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Introduction to Biomimetics 23

U.S. Air Force studies showed that these materials are sensitive to impact; a loss of about 80% in the
compression static strength was measured when an impact causes an easy-to-see damage to the
surface, whereas a loss of 65% when the damage is barely visible (Bar-Cohen, 2000). In order to
develop an impact damage indicator, paint was mixed with an encapsulated dye and developer, and
was applied to the surface of composite panels. The micro-capsules that were used had a diameter
of 1 to 10 mm and in this size they were easy to apply using conventional methods like spraying to
increase the practicality of this paint. Tests have shown the feasibility of this concept and the paint
was effective in indicating the location and intensity of the impact, where the larger the impacted
area, the larger the indication that appeared.

1.6.6 Mimicking Sea Creatures with Controlled Stiffness Capability

Certain sea creatures, such as the sea cucumbers, are capable of controlling the tensile properties of
their connective tissues by regulating the stress transfer between collagen fibrils (Trotter et al.,
2000; http://www.biochemsoctrans.org/bst/028/0357/0280357.pdf). Trotter et al. (2000) sought
to design a synthetic analog with similarly reversible properties, and have been able to demonstrate
a pair of synthetic molecules that selectively and reversibly associate with one another under
controlled physiological conditions.

1.6.7 Biology as a Source for Unique Properties and Intelligent Characteristics

Materials that are made by animals offer capabilities and properties that are often far superior to any
human-made imitations. These material properties include hardness, fracture resistance, and light-
weight — as can be found in pearls and shells of various marine species, including the abalone.
There are also many body parts (e.g., teeth and eye cornea) that are organized as layered assemblies,
which are now emulated by methods such as self-assembly and ink-jet printing. Smart materials
are increasingly evolving in various forms, with self-sensing and reaction capabilities that
cause them to stretch and contract in response to heat, light, and chemical changes. Another aspect
of biomaterials is self-healing, which is increasingly being adapted to polymer and composite
materials.

1.6.8 Multifunctional Materials

Nature has made great efforts to use its resources effectively, and besides the use of power in
efficient ways including its recycling, nature also assigned multifunctions to its materials and
structures. For example, our skin encases blood and other parts of our body, supports the regulation
of body temperature, has self-healing capability as well as many other functions. Also, our bones
provide the required body stiffness to support it allowing us not only to stand, walk, and conduct
various critical mobility functions, but it also produces our blood in the bone marrow. The use of
materials that perform multiple tasks allowed nature to make its creatures with a lower body weight.
The concepts of multifunctional materials and structures are being studied by many researchers and
engineers (see more details in Chapter 12) and has been the subject of a DAPRA program at the end
of the 1990s. Increasingly efforts are made to emulate this characteristic, where multiple disciplines
are used, for example, applied mechanics (elasticity or plasticity, fracture mechanics, aerodynam-
ics), materials sciences (metallurgy, composites, polymers), electronics (sensors, actuators, con-
trols), photonics (fiber optics), and manufacturing (micro- or macro-structure processing).

1.6.9 Biomimetic Processes

There are many biomimetic processes that were learned from studying the activity of the body
of living creatures. The imitation of biological processes ranges from operations at the level of