![Soft Robotics 1](https://upload.wikimedia.org/wikipedia/commons/8/86/Soft_Robotics_1.jpg)
Soft robotics is a subfield of
robotics
Robotics is an interdisciplinary branch of computer science and engineering. Robotics involves design, construction, operation, and use of robots. The goal of robotics is to design machines that can help and assist humans. Robotics integrat ...
that concerns the design, control, and fabrication of robots composed of
compliant materials, instead of
rigid links.
In contrast to rigid-bodied robots built from metals, ceramics and hard plastics, the compliance of soft robots can improve their safety when working in close contact with humans.
Types and designs
![Soft Robotics](https://upload.wikimedia.org/wikipedia/commons/8/84/Soft_Robotics.png)
The goal of soft robotics is the design and construction of robots with physically flexible-bodies and electronics. Sometimes softness is limited to part of the machine. For example, rigid-bodied robotic arms can employ soft end effectors to gently grab and manipulate delicate or irregularly shaped objects. Most rigid-bodied mobile robots also strategically employ soft components, such as foot pads to absorb shock or springy joints to store/release elastic energy. However, the field of soft robotics generally leans toward machines that are predominately or entirely soft. Robots with entirely soft bodies have tremendous potential. For one their flexibility allows them to squeeze into places rigid bodies cannot, which could prove useful in disaster relief scenarios. Soft robots are also safer for human interaction and for internal deployment inside a human body.
Nature is often a source of inspiration for soft robot design given that animals themselves are mostly composed of soft components and they appear to exploit their softness for efficient movement in complex environments almost everywhere on Earth.
Thus, soft robots are often designed to look like familiar creatures, especially entirely soft organisms like octopuses. However, it is extremely difficult to manually design and control soft robots given their low mechanical impedance. The very thing that makes soft robots beneficial—their flexibility and compliance—makes them difficult to control. The mathematics developed over the past centuries for designing rigid bodies generally fail to extend to soft robots. Thus, soft robots are commonly designed in part with the help of automated design tools, such as evolutionary algorithms, which enable a soft robot's shape, material properties, and controller to all be simultaneously and automatically designed and optimized together for a given task.
Bio-mimicry
Plant cells can inherently produce
hydrostatic pressure
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
due to a solute concentration gradient between the cytoplasm and external surroundings (osmotic potential). Further, plants can adjust this concentration through the movement of ions across the cell membrane. This then changes the shape and volume of the plant as it responds to this change in hydrostatic pressure. This pressure derived shape evolution is desirable for soft robotics and can be emulated to create pressure adaptive materials through the use of
fluid flow.
The following equation
models the cell volume change rate:
:
:
is the rate of volume change.
:
is area of the cell membrane.
:
is the
hydraulic conductivity
Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on th ...
of the material.
:
is the change in hydrostatic pressure.
:
is the change in
osmotic potential
Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane.
It is also defined as the measure of the tendency of a solution to take in a pure s ...
.
This principle has been leveraged in the creation of pressure systems for soft robotics. These systems are composed of soft resins and contain multiple fluid sacs with semi-permeable membranes. The semi-permeability allows for fluid transport that then leads to pressure generation. This combination of fluid transport and pressure generation then leads to shape and volume change.
Another biologically inherent shape changing mechanism is that of hygroscopic shape change. In this mechanism, plant cells react to changes in humidity. When the surrounding atmosphere has a high humidity, the plant cells swell, but when the surrounding atmosphere has a low humidity, the plant cells shrink. This volume change has been observed in pollen grains
and pine cone scales.
Similar approaches to hydraulic soft joints can also be derived from
arachnid locomotion, where strong and precise control over a joint can be primarily controlled through compressed hemolymph.
Manufacturing
Conventional manufacturing techniques, such as subtractive techniques like drilling and milling, are unhelpful when it comes to constructing soft robots as these robots have complex shapes with deformable bodies. Therefore, more advanced manufacturing techniques have been developed. Those include Shape Deposition Manufacturing (SDM), the Smart Composite Microstructure (SCM) process, and 3D multi-material printing.
SDM is a type of rapid prototyping whereby deposition and machining occur cyclically. Essentially, one deposits a material, machines it, embeds a desired structure, deposits a support for said structure, and then further machines the product to a final shape that includes the deposited material and the embedded part.
Embedded hardware
An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...
includes circuits, sensors, and actuators, and scientists have successfully embedded controls inside of polymeric materials to create soft robots, such as the Stickybot
and the iSprawl.
SCM is a process whereby one combines rigid bodies of
carbon fiber reinforced polymer
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...
(CFRP) with flexible polymer ligaments. The flexible polymer act as joints for the skeleton. With this process, an integrated structure of the CFRP and polymer ligaments is created through the use of laser machining followed by lamination. This SCM process is utilized in the production of mesoscale robots as the polymer connectors serve as low friction alternatives to pin joints.
3D printing
3D printing or additive manufacturing is the Manufacturing, construction of a three-dimensional object from a computer-aided design, CAD model or a digital 3D modeling, 3D model. It can be done in a variety of processes in which material is ...
can now be used to print a wide range of silicone inks using
Robocasting Robocasting (also known as robotic material extrusion) is an additive manufacturing technique analogous to Direct Ink Writing and other extrusion-based 3D-printing techniques in which a filament of a paste-like material is extruded from a small nozz ...
also known as direct ink writing (DIW). This manufacturing route allows for a seamless production of fluidic elastomer actuators with locally defined mechanical properties. It further enables a digital fabrication of pneumatic silicone actuators exhibiting programmable bioinspired architectures and motions.
A wide range of fully functional soft robots have been printed using this method including bending, twisting, grabbing and contracting motion. This technique avoids some of the drawbacks of conventional manufacturing routes such as delamination between glued parts. Another additive manufacturing method that produces shape morphing materials whose shape is photosensitive, thermally activated, or water responsive. Essentially, these polymers can automatically change shape upon interaction with water, light, or heat. One such example of a shape morphing material was created through the use of light reactive ink-jet printing onto a polystyrene target.
Additionally,
shape memory polymers have been rapid prototyped that comprise two different components: a skeleton and a hinge material. Upon printing, the material is heated to a temperature higher than the
glass transition
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rubb ...
temperature of the hinge material. This allows for deformation of the hinge material, while not affecting the skeleton material. Further, this polymer can be continually reformed through heating.
Control methods and materials
All soft robots require an actuation system to generate reaction forces, to allow for movement and interaction with its environment. Due to the compliant nature of these robots, soft actuation systems must be able to move without the use of rigid materials that would act as the bones in organisms, or the metal frame that is common in rigid robots. Nevertheless, several control solutions to soft actuation problem exist and have found its use, each possessing advantages and disadvantages. Some examples of control methods and the appropriate materials are listed below.
Electric field
One example is utilization of
electrostatic force
Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
that can be applied in:
*
Dielectric Elastomer Actuators (DEAs) that use
high-voltage
High voltage electricity refers to electrical potential large enough to cause injury or damage. In certain industries, ''high voltage'' refers to voltage above a certain threshold. Equipment and Electrical conductor, conductors that carry high ...
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
in order to change its shape
example of working DEA. These actuators can produce high forces, have high specific power (W kg
−1), produce large strains (>1000%), possess high energy density (>3 MJ m
−3), exhibit self-sensing, and achieve fast actuation rates (10 ms - 1 s). However, the need for high-voltages quickly becomes the limiting factor in the potential practical applications. Additionally, these systems often exhibit leakage currents, tend to have electrical breakdowns (dielectric failure follows
Weibull statistics therefore the probability increases with increased electrode area ), and require pre-strain for the greatest deformation.
Some of the new research shows that there are ways of overcoming some of these disadvantages, as shown ''e.g.'' in Peano-HASEL actuators, which incorporate liquid dielectrics and thin shell components. These approach lowers the applied voltage needed, as well as allows for self-healing during electrical breakdown.
Thermal
*
Shape memory polymers (SMPs) are smart and reconfigurable materials that serve as an excellent example of thermal actuators that can be used for actuation. These materials will "remember" their original shape and will revert to it upon temperature increase. For example,
crosslinked polymer
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
s can be strained at temperatures above their
glass-transition (T
g) or melting-transition (T
m) and then cooled down. When the temperature is increased again, the strain will be released and materials shape will be changed back to the original. This of course suggests that there is only one irreversible movement, but there have been materials demonstrated to have up to 5 temporary shapes. One of the simplest and best known examples of shape memory polymers is a toy called
Shrinky Dinks
Shrink art, Shrinky Dinks, or Shrinkles is a toy and activity kit consisting of sheets of polystyrene which can be cut with standard household scissors. When heated, the cut shapes become about nine times thicker while their horizontal and verti ...
that is made of pre-stretched
polystyrene
Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...
(PS) sheet which can be used to cut out shapes that will shrink significantly when heated. Actuators produced using these materials can achieve strains up to 1000% and have demonstrated a broad range of energy density between <50 kJ m
−3 and up to 2 MJ m
−3. Definite downsides of SMPs include their slow response (>10 s) and typically low force generated.
Examples of SMPs include
polyurethane
Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...
(PU),
polyethylene teraphtalate (PET),
polyethyleneoxide (PEO) and others.
*
Shape memory alloys are behind another control system for soft robotic actuation.
Although made of metal, a traditionally rigid material, the springs are made from very thin wires and are just as compliant as other soft materials. These springs have a very high force-to-mass ratio, but stretch through the application of heat, which is inefficient energy-wise.
Pressure difference
*
Pneumatic artificial muscles
Pneumatic artificial muscles (PAMs) are contractile or extensional devices operated by pressurized air filling a pneumatic bladder. In an approximation of human muscles, PAMs are usually grouped in pairs: one agonist and one antagonist.
PAMs wer ...
, another control method used in soft robots, relies on changing the pressure inside a flexible tube. This way it will act as a muscle, contracting and extending, thus applying force to what it's attached to. Through the use of valves, the robot may maintain a given shape using these muscles with no additional energy input. However, this method generally requires an external source of compressed air to function. Proportional Integral Derivative (PID) controller is the most commonly used algorithm for pneumatic muscles. The dynamic response of pneumatic muscles can be modulated by tuning the parameters of the PID controller.
Sensors
Sensors are one of the most important component of robots. Without surprise, soft robots ideally use soft sensors. Soft sensors can usually measure deformation, thus inferring about the robot's position or stiffness.
Here are a few examples of soft sensors:
* Soft
stretch sensors
* Soft bending sensors
* Soft pressure sensors
* Soft force sensors
These sensors rely on measures of:
* Piezoresistivity:
** polymer filled with conductive particles,
** microfluidic pathways (liquid metal, ionic solution),
* Piezoelectricity,
* Capacitance,
* Magnetic fields,
* Optical loss,
* Acoustic loss.
These measurements can be then fed into a
control system
A control system manages, commands, directs, or regulates the behavior of other devices or systems using control loops. It can range from a single home heating controller using a thermostat controlling a domestic boiler to large industrial c ...
.
Uses and applications
Surgical assistance
Soft robots can be implemented in the medical profession, specifically for
invasive surgery
Minimally invasive procedures (also known as minimally invasive surgeries) encompass surgical techniques that limit the size of incisions needed, thereby reducing wound healing time, associated pain, and risk of infection. Surgery by definitio ...
. Soft robots can be made to assist surgeries due to their shape changing properties. Shape change is important as a soft robot could navigate around different structures in the human body by adjusting its form. This could be accomplished through the use of fluidic actuation.
Exosuits
Soft robots may also be used for the creation of flexible exosuits, fo
rehabilitation of patients assisting the elderly, or simply enhancing the user's strength. A team from Harvard created an exosuit using these materials in order to give the advantages of the additional strength provided by an exosuit, without the disadvantages that come with how rigid materials restrict a person's natural movement. The exosuits are metal frameworks fitted with motorized muscles to multiply the wearer's strength. Also called exoskeletons, the robotic suits' metal framework somewhat mirrors the wearer's internal skeletal structure.
The suit makes lifted objects feel much lighter, and sometimes even weightless, reducing injuries and improving compliance.
Collaborative robot
A cobot, or collaborative robot, is a robot intended for direct Human–robot interaction, human-robot interaction within a shared space, or where humans and robots are in close proximity. Cobot applications contrast with traditional industrial rob ...
s
Traditionally, manufacturing robots have been isolated from human workers due to safety concerns, as a rigid robot colliding with a human could easily lead to injury due to the fast-paced motion of the robot. However, soft robots could work alongside humans safely, as in a collision the compliant nature of the robot would prevent or minimize any potential injury.
Bio-mimicry
An application of bio-mimicry via soft robotics is in ocean or space exploration. In the search for extraterrestrial life, scientists need to know more about extraterrestrial bodies of water, as water is the source of life on Earth. Soft robots could be used to mimic sea creatures that can efficiently maneuver through water. Such a project was attempted by a team at Cornell in 2015 under a grant through
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
's Innovative Advanced Concepts (NIAC).
The team set out to design a soft robot that would mimic a
lamprey
Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineage of jawless fish of the order Petromyzontiformes , placed in the superclass Cyclostomata. The adult lamprey may be characterized by a toothed, funnel-like s ...
or
cuttlefish
Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
in the way it moved underwater, in order to efficiently explore the ocean below the ice layer of Jupiter's moon, Europa. But exploring a body of water, especially one on another planet, comes with a unique set of mechanical and materials challenges. In 2021, scientists demonstrated a
bioinspired self-powered soft robot
for deep-sea operation that can withstand the pressure at the deepest part of the ocean at the
Mariana Trench
The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum know ...
. The robot features artificial muscles and wings out of pliable materials and electronics distributed within its silicone body. It could be used for
deep-sea exploration
Deep-sea exploration is the investigation of physical, chemical, and biological conditions on the sea bed, for scientific or commercial purposes. Deep-sea exploration is considered a relatively recent human activity compared to the other areas of ...
and
environmental monitoring
Environmental monitoring describes the processes and activities that need to take place to characterize and monitor the quality of the environment. Environmental monitoring is used in the preparation of environmental impact assessments, as well a ...
. In 2021, a team from Duke University reported a dragonfly-shaped soft robot, termed DraBot, with capabilities to watch for acidity changes, temperature fluctuations, and oil pollutants in water.
Cloaking
Soft robots that look like animals or are otherwise hard to identify could be used for surveillance and a range of other purposes. They could also be used for ecological studies such as amid wildlife. Soft robots could also enable novel artificial camouflage.
Robot components
Artificial muscle
Robot skin with tactile perception
=Electronic skin
=
Qualitative benefits
Benefits of soft robot designs over fully conventional robot designs may be lighter weight -- heavy payloads are expensive to launch -- and increased safety -- robots may work alongside astronauts.
Mechanical considerations in design
Fatigue failure from flexing
Soft robots, particularly those designed to imitate life, often must experience cyclic loading in order to move or do the tasks for which they were designed. For example, in the case of the lamprey- or cuttlefish-like robot described above, motion would require electrolyzing water and igniting gas, causing a rapid expansion to propel the robot forward.
This repetitive and explosive expansion and contraction would create an environment of intense cyclic loading on the chosen polymeric material. A robot in a remote underwater location or on a remote planetary body like Europa would be practically impossible to patch up or replace, so care would need to be taken to choose a material and design that minimizes initiation and propagation of fatigue-cracks. In particular, one should choose a material with a
fatigue limit
The fatigue limit or endurance limit is the stress level below which an infinite number of loading cycles can be applied to a material without causing fatigue failure. Some metals such as ferrous alloys and titanium alloys have a distinct limit, ...
, or a stress-amplitude frequency above which the polymer's fatigue response is no longer dependent on the frequency.
Brittle failure when cold
Secondly, because soft robots are made of highly compliant materials, one must consider temperature effects. The yield stress of a material tends to decrease with temperature, and in polymeric materials this effect is even more extreme.
At room temperature and higher temperatures, the long chains in many polymers can stretch and slide past each other, preventing the local concentration of stress in one area and making the material ductile. But most polymers undergo a
ductile-to-brittle transition temperature
below which there is not enough thermal energy for the long chains to respond in that ductile manner, and fracture is much more likely. The tendency of polymeric materials to turn brittle at cooler temperatures is in fact thought to be responsible for the
Space Shuttle Challenger disaster
On January 28, 1986, the broke apart 73 seconds into its flight, killing all seven crew members aboard. The spacecraft disintegrated above the Atlantic Ocean, off the coast of Cape Canaveral, Florida, at 11:39a.m. Eastern Time Zone, EST (1 ...
, and must be taken very seriously, especially for soft robots that will be implemented in medicine. A ductile-to-brittle transition temperature need not be what one might consider "cold," and is in fact characteristic of the material itself, depending on its crystallinity, toughness, side-group size (in the case of polymers), and other factors.
International journals
* Soft Robotics (SoRo)
* Soft Robotics section of Frontiers in Robotics and AI
*Science Robotics
International events
* 2018 Robosoft, first IEEE International Conference on Soft Robotics, April 24–28, 2018, Livorno, Italy
* 2017 IROS 2017 Workshop on Soft Morphological Design for Haptic Sensation, Interaction and Display, 24 September 2017, Vancouver, BC, Canada
* 2016 First Soft Robotics Challenge, April 29–30, Livorno, Italy
* 2016 Soft Robotics week, April 25–30, Livorno, Italy
* 2015 "Soft Robotics: Actuation, Integration, and Applications – Blending research perspectives for a leap forward in soft robotics technology" at ICRA2015, Seattle WA
* 2014 Workshop on Advances on Soft Robotics, 2014 Robotics Science and Systems (RSS) Conference, Berkeley, CA, July 13, 2014
* 2013 International Workshop on Soft Robotics and Morphological Computation, Monte Verità, July 14–19, 2013
* 2012 Summer School on Soft Robotics, Zurich, June 18–22, 2012
In popular culture
![Inflatable Robotic Arm](https://upload.wikimedia.org/wikipedia/commons/1/10/Inflatable_Robotic_Arm.jpg)
The 2014 Disney film ''
Big Hero 6'' features a soft robot,
Baymax
Baymax is a fictional superhero character appearing in American comic books published by Marvel Comics. Created by Steven T. Seagle and Duncan Rouleau, Baymax first appeared in ''Sunfire & Big Hero 6'' #1 (September 1998). Baymax begins his exis ...
, originally designed for use in the
healthcare industry
The healthcare industry (also called the medical industry or health economy) is an aggregation and integration of sectors within the economic system that provides goods and services to treat patients with curative, preventive, rehabilitative, a ...
. In the film, Baymax is portrayed as a large yet unintimidating robot with an inflated vinyl exterior surrounding a mechanical skeleton. The basis of Baymax concept comes from real life research on applications of soft robotics in the healthcare field, such as roboticist
Chris Atkeson's work at
Carnegie Mellon's Robotics Institute
The Robotics Institute (RI) is a division of the School of Computer Science at Carnegie Mellon University in Pittsburgh, Pennsylvania, United States. A June 2014 article in ''Robotics Business Review'' magazine calls it "the world's best robo ...
.
The 2018 animated Sony film ''
Spider-Man: Into the Spider-Verse'' features a female version of the supervillain
Doctor Octopus
Doctor Octopus (Dr. Otto Gunther Octavius), also known as Doc Ock for short, is a fictional character appearing in American comic books published by Marvel Comics. The character was created by Stan Lee and Steve Ditko and first appeared in ''Th ...
that utilizes tentacles built with soft robotics to subdue her foes.
In episode 4 of the animated series ''
Helluva Boss
''Helluva Boss'' is an American adult animated web series created by Vivienne "VivziePop" Medrano. It revolves around the misadventures of the employees of I.M.P., an assassination company in Hell. The pilot was released on November 25, 2019 ...
'', inventor Loopty Goopty uses tentacles with soft robotics tipped with various weapons to threaten the members of the I.M.P into murdering his friend, Lyle Lipton.
See also
*
Articulated soft robotics The term “soft robots” designs a broad class of robotic systems whose architecture includes soft elements, with much higher elasticity than traditional rigid robots. Articulated Soft Robots are robots with both soft and rigid parts, inspired to ...
*
Octobot (robot) Octobot is a proof of concept soft-bodied autonomous robot. The project team included Harvard University faculty members Robert Wood and Jennifer A. Lewis, along with researchers with backgrounds in mechanical engineering, 3D printing, microfluid ...
*
Bio-inspired robotics
Bio-inspired robotic locomotion is a fairly new subcategory of bio-inspired design. It is about learning concepts from nature and applying them to the design of real-world engineered systems. More specifically, this field is about making robots th ...
*
Bionics
Bionics or biologically inspired engineering is the application of biological methods and systems found in nature to the study and design of engineering systems and modern technology.
The word ''bionic'', coined by Jack E. Steele in August 1 ...
*
Biorobotics
Biorobotics is an interdisciplinary science that combines the fields of biomedical engineering, cybernetics, and robotics to develop new technologies that integrate biology with mechanical systems to develop more efficient communication, alter g ...
*
Home robot
A domestic robot is a type of service robot, an autonomous robot that is primarily used for household chores, but may also be used for education, entertainment or therapy. While most domestic robots are simplistic, some are connected to Wi-Fi ...
*
Robotic materials
*
Soft Growing Robotics
*
Magnetic slime robot
*
Wetware computer
A wetware computer is an organic computer (which can also be known as an artificial organic brain or a neurocomputer) composed of organic material " wetware" such as "living" neurons. Wetware computers composed of neurons are different than conve ...
*
Biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...
*
Robotic sensing
Robotic sensing is a subarea of robotics science intended to give robots sensing capabilities. Robotic sensing mainly gives robots the ability to see,Roh SG, Choi HR (Jan 2009).3-D Tag-Based RFID System for Recognition of Object" IEEE Transaction ...
External links
Soft Robot - A Review (Elveflow)Dielectric elastomer actuators (softroboticstoolkit.com)
References
{{Robotics
Robotics
Robot kinematics
Biorobotics
Articles containing video clips