Goophene: Hypersensitive Graphene Sensors

This tutorial will show you how you can quickly make a DIY version of GPutty (Graphene Putty) which is a super pressure -sensitive polymer you can use as a robust sensor for a variety of projects and interfaces. This sensor is so multi-purpose and sensitive it could do things like monitor your breathing and pulse just by sitting on your neck. This is a knock-off version (I'll call it "Goophene," -get it?) of the "GPutty" idea made by Coleman et al at Trinity College (https://www.npr.org/sections/health-shots/2016/12/...)

This DIY type seems nearly as sensitive as the lab-made stuff these guys describe (i tried re-creating the examples they gave). I've already succeeded in hooking it up to a microcontroller and sensing:

• Heartbeats
• Breathing
• Swallowing
• Talking
• Loud sounds
• light touches with a napkin
• ultra light touches with .012" monofilament
• Typing on the table nearby
• June-bugs walking around
• ANGRY ants (but not calm ants- more on this later)
• Inertial gestures (like sticking it on your finger an wagging it around)

So check out this how-to, and join me in playing with this new cool substance and figuring out fun new types of interfaces to make with it! I'm also a professor, and if anyone wants to write a paper with me to send to something like TEI (Tangible, Embedded and Embodied Interaction), Ubicomp, or CHI (Computer Human Interaction), types of places might be interested in fun new interactions we can design with this!

It can still be a bit quirky and unpredictable, but we are getting there! Maybe you will be able to help me improve this!

Here's a quick example of me playing with it with some hairs in it.

Background

I've been working for several years trying to design a cheap modular sensor for studying ant traffic on trees in jungles. It's a difficult challenge, in that it needed to be cheap enough to have several sensors in multiple parts of a tree, that could fit around arbitrary geometry of thick bark, and irregular sized branches. Most of all, it had to be sensitive enough to detect something as light weight as a little insect.

Last year, my ears perked up when I saw all the news about this cool sensor, GPutty, some folks at Trinity College made using a "silly putty"-like substance mixed with Graphene: https://hackaday.com/2016/12/18/flexible-sensitive... In the articles describing their cool new invention, they mentioned all the cool things it could do like sensing heartbeats, and also sensing the footsteps of a spider. Since then, I've been working in my house mixing various goos and messy powders together until i got something that seems to get pretty close to what they made!

Theory

By combining Graphene (a super conductive form of Graphite, or carbon powder (soot)) with a soft silicone polymer, they made a sensor that has a range of sensing about 250 times greater than current pressure sensors. This means you could sense somewhat large events (like shaking, or a finger poke), at the same time as very small, subtle interactions (like a heartbeat through your skin).

Silly Putty is SILICONE PUTTY

The reason behind this sensitivity seems to be due to the strange physicality of silly putty. Something I did not know before hand (despite working with silicone for the past 5 years) is that SILLY Putty is not named so because it is a goofy type of putty, but rather because it is a SILIcone Putty!

(Note I'm a terrible chemist, here's my basic understanding of what's going on. I basically just mix stuff together until things work)

Silly putty is basically silicone oil mixed with Boric acid. These ingredients have a special relationship where they form these special "cross-link" bonds between long polymer strands of the silicone oil. These are chemical bonds are neat because they can detach easily and recconect again. BUT it takes takes them a little bit of time to disconnect these bonds. So when you strike the silly putty, or bounce it, or do some kind of quick forceful action, the silly putty doesn't have time to re-arrange its bonds and they lock up all the long polymer molecules and it feels stiff! If you do a slow action, these bonds easily detach, and the putty acts like a slimey goo!

This dude has this great old timey site that gives a quick overview of the history of silly putty.

http://www.waynesthisandthat.com/sillyputty.htm

Graphene fits into the Putty

The cool thing happens when you introduce Graphene into these dynamic chemical matrices of the silly putty. Graphene is basically carbon powder that has been super refined so that it's entirely platelets of only a couple atoms. When it is in this state it has special properties where it can line up these little plates and become really conductive!

When it is mixed into the silly putty the graphene apparently settles into these layers of conductive plates that can be shattered by the slightest touch to the putty. When this happens, the resistance of the Graphene-Silly putty drastically changes, but then returns back to its original state.

So if you pop this variable resistor into a circuit like many others you might use with an Arduino (toss it in a voltage divider), and you have a cool new sensor!

Ingredients

• Graphene Powder (not Graphite, not Graphene Oxide, you need actual graphene)
  • https://www.ebay.com/itm/20-grams-99-Graphene-Powd...
  • you need about 2 times as much graphene (by volume) as silly putty
  • Graphene is pricey, but a little bit goes a long way with these sensors
  • Different places have different prices that really vary, shop around!
• Silly putty (can buy it in those little eggs, or even giant eggs!
  • https://www.amazon.com/Crayola-Giant-Silly-Stockin...
• Acetone (i used nail polish remover)
• Silicone Oil
  • You can find this as a silicone spray at hardware stores (make sure it's silicone spray) https://www.wd40specialist.com/products/silicone-...
  • or there's certain stores that sell silicone oil for body parts like this one that the military in switzerland seems to use. These work alright too https://www.amazon.com/SWISS-NAVY-Premium-Silicon...

All done, I'd say a single little blob sensor doesn't cost more than 2-3$ of raw materials.

Tools

• Safety Gloves
• Safety Goggles
• Stirring Sticks
• Napkins
• Aluminum foil to lay down (it's gonna get messy)

Their Recipe (Fancy lab, equipment, chemicals, and time)

The authors of the original GPutty fully describe their entire process for making Graphene Putty. This paper describes the full process of how they actually mixed and made their GPutty): http://science.sciencemag.org/content/sci/suppl/20...

They are hardcore scientists who know what they are doing. I even asked them for tips and they were quite nice, but issued the warning (or challenge?) "To properly make g-putty and to a lesser extent graphene, you would need a full lab setup and the prerequisite lab and safety training"

In short, to make their putty, you need

1. Mix boric acid with polydimethylsiloxane on a hotplate for 1 hour and let cool (makes pristine putty)
2. Mix putty into a graphene/chloroform dispersion
3. Stir for 2 hours and sonicate at 42 kHz for another 2hrs
4. Stir this on a hot plate at 45C for 6 hours
5. Let dry for 12 hours
6. Mix for 45 mins, folding and kneading the putty

So overall it takes like a whole day, and some nice equipment and materials (but still not that bad compared to other industrial things).

Our Recipe (cups, sticks, goo, nail polish remover, putty)

In contrast our recipe is a bit quicker and easier to do at home! It will take you about an afternoon. The key is making use of commercially available silly putty, and finding out that acetone (readily available as nail polish remover) will dissolve both the putty and the graphene together pretty well before evaporating away. Here's the quick recipe

1. In one cup, Mix a glob of silly putty with acetone. Stir it up for about 10 minutes until it gets nice and gooey. Let sit for maybe 15 more minutes.Shake it around to help break it up
2. In another cup stir in some graphene powder to acetone. (use about 2 times as much Graphene (by volume) as your silly putty). Shake it up good!
3. Scoop out your silly putty goo, and add it to the acetone cup full of graphene.
4. Shake up this cup really vigorously for like 10 minutes (Advanced: you can also set it on a large vibrating wand and let it mix for like 20 minutes)
5. After it's gotten shaken up really good, it will look like a clear liquid with tiny dots suspended in it.
6. Open up your cup in a well ventilated area, and let the acetone evaporate off
7. Give a brief spritz of silicone oil
8. mix up the dried up goo + oil until it forms a nice putty-like substance.

Check your resistance , if it's mixed up good, a little blob with plenty of graphene in it should have a resistance somewhere between 50-200 ohms. (The best i've gotten so far is about 30 ohms!). Some end up being around 2K ohms and they still work pretty good!

In one cup, Mix a glob of silly putty with acetone. Stir it up for about 10 minutes until it gets nice and gooey. Let sit for maybe 15 more minutes.Shake it around to help break it up

In another cup stir in some graphene powder to acetone. Use about 2 times as much Graphene (by volume) as your silly putty. The meant about 4 of those heaping spoonfuls for this mixture.

Shake it up good! It will be a dark black slime.

Dump your now gooey silly putty into the vat of dispersed Graphene.

Stir it around and then shake it up a lot.

You might need to "burp" your container every now and then as the acetone evaporates and creatures a pressurized vessle that could explode black fluid all over and make a mess.

In the official recipe, they use centrifuges and ultrasonic mixers. If you don't have that, try using a simple high-powered back massager! You can get them online for like 10 bucks!

Slip a glove over it to keep it clean if some of your goo leaks out. You will notice your mixture immediately starts clearing up.

Notice right now the goo has a resistance of 200Kohms, this will drop significantly (to 200ohms), as the acetone goes away.

Open up your cup in a well ventilated area, and let the acetone evaporate off. It should be quite hard and dry before you are ready for the next step. Setting it in a warm area helps!

You should have a little jar of dried crud. Don't wad it up yet.

A difference between our silly putty and the lab grown pristine putty is theirs is much less viscous. Adding the graphene to our harder putty makes it even a bit stiffer (people make carbon composites this way). So we need to mix in some silicone oil

You can hook it right up to a regular old arduino (like an arduino uno) in the fashion of a common voltage divider. You will notice some responsiveness for touching the graphene blob, but most of the sensitivity will be missing.

This is because the regular arduinos can only read a resolution of about 0-1023.

Instead i use a Teensy 3.2 which has a resolution it can read of 0-65535

It also has a DAC which can give you a very specific voltage with high resolution and less noise than other voltage sources onboard.

plus the Teensy 3.2 has LOTS of options for refining your readings of very small changes: https://cache.nxp.com/docs/en/application-note/AN5...

There's probably better ways to do this using fancy wheatstone bridges and such, but here's how I hooked it up:

1. set up a voltage divider circuit with your blob and a resistor of about equal value as the blob.
2. Use your DAC (pin A14) as the voltage source (as it gives me less noise than connecting to the regulator or Vin slots on the Teensy)
3. connect the sensor side of your voltage divider to pin A10.
4. Connect pin A11 (the other side of the differential measurer) to the DAC

Then just load some of this example code i have tweaked for these purposes:

https://github.com/quitmeyer/Goophene-Code

and start playing around with it!

The coolest thing about this sensor is how many different ways we can use it! I was just excited to get something working, so I haven't yet scratched the surface of all the things we could do with it!

Here's some things I have tried and some ideas of things to try!

Biometrics

• Hold it up to your neck and watch your pulse (or tie it on with a scarf)
• Notice it measures your breathing quite well when stuck on your neck

Motion

• stick multiple globs on a glove, use to measure basic movement of different digits
• Add to other types of clothing and accessories!

Hairy sensors

• stick pieces of thin monofilament into the goo to make ultra-sensitive hairs!

Composites

You can cover your gputty with other substances. Platinum cure, and acetic acid cure silicone will both bond to the putty. This could be used to insulate the goo from other conductive surfaces, or to embed them in silicone prosthetics.

Insect detectors

• My goal is that medium sized ants would trigger it just by walking over it. I haven't gotten them to fully be able to do that. But if you frighten the ant, they grab on harder, and those footsteps actually do register!
• Larger insects (like beetles) will trigger the detector though!
• Make long strands of this goo and wrap it around a tree branch and hopefully track

I've been trying to refine this for a while, and tried out a whole bunch of other things on the way to this more successful route.

Here's some other experiments I did, and maybe they will lead to some inspiration for you. They aren't always a good idea.

1. Using methanol or rubbing alcohol to dissolve the graphene and silly putty
   1. (pretty good, not as good as the acetone)
2. Trying to make Borax Slime with kids school glue instead of silly putty
   1. (not as sensitive)
3. Mixing various silicone rubbers with graphene
   1. responsive in a sort of normal way, not like the silly putty)
4. Mixing in Gallium with your putty
   1. doesn't really mix, may release toxic gas (or i just got sick that day)
5. Mixing acetic acid cure silicone rubber with silly putty graphene (interestingly does not harden, but actually makes the silly putty a bit squishier! needs more research)
6. Mixing with solutions of borax or boric acid (i tried both)
   1. Doesn't seem to help or do anything
7. Mixing with coconut oil instead of silicone oil
   1. Surprisingly, comepletely kills the conductivity!
8. Mixing with magnetic fingerprinting powder
   1. Makes magnetic putty (which is think is stupid, what's the point? it slowly eats magnets?). Doesn't seem to make the sensitivity any better.