Improved characterization of cartilage mechanical properties using a combination of stress relaxation and creep

Mechanical characterization of cartilage, other soft tissues and gels has become a ubiquitous and essential aspect of biomechanics and biomaterials research. Current progress in theoretical modeling and tools for data analysis often exceed what is required for routine mechanical characterization assays in experimental studies, making selection of methodologies difficult for the nonspecialist. We have therefore developed an approach for measurement of confined compression modulus and hydraulic permeability based on simple poroelasticity theory and requiring only linear regression tools for data analysis. This technique involves a new application of an early-time solution for creep combined with stress relaxation measurements to characterize soft tissue mechanical parameters as a function of compressive strain or water content. This combined methodology allows measurement of hydraulic permeability by two different techniques with only a modest increase in experimental duration, providing a more precise assessment of permeability and associated measurement error.     

Modeling the long-term effect of PCBs on Everglades fish communities

Fish communities are regularly exposed to a large number of natural and synthetic toxic chemicals, whose harmful effects, both short-term and long-term, need to be studied. Mathematical simulation models are known to be useful tools in the assessment of both acute and chronic effects. A time-concentration-effect model has been developed to assess the long-term effect of PCBs on multi generations of fish communities in the Florida Everglades. The term fish is used here generically to refer to five functional fish groups (FGs), two having small and two having large adult size, and a crustacean (crayfish), each of which are further composed of several age classes. Several contaminant concentration scenarios are analyzed in the simulations, subject to mortality and hatchability failure. The model predicts the population response time series for each FG, based upon exposure concentration and duration. Model results indicate that a concentration of 0.1 gl^–1 could have adverse effects on survival and reproduction success for Everglades fish communities. For comparison, the USEPA has stipulated a standard of 0.014 gl^–1 for the protection of freshwater fish from chronic effects and a standard of 1.0 gl^–1 from acute effects. Hopefully, the model may be further developed to assist in the formulation of policy or regulations concerning aquatic ecological risk to protect the aquatic ecosystem of the Everglades.