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Biological Materials in Engineering Mechanisms 371

under study in a number of laboratories and uniformly thin layers over inorganic surfaces have been
generated (Tamerler et al., 2003).

14.2.3 Shark Skin — Biological Approaches to Efficient Swimming
Via Control of Fluid Dynamics

Background — Sharks are in the class Chondrichthyes, or cartilaginous fish that includes rays,
skates, and others. The dermis is composed of collagen type I fibers organized in helices around the
shark’s body in alternating layers that form 50 to 708 angles with each layer between the pectoral
and anal fins and 45 to 508 angles in the thin caudal peduncle just in front of the tail. The epidermis
is covered with placoid scales called dermal denticles, which are like thousands of teeth embedded
in the skin. Unlike the scales of fish, which in most species tend to be broad and flat, placoid scales
of sharks are pointed with a basal plate, a pedicel, and a crown enclosure. The denticles vary among
species and as sharks age, the number of scales increases. The scales are compared to teeth, given
that each is covered by dentine and composed of enamel and have a pulp cavity. Denticles vary
widely in size among species. For example, the nurse shark has denticles that are so large and so
closely spaced that they can form a barrier against even harpoons. The morphologies vary including
blunt, scalloped, spade-shaped, thorn-like, geometric, and heart-shaped. Occasionally, denticles
develop independently and become comparatively gigantic structures as in the fin spine, a thorn-
like quill, in the spiny dogfish and Port Jackson shark, or the tooth found in the sawfish and the
stinger of stingrays.
Mechanism — The roughness of shark skin is paradoxical to principles of fluid dynamics since
rough surfaces increase drag, and shark skin is considered rough due to the denticles. However, the
rough texture of shark skin reduces drag due to the presence of microscopic riblets on the surface of
the skin. Riblets channel the laminar flow over the skin to further reduce drag after the larger
structures, the denticles, create a boundary against turbulent flow. The water is channeled through
the small valleys created by the microscopic ridges, speeding up the flow of water over the surface
of skin. Without the riblets and denticles, the water would flow over smooth skin and suffer the full
effects of friction. The ridges on the denticles, like the ridge that runs longitudinally along the
shark’s body, help in drag reduction and in the smoothing boundary layer turbulence. The efficiency
of shark skin and shark swimming in water originates in principles of fluid dynamics. Body
geometries, movements, and wake evolution have been modeled (Cheng and Chahine, 2001).
The structure and dynamic behavior of the vortex wakes generated by a swimming body are
responsible for the highly efficiency propulsion and maneuverability.
Hydrostatic pressure under the skin of sharks varies with the swimming speed (Wakling,
2001). The stress in the skin varies with internal pressure and this stress controls skin stiffness.
The inertial pressure on sharks increases tenfold between slow and fast swimming. The skin
acts as an external tendon by transmitting muscular force and displacement to the tail. Hydro-
2
static pressures of 7 to 14 kN m occur just under the skin when swimming slowly and with
2
bending pressure vary between 20 and 35 kN m . During bursts of swimming, tighter
2
bends generate pressures up to 200 kN m . To bend sharply as in fast swimming the muscles
on one side shorten and increase in cross-sectional area, causing the fibers in the skin overlying
the contracting muscles to increase their angle. The changes in fiber angle cause the skin to
remain taut in and avoid wrinkling or loss of tension.
As a variation, some sharks have special arrangements of riblets that converge or diverge in a V
pattern on the skin surrounding the shark’s sensory organs. One set angles in toward the shark’s pit
organ and others angle away from the lateral-line organ. The function of the pit organ is unclear but
the lateral-line organ functions similarly to the human ear (Koeltzsch et al., 2002). It is suspected
that the diverging riblets draw water away from a shark’s ‘‘ears’’ to prevent the noisy sound of
rushing water, which would otherwise inhibit hearing. At the rostrum and on the leading edges of
the fins, the skin is almost totally devoid of riblets. This arrangement promotes smooth water flow to