Nano-Machines Mimic Muscles

In a recent paper, scientists from France's National Centre for Scientific Research (CNRS) reported that they were successful in the development of an artificial muscle construct.This innovative work, headed by Nicolas Giuseppone, a professor at the Université de Strasbourg, and involving researchers from the Laboratoire de Matière et Systèmes Complexes (CNRS/Université Paris Diderot) uses tens of thousands of individual molecular nano-machines. The nano-machines work together to extend and contract a polymer chain over a length of 10 micrometers. The main compound in the polymer chain is rotaxane, which is a molecule that is commonly studied with regard to molecular machine construction. 

Rotaxane is an example of a mechanically-interlocked molecular architecture. What this means is that rotaxane is compound that is a combination of molecules that are not connected by bonding but rather by geometry. An analogy, which is explains this is that of a key and a key chain; the keys are not directly attached to the key chain, yet the keys cannot be removed from the chain until the chain is broken. In this case, the blue part of the rotaxane, which is a dumbbell shaped molecule, plays the role of the key while the cyclobis macrocycle is the key chain.

Crystal Structure of Rotaxane with a Cyclobis Macrocycle

The French researchers were able to extend and contract the chain by varying the pH levels of the environment. A change in pH causes the thousands of individual components to either compress or elongate by one nanometer, and these changes in size in the individual molecules add up to cause a drastic change in the size of the polymer chain. This movement in the polymer chain is significant enough and easily controllable so the chain can be used to mimic the function of real muscles and can also function as a nano-scale mechanical machine.

Image depicting the contraction and expansion of the polymer chain in a way that mimics human muscle movement.

The way scientists were able to connect thousands of rotaxane together to form a polymer chain is through supramolecular bonding.  A supramolecular bond is an interaction between different molecules that is not based on a traditional “covalent” chemical bond but instead on what are known as “weak interactions”, thereby constituting complex molecular structures. Supramolecular bonding can consist of a variety of interactions like hydrogen bonding and van der Waals forces. The strength of supramolecular bonds ranges from very weak like hydrogen bonding to very strong like covalent bonding.  It is the supramolecular bond that connects all the elements in the polymer chain together and makes the contracting and extending of the chain possible.