For many years, the idea of a human with bionic muscles immediately conjures up science fiction images of a TV series superhuman character that was implanted with bionic muscles and portrayed with strength and speed far superior to any normal human. As fantastic as this idea may seem, recent developments in electroactive polymers (EAP) may one day make such bionics possible. Polymers that exhibit large displacement in response to stimulation that is other than electrical signal were known for many years. Initially, EAP received relatively little attention due to their limited actuation capability. However, in the recent years, the view of the EAP materials has changed due to the introduction of effective new materials that significantly surpassed the capability of the widely used piezoelectric polymer, PVDF. As this technology continues to evolve, novel mechanisms that are biologically inspired are expected to emerge. EAP materials can potentially provide actuation with lifelike response and more flexible configurations. While further improvements in performance and robustness are still needed, there already have been several reported successes. In recognition of the need for cooperation in this multidisciplinary field, there is a series of international forums that are leading to a growing number of research and development projects and to great advances in the field.. In this paper, the field of EAP as artificial muscles will be reviewed covering the state of the art, the challenges and the vision for the progress in future years.
How do you a build the musculoskeletal system a humanoid robot If you're Sony, you use a system of servo motors, gears, metal rods and cables. But that's a poor imitation of the human body where the movement of limbs is dictated by the smooth, coordinated contraction of muscle fibers. Enter the breakthrough: electroactive polymers, also called artificial muscles. Recently unveiled by a scientist in Albuquerque, these artificial muscles contract when exposed to an electric current. Attach one end to the pelvis of a humanoid robot and the other end to the back of its knee and you have a robot that can do leg curls. Strap together enough such fibers, couple them with a smart contact feedback system, and you can teach a robot to walk using its own artificial muscles. Imagine it: no motors to wear out, no cables to snap, no rods to break: just muscle-like fibers that contract in response to an electric current. It's nothing short of a revolution in robotics. No doubt, the industry will rely heavily on this technology in the years ahead.
There's even hope that such fibers might somehow be used in human patients to aid those who have, for one reason or another, lost the use of their limbs.
Hope for human patients! As humansâ„¢ livelonger there is a growing need for availability of organs for transplant however shortage in donations necessitates the development of artificial alternatives. Advances in medicine have led to the availability of artificial blood, replacement joints, heart valves, and heart-lung machines that are common implanted. In the United States, nearly one in ten individuals are using some type of an implanted medical device. Muscle is a critically needed organ and its availability in an artificial form for medical use can greatly contribute to the improvement of the quality of life of many humans. The emergence of effective electroactive polymers (EAP) that are also known as artificial muscles can potentially address this need. These materials are human made actuators that have the closest operation similarity to biological muscles. While these actuation materials are far from being ready for use as implants enormous progress has been made in recent years turning them into a promising technology for consideration in medical applications.
Nature as a Biologically Inspiring Model
Evolution over millions of years made nature introduce solutions that are highly power-efficient and imitating them offers potential improvements of our life and the tools we use. Human desire and capability to imitate nature and particularly biology has continuously evolved and with the improvement in technology more difficult challenges are being considered. Imitation of biology may not be the most effective approach to engineering mechanisms using man-made capabilities. It is inconceivable to imaging flying with a machine that has feathers and flapping wings, where obviously a machine like that will not allow us to reach the distances and carry the loads that aircraft are doing today.
The introduction of the wheel has been one of the most important inventions that human made allowing to travel great distances and perform tasks that would have been otherwise impossible within the life time of a single human being. While wheel based locomotion mechanisms allow reaching great distances and speeds, wheeled vehicles are subjected to great limitations with regards to traversing complex terrain with obstacles. Obviously, legged creatures can perform numerous functions that are far beyond the capability of an automobile. Producing legged robots is increasingly becoming an objective for robotic developers and considerations of using such robots for space applications are currently underway. Making miniature devices that can fly like a dragonfly; adhere to walls like gecko; adapt the texture, patterns, and shape of the surrounding as the octopus (can reconfigure its body to pass thru very narrow tubing); process complex 3D images in real time; recycle mobility power for highly efficient operation and locomotion; self-replicate; self-grow using surrounding resources; chemically generate and store energy; and many other capabilities are some of the areas that biology offers as a model for science and engineering inspiration. While many aspects of biology are still beyond our understanding and capability, significant progress has been made and the field of biomimetics is continuing to evolve.
EAP actuated robotic mechanisms are enabling engineers to create devices that were previously only imaginable in science fiction. One such commercial product has already emerged in Dec. 2002 is a form of a Fish-Robot. It swims without batteries or a motor and it uses EAP materials that simply bend upon stimulation. For power it uses inductive coils that are energized from the top and bottom of the fish tank. This fish represents a major milestone for the field, as it is the first reported commercial product to use electroactive polymer actuators
The evolution of artificial muscles in robotics
The introduction of the wheel has been one of the most important inventions that human made allowing to travel great distances and perform tasks that would have been otherwise impossible within the life time of a single human being. While wheel based locomotion mechanisms allow reaching great distances and speeds, wheeled vehicles are subjected to great limitations with regards to traversing complex terrain with obstacles. Obviously, legged creatures can perform numerous functions that are far beyond the capability of an automobile. Producing legged robots is increasingly becoming an objective for robotic developers and considerations of using such robots for space applications are currently underway.
Making miniature devices that can fly like a dragonfly; adhere to walls like gecko; adapt the texture, patterns, and shape of the surrounding as the octopus (can reconfigure its body to pass thru very narrow tubing); process complex 3D images in real time; recycle mobility power for highly efficient operation and locomotion; self-replicate; self-grow using surrounding resources; chemically generate and store energy; and many other capabilities are some of the areas that biology offers as a model for science and engineering inspiration. While many aspects of biology are still beyond our understanding and capability, significant progress has been made and the field of biomimetics is continuing to evolve.
The evolution in the capabilities that are inspired by biology has increased to a level where more sophisticated and demanding fields, such as space science, are considering the use of such robots. At JPL, four and six legged robots are currently being developed for consideration in future missions to such planets as Mars. Such robots include the LEMUR (Limbed Excursion Mobile Utility Robot). This type of robot would potentially perform mobility in complex terrains, perform sample acquisition and analysis, and many other functions that are attributed to legged animals including grasping and object manipulation.
This evolution may potentially lead to the use of life-like robots in future NASA missions that involve landing on various to planets. The details of such future missions will be designed as a plot, commonly used in entertainment shows rather than conventional mission plans of a rover moving in a terrain and performing simple autonomous tasks. Equipped with multi-functional tools and multiple cameras, the LEMUR robots are intended to inspect and maintain installations beyond humanity's easy reach in space. This spider looking robot has 6 legs, each of which has interchangeable end-effectors to perform the required mission. The axis-symmetric layout is a lot like a starfish or octopus, and it has a panning camera system that allows omni-directional movement and manipulation operations.
EAP as Artificial Muscles
One of the key aspects of driving mechanisms that emulate biology is the development of actuators that mimic the capability of biological muscles. The potential for such actuators is continuously growing as advances are being made leading to more effective electro active polymers (EAP). These materials have functional similarities to biological muscles, including resilience, quiet operation, damage tolerance, and large actuation strains (stretching, contracting or bending). They can potentially provide more lifelike aesthetics, vibration and shock dampening, and more flexible actuator configurations.
These materials can be used to make mechanical devices and robots with no traditional components like gears, and bearings, which are responsible to their high costs, weight and premature failures. Also, they could potentially be used as artificial organ to assist or operate the heart and/or its valve, the eyelid and/move the eyeball as well as control the focal length of its length, and allow mobility of the legs and/or hand as well as provide smart prosthetics (also known as cyborgs). As an example of a capability of EAP materials that is inspired by biology, a team developed a miniature robotic arm. This robotic arm illustrates some of the unique capabilities of EAP, where its gripper consists of four EAP fingers (made by Ionic polymer metal composite strips) with hooks at the bottom emulating fingernails.
This arm was made to grab rocks similar to human hand. Generally, there are many polymers that exhibit volume or shape change in response to perturbation of the balance between repulsive intermolecular forces, which act to expand the polymer network, and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the volume or shape change, can be controlled by subtle changes in parameters such as solvent, gel composition, temperature, pH, light, etc
The type of polymers that can be activated by non-electrical means include: chemically activated, shape memory polymers, inflatable structures, including McKibben Muscle, light activated polymers, magnetically activated polymers, and thermally activated gels. Polymers that are chemically stimulated were discovered over half-a-century ago when collagen filaments were demonstrated to reversibly contract or expand when dipped in acid or alkali aqueous solutions, respectively. Even though relatively little has since been done to exploit such Ëœchemo-mechanicalâ„¢ actuators, this early work pioneered the development of synthetic polymers that mimic biological muscles The convenience and practicality of electrical stimulation and technology progress led to a growing interest in EAP materials.
The movements of a LEMUR robot using artificial muscles
A Spiderbot robot climbing and walking with the help
of artificial muscles
MAKING ROBOTS ACTUATED BY EAP
With todayâ„¢s technology one can quite well graphically animate the appearance and behavior of biological creatures. However, in past years, engineering such biomimetic intelligent creatures as realistic robots was a significant challenge due to the physical and technological constraints and shortcomings of existing technology. Making such robots that can hop and land safely without risking damage to the mechanism, or making body and facial expression of joy and excitement are very easy tasks for human and animals to do but extremely complex to engineer.
The use of artificial intelligence, effective artificial muscles and other biomimetic technologies are expected to make the possibility of realistically looking and behaving robots into more practical engineering models. To promote the development of effective EAP actuators, which could impact future robotics, toys and animatronics, two test-bed platforms were developed. These platforms are available at the Principal authorâ„¢s lab at JPL and they include an Android head that can make facial expressions and a robotic hand with activatable joints. At present, conventional electric motors are producing the required deformations to make relevant facial expressions of the Android. Once effective EAP materials are chosen, they will be modeled into the control system in terms of surface shape modifications and control instructions for the creation of the desired facial expressions. Further, the robotic hand is equipped with tandems and sensors for the operation of the various joints mimicking human hand.
The index finger of this hand is currently being driven by conventional motors in order to establish a baseline and they would be substituted by EAP when such materials are developed as effective actuators. The growing availability of EAP materials that exhibit high actuation displacements and forces is opening new avenues to bioengineering in terms of medical devices and assistance to humans in overcoming different forms of disability. Areas that are being considered include an angioplasty steering mechanism, and rehabilitation robotics. For the latter, exoskeleton structures are being considered to augment the mobility and functionalities of patients with weak muscles.
Challenges to Developing EAP Materials as Artificial Organs
As polymers, EAP materials can be easily formed in various shapes, their properties can be engineered and they can potentially be integrated with micro-electro-mechanical-system (MEMS) sensors to produce smart actuators. As mentioned earlier, the most attractive feature of EAP materials is their ability to emulate the operation of biological muscles with high fracture toughness, large actuation strain and inherent vibration damping. Unfortunately, the materials that have been developed so far are still exhibiting low conversion efficiency, are not robust, and there are no standard commercial materials available for consideration in practical applications.
In order to be able to take these materials from the development phase to application as effective actuators, there is a need to establish adequate EAP infrastructure. Effectively addressing the requirements of the EAP infrastructure involves understanding and analytically model the behavior of EAP materials, as well as developing effective processing and characterization techniques. If one considers the use of EAP as artificial organs there are challenges that need to be addressed that are common to the use of any foreign objects as implants in the human body. Such issues include biological compatibility and avoiding rejection, chemically safe use, and ability to meet the stringent functional requirements to operate as a replacement organ.
Some of the issues related to the use of EAP as artificial organs include the fact that the electronic EAP group requires high voltage. At present, the materials in this group have the highest robustness and they induce the largest actuation forces however the required voltages in the range from hundreds to thousands of voltage are a concern that must be addressed. Even though the electric current is relatively low, the use of such voltage levels can cause such dangers as inducing blood clot or injury due to potential voltage breakdown. On the other hand, the ionic group of EAP materials is chemically sensitive and requires careful protection, further, it is difficult to maintain a static position because of the fact that these materials involve chemical reaction and even DC voltage causes a reaction. Interfacing between human and machine to complement or substitute our senses may enable important capabilities for medical applications.
A number of such interfaces, with some that employ EAP, were investigated or have been considered. Of notable significance is the ability to interface machines and the human brain. A development by scientists at Duke University enabled this possibility where electrodes were connected to the brain of a monkey, and, using brain waves, the monkey operated a robotic arm, both locally and remotely via the internet. It is envisioned that success in developing EAP actuated robotic arms that can win a wrestling match with human opponent can greatly benefit from this development by neurologists.
Using such a capability to control prosthetics would require feedback to allow the human operator to feel the environment around the artificial limbs. Such feedback can be provided with the aid of tactile sensors, haptic devices, and other interfaces. Besides providing feedback, sensors will be needed to allow the users to monitor the prosthetics from potential damage (heat, pressure, impact, etc.) just as we are doing with biological limbs. The development of EAP materials that can provide tactile sensing is currently under way.
Throughout history, new materials have played a key role revolutionizing many engineering disciplines. Light and strong materials have become enabling technologies in many industrial sectors. But engines, motors, actuators, and in general all the actual motion technologies haven't evolved significantly in the past decades, becoming very often the performance barrier. The proposed actuator offers certain advantages over contemporary motor technologies. As with real muscle, an artificial muscle could offer transduction efficiency far superior to that of electric motors, or shape memory alloys powered by battery or fuel cell. Although high powers can be achieved from gasoline engines, stealth quietness cannot. The development of the proposed new technology will influence many technical aspects of society. Due to the enormous range of possibilities we will proceed to analyze the commercial applications by sectors:
Bio-inspired robotics: Again reducing the actuator's weight becomes a key role for this industry. For robots to be introduced fully into society, experts agree that a technology revolution in actuators would be required. Artificial muscles would mimic natural muscles in flexibility and efficiency and would mean a change in design philosophy that would allow integration of sensing, moving, suspension, and even control systems in a single device: the actuator. Due to this enhanced maneuverability robots could then be introduced into new sectors depending on the engineer's needs and imagination. Gas powered autonomous robots are noisy and cumbersome. Humanoids and other biomimetics initiatives so far suffer from inflexibility. Consumer Applications: The materials under study could cost as little as one euro per kilogram to mass-produce. This is why is interesting to look at consumer applications like toys (moving action figures), cosmetic or toothpaste dispensers. Anything that requires motion is fair game, and could be on the market in one to two years.
Aerospace: The weight-performance ratio is closely watched in this sector, especially in the aerospace field. Artificial muscle technology could enhance considerably this figure and replace existing actuator technologies in the aeronautical sector. The flexible nature of artificial muscles would also allow development of highly efficient active suspension systems also useful to the automotive industry. A new commercial field still to be fully developed called Smart Structures could also benefit from this technology. As preliminary experiments show (further in this document), artificial muscles could be integrated into the structure of a plane enabling a shape-changeable and therefore self-repairable structure. Some other innovative possibilities are the design of anti-G suits for pilots, or the design of extremely small flying machines.
Biomedical: The possibilities in this sector are unlimited. Prosthetics (highly weight sensitive), massage systems to prevent venous thrombosis for people who are at risk due to long periods of immobility, artificial urinary sphincters, cardio wraparounds, and stearable catheters are some of the ideas to start with, but there are probably many others only constrained to a doctor's imagination.
Naval applications: a fish's swimming motion is highly efficient and could be copied with the help of artificial muscles. As the demonstration videos show, the movement of the muscles could be used as a flipper for nautical motion. The demonstrated performance in water would help to accelerate this research. Fabrication Processes: The deformation of this material with an electrical stimuli can be seen as a highly accurate shape fabrication procedure. For example, the fabrication of small steel devices could be enhanced with a moving mold that we can control electronically. Again, this new technology could solve many fabrication problems depending on the industry needs.
Military applications: Most of the above applications could be military oriented. The materials involved in the device's design so far are predicted to be valid for operation in the military temperature range.
Lines of work
Focus is now made on ionic EAP materials, in particular Ionic Polymer Conductor Composites (IPCC). The reason is that this material is a low voltage intelligent material. It requires very low voltage in order to behave as actuator (3 V), and also works as a mechanical transducer. The activation mechanism of this group of materials is a relocation of the ions inside the structure due to a change in the electrical charge of the material.
The movement of the ions and water molecules inside the material induces a change in the material's volume, which can be used to produce electrically controlled actuation. The opposite effect is observed when used as a mechanical transducer. We manufacture the material at our labs, and after a tedious work, we have engineered several types of specimens, varying different aspects of their manufacturing (electrodes, doping ions, ion-conducting medium and coating.)
In parallel with the chemical work on these materials we are currently working on the final mechanism design and control. The size and design of the actuator will depend on the application, but the fundamental will be unique. Encapsulation requirements, mechanical design, control, speed, and scalability are some of the problems that must be solved to ensure a reliable prototype.
A specific line of research is focused on the characterization of Electro active Polymer materials as intelligent materials. Both transducer and actuating properties of these materials are being fully characterized employing novel procedures and equipment. A specific Unit Tester for Electro active Polymer actuators is being developed, and a project to adapt the equipment to the environmental testing of the materials is on the way.
The material needs to be humid in order to move, although not necessarily with water. Other polar solvents have proved feasible. Controllability of the material and long time operation stability seem to be extremely related to the loss of solvent, either by prolonged operation, or by electrolysis. Novel solvents and special silicones are being tested in order to overcome such limitation.
Preliminary experiments show the material can lift 100 times its own weight, but there is not a clear idea of the circumstances under which it will be able to sustain the required force. We plan to conduct a environmental testing of the material in order to evaluate the feasibility of using the material in a very harsh environment such as space.
Characterizing the materials is a difficult task. There is a lack of instrumentation for the characterization of intelligent materials. We are currently developing our own Unit Tester for the characterization of Electro active Polymer materials
RESEARCH WORKS ON EAP BY MIT
Image: Here, a soliton (blob with red and blue stripes) moves along a conducting polymer chain (aqua and yellow for hydrogen and carbon). The soliton blob causes a localized bend in the chain. The traditional way to make polymer actuate is to dope the material with an ion such as sodium, represented by the red dot. Image courtesy / Yip lab
Currently, robotic muscles move 100 times slower than ours. But engineers using the Yip lab's new theory could boost those speeds - making robotic muscles 1,000 times faster than human muscles - with virtually no extra energy demands and the added bonus of a simpler design. This study appears in the Nov. 4 issue of the journal Physical Review Letters.
In this case, a robotic muscle refers to a device that can be activated to perform a task, like a sprinkler activated by pulling a fire alarm lever, explains Yip, a professor of nuclear engineering and materials science and engineering.
In the past few years, engineers have made the artificial muscles that actuate, or drive, robotic devices from conjugated polymers. "Conjugated polymers are also called conducting polymers because they can carry an electric current, just like a metal wire," says Xi Lin, a postdoctoral associate in Yip's lab. (Conventional polymers like rubber and plastic are insulators and do not conduct electricity.)
Conjugated polymers can actuate on command if charges can be sent to specific locations in the polymer chain in the form of "solitons" (charge density waves). A soliton, short for solitary wave, is "like an ocean wave that can travel long distances without breaking up," Yip adds. Solitons are highly mobile charge carriers that exist because of the special nature (the one-dimensional chain character) of the polymer.
Scientists already knew that solitons enabled the conducting polymers to conduct electricity. Lin's work attempts to explain how these materials can activate devices. This study is useful because until now, scientists, hampered by not knowing the mechanism, have been making conducting polymers in a roundabout way, by bathing (doping) the materials with ions that expand the volume of the polymer. That expansion was thought to give the polymers their strength, but it also makes them heavy and slow.
Lin discovered that adding the ions is unnecessary, because theoretically, shining a light of a particular frequency on the conducting polymer can activate the soliton. Without the extra weight of the added ions, the polymers could bend and flex much more quickly. And that rapid-fire motion gives rise to the high-speed actuation, that is, the ability to activate a device.
To arrive at these conclusions, Lin worked from fundamental principles to understand the physical mechanisms governing conjugated polymers, rather than using experimental data to develop hypotheses about how they worked. He started with SchrÃƒÂ¶dinger's equation, a hallmark of quantum mechanics that describes how a single electron behaves (its wave function). But solving the problem of how a long chain of electrons behaves was another matter, requiring long and complex analyses
Image: New MIT research has suggested another way to make polymer actuate, shown here. The new method is to shine light of a specific frequency, on the conducting polymer. The polymer in this image is a chain (neutral charge, green) that is naturally curved before exposure. The effect of light is to create positive charges (red) in a localized area. The positive charges enhance the chemical bonding between the polymerâ„¢s units and straighten out the curved chain in that area. (This straightening occurs where the red and blue striped lobe appeared in image above. The lobe can move along the polymer chain rapidly.)
Electronic braille readers, agile planetary rovers and solar-powered drones that mimic birds in flight are just some of the envisioned applications that could get a practical workout as developers flex the capabilities of an emerging concept called artificial muscle.
Generally, imitating nature offers many advantages, since nature came up with numerous inventions that work and last. But sometimes, it is better to be inspired by nature rather than make an exact copy. Examples of copying include the use of honeycomb, Velcro, fins for diving and many others. However, copying the wings of birds as a means of flying did not work, and we as humans had to learn the principles of aerodynamics in order to be able to fly. Once those principles were proved out in rudimentary aircraft, subsequent engineering improvements produced machines that far exceeded the capabilities of birds in terms of speed, distance and load capacity. Making artificial muscles at this point falls into the latter category of being inspired by nature rather than imitating nature.
INTERNATIONAL JOURNAL OF ADVANCED ROBOTIC SYSTEMS
MASSACHUSETTES JOURNALS ON ROBOTICS
TABLE OF CONTENTS
1. INTRODUCTION 1
2. NATURE AS A BIOLOGICALLY-INSPIRING MODEL 2
3. THE EVOLUTION OF ARTIFICIAL MUSCLES IN ROBOTICS 3
4. EAP AS ARTIFICIAL MUSCLES 5
5. MAKING ROBOTS ACTUATED BY EAP 9
6. CHALLENGES TO DEVELOPING EAP MATERIALS 10
AS ARTIFICIAL ORGANS
7. SCOPES 12
8. LINES OF WORK 14
9. RESEARCH CHALLENGES 15
10. LATEST RESEARCH WORK ON EAP 16
11. CONCLUSION 19
12. REFERENCES 20