Nature has developed cellular material, with advantages of light-weight yet remarkable toughness, such as bamboo, shell, and bone. Polymeric and metallic foams have been developed mimicking cellular biomaterials for structural or functional application. With the development of high-resolution additive manufacturing techniques, especially photopolymer-based stereolithography 3D printing, microlattice architecture with optimized geometries has been introduced into cellular architected materials, yielding programmable and desired mechanical properties than their stochastic counterparts. Multi-materials lattice, in which struts are composed of two or more different materials with distinguishing phases and mechanical properties, demonstrate unique mechanical and functional properties, which can be also referred to as mechanical metamaterials.

Projection micro stereolithography (PμSL) is a novel 3D printing technology with a high printing speed for complex structures providing tens of the centimeters printing area and ultra-high “micrometer-scale” printing resolution. The point has been made that the mechanical properties of printed micro polymer structures are dramatically different from their bulk ones. However, the mechanical properties of PμSL printed materials with microsized features are less studied. In this work, these correlations for the acrylate-based resin are experimentally established by the tension of PμSL printed fibers with diameters ranging from 20 μm to 60 μm. When the size of microfibre decreased to 20 μm, the brittle resin (the fracture strain of bulk polymer is 5%) performs like rubber with a fracture strain up to ~100% and fracture strength goes to ~100 MPa. Such size-dependent mechanical behavior of PμSL-printed acrylate-based resin structures enables the tailoring of the material strength and stiffness of microlattice units over a wide range. This knowledge also enables the following fabrication of microlattice scaffolds with desired/programmable mechanical properties for the development of novel microlattice mechanical metamaterials.

To create high toughness yet lightweight metamaterials, a novel core-shell microlattice metamaterial incorporating liquid metal (LM) and polymer was proposed. The hollow polymer shell was printed by PμSL 3D printing technology and the liquid metal (Ga) core was filled. The existence of Ga in the hollow lattice prevents the polymer scaffold from brittle fracture dramatically improving the fracture toughness. Moreover, due to the low melting point of Ga, the significant stiffness transformation of Ga core can be achieved by thermal stimulation, which leads to mechanical properties recovery and shape memory effect simultaneously. Furthermore, a damaged Ga-filled octet microlattice can restore its original shape with strength recover 50% of its initial yield stress, showing superior recoverability for engineering applications under extreme cases. The result offers new insights into the design and manufacturing of solid-liquid mechanical metamaterials with tunable properties and high recoverability for soft robots, flexible electronics, and biomedical applications.

For the above LM microlattices controlled by temperature, not only the response time is relatively slow, their mechanical properties can only be switched between “on” and “off” states corresponding to the liquid and solid-state of liquid metal. To create a more prompt responsive and continuously adjustable metamaterial, magnetorheological (MR) fluid, composed of carrier fluid, suspended micro-sized magnetically polarizing particles besides stabilizing additives, has been introduced and employed to fabricate MR fluid-filled microlattice. Incorporating with flexible elastomer polymer shell, the MR fluid-filled microlattice demonstrates complete recovery after being compressed to 80%. Taking advantage of the field responsive properties of MR, the stiffness of MR-filled lattice can be altered distinctively between 20 KPa and 60 KPa when the magnetic field was increased from 0 mT to 60 mT, with a wide adjustable range of 200%. Compared with traditional magnetic active materials which are controlled by a high-power neodymium magnet, the flexible MR-filled microlattice metamaterials can be remotely controlled by a normal electromagnet and shows a much shorter response time at about 50 ms with continuously and precisely tunable mechanical properties.

In summary, this work explored the design and microfabrication technologies of solid-liquid microlattice metamaterials and characterized their mechanical properties in situ. The temperature-controlled LM-filled microlattices and magnetism-controlled MR-fluid microlattices were proposed and demonstrated with the aim to create more adaptive mechanical metamaterials. Besides outstanding and tunable mechanical properties, these mechanical metamaterials also show promising functional potential with their high degree of controllability and tunable stiffness for innovative flexible electronic, soft robots, and many other mechatronics applications.