Why Does Membrane Permeability Matter to Me?
“Water, water, everywhere,
Nor any drop to drink.”
Samuel Taylor Coleridge, The Rime of the Ancient Mariner, 1834.
With 71% of the Earth’s surface covered by water (Links to an external site.) , how then is the world facing a water crisis? Yes, water. The molecule of life. In crisis.
According to a 2019 World Resources Institute report, 17 countries, 25% of the world’s population, face “extremely high” water stress (when water demand exceeds water availability), drawing down 80% of their available supply per year. Another 44 countries, 1/3 of the population, face “high” levels of water stress, withdrawing more than 40% of their supply annually. Population growth, industrialization and climate change continue to intensify the demand. To help ensure water security, research is focusing on biologically-inspired nanotechnologies that mimic cell membranes, and a process vital in all cellular systems: osmosis.
Just under 3% of the world’s water supply is fresh- the kind needed for drinking, irrigation and municipal use. The remaining 97% is saltwater. However, large-scale systems for desalinating ocean water, turning saltwater into drinkable freshwater, have been operational for decades. As of 2018, almost 16,000 desalination plants are operating in 177 countries. Many use membrane-based, reverse-osmosis (RO) technologies.
Under normal osmotic conditions, water flows through a semipermeable membrane from a hypotonic solution (think of hypotonic as meaning “less solute”, in this case the freshwater) to a hypertonic solution (think of hypertonic as meaning “more solute”, in this case the saltwater) to balance out the salt concentration between the two. However, in an RO system, the water flow is reversed by applying high pressure, which forces seawater through a semipermeable membrane. The membrane, a thin composite permeable only to water molecules, rejects salts, microorganisms, and other contaminants. The result is pure, drinkable water. But, of course, it is not free and the process is not as productive as possible.
Efforts to maximize efficiency and minimize costs have driven the evolution of membrane desalination technologies, many of which focus on membrane permeability. Next-generation membranes include carbon nanotubes (CNTs), which are one-atom thick sheets of carbon rolled into cylinders, with diameters as small as one nanometer. CNT membranes have water permeability 20 times greater than traditional RO membranes. Additionally, they have a large surface area, are highly efficient at rejecting salt ions, are able to provide ‘gatekeeping’ functions much like proteins in cell membranes, and better withstand the harsh operating conditions by reducing chlorine interactions, one of the primary causes of degradation in RO membranes.
There is water, water, everywhere. And biologically-inspired nanotechnology is helping to make it safe, and more cost effective, to drink.
Other Membrane-inspired Applications
Biologically-inspired (biomimetic) synthetic membranes have applications beyond desalination technology. They are used heavily in medical and pharmaceutical fields for drug development and delivery. Some biomimetic membranes can be generated through 3-D printing and are capable of maintaining osmotic balance through engineered transmembrane proteins. There are biomimetic antifouling coatings. Even US national security agencies are funding research and development for biomimetic membrane materials.
One such effort involves “smart” second skin material for military uniforms. The material is made from a newly developed polymeric (remember polymers from Chapter 2) membrane embedded with CNTs that are capable of conducting moisture away from a soldier’s skin when a concentration gradient has developed. As with their use in RO desalination, the CNTs wick away water at rates that greatly exceed diffusion theory. Not only does this material help prevent heat-related stressors, it can protect soldiers against biological agents, like the Dengue virus, via size exclusion, and chemical contaminants, such as sulfur mustard (a blistering agent), through surface modification of CNT pores with chemical “gates” Scientists are even developing exfoliation capabilities-like a skin peel- for material that has encountered contaminants! (Think about about how these functions mimic those in a real cell membrane and what structures they mimic),
Research funding is becoming more and more limited. If funding becomes available for research and development of biomimetic membranes, to which application do you think it should be directed?
- Biomedical and pharmaceutical.
- National Security.
- Water Security and Management.
- Antifouling coatings
- It doesn’t matter to me.
Additionally, material scientists are even investigating ‘nanosponges’ that mimic human lung cells to ‘soak up’ SARS-CoV! The technology is being applied to many aspects related to COVID-19 (drug delivery, PPE, etc.). How exactly are cell membrane principles/biomimetic/nanotechnology being used in COVID-19 prevention/treatment/vaccination/other?
Do some research on one of the previously mentioned topics and provide your opinions/research. Make sure you can back up your statements with research from the literature (think numbers, study results, specific examples, etc.).