Keywords:polymer thin film, printed electronics, room-temperature device process
Recently, the structural evolution of wearable devices to be more conformable and imperceptible on the skin surface has been spotlighted. Such devices with various electronical components integrated on organic thin films, referred to as skin-contact electronics, require alternative technologies to conventional packaging methods for mounting those components without causing damage or shrinkage of flexible substrates due to heating processes. In this study, we utilized elastomeric thin films composed of SBS (polystyrene-block-polybutadiene-block-polystyrene) with unique physical properties such as high flexibility, stretchability and adhesiveness to biological surfaces as substrates and sealing materials of electronic circuits to develop sandwich method for fixation of electronic elements. First, we prepared SBS thin films with water-soluble sacrificial PVA (poly(vinyl alcohol)) layers on a PET film by a micro-gravure coating method. The free-standing SBS thin film peeled off from the substrate showed low elastic modulus (45 MPa, thickness ~212 nm) and high stretchability. Then, by coating the SBS thin film (thickness ~383 nm) with acrylic copolymer (thickness ~115 nm) to enhance wettability and ink-absorbing property, we successfully fabricated conductive lines (width >250 μm, interval >180 μm, thickness ~720 nm) on the SBS thin film by inkjet printing of silver nanoparticles aqueous suspension ink. The resistance value of the printed silver line was decreased to be 0.78 Ω/sq after the annealing process at 110 oC. Next, we mounted electronic elements such as chip resistors and chip LEDs on the printed silver lines by covering them with another SBS thin film. Owing to the conformability and adhesiveness of the SBS thin films, the printed silver lines were electrically connected to the electronic element with van der Waals interaction. After the sandwich fixation process, contact resistance between the chip resistor and the printed silver lines was stable to be ≤220 Ω for longer than one hour. When the electronic circuit-laden SBS thin film was attached on skin surface, the electrical connection was maintained stable against motion of the wearer, which indicated that the sandwich fixation process would be applicable for skin-contact electronics. Moreover, we applied the inkjet-printing method and the sandwich fixation method to equip wireless communication function (communication frequency: 13.56 MHz) on SBS thin film. The printed antenna coil worked for power supply to the mounted IC chip. In conclusion, elastomeric SBS thin films would be useful as sealing materials for low-temperature mounting process and substrates of soft and flexible electronics.