Zakaria Hsain1 James Pikul1

1, Mechanical Engineering and Applied Mechanics Dept., University of Pennsylvania, Philadelphia, Pennsylvania, United States

Numerous biological materials regenerate and self-heal in response to damage which allows them to strengthen areas under high mechanical stress and maintain their load-bearing capabilities for many decades. In bone, self-healing is enabled by the transport of nutrients and minerals through a vascular system embedded within a load-bearing cellular structure. In contrast, most human-made structural materials cannot autonomously heal or tune their mechanical strength. As a result, structures are designed with safety factors so that the strength of their constitutive materials is well above any stress they are expected to endure. Hence, not only are most human-made structures heavier and more voluminous than strictly necessary, but they also require regular monitoring and maintenance to guard against fatigue cracking, particularly in critical applications (aerospace vehicles and power plants, for example).

Self-healing materials have been developed to overcome the deficiencies of structural materials, but mostly rely on a “local healing” approach where the healing agent is available throughout the material so that cracks heal using material in the crack vicinity. Although successfully demonstrated in polymers, this method has proven impractical in metals without the external provision of heat, since metal atoms possess low diffusivities near room temperature.

Here, we overcome these limitations and report, for the first time, rapid and effective self-healing of metallic cellular materials at room temperature using electrochemistry as a method, and the structure of bones and their self-healing processes as an inspiration. In contrast to previous techniques, electrochemically-driven healing relies on the transport of metal as an ion through an electrolyte, which allows faster transport, as metal ion diffusivities in a room temperature electrolyte (~10-9 m2/s) are at least three orders of magnitude greater than metal atom diffusivities in a room temperature solid. Consequently, our self-healing technique uses at least three orders of magnitude less energy to heal a crack of a given length compared to techniques which rely on heat-driven diffusion or precipitation in metallic alloys. We use tensile testing of dog-bone shaped open-cell nickel foams coated with a passivating layer, followed by electrochemical healing to understand the relationship between material structure and healing capabilities. Our nickel open-cell foams show full recovery of tensile strength and toughness using an external nickel source during healing. The use of a passivating polymer coating on the nickel foams improves the probability of achieving full recovery of tensile strength compared to uncoated nickel foams. Moreover, we show that by varying the maximum strain of the passivating coating, self-healing metals with three distinct functionalities can be achieved: materials that increase their local stiffness before failure, materials that heal microscale cracks and materials that only heal macroscale cracks above a threshold size. Finally, we design a nickel foam composite which uses an electrolyte embedded within an internal synthetic vascular system to redistribute load-bearing nickel to damaged areas and recover lost mechanical strength. Pathways for enabling a truly autonomous self-healing metallic material are discussed.