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Biological Materials in Engineering Mechanisms 373

Mechanism — Geckos use van der Waal forces to adhere to different surfaces and do not use
secreted sticky materials for this function (Autumn et al., 2002). With such strong adhesive forces
the movement of a lizard could be problematic. However, to ‘‘unstick’’ their feet, lizards curl their
toes. This movement breaks the van der Waal forces and allows movement across substrates. The
critical angle needed to break the forces is 30.68+ 1.8 (Autumn et al., 2000).
Biomimetics — New tapes, similar in design to lizard feet, have been designed and developed as
a dry adhesive to eliminate the sticky residue normally left from more traditional tapes and glues.
The adhesive sticks to most surfaces. Various microfibers have been synthesized from polyester,
silicone, and polyimide materials. Their adhesive forces are 294 + 21 nN per spatula, 181 + 9nN
per spatula, and 70 nN per hair, respectively (Geim et al., 2003). These microfibers adhere in a
similar fashion to natural keratinous setae, but after repeated use, the microfibers lose their
stickiness due to bunching of the fibers (Geim et al., 2003). Further research studying the traits of
chitin microfibers, similar to those found in insects, should help resolve this problem. This type of
Gecko-like adhesive could eventually be used for a wide range of applications, from hanging
tapestries and rock climbing to wound closure in surgery.

14.2.5 Muscles — Efficient Biological Conversion of Chemical Energy into
Mechanical Energy

Background — Muscles are classified into (a) skeletal — often associated with bone, (b) cardiac —
found in the heart, and (c) smooth — lining of hollow organs in the gastrointestinal, urogenital, and
cardiovascular systems. Skeletal muscle functions in generating force and movement but also in the
support and maintenance of body structure. Cardiac muscle pumps and regulates flow of blood
throughout the body. Smooth muscle contractions function in general housekeeping activities, such
as the movement of food through the gastrointestinal system. The scaling and interactions of
proteins to form contractile muscle systems provide a classic example of the structural hierarchy
in biological materials to achieve remarkable function. Skeletal muscle contains myofibrils which
consist of a series of sarcomeres, the functional units of muscle contraction, composed of contract-
ile proteins, actin, and myosin. The thick myosin and thin actin filaments are arranged in a repeating
interlocking mesh structure, generating a striated appearance. In addition to contractile proteins,
there are also energy stores and signaling mechanisms associated with the myofibrils.
Mechanism — The mechanism of muscle contraction is based on the crossbridge cycle (sliding
filament model) and Lymn Taylor actomyosin ATPase hydrolysis kinetic scheme. Myosin consists
of a globular head with ATP and actin binding sites and a long tail that is involved in polymeriza-
tion into myosin filaments. Actin is polymerized into filaments and forms the ‘‘ladder’’ along which
the myosin filament climb to generate motion. Contraction and force generation result from the
shortening of the sarcomere as a result of thin actin filaments sliding over thick myosin filaments.
The formation of crossbridges between the filaments is ATP dependent. In sequence, (a) ATP binds
to myosin, which results in the dissociation of the myosin head, the crossbridge between the
filaments, from the actin filament, (b) the free myosin bridge moves into position to reattach to
the actin and ATP is hydrolyzed, (c) the free myosin bridge rebinds to the actin filament and the
working stroke, and (d) force is generated and the products of ATP hydrolysis are released from the
myosin crossbridge.
Skeletal muscles are attached to two or more bones by tendons. They work in antagonistic
groups such as flexing or bending vs. extending or straightening. Antagonist groups can also
contract together to stabilize joints or maintain body posture. Muscle arrangement is based on the
orientation of the fibers relative to the axis of force generation. The physiological cross-sectional
area (PCSA) of a muscle is the estimated sum of the cross-sectional areas of all the myofibers. The
PCSA is proportional to muscle force generation, increased PCSA results in increased force.
The extent and velocity of muscle contraction increases with myofiber length. Two other relation-
ships used to describe muscle properties are based on length–tension and force–velocity. The