It has been known for some time that surface integrity has an effect on the
fatigue life of metals and "brittle"
polymers. In cardiovascular applications of polymeric materials, emphasis is placed on
elastomers having extended flexure lifetimes (i.e.,
fatigue life). The effect of surface integrity on the performance properties of
Biomer (Ethicon, Inc, Somerville, NJ) a segmented
polyurethane used in many blood contacting devices, is being investigated using uniaxial tensile tests in air at room temperature, and biaxial
fatigue tests in deionized water at body temperature. Tensile tests were done using ASTM-D-882: Standard Test Methods for Tensile Properties of Thin
Plastic Sheeting. No significant differences were noted in the stress-strain curves for specimens with various surface finishes.
Fatigue tests were performed using an apparatus developed to allow for the exposure of thin-sheet
polymer specimens to fluid at body temperature, while being biaxially strained. Because no standard test method was available, a test protocol was developed with reference to ASTM-D-671-78: Standard Test Methods for the Flexural
Fatigue of Plastics by Constant Amplitude of Force. Stress versus life cycle data for specimens with differing surface finishes are being collected. Results to date suggest
fatigue life of thin flexing membranes will decrease with increasing order of surface roughness, and
fatigue properties are more sensitive to effects of changes in surface integrity than tensile properties measured by monotonic loading.