Water accessible for human consumption is estimated to be about 0.007% of fresh water, which itself amounts to barely 3% of water resources on Earth. The UN World Water Development Report of 2016 highlights the problem of water scarcity which is expected to affect about 1.8 billion people by 2025. This scarcity arises due to the predicted depletion of the Earth’s fresh water in the coming years and the current inability to ‘tap’ salt water resources. This poses new challenges for water purification that can only be addressed by development of specialized water purification methods.
The use of bioinspired materials has become fairly common for medical applications due to obvious reasons. Though not as common, these materials are also promising candidates for efficient water purification.
What makes up natural membranes?
An amphiphilic molecule, called phospholipid, forms the natural biological membranes. The word ‘amphiphile’ comes from Greek words amphis (meaning: both) and philia (meaning: love or friendship). A chemical compound is considered amphiphilic when it contains both a water-attracting (hydrophilic) part and a fat-attracting (lipophilic) part.
Common amphiphiles, like proteins and some block copolymers, can have many of both hydrophilic and lipophilic parts. The amphiphilic nature of such molecules influences the structure they assume by means of self-assembly. Examples of some common supramolecular structures are spherical micelles, bilayered vesicles and planar vesicles (Figure 1). Such structures are useful for the development of mesoporous materials, which in turn can be used in water purification.
Biomimetic membranes:
Biomimetic membranes are prepared in a way that mimics biological membranes. Various kinds of membranes have been used for decades in different separation methods. These membranes can be classified based on how homogeneous they are (isotropic and anisotropic) or its pore sizes in case of synthetic polymers (e.g. 1 nm for reverse osmosis, 2 nm for nanofiltration, etc.).
An economical and popular method of water purification is reverse osmosis. Filtration by reverse osmosis involves the application of pressure to the contaminated water. This drives water across a semipermeable membrane and removes up to 90-99% of impurities. Current reverse osmosis water purifiers use thin-film composite membranes which are anisotropic biomimetic membranes.
Aquaporin Z, a naturally occurring water channel, improves the water permeability as compared to conventional polyamide composite membranes when incorporated in a triblock copolymer and thus, forms a promising biomimetic material. Another possibility for improved water filtration is the use of aquaporins in conjunction with carbon nanotubes due to their high water flux. The major drawback here is the higher cost of preparing membranes with aquaporins or carbon nanotubes compared to the typical polyamide composite membranes. A recent study shows that the use of new biomimetic membranes with artificial water channels has a smaller characteristic size of 5-25 nm as compared to a commonly-used composite membrane consisting of one layer each of polyamide, a polysulfone and a polyester adding up to about 140-150 µm. This is clearly an advantage over the conventional counterpart for potential manufacture of small, portable filters.
What lies ahead for biomimetic membranes?
Although many new and promising candidates for efficient water purification exist, their commercialization will require further research in order to tackle all the associated challenges. As mentioned earlier, the cost of aquaporins is a major disadvantage. Additionally, carbon nanotubes might pose safety issues which require further investigation. In spite of the great promise shown by biomimetic membranes, a number of issues have to be resolved before they become acceptable as efficient, cost-effective and environment-friendly solutions to address the issue of water purification and eventually tackle water scarcity.
