Stem cells are encapsulated into niches made from materials designed to mimic the natural extracellular matrix. Hydrogels and macroporous sponges based on polymers are two examples of such materials and they have structures that encourage new tissue to grow.

Now, Mooney and colleagues have developed new void-forming hydrogels that help in bone regeneration, or osteogenesis. The researchers made the hydrogels by mixing together microbeads of one type of hydrogel that degrades very rapidly in a solution of another polymer that degrades much more slowly.

Continuous networks of pores

“Once injected into the body, the slowly-degrading polymer forms a continuous hydrogel around the rapidly-degrading microbeads,” explains Mooney. “These microbeads then degrade when exposed to water, leaving pores or empty spaces within the slowly degrading hydrogel,” he tells “When the initial density of the microbeads becomes high enough, a continuous network of pores forms, allowing biological cells to ‘crawl out’ of the hydrogels.”

Cells of many different types, included mesenchymal stem cells (MSCs), can simply be mixed with the solution of the slowly-degrading polymer before it is injected into the body, he says. “When this polymer forms a hydrogel, it will entrap the cells. This polymer forms the hydrogel by binding calcium – initially present in solution – so our approach is a very gentle way of encapsulating the stem cells.”

Controlling the rate of MSC osteogenesis

The researchers showed they could control the rate of MSC osteogenesis in vitro by modifying the elastic modulus of the hydrogel. Indeed, the hydrogel’s elasticity regulated bone regeneration, with an optimal amount of bone being formed at pressures of 60 KPa.

According to the team, which includes researchers from the Harvard-MIT Division of Health Sciences and Technology, the Julius Wolff Institute in Berlin, the Berlin-Brandenburg Center for Regenerative Therapies, Stanford University, Chung-Ang University in Seoul and Boston Children’s Hospital, says that its technique might be applied to many, if not all, stem cell types to promote tissue regeneration. “Our method provides new design criteria for materials used in regenerative medicine, and the work also provides more support to the general concept that stem cell fate can be regulated by mechanical cues,” adds Mooney.

And that is not all: the research, published in Nature Materials doi:10.1038/nmat4407, is the first to show that pore-forming materials can be used to control how transplanted cells interact with their host tissues. “This could be important in regulating how these cells actually alter the structure of the host tissue,” explains Mooney.

The team says that it is now busy studying how other types of stem cells interact with the hydrogels.