Material and structural design for multifunctional deformable electronics based on nanomechanics

Abstract

According to the explosive commercialization of flexible electronics, stretchable electronics are actively developing for application and commercialization. In the case of bending deformation, the mechanically weakest material among many constituent layers can be protected from severe deformation by using structural design with the neutral plane concept. On the other hand, all constituent layers in which stretching deformation occurs should be uniformly stretched according to iso-strain conditions. This characteristic of stretching deformation limits further development in stretchable electronics. There are two kinds of research for enhancing stretchability, and these are structure- dependent study and material-based study. It is important to research and combine not only the design of highly stretchable structures but also highly stretchable materials. Among many kinds of structures for stretchable electronics, it is known that an island- interconnect structure can easily enhance the stretchability of electronic devices by using stretchable interconnection without degrading electrical performance. However, this type of structure requires high stretchability and mechanical reliability on the structure and material of electrical interconnection. So far, copper and gold have been considered common interconnect materials due to their very high electrical conductivity. However, crystalline metals have a very small elastic deformation limit (below 0.5% strain), and it is a critical shortcoming for highly stretchable and highly reliable electrical interconnection despite various stretchable structural designs. In this research, material-based research is conducted to enhance the elastic deformation limit of highly conductive metal for stretchable interconnect by using a nanolaminate structure with nanocrystalline copper and metallic glass. These two kinds of layers can interact complementarily in mechanical and electrical properties, so the thickness of each layer is controlled to optimize them. Among the deformable electronic devices, there are bio-implantable electronics that consist of biocompatible materials and withstand biological motions with deformability. It can operate in the body with biocompatible materials for all components, and it is considered to be a key technology for the future. The general bio-implantable electronics use biocompatible materials, but they need extra removal surgery after the functional period. If all components of bio-implantable electronics are substituted with biocompatible and bioresorbable materials, it does not have to require extra removal surgery and it can be naturally resolved with the body fluid after the functional period. It means that the patient becomes free from the anesthesia and secondary infection through the extra surgery. The minimally invasive surgery concept is also actively researched in the bio-medical field. It pursues the minimization of a wound during surgery or the implantation process. It is possible to incise the minimum area to minimize the recovery time of the wound and greatly reduce the probability of infection and scarring when it uses this concept. In this research, we intend to fabricate a biocompatible and shape recoverable electronic device that can highly expand into a large area when injected into the body similar to an umbrella or tent works. The biocompatible shape memory polymers are a strong candidate for the shape expansion behavior near the body temperature. In addition, the electronic device must be spread through the narrow gap between the tissue and the skull to place the electronic device on the tissue. Therefore, during the process of packaging into the tube and injection into the body, the analysis of the mechanical properties of the entire electronic device and the optimization of the deformation mode and mechanism depending on the device structure are intended to be achieved at the same time. The deformation behavior will be analyzed based on the accurately measured mechanical properties of all constituent materials (biocompatible shape memory polymers, metals, oxides, etc.) to realize this concept. The packaging & injection process will be simulated through 3D finite element simulation based on the measured mechanical properties, and a general design rule for the structural design of mechanically reliable, biocompatible, shape expandable, minimally invasive implantable electronics can be presented. As a result, it is expected that this concept can open a new paradigm of bio-implantable electronics.