You’re shopping for a sweater, and you spot an adorable one. But there’s a problem: You’re right between a small and a medium, and neither size is quite right.

In 2019, you’ll be able to buy a sweater from the direct-to-consumer fashion company Ministry of Supply that will adjust to your size on the spot, using only heat. That means you could grab a medium off the shelf, and after Ministry of Supply takes your measurements, an in-store robotic machine will use a heat gun to adjust the sweater to your size right before your eyes.

The technology, developed by the Self-Assembly Lab at MIT, provides a clever solution to the problems of mass customisation. For years, fashion companies have explored how to mass-produce custom garments, but problems with efficiency and affordability have held up mass adoption. The collaboration between Ministry of Supply and the Self-Assembly Lab approaches the problem from a different perspective: Their sweater, coming to stores within a few months for a still-undisclosed price, can be mass-produced in standard sizes. The customisation process happens in-store, instead of during the production process.

This smart sweater doesn’t need a battery, nor is it robotically knitted using metal thread or shape memory alloy, which would traditionally be used to make a material change its form. Instead, the fabric shrinks when exposed to heat, thanks to both the structure of the knit and the combination of materials used. While the researchers wouldn’t get more specific about those materials, they say that the shape-shifting technology depends on the way that two different off-the-shelf materials interact together when they’re exposed to heat.

“People want to make smart garments and shoes, but they’re always putting batteries in your shoes,” says Skylar Tibbits, the founder and co-director of the Self-Assembly Lab. “We want them to be active and smart, but passive in the sense that it’s all based on materials.”

Tibbits and his team developed the fabric through hundreds of experiments, combining different types of materials together and then trying out different knitting patterns to see how they would react to each other when exposed to heat. The research, which is part of a grant from the MIT-based nonprofit organisation Advanced Functional Fabrics of America, was originally aimed at creating reversible transformations when it comes to shape and porosity, to make garments more breathable or waterproof based on temperature and moisture. But the team unexpectedly stumbled upon a way to create a permanent transformation, which is what enables this kind of custom tailoring.

Tibbits says that original research into reversible transformation–for instance, a jacket that could become waterproof when it comes into contact with moisture, and then become more breathable again when it’s dry–will continue. For now, his team is working with Ministry of Supply to turn their discovery about permanently changing a garment’s shape using heat into a full-fledged commercial product, starting with the company’s forthcoming in-store sweater customisation experience.