The multi-scale meso-mechanics model of viscoelastic dentin.

Human dentin is a hierarchical material with multi-level micro-/nano-structures, consisting of tubule, perti-tubular dentin (PTD) and intertubular dentin (ITD) as the major constituents at microscale; and the PTD and ITD are further composed of collagen and hydroxyapatite (HAp) crystals with different volume fractions at nanoscale. In most cases, the HAp is considered as elastic while the collagen as viscoelastic material. It is of great significance to study the hierarchical structure and viscoelasticity of human dentin to understand the mechanical properties of dentin for further development of restorative materials. Based on this, this paper focuses on multiscale modeling of the elastic properties and dynamic viscoelastic response of dentin and establishes a bottom-up micromechanics model from nano-to macro-scale. In order to study the nanostructural effect on the viscoelastic behavior of hierarchical structures, the homogenization theories of random platelets composites (HTRPC) and the locally-exact homogenization theory (LEHT) are introduced for the homogenization of heterogeneous materials of microstructures at different levels. The HTRPC, based on Eshelby Inclusion theory, is used to predict the effective modulus of PTD and ITD. The LEHT is a method for homogenizing multiphase dentin characterized by repeated unit cells (RUCs). The resulting predictions are in very good agreement with several experimental data from the literature. In addition, the results of nanostructrual effect on dentin show that the viscoelasticity of dentin is majorly contributed by collagen and the HAp greatly provide the strength and hardness of dentin. Furthermore, the ageing effect on dentin's viscoelasticity is considered from the proposed multiscale micromechanics model. It is demonstrated that the ageing effect is much more influential in affecting the loss moduli of dentin than the storage.

Limitations in activities of daily living increase the risk of stroke in older Chinese adults: a population-based longitudinal study.

It remains unclear whether limitations in activities of daily living (ADL) increase the risk of stroke in older Chinese adults. This longitudinal study used data from the Chinese Longitudinal Healthy Longevity Survey to investigate the effects of limitations in ADL on the incidence of stroke in older adults. Between 2002 and 2011, 46,728 participants from 22 provinces in China were included in this study. Of participants, 11,241 developed limitations in ADL at baseline. A 3-year follow-up was performed to determine the incidence of stroke. During the 3-year follow-up, 929 participants (8.26%) and 2434 participants (6.86%) experienced stroke in the ADL limitations group and non-ADL limitations group, respectively. Logistic regression was used to analyze the effect of ADL limitations on the risk of stroke. The results showed that after adjusting for the confounding factors gender, age, weight, hypertension, diabetes, heart disease, natural teeth, hearing impairment, visual impairment, smoking, alcohol abuse, exercise, ethnicity, literacy, residential area, and poverty, the ADL limitations group had a 77% higher risk of developing stroke than the non-ADL limitations group. After propensity score matching, the ADL limitations group still had a 33% higher risk of developing stroke than the non-ADL limitations group (OR = 1.326, 95% CI: 1.174-1.497). These findings suggest that limitations in ADL are a stroke risk factor.