Flexible Heaters for Combat Boot Insoles

Wearable heaters are low-profile, portable devices that are easily integrated into clothing or footwear and used to provide protection against frostbite in harsh environments. Fabrication of flexible heaters can be accomplished using a range of techniques, including direct incorporation into the fabric itself (e-textile) or printing the heater onto a patch or directly onto the fabric. Additive manufacturing (AM) techniques enable greater versatility when transitioning to a high throughput platform. Printing enables rapid prototyping and is an effective method for the development of soft, flexible devices. This versatility is necessary when investigating wearable substrates and negates the need for the development of new fibers, yarns, and fabrics. Although printed heaters are relatively more developed than those incorporated into e-textiles, several challenges still remain. Research focused on heater design, materials, and durability testing is still needed to ensure that the technology is suitable for commercialization using domestic manufacturing processes. In our previous work, we printed flexible conductive band stop metasurface arrays directly onto fabric. The metasurface retained more than 90% rejection at the target band stop frequency after 60 wash cycles, demonstrating excellent physical integrity.<br/> Herein, we present a printed heater suitable for a cold weather combat boot insole. Wearable heaters for footwear are expected to undergo regular mechanical stressors, and thus the design must compensate for faults including cracks or regions with inconsistent heating. Additionally, the design must minimize voltage and the power output should remain unaffected by any mechanical deformation. This can be ensured not only by the elasticity of the materials but also assisted by a perforated design. Durability assessment such as wash cycles, and flex and abrasion testing provide important metrics for device performance. Selected inks are used to print the heater design directly onto a woven fabric. Rather than addressing defects through modifying the initial design, they may be repaired by depositing an alternative ink composite ratio, ultimately creating an anisotropic trace. This novel methodology will be enabled by prioritizing careful materials selection, given that the heater can be defined as a set design, and ink properties can be tailored to develop a resistivity with the desired value. This will produce less waste, as small traces can be printed for testing prior to heater fabrication, rather than requiring a full printed design for each iteration of the prototyping process. Understanding the available materials and processes in the printed electronics ecosystem can enable the commercialization of novel, wearable devices.