The emerging field of soft robotics aims to improve robot/human interactivity promising to bring robots into all aspects our daily lives, including wearable robotics, surgical robots, micromanipulation, search and rescue, and others. Soft robotics differ from traditional counterparts in some important ways: Soft robots have little or no hard internal structures. Instead they use a combination of muscularity and deformation to grasp things and move about. Rather than using motors, cables or gears, soft robots are often animated by pressurized air or liquids. In many cases soft robotics designs mimic natural, evolved biological forms hence also called bio-inspired robots. This, combined with their soft exteriors, can make soft robots more suitable for interaction with living things or even for use as human exoskeletons. Ultimately, soft robots can perform various tasks beyond the limits of conventional robots, achieving instinctive characteristics in terms of safe for humans, geometric adaptation, and tunable camouflage.
For consolidating and pushing forward the abilities of soft robotic components and systems, a deep effort is necessary towards an advancement of materials chemistry, mechanics, as well as novel fabrication techniques for systems development and robust materials interfacing.
The secret of natural systems lies in the smart characteristics of how their body is designed and in how their intelligence is embodied and distributed in it, allowing them to effectively adapt, grow and survive. And here lays the link with soft robotics: similarly, soft robots benefit of the use of smart and multi-functional materials (gels, elastomers, biological materials, etc) and of a body compliant with the external environment. Intelligence is also integrated in the body (’embodied’) and co-develops with it, emerging from the interaction of the body itself with the world. This way, soft robots’ sensory-motor behavior becomes more efficient in responding to dynamic changes, as for living organisms.
Soft robotics has made leaps and bounds over the last decade as researchers around the world have experimented with different materials and designs to allow once rigid, jerky machines to bend and flex in ways that mimic and can interact more naturally with living organisms. A number of different physical mechanisms and actuator architectures have been reported in the last decade, in particular for powering soft robots.
Using soft, multi-functional, material structures for the design of new forms of actuation and sensing, together with new modeling and control architectures, soft robots have been allowed to achieve different abilities such as jumping, peristaltic locomotion, elongation and shortening, climbing, stretchability, adaptable grasping, combined bending and stiffening, flying, self-healing (SH), self-morphing and growing
Artificial muscles are the core components of the smart and interactive soft robotic systems, providing the capabilities in shape morphing, manipulation, and mobility. Traditional actuation technologies with electromagnetic motors and combustion engines have been the major approaches utilized to drive robotics and prosthetic devices. Though they can provide superior energy outputs (e.g., ≈10 kW kg–1 for jet engines) with excellent mechanical stabilities, their rigid, bulky, and heavy structures have become the limitations to achieve dexterous shape morphing, soft architecture, lightweight, and small footprint in the smart and interactive soft robotics.
Hydraulics and pneumatics actuators
The artificial muscles that move soft robots, called actuators, tend to rely on hydraulics or pneumatics, which are slow to respond and difficult to store. Existing soft actuator technologies are typically based on pneumatic or hydraulic inflation of elastomer skins that expand when air or liquid is supplied to them.
Power Soft Robotics
Suzumori proposed a new field in soft robotics, Power Soft Robotics. Many examples that show both power and flexibility can be found in nature. The elephant trunk deforms very flexibly while it works very powerfully, giraffes fight by attacking each other’s necks while the necks work flexibly at the same time to absorb the dynamic energy to protect themselves, and snakes kill animals that are bigger than their body by strangling them. Suzumori believes that power soft robotics is one of the most promising new fields in soft robotics, by which powerful and gentle rescue robots can be realized.
A key component in realizing Power Soft Robotics is a powerful and soft actuator. In particular, the hydraulic McKibben muscle is one of the most promising actuators currently in use. The McKibben muscle is one of the oldest artificial muscles and is well-known and widely used today. It consists of a rubber tube and braided cords around the tube. Applying fluid pressure to the rubber tube results in the expansion of the rubber tube in the radial direction, changing the blading angle of the bladed cords and causing the muscle to contract in the axial direction. Most specimens are driven pneumatically with air pressures lower than 0.7 MPa. Pneumatics higher than 0.7 MPa is not generally used because highly compressed air easily stores big energy in it, which would have potential dangers such as explosion and instantaneous unstable motion of actuator, while hydraulics can generally use a much higher working pressure, i.e. 7–35 MPa because of high incompressibility of water/oil. Applying high-pressure hydraulics to the McKibben muscles has great potential for pioneering Power Soft Robotics.
There are still several major issues and challenges. One issue is the material of the rubber tube, which requires physical strength and good elasticity: resistance to high pressure and large deformation ability, durability with respect to repeated deformations, small non-linearity, small hysteresis, and small creep behavior. In the case of oil hydraulics, oil resistance is also essential. In addition, chemical resistance to weatherability is also very important. For example, resistance to ultraviolet light or low/high temperature is required in some environments where power soft robots will work. Because soft materials used in soft robots show generally poorer performance with respect to physical/chemical durability in comparison with stiff materials used in conventional robotics, material science should play a large role in soft robotics.
Dielectric elastomers, soft materials that have good insulating properties, could offer an alternative to pneumatic actuators but they currently require complex and inefficient circuitry to deliver high voltage as well as rigid components to maintain their form— both of which defeat the purpose of a soft robot. Most dielectric elastomers have limited range of motion and need to be pre-stretched and attached to a rigid frame
Electro-fluidic actuators can be classified as follows: 1) capacitive electro-fluidic actuators and 2) resistive EHD actuators. Capacitive actuators use a compliant capacitor configuration: electrical forces between charges deform the system into a new equilibrium configuration, which depends on the applied voltage. Under a DC field, charges do not flow (except for leakage currents). AC driving is required for continuous power generation. Dielectric actuators, such as HASELS and HAXELS, zipping electro-ribbons, and devices based on dielectrophoresis and electrowetting are all capacitive electro-fluidic actuators. On the contrary, resistive EHD actuators generate power through a continuous flow of charges. Electrodes pairs are exposed to the dielectric liquid. Ions are formed and discharged at the electrodes, creating a continuous flow of charges. They work under DC fields. Examples are ECF actuators, EHD conduction, and injection pumps, including recently developed EHD stretchable pumps.
The external compressors and pressure-regulating equipment required for such technologies prevent miniaturization and the creation of robots that can move and work independently. Moreover increased flexibility and dexterity has a trade-off of reduced strength, as softer materials are generally not as strong or resilient as inflexible ones, which limits their use.
Natural muscles provide salient features of large stress of ≈0.35 MPa, high conformability with strain above ≈20%, high peak output power of 323 W kg–1, excellent mechanical stability with billions of repetitive contraction cycles, and utilization of renewable biological fuel energy with high efficiency (≈40%). To enable artificial muscles which can reproduce similar functionalities or even surpass the performances of their natural counterparts in some aspects is the major goal for the development of artificial muscles and their deployments in real‐world applications.
Artiﬁcial Muscles with Tethered Power Sources
Artiﬁcial muscles utilizing electric power and compressed ﬂuids required the tethered electrical wires or ﬂuid tubes to couple the energy inputs to the devices. Among them, dielec-tric elastomer actuators, electrohydrodynamic actuators, elec-trochemical actuators, and the ﬂuidic elastomer actuators are the most investigated devices. With the advantages of relatively high power output and high energy efficiency, artiﬁcial muscles with tether are preferable for applications in cooperative robotic systems in manufacturing, human–computer interfacing devices, and industrial automation etc. where high power output and high energy efficiency are important concerns.
It is possible to integrate onboard power sources for these devices and release cords in the systems to realize untethered applications. For instance, photovoltaic array can be integrated to generate electric power from solar energy to drive an untethered microscale aerial vehicle.On board electric power system can amplify the voltage output from battery with miniature high‐voltage converter to drive the DEA devices for untethered operations.
Actuators for untethered soft robots
In 2014, Researchers from U.S. Department of Energy’s (DOE) Lawrence Berkeley National Laboratory (LBL), fabricated a micro-muscle, a long “V-shaped” bimorph ribbon comprised of chromium and vanadium dioxide on a silicon substrate. A nano-scale muscle, thousand times more power than a human muscle, was able to catapult objects 50 times heavier than itself over a distance five times its length within 60 milliseconds, developed using vanadium dioxide.
Researchers at Columbia Engineering have solved a long-standing issue in the creation of untethered soft robots whose actions and movements can help mimic natural biological systems. A group in the Creative Machines lab led by Hod Lipson, professor of mechanical engineering, has developed a 3D-printable synthetic soft muscle, a one-of-a-kind artificial active tissue with intrinsic expansion ability that does not require an external compressor or high voltage equipment as previous muscles required. The new material has a strain density (expansion per gram) that is 15 times larger than natural muscle, and can lift 1000 times its own weight.
“We’ve been making great strides toward making robots minds, but robot bodies are still primitive,” said Hod Lipson. “This is a big piece of the puzzle and, like biology, the new actuator can be shaped and reshaped a thousand ways. We’ve overcome one of the final barriers to making lifelike robots.”
Despite that challenges still exist to realize muscle‐like performances which can be compared to that of the natural muscles in all aspects, artificial muscles have been able to surpass the performance of their natural counterparts in some particular properties. For instance, dielectric elastomer actuators (DEA) are capable of producing strains of >300%; thermal responsive coiled polymer fibers can achieve an impressive specific power of 27.1 kW kg–1 which is 84 times of the peak output power from natural muscle; pressurized fluid actuator can be programmed to transform into complex 3D texture and imitate natural stones and plants; soft magnetic actuators with small feature size can provide multiple locomotive modes of swimming, diving, walking, jumping, and crawling.
Characterizing and predicting the behavior of soft multi-material actuators is challenging due to the nonlinear nature of both the hyper-elastic material and the large bending motions they produce. Key challenges in the design and manufacture of soft robots include the complex fabrication processes and the interfacing of soft and rigid components.
While the initial investigations on artificial muscles have been focused on enabling soft actuators with improved mechanical performances, there is a clear shift in recent years to integrate soft functional electronic devices, including the sensing devices which can perceive external stimulus such as strain, pressure, and temperature etc. and the responding device which can provide interactive feedbacks to the users such as emissive surfaces, color changes, and acoustic outputs etc., to impart mechanical intelligence in the soft actuators.
The recent development in soft electronics has played a prominent role to circumvent the mechanical mismatch dilemma between the flat, rigid nature of conventional electronic devices and the curved, deformable surface of the soft actuators, promoting the integration of functional electronic components with artificial muscles to enable interactive soft robotic systems with bidirectional functionalities of actively sensing and responding.
Researchers, Jiangxin Wang, Dace Gao, and Pooi See Lee, have categorized the artiﬁcial muscles into tethered devices and devices without tether based on the kinds of the energy that are coupled to drive the artiﬁcial muscles and the tethered and untethered devices as they also have different targeted applications.
Electro-responsive elastomeric actuators
Dielectric Elastomer Actuators (DEA)
Among Electromechanically Active Polymers, which are ‘smart materials’ that deform in response to electrical stimuli, dielectric elastomer (DE) actuators show the greatest potential for soft mechatronics. They are capable of large electrically-induced strains and stresses, in some cases exceeding those of mammalian muscles.
As compared to electromagnetic motors, DE actuators (DEAs) have low specific weight, high energy density, fast response, self-sensing ability, scalable performance, low power consumption, and silent operation, as well as intrinsic mechanical compliance, which can also be electrically controlled
DEA has been intensively studied as a promising candidate for artificial muscle with its simple architecture, fast response, large strain output, and soft structure. The working mechanism of the DEA was based on the Maxwell stress p under high electric bias p = ε sq(E). Due to the operation mechanism of the DEA devices, their operation voltages are typically very high (in the range of kV) and the mechanical outputs are primarily in‐plane expansions. Further development in the approaches to reduce the operating voltages and couple the mechanical outputs to out‐of‐plane actuation is required.
Most of the active materials are made of highly deformable elastomers with low Young’s modulus, such as polyacrylates, polysiloxanes, and polyurethanes, giving large strains under the Maxwell stress. The dielectric constant and electrical strength of the elastomers are important factors for the selection of the material as the Maxwell stress is directly proportional to the dielectric constant of the elastomers and the electric field applied, which is limited by the electrical breakdown strength of the materials.
While it has been widely accepted to use solid materials as the dielectric layer in the DEA devices, recent works by Keplinger’s group have demonstrated a promising alternative material with liquid dielectric to achieve out‐of‐plane actuations with exceptional mechanical output. Under electrical bias, the electrostatic force (Maxwell stress) applied on the elastomeric shells can be coupled to the liquid dielectric encapsulated inside the soft device structure, converting the electrostatic force into hydraulic pressure with muscle‐like actuation behaviors.
Due to their large electrical deformability, DEAs are studied as ‘artificial muscles’, especially for bioinspired machines. Examples include robotic fingers, grippers and arms, tuneable lenses for robotic vision, as well as terrestrial (legged, crawling and hopping), aerial and underwater robots
The soft muscle‐like actuator can be scaled up to provide impressive force output. For instance, by combining six planar HASEL actuators, the device could lift a gallon of water (≈4 kg). In addition, while solid dielectric materials will easily form permanent leakage channel after electrical breakdown, the liquid dielectrics can immediately recover to the insulating state with their electrical self‐healing capability from the high mobility of the breakdown induced bubbles in the liquid materials.
The relatively high operating voltage is a main challenge, which will need to be carefully addressed in the further development of DEAs. High voltages can cause critical safety issues, especially when these devices are to be used in the interfacing systems with human bodies. A good insulating layer of the devices and limiting the applied currents below the human‐safety threshold (the ventricular fibrillation threshold current is <30 mA.
Due to non-linearities deriving from their hyper-visco-elastic properties, DEAs require special control strategies. High‐voltage operation will raise additional difficulties in the power source and control circuits for the systems. Promising solutions have been reported to resolve the problems. For instance, additional on‐board high‐voltage converter could be used to amplify the small voltage from a battery to high‐voltage outputs in the kV range, enabling the DEA operations without the requirement of the bulky high‐voltage power supply systems. Shea’s group has recently developed high‐voltage thin‐film transistors (HVTFTs) which can be used to drive the DEA devices. This offers an ideal solution to address and control the individual DEA device in the integrated systems with a number of actuators for soft robotics, tactile displays, and haptic interfaces.
In order to actuate systems that have a size in the millimeters-centimeters range and have to be compact, lightweight and energy efficient, DEAs have significant advantages over conventional actuators. However, when it comes to large-scale and large-force systems, DEAs cannot compete with traditional electromagnetic, pneumatic or hydraulic drives.
Electrohydrodynamic and Electroaerodynamic Actuators
Apart from DEAs, electrohydrodynamics (EHD) and electroaerodynamics (EAD) are also representative devices operating under high electrical biases with attractive mechanical performances. The underlying mechanism is to utilize the effects from high electrical biases (several kV) to induce dynamic motions in a fluidic or gaseous medium, converting the electrical energy into mechanical outputs.
In a conventional EHD actuator, a needle electrode is used for charge injection and a counter electrode with larger dimension is used to collect the free charges. The sharp needle electrode is specially used to accumulate high density charges at the edge of the needle, making it easy to induce free charges into the dielectric fluids and cause the EHD flows.
The EAD process is a promising alternative for the propelling systems in aerial vehicles with its simple and lightweight architecture, silent operation, and no requirement of moving components. Xu et al. has recently demonstrated an electrical solid‐state propelling system with the EAD process, which was consisted of an asymmetric electrode pair (one small electrode for ionization of the air and one large electrode for ion collection). The EAD engine provided a thrust‐to‐power ratio of 5 N kW–1, comparable to the conventional jet engine (3 N kW–1), which could be used to achieve a 40–50 m flight within 8–9 s on a fixed‐wing aeroplane.
Despite that the device demonstrated are still based on a rigid system structure, it shows great benefits from the solid‐state EAD actuation system. It is believed that the development of soft EAD actuators deserves significant research efforts, which can ultimately provide the technology to enable the “flight muscles” in miniature and soft aero robotic systems.
Compared to the devices discussed above, electrochemical actuators operate with a much lower voltage which is typically below 10 V. The working mechanism of the electrochemical actuators is based on the reversible ion migration at the electrode/electrolyte interface and/or in the electrodes via double‐layer formation, ion intercalation/deintercalation, or Faradic reaction. Their light weight structure, low working voltage, ease of fabrication, and fair strain output are the critical features which make them promising candidates for soft artificial muscles.
Ionic‐polymer–metal composites (IPMC) are representative electrochemical actuators, which consist of a simple device architecture with two thin electrode layers, an ion exchange membrane, and an electrolyte layer. The IPMC can operate with the low voltage of 1–5 V. Before the large deployments of the electrochemical actuators, their difficulties of poor cycling stability and low energy converting efficiency will need to be addressed. Active materials, which have excellent electrochemical cycling performance and efficient structural response to the electrochemical process, are the promising candidates.
In recent research, massive efforts have been dedicated to the exploration of nanomaterials, especially 2D nanomaterials as the active materials for the electrochemical actuators. 2D materials have the unique layer structures and high surface to volume ratios, which provide advantages for ion migration at the electrode/electrolyte interface and in the electrodes.
Nanomaterials with other structures, such as the 1D nanowires are also of great interest. Cheng et al. reported nickel nanowire‐forest as the active materials with electrochemical actuation by inducing strains in the nanowires with the oxidation/reduction process . The actuation can be triggered by the voltage within a range of 0.6 V with a relatively fast response time of 0.1 s.
Fluidic Elastomer Actuators (FEA)
Different from the actuators working with tethered electrical powers, the fluidic elastomer actuators (FEA), which use the external connected pressurized fluids to induce actuation of the deformable chambers, are among the most developed muscle‐like actuation technologies. FEAs possess the appealing characteristics of simple operation mechanism, inherent compliant device structures, high mechanical outputs (≈80 J g−1 for compressed air), and scalable manufacturing. Commercial products have been available for manufacturing and collaborative soft robots from the companies such as Soft Robotics Inc. and SoftGripping.
The efforts to develop the FEAs have been mainly focused on the designs of materials or structures to achieve controllable shape morphing behaviors. As the mechanical outputs are resulted from the materials deformation under the pressurized fluids, their mechanical properties including the Young’s modulus, stretchability, and mechanical strength are the important factors deciding the device performances.
A critical challenge in the FEAs is their requirement of bulky compressing systems or reservoirs to supply the pressurized fluid, which limits their practical applications in the development of fully compliant, small‐scale, and lightweight robotic systems. Recent research efforts have showed promising progress to tackle this issue by combining with other mechanisms to relieve the FEAs from the requirement of bulky peripheral components. For instance, Miriyev et al. reported the embedding of phase change materials (e.g., ethanol) into the polymer matrix (e.g., poly(dimethylsiloxane) (PDMS) elastomer) to achieve a self‐contained soft actuating composite.
Artificial Muscles with Untethered Power Sources
Artificial muscles with untethered structures are advantageous to enable soft robotic systems with high maneuverability in the complex and unstructured environments. The energy can be coupled to the soft actuators by magnetic field, light, thermal radiation, and chemicals. Though these devices generally showed a relatively lower power efficiency compared to that of the devices with tether due to the limitation in the energy coupling efficiency, they have promising applications in untethered systems with their high maneuverability, which is important in biomedical devices, small‐scale robotic systems operating in enclosed and confined spaces, and lightweight aerial vehicles etc.
Wireless powering and control of miniaturized robots has been demonstrated using magnetic fields. Recent efforts demonstrated the deformation of soft-bodied robots, built by dispersing magnetic powders into elastomers. However, magnetic actuators require large, expensive electromagnets and high electrical currents (≈10 A), leading to complex, high-cost, and energy-hungry systems. Acoustic fields can also be used to move and position objects wirelessly. Acoustic levitation offers high precision and freedom in selecting the object’s materials, but still requires a large number of ultrasonic transducers and generates low forces.
Magnetic actuators work with the interaction between the magnetization of the active materials with the external magnetic fields. Alignment of the magnetization of the active materials to the applied magnetic fields generates mechanical torque to deform the shapes of the devices. Magnetic actuators have attracted special interest for untethered applications by the virtue that shape transformations in the devices can be directly and effectively manipulated by the magnetic field, which is capable of penetrating many materials and can be spatially programmed. The advantage is critical for applications in an enclosed and confined spaces, such as biomedical applications in targeted drug delivery, invasive surgery, and cell manipulation.
A simple and effective approach to fabricate soft magnetic actuators is by embedding magnetic particles into elastomeric polymer matrix, which can then be magnetized by a high magnetic field. By control of the magnetization gradients in the composites, complex and dexterous shape morphing behaviors can be achieved in the devices.
Thermally Actuated Polymers
Thermally triggered actuations have been achieved using many different mechanisms, including thermal expansion of the polymers, humidity change in the materials by the thermal effect, and phase changes in the liquid crystal polymers, shape memory polymers, and shape memory alloys. In spite that some of the studies have demonstrated tethered thermal actuators with the thermal effects induced by electrothermal heating, interesting untethered devices have been reported with the thermal effects induced by direct heating, photothermal heating, and inductive heating.
Artificial muscles based on thermally actuated polymers have been limited to small strains due to their low thermal expansion coefficients. Development of material systems with high thermal expansion coefficients and structures to effectively enhance the mechanical output from the thermal expansion effects is critical to improve the device performance. Soft thermal actuators based on highly twisted fibers have gained extreme interest with their excellent mechanical output performance (5.26 kW kg–1, comparable to the work output of a jet engine and 100 times output of the human muscle)
Polymer Embedded With Metallic Nanoparticles Enables Soft Robotics
Researchers at North Carolina State University (NCSU), in Raleigh, have developed a technique through movement can be induced into polymer through application of magnetic field by embedding nanoparticles of magnetite—an iron oxide—into a polymer.
“Using this technique, we can create large nanocomposites, in many different shapes, which can be manipulated remotely,” said Sumeet Mishra, lead author of the paper, in a press release. “The nanoparticle chains give us an enhanced response, and by controlling the strength and direction of the magnetic field, you can control the extent and direction of the movements of soft robots.”
In research described in a paper published in the journal Nanoscale, the NCSU researchers describe a process that starts with dispersing the nanoparticles in a solvent. Next, a polymer is dissolved into the mixture and the resulting fluid is poured into a mold. Then a magnetic field is applied that arranges the magnetite nanoparticles into parallel chains. Once the solution dries in the mold, the chains of nanoparticles are locked into place.
“The key here is that the nanoparticles in the chains and their magnetic dipoles are arranged head-to-tail, with the positive end of one magnetic nanoparticle lined up with the negative end of the next, all the way down the line,” said Joe Tracy, an associate professor at NCSU and corresponding author of the paper, in the press release. “When a magnetic field is applied in any direction, the chain re-orients itself to become as parallel as possible to the magnetic field, limited only by the constraints of gravity and the elasticity of the polymer.”
Artificial muscles made from plastic
In 2019, The Army’s corporate research laboratory hosted a professor from Florida A&M University-Florida State University College of Engineering to collaborate on chemically-powered artificial muscles for the future Soldier. They studied how plastic fibers respond when they are twisted and coiled into a spring and found that different stimuli cause the spring to contract and expand, mimicking natural muscles.
The team’s expertise in polymer science and chemical engineering helped to identify optimal material property values to achieve the desired artificial muscle performance targets, and helped develop and implement techniques to measure those material properties.
According to the researchers, artificial muscles have several potential benefits to the Army, but more specifically to the Soldier. “They are needed to enable quiet, efficient artificial mules to support Soldiers in terrain where wheeled vehicles cannot go,” Hallinan said. “The added benefit will be to lighten the heavy load that Soldiers are currently required to carry. They could also potentially be used to augment human performance, allowing Soldiers to run faster, jump farther and wear heavier body armor to decrease fatalities. Perhaps most interesting is their possible use in active prosthetic devices.”
Discovery could lead to self-propelled robots, reported in feb 2021
Researchers at the University of Massachusetts Amherst discovered how to make materials that snap and reset themselves, only relying upon energy flow from their environment. This research, published in Nature Materials and funded by the U.S. Army, could enable future military robots to move from their own energy. Researchers uncovered the physics during a mundane experiment that involved watching a gel strip dry. The researchers observed that when the long, elastic gel strip lost internal liquid due to evaporation, the strip moved. Most movements were slow, but every so often, they sped up.
“This work is part of a larger multi-disciplinary effort that seeks to understand biological and engineered impulsive systems that will lay the foundations for scalable methods for generating forces for mechanical action and energy storing structures and materials,” said Dr. Ralph Anthenien, branch chief, Army Research Office, an element of the U.S. Army Combat Capabilities Development Command, now known as DEVCOM, Army Research Laboratory. “The work will have myriad possible future applications in actuation and motive systems for the Army and DOD.” Scientists discover how to make materials that snap and reset themselves, only relying upon energy flow from their environment. This research could enable future military robots that are able to move off their own energy.
These faster movements were snap instabilities that continued to occur as the liquid evaporated further. Additional studies revealed that the shape of the material mattered, and that the strips could reset themselves to continue their movements.”Many plants and animals, especially small ones, use special parts that act like springs and latches to help them move really fast, much faster than animals with muscles alone,” said Dr. Al Crosby, a professor of polymer science and engineering in the College of Natural Sciences, UMass Amherst. “Plants like the Venus flytraps are good examples of this kind of movement, as are grasshoppers and trap-jaw ants in the animal world.” Snap instabilities are one way that nature combines a spring and a latch and are increasingly used to create fast movements in small robots and other devices as well as toys like rubber poppers.
Breakthrough gives artificial muscles superhuman strength
UOW researchers from ACES joined with international partners from the U.S., China and South Korea to develop sheath-run artificial muscles (SRAMs), that can be used to create intelligent materials and fabrics that react by sensing the environment around them.
It builds on the work over the past 15 years by researchers from UOW and their international colleagues who have invented several types of strong, powerful artificial muscles using materials ranging from high-tech carbon nanotubes (CNTs) to ordinary fishing line.
The latest version of the muscles feature a sheath around a coiled or twisted yarn, which contracts (or “actuates”) when heated, and returns to its initial state when cooled. The outside sheath is like a close-fitting sock and absorbs energy to drive actuation of the muscle. The muscles can also operate by absorbing moisture from their surroundings.
The new SRAMs are made from common natural and man-made fibres, such as cotton, silk, wool and nylon, which are cheap and readily available. ACES Chief Investigator Senior Professor Geoffrey Spinks said the team wanted to improve upon its previous artificial muscle work, which relied on coiling and twisting more sophisticated materials like carbon nanotube (CNT) yarn.
“While there’s no doubt carbon nanotubes make wonderful artificial muscles, CNT is also a very expensive product. Our latest work utilises inexpensive, commercially available yarns with a CNT polymer coating for the sheath,” Professor Spinks said.
“Previously, we were applying energy to the entire muscle, but only the outer part of the fibre was responsible for actuation. By placing a sheath on the muscle, we can focus only that energy on the outer part of the fibre, and convert this input energy more quickly and efficiently.” ARC-DECRA Fellow and lead Australian researcher Dr. Javad Foroughi explained that the application possibilities for SRAMs are diverse.
“When we talk artificial muscles, we’re not just talking about a technology as a replacement for muscles in the body. These muscles offer some exciting opportunities for technologies where the artificial muscles intelligently actuate by sensing their environment,” Dr. Foroughi said.
“Picture these muscles being woven into comfort-adjusting textiles that cool in summer and warm in winter depending on their exposure to temperature, moisture (like sweat), and sunlight, or as smart controlled drug release devices for localised drug delivery through the actuation of valves that control the flow of liquids depending on their chemical composition or temperature.”
Variable stiffness technologies for soft robotics
The ability of varying the stiffness of a material or of a mechanism in a robot originates as a direct consequence of the use of soft and compliant materials in soft robotics. While on one side the use of soft materials enables a series of unprecedented capabilities, in general it also limits the maximum force that the robot can exchange with the environment. Excluding those cases where deformability, dexterity and intrinsic safety are important and sufficient for the intended task (e.g. morphing, inspection), usually it is necessary that the soft/compliant systems vary their stiffness. Tunable stiffness can be used to improve stability and force application, but it can also enable selective reconfigurability and variable kinematics (blocking/releasing degrees of freedom).
A stiffness variation can be obtained in three ways:
- Stiffening by deformation. An active soft actuator usually exploits its deformation to generate mechanical work and this deformation, in turn, causes a material stiffening;
- Stiffening by antagonistic activation. Active actuation technologies combined in pairs so that their action is equal and opposite to each other results in no deformation of the overall system, but with increase of stiffness;
- Stiffening (or softening) by variation of intrinsic properties. There exist some physical principles that can be exploited to develop semi-active actuation technologies.
The third option is gaining increasing attention, because it potentially provides higher compactness and it is decoupled from deformation. Moreover, these technologies can be also used as recoverable mechanical fuses and for mechanical energy dissipation. This group includes: shape memory materials (SMMs), electro- or magneto-rheological fluids (ERFs or MRFs), jamming transition (JT) and low melting-point materials (LMPMs). Note that, all these technologies increase the stiffness upon activation, except the last one, which on contrary softens.
EPFL’s Reconfigurable Robotics Lab (RRL) develops model of soft actuators
Soft robots, powered by muscle-like actuators, are designed to be used on the human body in order to help people move. They are made of elastomers, including silicon and rubber, and so they are inherently safe. They are controlled by changing the air pressure in specially designed ‘soft balloons’, which also serve as the robot’s body. EPFL’s Reconfigurable Robotics Lab (RRL) has developed a predictive model that can be used to carefully control the mechanical behavior of the robots’ various modules has been published in Scientific Reports.
We conducted numerous simulations and developed a model for predicting how the actuators deform as a function of their shape, thickness and the materials they’re made of,” said Gunjan Agarwal, the article’s lead author. “Elastomer structures are highly resilient but difficult to control. We need to be able to predict how, and in which direction, they deform. And because these soft robots are easy to produce but difficult to model, our step-by-step design tools are now available online for roboticists and students.”
A rehabilitation belt
In addition to these simulations, other RRL researchers have developed soft robots for medical purposes. This work is described in Soft Robotics. One of their designs is a belt made of several inflatable components, which holds patients upright during rehabilitation exercises and guides their movements.
“We are working with physical therapists from the University Hospital of Lausanne (CHUV) who are treating stroke victims,” said Matthew Robertson, the researcher in charge of the project. “The belt is designed to support the patient’s torso and restore some of the person’s motor sensitivity.”
The belt’s soft actuators are made of pink rubber and transparent fishing line. The placement of the fishing line guides the modules’ deformation very precisely when air is injected. “For now, the belt is hooked up to a system of external pumps. The next step will be to miniaturize this system and put it directly on the belt,” said Robertson.
Adaptable and reconfigurable robots
Potential applications for soft actuators don’t stop there. The researchers are also using them to develop adaptable robots that are capable of navigating around in cramped, hostile environments. And because they are completely soft, they should also be able to withstand squeezing and crushing.
“Using soft actuators, we can come up with robots of various shapes that can move around in diverse environments,” said Paik. “They are made of inexpensive materials, and so they could easily be produced on a large scale. This will open new doors in the field of robotics.”
Spider Silk Could Be Used As Robotic Muscle
Spider silk, already known as one of the strongest materials for its weight, turns out to have another unusual property that might lead to new kinds of artificial muscles or robotic actuators, researchers have found. Researchers recently discovered a property of spider silk called supercontraction, in which the slender fibers can suddenly shrink in response to changes in moisture. The new finding is that not only do the threads contract, they also twist at the same time, providing a strong torsional force. “It’s a new phenomenon,” Buehler says.
The researchers were able to decode the molecular structure of the two main proteins, that make up spider dragline silk. Spider dragline silk is a protein fiber,” Liu explains. “It’s made of two main proteins, called MaSp1 and MaSp2.” The proline, crucial to the twisting reaction, is found within MaSp2, and when water molecules interact with it they disrupt its hydrogen bonds in an asymmetrical way that causes the rotation. The rotation only goes in one direction, and it takes place at a threshold of about 70 percent relative humidity.
The findings were reported in March 2019 in the journal Science Advances, in a paper by MIT Professor Markus Buehler, head of the Department of Civil and Environmental Engineering, along with former postdoc Anna Tarakanova and undergraduate student Claire Hsu at MIT; Dabiao Liu, an associate professor at Huazhong University of Science and Technology in Wuhan, China; and six others.
“Silk’s unique propensity to undergo supercontraction and exhibit a torsional behavior in response to external triggers such as humidity can be exploited to design responsive silk-based materials that can be precisely tuned at the nanoscale,” says Tarakanova, who is now an assistant professor at the University of Connecticut. “Potential applications are diverse: from humidity-driven soft robots and sensors, to smart textiles and green energy generators.”
These researchers “have used silk’s known high sensitivity to humidity and demonstrated that it can also be used in an interesting way to create very precise torsional actuators,” says Yonggang Huang, a professor of civil and environmental engineering and mechanical engineering at Northwestern University, who was not involved in this work. “Using silk as a torsional actuator is a novel concept that could find applications in a variety of fields from electronics to biomedicine, for example, hygroscopic artificial muscles and humidity sensors,” he says.
“This could be very interesting for the robotics community,” Buehler says, as a novel way of controlling certain kinds of sensors or control devices. “It’s very precise in how you can control these motions by controlling the humidity.” “This is a fantastic discovery because the torsion measured in spider dragline silk is huge, a full circle every millimeter or so of length,” says Pupa Gilbert, a professor of physics, chemistry, and materials science at the University of Wisconsin at Madison, who was not involved in this work. Gilbert adds, “This is like a rope that twists and untwists itself depending on air humidity. The molecular mechanism leading to this outstanding performance can be harnessed to build humidity-driven soft robots or smart fabrics.”
“The protein has a rotational symmetry built in,” Buehler says. And through its torsional force, it makes possible “a whole new class of materials.” Now that this property has been found, he suggests, maybe it can be replicated in a synthetic material. “Maybe we can make a new polymer material that would replicate this behavior,” Buehler says.
Spider silk is already known for its exceptional strength-to-weight ratio, its flexibility, and its toughness, or resilience. A number of teams around the world are working to replicate these properties in a synthetic version of the protein-based fiber.
Self-contained soft actuator three times stronger than natural muscle, without the need of externals
In 2017, Researchers at Columbia Engineering reported to have solved a long-standing issue in the creation of untethered soft robots whose actions and movements can help mimic natural biological systems. A group in the Creative Machines lab led by Hod Lipson, professor of mechanical engineering, has developed a 3D-printable synthetic soft muscle, a one-of-a-kind artificial active tissue with intrinsic expansion ability that does not require an external compressor or high voltage equipment as previous muscles required. The new material has a strain density (expansion per gram) that is 15 times larger than natural muscle, and can lift 1000 times its own weight.
To achieve an actuator with high strain and high stress coupled with low density, the lead author of the study Aslan Miriyev, a postdoctoral researcher in the Creative Machines lab, used a silicone rubber matrix with ethanol distributed throughout in micro-bubbles. The solution combined the elastic properties and extreme volume change attributes of other material systems while also being easy to fabricate, low cost, and made of environmentally safe materials.
After being 3D-printed into the desired shape, the artificial muscle was electrically actuated using a thin resistive wire and low-power (8V). It was tested in a variety of robotic applications where it showed significant expansion-contraction ability, being capable of expansion up to 900% when electrically heated to 80°C. Via computer controls, the autonomous unit is capable of performing motion tasks in almost any design.
“Our soft functional material may serve as robust soft muscle, possibly revolutionizing the way that soft robotic solutions are engineered today,” said Miriyev. “It can push, pull, bend, twist, and lift weight. It’s the closest artificial material equivalent we have to a natural muscle.”
The researchers will continue to build on this development, incorporating conductive materials to replace the embedded wire, accelerating the muscle’s response time and increasing its shelf life. Long-term, they will involve artificial intelligence to learn to control the muscle, which may be a last milestone towards replicating natural motion.
Researchers have created origami-inspired artificial muscles that add strength to soft robots and can be made for less than $1
In 2017, researchers at the Wyss Institute at Harvard University and MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) created origami-inspired artificial muscles that add strength to soft robots, allowing them to lift objects that are up to 1,000 times their own weight using only air or water pressure, giving much-needed strength to soft robots.
“Artificial muscle-like actuators are one of the most important grand challenges in all of engineering,” adds Rob Wood, Ph.D., corresponding author of the paper and Founding Core Faculty member of the Wyss Institute, “Now that we have created actuators with properties similar to natural muscle, we can imagine building almost any robot for almost any task.”
Each artificial muscle consists of an inner “skeleton” that can be made of various materials, such as a metal coil or a sheet of plastic folded into a certain pattern, surrounded by air or fluid and sealed inside a plastic or textile bag that serves as the “skin.” A vacuum applied to the inside of the bag initiates the muscle’s movement by causing the skin to collapse onto the skeleton, creating tension that drives the motion. Incredibly, no other power source or human input is required to direct the muscle’s movement; it is determined entirely by the shape and composition of the skeleton.
“One of the key aspects of these muscles is that they’re programmable, in the sense that designing how the skeleton folds defines how the whole structure moves. You essentially get that motion for free, without the need for a control system,” says first author Shuguang Li, Ph.D., a Postdoctoral Fellow at the Wyss Institute and MIT CSAIL. This approach allows the muscles to be very compact and simple, and thus more appropriate for mobile or body-mounted systems that cannot accommodate large or heavy machinery.
Not only can the artificial muscles move in many ways, they do so with impressive resilience. They can generate about six times more force per unit area than mammalian skeletal muscle can, and are also incredibly lightweight; a 2.6-gram muscle can lift a 3-kilogram object, which is the equivalent of a mallard duck lifting a car. Additionally, a single muscle can be constructed within ten minutes using materials that cost less than $1, making them cheap and easy to test and iterate.
These muscles can be powered by a vacuum, a feature that makes them safer than most of the other artificial muscles currently being tested. “A lot of the applications of soft robots are human-centric, so of course it’s important to think about safety,” says Daniel Vogt, M.S., co-author of the paper and Research Engineer at the Wyss Institute. “Vacuum-based muscles have a lower risk of rupture, failure, and damage, and they don’t expand when they’re operating, so you can integrate them into closer-fitting robots on the human body.”
“In addition to their muscle-like properties, these soft actuators are highly scalable. We have built them at sizes ranging from a few millimeters up to a meter, and their performance holds up across the board,” Wood says. This feature means that the muscles can be used in numerous applications at multiple scales, such as miniature surgical devices, wearable robotic exoskeletons, transformable architecture, deep-sea manipulators for research or construction, and large deployable structures for space exploration.
The team was even able to construct the muscles out of the water-soluble polymer PVA, which opens the possibility of robots that can perform tasks in natural settings with minimal environmental impact, as well as ingestible robots that move to the proper place in the body and then dissolve to release a drug. “The possibilities really are limitless. But the very next thing I would like to build with these muscles is an elephant robot with a trunk that can manipulate the world in ways that are as flexible and powerful as you see in real elephants,” Rus says.
Lawrence Berkeley National Laboratory (LBL), fabricated a micro-muscle with vanadium dioxide
In 2013, A team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has demonstrated a micro-sized robotic torsional muscle/motor made from vanadium dioxide that for its size is a thousand times more powerful than a human muscle, able to catapult objects 50 times heavier than itself over a distance five times its length within 60 milliseconds – faster than the blink of an eye.
When the V-shaped ribbon is released from the substrate it forms a helix consisting of a dual coil that is connected at either end to chromium electrode pads. Heating the dual coil actuates it, turning it into either a micro-catapult, in which an object held in the coil is hurled when the coil is actuated, or a proximity sensor, in which the remote sensing of an object causes a rapid change in the micro-muscle’s resistance and shape that pushes the object away.
The vanadium dioxide micro-muscles demonstrated reversible torsional motion over one million cycles with no degradation. They also showed a rotational speed of up to approximately 200,000 rpm, amplitude of 500 to 2,000 degrees per millimeters in length, and an energy power density up to approximately 39 kilowatts/kilogram.”These metrics are all orders of magnitudes higher than existing torsional motors based on electrostatics, magnetics, carbon nanotubes or piezoelectrics,” according to researchers.
“Multiple micro-muscles can be assembled into a micro-robotic system that simulates an active neuromuscular system,” according to Junqiao Wu, one of co-authors. “The naturally combined functions of proximity sensing and torsional motion allow the device to remotely detect a target and respond by reconfiguring itself to a different shape. This simulates living bodies where neurons sense and deliver stimuli to the muscles and the muscles provide motion.”
Vanadium (IV) dioxide (VO2) , a dark blue solid inorganic compound has unique thermoelectric properties, when exposed to temperatures of above 68 degrees Celsius, the structure of vanadium dioxide changes and Its magnetic susceptibility as well as its electrical conductivity increases abruptly. This temperature-driven phase transition from insulator-to-metal is expected to one day yield faster, more energy efficient electronic and optical devices. However, this demonstration of temperature-driven structural phase transition of vanadium dioxide crystals whereby when warmed they rapidly contract along one dimension while expanding along the other two, makes vanadium dioxide an ideal candidate material for creating miniaturized, multi-functional motors and artificial muscles.
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