Natural Deep Eutectic Solvents in Skincare: The Green Solvent Rewriting Active Delivery — 2026 Formulation Science

Natural Deep Eutectic Solvents in Skincare: The Green Solvent Rewriting Active Delivery — 2026 Formulation Science

Every few years a solvent quietly changes what formulations can do. Glycols, ethanol, and petroleum-derived penetration enhancers have carried active skincare for decades, but they arrive with baggage: irritation, volatility, poor sustainability optics, and — for many actives — poor solubility. In 2026 the formulation conversation has shifted to a different class of liquid entirely: natural deep eutectic solvents (NADES). These are not exotic synthetics. They are blends of ordinary plant metabolites — organic acids, sugars, amino acids, polyols — that, when mixed at the right molar ratio, liquefy at room temperature through hydrogen bonding. The result is a tunable solvent that can dissolve stubborn actives, protect them from degradation, and improve skin permeation. This analysis breaks down the mechanism, the 2026 evidence, and the practical constraints formulators need to know before reaching for the hype.

What a Deep Eutectic Solvent Actually Is

A deep eutectic solvent forms when two or more solid components — a hydrogen bond acceptor and a hydrogen bond donor — interact so strongly that the mixture’s melting point drops far below that of either ingredient. The classic example is choline chloride plus urea, a combination liquid at room temperature despite both components being solids. When the components are primary metabolites (sugars, organic acids, amino acids, choline, betaine), the term becomes natural deep eutectic solvent, or NADES.

The key property is tunability. By swapping the donor and acceptor — lactic acid with glycerol, citric acid with sorbitol, betaine with malic acid — a formulator can dial polarity, viscosity, pH, and solubilizing power. That is precisely what makes NADES interesting for actives that are notoriously hard to handle in water.

Why Formulators Are Paying Attention

The push comes from three directions at once. First, clean-beauty reformulation pressure is squeezing out conventional synthetic glycols and petroleum-derived enhancers. Second, NADES can be compounded by simple cold mixing — no high-shear homogenization, no organic solvent recovery — which reduces processing energy and time. A 2026 study in Processes (Werner et al., DOI 10.3390/pr14172850) demonstrated that engineered NADES based on 1,3-propanediol and glycerin paired with citric, succinic, malic, and lactic acids at a 6:1 molar ratio could be incorporated into aqueous serums and O/W emulsions via seamless single-pot cold processing. The same work reported up to 89% DPPH radical scavenging and microbiological purity compliant with ISO 17516:2014.

Third — and most interesting scientifically — NADES do more than dissolve. They interact with the skin barrier itself.

The Penetration Mechanism: Lipid Lamellae, Not Corneocytes

The most important mechanistic finding comes from X-ray diffraction work by Sakuragi and colleagues (Sojo University), published in ChemistryOpen (2020, DOI 10.1002/open.202000114). Using wide- and small-angle X-ray scattering, the team showed that a choline chloride–glycerol deep eutectic solvent penetrated the stratum corneum through the intercellular lipid pathway, not through the corneocytes. Specifically, the solvent extracted a portion of the lipids forming the short lamellar structures of the barrier. A hydrated DES at a 9:1 solvent-to-water weight ratio produced the strongest lipid interaction and, in a fluorescent lysozyme penetration test, doubled penetration through the stratum corneum versus HEPES buffer.

That mechanism matters because it is the same route exploited by conventional chemical enhancers — but achieved with food-grade metabolites rather than aggressive synthetics. In transdermal research, choline chloride–glycerol and choline chloride–ethylene glycol DES outperformed choline chloride–urea in delivering insulin across rat skin (Khamoushian et al., Journal of Pharmaceutical Sciences, 2023, DOI 10.1016/j.xphs.2023.03.005), with cumulative permeation of 89.4 and 131.0 µg/cm² respectively at 24 hours. The authors attributed the differences to how effectively each solvent treated the epidermal layer.

Stabilizing the Actives That Water Destroys

The formulation payoff is largest for oxidation- and hydrolysis-sensitive actives. L-ascorbic acid is the textbook case: it degrades in water within days to weeks. A landmark study in Journal of Molecular Liquids (Gomez-Urios et al., 2024, DOI 10.1016/j.molliq.2024.125864) monitored ascorbic acid degradation across fourteen NADES systems over 30 days at 4 °C and 25 °C. The findings were nuanced but actionable: choline chloride: xylose gave the longest half-life, while organic-acid-based NADES such as betaine:malic acid and choline chloride:lactic acid stabilized ascorbic acid worse than a water control. In other words, not all NADES protect equally — polarity and pH drive the outcome.

The betaine route has proven especially practical for vitamin C. A therapeutic deep eutectic system of ascorbic acid and betaine, reported in Pharmaceutics (2024, PMC11124945), dissolved ascorbic acid at approximately 40% in various polyols and significantly enhanced delivery through porcine skin compared with free ascorbic acid. In a human in vivo study, a serum built on this system reduced aging markers and evened skin tone. An earlier patent (EP4011353A1) described the same concept — ascorbic acid, betaine, and a third component such as 1,3-propanediol — showing that vitamin C remained stable in closed containers over months, where an equivalent aqueous solution discolored.

NADES also serve as extraction media that concentrate actives while protecting them. A NADES-in-oil emulsion study from the University of Minho (2024, DOI 10.1016/j.molliq.2024.126413) reported tyrosinase inhibition with an IC50 of 0.18 mg/mL for a eucalyptus extract carried in a lactic-acid/glycerol NADES, alongside measurable transdermal permeation (about 15%) and no cytotoxicity up to 5% in skin cells.

The Constraints No One Should Ignore

NADES are not a free upgrade, and the 2026 literature is refreshingly candid about the limits.

Viscosity. Many eutectic mixtures are thick and tacky, which harms spreadability. The 2026 Processes work solved this with 1,3-propanediol-based systems that eliminated the characteristic polyol stickiness in sensory testing — but this must be engineered, not assumed.

Regulatory status of components. Choline chloride, the most studied hydrogen bond acceptor, is prohibited in EU cosmetics under Annex II entry 168 of Regulation 1223/2009. Betaine is the clean alternative — permitted in the EU and food-grade. Formulators targeting the EU or ASEAN should screen every NADES component against the target market’s positive and negative lists before development.

Emulsion stability. The same 2026 study found that while liquid serums stayed stable, O/W emulsions were highly dependent on the specific acid structure of the NADES. Acid choice is not a detail; it is a stability variable.

pH drift. Organic-acid NADES lower formulation pH, which can protect some actives but destabilize others and shift preservative efficacy. Buffering and preservative challenge testing are non-negotiable.

The Formulator’s Takeaway

NADES represent a genuine step change in how brightening and antioxidant actives can be delivered — better solubility, better stability for oxidation-prone molecules like vitamin C, and a lipid-pathway penetration mechanism achieved with benign metabolites. But the evidence also shows the outcome is component-specific. The winning approach in 2026 is not “add a NADES” — it is to select the donor–acceptor pair against the active’s polarity and pH tolerance, then validate viscosity, emulsion stability, and preservative efficacy before scale-up. Solvent selection has become a formulation strategy in its own right.