Drexler saw unlimited potential for nanotechnology. He postulated tiny little machines using gears and motors and rotary motion, molecular analogs of macromachinery. Actually that isn't likely to happen. In fact, nature has been there ahead of us. Nature has already solved the problem of rotary motion in the cilia of certain bacteria.
Nature also has much better power efficiency.
The rest is batteries. Nature uses glucose oxidation to get much better power efficiencies. The bacilli have rotary cilia for rapid swimming, and that's a better model than tiny gears and motors. A human being is generated by a genome of some 30,000 genes. Cells are RNA machines with proteins.