  Diffusion metamaterials with artificial spatial structures have significant
potential in controlling energy and mass transfer. Those static structures may
lead to functionality and tunability constraints, impeding the application
scope of diffusion metamaterials. Dynamic structures, adding the temporal
dimension, have recently provided a new possibility for electric charge and
heat diffusion regulation. This perspective introduces the fundamental theories
and practical constructions of spatiotemporal diffusion metamaterials for
achieving nonreciprocal, topological, or tunable properties. Compared with
traditional static design, spatiotemporal modulation is promising to manipulate
diffusion processes dynamically, with applications of real-time thermal coding
and programming. Existing spatiotemporal diffusion explorations are primarily
at macroscopic systems, and we may envision extending these results to
microscale and other physical domains like thermal radiation and mass diffusion
shortly.
