Polylysine: INCI functions, identifiers (CAS/EC), cosmetic uses, and formulation notes

Polylysine – polymer/chain of L-lysine (technical note: in technical/food contexts you may also encounter ε-poly-L-lysine, often as the hydrochloride salt)

Synonyms: polylysine, poly-L-lysine, lysine homopolymer, ε-polylysine (technical use), ε-poly-L-lysine hydrochloride (salt form)
INCI / Functions: hair conditioning (hair-conditioning function as reported in EU cosmetic databases)

Definition

Polylysine is a cationic polypeptide made of repeating L-lysine units; in practical terms, it is an amino-acid “chain” that, in aqueous solution, carries a pronounced positive charge due to its side-chain amine groups. This property explains most of its formulation behavior: it tends to interact electrostatically with negatively charged surfaces and materials (hair keratin, certain dispersed particles, and several anionic polymers) and can contribute to improved combability and sensoriality, while also influencing system stability and haze/turbidity.

It is useful to distinguish two overlapping “families”/use areas that can coexist in practice:

• Polylysine (INCI): cosmetic use mainly as a conditioning agent (especially in hair care).

• ε-poly-L-lysine (often supplied also as the hydrochloride): technical use as an antimicrobial agent in various contexts (well known outside the EU in food applications); in cosmetics it may also appear as a functional component in some protective systems or as support to microbiological robustness strategies, but the official INCI function may not be “preservative” depending on the ingredient’s classification/registration and the market.

Main uses

Food. In several non-EU markets, ε-polylysine is used as an antimicrobial at very low levels (ppm range) to support microbiological stability of specific food matrices. From a technological standpoint, its value lies in activity across a broad microbial spectrum and the possibility of working across relatively wide pH ranges. In the European Union, food labeling logic is linked to the E-number system under Regulation (EC) 1333/2008: this is a critical point because the global market sometimes shows “E-xxx” claims that do not align with the EU framework and can therefore be misleading if used in EU labeling.

Cosmetics. In cosmetic formulation, polylysine is primarily associated with hair care products (rinse-off and, in some cases, leave-on) as a conditioning agent: it can help reduce post-wash roughness, improve combability, reduce static, and support a more “compact” fiber feel. Its cationic nature makes it conceptually aligned with conditioning technologies: it is attracted to keratin and can support the deposition/anchoring of other actives and polymers. In industrial practice, polylysine may also be used where a functional contribution to microbiological robustness is sought (often in synergy with polyols or other components), but real efficacy must be interpreted at the full-system level and verified via challenge testing on the finished product.

Medicine. In biomedical and research contexts, polylysine polymers (especially poly-L-lysine) are known as cationic materials useful for interactions with surfaces and biomaterials; they are widely referenced in laboratory/research settings (e.g., supports, coatings, biointerface applications). In an “ingredient for cosmetics” profile, this area is more technical background than a direct use indication.

Pharmaceutical. It may appear as a polymeric auxiliary in technical or development settings, but any real pharmaceutical use is dossier-dependent (specifications, purity, grade, impurities, compliance). For true pharma use, critical aspects include quality, control of endotoxins/bioburden (where relevant), and lot-to-lot consistency.

Industrial use. Beyond cosmetics and (in some markets) food, polylysine is a cationic polymer of interest in technical applications requiring electrostatic interactions or an antimicrobial contribution in specific systems. Here too, the determining factor is compatibility with the matrix (pH, salinity, surfactants, polymers) and the regulatory profile of the sector/country.

Calories (energy value)

Identification data and specifications

Chemical-physical properties (indicative)

Functional role and practical mechanism

Formulation compatibility

Because polylysine is cationic, it should be treated as a “charge-active” ingredient: it is generally easier to incorporate into systems where the anionic component is controlled or balanced. Compatibility is often good with many nonionic bases and a range of emulsifying/surfactant architectures, but it can become critical in the presence of: (i) anionic polymers (e.g., non-neutralized carbomer or certain anionic acrylates), (ii) high levels of anionic surfactants, and (iii) high ionic strength (salt), which can shift solubility/association equilibria. The typical risks are haze, erratic viscosity, or precipitation due to anionic–cationic complexation.

In hair care, where deposition onto the fiber is often desired, the practical goal is to reach a balance: enough interaction to provide conditioning, but not so much as to cause build-up, heaviness, or physical instability. The real working window depends strongly on pH, surfactant ratio, presence of silicones/film formers, and the addition sequence (pre-dissolution in water, cold vs hot addition, hydration time).

Use guidelines (indicative)

Quality, grades, and specifications

Safety, regulatory, and environment

From a cosmetic safety standpoint, polylysine is generally considered manageable at typical use levels when incorporated into finished products that have been properly assessed. The most frequent criticalities are technical rather than toxicological: charge incompatibilities, haze, and physical instability can drive performance variability and, indirectly, reformulation decisions. As always, assessment must be carried out on the finished product (real exposure, target population, application area, potential eye contact).

From a regulatory perspective, in EU cosmetics the practical framing is via INCI naming and the associated functions in sector databases. In EU food, labeling is tied to the list of authorized additives under Regulation (EC) 1333/2008 with the E-number system: it is essential not to confuse numbers or naming used in other markets with those valid in Europe (for example, in the EU framework “E 239” corresponds to a different substance). In manufacturing and QC, adoption of GMP (Good manufacturing practice; benefit: improves consistency and reduces variability/contamination risk) and, where relevant, a HACCP approach (Hazard analysis and critical control points; benefit: strengthens preventive control at critical process points) helps maintain consistency and reduce contamination or variability risks.

Formulation troubleshooting

Conclusion

Polylysine (Polylysine) is an L-lysine polymer with a positive charge that, in cosmetics, is positioned primarily as a hair conditioning ingredient supporting sensoriality and manageability. The most relevant technical point is charge compatibility: the same property that drives deposition can also cause complexation and instability if the system contains anionics under unfavorable conditions. Effective use therefore requires a balanced formulation approach (pH, ionic strength, surfactant/polymer architecture) and practical validation on the finished product.

Mini-glossary

INCI: standard nomenclature for cosmetic ingredient labeling.
Cationic polymer: polymer that in water carries a positive charge and interacts with negatively charged species.
Anionic–cationic complex: electrostatic association between oppositely charged species, which can cause haze or precipitation.
E-number: EU code identifying food additives authorized in the European Union.
GMP: Good manufacturing practice; supports quality and process control, reducing variability and contamination risk.
HACCP: Hazard analysis and critical control points; preventive risk analysis and control of critical points (relevant as a control approach in regulated contexts).

References__________________________________________________________________________

Hua T, Wan R, Chai C, Li R, Wang S, Tang Y, Zhang T, Wu H. Polylysine Derivatives with a Potent Antibacterial Ability for Effectively Treating Methicillin-Resistant Staphylococcus aureus-Induced Endophthalmitis. ACS Biomater Sci Eng. 2025 Jun 9;11(6):3752-3761. doi: 10.1021/acsbiomaterials.5c00422. 

Abstract. Bacterial endophthalmitis (BE) is a severe ocular infection that can lead to irreversible blinding ocular disease. When diagnosed with BE, the main treatment approach is empirically administering intravitreal antibiotic injections. However, the excessive use of antibiotics leads to increased drug resistance in pathogens, and the retinal dose-limiting toxicities greatly limit its application in clinic. In this work, we present a series of polylysine derivatives (PLL-n) for the treatment of bacterial endophthalmitis. By precisely adjusting the balance of hydrophilic/hydrophobic, the optimal polymer, PLL-2, demonstrates high efficacy against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and various clinically isolated drug-resistant bacteria. The antibacterial mechanism showed that PLL-2 could effectively destroy the bacterial membrane and lead to bacterial death. Due to its unique antibacterial mechanism, PLL-2 exhibits rapid bactericidal kinetics and does not induce bacterial resistance up to 16 generations. More importantly, PLL-2 showed a significant therapeutic effect on a methicillin-resistant S. aureus-induced rat endophthalmitis model, which presents a promising therapeutic approach for managing endophthalmitis.

Takano S, Hu Q, Amamoto T, Refinetti P, Mimori K, Funatsu T, Kato M. Extraction of cell-free DNA from urine, using polylysine-coated silica particles. Anal Bioanal Chem. 2017 Jun;409(16):4021-4025. doi: 10.1007/s00216-017-0345-3. 

Abstract. DNA analysis is used for a variety of purposes, including disease diagnosis and DNA profiling; this involves extracting DNA from living organisms. In this study, we prepared polycationic silica particles to extract DNA that has the negatively charged phosphate backbone from solution. The coated particles were prepared by mixing conventional silica gel particles and poly-Lys; these particles could efficiently extract 1.3 μg of cell-free DNA from 50 mL of (male) urine. It is expected that these easily prepared particles (just a mixture of two commercially available chemicals) can be used as a noninvasive diagnostic tool for genetic disorders such as cancer, diabetes, and hypertension. Graphical abstract Effective extraction method of cfDNA from urine was developed that used commercially available silica gel particles and poly-Lys.

Kim SW. Polylysine copolymers for gene delivery. Cold Spring Harb Protoc. 2012 Apr 1;2012(4):433-8. doi: 10.1101/pdb.ip068619. 

Abstract. Polylysine and its copolymers have been extensively used as nonviral polymeric gene carriers. Although polylysine on its own is toxic to cells, when polyethylene glycol is covalently linked to polylysine, toxicity is reduced and DNA transfection efficiency is increased. A degradable polylysine analog, polyaminobutyl glycolic acid, has been synthesized. Stearyl polylysine shows strong hydrophobic interactions with low-density lipoprotein and these components can be combined with DNA to form a "terplex" system that allows delivery of DNA to targeted cells and significant levels of transfection both in vitro and in vivo.