|
We envision a portable,fully automated, fluidic device that can process biological sample in the field. We need to consider that every part of our device that gets in contact with our sample needs to be disposable and replaceable. Therefore we designed an replaceable fluidic chip out of PDMS that can be controlled via an external device. To achieve this we design so called Quake valves that are controlled via externally applied air pressure. To pump fluids we us air balloons being a super cheap supply for low pressure. To control our Quake valves we use a bicycle as a very cheap and refillable supply for air pressure up to 5 bar. We use electrically powered air valves to control the Quake valves and build a circuit to control the air valves with a microcontroller. The valves can be easily downscaled and require now 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".
The valve is made of three PDMS layers. The lower layer carries the water channel with a sinus shaped dome at the valve position. The middle layer is just a thin and therefore elastic PDMS film. The upper layer carries the air channel which as a cylindric air reservoir at the valve position. The three layers are shown in the explosive drawing below.
Explosive drawing of a quake valve.
If air pressure is applied to the upper channel the middle layer will be pressed in the sinus shaped dome of the upper channel and block the water channel. We chose a sinus shape for the lower channel to enable a smooth contact between the thin layer and lower layer. This mechanism is illustrated in the figure below
Function of a quake valve.
The Air channel is connected via tubes to two normally closed electric air valves. One to release pressure for opening the water valve and one to apply new pressure for closing the water valve.
We need a control circuit for the air valves because they are powered with high currents and consist of an electric coil that causes voltage pikes which can be harmful to a microcontroller
The electric valve is connected to the drain of drain of a N Channel MOSFET. The drain is connected via a voltage divider to a digital pin. 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 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.
|
