Stretchable Heater Research Articles

Stretchable, transparent and multifunctional PVC-gel heater: a novel approach to skin-mountable, wearable thermal devices

Electric heaters based on functional materials and innovative designs have been developed for various applications. In this paper, we propose a soft dielectric heater (SDH) using polyvinyl chloride-gel (PVC-gel) as the dielectric heater and hydrogel as stretchable electrodes. Under an AC voltage, the leakage current in the PVC-gel leads to continuous injection and discharge of charges, causing the polarized plasticizers and flexible PVC chains to vibrate and collide, thereby generating heat through dielectric heating. Furthermore, the SDH generates a uniform temperature distribution even under strains up to 400%. Besides, high transmittance over 86% across the visible range renders it suitable for wearable or skin-mountable heaters from an esthetic viewpoint. Its capacitor-like structure achieves a scalable design, enabling extension from a singular cell to a row/column addressable and pixelated array of heaters. The 5 × 5 SDH array can deliver varied thermal information and sensations while maintaining performance even when stretched.

3D printing of liquid metal circuits on fabric for stretchable heater application

Abstract Liquid metals (LMs), due to their excellent stretchability, low resistance, and high thermal conductivity, has become an ideal material for preparing stretchable circuits. 3D printing technology can produce patterns with complex structures with high precision and is suitable for low-cost large-scale production of circuits. However, the high surface tension of liquid metals limits the practical application of 3D printing of LMs. In this work, we proposed a method for preparing a mixed ink based on thermoplastic polyurethane, Cyrene solvent, and LMs, and producing stretchable LMs-based electrodes through 3D printing on a fabric substrate. The stretchable LMs-based electrode prepared by this method shows a wide range of strain response and excellent tensile recovery performance, providing theoretical guidance for the application of LMs electrodes. In addition, the LMs-based heaters showed good Joule thermal response at low voltages, as well as good tensile stability, providing a new strategy for wearable heaters.

Fabrication of superior conductive composite yarn with 3D continuous CNT/WPU structure for high-performance stretchable heater and sensor

Composite yarns based on coating are suitable for broad application fields and massive production due to the flexibility, efficient fabrication, and economic feasibility. However, challenges remain for composite yarns to simultaneously meet the stringent requirements for stretchability, durability, and functionality in practical application. Herein, we report the fabrication of carbon nanotube (CNT)/waterborne polyurethane (WPU) composite yarn (C/WCY) by inducing CNT/WPU composite ink to construct a 3D continuous conductive structure in draw texturing yarn (DTY) through the capillary effect. Consequently, a high loading of 49 wt% CNTs and conductivity of over 2.3 × 103 S/m were achieved, which enabled C/WCY fast thermal response (120 °C within 8 s), long-term durability, superb electrothermal conversion efficiency (hr+c = 1.38 mW/°C), and excellent electrical properties. The stretchable heater and wearable sensors based on C/WCY were further achieved by braiding and weaving technology. This work offered new opportunities for scalable, fast, and low-cost fabrication of wearable electric devices.

Recyclable and leakage-suppressed microcellular liquid–metal composite foams for stretchable electromagnetic shielding

The metallic conductivity and high liquidity of liquid metal (LM) make LM-polymer composites exhibit high electrical conductivity, large stretchability, and excellent mechanical–electrical decoupling, which provide significant advantages in the fabrication of stretchable electromagnetic interference (EMI) shielding materials. However, problems such as undesired LM leakage, high density, and poor recyclability due to the frequent use of chemically cross-linked polymers limit the applications of LM-polymer composites. Herein, we report the fabrication of stretchable and recyclable microcellular thermoplastic polyurethane (TPU)/LM composite foams (TLFs) with controllable LM distribution through a water–vapor-induced phase separation (WVIPS) process. The sedimentation of LM droplets helps to improve the shielding effectiveness (SE) of TLF, while the microcellular structure enables TLF with more uniform LM dispersion to achieve high leakage resistance. Moreover, the stretch-activated TLF shows high electrical stability with only slight resistance changes during stretching and exhibits a strain-insensitive shielding behavior. As a stretchable joule heater, the sample has outstanding Joule-heating performance, with stable temperature dropping only slightly under large tensile strains. More importantly, the excellent solubility of TPU, combined with a solution-based WVIPS process, allows our TLFs to be easily recycled into new products with very similar structure and performance to those before recycling, indicating the ideal recyclability of such materials.

Stretchable and Transparent Heaters Based on Hydrophobic Ionogels with Superior Moisture Insensitivity.

Flexible and stretchable transparent heaters (THs) have been widely used in various applications, including deicing and defogging of flexible screens as well as thermotherapy pads. Ionic THs based on ionogels have emerged as a promising alternative to conventional electronic THs due to their unique advantages in terms of transparency-conductance conflict, uniform heating, and interfacial adhesion. However, the commonly used hydrophilic ionogels inevitably introduce a moisture-sensitive issue. In this work, we present a stretchable and transparent hydrophobic ionogel-based heater that utilizes ionic current-induced Joule heating under high-frequency alternating current. This ionogel-based TH exhibits exceptional multifunctional properties with low hysteresis, a fracture strain of 840%, transmittance of 93%, conductivity of 0.062 S m-1, temperature resistance up to 165 °C, voltage resistance up to 120 V, heating rate of 0.1 °C s-1, steady-state temperature at 115 °C, and uniform heating even when bent or stretched (up to 200%). Furthermore, it maintains its heating performance when it is directly exposed to water. This hydrophobic ionogel-based TH expands the range of materials available for ionic THs and paves the way for their practical applications.

Surface Embedded Metal Nanowire-Liquid Metal-Elastomer Hybrid Composites for Stretchable Electronics.

Both liquid metal (LM) and metallic filler-based conductive composites are promising stretchable conductors. LM alloys exhibit intrinsically high deformability but present challenges for patterning on polymeric substrates due to high surface tension. On the other hand, conductive composites comprising metallic fillers undergo considerable decrease in electrical conductivity under mechanical deformation. To address the challenges, we present silver nanowire (AgNW)-LM-elastomer hybrid composite films, where AgNWs and LM are embedded below the surface of an elastomeric matrix, using two fabrication approaches, sequential and mixed. We investigate and understand the process-structure-property relationship of the AgNW-LM-elastomer hybrid composites fabricated using two approaches. Different weight ratios of AgNWs and LM particles provide tunable electrical conductivity. The hybrid composites show more stable electromechanical performance than the composites with AgNWs alone. In particular, 1:2.4 (AgNW:LMP w/w) sequential hybrid composite shows electromechanical stability similar to that of the LM-elastomer composite, with a resistance increase of 2.04% at 90% strain. The sequential approach is found to form AgIn2 intermetallic compounds which along with Ga-In bonds, imparts large deformability to the sequential hybrid composite as well as mechanical robustness against scratching, cutting, peeling, and wiping. To demonstrate the application of the hybrid composite for stretchable electronics, a laser patterned stretchable heater on textile and a stretchable circuit including a light-emitting diode are fabricated.

Flexible, Transparent and Conductive Metal Mesh Films with Ultra-High FoM for Stretchable Heating and Electromagnetic Interference Shielding.

Despite the growing demand for transparent conductive films in smart and wearable electronics for electromagnetic interference (EMI) shielding, achieving a flexible EMI shielding film, while maintaining a high transmittance remains a significant challenge. Herein, a flexible, transparent, and conductive copper (Cu) metal mesh film for EMI shielding is fabricated by self-forming crackle template method and electroplating technique. The Cu mesh film shows an ultra-low sheet resistance (0.18 Ω □-1), high transmittance (85.8%@550nm), and ultra-high figure of merit (> 13,000). It also has satisfactory stretchability and mechanical stability, with a resistance increases of only 1.3% after 1,000 bending cycles. As a stretchable heater (ε > 30%), the saturation temperature of the film can reach over 110°C within 60s at 1.00V applied voltage. Moreover, the metal mesh film exhibits outstanding average EMI shielding effectiveness of 40.4dB in the X-band at the thickness of 2.5μm. As a demonstration, it is used as a transparent window for shielding the wireless communication electromagnetic waves. Therefore, the flexible and transparent conductive Cu mesh film proposed in this work provides a promising candidate for the next-generation EMI shielding applications.

Stretchable and wearable polymeric heaters and strain sensors fabricated using liquid metals

Stretchable and wearable polymeric heaters and strain sensors fabricated using liquid metals

A novel modeling and simulation for the electro-thermal conversion analysis of graphene-coated knitted fabric

Electrical heating fabrics that possess the excellent characteristics of lightweight, soft, and rapid heating were exploited to address the warmth problem in the cold winter. In this study, a stretchable and effective graphene-decorated weft-knitted fabric heater was developed by spray-drying approach. The graphene-decorated weft-knitted fabric has good stretchability with the strain of 124%, and a high thermal response that reached 206°C with voltage of 10 V. The heating performance of the graphene-decorated weft-knitted fabric remained unchanged at 20 cycles. Furthermore, a three-dimensional geometric model of graphene-decorated weft-knitted fabric was constructed. Mathematical modeling and visual simulation analysis were carried out to explore the electro-thermal coupling behavior. The electro-thermal conversion simulation results are basically consistent with the experimental data. The results offer an approach to design flexible and wearable electrical heating fabrics for thermal management.

AgNWs decorated laser-induced graphene heater for performance enhancement

Film heaters are attracting growing attention as promising wearable electronic skin for a multitude of applications. Elastomer transfers laser-induced graphene (LIG) is a popular approach to preparing stretchable film heaters; however, they fall short of performance. Here, silver nanowires (AgNWs) were employed to decorate polydimethylsiloxane (PDMS) transferring LIG by using CO2 infrared laser direct-writing technology. This heater’s electrothermal performance was substantially enhanced at different voltages, with a 35.28% enhancement when the voltage was 12 V. And response and recovery times were also improved. The results of repeated heating and cooling cycles also showed long-term reproducibility and reliability. The better performance could be attributed to the strengthened connection between the porous holes after long AgNWs coating on the porous structure of LIG, consequently enhancing the film conductivity. PDMS transferring LIG/AgNws have the extensive potential for future applications in artificial intelligence.

Hierarchical and coaxial yarn with combined conductance stability and sensing capability for wearable electronics

Yarn-based sensors are essential units for smart textiles and wearable devices. Most reported fiber sensors, however, are sensitive or stable in conductivity under stretching, limiting their applicability. Herein, we report a strategy for fabricating a specific sensor based on liquid metal/tube/CNT polyester yarn (LTCPY) via friction spinning. The inside of the multilayer composite yarn served as a wire with a broad strain range (0%–300%) and ultrahigh conductivity (3.78 × 105 S/m). The conductivity of the yarn remains at a relatively high level when it is stretched to 250% strain. The external structure endows the sensor with good sensing performance (GF of 6.73 under an elongation of 175%-203%) and recoverability (1000 cycles). The combination of conductance stability and good sensing properties contributes to the application potential of LTCPY as portable conductive wires, stretchable heaters, and wearable sensors to monitor human activities. Moreover, LTCPY can be easily woven into fabrics, introducing a new strategy for the mass production of electronic textiles.

A highly stretchable and conductive continuous composite filament with buckled polypyrrole coating for stretchy electronic textiles

Highly stretchable and conductive fibers as crucial components for stretchy electronic textiles have developed rapidly in recent years, benefiting from the design of stretchable structures. However, routes toward continuous production via simpler fabrication processes remain a huge challenge that has greatly limited its widespread application. In this work, we reported an intrinsically stretchable multifilament with both highly stretchable and conductive (537 S m−1 under 100 % strain) polypyrrole (PPy) coating enhanced by sodium sulfosalicylate (NaSSA) as a dopant. Based on this, a post-drawing method was further proposed to continuously fabricate stretchable conductors with PPy buckling on surface. The PU multifilament with buckle PPy coating (PU@bPPy) exhibited a high and ultra-stable conductance, showing initial conductivity at 529 S m−1 and only 3.5 % change in resistance after > 6500 stretching cycles at 50 % strain. Moreover, the PU@bPPy could be easily integrated into electronic textiles as stretchable electronic circuits and stretchable joule heaters.

Characterization of the Variable Stiffness Actuator Fabricated of SMA/SMP and MWCNT/IL: PDMS Strain-Sensitive Heater Electrode

A microactuator that exhibited the variable stiffness function and the strain control was demonstrated by using the shape memory alloy (SMA), shape memory polymer (SMP), and the strain-sensitive stretchable heater polymer of multi-walled carbon nanotube/ionic liquid (IL): polydimethylsiloxane. The SMA filaments exerted the principal driving force and the SMP film modulated the elasticity of the actuator. To make the microfabrication of the actuator and the control feasible, we first demonstrated the use of a stretchable heater electrode, which accommodates 100% of the strain and low resistivity of 3.75 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m\Omega \cdot cm$</tex-math></inline-formula> for the temperature control. The elasticity change of the SMP film was successfully realized by electrical heating, and the response was characterized. To obtain the strain without any additional implementation, we performed the self-sensing actuation. By developing the specified driver circuit, strain measurement was achieved while modulating the elasticity. Finally, we demonstrated the weight lifting of a microarm, a typical example to control the variable stiffness actuation.

Transient Heat Conduction in the Orthotropic Model with Rectangular Heat Source.

Epidermal electronic systems (EESs) are a representative achievement for utilizing the full advantages of ultra-thin, stretchable and conformal attachment of flexible electronics, and are extremely suitable for integration with human physiological systems, especially in medical hyperthermia. The stretchable heater with stable electrical characteristics and a uniform temperature field is an irreplaceable core component. The inorganic stretchable heater has the advantage of maintaining stable electrical characteristics under tensile deformation. However, the space between the patterned electrodes that provides tensile properties causes uneven distribution of the temperature field. Aiming at improving the temperature distribution uniformity of stretchable thermotherapy electrodes, an orthotropic heat transfer substrate for stretchable heaters is proposed in this paper. An analytical model for transient heat conduction of stretchable rectangular heaters based on orthotropic transfer characteristics is established, which is validated by finite element analysis (FEA). The homogenization effect of orthotropic heat transfer characteristics on temperature distribution and its evolutionary relationship with time are investigated based on this model. This study will provide beneficial help for the temperature distribution homogenization design of stretchable heaters and the exploration of its transient heat transfer mechanism.

Super-stretchable polymer-AgPdCu superlattice electrodes for high-performance wearable electronics

Highly-stretchable and semi-transparent polytetrafluoroethylene (PTFE)/AgPdCu (APC)-PTFE-based superlattice electrodes were realized by alternately sputtering very thin APC-PTFE hybrid layers on a polyurethane (PU) substrate at room temperature. The electrical and mechanical properties of the superlattice electrodes were optimized by controlling the PTFE thickness and the number of layers of the stacked superlattice. The optimal PTFE/APC-PTFE/PTFE/APC-PTFE/PTFE superlattice electrode showed a sheet resistance of 8.5 Ohm/square and maintained a high conductivity, even at a strain of 100%. The outstanding mechanical stretchability of the superlattice electrode is attributed to the 3-dimensional conductive APC network structure in the PTFE polymer matrix . In addition, the external strain applied to the superlattice electrode led to a change in the optical transmittance that could be used for optical switching in stretchable electronics. The superlattice electrodes can be utilized to fabricate stretchable electroluminescent devices, stretchable thin film heaters, and wearable temperature sensors . This successful demonstration of stretchable devices that include highly stretchable superlattice electrodes demonstrated the feasibility of this manufacturing strategy for use in next-generation stretchable and wearable electronics . • Fabrication of highly conductive and stretchable metal-polymer 3D nanocomposites by continuous co-sputtering of polymer and metal target. • The number of stacked layers and thickness of the polymer and polymer-metal hybrid films were controlled to form an alternating superlattice structure. • The optimized electrode showed a sheet resistance of 8.5 Ohm/square, an average transmittance of 34.0% for visible light, and stretchability that reversibly maintained a high conductivity at up to 100% strain. • The optimized electrode was applied as stretchable thin-film heater, electroluminescent device, and wearable temperature sensor.

Multistable shape programming of variable-stiffness electromagnetic devices.

Programmable shape morphing enables soft machines to safely and effectively interact with the environment. Stimuli-responsive materials can transform 2D sheets into 3D geometries. However, most solutions cannot hold their shape at zero power, are limited to predetermined configurations, or lack sufficient mechanical stiffness to manipulate common objects. We demonstrate here segmented soft electromagnetic actuators integrated with shape memory polymer (SMP) films, capable of deforming and latching into a broad range of configurations. The device consists of liquid metal (LM) coils in an elastomer shell, laminated between two SMP films. The coils are linked by narrow joints, on which stretchable heaters are patterned. Heating the SMP greatly reduces its stiffness. Driving current through an LM coil in the presence of a magnetic field then leads to large bending or twisting. Cooling the SMP locks in the shape, leading to load-bearing capacity. Complex shapes are obtained from an initially flat device.

Thermomechanical analysis of the stretchable serpentine heaters considering finite deformation

Stretchable heater, as a newly developed heating device, can perfectly fit with undevelopable curved surfaces via mechanical deformation. Inspired by serpentine interconnects, we developed a new design of stretchable serpentine heater (SSH), where the serpentine heat source embedded into the softer and highly stretchable materials (such as Ecoflex and Silbione) provides an effective approach to achieving both high stretchability and thermal uniformity. It can provide uniform heating for objects with curvilinear contours, such as uniform thermal manikins, defogging curved windows and thermotherapy. However, under large mechanical deformation, the thermal field of SSH will drastically change on account of the morphology change of embedded serpentine heating wire. Therefore, the present study presents an accurate theoretical model to predict the thermomechanical behaviors of SSH considering finite deformation. After a validation via finite element analysis (FEA) and experiments to verify the present theory, the effects of geometric parameters of SSH on the mechanical and thermal behaviors are investigated to identify the most effective way to achieve both better stretchability and thermal uniformity in the process of stretching. This study can help guide the design and fabrication of SSH based on Ecoflex substrate to achieve the goal of uniform heating on complex curved surfaces in future applications.

A Biaxially Stretchable and Self-Sensing Textile Heater Using Silver Nanowire Composite.

Wearable heaters have garnered significant attention from academia and industry for their great potential in thermotherapy. Silver nanowire (AgNW) is a promising conductive material for flexible and stretchable electrodes. Here, a resistive, biaxially stretchable heater based on AgNW composite is reported for the first time, where a AgNW percolation network is encased in a thin polyimide (PI) film and integrated with a highly stretchable textile. AgNW/PI is patterned with a 2D Kirigami structure, which enables constant resistance under a large tensile strain (up to uniaxial 100% strain and 50% biaxial strain). The heater can achieve a high temperature of ∼140 °C with a low current of 0.125 A, fast heating and cooling rates of ∼16.5 and ∼14.1 °C s-1, respectively, and stable performance over 400 heating cycles. A feedback control system is developed to provide constant heating temperature under a temperature change of the surrounding environment. Demonstrated applications in applying thermotherapy at the curvilinear surface of the knee using the stretchable heater illustrate its promising potential for wearable applications.

Transparent Thermotherapeutic Skin Patch Based on Highly Conductive and Stretchable Copper Mesh Heater

AbstractIn the present work, a stretchable transparent heater (STH) is developed consisting of a free‐standing serpentine copper (Cu) mesh attached to a semipermeable polyurethane (PU) film. The as‐prepared Cu mesh STH is of extremely low sheet resistance of 0.15 Ω sq−1 at an optical transmittance of 82.5%. The electrothermal and mechanical properties of the fabricated STHs are systematically studied by both experimental measurements and theoretical simulations. The Cu mesh STH proves to be highly effective. For a size of 1 cm × 2 cm skin patch, it can generate temperature up to 120 °C in less than 30 s at only 0.7 V supplied voltage. The average temperature drop of the Cu mesh STH is only about 10% under 50% strain at a constant voltage. In addition to the efficient heating, the Cu mesh STH demonstrates high stability during long‐term repetitive heating and cooling cycles. Moreover, the STH is air permeable and highly conformable to skin. Its high transparency allows constant monitoring of skin condition during a healing process. As a proof‐of‐concept, a skin patch consisted of the Cu mesh STH is successfully applied to cure the epidermal tumor of lab mice, demonstrating its great potential for skin‐patched thermotherapy.

Design and Construction of Deformable Heaters: Materials, Structure, and Applications

AbstractConventional heaters have evolved from traditional rigid heaters to flexible heaters, stretchable heaters, and deformable heaters for the development of soft electronics. Film‐based or filament‐based heaters can be designed and engineered to fulfill the deformable, adaptable and stretchable applications. Compared with conventional heaters, deformable heaters can maintain stable electrothermal property even with large deformation. Based on this, deformable heaters have shown great potential to be used in the field of Internet of Things and work together with other electronic devices. This review summarizes the structure design strategies to achieve deformable heaters. Parameters, such as electrical conductivity, stretchability, thermal capacity, and thermal diffusivity, play essential roles in the effective design and development of heating devices. New methodologies and fabrication processes are developed to obtain deformable heaters that are expected to be highly conductive, adaptable, and conformable, as well as can be readily integrated with wearable fabrics. This review introduces the latest advances from conventional heaters to deformable heaters, emphasizing key recent developments on methodologies and fabrication processes and strategies geared toward applications in electronic skin, healthcare, deicing, sensors, and actuators. The challenges in this area are critically evaluated, and the perspectives for developing future deformable heaters are presented.