Development of a fatigue tester and material model for flexible heart valve applications

The leaflet material in heart valve prostheses is required to be both flexible and durable to eliminate the need for chronic anticoagulation medication and accommodate younger patients with longer life expectancies. This investigation aims to provide two of the necessary tools to design and test suitably flexible and durable materials for heart valve replacement. These tools address the question of how to model the stress-strain behaviour of polymer networks and thermoplastic polyurethanes in particular, as well as how to practically evaluate the durability of the proposed material. A model for polyurethane stress-strain behaviour is proposed, whereby the number of monomers between crosslinks is suggested to evolve with macroscopic strain. Following the polymer chain entanglement theory, the increase in the number of monomers between crosslinks is further extended to be a function of strain rate, incorporating the viscous effect observed in polyurethanes. A multistation, micro-tensile specimen fatigue tester was developed to evaluate material durability. The proposed equilibrium polyurethane model accurately predicts the experimental data across the full material strain range. The proposed model extension sufficiently captures the rate dependence of polyurethane, however, fails to account for the raised specimen temperatures at high strain rates. The developed fatigue tester is verified to successfully feature selectable variables including test frequency (1 - 20 Hz), amplitude (1 - 6 mm), waveform (Triangular, Sinusoidal, Square and Custom) and environmental temperature control (23 - 50 oC). Less than 10% error in measured force is observed when compared to a commercial tensile tester. The proposed model successfully provides a platform to aid the design of flexible materials suitable for heart valve leaflets. The developed fatigue tester enables the assessment of material durability across a range of test conditions, successfully providing a tool for leaflet material durability analysis and verification

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb29

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least 4 m. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the 6.5 m James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 yr, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit