3D PRINTING AND NANOMANUFACTURING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 31

schematic of a typical 2PP fabrication system ( Zhang and Chen, 2011 ). The laser used in the system
is a Ti:sapphire femtosecond laser (100-fs pulses at a repetition rate of 80 MHz and wavelength of
800 nm). The laser beam is expanded and guided by a group of mirrors into an inverted microscope.
It is then focused by an oil-immersion objective lens onto the sample that is mounted on a motor-
ized stage with high resolution ( < 20 nm), which moves three-dimensionally to draw a defined 3D
nanostructure in the sample. The 3D structure is first designed in AutoCAD and then imported to
the software of the motorized stage, which controls the motion of the stage in xyz directions. A CCD
camera is used to monitor the fabrication process. The laser power can be continuously adjusted by
an attenuator. With this femtosecond laser fabrication system, Zhang et al . were able to fabricate
defined and complex 3D structures with a resolution of 100 nm ( Zhang and Chen, 2011 ).
2.2.7 CLASSIFICATION OF ADDITIVE BIOMANUFACTURING TECHNIQUES
Based on their working principles, the existing AM systems can mainly be categorized into three
groups: (1) powder-fusion-based techniques, such as SLS; (2) particle- or cell-deposition-based tech-
niques, such as LGDW, LIFT, and MAPLE DW; and (3) photo-polymerization-based techniques, such
as stereolithography and 2PP ( Duan and Wang, 2013 ).
These additive manufacturing techniques can also be categorized based on their process con-
figurations: top-down and bottom-up. Most forms of laser-assisted additive manufacturing, such as
SLS, LG DW, LIFT, MAPLE DW, and BioLP, are classified as bottom-up approaches. The accu-
mulation starts from the bottom of the platform and prepolymer materials are supplied for each lay-
er. This approach has the advantage of fabricating multiple layers with different materials in each
layer ( Mapili et al . , 2005; Nahmias et al . , 2005; Odde and Renn, 1999; Koch et al . , 2009, 2012;
Pirlo et al. , 2006; Ovsianikov et al . , 2010; Chan et al . , 2010 ). SLA employs both top-bottom and
bottom-up approaches, as shown in Figure 2.6 . The top-down configuration consists of a container
and a movable platform that is located in the container. The platform is immersed just below the
surface of a prepolymer solution. The laser beam is focused onto the surface ( x - y plane) of liquid
resin to polymerize the resin. Once a layer is photo-polymerized, the platform is lowered by a
specific distance to fabricate a new layer. In the bottom-up approach, the container is a movable
platform on which a polymerized resin layer is created. Liquid prepolymer is supplied into the
container for one layer from the bottom to the top. Table 2.1 lists the type of approach that each
technique belongs to.
2.3 BIOMATERIALS USED WITH ADDITIVE BIOMANUFACTURING
TECHNIQUES
Over the past two decades, various biocompatible or biodegradable materials, including polymeric
materials ( Puppi et al . , 2010; Nair and Laurencin, 2007 ), bioceramics ( Best et al . , 2008 ), and hydrogels
( Fedorovich et al . , 2007 ), have been employed and developed for use with laser-based additive bio-
manfacturing techniques in bioapplications, such as drug delivery, regenerative medicine, and tissue
engineering. In this section, we briefly review typical biomaterials that have been used with laser-based
additive biomanufacturing techniques. Table 2.2 provides a summary of the biomanufacturing tech-
niques listed with corresponding biomaterials used.