Kathryn A. Whitehead

and a tail that likes other fatty things.

So when you throw a bunch of phospholipids together in water, they form this beautiful structure called a lipid bilayer.

In lipid nanoparticles, phospholipids have a similar role of keeping all of the other ingredients organized.

Second, there's a lipid called cholesterol.

Why, if cholesterol has a bad reputation, would we want to use it in a therapeutic nanoparticle?

It turns out that while cholesterol can be bad when it's in our bloodstream, it's actually a really good thing for our cell membranes.

And that's because those phospholipids I just told you about, they are entirely too free with themselves, and they are prone to falling apart.

Cholesterol is a stiff molecule that wedges itself in between the other lipids to fill in the gaps and hold them all together.

It plays a similar role in our lipid nanoparticles.

It provides structural support so the nanoparticles don't fall apart in between the injection and when they get into our cells.

Third, there's a lipid called an ionizable lipid.

Here, ionizable means that when these particles are in the bloodstream, they're neutrally charged, which helps with their safety.

Then they switch to a positive charge inside of our cells, which helps them release the mRNA.

Ionizable lipids are special because they have to be made in a lab, and scientists around the world have tested tens of thousands of these materials to find ones that are good at delivering mRNA safely.

And because they're made in the lab,

they tend to be proprietary to the company that invented them.

So, for example, Moderna and BioNTech, the company that partnered with Pfizer, they discovered different ionizable lipids, and that is the only important ingredient in their COVID-19 vaccines that differ.

And even then, their ionizable lipids aren't even that different.

which is reassuring because when independent groups of scientists converge on similar solutions, it's easier to trust the result.

Finally, one more ingredient.