How to make soft artificial muscles ?

Actuation in soft robotics

A soft robots can only be soft if all or most of it's body parts are soft. Furthermore, we already know that robots are usually expected do move. To move a robot, we use what is called an "actuator", also colloquially known as a "motor". 

But here lies the issue, most of the actuators that we use are not soft at all! Whether it's the internal combustion engine form your car, or the  electromagnetic motor from the latest drone, both are made using very rigid materials. As a consequence, they hardly match our soft robots' requirements. 

Hence the following question: how do we make soft actuators for our soft robots to move ?

Research in this particular part of soft robotics is alive and well. Actuators based on various principles have been proposed, and their design subsequently refined. Although there is no way to make an exhaustive list, I propose to give an overview of the current main technologies being investigated, classified based on the physics they rely on for actuation. For further information I will refer the interested reader to the reference section for scientific papers treating more in depth the various actuators. 

Fluidic actuators

This type of actuator is probably the most known and the oldest type of actuator used in soft robotics. There are various subtypes of fluidic actuators. 

McKibben actuators, popularized by the eponymous physicist Dr. McKibben in 1957 [1,2] are pneumatic linear actuators [3]. This means that they linearly contract when a compressed gaz enters the actuator. Inverse pneumatic linear actuators also exist [4], but contrary to McKibben actuators, they contract when pressure is released and extend when pressurized with a gaz. Both can also be referred as Pneumatic Actuator Muscles (PAMs). 

Hydraulic actuator muscles (HAMs) are very similar to PAMs, although the use of liquids instead of gaz tend to increase the possible actuator force, weight, as well as overall actuator rigidity when contracted [5,6,7]. 

Fluidic actuators do not necessarily have to be separated from the soft robot body. Harvad's PneuNet actuators for example work by inflating an internal channel or cavity and can be integrated within the soft robot's body [3].

The Harvard soft robotic toolkit webpages contains information regarding both McKibben and PneuNets bending actuators [8].

Soft actuators based on electric fields

Dielectric Elastomer Actuators (DEAs) are also one of the most popular type of soft actuators. DEAs actuate thanks to the application of a voltage (several kV) across a soft elastomeric membrane [14]. 

To manufacture such an actuator, one needs to sandwich the soft non-conductive membrane between to soft conductive electrodes. These are often made using thin elastomer layers bonded one on top of the other and pre-stretched using a rigid frame. When a high voltage is applied, the electrodes attract each other and since the non-conductive elatomer membrane placed in the middle has a constant volume, the middle layer thins and expands orthogonally to the contraction. Although DEAs have been quite popular in research laboratories, their at scale reliable manufacture and use is a challenge [19].  The Harvard soft robotic toolkit demonstrates how to manufacture some of these actuators [8].

Hydrolically amplified electric actuators such as Hasels [15], Peano-hasels [16] and Haxels [17] all work following similar principles. In this case, a non conductive liquid is placed between two electrodes. When a high voltage is applied, the electrodes attract each other and displace the fluid that mechanically deform the pouch or cavity it is in. These actuators are quite novel but can move fast and, contrary to DEAs, do not rely on membrane pre-stretch and framing [18]. 

Soft magnetic actuators

Magneto-active elastomers (MAEs) are usually made by trapping either magnet powder or iron powder into into a soft elastomer. Magnets are "hard" ferromagnetic materials, while iron is typically considered a "soft" ferromagnetic material. Both "hard" and "soft" ferromagnetic create magnetic fields when exposed to an external magnetic field, but "hard" ferromagnetic materials are much harder to magnetize but conversely stay magnetized while "soft" ferromagnetic materials are easy to magnetized but also lose their magnetic field easily. 

Iron is typically used to focus and amplify magnetic fields created by coils for example, while hard ferromagnetic materials such as Neodymium-Iron-Boron (NdFeB) magnets are usually magnetized and then used as permanent magnetic fields sources in electromagnetic motors for example. 

Obviously, both iron and NdFeB ceramic are very rigid materials. Elastomers imbued with NdFeB or iron powders can be soft while also displaying some of the original materials magnetic properties. Robots made using such materials may eventually be used for medical applications where controlling the robot at a distance and through the human body and overall robot softness are both obvious advantages. More information about this subject may be found in this very complete scientific review paper [14].

Shape memory materials

Shape memory alloys (SMAs) and shape memory polymers (SMPs) are materials that change shape when exposed to an external stimuli. In both cases this is due to a change that occurs in the atomic structure of the material usually when heating the material passed a certain temperature, and which is referred to as a "phase transition" [9]. SMPs and SMAs can be used in various ways depending on the shape "memorized" within the material, and the mechanism it is used in. 

As their name implies, SMAs are metal alloys, most commonly Nickel-Titanium (NiTi) in which a shape is "memorized" at high temperature [12]. Once cooled down, the SMA can be deformed, when exposed to a higher temperature again, it tries to change back to the "memorized" shape as demonstrated in this video [10]. Typically, NiTi wires can be shaped into small springs which makes them relatively soft. They may be heated in a variety of ways, one of the more popular being the Joule effect [11] where an electrical current is passed through and heats the wire, making contract back to it's original shape.  

SMPs behave very similarly to SMAs. However, being-based polymers instead of metal-based, they tend to be lighter, to be able to undergo larger deformations, and to dissipate heat slower, which may reduce the actuation cycle (elongation and contraction) compared to SMAs [13].