Bidisperse Perfluorinated Polyether (PFPE)-Based Magneto-Rheological Fluids in a Prosthetic Knee

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.

Cellular responses to novel, micropatterned biomaterials

Two UV-curable polymers, i.e., a star-shaped poly(ethylene glycol) (PEG) and a linear perfluorinated polyether (PFPE), are investigated as novel biomaterials in a systematic study of the cellular responses to surface chemistry, topography, and elasticity. Based on the wettability it was expected that the two novel biomaterials were too hydrophilic or -phobic, respectively, to support cell adhesion. Indeed, no cell adhesion was observed on the smooth, unstructured elastomers, whereas the materials showed no cytotoxicity. However, when the materials bear defined, topographic patterns (prepared by UV-based imprinting), cells do react strongly to the surfaces; they adhere, spread, and change their shape depending on the geometry of the features. Typically, cells were found to align along line patterns and "float" on pillar structures. It should be noted that the chemistry of the surface is not altered by the imprinting process, hence, there are no biofunctional molecules present at the surface to aid the cell adhesion. Finally, a remarkable effect of elasticity on the cellular behavior was discovered. Thus, the three parameters of chemistry, topography, and elasticity were investigated in- and interdependently, and it was found that the biomaterials may lose their resistance to protein adsorption and cell adhesion depending on the surface topography.

Wear of Spur Gears Having a Dithering Motion and Lubricated With a Perfluorinated Polyether Grease

Gear contact surface wear is one of the important failure modes for gear systems. Dedicated experiments are required to enable precise evaluations of gear wear for a particular application. The application of interest for this study required evaluation of wear of gears lubricated with a grade 2 perfluorinated polyether grease and having a dithering (rotation reversal) motion. Experiments were conducted using spur gears made from AISI 9310 steel. Wear was measured using a profilometer at test intervals encompassing 10,000 to 80,000 cycles of dithering motion. The test load level was 1.1 GPa maximum Hertz contact stress at the pitch-line. The trend of total wear as a function of test cycles was linear, and the wear depth rate was approximately 1.2 nm maximum wear depth per gear dithering cycle. The observed wear rate was about 600 times greater than the wear rate for the same gears operated at high speed and lubricated with oil.