Paper Electronics: Conductive Paints, Inks, and More

     This Instructable is all about the amazing technology of paper electronics and conductive materials. Instead of using stubborn wires and your rusty soldering skills to painstakingly connect components, why not use paint and glue? This tutorial will share various recipes to create your very own conductive paint, tape, glue, and ink. Using a maximum of 3 easy to find ingredients, these simple materials are easy to make. All of the conductive materials explained in this Instructable are based around the conductive paint (step 1). Using increments of different chemicals, the consistency of the paint can be changed from thick to thin (glue to ink). At the end of the Instructable a simple project will be shown using conductive materials (step 5). There is also a step that is dedicated interfacing conductive materials with kits and teaching classes and workshops.
Even though commercially available conductive materials work great, they are a wee pricey and often need to be ordered online. Another disadvantage of commercial products is that they are usually only available in conductive paint form.
To start off, I would like to share some of the science behind the main ingredient used in the conductive materials; graphite. Graphite is a mineral and a form of pure carbon. Graphite is very conductive and is sometimes used in arc-lamp electrodes. Because of its conductivity, graphite is the primary candidate for making conductive materials. Its other bonuses include being easy to obtain, mixing well with paint, and coming in a very fine powdered form. 

     Here are the supplies needed to make the conductive materials in this Instructable:

- powdered graphite lubricant- from from Ace Hardware or other hardware store
- black poster paint- from local craft store or from Amazon
- paint thinner- from local hardware store or from Amazon   
- popsicle/mixing sticks- from local craft store or Walmart 
- mixing cups- I used styrofoam cups from grocery store
- measuring spoon- baking spoons will work 

     To complete the ink and stamping steps and the project, you will need these additional supplies:
- light emitting diode (led) available at local Radioshack
- 3-volt coin cell battery- available at grocery store
- sponge- some type of sponge, I got mine out of an old printer ink cartridge
- airtight vial- any type of airtight container will work
- airtight plastic container- possibly tupperware or old food container  
- paper

     Now that the materials have been gathered, it is time to begin making conductive materials!

Warnings: Some of the projects and instructions in this Instructable use paint thinner. This chemical produces nasty fumes that probably aren't very healthy for you. Perform all projects using paint thinner in a well-ventilated room. I am not responsible for any accidents that may occur while using this Instructable. 

Please don't forget to vote for this Instructable in the Pocket Sized Electronics Contest and all of the other contests.

This Instructable is submitted for Powell Cubs for the Instructables Sponsorship Program.

     The first part of this tutorial features conductive paint. The paint will be the base of all of the other materials that are explained in this Instructable. The paint makes an ideal base because of its consistency. To make glue you add slightly more graphite powder, and to make ink a few drops of paint thinner is added... but onto that in later steps.
Conductive paint sticks well to most materials, especially paper and cardboard. Anything that the poster paint will bond to, the conductive paint will as well. I have found that this mixture of conductive paint flexes well on paper. However, sharp creases and folds will lead to a crack, usually causing a shaky connection. The conductive paint will turn out to have the same flexibility as the paint that is mixed in. 
As a general rule of thumb, use this paint in low-medium areas of stress to ensure a reliable connection. For higher stress applications resort to one of the conductive inks mentioned later in the Instructable.
The two materials used in conductive paint are powdered graphite and the poster paint. After much experimentation, I found that a mixture of 2 parts powdered graphite to 1 part black paint worked exceptionally well. 
Instructions: Using the measuring spoon, measure out two spoonfuls of powdered graphite and pour it into a mixing cup. Add 1 spoonful of the poster paint. Mix well, making sure all of the graphite is added. 

Application: Slather heavily onto the material (i.e- paper, cardboard) in the desired pattern using a paintbrush. Make sure that all lines are coated evenly. Allow a few hours to dry before using. Store remaining paint in an airtight container.
When dry, the conductive paint has a very low amount of resistance. After several tests, I calculated the resistance to be around  
115 ohms per centimeter. Even though 1 centimeter of standard 22 gauge hookup wire has a resistance of about 2 ohms, the 113 ohms difference will barely make a difference in the circuit. If the circuit schematic calls for a small resistor  (<115 ohms), it would be alright to omit it. I found that when you connect a new 9-volt battery to a painted line (of conductive paint) and attach a digital multimeter on the other side, the voltage detected is 9.27-9.28 volts out of 9.29-9.30 volts. In sum, there is a very minimal amount of voltage lost when using conductive paint. 

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Paper Electronics: Make Interactive, Musical Artwork with Conductive Ink

Before you go crazy with your conductive ink, there are a few constraints to your artwork.

1. There must be four separate sections of conductive ink. One acts as a distance sensor and will eventually control the pitch of the audio. This part of the artwork should ideally be as large as possible as the larger it is, the more sensitivity the sensor will have. The other three sections  will act as buttons that will allow us to control the frequency of the audio and don't need to be as large. It is important that none of these sections touch each other.

2. The four sections should have traces (a painted/printed line no thinner than 1mm) taking them to the edge of the paper terminating in a 5mm x 5mm square of ink. These squares of ink should be side by side with a 5mm gap in-between. This is clearly shown in the image of my print on the bottom right hand corner. This print was A5 in size.

When it comes to creating your artwork it doesn't really matter how you do it, but the two easiest ways are to either paint by hand or to screen print. Screen printing means that not only do you get a high quality print, but you can print as many as you like easily.

Painting is easy, just remember to follow the constraints above.

Screen printing with Bare Conductive is a little trickier as it tends to dry quickly in the screen. To get around this I found it best to dilute Bare Conductive with roughly 1 part water 10 parts Bare Conductive. This makes the whole process a hell of a lot less stressful. When it comes to selecting a good screen for the ink, I recommend using a textiles screen with a mesh of around 90t.

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Estimate pi using conductive paint

Given an "infinite" 2D square grid of resistors with resistance R, the equivalent resistance across the diagonal of one square is 2R/pi [1,2]. Working out this solution theoretically has sniped many a nerd. Furthermore, it is prohibitively laborious for an amateur to attempt to verify this experimentally -- until now. Using Bare Paint, I drew a square grid of resistors on a page of normal copy paper, and measured resistances with a multimeter. My wishful thinking was that, with a large enough array of resistors (14 x 14), I could approximate "infinity" sufficiently from the perspective of squares in the middle of the page. Drawing the resistors with the Bare Paint seemed easier than gathering and connecting 196 resistors.

While one set of measurements yielded a value of pi as impressive (to me) as 3.38, overall I observed poor consistency of measurements among adjacent squares near the center of the page, and poor repeatability of measurements for the squares tested. Nevertheless, I hope that you, dear reader, will consider trying this experiment for yourself. I will tell you how I did it, and perhaps you will take more care than I to apply the paint accurately and precisely; otherwise, you will too observe a high variance of resistance among your resistors. Note also that the resistance of the Bare Paint decreases as it dries, so be sure to allow ample time for drying (tens of minutes) before measurements.

I used one tube of Bare Paint, a multimeter with leads (pictured in the intro section), and an 8 1/2" x 11" sheet of paper.

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