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374 Biomimetics: Biologically Inspired Technologies

length–tension relationship illustrates that the isometric tension generated in skeletal muscle is a
function of the magnitude of overlap between actin and myosin filaments. The greatest force is
generated at an intermediate ‘‘optimal’’ sarcomere length. Long lengths result in decreased myosin
cross-bridges with actin filaments and short lengths create actin filament overlaps, which interfere
with crossbridges. Passive tension increases as a muscle is stretched, providing a resistive force in
the absence of muscle activation. The force–velocity relationship demonstrates that the maximum
force generated by a muscle is a function of its velocity and is used to define the kinetic properties of
the cross-bridge cycle during contraction.
Cardiac muscle has actin and myosin assemblies similar to that of skeletal muscle, but
the muscle cells are mononucleated and arranged in a continuous network of branching and
anastomosing cells. Cardiac muscles maintain constant, continuous contractions to pump blood
throughout the body. Smooth muscles also contain actin and myosin filaments that produce
contractions but not in a striated pattern. The cells are spindle shaped and mononucleated. Smooth
muscle contractions are slower and can be sustained for longer periods compared with skeletal
muscle. Smooth muscles are responsible for the contractability of hollow organs and have a role
in various functions such as regulating flow through blood vessels. Smooth muscle fibers
are organized into sheets that are arranged into two layers, which alternately contract. The fibers
have a longitudinal arrangement in the outer layer and fibers of the inner layer are organized
in a circular pattern. Unlike skeletal muscle, cardiac muscle and smooth muscle control is
involuntary.
Biomimetics — Muscles are natural actuators. Actuators are controllable machines that convert
energy to mechanical work by means of a shape or dimensional change. Braided pneumatic muscle
actuators, also known as McKibben artificial muscles, were first developed in the 1950s specifically
for use in artificial limbs. The device consists of an expandable fiber mesh wrapped around an
inflatable bladder. When the bladder is filled with pressurized air, its volume increases. A muscle-
like contraction is produced as the actuator expands radially while contracting along its axis due to
the constant length of the mesh fibers. Pneumatic muscles produce a large force when shortened and
can achieve greater power or weight ratios than natural muscle. They can operate in antagonistic
modes like natural muscles and are capable of more natural motion and control. The system is
lightweight, adaptable, and easily powered. Pneumatic actuators have high power efficiency and are
capable of amplifiable force output. Precise control of pneumatic muscles can be difficult because
of their nonlinear and time varying characteristics. Actuator properties can also vary with tempera-
ture and use. Another limitation is that current pneumatic actuators have a short fatigue life in
the order of 10,000 cycles, but a new pneumatic device in which the fiber mesh is impregnated
inside the bladder has demonstrated a fatigue life of 10,000,000 cycles. Force output is dependent
on the thickness of the actuator and the speed of the McKibbean muscle is slow relative to natural
muscle response.
Other examples of mimetic structures include shape memory alloys, such as Ni–Ti (Nitinol).
These are a class of metallic materials capable of returning to a predetermined size and shape when
subjected to a thermomechanical load. Macroscopic deformations of the material are caused by
microscopic crystalline structure changes between the hard, high temperature austenite phase and
soft, low temperature martensite phase. Shape memory alloys exhibit typical thermomechanical
properties of psuedoelasticity and shape memory effects. Nitinol wires have been used as artificial
muscles for robotic and prosthetic applications (De Laurentis and Mavroidis, 2002). Actuation
occurs by a voltage drop across the wire causing current flow through the material, which results in
heating that causes a crystalline transformation and accompanying shape change such as bending.
The advantages of using shape memory alloys are ease of actuation, small size and weight,
noiseless operation, and low cost. Shape memory alloys have high strength to area ratios and are
capable of high grasping strength. Disadvantages include slow response, high power consumption,
dependence of attainable motion and force on wire length and thickness, and heat generation.
Actuation effects are also nonlinear and have short life cycles.