They make the point that rotary joints are for conceptual illustration, and it could equally use hinge or flex joints. This would be relevant at the nano scale,

> In particular, a molecular version of this architecture could use stiff covalently-bonded nanotubes for the links and single bonds for the joints.

In that case joint friction would not be a consideration, or at least I would imagine things like electrostatic or molecular bonding forces would be larger.

I was going to submit the same paper, and had the same question, along with another one. The abstract says that the system can be scaled down to molecular level, much like electronics has been scaled to sub mm or nm level. But at the sub nanometer level, we face quantum effects which are hard to overcome. Would mechanical based computing systems give us the same headache as we try to scale them further down?

I think we know how to do links and rotary joints at that scale, and that's all you need for these machines.