Printable multi-stage variable stiffness material enabled by low melting point particle additives

A majority of biological organisms in nature can adjust their biomechanical energy to adapt to complex environments, but most of the current synthetic composites have limited rigid and flexible states that cannot achieve multi-level and continuous regulation on altering mechanical stiffness. Herein, a direct ink printing (DIW) approach for forming a 4D printable phase-changing elastomer that achieves multiple stable stages in response to a thermal stimulus has been developed. This composite consists of low melting point alloy (LMPA) microparticles incorporated into a silicone elastomer (PDMS) using a facile composite manufacturing process. The particles with different melting points amplify the steady stage in flexural modulus under a thermal stimulus, which is desirable for stiffness-changing applications, particularly relevant to soft robotics. Moreover, the composites exhibit improved printability for three-dimensional direct printing via adjusting the volume ratio of the raw materials, which circumvents the dilemma that most sample structures are restricted between one- and two-dimensional transformations and the conventional craftsmanship is limited by complex production. It is demonstrated as well that the utility of LMPA/PDMS composites has an advantage of multiple stiffness changes at the set-transition temperature for unveiling their brilliant prospects for soft actuators with 4D printing technology.