The concept of a "constituent part" can be extended further to include liquids and gases. This is the approach taken by Ashby , who has considered cellular materials as hybrids of 1) solid material and 2) air. Like composite materials consisting of two interwoven solid materials, the arrangement of the "air" component in cellular materials can also be designed through the wide range of manufacturing techniques :
|Variety of metal foams and their fabrication methods (from Wadley  - click to enlarge).|
As the architecture of these materials is designed, so are the resultant characteristics (e.g. density, strength, energy absorption, resonant frequency, and the list goes on). There is a great deal of very interesting on-going work which focuses on processing, properties, and applications of these cellular materials - most of which focuses on the light-weight structural benefits. For more information, see bi-annual conferences Cellular Materials (CellMat) and Porous Metals / Metalic Foams (MetFoam).
Natural Structures and Sustainable Design
Some of you may have already noticed now closely the above cellular materials mimic those of natural structures, of which a well-known example is spongy (trabecular) bone:
|The internal cellular structure of spongy (trabecular) bone. (Source: National Cancer Institute.)|
The employment of this cellular structure illustrates mechanical benefits such as light weight and high load-bearing structural efficiency. However, as recently studied by Doube et al  (also from ScienceDaily), this cellular design is not restricted to select organisms; in fact, the adoption of a cellular bone structure is wide-spread and is allometric (i.e. scalable with organism size) across a large number of mammals and birds from elephant to shrew.
Such a prevalent example of natural materials design is therefore not as much a niche evolutionary development as it is a generally-adopted (and powerful) design tool. And because tools such as these came about through evolutionary means, they therefore also offer insight into new dimensions for human-powered sustainable materials design and development.
 M.F. Ashby. Hybrids to fill holes in material property space. Philosophical Magazine, Vol. 85 (2005), pp. 3235-3257.
 H.N.G. Wadley. Cellular metals manufacturing. Advanced Engineering Materials, Vol. 4 (2002), pp. 726-733. (Available On-Line.)
 M. Doube, et al. Trabecular bone scales allometrically in mammals and birds. Proceedings of the Royal Society B, Vol. 278 (2011), DOI: 10.1098/rspb.2011.0069