UNDERskin™

The first underwear with prebiotics and probiotics infused into the fabric to support your natural skin microbiome.
The Mix Starter Kit

The Underdays Starter Kit

$93.00

Style :

Mix Brief Thong

Size :

Size guide

XS S M L XL XXL

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Free UK Delivery over £60+
Free UK Returns & Exchanges

Complete the set

Meet the Underdays Starter Mix Set — the easiest way to discover our cult-favourite underwear. Inside, you'll find three bestsellers: the Everyday Thong, our Brazilian-cut Cheeky, and the classic Brief. Each pair is thoughtfully designed for all-day comfort and support — luxuriously soft, breathable, and made from all-natural TENCEL™ Modal. Paired with our Scuba Cloud Pouch, designed to go everywhere you do. Everything you need to start a fresh, intentional underwear drawer.

UNDERskin™: 100% biobased synbiotic technology, supports your skin's natural microbiome, lasts up to 40 washes. Data provided by HeiQ based on testing conducted on other materials.

Crafted from buttery-soft, breathable TENCEL™ Modal — designed for all-day comfort.

An antimicrobial gusset designed to support your body — from daily wear to your most active days.

Designed to flatter every body and hold its shape wash after wash.

Comes in our Scuba Cloud Pouch — designed to go everywhere you do.

•92% Micromodal, 8% Elastane
•Gusset: 92% Tencel Micromodal, 8% Elastane
• UNDERskin™: Prebiotics + probiotics infused in the fabric transfer to your skin on contact, supporting a balanced, healthy-looking microbiome. 100% biobased. Powered by HeiQ Skin Care™.
•All fabrics are OEKO-TEX certified (SH015 210990 Testex)

•Care: To preserve the HeiQ Skin Care™ treatment, wash at 30°C on a gentle cycle. The synbiotic technology is designed to last up to 40 home washes. Data provided by HeiQ based on testing conducted on other materials.
We also recommend to wash your Underdays in a mesh laundry bag. Air-dry flat in a shaded area. Do not bleach. Do not iron or tumble dry.

  • We offer both Next Day Delivery and Standard Delivery

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Meet UNDERskin™

Prebiotics and probiotics, infused into the fabric, transferring to your skin all day. Supports your microbiome. Strengthens your skin barrier. Promotes a healthier appearance. 100% biobased. Developed in Switzerland.

Skincare. Now also underwear. No extra steps, just get dressed and get on with it.

The Problem

Your skin is under constant attack

Stress, pollution, hormones, over-washing — all of it disrupts your skin's natural microbiome. The result? Dryness, sensitivity, irritation, uneven texture.

The Solution

UNDERskin™ fights back, while you do nothing

Prebiotics + probiotics infused in the fabric transfer to your skin on contact. They restore balance, boost your native good bacteria, and promote a healthier, smoother skin appearance. Powered by HeiQ Skin Care™, Swiss synbiotic technology.

key benefits:

rebalance

rebalance

Helps restore the natural balance of your skin microbiome.
radiance

radiance

Promotes a healthy skin appearance thanks to pre+probiotics.
protect

protect

Strengthens your skin's natural protective barrier.
nourish

nourish

Feeds your own, native good microorganisms.
consistent

consistent

No change to handfeel, breathability, colour, or wicking.
effortless

effortless

No extra steps, just wear it as normal.

STEP 1

We infuse it in

Prebiotics + probiotics are embedded into the fabric within a 100% biobased slow-release matrix during production.

STEP 2

You put it on

When the fabric touches your skin, the synbiotics transfer naturally. No activation needed, just skin contact.

STEP 3

Your skin benefits

A more balanced microbiome means healthier-looking, smoother, softer skin. And it lasts up to 40 washes.

  • 100% biobased

  • Certified biodegradable

  • Clinically tested

  • Compliant with EC Cosmetic Regulation No 1223/2009

underwear that cares for your skin

product details

frequently asked questions

Our Everyday Collection is treated with HeiQ Skin Care™ — Swiss synbiotic technology that supports your skin's microbiome through fabric-to-skin contact. Underwear that works like skincare. It's a cosmetic treatment that promotes promotes healthy skin appearance, not a medical product, it doesn't diagnose, treat, or prevent disease.

No, it's already built in. HeiQ Skin Care™ is embedded directly into the fabric during production, so it's working from the very first wear. No creams, sprays, or steps required.

No change to handfeel, breathability, colour, or fit, it feels exactly like the Everyday you already love. Non-irritant, human skin patch tested, and all ingredients INCI-listed.

Up to 40 washes. Wash at 30°C on a gentle cycle. Data provided by HeiQ based on testing conducted on other materials.

Not at all. UNDERskin™ is an addition to your routine, not a replacement, use it alongside your existing products.