Extremely flexible and nearly transparent circuits can be made using conductive fabrics. Here are some of the experiments I've done with conductive fabrics. They can be painted or drawn on with resist and then etched like a standard circuit board. Conductive glue or conductive thread is then used to attach the components to the fabric circuit board.
To make this clear, the inkjet printer is not used to directly print resist onto the fabric. Instead, it is only used to print the circuit design onto the conductive fabric. You will then have to hand paint a clear resist over the inkjet image before the circuit can be etched.
See step 1 for details on a printer that may work to directly print the resist onto conductive fabric.
Alternately--an inkjet printer is not necessary--you can just freehand paint or draw on the resist where you want the conductive traces to be.
Pic 1 shows a simple circuit that that lights 3 LEDs. I made some of the traces circular to find out if they would conduct well at angles to the warp and weft of the fabric.
Performix (tm) liquid tape, black-Available at Wal-Mart or http://www.thetapeworks.com/liquid-tape.htm
Carbon Graphite, fine powder- Available in larger quantities at http://www.elementalscientific.net/
Available in smaller quantities at your local hardware store. It's called lubricating graphite and comes in small tubes or bottles.
Conductive thread-Available in small spools at: http://members.shaw.ca/ubik/thread/order.html
or at: http://www.sparkfun.com/commerce/categories.php?cPath=2_135
Conductive fabrics available from: http://www.lessemf.com/fabric.html
Clear Nail Polish
Ferric Chloride etchant available at: http://www.allelectronics.com/cgi-bin/item/ER-3/445/DRY_CONCENTRATED_ETCHANT_.html
or at: http://www.circuitspecialists.com/search.itml?icQuery=ferric+chloride
toluol solvent-available at hardware stores
Pic 2 shows the three conductive fabrics that were used in this instructable.
1-VeilShield-A mesh polyester plated with a blackened copper. Very light and 70% transparent.
2-FlecTron-copper plated nylon ripstop.
3-Nickel Mesh-Semi-transparent copper and nickel coated polyester.
pic 3 shows the back of the circuit and the glued components.
Step 1: Print the Circuit Pattern With an Inkjet Printer
Create a black and white image in a drawing or image program that will be the pattern for your circuit. Print it out onto the center of a piece of copy paper and adjust the image size until you get the exact printed circuit size you are looking for. The final traces should be 1'/8" to 1/4" wide. Make them wider if you plan on carrying more than 100ma of current through them.
Next is to glue a square of conductive fabric onto the center of a standard piece of copy paper (pic 4). Clear nail polish works well as it dries thin and fast (about 5 minutes). Glue all the way across the top of the fabric (the side that feeds into the printer) and then put a blob of glue on the bottom of the fabric to keep it stretched tight.
Then, print the pattern of your circuit board (pic 5) onto the conductive fabric. Sometimes it takes a couple of passes to easily see the pattern on the fabric.
An Experiment That Failed
I originally tried to directly print on the resist with an inkjet printer. I printed the pattern seven times on the front and then seven times on the back to make sure the fabric was well saturated with ink. Unfortunately the ink in my printer (a Canon Pixma MP500) was too porous or not waterproof enough to work as a resist. Perhaps there is some brand of inkjet printer that has an ink that would work.
Wax is very hydrophobic. As you can see in the next step, even wax crayons can be used as resist on conductive fabrics. So, one good possibility for printing resist directly onto the fabric, is a Xerox Phaser or Tektronix Phaser printer that uses a melted wax ink. This very good instructable https://www.instructables.com/id/DIY-Flexible-Printed-Circuits/ by ckharnett shows how he used such a printer to print wax ink resist on special copper-clad polyimide plastic sheet to create flexible circuits. These are expensive, hard to find business printers, but if you can get access to one, it may just work to directly print resist onto conductive fabrics.
Step 2: Paint or Draw on the Resist
Nail Polish Resist
For putting resist on the FlecTron (pic 6) or VeilSheild, I painted on clear nail polish. If you put it on thick enough, it will saturate the fabric and resist the etchant on both sides. To keep it from sticking, I painted it on a flat surface with wax paper underneath the fabric. After about 5 minutes it should be dry enough to flip over and touch up any dry spots on the back side.
Draw A Circuit With Crayons As The Resist
See pic 7. It turns out that you can simply draw your circuit pattern on either the FlecTron or Nickel Mesh fabric with crayons. The wax in the crayons is water resistant enough, that even though coverage is not 100 per cent, it works extremely well. The nickel fabric works best with crayons as it is stiffer and fairly transparent. You can place it like tracing paper, over a pencil drawing or printout of your circuit pattern and then draw on it. The traces should be 3/16" or wider. After you have solidly drawn on one side, flip it over and draw in the back side. It must be coated with crayon on both sides to resist the etchant well.
Step 3: Etch the Conductive Fabric
For those who have never etched a circuit board, here is how it works.
Ink, paint, tape, or some other material (called a resist) is used to cover parts of the copper clad circuit board and seal it from the etchant. The etchant (usually Ferric Chloride) reacts with any copper that is uncoated and chemically removes it. So, wherever there is resist, the copper will remain. The resist is put on in the pattern of conductive traces that you want your circuit board to end up with.
The process is the same with conductive fabrics, with the exception that we are dealing with a porous woven material that is plated with copper and/or nickel. Conductive fabrics have an extremely thin plating of metal, usually over nylon or polyester. It is so thin that they can be etched in from 5 to 60 seconds. This is with a strong Ferric Chloride solution at room temperature.
Soak the fabric in the Ferric Chloride solution for the following times:
FlecTron 30-60 seconds
Nickel Mesh-60 seconds
Remove the etched fabric and rinse VERY WELL with lots of water and then blot on paper towels and hang to dry.
pic 9 shows VeilSheild fabric that has been etched with 3 conductive traces to form an almost transparent, flexible cable. Pic 9b shows the cable with conductive glue and conductive thread.
Pic 8 shows the nickel fabric with crayon resist after etching. The Ferric Chloride etches nickel nicely. Even though there are tiny gaps in the conductive traces, they conduct extremely well. The fabric was soaked in toluol solvent to remove the crayon. Soak in a glass container for about an hour and agitate it occasionally.
Step 4: Completing the Circuit
It turns out that nail polish resist puts on a very thin insulating layer over the conductive traces. You can make a simple conductive paint that will melt through this insulating layer to create a conductive glue joint. This means you can glue components such as LEDs, Integrated Circuits, resistors, conductive thread or wire anywhere on the conductive traces.
Make Conductive Paint
It is easy to make a conductive paint that is simply conductive glue that has been thinned down. It is thinned down with a solvent in order to stick well to the fabric and melt through the nail polish resist. For more details on mixing conductive glues and paints see:
Mix the paint 1-1/2 powdered Graphite to 1 Liquid Tape to 1 Toluol by volume. Mix it fast and mix it well. Have everything you need to glue set up and ready to go as this paint dries fairly fast. Because it is so thinned out, you may have to apply two or three coats to get a thick enough connection to your components. This mix has strong solvent fumes. Do this in a VERY WELL VENTILATED ROOM or do it outdoors.
The conductive traces themselves will usually only add an ohm or less to the resistance. Each conductive glue joint to a component will add about 3 to 5 ohms.
Pic 10 shows the crayon resist circuit with one led lit. the Nickel Mesh fabric is transparent enough that the LEDs can be mounted on the back and the LED glow will come through.
Pic 11 shows the back of the crayon resist circuit.