There are all sorts of devices designed to make virtual worlds feel more real by mimicking physical sensations: full-body suits , gloves , robotic hands , and exoskeletons. These products mimic sensations, but they don’t create the realistic impression of spaces or structures, like walls or doors. As adoption of AR and VR becomes more pervasive, being able to walk through walls won’t seem like a neat trick–it’ll be a significant limit to user experience, jarring you out of a world that may otherwise seem realistic.

But how do you create a virtual wall, a physical obstacle that you can touch and push against, without using unwieldy mechanical hardware?

For human-computer interaction researcher Pedro Lopes, the answer lies beneath the skin. Lopes, along with Patrick Baudisch, Sijing You, Lung-Pan Cheng, and Sebastian Marwecki at the Hasso Plattner Institute in Potsdam, Germany, created a simple wearable that uses electrical muscle stimulation–small electric shocks–to directly deliver sensation to the user’s muscles. It’s similar to the type of stimulation used in physical therapy.

“We were really interested in trying to explore one of the hardest things to recreate in terms of physical sensation, which is a wall,” he says.

Lopes and his team found that they could use the wearer’s own muscles against them, artificially creating a counter force, or the feeling of resistance to a physical object, by triggering the opposing muscles. The same setup can also enable the wearer to “hold” a large cube in virtual reality or press a virtual button–while using only a small wearable device hooked up to electrodes that are stuck to the user’s skin.

The wearable setup includes up to eight electrodes stuck to the forearm, bicep, tricep, and shoulder of each arm. The electrodes are then hooked up to a medical eight-channel muscle stimulator, which is controlled by the VR simulator. The entire thing can be worn in a small backpack and serves Lopes’s research purposes–though he does note that he’s not a product designer, and envisions a much more streamlined design for a commercial version of the technology.

A wearable that creates the impression of physical structures would let users explore a virtual world in a way that feels genuine. Instead of your fingers passing right through every virtual wall you encounter in a particular game, you’d feel solid barriers. Or, if you’re exploring a virtual world that included a hazardous design element, the world’s designers could code the electrodes to fire, pushing your fingers away more forcefully.