A team of materials scientists have developed a new material that behaves like a cell membrane found in nature. Researchers have been seeking similar materials for applications such as drug delivery and water purification.
The material can assemble itself in a sheet that is more stable yet thinner than a soap bubble and is referred to as a lipid-like peptoid. The assembled sheet can repair itself after damage and also has the capability to withstand when immersed in a various liquids.
Nature is very smart. Researchers are trying to make biomimetic membranes that are stable and have certain desired properties of cell membranes. We believe these materials have potential in water filters, sensors, drug delivery and especially fuel cells or other energy applications.
Chun-Long Chen, Chemist, Department of Energy's Pacific Northwest National Laboratory
The Amazing Membrane
Cell membranes are remarkable materials. They develop from thin sheets of fatty molecules called lipids. Although they are ten times thinner than a soap bubble they allow cells to inclusively form organisms as distinct as trees, people and bacteria.
Cell membranes do not allow everything to easily pass through with its tiny embedded proteins acting as gatekeepers. Membranes have the ability to automatically repair damages to their structure and alter thickness to transmit signals from the outside environment to interior of the cell, where most of the process takes place.
Scientists are planning to take advantage of membrane properties such as gatekeeping to design filters or signaling to develop sensors. A material like cell-membrane would be beneficial compared to other thin materials such as graphene. For instance, imitating a cell membrane's efficient gatekeeping property could produce water purifying membranes that will require minimal energy or pressure to push the water through.
Researchers are interested in synthetic molecules known as peptoids because they are inexpensive, customizable and versatile. These molecules resemble natural proteins, like those that implant themselves in cell membranes, and can be developed to have very specific functions and forms. Chen and his colleagues decided to see if they could design lipid-like peptoids.
Lipid molecules are elongated and usually straight. Its fatty end prefers to be in the vicinity of other fats, while the other end is water-like and is comfortable in water. Due to this chemistry, lipid molecules tend to arrange themselves with the fatty ends pointed to each other with the water ends pointed out. Scientists refer this to as a lipid bilayer, which is a sheet that wraps around the contents of a cell. Protein or carbohydrate molecules implant themselves in the membranous sheet.
Chen and his colleagues were motivated by this, and they developed peptoids where each base peptoid was a long molecule with one fat-loving end and one water-loving end. Chemical characteristics that the researchers hoped would encourage the individual molecules to pack together once they were chosen. They tested the resulting structures with various methods of analysis including some at two DOE Office of Science User Facilities at Lawrence Berkeley National Laboratory, the Advanced Light Source and the Molecular Foundry.
The team discovered that after adding these lipid-like peptoids into a liquid solution, the molecules rapidly crystallized to form what the scientists refer to as nanomembranes, which are straight-edged sheets as thin as cell membranes, floating inside a beaker. These nanomembranes retained their structure in alcohol or water, even when exposed to different temperatures, and in solutions with low or high pH, or high concentrations of salts, a property that only few cell membranes could achieve.
A View From the Middle
To understand the nanomembranes in a better way, the group simulated the method followed by single peptoid molecules to communicate with each other using molecular dynamics software. The simulated peptoids formed a membrane resembling the lipid bilayer where the fat-loving ends assembled in the center, and the water-loving ends pointed outward either below or above.
To analyze if synthetic membranes had the potential to create signals like cell membranes, the researchers added a bit of sodium chloride salt. Salt is involved in the final step in various signaling series and thickens the real cell membranes, leading to thickening of the peptoids. A Higher amount of salt the thicker the nanomembranes became, increasing almost 125% of their original thickness in the range of salt concentrations they analyzed.
Real membranes also contain proteins that have specific functions, like the ones that allow only water to pass through. In order to test the ability of peptoids Chen's group introduced a variety of side chains. Side chains are materially small molecules of varied sizes chemical natures and shapes combined to the longer lipid-like peptoids. They tried ten unique designs in every case, the peptoids gathered into the nanomembranes with the core structure unchanged. The team was also able to construct a carbohydrate into nanomembranes, depicting that the material can be developed to have versatile functions.
The group later analyzed the nanomembranes to check whether they had the capability to repair themselves, a functional feature for membranes that could get damaged during use. After slitting a membrane, they added more of the lipid-like peptoid. The membrane was observed under a microscope within few hours, and it was found that the scratches filled up with more peptoid resulting in a complete nanomembrane. This is unlike tears in paper that do not repair themselves even after being patched together using tape.
All of the results proved to the researchers that they were on the right track to develop synthetic cell membrane-like materials. However, there are few other challenges to be resolved before application. For instance, the researchers are interested in learning more about the formation of membranes, so that it is possible to create various desirable sizes.
Chen said that the next step involves building biomimetic membranes by implementing natural membrane proteins or other synthetic water channels including carbon nanotubes to these sheet matrices. The team is also concentrating on methods to make the peptoid membranes conductive for energy uses.
The Department of Energy Office of Science and PNNL supported this study.