HEMA, TPO, and TPO-L

Everyone has heard of HEMA, HEMA-free, TPO-free, but few know how to clearly distinguish between HEMA, TPO, and TPO-L. To have a technical nail product that polymerizes under a UV LED lamp, we need 3 types of ingredients:
1) monomers (liquid substances with small molecules that adhere to the natural nail, helping the adhesion of underlying layers – for top coats);
2) photoinitiators (liquid or solid substances that, in the presence of UVA and blue light radiation from the lamp, break into pieces and bind to monomers, forming long chains of polymers, resembling a fishing net structure)
3) ingredients for structure and durability (oligomers and copolymers, more viscous substances or powders, which provide strength but also some flexibility; oligomers also have very good adhesion properties to the nail plate and other layers in the manicure). In addition to all the above classes of ingredients, there are pigments (for special effects and coloring), as well as preservatives, to keep the product intact and prevent contamination with microorganisms or yellowing/discoloration of the products.

HEMA (2-hydroxyethyl methacrylate) is a monomer with a small molecule and short chain, which allows it to easily (superficially) penetrate the prepared layer of the nail. In general, almost all ingredients found in nail products are oily ("greasy"), viscous, and do not penetrate the nail well. But HEMA, because it has a small molecule and this OH- group, like a "tail," can penetrate well into the first buffed/filed layers of the human nail, interact with the nail, and "bind" very well.

Also thanks to this "tail," HEMA is very liquid, transparent, flows easily, and combines very well with ... water. And when it accidentally gets on human skin (which has a lot of water and fats in it), HEMA can penetrate deeper into the skin and cause irritation and allergies. For this reason, the European Union has decided that HEMA can only be purchased and used by specialized personnel, from specialized stores, and carefully labeled, indicating the potential allergenic risk. But HEMA is not banned!!!. And when used correctly and in low concentrations in products, HEMA offers good adhesion and fluidity. The human nail does not have an immune system, so applying products with HEMA only to the nail does not inherently lead to the development of an allergy.

A major misunderstanding also arises between these 2 different substances, even though they are from the same family. Think of TPO as the "older cousin" and TPO-L as the "younger cousin." Although they belong to the same family of ingredients (photoinitiators, which break into 2 under UVA radiation and bind to other ingredients in products), they are still different.

TPO – in brief

Diphenyl(2,4,6 trimethylbenzoyl) phosphine oxide: a whitish-yellow, waxy powder used in gels to initiate polymerization. It requires both UVA (365 nm) and visible radiation (405 nm) to be fully activated and create 2 radicals. When TPO molecules break into 2 radicals, they become completely transparent, and that polymeric network is created – the construction itself or the base/top/color layer, etc. Due to the fact that it is also used in other industries (inks for packaging, industrial adhesives, etc.), where polymerization should occur almost instantly and lamps are not rigorously changed as in salons, these packages, when they reach the environment and come into contact with small living creatures, can impact their reproduction. Toxicological tests were conducted on small animals with enormous doses compared to those used in technical manicures. Here, it is used in small concentrations, so that polymerization proceeds controlled for 1-2 minutes, without releasing too much heat.

TPO-L

Ethyl(2,4,6 trimethylbenzoyl) phosphinate – compared to TPO, TPO-L is liquid, incorporates much better into products, is more biodegradable, and has lower potential sensitization risks than TPO. TPO-L retains the benefits of TPO (fast polymerization, deep efficiency, no excessive yellowing). Studies to date show that TPO-L is not carcinogenic, does not affect reproduction, and has a much better safety profile for use in cosmetic products. Therefore, TPO-L is not banned by EU regulations – it is permitted and considered a safe ingredient within the usual concentration limits in nail products (max 5%, usually 1-2%, a low percentage compared to other industries).

Mimona products contain TPO-L and have been reformulated to comply with European standards long ago, to familiarize you with these changes.

• https://cosmileeurope.eu/inci/detail/6327/hema/
• https://health.ec.europa.eu/publications/safety-cosmetic-ingredients-hema-and-di-hema-trimethylhexyl-dicarbamate-submission-i_en
• Angew. Chem. Int. Ed. 2025, e202425598 , doi.org/10.1002/anie.202425598
• Safety Assessment of Trimethylbenzoyl Diphenylphosphine Oxide as Used in Cosmetics , Cosmetic Ingredient Review , march 2025 : https://www.cir-safety.org/sites/default/files/Trimethylbenzoyl_Diphenylphosohine_Oxide.pdf
• Cytotoxicity, Colour Stability and Dimensional Accuracy of 3D Printing Resin with Three Different Photoinitiators Gi-Tae Kim , Hye-Bin Go  , Jae-Hun Yu , Song-Yi Yang  , Kwang-Mahn Kim  , Sung-Hwan Choi , and Jae-Sung Kwon
• Impact of light intensity on the polymerization kinetics and network structure of model hydrophobic and hydrophilic methacrylate based dental adhesive resin , Farhana Abedin, Qiang Ye Kyle Camarda, Paulette Spencer
• https://www.industrialchemicals.gov.au/sites/default/files/Phosphine%20oxide%2C%20diphenyl%282%2C4%2C6-trimethylbenzoyl%29-_Human%20health%20tier%20II%20assessment.pdf
• UV-cured polymer aided phase change thermal energy storage: Preparation, mechanism and prospects, Tong Xiao, Long Geng, Yucheng Dai, Jiateng Zha, Changhui Liu

Photoinitiators : Structures, Reactivity and Applications in Polymerization, Volume 1 ,Jean-Pierre Fouassier, Jacques Lalevée, ISBN-10 3527346090 , cap 3, pag : 68 - 76