(Figure Presented) Even better than the real thing? As host molecules, long synthetic nanotubes may be reasonable alternatives to single-walled nanotubes. For example, calixarene-based nanotubes effectively pack into infinite tubular bundles in the solid state (see picture), and they can be easily filled with guest molecules to form stable, but reversible, encapsulation complexes. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA.