Porous Hydrogels Boost Stem Cell-Based Bone Production

Regenerative medicine relies upon the ability to isolate, manipulate, and exploit stem cells from our own bodies or from the bodies of stem cell donors. A present obstacle to present therapeutic strategies is the poor survival of implanted stem cells. There are also worries of about properly directing the differentiation of transplanted stem cells. After all, if implanted stem cells do not differentiate into the terminal cell types you want to be replaced, the use of such cells seems pointless.

To address this problem, David Mooney from the Wyss Institute and his colleagues have designed a three-dimensional system that might keep transplanted stem cells alive and happy, ready to heal.

Mooney’s group has adopted a strategy based on the concept of stem cell “niches.”. In our bodies, stem cells have particular places where they live. These stem cell-specific microenvironments provide unique support systems for stem cells and typically include extracellular matrix molecules to which stem cells attach.

Mooney and others have identified chemical and physical cues that act in concert to promote stem cell growth and survival. The chemical cues found in stem cell niches are relatively well-known but the physical and mechanical properties are less well understood at the present time.

Stem cells in places like bone, cartilage, or muscle, when cultured in the laboratory, display particular mechanical sensitivities and they must rest on a substrate with a defined elasticity and stiffness in order to proliferate and mature. As you might guess, reproducing the right physical properties in the laboratory is no mean feat. However, several laboratories have used hydrogels to generate the right combination of chemical and physical properties.

Mooney and his colleagues have made two hydrogels with very different properties. A stable, “bulk” gel is filled with small bubbles of a pore-making molecule called a “porogen,” which degrades quickly and leaves porous cavities in its wake. When the bulk hydrogel is combined with extracellular matrix molecules from stem cell niches and filled with tissue-specific stem cells, and the porogen, Mooney and his team can make an artificial bone-forming stem cell niche. The porous cavities in the hydrogel, in combination with the chemical signals, drive the stem cells to grow, and divide while expanding into the open spaces in the gel. Then the cell move from the hydrogel to form mineralized bone.

In small animal experiments, Mooney and his colleagues showed that a porous hydrogel with the correct chemical and elastic properties provides better bone regeneration than transplanting stem cells alone. The maturing stem cells deployed by the hydrogel also induce neighboring stem cell populations to contribute to the bone repair, which further amplifies their regenerative effects.

This study provides the first demonstration that adjusting the physical properties of a biomaterial can not only help deliver stem cells but also tune the behavior of those cells in a living organism. Even though Mooney has primarily focused on bone formation, he and his group believe that the hydrogel concept can be broadly applied to other regenerative process as well.

This work was published in Nature Materials 2015; DOI: 10.1038/nmat4407.