Relaxation of ceramic tile stresses generated by fast drying: a kinetic model

Unfired tile mechanical properties are very important in the ceramic tile manufacturing process. Inadequate mechanical properties lead to rejects (both in unfired and fired tiles). Unfired tile mechanical strength changes significantly after the tiles exit the industrial dryer. This behaviour can be explained by assuming that the fast-drying process generates stresses in the tile, which subsequently relax. A kinetic model has been derived, based on Maxwell’s viscoelastic elements, which explains the development of dried tile mechanical strength. This increases asymptotically when the dried tiles are stored in dry conditions. However, if tiles adsorb humidity (upon exiting the dryer), tile mechanical strength rises and then decreases. This is the result of two opposing phenomena: stress relaxation raises mechanical strength while the concurrent rise in moisture content lowers mechanical strength. The developed model successfully describes this joint mechanical behaviour. Keywords: ceramic tiles, fast drying, stress relaxation, kinetic model

Abstract 57: Myofibrils From Pediatric IDC Patients Display Faster Relaxation Kinetics and Reduced Tension Generation

Objective: Pediatric patients with heart failure, compared to adults, respond differently to current armamentarium of heart failure (HF) therapeutics, indicating differences in the hearts of children and adults. We examined myofibril mechanics in children and adults with and without HF to determine: 1) baseline myofibril characteristics, 2) differences between children and adults, and 3) myofibril mechanics in the setting of HF. Methods: Myofibrils were obtained from pediatric and adult patients with IDC undergoing cardiac transplantation. Unused donor hearts were used as non-failing control (NF). Written informed consent was obtained. Myofibrils were stretched from slack length and set 5-10% above slack sarcomere length (SL), and Ca 2+ -activated and relaxed at 15°C. Mechanical and kinetic parameters: maximal tension (T max , mN/mm 2 ); rate constant of tension development ( k ACT , S -1 ); duration of slow relaxation phase ( t LIN , mSec), rate constant of fast phase relaxation ( k REL , S -1 ). Mechanical parameters ± SEM are shown. Results: Compared to NF adults, myofibrils from NF pediatric hearts had shorter SL (2.04±0.01 μm vs 2.11±0.01, p < 0.0001), slower t LIN (202.7+9.0 msec vs 152.9+4.1, p < 0.0001) and slower k REL (7.8+0.4 S -1 vs 15.7+0.8, p < 0.0001). T max and k ACT were similar. In the adult cohort, myofibrils from adult IDC patients showed slower k ACT (0.45+0.02 S -1 vs 0.57+0.03, p < 0.005) and slower k REL (11.9+0.7 S -1 vs 15.7+0.8, p < 0.005). T max and t LIN were similar. In the pediatric cohort, myofibrils from IDC developed lower T max (35.3+2.5 mN/mm 2 vs 57.5+4.4, p < 0.0001), and faster t LIN (136.6+3.2 msec vs 202.3+8.9, p < 0.0001) and faster k REL (9.1+0.4 S -1 vs 7.8+0.4, p < 0.05). k ACT was similar. Conclusion: NF myofibrils from children have shorter resting SL and slower relaxation kinetics than NF adults. While myofibrils from adults IDC result in slower activation and exponential relaxation kinetics, children with IDC display faster relaxation kinetic and reduced tension generation. These results show that myofibrils from children have fundamental differences in mechanical characteristics at baseline and in the setting of heart failure.

Rapid Reaction Kinetics: Lessons Learnt from Ion Pumps

Chemical kinetics underwent a revolution in the 1950–60s with the development by Manfred Eigen of relaxation kinetic techniques and theory for the analysis of the results obtained. The techniques he introduced extended the time scale of measurable reactions into the microsecond range and beyond. Since then, computing power has increased astronomically. Some of the approximations traditionally used in the analysis of relaxation kinetic data to reduce mathematical complexity are, therefore, now no longer a necessity. Numerical integration of coupled series of differential rate equations can be performed in seconds or less on desk-top computers. In research on the mechanism of the Na+,K+-ATPase, it has been found that traditional approaches to relaxation kinetic data can sometimes lead to erroneous conclusions or to an incomplete description of the mechanism. Therefore, one needs to be flexible in one’s approach to kinetic data analysis and carefully consider the validity of any approximations used.

Coordinated control of volume regulatory Na+/H+ and K+/H+ exchange pathways in Amphiuma red blood cells

The Na+/H+ and K+/H+ exchange pathways of Amphiuma tridactylum red blood cells (RBCs) are quiescent at normal resting cell volume yet are selectively activated in response to cell shrinkage and swelling, respectively. These alkali metal/H+ exchangers are activated by net kinase activity and deactivated by net phosphatase activity. We employed relaxation kinetic analyses to gain insight into the basis for coordinated control of these volume regulatory ion flux pathways. This approach enabled us to develop a model explaining how phosphorylation/dephosphorylation-dependent events control and coordinate the activity of the Na+/H+ and K+/H+ exchangers around the cell volume set point. We found that the transition between initial and final steady state for both activation and deactivation of the volume-induced Na+/H+ and K+/H+ exchange pathways in Amphiuma RBCs proceed as a single exponential function of time. The rate of Na+/H+ exchange activation increases with cell shrinkage, whereas the rate of Na+/H+ exchange deactivation increases as preshrunken cells are progressively swollen. Similarly, the rate of K+/H+ exchange activation increases with cell swelling, whereas the rate of K+/H+ exchange deactivation increases as preswollen cells are progressively shrunken. We propose a model in which the activities of the controlling kinases and phosphatases are volume sensitive and reciprocally regulated. Briefly, the activity of each kinase-phosphatase pair is reciprocally related, as a function of volume, and the volume sensitivities of kinases and phosphatases controlling K+/H+ exchange are reciprocally related to those controlling Na+/H+ exchange.