3D PRINTING AND NANOMANUFACTURING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 30

of microsized mirrors that can be controlled independently to on and off state ( Melchels et al. , 2010 ).
In this approach, the structure is not created via point-by-point scanning, but in a layer-wise fashion by
curing the entire layer simultaneously. After one layer is fabricated, the platform is lowered or raised
to cumulate a new layer. The thickness of each layer is controlled by the distance between the surface
of the platform and the liquid resin surface. Based on the basic concept just mentioned, Zhang et al .
developed a dynamic optical projection stereolithography (DOPsL) system for the rapid fabrication of
complex 3D extracellular scaffolds ( Zhang et al . , 2012 ). The setup of the DOPsL system is shown in
Figure 2.4 . Since this DMD-based stereolithography technology simultaneously utilizes a million mi-
cromirrors rather than one single focused point, the DOPsL system offers superior processing speed
compared to other nanofabrication techniques, thus making it more suitable for manufacturing large
structures with complex details with a submicron resolution ( Zhang et al . , 2012 ). With many advan-
tages, including rapid fabrication speed, maskless, flexibility, and relatively high resolution, the DOPsL
system is an appealing platform for the manufacture of complex 3D designer scaffolds for in vitro tis-
sue engineering as well as functional cellular constructs for in vivo implantation ( Zhang et al . , 2012 ).
A special laser directing technique is two-photon polymerization (2PP) ( Zhang and Chen, 2011;
Zhang et al . , 2013; Ovsianikov et al . , 2011a, 2011b; Gebinoga et al . , 2013 ), which has been used to
produce nanoscale features due to its high fidelity and resolution. 2PP systems usually utilize a fem-
tosecond laser to induce two-photon absorption (2PA). 2PA is a process by which one molecule is
excited to a higher energy electronic state by the simultaneous absorption of two photons ( Zhang and
Chen, 2011 ). 2PA was described theoretically in 1931 by Goeppert-Mayer ( Göppert-Mayer, 1931 )
and first demonstrated experimentally in 1961 by Kaiser and Garrett ( Kaiser and Garrett, 1961 ) in
a CaF2:Eu2+ crystal, and in 1962 by Abella in caesium vapor ( Abella, 1962 ). Figure 2.5 shows a
FIGURE 2.5
Schematic setup of a TPP fabrication system ( Zhang and Chen, 2011 ).