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Medical product manufacturers share the ideals of a circular economy, in which high value is given to the recovery and recycling of resources whenever possible. Zeus Industrial Products Inc., Orangeburg, SC, has recently extended its achievements in this area by developing a new method for recycling polylactide (PLA)-based bioabsorbable/biodegradable polymers. To find out more about the method, which promises to improve the useful range and cost profile of such polymers, Medical Design Briefs recently spoke with Bruce Anneaux, PhD, corporate director of research and development at Zeus.

MDB: How do the makeup and production of polylactide (PLA)-based polymers differ from other polymers used in medical product manufacturing?

Bruce Anneaux, PhD: PLA was among the first commercially produced bioplastics. Grades used in industrial applications are typically derived from crop sources such as corn or sugar beets, while grades used in medical applications are usually produced from high-purity synthetic monomers. PLA differs from other polymers in that it is quite sensitive to hydrolysis and thermal degradation, making it a poor candidate for conventional mechanical recycling.

MDB: What are the advantages of using PLA in medical products? Are there any significant disadvantages or limitations?

Anneaux: PLA has mechanical properties comparable to traditional polyesters. In addition, such degradable and bioabsorbable polyesters break down at reasonably predictable rates, producing molecules that are readily metabolized by the human body. Depending on the size of the implant and the tissue bed in which it is implanted, however, there are complex interactions that affect in vivo degradation of therapeutic devices.

MDB: For what types of medical products is PLA best suited?

Anneaux: PLA has gained extensive use in a wide range of therapeutic medical devices, including applications in cardiovascular surgery, drug delivery, orthopedics, sports medicine, and tissue engineering. PLA has also found use in many industrial applications, including agriculture and food packaging, with the number of potential uses continuing to show strong growth.

MDB: Many industries place a high value on the use of polymers (plastics) that can be recycled, both to reduce waste and to recapture the value of the polymer for onward use. Are most plastics used in medical products recyclable in this way?

Anneaux: Polymers used in medical applications can be classified as bioabsorbable/biodegradeable or permanent. Bioabsorbable polymers are largely metabolized once implanted. Scrap from the processing of these special polymers is difficult to recycle by conventional means because of its sensitivity to heat and hydrolysis.

Nonbiodegradeable or permanent plastics that come into contact with human tissue or fluids for acute interventions can be disposed of or incinerated as biohazardous waste. Scrap from manufacturing processes can be recycled at the source as relatively ‘clean’ waste. However, such recycled materials can rarely be used again for their initial purposes, because of the loss in properties caused by mechanical recycling. Collection of used medical products for recycling purposes is problematic because of the risk of biological contamination, and because the sources tend to be multipoint in nature (e.g., hospitals, clinics, doctors’ offices) with few central collection facilities.

MDB: What about PLA specifically? What challenges are involved in recycling PLA?

Anneaux: When using traditional mechanical recycling methods, PLA is particularly susceptible to thermal degradation, which significantly limits recycling of PLA. As an alternative to mechanical recycling, chemical recycling of PLA scrap from synthesis or processing recovers the building blocks of the polymer. However, the use of such chemical recycling has heretofore been limited by the requirement of high temperatures and high-pressure reaction vessels, making the process quite expensive and effectively negating the value of recovery.

MDB: Zeus has been working to develop a novel approach to overcome those challenges. How does the new process for recycling PLA differ?

Anneaux: Yes, Zeus has created a novel recycling approach using chemical degradation or depolymerization of PLA. This new biopolymer recycling process allows low-temperature and ambient pressure degradation of PLA into its component monomers or other valuable oligomeric products. The method is carried out at significantly lower temperatures than earlier methods, and does not require pressurized vessels to accomplish chemical conversion. This method of chemical depolymerization results in a very high percent recovery and is more cost-efficient than alternative methods. It also requires significantly less investment on the part of those choosing chemical recycling over traditional recycling options. Further, the chemicals used in the process are easily recoverable by traditional means, and can be reused in a closed-loop system.

MDB: How long does it take to complete the process of depolymerizing PLA into useful oligomers and monomers?

Anneaux: For complete depolymerization, the process can be accomplished within a few hours.

MDB: In practical use, how do you anticipate the manufacturers of PLA used in medical products will be able to recapture these waste plastics and separate them from other materials for recycling?

Anneaux: Medical grade PLA-based polymers are rather costly, so any means of recovering scrap from synthesis, extrusion, molding, or other forming methods is quite valuable. We anticipate the greatest value for medical product manufacturers will be in the recycling of ‘clean’ process scrap—for example, the waste generated during start-up, shut-down, transitions, and from trim loss. PLA scrap can also be separated from other polymers in a variety of ways, including air classifiers, electrostatic methods, or by density. Nonsoluble polymers can be removed from the dissolved PLA in the solvent contacting vessel.

MDB: Is it also possible to recover and reuse the solvents and other materials used in recycling PLA?

Anneaux: Yes, the solvents we use, along with residual alcohol from the reaction, are easily recoverable by conventional distillation methods, and can be reused in the process.

MDB: What limitations might be applicable to the new recycling process? For instance, is it necessary to exclude contaminated materials? Are there any limitations on reuse of the monomers and oligomers resulting from the process?

Anneaux: Once monomers and/or oligomers have been recovered from the process and purified, they can be treated in the same way as chemical feedstocks from traditional sources, and can be used to make new products with the same properties as the initial ones.

MDB: What additional refinements of the process is Zeus exploring?

Anneaux: The next steps for us involve exploring larger scale implementation of our process through industrial collaboration, and accelerating the depolymerization reaction by using catalysts and staged addition of alcohol.

Work to date has demonstrated the process is very effective at removing PLA-based polymers from a traditional mixed industrial waste stream that includes other mechanically recyclable polymeric products.

MDB: How does Zeus expect to make this new process available to its customers?

Anneaux: We look forward to sharing the generic process and the composition of the ternary systems utilized, as well as the kinetic models developed, with interested parties. Zeus’ primary goal for this technology is to make it available across multiple industries for a nominal licensing fee.

The lack of an effective recycling technology for PLA-based polymers has greatly limited their adoption in consumer markets. We expect that adoption of our process will reduce plastic waste, and increase the utilization of PLA-based products in consumer markets, by permitting manufacturers to remove the #7 (‘other plastics’) label from such products.