does not require an activation energy but it is certainly characterized by dissipation associated with the viscosity at the boundary between the crystalline domains and the amorphous matrix. A detailed description of this process goes well beyond the scope of this introductory communication.The recording speed of the holograms in our samples is of the order of tens of seconds, much slower than in the photorefractive composites with the smallest response times, [18,19] and this represents the main limit of our materials. Improvement in response time may require tighter control of the shape of the dispersed domains and of their surface dissipative properties. An increase in the holographic writing time was also observed as the AZPON content was lowered. This can be attributed to an increase in the surface/volume ratio of smaller domains, corresponding to an increased effect of the viscosity torques relative to the reorienting field torques. The stability towards further phase separation, corresponding to an increase in domain size beyond the limit of scattering in the visible, is relatively good, with little or no decrease of efficiency over a period of several months. An increase in the scattering is however observed after a 5±10 months period at room temperature, although the original performance is regained after a heating±cooling cycle during which phase separation is repeated. An improvement in the stability of the nanodispersions can, in principle, be obtained through well established techniques (vulcanization, reinforcement, etc.) of polymer stabilization. In summary, our data indicate that, at a given applied electric field, controlled phase separation can increase the orientational contribution to the dynamic range by orders of magnitude. As the improvement in performance that we describe here can in principle be obtained in most photoconductive composites, this method considerably broadens the range of potentially useful photorefractive materials for applications ranging from holographic storage to medical imaging. ExperimentalSamples consisted of thin amorphous films between two indium tin oxidecoated conducting glasses. They were obtained by mixing the required amounts of substances in chloroform and evaporating the solvent. Empty cells containing only glass spacers to control the thickness were prepared, placed on a hot plate, filled by capillary action, and cooled rapidly to room temperature. Alternatively, for solutions with higher viscosity, the hot melt was sandwiched between two hot glasses and then rapidly cooled.Light transmission was measured for a p-polarized laser beam at 633 nm, incident at a 60 angle with respect to the sample normal.Diffraction efficiency was measured by degenerate four-wave mixing experiments, carried out by overlapping two writing beams on a circular area of the sample with a 1 mm diameter. Writing beams had equal intensity and were s-polarized, with a power density of 0.4 W cm ±2, and the beam ratio was b = 1. The grating period was K = 3 lm in all cases. A reading bea...
Transfemoral amputees demand a mechatronic lower limb prosthesis as technical substitute for restoring their gait functions. Prosthetic knee is the key component of a transfemoral prosthesis. The performance of the prosthetic knee determines the walking ability of the transfemoral amputee. This study proposed a novel microprocessor-controlled prosthetic knee with hydraulic damper and evaluated the performance of the prosthetic knee by function simulation and evaluation platform. The prosthetic knee with electrical-controlled hydraulic cylinder that could modulate knee flexion and extension damping properties independently and continuously by single motor was designed. Gait phase identification system based on knee angle sensor, inertial measurement units mounted on thigh connector and shank and force transducer embedded in shank was proposed. Gait phase identification and damping control strategy were determined by typical gait events during walking. Speed adaption and gait symmetry tests were conducted with a customized gait simulator to evaluate the performance of the proposed microprocessor-controlled prosthetic knee. The angle trajectories of the prosthetic knee were similar under a range of walking speeds. While the symmetry index values indicated that the stance phase was more asymmetry than swing phase, the peak swing flexion knee angles were consistently controlled between 60-70 degrees under different speeds. The knee angle symmetry was observed in different speeds during swing phase. It is suggested that the proposed microprocessor-controlled prosthetic knee could meet the fundamental demands of walking with smooth angular transition across different walking speeds.INDEX TERMS Microprocessor-controlled prosthetic knee, hydraulic damper, gait phase identification, gait symmetry, peak swing flexion knee angle.
BackgroundsThere has long been a consensus that shortening of the first metatarsal during hallux valgus reconstruction could lead to postoperative transfer metatarsalgia. However, appropriate shortening is sometimes beneficial for correcting severe deformities or relieving stiff joints. This study is to investigate, from the biomechanical perspective, whether and how much shortening of the first metatarsal could be allowed.MethodsA finite element model of the human foot simulating the push-off phase of the gait was established. Progressive shortening of the first metatarsal from 2 to 8 mm at an increment of 2 mm were sequentially applied to the model, and the corresponding changes in forefoot loading pattern during push-off phase, especially the loading ratio at the central rays, was calculated. The effect of depressing the first metatarsal head was also investigated.ResultsWith increasing shortening level of the first metatarsal, the plantar pressure of the first ray decreased, while that of the lateral rays continued to rise. When the shortening reaches 6 mm, the load ratio of the central rays exceeds a critical threshold of 55%, which was considered risky; but it could still be manipulated to normal if the distal end of the first metatarsal displaced to the plantar side by 3 mm.ConclusionsDuring the first metatarsal osteotomy, a maximum of 6 mm shortening length is considered to be within the safe range. Whenever a higher level of shortening is necessary, pushing down the distal metatarsal segment could be a compensatory procedure to maintain normal plantar force distributions.
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