Sponge-like polymer graft may transform spinal surgery

If you’re of a certain age, you might remember ads for those magic sea animals in the back pages of the Fantastic Four and other comic books. They came in a capsule that, once immersed in water, transformed into marine creatures “before your very eyes.” Scientists have taken that idea, which lost a bit of its magic when you realized that the sea creatures were really just foam that expanded into its preordained shape as it absorbed water, to engineer biodegradable polymer grafts designed to repair damaged vertebrae. The researchers presented their work today in San Diego at the 251st National Meeting & Exposition of the American Chemical Society (ACS).
“The overall goal of this research is to find ways to treat people with metastatic spinal tumors,” says Lichun Lu, PhD. “The spine is the most common site of skeletal metastases in cancer patients, but unlike current treatments, our approach is less invasive and is inexpensive.”

Removing extensive spinal tumors typically requires taking out the entire bone segment and adjacent intervertebral discs from the affected area. In this case, something must fill the large void to maintain the integrity of the spine and protect the spinal cord, explains a news release on the ACS site.

Surgeons have two options: They can open the chest cavity and insert metal cages or bone grafts to replace the missing fragment or use minimally invasive surgical (MIS) techniques to insert expandable titanium rods through a small incision in the back. The latter method is quite expensive. Lu, who works at the Mayo Clinic, and her postdoctoral fellow, Xifeng Liu, PhD, sought a material that would work with the MIS approach but would be more affordable than titanium. That’s where the sea monkey analogy comes into play: They found a material that could be dehydrated down to a size compatible with posterior spinal surgery, and, once implanted, would expand to replace the missing vertebrae through fluid absorption.

The researchers started by crosslinking oligo[poly(ethylene glycol) fumarate] to create a hollow hydrophilic cage that would serve as a scaffold, which could then be filled with stabilizing materials, as well as therapeutics. “When we designed this expandable tube, we wanted to be able to control the size of the graft so it would fit into the exact space left behind after removing the tumor,” Lu says in the news release. The researchers also needed to control the kinetics of the expansion, because if the cage expands too quickly, a surgeon may not have enough time to position it correctly, while a slow expansion could mean a longer-than-necessary surgery.

Modifying the degree and timing of the polymer graft’s expansion was a matter of chemistry, Liu says. “By modulating the molecular weight and charge of the polymer, we are able to tune the material’s properties,” he says. The researchers studied the effects of these chemical changes by observing the polymer grafts’ expansion rates under conditions that mimic the spinal column environment in the lab. This information is key for determining the optimal size of a spinal implant for use in restorative surgery. The team identified a combination of materials that are biocompatible in animals and that they believe will work in humans.

Lu says her lab’s next step is to study the grafts in cadavers and simulate an in-patient procedure. Their goal is to initiate clinical trials within the next few years.

She acknowledges funding from the National Institutes of Health.

The video below shows how the technology will work.