The nail plate is hydroxyapatite — a calcium-containing inorganic surface with high oil content and low surface energy. Ordinary acrylates rely on physical attachment and lift at the edges within a week.
HEMA worked because it is small, penetrates quickly and forms hydrogen bonds, but it is a recognized contact sensitizer. The SCCS has documented its sensitization risk, and HEMA-free has become the industry direction.
The difficulty is that removing HEMA costs both adhesion and flexibility, so formulations either do not bond or become brittle.
Phosphate groups coordinate with Ca²⁺ on the hydroxyapatite surface — far stronger than hydrogen bonding and independent of small-molecule penetration.
The low acid value of KM2140 barely disturbs formulation pH and resists reaction with basic pigments or amines, while its flexible segment buffers the stress of nail flexing.