By infusing a hydrogel with live yeast cells and then using the gel as ink, researchers used three-dimensional printing to make a bioreactor that can ferment sugar into ethanol continuously for days (ACS Appl. Mater. Interfaces 2018, DOI: 10.1021/acsami.8b02719). The printed material, a gel lattice the size of a sugar cube, could make it easier, faster, and cheaper to produce biofuels, brew beer, and synthesize drugs and other chemicals.
Embedding living cells into hydrogels is not a new idea. Biotechnologists are having success doing this with mammalian cells to make tissue and entire organs. Since the 1980s, researchers have been trying to embed other types of cells, such as yeast, bacteria, and algae, into hydrogels for use in chemical syntheses. Such gels could speed up and reduce the cost of industrial manufacturing processes, which are typically done in batches in large bioreactors. To make ethanol, for instance, yeast is fed sugar, and after a few days, the reactor is shut down so that the alcohol product can be separated from the mixture and the dead cells cleaned out.
Trapping the yeast inside hydrogels “eliminates the need to separate the cells so the product is able to flow out easily,” says Alshakim Nelson, a chemist at the University of Washington. It also keeps cells safe from potentially toxic chemical products that might form during the reactions, allowing them to keep chugging away for longer.
Three-dimensional printing with cell-laden gels is a fast way to make biological materials and devices. But it has been hard to integrate the cells evenly into the structures and keep them viable. That’s because most previous work has used biopolymers such as calcium alginate or hyaluronic acid, Nelson says. Those gels start degrading after a few days, so the cells die prematurely or seep out of them.
Nelson and his colleagues made a synthetic polymer for the ink by adding dimethacrylate to Pluronic F127, a commercially available triblock copolymer. The polymer has three properties that make it a good 3-D-printing bio-ink. First, it is nondegradable. Second, it is temperature responsive: It’s a liquid at 5 °C but becomes a gel at 23 °C, so the researchers could add yeast cells into the cold liquid and warm into a gel to disperse the cells evenly. Lastly, the gel becomes slightly thinner under pressure so it was easy to squeeze out from a 3-D printer nozzle.
Using the yeast-hydrogel ink, the researchers printed cubic lattices that were 15 mm on each side and shined ultraviolet light on the lattices to cross-link the hydrogel.
To test the lattices’ fermentation performance, the researchers immersed the structures in a culture medium containing 2% glucose for two weeks, replacing the solution with a fresh one every three to four days. The cells converted on average 90% of the glucose into ethanol over the two-week period.
And they could go even longer. The hydrogel physically limits yeast from growing but they stay metabolically active. Nelson says the yeast could potentially ferment for months. “Preliminary results are promising,” he says. “The idea is to print these lattices, put them in columns, and have feedstocks flowing through them.” Such a continuous process would be more cost effective and efficient than the batch processes used today for industrial fermentation and chemical production, he adds. And by using other types of cells and engineered microbes, the process could be developed to produce other valuable compounds like antibiotics and proteins.
But it is early to tell how economical this will be. Creating the living materials on a large scale at low cost and making them stable enough for use in industrial-scale applications could be challenge, says Jason B. Shear of the University of Texas, Austin. The technology could be cost effective for high-value products such as certain drugs. “This is nice work that could have high value provided that the researchers identify the right reactions to focus on,” he says.
Nelson says that his group is now trying to assess how the productivity of reactions using the 3-D-printed material compares with traditional batch reactors.