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Electrospun Multifunctional Composite Fibers for Improved Warfighter Insect Protection
Jeffrey Lundin | U.S. Naval Research Laboratory
Fabrics and garments that exhibit similar feel of existing uniform fabrics, while also exhibiting superior performance via the incorporation of safer insect repellents (i.e. picaridin) have potential to greatly reduce the environmental and toxic burden of functional textiles. By improving the durability of the insect repellent fibers to laundering and degradation due to normal wear, sustainability can be improved since textiles will exhibit longer lifecycles in the field. The objective of this research was to develop novel multifunctional fibers containing environmentally friendly low toxicity insect repellents intelligently localized in textile-relevant polymeric fibers, such as nylon, with core-shell morphology via coaxial electrospinning. Coaxial electrospinning afforded the potential to create hierarchically-structured functional micro- to nano-scale fibers by control over the composition of specific areas of the fiber (core vs. surface).
The general approach of this work employed coaxial electrospinning to physically embed active additives into the core of textile fibers. First, insect repellents were uniformly distributed through monofilament Nylon fibers and then incorporated separately into the core of nylon, by coaxially electrospinning. Coaxial electrospinning afforded control over the design of the polymer shell (Figure 1) to tune the permeability of the polymer fiber to the additive in the core and control its release. This is critical for the insect repellent application, in which the release should be slowly occurring during the lifetime of the garments. Thorough material characterization was performed to evaluate fiber designs and compare the release of two insect repellents, DEET and picaridin.
Repellent nanofibers composed of picaridin in Nylon-6,6 were successfully developed. Electrospinning proved successful to demonstrate blends and coaxial nylon fiber designs that exhibited tunable and delayed release performance. The results described herein represent a significant proof of concept of the validity of the technical approach and identifies significant potential for the development of durable insect repellent textiles. Fabrication of electrospun repellent fibers containing the insect repellent picaridin was demonstrated and thermogravimetric analysis (TGA) was most effective in demonstrating the tunable release of repellent. A comparison of monofilament and coaxial structures provided evidence of another design tool by which to encapsulate volatile liquids and simultaneously control their release through fabrication inputs. TGA profiles of monofilament repellent fibers elucidated information on activation energies and release kinetics that cemented the importance of environmental conditions (i.e. ambient temperature, humidity) important to designing high durability fibers for improved warfighter uniforms.
Coaxial fibers demonstrated the ability to withstand high temperatures by physically limiting diffusion of the repellent-rich core through the fiber, further confirming the durability and performance of a coaxial fiber and its advantage compared to traditional monofilament fibers and textiles.
Electrospinning demonstrated the ability to prototype multifunctional composites that can be easily implemented and transferred to commercial production facilities. The ability to encapsulate volatile repellents like DEET and picaridin in textile-relevant materials increases the likelihood of deploying such systems across a wide range of battlespace environments. Thermal analysis was the most effective in characterizing and iterating the design of such fibers for generating platforms with tunable release profiles via bottom-up approach. Calculation of activation energies via Arrhenius equations provided further insight into small-molecule diffusion, impacting the design of electrospun repellent fibers. Scanning electron microscopy demonstrated that monofilament fiber morphology was largely unaffected by repellent incorporation, even at extremely high loadings. Similarly, coaxial morphology was largely independent of repellent loading, though at very high loading levels, more defects in fiber morphology were identified, which would be expected to compromise mechanical integrity. Further investigations of these systems would focus on optimization of thermomechanical performance as related to both morphology and composition of the insect repellent fibers. Fourier transform infrared spectroscopy demonstrated that even though no specific binding occurred between the relatively polar repellents and Nylon, physical encapsulation provided an adequate barrier that enabled release of repellent over very long times compared to more traditional topical approaches. This suggests that there is opportunity for further research to investigate more highly interacting systems, through exploitation of specific binding (i.e. ionic, H-bonding, etc.) to create ultra-durable, long-lasting multifunctional textiles. Additionally, because of the simplicity of electrospinning, this fabrication approach lends itself to potential implementation toward additional systems of interest including flame retardants, energetics, and supported catalysts. The flexibility afforded by electrospinning provides the opportunity for the generation of a library of multifunctional fibers with different additives, chemistries, and physical properties. Individual fibers may be tuned to encapsulate varying functionalities and at different concentrations; or, monofunctional fibers may be blended to form multifunctional fibers or yarns depending upon the use case (Figure 2).