Two Hands Are Better Than One (up to constant factors)
  We study the difference between the standard seeded model of tile self-assembly, and the "seedless" two-handed model of tile self-assembly. Most of our results suggest that the two-handed model is more powerful. In particular, we show how to simulate any seeded system with a two-handed system that is essentially just a constant factor larger. We exhibit finite shapes with a busy-beaver separation in the number of distinct tiles required by seeded versus two-handed, and exhibit an infinite shape that can be constructed two-handed but not seeded. Finally, we show that verifying whether a given system uniquely assembles a desired supertile is co-NP-complete in the two-handed model, while it was known to be polynomially solvable in the seeded model. 
Sarah Cannon, Erik D. Demaine, Martin L. Demaine, Sarah Eisenstat,   Matthew J. Patitz, Robert Schweller, Scott M. Summers, ...
cs.CC cs.CG