Magnetic refrigeration

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https://en.wikipedia.org/wiki/Magnetic_refrigeration

Gadolinium alloy heats up inside the magnetic field and loses thermal energy to the environment, so it exits the field and becomes cooler than when it entered.

Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.

A magnetocaloric material warms up when a magnetic field is applied. The warming is due to changes in the internal state of the material releasing heat. When the magnetic field is removed, the material returns to its original state, reabsorbing the heat, and returning to original temperature. To achieve refrigeration, the material is allowed to radiate away its heat while in the magnetized hot state. Removing the magnetism, the material then cools to below its original temperature.

The effect was first observed in 1881 by a German physicist Emil Warburg, followed by French physicist P. Weiss and Swiss physicist A. Piccard in 1917. The fundamental principle was suggested by P. Debye (1926) and W. Giauque (1927). The first working magnetic refrigerators were constructed by several groups beginning in 1933. Magnetic refrigeration was the first method developed for cooling below about 0.3 K (a temperature attainable by pumping on ³
He vapors).

Magnetocaloric effect

The magnetocaloric effect (MCE, from magnet and calorie) is a magneto-thermodynamic phenomenon in which a temperature change of a suitable material is caused by exposing the material to a changing magnetic field. This is also known by low temperature physicists as adiabatic demagnetization. In that part of the refrigeration process, a decrease in the strength of an externally applied magnetic field allows the magnetic domains of a magnetocaloric material to become disoriented from the magnetic field by the agitating action of the thermal energy (phonons) present in the material. If the material is isolated so that no energy is allowed to (re)migrate into the material during this time, (i.e., an adiabatic process) the temperature drops as the domains absorb the thermal energy to perform their reorientation. The randomization of the domains occurs in a similar fashion to the randomization at the curie temperature of a ferromagnetic material, except that magnetic dipoles overcome a decreasing external magnetic field while energy remains constant, instead of magnetic domains being disrupted from internal ferromagnetism as energy is added.

One of the most notable examples of the magnetocaloric effect is in the chemical element gadolinium and some of its alloys. Gadolinium's temperature increases when it enters certain magnetic fields. When it leaves the magnetic field, the temperature drops. The effect is considerably stronger for the gadolinium alloy Gd
₅(Si
₂Ge
₂). Praseodymium alloyed with nickel (PrNi
₅) has such a strong magnetocaloric effect that it has allowed scientists to approach to within one millikelvin, one thousandth of a degree of absolute zero.

Equation

The magnetocaloric effect can be quantified with the following equation:

$${\displaystyle \mathrm{\Delta }{T}_{ad}=-{\int }_{H_0}^{H_1}{\left(\frac{T}{C(T,H)}\right)}_H{\left(\frac{\mathrm{\partial }M(T,H)}{\mathrm{\partial }T}\right)}_{H}dH}$$

where ${\displaystyle \mathrm{\Delta }{T}_{ad}}$ is the adiabatic change in temperature of the magnetic system around temperature T, H is the applied external magnetic field, C is the heat capacity of the working magnet (refrigerant) and M is the magnetization of the refrigerant.

From the equation we can see that the magnetocaloric effect can be enhanced by:

• a large field variation
• a magnet material with a small heat capacity
• a magnet with large changes in net magnetization vs. temperature, at constant magnetic field

The adiabatic change in temperature, ${\displaystyle \mathrm{\Delta }{T}_{ad}}$, can be seen to be related to the magnet's change in magnetic entropy (${\displaystyle \mathrm{\Delta }S}$) since

$${\displaystyle \mathrm{\Delta }S(T)={\int }_{H_0}^{H_1}\left(\frac{\mathrm{\partial }M(T,H^{\prime })}{\mathrm{\partial }T}\right)d{H}^{\prime }}$$

This implies that the absolute change in the magnet's entropy determines the possible magnitude of the adiabatic temperature change under a thermodynamic cycle of magnetic field variation. T

Thermodynamic cycle

Analogy between magnetic refrigeration and vapor cycle or conventional refrigeration. H = externally applied magnetic field; Q = heat quantity; P = pressure; ΔT_ad = adiabatic temperature variation

The cycle is performed as a refrigeration cycle that is analogous to the Carnot refrigeration cycle, but with increases and decreases in magnetic field strength instead of increases and decreases in pressure. It can be described at a starting point whereby the chosen working substance is introduced into a magnetic field, i.e., the magnetic flux density is increased. The working material is the refrigerant, and starts in thermal equilibrium with the refrigerated environment.

• Adiabatic magnetization: A magnetocaloric substance is placed in an insulated environment. The increasing external magnetic field (+H) causes the magnetic dipoles of the atoms to align, thereby decreasing the material's magnetic entropy and heat capacity. Since overall energy is not lost (yet) and therefore total entropy is not reduced (according to thermodynamic laws), the net result is that the substance is heated (T + ΔT_ad).
• Isomagnetic enthalpic transfer: This added heat can then be removed (-Q) by a fluid or gas — gaseous or liquid helium, for example. The magnetic field is held constant to prevent the dipoles from reabsorbing the heat. Once sufficiently cooled, the magnetocaloric substance and the coolant are separated (H=0).
• Adiabatic demagnetization: The substance is returned to another adiabatic (insulated) condition so the total entropy remains constant. However, this time the magnetic field is decreased, the thermal energy causes the magnetic moments to overcome the field, and thus the sample cools, i.e., an adiabatic temperature change. Energy (and entropy) transfers from thermal entropy to magnetic entropy, measuring the disorder of the magnetic dipoles.
• Isomagnetic entropic transfer: The magnetic field is held constant to prevent the material from reheating. The material is placed in thermal contact with the environment to be refrigerated. Because the working material is cooler than the refrigerated environment (by design), heat energy migrates into the working material (+Q).

Once the refrigerant and refrigerated environment are in thermal equilibrium, the cycle can restart.

Applied technique

The basic operating principle of an adiabatic demagnetization refrigerator (ADR) is the use of a strong magnetic field to control the entropy of a sample of material, often called the "refrigerant". Magnetic field constrains the orientation of magnetic dipoles in the refrigerant. The stronger the magnetic field, the more aligned the dipoles are, corresponding to lower entropy and heat capacity because the material has (effectively) lost some of its internal degrees of freedom. If the refrigerant is kept at a constant temperature through thermal contact with a heat sink (usually liquid helium) while the magnetic field is switched on, the refrigerant must lose some energy because it is equilibrated with the heat sink. When the magnetic field is subsequently switched off, the heat capacity of the refrigerant rises again because the degrees of freedom associated with orientation of the dipoles are once again liberated, pulling their share of equipartitioned energy from the motion of the molecules, thereby lowering the overall temperature of a system with decreased energy. Since the system is now insulated when the magnetic field is switched off, the process is adiabatic, i.e., the system can no longer exchange energy with its surroundings (the heat sink), and its temperature decreases below its initial value, that of the heat sink.

The operation of a standard ADR proceeds roughly as follows. First, a strong magnetic field is applied to the refrigerant, forcing its various magnetic dipoles to align and putting these degrees of freedom of the refrigerant into a state of lowered entropy. The heat sink then absorbs the heat released by the refrigerant due to its loss of entropy. Thermal contact with the heat sink is then broken so that the system is insulated, and the magnetic field is switched off, increasing the heat capacity of the refrigerant, thus decreasing its temperature below the temperature of the heat sink. In practice, the magnetic field is decreased slowly in order to provide continuous cooling and keep the sample at an approximately constant low temperature. Once the field falls to zero or to some low limiting value determined by the properties of the refrigerant, the cooling power of the ADR vanishes, and heat leaks will cause the refrigerant to warm up.

Working materials

The magnetocaloric effect (MCE) is an intrinsic property of a magnetic solid. This thermal response of a solid to the application or removal of magnetic fields is maximized when the solid is near its magnetic ordering temperature. Thus, the materials considered for magnetic refrigeration devices should be magnetic materials with a magnetic phase transition temperature near the temperature region of interest. For refrigerators that could be used in the home, this temperature is room temperature. The temperature change can be further increased when the order-parameter of the phase transition changes strongly within the temperature range of interest.

The magnitudes of the magnetic entropy and the adiabatic temperature changes are strongly dependent upon the magnetic ordering process. The magnitude is generally small in antiferromagnets, ferrimagnets and spin glass systems but can be much larger for ferromagnets that undergo a magnetic phase transition. First order phase transitions are characterized by a discontinuity in the magnetization changes with temperature, resulting in a latent heat. Second order phase transitions do not have this latent heat associated with the phase transition.

In the late 1990s Pecharksy and Gschneidner reported a magnetic entropy change in Gd
₅(Si
₂Ge
₂) that was about 50% larger than that reported for Gd metal, which had the largest known magnetic entropy change at the time. This giant magnetocaloric effect (GMCE) occurred at 270 K, which is lower than that of Gd (294 K). Since the MCE occurs below room temperature these materials would not be suitable for refrigerators operating at room temperature. Since then other alloys have also demonstrated the giant magnetocaloric effect. These include Gd
₅(Si
_xGe
_1−x)
₄, La(Fe
_xSi
_1−x)
₁₃H
_x and MnFeP
_1−xAs
_x alloys. Gadolinium and its alloys undergo second-order phase transitions that have no magnetic or thermal hysteresis. However, the use of rare earth elements makes these materials very expensive.

Current research has been used to describe alloys with a significant magnetocaloric effect in terms of a thermodynamic system. Literature says that Gd₅(Si₂Ge₂) for example may be described as a thermodynamic system provided it satisfies the condition of being “a quantity of matter or region in space chosen for study”. Such systems have become relevant to modern research in thermodynamics because they serve as plausible materials for the creation of high performance thermoelectric materials.

Ni
₂Mn-X (X = Ga, Co, In, Al, Sb) Heusler alloys are also promising candidates for magnetic cooling applications because they have Curie temperatures near room temperature and, depending on composition, can have martensitic phase transformations near room temperature. These materials exhibit the magnetic shape memory effect and can also be used as actuators, energy harvesting devices, and sensors. When the martensitic transformation temperature and the Curie temperature are the same (based on composition) the magnitude of the magnetic entropy change is the largest. In February 2014, GE announced the development of a functional Ni-Mn-based magnetic refrigerator.

The development of this technology is very material-dependent and will likely not replace vapor-compression refrigeration without significantly improved materials that are cheap, abundant, and exhibit much larger magnetocaloric effects over a larger range of temperatures. Such materials need to show significant temperature changes under a field of two tesla or less, so that permanent magnets can be used for the production of the magnetic field.

Paramagnetic salts

The original proposed refrigerant was a paramagnetic salt, such as cerium magnesium nitrate. The active magnetic dipoles in this case are those of the electron shells of the paramagnetic atoms.

In a paramagnetic salt ADR, the heat sink is usually provided by a pumped ⁴
He (about 1.2 K) or ³
He (about 0.3 K) cryostat. An easily attainable 1 T magnetic field is generally required for initial magnetization. The minimum temperature attainable is determined by the self-magnetization tendencies of the refrigerant salt, but temperatures from 1 to 100 mK are accessible. Dilution refrigerators had for many years supplanted paramagnetic salt ADRs, but interest in space-based and simple to use lab-ADRs has remained, due to the complexity and unreliability of the dilution refrigerator.

At a low enough temperature, paramagnetic salts become either diamagnetic or ferromagnetic, limiting the lowest temperature that can be reached using this method.

Nuclear demagnetization

One variant of adiabatic demagnetization that continues to find substantial research application is nuclear demagnetization refrigeration (NDR). NDR follows the same principles, but in this case the cooling power arises from the magnetic dipoles of the nuclei of the refrigerant atoms, rather than their electron configurations. Since these dipoles are of much smaller magnitude, they are less prone to self-alignment and have lower intrinsic minimum fields. This allows NDR to cool the nuclear spin system to very low temperatures, often 1 µK or below. Unfortunately, the small magnitudes of nuclear magnetic dipoles also makes them less inclined to align to external fields. Magnetic fields of 3 teslas or greater are often needed for the initial magnetization step of NDR.

In NDR systems, the initial heat sink must sit at very low temperatures (10–100 mK). This precooling is often provided by the mixing chamber of a dilution refrigerator or a paramagnetic salt.

Commercial development

Research and a demonstration proof of concept device in 2001 succeeded in applying commercial-grade materials and permanent magnets at room temperatures to construct a magnetocaloric refrigerator.

On August 20, 2007, the Risø National Laboratory (Denmark) at the Technical University of Denmark, claimed to have reached a milestone in their magnetic cooling research when they reported a temperature span of 8.7 K. They hoped to introduce the first commercial applications of the technology by 2010.

As of 2013 this technology had proven commercially viable only for ultra-low temperature cryogenic applications available for decades. Magnetocaloric refrigeration systems are composed of pumps, motors, secondary fluids, heat exchangers of different types, magnets and magnetic materials. These processes are greatly affected by irreversibilities and should be adequately considered. At year-end, Cooltech Applications announced that its first commercial refrigeration equipment would enter the market in 2014. Cooltech Applications launched their first commercially available magnetic refrigeration system on 20 June 2016. At the 2015 Consumer Electronics Show in Las Vegas, a consortium of Haier, Astronautics Corporation of America and BASF presented the first cooling appliance. BASF claim of their technology a 35% improvement over using compressors.

In November 2015, at the Medica 2015 fair, Cooltech Applications presented, in collaboration with Kirsch medical GmbH, the world's first magnetocaloric medical cabinet. One year later, in September 2016, at the 7th International Conference on Magnetic Refrigeration at Room Temperature (Thermag VII)] held in Torino, Italy, Cooltech Applications presented the world's first magnetocaloric frozen heat exchanger.

In 2017, Cooltech Applications presented a fully functional 500 liters' magnetocaloric cooled cabinet with a 30 kg (66 lb) load and an air temperature inside the cabinet of +2 °C. That proved that magnetic refrigeration is a mature technology, capable of replacing the classic refrigeration solutions.

One year later, in September 2018, at the 8th International Conference on Magnetic Refrigeration at Room Temperature (Thermag VIII]), Cooltech Applications presented a paper on a magnetocaloric prototype designed as a 15 kW proof-of-concept unit. This has been considered by the community as the largest magnetocaloric prototype ever created.

At the same conference, Dr. Sergiu Lionte announced that, due to financial issues, Cooltech Applications declared bankruptcy. Later on, in 2019 Ubiblue company, today named Magnoric, is formed by some of the old Cooltech Application's team members. The entire patent portfolio form Cooltech Applications was taken over by Magnoric since then, while publishing additional patents at the same time.

In 2019, at the 5th Delft Days Conference on Magnetocalorics, Dr. Sergiu Lionte presented Ubiblue's (former Cooltech Application) last prototype. Later, the magnetocaloric community acknowledged that Ubiblue had the most developed magnetocalorics prototypes.

Thermal and magnetic hysteresis problems remain to be solved for first-order phase transition materials that exhibit the GMCE.

One potential application is in spacecraft.

Vapor-compression refrigeration units typically achieve performance coefficients of 60% of that of a theoretical ideal Carnot cycle, much higher than current MR technology. Small domestic refrigerators are however much less efficient.

In 2014 giant anisotropic behaviour of the magnetocaloric effect was found in HoMn
₂O
₅ at 10 K. The anisotropy of the magnetic entropy change gives rise to a large rotating MCE offering the possibility to build simplified, compact, and efficient magnetic cooling systems by rotating it in a constant magnetic field.

In 2015 Aprea et al. presented a new refrigeration concept, GeoThermag, which is a combination of magnetic refrigeration technology with that of low-temperature geothermal energy. To demonstrate the applicability of the GeoThermag technology, they developed a pilot system that consists of a 100-m deep geothermal probe; inside the probe, water flows and is used directly as a regenerating fluid for a magnetic refrigerator operating with gadolinium. The GeoThermag system showed the ability to produce cold water even at 281.8 K in the presence of a heat load of 60 W. In addition, the system has shown the existence of an optimal frequency f AMR, 0.26 Hz, for which it was possible to produce cold water at 287.9 K with a thermal load equal to 190 W with a COP of 2.20. Observing the temperature of the cold water that was obtained in the tests, the GeoThermag system showed a good ability to feed the cooling radiant floors and a reduced capacity for feeding the fan coil systems.

History

The effect was discovered first observed by German physicist Emil Warburg in 1881, subsequently by French physicist Pierre Weiss and Swiss physicist Auguste Piccard in 1917.

Major advances first appeared in the late 1920s when cooling via adiabatic demagnetization was independently proposed by chemistry Nobel Laureates Peter Debye in 1926 and William F. Giauque in 1927.

It was first demonstrated experimentally by Giauque and his colleague D. P. MacDougall in 1933 for cryogenic purposes when they reached 0.25 K. Between 1933 and 1997, advances in MCE cooling occurred.

In 1997, the first near room-temperature proof of concept magnetic refrigerator was demonstrated by Karl A. Gschneidner, Jr. by the Iowa State University at Ames Laboratory. This event attracted interest from scientists and companies worldwide who started developing new kinds of room temperature materials and magnetic refrigerator designs.

A major breakthrough came 2002 when a group at the University of Amsterdam demonstrated the giant magnetocaloric effect in MnFe(P,As) alloys that are based on abundant materials.

Refrigerators based on the magnetocaloric effect have been demonstrated in laboratories, using magnetic fields starting at 0.6 T up to 10 T. Magnetic fields above 2 T are difficult to produce with permanent magnets and are produced by a superconducting magnet (1 T is about 20.000 times the Earth's magnetic field).

Electronic skin

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Electronic_skin 

Electronic skin refers to flexible, stretc hable and self-healing electronics that are able to mimic functionalities of human or animal skin. The broad class of materials often contain sensing abilities that are intended to reproduce the capabilities of human skin to respond to environmental factors such as changes in heat and pressure.

Advances in electronic skin research focuses on designing materials that are stretchy, robust, and flexible. Research in the individual fields of flexible electronics and tactile sensing has progressed greatly; however, electronic skin design attempts to bring together advances in many areas of materials research without sacrificing individual benefits from each field. The successful combination of flexible and stretchable mechanical properties with sensors and the ability to self-heal would open the door to many possible applications including soft robotics, prosthetics, artificial intelligence and health monitoring.

Recent advances in the field of electronic skin have focused on incorporating green materials ideals and environmental awareness into the design process. As one of the main challenges facing electronic skin development is the ability of the material to withstand mechanical strain and maintain sensing ability or electronic properties, recyclability and self-healing properties are especially critical in the future design of new electronic skins.

Rehealable electronic skin

Self-healing abilities of electronic skin are critical to potential applications of electronic skin in fields such as soft robotics. Proper design of self-healing electronic skin requires not only healing of the base substrate but also the reestablishment of any sensing functions such as tactile sensing or electrical conductivity. Ideally, the self-healing process of electronic skin does not rely upon outside stimulation such as increased temperature, pressure, or solvation. Self-healing, or rehealable, electronic skin is often achieved through a polymer-based material or a hybrid material.

Polymer-based materials

In 2018, Zou et al. published work on electronic skin that is able to reform covalent bonds when damaged. The group looked at a polyimine-based crosslinked network, synthesized as seen in Figure 1. The e-skin is considered rehealable because of "reversible bond exchange," meaning that the bonds holding the network together are able to break and reform under certain conditions such as solvation and heating. The rehealable and reusable aspect of such a thermoset material is unique because many thermoset materials irreversibly form crosslinked networks through covalent bonds. In the polymer network the bonds formed during the healing process are indistinguishable from the original polymer network.

Figure 1. Polymerization scheme for formation of polyimine-based self-healing electronic skin.

Dynamic non-covalent crosslinking has also been shown to form a polymer network that is rehealable. In 2016, Oh et al. looked specifically at semiconducting polymers for organic transistors. They found that incorporating 2,6-pyridine dicarboxamide (PDCA) into the polymer backbone could impart self-healing abilities based on the network of hydrogen bonds formed between groups. With incorporation of PDCA in the polymer backbone, the materials was able to withstand up to 100% strain without showing signs of microscale cracking. In this example, the hydrogen bonds are available for energy dissipation as the strain increases.

Hybrid materials

Polymer networks are able to facilitate dynamic healing processes through hydrogen bonds or dynamic covalent chemistry. However, the incorporation of inorganic particles can greatly expand the functionality of polymer-based materials for electronic skin applications. The incorporation of micro-structured nickel particles into a polymer network (Figure 2) has been shown to maintain self-healing properties based on the reformation of hydrogen bonding networks around the inorganic particles. The material is able to regain its conductivity within 15 seconds of breakage, and the mechanical properties are regained after 10 minutes at room temperature without added stimulus. This material relies on hydrogen bonds formed between urea groups when they align. The hydrogen atoms of urea functional groups are ideally situated to form a hydrogen-bonding network because they are near an electron-withdrawing carbonyl group. This polymer network with embedded nickel particles demonstrates the possibility of using polymers as supramolecular hosts to develop self-healing conductive composites.

Figure 2. Self-healing material based on hydrogen bonding and interactions with micro-structured nickel particles.

Flexible and porous graphene foams that are interconnected in a 3D manner have also been shown to have self-healing properties. Thin film with poly(N,N-dimethylacrylamide)-poly(vinyl alcohol) (PDMAA) and reduced graphene oxide have shown high electrical conductivity and self-healing properties. The healing abilities of the hybrid composite are suspected to be due to the hydrogen bonds between the PDMAA chains, and the healing process is able to restore initial length and recover conductive properties.

Recyclable electronic skin

Zou et al. presents an interesting advance in the field of electronic skin that can be used in robotics, prosthetics, and many other applications in the form of a fully recyclable electronic skin material. The e-skin developed by the group consists of a network of covalently bound polymers that are thermoset, meaning cured at a specific temperature. However, the material is also recyclable and reusable. Because the polymer network is thermoset, it is chemically and thermally stable. However, at room temperature, the polyimine material, with or without silver nanoparticles, can be dissolved on the timescale of a few hours. The recycling process allows devices, which are damaged beyond self-healing capabilities, to be dissolved and formed into new devices (Figure 3). This advance opens the door for lower cost production and greener approaches to e-skin development.

Figure 3. Recycling process for conductive polyimine-based e-skin.

Flexible and stretchy electronic skin

The ability of electronic skin to withstand mechanical deformation including stretching and flexing without losing functionality is crucial for its applications as prosthetics, artificial intelligence, soft robotics, health monitoring, biocompatibility, and communication devices. Flexible electronics are often designed by depositing electronic materials on flexible polymer substrates, thereby relying on an organic substrate to impart favorable mechanical properties. Stretchable e-skin materials have been approached from two directions. Hybrid materials can rely on an organic network for stretchiness while embedding inorganic particles or sensors, which are not inherently stretchable. Other research has focused on developing stretchable materials that also have favorable electronic or sensing capabilities.

Zou et al. studied the inclusion of linkers that are described as "serpentine" in their polyimine matrix. These linkers make the e-skin sensors able to flex with movement and distortion. The incorporation of alkyl spacers in polymer-based materials has also been shown to increase flexibility without decreasing charge transfer mobility. Oh et al. developed a stretchable and flexible material based on 3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and non-conjugated 2,6-pyridine dicarboxamide (PDCA) as a source of hydrogen bonds (Figure 4).

Figure 4. A stretchable and self-healing semiconducting polymer-based material.

Graphene has also been shown to be a suitable material for electronic skin applications as well due to its stiffness and tensile strength. Graphene is an appealing material because its synthesis to flexible substrates is scalable and cost-efficient.

Mechanical Properties of Skin

Skin is composed of collagen, keratin, and elastin fibers, which provide robust mechanical strength, low modulus, tear resistance, and softness. The skin can be considered as a bilayer of epidermis and dermis. The epidermal layer has a modulus of about 140-600 kPa and a thickness of 0.05-1.5 mm. Dermis has a modulus of 2-80 kPa and a thickness of 0.3-3 mm. This bilayer skin exhibits an elastic linear response for strains less than 15% and a non linear response at larger strains. To achieve conformability, it is preferable for devices to match the mechanical properties of the epidermis layer when designing skin-based stretchy electronics.

Tuning Mechanical Properties

Conventional high performance electronic devices are made of inorganic materials such as silicon, which is rigid and brittle in nature and exhibits poor biocompatibility due to mechanical mismatch between the skin and the device, making skin integrated electronics applications difficult. To solve this challenge, researchers employed the method of constructing flexible electronics in the form of ultrathin layers. The resistance to bending of a material object (Flexural rigidity) is related to the third power of the thickness, according to the Euler-Bernoulli equation for a beam. It implies that objects with less thickness can bend and stretch more easily. As a result, even though the material has a relatively high Young's modulus, devices manufactured on ultrathin substrates exhibit a decrease in bending stiffness and allow bending to a small radius of curvature without fracturing. Thin devices have been developed as a result of significant advancements in the field of nanotechnology, fabrication, and manufacturing. The aforementioned approach was used to create devices composed of 100-200 nm thick Si nano membranes deposited on thin flexible polymeric substrates.

Furthermore, structural design considerations can be used to tune the mechanical stability of the devices. Engineering the original surface structure allows us to soften the stiff electronics. Buckling, island connection, and the Kirigami concept have all been employed successfully to make the entire system stretchy.

Mechanical buckling can be used to create wavy structures on elastomeric thin substrates. This feature improves the device's stretchability. The buckling approach was used to create Si nanoribbons from single crystal Si on an elastomeric substrate. The study demonstrated the device could bear a maximum strain of 10% when compressed and stretched.

In the case of island interconnect, the rigid material connects with flexible bridges made from different geometries, such as zig-zag, serpentine-shaped structures, etc., to reduce the effective stiffness, tune the stretchability of the system, and elastically deform under applied strains in specific directions. It has been demonstrated that serpentine-shaped structures have no significant effect on the electrical characteristics of epidermal electronics. It has also been shown that the entanglement of the interconnects, which oppose the movement of the device above the substrate, causes the spiral interconnects to stretch and deform significantly more than the serpentine structures. CMOS inverters constructed on a PDMS substrate employing 3D island interconnect technologies demonstrated 140% strain at stretching.

Kirigami is built around the concept of folding and cutting in 2D membranes. This contributes to an increase in the tensile strength of the substrate, as well as its out-of-plane deformation and stretchability. These 2D structures can subsequently be turned to 3D structures with varied topography, shape, and size controllability via the Buckling process, resulting in interesting properties and applications.

Conductive electronic skin

The development of conductive electronic skin is of interest for many electrical applications. Research into conductive electronic skin has taken two routes: conductive self-healing polymers or embedding conductive inorganic materials in non-conductive polymer networks.

The self-healing conductive composite synthesized by Tee et al. (Figure 2) investigated the incorporation of micro-structured nickel particles into a polymer host. The nickel particles adhere to the network though favorable interactions between the native oxide layer on the surface of the particles and the hydrogen-bonding polymer.

Nanoparticles have also been studied for their ability to impart conductivity on electronic skin materials. Zou et al. embedded silver nanoparticles (AgNPs) into a polymer matrix, making the e-skin conductive. The healing process for this material is noteworthy because it not only restores the mechanical properties of the polymer network, but also restores the conductive properties when silver nanoparticles have been embedded in the polymer network.

Sensing ability of electronic skin

Some of the challenges that face electronic skin sensing abilities include the fragility of sensors, the recovery time of sensors, repeatability, overcoming mechanical strain, and long-term stability.

Tactile sensors

Applied pressure can be measured by monitoring changes in resistance or capacitance. Coplanar interdigitated electrodes embedded on single-layer graphene have been shown to provide pressure sensitivity for applied pressure as low as 0.11 kPa through measuring changes in capacitance. Piezoresistive sensors have also shown high levels of sensitivity.

Ultrathin molybdenum disulfide sensing arrays integrated with graphene have demonstrated promising mechanical properties capable of pressure sensing. Modifications of organic field effect transistors (OFETs) have shown promise in electronic skin applications. Microstructured polydimethylsiloxane thin films can elastically deform when pressure is applied. The deformation of the thin film allows for storage and release of energy.

Visual representation of applied pressure has been one area of interest in development of tactile sensors. The Bao Group at Stanford University have designed an electrochromically active electronic skin that changes color with different amounts of applied pressure. Applied pressure can also be visualized by incorporation of active-matrix organic light-emitting diode displays which emit light when pressure is applied.

Prototype e-skins include a printed synaptic transistor–based electronic skin giving skin-like haptic sensations and touch/pain-sensitivity to a robotic hand, and a multilayer tactile sensor repairable hydrogel-based robot skin.

Other sensing applications

Humidity sensors have been incorporated in electronic skin design with sulfurized tungsten films. The conductivity of the film changes with different levels of humidity. Silicon nanoribbons have also been studied for their application as temperature, pressure, and humidity sensors. Scientists at the University of Glasgow have made inroads in developing an e-skin that feels pain real-time, with applications in prosthetics and more life-like humanoids.

A system of an electronic skin and a human-machine interface that can enable remote sensed tactile perception, and wearable or robotic sensing of many hazardous substances and pathogens.

Vibration isolation

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Vibration_isolation 

Vibration isolation is the prevention of transmission of vibration from one component of a system to others parts of the same system, as in buildings or mechanical systems. Vibration is undesirable in many domains, primarily engineered systems and habitable spaces, and methods have been developed to prevent the transfer of vibration to such systems. Vibrations propagate via mechanical waves and certain mechanical linkages conduct vibrations more efficiently than others. Passive vibration isolation makes use of materials and mechanical linkages that absorb and damp these mechanical waves. Active vibration isolation involves sensors and actuators that produce disruptive interference that cancels-out incoming vibration.

Passive isolation

"Passive vibration isolation" refers to vibration isolation or mitigation of vibrations by passive techniques such as rubber pads or mechanical springs, as opposed to "active vibration isolation" or "electronic force cancellation" employing electric power, sensors, actuators, and control systems.

Passive vibration isolation is a vast subject, since there are many types of passive vibration isolators used for many different applications. A few of these applications are for industrial equipment such as pumps, motors, HVAC systems, or washing machines; isolation of civil engineering structures from earthquakes (base isolation), sensitive laboratory equipment, valuable statuary, and high-end audio.

A basic understanding of how passive isolation works, the more common types of passive isolators, and the main factors that influence the selection of passive isolators:

Common passive isolation systems

Pneumatic or air isolators

These are bladders or canisters of compressed air. A source of compressed air is required to maintain them. Air springs are rubber bladders which provide damping as well as isolation and are used in large trucks. Some pneumatic isolators can attain low resonant frequencies and are used for isolating large industrial equipment. Air tables consist of a working surface or optical surface mounted on air legs. These tables provide enough isolation for laboratory instrument under some conditions. Air systems may leak under vacuum conditions. The air container can interfere with isolation of low-amplitude vibration.

Mechanical springs and spring-dampers

These are heavy-duty isolators used for building systems and industry. Sometimes they serve as mounts for a concrete block, which provides further isolation.

Pads or sheets of flexible materials such as elastomers, rubber, cork, dense foam and laminate materials.

Elastomer pads, dense closed cell foams and laminate materials are often used under heavy machinery, under common household items, in vehicles and even under higher performing audio systems.

Molded and bonded rubber and elastomeric isolators and mounts

These are often used as machinery (such as engines) mounts or in vehicles. They absorb shock and attenuate some vibration.

Negative-stiffness isolators

Negative-stiffness isolators are less common than other types and have generally been developed for high-level research applications such as gravity wave detection. Lee, Goverdovskiy, and Temnikov (2007) proposed a negative-stiffness system for isolating vehicle seats.

The focus on negative-stiffness isolators has been on developing systems with very low resonant frequencies (below 1 Hz), so that low frequencies can be adequately isolated, which is critical for sensitive instrumentation. All higher frequencies are also isolated. Negative-stiffness systems can be made with low stiction, so that they are effective in isolating low-amplitude vibrations.

Negative-stiffness mechanisms are purely mechanical and typically involve the configuration and loading of components such as beams or inverted pendulums. Greater loading of the negative-stiffness mechanism, within the range of its operability, decreases the natural frequency.

Wire rope isolators

Coiled Cable Mount

These isolators are durable and can withstand extreme environments. They are often used in military applications.

Base isolators for seismic isolation of buildings, bridges, etc.

Base isolators made of layers of neoprene and steel with a low horizontal stiffness are used to lower the natural frequency of the building. Some other base isolators are designed to slide, preventing the transfer of energy from the ground to the building.

Tuned mass dampers

Tuned mass dampers reduce the effects of harmonic vibration in buildings or other structures. A relatively small mass is attached in such a way that it can dampen out a very narrow band of vibration of the structure.

Do it Yourself Isolators

In less sophisticated solutions, bungee cords can be used as a cheap isolation system which may be effective enough for some applications. The item to be isolated is suspended from the bungee cords. This is difficult to implement without a danger of the isolated item falling. Tennis balls cut in half have been used under washing machines and other items with some success. In fact, tennis balls became the de facto standard suspension technique used in DIY rave/DJ culture, placed under the feet of each record turntable which produces enough dampening to neutralize the vibrations of high-powered soundsystems from affecting the delicate, high-sensitivity mechanisms of the turntable needles.

How passive isolation works

A passive isolation system, such as a shock mount, in general contains mass, spring, and damping elements and moves as a harmonic oscillator. The mass and spring stiffness dictate a natural frequency of the system. Damping causes energy dissipation and has a secondary effect on natural frequency.

Passive Vibration Isolation

Every object on a flexible support has a fundamental natural frequency. When vibration is applied, energy is transferred most efficiently at the natural frequency, somewhat efficiently below the natural frequency, and with increasing inefficiency (decreasing efficiency) above the natural frequency. This can be seen in the transmissibility curve, which is a plot of transmissibility vs. frequency.

Here is an example of a transmissibility curve. Transmissibility is the ratio of vibration of the isolated surface to that of the source. Vibrations are never eliminated, but they can be greatly reduced. The curve below shows the typical performance of a passive, negative-stiffness isolation system with a natural frequency of 0.5 Hz. The general shape of the curve is typical for passive systems. Below the natural frequency, transmissibility hovers near 1. A value of 1 means that vibration is going through the system without being amplified or reduced. At the resonant frequency, energy is transmitted efficiently, and the incoming vibration is amplified. Damping in the system limits the level of amplification. Above the resonant frequency, little energy can be transmitted, and the curve rolls off to a low value. A passive isolator can be seen as a mechanical low-pass filter for vibrations.

negative-stiffness transmissibility

In general, for any given frequency above the natural frequency, an isolator with a lower natural frequency will show greater isolation than one with a higher natural frequency. The best isolation system for a given situation depends on the frequency, direction, and magnitude of vibrations present and the desired level of attenuation of those frequencies.

All mechanical systems in the real world contain some amount of damping. Damping dissipates energy in the system, which reduces the vibration level which is transmitted at the natural frequency. The fluid in automotive shock absorbers is a kind of damper, as is the inherent damping in elastomeric (rubber) engine mounts.

Damping is used in passive isolators to reduce the amount of amplification at the natural frequency. However, increasing damping tends to reduce isolation at the higher frequencies. As damping is increased, transmissibility roll-off decreases. This can be seen in the chart below.

Damping effect on transmissibility

Passive isolation operates in both directions, isolating the payload from vibrations originating in the support, and also isolating the support from vibrations originating in the payload. Large machines such as washers, pumps, and generators, which would cause vibrations in the building or room, are often isolated from the floor. However, there are a multitude of sources of vibration in buildings, and it is often not possible to isolate each source. In many cases, it is most efficient to isolate each sensitive instrument from the floor. Sometimes it is necessary to implement both approaches.

In Superyachts, the engines and alternators produce noise and vibrations. To solve this, the solution is a double elastic suspension where the engine and alternator are mounted with vibration dampers on a common frame. This set is then mounted elastically between the common frame and the hull.

Factors influencing the selection of passive vibration isolators

1. Characteristics of item to be isolated
   • Size: The dimensions of the item to be isolated help determine the type of isolation which is available and appropriate. Small objects may use only one isolator, while larger items might use a multiple-isolator system.
   • Weight: The weight of the object to be isolated is an important factor in choosing the correct passive isolation product. Individual passive isolators are designed to be used with a specific range of loading.
   • Movement: Machines or instruments with moving parts may affect isolation systems. It is important to know the mass, speed, and distance traveled of the moving parts.
2. Operating Environment
   • Industrial: This generally entails strong vibrations over a wide band of frequencies and some amount of dust.
   • Laboratory: Labs are sometimes troubled by specific building vibrations from adjacent machinery, foot traffic, or HVAC airflow.
   • Indoor or outdoor: Isolators are generally designed for one environment or the other.
   • Corrosive/non-corrosive: Some indoor environments may present a corrosive danger to isolator components due to the presence of corrosive chemicals. Outdoors, water and salt environments need to be considered.
   • Clean room: Some isolators can be made appropriate for clean room.
   • Temperature: In general, isolators are designed to be used in the range of temperatures normal for human environments. If a larger range of temperatures is required, the isolator design may need to be modified.
   • Vacuum: Some isolators can be used in a vacuum environment. Air isolators may have leakage problems. Vacuum requirements typically include some level of clean room requirement and may also have a large temperature range.
   • Magnetism: Some experimentation which requires vibration isolation also requires a low-magnetism environment. Some isolators can be designed with low-magnetism components.
   • Acoustic noise: Some instruments are sensitive to acoustic vibration. In addition, some isolation systems can be excited by acoustic noise. It may be necessary to use an acoustic shield. Air compressors can create problematic acoustic noise, heat, and airflow.
   • Static or dynamic loads: This distinction is quite important as isolators are designed for a certain type and level of loading.
   • ; Static loading

     is basically the weight of the isolated object with low-amplitude vibration input. This is the environment of apparently stationary objects such as buildings (under normal conditions) or laboratory instruments.

   • ; Dynamic loading

     involves accelerations and larger amplitude shock and vibration. This environment is present in vehicles, heavy machinery, and structures with significant movement.

3. Cost:
   • Cost of providing isolation: Costs include the isolation system itself, whether it is a standard or custom product; a compressed air source if required; shipping from manufacturer to destination; installation; maintenance; and an initial vibration site survey to determine the need for isolation.
   • Relative costs of different isolation systems: Inexpensive shock mounts may need to be replaced due to dynamic loading cycles. A higher level of isolation which is effective at lower vibration frequencies and magnitudes generally costs more. Prices can range from a few dollars for bungee cords to millions of dollars for some space applications.
4. Adjustment: Some isolation systems require manual adjustment to compensate for changes in weight load, weight distribution, temperature, and air pressure, whereas other systems are designed to automatically compensate for some or all of these factors.
5. Maintenance: Some isolation systems are quite durable and require little or no maintenance. Others may require periodic replacement due to mechanical fatigue of parts or aging of materials.
6. Size Constraints: The isolation system may have to fit in a restricted space in a laboratory or vacuum chamber, or within a machine housing.
7. Nature of vibrations to be isolated or mitigated
   • Frequencies: If possible, it is important to know the frequencies of ambient vibrations. This can be determined with a site survey or accelerometer data processed through FFT analysis.
   • Amplitudes: The amplitudes of the vibration frequencies present can be compared with required levels to determine whether isolation is needed. In addition, isolators are designed for ranges of vibration amplitudes. Some isolators are not effective for very small amplitudes.
   • Direction: Knowing whether vibrations are horizontal or vertical can help to target isolation where it is needed and save money.
8. Vibration specifications of item to be isolated: Many instruments or machines have manufacturer-specified levels of vibration for the operating environment. The manufacturer may not guarantee the proper operation of the instrument if vibration exceeds the spec.
9. Not For Profit Organizations such as ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) and VISCMA (Vibration Isolation and Seismic Control Manufacturers Association) provide specifications / standards for isolator types and spring deflection requirements that cover a wide array of industries including electrical, mechanical, plumbing, and HVAC.

Comparison of passive isolators

Type of Passive Isolation	Applications	Typical Natural Frequency
Air Isolators	Large industrial equipment, some optics and instruments	1.5 – 3 Hz, large systems customized to 0.5 Hz
Springs or spring dampers	Heavy loads, pumps, compressors	3 – 9 Hz
Elastomer or cork pads	Large high-load applications where isolation of medium to high frequency noise and vibration is required	3 – 40 Hz, depending on size of pad and load
Molded or bonded elastomer mounts	Machinery, instruments, vehicles, aviation	10 - 20+ Hz
Negative-stiffness isolators	Electron microscopes, sensitive instruments, optics and laser systems, cryogenic systems	0.17 - 2.5 Hz
Wire rope isolators	Machinery, instruments, vehicles, aviation	10 - 40+ Hz
Bungee cord isolators	Laboratory, home, etc.	Depends on type of cord and the mass they support
Base isolators	Buildings and large structures	Low, seismic frequencies
Tuned Mass Dampers	Buildings, large structures, aerospace	Any, but usually used at low frequencies

Negative-stiffness vibration isolator

Negative-Stiffness-Mechanism (NSM) vibration isolation systems offer a unique passive approach for achieving low vibration environments and isolation against sub-Hertz vibrations. "Snap-through" or "over-center" NSM devices are used to reduce the stiffness of elastic suspensions and create compact six-degree-of-freedom systems with low natural frequencies. Practical systems with vertical and horizontal natural frequencies as low as 0.2 to 0.5 Hz are possible. Electro-mechanical auto-adjust mechanisms compensate for varying weight loads and provide automatic leveling in multiple-isolator systems, similar to the function of leveling valves in pneumatic systems. All-metal systems can be configured which are compatible with high vacuums and other adverse environments such as high temperatures.

These isolation systems enable vibration-sensitive instruments such as scanning probe microscopes, micro-hardness testers and scanning electron microscopes to operate in severe vibration environments sometimes encountered, for example, on upper floors of buildings and in clean rooms. Such operation would not be practical with pneumatic isolation systems. Similarly, they enable vibration-sensitive instruments to produce better images and data than those achievable with pneumatic isolators.

The theory of operation of NSM vibration isolation systems is summarized, some typical systems and applications are described, and data on measured performance is presented. The theory of NSM isolation systems is explained in References 1 and 2. It is summarized briefly for convenience.

Vertical-motion isolation

A vertical-motion isolator is shown . It uses a conventional spring connected to an NSM consisting of two bars hinged at the center, supported at their outer ends on pivots, and loaded in compression by forces P. The spring is compressed by weight W to the operating position of the isolator, as shown in Figure 1. The stiffness of the isolator is K=KS-KN where KS is the spring stiffness and KN is the magnitude of a negative-stiffness which is a function of the length of the bars and the load P. The isolator stiffness can be made to approach zero while the spring supports the weight W.

Horizontal-motion isolation

A horizontal-motion isolator consisting of two beam-columns is illustrated in Figure. 2. Each beam-column behaves like two fixed-free beam columns loaded axially by a weight load W. Without the weight load the beam-columns have horizontal stiffness KS With the weight load the lateral bending stiffness is reduced by the "beam-column" effect. This behavior is equivalent to a horizontal spring combined with an NSM so that the horizontal stiffness is ${\displaystyle K=K_S-K_N}$, and ${\displaystyle K_N}$ is the magnitude of the beam-column effect. Horizontal stiffness can be made to approach zero by loading the beam-columns to approach their critical buckling load.

Six-degree-of-freedom (six-DOF) isolation

A six-DOF NSM isolator typically uses three isolators stacked in series: a tilt-motion isolator on top of a horizontal-motion isolator on top of a vertical-motion isolator. Figure 3 (Ref. needed) shows a schematic of a vibration isolation system consisting of a weighted platform supported by a single six-DOF isolator incorporating the isolators of Figures 1 and 2 (Figures 1 and 2 are missing). Flexures are used in place of the hinged bars shown in Figure 1. A tilt flexure serves as the tilt-motion isolator. A vertical-stiffness adjustment screw is used to adjust the compression force on the negative-stiffness flexures thereby changing the vertical stiffness. A vertical load adjustment screw is used to adjust for varying weight loads by raising or lowering the base of the support spring to keep the flexures in their straight, unbent operating positions.

Vibration isolation of supporting joint

The equipment or other mechanical components are necessarily linked to surrounding objects (the supporting joint - with the support; the unsupporting joint - the pipe duct or cable), thus presenting the opportunity for unwanted transmission of vibrations. Using a suitably designed vibration-isolator (absorber), vibration isolation of the supporting joint is realized. The accompanying illustration shows the attenuation of vibration levels, as measured before installation of the functioning gear on a vibration isolator as well as after installation, for a wide range of frequencies.

The vibration isolator

This is defined as a device that reflects and absorbs waves of oscillatory energy, extending from a piece of working machinery or electrical equipment, and with the desired effect being vibration insulation. The goal is to establish vibration isolation between a body transferring mechanical fluctuations and a supporting body (for example, between the machine and the foundation). The illustration shows a vibration isolator from the series «ВИ» (~"VI" in Roman characters), as used in shipbuilding in Russia, for example the submarine "St.Petersburg" (Lada). The depicted «ВИ» devices allow loadings ranging from 5, 40 and 300 kg. They differ in their physical sizes, but all share the same fundamental design. The structure consists of a rubber envelope that is internally reinforced by a spring. During manufacture, the rubber and the spring are intimately and permanently connected as a result of the vulcanization process that is integral to the processing of the crude rubber material. Under action of weight loading of the machine, the rubber envelope deforms, and the spring is compressed or stretched. Therefore, in the direction of the spring's cross section, twisting of the enveloping rubber occurs. The resulting elastic deformation of the rubber envelope results in very effective absorption of the vibration. This absorption is crucial to reliable vibration insulation, because it averts the potential for resonance effects. The amount of elastic deformation of the rubber largely dictates the magnitude of vibration absorption that can be attained; the entire device (including the spring itself) must be designed with this in mind. The design of the vibration isolator must also take into account potential exposure to shock loadings, in addition to the routine everyday vibrations. Lastly, the vibration isolator must also be designed for long-term durability as well as convenient integration into the environment in which it is to be used. Sleeves and flanges are typically employed in order to enable the vibration isolator to be securely fastened to the equipment and the supporting foundation.

Vibration isolation of unsupporting joint

Vibration isolation of unsupporting joint is realized in the device named branch pipe a of isolating vibration.

Branch pipe a of isolating vibration

Branch pipe a of isolating vibration is a part of a tube with elastic walls for reflection and absorption of waves of the oscillatory energy extending from the working pump over wall of the pipe duct. Is established between the pump and the pipe duct. On an illustration is presented the image a vibration-isolating branch pipe of a series «ВИПБ». In a structure is used the rubber envelope, which is reinforced by a spring. Properties of an envelope are similar envelope to an isolator vibration. Has the device reducing axial effort from action of internal pressure up to zero.

Subframe isolation

Subframe vibration isolation graph: force transmission on suspended body vs. frequency for rigidly and compliantly mounted subframes.

Another technique used to increase isolation is to use an isolated subframe. This splits the system with an additional mass/spring/damper system. This doubles the high frequency attenuation rolloff, at the cost of introducing additional low frequency modes which may cause the low frequency behaviour to deteriorate. This is commonly used in the rear suspensions of cars with Independent Rear Suspension (IRS), and in the front subframes of some cars. The graph (see illustration) shows the force into the body for a subframe that is rigidly bolted to the body compared with the red curve that shows a compliantly mounted subframe. Above 42 Hz the compliantly mounted subframe is superior, but below that frequency the bolted in subframe is better.

Semi-active isolation

Semiactive vibration isolators have received attention because they consume less power than active devices and controllability over passive systems.

Active isolation

Active vibration isolation systems contain, along with the spring, a feedback circuit which consists of a sensor (for example a piezoelectric accelerometer or a geophone), a controller, and an actuator. The acceleration (vibration) signal is processed by a control circuit and amplifier. Then it feeds the electromagnetic actuator, which amplifies the signal. As a result of such a feedback system, a considerably stronger suppression of vibrations is achieved compared to ordinary damping. Active isolation today is used for applications where structures smaller than a micrometer have to be produced or measured. A couple of companies produce active isolation products as OEM for research, metrology, lithography and medical systems. Another important application is the semiconductor industry. In the microchip production, the smallest structures today are below 20 nm, so the machines which produce and check them have to oscillate much less.

Sensors for active isolation

• Piezoelectric accelerometers and force sensors
• MEM accelerometers
• Geophones
• Proximity sensors
• Interferometers

Actuators for active isolation

• Linear motors
• Pneumatic actuators
• Piezoelectric motors

Artificial muscle

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Artificial_muscle

Artificial muscles, also known as muscle-like actuators, are materials or devices that mimic natural muscle and can change their stiffness, reversibly contract, expand, or rotate within one component due to an external stimulus (such as voltage, current, pressure or temperature). The three basic actuation responses– contraction, expansion, and rotation can be combined within a single component to produce other types of motions (e.g. bending, by contracting one side of the material while expanding the other side). Conventional motors and pneumatic linear or rotary actuators do not qualify as artificial muscles, because there is more than one component involved in the actuation.

Owing to their high flexibility, versatility and power-to-weight ratio compared with traditional rigid actuators, artificial muscles have the potential to be a highly disruptive emerging technology. Though currently in limited use, the technology may have wide future applications in industry, medicine, robotics and many other fields.

Comparison with natural muscles

While there is no general theory that allows for actuators to be compared, there are "power criteria" for artificial muscle technologies that allow for specification of new actuator technologies in comparison with natural muscular properties. In summary, the criteria include stress, strain, strain rate, cycle life, and elastic modulus. Some authors have considered other criteria (Huber et al., 1997), such as actuator density and strain resolution. As of 2014, the most powerful artificial muscle fibers in existence can offer a hundredfold increase in power over equivalent lengths of natural muscle fibers.

Researchers measure the speed, energy density, power, and efficiency of artificial muscles; no one type of artificial muscle is the best in all areas.

Types

Artificial muscles can be divided into three major groups based on their actuation mechanism.

Electric field actuation

Main article: Electroactive polymers

Electroactive polymers (EAPs) are polymers that can be actuated through the application of electric fields. Currently, the most prominent EAPs include piezoelectric polymers, dielectric actuators (DEAs), electrostrictive graft elastomers, liquid crystal elastomers (LCE) and ferroelectric polymers. While these EAPs can be made to bend, their low capacities for torque motion currently limit their usefulness as artificial muscles. Moreover, without an accepted standard material for creating EAP devices, commercialization has remained impractical. However, significant progress has been made in EAP technology since the 1990s.

Ion-based actuation

Ionic EAPs are polymers that can be actuated through the diffusion of ions in an electrolyte solution (in addition to the application of electric fields). Current examples of ionic electroactive polymers include polyelectrode gels, ionomeric polymer metallic composites (IPMC), conductive polymers, pyromellitamide gels, and electrorheological fluids (ERF). In 2011, it was demonstrated that twisted carbon nanotubes could also be actuated by applying an electric field.

Pneumatic actuation

Main article: Pneumatic artificial muscles

Pneumatic artificial muscles (PAMs) operate by filling a pneumatic bladder with pressurized air. Upon applying gas pressure to the bladder, isotropic volume expansion occurs, but is confined by braided wires that encircle the bladder, translating the volume expansion to a linear contraction along the axis of the actuator. PAMs can be classified by their operation and design; namely, PAMs feature pneumatic or hydraulic operation, overpressure or underpressure operation, braided/netted or embedded membranes and stretching membranes or rearranging membranes. Among the most commonly used PAMs today is a cylindrically braided muscle known as the McKibben Muscle, which was first developed by J. L. McKibben in the 1950s.

Thermal actuation

Fishing line

Artificial muscles constructed from ordinary fishing line and sewing thread can lift 100 times more weight and generate 100 times more power than a human muscle of the same length and weight.

Artificial muscles based on fishing line already cost orders of magnitude less (per pound) than shape-memory alloy or carbon nanotube yarn; but currently have relatively poor efficiency.

Individual macromolecules are aligned with the fiber in commercially available polymer fibers. By winding them into coils, researchers make artificial muscles that contract at speeds similar to human muscles.

A (untwisted) polymer fiber, such as polyethelene fishing line or nylon sewing thread, unlike most materials, shortens when heated—up to about 4% for a 250 K increase in temperature. By twisting the fiber and winding the twisted fiber into a coil, heating causes the coil to tighten up and shorten by up to 49%. Researchers found another way to wind the coil such that heating causes the coil to lengthen by 69%.

One application of thermally-activated artificial muscles is to automatically open and close windows, responding to temperature without using any power.

Tiny artificial muscles composed of twisted carbon nanotubes filled with paraffin are 200 times stronger than human muscle.

Shape-memory alloys

Main article: Shape-memory alloy

Shape-memory alloys (SMAs), liquid crystalline elastomers, and metallic alloys that can be deformed and then returned to their original shape when exposed to heat, can function as artificial muscles. Thermal actuator-based artificial muscles offer heat resistance, impact resistance, low density, high fatigue strength, and large force generation during shape changes. In 2012, a new class of electric field-activated, electrolyte-free artificial muscles called "twisted yarn actuators" were demonstrated, based on the thermal expansion of a secondary material within the muscle's conductive twisted structure. It has also been demonstrated that a coiled vanadium dioxide ribbon can twist and untwist at a peak torsional speed of 200,000 rpm.

Control systems

Main article: Control theory

The three types of artificial muscles have different constraints that affect the type of control system they require for actuation. It is important to note, however, that control systems are often designed to meet the specifications of a given experiment, with some experiments calling for the combined use of a variety of different actuators or a hybrid control schema. As such, the following examples should not be treated as an exhaustive list of the variety of control systems that may be employed to actuate a given artificial muscle.

EAP Control

Electro-Active Polymers (EAPs) offer lower weight, faster response, higher power density and quieter operation when compared to traditional actuators. Both electric and ionic EAPs are primarily actuated using feedback control loops, better known as closed-loop control systems.

Pneumatic control

Currently there are two types of Pneumatic Artificial Muscles (PAMs). The first type has a single bladder surrounded by a braided sleeve and the second type has a double bladder.

Single bladder surrounded by a braided sleeve

Pneumatic artificial muscles, while lightweight and inexpensive, pose a particularly difficult control problem as they are both highly nonlinear and have properties, such as temperature, that fluctuate significantly over time. PAMs generally consist of rubber and plastic components. As these parts come into contact with each other during actuation, the PAM's temperature increases, ultimately leading to permanent changes in the structure of the artificial muscle over time. This problem has led to a variety of experimental approaches. In summary (provided by Ahn et al.), viable experimental control systems include PID control, adaptive control (Lilly, 2003), nonlinear optimal predictive control (Reynolds et al., 2003), variable structure control (Repperger et al., 1998; Medrano-Cerda et al.,1995), gain scheduling (Repperger et al.,1999), and various soft computing approaches including neural network Kohonen training algorithm control (Hesselroth et al.,1994), neural network/nonlinear PID control (Ahn and Thanh, 2005), and neuro-fuzzy/genetic control (Chan et al., 2003; Lilly et al., 2003).

Control problems regarding highly nonlinear systems have generally been addressed through a trial-and-error approach through which "fuzzy models" (Chan et al., 2003) of the system's behavioral capacities could be teased out (from the experimental results of the specific system being tested) by a knowledgeable human expert. However, some research has employed "real data" (Nelles O., 2000) to train up the accuracy of a given fuzzy model while simultaneously avoiding the mathematical complexities of previous models. Ahn et al.'s experiment is simply one example of recent experiments that use modified genetic algorithms (MGAs) to train up fuzzy models using experimental input-output data from a PAM robot arm.

Double bladder

This actuator consists of an external membrane with an internal flexible membrane dividing the interior of the muscle into two portions. A tendon is secured to the membrane, and exits the muscle through a sleeve so that the tendon can contract into the muscle. A tube allows air into the internal bladder, which then rolls out into the external bladder. A key advantage of this type of pneumatic muscle is that there is no potentially frictive movement of the bladder against an outer sleeve.

Thermal control

SMA artificial muscles, while lightweight and useful in applications that require large force and displacement, also present specific control challenges; namely, SMA artificial muscles are limited by their hysteretic input-output relationships and bandwidth limitations. As Wen et al. discuss, the SMA phase transformation phenomenon is "hysteretic" in that the resulting output SMA strand is dependent on the history of its heat input. As for bandwidth limitations, the dynamic response of an SMA actuator during hysteretic phase transformations is very slow due to the amount of time required for the heat to transfer to the SMA artificial muscle. Very little research has been conducted regarding SMA control due to assumptions that regard SMA applications as static devices; nevertheless, a variety of control approaches have been tested to address the control problem of hysteretic nonlinearity.

Generally, this problem has required the application of either open-loop compensation or closed-loop feedback control. Regarding open-loop control, the Preisach model has often been used for its simple structure and ability for easy simulation and control (Hughes and Wen, 1995). As for closed-loop control, a passivity-based approach analyzing SMA closed loop stability has been used (Madill and Wen, 1994). Wen et al.'s study provides another example of closed-loop feedback control, demonstrating the stability of closed-loop control in SMA applications through applying a combination of force feedback control and position control on a flexible aluminum beam actuated by an SMA made from Nitinol.

Chemical control

Chemomechanical polymers containing groups which are either pH-sensitive or serve as selective recognition site for specific chemical compounds can serve as actuators or sensors. The corresponding gels swell or shrink reversibly in response to such chemical signals. A large variety of supramolulecular recognition elements can be introduced into gel-forming polymers, which can bind and use as initiator metal ions, different anions, aminoacids, carbohydrates, etc. Some of these polymers exhibit mechanical response only if two different chemicals or initiators are present, thus performing as logical gates. Such chemomechanical polymers hold promise also for targeted drug delivery. Polymers containing light absorbing elements can serve as photochemically controlled artificial muscles.

Applications

Artificial muscle technologies have wide potential applications in biomimetic machines, including robots, industrial actuators and powered exoskeletons. EAP-based artificial muscles offer a combination of light weight, low power requirements, resilience and agility for locomotion and manipulation. Future EAP devices will have applications in aerospace, automotive industry, medicine, robotics, articulation mechanisms, entertainment, animation, toys, clothing, haptic and tactile interfaces, noise control, transducers, power generators, and smart structures.

Pneumatic artificial muscles also offer greater flexibility, controllability and lightness compared to conventional pneumatic cylinders. Most PAM applications involve the utilization of McKibben-like muscles. Thermal actuators such as SMAs have various military, medical, safety, and robotic applications, and could furthermore be used to generate energy through mechanical shape changes.