Over the course of this program, the project team, comprising the U.S. Naval Research Lab, U.S. Army Combat Capabilities Development Command (DEVCOM) Soldier Center, DEVCOM Army Research Laboratory, and the Naval Entomology Center of Excellence, has designed, developed, and demonstrated multiple novel polymer-insect repellent composite technologies in various form factors (microfibrous, filament, gel, and knit), developed new insect bioassays to enable evaluation of these materials, and demonstrated new fire-retardant nylon composites using bioderived additives.

The main objective of this project was to develop novel multifunctional fibers for the controlled delivery of environmentally friendly, low toxicity insect repellents encapsulated in the core of polymer fibers via electrospinning, dry jet wet spinning, and melt extrusion to deliver safe insect repellent materials for warfighter vector protection and demonstrate feasibility for technology transition. The project employed electrospinning as a rapid prototyping method to produce polymer fibers for long-term release of insect repellents and evaluated for performance and efficacy to down-select designs for live mosquito testing and more scalable production methods, such as dry jet wet spinning and melt extrusion. The approach yielded multiple long-release technologies that represented manifold paths to produce yarn and/or fabric composites, as well as standalone gels and patches, for environmentally sustainable multifunctional textiles.

This program was organized into four tasks. In Task 1, dual insect repellents were incorporated into electrospun nylon fibers to evaluate potential additive or synergistic effects uniquely enabled by the electrospinning process. In Task 2, electrospun fibers were physically spun into threads to demonstrate direct transition capability for improved performance for multifunctional uniform applications. In Task 3, insect repellent composite fiber designs developed under Tasks 1 and 2 were transitioned to conventional fiber drawing techniques (dry jet spinning and melt extrusion), and efficacy (mechanical, release properties) was evaluated to demonstrate feasibility for future transition. In Task 4, additional additives (fire retardant and antimicrobial) were separately incorporated into the most successful insect repellent designs via electrospinning to demonstrate potential for dual functionality. Ultimately, this research produced several different fiber designs with well-proven transition potential that could be used to design new multifunctional fabrics.

The flexibility afforded by electrospinning enabled the generation of a library of multifunctional fibers with different additives, chemistries, and physical properties. Individual fibers could be tuned to encapsulate varying functionalities and at different concentrations, or monofunctional fibers may be blended to form multifunctional yarns depending upon the use case. This approach had the potential to enable new textile manufacture methods that generated numerous multifunctional products that could impact multiple applications, including intelligently designed ‘smart’ uniforms, including garments with localized behavior (e.g., collars with high loading of insect repellent). Furthermore, the project showed that electrospinning could effectively serve as a testbed technology to allow for rapid prototype and demonstration of the coaxial designs, from which more conventional coaxial processing techniques could be easily adapted for transition to large scale production. The insect repellent-polymer composite materials produced using conventional, scalable, drawing methods successfully demonstrated long-term insect repellent release and live mosquito efficacy.

Electrospinning was used to fabricate nanofibers and form multiple series of mixed insect repellent nylon fiber composites, including mixtures of N,N-diethyl-meta-toluamide (DEET), Picaridin, and Permethrin at various loading ratios. Electrospun nylon fibers with incorporated DEET were fabricated and were shown to retain ca. 80% of the initial amount of repellent. Incorporation of up to 50 rel. wt% DEET relative to nylon had no detrimental effect on fiber morphology. The effective lifetime was dependent on the amount of sample (i.e., the weight) and the concentration of nylon. For samples akin to ultra-lightweight fabric, DEET repellency would be active for estimated >72 hours, which far outperformed common topical application. The project also incorporated Permethrin up to 10 rel. wt% in electrospun nylon fibers. The morphology of the fibers was unaffected by Permethrin loading and thermal properties largely maintained. This preliminary evaluation demonstrated the potential to incorporate Permethrin into electrospun nylon fibers at relatively high loading concentrations. The project designed and fabricated dual repellent nylon electrospun fibers containing both DEET and Picaridin at several ratios. Both repellents were successfully incorporated into the nylon fibers, with slightly more Picaridin retained than DEET. All formulations exhibited very long half-lives, with 75:25 DEET/Picaridin exhibiting a particularly long half-life that was attributed to potential synergistic effects. The project developed mixed insect repellent-doped recycled polyethylene terephthalate (rPET) electrospun microfibers and demonstrated their efficacy using a novel live insect bioassay specifically designed for insect repellent loaded textiles. The insect repellent-loaded rPET fibers showed effective repellency against live mosquitos in bioassay evaluation for two weeks due to the favorable polymer-repellent interactions. The project designed, formulated, and tested flame retardant tannic acid-nylon fibers that showed the natural product tannic acid was effective as a fire-retardant additive for nylon.

Preliminary electrospun yarns were produced from the insect repellent-loaded nylon electrospun fibers. The individual nanofibers were retained when incorporated into individual filaments and maintained their morphology throughout the twisting process. The yarn was ca. 150 µm in diameter, which was comparable to commercially available polymer filaments used in conventional fabric manufacturing. The electrospun insect repellent nylon fibers proved compatible with mechanical yarn fabrication techniques.

The project made progress in two parallel scalable fiber fabrication pathways, dry jet wet spinning and melt extrusion. The project identified and employed the benefits of dry jet wet spinning, which extrudes polymer gels at relatively low temperatures that enabled the incorporation of volatile repellents, to a novel polymer-DEET gel formulation that had significant promise to serve as a fiber core in coaxial polymer fibers, as well as standalone repellent material.

Notably, the project team successfully designed insect repellent-loaded physical gels with live mosquito repellency duration exceeding any known material to date (>30 weeks). Hansen Solubility Parameter modeling accurately predicted miscibility between the modacrylic copolymer poly(acrylonitrile-co-vinyl chloride) (P(AN-VC)) and DEET, the most common insect-repelling agent, to form physical gels with tunable gel temperatures proportional with polymer concentration. P(AN-VC)/DEET composites with gelation temperatures of 45°C were targeted to withstand a “warm” water setting during laundering for compatibility with potential textile applications. The resulting polymer-insect repellent gel exhibited physical properties to that of a semi-solid gel to enable compatibility with facile processing techniques.

For melt extrusion, the project identified a promising melt depressant that effectively lowered the processing temperature of nylon to enable compatibility with insect repellents and developed Nylon-12/poly(lactic acid) (PLA) blends to incorporate the insect repellent nootkatone. Adding PLA to Nylon-12 greatly improved processability when introducing nootkatone to the melt for filament extrusion. Increased processability was attributed to nootkatone relegating to PLA domains which effectively mitigated previously observed liquid-liquid phase separation from Nylon-12 and nootkatone, resulting in polymer slippage and shear loss. Finally, knitting trials successfully demonstrated knitting capability of unmodified Nylon-12, melt reduced Nylon-12/ para-toluenesulfonamide formulations, and nootkatone-loaded Nylon-12/PLA filaments. These knits showed acceptable strength, feel, and quality to demonstrate feasibility for potential textile-based applications.

The encapsulation of insect repellent (picaridin and DEET) into textile fibers via a bottom-up approach afforded the potential to create fabrics and garments that offer long-term protection to the warfighter from insect-borne diseases. Incorporation of the active materials into the core of the fibers greatly enhanced the durability of these functionalities to laundering, especially when compared with surface treatments, strongly reducing the current health hazards present for surface treated fibers and increasing their environmental sustainability. The insect repellent fibers and gels produced in this work have the potential to greatly reduce environmental and health risks during their lifecycle by: 1) increasing the longevity of functionalities after laundering; 2) reducing direct skin contact of active additives by encapsulation within the core of a benign material; and 3) generating novel fibers from which textiles and garments could be intelligently designed with functionalities localized and limited only to the areas in which they are needed.

The project presented several technology advancements in the program that have potential to serve as standalone insect repellent materials. Long-duration insect repellent materials and textiles have the potential to protect warfighters and civilians from widespread arthropod-borne diseases (e.g., Malaria, Dengue, Lyme, etc.). The development and demonstration of a novel bioassay specifically designed to evaluate the repellency of insect repellent-loaded fibers and textiles will have significant implications for providing live insect performance of future materials, as well as impact others in the field. The insect repellent gel designs are compatible with dry jet wet spinning, which is a scalable manufacturing compatible extrusion method that represents a facile potential transition pathway. The project also identified a promising melt depressant to lower the processing temperatures of nylon to enable volatile repellent incorporation to improve the compatibility of insect repellents with conventional melt extrusion manufacturing, which represents a promising transition pathway for textile applications. Importantly, the project demonstrated not only unprecedented sustained efficacy, but also feasible production pathways using commercially available and Environmental Protection Agency-registered materials.