First assumptions

1. Immobilized aptamers on a PDMS surface. In order to create an electrostatic and mechanical trap for our targeted protein, we planned to work in a PDMS environment. PDMS is a well-known manufacturing material for electronics. So we could easily integrate PDMS in our device.

2. Optical measurement sensor. The materials required to test our sensor were a 280nm UV LED emitter and an LDR. The amount of light traversing the solution was quantified by a drop in voltage across the LDR: with higher protein concentrations, higher absorption is expected together with an increased drop in voltage.

3. Microfluidics: for channeling fluids through the chip. Microfluidics allows us to move microliters of samples, minimizing the dead volumes and the waste through the chip.

4. Modular design and normalization: We needed to standardize the protocols related to hardware to reduce the number of variables involved. This would restrict the design and manufacture and help us a lot when playing with certain design parameters.

5. Enable the DIY: We had the need of developing everything in a way such that anyone, regardless his/her origin could replicate our experiments in an affordable and creative way.

How it works

imagen 4 subsistemas

The device is divided into four parts. Every subsystem has been conceived to be integrated in a bigger organic whole: the device.

1. Electronics

   1. Custom modules oriented to experimentation.

   2. Custom PCB created specifically for our device.

2. Microfluidics and aptasensor

   1. Affordable PDMS chip manufacturing.

   2. Input/output and chambers of measurement normalised for a chip design oriented to manufacture.

   3. Immobilised aptamers on PDMS surface.

3. System of measurement

   1. Optic system of measurement based on protein absorbance at 280 nm of wavelength.

   2. LDR as light receiver.

4. Pressure system

   1. Pressure pump regulated manually

From the point of view of the protein sample, the fluid would follow this stream:

1. The protein sample starts inside the syringe, suspended in a buffer solution. And it is manually pumped into the PDMS chip.

2. From the inlet, it is displaced to the sensor chamber, where the surface is almost covered by aptamers.

3. The circulating proteins get trapped in the aptamers chamber.

4. A clean buffer solution is injected to the chip, to wipe any other molecule out.

5. A saline solution is pumped into the chip, to detach the protein from the aptamer. The process has been previously described in this paper.

6. The saline solution with the remaining protein is taken to another chamber, where absorbance is measured at the 280 nm wavelength.

7. Once the measurement is taken, we get the information and analyze it.

8. The chip is cleaned and the process can start over again.

Further details

The caset

1. The caset is a two part PMMA structure, able to allow 4 fixed inputs (1 mm of diameter) and 4 fixed outputs (1 mm of diameter), 4 chambers for 280 nm UV LED modules and 4 chambers for LDR modules. An open module was designed for inserting other components if required by the user. Find more information on the modules in our GitHub.

2. The control electronics was designed to govern 4 LED modules and 4 LDR modules.

3. It has a frame for the 40x40 mm PDMS chip, which relates to the bed of the laser cutter. PDMS chip should be 3 mm of thickness.

4. These parts are fixed together with screws and spacers. Although not a large pressure is required.

The chip design

1. The chip has dimensional restrictions (40 x 40 mm) due to the boundary condition of manufacturing: the laser cutter bed maximum dimensions.

2. We created polymerization chambers for this purpose.

3. The input and output have been fixed for manufacturing concerns.

4. The last chip that we designed has two parallel circuits. Therefore, two inputs and two outputs and four chambers (two per circuit).

5. The chambers were conceived to be surrounded by an emitter and a receiver, facing one another.

The electronics for an absorbance related measurement

1. The electronics are governed by an Arduino Nano board. It links the analogue electronic board inside the device and the Arduino IDE. So the user can see the data related to the signal in the serial monitor.

2. The light emitter is controlled by the Arduino through a 2N2222 transistor for providing the module with 6.5V and enough current.

3. The circuit which receives the data related to the signal comes from the light receiver. When the protein sample contains a high concentration of protein, the voltage drops proportionally to the amount of protein. Then, the signal is amplified and corrected via an OpAmp and an Instrumental Amplifier.

4. The voltage is measured by the built-in Arduino Analogue to digital converter.

5. Further information can be find in our GitHub.

Measurement results

Things we learned while doing

1. The most important lesson we obtain was about the system of measurement. As we had no relevant results in this part, we proceed to change our approach, due to the following reasons:

- The sensor, at 280 nm, did not have a detection limit high enough for our necessities. It could not trace our concentrations.

- Optical-based sensors are too sensitive to ambient conditions. So we should refuse to use it in our final design.

- Optical-based sensors are too sensitive to metrologic precision parameters, as emitter-receiver alignment for instance.

2. The initial PDMS chips we made were very unstable. Troubleshooting leakages and integrating feasible input/output fittings require patience and creativity. We learnt the following:

- Our system should integrate a straightforward way of experimenting with microfluidics.

- We needed a smooth way of pumping microvolumes into the microfluidic system. PDMS is too sensitive to mechanical parameters, as pressure or input/output torques.

3. Modular design made our lives much easier. Designing a chip, cutting it with the laser, curing the PDMS in the polymerization chamber and integrating the fittings was very easy, as we had developed an standard way of doing it.

4. It was not worth it attaching aptamers to the PDMS surface, as the range of detection of the sensor was too far from tracing our concentrations.

5. DIY was the way to go, as we could not spend time and money in buying commercial equipment and learning how to use it. /p>