How do you make a nanobot?

Here is an essay I wrote while reflecting on and researching a lecture I heard by Yale Goldman, Professor of Physiology at the Pennsylvania Muscle Institute, and associate director of the NBIC at Penn.

How do you make a nanobot?

Imagine you are playing with a set of Legos. This set is one of the newer

ones. It has all kinds of blocks, gears, wheels, pulleys, batteries, etc. Put different

combinations of them together and you can build a house or maybe if you’re feeling

clever, a machine. Suppose you make a small battery-powered car. Imagine shrinking

this car. As it shrinks, the number of atoms in each Lego block must become fewer.

Eventually, you may reach the limit where each block is one atom. What if we could

make machines like this?

Eric Drexler had this vision in his 1987 book Engines of Creation: The Com-

ing Era of Nanotechnology. He imagined if we could manipulate atoms like Legos,

we could build nanomachines from the ground up, atom by atom. One could create

different kinds of machines by simply using different assemblies of atoms. Drexler

hoped that these “nanobots” could be used to do all sorts of things, like deliver drugs

to specific sites within the body. Micheal Crichton in his 2002 novel Prey envisioned

these nanobots existing, but in a typical Crichton ending the bots take over and kill

us. While Drexler’s vision is intriguing (and Crichton’s vision is scary), these hopes

(fears) should be looked at skeptically. For starters, it’s simply not possible to build

nanomachines using Drexler’s scheme.

Why is this? Consider a Lego block floating in a cup of water. This block is

surrounded by water molecules. The water molecules are constantly jostling about

due to a phenomenon called Brownian motion. You won’t see the effect of this phe-

nomenon with the Lego. Change the Lego to a speck of dust. The dust is much

smaller than the Lego and so is more affected by the jostling of the water molecules.

(You can actually see the dust being randomly jiggled around if you do this experi-

ment.) Now, imagine changing the speck to a single sugar molecule, comparable in

size to the water molecules. What was gentle jiggling of the dust translates into vio-

lent jostling of the sugar molecule. In fact, the sugar molecule collides with the water

molecules about a trillion times per second! So we really can’t place atoms together

in an orderly fashion like we can with Legos: the wind is too strong. In brief, the

nanoscale is a much harsher environment than the macroscale.

Despite the seemingly impossible construction challenge of Brownian motion,

nanomachines are ubiquitous. They are around us and already inside of us! Fear not,

as they are supposed to be there. They are naturally occurring biological molecules

that keep us functioning. So in order to make artificial nanomachines, we might need

to steal ideas that nature is already using. Biological nanomachines make use of a

few tricks, that scientists one day hope to copy. First, their molecular makeup is pre-

programmed by a DNA sequence. Secondly, they undergo self-assembly to make up

their final shape. Lastly, they use the Brownian motion of the surrounding molecules

to their advantage rather than detriment.

One example of a natural nanomachine is myosin. There are several different

varieties of myosin. They are all motor proteins that “walk” along long actin fibers.

All myosins are actuators, machines that convert one form of energy to another. Like

the motor in a battery-powered car converts electrical energy into motion, myosin con-

verts the energy of a chemical reaction (ATP to ADP) into motion. Clearly myosin

is a great model nanomachine to study for future technology. One common variety

of myosin (II) is responsible for flexing our muscles by sliding the filaments against

each other. One variety (V), is responsible for moving large amounts of cargo along

actin fibers. Myosin V has two “hands”, and seems to move hand-over-hand along

actin, similar to a child on monkey bars [Corrie, J.E.T., et. al. Nature 422, 399

(2003)]. However there does seem to be a key difference. The child on monkey bars

releases one hand, then does a power stroke with the free hand to get to the next

rung. Myosin does the same motion, but its power stroke only takes it 2/3 of the

way. It then lets the Brownian motion of the surrounding water push the rest of the

way [Shiroguchi, K. et. al. Science 316, 1208 (2007)]. By taking advantage of this

Brownian motion the efficiency of the system is about 50%, very comparable to an

electric motor!

While artificial nanomachines are still only a vision, it’s clear that natural

nanomachines are excellent prototypes. Unfortunately, our technology is not at the

stage where we can really imitate them. What scientists can do (and are doing now)

is try to understand more deeply how these fascinating natural nanomachines work,

and use this understanding to inform future technologies.

[1] Drexler, Eric. Engines of Creation: The Coming Era of Nanotechnology, 1987

[2] Crichton, Micheal. Prey, 2002

[3] Goldman, Yale. “Nature’s Nanotechnology: Biomolecules Explored One at a Time” Penn Science Café Lecture, 20th October 2010.

[4] Corrie, J.E.T., et. al. Nature 422, 399 (2003).

[5] Shiroguchi, K. et. al. Science 316, 1208 (2007).

Cartoon of still SEM images of myosin processing along actin from: Matthew L. Walker, Stan A. Burgess, James R. Sellers, Fei Wang, John A. Hammer III, John Trinick & Peter J. Knight. Two-headed Binding of a Processive Myosin to F-actin. Nature, 405, 804-807 (2000).