Carbon nanotube-integrated stretchable phase change actuator and method for manufacturing the same

Abstract

A stretchable phase change actuator incorporating carbon nanotubes and a method for manufacturing the same are disclosed. The phase change actuator may comprise: a substrate; a heating element formed on the substrate and comprising a material having conductivity and porosity; a first electrode connected to the heating element; a second electrode connected to the heating element and spaced apart from the first electrode; a phase change liquid contained within the heating element; and a stretchable membrane that closes to surround at least the heating element.

Description

Technology Field

[0001] The present invention relates to a stretchable phase change actuator incorporating carbon nanotubes and a method for manufacturing the same. Background Technology

[0002] Flexible electronic devices can operate stably even under conditions where mechanical bending or deformation is applied, making them suitable for various applications that inevitably deform according to user movements, such as wearable or implantable devices. In particular, thin and flexible structures can adhere closely to curved surfaces like skin, enabling seamless tactile interaction with the user while minimizing discomfort. Tactile interfaces include tactile sensors capable of measuring tactile information such as contact or pressure, and tactile displays capable of transmitting tactile information, such as static pressure or vibration, to the user. Recently, flexible tactile displays have garnered attention as a means to deliver sophisticated tactile stimuli to users in VR / AR applications and human-machine interfaces.

[0003] To be utilized as a flexible haptic display, it is necessary to develop actuators that are mechanically flexible and capable of providing haptic information perceptible to the user. Accordingly, flexible actuators based on various operating mechanisms, such as pneumatic, capacitive, and phase-change based actuators, have been reported. For example, an ultra-thin dielectric elastomer actuator with a thickness of 18 micrometers has been developed; this device can be directly attached to a user's finger to deliver precise vibrational stimuli in a frequency range of 1 to 500 Hertz, while also allowing the user to perceive haptic information from external objects. Such thin and lightweight actuators have various potential applications, such as non-visual notifications and haptic-based character recognition. However, since the maximum actuation displacement is limited to 10 micrometers or less, it may not be sufficient for the user to perceive static pressure.

[0004] Recently, haptic actuators with flexibility and elasticity based on an electrostatic zipper mechanism have been developed. These actuators utilize a pillar structure for pressure amplification to generate sufficient pressure to transmit tactile information to the skin, and a high-resolution actuator array configuration suitable for sensitive areas such as fingers is also possible through an array spaced 3 millimeters apart. Additionally, tactile interfaces have been demonstrated in which a robot detects external environmental information and transmits it as tactile feedback to the user's skin. However, these actuators have a limitation in that they require a pouch structure containing a dielectric liquid, which increases the volume of the device. The problem to be solved

[0005] The problem that this invention aims to solve is to utilize the expansion force resulting from the vaporization of a phase change material while ensuring that the overall structure of the actuator maintains flexibility. In particular, it seeks to realize a miniaturized and simplified structure by integrating the heating element and the chamber structure into a single carbon nanotube bundle. Furthermore, the focus is on a structural design capable of maintaining stable electrical contact and mechanical sealing even under repetitive tensile deformation environments. means of solving the problem

[0006] In one aspect, the present invention provides a flexible phase change driver comprising: a substrate; a heating element formed on the substrate and comprising a conductive and porous material; a first electrode connected to the heating element; a second electrode spaced apart from the first electrode and connected to the heating element; a phase change liquid contained within the heating element; and a flexible membrane that closes to surround at least the heating element.

[0007] In one embodiment, the heating element may include a carbon nanotube bundle.

[0008] In one embodiment, the carbon nanotube bundle may include vertically aligned carbon nanotubes (VACNT) formed perpendicularly to the substrate.

[0009] In one embodiment, the phase change liquid may include water or a liquid containing water.

[0010] In one embodiment, the vertically aligned carbon nanotubes can be captured with the phase change liquid contained within them because the periphery is hydrophobic and the center is hydrophilic.

[0011] In one embodiment, the elastic membrane may include an elastomer.

[0012] In one embodiment, the first electrode or the second electrode may include a liquid metal.

[0013] In another aspect, the present invention provides a method for manufacturing a flexible phase change actuator, comprising: a heating element forming step of forming a heating element comprising a conductive and porous material on a substrate; an electrode connecting step of connecting a first electrode and a second electrode to the heating element so as to be spaced apart from each other; a liquid impregnation step of forming the heating element to contain a phase change liquid; and a membrane closing step of wrapping at least the heating element with a flexible membrane.

[0014] In one embodiment, the heating element may include a carbon nanotube bundle.

[0015] In one embodiment, the carbon nanotube bundle may include vertically aligned carbon nanotubes (VACNT) formed perpendicularly to the substrate.

[0016] In one embodiment, the phase change liquid may include water or a liquid containing water.

[0017] In one embodiment, the liquid impregnation step can be performed by applying a hydrophilic treatment to the central portion of the vertically aligned carbon nanotube and then impregnating it with the phase change liquid.

[0018] In one embodiment, the hydrophilic treatment can be performed by treating the central part of the vertically aligned carbon nanotube with oxygen plasma or ozone plasma.

[0019] In one embodiment, the elastic membrane may include an elastomer.

[0020] In one embodiment, the first electrode or the second electrode may include a liquid metal.

[0021] In another aspect, the present invention provides a wearable tactile display comprising: a plurality of actuators arranged to generate displacement toward a surface facing the skin when the wearable tactile display is worn; a power unit capable of supplying power to at least some of the actuators; and a control unit capable of individually controlling at least some of the actuators, wherein at least some of the actuators are elastic phase change actuators according to an embodiment of the present invention.

[0022] In this invention, a stretchable actuator utilizing a phase transition from liquid to gas was developed. This actuator consists of a vertically aligned carbon nanotube (VACNT)-based heating element and a liquid metal (EGaIn) electrode, both of which are integrated into a stretchable Ecoflex substrate. Since all components are inherently stretchable, it can be utilized as a wearable device that adheres closely to the skin. This actuator can generate large actuation displacements of more than 1 millimeter and demonstrates high durability, showing almost no change in actuation performance even after 300 repeated acts under 100% tensile conditions. Effects of the invention

[0023] The advantage of the present invention lies in the fact that vertically aligned carbon nanotube bundles are simultaneously utilized as a heating element and a storage structure for phase change materials, thereby enabling both structural simplification and functional integration. This reduces the overall volume of the actuator and provides the possibility of generating self-contained expansion pressure without the need for external devices. Furthermore, since all components are made of flexible materials, there is potential to maintain stable operational characteristics even under repeated tension or bending. These characteristics can help expand the scope of application to various wearable systems, such as skin-attached interfaces and garment-integrated devices. Brief explanation of the drawing

[0024] FIG. 1: Side view of an elastic phase change actuator according to an embodiment of the present invention. FIG. 2: Flowchart of a method for manufacturing a flexible phase change actuator according to an embodiment of the present invention. Fig. 3: (a) Schematic of the proposed stretchable phase change material (PCM) actuator. This actuator consists of a stretchable elastomer substrate and a heating element based on VACNT bundles, and is filled with liquid phase change material to function as a PCM chamber. (b) Driving mechanism of the liquid-gas phase change-based actuator. When a bias voltage is applied to the CNT bundles, the phase change material vaporizes. During the vaporization process, a large volume expansion occurs, which causes the membrane to expand and generates driving displacement. Fig. 4: Fabrication process of a stretchable phase change actuator. (a, b) A vertically aligned carbon nanotube (VACNT) bundle synthesized via a CVD process is transferred onto an Ecoflex substrate. (c, d) Subsequently, a liquid phase change material (water) is selectively injected into desired areas of the VACNT bundle through a hydrophilic surface treatment process using a shadow mask. (e, f) After connecting the wiring with liquid metal, uncured Ecoflex is spin-coated to form a sealing layer and a membrane. Fig. 5: (a) Optical image of the fabricated stretchable phase change actuator. The device consists of a VACNT-based heating element, a phase change material chamber, and a liquid metal electrode, and is integrated into a stretchable Ecoflex substrate. (b) Optical images of the actuator before and after 100% stretching. Even after 100% stretching, the CNT bundle and the liquid metal electrode remain connected (scale bar: 10 mm). Fig. 6: A stretchable phase change actuator is mounted on a uniaxial stage to apply tension. A bias voltage is applied to the device using a DC power supply. During operation, a laser displacement sensor is used to measure displacement in real time. Fig. 7: Measured displacement of the elastic actuator. (a) Instantaneous displacement curve of the device driven at various power levels. (b) Displacement after driving for 10 seconds at different power levels. Fig. 8: Instantaneous displacement curve obtained through six cycles of driving for 10 seconds followed by cooling for 140 seconds under driving power conditions of 2 W. The result shows that the operating displacement is maintained stably even during repeated driving. Fig. 9: Instantaneous displacement curves of the PCM actuator after applying 100%, 200%, and 300% tension, respectively. Specific details for implementing the invention

[0025] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. Since the present invention is susceptible to various modifications and may take various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed forms, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the present invention. Similar reference numerals have been used for similar components in the description of each drawing. In the attached drawings, the dimensions of the structures are shown enlarged compared to the actual dimensions for the clarity of the present invention.

[0026] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, terms such as “comprising” or “having” are intended to specify the presence of the features, numbers, steps, actions, components, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, or combinations thereof. In the context of this specification, terms such as “about” may mean about ± 1%, about ± 2%, about ± 3%, about ± 4%, about ± 5%, about ± 6%, about ± 7%, about ± 8%, about ± 9%, or about ± 10% of the figures described in the specification.

[0027] In addition, the description of one aspect of the present invention may be applied identically or similarly to the description of other aspects for identical or similar configurations or terms.

[0028] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0029] FIG. 1 shows a side view of an elastic phase change actuator (100) according to an embodiment of the present invention.

[0030] Referring to FIG. 1, a flexible phase change driver (100) according to an embodiment of the present invention may include: a substrate (110); a heating element (120) formed on the substrate (110) and comprising a conductive and porous material; a first electrode (131) connected to the heating element (120); a second electrode (132) connected to the heating element (120) and spaced apart from the first electrode (131); a phase change liquid contained within the heating element (120); and a flexible membrane (140) that closes to surround at least the heating element (120).

[0031] The role of the substrate (110) is to form a base frame that supports the entire drive structure. For example, the substrate (110) may be composed of a stretchable or flexible material. If composed of a stretchable or flexible material, it can provide a foundation that allows the entire actuator to respond flexibly to external mechanical deformations such as tension and bending. Such a flexible substrate (110) can help increase adaptability in wearable environments or skin-adhesive applications and may provide a stable structure capable of maintaining positional alignment and electrical connections between components even under repetitive mechanical loads.

[0032] The role of the heating element (120) is to generate Joule heat by passing current when an external voltage is applied, while simultaneously maintaining a state in which a phase change material is contained within the porous structure. By performing this dual function, it is advantageous for simplifying and miniaturizing the structure, and a stable heat transfer path can be secured as a heat source to induce physical expansion. In addition, the porous structure can be designed to suppress the positional movement of the phase change material while having a fast thermal response, which can also contribute to improving the precision of repetitive operation.

[0033] In one embodiment, the heating element (120) may include a carbon nanotube bundle. A carbon nanotube bundle is a structure formed by a plurality of vertically aligned carbon nanotubes in a dense state, which can simultaneously possess high electrical conductivity and thermal conductivity. This structure can efficiently generate heat according to the flow of current, and due to its porous characteristics, it can also be advantageous for maintaining a state in which a phase change material is contained within. In particular, the flexibility and mechanical strength of the carbon nanotubes can help suppress structural damage and maintain consistency in heating characteristics even when the actuator is repeatedly deformed.

[0034] In one embodiment, the carbon nanotube bundle may include vertically aligned carbon nanotubes (VACNT) formed perpendicularly to the substrate (110). Vertically aligned carbon nanotubes (VACNT) refer to nanotube structures that are self-aligned and grown perpendicularly to the surface of the substrate (110), and possess characteristics of high density and alignment. This structure allows current to flow efficiently in the axial direction, and the heat transfer path can also be formed short and straight, which can be advantageous for localized heat control. In addition, the aligned structure can help stably retain phase change material within the bundle and tends to maintain a collective form even under external mechanical stimuli such as tension or compression, thus contributing to securing physical stability and functional continuity in a repetitive operation environment.

[0035] In particular, vertically aligned carbon nanotubes can possess properties that are insensitive to tension or deformation. This is because, within the vertically aligned structure, each carbon nanotube forms collective mechanical support through entanglement with one another or contact with adjacent nanotubes. These structural characteristics can induce individual nanotubes to distribute the load dispersively when external deformation, such as tension or bending, is applied, thereby preventing the sudden collapse of the bundle's shape or the interruption of electrical pathways. Furthermore, the high elongation and flexibility of the nanotubes themselves can also contribute to increased resistance to deformation, thereby increasing the likelihood that heat generation characteristics or structural consistency will be maintained even under repetitive deformation conditions.

[0036] Furthermore, vertically aligned carbon nanotubes are advantageous for rapid heat transfer. This is because individual carbon nanotubes have high axial thermal conductivity, and these characteristics can be collectively maximized in a vertically aligned arrangement. In particular, carbon nanotubes have a structure that allows for efficient heat transfer through both electrons and lattice vibrations compared to polycrystalline metals or general polymer materials, and the higher the degree of alignment, the faster the heat can travel along a straight path without diffusion. These characteristics can be advantageous for rapidly raising the local temperature of the heating element (120) and can help shorten the driving response time by rapidly inducing vaporization of the phase change material.

[0037] The role of the first electrode (131) and the second electrode (132) is to form electrical contacts at both ends so as to stably apply current to the heating element (120), thereby inducing Joule heat to be effectively generated inside the heating element (120). If both electrodes are made of a flexible material, the electrical connection can be maintained even when the entire actuator is stretched or repeatedly deformed, and thus there is a possibility of mitigating electrical instability or performance degradation during long-term use. In particular, if a liquid metal-based material is used, the change in contact resistance due to structural changes is small, so the electrical connection with the heating element (120) can be improved, and if arranged in a symmetrical structure, it can also contribute to suppressing the deviation of heat distribution generated during operation.

[0038] The first electrode (131) and the second electrode (132) may also be formed from a flexible material. In one embodiment, the first electrode (131) or the second electrode (132) may include a liquid metal. The advantage of the liquid metal is that it can freely change its shape according to external mechanical deformation while maintaining electrical conductivity similar to that of a solid metal. This characteristic can be advantageous in maintaining the continuity and contact stability of the electrode even in an environment where the entire actuator undergoes repetitive deformation such as tension, compression, and bending. In particular, in the case of a gallium-based liquid metal with a thin metal oxide film and excellent fluidity, the electrical path is not easily severed even during deformation, and it can also contribute to stably maintaining electrical coupling with carbon nanotubes. Furthermore, since micropatterning is possible, it can flexibly respond to various types of electrode designs, making it possible to apply it to complex shapes or high-density array structures.

[0039] The role of the above-mentioned phase change liquid is to act as a medium that causes volume expansion, which is the core of the driving force. By absorbing heat generated from the heating element (120) and changing its phase from a liquid state to a gas, it can induce a rapid increase in pressure within a sealed space. This pressure deforms the elastic membrane (140) to transmit mechanical movement to the outside, and if the expansion and contraction process is repeated, it can play a role in increasing the possibility of continuous operation as an actuator.

[0040] The characteristics that the above-mentioned phase change liquid must possess are thermodynamic properties that allow it to vaporize at a relatively low temperature while causing sufficient volume expansion upon vaporization. These characteristics enable the formation of driving pressure even with low power consumption and can be advantageous in improving response speed. Additionally, physical and chemical stability must be maintained during repeated vaporization and condensation cycles, and the possibility of chemical reaction or erosion with the internal structure of the heating element (120) must be low. It is desirable that the viscosity be neither excessively low nor high so that it can remain within the bundle and be properly distributed, and that it satisfy the condition of maintaining a sealed state without leakage even after operation.

[0041] In one embodiment, the phase change liquid may include water or a liquid containing water. Other non-limiting examples include ethanol, methanol, isopropanol, butanol, propylene glycol, glycerol, dimethyl sulfoxide (DMSO), acetone, acetonitrile, tetrahydrofuran, ethylene glycol, triethylene glycol, diethylene glycol, formamide, pyridine, triethylamine, 1,4-dioxane, chloroform, dichloromethane, benzene, toluene, xylene, hexane, heptane, octane, diethyl ether, methyltetrahydrofuran, n-methylpyrrolidone (NMP), dimethylformamide (DMF), trichloroethylene, tetrachloroethylene, perfluorohexane, hexafluoroisopropanol, fluorobenzene, bromobenzene, silicone oil, mineral oil, paraffin oil, phenol, cyclohexane, cyclopentane, trifluoroethanol, methyl ethyl ketone, It may include methyl isobutyl ketone, diethyl carbonate, propylene carbonate, and ethylene carbonate.

[0042] In one embodiment, the vertically aligned carbon nanotubes have a periphery (122) that is hydrophobic and a center (121) that is hydrophilic, so that the phase change liquid can be trapped inside. As a result, the phase change liquid can remain inside the carbon nanotube bundle without easily leaking out, so there is no need to structurally form a separate fixed chamber or boundary structure. This configuration can be advantageous in reducing the thickness and complexity of the entire device and can have the advantage of not compromising the functional integrity of the device even in environments requiring flexibility and stretchability. Furthermore, since spontaneous trapping of the liquid is possible by utilizing the spacing between nanotubes and the difference in surface energy, there is potential to contribute to the simplification of the fabrication process and the minimization of material usage.

[0043] The role of the above-mentioned elastic membrane (140) is to transmit pressure generated by the phase change to the outside and to act as a physical boundary that protects the internal components. In particular, the elasticity and elastic properties of the membrane allow it to accommodate repeated deformation due to expansion and contraction, and it can also perform a sealing function to prevent leakage of the phase change material from the external environment.

[0044] In one embodiment, the elastic membrane (140) may include an elastomer. For example, the elastic membrane (140) may include Ecoflex elastomer. An elastomer refers to a polymer material that can be easily stretched or deformed by an external mechanical load and returns to its original shape after the load is removed. Such physical properties can be usefully utilized in devices that aim to maintain structural stability even under repetitive operation or external impact. In particular, elastomers with low Young's modulus and high elongation are suitable for flexible and skin-friendly devices and can respond sensitively to minute pressure changes or expansions, which can help improve the mechanical response characteristics of actuators. Additionally, since it can be processed into various hardnesses and thicknesses, the ability to control the amount of deformation according to the application purpose can also serve as an advantage.

[0045] FIG. 2 illustrates a flowchart of a method (200) for manufacturing a flexible phase change actuator according to an embodiment of the present invention.

[0046] Referring to FIG. 2, a method (200) for manufacturing a flexible phase change actuator according to an embodiment of the present invention may include: a heating element forming step (S210) of forming a heating element comprising a conductive and porous material on a substrate; an electrode connection step (S220) of connecting a first electrode and a second electrode so as to be spaced apart from each other on the heating element; a liquid impregnation step (S230) of forming a heating element to contain a phase change liquid inside the heating element; and a membrane closing step (S240) of wrapping at least the heating element with a flexible membrane.

[0047] The role of the above-mentioned heat generation step (S210) is not only to generate heat but also to form a structural basis capable of holding a phase change liquid. In particular, when a material having both conductivity and porosity is utilized, heat can be generated rapidly when current is applied, and the generated heat can be efficiently transferred into the liquid to induce a vaporization reaction. The porous structure helps prevent the liquid from easily leaking out while maintaining a stable distribution state within, thus serving as a basis for maintaining consistent operation even in a repetitive operation environment.

[0048] In one embodiment, the heating element may include a carbon nanotube bundle. In one embodiment, the carbon nanotube bundle may include vertically aligned carbon nanotubes (VACNT) formed perpendicularly to the substrate.

[0049] The role of the electrode connection step (S220) is to form an electrical contact to allow current to flow stably through the heating part. When the first electrode and the second electrode are connected at appropriately spaced ends of the heating part, current can pass through the entire heating part and induce a uniform heat distribution. If a flexible electrode material is used in this step, it can help improve contact stability without compromising the elasticity of the entire actuator.

[0050] The role of the above liquid impregnation step (S230) is to properly retain the phase change liquid within the heating element, thereby establishing a physical foundation for the vaporization reaction required for future operation to occur. In particular, a method of selectively injecting or permeating the liquid into the central region of the heating element can be advantageous for stably fixing the position of the liquid while reducing structural complexity. Since the impregnated liquid is maintained within a limited space inside the bundle, it can contribute to reducing energy loss during operation and effectively inducing mechanical expansion.

[0051] In one embodiment, the phase change liquid may include water or a liquid containing water.

[0052] The role of the above membrane closure step (S240) is to protect the heating element and the phase change liquid contained therein from the external environment, and to form an outer shell structure that physically supports the gas expansion occurring during operation. The elastic membrane used in this step can accommodate repeated expansion and contraction, which can be advantageous for ensuring long-term durability. Furthermore, this membrane can function as part of a driving mechanism that effectively transmits internal pressure to the outside while preventing the penetration of external contaminants or moisture. By structurally performing both sealing and elastic functions simultaneously, it has the potential to contribute to ensuring the stability of the entire system and consistent driving operation.

[0053] In one embodiment, the liquid impregnation step (S230) may be performed by impregnating the phase change liquid after applying a hydrophilic treatment to the central portion of the vertically aligned carbon nanotube. In this specification, the term "hydrophilic treatment" refers to a process of imparting hydrophilicity to the surface of a carbon nanotube, which is originally hydrophobic, by changing the chemical properties of the surface so that a polar liquid, such as water, can easily spread and remain on the surface.

[0054] Such treatment can generally be performed using oxygen plasma, UV / ozone treatment, or chemical oxidation methods with oxidizing agents, and the treated area is transformed into an energy state where liquid can spontaneously penetrate or adsorb. If hydrophilicity is imparted only to selective regions within a vertically aligned structure, the phase change liquid can be stably trapped only at specific locations, which can help improve the controllability and positional precision of liquid distribution.

[0055] In one embodiment, the hydrophilic treatment can be performed by treating the central portion of the vertically aligned carbon nanotube with oxygen plasma or ozone plasma. This allows the phase change liquid to be selectively retained in the central portion of the vertically aligned carbon nanotube, thereby separating it from the hydrophobic region of the periphery and suppressing unnecessary diffusion or leakage of the liquid. This selective surface modification enables spontaneous positional alignment of the liquid, which can help improve impregnation efficiency without the need for a separate liquid injection structure or mechanical boundary. Furthermore, by concentrating the phase change liquid at the center of the heat transfer path, there is also the possibility of ensuring spatial consistency of the vaporization reaction during exothermic reactions and contributing to improved reproducibility and stability of the driving response.

[0056] In one embodiment, the elastic membrane may include an elastomer. In one embodiment, the first electrode or the second electrode may include a liquid metal.

[0057] Meanwhile, a wearable tactile display according to an embodiment of the present invention comprises: a plurality of actuators arranged to generate displacement toward a surface facing the skin when the wearable tactile display is worn; a power unit capable of supplying power to at least some of the actuators; and a control unit capable of individually controlling at least some of the actuators, wherein at least some of the actuators may be elastic phase change actuators according to an embodiment of the present invention.

[0058] The stretchable phase change actuator proposed in this invention possesses the following distinct advantages compared to existing research and inventions. First, by integrating vertically aligned carbon nanotubes synthesized at high temperatures onto a polymer substrate via a low-temperature transfer process, the possibility of application to various flexible substrates without thermal damage is secured. Second, as the carbon nanotube bundle functions as a chamber that simultaneously acts as a heating element and captures phase change materials, a stretchable device capable of structural simplification and miniaturization is realized. Third, the configuration integrating a polymer substrate and carbon-based nanomaterials enables the design of a device with high stretchability and mechanical flexibility, and allows the structure to operate while maintaining functionality under various deformation conditions.

[0059] Embodiments of the present invention are described below. However, the embodiments described below are merely partial embodiments of the present invention, and the scope of the present invention is not limited to the following embodiments.

[0060] Design and fabrication

[0061] Figure 3(a) is a schematic of a stretchable phase change material (PCM) driver. A VACNT-based heating element containing PCM is integrated into a stretchable substrate. Liquid PCM is filled inside VACNT bundles, and the edges of the bundles are surrounded by elastomer walls. Figure 3(b) illustrates the operating mechanism of the PCM driver. In the initial state, no voltage is applied, so the surface of the driver remains flat. When voltage is applied, current flows along the axially entangled VACNT strands, generating Joule heating. Heat is transferred from the CNTs to the PCM (water), causing it to vaporize. As a result, the PCM chamber expands and deforms the membrane upward.

[0062] Figure 4 illustrates the fabrication process of a stretchable PCM actuator. A VACNT bundle is transferred onto a stretchable Ecoflex substrate. First, uncured Ecoflex is spin-coated onto the substrate. A donor substrate formed with VACNT is placed on the Ecoflex layer so that CNT strands are partially embedded in the Ecoflex, and then cured. When the donor substrate is removed, some of the CNT strands remain attached to the Ecoflex and exposed on the surface (Figure 4(b)). The hydrophobic VACNT bundle is selectively converted into a hydrophilic region through oxygen plasma treatment (Figure 4(c)). In this state, when a water drop is dropped, the water is fixed only in the plasma-treated region (i.e., the hydrophilic region) (Figure 4(d)). After connecting a liquid metal electrode, the uncured Ecoflex is spin-coated again to cure. At this stage, the uncured Ecoflex penetrates only the VACNT region where there is no water, and an Ecoflex membrane is formed in the water-filled region.

[0063] Figure 5 shows an optical image of a stretchable PCM actuator. As shown in Figure 5(b), the connection between the liquid metal and the CNT heating element remains stable and does not break even under 100% tension. Since all components, including the heating element and the electrode, are made of elastic material, the phase change material can be effectively heated even under mechanical deformation.

[0064] Figure 6 is a schematic of the experimental setup for measuring the driving displacement of the device. After mounting the PCM actuator on a single-axis tension stage, the driving displacement was measured in real time using a laser displacement sensor while applying a bias voltage. In Figure 7, the driving displacement was measured when various driving powers of 1 to 4 W were applied for 10 seconds. It can be observed that within the same driving time, a larger displacement occurs as the power increases. Figure 8 shows the transient curve during 6 cycles, with each cycle involving driving at 2 W for 10 seconds followed by a 140-second rest period. It can be confirmed that a driving displacement of approximately 300 micrometers is repeatedly achieved. In Figure 9, the performance was evaluated by applying 100% tension to the device 300 times. This demonstrates that the device maintains stable operation despite repetitive mechanical deformation.

[0065] FIG. 10 is a diagram showing a two-dimensional profile and optical images measuring the change in surface height when the actuator power is turned on (On) and off (Off) under 0% and 100% strain conditions. Referring to FIG. 10, it can be seen that when the power is off, there is almost no change in surface height regardless of whether it is strained, whereas when the power is turned on, a driving displacement of about 3 mm occurs in the unstrained state (0% strain, On), and about 6 mm or more occurs in the strained state (100% strain, On). In the optical image inserted on the right, a dome-shaped expansion is clearly observed when the power is applied (On), and it can be visually confirmed that a flat state is maintained when the power is not applied (Off). Through FIG. 10, it can be confirmed that the stretchable phase change actuator of the present invention can induce large displacement according to electrical driving and maintain stable driving performance even under 100% strain conditions. This suggests the potential of the actuator as one capable of securing high responsiveness and reproducibility even in a repetitive strain environment.

[0066] Figure 11 is a diagram showing the results of driving pressure measurements for a stretchable phase change actuator. Figure 11a shows the change in pressure occurring according to driving power, and it can be observed that the output pressure tends to gradually increase as the power increases from 1 W to 3 W. Figure 11b shows the results of performing 10 repeated drives under the 1 W condition, demonstrating that a pressure of approximately 2 kPa is stably and repeatedly generated in each cycle. Figure 11c compares the results of repeating driving 3 times in the actuator's initial state (0% tension) and the tensioned state (100% tension); similar pressure responses are observed in both states, suggesting that the repeatable driving performance is maintained regardless of the presence or absence of tension. Through Figure 11, it can be confirmed that the stretchable phase change actuator of the present invention can precisely control the driving pressure according to the input power, has high output reproducibility even during repeated driving, and can maintain a stable pressure output without functional degradation even under tension conditions. This demonstrates that the structure is capable of providing highly reliable driving performance in various operating environments.

[0067] conclusion

[0068] In this invention, a PCM actuator with high elasticity and toughness is realized by integrating a VACNT-based heating element and a liquid metal electrode into a soft elastomer substrate. Unlike conventional actuators with limited displacement or elasticity, this device enables large deformations in the millimeter range through a liquid-to-gas phase transition mechanism, while simultaneously maintaining stable operation even under conditions of significant mechanical deformation. The utilization of entangled carbon nanotubes effectively suppresses changes in resistance, helping to maintain consistent heating performance even under repetitive driving or high tensile conditions. Experiments confirmed that the actuator responds reliably under various power input conditions and maintains its functionality even after extensive mechanical tension. These results demonstrate the potential for PCM-based actuator technology to contribute to the realization of next-generation skin-attachable tactile interfaces.

[0069] Although the present invention has been described above with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as set forth in the following claims. Explanation of the symbols

[0070] 100 phase change actuators 110 substrate 120 heating element 121 center 122 Periphery 131 First electrode 132 Second electrode 140 elastic membrane 200 Method for manufacturing a stretchable phase change actuator S210 heating element formation stage S220 Electrode connection step S230 Liquid impregnation step S240 membrane closure stage

Claims

Claim 1 A flexible phase change actuator comprising: a substrate; a heating element formed on the substrate and comprising a conductive and porous material; a first electrode connected to the heating element; a second electrode spaced apart from the first electrode and connected to the heating element; a phase change liquid contained within the heating element; and a flexible membrane that closes to surround the heating element; wherein the heating element comprises a carbon nanotube bundle, the carbon nanotube bundle comprises vertically aligned carbon nanotubes (VACNTs) formed perpendicularly to the substrate, the phase change liquid comprises water or a liquid containing water, and the vertically aligned carbon nanotubes have a periphery that is hydrophobic and a center that is hydrophilic, thereby capturing the phase change liquid contained within. Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 In claim 1, the elastic membrane comprises an elastomer, forming an elastic phase change actuator. Claim 7 In claim 1, the first electrode or the second electrode comprises a liquid metal, forming a flexible phase change actuator. Claim 8 A method for manufacturing a flexible phase change actuator, comprising: a heating element forming step of forming a heating element including a conductive and porous material on a substrate; an electrode connection step of connecting a first electrode and a second electrode to the heating element so as to be spaced apart from each other; a liquid impregnation step of forming the heating element to contain a phase change liquid; and a membrane closing step of wrapping at least the heating element with a flexible membrane; wherein the heating element includes a bundle of carbon nanotubes, the bundle of carbon nanotubes includes vertically aligned carbon nanotubes (VACNT) formed perpendicularly to the substrate, the phase change liquid includes water or a liquid containing water, and the liquid impregnation step is performed by impregnating the phase change liquid after hydrophilic treatment of the central part of the vertically aligned carbon nanotubes. Claim 9 delete Claim 10 delete Claim 11 delete Claim 12 delete Claim 13 A method for manufacturing a stretchable phase change actuator, wherein, in claim 8, the hydrophilic treatment is performed by treating the central portion of the vertically aligned carbon nanotube with oxygen plasma or ozone plasma. Claim 14 A method for manufacturing a stretchable phase change actuator, wherein the stretchable membrane comprises an elastomer in claim 8. Claim 15 A method for manufacturing a flexible phase change driver according to claim 8, wherein the first electrode or the second electrode comprises a liquid metal. Claim 16 A wearable tactile display comprising: a plurality of actuators arranged to generate displacement toward a surface facing the skin when the wearable tactile display is worn; a power unit capable of supplying power to at least some of the actuators; and a control unit capable of individually controlling at least some of the actuators; wherein at least some of the actuators are elastic phase change actuators according to claim 1.

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