Many materials have a cellular structure, with either a two-dimensional array of prismatic cells, as in a honeycomb, or a three-dimensional array of polyhedral cells, as in a foam.  Engineering honeycombs and foams can now be made from nearly any material:  polymers, metals, ceramics, glasses and composites, with pore sizes ranging from nanometers to millimeters.  Their cellular structure gives rise to a unique combination of properties which are exploited in engineering design: their low weight make them attractive for structural sandwich panels, their ability to undergo large deformations at relatively low stresses make them ideal for absorbing the energy of impacts, their low thermal conductivity make them excellent insulators, and their high specific surface area make them attractive for substrates for catalysts for chemical reactions.  Cellular materials are increasingly used in biomedical applications.  Open-cell titantium foams are used to replace trabecular bone.  Porous scaffolds for regeneration of damaged or diseased tissues often resemble an open-cell foam.  Cellular materials are also widespread in nature in plant and animal tissues:  examples include wood, cork, plant parenchyma, trabecular bone and lung alveoli.

Picture of voronoi honeycomb
A two-dimensional random Voronoi honeycomb.
(From Gibson and Ashby, 1997)