For years, scientists have been trying to develop a synthetic membrane-like material that mimics the structure and function of the membranes found in living organisms. In cells, membranes perform many important functions that would be highly desirable in a synthetic material. For a long time, the question has been how to reproduce the key properties of membranes from living organisms so that they can be used outside of cells for a variety of different applications.
Recently, scientists in the United States and China succeeded in designing such a membrane from easily-synthesizable building blocks. Their work, published this month in Nature Communications by Jin and colleagues , is a groundbreaking new step forward that could have a significant impact on many other fields, from water filters to fuel cells.
The cell membrane
So what is so special about cell membranes? In cells, fatty acid molecules form what is called a lipid bilayer to separate the inside of the cell from the outside environment. This bilayer is very thin and can interact freely with the watery environment on each side of itself, both inside and outside the cell. Almost nothing can pass freely through the cell membrane, however, because the inside between the two layers is a fatty environment. This fat-loving environment inside the membrane is formed because the fatty ends of all the fatty acid molecules point inwards toward the space between the two layers of the lipid bilayer. The water-loving ends all point outwards. To allow water, nutrients, and other substances into and out of a cell, the membrane is embedded with proteins that each let a specific item in or out. Other proteins embedded in the membrane can pass signals into or out of the cell, allowing the inner workings of the cell to adapt to and interact with the surrounding environment. The thickness of the membrane also changes based on external stimuli, as a type of signaling mechanism.
The cell membrane is virtually unbreakable, because its chemical makeup allows it to repair itself as soon as any small change in its shape occurs. Without the cell membrane, cells as we know them would not exist. We humans would not exist. Even though the number of cell membranes that exist in nature is so large we can barely wrap our head around it, mimicking cell membranes for use in other areas turned out to be rather difficult.
The science behind the mimicry
The new discovery was based on peptoids, synthetic molecules that have drawn interest for several reasons. First, they are easily synthesized and can be used as building blocks to quickly and easily assemble a large structure. Second, they are robust and resist degradation. They are also designed to be nontoxic. Finally, they are versatile in that they can be customized with a wide range of chemical properties for a wide range of applications. Research on peptoids was originally undertaken in the hopes of being able to mimic proteins for pharmaceutical applications, but now a different function has been discovered.
Jin and colleagues designed peptoids that mimic both the bilayer-forming aspect of the cell membrane and the embedded proteins that can transport substances. The base of the peptoids they designed is fatty acid-like, with one end attracted to water and the other end fatty. This encourages the peptoids to form a bilayer with the fatty ends on the inside, away from the watery environment.
To test their peptoids, the researchers performed experiments to see if the peptoids would form a membrane-like bilayer spontaneously, as well as to see how robust this bilayer would be under different conditions. They found that when they put the peptoid molecules into a watery liquid, the peptoids spontaneously crystallized to form a nanomembrane, a sheet as thin as a cell membrane. To top off this big discovery, the nanomembranes proved robust. They retained their form in different solutions like water and alcohol, at varying temperatures, at varying pH of the liquid, and at high salt concentrations. Compared to most real cell membranes, the synthetic peptoid membrane was more resistant to high salt concentrations in the liquid solution. In addition, the thickness of the membrane changed at higher salt concentrations, like the lipid bilayer in cells does. The nanomembrane can even repair itself when it is slightly damaged.
The team also looked at their peptoid nanomembrane under a microscope and performed computational analysis of their design using molecular dynamics. Combining these techniques allowed them to examine the structure of the nanomembrane they had created in more detail. They determined that their synthetic nanomembrane had high stability and flexibility, both of which are important for future applications of such a membrane.
The researchers are currently investigating how functional molecular objects could be added to the peptoids to mimic the function of proteins embedded in cell membranes. This would be another huge step forward, but the first hurdle of creating a self-assembling, robust, and flexible membrane-like material has already been overcome.
This scientific breakthrough has significant implications for a wide range of technologies and industries, from surface coatings to energy conversion for fuel cells. In the biological sphere, synthetic membranes could be useful for biological sensors, synthesizing other chemicals, and even drug delivery.
One of the most direct applications would be for water purification. A water filter made from a cell membrane-like material would have the key advantage of selective filtering that a cell membrane has. Other thin materials that had previously been developed were lacking such a simple selectiveness. For water purification, this cell membrane-like material could easily let water through and keep other molecules, bacteria, and metals out. An added bonus over current water filters would be that very little energy would be required to push the water through this water purification membrane.
The exciting new discovery of how man-made materials can be made to mimic the cell membranes found in nature has huge implications for so many important areas of research and development. This breakthrough promises to lead to even faster improvements with regard to its functionality and applicability to areas of development such as water purification.
- Haibao Jin, Fang Jiao, Michael D. Daily, Yulin Chen, Feng Yan, Yan-Huai Ding, Xin Zhang, Ellen J. Robertson, Marcel D. Baer, Chun-Long Chen. Highly stable and self-repairing membrane-mimetic 2D nanomaterials assembled from lipid-like peptoids. Nature Communications, 2016; 7: 12252 DOI: 10.1038/ncomms12252