Functional Electronic Screen Printing – Electroluminescent Smart Fabric Watch Marc De Vos, Dr Russel Torah, Prof Steve Beeby and Dr John Tudor Smart Textiles Salon 6th June 2013

Overview Motivation for screen printed smart fabrics. Introduce functional electronic screen printing on fabrics. Printed smart fabric watch design. Printing process for electroluminescent watch. Demonstration video. Conclusions and further work. Examples of other screen printed smart fabrics.

Motivation for a printed watch Digital watches are typically flexible straps with rigid displays. Key Advantages of a printed smart fabric watch: Increased comfort and flexibility through all fabric design. Washable – greater sporting applications. Can be easily integrated into other garments, furniture or curtains. The technology can be used to demonstrate electroluminescent applications for visual displays. Could be used alongside standard artwork for advertising displays.

Functional Electronic Screen Printing on Fabrics Screen printing requires a screen, squeegee and a printable paste. Key advantage: Any pattern can be printed, not restricted to warp and weft directions. Key advantage: Printed on top of the fabric so the fabric properties next to the skin are not affected. Key advantage: Roll to roll process so can be printed as part of the fabric manufacturing process. It can be used to deposit standard materials such as conductors, resistors and dielectrics. We have also developed more exotic printable materials such as piezoelectric, piezoresistive, thermochromic, sacrificial and electroluminescent. Typical print thickness after drying is between 5 and 50µm depending on the materials.

Printed smart fabric watch design Electroluminescence requires an electric field to be applied across a phosphor layer using. A capacitor sandwich structure is used with a semi- transparent top electrode layer to allow light emission. The layers are printed directly on to the fabric with no further processing required. Initial design is large to prove the concept. The design principle can be scaled up or down depending on application.

Printing process for EL watch on fabric 6 individually printed and cured layers for the EL watch: Interface layer: reduces surface roughness of the fabric. Bottom electrode layer: provides the ground plane. Dielectric layer: allows for a greater lamp brightness and reduced chance of short circuits. Phosphor layer: provides the light producing layer, the phosphor emits light under the influence of an electric field. Top/bus electrode layer: provides connection to the top electrode of the capacitor structure. Semi-transparent electrode layer: provides an even distribution of charge across the phosphor layer for improved light distribution. Could mention that because this was the first prototype and we wanted to see if we could get it working first, none of the layer designs are optimised, for example the interface only needs to go around the actual printed electronics parts and therefore a final design would have a much greater area of fabric still exposed.

Electrical characteristics Input voltage of 2.7-5.5v, 3v used for testing. If a standard “button cell” battery used, estimated lifetime of ~12 hours continuous use. Lifetime could be significantly improved through use of touch sensors to turn display on/off. Average resistance over a track of ~12 Ω – dependent on length and flexing. Average resistance across top semi-transparent electrode of ~10 kΩ/cm. Could implement a swipe feature to turn on. When you talk about the lifetime, mention something like “obviously the majority of the time you have a watch on your arm you’re not checking the time, so it doesn’t need to be always on, therefore this would significantly improve the lifetime of the battery”

Demonstration video Whilst the video is running you can mention things like John said about the fact you can combine this with standard printed colours to mask the wires if you don’t want to see them or combine the display into some other form of artwork. We’re not restricted to these colours it’s just what we used when we made it. The display itself can be white, green, blue or red electroluminescence depending on the material used and the frequency of the driving signals.

Photos of printed EL watch display prototype

Conclusions and further work World’s first all screen printed EL watch display on fabric. Can be printed on most fabrics. First prototype stage demonstrated, future devices will be smaller and with more multi-layered PCB approach. Integration of electronics possible into smaller packages on to fabric or as a detachable flexible circuit board. Additional functions such as a swipe across the touch pads to turn on the display. Smaller display and shorter wires would further increase the battery lifetime.

Examples of other UoS printed smart fabrics

Acknowledgments University of Southampton – Electronics and Computer Science – www.ecs.soton.ac.uk Smart Fabric Inks Ltd – www.fabinks.com