Soft Robot: A Review

Soft robotics is a growing field which relies on mimicking locomotion mechanisms of soft bodies existing in nature to achieve smooth and complex motion. Among those “soft bodies” that can move in complex environments, earthworms, snakes, larval insects, octopus and eels present a large range of different strategies, developed over years, that we can draw inspiration from.

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What is a Soft robot?

Soft robots are primarily composed of easily deformable matter such as fluids, gels, and elastomers that enable compliance matching with the environing materials. Compliance matching is the principle that materials that come into contact with each other should share similar mechanical rigidity in order to evenly distribute internal load and minimize interfacial stress concentrations.[1] However, this principle does not apply to rigid robots (E=109Pa) interacting with soft materials (E=102-106Pa), causing damage or mechanical immobilization (where E is the Young’s Modulus, which gives a measure of the stiffness of a solid material). These types of interactions with soft materials are widely spread, as for instance with natural skin, muscle tissue, delicate internal organs, but also organisms, artificial replications of biological functionalities, etc… Due to this dramatic mismatch in mechanical compliance, it is easy to conclude that rigid robots are not adapted and even dangerous for intimate human interaction. Therefore, there is a need of robots that match the elastic and rheological properties of materials and organisms found in nature, and this is where soft robots could bring the solution.

Designing soft robots calls for completely new models in their mechanics, power supply and control.[2] However, rethinking materials, design strategies and fabrication techniques should open up new areas of soft robotics in macro and micro length scales in a lot of fields (healthcare, human assistance, field exploration…)

This review addresses the different types of actuations that exist, as well as the principal method of fabrication of these soft robots, before delving deeper into their applications.

How to fabricate and actuate a Soft Robot?

1. Actuation

Pneumatic Networks actuators (PneuNets) are the most common soft robots, and are made up of a soft material, an elastomer, within which pressurized fluids can navigate through a series of channels and chambers. When these chambers are pressurized, the entrapped fluid generates stress from inside the material, causing the material to strain, to deform, and those enabling the motion of the actuator. The nature of this motion is controlled by modifying the geometry of the embedded chambers and the material properties of their walls. Generally, each segment of a fluidic elastomer robot bends, and this bending is due to material strain.

If a robot is composed of a single homogenous elastomer, most expansion will occur on the thinnest structures, and the motion of the robot will thus depend on the geometry of the microfluidic circuit. However, materials with different elastic behaviors can also be used to allow further control over actuator behavior.

For example, this figure illustrates how to obtain a unidirectional bending. This actuator is composed of two layers, one extensible and the other not, but both have the same size, L0. When pressurized, the top surface will become strained so that its new length will be L0+ΔL . Meanwhile, the bottom surface will remain unextended, causing the bending of the actuator. Bending is the basic, primitive motion of the fluidic elastomer robot. Usually, softrobots are composed of two parts like in this example, where the more rigid layer is called « constraint layer », and the layer containing the channels and chambers is called « outer layer ». However, more complex actuators possessing more pieces also exist.

This kind of softrobots exhibiting two materials, are usually made up of two silicone elastomers: the Ecoflex and the polydimethylsiloxane (PDMS). This two elastomers bond well to each other, present a different but well-suited rigidity, are easy to mold and relatively inexpensive. PDMS has a Shore A hardness* of 50, while Ecoflex has a hardness below the Shore A scale (it fractures only above a maximum strain of 900%).[3] As a result, PDMS is ideally suited for the constraint layer, as it has a limited range of deformation, while Ecoflex is more flexible and therefore more suitable to be used as the outer layer.

Instead of using elastomers with different rigidity, other techniques also allow to control the motion of the actuators. The actuator can be made of one single elastomer, but the constraint layer may contain fiber, paper, or a plastic film to produce the inextensibility property required for actuation[4]. Also, fiber-reinforced actuators exist, and consist of a simple PneuNet actuator wrapped with inextensible reinforcements, which enable to control their motion.

* Shore hardness is a durometer scale which enables to measure the hardness of materials, typically used for polymers, elastomers and rubbers. The A scale is for softer plastics, while the D scale is for harder ones.[5]

Other type of actuation:

The PneuNets actuators use the most developed technology in the domain of soft robotics, but other types of actuation are also explored:

Photosensitive actuators

Photosensitive actuators function with Liquid Crystalline Elastomers (LCEs)[6], a smart materials that can exhibit large shape change under illumination with visible light.

Dielectric elastomers actuators

Dielectric elastomers actuators (DEA)[7] work with electroactive polymers which deform when inducing an electric field. Common design of this type of actuators consists on trapping a soft insulating elastomer membrane between two compliant electrodes. When a voltage is applied between the electrodes, the arising electric field induces a slimming in thickness and an enlargement of the area of the membrane.

Combustion-driven actuators

Combustion-driven actuators (CDAs)[8] use the ignition of combustible mixtures to drive actuation. These actuators function in the same way as pneumatic-based actuators, except that they are powered by gas under pressure, which is directly created within the robot. This pressurized gas is obtained through reactions transforming small amounts of liquid fuel into large amount of pressurized gas. This type of actuation was for instance used to power the octobot[9], developed at Harvard.

2. Fabrication

To create soft robots, the most used manufacturing process consists in soft lithographic molding, which relies on casting elastomer in molds obtained by soft lithography or 3D printing. This fabrication process generally consists of three steps:

  • The robot is composed of two parts: the outer layer, containing the model of the desired channel structure, and the constraint layer, presenting inextensible property (due to a different rigidity) required for actuation. Each layer is molded by casting pourable silicone rubber in each mold, and cured.
  • Then, the two cured layers are removed from their molds, joined using a thin layer of uncured elastomer as glue, and cured together.
  • Finally, once the curing done, the final actuator is unmolded

Obviously, some soft robots are more complex and need more steps for their creation, but the process of fabrication stays the same, broadly speaking.

What are the applications of a Soft Robot

Besides being soft, these robots exhibit a lot of other major assets. Because they are composed of materials that match the compliance of biological matter (such as human skin and tissue), soft robots are mechanically biocompatible and capable of lifelike functionalities. Moreover, soft robots use materials that can change their shape and elastic rigidity, which are lightweight, but also adapted to intimate human contact: comfortable, soft enough to prevent injuries, biocompatible and compliant. These features will potentially lead to plenty of promising new technologies in a broad range of social, scientific, and industrial activities in the future.

Most of these applications should be part of the biomedical field. From large size robots like soft wearable robots, soft prosthetics, co-robots (assistive robots that cooperate with human partners) to miniaturized robots for field exploration, drug delivery, minimally invasive surgery, and medical implants, the number of applications of soft robotics in the medical field is broad.

The field of human motor assistance, using soft wearable robots, could be the first that will be successfully implemented. This new type of robots will assist patients with muscle weakness or patients who suffer from physical or neurological disorders (ex: stroke or traumatic brain injury) for fine motor tasks like moving or grasping.

For example, soft active ankle foot orthotic (AFO) could help prevent foot dragging for patients that suffer gait abnormalities such as drop foot.[10] As with the AFO, soft hand orthotics that contain artificial muscles have begun to be implemented to supply assistive mechanical aid in the fingers and wrist.

Even the domain of cardiac simulators[11], mimicking the motion of the heart, has been explored. These artificial muscles, are based on McKibben actuators[12], composed of an inflating balloon encased in a braided shell of woven inextensible fibers. Such assistive technologies, which effectively function like a second skin[13], compensate missing or impaired motor function by cooperating with the body’s healthy tissue. In this way, it gives the patient new opportunities to relearn or discover motor functions for grasping and gait.

McKibben actuator

Speaking of “second skin”, the development of some soft exoskeletons has begun, and their fields of application are wide. As mentioned before, this new type of robots would be useful for patients with movement disorders (Stroke, Multiple Sclerosis, Parkinson’s disease), for elderly, but also to help soldiers, firefighters, paramedics, and anyone who needs to carry heavy loads.

For decades, exoskeletons have been built with rigid links, in parallel with the biological anatomy, to increase the strength and endurance of the wearer while protecting them from physical stress and injury. However, these robots were not adapted enough to the smooth and complex movements of the human body, were heavy and took up place. Instead, these new types of soft wearable robots use pneumatic air artificial muscles (McKibben actuator) to work, are compliant with human motion and lightweight.

The exosuit, an exoskeleton developed at the Wyss Institute[14], is an example of such a lightweight device, using “soft clothing” and containing no rigid elements. It prevents restrictions for wearer’s motion and avoids articulations misalignment encountered with rigid exoskeleton.

Soft exoskeleton: “exosuit”

The advantages that present the materials used in soft robotics (soft, lightweight, biocompatible, compliant) also present an opportunity to create soft prosthetics, powered by artificial and natural muscle, and controlled through cognitive commands, body gestures, and onboard sensing.[15] Some research projects have already begun, like this soft prosthetics hand for amputees[16].

Soft prosthetics hand for amputees

This same properties present good advantages for evolutions in the field of assistive robots. Co-robots have often been imagined, in particular in science-fiction movies, but what would happen if this technology came to appear in real life? These humanoid robots, that must safely cooperate with humans in a broad range of medical, industrial and domestic tasks, could undergo the same universal integration that personal computers underwent. Taking advantage of these properties, these robots could live and interact with humans every day, and could help for applications that require carrying, lifting and other forms of intimate contact (ex: nursing and elderly care).

Of course, soft robots are not limited to large scale applications like humanoids or soft prosthetics, and, in a world where everything tends to be miniaturized, they also present a lot of opportunities at a small scale. Miniaturizing also calls for complexity, and at these length scales, the rheology of all materials used will impact the functionality of these robots. The domain of application of these small size robots is wide, but mostly resides in the capacity to make them completely autonomous. If successful, they will be capable of crawling and swimming in tight spaces that are impossible to navigate with rigid robots. The domain of swimmers (at the surface and underwater) has begun to be explored. They offer the versatility to accomplish a number of basic functions, including moving in tight spaces, climbing walls, grasping items and pushing objects as heavy as ten times their own mass[17]. This versatility comes naturally through actuation mechanisms that provide large forces and displacements.

This type of robots will also have a real role as field robots for search operations. Whether in case of natural disaster relief, for military reconnaissance, for the transportation of hazardous material or for data collection, they will prove to be very good assets.

However, they may also be used to go further in medicine. Knowing their properties of compliance and softness, they should be capable of navigating through the body without damaging vascular walls and tissue for drug delivery, minimally invasive surgery, and medical implants.[18]

Soft robotics presents an extraordinary solution for any application involving intimate human contact, or asking for the versatility and multifunctionality observed in nature. Although they are in their early stages, soft robots should have a myriad of uses in a lot of applications in the years to come.

Comparison of soft robotics vs classic robotics

Advantages Drawbacks
  • Achieve complex motion
  • Collision avoidance: absorb impacts, prevent injury
  • Elastically deformable and soft
  • Adaptable shape
  • Multifunctional
  • Compatible for human interaction (biocompatible and compliant)
  • Same elastic and rheological properties as soft biological matter (compliance matching)
  • Low cost materials
  • Difficult to control
  • Less powerful and precise than rigid robots
  • Support equipment (hardware) kept off-board: limit motion and autonomy
  • Today’s electronics not adapted (rigid supports)

From rigid and soft robots to hybrid robots

While natural neural tissues are soft and capable of extraordinary computational power, microengineered electronics, even though submicrometric in size, are nowadays constructed with rigid materials. Until the microelectronics are made with soft and elastic materials, soft robots will require rigid microprocessors for actuator control.

As long as this technology does not exist, and if we want soft-robots to be autonomous, they must wear their own support equipment, becoming hybrids robots. For now, these hybrid systems offer a unique solution to fulfilling tasks in real-world environments. With these hardware resources, it is possible to coordinate the two parts of the robots in order to fulfill the goals that either one of them would not be able to achieve alone.[19] This advancement in robotic symbiosis will broaden the use of soft robotics in the years to come.

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AUTHORS

Written by Wilfried Sire.

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References

[1] Majidi, “Soft Robotics: A Perspective—Current Trends and Prospects for the Future.”

[2] Rogóż et al., “Light-Driven Soft Robot Mimics Caterpillar Locomotion in Natural Scale.”

[3] “According to the Manufacturer’s Data Sheet: Ecoflex, http://www.smooth-On.com (Accessed April 2017).”

[4] Marchese et al., “A Recipe for Soft Fluidic Elastomer Robots.”

[5] “https://en.wikipedia.org/Wiki/Shore_durometer (Accessed May 2017).”

[6] Rogóż et al., “Light-Driven Soft Robot Mimics Caterpillar Locomotion in Natural Scale.”

[7] “http://Softroboticstoolkit.com/Book/Dielectric-Elastomer-Actuators (Accessed May 2017).”

[8]“http://Softroboticstoolkit.com/Book/Combustion-Driven-Actuators (Accessed May 2017).”

[9] “http://News.harvard.edu/Gazette/Story/2016/08/The-First-Autonomous-Entirely-Soft-Robot/ (Accessed May 2017).”

[10] Majidi, “Soft Robotics: A Perspective—Current Trends and Prospects for the Future.”

[11] “http://Softroboticstoolkit.com/Book/Case-Study-Cardiac-Simulator (Accessed May 2017).”

[12] Klute et al., “McKibben Artificial Muscles: Pneumatic Actuators with Biomechanical Intelligence.”

[13] Goldfield et al., “Bio-Inspired Design of Soft Robotic Assistive Devices.”

[14] “https://Biodesign.seas.harvard.edu/Soft-Exosuits (Accessed May 2017).”

[15] Majidi, “Soft Robotics: A Perspective—Current Trends and Prospects for the Future.”

[16] “https://www.tudelft.nl/io/onderzoek/research-labs/foundational-labs/soft-robotics (Accessed May 2017).”

[17] Rogóż et al., “Light-Driven Soft Robot Mimics Caterpillar Locomotion in Natural Scale.”

[18] Majidi, “Soft Robotics: A Perspective—Current Trends and Prospects for the Future.”

[19] “Soft Robotics Toolkit.”