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Ethylene gas, hexane, paraffin wax, and polyethylene all have the same chemical building block, namely an ethylene group (CH2-CH2). The difference in properties in these materials comes from the number of ethylene groups that make up a single molecule. Ethylene gas has a single group (or repeat unit), hexane has 3, and polyethylene can go up in excess of 70,000 repeat units. As the number of repeat units increase, the molecule moves from the idea of a rigid linear rod to a floppy random coil. When placed with other ethylene molecules of comparable size, the chains entangle, which causes the material to transition from a liquid to a solid, as the entangled chains resist disentanglement until sufficient energy, usually in the form of heat, is applied. Some portions of the chains will also align to form crystal structures, which further solidify and rigidify the material.

Although some techniques exist to count the number of repeat units in a material, such as MALDI, molecular weight is normally determined by inferred techniques that look at the size of the individual polymer chain in solution. The size of the chain depends both on its molecular weight and the solvent environment it is placed in (a better solvent and improved temperature will tend to expand the polymer chain more). As shown below, a rigid rod molecule’s size is the number of repeat units multiplied by the length of each repeat units. Most polymers will occupy space as a coil, shown on the right, with a size (the radius of gyration, Rg) dictated by the equation shown below for a good solvent. 

Researchers have derived relationships between measured properties of polymer solutions, such as viscosity, light diffraction, and osmotic pressure, to determine molecular weights. These techniques all report a mean value of molecular weight. Most polymers have a distribution of molecular weights, as the polymerization process will yield shorter and longer chains around the nominal desired chain length. Gel permeation chromatography (GPC) is a technique that allows determination of the distribution of molecular weights.

In GPC, a dilute solution of polymeric material is prepared using a good solvent for the polymer. This solution is passed through a chromatography column that contains a porous structure that is sized to allow penetration of smaller polymer chains while excluding the larger ones, which effectively separates the polymer chains by size as they exit the column. A detector (or multiple detectors) measures the quantity of the polymer chains at each elution time, forming a concentration vs. elution time plot. A series of calibration standards are injected through the same column, and the molecular weight vs. elution time is determined. By converting elution time to molecular weight for the unknown sample, a molecular weight distribution plot can be determined. As may have been obvious, the actual molecular weights are not measured; rather, the size of the polymer chains are, with the assumption that the size scales with molecular weight. 

CPG provides GPC analysis on a variety of polymer types, including water soluble, organic soluble, high temperature (for polyolefins), and biologic materials. We can also perform testing using viscometry detectors and light scattering.

The normal effects of beer consumption are well known.  The ethanol in beer enters the blood stream and eventually makes it to the brain, where the ethanol molecules can sit between brain cells and interfere with neurotransmission, the electrochemical process that controls the activities in the brain, such as body movement, communication, and general thought processes. Too much beer, or ethanol, in a short period of time can interfere with neurotransmission to the point that the imbiber loses control of one or more of these brain processes, leading to classic signs of drunkenness (unsteadiness, impulse control issues, memory lapses, and potential unconsciousness). Excessive drinking for prolonged periods of time can also lead to heart disease, generically termed cardiomyopathy, which is any disease that impairs the ability of the heart to circulate blood effectively.

Heart Disease Spike in Beer Drinkers

A study conducted in 1965 in Quebec and Nebraska of beer drinkers who showed unusually high levels of cardiomyopathy resulted in an unexpected source of drinking-related issues with the heart. In Omaha, Nebraska, in a study of 50 patients, a spike in cardiomyopathy was observed starting in 1964. A similar trend was observed in Quebec around the same time. During interviews, clinicians learned that both sets of patients were frequent beer drinkers, and that each set of patients were drinking the same beer local to their area. The one complicating factor in the Quebec case was that Montreal beer drinkers had access to the same brand of beer, but did not have the spike in cardiomyopathy. Clinicians discovered that breweries in both Omaha and Quebec started adding cobalt sulfate to the beer to stabilize the foam. This practice was employed to counter issues with beer foam dissipation in inadequately cleaned glasses due to poor rinsing of  detergent by the bar staff (yet another issue). 

Too Much of a Trace Metal

Cobalt is a metal commonly used in alloys, and is also part of the metabolism mechanism in all animals. However, too much cobalt can cause issues, such as interference with the Krebs cycle and aerobic cellular respiration. Studies where guinea pigs were administered cobalt into the myocardium resulted in diminished contractions of the papillary muscle.[1]  The clinicians felt that the beer drinkers in Quebec and Omaha were exposed to higher than normal levels of cobalt, which resulted in their cardiomyopathy.

So why the difference between Montreal and the rest of Quebec? For the larger breweries, such as those found in Montreal, separate batches were made for draft beer and bottled beer. Since the bottled beer normally was not put in glasses, no cobalt sulfate was added, hence fewer beer drinkers were exposed to high levels of cobalt sulfate. For smaller breweries, such as those found in Quebec, single batches were made for both draft and bottle, resulting in high levels of cobalt sulfate in both forms of beer. Researchers coined the phrase ‘beer drinkers’ cardiomyopathy’ based on this study.[2] Cobalt sulfate adulteration of beer was discontinued after this study came out.

Trace analysis of metals, such as cobalt, is normally measured in food and polymer products with inductively coupled plasma mass spectroscopy (ICP-MS). Contact CPG for assistance in trace metals analysis in your materials.

The durability of plastic materials is both a benefit and a liability. Many plastics can withstand harsh weather, salt spray, ultraviolet light, and mechanical stress for years without visible effect. Unfortunately, this durability also means that when we are done with the plastic, it persists in our waste stream for years, resisting breakdown. A view of any public landfill will show mountains of plastic material. The well-publicized Great Pacific garbage patch, a swirling field of plastic debris estimated at a minimum size of the state of Texas to twice the size of the United States, is the result of years of floating plastic particulate debris accumulating under the influence of ocean currents. Although some photodegradation will occur in some plastics, the debris can persist for decades.

Perhaps the solution to plastic accumulation can be found in the lowly caterpillar.  A recent news article suggests that the waxworm, a caterpillar who evolved to consume beeswax as a food source, also appears to digest polyethylene, at least according to one researcher. Waxworms were transported in polyethylene shopping bags, and were able to reduce the bags to shreds in a few hours, with no visible signs of particulate debris. The researchers believe that the polyethylene was being converted to ethylene glycol (e.g. antifreeze), although more data is required. The question is, what are the chemicals or enzymes in the waxworm that allows this process, assuming the observations are correct? If they can be identified, can they be produced and applied on a larger scale to reduce the persistence of plastic debris? 

Running shoe technology has come a long way since Bill Bowerman, a running coach at the University of Oregon, first made prototypes by melting rubber into treads with a waffle iron in the 1960s, starting what would become Nike. As material technology improved, along with capabilities in analyzing running kinematics and physiology, shoe designs have gotten more complex, with complicated viscoelastic cushioning, designs to correct pronation, and lightweight materials to allow greater turnover speed. A book published in 2009, Born to Run, started a short but passionate trend backwards towards less cushioning and more minimalistic shoes. Since then, runners have cycled back towards more cushioned designs.

In preparation for the Boston Marathon on Monday, we decided to evaluate a few pairs of running shoes, comparing and contrasting at their viscoelastic properties and chemical properties.  Running shoes are typically made from various types of polymers, and may provide a great deal of cushioning, or very little at all! It is apparent that some of our samples have been used to run many miles, while others are more gently used.

Dynamic Mechanical Analysis 

CPG used dynamic mechanical analysis (DMA) to measure the ability of the materials that make up the sole of the shoe to absorb and return mechanical energy. Storage modulus measurements are representative of the elastic portion of the stored energy, while the loss modulus measurements are representative of the viscous (or fluid-like) response of the material. Tan delta values may be interpreted as a “damping ratio,” characterizing the ability to which a material may absorb and dissipate energy. The above figure shows a comparison of the storage modulus, loss modulus, and tan delta for all shoe samples tested at a test frequency of 1Hz. 

Significant differences in storage modulus were observed among the tested samples. Generally, storage modulus values for shoe soles was significantly greater than the corresponding shoe’s insole, a measure of the increased stiffness of the sole and softer, more compliant nature of the insole. Significantly higher storage modulus values were observed for the “lightly used” Mizuno sole than for the “heavily used” Mizuno sole, suggesting that the mechanical properties (and underlying materials) have degraded with prolonged use and environmental exposure, likely due to permanent compression of the porous structure in the sole.

Tan delta values (representing damping) were higher for the insole as compared to the corresponding sole, providing a measure of the ability of these components to dissipate energy during running, ensuring a more comfortable running experience—an important factor over 26.2 miles!

HS-GC-MS

In order to perform a chemical comparison between the shoe samples, they were analyzed by headspace gas chromatography couple to mass spectrometry (HS-GC-MS). 

Measured chemicals may come from the constituent materials composing each sole, from environmental interferences[1] (all shoes were sampled from a mildly worn state), as well as from thermal degradation during the headspace heating and off gassing process.

Vibram

Some compounds identified in the Vibram sole include benzaldehyde, benzothiazole, decamethylcyclopentasiloxane, and BHT, suggesting the material contains significant silicone rubber elements. The benzothiazole is related to accelerators used for the crosslinking (vulcanization) of rubbers.

Nike

Some compounds identified in the Nike sole include n-butanol, α-methylstyrene, acetophenone, and butylated hydroxytoluene (BHT). These compounds suggest the sole is composed of a synthetic rubber rather than a natural rubber, due to the presence of styrene derivatives and possible degradation products. The BHT is likely added as an antioxidant to prevent material degradation during processing and usage. The n-butanol is likely used as a processing solvent during the rubber production.

Asics

Some compounds identified in the Asics sole include ethylhexanol, 1,2,3-trimethylbenzene, 2-pentyl-furan, acetophenone, 1-dodecene, 2,6-di-tert-butylbenzoquinone, and BHT. BHT is a widely used antioxidant and it is therefore not surprising to see its presence in each of the samples tested here.

Mizuno

The selection of a polymer for a medical device requires careful thought and knowledge of both the plastic and the target application, and how each will respond to the other. The range of choices of polymers for medical applications continue to increase as resin manufacturers synthesize novel homopolymers and copolymers, compounders create polymer blends and additives packages, and processors perform finishing steps such as crosslinking and surface treatments. Polymeric materials in medical devices range from rigid UHMWPE and PEEK, to flexible TPEs, films, and woven constructs, to soft hydrogels.

What is the Target Application of Your Medical Device?

When we assist clients in selecting a polymer for their device, the project always starts with a detailed description of the target application. Will the device be an implant with longer than 30 days in the body, will it be in the body for less than 30 days, or will it never contact the patient directly but instead be used in a medical setting (such as packaging or the housing on an infusion pump)? If the device is an implant, is it intended to be permanent or is deliberate degradation desirable? If the latter, over what time scale? And of course, where in the body will the device be used?

Following these questions, we then ask about the device design to get a sense of what material properties will be important. For example, in a hip or knee component wear behavior is important, along with resistance to fracture due to impact loads. For a spinal rod, creep behavior under physiological loads is relevant. For a heart valve, fatigue resistance for millions of cycles is relevant. For a drug release device, the microstructure of the material is important as it can influence the release kinetics. If known, target material properties, such as tensile strength, modulus in the relevant loading condition, wear rates, or pore size, are listed as required specifications. This information is often the result of computer simulation of the design in its target application area. Often times, however, the required specifications or material behavior inputs for simulation are not known, and some initial guesses have to be made, to be verified later during screening tests.

As part of this discussion, the potential manufacturing process is considered, which will further aid in polymer identification. In some designs, injection molding is required, while others may require film extrusion, compression molding or fiber spinning. The use of additive manufacturing has further expanded the consideration of material selection. Storage life of the plastic is relevant for inventory control, as is secondary sources of the material. Lastly, the cleaning and disinfection/sterilization process has to be considered, since both can affect some polymer materials.

This approach helps define the required material specifications for the device. This list may grow as more information is gathered through screening tests, design verification tests, or even through validation testing.

Material Selection & Testing

We then begin assembling a list of candidate materials, normally starting with materials that meet the known property requirements. This material list can be filtered by materials that already have clinical use, ideally in the same or similar clinical area. Any clinically relevant information, such as testing performed to ISO 10993, is considered. There are many cases where an off-the-shelf solution does not exist, but a custom formulation or modification of an off-the-shelf solution can be provided.

After the list of candidate materials has been reviewed with the client, selected candidate materials will be ordered. At this point, we will often perform specific tests on the materials relevant to the client’s target application that are beyond the standard mechanical property tests provided by the resin manufacturer. These tests may include standard or custom fatigue tests, oxidation resistance tests, biocompatibility tests including leaching and extractables, or processability tests. Based on results of these tests, the list of candidate materials can be further reduced.

From this reduced list, we or the client will then make prototype assemblies of the target device, in order to test manufacturing processes and to evaluate the potential failure modes of the device with the candidate materials. Based on these results, the material criteria may be further modified, the design may be adjusted, or a candidate material may rise to the top.

Of course, the cost of the polymer is an important consideration. Depending on the anticipated volume and price point of the device, pricing considerations may be a primary criterion, or a secondary one. The actual polymer pricing depends heavily on volume and form factors.

During this whole process, we encourage the client’s production team to be involved, along with the design team, as both parties have unique understanding of their capabilities and requirements.

If you need assistance in developing a new medical device, or in evaluating alternate materials for an existing device, please contact us.

Cambridge Polymer Group is pleased to announce ISO/IEC 17025:2005 accreditation by the American Association for Laboratory Accreditation (A2LA).

ISO/IEC 17025:2005 is the international standard by which a testing laboratory’s commitment to quality and technical competence is evaluated. ISO 17025 includes ISO 9001 standards and adds higher level requirements, specific to testing laboratories. CPG underwent a thorough assessment of its quality management processes and competency to perform chemical testing. 

Cambridge Polymer Group achieved ISO/IEC 17025:2005 accreditation by demonstrating its compliance with the standard and A2LA accreditation requirements. This accreditation is further evidence of CPG's commitment to quality and technical expertise.  Our dedication to quality is an integral part of our strategy of being recognized as the premier contract research organization in North America, offering a full range of contract research services, excellent technical support and innovative products.

In addition to the ISO/IEC 17025:2005 accreditation, CPG is certified to ISO 9001:2015.

About A2LA

A2LA is the largest U.S.-based, multi-discipline accreditation body with over 35 years of experience providing internationally recognized accreditation services and quality training. A2LA’s world-class accreditation services encompass testing and calibration laboratories, medical testing laboratories, inspection bodies, proficiency testing providers, reference material producers and product certification bodies. Organizations are accredited to international standards and field-specific requirements developed with government and industry collaboration.