immunoassay on-disk is the Enzyme linked Immunosorbent Assay or ELISA, this
technique requires a solid support with bound capture antibody (Sandwich Config-
uration) or antigen (Direct Configuration) which can then isolate target molecules
from solution before impurities are washed away [ 16 , 17 ]. Using a multi-valve
architecture to sequentially deliver reagents [ 18 ] at pre-defined times according to
an assay protocol, it is possible to implement a benchtop ELISA on the Lab-on-a-
Disc platform. More complex versions of protein analyte detection have been
integrated on-disk like multiplex immunoassays which utilize individual fluores-
cent tags for specific targets, and protein microarrays, which require surface
modification and precision to efficiently bind and array the proteins on the disc
[ 15 ]. The most common detection methods utilize either optical or electrochemical
set-ups. Such devices could replace conventional blood tests in the future due to
their ease of use, speed and overall cost.
3 Valving Technologies
As all disc-based liquids experience the centrifugal field, a set of valves is required
to orchestrate the spatio-temporal release sequence of on-board liquids along the
test protocol. A number of valving schemes have been devised for the Lab-on-a-
Disc platform which typically use the interplay of centrifugal forces with other
on-disc forces. These other forces typically depend on statically defined parameters
such as microchannel geometries, surface tensions and liquid viscosity.
Valving schemes are usually divided into two distinct types, passive and active
valving [ 19 ]. With passive valves being controlled by the interplay of both the
centrifugal and the capillary counter pressure and active valves require external
instrumentation, therefore needing an extra interfaces adding to the overall com-
plexity of the system [ 19 ]. Each valving scheme.
3.1 Passive Valving
3.1.1 Capillary Valves
On the microscale the surface-to-volume ratio is reduced such that the forces
governing the liquid are predominantly surface orientated, this allows the move-
ment of liquid through a narrow channel by capillary action [ 20 ]. Capillary valves
can take two forms. Geometry based valves use either an expansion or constriction
depending on if the surface is natively hydrophobic or hydrophilic. The other form,
a hydrophobic patch, locally modifies the channel surface tension to function as a
barrier to liquid movement [ 4 , 20 ]. Surface tension is typically characterised by the
surface contact angle,∅, which denotes the wettability of the surface to a given
liquid. A contact angle∅< 90 indicates a hydrophilic surface whereas a contact
angle∅> 90 indicates a hydrophobic surface (Fig.5.5).
5 The Centrifugal Microfluidic: Lab-on-a-Disc Platform 121