How do we make soft robots ?

What type of materials are commonly used in soft robotics ?

Many materials can be used to create soft robots. What usually qualifies a soft robots is it's ability to deform passively [7]. 

Intuitively, if you design a mechanism out of natural rubber, that mechanism will tend to be able to deform. Similarly, making a robot out of rubber will naturally make it soft. This is the reason why soft robotics makes heavy use of elastomers [1]. Elastomers are polymers that can easily deform and recover their original shape. This property is generally referred to as elasticity [2]. 

However, one can use fairly rigid materials and design soft mechanism out of them as well [12]. Picture a chain mail for example. Each element is made out of metal, yet the overall garment is fairly flexible. Soft robots as well can be made out of fairly rigid materials, as long as the structure is designed for deformations. 

Eventually there is a trade-off between a robot softness and the forces it can generate on it's environment. Furthermore, soft materials can deform in all axes, making pure elastomeric soft robots possibly hard to control. This is why multi-material soft robots, and/or complex structures might be leveraged in the future to create capable soft robots. 

In the end, "softness" maybe quite relative to the robot's task and environment. Softness may be seen not a goal, but rather as a property of the robot allowing it to behave in a way that is beneficial to a set of applications. 

How are soft robots designed and constructed ?

As discussed above, the materials and structure of soft robots may differ from their more traditional counterparts. However, regardless of the materials used, robots are usually understood as machines that can move, sense, and compute. That's why we are more likely to consider our autonomous vacuum cleaner as a robot than our microwave oven, our oven's sensing and computing capabilities are much more limited.

Soft robots' must then be capable of sensing, moving, and computing. It follows that we must design soft actuators, soft sensors, and soft computers. However, most of the technologies around us are made of rigid materials. 

Quite naturally, one of the best source of ideas for soft robotics has been the realm of living things. Animals and plants can move, sense and think, and are reasonably soft. On top of that, nature is incredibly diverse, granting us with varied design to take example from. Some researchers have looked at the octopus or the elephant trunk for their ability grasp and manipulate without relying on an internal skeleton  [6,8]. The octopus has beens studied for its use of camouflage as well [3]. Others have studied the giraffe neck [5] or even vines [4] to design long reaching soft robots.

Although soft robotics can take inspiration from the living world, it is not necessarily confined to it. Animals and plants have continuously evolved under a set of constraints that we may not find necessarily relevant. For example, evolution relies on some form of reproduction, but we may not need or want our robots to reproduce, possibly leading to designing soft robots differently from living organisms. Furthermore, evolution is not guaranteed to reach an optimal design. Living organisms are a tremendous source of inspiration but do not represent the only way to design a soft robot, nor necessarily the best one.  

In practice, soft robots are oftentimes made using elastomers that are either molded, spread in a layer by layer approach, or 3D printed [9]. 

Controlling an elastomeric soft robot's deformation is not a simple problem. Consequently, soft robots are oftentimes made out of various materials different mechanical properties. Inextensible threads [10], fabric [11] or more rigid polymers can be used to constrain inflating cavities [7].