Double Cap Design

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Motivation for Double Cap Design

About the previous kit

Last year bachelor project team has performed a kit composed by a cap with a silicon septum covering the bioreporter vial we can fill with a syringe. Thanks to properties of the septum material, the hole created by the syringe clogs by itself. It assures secure collection sample in the field but the syringe presents extra wastes and is a potential source of damage and contamination.
The workflow of this kit is presented in the following scheme :

Goals of the new kit

The new kit must assure secure and easy sample collection in the field and prevent GMO flowing out. We also have to replace the syringe.
The idea is to make a hermetic double cap which can link the two vials : one with the GMO and the other one with the water sample. It allows the mix of the two samples. After the experiment, the device can be brought back in a single piece to the lab to destroy the GMO.

Workflow of the double cap

The double cap has a side where we can screw the water sample vial and a side for the bioreporter. The latter presents a connector, responsible to pierce a seal which protects the bioreporter vial. This seal looks like a seal on a yoghourt, and prevents the GMO realising into the environment.
This principle is summarized in this scheme :
Principle doublecap.png
The workflow to use the new kit is the following :

  • The bioreporter is prepared in the lab and the vial is sealed. Then the vial is closed by a cap for the transport.
  • Water is collected with a second vial.
  • The double cap is screwed on the water collection vial (side without connector).
  • After removing the cap, the bioreporter vial is screwed to the other side (with connector).
  • The two substances are mixed, the two vials stays fixed to one another by the double cap.
  • The GMO are returned to the lab to be destroyed.

We can summarize it with the following scheme : Workflow.png

Design of the double cap

Defining the optimal dimensions

Dimensions of the connector part and of the double cap with a thickness of 1mm.

The first step of the design was to define the optimal dimensions of the double cap. This device presents two constraints :

  • The screw pitch adapted to the sample vials
  • The connector must pierce the aluminum seal after starting to screw

The first problem was solved by reverse engineering. We cut an existing cap in four to see the shape of the screw pitch and measure it with a calliper.
For the second one we measure the difference between the top of the screwed cap and the top of the vial to define the maximal height of the connector.

We finally take the biggest possible diameter for the connector, to reduce the Reynold number, i.e. allowing a better flow by reducing capillarity.

In the optics of making accessible for everyone the project, we realized the industrial drawing of the piece. It gives all the optimal dimensions of the double cap and especially of the connector. It had been realized with LibreCAD, which is a free and open source software.

Modelling the double cap

Top view of the whole double cap
Section of the double cap

After having defined the optimal dimensions, we can model the double cap with Blender. Blender is a free and open source software, which allows modeling, animation and 3D render.
Modeling is also a crucial step to prepare a file we can then 3D print and allows us to verify if our device is realisable.
To model the cap part, we started by designing the screw profile and we extruded it along a spiral. Then we realized the rest of the cap around the screw pitch. We got one cap we can duplicate by mirror (to have opposite screw pitches). The last thing to add was the connector between the two caps, respecting the dimensions previously defined.

By hand making of the double cap


After 3D modeling, we try to make a double cap by hand.
We started from the vial caps with a hole covered by a silicon septum. We cut a gasket in the septum, by removing its center. It is the basis for our connector, cut from a plastic tube. We took two caps to make the double cap.
Then we assembled the four pieces : the connector in the gasket, that we put between the two caps. We glued with neoprene glue to maintain them together. Neoprene glue is maybe not very optimal for this piece, it is well hermetic but it dries too slowly and stays elastic.

We could thus test it! There was a bit of leak, because the connector perforated the aluminum seal too early. To prevent this problem, we had to cut again the connector so that we first start to screw before the seal is perforated. It worked better like that.

This model was entirely make with recycled material (tube, silicon septum) and the kit cost just the price of the two vials : 2x 0.27CHF = 0.54CHF.

3D printing of the connector

We first tried to print only the connector of the double cap.

We performed it at the Hackuarium, with the help of Sam Sulaimanov from Octanis, with a Prusa Mendel I3 as 3D printer. It uses the “Fused Deposit Modeling” method. This technique involves melting a plastic filament (usually ABS or PLA) through a nozzle heated to a temperature between 160 and 270°C.

The 3D printed connector (thickness 1mm)

To print the connector, we used PLA filament. This polymer is entirely biodegradable.

We used Cura (free software) to define the caracteristics of the printing :

  • Temperature : 220°C (for PLA)
  • Material : PLA filament (1.75mm)
  • 4mm nozzle
  • 190*190 bed (temperature 70°C)

Although the piece is not very smooth because of the technology of 3D printer we use, the expected dimensions are respected, after verification with a caliper . So this method is precise enough for the connector making, despite its small dimensions.
The printing took 11min, for 0.46 meter of filament (1gram). The price, estimated from the cost of a spool of filament, is about 0.04CHF. We must add the price of the two vials, that is to say 0.54CHF.

The obtained double cap

We then have to glue the connector between two preexisting vial caps.
After having glued the two preexisting caps with the connector, we can make some tests.
We position the tip of the connector in front of the start of the screw pitch of the cap. As expected, the aluminum seal is pierced after we started to screw.

This proposition presents a disadvantage : we have to find a glue which resists to water, is strong enough and hermetic. For example, with Patex glue, the device was broken after about 20 uses.
We can also estimate that because it is in three parts, it is less hermetic.

3D printing of the whole double cap

Printing of the whole double cap (step of the connector printing)

The first printing of the double cap was also performed at Hackuarium with Sam Sulaimanov, with a printer which uses the “Fused Deposit Modeling”. This time we used ABS, a thermoplastic polymer having a good impact resistance and a relatively rigidity despite is low weight.
Here are the characteristics of the printing :

  • Temperature : 235°C (for ABS)
  • Material : ABS filament (1.75mm)
  • 5mm nozzle
  • 190*190 bed (temperature 70°C)

The major fear of printing this piece was the basis of the connector part. It is printed vertically and the 3D printer must create it without support. Finally, it was not a problem. The major problem came from the temperature of the piece. Because of the direction of the printing, the first layers are in contact of the bed at a temperature of 70°C, but the upper layers are in contact to air and the ABS tend to retract. So the diameter of the whole piece reduces with the height. Thus it is more difficult to screw the vial on the side with the connector, which becomes more complicated when the vial is protected by a seal. So the solution was to increase the scale of the printed piece by a few millimetres.

The positive part of this printing session is that the screw pitch matches the vial one. Finally we succeeded and we get a double cap with two sides that we can screw on the vial.

The result we get

However we can make some critics about the technology of printing we use (“Fused Deposit Modeling”). It produces pieces which are not very smooth and it presents many deposit errors : we had to file especially in the screw pitch. But this method stays precise enough for the dimension of the double cap.

The price of this whole printed double cap is 0.06CHF (2 gram of ABS, length 0.69 meter). This is low cost, because we can buy large spools of ABS filament, and our piece is small and ligth.

So we can test it in real conditions. As expected, the seal is pierced by the connector after starting to screw and the two vials communicate by the connector. The double cap assures to return the device in a single part to the lab to destroy the GMO present inside.

Potential improvements

After having printed the whole double cap and verified that our model matches the vial and pierces as expected the seal, we can improve the concept.

The double cap we get

First we have contacted the “STI atelier” at EPFL and especially Roland Dupuis to get somes advices about 3D printing. We wanted a smoother piece, especially for the screw pitch, than we get from the Prusa Mendel I3 3D printer. He advices us to print the double cap with the multi-jet machine. This type of 3D printer allows the deposit of liquid polymers like a classical printer and also is the most precise type of 3D printer. In fact, the result is perfect. The previous printing with the “Fused Deposit Modeling” method give us a result precise enough as well (especially the screw pitch matched the vial). But it remained some deposit, because of machine error, and the piece wass not very smooth. Here the piece we get presents no default. The shape and the screw pitch are smooth. There is no difference of diameter between the two sides of the double cap, because the temperature of printing is constant. That is why this double cap screws as easily as the injected cap given with the vials.

The weakness of the type of printing is the price. We pay the printing piece relatively to its weight (2CHF the gram of material). So for a whole double cap, the price is : 2.4CHF, whether 40 times the price of the previous whole printed double cap.

We also have the possibility to add a gasket between the screw pitch and the basis of the connector in the both sides of the double cap. It would strengthen the tightness of the double cap and prevent GMO flowing out.
This o-ring gasket must have an interior big radius of 8.20mm and a section radius of 1.9mm.

We have two option to make this gasket :

  • Printing a gasket at the right dimensions with a material that remains flexible after printing
  • Finding a gasket close to the optimal dimensions in a provider, like Angst+Pfister

The first solution has been rejected by Roland Dupuis, because of the too little dimensions and the fragility of the gasket, which could be removed by the washing step after printing. Thus we have looked at the gasket in provider offer, but no one has the optimal dimensions. If we take a non-optimal gasket size, it risks to preclude the screwing and the tightness of the double cap will be reduced. The other possibility is to adapt the double cap model, especially the connector part to the gasket size. But it is not optimal, because if :

  • We reduce the connector diameter, the flowing will be more difficult (Reynold number)
  • We increase the connector diameter, it will encroach in the screw pitch.

It can be a way of improvement, but for the moment we let the double cap without gasket.

Sealing the vials

This part of the project was to find an optimal way to seal the vials. The seal must be rigid enough to be pierced by the connector and prevent the vial sample from realising.

Pros and cons of the envisaged solutions

So first, we reject all the plastic material to concentrate on aluminum seal.

We found four solutions :

Solutions Advantages Disadvantages
Gluing a round cut aluminum foil on the top of the vial with liquid Uhu glue
  • Easy to produce
  • Not expansive
  • Close to what we want to finally have
  • The glue take hours to be completely dried
  • Glue can modify the optimal environment of the GMO in the vial, so we have to put glue just where aluminum is in contact with glass
  • Take long time to seal many vial
Use some aluminum scotch
  • Easy to find
  • Not expansive
  • The glue is not strong enough and the seal is snatched before it is pierced.
Restick a food packaging aluminum with an ironer
  • Do-it-yourself
  • No cost
  • The plastic which covers the aluminum is impossible to be pierced by the connector
  • The ironer temperature can modify the GMO optimal conservation temperature
  • Optimally we should remove the plastic which cover the aluminum but it is impossible
Induction sealing machine
  • Similar to the industrial way of proceed
  • Quite expansive

So we have also thought about a plastic material sealing solution :

Solutions Advantages Disadvantages
  • Easy to find (use in laboratories to seal vials)
  • Not strong enough to prevent from bacteria release during transport
  • Impossible to pierce it with the connector, the parafilm is removed when we screw

For the plastic solution, we have two options to be sure that the double cap connector pierces the seal :

  • Making a longer connector
  • Adding some teeth at the top of the double cap

Finally, the best solution seems to be gluing some aluminum at the head of the vial. To prevent the risk of toxicity of the glue for the bacteria, we have to put some glue just in the part in contact with the glass of the vial.
It stays possible to seal by head a few vials, but it is difficult if we have to perform this sealing hundreds of times.
In this case, the best solution would be the industrial way of sealing by induction.

Induction Sealing

We will try induction sealing used in food and pharmaceutical preparations, think yogurt container or medicine bottle seals, to seal the bacteria containing vial for field work. The advantage is that the sealing process does not heat the contents of the vial, and when properly done, the seal is hermetic.
Guide for induction sealing suggests we find the correct seal for the glass vials, which are fewer in options compared to seals for plastic containers.The Selig company has an extensive selection guide that allows you to choose the liners based on your sealing materials, contents, and the container material.

Legal aspect

Last year, the project has been approved by the authorities according to the Ordinance on Handling Organisms in Contained Systems» 814.912.

According to the safety expert Dr. Sabrina Leuenberger advices, the new kit do not need to be anew validated by the Federal Office for the Environment (FOEN). However, we contact Dr. Basil Gerber, a deputy head of this section, responsible for the implementation of the Biodiversity Convention in the area of genetic resources, to inform us of the advancement of our project.


In conclusion, this project started from a drawing on a paper and took shape during the semester. As excepted it proposes a device which allows the mixing of the water sample and the bioreporter. This double cap also prevents the GMO flowing out and assures secure and easy sample collection in the field.

Some improvements remaining

  • Test the induction sealing
  • Do we need to add some gasket?
  • Which type of sealing is the best?

Links for the double cap articles :