"You get this little machine you can control from the outside, and it has a sword. The machine gets in the water with a paramecium, and you try to stab it" 35

"You get this little machine you can control from the outside, and it has a sword. The machine gets in the water with a paramecium, and you try to stab it" 35 years after this statement these nano devices still do not exist even in laboratory settings. What has gone wrong? What is preventing this?