Most traditional robots are constructed with stiff materials such as steel, aluminum, and ABS plastic. They are usually powered by electric motors or pumps forcing hydraulic fluids through rigid tubes.
These machines can produce large forces, high speeds, and great precision, making them extremely productive in factory assembly lines. However, only a small number of these machines can operate in natural settings or close to humans. Aside from their safety concerns, these robots aren’t very good at adapting to different situations and aren’t well-suited to the materials they come across.
Therefore, there is an increasing interest in building robots from soft materials to overcome some of these obstacles. A soft robot is a mobile machine constructed from soft materials. Soft robots deform during normal use, ranging from simply flexible to extremely ‘squishy’ and capable of drastically changing the size and shape (morphing).
Why exactly do we need soft robots?
One goal is to make adaptable and more animal-like machines in their capabilities. We take for granted that humans can walk up and downstairs, navigate through a cluttered room, or move delicate objects. Even for the most advanced machines, these tasks are extremely difficult. Part of the issue is that stiff robots are extremely precise in their control; they constantly monitor their body posture/torques and plan out their movements strictly. This is essential because stiff robots can easily harm themselves or the environment if they become unstable.
Movement precision becomes extremely difficult when a robot has many joints (a high degree of freedom) or many ways to move its body. When the robot enters more natural or human-like environments, the problem is exacerbated by the variety and constantly changing conditions. Surface friction, hard and soft obstacles, uneven floors, gusting winds, and moving objects are just some variables that affect the robot’s performance. Such robots cannot calculate all required forces and displacements to maintain precision.
These calculations can be greatly reduced by designing the body to automatically exploit natural kinematics and dynamics. Passive dynamic walking robots, for example, can walk without a brain because their legs and torso interact mechanically to create a very natural-looking gait. This concept can be expanded to include the mechanical properties of structural materials.
Often, soft materials have non-linear responses to forces with properties such as pseudo-elasticity, viscoelasticity, anisotropy, yield, creep, and work softening or hardening. As a result, different soft materials for each body part can be chosen and matched to the robot’s function. One of the most significant differences between animals and current robots is the extensive use of soft materials. Even in animals like humans, the rigid skeleton accounts for less than 15% of total body weight; the rest is very soft tissue. Soft materials are excellent at absorbing energy from impacts, damping oscillations, and smoothing out irregular movements and forces. These features are expected to make robots more natural in their movements and more adaptable and robust in general.
Challenges of building soft robots
A basic body structure or the chassis, sensors, microprocessor (a central control system), actuators or motors, a power supply, and an overall program for its behavior are all required for an autonomous robot to work. Casting, injection molding, and multi-material three-dimensional printing can all be used to make a chassis out of soft materials. Even rigid components can now be incorporated into soft robots without compromising the robot’s overall soft properties because sensors and microprocessors can now be manufactured on such a small scale.
Electronic components can now be made flexible or stretchable thanks to new technologies. Finding appropriate actuators, control schemes, and power sources are difficult. Small motors are ineffective, and traditional electric motors cannot be miniaturized and distributed the same way that sensors can. Some recent soft robots have used hydraulic and pneumatic systems. Even so, stiff components are used, and the pumps are kept separate from the soft structure (usually off-board completely). Soft actuators like electro-active polymers (EAPs), macroporous gels, and other phase-transition materials are expected to become more widely available, but they have significant limitations as robot motors.
Miniature memory alloy wires or foils are used in some of the most widely used ‘soft’ actuators. These are metal alloys that change shape when heated. Threadlike actuators can be made by coiling fine wires and changing their length in response to the heating effect of an electric current which passes through them. Shape memory alloy coils act like muscles, but they’re notorious for being inconsistent, inefficient, and susceptible to environmental factors. Living muscle is the ideal soft linear actuator in many ways, and several research groups are working on ways to create muscle-powered machines.
The actuators’ energy supply is also a significant challenge. Anything that is powered by electricity requires batteries or capacitors to store energy. They are not yet commercially available and have a low energy density, even though they can be relatively flexible. This reduces the battery-powered device’s range of operation. Chemical energy, typically in hydrocarbons, is better to store energy. Organic molecules have a high energy density, which is why gasoline is a popular fuel for modern engines and why fat is the primary energy source for migrating animals. Several organizations are developing technologies to convert chemical energy into mechanical energy. Muscle is powered by the safe combustion of environmentally friendly sugars, fats, and proteins, so biological solutions are appealing once again.
The development of control systems suitable for highly deformable structures is the final major challenge in producing useful soft robots. Most of our current methods cannot control movements with a high degree of freedom, especially in unpredictable environments. This is a field of study that necessitates novel approaches. It will most likely benefit from the concept of morphological computation (embodiment), which was first developed in artificial intelligence and is now being applied to animals as ‘neuromechanics.’ Animals solved this control problem; and perhaps by studying their solutions, we will be able to design controls for our own machines more quickly.
How will soft robots be used?
Soft robots will be able to do things that no other machine can. They enter confined and complex spaces, follow cables, ropes, or wires, and climb branched three-dimensional structures by utilizing their ability to change size and shape. Robots will send themselves into dangerous situations to look for survivors or identify and repair damaged pipes and wires for the first time. These devices can be made of biocompatible materials and soft, ideal for diagnosis and therapy deep inside the body. They will be appealing for environmentally sensitive applications and bioremediation because of their biocompatibility. Soft robots could be extremely cheap (a few dollars each) with suitable manufacturing facilities and economies of scale, allowing them to be used in large numbers.
Swarms of soft robots could be used to locate and defuse landmines, which is an exciting possibility. Their low density, inherent safety, and high packing density will be useful in spacecraft applications like instrument and environmental monitoring. Soft robot technologies will also find their way into more traditional robot applications, which is an indirect benefit. Thanks to new materials and control systems, assistive robots for the home, hospital, and workplace will be much safer. Perhaps the most revolutionary aspect of these new developments is that we will learn to construct machines out of soft living tissues by learning to design, build, and control soft robots. These biorobots will be assembled in incubators rather than factories and powered by safe renewable fuels like fats and sugars and self-heal minor damage. Future robots can be organic, biodegradable helpers for the greater good.
