Desktop Fabrication: Wearable Electronics Circuitry

Star Simpson, an MIT student, gained notoriety when TSA agents at the Boston airport noticed her breadboard adorned sweatshirt with LED lights and visible wires. Luckily she lived through that ordeal and has created an Instructible titled, “Make a Breadboard Sweatshirt (Instant Wearable Electronics!)”. She believes more wearable electronics are in our future and I agree with her assessment.

I worked on a prototype recently that is going to be part of a wearable device. On this project I got to play with the Arduino Mini Pro. This is a great device, very compact and operated at the required 3.3V target voltage.

I needed a platform for attaching the electronic circuitry that would be flexible and heat resistant so I took a trip to the fabric store and bought some ironing board fabric. This fabric seems to be non-conductive although it has an aluminized appearance on one side, it wouldn’t register on my Ohm meter. I tried poking some wires through the fabric to see how well it would survive the heat from a soldering iron and it did great. Of course solder didn’t stick to the fabric but the wires could be soldered in place without scorching the material. I bought a needlepoint hoop to stretch out the fabric that helped keep everything in place while assembling this device. Once assembled the fabric will be cut to size and sewn into the clothing.

Arduino and wires soldered through ironing board fabric.

Figure 1: Arduino and wires soldered through ironing board fabric.

Back side showing soldering and proto board circuit.

Figure 2: Back side showing soldering and proto board circuit.

The pictures 1 & 2 show the front and back side of the circuitry mounted on the fabric. Its not evident from the photos that the wire used is solid core instead of stranded. Solid wire made it practical to punch the wire through fabric while pushing stranded wire through would not be much fun. There is a drawback to using solid core wire when soldering to the mini Arduino. Last summer I conducted a workshop with high school aged kids building robots using a different Arduino clone. Somehow one was handled rough enough to tear the copper through hole plating and solder pad off that device. Rough handling and vibration through relatively inflexible solid core wire could lead to a premature failure in a flexible environment.

Searching For a Flexible Platform.

This hacked together Arduino fabric platform will serve its purpose as a prototype but is only the tip of the iceberg in terms of time investigating flexible platform solutions. What I wanted was a flexible base with electronic traces to attach the Arduino with other components. I started looking for something similar to the Flexiforce sensor which has conductive traces sandwiched between layers of plastic. It is possible to get something fabricated commercially but I was looking for a DIY approach.

I was excited when my search landed on a web page for Conductive Inkjet Technology located in Cambridge England with their US sales office practically in my back yard. My hopes of refilling my inkjet printer cartridge with a special ink quickly faded. The process involved applying a digitally printed catalytic ink cured with UV. It is then immersed in a solution of metal ions and the metal film is grown by autocatalytic deposition. The technology used in their process doesn’t look like it will be commonplace for DIY’ers very soon.

My thoughts turned toward silkscreen printing and my laser printer. Since I began using the laser printer transfer method to make printed circuit boards, I’ve tried printing on a variety of materials. I tried printing on the ironing board fabric and it left a good image, of course it is not conductive. I wondered if I could print on screen printing fabric and get the holes filled with the toner to create a quick masking image. The picture on the left in Figure 3 shows some of the toner that bleeds through to the paper below. The toner did not clog the holes in the fabric.

Photo on left shows screen printing material raised off the paper surface. Photo on right shows screen printing material as run through the laser printer.

Figure 3: Photo on left shows screen printing material raised off the paper surface. Photo on right shows screen printing material as run through the laser printer.

Silkscreen Options

The most promising DIY approach looks like silkscreen using a Diazo photo emulsion to create the screened image. The next challenge is finding an ink compatible with the silkscreen process while remaining flexible. A trip to the art store didn’t provide any off the shelf solutions. Actually there are several challenges with this process.

  • Affordability of ink
  • Keeping the ink pliable after curing.
  • Maintaining a high level of conductivity
  • Attaching electronic components without solder

I went to the local Electroinics Supply store and bought some Nickel Print conductive paint, the store didn’t have any Silver Print. There are two drawbacks to the Nickel Print. When the paint dries it cracks when flexed. The other issue I have is this product has an Ohms rating of 5 to 6 ohms per square while the Silver Print is 0.1 ohms per square. The prices for these products are about $60 to $70 online for a two ounce bottle. Another commercially available product is CuPro-Cote Paint, this paint has a resistance of 5 ohm/sq.

I found some interesting projects that other people have created using conductive paint. The Paper Circuits artwork by Leah Buckley using conductive paint, magnetic paint, and magnets. Another article about pPCBs (Paper Printed Circuit Board) by Peter Blasser has an interesting approach that mentions using a product called Wire Glue. The Wire Glue sells for about $4 for a 0.3 oz bottle.

More searching found this web site on Instructables to Make Conductive Glue and Glue a Circuit. This approach or the product from Wire Glue could solve the problem of attaching electronic components without solder. The mechanical connectors like used on Flexiforce sensors are bulky and would have other drawbacks if used extensively throughout the circuitry for attachments. After reading the Instructible I was curious about mixing my own DIY paint.

Paint Mixing Experiment. The swatch of fabric on the left has conductive powders mixed with paint applied to test durability and conductivity.

Figure 4: Paint Mixing Experiment. The swatch of fabric on the left has conductive powders mixed with paint applied to test durability and conductivity.

I bought some water based screen printing paint for sweatshirts from the art store. I tested the graphite lubricant powder with the ohm meter and it conducted electricity. I tried mixing the graphite powder with the fabric paint and it remains flexible and conductive after curing.

I thought powdered aluminum and powdered copper would be good to try. I found it interesting that neither the aluminum or copper powders would show any conductivity with the ohm meter in dry powdered form. I mixed the copper powder with fabric paint and it did show conductivity with the ohm meter after dried. The copper was mixed in too heavy proportion to remain intact after dried. I need to continue experimenting with mixtures of powdered materials and paint while recording proportions and resistance values. This is more work than was necessary for the prototype so I haven’t gone any further yet.

Leftovers: Additional Search Results

Part of my reason for this blog post is to document my experience and a place to include some remaining search finds that are interesting but not applied to this project. The following links are not an exhaustive list but remnants of my research that I want to keep track of.

Conductive Thread

Silver Plated Nylon Thread.

Conductive thread that handles like ordinary thread but is highly conductive. The thing I find interesting is this thread has a resistance of 150 Ohms per foot. When the thread is stretched it increases to 250 Ohms per foot. I want to get a spool of this thread soon to play with and see how useful it would be in measuring the linear motion of an actuator. I’m interested in seeing how this would react when sewn into a piece of elastic and measure the resistance as an elastic band stretches.

DIY Copper Powder

I like this article: “How to make copper metal from dissolved copper compounds”. It describes using copper sulfate and steel wool to get a resulting copper powder. I bought the book “Illustrated Guide to Home Chemistry Experiments” earlier this year and copper sulfate is one of the chemicals used in the book so I had a reason to get some anyway. I’m looking forward to giving this a try.

Flexible Conductive Tracks

An article describing patents for (WO/2007/031711) METHOD OF FORMING CONDUCTIVE TRACKS FOR FLEXIBLE ELECTRONIC CIRCUITS

PRECO Inc.

A company with a local facility producing Close Tolerance Web Screen Printing. Their applications list identifies a number of interesting uses for flexible circuits.

Copper.org

Applications of Copper-Base Powder Metals.

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