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yjones
09-22-2010, 05:27 AM
Hi all,

A big thank you to all responded to my question and my apologies for the
delay in posting the summary. There were a variety of great suggestions and
we are currently reviewing all options for best approach....


Original Question:
___________________

Dear Biomech-l Subscribers,

We need to measure the force and angle applied by bandaid users to remove
the adhesive from the skin. These must be real applications, not
instron-type simulations. If needed, we have a motion capture system to get
the angle measurements but are unsure what the best approach would be in
measuring the force. The first thought was to instrument the bandaid
however there is concern that the rigidity of the sensor would affect the
mechanics of removal since bandaids are pliable. It also has to be small
enough that the mass will not have much of an effect. We've researched
sensors and found a few possibilities but without first purchasing and
trying them out it's hard to know what would work. Based on your
experience, can anyone make recommendations on sensors and a possible
approach? Any comments/suggestions are appreciated.

Regards,

Yvette Jones, MSc
Research Associate
British Columbia Institute of Technology
Burnaby, British Columbia
Canada



Summary of Responses
__________________

Could you add a small tab to the band-aid which had a strain gauge or
similar attached to it? I suspect you might have trouble fixing any sort of
sensing element to the band-aid itself. How about using an instrumented
pair of tweezers (or similar) to remove the band-aid?
Andrew


This reference may be useful to you ...

Karwoski AC, Plaut RH. Experiments on peeling adhesive tapes from human
forearms. Skin Res Technol 2004; 10:271-7

Cormac Flynn, Auckland Bioengineering Institute,


Can you do an in vivo test using an Instron-type machine? You might
build a seat to mechanically ground your subject with your tester. You
might need a pulley system to direct the pull of the tester. When
you're done making these things, you can set the rate of pull, the
amount of pull, and the angle of pull.
Your protocols should include orientation of the tape with respect to
the skin tension lines, pre- and post-exfoliation, and any
pre-conditioning of the skin or the tape. I would be concerned about
tests at a single site: how many tests can you perform on the same patch
of skin within a given time frame? What criteria do you use before
testing again?
You would want to do in vitro tests to understand the effect of tension
on the tape (band-aid).
I don't think size of the machine is an issue as compared to these other
concerns. However, after using the "Instron" to set up protocols and
determine the operating ranges, then think about smaller testers.
These are suggestions on my part. Another suggestion is to contact Dr.
Gary Grove at cyberDERM (www.cyberderm-inc.com). He has done or knows
how to do mechanical and chemical testing on skin. He may be able to
advise you.
Jim Furmato, TUSPM Gait Study Center


Tough problem. One approach to consider would be affixing (clamping,
strapping?) the body part (digit? limb?) to a force plate and isolating it
from internal forces (that's the really tough part). Then the force plate
will record the magnitude and direction of force applied to the limb by the
bandaid. The isolation would probably have to include a combination of
physical restraint of the more proximal body parts and instructions to
relax. May be worth a try, as I think it would be a lot simpler than
instrumenting the bandaid, particularly when direction is a free variable
that must be recorded, and you want a "natural" removal.
Larry Abraham


Why not use a strain gauged rod hooked or clamped into the end of the
bandaid plaster and record the strain at multiple pulling angles.
Dave Smith


Interesting problem. Does it have to be a human test? The reason I ask
is that it would be possible to measure the forces indirectly by using
an animal model. You could setup a piece of animal tissue, with the
skin intact on a force plate or weigh scale and then remove the bandaid
from the sample whilst it is on the scale. The reduction in weight of
the sample will correspond to the total force being exerted by the
bandaid on the sample as it is removed. You can obviously film it
simultaneously and then synchronize your video and force data using some
common event.. say tapping the specimen prior to pulling the bandaid
off.
I suppose you could use a human cadaver if you need it to be as
realistic as possible, but I suspect that the animal tissue would be a
close approximation.
Sean Osis, University of Calgary


I developed a mathematical model to correct for hysteresis and creep, and a
mechanical pre-loading procedure to eliminate shear loading effects on FSR
semi-conductive polymer sensors (Interlink Electronics), to allow for
accurate compressive force measurements (see attached). By using the
mathematical model and pre-loading procedure, you should be able to get
accurate and reliable compressive force measurements in your bandaid study.
I would recommend attaching a sensor to one end of the bandaid using a
strong adhesive to prevent sliding. There is a technique described in the
article for converting the sensor from a pressure-sensor to a force-sensor,
followed by the pre-loading procedure to eliminate shear loading effects.
Let me know if you have any questions regarding the mathematical model or
pre-loading procedure used.
Rick Hall


I guess the answer to your force measurement query will depend on how much
accuracy/precision is desirable in your application, how soon you want the
results and how much you are ready to pay for all of it. Also, from your
problem description below, I believe you want to compute the shear force
(along the bandaid plane) between the thumb/index finger and the bandaid.
We have used strain measurement devices from Vishay Precision Group (
http://www.vishaypg.com/micro-measurements/) on ligaments. Many
investigators have used their strain gages on bones (tibia,
anterior/lateral vertebral body, etc.) For your application, I guess you
could use one of their low profile shear stress sensors (Transducer Class
or Stress Analysis strain gages). However, you may have to buy their signal
conditioning unit as well.
A more fancy option could be their optical measurement option: (
http://www.vishaypg.com/docs/49913/photostr.pdf)
I am not sure about the cost but time to start collecting data should be
short in both cases.
Vikas


I recall from my tissue mechanics days that the way we measured strain in
skin was to draw a grid on it then measure the distortion of the lines. I
think this is probably the best approach for your problem. You would have
to measure the Young's Modulus of the band-aid so that you can calculate
stress from strain. As you say, any sensor will tend to distort the
band-aid, and I think you will be hard pressed to find a suitable sensor
anyway - Hall effect would probably be the only option, I guess.
Chris Kirtley, Sole Health Care Products Pty Ltd


There is a material called Quantum Tunnelling Composite manufactured by a
company in the UK called Peratech. The material is piezoresistive, rubber
like, and can be manufactured to almost any form. The material itself
is effectively a pressure sensor, exhibiting a resistance of 10 kOhms under
no load to under 1 Ohm under full load.