2.3 Biomimetic Movement

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A similar approach can be used to scale up artificial muscles (Figure 2.10) [24]. This is

another kinesin-microtubule microtubule/kinesin system that self-organizes from the

nanosize to tens of millimeters. Microtubule structures called “asters” (after the flower)

are held together by DNA bundles. These self-assemble via single-stranded DNA into

larger-scale structures. Multimeric kinesin linkers then walk on the microtubules in the

presence of ATP, thus contracting the “artificial muscles”.

To summarize, most applications of molecular motor proteins take advantage of

their movement. Several methods have been developed to control the direction of their

movement, even when more than one cargo is moving at the same time. An automatic,

more general assembly has now been reported as well. Cargo attachment, however, is

still a rather involved process, and large-scale preparation or long distance and long

term transport are still difficult. Initial work has been done to create self-organizing

structures across scales to create microscopic artificial flagella and muscles.

2.3 Biomimetic Movement

There are currently different approaches that use molecules for movement. One inter-

esting approach is to use random Brownian motion as the driving force (e. g., [25, 26]).

Another approach uses catalysts (e. g., [27]). These methods have not achieved a purpose-

ful direction or transport of other molecules yet and, therefore, will not be described in

detail here.

Instead, we will select approaches that are based on small molecules and have al-

ready achieved some movement control. The first approach works with rotaxanes. Ro-

taxanes are molecules that consist of two parts: a dumbbell-shaped molecule and a free-

floating ring around the dumbbell that is small enough in diameter that it cannot escape

at the ends of the dumbbell (Figure 2.11). Movement occurs when the ring moves from

one end of the dumbbell to the other. This can be controlled via several mechanisms. In

one example, this movement is initiated by a redox reaction on part of the dumbbell [28]

(Figure 2.11). The ring consists of positively-charged, aromatic compounds. The dumb-

bell contains an electron-rich and a neutral aromatic site. The ring will reside on the

electron-rich site and will only move after that site has turned positive via a redox reac-

tion. This is a reversible process, thus the redox reaction acts as a switch. This movement

could be used, e. g., for transport if the ring can be fixed to a surface.

The group of Huang has also worked with a rotaxane containing two rings [29] (Fig-

ure 2.12).

The two rings of the rotaxane are connected to an AFM cantilever. With an oxida-

tion, the two rings move from the outside to the inside of the molecule, thus bending the

cantilever (Figure 2.13). It has been demonstrated that the movement of the rings can be

translated into a force, in this case bending a cantilever. Unfortunately, after 20 cycles

or so the rotaxane degrades and the bending stops.