pupil instructions
theme:
fashion & textiles
activity:
medical compression – make a pressure sensor
introduction
Textiles are found in all sorts of medical applications – from
surgeon’s gowns to sutures for stitching wounds together.
Dr Lisa Macintyre, pictured, is a textile technologist. Lisa
has helped develop ‘pressure garments’ used to treat
people with serious burn injuries. By applying pressure
to the burn, it is possible to reduce itchy, lumpy, painful
scars.
Lisa has also worked on anti-embolism stockings. They
help prevent Deep Vein Thrombosis (DVT) by applying
pressure to people’s legs. This increases the blood flow
back to the heart which stops it from clotting.
In both of these applications, it’s essential that just the
right amount of pressure is applied – not too much, and
not too little.
So engineers have to develop ways of measuring precisely how much pressure is
applied to different parts of the body when different pressure garments and dressings
are worn or applied.
In this activity you will work with some really cool types of fabric to make a pressure
sensor of your own. You can then calibrate the sensor and try it out.
Your teacher will give you all the equipment you need.
Read more about Lisa here:
http://www.futuremorph.org/my-future-finder/fashion-textiles/medical-compression-products/
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pupil instructions
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activity: medical compression – make a pressure sensor
procedure
1
Start by cutting out the different fabrics. Use the diagrams as a guide.
You need two pieces of cotton (about 6 cm x 6 cm), one piece of piezoresistive material (about 4 cm x 4 cm) and two pieces of conductive
fabric (about 3 cm x 3 cm, each with a ‘lead’ about 1 cm x 4 cm).
6 cm
6 cm
4 cm
x2
cotton fabric
2
x1
piezo-resistive
material
3 cm
4 cm
3 cm
4 cm
x2
‘lead’
1 cm
conductive fabric
Your sensor
could be a
different sha
pe or size
if you want –
just make
sure that the
two pieces
of conductiv
e fabric
don’t touch.
Use fabric glue to stick your pieces of fabric together, in this order:
cotton – conductive – piezo-resistive – conductive – cotton
Put some fabric glue on
a piece of cotton fabric.
glue
Stick on a piece of conductive fabric.
Add more glue around the edge.
Stick on the piezo-resistive
material. Add more glue.
Stick on a piece of conductive fabric
(make sure its ‘lead’ is pointing in the
opposite direction). Add more glue.
Stick on a piece of cotton fabric.
You have made a pressure sensor!
How are you
r sewing
skills? You c
ould sew
the pieces o
f fabric
together, rath
er than
using fabric
glue.
pupil instructions
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activity: medical compression – make a pressure sensor
Try connecti
ng a 9 V
battery to th
e cir
set the multim cuit and
eter to read
current (Am
ps, A). The
current shou
ld increase
when you ap
ply pressure
.
Do you know
why?
3
Use crocodile clips to connect your pressure sensor to a
multimeter. Set the multimeter to read resistance (Ohms, W).
This tells you the resistance of the pressure sensor.
Record the resistance in a table, like the one below.
4
Push down on the pressure sensor. What happens to the
multimeter reading? It should decrease, showing that
pressure decreases the resistance of the pressure sensor.
Now you can calibrate the pressure sensor ...
5
Place a two-pence coin on the centre of your sensor and record the
resistance in your table.
6
Add another two-pence coin and record the resistance in your table.
Keep adding coins and recording the resistance. Stop when you’ve
added about ten coins, or when the multimeter reading doesn’t change.
results and calculations
number of two pence coins
0
1
2
3
4
resistance (W)
pressure (N/m2)
0
Complete the ‘pressure’ column of the table. You will need to work out:
zz the force (weight) of a 2p coin, in Newtons (N)
zz the surface area of a 2p coin, in metres squared (m2)
zz the pressure exerted by a 2p coin, in Newtons per metre squared (N/m2)
Remember:
zz 1000 g = 1 kg = 10 N
zz the area of a circle = πr2
zz pressure (N/m2) = force (N) ÷ area (m2)
Now draw a graph of resistance (W) against pressure (N/m2).
This is your calibration curve.
Try adding unknown masses to your pressure sensor. Use your calibration
curve to work out how much pressure is being exerted.
pupil instructions
activity: medical compression – make a pressure sensor
extension: using your pressure sensor
Try lightly taping the pressure sensor to your leg, so it touches your calf muscle.
Attach the wires and tape them so they run up your leg.
Now put on a compression sock and read the multimeter. Use your calibration
curve to work out the pressure being exerted by the sock.
zz Is the pressure the same if you attach the sensor to a different part of your leg?
zz Try doing the same thing with a normal sock. Does it exert less pressure?
zz You could also try with TubiGrips™, or ordinary bandages. Does the multimeter
reading change as you tighten the bandage?
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