Designing toughness and strength for soft materials.

Soft materials, such as hydrogels, elastomers, and plastics, are pervasive in nature and society. For example, except for teeth, nails, and bones, all other components of the human body are hydrogels, and we eat, wear, and use soft materials as foods, clothes, shoes, and car tires, just to name a few, in our daily life. Although amorphous polymer chains endow soft materials with high flexibility or “softness,” they can also lead to inferior mechanical performances, such as low fracture toughness and low strength, frequently hampering applications and innovations of soft materials. Over recent years, intensive efforts have been devoted to the development of softmaterials that possess extraordinary mechanical properties, especially by seeking inspiration from nature and biology. For instance, synthetic hydrogels can now be made much tougher than articular cartilages (1, 2) and more adhesive than mussel glues (3, 4). In PNAS, Wu et al. (5) report the fabrication of bioinspired fibers with both high toughness and high strength by drawing from a hydrogel at ambient temperatures and pressures. This nascent progress in the field raises a generic question. What are the fundamental principles for the design of soft materials to achieve certain mechanical properties in nature and engineered systems? Compared with hard materials such as steels and ceramics, which have been explored over centuries, such principles are still elusive in the field of soft materials. The goal of this commentary is to elucidate the design principles for soft materials that are both tough and strong, such as natural silks (6) and the hydrogel fibers synthesized byWu et al. (5). I will show that three design principles are required for soft materials to achieve both high toughness and high strength: (i ) high mechanical dissipation and stretchability, (ii ) below flaw-sensitive length scale, and (iii ) simultaneous stiffening and fracture of significant polymer chains at failure. By drawing from a multiphase self-assembled hydrogel, Wu et al. (5) exploited all three principles to fabricate fibers that were both tough and strong. In control cases, when any principle was violated, the resultant fibers turned out to be brittle and/or weak (5). In the following sections, I will elaborate on these principles and propose possible designs for future soft materials with extraordinary mechanical properties and beyond.