Dynamic evaluation of the viscoelastic properties of a biomedical polymer (biomer)

This paper deals with the experimental determination of the dynamic constants, storage and loss moduli of a viscoelastic material currently used as membranes in the artificial heart or other types of cardiac bypass pumps. A brief discussion of the viscoelastic theory is given, starting with the relaxation and creep function leading to the determination of the stress-strain relationship in integral equation form. It is shown that when dealing with periodical strain histories, the integral equation form may be reduced to a complex form. The complex moduli are used to describe the stress-strain relationship. The "Hysteresis Loop" method was used in this investigation to experimentally determine the material constants. This method was found to be convenient and direct for the experimental evaluation of the constants. These dynamic constants may then be used in a computer analysis, i.e., finite elements analysis, to determine the stress distribution in the membrane when subjected to loads or deformations.