What’s more, after the sac forms, it creates a huge imbalance in electric charge, which acts to pump any added HA through the sac’s membrane. This pumping action brings more HA molecules in contact with PA molecules, and as a result, the team found, the sac continued to grow for up to four days in solution. Stupp says that the team can tailor the sac’s size and thickness by simply leaving it in solution for various lengths of time.
In a second round of experiments, the team combined stem cells with the HA solution, then poured the mixture into a vial with PA molecules. This time, the PA molecules encapsulated both the HA molecules and the stem cells. Researchers added specific proteins to the solution and found that they penetrated the sac’s membrane despite its thickness. These proteins stimulated stem cells to differentiate into cartilage, effectively creating a miniature stem-cell laboratory inside the sac.
Stupp says that such sacs may provide safe, enclosed environments in which to grow stem cells before transplanting them into the body. Additionally, while proteins were able to traverse the sac’s membrane, Stupp says that immune cells would be too large to penetrate, preventing the sac, and its contents, from being destroyed before they can act on their target.
Stupp says that as a delivery vehicle, the sacs can be grown small enough to travel through the bloodstream, or robust enough to be sutured onto a target tissue or organ.
In the next year, the team plans to grow other cells within these sacs and study the growth of tumors, for example, in reaction to specific drugs or molecules.
“You can also have colonies of different cells in different sacs together–a raspberry of sacs–and you can expose them to multiple signals,” says Stupp. “Which might be valuable in cell biology, studying signals between cells in a three-dimensional environment.”
James Baker, director of the Michigan Nanotechnology Institute for Medicine and Biological Sciences, says that the team’s discovery may have important implications in tissue engineering. “A major advantage is the ability to potentially organize cells into unique structures,” he says. “It offers the potential to develop specialized tissue structures … a very impressive accomplishment.”