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

The study addresses the challenge of creating wearable electric heaters that are both highly stretchable and optically transparent, suitable for skin-mountable applications such as thermotherapy, thermo-haptics, actuators, camouflage, lab-on-a-chip, and AR/VR interfaces. Conventional approaches achieve stretchability and transparency by: (1) coating soft substrates with conductive nanomaterials (e.g., Ag nanowires/nanofibers), which suffer from connectivity loss and resistance increase at high strains (>100%), and often degrade optical transmittance due to dense loading; and (2) geometric designs (serpentine meshes and kirigami), which face limited stability above ~50% strain (serpentine) or suffer from non-uniform temperature due to large voids (kirigami), requiring extra heat-spreading layers. For localized and scalable thermal outputs used in haptics and information displays, two strategies dominate: pixelated Joule heaters with direct or row/column addressing (burdened by wire resistance, stretchability, and integration issues), or engineered anisotropic thermal pathways (thermal metamaterials or multilayer composites) that demand complex designs and fabrication, compromising wearability. Ionogel-based heaters offer transparency and toughness and operate under AC fields but are typically planar single-cell devices with non-uniform fields and challenges in scaling to arrays due to electrochemical compatibility. To overcome these limitations, the paper proposes a soft dielectric heater (SDH) that uses PVC-gel as a dielectric heating layer and hydrogel as compliant, transparent electrodes. Heating arises from AC-driven motion and collisions of polarized plasticizers and flexible PVC chains (dielectric loss), enabling uniform, localized, and scalable heating in a capacitor-like 3D configuration. The device targets high stretchability (up to 400%), high transmittance (>86% visible), and scalable, addressable arrays for diverse thermal outputs.

- Conductive nanomaterial heaters (Ag nanowires/fibers) on elastomers provide transparency and stretchability but exhibit percolation breakage, rising resistance/voltage under high strain, and optical haze when densified to maintain conductivity. Serpentine meshes fail to maintain stable performance beyond ~50% strain; kirigami reaches ~400% strain but sacrifices uniform heat due to large apertures and requires heat-spreading layers. - Pixelated Joule heaters (direct-wired or matrix-addressed) can render diverse patterns but need low-resistance, stretchable wiring to avoid parasitic heating and maintain wearability; wiring interfaces add mechanical/thermal complexity. - Thermal metamaterials and multilayer anisotropic structures enable high-resolution heat routing/patterning but demand intricate patterns, multilayer wiring, and rigid architectures incompatible with skin-wearable, highly compliant devices. - Ionogel heaters (transparent, tough, AC-driven) show promise but are typically 2D planar cells with non-uniform fields, limited localization, and scaling issues due to ion diffusion/electrochemistry between layers. - PVC-gel literature indicates plasticizer-mediated charge transport and field-driven deformation (solvent-rich layer near anode under DC), high dielectric dissipation under AC, and prior uses in actuators, lenses, sensors, and generators, motivating its new application as a dielectric heater.

Materials and preparation: - PVC-gel: PVC powder (Mw ≈ 233,000; Mn ≈ 99,000) plasticized with dibutyl adipate (DBA) in THF at weight ratio PVC:DBA = 1:6. Stirred 5 h, cast on Petri dish, solvent evaporated 4 days (RT) to yield stretchable, transparent gel (typical thicknesses ~350–800 µm). - Hydrogel electrodes: Pre-gel in DI water containing 19.8 wt% acrylamide (AAm), 0.021 wt% MBAA (crosslinker), 23.8 wt% LiCl (electrolyte), 0.056 wt% APS (initiator); stored at −20 °C to prevent premature gelation. TEMED (15 µL per 10 mL) added before casting to accelerate polymerization. Device fabrication and interfacial bonding: - PVC-gel sheet (~800 µm) on hydrophobic glass. PET mask (250 µm) laser-cut and applied to define hydrogel patterns. Exposed PVC-gel areas treated with 5 wt% benzophenone in ethanol via spray (100 kPa, ≲5 cm standoff). After ethanol evaporation, excess benzophenone was rinsed off (×3 with ethanol). - Hydrogel pre-gel poured on treated PVC-gel, covered with hydrophobic glass to block oxygen, UV-irradiated (365 nm) for 1 h to form covalently grafted PAAm hydrogel with robust interface. Process repeated on back side to form hydrogel/PVC-gel/hydrogel (capacitor-like) stack. - Wiring: 100 nm gold electrodes sputtered on PET and attached as leads to avoid electrochemical reactions with hydrogel. Array patterning and addressing: - Hydrogel electrodes patterned orthogonally (rows/columns) on PVC-gel using masks to form 5×5 array; cell size 10 mm with 20 mm pitch. - Row electrodes connected to HV amplifier output; column electrodes to ground via relay matrix (HE05-1A83-03) controlled by Arduino Mega 2560. AC 700 V, 100 Hz sinusoid applied; relays time-multiplexed each half-cycle (50 ms) to activate target pixels and form patterns (e.g., letters). Characterization: - Optical transmittance: UV-Vis-NIR spectrophotometer (200–1000 nm, 1 nm step). - Electrical/thermal: HV amplifier (Trek 623B); current via NI USB-6212; temperature by IR camera (FLIR E76). - Thermal properties: Specific heat by DSC (−20 to 160 °C, 10 °C/min); thermal conductivity by TPS (transient plane source) at ambient; thermal stability by TGA up to 600 °C in N2 (10 °C/min). - Mechanics: Tensile/stretch tests on motorized stage; interfacial toughness by 180° peel (Zwick/Roell, 20 mm/min) using PETG (150 µm) backing for PVC-gel and PET (50 µm) for hydrogel; abrasion by UMT TriboLab (carbide tip, 8 mN normal load, 5 mm stroke at 1 mm/s). - Dehydration and stability: Hydrogels with LiCl concentrations 2.5, 5, 10 mol/L tested at 50 °C, 20% RH for 30 h for weight and resistance changes; long-term cyclic heating (on 16 s/off 80 s) for 7 h. - Plasticizer migration tests: PVC-gel (1:6) placed on PDMS, Ecoflex 00-50, VHB 4910, and hydrogel (30×30×1 mm substrates); mass change for 100 h at 27 °C/55% RH; FT-IR (4000–500 cm⁻¹) to detect DBA migration. Modeling and analysis: - Impedance spectroscopy (1 Hz–100 kHz, 5 V) to identify zero-phase-angle frequency (~20–100 Hz) and equivalent circuit (parallel Rp–C with small Rs). Derived dielectric heating power P = ε0 ε″(T,f) ω V² (scaled by device geometry A/d) and separated ε″ into temperature- and frequency-dependent parts to explain frequency-independent heating above threshold. - Thermal model from energy balance: current I = εr ε0 (2πf) A/d · V; temperature rise ΔT(t) = (εr ε0 V² α / 2h) · (1 − e^(−ηt)) / d (with ε″ linear in T with slope β up to 60 °C). Time constant τ ≈ (Cρ d)/(η − επ α β)^(1/2) capturing thickness dependence. Wearable demonstrations: - Thermo-haptics on fingers with localized heating to ~40 °C under 1 kV, 100 Hz; skin-mounting without tape; elbow thermotherapy during flexion/extension with temperature tracking (minimal fluctuation).

- Mechanism and dielectric properties: • Under AC, continuous charge injection/discharge in PVC-gel drives polarized plasticizers and flexible PVC chains to vibrate/collide, generating volumetric dielectric heating localized to electrode-overlap regions. • Dissipation factor tanδ > 100 across 20–100 Hz, indicating heat dissipation >100× stored energy. • Dielectric loss ε″ ≈ 2000 (20 Hz–300 kHz), with optimal PVC:DBA = 1:6 exhibiting the highest tanδ and ε″. • ε″ increases linearly with temperature from ~1800 at ambient up to 60 °C (slope ~30/°C). - Frequency threshold and modeling: • Nyquist plots show zero-phase-angle near 20–100 Hz; effective heating occurs above this threshold (semi-circle region). • Derived P ∝ ε0 ε″(T,f) ω V² and, with ε″(f) ∝ 1/f, heating becomes frequency-independent above threshold. • Current scales linearly with voltage: I = εr ε0 (2πf) (A/d) V; model fits measured I–V data. • Temperature rise follows ΔT(t) = (εr ε0 V² α / 2h) · (1 − e^(−ηt)) / d; non-quadratic in V due to ε″(T) increase; data align with model. - Heating performance: • At 100 Hz, temperatures controlled by voltage up to ~75 °C at 1 kV AC. • Thickness effect: 350 µm samples heat faster than 800 µm; measured time constants τ350µm ≈ 30.1 s, τ800µm ≈ 69.3 s (model estimates 34.1 s and 73.6 s). • Spatial uniformity: >85% of heated area reaches ≥0.9·Tmax; uniform field due to capacitor-like geometry and low thermal conductivity (<0.2 W/m·K). - Mechanical stretchability and thermal response: • Operable up to 400% strain without delamination (robust benzophenone-grafted hydrogel/PVC-gel interfaces; peel strength ~5× higher than untreated, cohesive failure). • During stepwise stretching at 600 V, 100 Hz: average temperature increases from 46.6 °C to 49.6 °C at first elongation, reaches ~59 °C at 100% strain, ~61 °C at 200%, and changes only ~0.4 °C further to 300%; current increases with area expansion/thinning (e.g., ~3.8→6.86 mA at 100%, up to ~12.88 mA at 300%). - Optical transparency and electrodes: • Transmittance of hydrogel/PVC-gel/hydrogel (H-P-H) exceeds 86% across visible wavelengths and remains high under stretch. • Compared electrodes on PVC-gel: Hydrogel and AgNW yield similar heating (~62 °C vs ~60.5 °C at 600 V, 100 Hz, 600 s); CNT underperforms (~53 °C) due to higher electrode resistance (voltage drop along CNT reduces voltage across PVC-gel). • Double-sided AgNW/CNT coatings reduce transmittance (<70% and ~10%, respectively), whereas hydrogel maintains >86%. - Stability and materials interactions: • Hydrogel (10 mol/L LiCl) mitigates dehydration: after 30 h at 50 °C/20% RH, retains ~70% weight; resistance increases only ~4–5× (remaining ≲2 kΩ, negligible vs PVC-gel). Lower-salt hydrogels lose more weight, show drastic resistance increases (e.g., 2.5 mol/L ~20,000×) and stiffen. • PVC-gel plasticizer migration is blocked by hydrogel; significant migration into PDMS/Ecoflex/VHB (weight drops; FT-IR shows DBA signatures), but not into hydrogel (no DBA peaks). • Thermal stability: For PVC-gel (1:6), T5% and onset temperatures >200 °C (TGA); hydrogel onset ~90 °C; suitable for wearable temperature ranges. • Cycling: Stable heating for ≥150 cycles (29–40 °C, ~5600 s total) and long-term repeated operation to 50 °C over 7 h (268 cycles). - Pixelated array and thermal messaging: • 5×5 array with orthogonal hydrogel electrodes provides addressable localized heating; diverse patterns (single cell, rows, 15 cells, all cells) with <5.5 °C cell-to-cell variation over 10 min at 700 V, 100 Hz. • Current scales ~linearly with number of activated rows (e.g., ~1.77 mA for 1 row to ~8.85 mA for 5 rows). • Clear patterns (“H”, “E”, “A”, “T”) with on–off pixel temperature differences >15 °C due to low thermal diffusion and localization. • Array maintains function, localization, and transparency under biaxial stretch to area strain ε ≈ 300%, with faster heating due to thickness reduction (enhanced field).

The SDH leverages dielectric heating in a soft, capacitor-like stack to overcome limitations of conventional Joule heaters. By using PVC-gel with high dielectric loss under AC excitation, heat is generated volumetrically and localized within electrode-overlap regions, producing uniform temperature fields even under large strains. The hydrogel electrodes provide low resistance and high transparency, ensuring voltage division that concentrates most of the applied voltage across the PVC-gel layer, thereby localizing heat while preserving optical clarity. The robust benzophenone-induced covalent grafting ensures reliable interfaces that resist delamination during extreme deformations, a common failure mode in stretchable heaters. The impedance-guided operating window (above zero-phase-angle frequency) and the derived electro-thermal model accurately predict current–voltage behavior and transient/saturation temperatures, enabling real-time temperature inference from electrical signals. Scaling from a single cell to a row/column-addressable 5×5 array demonstrates uniform, addressable, and stretch-tolerant thermal patterning with low inter-pixel thermal crosstalk, enabling thermal information transfer and multi-modal displays. Collectively, the results validate the hypothesis that a dielectric-heating-based, transparent, and soft stack can deliver uniform, localized, scalable heating suitable for wearable and skin-mountable applications.

This work introduces a soft dielectric heater (SDH) that is simultaneously highly stretchable (up to 400% strain), optically transparent (>86% transmittance across visible), and thermally functional (uniform, localized heating with minimal crosstalk). The PVC-gel dielectric layer, energized under AC, exhibits high dissipation factor and dielectric loss (optimal PVC:DBA = 1:6), enabling efficient, uniform heating localized to electrode overlaps. A robust hydrogel/PVC-gel interface ensures mechanical integrity under extreme deformation, while low-resistance hydrogel electrodes support effective voltage division and transparency. The technology scales to a 5×5 passive-matrix array, delivering addressable thermal patterns with small inter-cell temperature variation and clear on–off contrast (>15 °C), and maintains performance under large biaxial strains (ε ≈ 300%). Applications demonstrated include thermo-haptics, wearable thermotherapy, and thermal messaging. Future research should focus on reducing operating voltage and improving heating rate via geometric optimization (thinner PVC-gel to increase field and reduce heat capacity), materials engineering to enhance dielectric loss at lower fields, and integration with thermally active materials (shape-memory polymers, liquid-crystal elastomers) for artificial muscles and rehabilitation robots, as well as multi-modal AR/VR interfaces.

- High operating voltage (hundreds of volts to ~1 kV AC) due to the intrinsic impedance of the dielectric heater; although wearable temperature ranges are achieved, lower-voltage operation is desirable. - Heating rate is limited by thermal capacity; thicker gels respond more slowly (τ scales with thickness). - Hydrogel dehydration at low salt contents degrades electrode performance; although 10 mol/L LiCl mitigates this, long-term hydration management and environmental sealing may be needed. - Hydrogel thermal onset (~90 °C) constrains maximum sustained operating temperature despite PVC-gel stability >200 °C. - System requires AC drive above a frequency threshold (~20–100 Hz) and HV amplification; portable, low-power drivers need development for practical wearables.