Chemistry Reference
In-Depth Information
pressure-driven flow, demonstrating that the Navier Stokes approach is still available while local shear rate at
the interface is lower than the critical shear rate [55].
18.3.2
Micro- and nanofabrication techniques
18.3.2.1 Chip fabrications
Several chip fabrication methodologies that allow the obtaining of different geometries according to the
micro- and nanofluidics have been reported. A brief description of the most important technologies is given
in the next sections.
Photolithography
is an optical methodology that allows transferring of patterns onto certain substrates.
It is essentially the same process used in lithographic printing. This technique consists in transferring a
landlord from a photomask to the surface of a substrate. Three-dimensional structures fabricated using this
process have potential applications in MEMS sensors/actuators, optical devices and microfluidics [56].
Soft Lithography
is a manufacturing technique using elastomers, moulds and conformable photomasks.
'Soft' is called because elastomeric materials are used, especially poly(dimethilsiloxane) (PDMS) and
recently poly[(3-mercaptopropyl)-methylsiloxane] (PMMS), demonstrating the successful pattern replication
from the micrometre to sub-100 nm scale [57]. Generally, it is used to build devices at micro- and nanoscale.
The process includes technology of impression by microcontact, moulding by replica, moulding by micro
transference and micromoulding in capillaries [58].
Micro manufacturing
is based on the use of a cutting tool to obtain predefined material geometries [59].
Although the sizes obtained by this technique are not as small as those obtained using lithography, it allows
the fabrication of platforms for the integration with smaller devices.
Ink-printing technology
was initially used for computer science and decorative objects. Now it is widely used
to print on different surfaces, such as aluminium, glass, plastic and paper. Nowadays, special inks like graphite
inks, silver, carbon nanotubes inks and polymer inks, among others are used. These allow printing geometries
with certain thicknesses and roughness to create three-dimensional structures, without using masks [60].
Polymerization based prototyping
is a layer-by-layer fabrication technique through a computer aided design
(CAD). Resin is deposited in crud and then using laser traces or an agglutinant cartridge it is polymerized and
solidified onto the surface. It allows the construction of 3D structures and assembled pieces [61].
Other processes used to fabricate microfluidic systems are: X-ray lithography [62], moulding by high
precision, hot embossing [63], micro-injection moulding
64
, and roll-to-roll embossing [65]. This technology
allows building three-dimensional complex structures with a great variety of materials.
18.3.2.2
Electrode/detector fabrication methods
Screen-printed technology
is based on the sequential transfer of different conductive or non-conductive inks
on a variety of inert substrates through a mask with the desired geometry. Nowadays, screen-printing
microfabrication technology is a well-established technology for mass production of thick film electrodes
and it is widely applied to build biological or chemical sensors [66]. Screen-printed electrodes (SPE) represent
one of the most important products of this technology because these are high mass production and low cost.
Generally, inert substrates used in screen-printed processes are ceramic or polymeric ones. In case of polymeric
substrates, polyester substrate is the most commonly used for its durability, thermal and hydroscopic stability,
clarity and stiffness. Exact composition of printing inks is uncertain, as they are commercial and patented.
Ink-jet printing is also being used as an alternative to screen-printing technology. Other micro and
nanofabrication techniques based on photolithography and other 'micro' and 'nano' imprinting techniques
are also extended for electrode/detector fabrication.
