Axially loaded whole teeth of Atlantic wolffish exhibit negative Poisson’s ratios due to their osteodentin microarchitecture

The Atlantic wolffish (Anarhichas lupus) is a teleost with prominent teeth highly adapted for crushing hard-shelled prey. The dentin type of wolffish teeth is osteodentin, which differs markedly in structure from the dentin of most vertebrates. This study aimed to investigate the three-dimensional (3D) deformation and strain fields that develop in intact wolffish teeth under load. Eight wolffish teeth (three caniniform and five molariform) were studied using low- and high-energy phase-contrast-enhanced x-ray tomography, imaged both statically and during in situ compression loading. Digital volume correlation was applied to sets of unloaded and loaded scans. Unexpectedly, 3D deformations and strains showed that all teeth exhibited auxeticity (negative Poisson's ratios), such that when a tooth is compressed longitudinally, it also contracts laterally. Strain fields were used to calculate effective Poisson's ratios in the x-z and y-z orientations (with z as the loading direction), and yielded negative values, mostly between -1 and -2. Auxeticity is rare in mineralized biomaterials and has previously been reported only in two invertebrates (limpet teeth and nacre). Nanoindentation showed that the solid osteodentinal components have bone-like mechanical properties. Microstructural analysis of osteodentin revealed a mineralized matrix with a vast array of canals that mostly run straight up from the base, and curve outward as they approach the tooth tip. We suggest that the internal architecture of osteodentin may explain its auxeticity through a mechanism similar to that of re-entrant auxetic metamaterials. Specifically, the externally curved canals and the mineralized columns between them bend inward under compression. STATEMENT OF SIGNIFICANCE: Here we report the surprising findings of negative Poisson's ratios (auxeticity) in osteodentin of wolffish (Anarhichas lupus) teeth. This means that when these teeth are compressed along their length, their width also contracts. This is a rare phenomenon which has not been reported so far in mineralized tissues of vertebrates. We propose that the cause of this unusual property is the internal micro-architecture of osteodentin, particularly its vast array of canals. Since materials with this property exhibit superior mechanical properties, such as resistance to wear and impact, this finding may lead to the development of new synthetic biomaterials with desirable features.