Hierarchically nanoporous nickel-based actuators with giant reversible strain and ultrahigh work density

Metallic actuators (metallic muscles) have attracted a great deal of interest because of their potential advantages over piezoelectric ceramics and conducting polymers. However, to develop high performance actuators using earth's abundant and inexpensive metallic elements is a formidable challenge so far. Here, we report the design and fabrication of nickel-based actuators with low material cost (< 1/2000 of gold), which demonstrate an unprecedented performance including giant reversible strain (up to 2%), ultrahigh work density (11.76 MJ m(-3), the highest among the known actuator materials), and long cycle life (70% strain retention after 10 000 cycles). This outstanding performance of the nickel-based actuators originates from their unique hierarchically nanoporous structure and the oxide-covered nature of the Ni surface.