Flexible and elastic thermal regulator for multimode intelligent temperature control

As nonlinear thermal devices, thermal regulators can intelligently respond to temperature and control heat flow through changes in heat transfer capacities, which allows them to reduce energy consumption without external intervention. However, current thermal regulators generally based on high‐quality crystalline‐structure transitions are intrinsically rigid, which may cause structural damage and functional failure under mechanical strain; moreover, they are difficult to integrate into emerging soft electronic platforms. In this study, we develop a flexible, elastic thermal regulator based on the reversible thermally induced deformation of a liquid crystal elastomer/liquid metal (LCE/LM) composite foam. By adjusting the crosslinking densities, the LCE foam exhibits a high actuation strain of 121% with flexibility below the nematic–isotropic phase transition temperature (TNI) and hyperelasticity above TNI. The incorporation of LM results in a high thermal resistance switching ratio of 3.8 over a wide working temperature window of 60°C with good cycling stability. This feature originates from the synergistic effect of fragmentation and recombination of the internal LM network and lengthening and shortening of the bond line thickness. Furthermore, we fabricate a “grid window” utilizing photic‐thermal integrated thermal control, achieving a superior heat supply of 13.7°C at a light intensity of 180 mW/cm2 and a thermal protection of 43.4°C at 1200 mW/cm2. The proposed method meets the mechanical softness requirements of thermal regulator materials with multimode intelligent temperature control.


INTRODUCTION
2][3][4] Unlike conventional static thermal management solutions, thermal switches with dynamic and variable thermal conductance, owing to their ability to tailor the heat flux on demand, hold great promise in numerous fields such as battery thermal control, [5][6][7] personal thermal management, 8,9 space conditioning, 10 and thermal energy storage. 112][23] Compared with thermal switches triggered by nonthermal stimuli, a thermal regulator, which is a nonlinear thermal device or material that responds directly to temperature and influences the temperature in turn by the self-feedback regulation of the heat flux, 24,25 exhibits characteristics of intelligent and self-adaptive thermal management and reduces energy consumption without repeated external intervention.
7][28] A high switching ratio (R on/off ) of ∼10 has been observed in crystalline polyethylene nanofibers based on the structural phase transition near T crit ≈ 400 K, 27 while this feature at the nanoscale limits their macroscopic applications.0][31] Chen et al. obtained a R on/off value of ∼3.2 in graphite and hexadecane suspensions, owing to the improved contact among graphite flakes under the internal stress generated during freezing 29 ; however, the solid-rigidity and liquid-fluidity of PCMs are detrimental issues that require attention.(iii) Solid-solid transitions of the crystalline phase (e.g., metal-insulator transition, 32,33 charge density wave phase transition, 34,35 and martensite-austenite transformation [36][37][38] ) in some metal alloys or transition metal oxides.A R on/off of ∼1.5 was observed in Ni-Mn-In alloys near room temperature through martensitic transformation 38 ; these transformations typically result in positive temperature coefficient changes in the thermal conductivity (a high-temperature phase has a higher κ than a low-temperature phase); however, their switching ratios are relatively low (R on/off < 2), and these materials are typically rigid or brittle.To sum up, the intrinsic brittleness may result in structural damage and functional failure under external mechanical stress or internal thermal stress. 17,39,40][43][44] Hence, thermal regulators with mechanical softness and elasticity are urgently required.
Because mechanically soft materials are generally amorphous and lack a high-quality crystalline structure and high-contrast transition of the crystalline phase, there is an inevitable trade-off between the mechanical softness and switchable thermal properties.A promising strategy to address this issue is the deformation of 3D porous polymer networks to modulate the heat transfer capacity through thermomechanical coupling.For example, Sun et al. used a slow-proton-release-modulating gelation and thermo-induced crosslinking strategy to fabricate hyperelastic Kevlar nanofiber aerogels, which showed a heat fulx R on/off of 7.5 at a compression of 80%. 18However, its switchable thermal conductance originates from the mechanical strain rather than spontaneous deformation.Recently, Liu et al. designed a liquid metal (LM) shape-memory polymer foam for a smart switch by loading LM on a deformable foam skeleton, and a thermal conductivity R on/off of 4.7 was obtained at a compression ratio of 80%. 45However, its switchable thermal conductance is not spontaneously reversible because of the one-way shape-memory effect.In summary, designing and fabricating thermal regulators with soft and thermally reversible characteristics remains a significant challenge.
In this study, inspired by the principle of thermomechanical coupling, we designed and demonstrated a soft thermal regulator based on the thermally reversible deformation of a liquid crystal elastomer/LM (LCE/LM) composite foam.LCE foams were prepared using the sacrificial salt template method and a two-step crosslinking process by controlling the crosslinking density in two stages.The LCE foam exhibited an unprecedented reversible actuation strain of 121% with flexibility below the nematic-isotropic phase transition temperature (T NI ) and hyperelasticity above T NI .The incorporation of LM further amplified the heat transfer difference of the LCE/LM composite foam during expansion and contraction, and an optimized thermal resistance R on/off of 3.8 was obtained with good cycling reliability, owing to the synergistic effect of the fragmentation and recombination of the internal LM network and the lengthening and shortening of the bond line thickness (BLT) during thermally reversible deformation.Accordingly, multimode intelligent thermal responses and temperature controls of LCE/LM composite foam were explored based on separate or combined functions such as variable thermal conductance, thermal actuation, and switchable electrical resistance.The method developed in this study can provide significant insights into the fabrication of soft regulators for intelligent thermal management applications.

Design principle and fabrication process of LCE/LM thermal regulator
To obtain a thermal regulator with mechanical softness based on the thermomechanical coupling strategy, the following are the prerequisites: (1) the matrix should deform dramatically and reversibly in response to the temperature; (2) the deformation results in a significant change in the thermal conductance, and (3) the filler is mechanically soft, and its incorporation does not impede the thermally deformable behavior of the matrix.To satisfy the first condition, LCEs, which combine polymer softness and mesogenic anisotropy, are particularly promising matrices because they exhibit the largest thermally reversible deformation among soft multifunctional materials. 46,47Density is an important factor affecting the thermal conductivity, and highly dense blocks typically have high thermal conductivities whereas loose networks with low densities have low thermal conductivities.To meet the second condition, an LCE can be designed as a porous structure to convert the bending deformation of local skeletons into a volumetric change of the foam entity.The thermally conductive filler composite further modulates the thermal regulatory properties of the LCE foam.As a low-melting-point alloy that combines the high thermal conductivity of metals and the fluidity of liquids, eutectic gallium-indium (EGaIn) can freely deform with the surrounding matrix and was selected as the filler to meet the third condition. 48,49he fabrication process of the LCE/LM composite foam included NaCl template fusion, LCE foam preparation, LCE foam programing, and LM incorporation (Figure 1A).First, to form a 3D through-hole structure in the LCE foam, the disconnected NaCl particles were fused into an integral whole prior to the LCE synthesis using vapor.Next, LCE was synthesized by a two-step thiol-acrylate Michael addition photopolymerization (TAMAP) method modified from a previous study (Yakacki et al.). 50The LC monomer solution with the designed formula was infiltrated into the gaps of the NaCl template under repeated vacuum application, and the first crosslinking network was formed by thermo-assisted thiol-acrylate Michael addition.After the solvent was dried, the initial LCE foam was obtained using the sacrificial NaCl template.Subsequently, the initial LCE foam was compressed to the designed strain and UV-irradiated to form a second crosslinking network by photopolymerization of the residual acrylate groups.Finally, the LCE/LM composite foam was obtained by extruding LM into the pores of the LCE foam through vacuum filling, and the additional LM was squeezed out at the designed loading ratio.Briefly, an LCE/LM composite foam that integrates the soft elasticity and thermal deformability of the LCE with the heat conduction and infinite deformability of the LM was developed to exhibit mechanical softness (flexibility below T NI and elasticity above T NI ) and serve as a thermal regulator (Figure 1B).

Actuation behaviors of LCE foams
As proposed in this study, the variation in the heatconducting performance of the LCE/LM composite foam depends on its volume deformation.Therefore, our first objective was to control the actuation behavior of the LCE foam and obtain the maximum possible thermally induced reversible deformation.The actuation of the LCE results from the coupling between the liquid crystal directors and the polymer network, which is influenced by the chemical formula design and mechanical shape programming.In this study, the LCE prepared by TAMAP undergoes a two-step crosslinking process, forming double networks.The first network formed by PETMP determines the shape at high temperatures (initial shape), and the second network formed by excess acrylate groups fixes the shape at low temperatures (programed shape).Although the concept of competitive dual networks has been proposed, [51][52][53] the decoupling control of network crosslinking densities lacks effectiveness, and the influence of crosslinking densities in two stages on the actuation strain of the porous LCE structure has not been explored.Based on the high conversion efficiencies of the thiol-acrylate Michael addition reaction and the photochemical reaction of acrylate groups, 50,54  (3) self-photopolymerization and formation of the second crosslinking points in Stage 2. To control the crosslinking density through a monomer formulation design, two parameters, X and Y, were introduced, where X is defined as the ratio of the acrylate groups that form the second crosslinking points to the first crosslinking points and Y is defined as the ratio of the acrylate groups that form crosslinking points to the total acrylate groups.The relationship between X and Y and the functional-group content can be deduced as follows: where N RM257 is the mole of acrylate groups in RM257, N EDDET and N PETMP represent the moles of thiol groups belonging to EDDET and PETMP, respectively.In addition, R c is defined as the degree of compression deformation in Stage 2 of mechanical programming.The effects of X, Y, and R c on the actuation strain were investigated using three orthogonal experiments.First, we kept Y and R c constant (Y = 0.3, R c = 70%) to explore the effect of X on the actuation strain.To verify whether the design of the monomer formula was effective in controlling the densities of the two-step crosslinking networks, the gel fraction, swelling experiments and differential scanning calorimetry were conducted separately.With an increase in X from 0.3 to 1.2, the swelling ratio of the LCE in Stage 1 increased from 161.7 to 192.4% (Supporting Information Figure S1b; Table S1) whereas T g decreased from 1.7 to −5.0 • C (Supporting Information Figure S2a; Table S1), suggesting a decrease in the density of the first crosslinking network.In addition, the difference in the gel fraction, swelling ratio, and T g of the LCE in Stages 1 and 2 widened with increasing X, indicating an increase in the density of the second crosslinking network.However, for different X values, there was little difference in the swelling ratio (154.1-157.3%)and T g (2.8-4.4 • C) of the LCE in Stage 2, confirming that the total crosslinking density remained unchanged with varying X.As for the thermo-induced deformability of the LCE foam, the thermomechanical tests (Figure 2A) showed that the reversible actuation strain first increased and then decreased with increasing X, and the maximum reversible actuation strain reached 121% for X = 1.0, consistent with the captured digital images (Supporting information Figure S3).Next, X and R c were fixed (X = 1.0,R c = 70%), and the effect of Y on the actuation strain was explored.With the increase in Y from 0.1 to 0.5, the swelling ratio of the LCE in Stage 2 decreased from 179.4 to 136.8% (Supporting Information Figure S4b; Table S2).T g increased from −4.5 to 14.8 • C (Supporting Information Figure S5b; Table S2), proving that the total crosslinking density increased monotonously with Y. Interestingly, the actuation strain of the LCE foam was significantly affected by Y as well.Figure 2B shows that for Y = 0.1, 0.2, 0.4, and 0.5, the actuation strains reached 34, 97, 28, and 22%, respectively, all of which were less than 121% when Y reached the middle value (Y = 0.3), consistent with the digital images (Supporting Information Figure S6).Furthermore, the influence of R c on the actuation strain was investigated using identical X and Y values (X = 1.0,Y = 0.3). Figure 2C shows that the actuation strain is maximum at R c = 70%.Supporting Information Figure S7 shows that the LCE foams can easily deform reversibly between the programmed shape and the initial shape for R c = 50, 60, and 70%, but for R c = 80%, the LCE form could not recover to the originally expanded state at 160 • C, and its deformation shape was irregular.
The evidence mentioned above shows that the actuation behaviors of LCE foams can be effectively controlled by three independent parameters, namely X, Y, and R c , instead of numerous attempts at monomers formulation without clear guidance, and that the adjustment of the two-step crosslinking density is a necessary condition for realizing a large reversible actuation strain.When X was low, the second crosslinking density was low, and the oriented mesogens were difficult to stabilize.When X was high, the degree of the second crosslinking significantly exceeded that of the first crosslinking, resulting in a locked-orientation structure of the mesogens and insufficient restoring forces.To achieve better reversible deformation between the programmed shape and the initial shape, a balance between the first and second crosslinking networks is required. 52Nevertheless, in addition to the relative levels of the two-step crosslinking density, the total crosslinking density has been verified to significantly affect the deformation of the LCE foam.Given that the LCE shape change relies on the coupling between the liquid crystal directors and the polymer network, when the total crosslinking density is low (low Y), which results in a weak coupling effect, it is insufficient for the nematic-to-isotropic transformation of the mesogens to drive the conformational change in the polymer network.In comparison, when the total crosslinking density is high (high Y), the mesogens are tightly bound to the polymer network, and it is difficult for them to undergo a phase transition.Therefore, the total crosslinking density must be moderately controlled to produce a large reversible deformation.Theoretically, the higher the R c value, the wider the range for the LCE foam to undergo reversible deformation.However, part of the first crosslinking network in bent areas may fall apart when it suffers from overlarge programmed strain, making it difficult for the LCE foam to return to its initial shape at high temperatures (Supporting Information Figure S8).
Supporting Information Figure 2D and Video S1 show the changes in the size and internal pore structure of the LCE foam at its cross-section during heating and cooling.When the temperature increased from 25 to 125 • C, its height increased by 127% (3.7 →8.4 mm), while the width decreased by only 6% (9.2 →8.6 mm), resulting in a 98% increase in the apparent volume of LCE foam, which will provide space for the 3D reconfiguration of LM distribution (will be discussed in Section 2.4).In contrast, the deformation of bulk LCE without pores is nearly volume-conserving, 55,56 which is because spontaneous and reversible contraction occurs upon heating and expansion occurs upon cooling along the director, while the two directions perpendicular to the director exhibit significant contraction upon cooling and expansion upon heating.Enlarged images show stretching of the skeletons and opening of the pores.After cooling, the deformation exhibited a good reversible recovery.Moreover, it could generate a thermally reversible deformation of approximately 120% even under a compression load (2 kg) 1000 times heavier than its own weight (Supporting Information Figure S9).As shown in Figure 2E, the LCE foam with X = 1.0,8][59][60][61][62][63] The macro-scopic shrinkage and expansion deformation of the LCE foam results from the microscopic bending and stretching deformation of the skeleton, which is driven by the partial mesogen monodomain in the bent regions (Supporting Information Figure S11).Additionally, the LCE foam exhibited intriguing mechanical properties, which can be confirmed by the stress-strain curves as shown in Figure 2F and Supporting Information Figure S12.At a low temperature (25 • C) below T NI , there was remarkable hysteresis in the compressing-releasing responses, and the recovery rate was only 62% for an applied strain of 25%.While at a high temperature (130 • C) above T NI , the compressing-releasing curve almost coincided, and nearly 100% of the recovery rate could be obtained for an applied strain of 50%.Rheological measurements also confirmed the change in the mechanical properties of LCE foam with temperature (Supporting Information Figure S13).At 25 • C, the storage modulus (G') is slightly higher than the loss modulus (G'') but in the same order of magnitude, and G'' gradually equalizes with G' with increasing angular frequencies (ω), implying the soft elasticity of the LCE foam at a low temperature.While at 130 • C, G' is nearly an order of magnitude larger than G'', and the G' of LCE/LM foam was less frequency-dependent than that at 25 • C, indicating that the elastic property was predominant at a high temperature.The transformation of thermomechanical properties mainly stem from the nematic-isotropic phase transition of LCE, which was investigated by a dynamic mechanical analyzer in Supporting Information Figure S14.The loss factor (tan δ) has a much broader peak than traditional elastomers that typically show a symmetric single peak, reflecting a semisoft elasticity of the nematic domain due to the reorientation of the liquid crystal directors upon loading and hysteresis in the director distribution evolution upon unloading. 64,65While the low storage modulus and tan δ above T NI reflect the inherent entropic elasticity of the crosslinking networks recovering at high temperatures.For an LCE in an isotropic phase at a higher temperature, the specific mesogen interaction becomes negligible under thermal agitation.[68]

Temperature-responsive heat-conducting properties of the LCE/LM composite foams
Based on the thermally induced reversible volume deformability of the LCE foam, the LM with a high thermal conductivity filled the pores of the LCE foam to modulate the temperature-responsive thermal conductance of the LCE/LM composite foam.For X = 1.0 and Y = 0.3, the thermal conductivity of LCE/LM (100 wt% LM) increased at temperatures of 25 and 130 • C with the decrease in R c (Supporting Information Figure S15a); this is because the LCE foams with a low R c could support more LM in the shrunken state (Supporting Information Figure S7).For a thermal regulator, R on/off is an important index for evaluating its thermal regulation capacity.For R c = 70, 60, and 50%, the R on/off values of the thermal conductivity reached 4.5, 3.1, and 1.3, respectively, suggesting that a larger deformation can lead to greater changes in the thermal conductivity.The differences are more significant in the thermal resistance owing to the considerable variation in the heat transfer distance, and R on/off of the thermal resistance reaches 9.5 for R c = 70% (Figure 3A).However, the significant change was irreversible in this case because the LCE/LM composite foam with R c = 70% could not return to its initial state after the heating and cooling cycle (Supporting Information Figure S16).We speculate that the compact LM network broke up during LCE expansion, and numerous thin but hard oxide layers were formed on the surface of the internal LM, which competed mechanically with the shrinkage force of the LCE skeleton. 69In addition, the LM loading ratio plays an important role in affecting the thermal conduction and R on/off of the composite foams.As shown in Supporting Information Figure S15b, with the increase in the LM loading ratio, the thermal conductivities of the LCE/LM composite foams increase at temperatures of 25 and 130 • C, while the maximum R on/off of the thermal conductivity reaches 2.0 at 50 wt%, where R on/off of the thermal resistance reaches 3.9 (Figure 3B).Notably, for pure LCE foam, the thermal conductivity decreases only slightly from 0.124 W/m/K at 25 • C to 0.115 W/m/K at 130 • C, with a change of less than 10%, suggesting that mere deformation cannot result in a significant change in the thermal conductivity without the participation of LM.For comparison, nondeformable PDMS/LM composite foams exhibit only a slight increase in the thermal conductivity from 25 to 130 • C (Supporting Information Figure S17), which is opposite to that observed in the case of LCE/LM composite foams.This may be attributed to the improved radiation heat transfer and thermal conductivity of the air inside the foam with increasing temperature, which also indicates that the temperature has a limited effect on the thermal conductivity and thermal resistance of the composite foams.The thermal regulation capacity of LCE/LM-50 originates mainly from the synergetic effect of LCE deformation and LM incorporation.
Figure 3C shows the specific thermal conductivity and thermal resistance of LCE/LM-50 as a function of the temperature, where the thermal conductivity of LCE/LM-50 decreases from 0.53 to 0.27 W/m/K, accompanied by an increase in the BLT from 4.2 to 8.3 mm (Supporting Information Figure S18), together leading to an increase in the thermal resistance from 79.09 to 306.27 K cm 2 /W.The transition mainly occurs in the temperature range of 40-100 • C, with a slight hysteresis resulting in a noncoincidence between the cooling and heating processes, which is similar to the actuation behavior of LCE foams (Figure 2A-C).LCE/LM-50 exhibited reliable reversibility during 100 heating and cooling cycles (Figure 3D), corresponding to an average thermal conductivity R on/off of 2.0 and an average thermal resistance R on/off of 3.8.The thermal conductivity can be divided in the horizontal direction (κ ‖ ) and axial direction (κ ⟂ ), as shown in Supporting Information Figure S19.The LCE/LM-50 is almost isotropic at 130 • C, whereas a slight anisotropy at 25 • C (κ ‖ = 0.568 ± 0.044; κ ⟂ = 0.492 ± 0.020 W/m/K), which may result from the orientation of LM caused by the contraction of the LCE skeleton.This resulted in a larger thermal conductivity switching ratio in the horizontal direction, but the significant variation of the lateral area makes it difficult to form stable thermal contact with the heat source or heat sink, which will severely limit its practical applica-tion.Thus, the heat transfer in the axial direction was the main focus.
To visually demonstrate the temperature-responsive thermal conductance of LCE/LM-50, the average apparent temperatures (T ave,top ) were identified using an IR thermal imager and compared with those of the PDMS/LM foams.In the experiment, the samples were placed on a temperature-controlled stage, and the steady state was recorded after 30 min at each stage temperature (T stage ) as shown in Figure 3E.The performance of LCE/LM-50 was consistent with that of PDMS/LM-4mm in the low-temperature region, and its T ave,top values were closer to T stage than those of PDMS/LM-8mm because of its higher heat transfer capacity.LCE/LM-50 behaved like PDMS/LM-8mm in the high-temperature region, and its T ave,top values were closer to room temperature because of its lower heat transfer capacity.The heat transfer capacity of LCE/LM-50 showed good switching ability, and the transition occurred in the region of 50-100 • C for the heating process and 90-40 • C for the cooling process (Figure 3F).Withregard to the respond rate, the LCE/LM-50 began to expand after approximately 5 s when it was placed on a hot stage at 160 • C, it took approximately 100 s to deform to the fully expanded state (Supporting Information Video S2), which means that the LCE/LM-50 may fail to deal with high heat flow in a very short period, but it is sufficient to respond to most cases of continuous heat output.In addition to the final steady state, the transient heating and cooling processes were tracked (Supporting Information Figure S20; Videos S3 and S4).When the temperature-controlled stage was rapidly raised to 160 • C, LCE/LM-50 showed slower heating than PDMS/LM-4mm, while LCE/LM-50 showed faster cooling than PDMS/LM-8mm when the bottom surface was cooled to −40 • C.This also confirms the switchable heat transfer capacity of the LCE/LM-50 depending on the temperature.Moreover, upon heating to 160 • C, the T ave,top -time curve of LCE/LM-50 almost coincided with that of nondeformable PDMS/LM-8mm, indicating that the deformation of LCE/LM-50 hardly delayed the transfer of heat flow because the response time is much lower than the time required for heat conduction to reach equilibrium.
In terms of the mechanical properties, the corporation of LM has little influence on the modulus of LCE foam both at 25 and 130 • C (Supporting Information Figure S13), that is due to the fact that liquid LM has a much lower modulus and therefore would not significantly impede the deformation of LCE foam.LCE/LM-50 remained flexible at a low temperature (25 • C) below T NI and hyperelastic at a high temperature (130 • C) above T NI (Supporting Information Figures S21 and S22).Moreover, as shown in Supporting Information Figure S23, the κ ⟂ of LCE/LM-50 at RT will increase when suffering from both out-plane compressive strain and in-plane tensile strain, which stems from the closer contact between LM droplets in the axial direction, and the R on/off can be further improved.Noteworthily, LM may risk of leakage under high pressure due to the porous structure (Supporting Information Figure S24).For LCE/LM-50 with Φ30 mm over 10 heating and cooling cycles, negligible LM leakage occurred at loads below 1000 g (13.8 kPa), which is 180 times its own weight, suggesting that the LCE/LM-50 can meet the thermal regulation requirements of numerous lightweight and soft devices under the gravity and assembly pressures.While the leakage occurs under 2000 g, which requires attention in practical applications.Overall, Figure 3G shows a comparison between the properties of LCE/LM-50 and other reported macroscopic thermal regulator materials, such as metals (Ni-Mn-In alloy 38 ), metal oxides (VO 2 32 ), polymers (PNIPAm hydrogel 70 and MMA-co-nBA colloid 71 ), and PCMs (PEG4000 72 and graphite/C 16 H 34 29 ).LCE/LM-50 exhibited an excellent overall performance.In addition to the relatively high R on/off and reversibility as universal thermal regulators, it exhibited the flexibility and elasticity characteristic of soft materials.Furthermore, the transition temperature in the range of 40-100 • C allowed LCE/LM-50 to continuously regulate the heat flow within a wide working temperature window.Moreover, the transition temperature is expected to be tuned by selecting different LC mesogens and monomers formulation to modulate the T NI of LCE. 73,74These advantages suggest that the LCE/LM composite foam is a promising candidate as a soft thermal regulator in the field of intelligent temperature response and thermal management.

Thermal regulation mechanism of the LCE/LM-50
Since the LM (26.4 W/m/K) has a significantly higher thermal conductivity than LCE (0.244 W/m/K) and air (0.02 W/m/K), it dominates the thermal conduction of the composite foam.To clarify the thermal regulation mechanism of LCE/LM-50, the 3D distribution of LM in the shrunken and expanded states was obtained from micro-CT.The LM droplets were tightly packed with abundant links between them in the shrunken state (Figure 4A), whereas in the expanded state, although there were local aggregations, the LM droplets were integrally discrete and separated from each other (Figure 4B).In addition, LCE/LM-50 exhibited a dramatic and reversible jump (∼10 5 times) in the electrical resistance during the temperature rise and fall (Supporting Information Figure S25), indicating a transition of the internal LM between the well-connected network and non-connected islands.When exposed to air, a thin oxide layer (mainly composed of Ga 2 O 3 ) will form instantly on the surface of EGaIn, which facilitates the LM droplets to adhere to other substrates.6][77] SEM images showed that the LM could deform freely to conform to the topological structure of the LCE skeleton, and well-defined but tightly bonded interfaces were foamed between the LM and the LCE (Supporting Information Figure S27).Due to the strong adhesion between LCE and LM, it can be inferred that the change in LM distribution (network fragmentation/recombination) is caused by the thermal deformation of the LCE skeleton.We supplemented a model experiment to further illustrate the effect of the deformation of the LCE skeleton on LM distribution.As shown in Supporting Information Figure S28, the LM droplet can be deformed by following the movement of LCE strips through strong interfacial adhesion, thus result in the separated or connected state.Moreover, the large volume change of LCE foam provides space for the 3D reconfiguration of LM distribution.Therefore, we can infer the process how the thermal deformation of LCE skeleton affects the LM distribution.When the temperature increases, the LCE skeleton dilates to drive the separation of LM droplets due to their strong adhesion, and the volume expansion of LCE foam provides space for LM fragmentation into non-connected islands; when the temperature decreases, the separated LM droplets recombine under the mechanical recovery force generated by the contraction of the LCE skeleton to form a connected network.
To further investigate the effects of the LCE skeleton and LM distribution on the heat transfer, finite element simulations were performed.Based on the above analysis and calculations of the porosity, LCE/LM-50 models in the shrunken and expanded states were established, as shown in Supporting Information Figure S29, where the LCE skeleton was generated using the stochastic reconstruction method with a porosity of 43% and a thickness of 4 mm in the shrunken state and a porosity of 69% and a thickness of 8 mm in the expanded state.The LM was subsequently generated using the quartet structure generation set method, aggregated and connected in the shrunken state and discrete and unconnected in the expanded state, with the same volume in both states.The simulation was conducted with a bottom temperature of 50 • C and an ambient temperature of 20 • C, as shown in Figure 4C and D. The shrunken state exhibited a higher heat transfer rate than the expanded state, and T ave,top in the steady state was 7.6 • C higher than that in the expanded state, consistent with the trend observed at a bottom temperature of 120 • C (Supporting Information Figure S30).The simulation results confirm the superior heat transfer capability of LCE/LM-50 in the shrunken state than in the expanded state, and that the change in the heat transfer performance of LCE/LM-50 at low and high temperatures is mainly caused by the transformation of the internal structure rather than temperature fluctuations.
In summary, according to R bulk = BLT/κ bulk , where BLT is the bond line thickness, R bulk and κ bulk are the intrinsic thermal resistance and thermal conductivity of the sample in bulk, respectively.The thermal regulation mechanism of LCE/LM-50 can be attributed to the following two factors: (1) during heating, the expansion of the LCE skeleton leads to a fragmentation of the LM network, thereby changing the thermal conductive pathway from the LM network to the LCE skeleton, that is, a decrease in κ bulk ; (2) the expansion of the entire composite foam in the thickness direction leads to an increase in the heat transfer distance, that is, an increase in the BLT, which further widens the difference in R bulk between the shrunken and expanded states of LCE/LM-50.

Two basic modes of the thermal regulator
Based on the temperature responsiveness of the intrinsic thermal resistance (model I), considering the heated object, LCE/LM-50 is expected to dynamically alter the heat transfer and adaptively regulate the temperature of the heated object to cope with the varying heat source powers, exhibiting the effect of heat conduction at low temperatures and heat insulation at high temperatures.Figure 5A shows a schematic in which the PI heater heats the weight through the samples.The thermal power of the PI heater was regulated by the input voltage.A weight of 200 g served as a counterweight to tightly contact the sample with the heat source and heated object, and the temperature at the top surface of the sample (T top ) was continuously monitored.As shown in Figure 5B, the performance of LCE/LM-50 is consistent with that of PDMS/LM-4mm at a low heating voltage and exhibits a trend similar to that of PDMS/LM-8mm at a high heating voltage.T top , with the heat being transferred through the PDMS/LM foams, linearly depended on the heat source power, whereas in the case of LCE/LM-50, it was nonlinear (Figure 5C).More specifically, at a heating voltage of 4 V, T top , when heated through LCE/LM-50 after 1 h, was 3.3 • C higher than that in the case of PDMS/LM-8mm (Figure 5D); whereas at 10 V, T top of LCE/LM-50 was 20.1 • C lower than that of PDMS/LM-4mm (Figure 5E).This suggests that LCE/LM-50 provides better heat transfer at a low heat source power, whereas the heat transfer capacity of LCE/LM-50 decreases at a high heat power, which can provide overheating protection for the heated object.
Conversely, the effects of heat insulation at low temperatures and heat conduction at high temperatures can be achieved using LCE/LM-50.Based on the temperatureresponsive actuation capability of LCE/LM-50 (Model II), a mechanical contact/separate thermal switch was designed from the perspective of the heat source, and the heat dissipation rate and real-time temperature of the heat source were adjusted autonomously to cope with the varying heat source power.As the testing platform, illustrated in Figure 5F, the PI heater dissipated heat toward the top Cu heat sink through the samples.Figure shows the real-time temperature of the PI heater (T heater ) at different thermal power levels.At low heating voltages, LCE/LM-50 shrank and separated from the heat sink, with T heater in the steady state being 11.0 • C higher than that of the PDMS/LM-8mm at 4 V.In comparison, at high heating voltages, LCE/LM-50 expanded and contacted the heat sink; the steady-state T heater was 52.1 • C lower than that of PDMS/LM-4mm at 10 V, with the heat dissipation being even superior to PDMS/LM-8mm.In this mode, for low thermal power, LCE/LM-50 facilitated the accumulation of heat from the heat source, which warmed up more quickly at a lower power consumption to reach the optimal operating temperature (e.g., for batteries operating in cold environments).With the increase in the power of the heat source, LCE/LM-50 helped the heat source to dissipate heat and prevent thermal failure.Notably, the working capacity of LCE/LM-50 was reversible and repeatable in both modes with no performance degradation over five cycles (Supporting Information Figures S31 and S32).

Photic-thermal and electrical-thermal integrated intelligent temperature control
As the main source of energy in space, sunlight can warm a spacecraft by a moderate amount; however, overexposure may cause thermal imbalance, exposing lives and equipment to high-temperature damage.Based on the combined capabilities of LCE/LM-50, such as its thermoinduced reversible deformation and variable thermal conductance (Model III), a "grid window" model utilizing photic-thermal integrated intelligent thermal control was designed as a proof of concept.As illustrated in Figure 6A, there are two main ways in which light transfers heat to the blackboard: indirect heat transfer to the blackboard through the sample and copper column after photothermal conversion of the center lighting-absorbing plate, and direct heat transfer of the light through the surrounding grids to the blackboard.As shown in Figure 6B, at a low light intensity, LCE/LM-50 shrinks, and the grid is open, allowing light to pass through the grid and directly irradiate the blackboard (Figure 6C), whereas at a high light intensity, LCE/LM-50 expands thermally, and the PI fibers drive the slidable baffles toward the center, causing the grid to close and significantly weaken the light reaching the blackboard (Figure 6D).
At different light intensities, the temperatures at point O at the center and at points A, B, C, and D in the surrounding midpoints of the blackboard were recorded.For point O heated indirectly through heat conduction, a temperature of 59.5 • C was reached using LCE/LM-50 after 1 h at a light intensity of 180 mW/cm 2 , which was comparable to PDMS/LM-4mm and 8.8 • C higher than that of PDMS/LM-8mm; in comparison, after 1 h at 1200 mW/cm 2 , LCE/LM-50 reached a temperature of 120.9 • C, which was 31.6 • C lower than that reached by nondeformable PDMS/LM-4mm (Figure 6E).For points A, B, C, and D heated directly through photothermal conversion, LCE/LM-50 could maintain the temperature variation at points A, B, C, and D within 10 • C under a light intensity gap of more than six folds, whereas the temperature change exceeded 25 • C and 50 • C with PDMS/LM-4mm and PDMS/LM-8mm, respectively (Figure 6F; Supporting Information Figure S33).Overall, at a light intensity of 180 mW/cm 2 for 1 h, the average temperature of the O, A, B, C, and D points with LCE/LM-50 was 13.7 ± 4.2 • C higher than that in the case of PDMS/LM-8mm; however, the tempera-ture drop of LCE/LM-50 reached 43.4 ± 9.8 • C compared with that of PDMS/LM-4mm after 1 h at a light intensity of 1200 mW/cm 2 .This showed that LCE/LM-50 could spontaneously regulate the heat flow input to the inner layer of the model with respect to the light intensity and effectively maintain the thermal balance, achieving heat supply at a low light intensity and protection against hightemperature damage at high light intensities.Moreover, LCE/LM-50 exhibited a switchable electrical resistance in response to the temperature (Supporting Information Figure S34a-c).Therefore, a self-feedback regulation of the thermal transport without the aid of logic electronics could be realized through electrical and thermal coupling (model IV) when LCE/LM-50 was connected in series with the electric heater (Supporting Information Figure S34d-f), demonstrating multimode intelligent thermal responses and temperature controls.

CONCLUSIONS
In summary, based on the thermomechanical coupling concept, a thermally reversible deformation strategy with a porous composite foam was proposed to fabricate a soft, reversible, and high-contrast thermal regulator, realized using a LCE/LM 3D interpenetrating network structure.The crosslinking densities in the two stages and the compression ratio were found to be crucial in controlling the reversible deformation of LCE foams.A maximum reversible actuation strain of 121% was obtained with intermediate design parameters (X = 1.0,Y = 0.3, and R c = 70%), the highest reported among two-way shape memory foams.The incorporation of LM increased the thermal switching contrast while ensuring flexibility below T NI and hyperelasticity above T NI , and an optimized thermal conductivity R on/off of 2.0 and a high thermal resistance R on/off of 3.8 could be achieved at an LM loading content of 50 wt% with continuous adjustment in a wide working temperature window of ∼60 • C and good cycle reliability.We experimentally and theoretically studied the regulatory mechanism by analyzing the variation in the structures and heat transfer of LCE/LM-50 in the shrunken and expanded states.The regulation of the thermal conductance can be attributed to the synergistic effect of the fragmentation and recombination of the internal LM network and the lengthening and shortening of the bond line thickness during thermally reversible deformation.Furthermore, mutimode intelligent thermal controls of LCE/LM-50 were demonstrated; it exhibited excellent adaptive modulation of the allowed heat transfer, in stark contrast to nondeformable PDMS/LM foams.These findings pave the way for the development of soft thermal regulator materials intended for flexible energy storage systems, soft robotics, and deployable aircrafts with autonomous thermal management.

EXPERIMENTAL SECTION
Experimental details and characterizations are provided as Supporting Information.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Design of soft thermal regulator based on LCE/LM composite foam: (A) Schematic of the fabrication process of the LCE/LM composite foam.(B) Characteristics (flexibility, elasticity, and thermal regulator) of the LCE/LM composite foam.

F I G U R E 2
Actuation behaviors of LCE foams: (A-C) Stress-free strain change versus temperature for LCE foams during cooling (solid line) and heating (dotted line) with orthogonal design parameters of monomer formula, (A) varying X with fixed Y = 0.3 and R c = 70%, (B) varying Y with fixed X = 1.0 and R c = 70%, (C) varying R c with fixed X = 1.0 and Y = 0.3.(D) Digital images and sizes of the cross-sectional LCE foam (dyed with black ink) with X = 1.0,Y = 0.3, and R c = 70% upon heating and cooling, top: overall view (scale bars, 2 mm), bottom: enlarged view (scale bars, 500 μm).(E) Comparison between the proposed LCE foam and reported two-way shape memory foams in terms of the actuation strain and transition temperature range.(F) Compressive stress-strain curves of the LCE foam with X = 1.0,Y = 0.3, and R c = 70% at 25and 130 • C, respectively.

F I G U R E 3
Temperature-responsive heat-conducting properties of LCE/LM composite foams: (A) Thermal resistance of LCE/LM composite foams with different R c at 25 and 130 • C; the foams were filled with 100 wt% LM and measured as prepared.(B) Thermal resistances of LCE/LM composite foams with different LM loading ratios at 25 and 130 • C; the foams were programmed with R c = 70% and measured after annealing.(C) Specific thermal conductivities and thermal resistances of LCE/LM-50 upon heating and cooling.(D) Thermal conductivities and thermal resistances of LCE/LM-50 during 100 cycles of heating and cooling.(E) IR thermal images and (F) T ave,top of LCE/LM-50, PDMS/LM-4mm, and PDMS/LM-8mm versus T stage in the steady state.(G) Comparison between LCE/LM-50 and other thermal regulator materials in terms of the thermal resistance switching ratio (R on/off ), working temperature window, reversibility, flexibility, and elasticity.

F
I G U R E 4 LM distribution change and thermal regulation mechanism of LCE/LM-50: the 3D reconstructed structure of the LM network by micro-CT for LCE/LM-50 in (A) shrunken state and (B) expanded state.Scale bars in the sectional view, 500 μm.(C) Finite element simulation of the heat transfer and (D) T ave,top evolution of LCE/LM-50 in the shrunken and expanded states, respectively.The temperature at the bottom was maintained at 50 • C. (E) Schematic illustration of the thermal regulation mechanism of LCE/LM-50.

F I G U R E 5
Two basic modes of a thermal regulator based on material change or physical gap, respectively: (A) Schematic of the performance measurement of the thermal regulator based on material change (Mode I).(B) T top versus time at different heating voltages and (C) corresponding T top after 1 h versus the input power density for cases with LCE/LM-50, PDMS/LM-4mm, and PDMS/LM-8mm as heat conductors, with detailed T top evolution at heating voltages of (D) 4 V and (E) 10 V in (B).(F) Schematic of the performance measurement of the thermal regulator based on the physical gap (Mode II).(G) T heater versus time at different heating voltages for cases with LCE/LM-50, PDMS/LM-4mm, and PDMS/LM-8mm as the thermal bridge.

F
I G U R E 6 Photic-thermal integrated intelligent temperature control: (A) Schematic of the "grid window" model based on the combined capabilities of thermal actuation and variable thermal conductance (Mode III).(B) Schematic of the "grid window" allowing light at low intensity and blocking light at high intensity.(C, D) Digital images of the "grid window" experimental setup equipped with LCE/LM-50 at light intensities of (C) 180 mW/cm 2 and (D) 1200 mW/cm 2 .(E, F) Temperature evolution at points (E) O and (F) A on the "grid window" at light intensities of 180 and 1200 mW/cm 2 , respectively.
This work was financially supported by National Key R&D Program of China (No. 2022YFB3805702), National Natural Science Foundation of China (Grant Nos.52173078, 52130303, 51973158, 51803151, 51973152, 52303101 and 52327802), the Science Foundation for Distinguished Young Scholars in Tianjin (No. 19JCJQJC61700), Young Elite Scientists Sponsorship Program by CAST (No. 2022QNRC001).