped firmly between the
thumb and finger pads (2). The usefulness of wrist-driven flexor hinge orthoses in the rehabilitation of C6/7 quadriplegics has been documented (3,4).
Variations of the flexor-hinge hand
orthosis in patients lacking good wrist
extension strength include externally
powered (McKibben muscle, C02 and
electric motor) passively positioned
ratchet hand orthoses and shoulder-driven (Bowden cable) hand orthoses
(5). These orthoses obtain prehension
by mechanically linking the motion of
joints through four-bar linkage (6).
The normal range of motion is from
full extension of the metaphalangeal
(MP) joints to the point where the finger and thumb pads touch. The patient
can obtain these positions voluntarily
or nonvoluntarily through such means
as Bowden cables, pneumatic cylinders, electric motors and linear ratchets that the patient activates by pushing
(5). Each method has its advantages
and disadvantages. The most obvious
disadvantages to these traditional
methods of actuation are the actuators'
bulkiness and unsightliness.
The SMART WHO orthosis uses
shape memory alloys (SMA) as the actuating element to convert electrical energy into mechanical work. Although
SMA have been around for some time,
applications have been slow to develop
due primarily to a lack of understanding
of how these materials behave. To design with SMA, an integrated approach
is required, considering aspects of mechanical, metallurgical and electrical engineering (7).
SMA's Shape Memory Effect (SME)
makes it a useful material for actuators. The SME allows the SMA to deform at low temperatures and recover
its original shape upon heating. This
remarkable recovery characteristic is
the result of a phase transformation in
the metal from a low-temperature disorganized crystal structure to a reorganized crystal structure at a high temperature.
Users may select the transformation
temperature within a certain temperature range determined by the composition of the metal. Consequently, if a
metal has been programmed with a certain memory shape and is deformed,
the metal will recover its memory
shape when heated above the transition temperature. As long as the deformation strain remains equal to or lower
than 8 percent, the metal will completely recover its memory shape with
no residual plastic deformation (8).
The alloy we studied was a nickel
titanium alloy dubbed "NITINOL."
This alloy offers corrosion resistivity
similar to that of 304 stainless steel and
possesses a high value of electrical resistance, making actuation by an electric current possible (8).
Device Description
The SMART WHO (patent pending)
appears in Figure 1
and Figure 2
. The principle of operation is as follows. If the
patient wishes to grasp an object, he or
she activates the SMA strand to close
the hand (see Figure 2
). The closing of
the hand is opposed by a bias spring
(component 1 in Figure 2
) as well as by
any object the patient is trying to grasp.
To minimize power consumption, the
position of the hand is maintained by a
rotary ratchet (component number 2 in
Figure 2
) mounted on the hand's center
of rotation.
If the patient wishes to release an
object, he or she engages the SMA
strand that pulls the spring-loaded
pawl (component number 3 in Figure 2
) away from the rotary ratchet, allowing the bias spring to redeform the
SMA strand used in the closing of the
hand. A schematic circuit diagram appears in Figure 3
.
Since the SMA chosen possesses a
high value of electrical resistivity,
electrical current may be used to induce the SME. The patient can control
how much electric current passes
through the SMA strands by using a
sip-and-puff switch. (In a sip-and-puff
switch positive pressure-puff-activates a switch closure and negative
pressure-sip-activates a second
switch closure.)
Sip-and-puff switches can be purchased with set pressures to activate
switch closure or can be made with
electronic circuitry and adjusted to
specific pressures using pressure transducers and electronic circuitry with a
relay to provide the switch closure.
Switch closure usually means two contacts are touching when activation occurs. Therefore, a patient may sip to
initiate closing of the hand and puff to
initiate opening of the hand.
In a simmilar manner, myoelectric signals from intact muscle groups may be
acquired using surface electrodes and
amplified to control the passage of current to the SMA strands. For example,
a C5 quadriplegic may initiate closing
of the hand by biceps flexion. A small
battery pack and associated electric circuitry provide power. The current supplied to the SMA must be regulated to
avoid overheating the SMA wire,
which would result in degradation of
the SME. In the prototype, the battery
pack and circuitry were approximately
the size of a portable Sony Walkman?(r)
and could be worn on a belt. The battery pack consisted of rechargeable NiCad batteries that could be plugged
into the wall socket when the orthosis
was not in use.
Presently, the orthosis can grab larger, rounder objects with ease (e.g., a
cup or tube of toothpaste). With minor
changes on the SMA actuator used to
close the hand, the SMART orthosis
will be capable of grasping large round
objects as well as small flat objects
(such as coins or paper). This could be
accomplished by changing the range of
rotation of the pawl mechanism. Since
the weight of the SMA strands is negligible, the orthosis' weight is essentially
that of the frame (~ 10 oz.)
Clinical Trial
The orthosis was tested on a CS quadriplegic. The patient had use of his deltoid and biceps muscles, which he used
to accomplish activities of daily living.
He could feed himself with the appropriate aids, accomplish grooming activities, help with upper-extremity dressing, help apply bracing, turn pages and
use an electric typewriter. He used a
joystick to activate and steer his electric wheelchair.
Preliminary results were very promising. For example, the patient was
able to grasp a tube of toothpaste firmly in his hand and undo the top with
his teeth. He could grasp thin objects without the awkwardness associated with ratchet-driven orthoses. The
light weight of the device's SMA actuators allowed for comfort, especially
during long-term use.
The device also allowed the patient
to use ordinary everyday objects. The
patient had been using an intercom system for telephone conversations, but
he said the SMART WHO let him
grasp a normal telephone receiver. He
verbalized his endorsement of the orthosis with an enthusiastic: "When can I
get one?"
Discussion
The SMART WHO design is lightweight and simple, combining ease of
use and good functionality. The use of
SMA actuators permits miniaturization, leading to improved cosmesis.
Reliability results from the simplicity
of the design and the high fatigue life of
the SMA (1O7 cycles to failure). SMA
actuators are also very inexpensive
compared with other alternatives. The
total cost of the SMA wire used in the
prototype was less than $15 (Cdn).
Several improvements to the prototype design are possible. While the response time of the orthosis in its present form is relatively fast (approximately two seconds to close the hand),
it may be improved by redesigning the
SMA actuators. Since heating time is
proportional to diameter, using thinner
SMA strands with shorter heating
times would result in faster opening
and closing times, providing ability to
perform repetitive tasks. (Instead of
using a single thick SMA wire to close
the hand, several wires of smaller diameter can be used.) Also, using plastic rather than aluminum in the orthotic frame will result in lighter weight and
eliminate the possibility of electrical
short circuits from the SMA to the
frame.
Conclusion
The SMART WHO prototype provides a new concept in the design
of wrist-hand orthoses, allowing for
greater independence in ADL. It uses
a long-life battery pack, and graduation of force is possible with lightweight SMA strands. Current research
to provide a "glove-like" look should
improve cosmesis. Future research
should focus on what factors are important to consumers in selecting assistive
devices (9).
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