The viscoelastic characteristics of contracted collagen gels populated with rat fibroblasts or cardiomyocytes were investigated by uniaxial tensile testing. Rat type I collagen-Dulbecco's modified Eagle's medium solution (each 2 ml in volume, 0.5 mg/ml collagen concentration) containing 2.0 million rat fibroblasts or cardiomyocytes were cast in a circular shape. After gelation and culture for 10 days the contracted gels were first stretched to a tensile strain of approximately 0.20 at 4.6 × 10-3/s strain rate, and then the strain was kept unchanged for 3 min. The tensile stress in the gels was recorded. The results were regressed against the equations of the Kelvin viscoelastic model. It was found that the two elastic coefficients in the model were 6.5 ± 1.7 and 10.2 ± 3.2 kPa, respectively, for gels with cardiomyocytes and 5.1 ± 1.6 and 4.5 ± 0.9 kPa for those with fibroblasts; the values for gels with cardiomyocytes were significantly higher than those for gels with fibroblasts. The viscous coefficient was 169.6 ± 60.7 kPa s for the cardiomyocytes and 143.6 ± 44.7 kPa s for the fibroblasts. The relaxation time constant for gels with cardiomyocytes was 19.6 ± 10.6 s, significantly smaller than for gels with fibroblasts (36.4 ± 13.3 s). This study is the first to obtain viscoelastic data for living cell-contracted collagen gels. These data show that the viscous effect has a vital effect on the mechanical behavior of the gels and cannot be neglected in the culture and function of artificial substitutes based on contracted collagen gels. Furthermore, the data may imply that viscous coefficient of the gels might be closely related to collagen density rather than to cross linking among collagen fibrils.
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