soft robotics

Industry 4.0 is changing the traditional way that manufacturing and production are accomplished, and a positive turn towards clean energy means that more and better air-driven components are available.

The Ultimate Pneumatics Guide

Festo pneumatic muscle actuator promotional image.

Advancements in pressurized fluid power systems may hold the key to the next generation of Biomimetic Robots.

Biomechanical Marvel-Fluidic Muscle Technology

The heart-on-a-chip is entirely 3D printed with built-in sensors that measure the contractile strength of the tissue, providing scientists with new possibilities for studying the musculature of the heart 

Technique paves the way for more complex, customizable devices

3D-printed heart-on-a-chip with integrated sensors

<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569983765878-HeartSleeveInVirtoFinal_0.gif" style="width: 633px;" class="fr-fic fr-dib">In vitro demonstration of the pumping of the soft robotic sleeve (Video courtesy of Ellen Roche/Harvard University)&nbsp;Harvard University and Boston Children&rsquo;s Hospital researchers have developed a customizable soft robot that fits around a heart and helps it beat, potentially opening new treatment options for people suffering from heart failure.The soft robotic sleeve twists and compresses in synch with a beating heart, augmenting cardiovascular functions weakened by heart failure. Unlike currently available devices that assist heart function, Harvard&rsquo;s soft robotic sleeve does not directly contact blood. This reduces the risk of clotting and eliminates the need for a patient to take potentially dangerous blood thinner medications. The device may one day be able to bridge a patient to transplant or to aid in cardiac rehabilitation and recovery.&ldquo;This research demonstrates that the growing field of soft robotics can be applied to clinical needs and potentially reduce the burden of heart disease and improve the quality of life for patients,&rdquo; said Ellen T. Roche, the paper&rsquo;s first author and former PhD student at the&nbsp;<a href="http://seas.harvard.edu/" target="_blank">Harvard John A. Paulson School of Engineering and Applied Sciences</a> (SEAS) and&nbsp;<a href="http://wyss.harvard.edu/" target="_blank">The Wyss Institute&nbsp;</a><a href="http://wyss.harvard.edu/" target="_blank">of</a><a href="http://wyss.harvard.edu/" target="_blank">&nbsp;Biologically Inspired Engineering at Harvard University</a>. &nbsp;Roche is currently a postdoctoral fellow at the National University of Ireland Galway.The research, published in&nbsp;<a href="http://stm.sciencemag.org/" target="_blank">Science Translational Medicine</a>, was a collaboration between SEAS, the Wyss Institute and&nbsp;<a href="http://www.childrenshospital.org/" target="_blank">Boston Children&rsquo;s Hospital</a>.&ldquo;This work represents an exciting proof of concept result for this soft robot, demonstrating that it can safely interact with soft tissue and lead to improvements in cardiac function. We envision many other future applications where such devices can delivery&nbsp;<a href="https://wyss.harvard.edu/disruptive-mechanotherapeutics-from-drugs-to-wearables-2/" target="_blank">mechanotherapy</a> both inside and outside of the body,&rdquo; said&nbsp;<a href="https://www.seas.harvard.edu/directory/walsh" target="_blank">Conor Walsh</a>, senior author of the paper and the John L. Loeb Associate Professor of Engineering and Applied Sciences at SEAS and Core Faculty Member at the Wyss Institute.Heart failure affects 41 million people worldwide. Today, some of the options to treat it are mechanical pumps called&nbsp;<a name="OLE_LINK2" id="OLE_LINK2" target="_blank"></a><a id="OLE_LINK1" name="OLE_LINK1" target="_blank">ventricular assist devices&nbsp;</a>(VADs), which pump blood from the ventricles into the aorta, and heart transplant. While VADs are continuously improving, patients are still at high risk for blood clots and stroke.To create an entirely new device that doesn&rsquo;t come into contact with blood, Harvard researchers took inspiration from the heart itself. The thin silicone sleeve uses soft pneumatic actuators placed around the heart to mimic the outer muscle layers of the mammalian heart. &nbsp;The actuators twist and compress the sleeve in a similar motion to the beating heart.The device is tethered to an external pump, which uses air to power the soft actuators.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569983812883-HeartSleeveFinalGif1.gif" style="width: 766px;" class="fr-fic fr-dib">In vivo demonstration of cardiac assist in a porcine model of acute heart failure (Video courtesy of Ellen Rouche/Harvard SEAS)The sleeve can be customized for each patient, said Roche. If a patient has more weakness on the left side of the heart, for example, the actuators can be tuned to give more assistance on that side. The pressure of the actuators can also increase or decrease over time, as the patient&rsquo;s condition evolves.The sleeve is attached to the heart using a combination of a suction device, sutures and a gel interface to help with friction between the device and the heart.The SEAS and Wyss engineers worked with surgeons at Boston Children&rsquo;s Hospital to develop the device and determine the best ways to implant the device and test it on animal models.&ldquo;The cardiac field had turned away from idea of developing heart compression instead of blood-pumping VADs due to technological limitations, but now with advancements in soft robotics it&rsquo;s time to turn back,&rdquo; said Frank Pigula, a cardiothoracic surgeon and co-corresponding author on the study, who was formerly clinical director of pediatric cardiac surgery at Boston Children&rsquo;s Hospital and is now a faculty member at University of Louisville and division chief of pediatric cardiac surgery at Norton Children&rsquo;s Hospital. &ldquo;Most people with heart failure do still have some function left; one day the robotic sleeve may help their heart work well enough that their quality of life can be restored.&quot;More research needs to be done before the sleeve can be implanted in humans but the research is an important first step towards an implantable soft robot that can augment organ function.Harvard&rsquo;s<a href="http://otd.harvard.edu/" target="_blank">&nbsp;Office of Technology Development&nbsp;</a>has filed a patent application and is actively pursuing commercialization opportunities.&ldquo;This research is really significant at the moment because more and more people are surviving heart attacks and ending up with heart failure,&rdquo; said Roche. &ldquo;Soft robotic devices are ideally suited to interact with soft tissue and give assistance that can help with augmentation of function, and potentially even healing and recovery.&rdquo;

Harvard researchers have developed a customizable soft robot that fits around a heart and helps it beat, potentially opening new treatment options for people suffering from heart failure. The device is tethered to an external pump, which uses air to power the soft actuators

Sleeve attaches directly around the heart

Soft robot helps the heart beat

Hybrid silicone-fabric sensor detects fine motor movements by flexing with the body

Soft and stretchy fabric-based sensors for wearable robots

Rear view of the ankle-assisting exosuits with its soft robotic elements and the cable that transfers mechanical power from actuators operated from the hip to the ankle join. (Image courtesy of the Rolex Awards/Fred Merz)

A soft wearable robotic suit promotes normal walking in stroke patients, opening new approaches to gait re-training and rehabilitation

Post-stroke patients reach terra firma with exosuit technology

Flexible endoscopes can snake through narrow passages to treat difficult to reach areas of the body. However, once they arrive at their target, these devices rely on rigid surgical tools to manipulate or remove tissue. These tools offer surgeons reduced dexterity and sensing, limiting the current therapeutic capabilities of the endoscope.Now, researchers from the&nbsp;<a href="http://seas.harvard.edu/" target="_blank">Harvard John A. Paulson School of Engineering and Applied Sciences</a> and&nbsp;<a href="http://wyss.harvard.edu/" target="_blank">the Wyss Institute for Biologically Inspired Engineering</a> at Harvard University have developed a hybrid rigid-soft robotic arm for endoscopes with integrated sensing, flexibility, and multiple degrees of freedom. This arm &mdash; built using a manufacturing paradigm based on pop-up fabrication and soft lithography &mdash; lies flat on an endoscope until it arrives at the desired spot, then pops up to assist in surgical proceduresThe research is described in <a href="http://onlinelibrary.wiley.com/doi/10.1002/admt.201700135/full" target="_blank">Advanced Materials Technologies</a>.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569906785018-ArmwithWritingWeb.jpg" style="width: 766px;" class="fr-fic fr-dib">Multi-articulated soft pop-up robotic arm. Concept of the system (left): An endoscope navigating in the GI tract and detail of the arm mounted at the tip of the endoscope. Soft pop-up arm (right) performing tissue counter-traction during an ex-vivo test on a porcine stomach (Image courtesy of Harvard University)Soft robots are so promising for surgical applications because they can match the stiffness of the body, meaning they won&rsquo;t accidentally puncture or tear tissue. However, at small scales, soft materials cannot generate enough force to perform surgical tasks.&ldquo;At the millimeter scale, a soft device becomes so soft that it can&rsquo;t damage tissue but it also can&rsquo;t manipulate the tissue in any meaningful way,&rdquo; said Tommaso Ranzani, a postdoctoral fellow at SEAS and the Wyss Institute and coauthor of the paper. &nbsp;&ldquo;That limits the application of soft microsystems for performing therapy. The question is, how can we develop soft robots that are still able to generate the necessary forces without compromising safety.&rdquo;Inspired by biology, the team developed a hybrid model that used a rigid skeleton surrounded by soft materials. The manufacturing method drew on previous work in origami-inspired, pop-up fabrication, developed by Robert Wood, the Charles River Professor of Engineering and Applied Sciences.Wood coauthored the paper and is a Core Faculty Member of the Wyss Institute. &nbsp;Previous pop-up manufacturing techniques &mdash; such as those used with the&nbsp;<a href="http://micro.seas.harvard.edu/research.html#flapping" target="_blank">Robobees</a> &mdash; rely on actuation methods that require high voltages or temperatures to operate, something that wouldn&rsquo;t be safe in a surgical tool directly manipulating biological tissues and organs.So, the team integrated soft actuators into the pop-up system.&nbsp;&ldquo;We found that by integrating soft fluidic microactuators into the rigid pop-up structures, we could create soft pop-up mechanisms that increased the performance of the actuators in terms of the force output and the predictability and controllability of the motion,&rdquo; said Sheila Russo, postdoctoral fellow at SEAS and Wyss and lead author of the paper. &ldquo;The idea behind this technology is basically to obtain the best of both worlds by combining soft robotic technologies with origami-inspired rigid structures. Using this fabrication method, we were able to design a device that can lie flat when the endoscope is navigating to the surgical area, and when the surgeon reaches the area they want to operate on, they can deploy a soft system that can safely and effectively interact with tissue.&rdquo;The soft actuators are powered by water. They are connected to the rigid components with an irreversible chemical bond, without the need of any adhesive. The team demonstrated the integration of simple capacitive sensing that can be used to measure forces applied to the tissue and to give the surgeon a sense of where the arm is and how it&rsquo;s moving. The fabrication method allows for bulk manufacturing, which is important for medical devices, and allows for increased levels of complexity for more sensing or actuation. Furthermore, all materials used are biocompatible. &nbsp;The arm is also equipped with a suction cup &mdash; inspired by octopus tentacles &mdash; to safely interact with tissue. The team tested the device ex vivo, simulating a complicated endoscopic procedure on pig tissue. The arm successfully manipulated the tissue safely.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569906823020-ArmPigWeb.jpg" style="width: 688px;" class="fr-fic fr-dib"><div class="inline-image">Soft pop-up arm performing tissue counter-traction during an ex-vivo test on a porcine stomach (Image courtesy of Harvard University)</div>&ldquo;The ability to seamlessly integrate gentle yet effective actuation into millimeter-scale deployable mechanisms fits naturally with a host of surgical procedures,&rdquo; said Wood. &ldquo;We are focused on some of the more challenging endoscopic techniques where tool dexterity and sensor feedback are at a premium and can potentially make the difference between success and failure.&rdquo;The researchers demonstrated that the device could be scaled down to 1 millimeter, which would allow it to be used in even tighter endoscopic procedures, such as in lungs or the brain.Next, the researchers hope to test the device&nbsp;in vivo.&ldquo;Our technology paves the way to design and develop smaller, smarter, softer robots for biomedical applications,&rdquo; said Russo.

SEM images of the hybrid soft pop-up actuators. The image has been colored in post processing to differentiate between the soft (in yellow) and the rigid structure (in blue) (Image courtesy of the Wyss Institute at Harvard University)

Bioinspired approach combines pop-up fabrication with soft robotics

Smaller, smarter, softer robotic arm for endoscopic surgery

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. However, 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.Now, researchers at the&nbsp;<a href="http://seas.harvard.edu/" target="_blank">Harvard John A. Paulson School of Engineering and Applied Sciences</a> (SEAS), the <a href="http://wyss.harvard.edu/" target="_blank">Wyss Institute at Harvard University</a> and MIT&rsquo;s Computer Science and Artificial Intelligence Laboratory (CSAIL) have 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. The study will be published this week in&nbsp;<a href="http://www.pnas.org/content/early/2017/11/21/1713450114.full" target="_blank">Proceedings of the National Academy of Sciences</a> (PNAS).&ldquo;We were very surprised by how strong the actuators [aka, &ldquo;muscles&rdquo;] were. We expected they&rsquo;d have a higher maximum functional weight than ordinary soft robots, but we didn&rsquo;t expect a thousand-fold increase. It&rsquo;s like giving these robots superpowers,&rdquo; said Daniela Rus, the Andrew and Erna Viterbi Professor of Electrical Engineering and Computer Science at MIT and one of the senior authors of the paper.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569469584573-Origami-inspired+artificial+muscles+snake+arm.jpg" style="width: 766px;" class="fr-fic fr-dib">The movement and shape of the artificial muscles is defined by the shape of their internal &ldquo;skeleton&rdquo; &ndash; in this case, made of notched blocks of foam. (Image courtesy of the Wyss Institute for Biologically Inspired Engineering)&nbsp;&ldquo;Artificial muscle-like actuators are one of the most important grand challenges in all of engineering,&rdquo; said <a href="https://www.seas.harvard.edu/directory/rjwood" target="_blank">Robert J. Wood</a>, corresponding author of the paper and&nbsp;the Charles River Professor of Engineering and Applied Sciences at the SEAS. &ldquo;Now that we have created actuators with properties similar to natural muscle, we can imagine building almost any robot for almost any task.&rdquo; Wood is also a Founding Core Faculty member of the Wyss Institute.Each artificial muscle consists of an inner skeleton&nbsp;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.&nbsp;A vacuum applied to the inside of the bag initiates the muscle&rsquo;s movement by causing the skin to collapse onto the skeleton, creating tension that drives the motion. No other power source or human input is required to direct the muscle&rsquo;s movement; it is determined entirely by the shape and composition of the skeleton.&ldquo;One of the key aspects of these muscles is that they&rsquo;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,&rdquo; said first author Shuguang Li, 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.&ldquo;When creating robots, one always has to ask, &lsquo;Where is the intelligence - is it in the body, or in the brain?&rsquo;&rdquo; said Rus. &ldquo;Incorporating intelligence into the body (via specific folding patterns, in the case of our actuators) has the potential to simplify the algorithms needed to direct the robot to achieve its goal. All these actuators have the same simple on/off switch, which their bodies then translate into a broad range of motions.&rdquo;The team constructed dozens of muscles using materials ranging from metal springs to packing foam to sheets of plastic, and experimented with different skeleton shapes to create muscles that can contract down to 10 percent of their original size, lift a delicate flower off the ground, and twist into a coil, all simply by sucking the air out of them.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 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.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569469620868-Origami-inspired+artificial+muscles+white+muscle.jpg" style="width: 766px;" class="fr-fic fr-dib">The zig-zag pattern of this muscle&rsquo;s interior &ldquo;skeleton&rdquo; allows the muscle to contract down to a fraction of its original width. (Image courtesy of the Wyss Institute for Biologically Inspired Engineering) These muscles can be powered by a vacuum, a feature that makes them safer than most of the other artificial muscles currently being tested. &ldquo;A lot of the applications of soft robots are human-centric, so of course it&rsquo;s important to think about safety,&rdquo; said Daniel Vogt, co-author of the paper and Research Engineer at the Wyss Institute. &ldquo;Vacuum-based muscles have a lower risk of rupture, failure, and damage, and they don&rsquo;t expand when they&rsquo;re operating, so you can integrate them into closer-fitting robots on the human body.&rdquo;&ldquo;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,&rdquo; said Wood. 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. &ldquo;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,&rdquo; said Rus.

Origami-inspired artificial muscles are capable of lifting up to 1,000 times their own weight, simply by applying air or water pressure. (Image courtesy of the Wyss Institute for Biologically Inspired Engineering)

Origami-inspired muscles are both soft and strong, and can be made for less than $1

Artificial muscles give soft robots superpowers

Even octopuses understand the importance of elbows. When these squishy, loose-limbed cephalopods need to make a precise movement &mdash; such as guiding food into their mouth &mdash; the muscles in their tentacles contract to create a temporary revolute joint. These joints limit the wobbliness of the arm, enabling more controlled movements. &nbsp;&nbsp;<a href="http://seas.harvard.edu/" target="_blank">Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences</a> (SEAS) and the&nbsp;<a href="http://wyss.harvard.edu/" target="_blank">Wyss Institute for Biologically Inspired Engineering</a> have shown how a multi-layered structure can allow robots to mimic the octopus&rsquo;s kinematics, creating and eliminating joints on command. The structure can also allow robots to rapidly change their stiffness, damping, and dynamics.The research was published in two papers in&nbsp;<a href="https://onlinelibrary.wiley.com/doi/10.1002/adfm.201707136" target="_blank">Advanced Functional Materials</a> and&nbsp;<a href="https://ieeexplore.ieee.org/document/8141952/" target="_blank">IEEE Robotics and Automation Letters.</a>&ldquo;This research helps bridge the gap between soft robotics and traditional rigid robotics,&rdquo; said Yashraj Narang, first author of both studies and graduate student at SEAS. &ldquo;We believe that this class of technology may foster a new generation of machines and structures that cannot simply be classified as soft or rigid.&rdquo;<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569465675108-artistRendering-640x278.jpg" style="width: 766px;" class="fr-fic fr-dib"><div class="inline-image">The structure consists of multiple layers of flexible material wrapped in a plastic envelope and connected to a vacuum source. (Image courtesy of Yashraj Narang/Harvard SEAS)</div>The structure is surprisingly simple, consisting of multiple layers of flexible material wrapped in a plastic envelope and connected to a vacuum source. Without vacuum, the structure behaves exactly as you would expect, bending, twisting and flopping without holding shape. But when a vacuum is applied, it becomes stiff and can hold arbitrary shapes, and it can be molded into additional forms.This transition is the result of a phenomenon called laminar jamming, in which the application of pressure creates friction that strongly couples a group of flexible materials.&ldquo;The frictional forces generated by the pressure act like glue,&rdquo; said Narang. &ldquo;We can control the stiffness, damping, kinematics, and dynamics of the structure by changing the number of layers, tuning the pressure applied to it, and adjusting the spacing between multiple stacks of layers.&rdquo;The research team, which also included&nbsp;<a href="https://www.seas.harvard.edu/directory/howe" target="_blank">Robert Howe</a>, the Abbott and James Lawrence Professor of Engineering,&nbsp;<a href="https://www.seas.harvard.edu/directory/jvlassak" target="_blank">Joost Vlassak</a>, the Abbott and James Lawrence Professor of Materials Engineering, and Alperen Degirmenci, a graduate student in SEAS, extensively modeled the mechanical behavior of laminar jamming to better control its capabilities.Next, they built real-world devices using the structures, including a two-fingered gripper that, without vacuum, could wrap around and hold onto large objects and, with a vacuum, could pinch and hold onto small objects about the size of a marble.<iframe src="https://www.youtube.com/embed/ecTOd6O9Beg?&wmode=opaque" frameborder="0" allowfullscreen="" class="fr-draggable" style="width: 767px; height: 466px;"></iframe> The researchers also demonstrated the structure&rsquo;s capabilities as shock absorbers by attaching them to a drone as landing gear. The team tuned the stiffness and damping of the structures to absorb the impact of landing.The structure is a proof-of-concept that could have many applications in the future, from surgical robots to wearable devices and flexible speakers.&ldquo;Our work has explained the phenomenon of laminar jamming and shown how it can provide robots with highly versatile mechanical behavior,&rdquo; said Howe, senior author of the paper. &ldquo;We believe that this technology will eventually lead to robots that can change state between soft, continuous devices that can safely interact with humans, and rigid, discrete devices that can meet the demands of industrial automation.&rdquo;

This drone’s landing gear is tuned the stiffness and damping to absorb the impact of landing. (Image courtesy of Yashraj Narang/Harvard SEAS) 

Harnessing friction gives soft robots new capabilities

Robot transitions from soft to rigid

This is the first article, in a series, exploring the vast landscape of artificial muscle actuators. In the subsequent articles, the quest to create and harness artificial muscle technology will be explored.Until recently, conventional robots have utilized rigid materials and fabrication methods. Ironically, traditional robots are relatively weak and brittle, when compared with biological organisms, which utilize soft adaptive tissues to produce movement. A new approach to robotics has taken shape in the form of soft robotics and biomimicry.So, what does “soft” robotics mean? If you ever stayed up late as a child and remember watching old science fiction movies, as I did, your immediate concept of a “Robot” is typically a large, metal creature, lumbering about. In sharp contrast to this classic robot concept, soft robots are flexible and agile; think of a ballerina or an athlete. Obviously, modern robots have advanced from this stereotype, however our best robot is still not capable of the nuanced movements and physical strength of a human.A key component to producing a robot that more closely replicates the movement of a human, starts with the actuation system. An actuator is any mechanism that can convert some form of energy into physical movement, similar to how our muscles work. Typically, servo motors are used to produce movement in most robotic systems. However, servo motors are bulky, have a low power to weight ratio(when compared to muscle tissue) and are non-compliant. Essentially, compliance of a material means that it has some give to it, like a rubber band. You may not realize it, but the human body is capable of withstanding incredible forces.As demonstrated at the 2015 DARPA Robotics Challenge, a humanoid robot struggled to accomplish physical tasks that any average human would be able to do with ease. Common bumps and bruises that humans experience, would be catastrophic to most modern robots. Our ability to adapt to different physical environments is the template that will be used to create future robots.Artificial Muscle Research Engineers have sought a multitude of materials and fabrication methods in order to replicate the attributes of biological muscle tissue. I have also spent close to 20 years attempting to create an artificial muscle actuator, with limited success. So, what makes muscle tissue so special and replication of its attributes so elusive? Well, for starters, nature has had a 560 million year head start. Additionally, each of our muscles is a compact and powerful actuator that is made of soft, self-healing tissue. Our muscle tissue affords us dynamic movement and uses an efficient chemical fuel as a power source, glucose. Muscle tissue is composed of thousands of contractile fibers-within each fiber are up to 100,000 molecular units that synchronously shorten in length with a tremendous amount of force and velocity. This force and velocity enables basic movement and locomotion.Extensive research has been conducted to create an artificial muscle actuator which can replicate or exceed biological muscle tissue, with varying levels of success. Several types of technologies that have been explored include: Electroactive polymers (EAPs), Shape-memory metal alloy wires/liquid crystalline elastomers (SMAs), the Mckibben air muscle (Pneumatic artificial muscles), nylon fiber (torsional actuators) and Carbon fiber (PDMS rubber/torsional actuators). While all of these types of artificial muscle actuators, possess some of the sought after attributes of biological muscle, none of them are a complete solution in and of themselves. In order to fully appreciate the vast landscape of artificial muscle research and current innovations-each of these technologies will be further discussed; including history, fabrication techniques, advantages, drawbacks and future applications.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1561300296387-materiability2.jpg" style="width: 752px;" class="fr-fic fr-dib">EAP(Electroactive Polymer) ActuatorElectroactive polymers (EAPs) EAP technology is based upon the principle of generating a motive force in polymeric materials via an applied electrostatic field. One of the basic principles behind this technology is static electricity, if you have ever heard the crackle when you remove laundry from the dryer or accidentally, “shocked” your friend when walking, you have experienced static electricity. The electroactive effect occurs when a high voltage is applied to a flexible material, the electric static field that is generated is strong enough to produce movement in the material.Another technique used for actuation is based on elastomeric, meta-materials that exhibit the piezoelectric effect. Piezoelectric materials are actually very common, they are the technology that enables, haptic feedback in your cell phone. Typically, Piezoelectric materials are in the form of quartz crystals, which have the incredible ability to convert physical stress into electricity and vice versa. Moreover, certain polymer compounds can be, “doped”, with certain metals to produce the piezoelectric effect.&nbsp;While EAPs can embody, several permutations, the main focus of this discussion will be on the most commercially viable configurations which include, dielectric elastomers and ionic polymer-metal composites. EAP research started as early as the 1800s, however it wasn't until the turn of the century that EAP technology was realized for practical applications.Physicist, Dr. Yoseph Bar-Cohen, of the Jet Propulsion Laboratory, further advanced EAP technology by inspiring a global research initiative which challenged research groups around the world to compete in the world's first, “Arm wrestling Match of EAP Robotic Arm against Human challenge”. In 2002, Japanese tech company Eamex, released one of the first commercial products-a realistic robotic fish toy, utilizing a viable EAP, artificial muscle. Continued research, funded by DARPA, resulted in the tech start-up; Artificial Muscle, Inc-which specializes in development and manufacture of products based on EAP technology.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1561300403609-image022.jpg" style="width: 748px;" class="fr-fic fr-dib">Arm wrestling Match of EAP Robotic Arm against Human challenge(human opponent, Panna Felsen)Dielectric Elastomeric Actuator Dielectric elastomeric architecture involves an elastomeric film or cross sectional area of polymeric material, coated with an electrically conductive layer on either side. This configuration yields a compliant capacitor, where in, the conductive coating serves as a pair of electrodes. When an electrical field is applied to the DEA, changes in the electric field distribution of the electrodes generate a mechanical strain via Coulombic charge interaction of the compliant electrodes. Essentially, the conductive electrodes become electrostatically attracted and repelled in specific regions, causing the material to compress and expand. Additionally, the strain deformation state can be held in a static position via a constant DC voltage; analogous to an isometric muscle contraction.Composition of a DEA is relegated to a suitable elastomeric material with high elastic modulus; typically silicone or acrylic elastomers are used. During the fabrication process, elastomeric film of a predetermined thickness, is stretched upon a rigid frame in order to provide bidirectional static prestrain(stretch) of the elastomeric film. This prestrain fabrication technique has been shown to significantly increase the &nbsp;mechanical efficiency and responsiveness of DEAs. The electrically conductive coating, typically comprises conductive carbon grease or paste, applied to either side of the elastomeric film to provide diametrically opposed electrodes. This fabrication process can also be repeated several times, with each elastomeric film/electrode layer, “stacked” onto the subsequent layer in order to provide an actuator capable of exerting much larger forces.Ionic Polymer-Metal CompositesIPMC actuators are characterized by synthetic polymers engineered with positively charged metal ions, which are randomly arranged in an aqueous, polymer matrix. Typically, a noble metal coating is applied to the polymer-metal matrix in order to provide positive/negative electrodes(orientation of cathode/anode is based upon the terminal electrical wires attachment). If you remember from Chemistry class, noble elements are those that do not play well with other elements or resist chemical interaction. The positive metal ions partially entangle with water molecules and any inherent, negatively charged ions within the polymer matrix. When an electrical field is applied, the conjugated metal ions are attracted to the negatively charged metal electrode. This ionic migration of entangled water molecules provides an osmotic pressure shift resulting in mechanical strain across the polymer matrix membrane. Conversely, mechanically bending or deformation of the polymer matrix can produce electrical potential and output voltage, which can be advantageous for energy harvesting and biomedical sensors.Edge Over Other ActuatorsPossessing many biomimetic attributes has warranted continued research into EAPs development. Similar to biological muscle tissue, Dielectric Elastomers have an inherent ability to store large amounts of mechanical energy and contract with a large amount of usable force. Another appealing attribute is measured by how well an actuator can operate under continued stress and adaptability under varied conditions. A crucial element when considering building a humanoid robot that can work along side humans. With this in mind, DEAs have been shown to operate within standard room conditions, without the need for additional considerations. A crucial element for any biomimetic actuator is the ability to respond quickly when activation stimuli is applied, DEAs continue to excel with rapid response activation times measuring in the milliseconds.Additionally, Ionic Polymer-Metal Composites bring a unique set of properties to the table, including bi-directional actuation. Dependent upon the polarity of the activation voltage applied, IPMCs can reverse actuation direction, contributing further to adaptable mobility. Due to the unique chemistry of IPMCs, the mechanism of movement is attributed mostly to molecular motion, which allows for low activation voltages. An attribute that is extremely appealing in the quest to create a sustainable, autonomous robot. Configuration of IPMCs is not only constrained to simple membranes. There complaint nature allows the material to be rolled up into compact cylinders, with the ability to bend or extend. A quality that is important in order to approximate various muscle shapes and sizes, inherent in biological organisms.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1561300566769-a-Optical-image-of-the-bending-actuation-of-the-i-EAP-actuator-under-AC-electric.jpg" style="width: 300px;" class="fr-fic fr-dib">(a) Optical image of the bending actuation of the EAP actuator under AC electric signals.(b) Schematic of bending actuation mechanism.Challenges In The FieldWhile EAPs do posses several remarkable properties, we must also evaluate the engineering challenges faced in the development of EAPs for practical applications. One such hurdle to overcome for any robotic system is power consumption, especially within the context of realizing a self-contained humanoid robot. Unfortunately, this is a major con for DEAs, which require thousands of volts of electricity to operate. With the current state of battery technology this is far from ideal. EAP longevity is also limited due to the high voltage field, often exceeding the breakdown threshold of the polymer material. While IPMCs require minimal activation voltage, additional considerations are required for typical usage. Ironically, not unlike a biological muscle, IPMCs also require encapsulation in an impermeable membrane to prevent dehydration of the polymer matrix. In addition to IPMCs low actuation force, the most critical disadvantage remains the current inability to commercially, mass produce, a consistent ionic polymer. Until these challenges can be overcome, a viable EAP artificial muscle actuator remains elusive.Looking AheadWhat's in store for EAP technology? While an EAP based artificial muscle actuator seems unlikely in the immediate future. EAP technology is currently in development towards, biomechanical energy harvesting. Specifically, IPMC films can be attached to key flexing areas of the human body in order to harvest kinetic energy. So, next time you go to the gym, you can burn calories and charge your cell phone. This technology could also turn out to be invaluable for Soldiers in the midst of combat with little to no recharge options. The structure and mechanism of DEAs has found a practical application within the realm of virtual reality systems. An array of actuator “pixels” can be utilized for graphic displays for the visually impaired and possibly for theme park applications, such as virtual-haptic, hybrid environments. The National Science Foundation is continuing research towards an artificial muscle application, however. With the aid of 3-D printing technology, IPMC technology may prove to one day be a suitable artificial muscle solution.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1561300675742-3D_printed_IPMC_hand.jpg" style="width: 750px;" class="fr-fic fr-dib">A small 3-D printed ionic polymer-metal composite soft robotic hand. National Science Foundation researchers are working to transform this material into artificial muscles. This is the first article in a series&nbsp;about artificial muscles, created by&nbsp;<a href="https://www.wevolver.com/profile/samuel.manriquez" rel="noopener noreferrer" target="_blank">Samuel Manriquez</a>.

Skeleton with an artificial bicep. Credit: Aslan Miriyev - Columbia Engineering

The current state of robotic actuators and mechanical control systems are failing to keep pace with advancing AI software, exploring novel actuator technologies may bridge the gap between man and machine.

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