Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure. Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives. Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported, but the only successful crack-healing methods that have been reported so far require some form of manual intervention. Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Our fracture experiments yield as much as 75% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).

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PPARa were determined by chemical-mediated fluorescence energy transfer assays using the AlphaScreen Technology from Packard BioScience 30. The experiments were conducted with 5 nM PPARa LBD of biotinylated peptide containing individual motifs (Fig. 3a), following the manufacturer's instructions for the hexahistidine detection kit in a buffer containing 50 mM MOPS, pH 7.4, 50 mM NaF, 0.05 mM CHAPS, 0.1 mg ml-1 bovine serum albumin, and 10 mM dithiothreitol (DTT). The binding signals were detected with the increasing concentrations of GW6471, and the results from four repeated experiments were normalized as a percentage of the binding in the absence of GW6471. The effects of GW6471 on the affinity of the SMRT or N-CoR peptides with purified PPARa LBD were determined by fluorescence polarization in a buffer containing 10 mM HEPES, pH 7.4, 0.15 M NaCl, 3 mM EDTA, 0.005% polysorbate-20, 5 mM DTT and 2.5% DMSO. Varied concentration of PPARa LBD in the presence or absence of 40 mM GW6471 were incubated at room temperature with 10 nM of a fluorescein-labelled peptide of N-CoR2 or SMRT2 (Fig. 3a). The fluorescence polarization values for each concentration of receptor were determined using a BMG PolarStar Galaxy fluorescence reader with 485 nm excitation and 520 nm emission filters. The apparent dissociation constant (K d) values were determined by the binding curves derived from a nonlinear least-squares-fit of the data for a simple 1:1 interaction. Mutational analysis of the SMRT co-repressor motif interaction with the PPARa and TRb LBDs was also performed by fluorescence polarization. To determine the importance of each amino acid in the SMRT motif for binding to nuclear receptors, SMRT peptides with alanine substitution at each position were added to inhibit the binding of 1 mM TRb LBD or 2 mM PPARa to the fluorescent N-CoR2 peptide. For the PPARa experiments we added 10 mM GW6471. The inhibition curves were constructed and IC 50 values were determined by nonlinear least-squares-fit of the data to a simple 1:1 interaction.

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