Optotune

Biological muscles are amazing actuators with properties, including large actuation strain, high stress and energy density, fast response, resilient and damage tolerant operation, unmatched by any conventional actuator technology such as combustion engines, electric motors, and piezoelectric actuators.

Researchers have realized that many highly anticipated systems such as surgical mini- and microrobots, and biomimetic devices, including limbs and artificial organs cannot be achieved with conventional actuators. Therefore, in the last two decades an increasing number of scientists started to develop novel actuation mechanisms and materials that imitate the functionality of natural muscles. The majority of these emerging artificial muscle technologies are based on polymers that expand or contract when varying electric or magnetic fields, light, pH and heat are applied.

For many years, electroactive polymers (EAPs), which use electrostatic forces, electrostriction, ion insertion, and molecular conformational changes, received relatively little attention. However, the increased demand for electrically controlled compact adaptive structures recently boosted the research efforts, resulting in novel electroactive materials and a large number of prototype devices including robot fish , catheter steering elements , lens positioners , robotic arms , grippers , loudspeakers , a blimp , dust-wipers , hopping robots , heel-strike generators and strain sensors .

Depending on their activation mechanism, EAPs are divided into two major classes:

1. Electronic EAPs are driven by Coulomb forces and can be operated in dry conditions for a long time. Electronic EAPs include dielectric elastomer actuators (DEAs), electrostrictive graft elastomers, electro-viscoelastic elastomers, ferroelectric polymers and liquid crystal elastomers (LCEs). They have high mechanical energy densities and can maintain strain under DC voltage. The main disadvantage of these materials is the high actuation voltage (> 1 kV).
2. Ionic EAPs, which are also known as wet EAPs, are materials requiring mobile ions for actuation. Normally, an electrolyte is sandwiched between two electrodes. When a low voltage (< 2 V) is applied to the electrodes, the ions in the electrolyte move, causing mostly a bending of the actuator. Ionic EAPs, such as ionic polymer-metal composites (IPMCs), carbon nanotubes (CNTs), conductive polymers (CPs), electrorheological fluids (ERFs) and ionic polymer gels (IPGs) produce large bending displacements and exhibit polarity dependent bi-directional actuation. However, most wet EAPs have low electromechanical coupling efficiency, slow response and operation in air is difficult due to evaporation of the electrolyte.

The following subsections cover, biological muscles and the most promising electronic and ionic EAPs in more detail. Furthermore, the mechanical properties of these actuator technologies are summarized in an overview table.