Metallic muscles and beyond: nanofoams at work
In this contribution for the Golden Jubilee issue commemorating the 50th anniversary of the Journal of Materials Science, we will discuss the challenges and opportunities of nanoporous metals and their composites as novel energy conversion materials. In particular, we will concentrate on electrical-to-mechanical energy conversion using nanoporous metal-polymer composite materials. A materials system that mimic the properties of human skeletal muscles upon an outside stimulus is coined an ‘artificial muscle.’ In contrast to piezoceramics, nanoporous metallic materials offer a unique combination of low operating voltages, relatively large strain amplitudes, high stiffness, and strength. Here we will discuss smart materials where large macroscopic strain amplitudes up to 10 % and strain-rates up to 10−2 s−1 can be achieved in nanoporous metal/polymer composite. These strain amplitudes and strain-rates are roughly 2 and 5 orders of magnitude larger than those achieved in common actuator materials, respectively. Continuing on the theme of energy-related applications, in the summary and outlook, we discuss two recent developments toward the integration of nanoporous metals into energy conversion and storage systems. We specifically focus on the exciting potential of nanoporous metals as anodes for high-performance water electrolyzers and in next-generation lithium-ion batteries.
KeywordsPANI High Occupied Molecular Orbital Strain Amplitude Sweep Rate Aqueous Electrolyte
With the emphasis on miniaturization stemming from the electronics industry , the same push has been seen in submicron- and nanosized mechanical systems. In medicine and biology, for example, there is a need for high-precision actuators and manipulators for work on fluid filtration and living cell manipulation . The increasingly popular lab-on-a-chip technology takes advantage of highly miniaturized mechanical systems—Micro-Electronic Mechanical Systems or MEMS—to fit efficient analysis systems in a very small space. For progress in these fields, there is a necessity for the continuous development of both materials with micro- and nanoscale functions and of tools that can facilitate the production and characterization of these materials.
Mechanical displacement that comes as a result of an electric signal passing through a material is called actuation. In materials that produce an actuation response, the reverse is often possible as well—an electric current can be induced to flow if the material is deformed. The most common type of material that shows such properties is described as piezoelectric, and of this class of materials, quartz is the most well-known. Indeed, it is the piezoelectric property of quartz that allows it to be used as an oscillating pace mechanism in the common wristwatch . The typical piezoactuator delivers a ~0.2 % strain at a high potential of 150 V . Considering that it is desirable to see the use of actuation in low-voltage devices, such as MEMS, much lower operational parameters are required for the modern actuating material. Polymer-based actuation materials have been developed, which offer extraordinary capacity for induced deformation, but have the drawback of being weak and compliant. In recent years, we have been exploring metallic nanofoams and have demonstrated the potential of nanostructured metals to act as actuators, creating so-called “metallic muscles,” with the ability to demonstrate the properties required of the modern actuator: low throughput voltage requirements, high extension yield, strength, and stiffness [5, 6, 7, 8, 9, 10, 11, 12]. In this respect, the beautiful work by Weißmüller et al. and the pioneering work by Herbert Gleiter and collaborators have to be mentioned [7, 8].
In this contribution, we will review some aspects of this fast growing field and highlight a couple of ideas of applications in the outlook section that may also have a great impact onto the field of energy-related materials. While the production of nanoporous metallic structures is well-documented, up until recently very little was known about their mechanical properties—at submicron scales, sample size has the possibility to produce a large effect on mechanical properties, where in macroporous foams cell size specifically does not have an influence on material strength [13, 14]. Indeed, it is highly uncertain that the behaviors of macroscopic and microscopic foams will be at all similar in principle and nature. Li and Sieradzki reported that porous Au undergoes a ductile–brittle transition that seemed to be influenced by the microstructural length scale of the material . Biener et al. have continued this investigation into the mechanical properties of nanoporous Au through nanoindentation [10, 16]. They reported the main deformation mechanism during nanoindentation as a ductile, plastic densification. Strong long-range stress fields, brittle fracture, and crack emission were not observed. They note that the scaling laws that are typically applied to macroporous foams apply poorly to nanoporous metals, as they observe experimental yield strength of 145 MPa instead of the expected 16 MPa.
Volkert et al. performed microcompression experiments on FIB-milled micronsized pillars of nanoporous Au with 15-nm diameter ligaments . They find that, while Young’s modulus values as determined experimentally and as predicted by scaling laws do not show significant difference, there is a major increase in yield strength as sample size decreases below 50 mm length scales. A yield strength of 1.5 GPa is predicted, which is several orders of magnitude above that of typical bulk Au. They interpret this effect as influenced by the increased required stress to activate dislocation sources as ligament size decreases, until theoretical shear strength is reached.
Further work by Biener et al. [10, 16] investigated this elevated yield strength whether its origin was the microstructure of disordered nanoporous Au ligaments or the specific size-dependent mechanical properties of Au. This was performed by preparing multiple samples with varying ligament sizes. It is established that in the production of a nanoporous material it is possible to tune ligament and pore size through varying dealloying conditions. They observed a clear influence of ligament size such that the strength of nanoporous Au increases with decreasing ligament diameter, and thus propose that the Gibson and Ashby scaling model of foam plasticity  needs to be adjusted to take into account ligament size for nanoporous systems. Recently, we have applied a novel approach to the investigation of deformation of nanoporous metals at the nanoscale by exposing nanoporous nanopillars to a Ga+ ion beam . It will be not the main topic of this review but it is interesting to note that the results we have obtained with Au nanopillars have also been observed in Cu, Al, and Ni nano(porous) pillars, i.e., a gradual massive deformation effect of the pillar during Ga ion beam exposure, where the pillar bends toward the ion beam. A relationship between the formation of defects due to ion collisions in the nanopillar and the pillar’s deformation was derived, and we find that the deflection is linearly related to ion fluence. The high degree of control over deflection and the variables that influence it open an opportunity for use of ion-beam-induced bending as a characterization technique of nano(porous) materials.
Porous systems come in two types—interconnected and noninterconnected (alternatively, open-cell and closed-cell, respectively), describing the relationship of the material’s pores: in the former, there exists a continuous pathway between every single pore in the material, and in the latter, the pores exist independently as separated islands. A porous system is typically characterized by a high surface area-to-volume ratio due to the high amount of air-to-solid interface area as well as by a lower density and, by connection, by a lower weight compared to its solid bulk counterpart. To briefly mention terminology, a porous material is made up of pores, struts, and nodes. Pores are the encompassing term for the volume of air within foam and struts are solid material that merge at nodes and connect nodes together.
Nanofoams share many properties with their macrofoam counterparts, such as the high surface-area-to-volume ratio, but also including the capacity for cheap production and easy machinability. In addition, however, nanoporous foams have seen usage in many applications beyond those of macrofoams, including nanofiltration systems, drug delivery platforms, catalysis, sensing, and actuation [26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. A major advantage that nanoporous metals have is the ability to hold a lattice of nanoscale features while being able to be easily handled and transported, something metallic nanoparticles, for example, cannot provide.
The contribution for this Golden Jubilee issue of the Journal of Materials Science will be focused on the actuating properties of metallic foams, although these materials turned out be also sensors [36, 37]. In the Discussion section, we also present an outlook onto two promising energy-related fields, involving oxygen evolution and Li-ion battery, in which porous metals may have a large impact.
Metallic muscles: why does it work ?
The charge distribution at the metal surface (dark blue ovals in Fig. 2) can be controlled in different ways, for instance, by bringing a layer of adsorbates at the interface . A positive charge of the adsorbate will result in an electronic charge redistribution at the metal surface. It generates a lower tensile surface stress and results in a relaxation of the surface atom positions by increasing the interatomic spacing. In order to preserve the mechanical equilibrium , bulk atoms experience less compressive stress and move in a positive direction outward, as illustrated in Fig. 2b. Therefore, since detection is due to the bulk atoms, a positive displacement is measured experimentally. Although these surface stress-induced bulk deformations are not detectable in macroscopic metals, they become significant in nanostructured metals where the properties are governed by the large surface area, rather than by the bulk volume as highlighted above. This phenomenon represents the basic operating principle of metallic muscles which consist of nanoporous metals with high surface-area-to-volume ratios.
The electronic charge distribution at a nanoporous metal interface can also be easily controlled in an aqueous electrolyte where an electrical voltage is used to bring positive or negative charge carriers (i.e., ions) from the electrolyte to the nanoporous metal interface. Typically, referring to the above-mentioned actuation mechanism in metals, injection of negative charge in the space–charge region at a nanoporous metal/electrolyte interface during electroadsorption of charge compensating positive ions enhance the tensile surface stress in the metal [8, 43], resulting in an increase in compressive stress in the bulk of the ligaments and in an overall macroscopic volume shrinkage of the nanoporous metal specimen [8, 44], i.e., a negative displacement is measured experimentally.
At first, an aqueous electrolyte limits the usage of metallic muscles to wet environments whereas most of the practical applications require artificial muscles that can operate in dry environments.
A second major concern is that the aqueous electrolyte limits the actuation rate because of its relatively low ionic conductivity. Simply replacing the aqueous electrolyte by a solid one is an obvious solution but the actuation rate of all-solid-state electrochemical actuators is more severely hampered by the low room-temperature ionic conductivity of solid-state electrolytes.
- Completing the trio of challenges is the fact that the ligaments in nanoporous metals suffer from severe coarsening (undesired growth) during electrochemical processes  including actuation via redox reactions. Coarsening causes metallic muscles to lose performance efficiency after many cycles, because the charge-induced strain is ligament size-dependent as shown in Fig. 3 where strain amplitudes are plotted as a function of the ligament size. This shows that the growth of the ligaments during performance of electrochemical actuation is undesirable.
In addition to these technical boundaries, it is emphasized that metallic muscles operating as electrochemical actuators require a three-component configuration to function; explicitly these are a working electrode, an electrolyte, and a counter electrode. In such a configuration, the working and counter electrodes, which may separately actuate as demonstrated by Kramer et al. , are placed at a relatively large distance from each other. This fixed distance represents a limitation for the integration of metallic muscles into miniaturized devices . In view of these various restrictions caused by the electrolyte, an electrolyte-free approach is desirable for actuation in nanoporous metals. In fact, the following features are a prerequisite for a breakthrough in the field of artificial muscles: (i) no usage of aqueous or solid electrolyte, (ii) a fast actuation rate, and (iii) a single actuating component as in piezoelectric materials.
Silver is alloyed with gold to form the so-called “white gold,” an alloy commonly used in jewelry. The versatility of the alloy stems from its ability to form a solid solution at any ratio of gold to silver allowing for fine control of porosity of a resultant pure gold system. While copper does not have alloy systems as simple as gold’s with silver, copper–manganese forms a reliable solid solution across a wide range of compositions [68, 69]. Unfortunately, except at very low percentages, Mn tends to segregate out of the solid solution and form a phase of pure Mn at low temperatures. This issue is solved through rapid quenching from solid solution temperature. This process prevents the formation of the pure Mn phase and, provided that the quenching had occurred successfully, yields an ingot with a microstructure comparable to that of Ag-Au alloy.
As is often the case for many novel materials, interest in producing a nanoporous structure with an ordered, anisotropic pore structure came from observing nature. Specifically, the opalescence effect—variation in color based on direction of observation—seen in butterfly wings and mother-of-pearl stems from the ordered chitinous scales for the former or calcium plates for the latter serving as photonic diffraction gratings. The brilliant color variation is a light effect described as opalescence and is a result of an ordered nanoporous lattice acting as a series of waveguides, only permitting through light of a particular frequency depending on viewing direction. The use of waveguide materials such as this has been proposed for use in optical circuitry [70, 71].
With clear applications across many industries, [72, 73, 74, 75], three-dimensionally ordered materials have garnered much attention. As with all nanostructured materials, two methods of approach are viable for their production: top-down and bottom-up. Top-down production focuses on reducing a bulk source sample down to the correct size, shape, and morphology through a variety of destructive production methods. While the top-down method boasts a very logical application with easy potential for iterative improvement, its greatest limitation is the scale down to which its production methods can reach—detail at the nanometer scale is beyond the capabilities of the typical top-down method [76, 77, 78]. The alternative is the bottom-up approach, where nanoscale features are assembled piece by piece into a full structure. This method allows for far finer detail control and overall quality of the assembled structure, but is typically challenging to implement due to it often being a multistage process.
The top-down process of production of nanostructured materials typically involves a starting bulk macroscale solid and then the use of one of several techniques to achieve a nanostructure through size reduction. In medical applications, a top-down method for the production of nanoparticle suspensions is high-pressure homogenization, consisting of the repeated forcing of a suspension through a very thin gap at high velocity or media milling, which is the mechanical attrition of suspended particles using glass or zirconium oxide . Mechanical attrition in general is a common procedure for the production of nanostructured materials, and one example of such a procedure is the method of ball milling, where powders are sealed in a strengthened container with a set of hard metal spheres and treated in a vibratory mill to elicit potential phase changes and the formation of nanostructured grains in the processed particles . More nonstandard procedures are also known: Yan et al. have proposed that, considering its simplicity and capacity for high-resolution spatial imaging, an atomic force microscopy (AFM) apparatus can be used to machine nanoscale features, and have demonstrated this capability in aluminum .
As a whole, lithography is considered a top-down method, and is common in the electronics industry for the production of microchips. However, it can be converted into a bottom-up method by reducing the size of the initial template. This idea can be applied for 3D nanostructures: a porous 3D template is constructed that allows for the introduction of a particular material into the pore spaces within the template. After the template is removed, the result is a 3D “image” of the template’s pore network. This is called inverse templating, and is the principal bottom-up method that allows for repeatable batch production in moderately large quantities, provided that the template is easily constructed. For this purpose, self-assembling templates are highly valued—templates whose component parts can, over time, arrange themselves into a desired 3D pattern with no further intervention aside from the initial process setup.
Nanosphere templating uses the natural ability of silica or polymer (PMMA, polystyrene, latex) nanospheres to reliably self-assemble into a template on a large scale. The nanospheres are suspended in a solution and are allowed to naturally settle over time, although centrifugation can be used to accelerate the process at the expense of quality of ordering. An alternative method is to allow the spheres to settle across a meniscus to improve ordering with the downside of the resultant template film being very thin. The spheres will preferentially settle into their most low-energy configuration, which is a crystalline face-centered cubic arrangement. As the nanospheres settle, the solution evaporates and eventually a dry nanosphere template remains.
It should be realized that the nanostructured metallic foams can show rather brittle behavior [83, 84] (although we found way to circumvent that problem by making layered stacks ). Recently, Weißmüller and co-authors  investigated the anomalous compliance and early yielding of nanoporous gold by molecular dynamics simulation pointing out that nanoporous gold can be deformed to large strain in compression, but with very high strain-hardening coefficient. As regards brittleness, we have investigated size effects of these nanoporous materials. It is not difficult to understand that upon decreasing diameter (aspect ratio 3 because of stability) a lower stiffness is expected since a larger fraction of loosely bound struts appears whereas under pure shear the opposite will be observed (because of increasing fraction of confined material). Focused Ion Beam was employed to fabricate pillars with a controlled taper free shape. The use of nontapered pillars prevents localization effects due to variation in diameter across the compression axis. Indentation experiments were carried out on the freestanding nontapered nanoporous metal pillars. Pillars were produced from both disordered nanoporous Au as well as ordered nanoporous Ni. Pillars produced in such a manner fully retained their porosity, as well as quality of ordering.
Electrolyte-free actuation in metallic muscles
As already aforementioned in Sect. 1, an important objective is to overcome the limitations that arise in metallic muscles when an aqueous electrolyte is used for actuation: existing metallic muscles made of nanoporous metals with high surface-area-to-volume ratios can exert work due to changes in their interface electronic charge density. However, they suffer from serious drawbacks caused by the usage of an aqueous electrolyte needed to modulate the interface electronic charge density by injection of electronic charge at the nanoporous metal/electrolyte interface. An aqueous electrolyte prohibits metallic muscles to operate in dry environments and hampers a high actuation rate due to the low ionic conductivity of electrolytes. Simply replacing the aqueous electrolyte by a solid is an obvious solution, but the actuation rate of all-solid-state electrochemical actuators is more severely hampered by the low room-temperature ionic conductivity of solid-state electrolytes. In addition, redox reactions involved in electrochemical actuation severely coarsen the ligaments of nanoporous metals, leading to a substantial loss in performance of the actuator. We have developed a new electrolyte-free concept to put metallic muscles to work via a metal/polymer interface : A nanocoating of polyaniline (PANI) doped with sulfuric acid was grown onto the ligaments of nanoporous gold (NPG). Dopant sulfate anions co-adsorbed in the polymer coating matrix were exploited to tune the electronic charge density at the NPG interface and subsequently generate macroscopic dimensional changes in NPG. Strain rates achieved in the single-component NPG/PANI bulk heterojunction actuator are three orders of magnitude higher than that of the standard three-component nanoporous metal/electrolyte hybrid actuator. Also the combination of a nanoporous metal and a polymer has been further exploited to add a new functionality to metallic muscles operating as electrochemical actuators: in addition to the reversible dimensional changes in the nanoporous metal, the polymer undergoes reversible changes in color when it is electro-oxidized/reduced. This results in a smarter hybrid actuation material.
It is emphasized that low sweep rates are required during actuation in nanoporous metals via an electrolyte; in fact dimensional changes in nanoporous metal/electrolyte composite actuators vanish at sweep rates beyond a few tens of mV/s. That behavior has two origins: (i) the low room-temperature ionic conductivity of electrolytes  does not favor a rapid transport of ions to the nanoporous metal/electrolyte interface; (ii) the equilibration of redox reactions involved in charged transferred at the nanoporous metal/electrolyte interface is not satisfied during fast sweep rates as highlighted in Ref. .
Discussion: origin of the dimensional changes in electrolyte-free actuation of metallic muscles
Referring to the nanoporous metal/electrolyte hybrid actuator, it is well-established that dimensional changes in this system are caused by changes in the nanoporous metal surface stress, when electronic charges are injected at the nanoporous metal/electrolyte interface during ions electroadsorption. In order to preserve the mechanical equilibrium, these changes in the nanoporous metal surface stress are compensated by opposite changes in the stress state in the bulk of the ligaments , resulting in an overall macroscopic dimensional changes in the nanoporous metal. For our NPG/PANI hybrid material, the situation is different because no electrolyte is used during actuation. In the absence of an electrolyte, changes in the NPG surface stress as a result of electronic charges accumulation in the space-charge region at the NPG interface are still possible, provided that an opposite space-charge builds up in the polymer coating during the voltage sweeps. However, as we have seen in the previous section, hole-transport in the PANI coating is governed by an Ohmic current (slope value of 1 in the double logarithmic plot), rather than a space-charge limited-current (slope value of 2 in the double logarithmic plot) . This excludes the possibility of having changes in the surface stress of NPG as result of the build-up of a space-charge in the polymer coating.
Another possible origin of the measured dimensional changes in the NPG/PANI hybrid material points toward actuation in PANI. PANI can undergo reversible dimensional changes during electrochemical oxidation/reduction . However, this option is not applicable to our electrolyte-free actuator because the electrochemical oxidation/reduction of PANI requires an electrolyte. Second, charge carriers in conducting polymers including PANI are susceptible to induce conformational changes in the polymer chains [105, 106, 107, 108]. This later deformation mode does not necessarily require the oxidation or reduction of the polymer . Conformational changes in PANI chains can therefore be responsible for the dimensional changes in our NPG/PANI composite material provided that stresses developed in the polymer chains during these conformational changes are fully transferred to the metal. This is not likely because mechanical adhesion at metal/polymer interfaces is commonly weak . In addition, the relatively small PANI content in the NPG/PANI composite material (~Au95(PANI)5 wt%) and the relatively low Young’s modulus of PANI (~2 GPa)  compared to that of the metallic ligaments (~79 GPa) lead to the conclusion that the measured strains do not come from actuation in the thin polyaniline coating.
The total amount of negative charges in the polymer matrix, arising from the co-adsorbed sulfate anions, was estimated for a 5-nm-thick PANI coating and was found to be ~3.2 C per m2 coating, assuming that each repeating unit of PANI contributes with two sulfate anions as illustrated in Fig. 12b. This amount of charge is comparable to the quantity of electronic charge involved in dimensional changes in nanoporous metal/electrolyte hybrid actuators. During the potential sweeps, this relatively large amount of negative charge dispersed into the thin polymer matrix electrostatically interacts with the positive NPG electrode . PANI molecular chains undergo conformational changes in order to bring the sulfate anions (i.e., negative charge carriers) in the proximity of the positive metal electrode; sulfate anions present in the first monolayer of PANI are eventually electroadsorbed onto the metal electrode as reported by Lee et al. and illustrated in Fig. 12d . The electrical potential-induced interactions between sulfate anions and the ligaments of NPG give rise to electronic charges redistribution at the ligaments interface . Typically, the delocalized free electrons in the metal move from the interface toward the bulk, leaving the metal interface with positively charged metal ions (see Fig. 12d). These metal cations consist of nuclei and inner-shell electrons of metal atoms. The delocalization of negative charges from the metal surface toward the bulk weakens the interatomic bounds between metal surface atoms, resulting in relaxation of these metal surface atoms. This gives rise to an increase in tensile stress at the surface of the ligaments. The bulk of the ligaments opposes with an increase in compressive stress in order to preserve the mechanical equilibrium . Due to the high surface-area-to-volume ratio of NPG, the dimensional changes in the ligaments result in an overall macroscopic volume change in the NPG electrode , which is experimentally measured during forward voltage sweeps in the NPG/PANI/Au configuration.
During the reverse voltage sweep where the applied electrical potential is gradually removed, electrostatic interactions between the negative sulfate ions and the positive metal electrode gradually vanish, charge redistribution takes place again at the metal interface, and the initial charge distribution is restored. This causes lesser tensile surface stress in the ligaments and accordingly a reduction of the compressive bulk counter stress [102, 114], resulting in ligaments expansion and in an overall macroscopic expansion of the NPG electrode back to its initial shape.
When the NPG/PANI hybrid actuator is connected in the configuration (Au/PANI/NPG), NPG is then used as negative electrode, and the sign of the strain is reversed (expansion during forward voltage sweeps), which suggests that in this later configuration conformational changes in the polymer chains take the negative sulfate anions away from the negative NPG electrode. Such a process will cause the delocalized electrons in the metal to move toward the metal surface, resulting in lesser tensile surface stress in NPG and, consequently, in a lesser compressive counter body stress in the bulk of the ligaments. In turn, the relaxation of the compressive stress in the bulk of the metal gives rise to a volume change.
Although the dimensional changes in the NPG/PANI hybrid actuator do not come from actuation in PANI as emphasized above, in the current understanding of the process it is believed that conformational changes in the polymer chains play an important role during actuation: (i) changes in molecular shapes of the polymer bring the sulfate anions in the proximity of the metal electrode, or take these counter anions away from the metal electrode depending on the sign of the potential applied at this electrode . This process can be compared with the diffusion ions toward a metal/electrolyte interface in the case of actuation in an aqueous electrolyte. (ii) The high rate of which conducting polymers undergo conformational changes as highlighted by Yip and co-authors  might justify the high actuation rate recorded on the NPG/PANI composite material: rapid shape changes in polymer chains favor a fast exposure of sulfate anions to the positive NPG electrode and, consequently, rapid charge redistribution at the NPG interface. In contrast, when ions are transported through an electrolyte, a high actuation rate is hampered because of the low ionic conductivity of electrolytes .
The work density W = 1/2Yε 2 of the NPG/PANI actuator (~113 kJ/m3), is comparable to the ~130 kJ/m3 achieved in piezoceramics  and 90 kJ/m3 reported for the nanoporous metal/electrolyte actuator in Ref. . In the above expression, W, Y, and ε represent the volume work density, effective Young’s modulus, and maximum strain amplitude, respectively [44, 115]. Although the work density is the standard measure for the mechanical performance of artificial muscles, it is pointed out that a high value of W does not necessarily mean that the corresponding actuation material is suitable for every application. In fact, each actuation material satisfies only specific applications depending on how Y and ε are combined: materials such as electroactive polymers can produce large actuation strokes (ε ~ 4.5 %), but they are weak (Y ~ 1.1 GPa); other like piezoceramics are strong (Y ~ 64 GPa) but their strain amplitudes are restricted to ~0.2 %. Metallic muscles are unique for a number of reasons, but none-more-so than in the sense that they can achieve a wide range of strengths as depicted by the effective Young’s modulus of NPG, which is tunable from ~5 to ~45 GPa through manipulation of the ligaments’ size . Additionally, they can also be designed to achieve a wide window of strain amplitudes ranging from the standard value of ~0.1 % up to large strains of ~1.3 % in binary nanoporous alloys .
As regards the small volumetric strain of about 0.1 %, we have designed a new type of nanoporous gold architecture consisting of a two-microscopic length scale structure and details are presented in . The nanoporous gold with the two-microscopic length scale structure consists of stacked gold layers with submicrometer thicknesses; in turn each of these layers displays nanoporosity through its entire bulk. This two-length scale structure strongly enhances the stain amplitude in metallic muscles up to 6 %, compared to the standard strain of ~0.1 % achieved in nanoporous metals with one-microscopic length scale structure (i.e., with uniform porous structure). The ratio between the work density of NPG actuator with a dual-microscopic length scale (volumetric strain 6 %) and that of NPG actuator with a one-length scale porous morphology (volumetric strain 0.3 %) was found to be ~215. It is concluded that the relatively low effective Young’s modulus of NPG with layered structure (compared to that of NPG with a uniform porous morphology) is largely compensated by the giant strain amplitudes. The large strains in NPG with layered structure give rise to an enhancement of the work density with at least two orders of magnitude.
Recently, various alternatives for achieving large displacements in actuation materials have been investigated , and several ideas were proposed for the displacement amplification including cantilever systems, hydraulic-piston devices, and piezoelectric motors. These techniques however are not always appropriate for microscale applications. Kramer et al. have achieved large relative displacements during cantilever bending experiments , up to ~3 mm over a length of ~35–40 mm, by using a nanoporous metal strip to design a 40 mm-long bilayer strips. One advantage of the actuation mechanism associated to the layered structure we developed is the possibility to achieve comparable large relative displacements at smaller scales: displacements up to ~4 µm can be achieved over a thickness of ~70 µm. Furthermore, the multilength scale layered nanoporous systems operate at low voltages compared to common artificial muscles. Exploiting a polymer skin augmentation of the muscle for actuation, as we have demonstrated, is expected to stimulate the development of metallic muscles into a new class of actuation materials that operate at low voltages and combine large strain amplitudes with high stiffness and strength.
Summary and outlook for next generation applications of nanoporous metals
In conclusion, although metal nanofoams share many properties with their macrofoam counterparts, they have many applications beyond those of macrofoams. One promising application corresponds to metallic muscles based on nanoporous metals with high surface-area-to-volume ratios. For that specific application, we have demonstrated a new electrolyte-free approach to generate work from metallic muscles by exploiting a nanoporous metal/polymer interface rather than the common nanoporous metal/liquid electrolyte interface. In this actuation concept, a doped polymer coating is grown onto the ligaments of a nanoporous metal, and dopant counter ions present in the polymer coating matrix are exploited to modulate the electronic charge distribution at the nanoporous metal surface, resulting in surface stress changes and dimensional changes in the nanoporous metal. With this actuation approach, many of the drawbacks encountered in metallic muscles operating in aqueous electrolytes have been circumvented. In particular, the electrolyte-free actuator consists of a single-component hybrid material, in contrast to the three-component configuration required in nanoporous metal/electrolyte composite actuators; the nanoporous metal/polymer hybrid actuator is an all-solid-state device, like piezoceramic actuators, and its actuation rate is about three orders of magnitude higher than that of metallic muscles operating in aqueous electrolytes.
An interesting observation is that a thin polymer coating grown onto the metallic ligaments of nanoporous gold can be exploited to add a new functionality to nanoporous metals operating as electrochemical actuators. For example, a metallic muscle becomes a smart material because in addition to its reversible dimensional changes, it also undergoes a reversible change in color. This combination of electromechanical and optical changes could open the door to new applications in artificial muscles. A straightforward application includes a metallic muscle that can give feedback on the progress on its work simply by changing its color [117, 118]. An interesting and rather new development in this field of actuation was recently published by Shih and co-authors  making an actuator made from botanic epidermal cells. This soft actuator changes its actuation direction by simply changing the magnitude of the applied voltage. In fact, the single-layered, latticed microstructure of onion epidermal cells after acid treatment became elastic and could simultaneously stretch and bend when an electric field was applied.
Overall we may conclude that metallic nanofoams sit at the centrepoint of a myriad of engineering disciplines, enabling a variety of applications because of their chemical and structural diversity. In this contribution, we have highlighted mainly the ‘metallic muscle’ performance, and as far as functional properties are concerned, modern actuating materials—i.e., materials that have the capacity for controlled deformation under an applied electric current—nanoporous metals make ideal candidates for such roles. Actuation in nanoporous metals is enabled by the fact that injection of charge into a metal causes a change of surface charge, which is amplified due to the high surface-to-volume ratio present in a nanoporous metal; this ultimately leads to measurable deformation. Clearly as porous materials, these systems display excellent surface-area-to-volume ratios and attractive commercial properties such as reasonable low-cost production. As nanostructured materials, they also offer many properties often sought-out for potential applications: good electrical and thermal conductivity, strong capacity as catalyst and catalyst carrier, and potentially exploitable optical properties, to name just a few. As metals, they boast the robust mechanical properties required of a structural material: strength, impact resistance, and resistance to aging [129, 130]. In particular, the exploitation of nanoporous metals in energy-related applications such as hydrogen fuel production and batteries opens novel avenues for fundamental and applied materials research that may result in many new publications during the next 50 years in the Journal of Materials Science.
The authors are thankful to the Netherlands Organization for Scientific Research (NWO-the Hague, Mozaıek Grant 2008 BOO Dossiernr: 017.005.026 and the Rubicon Grant Dossiernr: 680-50-1214) and the Zernike Institute for Advanced Materials, University of Groningen, the Netherlands. Fruitful discussions with Patrick Onck and the testing of NP-Sn by John Cook are gratefully acknowledged.
Compliance with ethical standards
Conflict of Interest
The authors declare that they have no conflict of interest.
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