UWE Bristol Engineering showcase 2015

Cameron Halpin
Mechanical Engineering
Project Supervisor
Dr. Appolinaire Etoundi
An investigation into the effectiveness of ‘smart’ materials in the
rehabilitation of stroke patients fingers
General:
In the UK there are estimated to be 1.2 million
stroke survivors. 77% of these survivors suffer
from some weakness in the arm or fingers.
Technologies used to rehabilitate these survivors
are 2 different types.
They can be assistive, where the user will wear the
device whilst performing a task that would
otherwise been much more difficult, or else they
can be entirely therapeutic, where the user will
wear the device for a certain period each day
running through a pre-defined cycle.
The Bioness H200, pictured above on the left, is
both therapeutic and assistive however there is
more emphasis on the assistive aspects. The
device actuates a grasp and release action via
small currents applied to the tendons of the lower
arm.
The SaeboFlex, pictured above on the right, is a
balanced device in terms of assistance and therapy
however user reviews have shown it to be difficult
to apply and slow in its effects.
The device has individual mechanisms for each
finger and is spring loaded. The springs reduce the
effect of spasticity on the hand, reducing stiffness
and muscle ache.
The design of the ‘soft’ actuated device was
decided to be similar to the SaeboFlex – spring
loaded with individual mechanisms – with the
ability to apply a ‘nudging’ force when required.
Experimentation:
This investigation tested 2 materials for properties
such as response time and power density.
These 2 materials were:
A Nickel-Titanium alloy, manufactured by Toki in
Japan. The SMA was manufactured as a coil in
order that the transition temperature caused
linear actuation.
The second material was VHB by 3M, a dielectric
elastomer, electro active polymer.
All factors considered, SMAs were chosen as the
most appropriate material to proceed to use in
design.
Areal Strain
Recovered Strain
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
VHB Max. Areal Strain vs Voltage
Voltage (V)
SMA Linear Recovered Strain vs Time
0 10 20 30 40 50 60 70
Frames
65g
Design:
The sketch shows the components of the
mechanism. Due to the pairing of the
compressive and torsional spring, the design of
the mechanism has the benefit of allowing the
user freedom in controlling the extent of
actuation; the user does not need to follow the
same cycle of movement every time. The device
has a closed loop control system.
The results of a ‘proof of concept’ model
showed that this mechanism, if slightly adjusted
can effectively assist in the operation of a
rehabilitative nudging device.
The image below shows each stage of the
device; from power supply to mechanism. It
details how the device can be mounted
effectively on to the user.
Project summary
Current rehabilitation technologies have been shown
to be less effective at combining assistance and
rehabilitation aspects. By applying alternative
materials there is potential to improve the
effectiveness, weight or cost performances of the
devices.
Project Objectives
To determine how effective ‘soft’ actuation
materials can perform in terms of rehabilitation of
fingers.
To design a mechanism that may better assist in
rehabilitation.
Project Conclusion
Depending on the mechanism, shape memory alloys
can provide a lighter, more effective method of
actuation than a motorised alternative.
The hysteresis loop s shown in the results section,
that SMAs exhibit is naturally similar to the hysteresis
loops of muscles and tendons. This similarity
indicates a high suitability for application in
prosthetics, as this is where the device is required to
mimic the behaviour of a finger most effectively.
For this mechanism however, the high cost incurred,
coupled with the pulse force required for a nudge
makes this a case of over engineering; a nudging
device does not require the unique loops available
from shape memory alloys.
In order to exploit the similarities, SMAs should be
used in full cycle simulations or in full prosthetics.
When able to be produced at a more affordable rate,
SMAs would make a highly suitable material for
artificial muscles.