Biodegradable exfoliating microbeads

Plastic exfoliating beads were banned rather than phased out, so the replacements were designed against a legal deadline. Understanding what was lost explains why the substitutes differ from each other so much.

What makes a particle exfoliate

Mechanical exfoliation removes corneocytes from the outermost stratum corneum by friction. Four particle properties set the result.

Hardness relative to the target. The particle must be harder than the corneocyte layer it is meant to lift and no harder than it needs to be. Too soft and it deforms and does nothing; too hard and it abrades beyond the dead layer.

Shape. This is the property that mattered most and is discussed least. A sphere contacts the skin at a point and rolls; an angular fragment presents edges that cut. Polyethylene beads were smooth spheres, which is precisely why they were mild — and why an angular replacement of the same size and hardness is a harsher product. Ground shells and stone fruit kernels are angular by nature and have been implicated in micro-tearing.

Size, conventionally in the 1–1000 µm range, sets how the particle sits against skin topography and how it feels.

Deformability. A particle that yields under pressure self-limits its own abrasion, which is a safety property rather than a performance one.

Why biodegradability is a property of the backbone, not the origin

“Natural” and “biodegradable” are not synonyms, and this category contains the clearest example of the gap.

Biodegradation requires that environmental microorganisms possess enzymes able to attack the polymer’s backbone, and that the backbone be physically accessible to them. Cellulose qualifies: its β-1,4-glucan chain is the substrate of cellulases, which are ubiquitous.

Cellulose acetate is cellulose whose hydroxyl groups have been acetylated. The degree of substitution — how many of the three hydroxyls per glucose unit carry acetyl groups — governs what happens next. At low substitution, esterases remove the acetyl groups and cellulases then reach the chain. At high substitution the acetyl groups sterically block cellulase binding, and degradation slows sharply. A cellulose-derived particle is therefore not automatically degradable, and the relevant number is the degree of substitution, not the feedstock.

Ready biodegradability is defined by test, not by assertion. The OECD 301 series measures mineralisation — typically CO₂ evolution or oxygen demand — against a pass threshold within 28 days, and separate marine protocols exist because seawater is colder, more dilute in nutrients and differently populated than an activated-sludge inoculum. A material passing a freshwater test has not been shown to degrade in the ocean, which is where the regulatory concern originated.

The three main replacement chemistries

Microcrystalline cellulose is cellulose hydrolysed to remove amorphous regions, leaving crystalline aggregates that can be shaped into near-spherical particles — the closest structural analogue to the PE bead.

Jojoba beads are made from jojoba oil, which is chemically not a triglyceride but a liquid wax ester of long-chain fatty acids and alcohols; hydrogenation raises its melting point to give a solid, soft, spherical particle that softens at skin temperature.

Konjac glucomannan forms a soft, water-swollen sponge rather than a hard particle, exfoliating by surface texture at very low abrasive load.

The regulations that drove all three are specific: the US Microbead-Free Waters Act of 2015 for rinse-off cosmetics, and EU REACH restriction (EU) 2023/2055 on intentionally added microplastics.