Multifunctional liquid metal lattice materials through hybrid design and manufacturing

Abstract

Multifunctional lattice materials exhibit functionalities beyond conventional load-bearing usage and are usually fabricated by additive manufacturing. This work introduces a new class of functional lattice materials called liquid metal lattice materials. These lattice materials consist of liquid metals and elastomers organized in a core-shell manner. This hybrid design induces a shape memory effect by harnessing the solid-liquid phase transition of liquid metals. Consequently, several remarkable functionalities are achieved such as recoverable energy absorption, tunable rigidity, and reconfigurable behaviors. These liquid metal lattice materials are fabricated by using a hybrid manufacturing approach, which integrates the 3D printing, vacuum casting, and conformal coating techniques. A variety of lattice structures are presented to demonstrate the capability of this hybrid manufacturing method and the functionalities of liquid metal lattice materials. This new class of lattice materials have promising applications in aerospace, robotics, tunable metamaterials, etc.

Introduction

Lattice materials [[1], [2], [3]] are composed of repetitive unit cells with artificially designed geometry to achieve weight reduction and/or desirable functionalities. Early development of lattice materials focused on simple structures such as honeycomb structure, mesh, and foam [4,5] due to the limitations of conventional manufacturing technologies. In contrast, the last decade has witnessed an eruption of lattice materials with complicated geometry [6,7], hierarchical structure [8,9], gradient design [10,11], multimaterial [12,13], and multifunctionality [1,12,14], due to the rapid development of the additive manufacturing (or 3D/4D printing) technology [15]. Considering the rapid growth of this area, we envision that more and more novel lattice materials will be developed in the near future. In addition, promising applications of lattice materials will also be achieved in aerospace, robotics, biomedical, sensing, and wave control, among others.

Multifunctional lattice materials exhibit functionalities beyond conventional load-bearing usage and are usually enabled by multiphysical fields such as thermal, electrical, magnetic, and acoustic ones. For instance, among the most intriguing functionalities are tunable shape and rigidity, shape memory, reconfigurability, sensing and actuation, wave filtering, color change, and so on. Most of the multifunctional lattice materials are fabricated by additive manufacturing. We classify multifunctional lattice materials into four categories: (1) Thermal. Much literature work falls into this category, such as lattice materials with the shape memory effect [14,16,17], tailored thermal expansion [12], and designed heat conductivity [18]. In particular, researchers are able to fabricate lattice materials with tunable properties, recoverable shapes, deployable and reconfigurable behaviors by using shape memory polymers [14,16,17]; (2) Electrical. Recently, Zheng et al. [19] manufactured piezoelectric lattice materials with potential applications in sensing and actuation. On the other hand, Hopkins et al. [20] designed and fabricated a few electrically activated lattice metamaterials by using hybrid manufacturing methods; (3) Magnetic. Most of magnetic lattice materials are based on elastomers filled with magnetic particles. By applying an external magnetic field, the lattice materials can be readily deformed or reconfigured [21,22]. Researchers also fabricated magneto-rheological lattice materials [23] with tunable rigidity by using 3D printing and liquid filling; (4) Acoustic. Lattice materials can be employed to manipulate acoustic fields. The additive manufacturing technology has been used frequently to fabricate such kinds of acoustic lattice metamaterials in recent years [21]. We classify the lattice metamaterials that control elastic waves [24] in this category as well. Recently, multifunctional lattice materials activated by more than one physical fields are also emerging, e.g., by combining the thermal field with an acoustic/magnetic field.

Overall, most published works on multifunctional lattice materials have focused on tunable shape and rigidity, programmability, and reconfigurability, possibly driven by the demanding applications in soft robotics [25], soft implants, tunable wave control, and deployable components, among others. The materials being employed are mostly shape memory polymers [14,16,17] due to their intrinsic flexibility and the ease of fabrication. However, it is known that shape memory polymers exhibit some limitations [[26], [27], [28]], e.g., low stiffness, slow response speed, low thermal conductivity, wide transition temperature, lack of precisely tunable stiffness and transition temperature, etc. In contrast, shape memory alloys [29], such as NiTi, do not suffer from these limitations, but lack flexibility for many modern applications. Therefore, there is a demanding need to develop shape memory materials that bridge the gap between traditional shape memory polymers and shape memory alloys for multifunctional lattice materials usage.

In this work, we introduce the design and manufacturing of a new class of shape memory lattice materials; namely, liquid metal lattice materials. By integrating liquid metals and elastomers together, the hybrid lattice material exhibits an extrinsic shape memory effect that originates from the phase transition of liquid metals, which will be explained in Section 2. Because direct additive manufacturing of such lattice materials is still challenging, we propose a hybrid manufacturing approach by combining 3D printing, vacuum casting, and coating, which is introduced in Section 3. The developed liquid metal lattice materials exhibit a variety of intriguing functionalities, which include recoverable energy absorption, shape and rigidity tuning, deployable and reconfigurable behaviors to be introduced in Section 4. Finally, Section 5 will be dedicated to further discussions of the liquid metal lattice materials.