Smart   hydrogel degrades when new bone grows Bioengineers have created a hydrogel to help people regrow lost bone and tissue. The material is liquid at room temperature but turns semisolid when it injected into a patient.

It built to degrade as bone and tissue seeded from the patient cells grow to take it place.

Long-term stability

A problem with thermogelling polymers is that once they harden, they begin to collapse and then force out water, says Brendan Watson, a graduate student at Rice University who helped create the hydrogel.

That process, known as syneresis, defeats the purpose of defining the space doctors hope to fill with new tissue.

�If the transition gellation temperature is one or two degrees below body temperature, these polymers slowly start to expel water and shrink down until they�re one-half or one-third the size. Then the defect-filling goal is no longer accomplished,� he says.

Watson and colleagues solved the problem by adding chemical cross-linkers to the gel molecules. �It a secondary mechanism that, after the initial thermogellation, begins to stabilize the gel,� he says.

The links begin to form at the same time as the gel, but crosslinking takes up to a half-hour to complete.

The hydrogel is designed for stability over its long-term use as a scaffold for cells to take root and proliferate. But it also designed for its own timely destruction. The new material is described in a paper published in the journal Biomacromolecules.
�Semi-smart� material

�I came up with the idea a few years ago, but it finally all come together,� says Watson, who is pursuing both a Rice doctorate and a medical degree in a joint program with nearby Baylor College of Medicine. �These chemical crosslinks are attached by phosphate ester bonds, which can be degraded by catalysts�in particular, alkaline phosphatase�that are naturally produced by bone tissue.

�The catalysts are naturally present in your body at all times, in low levels. But in areas of newly formed bone, they actually get to much higher levels,� he says. �So what we get is a semi-smart material for bone-tissue engineering. As new bone is formed, the gel should degrade more quickly in that area to allow even more space for bone to form.�

Watson expects that the material degradation can be tuned to match various bone growth rates.

�Optimizing the degradation kinetics is nontrivial and may be better suited for a biotech company,� he says. �We focus more on the performance of the hydrogels and the underlying molecular mechanisms�

The National Institutes of Health, the Keck Center Nanobiology Training Program of the Gulf Coast Consortia and the Baylor College of Medicine Medical Scientist Training Program supported the research.

Source: Rice University