Biological Materials in Engineering Mechanisms - Biomimetics: Biologically Inspired Technologies - page 372

each side. If the leading edges of the body and fins were covered by the same denticles as the
majority of the body, a swimming shark would deflect the boundary layer away from the body and
increase drag. The posterior edges of fins are flexible and denticle-free. This may help to reduce
turbulence, saving energy lost to the vortices occurring immediately behind the fins (Bargar and
Thorson, 1995). Three factors affect the drag reducing properties of riblets, sharp-edged riblets,
riblet protrusion height as there is an optimal height for riblets to protrude into the boundary layer
beyond which they would interfere with the flow of seawater, and the lateral spacing of the riblets to
affect the dynamics of the water passing over the skin (Bechert et al., 1986).
Biomimetics — The structure of shark skin has prompted swimsuit and wetsuit manufacturers
to develop new designs to reduce drag in water to improve times for competitive swimmers or to
improve navigation by scuba divers. Properties of shark skin have also been used as models for
movements of submersible and surface vessels in order to reduce the drag created by the speed
of solid boat structures through water. Finally, aeronautics research has keyed into the structures of
shark skin to reduce air resistance for planes. The human's body with smooth skin covered with hair
creates a great deal of drag. Speedo, Inc. has developed a swimsuit for competitive swimmers based
on shark skin designs. The Speedo Fastskin FSII suit reduces drag in water by as much as 4%.
Passive drag affects a swimmer in the streamline position, usually after the initial dive into the
water and following a turn. During a 50-m race, a swimmer is likely to be in the streamline position
for up to 15 m. Swimmers from more than 130 countries wore this biomimetic suit at the Sydney
Olympics and over 80% of the swimming medals and 13 out of the 15 world records set were with
swimmers in this new suit. Computational fluid dynamics were used to design the swimsuit which
directs water along grooves in the fabric, allowing the water to swirl in microscopic vortices,
reducing drag. This control of fluid flow creates greater efficiency in movement and up to 3%
improvement in overall speed. A similar design could be applicable to wetsuits to reduce transit
time to great depths. Other applications include the design of highly efficient, fast, and maneuver-
able underwater craft, and options for pipes in water distribution systems. Lining a pipe with riblet-
like grooves speeds flow by up to 10% (Koeltzsch et al., 2002).
Interest in these general features has also been seen in the aerospace industry for airplane design.
In 1997, two Airbus Industry A30 planes were designed to test a specially ribbed plastic film that
cuts aerodynamic drag when attached to aircraft surfaces and is expected to decrease fuel con-
sumption by 1% (Ball, 1999). The riblets are barely perceptible to the touch, and they appear like a
matte finish on the aircraft skin. Cathay Pacific and Lufthansa have already begun flying planes with
small percentages of their surfaces covered in riblets to test durability.

14.2.4 Gecko and Burrs — Biological Solutions to Sticking to Surfaces

Background — Gecko lizards do not have little suction cups on their feet but are able to climb
up walls and stick to ceilings. The feet of these animals have toe pads consisting of tiny hair-
like structures called setae, made of keratin (Autumn et al., 2000, 2002). The setae are arranged
in lamellar patterns and each seta has 400 to 1,000 microhair structures, called spatulae. These
tiny structures allow geckos to climb vertical walls or across ceilings. Lizards can cling to
hydrophilic or hydrophobic surfaces, although adhesion strength is related to the polarity of the
substrate with the more polar the better (Autumn et al., 2002). Setae range from 30 to 130 mm,
and there are 5,000 setae per mm 2 , thus the total number of setae per gecko foot is greater than
half a million (Autumn et al., 2000). The size of the spatulae that are attached to the setae ranges
from 0.2 to 0.5 mm, distances in which molecular interactions can occur and accounting for van der
Waals interactions (Autumn et al., 2002). The average adhesive force of a seta is ~194 + 25 mN
(Autumn et al., 2000). If the average lizard foot is 100 mm 2 , the total adhesive force by a lizard is
~400 N. If a human hand were covered in setae, similar to a gecko lizard, the total adhesive
force created from just human hands would be over 30,000 N (equivalent to 6,744 pound-force or
3,059 kg-force).