It was 1958 when the first artificial muscle was patented. It was the so-called Mckibben pneumatic artificial muscle, which was driven by air. Since then, a wide range of artificial muscles have been created, all of which have different advantages (and also disadvantages) and the materials of which they are composed differ .
Artificial muscles have different fields of application. These fields are diverse, from medical uses to industry and robotics, some of them being used in more than one area, and for different purposes. For instance, they are used in prosthetics, thus helping amputees to cope with everyday challenges and tasks. As a prosthesis, the artificial muscle literally replaces a missing body part, usually a limb. They can be adopted in biomedical applications and as tiny biomedical devices, such as orthotic or implantable devices, whose purpose is usually to support or enhance existing biological structures. They are equally used as actuators in robotics and in industrial automation, in order to reduce costs and increase productivity, to list just a few advantages.
However, despite their wide-ranging applications and the continuous development in the technologies involved in their creation, artificial muscles also have some disadvantages. A principal one is their cost. This is not a minor factor but a fundamental one, as it involves the initial investment for the device itself and that for human training. Moreover, costs for maintenance also have to be taken into account. Focusing on medical and prosthetic use, there are also other challenges to deal with. In fact, whilst artificial muscles are becoming more and more precise and similar to natural ones, the gap between them and human muscles is still undeniable. Moreover, some aspects have to be improved and among them, their heavy weight, slow response time, form of the robotic arm/limb, and noise.
Even though all of them have limitations, they all feature some strengths. For instance, researchers in Sweden have tested an artificial muscle which is able to contract thanks to a combination of glucose and oxygen. Made of polypyrrole, a polymer material, the muscle has been integrated with enzymes. Featuring high electroconductive properties, the polymer can be powered by a charge coming from an electrical current. This current can derive from either a battery or from glucose, which being burnt by the injected enzymes, makes the muscle contract. In order for the actuator to be powered, it is thus sufficient to put it in a glucose solution, scientists discovered. Considering that glucose is present in all the organs of our body, this means that this kind of artificial muscle can contract thanks to a natural electrical trigger, unlike traditional ones. Another type of artificial muscle is characterised by a compound of two polymers, coiled together to form a single strand of fiber. The two polymers have different thermal expansion coefficients, resulting in the more stretchable one curling up faster and bending towards the stiffer one. This can even occur under slight thermal changes. In fact, the experiment proved that contraction is possible with just the touch of a human hand and that the fiber response is quick. Moreover, in the future, this fiber-based system will allow the incorporation of heating elements (i.e. optical fibers, electrodes) that internally heat the muscle. Because of its structure, sensors could also be incorporated into the fiber itself, so as to get feedback. In addition to the medical field, such artificial muscles could thus be applied in robotic systems, where automated and precise control is required. According to a very recent piece of research, spider silk is expected to be a very good material for artificial muscles. In fact, it has been proved to have unusual properties: subjected to changes in moisture, the silk quickly twists while contracting, and with a strong force. Thanks to this discovery, muscle fiber made of spider silk will now be subjected to further research, planning future applications in soft robotics, in the creation of control devices, sensors, and artificial muscles. Finally, Orlon muscles, also known as ‘artificial silk’ muscles, are artificial muscles which are very similar to human ones in the way they move. Following a change in acidity, the polymer-based fiber is able to shrink as fast as biological muscles, thus being able to mimic their form. Preparing Orlon and understanding how it simulates human fibers is apparently simple:
1. Cook the Orlon. Orlon is a form of artificial silk.
2. Boil the Orlon until it turns into a liquid rubbery substance.
3. Pour the solution onto Plexiglas to form a thin film.
4. Vacuum away excess water from the film. Allow the film to dry.
5. Cut the dried film into 2 centimeter wide strips. Bake them in an oven set at 90°C. The material is ready for use after it has been baked.
6. Prepared Orlon has a structure similar to that of human muscle fiber and is naturally negatively charged with electricity.
7. If you apply acid to the material, you introduce a positive charge and you cause the ions to attract. This attraction contracts the material like a muscle.
8. If you apply a base material, you introduce a negative charge, the ions repel, and the muscle expands.
In spite of these apparently easy instructions, preparing Orlon material requires chemical treatment. Beyond that, Orlon muscle also requires a chemical trigger to shrink, this dependance is one of its limitations.
What has been shown here is not a comprehensive scenario, evidently. However, it can be inferred that all kinds of artificial muscles have weaknesses. In the medical field, in particular, although scientists have made great advances in their research, a lot of work has yet to be done. The scope is to achieve an artificial muscle which is similar to biological ones in all its physical potential. More precisely, the next frontier now is to provide artificial muscles with sensory feedback. It is what is known as neuroprosthetics, a branch of medicine where brain-controlled prosthetics are provided with the sense of touch. This is definitely the future challenge.
7 min read