With the rapid development of bio-integrated electronics, soft materials with bio-mimetic mechanical properties have been a topic of increasingly growing attention. Many biological tissues possess the type of J-shaped stress–strain responses, which can be attributed to the wavy microstructures that experience a bending-to-stretching transition of deformation mode under an external tension.
For the two-dimensional (2D) soft materials, my group reported a class of soft network materials that incorporate horseshoe microstructures into 2D periodic lattice constructions. This type of bio-mimetic soft 2D network materials can be tailored precisely to match the J-shaped stress-strain curves of human skins at diverse locations. Due to the limited expansibility of horseshoe microstructures and the difficulties in manufacturing lattices of complex 3D microstructures, the development of soft 3D architected materials that can mimic the nonlinear, anisotropic mechanical responses of 3D biological tissues remains very challenging. And we have been thinking of possible solutions to overcome this challenge since five years ago.
It is well known that many collagenous tissues consist of helix-shaped 3D microstructures. Inspired by these microstructure configurations, we try to create a 3D lattice material with different spatial topologies, where 3D helical microstructures connecting the lattice nodes serve as building blocks. To achieve this purpose, we need to exploit a spatially non-uniform helical geometry to ensure the structural periodicity and avoid the self-overlap. Employing the high-precision polyjet 3D printing techniques, we fabricated successfully soft 3D network materials with characteristic dimensions down to a few hundreds of micrometers, while offering stable mechanical performances.
Based on the combined approaches of experiments and numerical simulations, we also spent lots of efforts in understanding and summarizing the microstructure-property relationship of the proposed 3D network materials. The utility of validated simulation tools allows for a rational design of the helical microstructure, such that the desired stress–strain curves of biological tissues can be reproduced.
The developed soft 3D network materials hold potential for many applications in bio-integrated devices, if conductive and/or semi-conductive layers can be integrated. We explored a couple of fabrication options, such as electron beam evaporation, magnetron sputtering, solution immersion method, etc. In this work, we finally exploited the magnetron sputtering techniques that can maintain relative uniform coating across the different regions of the sample. The resulting conductive soft 3D network materials have a metallic thin layer (10nm Cr, 500nm Cu, and 100nm Au, from inside to outside) at all of the helical microstructures in the 3D network. Finally, we demonstrated the potential uses of the conducting soft 3D network materials as flexible pressure/strain sensors.
We believe that these findings provide systematic design guidelines for the 3D network materials and functional systems, with many application opportunities in soft robotics, bio-integrated electronics, tissue engineering, etc.
Dongjia Yan, Tsinghua University, China
Yihui Zhang, Tsinghua University, China
Yan, D., Chang, J., Zhang, H. et al. Soft three-dimensional network materials with rational bio-mimetic designs. Nat Commun 11, 1180 (2020).