Biomedical Engineering Reference
In-Depth Information
It has recently been used to obtain multilayer structures by bonding very different
materials like polyelectrolytes, metal colloids, biological molecules, conductive
polymers and light emitting polymers (Decher et al.
1992
).
The process begins by taking a clean substrate with a negative surface charge.
This material is submerged in a solution with polymer molecules dissolved in it that
have functional groups bonded to a polymer chain with a net positive charge. These
molecules are attracted to the surface of the substrate, which is left coated with a
layer that is neutral as a whole, but with a positive charge on its upper surface. When
this cationic layer has been deposited, the external charge then incites the deposit of
another anionic layer, and in this way a multilayer structure can be produced.
By adding appropriate functional groups to the substrate or using surface pat-
terns, deposition can be encouraged in certain zones. By adjusting immersion times,
solute concentration and dissolution temperature, 3D structures can be obtained by
depositing material layers of controlled thickness that generally have more stable
properties than those obtained by Langmuir-Blodgett technology (Madou
2002
). It
can then be subjected to chemical attack to eliminate zones of unwanted deposits.
This technology together with the production of multilayer deposits with electro-
active polymers has succeeded in optimising the performance of light-emitting
diodes (LEDs), as well as enhancing the stability of luminescent organic pigments
compared to fi lms obtained by spin coating (Bar-Cohen
2004
). Medical applica-
tions based on active material substrates are also promising, as this process can also
be used to functionalise substrates for biosensors and for improving the biocompat-
ibility of active implantable devices (Saliterman
2006
).
In fact ionic self-assembled monolayered techniques somehow resemble some
processes carried out by Nature itself to produce its (bio)materials and (bio)struc-
tures. Self-assembly processes, by directing the deposition of molecules through
charge-based mechanisms are common. Further research in the fi eld and the use of
biomimetic design principles will surely lead to more and more precise additive man-
ufacturing machines for even constructing molecule by molecule or atom by atom.
Laser ablation
. Laser ablation consists in eliminating the surface material of a sub-
strate, usually solid, using a laser beam to produce evaporation and sublimation or
to convert the zone exposed to the beam into a plasma. The process is performed by
laser pulses (that last from milliseconds to femtoseconds), which means the elimi-
nation of material is so precisely focused that the rest of the substrate remains prac-
tically unaltered.
It is therefore an extremely suitable technology for changing the surface of mate-
rials that cannot be subjected to high temperature processes (generally polymers
and organic matter), as the heat-affected zone “HAZ” in conventionally extremely
reduced.
This process has also been used to produce carbon nanotubes and as a support for
PVD processes in which a laser acts on the substance to be deposited and the plasma
generated is projected on to the substrate to be coated (Phillips
2006
).
Ion implantation
. The process consists in coating a substrate with the ions of another
material, in order to change the physical properties of the substrate. This has
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