Biological muscles are the engines of life. They efficiently convert high energy density fuels like sugars and fats into mechanical work. In humans, three types of muscles are distinguished: skeletal, smooth and cardiac muscles.
All muscles have a complex multi-level structure, exquisitely tailored for force generation and movement. A muscle consists not only of a bundle of muscle fibers (single, multi-nuclear cells), but it also integrates a complex blood circulation system, which delivers fuel (glucose and oxygen) and removes heat and waste. Additionally, satellite cells, responsible for the growth and repair of the muscle fibers, are present. Each muscle fiber (cell) contains many myofibrils, the contractile units of muscles, and is individually controlled by a motor neuron. Myofibrils are built of a chain of sarcomeres, which contain proteins that ultimately produce the force. The formation of cross-bridges between thin (actin molecules) and thick (myosin) filaments causes a contraction of the sarcomeres. This molecular mechanism is called sliding filament mechanism and is initiated by a calcium release, which is triggered by an electrical signal controlled by the motor neuron. Due to their complex hyrarchical design, biological muscles have several properties not yet achieved by man-made actuators (see also overview table). For example, the activation of individual muscle fibers, so called recruitment, enables a graded force control and a variable stiffness. Furthermore, an energy optimized operation over a wide range of loads and contraction velocities can be achieved. Additionally, muscles are self-regenerating, operate over billions of cycles and for more than 100 years, and work on the micrometer as well as on the meter scale.
In the following sections, several artificial actuator technologies are discussed that already surpass biological muscles in certain properties such as strain, stress and peak forces per cross-sectional area.