Integrated cooling (i-Cool) textile of heat conduction and sweat transportation for personal perspiration management

The study addresses how to unlock the cooling power of sweat for human thermoregulation, especially under moderate to profuse perspiration where liquid sweat is present. Conventional textiles prioritize sweat removal (wicking/absorption) but provide limited evaporative cooling of the skin because latent heat for vaporization is not efficiently drawn from the skin due to poor heat transfer. This leads to inefficient cooling, increased sweat accumulation, and potential dehydration. The authors propose an integrated cooling (i-Cool) textile that combines a heat-conductive matrix with sweat transport channels to enhance both evaporation rate and evaporative cooling efficiency, thereby improving skin cooling while reducing sweat consumption. The research tests whether integrating heat conduction paths with directional sweat transport can deliver faster evaporation, lower skin temperature, and higher cooling power per unit sweat than conventional fabrics.

Human body dissipates heat via radiation, convection, conduction, and evaporation. Prior personal thermal management textiles have targeted radiative properties (e.g., infrared-transparent or radiative cooling fabrics), and convective/conductive insulation approaches, performing well in mild scenarios. Moisture management fabrics (e.g., cotton, polyester microfibers) and designs with hydrophilicity/hydrophobicity modification, multilayers with differential wettability, and hierarchical capillarity gradients focus on removing liquid sweat and reducing wetness by absorption and directional transport. However, these approaches often do not maximize evaporative cooling efficiency at the skin because the heat of vaporization is not effectively supplied from the skin to the evaporation interface. Studies have shown reduced evaporative cooling effectiveness when evaporation occurs away from the skin, which can also slow evaporation and lead to sweat accumulation. Thus, an explicit integration of heat conduction with sweat transport to enhance both evaporation rate and the fraction of latent heat extracted from the skin remains underexplored.

• Concept and materials: The i-Cool textile integrates a highly heat-conductive matrix with sweat transportation channels. Proof-of-concept used a laser-cut copper (Cu) matrix (~400 W·m⁻¹·K⁻¹) with a pore array (2 mm diameter, 3 mm pitch) laminated with electrospun nylon-6 nanofibre film (~4.5 g/m², ~25 µm) serving as wicking channels. The assembled i-Cool (Cu) textile was ~45 µm thick and 107.7 g/m². Nylon nanofibres covered the Cu top and filled pores to promote one-way transport. For practical demonstrations, Ag-coated polymer matrices (Ag-coated PET fabrics and nanoporous polyethylene) and i-Cool structures on commercial Dri-FIT and CoolMax fabrics were fabricated via laser cutting pores, electroless silver plating (after polydopamine priming), cellulose fiber filling, and press lamination with nylon-6 nanofibres (i-Cool (Ag)).
• Characterizations and tests:
  • Wicking and one-way transport: Modified AATCC 198-based test measured wicking rate by time to reach a set radius on the top surface after placing water beneath. One-way transport index μ = S_outer/S_inner was evaluated by imaging 20 µL droplet transport from inner to outer side and vice versa.
  • Thermal resistance: Cut-bar method (adapted from ASTM 5470) measured dry thermal resistance under ~15 psi contact pressure; thermal resistor modeling apportioned resistance among layers.
  • Transient droplet evaporation: Artificial skin (heated polyimide heater on insulation) received 37 °C water (0.05–0.2 mL typical; reported example 0.1 mL), covered with textile. Skin temperature was recorded via thermocouple under fixed power density (e.g., 422.5 W/m²). Average evaporation rate = initial water/evaporation time; average skin temperature taken over the quasi-steady evaporation period.
  • Steady-state evaporation: Artificial skin with embedded thermocouples and a water inlet pumped 37 °C water at controlled rates. Skin temperature held at ~35 °C via adjusting power density. At steady state, water mass gain (wet mass – dry mass), evaporation rate, and required skin power density were recorded. Metrics: water mass gain ratio W (% of dry mass), its gradient dW/dv, cooling power density q, and dq/dv as a proxy for evaporative cooling efficiency η.
  • Artificial sweating skin with feedback: A 3D-printed water reservoir, perforated heater, and a Janus wicking layer with limited outlets created uniform "sweating" at 37 °C. Skin temperature measured in real-time controlled the syringe pump via a linear feedback rule (sweating rate proportional to skin temperature above a threshold of 34.5 °C), mimicking physiological feedback. Tests compared bare skin, i-Cool (Cu), i-Cool (Ag), and commercial textiles (cotton, Dri-FIT, CoolMax, Coolswitch) under various power densities (e.g., 750–1035 W/m²), ambient temperatures (22 °C and 40 °C), and high RH conditions.
  • Additional: Water vapor transmission and evaporative resistance tests (modified ASTM E96; ISO 11092/ASTM F1868), and coupled thermal-moisture-human simulations assessed skin and core temperature impacts with i-Cool vs conventional textiles.

• Wicking and one-way transport: i-Cool (Cu) showed comparable or higher wicking rates than cotton, Dri-FIT, CoolMax, and Coolswitch. Morphology-driven capillarity differences produced strong one-way transport from inner to outer surface, with large μ and S_outer >> S_inner, unlike cotton (μ ~1), facilitating faster evaporation on the heat-conductive matrix.
• Thermal resistance: i-Cool (Cu) exhibited ~14–20× lower dry thermal resistance than conventional textiles under ~15 psi. Modeling indicates the nylon-6 nanofibre layer dominates resistance; increasing Cu thickness adds little, supporting scalability across thicknesses.
• Transient evaporation (0.1 mL, 422.5 W/m², ~22 °C ambient): i-Cool (Cu) reduced average skin temperature by 2.3–4.5 °C and roughly doubled average evaporation rate versus conventional textiles. Over varied conditions, evaporation rate increased with initial water and approached saturation; for a given skin temperature, i-Cool achieved higher rates with less initial water and lower skin temperature than cotton.
• Steady-state evaporation: At the same evaporation rate v, i-Cool (Cu) consistently had lower water mass gain ratio W than cotton and Dri-FIT. Example: at 1.1 mL/h, W ≈ 20% (i-Cool) vs ≈130% (cotton). The gradient dW/dv for i-Cool was small and nearly constant, while cotton and Dri-FIT increased rapidly, implying i-Cool reaches higher evaporation rates with less sweat accumulation. Skin power density q at a given v was higher for i-Cool, and dq/dv (cooling power increment per unit evaporation) was ~3× that of cotton and Dri-FIT. Estimated evaporative cooling efficiency η ≈ 0.8–1 for i-Cool vs 0.2–0.4 for cotton/Dri-FIT.
• Artificial sweating skin with feedback (22 °C, 750 W/m²): i-Cool (Cu) performed similarly to bare skin and substantially better than conventional textiles: ~2.8 °C lower skin temperature than cotton; ~2.0 °C lower than Dri-FIT and Coolswitch; ~3.4 °C lower than CoolMax. To maintain temperature, conventional textiles required 2–3× higher sweating rates than i-Cool, indicating significant sweat savings and drier textiles (visual confirmation). Separating components (heat conduction alone or wicking alone) reduced performance, highlighting the need for integration.
• Robustness across conditions: At higher power densities (up to ~1035 W/m²), i-Cool maintained lower skin temperatures and reduced sweating rates vs conventional textiles. At 40 °C ambient and at high RH, i-Cool still outperformed others; even when skin temperature was set below ambient, i-Cool matched bare skin and surpassed cotton, indicating net benefits despite possible heat conduction from ambient.
• Practical feasibility: i-Cool (Ag) built on commercial Dri-FIT and CoolMax substrates, and polymer matrices with heat-conductive coatings (Ag-coated PET, NanoPE), achieved performance comparable to i-Cool (Cu) and markedly superior to the original fabrics in both steady-state and artificial skin tests. Variants with only transport channels (no integrated conduction) underperformed integrated i-Cool.
• Simulation: Coupled human-body simulations indicated that improved evaporation ability and efficiency with i-Cool can reduce both skin and core temperatures compared to conventional textiles.

Integrating a heat-conductive matrix with capillary sweat transport channels addresses the key limitation of conventional moisture-management textiles: insufficient heat delivery from skin to the evaporation interface. The i-Cool design accelerates evaporation by enlarging evaporation area while efficiently supplying latent heat from the skin, thereby cooling the skin more effectively and increasing cooling power per unit of sweat. This enhanced evaporative cooling efficiency translates into lower required sweating rates (reduced dehydration risk), drier textiles, and improved thermal comfort. The approach remains effective across a range of metabolic heat loads, high ambient temperatures, and high humidity. Control experiments confirm that neither conduction nor wicking alone achieves the observed benefits; their integration is critical. Practical demonstrations on commercial fabrics with conductive coatings show the concept is material-agnostic and scalable. Collectively, the findings validate that i-Cool textiles can more efficiently utilize sweat for thermoregulation, approaching bare-skin efficiency while maintaining the advantages of wearing a textile.

The work introduces an integrated cooling (i-Cool) textile architecture that synergistically combines heat conduction pathways with directional sweat transport. Across multiple tests, i-Cool exhibits faster evaporation, substantially higher evaporative cooling efficiency (η ≈ 0.8–1), lower skin temperatures, and significantly reduced sweating demand compared with conventional textiles. The structure mitigates sweat accumulation and maintains drier conditions during perspiration. Practical embodiments using Ag-coated commercial fabrics achieve performance comparable to copper-based prototypes, indicating feasibility for real-world use. Simulations further suggest skin and core temperature reductions for wearers. Future work includes scaling to full garments, long-duration human physiological wear trials, optimizing material combinations and geometries for durability and comfort, and exploring manufacturable conductive fiber or coating systems for mass production.

The artificial sweating skin platform, while enabling controlled, comparative testing and feedback regulation, does not fully replicate human physiology (e.g., lacks blood flow and vasomotor feedback, and differs in size, shape, and thermal capacity). No human subject trials are reported; thus, comfort, durability, and long-term wear effects remain to be validated. Measurements were conducted under controlled laboratory environments with specified ambient conditions; real-world variability in wind, posture, and activity could affect performance. Material choices in the prototypes (e.g., copper, silver coatings) may raise concerns about cost, weight, flexibility, and wash durability that require further engineering for consumer applications.