First question: what exactly are fuzzy nanoassemblies and what are layered polymeric multicomposites?

"Fuzzy" refers to a structure that is not perfectly well defined, in the sense that a crystal is perfectly well defined. But it does not mean that it is completely disordered either; what is meant is that there is positional order between molecules, but not between atoms. As for multicomposite, this is related to complex systems. If you look at complexity in nature, it is the evolutionary consequence of the need for more functionality.

In nature, new properties arise from the close spatial arrangement of smaller components—an aggregate of enzymes, for instance, responsible for a series of consecutive transformations. One of the most interesting properties evolving from the intricate interplay of thousands of subcellular components on the length scale of about one micron is life itself. Life is such a complex process requiring so many components that they cannot be fitted into a compartment much smaller than a cell.

In materials science most of today’s "workhorses" are single- or two-component systems. However, scientists are already working on the design of smart materials that require a much higher complexity. Our interest is to put as many different materials together as needed in order to create materials with new properties. This is similar to the attempt to synthesize molecules possessing multiple functional groups, which eventually should become impractical if the number of functions gets too high or the molecule gets too large. Layer-by-layer self-assembly may be an alternative way to create multimaterial composites in a simple fashion.

  

What it means is that multimaterial assemblies are created by several consecutive adsorption steps. For doing so, one needs components that interact with each other, for example electrostatically. If you have one molecule on the surface, this will attract a partner molecule from the solution to the surface and this can be done in a consecutive fashion such that it repeats from molecule to molecule, first a, then b, then a, then b. And then you can add molecule c, that is attracted to either a or b, so you can continue to vary the architecture in an easy way. At first layer-by-layer deposition started out with electrostatic interactions, which includes materials such as charged polymers, but there are other intermolecular interactions that can be used. Typical materials that are used by various research teams for making multilayer composites include polyelectrolytes, DNA, proteins or colloids.

    Why do you think your 1997 Science paper has had such tremendous impact?

Well, if we go back to layer-by-layer techniques that people have previously used; these date back to one very elegant technique developed in 1927 by Irving Langmuir and Kathleen Blodgett. This procedure is so attractive that it has been used by colleagues all over the world to fabricate so-called Langmuir-Blodgett (LB) films. However, the technique has not been successful in the commercial sector. There have been huge collaborations between scientific institutions and industry to develop the technology. While the scientific results have been quite exciting, there was never a commercial breakthrough. Some of the problems associated with the practical application of the Langmuir-Blodgett technique have been solved by us and this has kindled widespread interest among the LB community.

    So your layer-by-layer assembly approach works better and leads to products?

Well, the fact that layer-by-layer assembly by adsorption from solution is a robust technique and works for many different combinations of materials is most likely the reason why it had the impact that was shown in your survey. However, before we talk about applications one needs to discuss what kind of problems can potentially be addressed for use by commercial companies.

So why are scientists interested at all in making these surface layers and why should companies be interested? The answer is quite simple in the case of devices made from simple materials. Such an object has intrinsic properties such as shape or mechanical properties. However, every object interacts with its environment through its surface. So all the interactions with the environment are dictated by the properties of the surface. If we can control the surface of an object, we can control the interaction of this object with its environment. So that means, for instance, that we might put a thin film on the surface and that might provide corrosion protection, or anti-static coating. It might be sticky or non-sticky, hydrophilic or hydrophobic. It might make the surface biocompatible or anti-bacterial or suitable for sensing. Many interactions could ultimately be useful and we can do many of them with this technique.

Reason number two is that we can use it to fabricate surface-based devices, which means we can put materials onto the surface that would then function as a device. For example, in optics or bio-sensing or separation membranes. Today many groups in physics, chemistry or biology are working on fabricating multicomposite films by layer-by-layer assembly that may lead to prototypes of devices and hopefully real applications.

To put the matter in perspective, I should say that there is already a first commercial product on the market since early 2002.

First, the technique is robust; it helps that it is also cheap and environmentally friendly. Second, it is just very easy to put new combinations of molecules and thus new functionality into such layered systems. So far, for instance, more than 30 proteins have been put into multilayer films, some just as a tryout, some to characterize their function, some for sensing purposes. One can easily put DNA into such a film, and one can also put in inorganic compounds. One can put in colloids, which are solid objects of nanoscopic to microscopic dimensions. We've put gold colloids into these layers, which give the layers properties that would be very hard to engineer in otherwise.

    Where do you think this research is going in the near future?

In my opinion it will be going in two directions. In the biosciences, I would say the major objective would be to use the technique to enhance biocompatibility; to improve surface coatings for biomedical applications, for example. And in materials science it is going toward layer-based devices. At the last American Chemical Society meeting, a company announced a biomedical product that is equipped with this technology and was to be available on the consumer market in the beginning of 2002.

    Are you satisfied with the pace of research in your field?

On the one hand, when I look worldwide, there are now more than 40 independent research teams working on topics related to this technology. The growth rate has been exponential, which is very rewarding to see. As for our own research, we have been quite successful in applying for research grants in addition to what we can do with the resources in our laboratory. We are happy that Ph.D. students and postdocs are eager to join our team on topics related to multilayer assemblies. A first commercial product is on the market, which represents a technology being implemented much quicker than average. The pace is already amazing, yet there are still many more ideas than we can realize.

    What was the biggest obstacle in pursuing your research?

Well the biggest obstacle at first was to find the first system that worked. Looking back we had chosen a very difficult system to begin with. Luckily this changed quickly later on. After that it was not so easy to get accepted that things were like they were and especially that they were as easy as they were. The biggest obstacle remaining today is that layer-by-layer assembly is not a technology that is focused on a single goal. Rather it is a very fundamental technology, something very general that can be applied to many domains and many different kinds of problems. There is no clear answer for the question of what it should best be used for, thus one cannot easily go for the single most promising target with respect to research. There are huge barriers of belief before scientists and company researchers accept more general solutions, which actually is a good thing. However, pushing into many fields at the same time was something that was only possible by the community of independent research teams working now on the topic.

    Is there anything else you'd like to say about your work?

In advertising for the layer-by-layer assembly technique, I have always used phrases like "Multifunctional coatings for surfaces of almost any kind and any shape." This seems to become more and more of a reality. The biggest reward was to see that the technique works well in other people's hands and that now they are actually using it and taking the technology further along. It has been shown by colleagues that multilayer assemblies can even be put on colloids or onto cells like erythrocytes, something we had never anticipated ourselves. I am quite curious what else will emerge.

Professor Gero Decher
Université Louis Pasteur
Strasbourg, France