Berlin, June 2009
Dior, Versace, Gucci - all curry favor in our fashion-conscious society with flamboyant materials and offbeat designs. But none of their gowns sparkle like Fraunhofer IZM's latest creation. The dress features integrated diodes, an accelerometer and a microcontroller, which detect the wearer's movements and convert the dynamics into an LED-display pattern. The integrated LED display is made possible by the stretchable substrate, one of the most exciting developments in todays circuit board industry.
The manufacturing process for these stretchable circuit boards (SCB) was developed by scientists at Fraunhofer IZM and TU Berlin as part of the European research project STELLA (STretchable ELectronics for Large Area Applications). A flexible thermoplastic polyurethane (TPU) foil proved to be the most suitable substrate material. Thanks to its excellent properties, including wear and tear resistance, TPU has already been tested and is widely used in the textile industry, for example, as a breathable membrane in rainwear.
To render the highly conductive but basically rigid copper wires flexible, the wires are structured into the substrate in tiny meander patterns. Depending on the design of the meander shapes and the intended applications, the researchers are able to achieve elasticities of up to 300 percent.
Developing a manufacturing process that uses conventional circuit board industry techniques was a priority. An 18-35 m thick copper film is laminated onto the TPU, which can then be structured using conventional lithography and etching techniques. Currently the Fraunhofer researchers are developing a double-sided wiring substrate with through-vias and traces as thin as 100 m embedded in the polymer foil. At the same time, they are also working on a protective cover for the traces and encapsulation of the electronic components to safeguard the system against mechanical and climatic environmental influences. A particular challenge is making the system washable.
The biggest problem in assembling such circuit boards is connecting the flexible substrate to the rigid individual components (diodes, driver ICs, etc.). To suppress the elasticity at these points, stops are structured into the copper around the components. Conventional materials and processes such as low-temperature soldering, conductive adhesion, dispensing and injection molding are used for the assembly and subsequent encapsulation.
These processes are both cost-effective and long-established in the circuit board industry. The stretchable circuit board is then easily integrated into textiles using standard heat pressing at approximately 190 C. The advantage is that the stretchable TPU foil functions as circuit board, while at the same time adhering the electronics to the material. The transfer process does not damage the textiles properties, such as flexibility, tear resistance and bending stability, and the electronics literally disappear into the material.
An interdisciplinary cooperation has also been running between design students from the HTW and the UdK Berlin and Fraunhofer IZM scientists, using the IZM and TU Berlin innovations to develop new smart clothing designs. The group received the Avantex Innovation Award 2009 for one of their creations, an interactive dress.
However, possible applications for the stretchable circuit board are not limited to fashionable technologies, but also make new medical products possible. Especially for applications in which sensors have to be worn close to the body to measure and monitor vital signs, stretchable circuit boards are reliable and comfortable thanks to their ability to adapt to the bodys shape and movement.
Thus, new products being developed in the project STELLA together with the project partners include a baby jersey that monitors breathing to prevent sudden infant death syndrome (SIDS). Other developments include an intelligent bandage that can detect secretion and test that the bandage is not too tight, as well as a band-aid that can accelerate wound healing by means of electrostimulation.
