Lightweight Cellular Materials and Lessons from Nature

New hybrid or composite materials can be considered a combination of two or more constituent parts.  The end result is a material that possesses a set of properties not offered by either individual part alone.  A basic example, concrete reinforced with rebar is a composite material possessing both strength (provided by the concrete) and toughness (provided by the steel).

The concept of a "constituent part" can be extended further to include liquids and gases.  This is the approach taken by Ashby [1], 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 [2]:

Variety of metal foams and their fabrication methods (from Wadley [2] - 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 [3] (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.

[1] M.F. Ashby.  Hybrids to fill holes in material property space.  Philosophical Magazine, Vol. 85 (2005), pp. 3235-3257.
[2] H.N.G. Wadley.  Cellular metals manufacturing.  Advanced Engineering Materials, Vol. 4 (2002), pp. 726-733.  (Available On-Line.)
[3] 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

Self-Healing Concrete and Utility of Lessons from Nature

Decomposition of Reinforced Concrete (from Dornob)

To compliment my previous post, here I wanted to take a look at a recent article found via the design site Dornob: The design of a bacteria-concrete hybrid material which has crack self-healing properties.  In short: tiny bacteria (aptly called Bacilla Filla) are genetically programmed to seek out cracks in the concrete; there, the bacteria secrete a combination of calcium carbonate (one of the most basic and widespread building materials) and a "bacterial glue" to fill in the cracks.  When these secretions harden, they have properties essentially matching those of the concrete.

While the potential benefits are far-reaching and the concept intriguing, the science behind such hybrid materials is not new.  A recent review article written by De Muynck et al [1] outlines the science and its long history.  In general, all construction materials undergo degradation with age and exposure to environment ("weathering"), which eventually leads to losses in load-bearing properties.  Adapting lessons from nature, there are a number of known and fairly common bacteria which naturally secrete carbonate mineral compounds; and these bacteria can be controlled to do so depending on the local environment (i.e., acidity, available carbon and calcium, etc.).  Adding the bacteria to the concrete, the result is a self-healing property, or "biocementation", only one of the possibilities offered by "microbially-induced" carbonate formation.

Currently, these kinds of biocementation - i.e., autonomous remediation of cracks and self-healing - are still in the laboratory experiment stages [1] (although other techniques are commercial, such as the application of "calcinogenic" bacteria for surface protection of ornamental stone [2], and the ).  And according to De Muynck et al the largest hurdle towards full-scale application of such a technology would be its cost relative to traditional construction materials and their treatments; after all, typical concrete only costs between 3-30¢ per kg.  Self-healing building materials could, however, add value by reducing the necessity for manual inspection and repair [1].

This example provides an introduction to the advantageous utility of lessons learned from nature, or, more generally, "Biomimicry".  My personal field of expertise surrounds the design, structure, and properties of cellular materials; metal foams, for example, mimic natural cellular materials such as cork and bone, and offer light-weight properties not offered by their fully-dense counterparts.  There is a wide range of biomimicry examples - far too many to cover in detail at once - all of which add an extra dimension to the framework of sustainability and the design of new materials.   

[1] W. De Muynck, N. De Belie, W. Verstraete.  Microbial carbonate precipitation in construction materials - A review.  Ecological Engineering, Vol. 36 (2010) pp. 118-136.
[2] Calcite Bioconcept.