You don’t need much scaffolding for a one-story home, but if you’re putting up a high-rise, you’ll require plenty of it to create the rising edifice.
Similarly, a cell monolayer needs little support, but if doctors are going to grow complex 3-D tissues—hearts, lungs and bones—stem cells will need a supportive structure to grow into. “Without the scaffold, I don’t think the cells necessarily know what to do,” said Robert Langer, a biomedical engineer at MIT. Recent advances in scaffold technology include high-tech tooth models, low-tech gels molded around cotton candy, and scaffolds made from real organs.
Scaffolds come in all shapes and sizes, made out of materials ranging from gels to silk to ceramics (reviewed in Willerth and Sakiyama-Elbert, 2008). Simple hydrogels, made of artificial or natural polymers, are common scaffold materials. Researchers can also go the high-tech route, using use microfabrication techniques—some quite similar to those used in making computer chips—to precisely create a complex scaffold (reviewed in Sachlos and Czernuszka, 2003: PMID14562270).
At this point, there is no clear choice for the best scaffold. “It really depends on the tissue,” Langer notes. Some tissues will require a soft, spongy scaffold; others will need something more rigid. Some scaffolds melt away quickly; others can persist for months or years.
Each material offers advantages and disadvantages. Hydrogels are cheap and easy to make, but lack the precise structure of more expensive methods such as 3-D printing. Natural materials are a good match for living tissue, but also require purification to avoid any immune response in the recipient. In addition, researchers working with natural compounds are limited in how much they can vary their materials. With synthetics, on the other hand, scientists can tweak properties such as stiffness, degradation rate and shape at will. But for artificial scaffolds, researchers often have to add in sites for stem cells to sit down on; these sites are often included in natural materials.
Providing structure is only part of the challenge. Cells need plenty of nutrients and oxygen to survive. In the body, cells rely on the vascular system to drop off oxygen and nourishment. “Cells can live about a hair’s diameter distance from a capillary,” said Leon Bellan, a post-doctoral researcher in Langer’s lab. “That’s one of the major hurdles that still has to be overcome.”
Here, StemBook reviews a few recent advances in scaffold technology.
Jeremy Mao, a dentist and researcher at the Columbia University Medical Center in Manhattan, New York, took the high-tech route, as he and his colleagues reported in the August Journal of Dental Research (Kim et al., 2010: PMID20448245). Mao is looking to re-grow lost teeth from stem cells. Mechanical implants, he noted, do not integrate with the jawbone and can come loose. Stem cells, he hopes, can form teeth that hook up with the jaw.
Researchers have attempted to form dental tissue in hydrogels, Mao said, and made components such as dentin and enamel. But those ingredients fail to coalesce in an organized fashion. “You have multiple pieces, and you don’t have the anatomic shape,” Mao said.
Mao used a 3-D bioprinter to craft precise, tooth-shaped chemical scaffolds, including plenty of channels for cells and vascular structures to fill. The machine works much like a regular printer, he said, but instead of red, cyan and yellow inks, the cartridges hold biomaterials.
The researchers extracted teeth from rats and implanted the scaffolds. Endogenous stem cells migrated to the site. They formed tooth-like structures that connected to the jawbone and filled up with blood vessels. Mao is planning to try the same technique with people in a small safety study.
Advanced technologies such as bioprinters offer precision, but they have drawbacks. “They’re awfully slow, and they’re awfully expensive,” Bellan noted. He’s working on a cheap-and-easy way to leave space for blood vessels in a scaffold.
As he was finishing his PhD at Cornell University in Ithaca, New York, Bellan found himself pondering the vascularization problem. Cotton candy, he reasoned, looks a lot like the vascular system. The spun sugar goody—which Bellan doesn’t particularly enjoy—consists of 10-micron fibers, the same diameter as the smallest capillaries.
Bellan calls his technique “sacrificial cotton candy” (Bellan et al., 2009). With a $40 cotton candy machine and a bag of sugar from the supermarket, he whips up a sugary puff. He pours silicone or epoxy over the treat, and then dissolves the sugar in water. What’s left is material with a microchannel network that capillaries, he hopes, could fill.
Bellan is currently working on a way to use his technique with hydrogels. Or, as he put it, “I’m trying to put cotton candy inside of Jell-O.”
Kit Parker, of the Wyss Institute for Biologically Inspired Engineering and the Harvard Stem Cell Institute, is also using a cotton candy machine to spin polymer fibers for scaffold construction (Badrossamay et al., 2010: PMID20491499). “It’s a novel, cheap way to do it,” he says. “We have a tendency to over-engineer something…the simple way to template these things is great.”
Mother Nature’s Scaffold
Be they high- or low-tech, artificial scaffolds cannot yet duplicate the complex mix of proteins in the natural extracellular matrix, said Korkat Uygun of Massachusetts General Hospital. In the July issue of Nature Medicine, he and colleagues reported on their efforts to make a liver scaffold out of perhaps the most obvious ingredient—liver (Uygun et al., 2010: PMID20543851). The trick is to dissolve away the cells, leaving the extracellular matrix behind. Uygun and his team, led by first author Basak Uygun, followed on the work of other scientists who have decellularized tissues such as heart (Ott et al., 2008: PMID18193059) and lung (Ott et al., 2010: PMID20628374). Doctors have even reconstructed a trachea this way, and successfully transplanted it in a person (Macchiarini et al., 2008: PMID19022496).
“The good thing is you don’t have to re-engineer all the mechanical and physical aspects of it,” commented Kaushik Chatterjee, who works on scaffold mechanics at the National Institute of Standards and Technology in Gaithersburg, Maryland. He was not part of Uygun’s study.
Approximately thirty million Americans have liver disease (Heron et al., 2009: PMID19788058), with many waiting for a transplant. But the annual supply of perfect donor livers is 4,000 below the demand (Punch et al., 2007: PMID17428283). Uygun suggested his method could allow imperfect livers to serve as transplant scaffolds. “We basically show that you can take a liver that you are about to throw out in the garbage, wash the cells out, and you end up with a scaffold that you can do quite a few things with,” Uygun said.
The researchers used detergent to dissolve the cells in rat livers, leaving behind a ghost of the organ. “It looks like a liver, it almost feels like a liver, except it’s white and translucent,” Uygun said. The structure and vascular channels remain, but other researchers have shown that the immunogenic MHC markers—which could scupper any transplant utility—do not linger (Petersen et al., 2010: PMID20576850).
The researchers added primary rat liver cells to repopulate the structure. Then they transplanted the new tissue into living rats, where it survived, and allowed blood flow, for at least eight hours.
The process should easily scale up to human organs, Uygun said, and there is a ready supply of unused cadaver livers. He imagines that the decellularized matrices could be stored, then recellularized on demand.
Plenty of challenges remain, Uygun said, before researchers attempt to scale up to human tissues. One key goal, he noted, is to convince endothelial cells to line the vascular channels.
With further scientific advances, Uygun predicted, it is possible that materials engineers will be able to create an artificial scaffold that mimics the complexity of real tissue. But it might take a century, he speculated. Until then, real-tissue scaffolds bypass the difficulties in replicating organs in minute detail. “We need stuff that works right now.”
The vast variety of materials and approaches means researchers will likely be working on scaffolds for a good long time. “There are a lot of very big questions that still have to be answered,” Bellan said. Advances to watch for include new materials and new ways to seed them with cells.
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