Thanks to Emerson for supporting this episode of SciShow.

To learn more, visit Emerson.com/WeLoveSTEM. [♪ INTRO]. Thanks to modern medicines and vaccines, some of the worst diseases in history have become preventable.

Like, smallpox is totally eradicated. And we're within jabbing distance of eliminating polio, too. Getting here hasn't been easy, though.

These accomplishments are thanks to tremendous, global efforts to get medicine to essentially the entire human population, from American suburbs to the most remote inhabited regions. But getting medicine to everyone on the planet comes with some interesting challenges, because these drugs can be… finicky. Some of them are pretty stable, no matter the conditions.

But others are highly sensitive to light, humidity, and especially heat. Like, some medicines should be kept between two and eight degrees Celsius… which isn't a lot of wiggle room. In rich, developed nations, meeting these requirements is often doable thanks to elaborate, carefully-controlled supply chains — systems that get medicine from the lab to the patient while maintaining its quality.

But for less developed areas — especially those without easy, reliable access to an electrical grid — this can be a serious problem. Coolers and ice packs can be a good last step if you need to fly, drive, or hike a vaccine or medicine to a remote community. But to really address the problem, we need solutions that can last a long time and that can be more precisely controlled.

We need systems that can actually sense when conditions are getting too hot or too cold and can readjust. And luckily, engineers have come up with some options. One alternative that's been used for a long time is kerosene or gas-powered fridges.

And before you ask, these aren't just fridges hooked up to gas-powered generators. They work thanks to clever engineering and physics. First, you take a liquid refrigerant, like ammonia, and put it in a sealed container at low pressure.

The low pressure makes the liquid ammonia start to evaporate and turn into a gas. This change in its physical state absorbs heat from the surrounding environment — kind of like how evaporating sweat cools your skin. This sucks heat away from the inside of the fridge, chilling it down.

Then, the ammonia gas is piped into a different container filled with water, where it mixes and forms a solution. That solution is boiled with the help of burning gas or kerosene, then the resulting vapors are passed through a series of separators and condensers. Ultimately, you end up separating the ammonia and water, so the ammonia can flow back into its original chamber and start the cooling process all over again.

Different models might be set up slightly differently or use different chemicals, but that's essentially it. And this cycle can be repeated for about as long as you have gas to burn. Temperature, meanwhile, can be controlled manually or with a thermostat.

And a thermometer and recorder can warn doctors if the machine ever accidentally got too hot or cold . Kerosene fridges were actually some of the earliest models of refrigerators, and the old-school stuff sometimes works pretty well. This has been a common way to keep medicine cold in remote areas for the last 35 years or so, and these fridges are used in more than 60% of vaccine storage locations.

Of course, they aren't exactly simple. These things need consistent shipments of fuel, and they're not super-efficient, so they're now starting to be phased out in favor of better and simpler options. Like, for example, solar-powered fridges.

These are refrigerators powered by solar panels, and like kerosene fridges, they can come with thermostats and temperature readers. These devices first really came onto the scene in the 1980s. But unfortunately, while they did fix the problem of having to keep buying fuel, they also needed big battery systems to run at night, which could break or be expensive to repair and replace.

So since about 2010, the preferred technology is what are called solar direct-drives. They still use solar panels, but instead of using that energy to charge batteries, they use it to directly freeze something like water. That keeps the medicine cool, and the ice will last for up to a few days even when the sun goes away.

They can also use something called phase-change material, which freezes like water, but at a bit higher temperature — like 5 degrees Celsius, rather than 0. This still keeps things cool, but is less prone to accidentally getting too cold. Regardless, these materials don't keep refrigerators cool for as long as an array of batteries might, so these things might not be as good for places with regular, heavy cloud cover.

But when you're in an area with lots of sun, a short charge is good enough to keep everything frozen. These devices haven't been extensively tested in the field yet, so it's hard to make sweeping statements about their lifespan. But we have gotten promising results about their reliability and ease of use out of field tests in Tanzania, Colombia, and Kenya.

While these things are more expensive to buy up front, they seem to be simpler and cheaper in the long run. And with more being developed all the time, the medical community will probably see a lot more of them. Of course, solar direct-drives and kerosene or gas fridges aren't our only options for transporting medicines.

New research and design are happening all the time, and that's pushing us into some interesting territory. For example, one project invented what's essentially a keg-sized, heavy-duty super-thermos that could be carried during a hike or a field trip and that could stay cold for more than a month. Vaccines in the center of the device are surrounded by ice blocks, and the entire inside compartment is then surrounded by insulation and a vacuum chamber, which keeps heat from the outside getting in.

Small, onboard temperature sensors and electronics can also sound an alarm if the system gets too cold, which might alert doctors to open it up and re-pack things or move the vaccines somewhere else. This container was added to the World Health Organization's list of pre qualified products in 2015. If a country or NGO is looking to buy or donate medical equipment, this list tells them what the WHO thinks is up to the job.

It is not, however, the only option in development. Student and design competitions have also come up with coolers that could be run off things like hand-cranks — basically, by using mechanical energy to power a generator. And other designs miniaturize existing technology to portable sizes.

So whether it's tried and true, new and improved, or the cutting-edge, we're working on solving this transportation problem. Medically, we've already proven that we have what it takes to treat or even wipe out some diseases. Now, we just have to keep our cool to put the rest of them on ice.

Thanks for watching this episode of SciShow, and special thanks to Emerson for helping us make it! If you want to learn more about who Emerson is and what they're about, you can go to Emerson.com/WeLoveSTEM. [♪ OUTRO].