Regaining biomechanical function, comfort and quality of life is a prime consideration when designing prosthetic limbs. Recently, microprocessor-controlled prosthetic knees, which rely on magneto-rheological (MR) technology, have become available and have the potential to meet these needs. One of these promising products is a prosthetic knee manufactured by the company Ossur Inc. The knee is a synergy of artificial intelligence, advanced sensors and MR actuator technology. A critical factor in the success of the prosthetic knee is the composition of the MR fluid. In the prosthetic actuator, the fluid is used in shear mode in a micron-sized fluid gap. The characteristics of the MR fluid, such as, the off-state viscosity, the field-induced shear yield stress, the post-yield viscosity, and the particle sedimentation rate, determine the properties of the prosthetic knee. This paper describes a novel perfluorinated polyether (PFPE)-based MR fluid with properties that are tailored for the requirements of the prosthetic knee actuator. Rheological measurements of monodisperse and bidisperse PFPE-based fluid mixtures are presented. The monodisperse fluid consists of micron-sized carbonyl iron particles and the bidisperse mixture contains micron- and nano-sized particles. A few different concentrations of nano particles are investigated; first by holding the total solid concentration constant, and then by increasing the total solid concentration, to exceed that of the MR fluid containing only micron-sized particles. An MR fluid composition is sought that has a suitable balance between field-induced strength, off-state viscosity and sedimentation rate, for the proposed application. This balance is determined by desired qualities of the prosthetic knee and relate directly to the MR fluid. The field-induced shear stress of MR fluid samples is measured as a function of the magnetic flux density along with the off-state viscosity as a function of the shear-rate. The shear stress and off-state viscosity at high-shear rates are of particular interest, since the working shear rate in the prosthetic knee is high, due to the micron-sized gap between the blades in the fluid chamber of the actuator. Mathematical models are presented that describe how the MR fluid properties relate to the behavior of the prosthetic knee. The paper shows how a tailored design of an MR fluid can further the success of the MR prosthetic knee.

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