Engineering weft-knitted spacer structures for enhanced wicking in menstrual underwear

Abstract

Reusable menstrual underwear is an undergarment that is designed to absorb and hold blood during the menstrual cycle. They are usually made with multiple layers consisting of wicking and absorbing fabrics, including an additional barrier layer for leak-proofing [1]. In published literature [2], the function of the skin-contact or wicking layer is to promptly wick and transfer the fluids into the subsequent absorbent layers. Additionally, consumer reviews of existing reusable menstrual underwear emphasized the need to minimize the spread of fluid and prevent rewetting of the skin contact surface [3]. Wicking can be defined as the natural flow of liquid through a porous material that is triggered by capillary pressure. Contact angle and capillary radius play a significant role in determining the vertical, horizontal, and transverse wicking. When the contact angle and capillary radius are lower, capillary pressure becomes greater. Furthermore, when the contact angle is lower while the capillary radius is higher, the horizontal spread of the liquid becomes higher. Thus, how the moisture spreads and travels within a porous material can be engineered by controlling the above two parameters [4]. Higher net capillary pressure through the fabric ensures oneway fluid transfer. Simultaneously, higher horizontal spread in the outer surface compared to the inner surface can be achieved by making that surface more hydrophilic while lowering its capillary radius [5]. Only a few published studies were present on reusable menstrual underwear, especially with a focus on design and development [1]. Additionally, most of the research [2] and patents [6] did not disclose the technical specifications of the wicking layers. Moreover, no publications were available on wicking fabrics designed specifically for menstrual fluid transfer without the use of zonal or engineered finishes. Several studies have focused on wicking fabrics that focused on achieving unidirectional liquid transfer and increased horizontal diffusion for sweat management applications, which could potentially be adapted for menstrual underwear products. Those studies investigated three-thread fleece fabrics [8, 9] and plated weft-knitted fabrics [9, 10] that had layered structures. The skin-contact surfaces of those fabrics had less hydrophilic yarns with lower yarn counts and higher filament counts compared to their outermost surfaces to achieve the above-mentioned properties. Weft-knitted spacer fabric structures could also satisfy that requirement by incorporating yarns with different hydrophilicity, as well as yarn and fiber counts to the face, back, and spacer yarns. Spacer fabrics also possess the advantages of low area density, low bulk density, freedom of thickness modifications, layered structure capability, and design flexibility [12 - 14]. However, the use of spacer fabrics with the above-mentioned principles for enhanced wicking function has not been explored in any of the published literature. Therefore, the present study aims to address the knowledge gap in that area. In this study, fifteen weft-knitted spacer fabrics were developed following the aforementioned yarn and fiber requirements. The unidirectional liquid transfer and horizontal wicking performance of their outermost surfaces were evaluated to determine their suitability as the wicking layer in menstrual underwear.