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Limpet Teeth

Function and Formation

In order to obtain food, limpets rely on an organ called the radula, which contains iron-mineralized teeth. Although limpets contain over 100 rows of teeth, only the outermost 10 are used in feeding. These teeth form via matrix-mediated biomineralization, a cyclic process involving the delivery of iron minerals to reinforce a polymeric chitin matrix. Upon being fully mineralized, the teeth reposition themselves within the radula, allowing limpets to scrape off algae from rock surfaces. As limpet teeth wear out, they are subsequently degraded (occurring anywhere between 12 and 48 hours) and replaced with new teeth.

Biomineralization

Currently, the exact mechanism behind the biomineralization of limpet teeth is unknown. However, it is suggested that limpet teeth biomineralize using a dissolution-reprecipitation mechanism. Specifically, this mechanism is associated with the dissolution of iron stored in epithelial cells of the radula to create ferrihydrite ions. These ferrihydrite ions are transported through ion channels to the tooth surface. Build up of enough ferrihydrite ions leads to nucleation, the rate of which can be altered via changing the pH at the site of nucleation. After one to two days, these ions are converted to goethite crystals.

The unmineralized matrix consists of relatively well-ordered, densely packed arrays of chitin fibers, with only a few nanometers between adjacent fibers. The lack of space leads to the absence of pre-formed compartments within the matrix that control goethite crystal size and shape. Because of this, the main factor influencing goethite crystal growth is the chitin fibers of the matrix. Specifically, goethite crystals nucleate on these chitin fibers and push aside or engulf the chitin fibers as they grow, influencing their resulting orientation.

Strength

Looking into limpet teeth of Patella vulgata, Vickers hardness values are between 268 and 646 kg m^-1 m^-2, while tensile strength values range between 3.0 to 6.5 GPa. As spider silk has a tensile strength only up to 4.5 GPa, limpet teeth outperforms spider silk to be the strongest biological material. These considerably high values exhibited by limpet teeth are due to the following factors:

The first factor is the nanometer length scale of goethite nanofibers in limpet teeth; at this length scale, materials become insensitive to preexisting flaws that would otherwise decrease failure strength. As a result, goethite nanofibers are able to maintain substantial failure strength despite the presence of defects.

The second factor is the small critical fiber length of the goethite fibers in limpet teeth. Critical fiber length is a parameter defining the fiber length that a material must be to transfer stresses from the matrix to the fibers themselves during external loading. Materials with a large critical fiber length (relative to the total fiber length) act as poor reinforcement fibers, meaning that most stresses are still loaded on the matrix. Materials with small critical fiber lengths (relative to the total fiber length) act as effective reinforcement fibers that are able to transfer stresses on the matrix to themselves. Goethite nanofibers express a critical fiber length of around 420 to 800 nm, which is several orders of magnitude away from their estimated fiber length of 3.1 um. This suggests that the goethite nanofibers serve as effective reinforcement for the collagen matrix and significantly contribute to the load-bearing capabilities of limpet teeth. This is further supported by the large mineral volume fraction of elongated goethite nanofibers within limpet teeth, around 0.81.

Applications of limpet teeth involve structural designs requiring high strength and hardness, such as biomaterials used in next-generation dental restorations.