Comparative study of robotic artificial actuators and biological muscle

Braided PAMs

Braided PAMs are usually cylindrical in shape and composed of a cylindrical tube or bladder surrounded by a braided sleeving with a constant pitch angle (Figure 1(b)). ²¹ The McKibben PAM and sleeved bladder PAM are two basic types of braided muscle, in which the inner bladder is either attached to the end fitting or not attached to the end fitting, respectively.^28,29 The McKibben PAM is the most extensively applied artificial muscle in anthropomorphic robots with low cost (Figure 1(c)) and can be easily assembled in prostheses and orthotics due to the simple design. Its length-load curves between this type of PAM and biological muscle are similar.^22,30,31

Plated PAMs

Plated PAMs have a rearranging-type membrane, which means that the membrane does not stretch when inflated. ³² The rearranging-type membrane has numerous pleats in the axial direction, which can expand during inflation and causes the membrane to unfold without material strain (Figure 1(d)). ²³ No friction exists, and no energy is required to expand the membrane in the radial direction during the inflation process. Therefore, minimal hysteresis occurs, and a low threshold pressure is required to actuate plated PAMs. Plated PAMs can be applied in accurate and fast positioning devices and compliance actuation robots (Figure 1(e)).^24,32

Netted PAMs

Netted PAMs have a surrounding membrane with a low-density network. The low-density network consists of a mesh with relatively large holes and a tightly woven braid. This membrane is also the rearranging type due to the membrane structure. Netted PAMs include the Yarlott PAM, robotic muscle actuator (ROMAC) PAM and Kukolj Muscle.^25,33,34 A higher force and negligible hysteresis occur due to the absence of friction and membrane stretching. Thus, netted PAMs are usually employed in lifting devices (Figure 1(f)). ²⁵

Embedded PAMs

Embedded PAMs have a rubber tube (load-carrying structure) that is embedded in their thin membranes, which can reduce the friction and threshold pressure due to the absence of friction between the rubber tube and the membrane. Embedded PAMs include the Morin PAM, Baldwin PAM, Paynter knitted PAM and Kleinwächter PAM.^28,35–37 In addition to linear motion, embedded PAMs, such as the PAM shown in Figure 1(g), can also achieve torsional motion, which can be applied in torsional inflated robots (Figure 1(h)).^26,27,38

TSMAs

TSMAs return to their original shape if heated after deformation, in which the transformation from martensite to austenite has a central role. At low temperatures, a TSMA has a twinned martensite structure, which can be easily deformed by an external force and converted into detwinned martensite. When the TSMA is heated by external or internal heating (Joule heating), it begins to transform from martensite to austenite at the austenite-start (As) temperature and finishes this transformation at the austenite-finish (Af) temperature. The austenitic phase is a highly ordered state, and the TSMA contracts and recovers to the predefined high-temperature shape. Conversely, the TSMA reverts to martensite after cooling, which starts at the martensite-start (Ms) temperature, and is completed at the martensite-finish (Mf) temperature. The TSMA is elongated by the defined external force (Figure 2(a)).^39–41 Therefore, the shape memory effect is only observed in most TSMAs, when an external stress is applied. This type of SMA is also known as a one-way SMA. In addition, some TSMAs display a two-way shape memory effect, which can act without an external stress. ⁴² Nickel–titanium (NiTi) alloys are one of the most extensively applied TSMAs and have been applied to small robots such as fish robots (Figure 2(b)), climbing robots (Figure 2(c)), flying robots (Figure 2(d)) and hand prostheses (Figure 2(e)).^43–46

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Figure 2.

SMAs actuating principles and applications: (a) actuation principle of TSMAs, (b) NiTi SMA-actuated jellyfish robot, (c) NiTi SMA-actuated climbing robot, (d) NiTi SMA-steered flying robot, (e) NiTi SMA-actuated hand prosthesis, (f) three kinds of martensitic phases of FSMAs, (g) martensitic transformation and twinning of an FSMA and redistribution under a magnetic field in two dimensions and (h) FSMA-actuated fish robot.

FSMAs

FSMAs demonstrate the shape memory effect in response to a magnetic field, which drives the shape change faster than relatively slow heating and cooling processes. This phenomenon is generated by the transition among variants of the tetragonal martensite phase that formed during cooling from the cubic austenite phase. In three-dimensional (3D) space, the martensite unit cell has a shorter size along one axis and a longer size along the other two axes than the cubic cell (Figure 2(f)). ⁴⁹ The martensite unit cell can be easily, magnetically and mechanically deformed along its shortest axis (Figure 2(g)). ⁴⁷ Typically, a predefined force is applied in the short axis direction, which compresses the FSMA; it can return to its original shape when a magnetic field is applied orthogonally to the short axis direction. Due to the fast response and high specific power, FSMAs have also been employed in small-sized robots such as caudal peduncles for fish robots (Figure 2(h)) and flappers for flying robots.^48,50

DEAs

DEAs can generate deformations as a result of electrostatic interactions between two electrodes. DEAs act as capacitors when an external voltage is applied; opposite charges are attracted in the electric field direction, while like charges repulse each other in the direction perpendicular to the electric field. Therefore, the Maxwell stresses exerted in the dielectric materials squeeze the DEAs along the direction of the voltage and expand them in the other two directions (Figure 3(a)). A silicone or acrylic material often acts as the elastomer of a DEA, with a compliant electrode that is applied to achieve large deformation. For this reason, DEAs are extensively applied as actuators for robots: fish robots (Figure 3(b)), flying robots (Figure 3(c)), robot arms (Figure 3(d)) and legged robots (Figure 3(e)–(g)).^53–60

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Figure 3.

e-EAPs actuating principles and applications: (a) actuation principle of DEAs, ⁵³ (b) DEA-actuated fish robot, (c) flapping wings actuated by a DEA for flying robots, (d) DEA-actuated robot arm, (e) DEA-based six-legged robot, (f) DEA-actuated hexapod robot, (g) DEA-actuated quadruped robot, (h) actuation principle of RFPs, (i) RFP-actuated legged robot and (j) RFP-actuated fish robot.

RFPs

A ferroelectric polymer material that is often used as an actuator is poly(vinylidene fluoride-trifluoroethylene), abbreviated P(VDF-TrFE). The backbones of P(VDF-TrFE) are highly polar due to the electronegativity of fluorine. The reversible conformational changes caused by the alignment of polar groups are utilised in actuation. ⁵² As shown in Figure 3(h), when an electric field is applied perpendicular to the chain, the ferroelectric that is originally in the nonpolar alpha phase switches to the polar beta phase. ⁶⁴ This switching causes elongation along the chain length and contraction perpendicular to the chain direction. A problem with ferroelectric polymers is the existence of a massive hysteresis and substantial energy dissipation during the actuation process, when a large electric field is applied to reverse the polarisation. ⁶⁵ To eliminate the hysteresis behaviour, P(VDF-TrFE) can be irradiated or a small mass fraction of a bulky monomer can be added to disrupt the long-range order to form RFP.^66,67 Consequently, great reversible electrostriction is obtained in RFPs, which can be applied in millimetre-sized legged robots (Figure 3(i)) and swimming robots (Figure 3(j)).^62,63

CPs

CPs are also known as conjugated polymers. The electrochemically changing oxidation state causes charge addition to or removal from the electrolyte when a voltage is applied to the two electrodes, which is also known as doping. Dimensional changes are caused by electrochemical doping. ⁶⁸ As shown in Figure 4(a), the CP expands when it is oxidised as the anions (A-) enter the electrolyte to balance the charge, and the CP contracts when it returns to the reduced state, in which anions have been removed. ⁶⁹ The main advantage of CPs over e-EAPs is their low driving voltage, which typically ranges from 1–2 V to 10 V for higher speed and response. ⁷⁰ This characteristic combined with high stress renders CPs attractive as actuators. For example, CPs have been applied in rotational motors (Figure 4(b)). ⁷¹ In addition, CPs have also been employed to actuate micro-grippers (Figure 4(c)) and fish robots (Figure 4(d)).^72–75

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Figure 4.

i-EAPs actuating principles and applications: (a) actuation principle of CPs, (b) CP-actuated rotational motor, (c) CP-actuated soft gripper, (d) CP-actuated fish robot, (e) actuation principle of CNTs, (f) CNT-actuated quadruped robot, (g) torsional artificial muscle actuated by CNTs, (h) actuation principle of IPMCs, (i) IPMC-actuated manta ray robot, (j) IPMC-actuated fish robot, (k) IPMC-actuated joint for a robot manipulator and (l) IPMC-actuated joint for a swimming snake robot.

CNTs

A CNT is a single rolled sheet of graphite with a large surface area and high conductivity, whose size is on the micrometre scale. The main cause of actuation is electrostatic forces. As shown in Figure 4(e), CNTs act as electrodes and/or counter electrodes when they are immersed in electrolytes. ⁶¹ When a potential is applied, ions are attracted to the nanotubes, which cause charging and rearrangement of the electronic structure. The electrostatic repulsive forces between two charges on the CNT surface can elongate and expand the nanotubes.^83,84 Typically, actuation is generated in films or yarns that are composed of many nanotubes. For instance, CNT films that are often applied as bending actuators to drive a soft gripper can also be utilised as buckypaper actuators to actuate a centimetre-sized walking robot (Figure 4(f)).^76,85 CNT yarns perform as artificial muscles that can generate torsional and tensile motion (Figure 4(g)). ⁸⁶ In addition to the electrical method, this type of torsional artificial muscle can be chemically and thermally driven. ⁷⁷

IPMCs

An IPMC is composed of a polymer electrolyte sandwiched by two high-surface area metallic electrode layers, and the two electrodes interpenetrate the polymer electrolyte.^87,88 The polymer electrolytes that are often applied, such as Nafion, are neutralised with certain counterions, which are balanced by the electrical charge of the anions fixed to their backbone. ⁸⁷ When an electric field is applied perpendicular to the IPMC layers, the mobile cations in the polyelectrolyte move towards the oppositely charged electrode (Figure 4(h)), which cause swelling at the negative side and shrinkage at the positive side to achieve bending of the entire IPMC actuator.^78,89 Numerous robotic applications that are based on IPMC actuators have been reported. First, IPMCs can be readily employed as actuators of soft grippers due to their inherent bending motion.^90,91 Second, IPMCs are the preliminary technology for hydrodynamic propulsion such as IPMCs in manta ray robots (Figure 4(i)) and fish robots (Figure 4(j)).^79,80 Finally, IPMCs can perform as robotic joints that move in parallel such as IPMCs in a millimetre-sized manipulator (Figure 4(k)) and a snake robot (Figure 4(l)).^81,82

TFAs

Actuation principles: high-strength polymer fibres are transformed to artificial muscles by twist insertion. Twisted yarns of polymer fibres can generate linear and torsional actuation after volumetric expansion, which can be achieved via different mechanisms, including thermal expansion and volumetric expansion of absorption. The heat of thermal actuation is generated via two mechanisms: Joule heating and photo-thermal heating. ⁹² The absorption actuation employs moisture-sensitive silk fibre to exert deformation while changing the relative humidity. ⁹³ The one-end-tethered twisted yarn responds to the structural volumetric change by untwisting at its free end to form the torsional actuator. The two-end-tethered twisted yarn can only change the diameter by translating and contracting in length instead of untwisting in the end, which forms the linear actuator.

A typical feature of this kind of actuator is the twisted fishing line (e.g. nylon 6) or sewing thread (e.g. nylon 6.6). First, this thermally driven artificial muscle can be constructed by inserting an extreme twist into the fibre. Second, the twist is further inserted to form a self-coiled yarn (Figure 5(a)). ⁹⁴ Finally, an annealing process is applied to yarn to avoid untwisting after disengaging the fibre end-fixing. The nylon artificial muscle can achieve large output contraction and generate super-high mechanical power. The excellent muscle stroke and work capacity are retentive as the fibre diameter changes by orders of magnitude. This near-invariance of actuation performance indicates great scaling of structure and properties. Therefore, the nylon muscles can be converted to a two-ply (Figure 5(b)) configuration and braided structure (Figure 5(c)).^94,95 The scalable nylon artificial muscle enables a larger strain and stress output, which is applied in macroscopic applications, such as the light-weight prosthetic hand (Figure 5(d)), ⁹⁵ where the fingers are actuated by a twisted nylon artificial muscle and soft robotics, where two twisted nylon fibres are inserted into the substrate, which is represented by blue lines (Figure 5(e)). ⁹⁶

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Figure 5.

(a) A single twisted coiled nylon fibre, (b) two-ply twisted nylon fibre, (c) braided actuator of twisted nylon, (d) a hand prosthesis actuated by nylon artificial muscle, (e) different geometry of robot made from twisted and coiled nylon polymer, (f) actuation principle of hydrogel, (g) a small size hydrogel gripper; (h) four-dimensional (4D) printed hydrogel actuator driven by temperature, (i) basic structure of planar HASEL actuator, (j) a gripper constructed by multi-layer planar HASEL actuator, (k) basic structure of peano-HASEL actuator, (l) parallel configuration of peano HASEL actuator and (m) a lever arm actuated by peano HASEL.

HGAs

Actuation principles: hydrogel polymers with high water contents (≥90 wt%) can switch their sizes and shapes by the uptake and release of water to change the water content in the polymer work, and the polymer actuators perform deformation and movement in response to various environmental stimuli (Figure 5(f)). ⁹⁷ The main branch of the HGA family consists of pH-responsive hydrogels, which sensitively respond to pH changes in the ambient environment with swelling or shrinking deformation. When experiencing pH variation, the hydrogel polymer backbone will be ionised and the electrostatic repulsion is generated between adjacent polymer chains, which enables the absorption of water and causes hydrogel swelling.^102,103 Some hydrogel polymer can undergo reversible shifts in morphological and physical phases in response to temperature variations. The underlying mechanism of the thermally driven hydrogel is attributed to a solubility shift in aqueous solution. ¹⁰⁴ These two methods are limited by the uncontrollability on/off state of actuation. Thus, a precisely controllable method is introduced. Electric-responsive hydrogel can swell/deswell in an electric field, which is combined of coulombic, electrophoretic and electro-osmotic interactions.^105,106 Electrical-responsive hydrogels are often employed in an aqueous electrolyte, and the direction migration of mobile ions in the solution occurs in an applied electric field, which causes contraction of the hydrogel. An osmotic pressure difference is then exerted due to the concentration gradient of ions. The hydrogel polymers generate swelling and deswelling according to the osmotic pressure increase and decrease, respectively.

To respond to various stimuli, in this way, the hydrogel-based actuators could perform deformation in various environments. HGAs are soft and wet materials that can interact with the internal constituents of the human body in a safe and human-friendly manner. Therefore, HGAs are raising tremendous attention and showing their potential in applications such as grippers (Figure 5(g)), ⁹⁸ artificial muscles (Figure 5(h)) ⁹⁹ and drug delivery. ¹⁰⁷

HASEL actuators

Actuation principles: two types of HASEL actuators achieve linear actuation motion. First, the planar HASEL actuator, which has an elastomeric shell, is partially covered by a pair of opposing electrodes outside and filled with a liquid dielectric inside. ¹⁰⁰ An electric field exists through the liquid dielectric when voltage is applied to the electrode. The resulting electrostatic Maxwell stress presses and displaces the liquid dielectric from between the electrodes to the surrounding volume. Actuation strain increases with an increase in the input voltage, where the resulting hydraulic pressure causes the soft structure to deform into a toroidal shape and gain an elongation actuation movement, and this type of actuator could be stacked to achieve higher strain and force (Figure 5(i)). ¹⁰⁰ Second, a peano-HASEL actuator is constructed with a series of pouches made from a flexible and inextensible shell that is filled with a liquid dielectric. Electrodes cover a portion of each pouch on both side of the actuator. The electrostatic force causes electrodes to progressively close when a voltage is applied. The pressure forces dielectric fluid into an uncovered portion of the pouch, which causes the pouch to transition from a flat cross section towards a circular cross section. This transition causes linear contraction of the actuator as the shell is inextensible (Figure 5(k)). ¹⁰¹ Furthermore, the parallel configuration obtained from the peano-HASEL actuator has greatly amplified the output force (Figure 5(l)). ¹⁰¹

The HASEL actuator blends soft fluidic actuators with muscle-like performance of DEAs to achieve a variety of actuation modes without rigid frames or prestretched mechanisms. The hydraulic principles enable HASEL actuators to scale actuation output strain and force. The scalability gives HASEL actuators the capability to construct a soft gripper that can handle delicate objects (Figure 5(j)) ¹⁰⁰ and an artificial muscle that can lift more than 200 times their weight (Figure 5(m)). ¹⁰¹ Unlike traditional solid DEAs, which would be permanently damaged due to a high electric field, the use of a liquid dielectric enables HASEL actuators to self-heal from a dielectric breakdown. The self-healing property improves the durability and stability of HASEL actuators, which demonstrates promise for applications in robotics.

PAMs

Of all the PAM technologies, the McKibben PAM is the most prevalent due to its low cost and simple and compact structure. This muscle is constructed with an elastic inner tube surrounded by an expandable braided sleeve. The braided sleeve translates the volumetric increase in the inner bladder to axial contraction, which is capable of generating a pulling force. The length and diameter of the braid change as the pitch angle changes. The muscle would contract until a pitch angle of 54.7° is attained, ³⁸ which is the critical angle that corresponds to the maximum volume of the inner bladder and a zero contract force, and the muscle would stretch until the pitch angle decreases to its lower limit, which significantly depends on the braided fibre thickness and strand density. However, the tension force that is generated during contraction depends on the muscle diameter and pitch angle at rest, as well as the driving air pressure. The generated force decreases greatly with an increase in the initial angle to the critical angle and then the muscle produces negative forces. ¹⁴⁰ Consequently, the range of contraction–extension (i.e. strain) and tension force (i.e. stress) is determined by the muscle size, woven pattern and driving pressure. As shown in Figure 7(a), the strain and stress of this PAM are slightly higher than those of biological muscle. The strain of the McKibben PAM is typically 25% in practice, and a 1.16 MPa output stress is achieved at an input air pressure of 5 bar.^18,38 Based on the output strain, output stress and ease of assembly, McKibben PAMs can be extensively applied in task-based robot hands and arms such as the Festo Bionic Soft Hand, Shadow Biomorphic Arm, Festo Airic’s Arm and 7 degrees of freedom (7 DOF) robot arm developed by B. Tondu.^141–144

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Figure 7.

Comparisons of the comprehensive actuation performances between different artificial actuators and biological skeletal muscle. In these radar charts, each corner represents an actuation parameter. The centre of the radar figure represents the value of zero, while the boundaries of the radar chart represent the theoretical maximum value of reviewed actuator technologies, which is listed in parentheses. (a) Comparison between PAM, SMA and skeletal muscle; (b) comparison between e-EAPs and skeletal muscle; (c) comparison between i-EAPs and skeletal muscle; and (d) comparison between newly developed actuators and skeletal muscle.

The most remarkable advantage of the McKibben PAM is its high specific power, which is derived from the light weight of the actuator materials and compressed air, which is typically in the range of 0.5–2 kW/kg to a maximum of 10 kW/kg.^110–112 The work density is approximately 0.2 MJ/m³. ³⁸ With the relatively fast strain rate of approximately 800%/s, the McKibben PAM performs better in these aspects than biological skeletal muscle, and thus, is suitable for biped walking robots, humanoid robots and Kangaroo jumping robots,^22,145,146 where high power actuators are required. The McKibben PAM’s energy efficiency is approximately 0.32–0.49; the energy efficiency of human muscle is 0.4.^38,112

Despite the distinct positives, the McKibben PAM shows nonlinear strain-stress features, which are caused by viscoelastic properties of inner tube materials, the compressibility of air and complex behaviour of the braided configuration. The nonlinearity in combination with inherent dry friction between the sleeving strands and the inner tube causes actuation hysteresis, which increases the complexity of their model and control. Thereby, the McKibben PAM is not suitable for application to the scenario in which accurate positioning is required. The required compressor in the conventional McKibben PAM makes the pneumatic system heavy, bulky and unsuitable for use as a micro-actuator, where compact and weight minimisation configurations are desired.

SMAs

The NiTi SMA provides significant displacement as a hard material; the most prevalent method for manufacturing SMA is vacuum melting, where NiTi alloys are melted by vacuum induction melting in a graphite crucible, where the carbon content increases and TiC is mixed in the alloy. ¹⁴⁷ In addition, SMA can be produced by many other ways such as the heat treating method, powder metallurgy method and thermal spray method. ¹⁴⁸ The strain is typically 5%, with a maximum value of 10%.^108,112,113 This SMA exerts a large force per unit area of more than 200 MPa. Therefore, NiTi SMA is extensively employed as a micro-actuator or artificial muscle in robotics. The SMA reacts directly to an external stimulus, which significantly reduces the actuator’s complexity and size. Thus, the NiTi SMA has a very high work density of 10 MJ/m³, which is 25 times that of DC motors, and an unmatched specific power of 50 kW/kg, which means that it can lift 100 times its weight.^112,113 As shown in Figure 7(a), the stress, work density and specific power of this SMA are the highest among all the actuators. Furthermore, the NiTi SMA can act in a fully 3D manner, which allows the actuator to extend, bend and twist, separately or simultaneously. These excellent characteristics have enabled application of the NiTi SMA in micro-walking robots, climbing robots, flying robots, jumping robots, swimming robots and hand prostheses.^{43–46,149,150}

The main challenge is the low response time and low operational frequency of the NiTi SMA, which stem from a relative high heat capacity and density of the material. These disadvantages caused difficulties in the rapid transfer of heat from active elements. The response time of the NiTi SMA can be reduced to several milliseconds, with a maximum strain rate of 300%/s, when a very large electrical current is used for heating. ¹¹⁴ However, the relatively slow cooling process still limits the actuation cycle time, even with an active or passive cooling method. The response time is also affected by the size and shape: the smaller diameter actuator heats faster due to higher resistivity and cools faster due to a higher surface-to-volume ratio. Another challenge is the low efficiency, which is approximately 10% theoretically and usually less than 5%, in practice; these values are much lower than the efficiency of biological muscle (Figure 7(a)).^108,115 Most SMA actuators are based on a spring configuration but the stress distribution is not constant over the spring cross section, which require a larger volume of material to be heated or cooled for the same output force and further reduces the actuator’s efficiency. For optimal use of the NiTi SMA, a straight wire configuration is recommended due to a greater amount of work generated from less material volume.

e-EAPs

i-EAPs

Newly developed actuators

The newly developed actuators combine the strengths of different technologies to address many of the issues in actuators. As shown in Figure 7(d), these actuators achieve qualified actuation performances using different principles to linearly contract applications in a biological muscle-like fashion.

Summary

The comprehensive comparison results are summarised as follows:

Artificial actuators have advantages over biological muscle in terms of the actuation parameters but challenges always exist when these artificial actuator technologies are applied in practice. Thus, artificial muscles cannot replace or perform as biological muscle. Therefore, further exploration of actuators with high performances is needed.