Water desalination: Fresh for less.


state and activate the repair process. Any synthetic self-healing system must mimic this regulatory feedback mechanism: the site of damage needs to recruit the components that are responsible for the healing, and once they are in the vicinity of the damage, these components must release and activate the repair agents. This might seem like a tall order for inanimate synthetic components, such as organic polymers and nanoparticles, yet Balazs and co-workers have shown that properly designed capsules can demonstrate remarkable regulation of their behaviour. The basic premise is to disperse hydrophobic nanoparticles in oil and encapsulate them within a protective shell that is both deformable and amphiphilic (that is, it displays both hydrophobic and hydrophilic properties). The capsules are then dispersed in water. The Pittsburgh–Massachusetts team simulate a rigid hydrophilic surface that contains a crack, and assume that the interior of this crack is hydrophobic. Capsules that flow over the surface are attracted to the hydrophobic surface of the crack, and once they have been captured, entropy forces the nanoparticles out of the capsules and into the crack, ultimately leading to healing of the surface (Fig. 1). Performing three-dimensional computer simulations based on the lattice Boltzmann model for the fluid dynamics and a lattice spring model for the deformable shells of the capsules, Balazs and co-workers examine the conditions necessary for the effective delivery of nanoparticles into the cracks. They show that capsules can be sequestered within the crack even in the presence of constant shear flow. This surprising result depends on both the attraction of the capsule to the hydrophobic crack and the deformability of the capsule shell. For strongly attractive capsules that deform easily, the capsule nestles within the crack and conforms to its surface, making it easier for the nanoparticles to fill the crack. This coupling between deformability and function is common in biological systems, where cell adhesion and spreading regulates processes such as cell migration and tissue formation5. The situation is even better for pulsatile flow regimes, when the flow switches between high and low shear rates. Capsules can be captured within a crack during low-shear-rate conditions, and then flushed out of the crack for high values of the shear rate. Thus, fluctuations in flow conditions can switch the repair function on and off. Capsules flushed out of a crack go with the flow again, until they are captured by another crack and the repair process is repeated. This continues until there are no longer any nanoparticles in the capsule. The effectiveness of this ‘repair-and-go’ process is strongly affected by many factors including, among others, the geometry of the damage region, the interaction between the capsules and the flow, the delivery of the nanoparticles to the damage site, and the interactions between the capsules. Modelling these phenomena is particularly challenging given the lack of experiments in the field, but is also an exciting example of theory driving innovation and new ideas. Putting these ideas into practice will require experimentalists to develop capsules that are both robust and deformable, possess good adhesion characteristics, and able to release nanoparticles through their walls under appropriate conditions. What is most exciting about the repairand-go system simulated by Balazs and co-workers is that it seems to capture the essential nature of wound healing by white blood cells — a regulated delivery of healing components to specific sites of damage. Repair-and-go capsules could, for example, address the problem of capillary blockages in synthetic self-healing microvascular materials6. (These materials are repaired by healing agents that flow through a network of capillaries inside the material). Just as with the human arterial system, blockages can form within selfhealing microvascular materials when the repair process proceeds too far, too fast or without regulation. Capsules such as those described by Balazs and co-workers could be transported within the vascular network, captured at the site of damage, and then flushed downstream after the healing process is complete. These latest developments in self-healing concepts point towards future repair mechanisms that use simple principles — such as tuning the hydrophobicity of surfaces — to achieve high levels of control and regulation. Other surface interactions could also be used to target specific types of damage or specific materials needing repair in multicomponent systems. Eventually, when specific molecular units are grafted onto the capsules to improve the recognition capabilities even further, the self-healing equivalent of the highly targeted enzyme/substrate or antibody/ antigen pairings found in nature will be complete. ❐

DOI: 10.1038/nnano.2010.71

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Cite this paper

@article{Shannon2010WaterDF, title={Water desalination: Fresh for less.}, author={Mark A. Shannon}, journal={Nature nanotechnology}, year={2010}, volume={5 4}, pages={248-50} }