We have examined the tensile viscoelastic properties of circumferential and radial strips of porcine aortic valve leaflets following fixation in glutaraldehyde and formaldehyde, with or without pressure. After aldehyde treatment, the radial strips remained weaker and less stiff than circumferential strips and responded slightly differently to the treatment. After fixation, with or without pressure, the radial strips showed large changes in stress-strain and hysteresis responses due to initial loading, and there was a twofold reduction in tensile strength and final stiffness. For strips in both directions, fixation without pressure produced doubled extensibility and a ramping stress-strain curve. Permanent (plastic) deformation of 5-20% occurred as a result of cyclic loading, stress relaxation, and creep experiments. Pressure fixation, however, produced little change in stress-strain results other than a simple shift to lower strain and produced no plasticity. Both methods of fixation reduced stress relaxation and creep. Mechanical test results are consistent with a loss of ground substance matrix during fixation. Reductions in tensile strength after fixation may be due to "riveting" of collagen geometry, producing local stress concentrations.
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