Chain is made of separate chain links. I made three different chain link types:

• with circular cross-section (needs support material )
• with octagonal cross-section (does not need support material)
• with square cross-section with rounded sides (does not need support material. I prefer this type.)
  Each chain link has X-shaped slot in the middle. These slots make it possible to interlink chain links into a chain. But it is also almost impossible for chain links to separate on their own. The same method is used to lock handcuffs.

I also made normal sized version for hands and smaller sized version for fingers.
Normal sized version uses M4x35mm bolt and lock-nut to assemble the handcuffs.
Small sized version uses 1.75mm filament (length about 18mm). Then you have to heat up the ends and squish them. I have used heated bed at 120°C, but you can also try other methods (soldering iron, 3d pen, hot water,...)
I have also included Fusion360 files, so anyone can change and adjust the size and shape.

Because Chain links have slots in the middle, they are not as strong as they could be. If you need stronger links, you can weld the slots with 3d pen or with friction welding with rotary tool. (After you assemble the chain.)