María Dolores Martín Alonso, IMDEA MATERIALS

TikTok has an uncanny ability to turn everyday objects into viral miracles. One day it’s a cream that erases wrinkles in seconds, the next it’s a blanket that promises cool nights without air conditioning.

“Cooling blankets” are the algorithm’s latest obsession: videos with millions of views show influencers wrapping themselves in fabrics that, according to them, “absorb body heat.”

One of the most talked-about videos is from the portal SlashGear, which ran a hands-on test with one of the best-selling viral blankets. They placed that blanket and a conventional one in the sun. The result? The “cooling” blanket’s outer surface showed up to 6 °C less. At first glance, it looks like a win… but, as is often the case, physics demands a second opinion.

Why the temperature drop?

Why does it show that temperature drop even without human contact? The key lies in how each fabric absorbs, reflects, or dissipates heat from its environment. Some synthetic materials, like nylon or modified polyethylene, reflect more solar radiation or heat up less in the sun, something that may explain the surface temperature difference.

However, that doesn’t automatically guarantee a long-lasting cooling sensation when we touch the blanket. The initial feeling of thermal relief is mainly due to thermal conductivity. Some fabrics, like the aforementioned nylon or polyethylene, transfer the heat from our skin more efficiently than others, such as cotton. It’s the same reason why a metal handrail feels hotter in the sun than a wooden one, even if both are exposed to the same conditions.

That’s why many people who try these blankets in a mildly warm room say that “yes, it does feel cooler”, at least at first. But that initial sensation doesn’t last. On forums like Reddit, you’ll easily find experiences that contrast with the initial hype: “The first ten minutes, great. Then it felt like wrapping myself in cling film.”

The effect disappears once thermal equilibrium is reached

What happens is that, after absorbing our body heat, the fabric quickly reaches thermal equilibrium. If that heat doesn’t dissipate, for instance, if we’re lying down with no ventilation or the ambient temperature is high, the blanket stops feeling cool. In other words, if there’s no mechanism to maintain a thermal gradient, the effect vanishes.

However, there are blankets that do manage to maintain that gradient for longer. They do so thanks to materials specifically designed for that purpose. And this is where physics comes into play.

Basic physics: phase change

The trick isn’t in the fabric, the texture, or some Coca-Cola-style secret formula. It’s in a basic principle of thermal physics: phase change.

When a material changes state (for example, from solid to liquid), it needs to absorb a lot of energy without increasing in temperature. This energy is called latent heat of fusion. The most familiar example is ice: it can absorb a lot of heat while melting but stays at 0 °C until it has completely turned to water.

In the case of truly cooling blankets, materials called PCMs (Phase Change Materials) are used. These are designed to melt at temperatures close to human thermal comfort, between 18 and 21 °C. During this change of state, they absorb body heat without warming up until all the PCM has melted, allowing the user to stay cool for longer.

Imagine wrapping yourself in a blanket full of “invisible ice cubes” that melt at just the right temperature. While melting, they “drink up” some of the heat your body produces while you sleep. That’s quite literally the essence of a PCM-based blanket. And the best part is that once the material has finished melting, it can be “recharged” by leaving it in a cool place to solidify again.

The blanket of the future

To create truly effective cooling blankets, we need to turn to materials science. Not all solids melt at temperatures suitable for human comfort and absorb a significant amount of heat at the same time. The most common PCMs fall into three main categories: organic, inorganic, and eutectic.

Organic PCMs, such as paraffins, are popular for their stability and low cost. They’re composed of long hydrocarbon chains that absorb heat when melting and remain stable through many thermal cycles. Their melting point can be adjusted by selecting the number of carbon atoms.

In the context of blankets, these PCMs are encapsulated in microstructures, usually polymer capsules, that allow them to go from solid to liquid without leaking or damaging the textile. Encapsulation protects the material from degradation and ensures the blanket can withstand many cycles without losing effectiveness.

Are they already on the market or still in the lab?

Los cubitos invisibles ya son una realidad

The invisible ice cubes are already real

Though talking about “invisible ice cubes” may sound like science fiction, phase change materials are already used in real products, not just blankets but also sportswear, technical footwear, and bioclimatic architecture.

In the textile sector, several brands have started marketing fabrics that incorporate PCM microcapsules. One of the best-known is Outlast Technologies, born from collaborations with NASA, which uses these technologies in thermal clothing, bedsheets, and jackets.

Meanwhile, research continues. The most active lines focus on improving long-term stability, increasing thermal conductivity, and developing more sustainable materials with the highest possible latent heat of fusion per mass. The challenge is no longer to prove that they work, but to make sure they do so reliably, affordably, and comfortably.

Like many viral trends, cooling blankets have one foot in reality and the other in exaggeration. Some of them really do work, not through magic or a “secret formula sealed in a wax envelope,” but thanks to well-known principles of physics and materials engineering. And although the effect isn’t eternal or miraculous, it may be just enough to get through a summer night without breaking a sweat.

María Dolores Martín Alonso, Materials Science PhD, IMDEA MATERIALS

This article was originally published in The Conversation. Read the original (content in Spanish).