Stem-cell therapies are often touted as the future of tissue engineering and regenerative medicine. But one of the challenges to developing such therapies is creating an environment in which stem cells can grow. An additional hurdle involves designing a vehicle to deliver stem cells to their target, without being detected by the body’s immune system. Now scientists at Northwestern University have engineered a “miniature laboratory” in the form of a tiny, gel-like sac. They successfully grew stem cells within the sac, delivering proteins and nutrients to the cells through the sac’s membrane. Researchers say that the sac may act as a delivery system for stem cells and other drugs, shielding them until they reach their target. Samuel Stupp, lead researcher and board of trustees professor of materials science and engineering, chemistry, and medicine at Northwestern, says that the discovery may have promising applications in cell therapy and regenerative medicine.
“You could transplant these sacs inside a patient,” says Stupp. “And in the sac, the cells would be protected, until they get more established in an organ or tissue. Then the sac should be able to biodegrade.”
The team developed the sac after months of mixing various molecular solutions together.
“When we would mix solutions, we would sometimes get a cloudy solution or precipitates, but nothing we thought was interesting,” says Stupp. “And one good day, my postdoc walked into my office with a sac, and I knew we had something good. And then we spent more than a year trying to understand what happened.”
Researchers developed the sac from a combination of two molecules: a peptide amphophile (PA), a synthetic molecule that Stupp’s lab developed seven years ago, and hyaluronic acid (HA), a molecule found in joints and cartilage. The team first poured the PA solution in a large vial, then added the HA solution. Almost instantly, the two liquids began to solidify at the point of contact.
As Stupp looked at the interaction more closely, he found that the lighter PA molecules surrounded the HA molecules, sealing them in to create a single pouch, or sac. Interestingly, the sac continued to grow even after its formation, expanding and creating a thicker membrane the longer it remained in solution. Researchers stopped its growth by simply removing the sac from the vial with a pair of tweezers.
But why exactly do these molecules interact so strongly? Stupp explains that the PA molecules are particularly primed to form solid structures. In liquid solution, PA molecules hold a uniform positive charge, essentially repelling each other and remaining in liquid form. However, as soon as it comes in contact with a negatively charged solution such as HA, the PA molecules do not repel as much, and they automatically begin to form nanoscale fibers.
“This is a very potent reaction,” says Stupp. “These molecules want to crystallize, and when they see hyaluronic acid, they weave a fabric of fibers in the plane of contact between the liquids.”
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.”
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