The lab workflow for PDMS chips

1-Molding of the upper half

1. Negative:a laser cuts the tape that is adhered to an acetate. The remaining tape is removed carefully. The channels and the chambers, as well as the input and the output have been cutted and the negative has been created. More info about the protocols involved here.



2. Molding box: (Find the polymerization chamber in our github). Once the negative has been created, it is time to align the acetate with the marks in the polymerization chamber. Depending on the chosen configuration, it might be worth to place the perforated base on the bottom of the acetate.

3. PDMS casting: PDMS casting was made inside an lab oven most of times. Curing time depended on the drying method selected. More info about the protocols involved here.

2-Molding of the lower half

The process is repeated without the negative part of the mold.

3-Fixing the two halves

the selected method for fixing both halves was plasma bonding. More info about the protocols involved here.

4-Creating the input and the outputs

We usually used to hole punch the PDMS inlet/outlet with a needle. But we cured the PDMS with a needle inside as another negative volume for molding.More info about the protocols involved here.

5-Injecting fluids into the chip

Automatic controlled microvolume pressure pumps have been developed specifically for our microfluidic chips. Specific plans of the pumps design can be found in our github.

Manufacturing the PMMA chips

Although we are proud of having implemented an affordable workflow for developing functional PDMS chips, we manufactured PMMA chips with micromachining techniques.

Our University has a mechanical workshop that usually machines vacuum chambers, or metallic parts of machines, bending aluminum sheets, etc. We visited the workshop and asked the workers how to micromachine a PMMA chip with almost 0.2 mm height and 0.8 channel width. We purchased a 0.4 mm tip diameter and adapted the manufacturing to other available tools.

The input and output needed to be modified, and we used 21G needles (0.8 mm) as inlet and outlet. The fitting was made with High Performance Liquid Chromatography (HPLC) 0.8 mm tubes. They fitted tight enough to avoid leaks.

Fluid Mechanics behaviour inside the chip

Once the workflow was designed and implemented, we focused on designing microfluidic concepts that could prove our system right. In this regard, there were some Fluid Mechanics concepts that we wanted to experiment with. This is why we created the following experiments:

1. Our mixer: Inside the chip, the fluid behaves in a laminar way. There are many papers on this topic.We wanted to test this experimentally. And that is why we created a mixer. We could study how the fluid behaves in the conditions of a mixer. Our mixer is just an example on how microfluidic components can be small enough to be modularly assembled in series or in parallel as an electronic component.



2. Flow separation tests: We have designed four experiments to study the behaviour of our flow under different circumstances. The flow circulates towards a triangle, a circle, a throat and the shape of a heart. This will show us how the flow behaves under certain circumstances. Its immediate consequences affect the design of chambers or any microchannel widening.



3. Droplet generation tests: Generating droplets is one of the milestones of microfluidics. Droplets are small volumes of sample moving as small drops in an arranged and harmonic way. It is much more than just beautiful. The main task of this chip is to study how a fluid and air pressure gradients can work together in the same room. The design pushes to the limit the available capabilities of our device.



4. Tree and mixer test: We have designed a large PMMA chip to work as a sample on how fluid behaves when flow is separated into different branches of a tree. The aim of this experiment is to study the laminar flow, and how it behaves when it arrived to the central chamber. On the exact opposite side, a negative relative pressure will be generated to study how it behaves in an alternative “negative relative pressure” tree. In this experiment there are two sides of a chip. Both of them are experimentally equivalent.



5. A chip adapted to Dropsens GNP110 electrode: We manufactured via regular CNC milling, adapted to micromachining, the housing for a Dropsens GNP110 electrode. A paper proved our arrangement to be functional. We manufactured a two part chip. The upper side was micromilled with a 0.4 mm tool, with a custom made circuit for injecting the protein solution, ferricyanide and a buffer solution.We integrated the Dropsens electrode, looking forward to replicating the results obtained in the laboratory:


Binding the aptamers to the electrode

Plasma bonding

After setting the microwave up for treating the chips with plasma, we got some results that might serve as an illustration of the process. Other documentation can be found here. As we explain in the protocols section, we used a 700W microwave, modded to fit our requirements, as we explain in the protocols section:

We finally configured the microwave to half of its power approximately, inserted a 100ml glass of water and 20 seconds of treatment. After these parameters were established, we got the following results.

One of the indicators that show that plasma is treating the PDMS correctly is the modification of the surface tension of the water on a PDMS surface.

Injection

One of the improvements of the second prototype with respect to the initial is centered in the pressure system. It has the capability of displacing liquid volumes in the order of microliters. Our pressure pump has an unique arrangement, and it has been designed to be affordable and precise enough to govern the physical parameters involved in microfluidics mechanics.

Further information can be found in our Github.

Further improvements

Although the microfluidic chip is quite similar to what we consider a final version, there are many situations that we want to warn about to anyone who wants to replicate our setup.

Microfluidics does not always behave as we expect. DIY manufacturing is close to artisanry. Getting to a point in which replicability is expected is hard. It requires a lot of time and effort to master the technique.

PDMS has a very positive side for DIY manufacturers: it is affordable and resilient. It is easy to understand and a good way of learning microfluidics.

On the other hand, PMMA micromachining and precision manufacturing involve higher costs and a dependence to a mechanical workshop. You will not implement designs as fast as you can with the workflow that we have developed for PDMS, with the laser, the plasma bonding and the polymerization chamber.

We would love to share a project for anyone to replicate a microfluidics chip in the most affordable and optimal way. DIY tools are capricious and sometimes they do not behave as we expect them to do.

By repairing and refining DIY tools, we have learn a lot of machine design, manufacturing and biodevices design. We consider that DIY is the best way of learning anything. This is the reason why we would love to share our spirit and encourage any interested person to overcome these difficulties and experience the satisfaction of designing, manufacturing and searching beyond the immediate reality.