Based on these formulas and the relation between the material density (ρ), the
channel depth (D), channel width (W) and laser spot (L), the groove depth is
calculated as:
D¼ðÞ 2 kα=ρðÞPin=Wv 4 kQth=ρW^2This formula provides a linear correlation between groove depths, incident laser
power while reciprocal dependence on the cutting speed is observed. This simple
model has shown good agreement with experimental data performed earlier
[ 17 ]. Essentially, the ratio of the incident power to the scanning speed is the energy
densityε[J/m]. As it becomes obvious the second term of the model is the offset in
energy density due to dissipation into the material and surroundings.
The predictive power of the model makes it valuable for determination of the
laser parameters necessary to attain a feature size. Varying simply the cutting speed
and the incident power defines the energy density, hence the physical parameters of
the groove. The nominal power of the laser diminishes with time and must be
calibrated to make sure that the real values are implemented.
3 Packaging and Bonding
A separate section is dedicated to packaging and bonding as that is the final step of
fabrication that facilitates means of connectivity between auxiliary equipment (e.g.,
pumps, optical cables, signal amplifiers etc.) and the microfluidic device. Several
methods are presented that allow bonding in regular chemical lab.
3.1 Thermal Bonding
This technique is very similar to the embossing approach; namely, the layers (2 or
more) of the device a are placed in contact with one another and the assembled stack
is incubated at temperature above the glass transition temperature of the material. In
contrast to the hot embossing, feature alterations need to be avoided. The method
can be performed with sophisticated machine as the HEX02 (Fig.3.5); also using
static loads (e.g., weight) and standard lab oven. The process development involves
optimisation of the pressures (loads), temperatures and incubation time.
3.2 Chemical Bonding
The uppermost layer of the plastic surface is deplastified using chemical agents,
vapours (e.g., acetone, heptane), the substrates are aligned, placed in hard contact
3 Manufacturing Methods Overview for Rapid Prototyping 99