Molecular weight may best be explained by way of a basketball analogy: If two teams have an average height of 6’7″, that doesn’t mean the teams are identical – one may have all 6’7″ players, while the other may have some very tall and some very short players. The same is true of molecules: their average molecular weight is not indicative of the specific weight of individual molecules. Just as having two seven-footers on a basketball team will affect the way the team plays, molecules with higher- or lower-than-average molecular weight can influence how they behave. For polymers, molecular weight relates to mechanical properties and how well resin can be processed, though it is not the only factor. In the case of bioabsorbable polymers, for example, molecular weight establishes how well quickly those polymers will be absorbed by the human body. In short, molecular weight is a critical metric for determining how polymers function and ensuring they perform as intended. That’s also why it’s important to understand which processes have the potential to alter molecular weight and by how much.
Sterilization is one of those processes. Depending on the sterilization technique used, scientists may inadvertently alter a polymer’s molecular weight, thereby changing how it behaves. Still, sterilization is often a necessary process in and of itself. Materials analysis and molecular weight analysis are therefore necessary before and after sterilization to make sure polymers aren’t radically changed. Two common methods of sterilization, that can have an impact on molecular weight, are gamma radiation sterilization and ethylene oxide sterilization.
Gamma rays are a form of high-energy electromagnetic radiation with the power to kill microorganisms by shattering their covalent bonds, according to BioPharmInternational.com. Some elements emit gamma radiation during radioactive decay. By and large, humans shouldn’t spend too much time worrying about the dangers of gamma radiation. In fact, scientists used controlled amounts of this powerful force to sterilize plastics and other polymers. They do not generate heat or moisture, nor do they leave any radioactive traces. Gamma radiation is useful in terms of cost efficiency and level of sterility.
However, some studies have found gamma radiation to have potentially unwanted effects on molecular weight. In one report published in Science Direct, a group of scientists found gamma radiation to have significantly reduced the molecular weight of chitosan, a biocompatible antioxidant additive with film-forming properties. Chitosan has applications in food preservation for a variety of ingredients – if it has a lower molecular weight than anticipated, that could affect how well it performs as a preservative.
Ethylene Oxide Sterilization (ETO)
ETO is a flammable, explosive and colorless gas, according to the Centers for Disease Control and Prevention. The former two qualities make it more dangerous for workers than gamma radiation. ETO is known to cause eye and skin irritation, central nervous system depression, is linked to cataracts and cognitive impairment when inhaled, and is considered by many to be a carcinogen to humans. Additionally, it can be more expensive and require a lengthier cycle time. However, ETO is also a highly effective sterilization method for heat or moisture-sensitive medical equipment.
One drawback to the use of ETO sterilization is that it can potentially leave toxic residuals in the polymer. This must be carefully evaluated and monitored prior to use to ensure patient safety. There is also the potential for a reaction with the polymer molecules and additives which could alter the chemistry of the material. Perhaps the largest benefit to using ETO for sterilization is that it has less effects on the physical properties of the polymer, as compared to other methods. It can be used repeatedly on the same articles, making it a great option for things such as hospital products.
The Importance of Molecular Weight in Bioabsorbable Polymers
Polymers that are designed to be absorbed by the human body must pass highly-specific, detailed testing to ensure they will perform as expected and as needed once ingested or implanted. In the medical world, proper performance of bioabsorbable medical devices and drug delivery systems are necessary for positive patient outcomes and to ensure the medical Hippocratic oath of “do no harm” is upheld. Molecular weight is one property that plays a significant role in bioabsorbable polymer function. In general terms, if the molecular weight is too low, the device may hydrolyze and reabsorb faster than anticipated, while if the molecular weight is too high the device may take longer than anticipated to reabsorb. Verification of the proper molecular weight is essential before going forward with any procedure involving the use of bioabsorbables.
Molecular weight loss through various modes of processing – including sterilization and molding – can result in different behavior and function of the polymer device once it is implanted within the body than what is anticipated or desired.
An independent testing laboratory with molecular weight analysis capabilities is critical for companies that need to determine their products’ properties after sterilization and other processing conditions. These labs have the expertise and equipment to provide scientific data on the molecular weight of polymers, which is one factor in determining the safety of medical products, food packaging and pharmaceuticals.