High-performance compliant thermoelectric generators with magnetically self-assembled soft heat conductors for self-powered wearable electronics

The study addresses the challenge of achieving high-efficiency, self-powered wearable thermoelectric generators (TEGs) that can harvest heat from arbitrary 3D surfaces. While TEGs can convert waste heat to electricity, practical wearable implementations suffer from low temperature drop across the TE legs (ΔT_TE) due to parasitic heat loss in polymer substrates with high thermal impedance and poor thermal contact caused by rigid interconnects. Given that output power scales with energy conversion efficiency and the temperature difference across the TE legs, maximizing ΔT_TE for a given heat input and material zT is critical. Prior flexible TEGs based on Bi2Te3 have been limited by thick/rigid electrodes, polymer infiltration leading to heat losses, undesirable air gaps due to limited conformability, and low-yield manual fabrication. The research question is how to realize compliant TEGs that simultaneously provide superior heat transfer into TE legs (low thermal impedance, minimal parasitic loss) and excellent conformability (to eliminate air gaps) through scalable, automated manufacturing.

Bi2Te3-based materials are leading candidates for room-temperature thermoelectrics. Traditional rigid TEGs harvest heat from planar sources only. Efforts to develop flexible/wearable TEGs include PDMS infiltration for mechanical tolerance and the use of liquid metals (eutectic Ga-In) as soft interconnects. However, these approaches often suffer from high thermal impedance substrates, air gaps due to limited conformability, low device yields, and encapsulation needs for liquid metals that further impede heat transfer. Rigid Cu interconnects improve conductivity but compromise conformability and induce stress concentrations. Manual placement of bulk legs limits fill factor and scalability, reducing power density. Recent advances in printed and nanostructured TE films provide flexibility but can be strain-sensitive, affecting Seebeck performance. A gap remains for a compliant, scalable architecture that matches thermal impedance to TE legs, improves heat transfer through the elastomer, and maintains intrinsic stretchability for conformal 3D contact.

Architecture and materials: The authors developed a soft heat transfer and electrical interconnection platform (SHEP) embedding (i) intrinsically stretchable Ag nanowire (AgNW) electrodes in PDMS as interconnects and (ii) soft heat conductors (s-HCs) made by magnetically self-assembling Ag-coated Ni (Ag-Ni) particles in PDMS. Bulk Bi2Te3 p/n legs are integrated between top and bottom SHEPs. SHEP fabrication via simultaneous embedding/patterning/curing: A Ag-Ni/PDMS precursor (varied concentrations) is cast on FOTS-treated glass with spacers. A AgNW-deposited PEN film is placed atop the mixture. The stack is sandwiched between two aligned iron pillar arrays; magnets above and below focus vertical magnetic flux along pillar pairs, rapidly (≤~180 s) aligning Ag-Ni particles into vertical chains (percolation paths) in defined 1.2×1.2 mm² regions. After resting (~10 min), the composite is cured at 100 °C for 1 h under magnetic field to lock in the aligned structure, then the PEN is detached to yield the SHEP on glass. Pattern design is set by pillar arrays; particle loading and thermal properties are tuned by concentration and process parameters (magnetic flux, PDMS viscosity) optimized within a stable window. Automated TE leg integration: Conductive Ag epoxy is dispensed (programmable dispenser, 32G needle, 400 kPa, 2 s) at s-HC/AgNW pad locations to reduce contact resistance. Bi2Te3 legs are placed by a pick-and-place machine. Bottom joints are annealed at 170 °C for 1 h; the top SHEP is attached and annealed similarly. PDMS is infiltrated between SHEPs for mechanical robustness and cured (100 °C, 1 h). Supporting glasses are detached, yielding compliant TEGs. Total process time ~4.5 h (SHEP ~2 h, integration ~2.5 h). The process scales to large-area arrays (e.g., 440 legs in 3.9×4.3 cm²) with high yield and customizability. Thermal/mechanical characterization of s-HCs: Bulk (unpatterned) Ag-Ni/PDMS composites were measured for through-plane and in-plane thermal conductivities as a function of Ag-Ni wt%, with/without magnetic alignment. SEM and EDS confirmed vertical chain formation upon alignment. Mechanical stress–strain tests assessed Young’s modulus and fracture strain versus concentration and compared to commercial thermal pads. Device simulations and measurements: 3D finite element analysis (COMSOL) coupled heat transfer in solids, electric currents, and solid mechanics modules to evaluate temperature distribution, VOC, mechanical stress/strain under bending/stretching, and performance on curved substrates. Material parameters were taken from measurements or datasheets; s-HC K_thru-plane for 85 wt% was extrapolated. TE performance of 36-np-pair and 220/440-leg modules was measured using Peltier-based hot/cold plates with thermal pads to prevent electrical shorts; VOC, I–V, and power curves were recorded after thermal equilibrium. Dynamic response to abrupt temperature change was captured by time-resolved VOC while contacting with a hot aluminum cup. Mechanical reliability was tested via bending (up to 10,000 cycles) and stretching (up to 20% strain; cyclic at 10%), tracking resistance and TE performance. Humidity/temperature tolerance was evaluated up to 384 h at controlled RH/temperatures. Flexible circuit and system demo: A flexible PCB with a step-up converter (LTC3105) and five red LEDs was designed to operate solely from the TEG. Component values (capacitors, resistors, inductor) were optimized for ~1.6 V LED turn-on and low start-up voltage (~250 mV). The system was integrated into oven gloves with a light diffuser to display a bright “H” as a hot-surface warning when grasping hot objects.

- Magnetically self-assembled s-HCs substantially increased through-plane thermal conductivity of Ag-Ni/PDMS composites: K_thru-plane rose from ~0.15 to 0.53 W m⁻¹ K⁻¹ as loading increased to 70 wt%, and to ~1.1 W m⁻¹ K⁻¹ with magnetic alignment. For patterned s-HCs (~85 wt% estimated in patterned regions), K_thru-plane was extrapolated to ~1.4 W m⁻¹ K⁻¹, approaching Bi2Te3 leg values (~1.9 W m⁻¹ K⁻¹), enabling thermal impedance matching. s-HCs remained soft (Young’s modulus <10 MPa; fracture strain >90%). - For 36-np-pair TEGs at ΔT_Applied = 10 K, FEA predicted ΔT_TE rising from 5.1 K (no s-HC) to 8.6 K (with s-HC), with VOC increasing from 61.7 to 96.5 mV. Experiments matched: VOC 61.4 vs 89.5 mV (with s-HC +45%). At ΔT_Applied = 40 K, maximum power increased from 232 µW (no s-HC) to 828 µW (with s-HC), a ~260% improvement. The s-HC device also responded faster to step temperature changes and reached higher peak VOC. - Large-area compliant TEG (440 legs; area 3.9×4.3 cm²) delivered maximum power 7.02 mW and VOC 2.12 V at ΔT_Applied = 40 K. The 220-np-pair device achieved the highest normalized Seebeck voltage per unit area and, within stretchable TEGs, the highest normalized power density (0.26 µW cm⁻² K⁻²). On human skin, it produced record performance among wearable TEGs: maximum power density 6.96 µW cm⁻² and VOC 266 mV. - Mechanical reliability: The AgNW stretchable electrodes reduced stress concentration versus Cu plates, enabling bending and up to 20% tensile strain with maintained functionality. Electrical resistance remained stable over 1000 bending cycles (r ≈ 15 mm), and TE performance (VOC and power at ΔT = 10 K) was stable after 10,000 bending cycles in both axes. The bulk Bi2Te3 legs remained effectively strain-free due to strain absorption by soft interconnects, showing negligible VOC change under applied strain/bending. - Conformability on 3D surfaces: FEA on curved heat sources showed that conformal contact (AgNW/s-HC TEG) eliminated air gaps and increased VOC to ~243 mV, ~600% higher than a rigid Cu-electrode TEG. Experiments on a bell-shaped aluminum cup (78 °C water) yielded up to ~340 mV VOC regardless of position, indicating robust heat collection from anisotropic curvatures. - System demonstration: A 220-np-pair TEG powered a flexible step-up converter and five LEDs with no external power. At ΔT_Applied ~20 K, the TEG provided ~1.8 mW at 0.56 V; the converter output ~1.66 V at ~1.1 mW, sufficient to light LEDs. Minimum ΔT_Applied to turn on LEDs was ~13 K. Integrated “hot surface warning gloves” produced a bright “H” when grasping hot objects. - Resistance budget: Module resistance was dominated by junction resistance (~77%) and electrode resistance (~19%). Identified routes for improvement include reducing contact resistance between Ag epoxy and TE legs, between conductive epoxy and AgNW electrodes, optimizing AgNW length/diameter/coverage, and introducing interfacial layers or PDMS surface etching to expose more AgNWs.

The work demonstrates that combining soft heat conductors with intrinsically stretchable interconnects addresses the two core bottlenecks in wearable TEGs: parasitic thermal losses and non-conformal contact. Magnetically aligned Ag-Ni/PDMS s-HCs boost through-plane heat conduction and match thermal impedance to Bi2Te3 legs, maximizing ΔT across the legs. The AgNW/PDMS electrodes absorb strain, enabling the TEG to achieve intimate, air-gap-free contact with complex 3D surfaces and to maintain performance under bending and stretching. These design choices explain the large gains in VOC, power, dynamic response, and on-skin/curved-surface performance relative to prior flexible/stretchable TEGs. Automated, additive assembly improves fill factor and yield, translating material- and architecture-level gains into module-level power density suitable for wearables. The successful powering of an LED warning system without external energy underscores practical relevance for self-powered IoT and safety wearables.

This study introduces a scalable compliant TEG platform that integrates magnetically self-assembled soft heat conductors and intrinsically stretchable AgNW interconnects to achieve high thermoelectric performance and exceptional conformability. Key contributions include: (i) s-HCs with K_thru-plane ≈1.4 W m⁻¹ K⁻¹ enabling thermal impedance matching and ~260% power improvement over controls; (ii) robust, strain-absorbing electrodes providing up to 20% stretchability and long-cycle mechanical reliability; (iii) automated manufacturing of high-FF, large-area modules yielding up to 7.02 mW and 2.12 V at ΔT 40 K; and (iv) record on-skin performance and effective energy harvesting from curved 3D sources, enabling a self-powered LED warning glove. Future work should focus on reducing module resistance—particularly junction and electrode contributions—via interfacial engineering (e.g., exposing more AgNWs, adding metal interlayers, optimizing AgNW morphology) and exploring further enhancements in s-HC thermal conductivity and geometry, to push power densities toward practical continuous-power wearable electronics.

- The exact thermal conductivity of patterned s-HCs at high Ag-Ni loading (~85 wt%) could not be directly measured due to instrument and fabrication limits for bulk composites above 75 wt%; the reported K_thru-plane (~1.4 W m⁻¹ K⁻¹) is extrapolated from measured data. - Module output power is limited by electrical resistance dominated by junction resistance (~77%) and electrode resistance (~19%); contact resistance at epoxy/electrode interfaces is significant due to partial AgNW exposure through PDMS. - Liquid/ambient durability was assessed up to 384 h under controlled humidity/temperature; longer-term environmental and mechanical stressors in real-world use may require further validation. - While s-HC parameters were optimized balancing performance and softness, further increases in K_thru-plane or reductions in s-HC thickness could raise performance but may impact mechanical compliance or process stability.