Polymers
uranium, in the form of uranyl ions, via solid-phase extraction. 4 Scanning electron microscopy shows a spontaneous change from nanowire morphology to uniform nanosheets as uranyl ions are accommodated into the lattice. The resulting yellow solid may be removed, providing an easy and efficient process for extracting, and potentially recycling, uranium. As supramolecular polymers are developed with specificity for other ions, copolymers might follow that detect, or extract, a variety of ions, either simultaneously or in step-wise fashion. Such materials might be applied as supramolecular polymer coatings to vessels, changing colour if filled with contaminated solutions, or perhaps used to safely transport harmful materials in a stable, self-healing structure. Another promising field concerns the interface between supramolecular polymers and biology. One application is the shielding of molecules within polymer structures, with release occurring only when conformational change is induced. This could be exploited to control drug delivery in the body. Proof of concept has been demonstrated with fluorescent proteins held within supramolecular vesicles, formed by self-assembly of ternary complexes that comprise equimolar proportions of cucurbit[8]uril, alkyl- substituted methyl viologen and azobenzene bearing substitutions of oligo(ethylene glycol) and Arg- Gly-Asp peptide, an integrin receptor ligand. 5 The vesicles target tumour cells that over-express integrin receptors, leading to selective delivery of fluorescent protein. Another application for supramolecular polymers is in enhancing biocompatibility of vascular grafts through incorporation of bioactive groups. Cell-free vascular grafts formed fromUPy-modified polymer scaffolds, including UPy- facilitated stromal cell derived factor 1α (SDF1α) peptides, have been shown to induce rapid population of the polymer structure by progenitor cells. 6 A future can therefore be envisaged in which damaged body parts are easily replaced, and chemotherapy or other drugs are delivered more precisely than is possible at present, potentially decreasing dose requirements and side-effects. These examples barely scratch the surface of the vast array of applications envisaged for supramolecular polymers. In recent years, supramolecular inks, carbon fibre, self-healing coatings, organic electronics and hydrogel bone templates have all been developed, with potentially significant advantages over the products they might replace. Increasing sophistication of computer modelling will enable more varied and imaginative routes of synthesis, and cross-discipline collaboration will introduce new and ingenious applications. Furthermore, materials have been developed that combine covalent polymers with supramolecular end-groups. These hybrids may be more durable than purely covalent polymers, while also being simpler to recycle, satisfying consumer demand for high quality products with reduced environmental impact. In summary, supramolecular polymers are likely to play increasingly important roles in an extraordinarily wide range of applications. The likes of self-assembling eye-drop contact lenses, surface coatings that prevent the spread of disease by capturing pathogens, and glucose-sensitive insulin delivery mechanisms, are appealing depictions of the bright future that supramolecular polymers will 4 Li, B. et al . (2017) ‘Conversion of supramolecular organic framework to uranyl -organic coordination complex: a new ‘matrix - free’ strategy for highly efficient capture of uranium’, RSC Advances 7: 8985 – 93. 5 Cavatorta, E. et al . (2017) ‘Targeting protein -loaded CB[8]-m ediated supramolecular nanocarriers to cells’, RSC Advances 7: 54341 – 6. 6 Muylaert, D. et al . (2016) ‘Early in -situ cellularization of a supramolecular vascular graft is modified by synthetic stromal cell-derived factor- 1α derived peptides’, Biomaterials 76: 187 – 95.
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