NONPARAMETRIC BLOCK-STRUCTURED MODELING OF LUNG-TISSUE STRIP MECHANICS

Very large amplitude pseudorandom uniaxial perturbations containing fr equencies between 0.125 and 12.5 Hz were applied to five dog lung tiss ue strips. Three different nonlinear block-structured models in nonpar ametric form were fit to the data. These models consisted of (1) a sta tic nonlinear block followed by a dynamic linear block (Hammerstein mo del); (2) the same blocks in reverse order (Wiener model); and (3) the blocks in parallel (parallel model). Both the Hammerstein and Wiener models performed well for a given input perturbation, each accounting for greater than 99% of the measured stress signal variance. However, the Wiener and parallel model parameters showed some dependence on the strain amplitude and the mean stress. In contrast, a single Hammerste in model accounted for the data at all strain amplitudes and operating stresses. A Hammerstein model featuring a fifth-order polynomial stat ic nonlinearity and a linear impulse response function of 1 s duration accounted for the most output variance (99.84%+/-0.13%, mean+/-standa rd deviations for perturbations of 50% strain at 1.5 kPa stress). The static nonlinear behavior of the Hammerstein model also matched the qu asistatic stress-strain behavior obtained at the same strain amplitude and operating stress. These results show that the static nonlinear be havior of the dog lung tissue strip is separable from its linear dynam ic behavior. (C) 1998 Biomedical Engineering Society.