IntroductionStructural polymers, being attractive from mechanical and chemical points of view, are susceptible to deterioration in the form of cracks. This leads to degradation of their mechanical properties and decreasing lifetime of such materials. Self-healing of mechanical properties of polymeric materials can be useful in a variety of applications.Here we elaborate on a self-healing approach in which special healing capsules are embedded in the polymer matrix (1-4). When a crack propagates it ruptures the capsules, healing glue leaks out into the crack, sealing, and "curing" the crack. This repairs the crack, and to some extent recovers mechanical integrity of the matrix. The major problem of this approach is the necessity of using a rather expensive catalyst, which in addition has a rather short lifetime. Another natural candidate for self-healing glue would be a two component epoxy (resin and hardener) encapsulated in separate capsules. Apart from relatively low cost and good availability of various epoxies, the advantage of using healing epoxy capsules within the epoxy matrix is the good compatibility between such healing glue and the matrix. As a result, higher adhesion strength and better repair effect might be expected. While the epoxy resin is relatively easy to encapsulate (3,4), there is a problem with doing that for the hardener. Recently, there was a proposal (3) to use a latent curing agent for the encapsulated epoxy resin, which can be dispersed in the epoxy matrix and activated later by heating. However, this is not a true selfhealing, but rather an assisted healing. Tension tests revealed a slight decrease in Young's modulus of the specimens containing the microcapsules but improved elongation at break and unchanged tensile strength. Fracture testing, in the form of single-edge notched bending tests, show a healing efficiency of 111% when the concentration of microcapsules and latent hardener are optimized. Some preliminary tests on epoxy-based fabric laminates containing this self-healing system demonstrated a 68% recovery of virgin interlaminar fracture toughness.Yuan et al. (4) reported another promising combination of healing agent and catalyst for self-healing polymer composites. The healing agent, consisting of a mixture of diglycidyl ether of bisphenol A (DGEBPA) along with a catalyst made from 1-butyl glycidyl ether (BGE), are stored in poly(urea-formaldehyde) (PUF) microcapsules that were prepared by an oilin-water emulsion process. This process of preparing the PUF microcapsules promotes long shelf-life and good chemical stability at temperatures below 238 o C. This system is still in the early developmental stages and the selfhealing efficiency of the system within a composite material is yet to be tested.Here we use a "physical" encapsulation of both epoxy and hardener inside nanoporous silica capsules. Both components are protected inside long channels that comprise the structure of the silica capsules. When they are mixed with the polymer epoxy matrix, they are expected to withst...