Delft, Netherlands, 7-9-2015 — /EuropaWire/ — A team of researchers at Delft University of Technology has shown that synthetic materials can change into active materials if they are supplied with fuel. The underlying process, the so-called self-assembly of molecules, is an important natural mechanism. It’s a process that allows, for example, living materials to move or adapt to their surroundings. The research paves the way for the replication of the process with synthetic materials. The TU Delft scientists will publish their results in the journal Science on 4 September.
Living materials consist of thousands of different molecules connected by non-permanent links. The process whereby these molecules organise themselves is known as self-assembly. “Because they are not attached permanently to each other,” explain researchers Dr Rienk Eelkema and Prof Jan van Esch, “they can easily change, reorganise or repair themselves. Cells require energy in order to perform these functions: consuming energy is a fundamental characteristic of living materials. It’s these typical properties and processes that we now want to introduce into synthetic materials.”
The research group headed by Van Esch has now succeeded in inducing and controlling a synthetic, fuel-driven self-assembly process. The resulting active material is an artificial variant of the cytoskeleton of a cell, the system of tiny tubes – tubules and actin filaments – which keeps a cell rigid. The researchers have developed molecular building blocks that are able to react with another compound, the fuel, in order to be able to form synthetic fibres. These synthetic fibres are formed as long as fuel is added to the building blocks, but the network collapses as soon as the supply of energy is interrupted. In other words, the synthetic fibres exist as long as fuel is present, just like the cytoskeleton in living cells.
Once a chemical fuel is added, molecular building blocks assemble to form a network of fibres, changing the solution into a hydrogel. The fuel is eventually used up, the gel structure collapses, and the solution returns to that of the original building blocks. The cycle can be restarted by adding a new dosage of fuel.
The study being published in Science was mainly exploratory in nature, but the insights it reports pave the way for the production of adaptive and self-healing materials. Potential applications include robotics and autonomous systems. “Consider an autonomous robot which has to work under extreme conditions, with no human control or external source of energy,” says Eelkema. “Chemical fuel has a much higher energy density than batteries, especially when you can utilise that energy immediately. But you then also have to know how to use the chemical energy in order to trigger a self-assembly process or a change in the material. That’s what we’ve shown here.”
Another property of fuel-driven self-assembly is that the process can be well managed over time. After all, it stops as soon as the supply of fuel ends. The artificial tubules and actin filaments in this study form a hydrogel: the created fibres assemble as a network in the solution, a sort of ‘micro-sponge’ that then holds all the water in it. This same mechanism could be used to gel substances temporarily, with one possible application being to stop oil sloshing around inside tankers, which can have dramatic consequences for the stability of the ship. As such, gelling the oil for the duration of the voyage would reduce the risk to the ship.
“The next step, materials that use energy to actually do something, was one of the great challenges in our field,” says Van Esch. “There has been a lot of interest in the use of light, electricity or magnetism, but natural systems are always powered by chemical fuels because they can easily store it, removing their dependency on an energy supply from the environment for shorter or longer periods of time.” Energy consumption is also a product of another characteristic of living systems: they are out-of-equilibrium. “To keep something in a transient state,” Van Esch continues, “you constantly have to add energy. Otherwise it steadies itself naturally with the lowest energy level. Until now, the field of self-assembly research has focused mainly upon systems that are in thermodynamic equilibrium. That is, that are at a minimal energy level. This is the first time anyone has succeeded in achieving a transient self-assembly process using a chemical fuel, just like in living systems.”
The results published in the report pave the way for the development of active materials that can replicate functions seen in nature. Up until now, functions such as movement, self-healing and adapting to the environment were reserved for living materials. Van Esch: “We have learnt how to use a network of chemical reactions to create a synthetic self-assembly system. This knowledge is essential for imbuing other synthetic materials with the special properties of living materials.”
Prof Jan van Esch: tel. +31 (0)15 278 8826; email j.h.vanesch@.tudelft.nl
Ilona van den Brink, Science Information Officer, Delft University of Technology: tel. +31 (0)15 278 4259; email i.vandenbrink@.tudelft.nl
Publication: Science, published 4 September 2015
“Transient assembly of active materials fuelled by a chemical reaction”,
Job Boekhoven1,‡,#, Wouter E. Hendriksen1,#, Ger J. M. Koper1, Rienk Eelkema1,2,*, Jan H. van Esch1,2,*.
1. Advanced Soft Matter, Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
2. Delft Process Technology Institute, Delft University of Technology, Leeghwaterstraat 39, 2628 CB Delft, the Netherlands.
‡ Current address: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208, USA. # These authors contributed equally to this work.