Silc Circuits: High Performance Conductive Silicone

RUBBERY CONDUCTIVE FUN! Build Waterproof, Wearable, Bouncy, Heatproof, Low-Resistance, Transparent, Indestructible circuits!

Plus it's real easy to do, and requires few exotic materials!

The goal here is to open up an amazing material, Silicone, that is becoming more and more accessible to the DIY community to the wonderful possibilities of electronics. DIY Conductive materials (conductive thread, paint, glue, fabric) have let craftspeople incorporate electronics in their project in uncountable fun ways. In the way that soft circuits let makers explore digital technology while utilizing the amazing properties of textiles, I think silicone circuits (silc circuits) have the potential to open another arena of physical-digital crafting in sculpture, wearables, prosthetics, toymaking, and special effects.

For those who want to get right to it, here's the basic recipe, and I'll go into lots more details about this in the later steps (including materials, suggestions, and project ideas). It's pretty simple, you just need the right materials, and the right process. With this method you can utilize many of the properties of high performance silicone, and create shapes and designs both thick and thin.

Basic Recipe

(Contributed to the Public Domain 2015)

1. Mix Chopped Carbon Fiber (under 6mm lengths) with a bit of rubbing alcohol (to break it up)
2. Let mostly dry
3. Whip together the Carbon Fibers with the Part A of the silicone
   
4. Add Part B, and mold it!

Example Projects Described

• Rubber Breadboards
• Anemone Touch Sensors
• Rubber Finger Mold Styluses
• Conductive Rubber Film
• Cap-Touch Quiz Games
• And MORE!

What's awesome about Silicone?

Silicone is an incredible (but sometimes frustrating material) with many amazing properties that highly compliment many problems with the way people usually do electronics with metal:

• Waterproof: Your circuits can be inherently weatherproofed. Leave them in the jungle! Bring them into the ocean!
• Durable: Can stand up to large impacts! Make toys for pets to toss and smash!
• Flexible: Can be worn on the body, stretched, played with.
• Translucent: Which lets you add colorings that respond to heat, light, or electricity to change colors or glow!
• Body-Safe: Silicone is pretty inert and non-toxic. It's like rubbery glass. There's a reason they use it for food trays and sex toys. Also no horrid fumes when you mix it! You can mix it in your weird basement!
• Mold-Able: Unlimited shapes and sizes and textures. That's why the best movie props and halloween masks are made from it
• Grippy: You can stab things into it (like wires!) and they will stick, and be held in place!
• Insulative: Stops electrical shorts (unless of course you make it conductive!)

Previous Works

This is not a new idea, but it's one that has been generally only possible in big industrial processes. It has been quite difficult to figure out a formula that works for the DIY enthusiast who also doesn't want to work with my gross, potentially toxic chemicals. Folks have been trying to figure out how to make conductive silicone yourself in the DIY community for a long time with some success. The amazing instructables user mikey77, for instance, has the standard instructable for conductive rubber.

Most DIY techniques, however rely on incorporating graphite powder into the silicone which has many drawbacks. Examples of such drawbacks to these methods include: making the rubber brittle (falls apart, not rubbery), messy (leaves black stuff on you), high impedance (500kohms++), and being limited to tin cure silicones (e.g. GE silicone 1 which is not as easy to make thick objects, not as stretchy and not food and body safe). The tin cure silicones also prevent platinum cure silicones from curing, so you cannot incorporate this into bigger silicone projects. I have spent 2 years working with the fantastic open-source sex toy company Comingle (www.comingle.io), looking through many papers and projects with some limited details about various different techniques for doing this, but when implementing them, they have all come up short in do-ability or performance.

I owe many thanks to the incredible engineer Craig Durkin, for helping me with this project!

This new type of carbon fiber silicone (which sounds cool) solves a lot of these problems!

Advantages of Carbon Fiber Conductive Silicone

I ran across the idea of using carbon fibers in an old (thankfully) expired patent https://patents.google.com/patent/US3680027A/en?q=... They add 5% carbon fibers to a mix to increase the conductivity of already conductive silicone, but it works great just on its own!

This style of making your own conductive silicone has lots of advantages compared to other techniques I have seen including:

• Rubbery AND Conductive: Others tend to get brittle and not stretchy when made conductive
• Non-Toxic: Just silicone and graphite! The MSDS's of these are quite low on toxicity! No gross metals or solvents!
• Low Resistance: 40-150 Ohms compared to 100's of kilo ohms of other techniques. This means you can use it for capacitive sensing, electrical traces, and maybe even EEG sensing!
• Quick: You can mix and have ready to use parts in half an hour! No waiting weeks for your molds to de-gass
• Clean: Most graphite mixes leave gross crud, and leaves black smears on stuff. This one is nice and clean!
• Electrically Stable: Needs more testing, but it seems that slight changes in pressure or bending do not greatly change the resistance of the material.
• Tight-Connections: Making Hard-Soft interfaces from things like metal wires to soft conductive fabrics is always tricky, but this rubber offers a way to firm up connections both physically and electrically!
• Fully Mold-able: Doesn't pour quite as easy as regular silicone, but, since you can use platinum cure (additive-cure) silicone, this conductive rubber can be molded into any shape, thick or thin! And it can be combined into any other silicone projects you are working with!

Conductive Silicone totally already exists at the industrial level (e.g www.stockwell.com) but finding a way for the average person to make and mold their very own conductive silicone has been elusive. Instructables user Mikey77 has shared the standard solution for a long time now with his conductive oogoo. But it has several drawbacks: brittle, long cure times, high resistance, leaves black smears (at least when I make it).

Core Concept

The basic idea of conductive silicone is that, while silicone itself is super insulative, you can impregnate it with conductive particles. Often this is done with small metal or carbon particles (this has a decent breakdown http://link.springer.com/article/10.1007/s00779-01... ). For instance people mix in copper or silver particles. Graphite (like in conductive ink and glue) often in the form of Carbon powder or Carbon Black is probably the most common because it is cheap, non-toxic, and doesn't corrode.

Common Problems / Difficulties

Silicone is a fickle material though. Adding different substances can lead to many different problems. Some common ones encountered in this pursuit are:

• Cure Inhibition: The stuff added prevents the silicone from curing solid and it stays gooey (gross)
• Non-Rubber Like properties: the addition makes the rubber conductive, but it sacrifices the material properties that make silicone great, like bouncy, stretchy, and flexibility. Many additions (like the standard carbon powder technique) can leave the silicone
• Smears: The little bits of powder can stay on the outside and leave you with a rubbery bit that you don't really want to touch because it keeps getting nasty black smears everywhere.
• High Resistance: often with many of these DIY techniques, one can only get the rubber a little bit conductive, with quite high resistances (>100mOhms)
• Platinum Interference: This is one of the trickiest problems! Sometimes techniques that work for tin-cure silicones (like cheap Home Depot Caulk) don't work for nicer Addition Cure silicones (like Platinum Cure). You can keep adding Carbon powder, and while it will be conductive when still wet, once the silicone cures, it completely negates the conductive properties of the added element. My guess is that somehow the silicone is surrounding all the individual particles and insulating them from each other.

The Solution

The best solution I have found so far (and please share any solutions you have come across), has been to incorporate Chopped Carbon Fibers. These are short little bits that people add to epoxy to increase the strength of the glue, but it also happens to be quite conductive! Another important factor is that unlike the graphite particles, the platinum-cure silicone seems less able to render it non-conductive. The hairs are also thin and small enough to keep all the nice properties of the rubber intact!

Some papers and links that have helped me think about ways to make DIY rubber over the years:

http://link.springer.com/article/10.1007/s00779-01...

http://www.gillesbailly.fr/publis/BAILLY_ISkin.pdf

http://cmd-www.dowcorning.com/content/etronics/etr...
"Similarly, adding nickel-graphite, silver plated aluminum, silver plated copper or conductive carbon will make silicone electrically conductive. These various forms lend themselves to a sundry of applications including gaskets and pads for analytical instruments, handheld data devices, airflow management equipment, medical diagnostics devices, high tech gadgets, and other applications in the aerospace, defense, telecom and automotive industries."

http://siliconesolutions.com/electrically-and-ther...

https://books.google.com/books?id=bUdNzaeVGJkC&pg=...

http://www.currentpleasures.com/collections/conduc...

All about playing with silicone in general: https://www.artmolds.com/pdf/Silicone.pdf

The Recipe :

(Contributed to the Public Domain 2015)

Materials

• Platinum Cure Silicone ( Smooth-On Sorta Clear 40 $30) (will also work with cheaper, tin-cure silicone)
• Chopped Carbon Fiber (1 - 6mm max e.g Tenax 2 2lbs for $30) ***
• Rubbing Alcohol

Tools

• Butcher Paper (Lay it down to not get everything sticky)
• Disposable Cups (to mix in)
• Multimeter
• Mixing Stick (This stainless steel spatula from mcmaster is my favorite for silicone)
• (optional) Conductivity Testing Mixer (Make Your Own in Later Steps)

Process

1) Mix a small spoonful of chopped carbon fiber with a splash of rubbing alcohol. (just enough to get it wet)

2) Disperse the hairs. Stir it up real good, the alcohol will break apart all the little hairs, you can see them separate.

3) Let the alcohol evaporate

4) Add some of your dispersed carbon hairs to a cup of your Part A Silicone Mixture Goo

5) Mix REALLY WELL. Test the conductivity every-now-and-then to see when the mixture gets conductive. If you are mixing well, the goo should be gray with a metallic silver sheen.

6) Add Part B and Mix well again!

7) Mold into the shapes you want and let cure!

***Remember to use basic safety precautions when handling carbon fiber. The fibers themselves are non-toxic, but can be bad for you if loose ones get in your eyes or lungs. Wear a facemask when mixing up dry carbon fibers. More carbon fiber safety info here: http://www.protechcomposites.com/pages/Working-Wit...

Stirring / Felting

This recipe has two keys to making it work: 1) breaking apart the graphite fibers with a solvent (the ethanol) and 2) Mixing it thoroughly. If either one of these steps are omitted it totally doesn't work; it won't be conductive at all. To ensure you get a good stir on, i recommend making your own robotic mixing device (a.k.a. stick shoved into a drill).

You will know that you have achieved a correct amount of stirring when your goo starts having a more fabric-like consistency, and takes on a silvery sheen. From my experience, it's once you get this sheen that the good has finally become conductive!

Curing and Ethanol

If you put carbon fibers that are still very soggy with ethanol directly into your silicone it will tend to inhibit the curing. This inhibition is more noticeable if there is a LOT of excess ethanol and the mold does not have much contact with the air (for the ethanol to evaporate). I'd recommend just making sure your carbon fibers dry up a fair amount before mixing . That being said, if you let your fibers TOTALLY dry out, they will clump together again and you will have to re-dissolve them with some more ethanol.

Alternate Solvents

There's probably a MUCH better way to do all of this! This has just been the reliable, safe-seeming method I have found from a lot of tests with lots of different chemicals. When people usually need to add a chemical to Silicone that evaporates, it is usually something like Smooth-On's Novocs ( example of shrinking silicone with it here: https://www.smooth-on.com/gallery.php?galleryid=5... or mineral spirits, or odorless mineral spirits. The alcohol has worked much quicker and more reliably for me though.

General

In general, with this recipe, I get an average of about 500 ohms of resistance for a 5cm distance across a chunk of the rubber. I have seen this performance vary as low as 40 ohms, and as high as 2-3 kohms.

Most strange types of new conductive materials are measured in how much resistance they put up over a certain distance. For instance conductive threads may include a measurement of a certain number of ohms of resistance per centimeter. In theory the performance of this conductive rubber should vary with the volume being measured across. That is, the more rubber available, the more possible connections there are between the points where the electricity is measured across. The resistance should still probably keep increasing over larger and larger distances.

This doesn't totally seem to be the case right now. Sometimes measurements across larger chunks give smaller resistances than measurements when probes are right next to each other. Look at the third image in this step, and you can see resistance readings for multiple probe distances. The lowest resistance over at 13 cm span is when they are directly close to each other (around 100 ohms). There are spikes of 1kohm in places, but generally the stuff stays around 300-400 ohms.

Now there are oodles of different factors interplaying here: ratios of silicones (parts a and b), graphite stirring amounts, how much the ethanol has evaporated, and the shapes of the rubbers, the irregularities of the molds, and potential clumps of graphite or solid silicone. So at least without out much more rigorous experimentation, it is hard to say precisely what kind of performance you can expect out of this rubber. That's why I would suggest in your designs to plan for any of your conductive rubber parts giving you a resistance of about 500-1000 ohms.

Like I mentioned before, many other DIY ways of doing conductive rubbers tend to get you about 100K-1mOhm for 1-4 cm. So keeping your resistances down in just the ohm range opens up many more possibilities!

So there is definitely a voltage drop you will have to consider when making circuits with this material. This means in general conductive rubber is better suited for building unique sensors or novel interfaces than just transmitting current (you shouldn't plan on trying to make an all-rubber microcontroller for instance*)

*(though that would be fun)

Stretching/Compression

Many non-conductive materials that are made conductive by adding conductive bits generally exhibit changes in electrical properties in tandem with physical changes. For instance, stretching a piece of some conductive fabrics can make the resistance increase. This can be quite useful in designing sensors right from the materials themselves. This rubber does seem to change resistance when bent, compressed, or stretched, but not to the relative extent of many other materials.

Durometer of Rubber

In general the Sorta-Clear 40 seems to perform better than the Dragon Skin 10 (a softer stretchier silicone). If you stretch your conductive silicone too far it will permanently distend, and look kinda screwed up. I'd recommend the harder silicone when possible.

Silicone is strange, and infusing it with carbon fibers makes it an even stranger material to work with. It's a bit fluffy and pretty thixotropic. It doesn't just pour like other silicones, but with a few extra hints you can start making it do lots of things.

Get a badass caulk gun (we use the Newborn 626 at Comingle.io) (or a regular one with some empty tubes), and load your conductive goo in there. Then squeeze these liquid noodles into whatever mold you are creating. In this way you can acheive the kind of "pouring" that you would with normal silicone.

To get the basic idea across, I created a simple electronic circuit almost entirely in rubber.

This has an LED, a photo-sensor, and a pressure sensor. They are all embedded directly in clear (insulative) silicone with traces made from conductive rubber noodles.

You can start to see how flexy, bouncy, transparent circuits aren't that common, and have some potential uses!

To make it, I laid out the circuit first, and potted it into some degassed silicone.

I wanted to get this recipe out to you all as quickly as possible, but I also wanted to share the kinds of ideas that are possible with such a new material. In the next few steps I will share some of the projects I have already tried out, and several more that I have queued up! Some of the projects have only been designed on paper, but I will update this instructable when I get to these ideas (or someone else has tried them!). They are here to just give some initial inspiration to the possibilities of this type of material. Try it out on your own and report back!

One cool thing about this developing area is that we get a chance to try out lots of the great ideas that people have made in the soft circuit world (like those rad designers at Kobakant: http://www.kobakant.at/DIY/) in the new context of conductive silicone. Many of the standard concepts in e-textiles will serve this arena well such as: How to connect power, hard-soft connections, sensors from design. If you are looking for a project idea, go to http://www.kobakant.at/DIY/ and try to recreate one of their projects with rubber!

This concept is a rubber version of Plusea's Stroke Sensor: https://www.instructables.com/id/Stroke-Sensor/

The idea is the same, conductive and non-conductive lengths mixed together so that when the hairs are brushed past, it makes a connection. The idea would be to make a version that could maybe be used in air or water to detect the passing of a mammal or fish. You could also just make funky rubber noodles as wearable bracelets or parts of costumes.

First Trial:

The first principle of this comes from making fun silicone noodles, which are pretty easy to do and yeild fun wiggle chunks of rubber! Squeeze your conductive or non-conductive goo through a small tube (make sure it is not a latex tube). Fill it up, let it cure, and pull the noodle out. Simple!

Load your noodles into something that you can use to bond them into rows. I used a pen case. Make a couple rows of these.

Wrap them in chunks with some silicone tape then into whatever shape you need! Depending on how you organize it, you can use it for sensing brushes or movement in different directions.

Make your own drawing and pointing tools! I made a mold of my own finger and and now use it to "digitally" extend my own finger. This feels silly, but it is fun!

This project illustrates how detailed and thick you can get with this type of carbon-fiber conductive silicone. You can even see my thumbprints! Maybe you can use it to spoof thumbprint readers?

This shows how open the possibilities are for making neat conductive sculptures!

A useful item to have when making silicone circuits is conductive bits extruded into long thin wires. You can leave them bare, or dunk them in some regular silicone to make insulated wire (just like regular wires). To make them, you squeeze silicone through a caulk gun, into a small tube. Let it cure in the tube, and pull it out! It's a fun conductive noodle!

To make insulated wires, dip into non-conductive silicone and let cure.

A full rubber Bread-board would be nice because electronics can be pressed into the rubber nicely, and once you have your circuit set, you can adhere the breadboard directly to a silicone scultpture. It would be nice and flexible.

My first attempt to cast the silicone directly into a breadboard did not work because of the tiny areas involved. If I want to cast such small bits directly, I probably need to source a smaller length of chopped Carbon Fiber (like 1mm). http://www.tohotenaxamerica.com/shortfibers.php

Instead, I created some insulated conductive wires, loaded them up next to each other, and potted them in silicone. Now I have a fantastic breadboard that unlike a regular breadboard:

• doesn't fill up with water and corrode when left outside
• allows for nearly unlimited numbers of components connected in different rows (since no discrete holes)
• Holds the components real tight (no falling out when shaking the breadboard)
• can be really any sort of shape
• is bouncy and flexible!
• 
  

Ideally, to retain the rubbery-ness of your silicone, you want to add the least amount of additional material to it for the desired conductivity. When mixing your conductive rubber, there can be a sharp cutoff point between when the goo goes from perfectly insulative to quite conductive. Making this simple tool, the Conductivity Testing Mixer, lets you mix your goo, and tell right when it starts becoming conductive. This is easier than using your multimeter constantly to check it.

It's based off Plusea's project Tools We Want where they make tools to help them with Soft Circuits: http://www.plusea.at/?p=5531

This tool is to help with Silc Circuits!

Like many of Hannah's fabric speakers, you just need to make a decently tight coil, and pass alternating currents through it while the coil is over a magnet, and voila! you got a speaker!

Silicone is a great material for stenciling and squee-gee-ing onto fabric! You can use this basic idea: https://www.comingle.io/howto/print-with-silicone... to make circuits directly on clothing that remains flexible and bendy!

Full Instructable Here: https://www.instructables.com/id/Quick-Quiz-5-Min-N...

The rubber bits are fun to squeeze, and work great as capacitive touch sensors! This makes it great for a multi-person quiz-show like buzzer device. If you follow this setup, you can setup the quickest version of a quiz show type game I've seen. Plus if you are using cap-touch, you don't need to solder, add resistors, or run multiple wires like in other examples: https://www.instructables.com/id/Quiz-O-Tron-3000-A...

All you need is:

• Arduino Uno
• Gator Clips
• 4 Leds
• Piezo Buzzer (optional)

and here's the arduino code!

https://github.com/quitmeyer/QuickQuiz

A key consideration in soft circuits is how you can elegantly power your wearable device. This means building the battery holster into your design. Here's an example from Kobakant of a handy stretchy battery pouch: http://www.kobakant.at/DIY/?p=4432

Rubber will be a great tool for this! Make a mold so the slot holding the battery in place will be slightly smaller than the battery, so the stretchyness will hold the device in place.

This is an advanced idea that shows how the conductive rubber can be used simultaneously for sensing and as a tactile display.

Here's a video example of me testing the shocking abilities of this rubber. It's pretty intense because these were proofs of concept and I just wanted to see if it could be done:

In Mouth (and Brain)

You could have something like a rubber coated hand, with rubber-y capacitive touch sensors (like Joshua Smith's Pre-touch sensing he does for robotics), and these sensors can be mapped to conductive rubber pads with High-Voltage, low current electro-tactile display (e.g. http://www.ncbi.nlm.nih.gov/pubmed/2026426 ). You could imagine something like the Electric Eel getting a softer, nicer makerover :)

Silicone Masks and special effects are the fanciest kinds because they feel and look the most realistic. Generally they combine parts that are standalone sculptures (like a mask) which are then connected via temporary silicone "makeup" to make seamless transitions. One could imagine designs where the masks incorporate circuitry and electronics within them, and the makeup can be used to create interactive circuits and switches (like the blink switch i drew in the illustrations).

Rubber-soled shoes with built in moisture sensors!

Light up rubber bat toys!

Thermochromic Glowing Displays! (Use conductive rubber traces to act as heating elements with silicone embedded with phosphorous and thermochromic pigments!)

It's taken 2 solid years of me working on figuring out a decent recipe for conductive silicone while making toys for www.Comingle.io in my basement. It's resulted in tons of frustration and messy messy cups of black goo. But this seems to be a decent solution! yay!