Effect of Low Hydroxyapatite Loading Fraction on the Mechanical and Tribological Characteristics of Poly(Methyl Methacrylate) Nanocomposites for Dentures.

Denture base materials need appropriate mechanical and tribological characteristics to endure different stresses inside the mouth. This study investigates the properties of poly(methyl methacrylate) (PMMA) reinforced with different low loading fractions (0, 0.2, 0.4, 0.6, and 0.8 wt.%) of hydroxyapatite (HA) nanoparticles. HA nanoparticles with different loading fractions are homogenously dispersed in the PMMA matrix through mechanical mixing. The resulting density, Compressive Young's modulus, compressive yield strength, ductility, fracture toughness, and hardness were evaluated experimentally; the friction coefficient and wear were estimated by rubbing the PMMA/HA nanocomposites against stainless steel and PMMA counterparts. A finite element model was built to determine the wear layer thickness and the stress distribution along the nanocomposite surfaces during the friction process. In addition, the wear mechanisms were elucidated via scanning electron microscopy. The results indicate that increasing the concentration of HA nanoparticles increases the stiffness, compressive yield strength, toughness, ductility, and hardness of the PMMA nanocomposite. Moreover, tribological tests show that increasing the nanoparticle weight fraction considerably decreases the friction coefficient and wear loss.

Determination of damping coefficient of soft tissues using piezoelectric transducer.

Measuring viscoelastic properties of soft tissues becomes a new biomarker in the medical diagnosis field. It can help in early diagnosis and related fields, such as minimally-invasive-surgery (MIS) applications and cell mechanics. The current work presents a tactile sensor for measuring the damping coefficient of the soft tissues. The proposed sensor can be miniaturized easily and used in MIS applications. Besides the proposed sensor, a mathematical model, based on Jacobsen's approach, is built to calculate the damping coefficient of the specimens and the surrounding. These damping sources significantly influence the proposed sensor, such as air damping and hysteretic damping. The sensor system principally depends on a piezoelectric transducer, which is cheap, commonly available, and easily integrated into MEMS. To conceptually prove the sensor feasibility, silicon rubber samples with different stiffnesses have been fabricated and tested by the new sensor. The obtained results prove the newly proposed sensor's capability to differentiate the damping coefficients for soft materials effectively.