1. An apparatus comprising: a substrate; a plurality of carbon nanotubes disposed and fixed with said substrate to define a carbon nanotube network at the percolation threshold of the network, wherein the percolation threshold provides for one or more paths extending from an input region to an output region of the network, wherein the carbon nanotubes are disposed randomly on said substrate to define said network, the apparatus being configured to, upon receiving particular input signalling via the input region, provide particular predefined output signalling at the output via the one or more signal paths, the particular output signalling being predefined according to the one or more one signal paths, wherein the particular predefined output signalling comprises unique output signaling provided by virtue of the random distribution of the carbon nanotube network and wherein the apparatus is configured as a physically unclonable function provided by virtue of the random distribution of the carbon nanotube network.