A molecule that upon absorption of light at a specific wavelength produces reactive species capable of catalyzing polymerization or polycondensation reactions. These reactions area also called photopolymerization or radiation curing.
There are two types: cationic and radical photoinitiators. Cationic photoinitiators act upon molecules containing epoxies or molecules which lend themselves to polycondensation reactions. This type of photoinitiator is not compatible with biological systems since its reactivity is inhibited by water.
Radical photoinitiators are water-compatible and act on molecules containing an acrylate or styrene group. The range of wavelengths used is typically near UV (300-400nm) but recent progress in initiator chemistry is expanding the ranges of wavelengths that can be used. Commonly used photoinitiators in biology such as Irgacure 2959, 184, and 651 fall into this class.
- Photoinitiators are typically ketones which contain both aromatic and carbonyl groups. Arguably the most popular photoinitiator for tissue engineering applications is Irgacure 2959 (4-(2-hydroxyethoxy) phenyl-(2-propyl) ketone due to its high solubility in water and its minimal toxicity compared to other Irgacure species.
- Mode of action: Upon absorption of UV light, Irgacure 2959 dissociates into 2 primary radicals which then react with the vinyl (C=C) groups of PEGDA to initiate polymerization. There are three phases in photopolymerization: photoinitiation, propagation, and termination. Rate of reaction during the first step is dependent on the nature of the photoinitiator (quantum yield, photoinitiator efficiency) and intensity of light while the later steps (propagation and termination) are a function of vinyl bond concentration and the rate constants for propagation and termination.
- CIBA website: http://www.ciba.com/photoinitiator.htm
- Ma PX and Elisseeff J editors, Scaffolds in Tissue Engineering, CRC Press Boca Raton, FL 2006.
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