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Bovine pericardium (BP) has been used as leaflets of prosthetic heart valves. The leaflets are sutured on metallic stents and can survive 400 million flaps (~10-year life span), unaffected by the suture holes. This flaw-insensitive fatigue resistance is unmatched by synthetic leaflets. We show that the endurance strength of BP under cyclic stretch is insensitive to cuts as long as 1 centimeter, about two orders of magnitude longer than that of a thermoplastic polyurethane (TPU). The flaw-insensitive fatigue resistance of BP results from the high strength of collagen fibers and soft matrix between them. When BP is stretched, the soft matrix enables a collagen fiber to transmit tension over a long length. The energy in the long length dissipates when the fiber breaks. We demonstrate that a BP leaflet greatly outperforms a TPU leaflet. It is hoped that these findings will aid the development of soft materials for flaw-insensitive fatigue resistance. 

Among soft materials, hydrogels with dynamic bonds that can be activated by a range of stimuli including temperature, pH, and infrared or ultraviolet light, constitute a special class of materials with unusual properties such as self-healing, actuation, and controlled degradation. Here, we take a hydrogel with reconfigurable disulfide crosslinks as an example and investigate its mechanical behavior. We demonstrate that this material has excellent fracture and fatigue resistance when the disulfide crosslinks are activated by ultraviolet illumination. We propose a simple constitutive model that describes the mechanical behavior of the material under a broad range of conditions. 

In many applications, glass fabrics are subject to cyclic forces. Here we show that a glass fabric can tear under a much lower cyclic force than monotonic force. For samples of a given width, a threshold force exists below which the fabric does not tear under cyclic load, and a critical force exists at which the fabric tears under monotonic load. For example, for 80 mm wide sample, the threshold force is 12.78 N, and the critical force is 344.73 N. Under cyclic force of amplitude between the threshold force and critical force, tear initiates after some number of cycles. Under either cyclic or monotonic force, the fabric tears in three modes: pullout of transverse yarns, pullout of transverse yarns and break of longitudinal yarns, and break of transverse and longitudinal yarns. We summarize the observed tear modes on the plane of two axes: the amplitude of force and the width of sample. It is hoped that this study will aid the development of fatigue-resistant fabrics and fabric reinforced composites. 

Biological tissues, such as heart valves and vocal cords, function through complex shapes and high fatigue resistance. Achieving both attributes with synthetic materials is hitherto an unmet challenge. Here we meet this challenge with hydrogels of heterogeneous structures. We fabricate a three-dimensional hydrogel skeleton by stereolithography and a hydrogel matrix by cast. Both the skeleton and matrix are elastic and stretchable, but the skeleton is much stiffer than the matrix, and their polymer networks entangle topologically. When such a hydrogel is stretched, the compliance of the matrix deconcentrates stress in the skeleton and amplifies fatigue resistance. We fabricate a homogeneous hydrogel and a heterogeneous hydrogel, each in the shape of a human heart valve. Subject to cyclic pressure, the former fractures in  560 cycles but the latter is intact after 50,000 cycles. Soft materials of complex shapes and high fatigue resistance open broad opportunities for applications.