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A soap bubble crossed with a bulletproof vest

21 January 2021

A new strategy for molecular design takes self-assembled materials where they’ve never gone before.

Every cell in your body is encased in a lipid bilayer membrane. Because the membranes’ constituent “amphiphilic” molecules have polar, hydrophilic heads and nonpolar, hydrophobic tails, they organize themselves into bilayer structures—tails pointing inward, heads pointing outward toward the surrounding water—without too much prodding. Other such molecules include soap and other emulsifiers and surfactants, which form similarly intricate structures in water.

A structure assembled through hydrophobic interactions almost always requires the presence of water for its continued existence. The amphiphilic molecules in a self-assembled bilayer are held to one another only through weak van der Waals interactions. So when the water dries up, the structure falls apart.

Now MIT’s Julia Ortony, her graduate student Ty Christoff-Tempesta, and their colleagues have developed a new material made out of self-assembled amphiphilic molecules that retains its structure in a water-free environment. They drew their inspiration from Kevlar, an ultrastrong polymeric material used in body armor and protective clothing. Kevlar derives its extraordinary strength from the interactions between adjacent parallel polymer chains. In one direction, the chains connect via intermolecular hydrogen bonds; in the perpendicular direction, they cling together through the stacking interactions between rigid benzene rings.

Layered amphiphilic molecules
Image by Peter Allen and Ryan Allen

The researchers designed their amphiphilic molecules to incorporate three repetitions of the Kevlar monomer unit (shown in bright green and yellow in the figure). They then tailored the size and shape of the hydrophilic heads (purple) and small hydrophobic tails (darker green) so that the Kevlar groups have minimal room to move around.

When placed in water, the molecules assembled into long bilayer nanoribbons like the one shown in the figure. They remained perfectly locked together for months. That level of stability is otherwise unheard of among hydrophobically assembled structures, which usually see molecules diffusing in and out over time scales of hours. When the researchers drew a nanoribbon suspension into a macroscopic thread, it remained stable and sturdy even when dried.

The appeal of amphiphilic bilayers is that they always expose the same part of the molecule—the hydrophilic head—to the surrounding environment. Because the exact chemical properties of the head groups aren’t critical for holding the structure together, the head groups can be designed to perform tasks like pulling trace impurities out of the surrounding medium, releasing a cargo molecule, or catalyzing a surface reaction. Ortony and colleagues now hope to put their highly stable nanoribbons to work in places where water can’t go. (T. Christoff-Tempesta et al., Nat. Nanotechnol., 2021, doi:10.1038/s41565-020-00840-w.)

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