Instructables

Conductive Rubber: Make Touch Sensitive Robot Skin

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Picture of Conductive Rubber: Make Touch Sensitive Robot Skin
Make conductive rubber and use it to create a flexible touch sensitive robot skin. The resulting skin can be stretched across a frame or mounted on solid surfaces. It can also be mounted on curved surfaces or clothing.

It is touch sensitive and will produce an electrical signal no matter where you press on it.
 
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Step 1: How It Works

Picture of How It Works
edge.jpg
Step 1 pic shows a 3d exploded illustration of the layers that make up the robot skin. It was made using 123d. Thumb pic shows the edge 0f the finished robot skin.

Ideally, this robot skin could be 3d printed. With a more intricate pattern, the feel and resiliency of human skin could probably be achieved. Unfortunately, 3d silicone printing is beyond the reach of my limited budget, and as far as I know, no one so far is 3d printing conductive rubber.

Oogoo II-A Conductive Rubber
You can make your own conductive rubber from silicone caulk, corn starch, solvent, and graphite powder. This creates a silicone rubber which can be made thin enough to paint on or thick enough to cast into molds.

Oogoo II can be cast into sheets and then laminated to create a layered robot skin that conducts electricity when under pressure. The layers are designed to conduct between two layers, no matter where you press on the surface of the skin.

Step 2: Materials

Picture of Materials
100% Silicone Caulk from hardware store

Corn starch from grocery store

Carbon Graphite powder- Available from: http://www.elementalscientific.net/

Naphtha solvent from hardware store. Turpentine or Citrus solvent can also be used but they take longer for the fumes to dissipate.

Mixing cup

Popsicle stick

1/32" and 1/16 inch thick plastic sheets

Step 3: Make Conductive Rubber Sheets

Picture of Make Conductive Rubber Sheets
Make A Form
Step 3 pic shows the form which is composed of two 1/16" thick phenolic sheets that are taped to a plastic base about 2-3/8" apart. I used phenolic because I have surplus sheets of it, but any kind of plastic can be used. Oogoo and Oogoo II will stick to any plastic but can be peeled off cleanly using medium force.

Mix Conductive Rubber (Oogoo II)
Oogoo II is basically Oogoo with the addition of graphite to make it conductive and a solvent (Naphtha) to make it possible to mix.

To create a sheet of Oogoo II about 1/16" thick x 2-3/8" x 3-1/2", add 3-1/2 parts graphite powder (by VOLUME) to one part silicone caulk to 1/2 part corn starch . A mix of 5-1/4 Teaspoons Graphite to 1-1/2 Teaspoons Silicone Caulk to 3/4 Teaspoons of Corn Starch gives more than enough to cast one sheet.

First mix the graphite and corn starch together with enough Naphtha to make a thick paste of the powders. Then add the silicone caulk and mix it very well with a popsicle stick or stirring rod. You want to end up with a mix about the consistency of smooth peanut butter.

WARNING: Naphtha is a sickly-sweet smelling solvent that wants to dissolve your brain cells if you breathe too much of it. Make the mix outside with good ventilation or
inside under a vent hood. Wear safety glasses and nitrile gloves, as it can be absorbed by the skin.

I tried at least a dozen different solvents. While other solvents can be used instead of Naphtha, it has the advantage that it evaporates extremely fast and most of the odor is gone in a few hours. Other solvents can take a day or two to dissipate and increase the set up time.
Mix in small batches and use it quickly before it starts to skin up. It will typically set up enough in two or three hours to de-mold. Let it completely set up overnight before using it.

Other Mixes
At about two parts graphite to one silicone caulk, the mix starts to become usefully conductive. Adding more than 3-1/2 parts graphite (by volume) to 1 of silicone will reduce the flexibility of the final rubber.

Screed The Sheet
Spread the mix carefully to avoid bubbles and screed it with a stiff piece of metal or plastic and let it set up overnight.

Step 4: Make The Insulating Spacers

Picture of Make The Insulating Spacers
stencils.jpg
Make the Spacer Stencils
The Three conductive sheets are held apart by insulating spacers. Pushing between the spacers creates a conductive path. The insulating spacers are the white dots that are made by screeding Oogoo in the holes in the stencils. The stencils are made by drilling 1/4" holes in 1/16" plastic sheet. The holes are drilled 1" apart with the holes in the second stencil drilled in between the holes in the first.

Screed the Spacers
Step 4 pic shows a small mix of Oogoo (2 silicone caulk to 1 corn starch by volume) being screeded on one layer of Oogoo II.

Step 5: Layers To Create A Robot Skin

Picture of Layers To Create A Robot Skin
Step 5 pic shows the four layers in the order they are laminated:
1. 1/16" top sheet of standard Oogoo which can be colored with artists oil paint. This provides the necessary resilience to the skin and protects the conductive layers.
2. 1/16" Oogoo II with insulating spacers (white dots)
3. 1/16" Oogoo II with offset insulating spacers
4. 1/16" Oogoo II base layer.

Glue Wires On
Before gluing the layers together, wires or conductive thread should be attached with Oogoo II to the edge of the 3 conducting layers. Stranded wire that is flared out works the best for a good mechanical connection. Conductive thread can also be used.

Glue the Layers Together
The four layers are glued together using a thin layer of Oogoo on the top sheet and small amounts of Oogoo on the insulating spacer dots. Do this a flat surface with a weighted flat sheet on top to flatten everything out as the Oogoo sets up.

Step 6: Testing The Robot Skin

Picture of Testing The Robot Skin
The resulting robot skin has a resistance between 300 to 400 ohms when pressed with about 3 ounces of force anywhere on the surface. The top and bottom conductive layers are connected together to create a switch with the middle layer. The resistance is low enough that an led can be directly lit using five volts.

By varying the insulating dot spacing and thickness of the conductive rubber, the pressure of this touch sensor can be adjusted.

Step 7: Other Possibilities

Picture of Other Possibilities
Thinner Skin
This prototype robot skin came out at .4 inches thick. The five layers could be made thinner resulting in a skin about .25 inches thick. Even thinner skin might be made using conductive fabric instead of Oogoo II. The only limitation would be the thin plating on most conductive fabrics that might wear off fairly fast.

If you don't need skin that is touch sensitive everywhere, a three layer skin could be made that is only sensitive between the spacer dots.

Curved Skin
The layers can be glued up on a curved form such as a pipe. For compound curves, the sheets would have to be molded on a form with the proper curves.

Tile the Robot Skin
Oogoo II layers can be laminated behind a continuous sheet of Oogoo to create touch sensitive zones to give a higher resolution of touch sensitivity.

Human Touch Sensitive Skin
A two layer skin of laminated Oogoo and Oogoo II can be made that is sensitive to human touch. The capacitive touch detecting inputs of a Picaxe microcontroller can be used, or you can try this circuit: http://www.instructables.com/id/Curved-Circuit-Board-Art-Make-a-Touchless-Touch-S/

Touch Sensitive Clothing
Two layers of conductive rubber with spacers between, can be glued behind shirt fabric to make pressure sensitive switches for wearable electronics.

Human-Like Skin
Oogoo has a smoothness and texture very similar to human skin. By varying the thickness of the layers and the pattern of the spacers, a robot skin that has the feel and resiliency of human skin could be created.

Other uses
See here for another use for conductive rubber: http://www.instructables.com/id/Make-Conductive-Rubber-Transparent-stylus-iPodiP/
roboticarrow9 months ago

Just curious, do you think it would be possible to make magnetic oogoo using a process similar to this? http://www.instructables.com/id/magnetic-silly-put...

mikey77 (author)  roboticarrow9 months ago

Yes, I have tried it and it works.

Adding 3 Oogoo to 1 black iron oxide will make the Oogoo object become attracted to magnets. If you add more iron oxide it will be more attracted, but will also become less flexible.

thank you mikey for hopping over to my ible and now i just opened a pandora's box! wow, thank you for the goldmine of inspirations here!

Hi, do you have instructions an / or pictures (even better) of how you connected the wires? I'm not quite sure on what the circuit layout should be? I would have guessed (wrongly?) that you wired up the middle Oogloo2 skin with your active, and 1 or 3 with the other wire (so the circuit could go through 1 or the other)...?
mikey77 (author)  IBreakHeavyStuff2 years ago
You got it right.

Although it doesn't show the wires, If you look at the step 1 pic, the layer 2 and layer 4 wires are connected together and layer 3 makes up the other side of the switch.

The wires are simply glued to the edge of the layers using Oogoo II. But it would be tidier to embed them when the layers are being cast.
iceng2 years ago
Very neat product and ible :-)
valkgurl iceng2 years ago
Can I put in an "order" for some of this stuff to make a "feeling" prosthetic skin???? Would sure make walking easier for us gimps if we had a clue as to WHAT we were walking ON.

And imagine "fingers" that could tell you what they were FEELING.
d0vr2 years ago
Can this instructable project detect where on the 'skin' it is being touched and then provide an x.y co-ord?
rsangeethk d0vr2 years ago
if you want the xy cords..... there is lil harder job and simple technique..... just make this rubber smaller... so that you make them into pixels. this tech is mostly used in digital image processing..... better try this in your's!!!!!!!!! :)
d0vr rsangeethk2 years ago
I hope this is tongue in cheek. Why would you go to all that effort for such a poor result?
rsangeethk d0vr2 years ago
this is digital image processing... no issues... it didnt tried b'fore.! just said as idea.. idea does matters rite?
mikey77 (author)  d0vr2 years ago
Not the way it is configured now.

It a little more complicated than simple resistance everywhere. No matter where you press on the square of skin it has a resistance of 300-400 ohms. Less if you press real hard. Pressure expands one layer and compresses two, so the resistance does not vary in a linear fashion.

To get more precise detection, the conductive layers could be divided into smaller squares with their own wires and tiled side by side to give better resolution.
d0vr mikey772 years ago
Damn, I wonder how hard it would be to develop this into being able to be more aware?
joen2 years ago
In step 5, Where is the fifth layer? I only saw four.
mikey77 (author)  joen2 years ago
Thanks.

I have changed it to say four layers.
Katzsta2 years ago
If I could give it 10 stars I would. Not only do you make a useful material (Oogoo) but u make something potentially super useful with it. Great job.
Cool! You could even extend this to localize the touch:

-Measure the resistance from the 4 corners

-Using the difference between resistance as measured from the right and resistance as measured from the left, you can figure out how far right/left the touch is, by using the known resistance of the material. (Higher resistance when measured from the right means that the touch is on the left.)

-Same for up/down

This method only works for a single touch, but could still be useful.
PS1182 years ago
Excellent job!

I do have a couple of questions though (vaguely leading in the direction Hom3rSimpson was suggesting).

1) What is the approximate conductivity of the oogoo itsself in terms of ohms per inch?
2) Since the oogoo is squishy (scientific term), does its conductivity vary based on how much pressure is applied? If so, it might also be helpful to figure out the conductivity in terms of pressure per inch.
hammer98762 years ago
Cool! Congratulations on being "Featured."
deepcut712 years ago
So you basically want to make a sex doll :)

Seriously, though, this is very good, thanks for sharing.
WVvan2 years ago
Double Plus Good!
Geosync2 years ago
Conductive rubber? What's next, non-conductive wire?? :) Really nice job. Thanks for posting.
crackHacker2 years ago
could you add the wiring on the first pour? so it is in the goop while it dries? also does it need to be 3 layers deep with spacers or can you put a postive and negative lead on each side of one sheet and get kind of the same effect?
mikey77 (author)  crackHacker2 years ago
Yes.
Adding the wire or conductive thread on the first pour would be a more compact way to do it.

The skin conducts by pushing in between the insulating spacers. With only two layers and one set of spacers, there would be dead spots where you push directly on a spacer.
Orngrimm2 years ago
Plain and simple: Cool stuff! :)
Well isn't this the niftiest!
I must give it a try!!!

thanks!
belsey2 years ago
This is amazing! Such an incredibly cool idea with so many potential applications, made with simple, easy to find ingredients...
kelseymh2 years ago
Wow! What a great project, and very well presented.
boeietoch2 years ago
this is exactly what i was thinking for an use of the conductive Oogoo
just havent found the time to experiment with it
very neat job =]