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Morphing Metal and Elastomer Bicontinuous Foams for Reversible Stiffness, Shape Memory, and Self-Healing Soft Machines

Ilse M. Van Meerbeek , Benjamin C. Mac Murray , Jae Woo Kim , Sanlin S. Robinson , Perry X. Zou , Meredith N. Silberstein , and Robert F. Shepherd*

I. M. Van Meerbeek, Dr. J. W. Kim, P. X. Zou, Prof. M. N. Silberstein, Prof. R. F. Shepherd Department of Mechanical and Aerospace Engineering Cornell University Ithaca , NY 14853 , USA E-mail: rfs247@cornell.edu B. C. Mac Murray, S. S. Robinson Department of Materials Science and Engineering Cornell University Ithaca , NY 14853 , USA

DOI: 10.1002/adma.201505991

from smaller sub-components, and vi) strengthening along the primary axis of deformation after morphing.

We chose eutectic Sn–Bi–In (Field’s metal, RotoMetals) as the metal for its low melting temperature, low toxicity, and high elastic modulus ( E ≅ 9.25 GPaalloy ) [ 11 ] at room tem-perature. To allow a large range of shape morphing with little resistance to deformation, we used a silicone elastomer (Elas-tosil M4600, Wacker) for our matrix material due to its large

ultimate strain, L L

Lε = − ≈ 8ultsilicone 0

0

, and low elastic modulus,

E = 0.54 MPasilicone ; [ 19 ] additionally, silicone is stable over the temperature range required to melt the metal alloy. [ 20,21 ]

To fabricate this bicontinuous structure, we formed an open-cell network of pores in silicone (average diameter of 2 mm) ( Figure 1 A) via dissolution of a densely packed fugitive salt. [ 22 ] We then impregnated the silicone foam’s open-cell network with molten metal (Figure 1 B) using a microfl uidic channel outgas technique (Figure S1, Supporting Information). [ 23 ] This process produces uniformly distributed, interpenetrating sto-chastic networks of Field’s metal and silicone, resulting in an isotropic composite that can be cut or molded into a variety of 3D geometries. See the Supporting Information for more details about this procedure.

At above Tmalloy, the molten metal remains within the elasto-

meric foam even during deformation and contact with other composite surfaces. This strong impregnation is likely due to surface oxidation of the liquid metal, which adheres strongly to the silicone surface. [ 24–26 ] As the molten metal is now in con-tact with its own oxide, strong surface wetting results in high capillary forces. Table S1 (Supporting Information) shows the results of an experiment conducted to demonstrate the strong capillary forces. See the Supporting Information text for more details.

Although some commercially available elastomer foams exist, the selection is limited and tends to contain primarily small-pore or closed cell foams that have lower ultimate strains and higher elastic moduli than our selected silicone material. As we found that large pore sizes ease infi ltration of the metal, and we require an open cell foam network, we chose to form our own foams using the lost salt method with large grained salt (i.e., “Himalayan,” Pure Himalayan Salt). While directly 3D printing the pore network could enable precision control of the composite architecture, our stochastic process is simple, easily duplicated, and results in an isotropic structure.

Controlling the pore size and porosity allows us to tune the properties of our composite. For example, increasing the

Synthetic composites usually have fi xed internal structures and mechanical properties tuned for a particular application. Natural composite materials, however, can alter their struc-tures and mechanical properties in response to changing environmental conditions. Bone, for example, remodels itself upon induced mechanical stresses. [ 1,2 ] When touched, the sea cucumber can rapidly and reversibly increase the stiffness of its skin for protection. [ 3,4 ] Here, we present a synthetic composite material that demonstrates some of these abilities (i.e., stiffness variation and shape morphing) and incorporate this material into a soft robotic tentacle and morphing wing.

Recent work on variable stiffness composites based on inducing phase changes in polymers [ 5–9 ] and metals [ 10,11 ] has begun to show promise in overcoming the usual tradeoff between shape adaptability and load-bearing capability. These synthetic materials are often layered composites [ 9 ] or con-tain soft microchannels fi lled with a low melting temperature material; [ 8,11 ] when the phase change material is molten, these composites deform easily. Such structures are highly sensi-tive to cracks in the stiff material and their need for layered assembly results in anisotropic mechanical properties. In this communication, we describe the material design and simple processing of a bicontinuous [ 12 ] network of two foams—elas-tomeric and metallic. The metal (an alloy of indium, tin, and bismuth) melts at = °T 62 Cmalloy . When the composite’s tem-perature is below Tm

alloy, the mechanical properties of the solid metal foam dominate, and the material is stiff; above Tm

alloy, the metal is molten, and the elastomer foam dictates the compos-ite’s mechanical properties, making the composite rubbery. In this paper, we demonstrate six capabilities of this bicontinuous foam composite: i) reversible stiffness, ii) an ability to change shape by stretching to over 200% strain when heated, iii) shape-memory actuation, [ 13–16 ] iv) non-autonomic, heat-triggered self-healing, [ 17,18 ] v) assembly into larger continuous structures

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silicone porosity would result in a lower average cross-sectional area of the silicone and a higher average cross-sectional area of the metal. In this case, the effective elastic modulus of the silicone foam would decrease and the overall elastic modulus of the com-posite foam would increase. Presently, we have chosen an inter-

mediate elastomer foam porosity, V

V Vφ =

+=( 0.5)ElFoam

sil

sil metal

;

however, 3D printing strategies could increase this porosity even further and result in stronger metal foams that would likely improve the self-healing and welding capabilities of the composite. If the pores become too large, however, the capil-lary forces retaining the molten metal may become insuffi cient. Alternatively, increasing the volume fraction of elastomer in the composite will increase the composite’s restoring force for shape-memory actuation.

For mechanical testing, we molded dumbbell and cylindrical specimens using the ASTM International standards ASTM D412-06a(2013) and ASTM E9-09, respectively. We imbibed half of the samples and measured the mechanical properties of the resultant pure elastomer foams and composites by conducting uniaxial, monotonic tension, and compression tests on a Zwick & Roell Z010 testing system according to the ASTM standards. (See Supporting Information for more details.) For reference,

we tested sealed samples of empty elastomer foam and the foam imbibed with molten metal and found that the mechan-ical properties were nearly identical (Figure S2, Supporting Information); therefore, we present here only the empty elas-tomer foam. We note here that the mechanical properties of the sealed samples do not match those in Figure 1 E; this difference is likely due to silicone degradation as a result of salt exposure over the extended periods between testing. [ 27 ]

Bars (2.5 cm × 11.7 cm × 1.1 cm) of the composite sus-tained bending loads of 4.9 N without noticeable defl ection (Figure 1 C); upon melting of the metal, the bars immediately yielded (Figure 1 D). Tensile testing data (Figure 1 E; see the Supporting Information) show that at room temperature, the composite has an elastic modulus of E ≈ 1.8 MPatensileBiFoam , which decreases to E ≈ 0.1 MPatensileElFoam at T T> malloy, measured at strains

ε<

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them to ≈70 °C, deforming them into bent, twisted, and elon-gated shapes, then allowing them to cool below Tm

alloy to freeze the material into these confi gurations ( Figure 2 A–D). Above Tm

alloy, the molten metal deforms and reorients with the foam structure under tensile loading (Figure S3A,B, Supporting Infor-mation). We note, however, that during signifi cant compres-sion, the pressure component of the stress forces some of the molten metal out of the elastomer matrix (Figure S3C,D, Sup-porting Information). In situations where signifi cant compres-sion was expected to occur, we sealed the composite with a thin (≈2–3 mm) silicone skin (Figure S2, Supporting Information).

When liquid, the cells of the metal foam align and lengthen in the direction of tensile loading; upon crystallizing, the metal foam increases the composite’s strength along the axis of defor-mation. We characterized this capability by heating dumbbell

samples to 70 °C, stretching them to three different strains ( ε ≈ 0.27, 0.50, 0.71), and allowing the metal to cool below Tm

alloy (Figure 2 E). We found that increasing prestrain caused the ultimate strength to increase. By prestraining the com-posite to ε ≈ 0.7, the yield stress increases from yσ ≈ 0.080 to

yσ ≈ 0.27 MPa0.7 . Commensurately, the residual extensibility of the composite reduced from εult0 ≈ 2.9 to εult0.7 ≈ 1.6. Though not previously reported in a dynamic system, aluminum foams have shown similar dependence on pore structure. [ 28 ] Nieh et al. report that when a cell experiences load along the longitu-dinal direction, the effective elastic modulus is proportional to the length of the cell’s primary axis and inversely related to the cross-sectional area. Therefore, as the cells elongate and their cross-sectional areas shrink, the effective elastic modulus of the composite increases.

The elastomer foam acts as an entropic spring [ 29 ] that stores energy when deformed, allowing it to be used as a shape-memory actuator when a shape is locked in using the frozen metal foam. We took advantage of this resilience to enable shape-memory actuation (Video S1, Supporting Informa-tion). As a demonstration, we morphed a rectangular cuboid (27 mm × 27 mm × 9 mm) into a cylinder (diameter, d = 25 mm; Figure 3 A, left). By heating the composite to above Tm

alloy, the metal melts and the elastomer foam relaxes and returns to its cuboid form (Figure 3 A, right). When the strain of the elastomer foam remains below ε ≈ 2.5 (Figure S4A, Sup-porting Information), the force at a given strain is linearly pro-portional to Ecompr

ElFoam. Exposed to a stream of hot (500 °C) air, the compressed cube took 78 s to recover 89% of its original shape (Video S1, Supporting Information). By encapsulating the com-posite in an elastomer membrane, we prevented leaking during compression and, within the limits of our imaging system, recovered 100% of the original shape (Video S2, Supporting Information).

In addition to characterizing the composite’s mechanical properties and intrinsic shape-memory actuation, we incorpo-rated this material into soft machines for “on-demand” skeletal networks. We demonstrate two soft machines using pneumati-cally actuated structures: a tentacle and a morphing wing. The tentacle design was inspired by a previously reported soft actu-ator [ 30 ] with three degrees of freedom, allowing it to bend in any direction. At the core of this tentacle, we inserted the metal elastomer composite to act as a variable stiffness endoskeleton and enable stiffness even when unpressurized. Figure 3 B (left) shows the tentacle relaxed, (middle) heated, and bent through actuation of two of the chambers, (right) then frozen in this position while the chambers are defl ated. The wing used a sealed pneumatic foam chamber [ 31 ] with a nylon strain-lim-iting layer to direct the mode of actuation to achieve bending (Figure 3 C). When unpressurized, the wing has a retracted and curved motif, yet remains stiff due to the exoskeleton of crystal-line metal foam. To extend and unfurl the wing, we melt the metal and pressurize the foam actuator; in this mode, the wing is stiff due solely to pneumatic infl ation.

Since the molten metal can recrystallize with itself, it is pos-sible for cracks in the metal foam to self-heal. We used this ability to heal damaged foam composites. We quantifi ed the results by cutting tensile testing bars in half and rejoining them by melting and refreezing the metal foams (see the Supporting

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Figure 2. A–D) Morphed confi gurations that demonstrate the compos-ite's ability to hold bent (A), twisted (B), relaxed (C), and elongated (D) positions at room temperature. E) Stress–strain plot of “pre-strained” tensile samples.

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Information). Though the silicone foam does not rebond, Figure 4 A shows that the self-healed composite has an elastic modulus that is very close to that of a virgin sample. Self-healed samples also achieve 78% of the undamaged samples’ inelastic limit (measured at ε = 0.04). The decrease in toughness and ultimate tensile strength comes from the irreversible damage done to the elastomer. The severed elastomer foam cannot heal and therefore provides no resistance to deformation; once the metal foam in a healed sample is damaged, the elastomer foam cannot provide structural support as it does in virgin samples. Figure 4 B–D shows the cutting and healing of a composite bar, where the cut becomes diffi cult to see after repair—visually demonstrating the remarkable degree of healing achieved.

We conducted mechanical tests on self-healed samples with the worst possible damage: complete severing of the metal and elastomer foams. There are, however, more minor types of

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Figure 3. A, left to right) A compressed cylinder of metal–elastomer com-posite expanding into a cuboid after melting the metal foam. B) Pneu-matic tentacle with (left) solid endoskeleton being (middle) actuated after melting the metal foam and then (right) frozen in position by the solidifi -cation of the composite. C) Pneumatic wing with (left) solid exoskeleton that is (right) liquefi ed and caused to bend by pneumatic actuation.

Figure 4. A) Mechanical testing of self-healed, welded, and virgin composite specimens. B–D) Composite bar being cut with scissors (B); damaged composite bar (C); self-healed composite bar (D). E) Two independent composite samples (left) before welding and (right) after.

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damage from which this composite may heal more completely. Most impacts will damage only the metal foam. In such cases, the elastomer foam remains intact and the metal foam can be melted and recrystallized. This material could also suffer punc-ture damage, where some of the elastomer foam remains intact and able to support load. In this case, self-healing could restore more of the original ultimate toughness.

Using the same method as for self-healing, we also welded two independent composite pieces into a monolithic structure (Figure 4 E). Each separate composite surface has both silicone and metal present (Figure 1 B); thus, when they touch, there is likely metal–metal contact. By heating the two composites while touching, the metallic surfaces mix and, upon freezing, the Field’s metal recrystallizes into a single form. We measured the mechanical properties of welded samples and compared them to virgin and self-healed ones (Figure 4 A). The elastic modulus and inelastic limit of the welded samples were nearly identical to those of the self-healed ones. Although cell alignment is not guaranteed in the welded samples, the similar moduli are likely due to the molten metal layer that forms along the cut surface during rebonding. Remarkably, self-healed and welded surfaces can support loads almost as great as virgin samples before plas-tically deforming. As in the self-healed tests, the welded sam-ples exhibited decreased toughness due to a plane of disconti-nuity in the elastomer phase.

The composite material we designed has a stiffness that varies dramatically through changes in temperature. This mate-rial can be stretched and reshaped into different rigid struc-tures and can use stored energy in the elastomer to enable shape-memory actuation. Under the application of heat, the same composite can self-heal after sustaining damage and can be assembled from a few subcomponents into myriad super-structures with a continuous metal skeletal system. These capabilities could be used to create multifunctional tools by reforming rigid structures into new shapes, (e.g., a hook into a spear). Like bone, this shape-changing capability could also be used to make adjustments to the geometry and strength of a structure in response to its environment. Finally, we demon-strated that these composites can be used for skeletal networks in soft machines without sacrifi cing their shape adaptability.

The strength and shape adaptability of this class of com-posites can be improved by employing existing techniques. By using different porogens [ 32 ] or a 3D-printed lost-wax mold, [ 33,34 ] we could directly control the foam structure and optimize it for various applications. Designing the microstructure such that the metal grains have larger contact areas and therefore fewer areas of stress-concentration would increase the strength. We could also spatially vary the pore structure to form gradient mechanical properties that intentionally induce anisotropic behavior. Additionally, it is possible that the interpenetration of the metal is not total—future X-ray µCT studies will allow analysis and improvement of the metal foam’s interconnec-tivity. [ 35 ] The melting and freezing of the metal in this paper was generally achieved via external heating; however, the com-posite’s stiffness can be controlled via Joule heating the metal directly [ 10 ] or embedding soft, stretchable heaters, [ 11 ] which will enable shorter melting times. While we report variable stiffness caused by melting and freezing of low melting temperature metal alloys, similar mechanical behavior could be achieved by

replacing the phase-changing material—the metal foam—with a thermoplastic. [ 5,6,8 ]

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported in part by the Air Force Offi ce of Scientifi c Research under award number FA9550-15-1-0160, by the National Science Foundation Graduate Research Fellowship under grant no. DGE-1144153, and by a grant from the Alfred P. Sloan Foundation. The authors also thank Bryan Peele, who aided with the electronics for the embedded heating elements in our soft robotic devices.

Received: December 2, 2015 Revised: December 18, 2015

Published online: February 12, 2016

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