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Material Selection in Dental Fillings

So you indulged in a little too much holiday candy, and now need a cavity filled? There are several material choices, including amalgam alloy and composite resin.

Metal Amalgam vs. Polymeric Composite

If you had a filling prior to 1986, it was likely composed of an amalgam alloy, which is an alloy of mercury, silver, tin, and copper. The composition is carefully regulated by ISO 1559, to account for corrosion and dimensional changes. First used in the 1500s, amalgam remains a very successful material to this day, although concerns about its appearance and perceived issues with mercury toxicity have diminished enthusiasm for its use. Polymeric composite materials based on methacrylate chemistries have found greater use in restorative dental procedures, as these resins can be color matched to the native tooth. 

Put On the Gloves 

Earlier in the days of dentistry, some dentists would mix the amalgam in the palms of their hands, without gloves. Multiple studies have shown that dentists had higher levels of mercury in their bodies than the regular population. Mercury poisoning affects the central nervous system and the kidneys, resulting in tremors in the face and eyelids, and then affecting the limbs and handwriting. The regular use of gloves in dentistry is believed to have reduced issues with mercury accumulation in dentists, along with the increased use of non-mercury alloys and dental composites.

The Mad Hatter, by the way, is a reference to chronic mercury poisoning found in Victorian-aged hatmakers, which was used in the manufacturing of felt hats. Hatmakers of that era often had tremors, paranoia, shyness, and irritability.

Sweetheart Conversation Hearts are an iconic symbol of Valentine's Day. First created by the New England Confectionery Company in 1902, Sweethearts are Necco Wafers cut in heart shapes and stamped with romantic messages. The recipe hasn't changed much since the early 20th century, but the messages are updated as popular vernacular evolves, and now include "Text Me" and "Tweet Me." Although Necco makes more than 8 billion hearts a year, some candy aficionados aren't impressed.

Complaints about Sweetheart's chalk-like texture abound throughout popular culture and the blogosphere. CPG scientists decided to characterize candy hearts to see if they deserve their chalky reputation. We examined the chemical composition, surface topography, flavor and odor of candy hearts, using SEM, EDS, and HS-GC-MS.

Not surprisingly, EDS analysis (see Figure 2) showed the candy consisted of carbon and oxygen, the two main elements in sugar (aside from hydrogen, which is not detectable by EDS). The spectrum showed a complete lack of calcium signal, indicating the absence of calcium carbonate (chalk) in the candy.

Read more in our Material Characterization of Candy Hearts application note.

As the Boston area cleans up after another Nor’easter (New England’s name for a blizzard), we considered the practice of salting roads during and after these winter storms. If you have observed this practice, you will first notice that the salt used on roads bears little resemblance to the salt on your dining room table. The latter is mostly sodium chloride (NaCl), with the occasional trace amounts of other salts that provide pink hues or subtle flavors, a trend more popular in recent times (region-specific salts). Road salt, on the other hand, is a mix of sodium chloride, calcium chloride, as well as other chloride-based salts (potassium, magnesium). Since sodium chloride is fairly corrosive to roads, cars, and plant life, calcium chloride is used in combination more often. Road salt is usually  blue or yellow in color. The actual salts used in road salt are all white. Manufacturers likely add a chemical indicator to provide a color tint, so that road work crews can see where they have salted.

The salts in road salt are highly soluble in water. When salt is placed in contact with ice, the local contact of the salt depresses the freezing temperature of the ice, which is normally 0°C (32°F). By freezing at a lower temperature, this means that the ice has to be held at a lower temperature to remain solid. For sodium chloride, this reduced temperature is -21°C (-6°F) under controlled conditions. Practically, sodium chloride will only melt ice when the roads are around -10°C or higher.  The application of salt is the same as locally heating the ice above its melting point.

What is happening on a molecular level is that the ions from the salt (say Na+ or Cl-) want to associate with the water molecules in the ice. In the thin layer of water that sits on top of the ice, salt ions are sitting in solution at a fairly high concentration. Since nature does not like a concentration gradient, it sends more H₂O molecules from the crystal structure into the liquid layer to try to dilute the salt concentration. Aside from the solubility of the salt in water, the amount of melting only depends on the concentration of salt, not its chemical nature, which is why this process is called a colligative property (e.g. it only depends on the concentration of species, not their chemistry). Elevation of boiling point is another colligative property. So as long as the salt concentration remains sufficiently high, it will continue to melt the ice underneath, and make the drive through New England towns a bit safer.  

Highly engineered thermoplastic elastomers are finding broad application use these days. Traditionally, elastomers often involved silicones, polyurethanes, or crosslinked rubbers. For applications requiring greater mechanical properties, such as impact strength, modulus, and fatigue strength, block copolymers comprised of polyether amide (PEBA) are often found.

PEBAs are formed from the condensation polymerization of a carboxylic polyamide with a polyether (often a polyethylene glycol terminated by alcohols). Varying the relative lengths and amounts of the blocks results in a range of mechanical properties, including elasticity and energy damping.

PEBA in Sports Equipment

PEBAs are often used in sports equipment. Its fatigue resistance and relative immunity to temperature-related property change makes it a good candidate for the shells of ski boots, and the energy damping behavior and low density makes PEBAs attractive for the damping system in running shoes.

PEBA in Medical Devices

PEBAs can be injection molded and extruded, which permits forming into narrow wall constructs. This behavior, coupled with its biocompatibility and the lack of a need for plasticizers, has resulted in PEBAs use in catheters, tubing, and cannula.

Contact CPG for assistance in selecting polymer materials for your specific application. 

Material Characterization of Synthetic vs. Leather Balls 

The argument of synthetic over natural leather in football and other sports, such as rugby or basketball, ultimately comes down to ball feel and grip. Rugby has transitioned to synthetic surfaces (and anyone who ever caught a high ball made from leather in wet conditions is grateful for that), but in football the preferred elite ball composition is leather.  Is that choice advantageous or is it purely a preference for the traditional? CPG scientists sought to determine how different the two types of footballs really are. 

Leather vs. Pleather: Chemical Composition & Surface Topography

Fourier Transform Infrared Spectroscopy (FTIR) analysis confirmed that the leather football is comprised of animal hide, while the synthetic football is polyurethane-based. Not surprisingly, the FTIR spectra for the leather and synthetic material are markedly different.

Scanning Electron Microscopy (SEM) showed differences in the surfaces of the two footballs.  Natural animal hide is embossed to add the raised features into the leather football for improved friction.  The added topography increases the surface area of the football, making it easier to catch. Synthetic materials were designed to mimic the same embossed texture, yet the details of the microscopic fibrous nature of the leather are not captured in the man-made football (see below).

SEM micrographs of the surface of a leather (left) and synthetic (right) football. At higher magnification, the leather football appears fibrous while the synthetic surface is covered with small bumps on the order of 10 µm in diameter.

Ranking Football Friction Properties

CPG scientists determined the friction on a synthetic and natural leather ball using a conventional method to yield a coefficient of friction (CoF) that allows ranking of relative frictional properties, and also how those frictional properties vary with speed and wet versus dry.

Images of ball testing configuration (left) on AR-G2 rheometer, and leather (middle) and synthetic (right) balls. Blue material is silly putty used as a barrier to enable water to be trapped in the contact region.

Dry

Under dry conditions, the synthetic material appears to have higher friction than the leather at all speeds.  In both cases, the friction force increases with compression load, suggesting the grip gets better the harder the ball is held.

Wetted

The picture changes somewhat when the balls are wetted with distilled water.  The synthetic ball actually exhibits a marked drop in frictional force at higher loads (the leather ball also sees a slight decrease).  The leather ball would therefore have fairly consistent levels of grip, irrespective of how hard the ball were held, but the synthetic ball would have less grip at higher loads.  Although minor, this observation already indicates that the leather ball may have an advantage in less-optimal playing conditions.

Soaked

When the balls are soaked in water, the difference is even more dramatic. The synthetic ball is impacted by the soak, but to the detriment of grip, with a gradual decrease in frictional force. In contrast, the frictional force on the leather ball goes up as the ball gets wet, suggesting in fact that this ball should have improved grip in wet conditions.

The weakness in this friction discussion is the choice of counterface.  Most players either use bare hands (in the case of the quarterback) or silicone coated gloves (in the case of receivers).  The steel counterface used here was a pragmatic choice, and may not truly represent the frictional force, which can be very sensitive to both the surface chemistry, and the conformability of the material.  Nonetheless, this simple non-destructive ranking experiment yields insights into why the leather ball is still the choice of the elite sport divisions. GO PATS!

Read more about these results in this application note.

Join CPG scientists Stephen Spiegelberg and Gavin Braithwaite for a webinar on medical device cleanliness. The discussion will include examples of what happens when cleaning processes are not properly verified and validated, how to establish the number of samples to test, how to test for device cleanliness, and how to establish acceptable residue limits.

Dr. Spiegelberg is the chairman of the ASTM task group on Medical Device Cleanliness, and Dr. Braithwaite regularly consults on cleaning issues in the medical device area. 

This webinar is targeted towards:

• Medical device manufacturers
• Medical device engineers
• Process engineers
• Quality engineers
• Regulatory personnel

Duration: 30 minutes

Cleanliness in Medical Devices Webinar

Thursday, February 23, 2 p.m., Eastern Standard Time

To register, click here