Wearable Electronics Made With A 1000-year Old Technology
Yes, you read it right. A 1000-year old technology from Song Dynasty is being used to make smart textiles as we speak.
It is the year 989 AD, silk has gained the highest place amongst textiles in Song dynasty. Worn exclusively by the elites, silk is the new “gold” for the Chinese and is being exported to countries as far as Europe. The experts in the Song dynasty have noticed a some great properties about silk while making clothes.
Silk fibres can be woven into thin and consistent mesh patterns which is useful for intricate designs. Despite its thinness, silk fibre has high tensile strength — meaning they can be used multiple times without tearing or distorting. Silk fibres can also be consistently produced, making silk clothes flawless and ideal as a status symbol. But engineers, being engineers, figured out a way to turn this art into a technology.
The Song dynasty textile gurus used this silk to create silk screens for transferring patterns on textiles. Instead of painstakingly applying patterns on every cloth, “stencils” made of silk (it was human hair before silk) made the process simpler. A stencil was made using silk because of its wonderful properties (thinness, strength and uniformity) discussed just some time ago. Brushes were used to force ink through these silk stencils and patterns could be made in no time. This process is also known as serigraphy, literally meaning “written by silk”.
Screen printing today
The fundamentals of screen printing remain the same as 1,000 years ago. Instead of human hair or silk; plastic or stainless steel meshes are used. Plastic meshes give a good value for money, but the quality obtained by stainless steel meshes are unmatched. Earlier, the screens were handmade and making them was not so easy. But today, with the rise of photo chemicals, we can easily create patterns without much effort. So how do we screen print today?
- Create the design or pattern which we want to print. This can be done in any 2D CAD software like Illustrator or Inkscape.
- Once you’ve got the pattern, print it on a transparent plastic sheet with an opaque black ink. This is your “positive” film.
- For the following steps you need to be in a room with yellow light. This is because you will be working with photosensitive chemicals. Now you must coat your screen with a photosensitive chemical, known as “emulsion”. This can be done with a coater tool. This chemical by itself can be washed away with water. But, when UV light falls on the chemical, it hardens and cannot be removed with water anymore. Hence the word photosensitive.
- Place the positive film on the coated emulsion.
- Now use a UV lamp to “expose” your screen. The UV light falls wherever the positive is transparent. Parts of your positive which were printed with black opaque ink (your pattern) remain unexposed, as UV light is stopped by the black opaque ink.
- Take the exposed screen to your sink and wash it. The pattern, which was unexposed, washes away and remains “open”. The other areas of the screen remain “closed”, as the emulsion is hardened and is undisturbed by water.
- After the screen is dried, you can pour some ink onto screen. Use a squeegee to force the ink through the mesh. The ink passes through the open pattern area, printing the pattern exactly as you had designed.
- Repeat Step 7 to make multiple copies of the designed pattern in a fraction of time compared to manual painting or even inkjet printing.
So, what about electronics?
Just use conducting inks instead of graphic inks and you are able to make circuits. Our experience with silver, carbon and copper inks has been great. With screen printing we can prototype faster and later scale to manufacturing scale. Since screen printing only uses (or adds) material where it is needed, it is an additive manufacturing technique. This means that the process is economically viable even though we use metals like silver.
Screen printed electronics are greener, cost-effective and can promote onshoring of manufacturing for North American and European countries.
Hmm, ok, but you said “wearable electronics”?
The process still remains the same. Instead of rigid plastics, we use thermoplastic polyurethane (TPU) to make soft and stretchable electronics. The beauty of TPU is that it can be hot laminated on clothes and is widely used for logo printing on apparel.
The GIF above shows the stretchability of an electrical heater printed on TPU. This heater is embedded in our first Kickstarter, idoona. Unlike other heated jackets which come with carbon fibre heaters and long wires, idoona is made of soft TPU heaters which are just 0.3 mm thick. The result is comfortable winter clothing which encourages you to move freely.
Being our first Kickstarter, we really appreciate any feedback or suggestions regarding the campaign. You can know more about our story on LinkedIn or our website.
Disclaimer: The year 989 AD is chosen out of a period of 960–1279 AD for clarity.
