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Multiple degradable thermoplastics are being used for implant or other in vivo use in the medical industry. Polycaprolactone, polylactic acid, poly lactic-co-glycolic acid and polydioxanone are just four examples of polymers that will biodegrade when placed in the body through a hydrolysis reaction. The benefits of these polymers for biomedical applications depend on their degradation rates, properties during degradation, and the degradation compounds. Whereas there are fairly well established methods for assessing degradation rates and the resulting properties of the degrading polymer, determination of the degradation products can be more challenging. Often, researchers will perform in situ degradation studies, using a simulated environment such as phosphate buffered saline, enzymatic solutions, or similar, with the assumption that these in situ environments will result in the same degradation pathway as an in vivo environment.

CPG has developed assays that allow the identification and quantification of degradation products of biodegradable polymers from animal studies. En bloc tissue samples containing the device and surrounding tissue are analyzed for degradation products amongst the biological tissue. The results can then be compared to in vitro degradation samples to assess if the in vitro assay is faithfully generating the same degradation products as the in vivo test.

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CPG researcher Adam Kozak was a co-author on a recently published article in the Journal of the Mechanical Behavior of Biomedical Materials. Along with co-authors from UC-Berkeley (Ansari, Gludovatz, Ritchie, and Pruitt), these researchers investigated the sensitivity of ultra high molecular weight polyethylene (UHMWPE) to fatigue when stress concentration sites are present in the form of notches. The authors investigated 3 formulations of UHMWPE commonly used in hip or knee implants today, and found that the sensitivity to crack propagation resulting from fatigue was more sensitive to microstructure, such as crosslink density, rather than specifics of the notch geometry.

Link to Article

Formaldehyde, or CH2O, is commonly used in producing resins for coatings and adhesives, automotive materials, as well as materials for the textile industry. In these applications, the formaldehyde is normally incorporated into the material through a chemical reaction, and hence loses its chemical identity. Formaldehyde is also a by-product of some chemical reactions.

Aqueous solutions of formaldehyde, also known as formalin, are used as disinfecting agent for biological tissues, in that it is effective in killing bacteria and fungi. This toxicity to bacteria applies to humans as well, prompting manufacturers to reduce residual formaldehyde to safe levels in manufactured products.

CPG has developed sensitive techniques to measure trace levels of formaldehyde in a variety of matrices. This approach allows manufacturers to determine whether their formaldehyde levels have been reduced below threshold levels.

Ever wonder what those little blue particles are in some toothpastes? One of our scientists did, and started to investigate their effects on teeth brushing. Known as microbeads or microplastics, these blue particles are usually polyethylene or polypropylene. They are commonly included as exfoliants in face cleansers, and may be added to toothpaste for visual effects.

CPG researcher Lucas Rossier performed a bench top study comparing enamel wear using toothpastes with and without microbeads. A model enamel substrate was used in place of an actual tooth surface. Through optical and electron microscopy, Rossier showed greater scratching and abrasive wear in the toothpaste containing microbeads. Although the results may not be directly comparable to actual tooth enamel wear, they do demonstrate that the microparticles have an abrasive effect that is greater than the toothpaste alone.


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Okay, so my wife may disagree. However, the long chain nature of a lot of food products (i.e. their polymeric nature) allows us to at least understand the physics behind the behavior of food.

Cooks have known the secret of thickening sauces and gravies with a fairly small amount of flour or cornstarch. A key aspect of this process is to add the flour or starch when the sauce is fairly cool, mix well, and THEN apply heat. At some point in the heating process, the sauce will magically start thickening (i.e. the viscosity increases).

So what is happening? First off, the key material is starch, which is found in flour and of course corn starch. Starch is a polysaccharide, or a polymer made up of glucose (sugar) repeat units with glycosidic bonds. Starch is derived from various types of plants, and can have varying degrees of amylose and amylopectin, the two types of molecules making up starch. At room temperature, starch is not soluble in water, due to its crystalline nature. The molecules in the starch can fold themselves into tightly ordered sections, with thousands of the amylose and amylopectin molecules tightly bound into small micron-sized particles. This is how they initially start in the sauce. The small particles do not affect the viscosity in any measurable way. Once the sauce is heated, however, sufficient energy is put in the starch granules to overcome the melting point, and the chains start to unfold, stretching out and entangling with other chains. The long, entangle molecules of the amylose and amylopectin increase the viscosity of the sauce, thereby 'thickening' it.

Having problems with lumps in your gravy? The gravy was too warm when you added the flour or corn starch, and the starch granules began to swell and expand while they were still in a bundle, preventing them from spreading out in the gravy.  By adding them to cool gravy, you can distribute the granules before they expand and entangle.

This thickening behavior of gravy with temperature can be nicely captured by shear rheometry.

A common science experiment is to have a student place a rubber band against their lips while rapidly stretching the rubber band. The student will feel the rubber band heat up. Rapidly relaxing the rubber band will result in the band cooling. What is happening is that in an unstretched state, the rubber molecules are randomly organized; stretching the rubber band orients the molecules, thereby reducing their entropy (state of randomness). If this is done quickly, the rubber band heats up as the heat generated internally by this decrease in entropy cannot be dissipated rapidly enough to the surrounding atmosphere. Likewise, when the entropy is rapidly increased by relaxing the rubber band, energy is consumed by the rubber band, resulting in its cooling.

This effect has been termed the elastocaloric effect, and is being investigated as a means of providing cooling as an alternative to the typical vapor compression cycle used in most refrigerator and air conditioner units. This change in molecular level ordering can be accomplished through mechanical means, magnetic means, or electrical means (the latter two termed magnetocaloric or electrocaloric cooling). A key technological challenge is identifying materials that can handle the millions of cycles of fatigue behavior without change.

The Department of Energy is exploring these alternative technologies as part of their interest in greener building technologies.  The key metric for success is an improved coefficient of performance (COP) of the new technologies (ratio of the delivered cooling energy to the total input wattage of the device) vs. existing vapor compression cycles, which are around 3-4.