We envision a portable, fully automated, fluidic device that can process biological samples in the field. We considered that every part of our device that gets in contact with our sample needs to be disposable and replaceable. Therefore we designed a replaceable fluidic chip out of PDMS that can be controlled via an external device.
To achieve this we build so called Quake valves1 that are controlled via externally applied air pressure. To pump fluids we use an air balloon which is a super cheap supply for low pressure. To control our Quake valves we use a bicycle tube as a cheap and refillable supply for air pressure up to 5 bar. We use electrically powered air valves to control the Quake valves and build an electric circuit to control the air valves with a microcontroller. The valves can be easily downscaled and require no special equipment for their manufacture. We constructed the Quake valves by using 3D printed negative via soft lithography. A detailed protocol for manufacturing macroscopic fluidic chips with 3D Printed negatives can be found here at the subitem "Soft lithography".
Function and Composition of a Quake Valve
The valve is made of three PDMS layers. The lower layer carries the flow channel with a sinusoidal dome at the valve position. The middle layer is just a thin and therefore elastic PDMS membrane. The upper layer carries the air channel which has a cylindrical pressure camper at the valve position. The three layers are shown in the explosive drawing below.
Explosive drawing of a Quake valve.
When air pressure is applied to the pressure Chamber the thin membrane will be pressed in the sinusoidal dome of the flow channel and blocks the water flow. We chose a sinusoidal shape for the lower channel to enable a smooth contact between the the membrane and lower layer. This mechanism is illustrated in the figure below.
Function of a Quake valve.
The air channel is connected to two normally closed electric air valves via tubes. One valve to apply pressure, for opening the water valve and one to release pressure, for closing the water valve.
An image of an operational quake valve can be seen in the image below. For better contrast, we filled the flow channel with black ink and the pressure chamber with yellow inc.
Operational Quake valve.
Control Circuit for the air valves
We need a control circuit for the air valves because they are powered with high currents and consist of a solenoid that causes voltage spikes which can be harmful to a microcontroller
The solenoid valve is connected to the drain of an N-channel MOSFET. The drain is connected to a digital pin via a voltage divider. When the digital pin is set to 5V, the voltage at the gate is above the threshold voltage, current flows through the valve and the air channel is opened. A diode in blocking direction parallel to the valve damps the voltage spike caused by turning of the valve. The circuit is shown in the figure below.
Control circuit for the electric valves.