On Soft Matter

FASCINATING Research Soft MATTER Hard Work on Soft Matter

What do silk blouses, diskettes, and the membranes of living cells have in common? All three are made barked on an unparalleled triumphal between the ordered structure of building blocks.“ And Prof. Helmuth march. Even information, which to- solids and the highly disordered gas. Möhwald, director of the “Interfaces” from soft matter: if individual molecules are observed, they appear disordered, however, on a larger, day frequently tends to be described These are typically supramolecular department at the MPIKG adds, “To as one of the most important raw structures and colloids in liquid me- supramolecular scale, they form ordered structures. Disorder and order interact, thereby affecting the put it in rather simplified terms, soft materials, can only be generated, dia. Particularly complex examples matter is unusual because its struc- properties of the soft material. At the MAX PLANCK INSTITUTES FOR POLYMER RESEARCH stored, and disseminated on a mas- occur in the area of biomaterials,” ture is determined by several weaker sive scale via artificial soft materials. says Prof. Reinhard Lipowsky, direc- forces. For this reason, its properties in Mainz and of COLLOIDS AND INTERFACES in Golm, scientists are focussing on this “soft matter”. Without light-sensitive coatings, tor of the “Theory” Division of the depend very much upon environ- there would be no microchip - and Max Plank Institute of Colloids and mental and production conditions.” nor would there be diskettes, CD- Interfaces in Golm near Potsdam The answers given by the three ROMs, and videotapes, which are all (MPIKG). Prof. Hans Wolfgang scientists are typical in that their made from coated plastics. Spiess says, “Soft material reveals its perspectives all differ. Some scien- It is nature, however, which has complex combinations of properties tists define soft matter as being al- really perfected the art of creating due to the precise interplay between most liquid, whilst others also in- soft matter. For billions of years, areas of order and disorder of the clude very strong materials such as natural or artificial fibres. However, whether liquid or solid – there are properties that all forms of soft materials have in common. What the three scientists have outlined in

Hans Wolfgang Spiess with the probe-head of a NMR spectrometer (in the background) Klaus Müllen also achieves mastery over polymers in the form of a football…

he cultural development of since the existence of life on earth, „T mankind is inseparably linked highly complex structures have orig- to the development of new, high- inated from soft matter. The ways in performance materials,” says Prof. which they arise are typically very Hans Wolfgang Spiess, Managing elegant; the molecules organise Director of the Max Planck Institute themselves into complex cellular for Polymer Research in Mainz; “it is membranes which perform vital not without reason that we speak of functions. Or they form extremely the Stone, Bronze or Iron Age.” After strong fibres that give plants shape this hard prehistoric past, we have and support. Biologists, chemists, now arrived in the age of soft mat- material scientists, and physicists are ter: leather and natural fibres such trying to understand and emulate ILSER as wool and silk or paper have been F nature’s incredible structures and

important economic commodities for OLFGANG production processes.

centuries. And over the past fifty : W “The term soft material relates to HOTOS

years, synthetic materials have em- P the states of aggregation which lie

52 M AXP LANCKR ESEARCH 4/2001 4/2001 M AXP LANCKR ESEARCH 53 FASCINATING Research Soft MATTER

technical terms (explained on page tremendous variety of polymers. The Despite the simulation being virtual, excited by an NMR spectrometer to 60) can be roughly expressed in lay- carriers of the hereditary informa- they are able to zoom into the mole- receive and transmit electromagnetic man’s terms as follows: tion of life, the nucleic acids, are cular processes that take place in re- signals. From this, the scientists are ❿ Firstly, the molecules form a polymers. Proteins are polymers, as al polymers. able to deduce how the material is much more disordered structure in is the starch from potatoes or grains. For the computer simulations to be constructed. Spiess and his col- soft matter than the atoms or mol- Polymers also lend animal tissue able to draw a realistic picture of the leagues have developed new solid ecules in the crystal lattice of a elasticity and stability in the form of material being studied, they must al- state NMR methods, which are par- true solid. On the other hand, they Fig. 1: This three-dimensional collagen fibres. Other polymers, cel- so be provided with reliable data ticularly suitable for explaining the do not swirl around in a disor- At the Max Planck Institute for “snapshot” from a Mainz computer lulose fibres, perform this function from measurements in order to have structure of polymer materials. simulation shows how the long dered fashion as they do in a gas. Polymer Research in Mainz, the sci- in plant tissue. firm ground to stand on. This data is By working with the theorists and polymer chain molecules move in Soft materials are also not true liq- entists are investigating a type of a polymer melt. In Mainz, two research groups are being provided by the “Polymer spectroscopists, the material scien- uids, although areas that are liquid soft material without which life – playing a special role in the analysis Spectroscopy” research group under tists in Mainz are able to experiment can affect their properties signifi- and our everyday existence as we of the properties of polymers. They Hans Wolfgang Spiess. Spectroscopy very effectively. Besides, pure re- cantly. know it today – would be unimagin- are developing increasingly precise is the second tool for the developer search, applied research – the aim of ❿ Secondly, the structures of soft able: natural and artificial materials instruments which allow them to of new materials. This is where the which is to develop marketable prod- matter are both flexible and stable. consisting of polymers. Polymers are look into the world of molecules. new samples are investigated using ucts – is also important at the Mainz ESEARCH ❿ Thirdly, soft matter can form large molecules in which hundreds R Thanks to these tools, the researchers various analytical methods. NMR Institute. The scientists at Mainz are

supramolecular structures sponta- or thousands of similar, fundamental OLYMER in the laboratories are far better able (Nuclear Magnetic Resonance) spec- even able to further develop conven- P

neously through self-organisation. building blocks, the monomers, are FOR to understand the properties of the troscopy is particularly important. tional mass-produced plastics so that Without this particularly interest- strung together. Every artificial ma- polymer molecules. They are in a NMR makes use of the fact that they get completely new properties; .: MPI IG ing characteristic, no natural or- terial, from the contact lens to the F much better position to investigate many atomic nuclei behave like tiny for example, pipes made from poly- ganism would be able to exist or most modern woven fibre, is a poly- Fig. 2: The pushed-in repair tube materials and develop new ones with magnets. Placed in a strong magnet- ethylene, the artificial material from survive. mer. Nature conceals an even more before inflation. greater accuracy than previously. ic field, these atomic magnets can be which plastic bags are made. One successful example of industrial co-operation by the Max Planck Institute for Polymer Research is the development of an artificial material Joachim O. Rädler is head of the polymer physicists in Mainz. His group is for renewing pipes in collaboration studying the interaction between molecules in order to use them for building nano-systems, and is hot on the trail of nature’s fascinating strategies, one of which is the self-organisation of molecules into complex structures. Kurt Kremer is head of the Theory Division in Mainz.

One of these tools is computer simulation. The experts in this area are members of the “Theory of Poly- mers” research group, headed by one of the Institute directors, Prof. Kurt

Intact material Formation of cracks Kremer. The scientists construct the simulated polymer material from individual, virtual molecules and cal- culate its properties. Soft matter is still too complex to be easily repro- duced in all its detail on a computer. But the theorists are able to simulate

Partially crystalline polymer the most important properties amazingly well (Fig. 1). In the process, Fig. 3 left: The long chain molecules they are learning how the individual (red) hold the crystalline (blue) together in the undamaged plastic. molecules behave in the polymer. Right: Under stress the chains 54 M AXP LANCKR ESEARCH 4/2001 tear and a crack opens. 4/2001 M AXP LANCKR ESEARCH 55 Fig. 4: Dumbbell-shaped graphite FASCINATING Research islands arrange themselves on a Soft MATTER graphite surface. Two neighbouring islands are approximately 3 nm away from each other.

with a subsidiary of the former to the Max Planck Society’s annual great pressure. As it must not deteri- the crystallites. The industry is now dumbbell or a figure of eight. Fig. 4 Hoechst Corporation. Who hasn’t budget”, Hans Wolfgang Spiess men- orate with time over decades, flexi- using the modified plastic to produce shows the regular pattern formed by had big problems with pipes at one tions in passing. bility creates a problem. Flexibility new, flexible pipes with a life up to these islands on the surface. time or another? And not just with does in fact depend on high mobility fifty times longer than before. Now the island structures develop HIGH-TECH-PLUMBERS ceramic or metal pipes: plastic pipes of the molecules in the artificial ma- It may be that soft materials of the remarkable behaviour: they stack can also crack or become porous. His group made a significant con- terial. For this reason, the molecules future will have completely new themselves up into microscopic twin “American pipe manufacturers have tribution to the development of a do gradually tend to give way under properties. For example, they might columns. These columns can convert already had to pay a billion US dol- product that cleverly simplifies the stress. The pipes age and can crack convert light into electric power. So- the captured light into electric pow- lars in compensation, a figure equal repair of gas and water pipes. It is a at places subjected to intense pres- lar cells have enormous potential for er. The shape of the columns is cru- flexible polyethylene pipe. The re- sure. Fig. 5: How the Mainz chemists growth. Today they are made from cial to their efficiency. The scientists manufacture nano-objects: from the pairers first push it, folded up, into The scientists in Mainz first had to silicon, a hard, brittle material with once again struck on the characteris- starting material, left above, blocks the old pipe. Once it is in position, establish how these cracks develop develop through differently controlled limited applications. If it were also to tic property of soft matter: a well de- A FIELD OF RESEARCH they inflate it – and that’s it! The in the polyethylene material (PE) sources, which contain different prove possible to manufacture solar fined “ordered disorder” in the stacks IN ITS INFANCY new pipe nestles up to the wall of used. To this end, they developed a nano-structures (yellow). Solvents cells in the form of light, pliable makes the columns such good elec- the broken pipe, bridges leaky seals special NMR experiment that showed dissolve the polymer block (blue) and films, it would open up completely trical conductors that they are ideal- When the French physicist Pierre-Gilles de Gennes release the nano-building blocks. was awarded the Nobel Prize for Physics in 1991, and cracks, thus sealing it complete- them what happens to the molecules new areas of application. ly suited for solar cells. This devel- a scientist commonly described as the “father of ly (fig. 2). during the ageing process. PE is a opment is still in its early stages, but soft matter“ was honoured. He had made signifi- Despite the incredible simplicity of typical form of soft matter that owes SOLAR CELLS ON FILM perhaps one day we will even wear cant theoretical contributions to a young field this idea, it does impose strongly its properties to the interaction be- The research group headed by jackets which are capable of charg- of research that only became more defined in the conflicting requirements on the plas- tween order and disorder. It consists Prof. Klaus Müllen, another director ing mobile phones or Walkman bat- eighties. Besides new theoretical models, development also progressed due to better measuring tic used; on the one hand, it must be of crystalline areas in which the long at the Mainz Institute, is developing teries using solar power. techniques and the growing capabilities of com- very flexible, whilst on the other, it molecular chains of polymers lie films from organic materials which There is little that fascinates scien- puters. This resulted in the scientists discovering must be capable of withstanding highly ordered, just like spaghetti in are capable of converting light into tists more today than nano-objects, that the properties of very varied and structurally electric power. The basic components which measure just a few billionths complex materials all had a common factor: the of the new materials are carbon up to several hundred billionths of a interaction of order and disorder on a molecular level. Whether virtually liquid or very strong - atoms in the form of graphite, which metre. Work on nano-objects is ex- these materials are always flexible. It was for this we commonly come across as soot or panding our knowledge of the chem- reason that they were christened “soft matter”. in pencils. The carbon atoms form ical, biochemical, and physical pro- honeycombed lattices in the graphite cesses which are responsible for the Wolfgang Knoll, Mainz Materials Research Director, Fig. 6: A multi-layer nano-capsule which can be easily pushed together. diverse nature of our world. And yet, must, like all other scientists, conquer mountains of paperwork. is formed: the nucleus is coated with First the scientists synthesise tiny the scientists are only really just be- oppositely charged polymer molecules (red or blue) (A to D). Next, acid graphite islands on a graphite sur- ginning to get into the nano-world dissolves the nucleus (E), and the face. These are in the shape of a (MAX PLANCK RESEARCH 3/2000). And completed hollow capsule is left (F). it is therefore hardly surprising that today’s research is sometimes remi- niscent of small children at play: by playing around with building blocks, a bag. Long chain molecules wind children “come to grips” with the themselves round these crystallites, world - by playing with tiny nano- just like tangled knots of thread. A B C building blocks, scientists become They support the structure. Under acquainted with and master the stress – such as excessive pressure – Gradual adsortion of polyelectrolyte molecules nano-world. Although even the man- these chain molecules can tear. If ufacture of the nano-building blocks several neighbouring chains give themselves is a challenge to today’s way, a weak spot develops in the scientific technology. material. With time, these weak spots In collaboration with colleagues grow until cracks open up (fig. 3). from the group led by Hans Wolf- After the Max Planck scientists gang Spiess, Dr. Ulrich Wiesner, cur- had explained the mechanism of rently at Cornell University in the ageing, the industrial scientists were F E D USA, has produced nano-objects able to make specific improvements such as these from organic/inorganic Finished product: Removal of the nucleus to the artificial material by strength- a polyelectrolyte capsule hybrid materials. Such materials are ening the chain compounds between used to manufacture contact lenses,

56 M AXP LANCKR ESEARCH 4/2001 4/2001 M AXP LANCKR ESEARCH 57 FASCINATING Research Soft MATTER

scratch resistant coatings, and dental ganic solvent, and add a given chain molecules stand out from their searchers are able to build up many capsule walls can be formed which fillings. They are based on two phas- quantity of A. Then they condense surfaces like hairs. Who knows what alternately charged coatings like remain stable for years or, by con- es, which are mixed together. The the resulting composite to make it surprises these hairs might have in onions skins, thereby greatly varying trast, can be made to readily decom- first phase, termed “organic“ in the solid. Amazingly, self-organisation store for us… the properties of the capsule. pose. chemical world, consists of long- results in the formation of very reg- Fig. 7: Scanning force microscope At the Max Planck Institute of What happens to the nucleus in- The permeability to different ac- chain polymer molecules. For the ular nano-structures. Even more in- photograph of a completed nano- Colloids and Interfaces in Golm, side the capsule now? If it is not tive substances can also be regulat- capsule. This requires a vacuum sake of simplicity we shall call this credible: the shape of the structures there is also close collaboration be- composed from the substance with ed. If, for example, the scientists in which the capsule caves into folds. phase O. The other phase is “inor- can be precisely controlled by the tween experimenters and theorists which the capsule is to be filled, it vary the surfaces of the layers, they ganic”, and we shall call this phase mixing ratio of phases A and O. Pro- from the chemical, physical and ma- must be removed from the capsule can cause certain chemical sub- A. A is formed from smaller mole- vided that phase A is not dominant, terial sciences. Helmuth Möhwald’s without it being destroyed. To do stances to accumulate within the cules that contain metal atoms (alu- a polymer block is formed. It con- team are among the experimenters. this, the scientists dissolve it into capsule interior. The capsules fill minium) and a compound called hy- tains the nano-structures produced They invented a modular system for small molecules. The wall of the themselves up, so to speak, with the drosilicon. Hydrosilicons consist of from A and O. Depending upon the nano-capsules, which can be used polymer capsule is in fact permeable desired active substance. hydrogen and the basic element of mixing ratio of the phases during for nano-packaging. In recognition to such small molecules and the cap- The nano-capsules even make it sand, silicon. They are, incidentally, synthesis, these can be spheres, of this groundbreaking work, one of sule empties itself. possible for the scientists in Golm to the inorganic counterparts of the hy- cylinders or cuboid layers (fig. 5, the young scientists involved, Dr. All that is left is the completed imitate the properties of biological drocarbons in organic chemistry. above). In order to release these Gleb Sukhorukov, was awarded the nano-capsule (Figs. 6, 7 and 8). The cells in a simple way. In this case, it Phase O is a soft material, whereas nano-objects, the chemists now only 2.25 million Mark Sofja Kovalevska- scientists at Golm can regulate the is a question of the outer layers of A is hard, like glass or pottery. In have to dissolve the polymer block ja prize by the Alexander von Hum- wall thickness of their hollow cap- the cells acting as membranes to simplified terms, the clever synthesis (fig. 5, below). boldt Trust (see page 94 of this edi- sule with great precision to a bil- protect the interior of the cell. These process developed in Mainz is car- Besides being minuscule, the tion). The manufacture of the Golm lionth of a metre and can also great- membranes can make themselves ried out as follows. First, the Mainz nano-building blocks have capsule is based on an idea, which is ly vary the chemical composition of permeable to certain substances. This chemists dissolve phase O in an or- another characteristic feature: long as irresistibly simple as it is experi- the capsule wall. By this process, is how they control the exchange of essential substances between the inside of the cell and the environment. To emulate this, the Max Planck scientists coat the walls of their nano- capsules with a double layer of lipid Reinhard Lipowsky and his colleagues design tiny membrane machines. Helmuth Möhwald and his department manufacture nano-capsules. Fig. 8: Two sectional drawings through different levels of a capsule, created by coating a red corpuscle in the “thorn-apple form” (erythrocyte).

mentally demanding. The fundamental building blocks are microscopic nuclei. These can be inorganic particles, or alternatively polymer particles, crystals from a pharmaceu- tical agent or even biological cells. The researchers coat these nuclei with electrically charged polymer molecules, which are taken up in a solution. Once the first coating is completed, the resulting nano-capsule is put into a second solution with oppositely charged molecules. Fig. 9: These capsules measuring approximate- The molecules are attracted by the ly ten micrometres (millionths of a metre) are first coating and form a second lay- filled with a polymer (red). Chemical reactions resulted in the formation of the polymer er. Using this method, the re- which was built up from smaller molecules 58 M AXP LANCKR ESEARCH 4/2001 that had penetrated the wall of the capsule. 4/2001 M AXP LANCKR ESEARCH 59 FASCINATING Research Soft MATTER

molecules (lipids are fats). The cap- nature using their high power com- lent molecules would be exposed and – the “cis isomer“ – to another – the STATES OF AGGREGATION sule walls are then able to recognise puters. The computer simulation in be in danger of coming into contact “trans isomer”. Infrared light switch- Materials can exist in three states of aggregation: selected molecules and make them- fig. 10 shows snapshots of how mol- with the surrounding water. The es them back again. If such mole-

5 5 5 solid, liquid, and gaseous. The fundamental build- selves permeable to them. In this ecules form a simple membrane. To A t= 0.5 . 10 B t= 1.5. 10 C t= 7 . 10 membrane is only able to avoid this cules are present on the membrane ing blocks of the substances - the atoms or mole- way they become a simple model for do this, the scientists place virtual by simply forming no edges. It dou- surface, they change its curvature. cules - are most ordered in solids. They form three-dimensional crystal lattices in which they the membrane functions of a living amphiphilic molecules in virtual wa- bles itself up and closes itself up into The cis isomer bends the membrane have fixed places. If the solid matter becomes liq- cell, which are, of course much more ter and instruct the computer to cal- a fluid filled blister, a vesicle. differently from the trans isomer. If a uid, these rigid structures break up and the degree complex. culate what happens. This kind of vesicle can assume vesicle were equipped with these of order diminishes. The molecules are mobile and Amphiphilic molecules have one many different shapes. However, it molecules, the scientists really would glide past each other. In gases, the atoms or mole- FROM THE NANO-CAPSULE end that attracts water, and one that 5 5 5 always tends towards the shapes for be able to use light to switch them cules produce enough energy to detach themselves TO THE VESICLE MEMBRANE D t= 7.5 .10 E t= 8 . 10 F t= 10 .10 from each other and to fly through space (virtual- repels it. The water-repellent end which its membranes need to gener- from one shape to another. ly) freely. Gases are therefore the least ordered. The Golm capsules are an example does its best to avoid contact with ate the least bending energy. The Fig. 10: A bilayer membrane SWIMMING LESSONS of how pure research can lead to the water molecules and it is precise- organises itself. The computer minimum level of energy – and FOR MEMBRANE MACHINES SUPRAMOLECULAR STRUCTURES completely new fundamental tech- ly this that drives the amphiphilic simulation calculated a system of hence the shape of the vesicle – de- 100 amphiphilic molecules (in How do supramolecular structures come into be- nology. They have tremendous po- molecules to organise themselves in- pends upon the ambient tempera- This gave Reinhard Lipowsky a this case they consist of four green ing? Let us imagine that the molecules are cars. tential for application in industry. to a bilayer membrane. Both layers ture, the volume, and the nature of fascinating idea. He designed a mod- spheres with a red head) in 840 The drivers of these cars have completely different For example, Möhwald highlights the of the membrane arrange themselves water particles (blue spheres). the vesicle’s filling. Even molecules el of a vesicle that can actively swim destinations. If the drivers were simply able to dri- fact that many cosmetics and phar- so that the ends that shy away from There is approximately a that cling to the two surfaces of the through an aqueous liquid. To do ve to their destination in a straight line, there femtosecond between snapshots would be chaos. Individual traffic is fundamentally maceuticals are not readily soluble in the water sit on the inside of the membrane can bring about a change this, the microscopic membrane ma- A, B and C to F (a millionth extremely disordered. Order is only created from water; for this reason our body can- membrane. in its shape. chine does however require a special part of a billionth of a second). the chaos by a superstructure of roads, traffic not absorb them properly. The cap- The computer simulation also What happens, for example, if swimming lesson: two different lights, and crossroads, which then guarantees a sules could act like microscopic sub- clearly illustrates the interplay be- there are two different types of mol- shapes, A and B, are not enough. The functioning traffic network. Supramolecular struc- marines to transport such substances tween order and disorder which is ecule in one vesicle membrane vesicle would just bob around in the tures perform a similar role. In nature, these supramolecular structures come into being through into the body. Their shells could be characteristic of soft matter. The ran- which repel each other? These might water, but not move. To really cover the self-organisation of the molecules. The mole- configured to take them to precisely domly distributed molecules are perhaps be lipids and cholesterols, any distance, it must go through a cules behave as if they know their place within a the place where their contents were more ordered within the membrane which are present in the membranes cycle of several changes of shape, as complex structure and migrate there. For example, to act and release the active sub- than previously. In fig. 10, it can be a b of red blood corpuscles. Fig. 11a depicted in figure 12. big molecules organise themselves in flowing liq- stance in a controlled manner. seen how the order increases from shows what happens according to This is where biomimetics come on uids in such a way that they can flow faster. Or red blood corpuscles change their shape so as to be The system developed in Golm also snapshot to snapshot. But a remnant the computer simulation. The mutu- the scene again, because the princi- able to pass through the narrowest blood vessels. opens up completely new avenues for of disorder remains even in the fin- ally “disagreeable” molecules collect ple of this swimming technique has When they change their shape, the molecules of pure research. Helmuth Möhwald ished membrane. The molecules are in separate membrane domains been used successfully by single- their membrane form a different supramolecular cites as an example the study of in fact not fixed firmly into the c d which are coloured red or blue on celled organisms for hundreds of structure from before. Self-organisation is there- chemical reactions or crystallisation membrane structure, but can wander the illustration. As the molecules mi- thousands of years. If, one day, it fore a fundamental characteristic of nature, without which no life could exist. Self-organisation al- under the special conditions offered freely within the membrane surface grate, these zones become larger. As were to become possible for such a lows those big molecules that form the building by nano-cavities. Fig. 9 shows an ex- and swap places. With its two di- this happens, the red zones behave membrane machine to be built in the blocks of living organisms to come into being. It ample from the Golm “box of tricks”. mensions the membrane surface be- extremely peculiarly. They form laboratory, the researchers would also shapes the biochemical processes of all vital Here, the Golm scientists demonstrat- haves almost as if it were liquid. The buds. Neighbouring buds are in turn again have unravelled another mys- functions. ed that they can not only fill the cap- membrane owes its flexibility to this Fig. 11: A vesicle forms buds. attracted to each other and form tery. sules with smaller molecules – by us- mechanism. even bigger buds (figs. 11b-d). In this Throughout the ages, soft matter COLLOIDS ing chemical reactions, they are even A bilayer membrane like this is on- way a “nano-blackberry”, which was and is a motor of human cul- The term “colloid” is derived from the Greek word able to produce larger molecules ly four to five nanometres thick. Its looks as if it’s been pecked at by ture. Perhaps nanotechnology based for “gluey”. Colloids consist of microscopically from them within the capsule. surface can however have a signifi- birds, develops. The behaviour of on soft matter will give 21st century small particles. They can be dispersed throughout another substance, the “dispersion medium”. Ex- When talking to Reinhard Lipows- cantly greater diameter. It can extend this simple system gives an idea of civilisation a big push as artificial amples of colloids are particles of cigarette smoke ky, the term biomimetics keeps crop- over several micrometres, i.e. mil- how living cells form protuberances, materials did in the 20th century. (II) (III) in the air, or of varnish, in which fine colour pig- ping up. This is how scientists de- lionths of a metre, even up to mil- for example at the start of a cell di- Material scientists can still learn a ments are “dispersed” in a solvent. A colloid owes scribe the method of reconstructing limetres. In this case, the membrane vision. lot from nature, as Hans Wolfgang its properties to the fact that the particles are biological subsystems in a simplified is a million times more extensive Can a vesicle deliberately be Spiess emphasises: “Nature achieves much larger than atoms or “normal” molecules, ESEARCH but still remain microscopic. Their diameter varies form and at the same time are learn- than it is thick! The membranes of a R switched between different shapes? tremendous complexity using just a (I) (IV) between a few nanometres (one billionth of a me- ing something about them. Reinhard nerve cell can even reach dimensions Light-sensitive polymer molecules, few building blocks. In our work NTERFACES tre) and several tens of micrometres (millionth of Lipowsky and his colleagues in Golm in the region of a decimetre. But I termed azobenzene chromophores, with synthetic materials, we use an

a metre). Colloids have enormous economic and AND Fig. 12: A vesicle that assumes are specialists in theoretical bio- however far the membrane might - do have this interesting potential. amazing array of building blocks, technical importance. In nature, for example, they different shapes (schematic).

mimetics. They explore the fascinat- stretch, it has a problem, namely its OLLOID Ultraviolet light allows these mole- but, at the moment, achieve relative- play an important role in living cells. C If it runs through these forms

ing world of membranes and imitate edges. On one side the water-repel- FOR cyclically, it will swim through cules to jump from one spatial form ly little.” ROLAND WENGENMAYR the surrounding liquid like .: MPI IG

60 M AXP LANCKR ESEARCH 4/2001 F a living unicellular organism. 4/2001 M AXP LANCKR ESEARCH 61