Polymers May Fill the Gap for Bone Growth

Polycaprolactone (PCL), the type of polymer used in "bone foam."
A sample of polycaprolactone (PCL), the type of polymer used in “bone foam.”

Many people have to live with facial deformities due to a gap in their bone structure. These gaps can be caused by an injury, tumor removal, or birth defect. If the holes are too big, bone cannot grow to span the gap and close it. In this situation, a bone graft can be done to fill the hole, but the grafts don’t always take and can be ineffective at replacing the intricate structure of facial bones.

The bone graft must also be harvested from the patient — a challenging procedure. Then the bone must be formed to fill the gap, and it’s difficult to create a perfect fit. There are alternatives to bone grafting such as bone cement (a combination of polymethyl methacrylate (PMMA) and methyl methacrylate (MMA), or bone putties. The major downside to these materials is they lack the porous nature needed to proliferate natural bone growth, which inhibits regeneration of the host’s bone.

Turning to Polymers

Enter Melissa A. Grunlan, Ph.D., of Texas A & M University. Dr. Grunlan and her team have created a foamlike polymer sponge that can fill in bone gaps for those with facial deformities. The foam is made of polycaprolactone (PCL), a known biomaterial used in sutures and other applications. PCL is stiff at 98.6°F (body temperature) and pliable at 140°F or above, and it degrades in the body at a safe and slow rate.

The “bone foam” would be heated by the surgeon in salt water; when optimal temperature is reached, it would be formed to into the shape of the missing piece of bone. After the foam is put in place, it would stiffen at the lower body temperature. It would then act as a scaffold, with bone cells gradually moving in to replace the foam, which would slowly and naturally degrade and be absorbed by the body, then expelled. Grunlan estimates that considering the rates of bone growth and degradation of the PCL, an implanted patient should have full bone growth after approximately a year.

Boning Up

Grunlan points out that during experiments, she and her team coated the PCL with another polymer, polydopamine, which encourages bone development. They then seeded the polymers with human bone cells. After a couple of days of observation, they noticed not only the growth of new bone tissue, but also the proliferation of proteins vital to bone formation.

So far the only tests have been executed in a lab setting. According to Grunlan, the next phase would be to use mice to determine bone growth in a live specimen before moving on to clinical trials. There is still quite a bit of testing to go regarding this new foamy wonder, but with FDA approval, doctors may have a new, easier, and more precise medical product to treat patients with facial bone gaps. Other possible applications for this type of bone-regenerating technique may include aiding those with a slower rate of bone development such as the elderly.

So, you might say this polymer is “Bad to the Bone,” because it’s good for bone growth!

Image by Steve Jurvetson.
Source: “This Sponge-Like Polymer Could Fix Facial Deformities,” by Nick Stockton, www.wired.com, August 14, 2014.
Source: “This Sponge Could Help Fill Gaps Where Bone Can’t Regrow Itself,” by Robert Sorokanich, www.gizmodo.com, August 14, 2014.
Source: “Continuous Gradient Scaffolds for Rapid Screening of Cell-Material Interactions and Interfacial Tissue Regeneration,” by Brennan M. Bailey, et al., Acta Biomaterialia, September 2013, DOI: 10.1016/j.actbio.2013.05.012.