Flaw-insensitive fatigue resistance of chemically fixed collagenous soft tissues

Fig. 1.. Transcatheter heart valves. (A) BP is a collagenous membrane covering the bovine heart. (The figure was partly generated using Servier Medical Art, licensed under a Creative Commons Attribution 3.0 unported license.) (B) A photo of BP, thickness ~ 200 μm, harvested from the heart. (C) Photographs of a suture thread of a diameter of ~150 μm, BP leaflet, and metallic stent. (D) A schematic of BP leaflets sutured on a metallic stent to assemble into a transcatheter heart valve. (E) The transcatheter heart valve is compressed in a catheter and is pushed from a leg to the heart through an artery using a guide wire. Upon reaching the site of the aortic valve, the material expands into the shape of the heart valve, and then, the catheter is removed.

Fig. 2.. In BP, strong collagen fibers bear high tension, and soft matrix deconcentrates the high tension over long length. (A) In an undeformed BP membrane, a network of strong collagen fibers is embedded in a soft matrix. (B) When the membrane is stretched, the soft matrix deconcentrates high tension in an individual collagen fiber over a long length. (C) When the collagen fiber ruptures, the elastic energy stored in the long length of the fiber dissipates.

Fig. 3.. Structure of the BP. (A) Schematics of the structure at several length scales. A BP membrane is observed in transmission and scanning electron microscopes (TEM/SEM) with various resolutions. (B) A TEM image shows individual collagen fibrils with a diameter of 100 nm and d-bands with a spacing of ~65 nm. SEM images: (C) A bundle of collagen fibrils constitute a collagen fiber, which is surrounded by elastin fibers. (D) Collagen fibers are wavy, and multiple collagen fibers are aligned. (E) Collagen fibers are embedded in a soft matrix.

Fig. 4.. Tension and pullout of collagen fibers. (A) Stress-stretch curve. (B) Snapshots of four states of deformation. Polarized light micrographs of (C) an undeformed sample and (D) the rupture surface of the sample. In the uniaxial tensile test, the number of samples, n = 6. Thickness of samples, t = 0.28 ± 0.03 mm (means ± SD).

Fig. 5.. Fracture and deconcentration of strain. (A) The stress-stretch curve. (B) Snapshots of a membrane with a precut in four states of deformation. (C) A polarized light micrograph of the crack tip. (D) The resistance curve. (E) Contours of strain EYY observed using DIC. Both BP and TPU are imaged for comparison. (F) The concentration factor of strain EYY at distance X from the crack tip measured in the undeformed state. In the fracture test, the number of samples, n = 3. Thickness of samples, t = 0.31 ± 0.04 mm (means ± SD).

Fig. 6.. Flaw sensitivity of materials under monotonic stretch. Various materials are plotted on the plane of fracture toughness Gc and work of fracture Wc. The ratio Gc/Wc defines the fractocohesive length, several values of which are marked. HA-PVA, hierarchical PVA; DA-PVA, dry-annealed PVA; PAAm, polyacrylamide; DN, double network.

Fig. 7.. Cyclic stretch of membranes without precut. (A) Stress-stretch curves under cyclic stretch. (B) The number of cycles to rupture is a function of the amplitude of work per volume. (C) Patterns of small-angle x-ray scattering (SAXS) before and after cyclic stretch. (D) Intensity-q spectra lines before and after 30,000 cyclic stretches. Inset: Periodic d-band structure of collagen fibrils. In the fatigue test of samples without precut, the number of samples for each stretch amplitude, n = 1. Thickness of samples in the whole fatigue test group, t = 0.27 ± 0.04 mm (means ± SD).

Fig. 8.. Cyclic stretch of membranes with precut. (A) Schematic of a membrane with precut subjects to cyclic stretch, with a prescribed amplitude of stretch. (B) The plane of the energy release rate and the number of cycles to the onset of crack growth. A point represents a failed sample, and an arrow represents a sample surviving 30,000 cycles without crack growth. (C) A membrane with precut is subject to cyclic stretch of amplitude λ = 1.45 and the energy release rate being G = 23.5 kJ/m2. (D) A membrane with precut is subject to cyclic stretch of amplitude λ = 1.35 and G = 8.6 kJ/m2. (E) Polarized light micrograph of the crack tip. In the fatigue fracture test, the number of samples for each stretch amplitude, n = 1. Thickness of samples in the whole fatigue test group, t = 0.30 ± 0.03 mm (means ± SD).

Fig. 9.. Flaw sensitivity of materials under cyclic stretch. Various materials are plotted on the plane of fatigue threshold Gth and work of endurance We. The ratio Gth/We defines the endurance fractocohesive length, several values of which are marked. ML-PVA, muscle-like PVA.

Fig. 10.. Accelerated fatigue test of transcatheter heart valves. (A) Schematic of accelerated fatigue test. A transcatheter heart valve is mounted in an accelerated wearing test machine to endure cyclic pulsive flow. (B) During the test, the transvalvular pressure gradient of the pulsive flow is controlled at an amplitude of 100 mmHg and a frequency of 15 Hz. (C) A BP valve remains intact after 2 × 108 cycles of pulsive flow. (D) A TPU valve grows cracks after 2.7 × 106 cycles of pulsive flow.