3.1.3 Centrifugo-Pneumatic Valving
This type of Lab-on-a-Disc platforms create structures between layers of different
materials bonded together to form a continuous network of microchambers and
microchannels; due to this feature it is possible to insert dissolvable film between
layers of adhesive material to form valves [ 22 ]. The valve is be dissolved upon
contact with an aqueous solution and also blocks the passage of air meaning that on
the addition of liquid upstream a sealed channel of air is created between the valve
and the front of the liquid plug [ 22 ]. When the frequency of rotation is increased the
fluid plug advances down the channel, this is due to the centrifugal force exerted on
the liquid as described above; however, due to the now decreasing volume of air
there is increasing centrifugally induced pressure which contracts that centrifugal
force attempting to advance the liquid [ 22 ]. This induced pressure scales according
to the following equation [ 22 ],
ΔPω¼ρΔrrω^2 ð 5 : 11 ÞwhereΔPωdescribes the pressure at a specific frequency of rotation and all other
terms are consistent with above descriptions. The advancing liquid plug is halted as
Δρωbecomes balanced by the air pressure within the microchambers which can be
calculated according to the following equation,
P¼P 01
1 ΔVV 0ð 5 : 12 ÞwherePis the pressure within the microchannel,P 0 is the initial pressure within the
microchannel, V 0 is the initial volume of the microchannel andΔV is the change in
Volume of the microchannel [ 22 ].
Using the above information, it is possible to design pneumatic chambers with
calculated dimensions that will result in a specific burst frequency.
3.1.4 Event Triggered Valving
A key disadvantage of rotationally-actuated capillary burst valves is that their
minimum achievable feature size requires a relatively large amount of disk real
estate as well as large differences in rotational frequency [ 23 ]. This limits the
number of discrete LUOs which can be automated on-disk. These specific limita-
tions can be circumvented through the use of so called “Event Triggered Valves”.
This mechanism of valving builds upon that of dissolvable film valves described
above, an extra degree of control is introduced by utilizing a second dissolvable film
which controlled the pressure in the pneumatic chamber, in this way the liquid is
restricted by the counter acting pressure even at high rotational frequency’s
[ 18 ]. The created valve requires a pneumatic chamber which is sealed by two
dissolvable film (DF) tabs, one DF is connected to the reservoir and leads to further
124 B. Henderson et al.