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Manufacturers of medical devices and food products will often use packaging with a reduced oxygen level in the region of the product to minimize oxidation of the product and increase shelf-life. Inert gas, such as nitrogen or argon, may be flushed through the packaging, or vacuum may be pulled on the packaging.

Alternatively, an iron-based oxygen scavenger can be placed in the packaging. The packaging often uses a barrier film to inhibit the diffusion of oxygen through the packaging. This film is often a composite structure of multiple layers of plastics and metals.

Determination of the oxygen level in the packaging is a useful way to determine the efficacy of the
packaging process, and to monitor the packaging integrity over time. CPG has developed a technique to quantify the oxygen content in packaged components down to oxygen concentrations in the parts per million.  This technique is discussed in this application note.

Some of the staff at Cambridge Polymer Group and their spouses participated in Boston's annual Hub on Wheels, a 50 mile bike ride that winds through the Boston and its surrounding communities. The ride raises money for the Special Olympics, the Boston Parks and Recreation Fund, and Boston Bikes, an organization set up to encourage cycling in Boston.

An article recently appeared that discuss the use of extensional rheometry to characterize natural polymer systems. The study, published in Food Hydrocolloids by Choi and co-workers, considers the effect of saliva on food products thickened with either xantham gum or carboxymethyl cellulose. These authors used a CaBER extensional rheometer, developed by Cambridge Polymer Group, to demonstrate that filament breakup kinetics of xantham gum is greatly influenced by the presence of saliva, whereas carboxymethyl cellulose is not as affected.


The significance of this study relates to the concept of psychorheology, or how our perception of a product can be influenced by its rheology. In the present study, the extensional viscosity can influence our perception of taste and flavor, as well as general mouth feel, when the xantham gum's viscosity is modified by the presence of saliva.

Scanning electron microscopy (SEM) is a powerful imaging technique that can be used to discern morphological features down to nanometers. High energy electrons are focused into a narrow beam with electro-magnets, which then impinge on the sample and scatter backward off the surface.  This beam is rastered across the surface of the sample in a similar manner to old television cathode ray tubes.  Detectors then create an image by collecting these scattered electrons at each raster point, either from the source electrons that are backscattered from the surface, or from secondary electrons that are stripped from the atoms by the source electrons on and below the surface of the sample. Because of the flood of electrons (essentially a current) on the surface of the samples, a conductive path is required to prevent charge buildup. Charge buildup will alter the appearance of the surface by deflecting the paths of the electrons. Normally non-conductive materials such as polymers and other organic species such as tissue must be sputter-coated with a conductive coating, such as gold or carbon, in order to prevent this charge buildup. This process will hence alter the surface properties of the sample. Additionally, high vacuum is required with this approach, which will dehydrate samples containing a volatile material such as water. The surface coating will also interfere with energy dispersive spectroscopic analysis of the surface of the sample (EDS), whereby the metallized coating may obscure the elements actually present in the sample.




As an alternative method, variable pressure SEM, which is also sometimes called environmental SEM, can be used.  Here a controlled amount of gas, which can be inert, such as nitrogen, or contain water vapor, is maintained in the sample chamber, while an aperture inhibits the flow of this gas into the gun chamber. The gas in the chamber is ionized by the incoming source electrons, and hence will neutralize charge buildup on the sample, negating the need for gold coating. Additionally, the increased gas pressure slows the evaporation of liquids, allowing the visualization of water-containing samples in a hydrated state, all be it at a reduced resolution.



CPG recently acquired a variable pressure scanning electron microscope (SEM). This system is useful for failure analysis of components, in that the imaging is non-destructive. Additionally, the CPG system has a cold stage, which allows the control of relative humidity and temperature in the chamber, which is useful for imaging water-containing samples such as tissue and hydrogels.  The system acquired also has a large capacity chamber enabling in most cases visualization on entire devices and components. 




More information can be found here.

Most polymeric materials exhibit non-Newtonian behavior, meaning that their properties do not behave linearly, and are often strongly rate-dependent. This behavior is strikingly demonstrated in Silly Putty, which flows like a liquid a low deformation rates, and breaks like a brittle solid at high deformation rates. Non-Newtonian behavior in shear flow is often seen as shear-thinning, where the viscosity decreases with increasing shear rate. In contrast, when polymer materials are subjected to an extensional flow, such as that found in fiber spinning, blow molding, contraction flow, and some injection molding processes, the polymer chains are stretched out, resulting in increases in viscosity and elasticity that can reach several orders of magnitude. These properties changes can radically change the polymer's behavior in these processes, either beneficially or detrimentally. Extensional flow characterization will help predict this behavior and allow processes to determine optimal process conditions.



The best way to determine extensional flow properties is through filament stretching extensional rheometry. This technique has been around for several decades, although most extensional rheometers are home-made.  Filament stretching extensional rheometers, or FiSERs, look similar to load frames used to determine the tensile properties of polymer solids. A set of motors stretches a small volume of fluid while simultaneously measuring the tensile force and cross-sectional area of the fluid strand. What makes this test challenging are the small forces and high rates of deformation typically required, along with a non-standard deformation profile. In the images above, a non-Newtonian fluid filament was stretched in a FiSER, and underwent an elastic instability at the endplate, causing the single filament to split into multiple filaments. This instability is discussed in greater detail in the following publication.



Cambridge Polymer Group has developed several FiSER systems in the past for clients, in areas ranging from polymer melts, food products, and polymer solutions. Each FiSER system is custom made based on client requirements.



More information on buying a FiSER
Several publications on FiSER testing of polymer materials are found here.
Application note on FiSER testing

Radiopacity (or radiodensity) is the ability of a material or device to block or obstruct the passage of electromagnetic photons, normally in the form of X-rays. On an photographic X-ray image, a material with more radiopacity than the background will appear brighter than the background due to the unexposed emulsion not developing on the image. For historical reasons this relationship is preserved for modern digital images as well.  In general, the more dense a material is, the higher its radiopacity, although the nature of the specific atoms present (how electron dense they are) also plays a role. As such, metals and ceramics tend to have higher radiopacity than plastics and fluids. Lead, which has a density of 11.8 g/ml, is one of the more dense metals, and is why it is used as a shielding material for X-rays. The opposite of radiopacity is radiolucency.


Device manufacturers will often incorporate metals such as tantalum, tungsten, and stainless steel into devices for temporary or permanent implantation. Salts such as barium sulfate, zirconium oxide, and bismuth are also used to render plastics radiopaque.  Increasingly, regulatory agencies and device manufacturers are requiring quantification of the degree of radiopacity in medical devices to assure that these devices exhibit sufficient radiodensity for their application.

More information can be found on this application note