Biofoams are biological or biologically derived
foam
Foams are materials formed by trapping pockets of gas in a liquid or solid.
A bath sponge and the head on a glass of beer are examples of foams. In most foams, the volume of gas is large, with thin films of liquid or solid separating the r ...
s, making up lightweight and porous cellular solids. A relatively new term, its use in academia began in the 1980s in relation to the scum that formed on activated sludge plants.
Biofoams is a broad umbrella term that covers a large variety of topics including naturally occurring foams, as well as foams produced from biological materials such as
soy oil and
cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
. Biofoams have been a topic of continuous research because synthesized biofoams are being considered as alternatives to traditional petroleum-based foams.
Due to the variable nature of synthesized foams, they can have a variety of characteristics and material properties that make them suitable for
packaging,
insulation, and other applications.
Naturally occurring foams
Foams can form naturally within a variety of living organisms. For example, wood, cork, and plant matter all can have foam components or structures.
Fungi are generally composed of
mycelium, which is made up of hollow filaments of
chitin nanofibers bound to other components.
Animal parts like
cancellous bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...
,
horseshoe crab shells,
toucan beaks,
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...
, coral, feathers, and
antler
Antlers are extensions of an animal's skull found in members of the Cervidae (deer) family. Antlers are a single structure composed of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are generally found only on males ...
s all contain foam-like structures which decrease overall weight at the expense of other material properties.
[Chen PY, Lin AYM, Meyers MA, McKittrick JM. J Mech Behav Biol Mater, submitted for publication.]
Structures like bone, antlers, and shells have strong materials housing weaker but lighter materials within. Bones tend to have compact, dense external regions, which protect the internal foam-like cancelous bone.
The same principle applies to horseshoe crab shells, toucan beaks, and antlers.
The barbs and shafts of feathers similarly contain closed-cell foam.
Protective foams can be formed externally by parent organisms or by eggs interacting with the environment:
tunicate egg mix with sea water to create a liquid-based foam; tree frog eggs grow in protein foams above and on water (see Figure 1); certain freshwater fish lay eggs in surface foam from their mucus; deep sea fish produce eggs in
swimbladders of dual layered foams; and some insects keep their larvae in foam.
Biomimetic synthetic foams
Honeycomb
Honeycomb refers to bioinspired patterns that provide a lightweight design for energy absorbing structures. Honeycomb design can be found in different structural biological components such as
spongy bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and ...
and
plant vasculature. Biologically inspired honeycomb structures include
Kelvin,
Weaire and Floret honeycomb (see Figure 2); each with a slightly different structure in comparison to the natural hexagonal
honeycomb. These variations on the biological design have yielded significantly improved energy absorption results in comparison to traditional hexagonal honeycomb biofoam.
Due to these increased energy absorption performances, honeycomb inspired structures are being researched for use inside vehicle
crumple zone
Crumple zones, crush zones, or crash zones are a structural safety feature used in vehicles, mainly in automobiles, to increase the time over which a change in velocity (and consequently momentum) occurs from the impact during a collision by a ...
s. By using honeycomb structures as the inner core and surrounding the structure with a more rigid structural shell, these components can absorb impact energy during a crash and reduce the amount of energy the driver experiences.
Aerogel
Aerogel
Aerogels are a class of synthetic porous ultralight material derived from a gel, in which the liquid component for the gel has been replaced with a gas, without significant collapse of the gel structure. The result is a solid with extremely low ...
s are able to fill large volumes with minimal material yielding special properties such as low density and low
thermal conductivity. These aerogels tend to have internal structures categorized as open or closed cell structures, the same cell structure that is used to define many 3-dimensional honeycomb biofoams. Aerogels are also being engineered to mirror the internal foam structures of animal hairs (see Figure 3). These biomimetic aerogels are being actively researched for their promising elastic and insulative properties.
Material properties
Foam cell structures
A foam is considered open-celled if at least two of its facets are holes rather than walls.
In this case the entirety of the load on the foam is on the cross-beams that make up the edges of the cell.
If no more than one of the walls of the cell are holes, the foam is considered closed-celled in nature.
For most synthetic foams, a mixture of closed cell and open cell character is observed due to cells rupturing during the foaming process and then the matrix solidifying.
The mechanical properties of the foam then depend on the closed cell character of the foam as derived by Gibson and Ashby: