Water-Soluble Photoinitiators

Introduction

Photoinitiators are compounds added to a formulation to convert absorbed light energy, ultraviolet or visible light, into chemical energy in the form of initiating species, typically free radicals. Traditionally these molecules are engineered for non-polar environments with hydrophobic moieties rendering them water-insoluble. The growth of hydrophilic polymerization conditions, especially in hydrogel systems has generated a need for a new class of water-soluble photoinitiators.

Merck KGaA, Darmstadt, Germany is proud to introduce lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP) and water-dispersible photoinitiator nanoparticles of diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) as cutting edge products that allow aqueous photopolymerization. This enables the development of novel formulations for 3D bioprinting, tissue engineering applications, and device manufacturing.

Lithium phenyl-2,4,6-trimethylbenzoylphosphinate (LAP)

Background

This compound was first reported by Majima et al. in 1991¹. Further work by Benedikt et al. in 2015² explored optimal usage for this photoinitiator and compared the performance to alternative structures. This compound stands out as a leading option for many applications due to the high reactivity and the ability to initiate under both ultraviolet and violet light sources.

Properties

Water solubility: 4.7 wt%
λ_max: 380 nm (UV and 350-410 nm violet light curing)
High reactivity formulations
Biocompatible
Recommended addition level: 0.1 – 0.8 wt%
Avoid contact with alkaline additives

Water-soluble TPO based nanoparticle photoinitiator

Background Type I photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) is highly efficient but water-insoluble. Water-dispersible TPO nanoparticles have been developed by Professor Shlomo Magdassi group at the Hebrew University of Jerusalem. This water-soluble version of IPO enables the rapid 3D printing of hydrogels in the aqueous solution, which is desired in tissue engineering applications³. The TPO nanoparticles absorb significantly in the range from 385 to 420 nm, making them suitable for use in commercially available, low-cost, light-emitting diode – based 3D printers and UV-curing devices, for any UV-curing aqueous formulation.

Properties

Water solubility: 3.0 wt% TPO nanoparticles
λ_max :366-381 nm (UV and 385-420 nm violet light-curing)
High reactivity formulations
Biocompatible

Recommended addition level: above 2 wt%, further concentration optimization is required. (Example for 3D printing bioink formulation: Weigh 20 g of Acrylamide and 2 g of PEGylated diacrylate 600 (SR610) together. Add 80 g aqueous dispersion of the photoinitiator nanoparticles, (78 gram water + 2 gram TPO-NP). Stir the dispersion to obtain a clear ink.)