Using a laser and a light-reactive gel, a research team led by Hao Liu at ETH Zurich produced highly aligned microfilament structures for growing connective, nerve, and muscle tissue in a lab. The optically-based approach developed by the team could open new possibilities for building lifelike tissue models for biomedical research and translational medicine. To successfully fabricate tissues in the laboratory, researchers must replicate the way that the cells in biological tissues are aligned. In many cases, this is achieved by producing a 3D, biocompatible tissue scaffold with aligned microstructures to serve as a platform on which to grow the tissues. Liu first worked with lab-grown tissue as a graduate student in Osaka, when he participated in a project to develop artificial meat. “Back then, I learned that you can develop something relevant and make a difference by growing tissue,” he said. The filamented light 3D bioprinter from ETH Zurich can be used to produce aligned tissue constructs. Courtesy of Zurich University of the Arts/Samuel Thalmann. At ETH Zurich, Liu and his team adopted an existing process for producing a tissue scaffold and used it with a chemically modified gelatin that reacts to light. The gelatin is in liquid form before irradiation, but hardens when exposed to light. “Where we expose it with a laser, it solidifies into hydrogel,” Liu said. “Wherever the laser can’t reach, the gelatin remains liquid.” By targeting the laser beam, Liu and his team found that they could produce customized, 3D hydrogel structures. Liu was tempted to discard some of the hydrogel workpieces that resulted from the tests that he and the team conducted, but instead, he set them aside. When he revisited the pieces, he observed that the hydrogel structures were not uniform and consisted of extremely fine filaments. To confirm this finding, he examined the structures with a microscope. The microfilaments in the hydrogel scaffolds were due to variations in the intensity of the light that the gel was exposed to. Light from a laser beam is not equally intense everywhere it shines; instead, its intensity resembles a spot pattern. In some areas the intensity is extremely high, and in others it is quite low. Because the intensity of the beam varied, the light-sensitive gel did not harden evenly when it was irradiated with the laser beam. Instead, the gel formed parallel, thread-like filaments with channel-like spaces between them. Liu observed that the microfilaments in the hydrogel were similar in diameter to the fiber components found in many biological tissues. Both the filaments and the channels in the gel had an approximate diameter of between 2 and 20 μm. When Liu encapsulated cells in the hydrogel scaffold, tissue grew in the channels between the gel filaments. Using the cells, he created aligned tissue constructs similar to the natural structure of many tissues in the body. “If I had thrown the workpieces away back then, I wouldn't be where I am today,” he said. Although physicists and material scientists have long known about the optical phenomenon that causes the filament microstructures in the gel, Liu and his team are the first to apply this phenomenon to biology. The image shows hydrogel scaffolds and cells that can be used to produce (from left to right) connective tissue, nerve tissue, and muscle tissue. Courtesy of ETH Zurich. Working with industrial design students from the Zurich University of the Arts, the team designed a compact prototype printer to produce filamented hydrogel scaffolds for growing aligned tissues. Using the new printer and the light-activated gel, the team has generated muscle, tendon, nerve, and cartilage tissue constructs. Liu envisions the use of the technology in a broad range of applications. “As a first step, we want to make the technology and the printer available to other scientists so that they, too, can produce such aligned tissues and use them in their research,” he said. Several labs have expressed interest in the new approach to lab-grown tissue. Liu also wants to use the technology to develop different tissue models, like muscle tissue or tendons, with the goal of creating human tissue models for high-throughput drug screening and other applications. The tissue constructs could be used in place of animal testing to research diseases and test drugs in vitro. Liu is a recipient of an ETH Pioneer Fellowship, which he intends to use to develop the bioprinter and bring it to market. The tissue generation technology has been patented by ETH Zurich. In the future, scientists could use Liu’s approach to generate tissue as a substitute material used in surgeries. “It’s even conceivable that this could be used in the future to produce nerve conduits that can be transplanted into patients suffering from nerve injuries,” he said.
