FridayAFM - toothbrush degradation

A few months ago I shared with you a few images taken on my toothbrush. On them you can see the way I mount the fibers for AFM imaging, how a brand-new toothbrush looks like, and the amount of bacteria accumulated on a used toothbrush. (Yes, I push too hard when using the toothbrush).

FridayAFM as seen on the 2022 11 04. Toothbrush before/after a couple of weeks of use. 

From the images we conclude that the fiber material is likely to be Nylon, and that the bacteria are likely to be dried/collapsed gram-negative bacteria.

The information that was missing was how the toothbrush was stored and how it was cleaned/disinfected. The question that arises is how much the storage and disinfecting techniques can change the outcome of this experiment. 

The information is that I used the toothbrush, rinse it in tap water, and then keep it in my bedroom in dry conditions, in the dark.

The question, is what we try to answer in this week fridayAFM. For this, I bought 2 new toothbrushes, one UV disinfection machine, and one bottle of mouthwash. In the first part, I expose a toothbrush to a number of UV-light sessions, and at regular intervals I take a fiber and inspect it with the AFM system. 

Toothbrush degradation 1 of 2
Effect of UV light exposure on the toothbrush fibers after a number of doses with the UV disinfecting machine.

According to the bibliography I had access to, Nylon (our best guess for the toothbrush fibers), should degrade significantly after 4h of UV-C exposure, with chunks of material falling off. However, we don't see such degradation here, meaning that either the material is not Nylon, the intensity of the UV light is not high enough, or the role of water (seawater to match the literature) is critical for the degradation to takes place.

On the second part of the experiment, I took a brand-new toothbrush, dip it in mouthtwash, keept it in the dark, and after two months I took it out, rinsed it with tap water and imaged it.

Toothbrush degradation 2 of 2
Effect of mouthwash liquid exposure on the toothbrush fibers after two months of exposure.

In this case, there is some minor pitting in the images, but most of the surface remains unaffected. What remains an incognita (hopefully you will have the answer), is which ingredient of the mouthwash produces this effect.

Mouthwash ingredients: Water (Aqua), Alcohol, Glycerin, Polysorbate 20, Flavour (Aroma), Citric Acid, Sodium Citrate, Sodium Hydroxide, Sodium Saccharin, Cetylpyridinium Chloride, Zinc Chloride, Sodium Fluoride, Colours (CI 42051, CI 16035), Contains 0.05% w/w Sodium Fluoride (225 ppm F).

As usual, this week fridayAFM answers some questions and opens others. In this case, this is a control experiment needed to be able to differentiate between different effects when trying different disinfection techniques. Will the UV exposure kill the bacteria? Does it need rinsing in mouthwash or is it tap water enough? Will the water, combined with the UV light degrade the fibers to the point that chunks of plastic drop?

Stay in touch, as we will try to answer these questions in future friday afternoon experiments.

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