Maintaining human body temperature within a narrow range (36-38°C at rest, up to 41°C during heavy exercise) is crucial for health and performance. Heat-related problems negatively impact health, productivity, and the economy. Personal thermal management, focusing on the human body and its local environment, offers an energy-efficient solution. Human body heat dissipation occurs through radiation, convection, conduction, and evaporation. While textiles with engineered radiative, convective, and conductive properties show promise for mild scenarios, textiles for intense scenarios (moderate/profuse perspiration) remain lacking. Evaporation plays an indispensable role in thermoregulation, accounting for about 20% of heat dissipation in mild conditions and becoming the dominant route during intense activity or hot/humid environments. Current textiles, including moisture management fabrics, often focus on sweat removal to avoid skin wetness discomfort. Natural fibers (cotton) exhibit high water absorption, while synthetic fibers (polyester) are engineered for enhanced moisture transport. Microfibers, surface modifications, multi-layer designs, and hierarchical pore structures are explored to improve water transport. However, these textiles largely ignore the efficient utilization of sweat's cooling power. Sweat is secreted for evaporation, drawing heat from the skin; however, conventional textiles hinder efficient heat transfer from the skin to the evaporation interface, leading to decreased evaporative cooling efficiency and potentially excessive perspiration. The inefficiency leads to increased perspiration, skin wetness, and risks of dehydration. This research proposes an integrated cooling (i-Cool) textile that addresses these limitations by integrating heat conductive components and water transport channels to enhance sweat wicking, evaporation, and evaporative cooling.

Existing literature highlights the importance of personal thermal management for human health and productivity. Studies have explored textiles with engineered radiative, convective, and conductive properties for mild scenarios. However, there's a gap in addressing the need for efficient evaporation management in intense perspiration scenarios. Previous work on moisture-wicking textiles focuses on improving water transport via modifications to fiber types, surface properties, and fabric structures. However, these studies largely overlook the crucial aspect of efficient heat transfer from the skin to the evaporation site, limiting the cooling effectiveness of sweat evaporation. The lack of textiles effectively leveraging the cooling power of sweat is identified as a significant research gap. The current study aims to address this limitation by designing a textile that combines efficient sweat transportation with enhanced heat conduction to maximize the cooling effect of sweat evaporation.

This study introduces an integrated cooling (i-Cool) textile designed to improve personal perspiration management. The i-Cool textile integrates heat-conductive components (copper (Cu) in the proof-of-concept design, later replaced by silver (Ag) coated polyester (PET) and nanoporous polyethylene (NanoPE)) and water transport channels (nylon 6 nanofibers). The fabrication process involved electrospinning nylon 6 nanofibers and laser-cutting a copper matrix, followed by press lamination to integrate the components. For practical application, silver coating on commercial fabrics (Dri-FIT and CoolMax) was utilized to create a scalable and practical i-Cool textile. Several experimental methods were employed to characterize the i-Cool textile's performance and compare it to conventional textiles (cotton, Dri-FIT, CoolMax, Coolswitch): 1. **Liquid water transport characterization:** Mimicking sweat transport from skin to the textile's outer surface, wicking rates were compared. One-way water transport was assessed by observing water droplet movement. 2. **Thermal resistance measurement:** A cut-bar method was used to measure the thermal resistance, quantifying the heat transport enhancement of i-Cool. 3. **Transient droplet evaporation test:** This test measured evaporation rate and skin temperature using a heated platform simulating human skin. 4. **Steady-state evaporation test:** A continuous water supply system simulated steady perspiration, measuring water mass gain ratio and power density under various evaporation rates. 5. **Artificial sweating skin platform with feedback control loop:** A system mimicking human thermoregulation was developed. The sweating rate adjusted based on skin temperature, simulating the body's feedback mechanism. Experiments compared skin temperature and sweating rate using i-Cool and conventional textiles under various power densities and ambient conditions (high temperature, high humidity). This system also allowed for comparison of i-Cool to bare skin, a gold standard for efficient cooling. 6. **Thermal simulation:** The authors used a coupled heat and moisture transfer model to simulate the effects of i-Cool textile on human body temperature. 7. **i-Cool practical application demonstration:** The feasibility of i-Cool using commercial fabrics and electroless plating of silver was also demonstrated. The performance of this modified i-Cool was tested using the steady-state evaporation and artificial sweating skin tests.

The i-Cool textile significantly outperformed conventional textiles across all tests. Key findings include: 1. **Enhanced Wicking:** i-Cool exhibited comparable or superior wicking rates to conventional textiles, rapidly transporting liquid water from the bottom to the top surface. 2. **Reduced Thermal Resistance:** i-Cool showed 14-20 times lower thermal resistance than conventional textiles, indicating significantly improved heat transport. 3. **Faster Evaporation:** Transient droplet evaporation tests showed that i-Cool achieved about twice the evaporation rate and 2.3-4.5°C lower average skin temperature than cotton. 4. **Lower Water Mass Gain Ratio:** In steady-state evaporation tests, i-Cool maintained a much lower water mass gain ratio (e.g., ~20% compared to ~130% for cotton at 1.1 mL/h evaporation rate), demonstrating reduced sweat accumulation. 5. **Higher Sweat Evaporative Cooling Efficiency:** i-Cool exhibited a threefold increase in skin power density increment per unit of sweat evaporation compared to cotton, indicating superior cooling effectiveness per unit of sweat. 6. **Reduced Sweat Consumption for Cooling:** Artificial sweating skin tests under different power densities and ambient conditions showed i-Cool achieving similar cooling effects to bare skin with significantly less sweat consumption compared to conventional textiles, a 3°C cooling effect with less than half the sweat requirement of cotton. 7. **Scalable and Practical Application:** i-Cool design principles were successfully demonstrated using commercially available fabrics with electroless silver plating, preserving the superior performance observed in the proof-of-concept design.

The results demonstrate that the integrated design of i-Cool, combining heat conduction and sweat transport, is crucial for effective cooling. Separately testing the heat conduction and water transport components showed significantly reduced cooling performance, highlighting the synergistic effect of the integrated design. The i-Cool textile’s ability to achieve comparable cooling to bare skin while requiring significantly less sweat addresses the limitations of current textiles and their inefficient use of sweat's cooling potential. The findings suggest that i-Cool can prevent overheating and reduce the risk of dehydration during strenuous activity or in hot and humid environments. The successful demonstration using commercial fabrics expands the potential for practical implementation in clothing and other applications. The thermal simulation, showing temperature reduction in both skin and core temperature, further supports the potential of this design for practical human body cooling.

This study presents a novel i-Cool textile design that effectively manages personal perspiration. The integrated design enhances sweat wicking, evaporation, and evaporative cooling. The superior performance was consistently demonstrated across various tests and conditions. The successful translation of the i-Cool concept to commercially available fabrics suggests a promising path for next-generation performance textiles. Future research could explore optimizing the material choices, fabric structures, and integration with other thermal management strategies to further improve performance and expand the range of applications.

While the study demonstrates the effectiveness of the i-Cool textile in laboratory settings using artificial skin, further research is needed to fully validate its performance in real-world scenarios with human subjects. The current study primarily focuses on the cooling effect during active perspiration. Further research could investigate the comfort and performance of the i-Cool textile under different conditions, such as passive heat dissipation and varying humidity levels. The long-term durability and potential for degradation of the materials used in i-Cool textile under repeated washing and wear also warrant further investigation. The scalability of the fabrication methods for mass production also requires additional studies.