Bio-based activewear

Activewear is a narrow engineering brief dressed as a product category. A garment worn during sustained exertion has to move sweat away from skin faster than the wearer produces it, recover its shape after several hundred thousand strain cycles, and resist abrasion at the seams and shoulder. Each of those is a materials constraint with a measurable ceiling, and bio-based fibres meet them unevenly.

Sorption is not wicking

The two mechanisms are routinely conflated. Wicking is capillary flow through the spaces between fibres and yarns; it is driven by pore geometry and surface energy, and it moves liquid water along the fabric plane where it can evaporate. Sorption is water taken up into the fibre substance itself, and it is a property of the polymer. Measured at the standard atmosphere of ISO 139 — 20 °C, 65 % relative humidity — moisture regain is about 0.4 % for polyester, around 8.5 % for cotton and typically 11–13 % for viscose.

That difference decides how a base layer behaves. A hydrophobic synthetic holds almost no water inside the fibre, so what the wearer sweats stays in the interstices and either wicks outward or evaporates. A cellulosic fibre absorbs it, swells, gains weight, loses tensile strength wet — viscose loses a large fraction of its dry strength when saturated — and cools by evaporating from the fibre surface, which is slower and continues after exertion stops. The resulting after-chill is not a manufacturing defect; it is the sorption isotherm doing what it does. Lyocell and hemp blends mitigate it by structure rather than chemistry: a knit that keeps a hydrophobic layer against the skin and a hygroscopic layer outside can be built from either fibre family, and the yarn geometry matters more than the feedstock.

The elastane problem

Stretch garments depend on a segmented polyurethane-urea: a soft polyether or polyester block that extends, and hard urea segments that microphase-separate into physical crosslinks and pull the chain back. Recovery — not extension — is the demanding property, and it comes from that two-phase morphology. Nothing plant-derived reproduces it. Bio-based elastane exists only in the sense that part of the soft-segment diol can be fermentation-derived, which changes the feedstock and not the mechanism. The alternative, mechanical stretch built from the loop geometry of the knit, gives extension without comparable recovery, so growth and bagging at knees and elbows appear sooner.

What the microplastic argument actually says

The strongest case for bio-based activewear is shedding: synthetic knits release fibre fragments during laundering, now measurable under the ISO 4484 series. The honest qualification is that cellulosic fabrics also shed, and that regenerated cellulose fragments have been recovered in environmental samples; their advantage is that they are enzymatically degradable, not that they are absent. Dye and finish chemistry travels with the fragment either way. A durable water-repellent finish or an antimicrobial treatment can dominate a garment’s environmental profile regardless of what the fibre was made from.