High-resolution mechanical mapping of the adhesive-dentin interface: The effect of co-monomers in 10-methacryloyloxydecyl dihydrogen phosphate.

The presence of 10-methacryloyloxydecyl dihydrogen phosphate (MDP) at the adhesive-dentin interface enables ionic binding to calcium salts, which results in rigid nano-layering within the submicron scale resin-dentin interdiffusion zone. MDP has been used with additional co-monomers, such as hydroxyethyl methacrylate (HEMA) and/or 4-methacryloyloxyethyl-trimellitic acid (4-MET), mainly to enhance the chemical bonding properties. However, the use of co-monomers may compromise the rigidity of the adhesive-dentin interface. In this study, we use high-resolution mechanical mapping across the interface to discern the in situ mechanical properties of each target region at the nanoscale. Visualization by modulus mapping demonstrated that HEMA increases the diffusion properties of MDP into dentin structures. However, the rigidity of the adhesive-dentin interface indicated by the storage modulus was markedly lower in MDP containing HEMA than in MDP containing 4-MET. Dynamic indentation testing revealed that the bonding layer was more deformable in the presence of HEMA. Moreover, the presence of MDP in the bonding layer might also increase the deformability because the polymerization linearity allows a large degree of viscoelasticity. These factors also diminish the rigidity of the adhesive-dentin interface. Within the limitations of this study, our findings demonstrated that 4-MET is a better co-monomer than HEMA in MDP-based dental adhesives. Modulus mapping and nanoindentation are introduced as new tests for the adhesive-dentin interface to address queries about the effectiveness of dental adhesives.

Exceptional contact elasticity of human enamel in nanoindentation test.

OBJECTIVE: Tooth enamel has unsurpassed hardness and stiffness among mammalian tissue structures. Such stiff materials are usually brittle, yet mature enamel can survive for a lifetime. Understanding the nanoscale origin of enamel durability is important for developing advanced load-bearing biomaterials. Here, nanoscale exceptional contact elasticity of the human tooth enamel, based on nanoindentation tests, is reported. METHODS: Spherical indenter tips with radii of 243 and 1041nm were used to determine stress-strain curves of enamel. Force-displacement curves were recorded using quasi-static loading strain rates of 0.031, 0.041, and 0.061s-1. The storage moduli from a superimposed signal amplitude (dynamic strain at 220Hz) embedded during primary quasi-static loading and from quasi-static elastic theory were simultaneously measured. Modulus mapping was considered to be an extremely low quasi-static loading strain rate indentation test. RESULTS: The elastic limits were 7-9GPa and 5-6GPa for the small and large indenters, respectively. The elastic-plastic transition point and elastic modulus value increased with substantially increased quasi-static loading strain rate. The results suggested that the increase of the elastic limit during high-loading strain was associated with exceptional contact elasticity at the nanoscale of the enamel structure and the consequent extension of the contact area (i.e., a temporary pile-up response, dependent on the enamel nanocrystals and protein glue). SIGNIFICANCE: Structural modification at this scale effectively prevents the initiation of cracking from localized strain, thus reinforcing the bulk structure. These results may provide valuable insight for conceptualizing bio-inspired nanocomposites.

Nanomechanical characterization of time-dependent deformation/recovery on human dentin caused by radiation-induced glycation.

An increase in non-enzymatic collagen matrix cross-links, such as advanced glycation end-products (AGEs), is known to be a major complication in human mineralized tissues, often causing abnormal fractures. However, degradation of mechanical properties in relation to AGEs has not been fully elucidated at the material level. Here, we report nanoscale time-dependent deformation and dimensional recovery of human tooth dentin that has undergone glycation induced by x-ray irradiation. The reduction in enzymatic collagen cross-linking and the increased level of AGEs with concomitant growth of disordered collagen matrix diminished creep deformation recovery in the lower mineralized target region. However, the elevated AGEs level alone did not cause a reduction in time-dependent deformation and its recovery in the higher mineralized target region. In addition to the elevated AGEs level, the degradation of the mechanical properties of mineralized tissues should be assessed with care in respect to multiple parameters in the collagen matrix at the molecular level.