Stretchable and reliable circuit board based on strain-controllable polymer and functional composite

Abstract

In this study, a stretchable circuit board capable of stably integrating electronic devices was implemented using micro/nanomaterial-based functional composites and strain-controllable polymers. Unlike previous rigid-soft hybrid electronic devices, the proposed circuit board achieves both a device mounting stability and structural stability itself by minimizing the problem of interfacial instability by utilizing and incorporating various micro/nanoscale fillers inside a stretchable polymer matrix. First, a mechanically reinforced polymer composite was prepared by a glass fiber reinforcement method and selectively utilized for partial modification of the elastic modulus and control of the stress/strain distribution of the stretchable substrate. The modulus of the polymer composite with discontinuous glass fibers was shown in a variety of ranges from 80 kPa to 80 MPa depending on the concentration of glass fibers. The stretchable substrate in which the concentration of glass fibers contained is gradually changed was implemented by controlling and modifying patterning methods and curing conditions of the reinforced polymeric composite within the stretchable substrate. The gradual change in mechanical properties prevented abrupt stress changes occurring at the interface between the reinforced and pristine domains and improved the structural stability of the substrate against deformations. In addition, in the case of embedding continuous glass fiber sheets inside the polymer, the difference in the modulus between the reinforced and the pristine domains could be realized up to 167,000 times. The embedded glass fiber sheet with a mechanical meta-structure led to an abnormal deformation behavior of the stretchable substrate, which was a negative Poisson's ratio. This controllability of the strains and stresses at selective regions in which electronic components are to be formed/integrated through the glass fiber reinforcement resulted in highly improved integration reliability. In addition, despite the huge disparity in mechanical properties within the stretchable substrate, it was possible to secure intra-structural stability since the overall substrate was made of a homogeneous polymer matrix. As the electrodes of the stretchable circuit board, liquid metal amalgam (LMA) that has high electrical conductivity and stability under deformation was utilized. The LMA that was produced through redox reactions between Ga-based liquid metal and internalized Cu particles showed good wettability and processability due to its semi-solid state. In order to prevent leakage, which is a fatal problem of liquid metal-based stretchable devices, Ag nanowire (AgNW) percolation networks that were embedded in the substrate were used for interconnections. An electrical conductivity of the LMA that was hermetically sealed inside the substrate material could be connected to the outside by the AgNW networks, which led to the integration of other electronic components. The interconnection structures composed of the LMA and AgNW were formed by a brush printing and a spray coating on the polymeric substrate, respectively. The AgNW-LMA interconnections exhibited a linear resistance of ~ 10 ohm/mm and showed excellent strain-insensitive characteristics when conjugating with the glass fiber reinforced polymeric substrate, proving electrical stability as the stretchable interconnections. Based on the modulus-controllable polymeric substrate and functional micro/nanomaterial-internalized composites, stretchable and highly reliable circuit boards were fabricated. A 3-dimensional stretchable printed circuit board (s-PCB) including electrical traces, contact pads, and via hole structures was fabricated using modulus-gradient substrate and AgNW-LMA interconnections. It was confirmed that the s-PCB was used as a general-purpose platform for stable integration of various individual electronic components by successfully integrating soft material-based sensors as well as conventional electronic devices. In addition, a stretchable sensor system capable of independently sensing pressure and the strain was implemented by a circuit board based on the negative Poisson’s ratio stretchable substrate (NPRSS) with special stress/strain distribution characteristics. It is expected that this research about the functional micro/nanomaterials and polymeric composite and development of the circuit boards having both the integration reliability and the structural stability owing to the minimized heterogeneous interfaces can realize practical uses and applications of rigid-soft hybrid stretchable electronics and in various fields such as personal health care systems, human-machine interactions, soft robotics, smart clothing, and prosthetics.