how do slow release medicines like the contraceptive implant work?

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It’s the slow release bit I’m confused by-what stops it all going into your system at once?

In: Biology
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There are Seveal Methods to achive this. In Generel your pill/ implant doesn’t degrade or degrade slow, so the Molecule has to diffuse slowly out of the drug. The Molecul could also be very unsoluable.
But the exact mechanism depends on the exact drug.

Going off your example of slow release contraceptives:

In the arm implant and the vaginal ring, the plastic that you can see and touch is a copolymer called *ethylene-vinyl acetate*, or EVA. The drug is loaded *inside* the plastic (like it’s now a mixed-in component of the plastic; it’s drug-plastic) and the drug moves from inside the plastic to inside your body by the process of *diffusion.*

Diffusion is a chemistry term for substances naturally moving from areas of higher concentration to areas of lower concentration, like how a fart smell eventually dissipates because the fart particles (farticles, if you will) achieve such a low concentration by moving to the formerly fartless areas throughout the room and away from the high concentration area it started out in (the area right by your butt).

Basically, the drug is VERY VERY SLOWLY moving from its area of high concentration (the plastic rod or ring it’s been loaded into) to the surrounding area of lower concentration (the inside of your body). It “wants” to do this naturally, as a property of how things work on Earth at a molecular level, and so we can take advantage of this and just control its rate.

Now we can get to your actual question, which was how it doesn’t do this all at once. In the fart room example, there’s no barrier between your fart and people’s noses, but imagine if there were a special membrane sealed over everyone’s noses that was engineered to only let certain amounts of fart particles through at a certain rate.

The birth control device has a membrane like that, that only lets a certain amount through at a time. They spend a LOT of effort in the labs determining how to get that right, involving a LOT of chemistry equations and math around the drug concentration and solubility and diffusivity, membrane thickness, permeability, level of actual vinyl-acetate content in the polymer, and more.

Tldr: concentration of drug inside plastic is engineered, along with permeability of device membrane, to work with the natural process of diffusion.

Adapted from:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7076478/