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Conductive polymer substrate creates platform for biosensing applications

By Rachel Berkowitz October 21, 2016
Lipid Bilayer
Lipid bilayer on conductive thin film built into a transistor. The dark material on either side of the 100 × 10 µm channel is an insulator, necessary for the transistor fabrication. Magnification: (top) 10×, (bottom) 100×. Credit: Advanced Functional Materials

If you want to know how a living cell exchanges information with its surroundings, or how a virus infects a live host, you have to understand how materials pass through the cell wall. At the molecular scale, any material that travels into a cell first encounters the lipid bilayer, which comprises the main structure of the membrane that separates the interior of a cell from the external environment. In a new study, researchers have synthesized lipid bilayers on a flexible, conductive substrate that supports the cell lipid bilayers and allows signals to transduce the model membrane. This system can sense both electrical and optical signals, enabling researchers to investigate how ions, proteins, and pathogenic materials pass across host cell membranes.

“By synthesizing a biomimetic system such as a lipid bilayer, where you can control what you present to the ‘cell’ exterior, you can model what’s happening with the cell,” says Róisín Owens, Associate Professor of Bioelectronics at École des Mines de St. Étienne (EMSE), and co-leader of the study.

Scientists have been developing biomimetic systems for decades, but they have typically built them on silica-based substrates. The transparent glass surfaces have proven successful for interfacing with standard analytical techniques like microscopy, to observe how proteins move into and out of a cell. But these surfaces are rigid and electrically insulating.

The new method of assembling lipid bilayers on flexible, conductive thin films of a conducting polymer expands the capability for studying interactions with host cells by offering the added tool of electrical readout.

“You get two pieces of information, optical and electrical, plus the material is softer. This lets us move closer to a realistic physiological system,” says Susan Daniel, Associate Professor of Biomolecular Engineering at Cornell University, who jointly led the work.

To assemble a lipid bilayer onto a conducting polymer film, Daniel and Owens optimized a self-driven process called vesical fusion by which lipid- and protein-containing vesicles are attracted to a thin film surface where they rupture and flatten out. By finessing the vesicle composition and conditions for creating the polymer, they ensured that the vesicles would reliably rupture and form planar sheets.

“They...managed to find conditions for fusing a lipid bilayer on a quite challenging PEDOT [conductive polymer] surface, and show evidence of bilayer formation,” says Aleksandr Noy, Associate Professor of Bioelectronics at the University of California at Merced, who was not involved in this study.

Then the team built the film into a transistor device which amplifies output signals. “Even if you have a very small biological signal, it can be electrically amplified so it’s readable,” Owens says. “This sensitivity makes the platform more forgiving for imperfections or defects in the bilayer coverage.”

Daniel’s research group investigates events like a virus entering a bilayer, using optical techniques. She observes through a microscope how fluorescent-tagged viral genomes disperse across a membrane. With the new thin film system, electrical signals offer information complementary to the optical readout about the opening or closing of a pore in the membrane during this process, and the passage of viral genomes across the membrane.  

Owens’s research group plans to develop devices that convert multiple types of signals for measuring biomolecular activity within a cell. At this molecular level, individual ion channels suspended in the membrane are best studied using combined optical and electronic monitoring.

Now, they can do just that: by inserting a bacterial protein that mimics an ion channel into a lipid bilayer, they proved the ability to distinguish electrically between a bilayer without the protein, and one with the protein that lets ions through.

“Hybrid systems between polymers and lipid membranes [demonstrate] the potential of bridging the gap between natural and synthetic architectures,” says Eva-Kathrin Sinner, Head of the Institute for Synthetic Bioarchitectures at the University of Natural Resources and Life Sciences in Vienna, who was not involved in this study. “Membrane protein-based devices are indispensable readout platforms, [offering] robust alternatives to cell-based assays for characterizing the activity of ion channels in the context of organic electrochemical transistor-based drug screening.”

The new system takes lipid and protein samples from bacteria and archaea, but Owens and Daniel plan to develop a more human-like biomimetic system with mammal cell-derived materials. Not only does this study provide a useful platform for studying host-pathogen interactions, but it paves the way for devices that could help distinguish between strains of viral and bacterial infections.

Read the abstract in Advanced Functional Materials.