Using ultra-thin membranes to solve environmental problems across the world
15 June 2007 (Volume 2 Issue 6)
It is said that in the 21st century we will face a shortage of water resources, which will be more serious than the problems with oil shortages. The Topochemical Design Laboratory successfully created a new material with the potential to contribute in a major way to solving such resource and environmental problems. The material is only 20 nm thick (two millionths of a centimeter), in the form of a huge nanomembrane, with an area of several square centimeters. It is expected that we will be able to produce inexpensive fresh water by using the huge nanomembrane as a reverse osmosis membrane in seawater desalination plants, and also improve power generation efficiency by using the film as an electrolyte membrane, which is the heart of the fuel cell.
The essential function of huge nanomembrane is the ‘separation’
The huge nanomembrane created by Toyoki Kunitake, who is Head of the Laboratory, is about 20 nm thick. Ten thousand films of this membrane laid on top of each other reach only the thickness of a postcard (about 0.2 mm). In fact, the human body has the same kind of ultrathin membrane with exquisite capabilities, namely the cell membrane. This is 5-10 nm thick, surrounds the cell, and is equivalent in thickness to two lipid molecules (Fig. 1).
Figure 1: The structure of a cell membrane.enlarge image
Kunitake says, “The cell membrane plays the role of separator between the internal and external regions, taking in substances and information necessary for the cell, and discharging unnecessary substances. In other words, the cell membrane separates necessary compounds from unnecessary compounds. Both in living things and as an artificial material, ‘separation’ is the essential function of a membrane.”
Kunitake successfully created this, the world’s first artificial membrane, at Kyushu University, with a structure and thickness similar to those of the cell membrane. When molecules such as soap are dissolved in water, they naturally aggregate together and create a membrane structure. The phenomenon in which molecules naturally gather together and create a structure like this is called ‘self-organization’. This phenomenon is actively applied in nanotechnology to manipulate molecules and atoms to produce new materials and devices with unconventional functions. Kunitake has used this self-organizational approach to create thin films with various kinds of properties. In 1999, he started the Topochemical Design Laboratory at RIKEN to work on the challenge of creating nano-structures, which were considered impossible to achieve in those days.
Creating a huge nanomembrane!
“All the other researchers knew that a huge nano-precision nanomembrane, with a large area, could drastically contribute to an expanding range of applications. Researchers had been able to create huge nanomembranes, but they were easily torn. A good example is a huge soap bubble. However, a practical membrane material to be used for separating substances should be strong and free from pores. The vast majority of scientists thought that it would be a major challenge to create a huge membrane that could be used practically in industry.”
Then, how did Kunitake succeed in creating these huge nanomembranes?
“We started to investigate how to create thin ceramic films at RIKEN. They are created on a substrate such as glass. It is comparatively easy to create even a nano-precision membrane with a huge area on a substrate. However, the membrane usually tears when it is peeled off the substrate. We therefore tried inserting a layer between the ceramic film and the substrate so that the ceramic film could be easily peeled off. This technique allowed us to create a strong membrane that was just barely suitable as a material, and we thought that we would be able to create a stronger, huge nanomembrane successfully if we could add a little more flexibility to the membrane. We then tried to incorporate flexible organic molecules into a strong ceramic film to create a hybrid membrane.”
Many researchers have conducted research into creating inorganic-organic hybrid membranes, but they came up against a wall. To create a strong membrane, we need to entwine inorganic molecules with organic molecules. However, it was very difficult to entwine them in a thin film because inorganic and organic molecules have different properties. We made repeated attempts to choose good combinations of inorganic and organic molecules and were eventually successful in creating a huge nanomembrane.”
In this huge nanomembrane, an inorganic material called zirconia, a kind of ceramic, and an organic material called acrylic polymer, are used to create individual web structures. These two structures, in turn, create a combined web structure (Fig. 2). It is this combined web structure that contributes to strength and flexibility (Fig. 3). The first huge nanomembrane was 35 nm thick, but we have successfully reduced the thickness of the nanomembrane down to about 20 nm.
Figure 2: The structure of a huge nanomembrane.enlarge image
Kunitake and members of the Laboratory have clarified that huge nanomembranes exclusively composed of organic molecules can be created by using a much finer web structure. In practice they have succeeded in creating a huge nanomembrane composed of two kinds of organic molecules: epoxy oligomer and polyamine.
Producing inexpensive fresh water from seawater
What specific applications can we expect from huge nanomembranes? “A study in applications has not been conducted yet, but I think that huge nanomembranes can contribute in a major way, especially in environmental fields related to resources and energy, because technology that can separate necessary chemicals from unnecessary chemicals is important in these fields. For example, we may be able to produce inexpensive fresh water from seawater.”
There is a fear that we will face more serious shortages in water resources because of rapid population growth, economic progress, changing climate due to global warming, and especially desertification. The worldwide water shortage is also a matter of life and death for Japan, which imports food produced overseas using water resources in large quantities; water resource shortages are directly linked to food production, which requires large volumes of water.
Figure 3: A huge nanomembrane.
Left, a huge nanomembrane only 35 nm thick. This huge nanomembrane is so strong and flexible that it does not tear off, even if it is sucked into a pipette and discharged again (shown on the right).
It is hoped that technology to produce fresh water from seawater can solve water resource shortages. Oil-producing countries in the Middle East have tried various approaches, in which large amounts of energy were consumed to produce fresh water from seawater.
However, many countries are beginning to adopt an alternative approach based on a less energy-consuming reverse osmosis membrane technique, in which seawater under pressure is used to drive fresh water through a reverse osmosis membrane. This approach not only greatly helps to sanitize environments because of its applicability to purifying polluted water, but is also used in wastewater reclamation and producing the high-purity water necessary for semiconductor manufacturing.
“The world needs technology that can produce fresh water or clean water from seawater or polluted water with the least amount of energy, and at the lowest cost. The approach based on the reverse osmosis membrane technique also uses a certain amount of energy to pressurize the seawater or polluted water, but the thinner the reverse osmosis membrane, the less pressure needed, exerted by the seawater or polluted water.”
The lowest thickness of reverse osmosis membrane currently used is several hundred nanometers. Thus, if we can use a huge nanomembrane that is one-tenth of that thickness as the reverse osmosis membrane, we will be able to greatly reduce energy consumption, and produce fresh water or clean water at low cost.
“The use of huge nanomembranes may lead to efficient recovery of scarce resources, such as uranium or gold dissolved in seawater in minute amounts,” says Kunitake, expanding his dream.
The use of huge nanomembranes will make it possible to improve the power generation efficiency of fuel cells, which have been under development for clean power generation systems. Several types of fuel cells are available, but the fuel cell based on electrolyte membranes has been attracting attention as one type suitable for automobiles and household use.
“The electrolyte membrane is a membrane that allows only hydrogen ions to pass through it. The thinner this electrolyte membrane, the higher the power generation efficiency of the fuel cell.”
Electrolyte membranes currently used are about 10 to 100 µm thick. Application of large-area nanomembranes as electrolyte membranes can reduce this thickness by three to four orders of magnitude or more, and significantly improve the power generation efficiency.
Using the advanced capabilities of the cell membrane
The cell membrane functions in an advanced way because membrane proteins called ‘ion channels’ or ‘receptors’ penetrate through the membrane (Fig. 1). The membrane protein acts as a sensor that accepts information from outside, or a pathway that allows specific substances to move in and out of the membrane.
“A single membrane protein several nanometers in size has this kind of advanced capability. If we can embed many membrane proteins in a huge nanomembrane, we will be able to use these advanced capabilities of the membrane protein on a large scale.”
It has been very hard to extract pure membrane proteins from the body and to use their capabilities because we could not produce a membrane with an area large enough, which is thin enough for a membrane protein to penetrate.
What applications can we expect from huge nanomembranes that replicate the capabilities of the cell membrane?
“For example, a huge nanomembrane could be used as a high-sensitivity sensor to detect signs of disease,” says Kunitake. When a person gets sick, certain molecules of the person’s blood may increase. Thus, the development of a highly sensitive sensor capable of detecting extremely small quantities of molecules would greatly contribute to early disease detection.
The great potential of huge nanomembranes
A lipid molecule that constitutes a cell membrane has hydrophilic and hydrophobic repgions. In water, lipid molecules create a membrane structure on the basis of the self-organization principle, keeping each of their hydrophobic regions enclosed (Fig. 1).
“When we create an artificial membrane based on self-organization, the governing chemical principle itself is very simple. However, creating structures envisioned on the basis of such a simple principle requires sophisticated condition settings for molecular design and synthesis. To begin with, we make a hypothesis, and carry out experiments under various conditions. Of course, we will face repeated failure at first, but in the face of these experimental difficulties, the moment will come when we realize, “This is what I am looking for.” That is the moment when the sophisticated conditions come into harmony with a simple principle and a new material is created, and also the most interesting moment in the research process. Once the material is created, we will soon find that what we did is only a simple thing. However, it is this simple thing that we cannot easily anticipate.”
Finally, Kunitake talked about the significance of these findings as follows: “The significance lies in the fact that once we can create huge nanomembranes, many other researchers will start creating similar nanomembranes using different materials. The current Spatio-Temporal Function Materials Research Project is to end in the fall of 2006, but we want to put this potentially promising material to practical use in some way.”
A new material is created when a simple principle is combined into harmony with sophisticated experimental conditions for molecular design and synthesis.