Engineering Surface Patterns with Shape Memory Polymers: Multiple Design Dimensions for Diverse and Hierarchical Structures.

Deterministic design of surface patterns has seen a surge of interests due to their wide applications in flexible and stretchable electronics, microfluidics, and optical devices. Recently, instability of bi-layer systems has been extensively utilized by which micro/nano-patterns of a film can be easily achieved through macroscopically deforming the underlying substrate. For bi-layer system with traditional thermostable substrates, the pattern morphology is only determined by initial strain mismatch of the two layers, and the realization of localized patterns appears to be particularly challenging because of the difficulties associated with manipulating inhomogeneous deformations. In this work, we exploit cross-linked polyethylene (cPE), a shape memory polymer (SMP), as the flexible substrate for building micro/nanostructures of sputtered gold film. We find that the shape memory effect can offer new dimensions for designing diverse and hierarchical surface structures by harnessing film thickness and heating time, and by globally and locally controlling the thermal field. By combining those strategies, we further demonstrate versatile hierarchical, superimposed, and local surface patterns based on this cPE-gold (Au) system. Piezoresistive pressure sensors are assembled with the obtained patterned surface, which have high sensitivity, operational range and cyclic stability. These results highlight the unique advantages of SMPs for building arbitrary surface patterns.