Self-actuation enabled by anisotropic van der waals thermal conduction and expansion

Abstract

Smart materials that can self-activate based on the material interface, generating a mismatch in the thermal and mechanical characteristics in response to variable environmental energy levels, allow for the development of intelligent and sustainable devices and systems. However, due to symmetric strain coupled with intrinsic isotropic heat transport at the material interface, their capacity to self-activate is fundamentally constrained, resulting in a restricted physical and sensory range as well as low mechanical durability. Unlike current 3-D materials frequently used in self-activation, 2-D materials offer a new way for in-plane thermal expansion without suffering from interfacial slip or structural deformation. This disclosure reports an anisotropic van der Waals thermal transfer and asymmetric expansion at a material interface enabling significantly enhanced self-actuation by interfacial a molybdenum disulfide (MoS2) layer (IML) within a metal-elastic structure.

Description

Attorney Docket No.2115-008389-WO-POA SELF-ACTUATION ENABLED BY ANISOTROPIC VAN DER WAALS THERMAL CONDUCTION AND EXPANSION CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 623,799, filed on January 22, 2024. The entire disclosure of the above application is incorporated herein by reference. FIELD

[0002] The present disclosure relates to self-actuated materials and structures. BACKGROUND

[0003] As the global energy demand and climate change present critical challenges, there is an urgent need for novel and smart materials to foster the development of sustainable technologies. Smart walls for example, are emerging building enclosures that enable self-regulation of solar energy and moisture, allowing to increase the comfort while reducing the energy needs. To this end, smart materials have shown great promise in a variety of applications in smart walls, sustainable energy and environmental systems, smart robots, and biomedical devices. These materials have the capacity to self-activate depending on their interface structure, leading to a mismatch between their thermal and mechanical characteristics in response to variable ambient energy levels. Self-activation has been studied and used to create micro / nano-sized systems showing complicated responses to simple changes in scalar signals, such as a thin layer, promoting a range of three-dimensional (3D) structures since it allows for an energy-free and closed-loop control system for sensors, actuators, micro robotics, nanoelectromechanical systems, active materials, optoelectronics, stretchy electronics, and drug-delivery systems. The essential steps in the self-activation process include perceiving or capturing the environmental stimulus, transmitting the stimulus to the interface of the bilayer structure, and asymmetrically releasing the energy at the interface. In the presence of surface stresses, misfit strains, residual strains, thermal stresses, swelling and shrinkage, and differential growth, the stimulus induces an asymmetric release of potential energy at the bilayer interface. The interface of self-activated smart materials mainly suffers from large stresses caused by excessive expansion and contraction of the actuation layer, while the symmetric heat transfer over the interfaceAttorney Docket No.2115-008389-WO-POA results in limited actuation. These fundamental issues of strain and heat transfer at the interface of two materials cannot be resolved unless the structural stress and heat transfer are addressed simultaneously at nanoscale.

[0004] It is worth noting that anisotropic stress–strain management at the interface layer is substantially related to self-activation. To build self-activated bending materials, an advanced design approach is needed. Three major criteria must be considered: (1) two candidate materials with intrinsically large differences in Young’s modulus and energy capacity; (2) a method to substantially increase the stress at the interface without affecting the heat transfer; (3) facile, scalable production and integration of such materials with precise control of the material dimensions e.g. film thickness. Accordingly, extensive efforts have been made in producing micro- and nano-thin layers that encompass self- activation capabilities under a uniform environmental change by controlling the interfacial stress in the self-bending material. However, current self-actuated bending materials have a limited range of structural changes, low sensitivity, and poor strength. Significantly improved performance can be realized by enabling anisotropic high thermal stress at the material interface accompanied by structural tunability. However, a strategy for significantly lowering the out-of-plane thermal conductivity (κ⊥) (to reduce 3D stresses) while preserving a high in plane thermal conductivity (κ||) (to maintain high 2D stress / strain) is needed.

[0005] Transition metal dichalcogenides (TMDs), for example, which show layered van der Waals forces, have drawn a lot of interest due to their distinct optical, chemical, and mechanical characteristics. Such materials provide an ideal material platform for the design of highly anisotropic electronic / thermally conductive materials. In particular, these materials have shown excellent intrinsic in-plane thermal conductivities (κ||) and a significant decrease in out-of-plane thermal conductivity (κ⊥) (Asymmetric ratio of heat transfer = ~ 400). Although such fundamental understanding has been actively achieved, realization of the anisotropic thermal properties in sensor and actuator systems has remained at the early stage due to the limited integration and design principles for the smart materials and structures.

[0006] This section provides background information related to the present disclosure which is not necessarily prior art.Attorney Docket No.2115-008389-WO-POA SUMMARY

[0007] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0008] A self-actuated bending structure is presented. The bending structure includes: a base layer comprised of an elastomer; an interface layer disposed directly on the base layer; and a temperature sensing layer disposed directly on the interface layer. The interface layer has an anisotropic heat transfer property in the range of 100 to 400 with an asymmetric ratio between the in-plane thermal conductivity (κ||) and out-of-plane thermal conductivity (κ⊥). In one embodiment, the interface layer is a transition metal dichalcogenide, such as molybdenum disulfide. The temperature sensing layer is a metal, such as aluminum.

[0009] In some embodiments, a layer of titanium is deposited between the interface layer and the temperature sensing layer.

[0010] In another aspect, a method for fabricating a bending structure is presented. The method includes: depositing an elastomer onto a substrate; depositing an interface layer onto the elastomer, where the interface layer has an anisotropic heat transfer property in the range of 100 to 400 with an asymmetric ratio between the in- plane thermal conductivity (κ||) and out-of-plane thermal conductivity (κ⊥); and depositing a metal onto the interface layer, thereby forming a bending structure.

[0011] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. DRAWINGS

[0012] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0013] Figure 1 is a diagram depicting a bending structure in accordance with this disclosure.

[0014] Figure 2A shows the side and top view of the lattice structure of a MoS2 monolayer that belongs to D3h point group and the phono modes of the MoS2 monolayer.Attorney Docket No.2115-008389-WO-POA

[0015] Figure 2B is a schematic illustrating how in-plane thermal expansion of MoS2 layer depends on the α of the substrate.

[0016] Figure 2C is a schematic illustrating the shift in Raman spectra of MoS2 layer on low and high α substrates at low and high temperatures, respectively.

[0017] Figures 2D and 2E are graphs showing Raman spectra and peak positions of the E’ mode in the MoS2 layer supported on aluminum at different temperatures.

[0018] Figures 2F and 2G are graphs showing Raman spectra and peak positions of the E’ mode in the MoS2 layer supported on PDMS at different temperatures.

[0019] Figure 2H is a graph showing Raman peak position of the E’ mode in the MoS2 layer supported on PDMS as a function of the layer number at different temperatures.

[0020] Figure 3 is a flowchart depicting a technique for fabricating a bending structure.

[0021] Figure 4A are schematics showing the effect of thermal anisotropy of IML on the self-actuation, where the focused temperature gradient at the interface between the hat collector and the elastomer leads to the larger thermal stress at the interface.

[0022] Figure 4B illustrates the temperature gradient at the interface layer of the bending structure.

[0023] Figure 4C illustrates the stress profiles at the interface layer of the bending structure.

[0024] Figure 4D illustrates the strain at the interface layer of the bending structure.

[0025] Figure 5A is a schematic of the self-activation of the bending structure in response to temperature change.

[0026] Figure 5B are dynamic photo images of the bending structure and the control sample, respectively, over time.

[0027] Figure 5C is a graph showing the change of self-activated bending angle as a function of time with δIML= 10-5between Thigh= 100°C and Tlow= 25°C.

[0028] Figure 5D is a graph showing activated bending angle as a function of temperature for 25°C to 100°C at different δIML.

[0029] Figure 5E is a graph showing activated bending angle as a function of δIMLfrom 0 to 100 nm at T = 100°C.Attorney Docket No.2115-008389-WO-POA

[0030] Figure 6A is a graph showing the self-actuate bending angle of the bending structure and the control sample with δHeat / δEL = 0.05 in different mediums including air, water, and glycerol at T = 100 °C in 120 sec.

[0031] Figure 6B is a graph showing the self-actuated bending angle of bending structure (δIML = 10-5and δHeat / δEL = 0.05) and control sample as a function of viscosity from 0.0001 Pa⋅s to 0.014 Pa⋅s at T = 100 °C in 120 sec.

[0032] Figure 7A depicts the geometric parameters of self-activated micropore actuator including node width (Wp = wp / ws), node length (Lp = lp / ws), and layer thickness (ta=tp / ws).

[0033] Figure 7B is a schematic of a self-activated micropore actuator consisting of the interface layer, elastomer (PDMS), and heat collector (Al) (δIML = 10-5and δHeat / δEL = 0.05) enabling self-activated open and close operation of the micro pore structure in response to temperature change.

[0034] Figure 7C is a graph showing the pore open / close ratio of self-activated micropore actuator with δHeat / δEL = 0.05 and δIML = 10-5between Thigh = 100 °C and Tlow = 25oC as a function time.

[0035] Figure 7D is a graph showing the temperature cycling of the self-activated micropore structure with δHeat / δEL= 0.05 and δIML= 10-5between Thigh= 100 °C and Tlow= 25 °C, the pore open / close ratio is shown as a function time.

[0036] Figure 7E is a graph showing the temperature effect on the pore open / close ratio of the self-activated micropore structure (ta = 0.01, δHeat / δEL = 0.05, and δIML = 10-5) as a function of layer thickness.

[0037] Figure 7F is a node width and length effect on the pore open / close ratio of the self-activated micropore structure (δHeat / δEL = 0.05 and δIML = 10-5) at T = 100oC.

[0038] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0039] Example embodiments will now be described more fully with reference to the accompanying drawings.

[0040] Figure 1 depicts a bending structure 10 in accordance with this disclosure. The bending structure 10 is comprised of a base layer 12 comprised of an elastomer and a temperature sensing layer 14. In one example embodiment, the base layer 12 is madeAttorney Docket No.2115-008389-WO-POA of silicone elastomers, such as polydimethylsiloxane, and the temperature sensing layer 14 is a metal, such as aluminum or gold. Other types of elastomers, such as polytetrafluoroethylene, polyvinylidene fluoride, ethylene tetrafluoroethylene and polycarbonate, as well as other types of metals are contemplated by this disclosure.

[0041] Structure 10 can be subject to different energy levels when varying its temperature. Structure 10 can be heated by its surrounding media through conduction and / or convection and in other embodiments structure 10 can be heated indirectly by radiation from a radiant source.

[0042] In one embodiment, structure 10 can be heated through friction generated from the relative movement of the structure and its surrounding. In another embodiment, the temperature sensing material can encompass a predefined electrically conductive material that can allow for the heating of the structure 10 when an electric potential differential is applied. In yet another embodiment, the structure 10 can encompass specific particles in either of the base layer or the temperature sensing layer that would generate heat when subject to a magnetic field. Examples of particles that can be heated using induction heating are iron and its alloys, magnetic materials, as well as some semi- conductors, such as silicon carbide.

[0043] Significantly enhanced self-actuation of the bending structure is achieved by integrating anisotropic van der Waals thermal conduction and asymmetric expansion at the material interface. That is, an interface layer 13 of a transition metal dichalcogenides is disposed between the base layer 12 and the temperature sensing layer 14. In the example embodiment, the interface layer 13 is molybdenum disulfide. Other suitable materials for the interface layer 13 may include but are not limited to transition metal dichalcogenide materials such as WS2, MoSe2, WSe2 and MoTe2. Suitable materials exhibit an anisotropic heat transfer property of intrinsic in-plane thermal conductivity and a significant decrease in out-of-plane thermal conductivity, for example in the range of 100 to 400.

[0044] Properties and benefits of molybdenum disulfide as the interface layer 13 are further explained below. Asymmetric thermal expansion of the interface layer is investigated. The thermal expansion of the interface layer originated from the 2D MoS2 layer supported on a substrate leads to 2D constrained or expanded structure as shown in Figures 2A-2G. Because of the in-plane strain change, three dimensional stress and strain that mainly induces structural deformation can be minimized while maintaining a large 2D strain.Attorney Docket No.2115-008389-WO-POA

[0045] Figure 2A shows the schematic of MoS2 layer, a member of the D3hpoint group, from both the top and the side, and the phonon modes. With the backscattering arrangement, Raman spectroscopy is used to acquire the in-plane shear mode E′ and the out-of-plane mode A'1. When the 2D MoS2 layer is supported on substrate which has the intrinsic thermal expansion properties, at room temperature, the 2D MoS2 layer initially would have thermal equilibrium with the substrate (Figure 2B). As the ambient temperature rises, the 2D MoS2 layer expands by following the supported substrate as both expand. Depending on the thermal expansion property of the supported substrate, the expansion of the 2D MoS2 is expected to be varied.

[0046] First, a model is established representing a correlation between the in- plane thermal expansion and Raman peak positionwhere Δωs(n), n, A(n), ∆T,and σs denote the shift of Raman peak position, the Raman- active vibrational mode, the self-standing temperature coefficient, the temperature gradient, the stress coefficient, and the in-plane thermal stress within the 2D MoS2 layer due to thermal mismatch, respectively. The σs is determined by the temperature dependent stress-strain relationship aswhere εs, α, v, E, αs are the in-plane thermal strain within the MoS2 layer when being supported by the substrate, the thermal expansion coefficient (TEC) of the MoS2 layer, the Poisson’s ratio, the Young’s modulus of the MoS2 layer, and TEC of the 2D MoS2 layer supported substrate, respectively. From equation 2,σs can be expressed by∆T. When the σs is plugged into equation 1, ∆ωsncan also be represented aswhere ∆ωs(n)and As(n)stand for the change of Raman peak position and the temperature coefficient measured on substrate supported 2D MoS2 layer, which includes the substrate effect. The value of As(n)depends on the substrate type. Therefore, the Δωs(n)is function of substrate type and the temperature gradient. Referring to Figure 2C, the schematic Raman spectra of the 2D MoS2 also show the combined influence of temperature and substrate. For two systems of 2D MoS2 layers supported by aluminum and polydimethylsiloxane (PDMS), the change of Raman spectra of the 2D MoS2 is compared at different temperatures. It is noted that the aluminum reveals low TEC (αPDMS = 310 µm m-1°C−1at 25°C) while PDMS is 100 times larger thanAttorney Docket No.2115-008389-WO-POA that (αAl= 2 µm m−1°C−1at 25°C) of aluminum. The length of the arrows represents the shift caused by photon softening in the Raman peak positions of the 2D MoS2 layer at high temperature. This shift is dictated by the temperature coefficient As(n) and the thermal expansion of each substrate. The Raman shift of E′ of 2D MoS2 layer representing in-plane expansion is expected to be dependent on the TEC of each substrate. The low α substrate (Al) and high α substrate (PDMS) produce different tensile thermal stresses, respectively. This ultimately reduces and amplifies the temperature dependence of Raman peak position change, respectively, it is evident that that> ∣Δωs0∣ > ∣Δωs_1ow∣ and ∣As_high∣ > ∣A∣ > ∣As_low∣. The dotted lines show that Raman red shifts of the E′ mode eludes to only in-plane strain with temperature change. To confirm the 2D expansion of MoS2 layer, Raman spectra were acquired as a function of temperature from 20°C to 100°C for Al and PDMS respectively (Figure 2D-2G). A strong substrate dependence was confirmed by the shift in Raman peak positions as a function of temperature. Estimated temperature coefficients with an R2> 0.99 revealed a good linear trend. The linear relationship between the shift in Raman peak positions and the temperature change indicates that no slip nor structural deformations occurred between the MoS2 layers and the substrates in the temperature range between 20 and 100 °C. The van der Waals (vdW) interactions in the MoS2 layers is attributed to such results. The estimated temperature coefficient of the MoS2 monolayers supported on aluminum (AAlE′) was −0.0043 ± 0.0003 cm−1K−1. However, in case of the PDSM substrate, because of the larger tensile stress from the highly thermal-expanding PDMS substrate, the temperature coefficient of the MoS2 monolayer on the PDMS substrate (APDMSE′= −0.0188 ± 0.0002 cm−1K−1) was larger than that of ∣AAlE′∣ in magnitude. The A′1mode moves independently in the 2D plane of the MoS2 layer. The A′1mode's Raman spectra changes at about 416 cm−1of MoS2 and linearly increases with temperature. The A′1 mode of MoS2 was then used to quantify the thermal extension, which displayed a good agreement with the E′ mode.

[0047] In addition, the effect of MoS2 layer thickness (δMoS2) on thermal expansion was characterized. Raman signals of a 2D MoS2 layer were measured as a function of temperature and number of MoS2 layers (Figure 2H). The change of Raman shift for the E′ mode of monolayer and 10 layers MoS2 on the substrate increases linearly as temperature rises, but the change of Raman shift for 20 layers is not apparent within the same temperature range. Up to the 10 layers, depending on the substrates, the thermal expansion linearly increases with layer numbers and temperature. However, whenAttorney Docket No.2115-008389-WO-POA increasing the number of MoS2 layers beyond 10 layers a reduction in the thermal extension is observed.

[0048] Unlike most materials that are frequently used in self-activation, such as polymers and metallic materials, which exhibit symmetric and 3D thermal expansion, the Raman spectra of the supported MoS2 layer clearly demonstrated that the thermal expansion of MoS2 varies in accordance with the in-plane thermal expansion of the substrate without suffering from interfacial slip and structural deformation. The in-plane thermal expansion of a 2D TMD layer enables the reduction of strain mismatches independently of the support type because the 3D thermal expansion of integrated materials for self-actuation is the main cause of strain mismatches.

[0049] Figure 3 illustrates an example method for fabricating the bending structure. Fabrication begins with depositing an elastomer onto a substrate as indicated at 31. In the example embodiment, polydimethylsiloxane is deposited onto glass. For example, a clean glass substrate was prepared by first treating them with piranha solution (H2SO4:H2O2 = 3:1 v / v) for 10 minutes, rinsing them well with DI water, and then placing them in an ethanol-filled ultrasonic bath for 30 minutes. Polydimethylsiloxane (PDMS) (at a 10: 1 prepolymer to curing agent ratio) was then spun onto the glass substrate. As an alternative, the PDMS film's thickness and modulus were changed by spinning a 50% prepolymer in hexane solution (with the same 10 :1 ratio). The PDMS was incubated on the glass substrate for a whole night at room temperature.

[0050] Next, an interface layer was deposited at 32 onto the elastomer. In this example, the interface layer is molybdenum disulfide. More specifically, MoS2 layers were produced, for example using the chemical vapor deposition (CVD) method or liquid exfoliation on a SiO2 / Si substrate. For the CVD method, the growth of the MoS2 films was carried out in a furnace with a 4-inch quartz tube. Prior to the CVD process and to ensure strong bonding, the SiO2 / Si substrate (e.g., 300 nm thick) with a size of 2 cm × 2 cm was treated with oxygen plasma (100W) for 60 seconds. Before loading the SiO2 / Si substrate, an alumina boat containing MoO3 powder was loaded close to the furnace's heating zone center. Another alumina boat, filled with sulfur powder, was placed 30 cm distant from the MoO3 powder, in the direction of the gas flow entrance. The SiO2 / Si substrate was then loaded 10 cm distance from the edge of the alumina boat containing MoO3 powder. After being purged with Ar for 5 min, the furnace was heated to 850 °C at a rate of 10 °C min−1 using 100 sccm Ar flow. The MoS2 layers were then synthesized at 850 °C for 20 min. In this CVD method, the MoS2 layer thickness was controlled inAttorney Docket No.2115-008389-WO-POA this CVD approach by adjusting the treatment duration, which ranged from 90 to 1200 seconds longer time of treatment time led to thicker MoS2 layers.

[0051] For the liquid exfoliation, a 0.5-mg / ml aqueous solution of mono-or few- layer MoS2 was prepared using 2% sodium deoxycholate as a surfactant. After 400 min of sonication, large MoS2 aggregates, metal catalyst particles and other impurities were removed by centrifuging the MoS2 dispersion for 10 min at 13,000 rotations per minute. Liquid-phase exfoliation was performed following the plasma treatment with the PDMS layer. After drying for 24 h in air, the same process was repeated to form the next MoS2 layer.

[0052] To transfer the MoS2 layer onto the PDMS on glass substrate, water transfer method was used. A single droplet of DI-water was placed on the MoS2 layer of the SiO2 / Si substrate. Next, in the water droplet on the SiO2 / Si substrate, the MoS2 layer floated to the top surface. By making contact between the top side of a water droplet and the B-staged PDMS layer on an upside-down glass substrate, the floated MoS2 was transferred to the PDMS layer. By repeating this MoS2 transfer process, MoS2 layer film number was controlled. The MoS2 / B-staged PDMS layer was then post-cured for 3 min at 100 °C to form MoS2 / PDMS on glass substrate.

[0053] Lastly, a temperature sensing layer is deposited at 33 onto the interface layer. In this example embodiment, the temperature sensing layer is aluminum. To deposit the aluminum, the MoS2 / PDMS on glass substrate sample was adhered to a larger, clean glass slide, for example on a 6 inch wafer using Kapton tape. The sample was then loaded in the dome of the electron beam evaporator and the vacuum pump was activated to reach less than 3 x 10-6torr pressure. Metallic layers of Ti (2 nm) and Al (25–1000 nm) were then deposited. The titanium layer is optional and was used to enhance the adhesion between MoS2 / PDMS and the Al layer, preventing delamination of the two layers during the release of the residual stress. During the Ti and Al deposition (deposition rate = 0.5 nm / s), the substrate was placed above the sources at a constant rotating speed (100 rpm). Once the sample reached atmospheric pressure following deposition, it was removed from the vacuum chamber. The Kapton tapes were removed to obtain the Al-deposited MoS2 / PDMS sample on glass substrate. Lastly, the glass substrate was manually removed from the PDMS side. While exemplary fabrication technique has been described above with specific materials having specific values and arranged in a specific configuration, it will be appreciated that the bending structure 10 may be constructed with many different configurations, materials, and / or values asAttorney Docket No.2115-008389-WO-POA necessary or desired for a particular application. The above configurations, materials and values are presented only to describe one particular embodiment that has proven effective and should be viewed as illustrating, rather than limiting, the present invention.

[0054] The extreme anisotropy of the interface layer 13 enables in-plane heat distribution at the interface while the in-plane thermal expansion leads to in the interfacial stress reduction. The asymmetric heat transfer of the interface layer 13 results in breaking the through-plane translational symmetry while retaining the in-plane layers, thereby providing effective means for suppressing only κ⊥. The anisotropic and uniform heat transfer at the interface, enabled by the interface layer 13, induce a thermal mismatch at the interface, leading to enhanced actuation at the elastomer layer.

[0055] To validate the anisotropic heat transfer effect on the actuation, thermomechanical analyses of the interface layer 13 was performed between the elastomer and the temperature sensing layers using finite element analysis (FEA) as seen in Figure 4A. Since the thermal resistance of each layer affects the heat transfer, the key parameters investigated are the effect of the thicknesses of elastomer (δEL) and the heat collection layers (δHeat), and relative thickness of IML, δIML(= δMoS2 / δEL) with constantδHeaton the actuation.

[0056] To clarify the effects of isotropic heat transport and the in-plane thermal expansion effect, independently, sequential FEA was employed, doing a heat transfer analysis before proceeding to a thermal mechanic analysis. When applying a constant external temperature of 100 °C to the assembled structure, the temperature gradient (ΔTIML) at the interface area along the x-axis is considerably uniform, with ~ 1 °C fluctuation, compared to ΔTcon of the control structure showing ± 8 °C. Along the y-axis, there is a significant temperature drop in the interface area, whereas the control sample displays a gradual temperature reduction from the aluminum to the PDMS layer. This indicates the effectiveness of the interface layer 13 in thermal accumulation because of its excellent κ|| (Figure 4B). The larger interfacial ΔTIML creating higher thermal accumulation induced 10 times enhanced interfacial stress compared to that of the control with the same δELand δHeat(Figure 4C). As a result, a significantly increased strain was achieved in the IML-integrated bending structure (Figure 4D). For the IML assembled bending structure, the measured bending angle (∆θIML) was 100% larger than that (∆θcont) in the control sample. The effect of interface layer thickness on the thermal stress and strain changes was also tested. The highest stress in the bending structure is shown at δIML = 10-5. For δIML > 10-5, larger temperature gradients along the y-axis wereAttorney Docket No.2115-008389-WO-POA present, whereas for thinner IML, the insulation effect was larger. As δIMLbecomes larger, the thermal accumulation effect at the interface decreases.

[0057] The performance of the bending structure 10 in response to temperature changes is shown in Figures 5A-5E. A schematic of an assembled bending structure exposed to different biaxial strains is shown in Figure 5A. For the self-actuation tests, the constructed structure was in a state of unrelaxed tension over its entire length (L) and it was free hanging. Only the fixed boundary, where the film was attached to the substrate layer with a certain length (Lhold), prevents the hanging segment of the film from elastic relaxation. Prior to the construction of bending structure, δELand δHeatare optimized.

[0058] Subsequently, time-lapse images are acquired for the integrated structure and the control sample at T = 100°C (Figure 5B and 5C). By measuring the dynamic change in the bending angle (∆θIML) at T = 100°C, the self-activation performance was quantified. With the constructed bending structure consisting of Al and PDMS for the heat collection and elastomer, respectively, ∆θIML increases as a function of time. A maximum ∆θIML of ~50° was obtained in one minute, while ∆θcont (without the interface layer) reached 35° during the same time frame. ∆θIML was maintained at ~60° after the 1 min mark without further change in the bending angle, while a further ∆θcont = ~ 3° was observed in the control sample on the two-minute mark. When T was reduced to ~ 25 °C, both the bending structure with an interface layer and the control structure showed reverse bending, and within ~ 2 min, the structures returned to their initial geometry. Furthermore, cycling at temperatures between T = 25°C and 100°C for 10 cycles, the integrated bending structure shows a repetitive behavior with consistent bending / unbending cycles performance. These results imply that the self-activation mechanism of the integrated bending structure has a consistent response to the environmental temperature.

[0059] Furthermore, the effect of the interface layer on the self-activation performance was investigated by varying the temperature and interface layer thickness. ∆θ IML of the integrated bending structure was characterized at different temperatures from 25 to 100°C (Figure 5D). Overall, ∆θIML increased with temperature; at T = 60 °C, ∆θ IML reached ~ 35° within 2 min, while it increased to ∆θ IML ~ 60° at T = 100 °C for a similar time range. The combined effect of the anisotropic heat transfer and in-plane strain of the interface layer on the self-activation bending performance varies with the interface layer thickness. At T = 100 °C, ∆θIMLincreased from 45 to 55°, when interfaceAttorney Docket No.2115-008389-WO-POA layer thickness increased from δIML= 0 to 10-4. However, for the same temperature, ∆θ IML then decreased by reaching 50° at δIML= 10-5. The effect of the interface layer thickness on the bending angle occurred regardless of the temperature change.

[0060] Numerous self-activation structures have been reported to be performant in particular medium phases. For instance, hydrogel structures are operational in the aqueous phase because of their temperature-dependent charge-induced interactions with the water molecules. Ideally, the self-activation performance should be attained independently of the medium type, whether it is air or an aqueous phase, for example. With references to Figures 6A and 6B, how different medium’s phases and viscosities affect the self-activation capability of the integrated bending structure were examined. The temperature-sensitive ∆θIML was characterized for different medium phases: air, water, and glycerol as seen in Figure 6A. The integrated bending structure showcased a near constant ∆θIML at T = 100 °C, for all the tested mediums, with actuation times for air, water, and glycerol of 1, 1.5, and 1.7 min, respectively. On the other hand, the control sample showed ∆θcontof about 33°, 30°, and 25° with actuation times of 1.5, 2.5, and 3.7 min for air, water, and glycerol, respectively. The integrated bending structure revealed consistent self-activation performance independent of the medium type.

[0061] The viscosity of the medium (air), where the integrated interface layer results in most effective self-activation, is lower (1.81× 10-5Pa·s) than that of water and glycerol at 20°C (10-3Pa·s and 1.4× 10-2Pa·s, respectively). In Figure 6B,and ∆θcont are depicted as a function of viscosity at 100⁰C and in 120 seconds. The integrated bending structure provides consistent self-activation performance for various viscosities ranges and up to ~ 10-3Pa*s, making it a reliable option for different medium. The thermal anisotropy of the interface layer provided both accumulation and uniformity at the interface in the through-plane direction. The assembly provides the benefit of increased structural stability while delivering consistent bending performance in a range of medium conditions.

[0062] The bending structure described above was then applied to a self-activated pore structure as seen in Figure 7A and 7B. The pore structure is closed at low temperatures and opens at high temperatures. The pore structure consists of the self- actuation hatch integrated with the bending structure and a supporting layer made of polytetrafluoroethylene (PTFE). To optimize the self-actuated motion considering the mechanical stability and a large pore size, three geometric parameters wee studied: node width, (Wp= wp / ws, where Wp is normalized node width, wpis node width, and wsisAttorney Docket No.2115-008389-WO-POA actuator width), node length (Lp= lp / ws, where Lpis normalized node length and lpis node length), and layer thickness (ta= tp / ws, where ta is normalized actuator thickness, and tp is actuator thickness). Subsequently, the pore open / close function was evaluated at 100 °C. The pore structure closed at t =0, opens rapidly within 60 sec, when exposed to high temperature and remains fully open. When the environmental temperature is reduced to 25 °C, the pore structure closes in 120 sec as seen in Figure 7C. The open / close actuation of the constructed pore structure shows highly consistent and stable motion over time, with monotone cyclic performance over various temperature cycles.

[0063] In addition, the temperature effect on the pore open / close ratio was characterized for various Wp as shown in Figure 7E. For a tpof 0.1, as temperature increases from room temperature to 100°C, the pore open / close ratio increases from 0 to 100%. The pore open / close ratio seems to be generally highest when Wp= 0.5. The effect of geometry parameters tp and Lp were characterized on the bending angle, representing the pore open / close ratio as shown in Figure 7F. The bending angle was the highest at tp of 0.1 regardless of Lpat Wpof 2. No structural change was observed during the pore open / close operation.

[0064] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0065] When an element or layer is referred to as being "on," “engaged to,” "connected to," or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," “directly engaged to,” "directly connected to," or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describeAttorney Docket No.2115-008389-WO-POA the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0066] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0067] Spatially relative terms, such as “inner,” “outer,” "beneath," "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0068] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

Attorney Docket No.2115-008389-WO-POA CLAIMS What is claimed is:

1. A bending structure, comprising: a base layer comprised of an elastomer; an interface layer disposed directly on the base layer, where the interface layer is a transition metal dichalcogenides; and a temperature sensing layer disposed directly on the interface layer and comprised of a metal, where the bending structure alters its shape reversibly as a function of an external stimulus.

2. The bending structure of claim 1 wherein the interface layer is selected from one of molybdenum disulfide, molybdenum diselenide, molybdenum ditelluride, tungsten disulfide and tungsten diselenide.

3. The bending structure of claim 1 wherein the base layer is further defined as polydimethylsiloxane.

4. The bending structure of claim 1 wherein the temperature sensing layer is further defined as aluminum.

5. The bending structure of any of claims 1 to 4 further comprising a layer of titanium deposited between the interface layer and the temperature sensing layer.

6. The bending structure of claim 1 wherein the external stimulus is temperature provided by friction, conduction, convection, radiation or a combination thereof.

7. The bending structure of any of claims 1 to 6 wherein the structure encompasses particles that would heat when subject to electricity, a magnetic field or a combination thereof.Attorney Docket No.2115-008389-WO-POA 8. A bending structure, comprising: a base layer comprised of an elastomer; an interface layer disposed directly on the base layer, where the interface layer has an anisotropic heat transfer property in the range of 100 to 400 with an asymmetric ratio between the in-plane thermal conductivity (κ||) and out-of-plane thermal conductivity (κ⊥); and a temperature sensing layer disposed directly on the interface layer and comprised of a metal, where the bending structure alters its shape reversibly as a function of an external stimulus.

9. The bending structure of claim 8 wherein the interface layer is a transition metal dichalcogenide.

10. The bending structure of claim 8 wherein the interface layer is selected from one of molybdenum disulfide, molybdenum diselenide, molybdenum ditelluride, tungsten disulfide and tungsten diselenide.

11. The bending structure of claim 8 wherein the base layer is further defined as polydimethylsiloxane.

12. The bending structure of claim 8 wherein the temperature sensing layer is further defined as aluminum.

13. The bending structure of any of claims 8 to 12 further comprising a layer of titanium deposited between the interface layer and the temperature sensing layer.

14. The bending structure of claim 8 wherein the external stimulus is temperature provided by friction, conduction, convection, radiation or a combination thereof.

15. The bending structure of any of claims 8 to 14 wherein the structure encompasses particles that would heat when subject to electricity, a magnetic field or a combination thereof.Attorney Docket No.2115-008389-WO-POA 16. A method for fabricating a bending structure, comprising: depositing an elastomer onto a substrate; depositing an interface layer onto the elastomer, where the interface layer has an anisotropic heat transfer property in the range of 100 to 400 with an asymmetric ratio between the in-plane thermal conductivity (κ||) and out-of-plane thermal conductivity (κ⊥); and depositing a metal onto the interface layer, thereby forming a bending structure whereas the bending structure alters its shape reversibly function of an external stimulus.

17. The method of claim 16 wherein the elastomer is further defined as polydimethylsiloxane and the metal is aluminum.

18. The method of claim 16 wherein the interface layer is a transition metal dichalcogenide.

19. The method of claim 16 wherein the interface layer is selected from one of molybdenum disulfide, molybdenum diselenide, molybdenum ditelluride, tungsten disulfide and tungsten diselenide.

20. The method of claim 16 wherein the interface layer is deposited using chemical vapor deposition.

21. The method of any of claims 16 to 20 further comprising depositing titanium onto the interface layer before depositing the metal.

22. The method of claim 16 wherein the metal is deposited using electron- beam vapor deposition.

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