Navid Kazem1 Michael Bartlett2 Carmel Majidi1

1, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
2, Iowa State University, Ames, Iowa, United States

Soft and tough materials are critical for engineering applications in medical devices, stretchable and wearable electronics, and soft robotics. Toughness in synthetic materials is mostly accomplished by increasing energy dissipation near the crack tip with various techniques from mesoscale approaches like particle-filled composites to molecular scale techniques including hybrid and double network gels and polymers. However, bio-materials exhibit extreme toughness by combining multi-scale energy dissipation with the ability to deflect and blunt an advancing crack tip. Here, we demonstrate a synthetic material architecture that also exhibits multi-modal toughening, where by embedding a suspension of micron sized and highly deformable liquid metal (LM) droplets inside a soft elastomer, the fracture energy dramatically increases by up to 50x (from 250 ± 50 J/m2 to 11,900 ± 2,600 J/m2) over an unfilled polymer. For some LM-embedded elastomer (LMEE) compositions, the toughness is measured to be as high as 33,500 ± 4,300 J/m2, which far exceeds the highest value previously reported for a soft elastic material. This extreme toughening is achieved by means of (i) increasing energy dissipation, (ii) adaptive crack movement, and (iii) effective elimination of the crack tip. Such properties arise from the deformability and dynamic rearrangement of the LM inclusions during loading, providing a new mechanism to not only prevent crack initiation, but also resist the propagation of existing tears for ultra tough, highly functional soft materials.