Stronger, longer-lasting PLA could expand implantable medical applications

Polylactic acid (PLA) is a biodegradable polymer that is used to fabricate medical devices that dissolve in the body once they have performed their function. Resorbable polymers are popular in such applications as dissolvable and drug-eluting stents and implantable fixation devices. In all of these applications, the rate of resorption is a key property. Researchers at Brown University (Providence, RI) have discovered that the degradation rate of PLA can be decreased by treating it at various temperatures and pressures. By tuning the material’s rate of resorption, the researchers envisage broadening the scope of potential medical applications.

PLA is a semi-crystalline material with a molecular structure that is partly ordered into crystals while the rest is disordered, or amorphous, like glass. Work by previous researchers had shown that treating PLA with heat could increase the material’s crystalline makeup, which could help to increase its strength. Researchers led by Brown University doctoral candidate Christopher Baker wanted to see if adding pressure to the treatment process would further influence the material’s structure, explains a press release published on the university website.

Baker treated PLA samples under different temperature and pressure conditions for varying amounts of time. He showed that the treatments increased the amount of crystalline area in the material, but there was another, more surprising finding. At higher temperatures and pressures, the amorphous parts of the material became birefringent, meaning that they bend light differently depending upon how the light is polarized. Birefringence is typically observed in crystalline materials, so seeing it in the amorphous regions of PLA was a surprise, notes Edith Mathiowitz, a professor of medical science and engineering at Brown. “We didn’t expect it to have such properties,” Mathiowitz said. “So to see it in the amorphous phase was really amazing.”

“The polymer strands [in the amorphous region] are normally a tangled mess,” Baker explains. “But we found when we processed the material that they became less entangled and much more oriented in a particular direction.” Further thermal analysis showed that the more ordered sections had a higher glass transition temperature, resulting in material degradation at significantly slower rates.

The new amorphous phase combined with the overall increase in crystallinity in the treated samples could have significant implications for the material’s mechanical properties, according to the researchers. The higher crystallinity could make it stronger, while the more ordered amorphous sections could make it last longer. That slower rate of degradation could be particularly useful in medical applications, an area in which Mathiowitz’s lab specializes.

For example, PLA is used as a coating for time-release pills and implantable drug delivery systems. If the rate at which PLA degrades can be controlled, the rate at which it delivers a drug can be altered. There is also interest in using PLA for plates and screws used to stabilize broken bones. The advantage of PLA-based implants is that they degrade over time, so a patient would not need a second surgery to remove them. PLA may degrade too quickly for some of these applications, but if this new polymer phase slows degradation, it may become a better option.

“Now that we’ve shown that we can intentionally induce this phase, we think it could be very useful in many different ways,” says Mathiowitz.

The researchers plan more research aimed at quantifying changes in material properties as well as investigating whether this phase can be induced in other semi-crystalline materials.

The findings are published in the Polymer journal.