# Bio-based underwear

Why viscose and lyocell diverge sharply when wet, how skin bacteria convert odourless secretions into thioalcohols and branched acids, and why an antimicrobial finish is a regulated biocide rather than a fibre property.

Next-to-skin garments are governed by three things a fibre datasheet does not show: wet strength, the biochemistry of body odour, and what regulators allow you to put on a treated article.

Source: https://en.bioecon.ru/docs/forestry-biomaterials/fibers-textiles/bio-underwear/
Updated: 2026-09-04



Underwear looks like the least technical garment in a wardrobe and is in fact tightly constrained. It is worn against skin at 32–34 °C in a humid, salted, bacterially colonised microclimate, laundered more often than anything else, and expected to keep dimensional stability through hundreds of wash cycles. Three mechanisms decide whether a bio-based fibre survives that.

## Wet strength divides the cellulosics

Regenerated cellulose fibres are chemically nearly identical and mechanically very different, because the difference is morphological. In viscose, cellulose is regenerated from a xanthate solution with low crystallinity and poorly ordered chains; water penetrates the amorphous regions, swells the fibre and disrupts the hydrogen bonding that carries load, so wet tenacity falls to roughly half the dry value. Lyocell is spun directly from an amine-oxide solvent without derivatising the cellulose, retains far higher crystallinity and orientation, and holds most of its strength wet. In a garment washed constantly this is the difference between shape retention and progressive elongation at waistband and seat.

Lyocell's own defect follows from the same order: highly oriented fibres fibrillate, splitting lengthwise under wet abrasion into microfibrils that show as surface fuzz. Crosslinking finishes suppress it, and those finishes are then themselves a skin-contact chemistry question.

## Odour is a bacterial reaction, not a smell in the sweat

Apocrine secretion in the axilla is essentially odourless when produced. Resident skin bacteria, mainly staphylococci and corynebacteria, carry the enzymes that liberate the volatiles: a cysteine-S-conjugate β-lyase releases volatile thioalcohols such as 3-methyl-3-sulfanylhexan-1-ol, and specific aminoacylases release branched short-chain acids including 3-methyl-2-hexenoic acid. The garment's role is to hold or not hold the substrates and the organisms.

This is where synthetic and cellulosic fibres genuinely diverge next to skin. Hydrophobic polyester surfaces adsorb the sebum-derived precursors and are readily colonised, and the odour builds; hydrophilic cellulose absorbs the aqueous secretion into the fibre and releases the precursors more completely in laundering. The advantage is real but it is about substrate partitioning, not antimicrobial activity — the fibre is not killing anything.

## An antimicrobial finish is a biocide

Silver, zinc pyrithione and quaternary ammonium finishes are frequently added to bio-based underwear and marketed as part of its natural credentials. Legally they are not a fibre property. In the EU, a fabric treated to protect the wearer is a treated article under the Biocidal Products Regulation (EU) 528/2012, and the active substance must be approved for that product type; in the United States the equivalent claim falls to EPA registration under FIFRA. The relevant durability question is leaching: an active that washes out stops working and enters the effluent. Skin-contact chemical limits are separately addressed by voluntary schemes such as OEKO-TEX Standard 100, whose strictest product class covers articles for babies. A fibre grown from wood or hemp says nothing about whether the finish on it passes any of these.