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This paper explores and demonstrates the potential of using pyrolytic carbon as a material for coronary stents. Stents are commonly fabricated from metal, which has worse biocompatibilty than many polymers and ceramics. Pyrolytic carbon, a ceramic, is currently used in medical implant devices due to its preferable biocompatibility properties. Micropatterned pyrolytic carbon implants can be created by growing carbon nanotubes (CNTs), and then filling the space between with amorphous carbon via chemical vapor deposition (CVD). We prepared multiple samples of two different stent-like flexible mesh designs and smaller cubic structures out of carbon-infiltrated carbon nanotubes (CI-CNT). Tension loads were applied to expand the mesh samples and we recorded the forces at brittle failure. The cubic structures were used for separate compression tests. These data were then used in conjunction with a nonlinear finite element analysis (FEA) model of the stent geometry to determine Young's modulus and maximum fracture strain in tension and compression for each sample. Additionally, images were recorded of the mesh samples before, during, and at failure. These images were used to measure an overall percent elongation for each sample. The highest fracture strain observed was 1.4% and Young's modulus values confirmed that the material was similar to that used in previous carbon-infiltrated carbon nanotube work. The average percent elongation was 86% with a maximum of 145%. This exceeds a typical target of 66%. The material properties found from compression testing show less stiffness than the mesh samples; however, specimen evaluation reveals poorly infiltrated samples. [ABSTRACT FROM AUTHOR]

Cross-axis flexural pivots, formed by crossing two flexible beams at their midpoints, have been used in compliant mechanisms for many years. However, their load–deflection behavior has yet to be appropriately modeled to allow easy analysis and synthesis of mechanisms containing them. This paper uses results of non-linear finite element analysis to investigate this behavior. Based on the analysis, two models for the pivots are presented – one simple and one more complex. The accuracy of the models is demonstrated by comparing results to those measured for pivots made from polypropylene and steel. [Copyright &y& Elsevier]

The cylindrical cross-axis flexural pivot (CCAFP) is proposed as an ultra-compact flexure capable of being integrated into hollow cylindrical shafts, enabling shaft motion without inhibiting cables or other components inside the shaft. Mechanism geometry, materials, and manufacturing are proposed and the results analyzed and tested. A parametric finite element model of the CCAFP was created to analyze the force-deflection and strain-deflection relationships and the predicted behavior was verified by experiment. Analytic models of stress-limiting cam-surfaces suggest even larger motions may be possible when not limited by current practical constraints. The CCAFP is demonstrated and tested at multiple size scales and in multiple materials, ranging from 28.6 mm diameter 4130 steel (achieving 9 degrees of angular deflection) to 3 mm diameter NiTi (achieving an angular deflection of 85 degrees). The results are generalized to apply to a range of applications, and the CCAFP particularly shows promise for implementation in minimally invasive surgical instruments to decrease instrument size while maintaining instrument performance. [ABSTRACT FROM AUTHOR]

An underactuated or underconstrained compliant mechanism may have a determined equilibrium position because its energy storage elements cause a position of local minimum potential energy. The minimization of potential energy (MinPE) method is a numerical approach to finding the equilibrium position of compliant mechanisms with more degrees of freedom (DOF) than inputs. Given the pseudorigid-body model of a compliant mechanism, the MinPE method finds the equilibrium position by solving a constrained optimization problem: minimize the potential energy stored in the mechanism, subject to the mechanism’s vector loop equation(s) being equal to zero. The MinPE method agrees with the method of virtual work for position and force determination for underactuated 1-DOF and 2-DOF pseudorigid-body models. Experimental force-deflection data are presented for a fully compliant constant-force mechanism. Because the mechanism’s behavior is not adequately modeled using a 1-DOF pseudorigid-body model, a 13-DOF pseudorigid-body model is developed and solved using the MinPE method. The MinPE solution is shown to agree well with nonlinear finite element analysis and experimental force-displacement data. [ABSTRACT FROM AUTHOR]

This paper explores the deflection and buckling of fixed-guided beams used in compliant mechanisms. The paper’s main contributions include the addition of an axial deflection model to existing beam bending models, the exploration of the deflection domain of a fixed-guided beam, and the demonstration that nonlinear finite element models typically incorrectly predict a beam’s buckling mode unless unrealistic constraints are placed on the beam. It uses an analytical model for predicting the reaction forces, moments, and buckling modes of a fixed-guided beam undergoing large deflections. The model for the bending behavior of the beam is found using elliptic integrals. A model for the axial deflection of the buckling beam is also developed. These two models are combined to predict the performance of a beam undergoing large deflections including higher order buckling modes. The force versus displacement predictions of the model are compared to the experimental force versus deflection data of a bistable mechanism and a thermomechanical in-plane microactuator (TIM). The combined models show good agreement with the force versus deflection data for each device. [ABSTRACT FROM AUTHOR]