In recent years, the research and development of stretchable electronic devices, an evolution beyond flexible electronics, have been actively pursued. These next-generation devices can operate without losing their characteristics even when their substrate is stretched or severely bent, and maintain their performance once the external force is removed.
Flexible electronic components based on substrates such as glass, PET (polyethylene terephthalate), and PI(polyimide) are already ahead of their rigid counterparts. To create wearable devices and stretchable printed circuit boards(PCBs), considerable research has been done on various stretchable substrates(films) and electrodes. Stretchable substrates are broadly classified into two categories: urethane-based and silicon-based. Clear Flex and TPU(thermoplastic polyurethane) are representative of urethane-based stretchable films.
However, Clearflex is not suitable for contact printing methods like screen printing due to its excessively sticky surface. To apply Clearflex to screen printing, a primer layer must be coated onto the film to reduce its stickiness. Hence, TPU film is considered preferable in the urethane category.
PDMS(polydimethylsiloxane) and EcoFlex are typical silicon-based materials. However, when a film is made using silicone-based resin, the adhesion is weak when a non-silicone-based electrode binder is used. Therefore, a special surface treatment, like plasma treatment, must be applied, or the resin used in film production must be used as a binder for the conductive paste. For this reason, it's best to print the PDMS film with a silver paste that can form a stretchable electrode.
Many methods for creating stretchable substrates and electrodes have been studied, but most of these studies are for research purposes rather than product commercialization. Hence, for commercialization and mass production, there's a need for simpler, cost-effective stretchable substrates and an easy method to form electrode patterns. In this research, we aimed to resolve these challenges by optimizing screen printing conditions and silver particle size for TPU and PDMS films currently in mass production. We used a 150 μm TPU film from Matai, Japan.