Rutgers researchers test polymer reliability for medical implants

Sascha Abramson has been investigating new methods to ensure that polymer medical implants in the human body don’t fail. Abramson looked at degradable polymers, ones the body can ultimately absorb, to gain a deeper understanding of how and why their structures change – crucial parts of a puzzle that must be solved for polymers to perform predictably and successfully in medical implants.

Her research was conducted as a postdoctoral associate at Rutgers’ New Jersey Center for Biomaterials in the laboratory of Joachim Kohn, Board of Governors Professor of Chemistry and Chemical Biology, at Rutgers, The State University of New Jersey. Kohn and Abramson co-authored a paper on her findings presented in New Orleans today at the 225th American Chemical Society (ACS) national meeting.

Abramson points out that polymers or plastics are different from other materials that have solid, liquid and gaseous phases. Some polymers exhibit two solid states – a rubbery state and a glassy state. “There is a transition in polymers where they go from a hard, glassy state to a rubbery state. They leave their glassy state when they cross a threshold temperature we call the glass transition temperature,” said Abramson.

She cited the example of cold chewing gum being hard in the package, but softening in the mouth as it warms up above its glass transition temperature and goes into its rubbery state.

The research discussed in the ACS presentation focused on how changes in a polymer’s immediate environment and alterations in its glass transition temperature might affect the material’s stability once it becomes part of a medical implant.

Abramson said that traditionally polymer testing has often been done on dry materials. “The point I am making is that we can’t look only at the dry glass transition temperature. That’s not going to be relevant once you put polymers in the body,” Abramson stated. “In the body they get wet, they hydrate or absorb water, and their glass transition temperature can drop to or below body temperature. What was a very hard glassy material outside the body, now becomes soft.

“We know that hydration can also affect the degradability of the polymer,” Abramson observed. “If we use these polymers for tissue engineering where we want the material to eventually be absorbed harmlessly into the body, hydration-induced shifts in the glass transition temperature can affect how fast the material degrades, in addition to its structural stability in the body.”

She stressed that the glass transition temperature needs to be above body temperature for a load-bearing implant, while a lower glass transition temperature may be more desirable in applications, such as artificial skin, where the material needs to be pliable.

Abramson said her studies of hydrated degradable polymers are preliminary but contends that their importance lies in the fact that this was the first time anyone had ever looked at these materials in this context. “I think we need to understand what is going on in a hydrated material because that’s essentially what is happening in the body,” she said.

Another aspect of her investigations considered the phenomenon of enthalpic relaxation. In these studies, Abramson again focused on the glass transition temperature, using its rise to track the relaxation of molecules over time.

Abramson explained that polymers are made up of very long chain molecules, and these molecules move and they twist around each other. “Over a period of time, these molecules can relax, releasing their pent-up energy in a process called enthalpic relaxation,” she said. “This can lead to a polymer becoming brittle and fracturing easily, which is something we do not want to see in load-bearing medical implants, such as artificial hips.”