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Interactive development of the hardware

Reactor Vessel

Fig 1. 3D CAD illustration of 1L GLS-45 as a reactor

Yet, we noticed that thin pH sensors are not common nor are cheap. So, we tried this glassware with a benchtop pH sensor, and realized it wouldn’t fit many things beside the sensor, also the cap would be fragile with too many holes and thin spacing. To solve this, we changed to the same glassware, but with a wider mouth: 1L GLS-80 (wide mouth).

Fig 2. 3D CAD illustration of 1L GLS-80 as a reactor

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Reactor Lid

FIRST CYCLE ROUND

At first, we considered a commercial lid with 5 multiple input-outputs (as shown above), nonetheless, this is too expensive and may not fit all tubing and sensors, reducing flexibility. So we considered designing a lid to be 3D printed using additive deposition manufacturing, yet available plastics for this technique doesn’t handle well autoclave, nor have good sealing between printed layers. Our next option was to build the top part using other material that can stand autoclave, is nontoxic, is resistant to acid and basic agents, and holds the form. The material that suits it all and it is not expensive nor require many resources to create is the polymerizable silicon rubber.

Yet, the platinum silicone was out of stock in our country due to pandemics and probably to high demand in medical-industrial sector. To circumvent, we migrated to non-platinum silicone rubber, that contains a small quantity of components that may interact with cultured cells. Nonetheless, due the minimal exposure of the lid to the culture medium, the risk was accepted. We have chosen a silicone rubber that is capable to withstand up to 250°C and is red colored.

We first designed a shaping mold on 3D CAD software and printed it:

Fig 3. 3D CAD design

But results weren’t good: one plastic pin broke due to bad printing configuration, the silicone leaked due to inadequate connection design, and the lid wall had almost no contact with the glass.

Fig 4. First silicone iteration

SECOND CYCLE ROUND

So, we made a new mold with inside walls more conic shaped, we designed a better wall junction, and printed again.

Fig 5. Second lid iteration

Most problems were solved, and we decided it was time to our first test with living organisms. But again, a new problem appeared: when testing the lid, we noticed it started to crack. Our supposition was that the proportion of silicon to catalyzer was incorrect and a new attempt with the same mold was made.

Fig 6. First and second attempt with lid mold version 2

As the material was as brittle in the first attempt, we discovered that the rubber was too hard to this function! So, we searched again for a new rubber, with hardness bellow the red silicone, which has a hardness grade of shore A 45. We were not able to find any rubber in stock that has a hardness bellow shore A 45 and was able to stand up to 120°C of an autoclave cycle.

THIRD CYCLE ROUND

One silicone supplier informed that one of his silicones, a white rubber with shore A 10 , probably would handle the autoclave cycle, and we tried it. And finally, we could test with a living organism.

Also, on this first test, we planned to use a syringe directly connected to the tubbing, but we soon discovered this wasn’t the best scenario as it was hard to attach the syringe nozzle to the tube, and it scaped twice of it too. On the second attempt with the white rubber, we also created a sealed sampling port using the same white rubber and pipette tips.

Fig 7. First and second attempt with white silicone rubber, and a new tool created for the reactor, a sealed sampling port using pipette tips and silicone rubber.

Meanwhile, we worked on the mold too. As we chose to work with standard pH sensor to avoid the high-cost long pH sensors, we found that the minimum working volume was 800mL in a 1000mL flask. As we noted that there were room on the rubber lid, we designed a simple attachment that enabled us to move the pH sensor 2.5 cm down on the flask.

Fig 8. A simple mod was developed to the mold, so we gained 2,5cm of pH movement, and reduced the minimum working volume from 800mL to 700mL.

Yet, the white rubber became softer and sticky after just a few trials, it started to degrade due to autoclave cycles.

Fig 9. Picture showing white silicone degradation after some cycles of use.

FOURTH CYCLE ROUND

A new solution was needed. We simply tested another silicone with hardness shore A 15 green colored, besides the datasheet describing a maximum temperature of 80°C.

The green silicone worked better then expected, with no signs of degradation even after more than 6 cycles.

Fig 10. The third silicone rubber material tested.

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Additional information about hardware design can be found HERE.

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