Biomedical Engineering Reference
In-Depth Information
polysaccharides [19]. One of the main advantages of laser sintering is the ability to incorporate par-
ticles such as hydroxyapatite into the scaffold with minimum effort where the particles are simply
added to the polymer, which fuses around them [6].
In laser sintering, the resolution of the system depends on laser spot size, thermal conductivity
and absorption of the polymer, and the grain size. A commercial device, Sinter Station 2500 (DTM,
USA), has a laser beam with a 400 µm spot. Heat conduction inevitably makes the precision of the
system rather larger than the spot size. At present, new systems with better resolutions are being
developed. Much effort is also being dedicated to resolve the problem of the presence of excess
powder by using ultrasonic vibrations, compressed air, and particular solvents.
4.6.4 P
HOTOPOLYMERIZATION
Photopolymerization is a method based on the polymerization of photopolymeric resins in a site-
specifi c manner. The photopolymeric resins are mixtures of simple monomers with low molecular
weight that form a long chain polymer when they are activated by light. An initiator or a catalyst is
often required for polymerization to occur (these are toxic to cells) and the light source must have a
high frequency (usually ultraviolet).
Site-specifi c polymerization takes place either by scanning a laser beam over the surface of the
liquid prepolymer—a process known as stereolithography—or by irradiating a mask placed above
the surface. In both methods, irradiated zones become solid whereas the other areas remain liquid.
Once the layer has polymerized, a fresh layer of prepolymer is applied. In theory, the laser spot
could also be focused at different heights, but the depth of action is limited by the absorption of the
liquid. The second technique requires the fabrication of a different mask for each layer and cannot
strictly be defi ned as RP. Using the data in Ref. 20, an RTM ratio of about 0.5 can be estimated,
which places this method among the more effi cient techniques.
The main limitation of RP by photopolymerization is the use of acrylic or epoxy photopoly-
meric materials, which are often not biocompatible. Currently attempts are being made to extend
this methodology to bioerodable polymers and biological polymers. For example, Dhariwala et al.
[20] have used a commercial stereolithography machine to polymerize poly(ethylene oxide) and
poly(ethylene glycol) dimethacrylate hydrogel using a photoinitiator. In their setup, a 250 µm ultra-
violet light spot was rastered over a layer of prepolymer containing living cells, as schematized in
Figure 4.9. The results demonstrated that the cells were very sensitive to the initiator and remained
viable only at very low initiator concentrations.
4.7 OTHER RP METHODS
Two further techniques that are worth mentioning are the sacrifi cial mold method and fi ber spin-
ning. The former uses rapid-prototyped molds to cast melts, solutions, or even powders, whereas
the latter is not strictly RP at present but can be used to build nanoscale features on microfabricated
scaffolds.
4.7.1 S
ACRIFICIAL
M
OLDS
A limited number of techniques using sacrifi cial molds have been reported, and this is likely due
to the complexity of the fabrication process. Firstly, a plastic mold of the microstructure to be real-
ized is constructed through one of the RP methods described above, usually ink-jet printing or
3-DP. Secondly, a deposition head then fi lls up the empty spaces present in the microstructure with
a selected biopolymer thereby forming the scaffold. The polymer mold that simply constructs the
support for the 3-D structure is then removed or sacrifi ced by bathing it in an appropriate solvent. In
theory, using more heads, complex and multipolymeric structures can be rapidly realized.
Sachlos et al. [3] reported a two-phase ink-jet-based method to prepare molds of Protobuild
(phase 1) and Protosupport (phase 2). The phase 2 material was dissolved away leaving a 3-D
