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Prior to fracture, a polyacrylamide hydrogel has a stress-stretch curve of nearly perfect elasticity, but it has been suggested that an inelastic zone exists around a crack tip. This inelastic zone, however, has never been observed directly in a polyacrylamide hydrogel. Here we identify the inelastic zone using digital image correlation (DIC). We prepare a polyacrylamide hydrogel with a precut crack. While a sample of the hydrogel is stretched, the speckle patterns are recorded using a microscope or a camera, with pixel size 2.3 μm and 22.7 μm, respectively. The speckle patterns recorded by the microscope and camera are processed using the DIC software, and merged to provide the deformation field over the entire sample. The measured field of deformation is used to calculate the field of energy density according to the neo-Hookean model. When the body is perfectly elastic, the field of energy density around the crack tip is inversely proportional to the distance from the crack tip. The difference between the measured field and the predicted elastic field identifies the inelastic zone. The measured size of the inelastic zone is ∼ 0.6 mm. We further confirm that, when a sample is much larger than the inelastic zone, an annulus exists, in which the elastic crack tip field prevails. 

S ubmerged in an aqueous solution of sodium chloride (NaCl), a poly(N-isopropylacrylamide) (PNIPAM) hydrogel c an be in one of two phases: swollen phase and collapsed phase. We measure the equilibrium volume of the h ydrogel as a function of temperature T and ionic concentration y. The hydrogel is in the swollen phase when T a nd y are low, and is in the collapsed phase when T and y are high. We develop a thermodynamic model in which t he free energy is a function of volume, temperature, and ionic concentration. The free energy also contains s everal adjustable parameters, which we best-fit to the experimental data of volume as a function of T and y. For a given pair of T and y, the free energy is a function of volume. This function has a single minimum for some pairs of (T, y), but two minima and a maximum for other pairs of (T, y). In the former, the single minimum corresponds t o either a swollen or a collapsed state. In the latter, the lower minimum corresponds to a state of equilibrium, t he higher minimum corresponds to a metastable state, and the maximum corresponds to an unstable state. When t he two minima are equal, the hydrogel undergoes phase transition. The condition of phase transition is rep - r esented as a curve on the (T, y) plane. The thermodynamic model represents the experimental data well. 

The conditions for rupture of a material commonly vary from sample to sample. Of great importance to applications are the conditions for rare-event rupture, but their measurements require many sam- ples and consume much time. Here, the conditions for rare-event rupture are measured by developing a high-throughput experiment. For each run of the experiment, 1,000 samples are printed under the same nominal conditions and pulled simultaneously to the same stretch. Identifying the rupture of individual samples is automated by processing the video of the experiment. Under monotonic load, the rupture stretch for each sample is recorded. Under cyclic load, the number of cycles to rupture for each sample is also recorded. Rare-event rupture is studied by using the Weibull distribution and the peak-over-threshold method. This work reaffirms that predict- ing rare events requires large datasets. The high-throughput exper- iments enable the prediction of rare events with high accuracy and confidence. 

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.