The ability to heal wounds is one of the truly remarkable properties of biological systems. A grand challenge in materials science is to design "smart" synthetic systems that can mimic this behavior by not only "sensing" the presence of a "wound" or defect, but also actively re-establishing the continuity and integrity of the damaged area. Such materials would significantly extend the lifetime and utility of a vast array of manufactured items.
Nanotechnology is particularly relevant to both the utility and fabrication of self-healing materials. For example, as devices reach nanoscale dimensions, it becomes critical to establish means of promoting repair at these length scales. Operating and directing minute tools to carry out this operation is still far from trivial. An optimal solution would be to design a system that could recognize the appearance of a nanoscopic crack or fissure and then could direct agents of repair specifically to that site.
Even in the manufacture of various macroscopic components, nanoscale damage is a critical issue. For instance, nanoscopic notches and scratches can appear on the surface of materials during the manufacturing process. Because of the small size of these defects, they are difficult to detect and consequently, difficult to repair.
Such defects, however, can have a substantial effect on the mechanical properties of the system. For instance, significant stress concentrations can occur at the tip of notches in the surface; such regions of high stress can ultimately lead to the propagation of cracks through the system and the degradation of mechanical behavior.
Thus, one of the driving forces for creating self-healing materials1-9 is in fact the need to affect repair on the nanoscale. On the positive side, advances in nanotechnology could also provide routes for realizing the creation of these materials. In particular, scientists can now produce a stunning array of both soft and hard nanoscopic particles and have become highly adapt at tailoring the surface chemistry of these particles.
Below, we describe two recent computational studies on designing self-healing materials that exploit the unique properties of nanoscopic particles. As we note below, both these studies take their inspiration from biological systems.
In a recent study involving soft nanoparticles,10 we focused on nanoscopic polymer gel particles, or "nanogels"11 as the primary building blocks in our system. New methodologies have recently enabled the well-controlled synthesis of such colloids.12 Furthermore, the surface of these particles can be functionalized with various reactive groups, which allow the individual nanogel particles to be cross-linked into a macroscopic material.11 Using a coarse-grained computational model, we examined systems of such cross-linked, soft nanogel particles and designed a coating that undergoes structural rearrangement in response to mechanical stress, and thereby prevents the catastrophic failure of the material.10
We assumed that the particles are connected via a fraction of labile bonds (e.g., thiol, disulfide or hydrogen bonds3). The particles are also interconnected by stronger, less reactive bonds (e.g., C-C, bonds), which we refer to as the "permanent" bonds, and thus, the system exhibits a so-called "dual cross-linking".
Within this system, the stable, "permanent" bonds between the nanogels play an essential role by imparting structural integrity. It is the reactive, labile bonds, however, that improve the strength of the material. In particular, when the material is strained, the labile bonds break before the stronger connections; these broken bonds allow the particles to slip and slide, come into contact with new neighbors and make new connections that maintain the continuity of the film.
In this manner, the labile bonds postpone catastrophic failure and thereby, impart self-healing properties to the material. Through the computer simulations, we isolated the parameter range for optimizing this self-healing behavior. In fact, we found that just a relatively small volume fraction of labile bonds within the material can dramatically increase the ability of the network to resist catastrophic failure.10
The above behavior is conceptually analogous to the properties that contribute to the strength of the abalone shell nacre, where brittle inorganic layers are interconnected by a layer of cross-linked polymers.13 Under a tensile deformation, the weak cross links or "sacrificial bonds" are the first to break. These ruptures dissipate energy and thereby mitigate the effects of the mechanical deformation. Consequently, the breakage of these sacrificial bonds helps maintain the structural integrity of the material.
In another recent study,14 we also took our inspiration from the functionality of biological leukocytes, which localize at a wound and thereby facilitate the repair process. In our synthetic system, the "leukocyte" is a polymeric microcapsule, the healing agents are encapsulated solid nanoparticles and the "wound" is a microscopic crack on a surface. In the simulation, the nanoparticle-filled microcapsules are driven by an imposed fluid flow to move along the cracked substrate (see Fig. 1).
The simulations revealed that these capsules can deliver the encapsulated nanoparticles to specific sites on the substrate, effectively generating an alternate route to repairing surface defects. Once the healing nanoparticles were deposited on the desired sites, the fluid-driven capsules could move further along the surface and for this reason, the strategy was termed "repair-and-go". The latter strategy could be particularly advantageous since it would have negligible impact on the precision of the non-defective regions and involves minimal amounts of the repair materials.
Figure 1. Graphical output from the simulation showing the fluid-driven motion of a capsule on a damaged surface; time increases going from left to right. The images depict the capsule's movement from its initial position (top) to the interior of the crack (middle) and its re-emergence onto the undamaged portion of the surface (bottom). The gray shaded areas mark the substrate and the blue points correspond to the nanoparticles. Red arrows indicate the direction of the imposed shear flow.
It is noteworthy that micron-sized capsules filled with dissolved particles can encompass very high payloads, allowing them to rapidly carry and deliver large amounts of nanoparticles to a desired location. Furthermore, the continued, flow-driven motion of these micro-carriers potentially allows multiple damaged regions to be healed by the capsules.
In addition to healing surface cracks, the nanoparticle-filled microcapsules could provide an effective means of assessing the integrity of the surface. The fluid-driven microcapsules would continue to move along a "healthy", undamaged system, but become trapped or localized at a damaged site and thereby, deliver a visible chemical "marker", such as fluorescent nanoparticles. Such markers would enable one to non-destructively locate and track the damaged regions.
The above examples indicate how concepts from biology can be utilized to design synthetic systems that adapt to mechanical stress in beneficial ways. By incorporating such biomimetic mechanisms into the fabrication of components, one can extend the sustainability of the system. Thus, these new design concepts can ultimately prove to be economically advantageous.
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