Editorial Feature

Artificial Muscle Technology

The human muscle is a contractile tissue and crucial for survival. In the biomechanical world, the muscle is playing a major role in research on developing autonomous humanoid robots and artificial limbs that can behave as exoskeletons for patients with an amputated limb, especially war veterans who have lost a limb in combat and can then benefit from such bio-inspired technology. The concept of artificial muscles opens an array of structural and functional possibilities, but could this technology ever recreate what nature can perform so perfectly well?

There are a couple of issues with imitating the biological mechanisms of a human muscle into an artificial system that can perform in a similar fashion. Firstly, the historical evolution of muscle tissue means that it can self-repair and grow whilst maintaining all functional capacity. Metabolic regulation of muscles means that this tissue can re-fuel to help perform at an optimal level. It is also important to understand the sensory network and control system that works alongside muscle tissue to animate a particular movement.

Designing an Artificial Muscle

Inspiration for the design and engineering of an artificial muscle has come from studying the human body at great detail which has allowed for the development and integration of natural mechanisms into the development of biomechanical parts.  

An artificial muscle is made up of lightweight polymers with a great degree of power density. The fascinating characteristic of such polymers is their ability to take any shape and they can be adapted to multiple applications. The most common type of polymer in the context of building an artificial muscle is the electroactive polymer (EAP). Electroactive polymer material responds to electrical impulses when the muscle changes shape. Typically, EAPs fall under two distinct categories: Ionic and Electronic. The table below compares both types of EAP actuators.

Table 1. Ionic vs. Electronic Polymer Material

EAP Advantages Limitations
Ionic electroactive polymers Capable of shear force and bending capacity in varying directions based on the flow of electrons. Ionic EAPs also function on a low voltage (approximate range between 1 and 7 volts) These polymers do not work well to maintain strain during a unidirectional flow of electrical current. There is a slow response to a low voltage resulting in low tolerance control of the actuator
Electronic electroactive polymers Work well under room temperature and can maintain a strain in the absence of actuation force A higher energy supply in the form of kilovolts is required to power electronic EAPs

Electronic EAPs

Electro-active polymers exhibit a range of electromechanical coupling mechanisms, which allows this material to demonstrate strain when introduced to an electrical charge and to then generate an electrical stimulus in response to the strain. This electromechanical quality to such polymers means that a variety of modelling techniques can be used and applied of demonstrate a range of actuation behaviour. Basic structural composition of the electroactive polymer involves monomers (single molecules). For example, polyacrylonitrile is a common electroactive polymer actuator chemically known as CH2=CHCN and made up of multiple repeating monomers called acrylonitrile (see Figure 1). Branched electroactive polymers will have multiple connections with additional polymer chains and this will increase the crosslink density of the chemical structure.

Induction of electrostrictive force to EAP material. Source: Yan, X., Jiang, C., Eynard, B. (2008). Advanced Design and Manufacture to Gain a Competitive Edge: New Manufacturing Techniques and their Role in Improving Enterprise Performance. UK, London: Springer.

Figure 1. Induction of electrostrictive force to EAP material. Source: Yan, X., Jiang, C., Eynard, B. (2008). Advanced Design and Manufacture to Gain a Competitive Edge: New Manufacturing Techniques and their Role in Improving Enterprise Performance. UK, London: Springer.

The introduction of an electrical charge across an electroactive polymer chain results in rotation of a bound charge (as a consequnce of polarization) and this collectively bound energy remains in the electroactive material. It is this change in the chemical structure of such polymers that makes this material an electrical conductor. However, ionically conducting material behaves in a slightly different manner: as opposed to attracting electrons, the ionic conductor works by delivering charged atomic particles across the polymer structure to enforce a structural change in shape.

Maxwell Stress

The concept of Maxwell Stress is used to explain Electronic EAP behaviour. Dielectrict elastomers, a large strain of soft electronic EAPs, bridge a gap between two conducting metal electrodes. The basic notion states that a force (stress) is produced between the two conducting metal electrodes when there is an open cavity between these plates. By activating the conducting electrodes using a direct current voltage, this stress forces the electrodes of opposite charge to interact and this pushes the polymer structure together, followed by expansion of the conducting material in response to the pressure. The stress created from the electrostatic interaction results in a tension or strain, a pull which changes the shape of the conducting material.

In the video below, Professor Ray Baughman, Director at the Nanotech Institute, The University of Texas at Dallas, explains the mariage between nanotechnology and robotic technology by focusing on the creation of artificial muscles. Professor Baughman and his team have developed an artificial muscle made out of nanotubes that can work like a human muscle. One of the most important applications for this technology is prosthetic limbs that can capture the intricate function of a natural limb. Professor Baughman puts forward for consideration the question of future research into creating nanotube muscles that are much stronger. He expands on the idea of stronger conducting nanotube actuators to also suggest that the biggest challenge with this technology will be to focus on how we can make this structure into yarns without losing the original properties of the nanotubes.  

The basic functional principle of carbon nanotube material invovles nanotube electrodes that can catalyze the conversion of chemical energy into electrical energy, together with a muscle electrode that can then tranform the electrical energy into mechanical energy. It is the change in mechanical energy that brings about movement in an artificial muscle. There is also the idea of converting chemical energy to heat energy using oxygen and chemical fuel. This catalytic reaction will induce a change in temperature, which will force a change in the shape of a conducting carbon nanotube polymer chain. This conducting reaction is an example of how nanotechnology is working alongside biomechanics to try and imitate, or at least understand the metabolic reactions that do take place in a natural muscle to allow it to function.

In the Lab

The nanotech industry is starting to invest quite heavily in the design and engineering of conducting nanotube materials for the purpose of prosthetics. Decker Yeadon, a firm based in New York that specializes in material technology, demonstrates the creation of an artificial muscle prototype that can function in liquid and is engineered out of carbon nanotubes (see video below).

Now let’s view an example of this technology by looking at the idea of fluidic muscle mechanics as introduced by Festo.

Challenges Ahead

Nanotechnology has clearly found its place in being able to advance the world of robotics and it is hoped that this can only help progress research into actuators and electroactive polymers for their application in the modelling of artificial muscles. However, there are major challenges ahead in this research. For example, these actuators are difficult to mass produce, which becomes a problem when considering demand for the end product. Carbon nanotube material is great for demonstrating strength at low voltage, but this material is also coupled with low strain at a level of 1% and is a long way away from its application in the real world.

Sources and Further Reading

  • Yan, X., Jiang, C., Eynard, B. (2008). Advanced Design and Manufacture to Gain a Competitive Edge: New Manufacturing Techniques and their Role in Improving Enterprise Performance. UK, London: Springer.
  • Leo, D.J. (2007). Engineering Analysis of Smart Material Systems. USA, Hoboken, New Jersey: John Wiley & Sons, Inc.
  • Schaefer, H. (2010). Nanoscience: The Science of the Small in Physics, Engineering, Chemistry, Biology and Medicine. Germany, Berlin: Springer Science and Business Media.
  • Mahalik, N.P. (2006). Micromanufacturing and Nanotechnology. Germany: Springer Science.
  • Bar-Cohen, Y. (2004). Electroactive Polymer (EAP) Actuators as Artificial Muscle: Reality, Potential, and Challenges. 2nd Edition. USA, Washington: The Society of Photo-Optical Instrumentation Engineers.
  • Kim, K.J., Tadokoro, S. (2007). Electroactive Polymers for Robotic Applications: Artificial Muscles and Sensors. UK, London: Springer Science and Business Media.

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