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CPG researchers, along with orthopedic surgeons from the Massachusetts General Hospital, received a U.S. patent for a low cost navigation system designed to assist surgeons in aligning the acetabular cup used in hip replacement surgeries. The USTPO issued the patent as number 9,554,731.

Hip replacements now have a possible life expectancy of over 20 years, the duration enhanced if the replacement is positioned correctly during implantation. After initial positioning on the operating table, the patient is secured with rigid posts or bags, and then covered in sterile drapes. Despite the restraints, the patient often moves, due to the sometimes drastic motions during surgery. The drapes make it difficult for the surgeon to find the true orientation of the hip, and even a small positioning error could lead to decreased implant longevity.

CPG's tool allows surgeons to overcome the challenge of the draped, moving patient without radical changes in surgical procedures. The device streams patient orientation data to either a monitor or hand-held device, reporting an acceptable range of position angles and warning of incorrect orientation. CPG's patented sensor is built on an adhesive pad, slightly larger than a quarter - cheap and disposable.

Optimize Your Patient Positioning

• Improved implant position decreases wear and increases longevity.
• Additional feedback is beneficial to all surgeons, but is especially helpful to low-volume and inexperienced surgeons.

This device is available for license. Please contact us for more information.

We continue our discussion of polymers in sports equipment by looking at football helmets. The well-publicized dangers of head injuries have put helmet design back in the forefront of sports equipment manufacturers. Modern day football helmets bear little resemblance to the original leather helmets first introduced in the early 1900s. These leather helmets had no face guards, little padding, and were primarily used to protect the players’ ears. In the late 1930s, plastic helmets were introduced, which incorporated the chin strap for the first time. In the 1940s, the first face guards made their appearance, although they were much more minimal than the modern face guards seen today.

Material Composition of a Football Helmet

The mechanical requirements of football helmets requires a material that is lightweight, tough, and exhibits good impact strength. Most modern helmets use polycarbonate for the outer shell. Polycarbonate is a high impact resistant polymer that can be injection molded, painted, and can maintain its properties when subjected to cold temperatures, an important feature for teams playing in the northern states. Less expensive helmets sometimes use acrylonitrile-butadiene-styrene (ABS) copolymer, another impact resistant material.

The padding in helmets, critical for absorbing and dampening impact loads, is often made from nitrile-based foam, and reinforced with polyethylene foam. The pore size and chemistry of the foam will affect its viscoelastic response (modulus vs. frequency response), which is the most important behavior for a helmet application, so proper testing of potential materials is important, often requiring dynamic mechanical analysis.

Minimizing Impact Load

In addition to material selection, helmet design plays an important role in its ability to minimize direction transmission of impact load from the helmet to the athlete’s brain. Some helmet designs contain a hexagonal-shaped cutout near the forehead of the helmet. This cutout is a cantilever system which is intended to absorb some of the impact force.

Researchers are also considering integrated helmet-shoulder pad designs to further dissipate impact loads from the head down into the body. Accelerometers built into the helmets provide side-line monitoring of impact loads in players. 

The Antimicrobial Properties of Silver

The phrase ‘blue bloods’ is associated with nobility or wealthy upper class. The etymology of the phrase has a few sources. One is that the pale skin associated with a non-laboring class of citizens allowed the blue veins to be better observed. A second, and more scientifically intriguing, source is that the silver cutlery, place settings, and goblets used by the wealthy and nobility imparted the users with a rich influx of silver ions, resulting in a healthy blue-ish patina on their skin.

Microbe Slayers in Medical Devices

Silver ions also impart some antimicrobial properties. Multiple researchers and companies have incorporated silver into wound dressings, salves, and medical devices to provide antimicrobial benefits. A potential issue of silver is that for efficacy against microbial agents, silver dose levels can often result in a toxic response in tissue. Fortunately for the 'blue bloods,' toxicity from ingested silver is fairly low, usually consisting of the cosmetic symptom of blue-grey skin. 

Skinny Silver For a Better Bandage

Research from the University of Wisconsin-Madison has led to the development of a wound dressing comprised of a porous polymer nanofilm that contains elemental silver nanoparticles. The nanofilm is less than one cell thick, and is very conformable to the wound surface, allowing intimate contact with the wound. This intimate contact allows better material localization to the skin cells rather than the wound fluid, allowing lower silver concentrations, which may address the issue of silver ion toxicity in the skin cells.

While in past centuries the upper class may have been the sole beneficiary of silver's antimicrobial properties, the University of Wisconsin-Madison researchers envision a day when silver nanoparticle band-aids are sold in drugstores. 

Social scientists have opined that social behavior is influenced both by environment and genetic makeup, the latter relating to our body chemistry. Interestingly, many polymers' resistance to cracking follows these same two influences. Whereas some cracking results from excessive loads, or from thermal or UV degradation, a large percentage of plastic part failure results from a phenomenon termed environmental stress cracking (ESC), which is the result of an external chemical acting on a plastic part that contains internal stresses. The stress at which ESC cracking occurs is often well below the nominal fracture stress of the material.

ENVIRONMENTAL STRESS CRACKING EXAMPLES

We often see examples of ESC in components that are subjected to regular exposure to cleaning agents. Hospital equipment, which will often see alcohols, surfactants, and disinfectants during reprocessing, often experience ESC after multiple cleaning cycles. Whereas ESC may not lead to parts failure or loss of functionality, they do result in a marred appearance, and may also lead to disinfection issues due to the generation of internal surfaces that may harbor microorganisms.

Polycarbonate is another thermoplastic that is susceptible to environmental stress cracking. Windows made from polycarbonate often show evidence of crazing (fine, silvery cracks) over time, which is the result of ESC (look at an airplane window next time you fly; you will likely see some crazing). Similar to the description above, crazing does not necessarily mean that the window’s integrity is compromised. Rather, the crazes have relieved localized stresses and may have stopped growing.

ESC PROCESS

ESC occurs when a polymer component is exposed to a chemical when under stress. The stress may be residual stress resulting from a molding or machining process, or can be an externally applied stress during its application use. The chemical, almost always a liquid, does not result in bulk solvation or chemical degradation of the polymer. Rather, typical ESC chemicals are weak solvents for the polymer in question, and result in partial chain disentanglement in the regions of high stress. This chain disentanglement leads to localized plastic flow at stresses below the normal yield or fracture stresses, resulting in craze formation followed by cracking. The kinetics of ESC depend on the rates of the ESC chemical absorption and the relative rates of craze and crack formation, which are also influenced by the mechanical stresses in the samples.

ESC MORPHOLOGY

ESC is characterized by brittle fracture surfaces, even occurring in polymers that normally exhibit ductile behavior. The crack morphology usually shows a smooth fracture plane, indicating a slow growing crack, as opposed to a striated surface indicating fatigue behavior. Since ESC often results in craze formation ahead of the crack, residual craze fibrils are sometimes evident on the fracture surface. ESC cracks often initiate from the surface, which is the source of the chemical cracking agent.

NATURE VS. NURTURE

So what materials (nature) and what environmental conditions (nurture) lead to situations where ESC is more likely? On the nature side, lower molecular weight polymers are more susceptible, since chain disentanglement is more likely. Additionally, polymers that have a lower amount of crystallinity, or that are amorphous, are more likely to exhibit ESC behavior, since crystalline domains have less free volume available for an ESC chemical to occupy. For this reason, polystyrene and polymethyl methacrylate (amorphous) are more susceptible to ESC than polyethylene (semicrystalline).

On the environmental side, the chemical makeup of the polymer is the primary determinant of which chemicals may act as ESC agents for a given polymer. Potential ESC agents include chlorinated hydrocarbons, aromatics, carbonyls, fatty acid esters, alcohols and aliphatic hydrocarbons, to name a few. Additionally, lower molecular weight ESC agents are more active than higher molecular weight due to improved ability to diffuse into the polymer structure.

Also on the environmental side, the nature of the loading is important. Tensile stresses are required to induce ESC; compression will not cause ESC, unless they lead to a component of tensile orientation in another plane. Additionally, residual stresses resulting from material orientation during injection molding can result in ESC.

ESC TESTING

There are multiple approaches to testing materials and components for their propensity to exhibit ESC. ASTM F484 “Standard Test Method for Stress Crazing of Acrylic Plastics in Contact with Liquid or Semi-Liquid Compounds" describes a method where plaques of acrylic materials are flexed to specific levels of strain, and hence stress, while exposed to potential ESC agents for periods of time, while examining the plaques for evidence of craze or crack formation. Strain (stress) can be applied to samples in many ways, including a circular strain jig (detailed in ASTM D543 “Evaluating the Resistance of Plastics to Chemical Reagents”) or an elliptical Bergen jig, which applies a range of strains to a single sample bar.  Chemical exposures can also be performed in several ways including immersion, wet patch, wipe, or spray.

CPG has many years of experience investigating cracks in polymeric devices, and can assist in determining the root cause and preventing their appearance. 

Some of Cambridge Polymer Group's staff spent last Friday’s lunch hour working with polymers for a good cause. Unlike our usual analytical testing and contract R&D, this particular work did not require a lab. We braided polyethylene terephthalate (PET) into dog toys for PetsEmpower, a non-profit that fosters and reunites pets with survivors of domestic violence. The polar fleece toys will be given to animals during and after their foster experience. 

PET - A Warm and Fuzzy Material 

From dog toys to astronaut underwear, polar fleece has infiltrated every aspect of our textile lives. It's hard to believe this ubiquitous fabric was invented as recently as 1979. Two factors contributed to polar fleece's widespread adoption. Firstly, extruded and then woven PET is a good replacement for wool; it is soft and warm, but not as scratchy as wool and certainly not as smelly when wet. Secondly, Malden Mills, the fabric's inventor and manufacturer, chose not to patent the new technology, allowing other vendors to make the wool alternative. Although originally seen as an outdoor material, garment makers quickly recognized polar fleece's potential benefits in applications beyond backpacking gear.

Polyethylene terephthalate is made by heating terephthalic acid with ethylene glycol (a.k.a. antifreeze). As the PET cools, it forms a viscous liquid. That liquid is then extruded through a nozzle, dried, and cut into plastic flakes. To make polar fleece, those flakes are heated and extruded through fine holes. As the liquid sprays out of the holes, it hardens into a fiber that is wound onto heated spools. In addition to making polar fleece out of virgin PET, it is also possible to use chopped up soda bottles.

PET Toys for PetsEmpower

The polar fleece toy is soft on the dog's mouth, yet strong enough to withstand some pulling. Since the fabric is machine washable, it doesn't matter how dirty it gets. These toys provide enrichment and chewing stimulation during and after the dog's foster stay.

Why is the foster stay necessary? Some domestic violence survivors are reluctant to leave abusive living situations because of what will happen to their pets; most human shelters do not allow animal family members. PetsEmpower finds foster families for those animals so that domestic violence survivors can access resources without worrying about the safety and comfort of their pets. Once survivors have reestablished themselves in new environments, they are reunited with their pets.

Cambridge Polymer Group is proud to support PetsEmpower. We are grateful to Polartec (previously known as Malden Mills) for providing the navy polar fleece for the dog toys and to Building Impact for supplying the other colors and for facilitating this warm and fuzzy volunteer opportunity.

Warming from the sun occurs through the absorption of visible light and infrared light into materials whose molecular groups become energized at specific wavelengths, and the material warms. These molecules then can lose the energy by emitting it at different wavelengths, often in the infrared spectrum, cooling the material.

Passive Cooling

Creating surfaces that do not heat up as much has been a goal of material scientists interested in passive cooling systems, which are more cost and energy effective than active cooling systems like air conditioning and thermoelectric coolers.

Researchers have tried to make materials that do not absorb much infrared light, but are capable of absorbing heat from adjacent surfaces and then emitting the absorbed energy in the form of infrared radiation. Films of materials like this could be put on solar panels to drop their temperature to a level where they operate more efficiently, or could be used on the outside walls of buildings to reduce the internal temperature by a few degrees.

Cool as a Glass-Polymer Hybrid

Researchers at the University of Colorado-Boulder recently developed a composite structure made from a thin sheet of transparent plastic (polymethylpentene) which was infused with 8 micron glass beads. The sheet was coated on the back with silver. When put on another surface to be cooled, the silver reflected most of the visible light that contacted the sheet. The film also removed the heat from the underlying layer and reflected it in the mid-IR range. The glass beads act as IR resonators  to make them efficient IR emitters. The Colorado researchers estimate that the films can reduce the surface temperatures by 10°C and can be made cost-efficiently.