Soft Artificial Tissues based on Droplet Interface Bilayers and Their Application as Membranes for Electrically Driven Separations

Soft tissue-like materials mimicking biological tissues' functionality, responsiveness, and reconfigurability hold immense potential for revolutionizing robotics, sensing, computing, biomanufacturing, and separations. Key features involve replicating cell structure by compartmentalizing aqueous environments with lipid membranes and incorporating membrane proteins for ion and solute transport. Droplet interface bilayers (DIBs) emerge as a promising platform, forming cell-like compartments enclosed by lipid membranes, serving as building blocks for 2D and 3D soft, responsive tissues. Conductive pathways are established by associating membrane proteins with lipid bilayers, providing tunable transport properties.<br/><br/>A rapid, scalable protocol combining emulsification and centrifugation has been devised for creating artificial tissues based on DIBs. This process successfully produces tissues from lipids, and polymers, and integrates ion channels like self-inserting peptides, integral membrane proteins, and artificial channels. Additionally, a platform has been designed to employ these tissues as membranes for electrically driven separations. The process hinges on three main components: an aqueous solution, hydrophobic media, and a bilayer-forming amphiphile as the emulsifier. Emulsification leads to aqueous droplet formation enclosed by a monolayer. Centrifugation creates a tightly packed system and displaces the excess oil. The contact of monolayer-enclosed droplets creates a bilayer, forming a network of communicating water compartments. This process enables the formation of tissues from microliter to liter scale, with resulting material exhibiting exceptional properties including high electrical resistance, viscoelasticity, and self-healing behavior.<br/><br/>Modifications to the process can significantly alter tissue's electrical properties. By incorporating ion channels during emulsification, the tissue can become electrically conductive. This enables the creation of biomimetic membranes that replicate and amplify channel properties, such as selectivity and pore size exclusion. The system allows for the incorporation of various natural or artificial channels, as they self-arrange in the monolayer/bilayer due to the hydrophobic effect. Channels like gramicidin (self-inserting peptide), Outer Membrane Protein (OmpF), Ammonium Transporter Protein (AmtB), and artificial channels like LAP5n10, a highly selective lithium channel, have been successfully incorporated through this method.<br/><br/>The use of DIB-based artificial tissues as membranes for water treatment has been proposed and demonstrated at the laboratory scale. To fabricate the membrane, the tissue is placed in a glass or plastic container, separated by two hydrogel layers on each side, which are in contact with an aqueous media. The assembly order is aqueous solution/hydrogel/tissue/hydrogel/aqueous solution. Applying voltage between the two aqueous solutions drives the transport of charged molecules or ions across the membrane through ion channels. The membrane's selectivity is controlled by the tissue's width and the selectivity of the channels used, as each layer of droplets acts like a membrane. For example, two layers of droplets containing a channel with 90% selectivity for a specific molecule will behave like two 90%-selectivity membranes in series, yielding an overall 99% selectivity provided all droplets contain the selective channel. This feature allows fine-tuning of membrane selectivity even when using low-selectivity channels. Overall, this innovative platform for fabricating soft membranes holds tremendous promise for advancing water treatment separations.