A rate-controlled indentor for in vivo analysis of residual limb tissues

A tissue tester was designed to enable rate-controlled indentation of the bulk soft tissues of lower extremity residual limbs. The tissue tester employs a digital linear actuator that implements rate-controlled indentation, and a load cell that measures the reaction force resulting from tissue indentation. Viscoelastic phenomena such as preconditioning, hysteresis and force relaxation can be assessed, and the effect of varying indentation rates on soft tissue stiffness can be investigated. The device accommodates indentor excursions up to 30 mm, indentation at rates of 0 to 10 mm/s, reaction forces up to 44 N, and multiple loading/unloading cycles. The tissue tester is controlled via a notebook personal computer with a PCMCIA data acquisition card. The tissue testing trials are automated and the entire test system is portable and amenable for use in a clinical or research environment. System output consists of force-displacement curves from cyclic loading, and force-time curves following ramped-step indentation. The mean indentor positioning error was 0.071 (+/-0.75)% of the desired displacement. This error varied as a function of indentation and was approximately independent of the indentation rate. Indentation rates were accurate to within 0.94(+/-0.68)% of the desired value and also varied with indentation. Indentation of a viscoelastic foam yielded force-displacement curves that were consistent with that obtained from an Instron universal testing machine.