Adaptability is one of the hallmarks of living systems that provide resilience to survive and flourish in a dynamically changing environment. I will present our past, current, and future efforts to study coupled material systems toward mechanically adaptive materials for extreme environments. I will start with the overview of my research, then focus on two recent efforts, followed by future directions.
First, I will present self-adaptive materials that can change their mechanical properties depending on loading conditions by the coupling between stress and material synthesis . Nature produces outstanding materials for structural applications, such as bone and wood, that can adapt to their surrounding environment. For instance, bone regulates mineral quantity proportional to the amount of stress. It becomes stronger in locations subjected to higher mechanical loads. This capability leads to the formation of mechanically efficient structures for optimal biomechanical and energy-efficient performance. However, it has been challenging for synthetic materials to change and adapt their structures and properties to address the changing loading condition. To address the challenge, we are inspired by the findings that bones are formed by mineralizing ions from blood onto collagen matrices. I will present a material system that triggers proportional mineral deposition from electrolytes on piezoelectric matrices upon mechanical loadings so that it can self-adapt to mechanical loadings. For example, the mineralization rate could be modulated by controlling the loading condition, and a 30-180% increase in the modulus of the material was observed upon cyclic loadings whose range and rate of the property change could be modulated by varying the loading condition. Moreover, the material system showed improved fatigue resistance from its damage mitigation mechanism with one order of magnitude slower crack propagation speed. Our findings can contribute to new strategies for making resilient and sustainable materials for dynamically changing mechanical environments, with potential applications including healthcare, infrastructure, and vehicle.
Second, I will present adaptive energy-absorbing materials with extreme energy dissipation and improving energy absorption with increasing strain rate by the coupling between viscoelastic properties of materials and nonlinear geometrical effects . An architected material (or metamaterial) is a class of materials that provide new properties not observed in natural materials or from a bulk material that its constituent is made of. We utilize energy dissipation mechanisms across different length scales by using architected liquid crystalline elastomers. As a result, our energy-absorbing materials show about an order of magnitude higher energy absorption density at a quasi-static condition compared with the previous studies and even higher energy dissipation at faster strain rates with power-law relation, whose exponent can be tuned by controlling the mesoscale alignment of molecules using a simple strain control-based approach. Thus, the material exhibits up to a 5 MJ/m3 energy absorption density at a strain rate of 600 s-1, which is comparable to the dissipation from irreversible plastic deformation exhibited by denser metals. Our findings have the potential to realize extremely lightweight and high energy-dissipating materials, which will be beneficial for a wide range of applications, including automotive, aerospace, and personal protection.
We envision that our research can contribute to intelligent, resilient and sustainable material systems for harsh environments, with applications including defense, environment, and healthcare.
 S. Orrego, Z. Chen, U. Krekora, D. Hou, S.-Y Jeon, M. Pittman, C. Montoya, Y. Chen, S. H. Kang*, “Bioinspired materials with self-adaptable mechanical properties,” Advanced Materials, 32, 1906970 (2020).
 S.-Y. Jeon, B. Shen, N. A. Traugutt, Z. Zhu, L. Fang, C. M. Yakacki, T. D. Nguyen, S. H. Kang*, “Synergistic energy absorption mechanisms of a bistable architected liquid crystal elastomers,” Advanced Materials, 2200272 (2022).