Effect of Specimen Size and Aspect Ratio on the Tensile Properties of Porcine Aortic Valve Tissues E. O. CAREW, 1 J. PATEL, 1 A. GARG, 1 P. HOUGHTALING, 2 E. BLACKSTONE, 2 and I. V ESELY 1 1 Department of Biomedical Engineering, Lerner Research Institute and 2 Department of Biostatistics and Epidemiology, The Cleveland Clinic Foundation, Cleveland, OH (Received 13 May 2002; accepted 28 January 2003) Abstract—The measurement of mechanical properties of bio- logical tissues is subject to artifacts such as natural variability and inconsistency in specimen preparation. As a result, data cannot be easily compared across laboratories. To test the ef- fects of variable specimen dimensions, we systematically modi- fied the size and aspect ratio AR of porcine aortic valve tissues and measured their stiffness and extensibility. We found that: i as the AR of circumferential specimens increased from 1:1 to 5:1, their stiffness increased by 36% ( p 0.001) and their extensibility decreased by 21% ( p 0.001); ii as the AR of radial specimens increased from 0.8:1 to 4:1, their stiffness increased by 36% ( p 0.001) and their extensibility decreased by 34% ( p 0.001); iii as the size of circumferential speci- mens was reduced from 128 to 32 mm 2 at fixed AR 2:1, their stiffness decreased by 6% ( p =0.05), and their extensibility increased by 17% ( p 0.001); and iv as the size of radial specimens was reduced from 72 to 32 mm 2 at fixed AR 2:1, their stiffness decreased by 7% ( p =0.03) and their extensibil- ity increased by 16% ( p =0.005). Thus, as specimens of con- stant length became narrower, they became stiffer and less extensible, and as specimens of fixed aspect ratio became smaller, they became less stiff and more extensible. Statistical models of these trends were predictive and can thus be used to integrate materials test data across different laboratories. © 2003 Biomedical Engineering Society. DOI: 10.1114/1.1568116 Keywords—Stiffness, Extensibility, Predictive model, Speci- men area. INTRODUCTION The ability to accurately characterize the mechanical properties of heart valve tissues is important for a num- ber of reasons. First, since the mechanical properties of replacement materials should mimic those of native tissues, 12–14,17 precise measurement of mechanical prop- erties is required. Second, finding differences in me- chanical properties between intact and failed biopros- thetic valves may help identify the mechanism of their failure. Third, identifying the subtle changes in tissue mechanics that accompany disease and aging may help researchers develop strategies to treat these conditions in patients. It is, therefore, critical that the mechanical prop- erties of biological tissues, such as heart valves, be mea- sured accurately in any study of their function. Uniaxial tensile testing is perhaps the most popular means of measuring the mechanical properties of soft tissues. Although it is used to quantify one-dimensional mechanics, it can be adapted to analyze anisotropic ma- terials by testing specimens at different orientations. Multiaxial testing is more appropriate for anisotropic tis- sues but unidirectional testing is easier to perform and control, and it produces data that can be easily analyzed and interpreted. This viewpoint is supported by the wealth of data in the literature on the one-dimen- sional mechanical properties of soft biological tissues. 1,2,5,7,9–11,15–18 These reports, however, are full of inconsistent and contradictory data. For example, the elastic modulus of circumferentially oriented native porcine aortic valve tis- sues has ranged from 6.7 MPa 11 to 17.97 MPa, 10 and the extensibility has ranged from 12.79% 10 to 50%. 15 Simi- larly, the modulus of radially oriented tissues has ranged from 1.85 MPa 10 to 4 MPa, 15 and the extensibility has ranged from 33% 10 to 83%. 15 Normal biological variabil- ity is believed to account for the majority of the scatter of data within test samples in any given laboratory. In- consistency in extracting material parameters from highly nonlinear material test data is another major source of the variation, particularly when data are com- pared between laboratories. Inconsistent data may also result from the use of different strain rates and target loads or variable specimen size and dimensions. The tensile mechanics of inhomogeneous and aniso- tropic materials e.g., heart valves are expected to vary with test specimen dimensions as different fibers are sev- ered and others retained in the selected test specimen. The absolute size and aspect ratio of the selected speci- Address correspondence to Ivan Vesely, PhD, Department of Bio- medical Engineering/ND20, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195. Electronic mail: vesely @bme.ri.ccf.org Annals of Biomedical Engineering, Vol. 31, pp. 526–535, 2003 0090-6964/2003/315/526/10/$20.00 Printed in the USA. All rights reserved. Copyright © 2003 Biomedical Engineering Society 526