Bioinspired self-healing materials: lessons from nature.

Healing is an intrinsic ability in the incredibly biodiverse populations of the plant and animal kingdoms created through evolution. Plants and animals approach healing in similar ways but with unique pathways, such as damage containment in plants or clotting in animals. After analyzing the examples of healing and defense mechanisms found in living nature, eight prevalent mechanisms were identified: reversible muscle control, clotting, cellular response, layering, protective surfaces, vascular networks or capsules, exposure, and replenishable functional coatings. Then the relationship between these mechanisms, nature's best (evolutionary) methods of mitigating and healing damage, and existing technology in self-healing materials are described. The goals of this top-level overview are to provide a framework for relating the behavior seen in living nature to bioinspired materials, act as a resource to addressing the limitations/problems with existing materials, and open up new avenues of insight and research into self-healing materials.

Hydrogel Inverse Replicas of Breath Figures Exhibit Superoleophobicity Due to Patterned Surface Roughness.

The wetting behavior of a surface depends on both its surface chemistry and the characteristics of surface morphology and topography. Adding structure to a flat hydrophobic or oleophobic surface increases the effective contact angle and thus the hydrophobicity or oleophobicity of the surface, as exemplified by the lotus leaf analogy. We describe a simple strategy to introduce micropatterned roughness on surfaces of soft materials, utilizing the template of hexagonally packed pores of breath figures as molds. The generated inverse replicas represent micron scale patterned beadlike protrusions on hydrogel surfaces. This added roughness imparts superoleophobic properties (contact angle of the order of 150° and greater) to an inherently oleophobic flat hydrogel surface, when submerged. The introduced pattern on the hydrogel surface changes morphology as it swells in water to resemble morphologies remarkably analogous to the compound eye. Analysis of the wetting behavior using the Cassie-Baxter approximation leads to estimation of the contact angle in the superoleophobic regime and in agreement with the experimental value.

Interaction of oil drops with surfaces of different interfacial energy and topography.

During a marine oil spill, the oil can interact with and potentially wet a variety of surfaces such as corals, skin/shells of marine animals, and bird feathers. We present both qualitative and quantitative data for the interaction of a dodecane droplet submerged in water with surfaces varying in both surface energy and roughness. Flat, unstructured silicon surfaces with water in air contact angles of 0°, 43°, 66°, 87°, 96°, and 108° were tested first to obtain base readings, after which photolithography was used to introduce structured surfaces representative of marine biological systems. We find that the more hydrophilic a surface, the less prone it is to oil contamination. Also, the Cassie-Baxter approximation holds up for submerged oil in water systems and can be used to predict contact angles of oil on solid rough surfaces submerged in an aqueous environment. Furthermore, the addition of surface structure, even on strongly hydrophobic (oleophilic) surfaces, greatly reduced (≈75% reduction in F(adhesion)) a surface's affinity for oil.