Biomedical Engineering Reference
In-Depth Information
subject the structure to larger mechanical forces but on the other hand the vat
content can be minimized, the irradiated surface will not be exposed to the atmo-
sphere (crosslinking efficiency limitation due to oxygen will be minimized), recoat-
ing the structure with a new resin layer is not required and the illuminated area is
always smooth [
131
]. Another recent and more fundamental trend in the field of
SLA is the emerging use of digital light projection (DLP) technology [
133,
139-
141
]. The working principle is illustrated in Fig.
9.3
(right) in the top-down scheme,
but is also applicable for bottom-up set-ups. Projection technology enables the cur-
ing of a complete layer of resin in one go, which obviously reflects on the building
time. A Digital Micro-mirror Device™ (DMD) consists of an array of mirrors,
which can independently be tilted in an on/off state. In this way the DMD serves as
a dynamic mask that projects a 2D pattern (often designed in PowerPoint slides) on
the surface. Instead of DMD, LCD displays have also been employed as a dynamic
mask projector [
142,
143
]; however, DMD offers better performance in terms of
optical fill factor and light transmission [
139
] .
Solid ground curing (SGC).
A projection technology somewhat similar to DMD
technology is SGC developed by Cubital Inc. [
144,
145
] . Coating the fabrication
platform by spraying a photosensitive resin is the first step in the SGC workflow.
Meanwhile, the machine prints a photomask of the layer to be built on a glass plate
above the fabrication platform. The printing process resembles the one applied in
commercial laser printers. Solidification of the sprayed layer occurs when the mask
is exposed to UV light, only permitting irradiation of the transparent regions. After
the layer is completed, excess liquid resin is removed by vacuum and replaced by a
wax to support the next layer. Before repeating the cycle, the layer is milled flat to
an accurate, reliable finish for the next layer.
Two-photon polymerization (2PP).
2PP is an emerging state-of-the-art laser-based
technique. In this process, light is used to trigger a chemical reaction leading to
polymerization of a photosensitive material. Unlike other light curing systems
(single photon polymerization), 2PP initiates the polymerization through radia-
tion with near-infrared femtosecond laser pulses of 800 nm (Fig.
9.4
). In the focal
point, a suitable photoinitiator absorbs two photons, with a wavelength of 800 nm,
Near-IR light
Crosslinked resin
Fig. 9.4
Working principle
of two-photon
photopolymerization. In the
focal point of the near-IR
laser beams, the
photosensitive polymer is
crosslinked. A “true” 3D
object is obtained
Liquid photocurable resin
Near-IR light
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