flow, which then automatically triggered the fluidic operations to complete the
immunoassay. However, the system was unable to sequentially deliver multiple
reagents required to amplify the signals and enhance the detection limit of the
assay.
Safavieh and Juncker [83] introduced two novel capillary elements: trigger valve
and retention burst valve. A trigger valve consists of a shallow channel, intersecting
another channel that is deeper, covered by a hydrophobic layer. The abrupt increase
in the cross-section of the microchannel making it energetically unfavourable for
the liquid to flow from the shallow to the deep channel due to the large increase in
liquid-air interface at the liquid front. The trigger valve is triggered by flowing a
liquid through the larger channel. A retention burst valve is formed by constrictions
in the microchannel that produce high capillary pressure. When the pressure in the
capillary circuit near the retention burst valve is weaker that the valve’s burst
pressure, liquid remains pinned by the retention burst valve. When the pressure in
the circuit builds up and rise above the valve’s burst pressure, the liquid begins
draining the channel of the valve until it eventually bursts, and as a result the
reservoir downstream of the retention burst valve beings to drain rapidly (Fig.9.4).
A novel capillary circuit was built by combining a variety of capillary elements,
and achieved completely autonomous delivery of multiple liquid reagents
according to a defined sequence that followed a preprogrammed and predetermined
flow rate and timing. The capillary circuit was able to perform a sandwich immu-
noassay to measure the concentration of C-reactive protein. The authors defined
such circuit as capillarics and introduced symbolic representations reminiscent of
those used in electrical circuits.
Fig. 9.4Optical micrograph of a microfluidic capillaric circuit containing 4 side-reservoirs made
in PDMS.CPcapillary pump,RBVretention burst valve,CRVcapillary retention valve,TVtrigger
valve. Taken from [83]. Copyright©The Royal Society of Chemistry 2013
236 A. Ng