It is an old t-shirt with an electronic circuit which blinks some LED when triggered by an infrared remote control.
The circuit is very simple and consists of few main components :
The conducting wires used were sewed with a sewing machine, they took the place of the bottom thread. It might not have been the best choice as it made the sewing process quite tricky. But I think it is worth another try with thinner and more flexible wire.
The wires used are not particularly well choosen for this application as they were single thread wires from old IDE data cable. They are easily broken when folded, particularly near soldering points.
The infrared receiver is located inside the Tshirt (in a small pocket), thus the infrared signal is received through the cloth. The range is therefore limited to a few meters which is more than enough in this case.