unit operations downstream, this is known as the Load Film (LF). The second DF is
located within a second chamber which is connected to the pneumatic chamber by a
pressurised microchannel, when this chamber fills with an ancillary liquid the DF is
dissolved and the pressure holding back the liquid is released, this DF is known as
the Control Film (CF) [ 18 ]. The main liquid in the reservoir can then advance down
the microchannel connecting the reservoir to the LF where it dissolves and releases
the liquid [ 18 ].
3.1.5 Paper Imbibition Valves
A further improvement with respect to event triggered valving is realised through
the use of low cost paper imbibition valving. This iteration utilizes the well-defined
mechanism of liquid “Wicking” through absorbent paper, an ancillary liquid wicks
from a liquid field chamber through the absorbent paper which is in contact with a
number of sequential Control Films (CFs) [ 23 ]. As the liquid wicks across the paper
at a well-defined rate the control films are dissolved which in turn releases the liquid
held in the corresponding reservoir towards further LUOs in the same manner as
described above [ 23 ].
3.2 Active Valving
3.2.1 Phase-Change Micro-valves
Park et al. introduced a nanocomposite material of 10 nm-sized iron oxide
nanoparticles dispersed in paraffin wax which could be used to create a unique
phase change microvalve controlled by an external laser diode [ 24 ]. The integration
and operation of the Laser Irradiated Ferrowax Microvalve (LIFM) is achieved by
including a small chamber called an Assistant Valve Chamber (AVC) which is
located next to the ferrowax plug [ 24 ]. The microvalve can be installed on disk in a
Normally Open (NO) or Normally Closed (NC) configuration which can then be
reversed by irradiating the iron nanoparticles, on irradiation the iron partials
quickly heat causing the paraffin wax to melt thereby opening or closing the
channel [ 24 ].
The ferrowax plug is located within the microchannel in the case of the NC
configuration, as the laser diode transfers heat to the iron nanoparticle the paraffin
wax melts and flows into the AVC thereby unblocking the microchannel and
opening the valve [ 24 ]. In the case of the NO configuration the reverse occurs,
the ferrowax is contained within the AVC in a solid state until the laser diode
irradiates the iron nanoparticles, this then causes the ferrowax to burst from the
AVC and block the microchannel [ 24 ].
5 The Centrifugal Microfluidic: Lab-on-a-Disc Platform 125