Learning Thru Research: ProDiehlene Glycol project

@TeamDFDV

The electronic cigarette was first considered as a fashion phenomenon. But this “gadget”, which changed many smokers’ daily life, became a real long term solution. More than 12 million of French people has already tested it, and 400 000 stopped smoking thanks to this electronic device.

However, there is one point that is controversial. Indeed, a lot of people worry about its possible toxicity, and one component is of central concern: propylene glycol. This chemical, which represents 50-70 % of the cigarette liquid, is supposedly toxic, and is the subject of many studies. That is why we wanted to test its toxicity both as a poison, and as a hygroscopic substance (a substance that suck up the humidity in the air).

Fig 1 - Comparison between electronic cigarette and cigarettes

Is propylene glycol toxic?

So, we want to evaluate if the propylene glycol is toxic. Several studies on the electronic cigarette and its toxicity were already made, such as in this article.

To try to answer this question, we run a biological experiment with yeasts to observe if the propylene glycol has an impact on their growth. It’s an organism much used in research because it’s easy to grow and observe. Moreover, it’s a eukaryotic organism (ADN contained into a nucleus) like humans, so it can be used as a model. As for the hygroscopic properties, we run an electronic experiment with a humidity sensor of to see if the propylene have an effect on the humidity of the body.

Fig 2 - An electronic cigarette

The biological experiment - How does different concentrations of propylene glycol affect yeasts growth?

To begin our biological experiment, we put yeasts in culture for one night. We chose two different wild-type yeast strains, to see if there was a change. We called these strains: ALA 1 & ALA 2. Then, we used a 96 well plate to put our yeasts in various concentrations of propylene glycol. We determined these concentrations from a series of calculation by taking into account the mass of our yeasts after the night culture and the lethal dose (referred to as LD50) of propylene glycol for an organism (20g/kg).

We have 10 replicates for every concentration of propylene glycol (5 for each strain). We also realized two lines of control for our media of culture (YPD) to know if it is contaminated.

Fig 3 - How did we put the yeasts at different propylene glycol concentration

Then we put this plate into a Tecan, a machine that measure the Optical Density (or Absorbance) in each well. The higher the Optical Density is, the more there are yeasts. Thanks to this data, we can determine the growth or the decay of the yeasts.

The electronic experiment - What are the effect of Propylene Glycol concentration and volume on air humidity?

For our electronic experiment, we used an Arduino, which is a microcontroller, and one sensor of humidity DHT 22. We made this assembly and used the library available on this link to be able to take our measures.

Fig 4 - The electronic assembly

We took a tube and drilled the cork three times to pass each connections. We put parafilm and tape on the cork to have a better insulation. Afterwards, we put the solution in the falcon. We have different solutions. We did this with experience with concentrations of propylene glycol at 33 %, 66 %, 70 %, 80 %, 90 %, 95 % and 100 % for volumes of 10, 20 and 30mL.

Then, we put the humidity sensor in the tube and closed the tube with the pierced cork. Finally, we took our measures every 2 seconds for 12min.

Fig 5 - The electronic experimental setup

What did we obtain at the end?

Finally, for the yeast experiment we obtained these two graphs:

Fig 6 - Influence of propylene glycol on ALA1 yeast strain

Globally, ALA1 grew better with propylene glycol than without. Indeed, the more propylene glycol there is, the better yeast grew. It’s the opposite of what we expected.

Fig 7 - Influence of propylene glycol on ALA2 yeast strain

Regarding ALA2, all the curves all really close to one another and we obserbe that yeast grew better witthout propylene glycol than with it.

For the electronic sensor experiment we obtained this:

Fig 8 - Influence of concentration of propylene glycol on air humidity at different volumes

We see easily that, the higher the concentration is, the lower the humidity is. We also observed that the volume of the samples don’t modify the humidity rate. Indeed the curves are close to each others. We obtained some inconsitent values, especially for the 30 mL.

So, what can we conclude?

Our study shows that propylene glycol is efficient at absorbing humidity. However it’s not obvious that Propylene glycol is really toxic. There is no significant decreases of yeast growth when there are in contact with this product. And we saw two different behavior from the 2 strain.

This study presents some biaises. Indeed, we don’t have enough data. To be more precise, it would be better to take measure every 10 minutes (and not every 30min like we did). And to use another 96 well plate to obtain to repeat the experiment. Finally, we could have uses another humidity captor for our results to be more reliable. We also had problems with the ALA1 strain, wich didn’t grow much, maybe that’s why we got these strange results for them.

Curaudeau Manon, Touboul Nathan, Diehl Aurélien and Hermant Loïc

L2 students from LFDV

If you want to know more about propylene glycol and electronic cigarette, here are some interesting links, that are our ressources: