Photocrosslinkable Materials for the Fabrication of Tissue-Engineered Constructs by Stereolithography - Tissue Engineering: Computer Modeling, Biofabrication and Cell Behavior - page 149

Fig. 2 Jablonski energy diagram

reaction proceeds, the material becomes more cross-linked, its molecular weight
increases and a glassy solid material is formed [ 13 ]. The curing reaction is highly
dependent on the light intensity, temperature, irradiation time, and photo-initiator
concentration [ 13 ]. Either a single-photon polymerization or a two-photon polymer-
ization (2PP) can be used to induce the curing reaction. The chemical principle of
these two processes is similar, though they differ in the number of absorbed photons
required to induce the polymerization process [ 13 , 82 ]. When a molecule absorbs
light, the electrons are set into motion by the oscillating electric field, which is pro-
moted from the highest occupied molecular orbital to an unoccupied molecular or-
bital with the formation of an excited singlet state molecule. However, this excited
molecule is a short living species (less than 10 − 8 s), which disappears by various
competitive processes dissipating the excited energy. The absorption of light by a
molecule and the subsequent evolution of its excited states can be observed in Fig. 2
through the Jablonski energy diagram.
Two processes can be identified, the photophysical and the photochemical ones,
as follows:

• Photophysical processes:

- Radiative
- Non-radiative

• Photochemical processes.

Radiative mechanisms involve both the absorption of a photon or more by a
molecule in its ground state ( S 0 ) and the emission of energy from an electroni-
cally excited state, by either fluorescence (de-excitation of an excited state with the
same spin multiplicity as the ground state) or phosphorescence (de-excitation of