The development of electric heaters with enhanced mechanical, optical, and thermal functionalities has driven significant research. Wearable and skin-mountable heaters require high stretchability and transparency for conformal contact with the skin and seamless integration. Current approaches, such as coating with conductive nanomaterials or utilizing stretchable geometries like serpentine meshes or kirigami patterns, face challenges. Nanomaterial coatings can crack under strain, increasing resistance, while serpentine meshes are unstable above 50% strain. Kirigami patterns, though highly stretchable, struggle with uniform temperature distribution. Furthermore, localized heat generation and scalable heating elements are crucial for applications like haptic displays, surface rendering, and thermal messaging. Existing methods using pixelated elements or anisotropic thermal conductivity materials face challenges in achieving both thermal patterning and skin-adaptability. Ionogel-based heaters offer transparency and toughness, but their 2D planar design hinders localized heating and scalability. This research proposes a novel soft dielectric heater (SDH) to overcome these limitations.

Existing stretchable and transparent heaters primarily rely on two methods: conductive nanomaterial coatings and stretchable geometries. Conductive nanomaterials like silver nanowires and nanofibers, while offering electrical conductivity, suffer from connection breakage under high strain (>100%), leading to increased resistance. To mitigate this, dense nanoparticle coatings can be employed but often compromise optical transparency. Stretchable geometries, such as serpentine meshes and kirigami patterns, are used to accommodate strain, but serpentine meshes struggle to maintain performance above 50% strain. Kirigami, while highly stretchable, needs additional heat-spreading layers for uniform temperature distribution. For diverse thermal outputs, pixelated heaters and materials with anisotropic thermal conductivity have been explored, but these approaches often compromise flexibility or involve complex fabrication processes. Recent ionogel-based heaters offer improved transparency and toughness, but their 2D planar structure restricts localized heating and scalability. This paper's novel SDH addresses these existing limitations.

The SDH consists of three layers: a PVC-gel dielectric layer sandwiched between two hydrogel-based stretchable electrodes. To ensure robust bonding even under high strain, the PVC-gel surface was treated with benzophenone before hydrogel application, enabling covalent crosslinking. Hydrogels were chosen as electrodes for their high stretchability, transparency, and ionic conductivity, which is higher than that of PVC-gel, ensuring efficient voltage division. The fabrication involved dissolving PVC powder and DBA plasticizer in THF, followed by solvent evaporation to form the PVC-gel. The hydrogel was prepared by dissolving acrylamide, N,N'-methylenebisacrylamide, lithium chloride, ammonium persulfate, and TEMED in DI water. The hydrogel precursor was then applied to the benzophenone-treated PVC-gel and cured under UV light. A similar process was repeated on the other side of the PVC-gel. Gold electrodes were deposited on PET films to avoid electrochemical reactions. The characterization involved measuring optical transmittance, electrical impedance, temperature profiles (using an IR camera), and current flow. The mechanical properties were evaluated via tensile tests, and thermal stability was assessed using thermogravimetric analysis. A 5x5 pixelated array was fabricated using a masking technique during hydrogel polymerization to achieve localized heating and thermal patterning. A row/column addressing scheme was implemented using relay switches to control individual cells.

The SDH demonstrated exceptional properties. It exhibited a dissipation factor exceeding 100 and dielectric loss close to 2000 at 20-100 Hz, indicating efficient heat generation via dielectric heating. The heating performance was found to be frequency-independent above a threshold of 20-100 Hz. A thermal model, validated by experimental data, accurately predicted the temperature change as a function of applied voltage. The heating rate was found to be inversely proportional to thickness. The SDH maintained uniform temperature distribution even under 400% strain, with temperature fluctuations below 2 °C at 200% strain and 0.4 °C at 300% strain. Transmittance exceeded 86% across the visible spectrum. The 5x5 pixelated array successfully generated diverse thermal patterns, showcasing the potential for thermo-haptic and thermal messaging applications. The SDH exhibited long-term thermal stability through 150 heating cycles (29-40°C). The hydrogel electrode demonstrated superior performance and transparency compared to alternatives like AgNWs and CNTs. The SDH showed excellent performance in wearable applications (thermo-haptics and thermotherapy).

The SDH successfully addresses the limitations of existing stretchable and transparent heaters by combining high transparency, high stretchability, and uniform heating. The use of PVC-gel for dielectric heating and hydrogels for stretchable electrodes provides a unique approach, overcoming the challenges associated with nanomaterial-based coatings and complex geometries. The localized heating, enabled by the voltage division principle and the low thermal conductivity of PVC-gel, allows for precise thermal control and patterning. The scalability demonstrated by the 5x5 array opens up numerous applications, including advanced thermo-haptics, thermal messaging, and thermotherapy. The results suggest the SDH’s potential for creating immersive thermal experiences in virtual and augmented reality, and its superior optical properties make it suitable for applications combining thermal and visual displays. The high operating voltage is a limitation, but reducing PVC-gel thickness could significantly improve the heating rate and reduce the required voltage.

This research presents a novel SDH with exceptional mechanical, optical, and thermal properties, overcoming limitations of existing technologies. Its high transparency and stretchability, coupled with localized heating and scalable array design, open new possibilities for wearable and skin-mountable thermal devices. Future work should focus on reducing the operating voltage and further enhancing the heating rate through geometric optimizations. Integration with shape-memory polymers and liquid-crystal elastomers could expand applications to robotics and artificial muscles. The combination of thermal and visual information transfer offers significant potential for advanced human-computer interfaces.

The high operating voltage required by the SDH is a limitation. While reducing the thickness of the PVC-gel layer is suggested to mitigate this, further research is needed to optimize the design for lower voltage operation. The current study focuses on a specific range of temperatures; further investigation is needed to determine its performance across a broader temperature range. Long-term stability under extreme conditions, such as exposure to high humidity and temperature fluctuations, should be further explored.