Three-dimensional printing is revolutionizing art, manufacturing, and now biomaterials. Researchers have automated the printing of smart, programmable networks of lipid-bound droplets that may be the future in creating synthetic, tissue-like biomedical materials. These soft tissues can conduct electrical signals like neurons and contract like muscle. Tens of thousands of aqueous droplets are judiciously ejected into oil to “print” a lattice of microcompartments bound by cell-like lipid bilayers. While most engineered-cell mimics, such as lip-osomes, fail to work together to exhibit teamwork-dependent functionality, these printed droplet networks display cooperative tissue-like emergent properties.
We want to “do something where the sum is greater than the individual parts,” said chemist Hagan Bayley, a professor of chemical biology at the University of Oxford. As reported in the April 5 issue of Science (DOI: 10.1126/science.1229495; p. 48), Bayley’s group used a modified commercial printer to pattern picoliter lipid-coated droplets to form a soft, tissue-like material. The droplets are formed from phospholipid bound monolayers where the inside is an aqueous solution. Precise control over droplet deposition and definition of the composition of the droplet contents and supporting membrane then enables the engineering of structures exhibiting tissue-like cooperative properties.
This assembly technique affords biotechnologists the ability to create a wide variety of materials. Through control of the position and properties of each individual droplet in the assembly, Bayley’s group can “confer upon simple materials the properties of complex biological tissues.” For example, by outfitting only certain droplets with an ionically conductive pore membrane protein (αHL), a pathway can be created within the material that allows for nearly instantaneous conduction of current. This controlled conduction process mimics the activity of nerve axons by allowing long-distance electrical communication.
“Cooperative action” can also enable macroscopic conformational change, allowing researchers to design complex shapes through folding. By patterning high and low osmolarity droplets within the structure, folding can be programmed through osmosis. As an aqueous medium flows up the salt gradient, droplets respectively swell and shrink; small volume change at the cellular level evolves to macroscopic folding. Such an emergent property could allow design of nonprintable structures or muscle-like flexing activity.
We want to “build materials that behave like biological tissues,” Bayley said when asked to define his group’s aspirations. He sees a tremendous future in this approach, starting, for example, by modulating the mechanical properties of these structures by inserting hydrogels or polymers into the microcompartments. This system could also be used for advanced drug delivery and as a highly engineered three-dimensional scaffold for tissue engineering.
