## 1 Introduction  
Architected metamaterials represent a class of artificially-designed materials that consist of periodically arranged microstructures [1]. Desirable properties can be implemented in architected metamaterials that previously unachievable in conventional solids [2], such as ultra-lightweight [1; 3], excellent energy absorption [4], negative Poisson's ratio [5] and heat dissipation [6]. Consequently, this novel group of materials has been receiving considerable attention as structural materials for a myriad of applications in the industry of aerospace [7] and medicine [8]. At present, the rapid development of additive manufacturing (AM) technologies opens up the possibilities of fabricating metamaterials with complex microstructures and unmatched capabilities [9; 10]. The architecture of the unit cells (the thickness and configuration of struts, geometry, etc.) plays a significant role in the mechanical performance of architected metamaterials [2]. Typical 2D and 3D unit cell structures can be divided into stretching-dominated and bending-dominated lattices [11]. While the former such as Octet and Octahedral lattices exhibits higher strength and stiffness, the latter is suitable for energy absorption [12]. To meet the demand of various applications, a great deal of unit cell structures has been designed and investigated. Extensive attention has been recently paid to the prime properties of architected metamaterials with periodic arrangement of struts (similar to atomic bonds) and nodes (equivalent to atoms), which are constructed to mimic the microstructure of perfect metals/alloys. Typical examples are: simple cubic structure (SC), face-centered cubic structure (FCC) and body-centered cubic structure (BCC) [13]. [PERSON] et al. [14] have conducted a theoretical analysis to predict the initial stiffness and plastic collapse strength of architected metamaterial with a BCC structure, which illustrated a nonlinear relationship between plastic collapse strength and strut aspect ratio. [PERSON] and [PERSON] [15] have designed elastically-isotropic truss lattices through a combination of SC, BCC and FCC structures, and a theoretical limit of stiffness could be achieved at the cost of a 40% reduction of theoretical Young's modulus due to periodic homogenization. [PERSON] et al. [16] have conducted both dynamic experiments and numerical simulations on metallic BCC structures, and figured out that the strength and energy absorption capability are proportional to the square of relative density. Several unit-cell-based topology optimization methods for architected metamaterials are proposed to help fabricate metamaterial structures with designed properties. [PERSON] et al. [3] have fabricated ultralightweight metallic metamaterial with nickel-plating thin film, thus improved mechanical efficiency was obtained via optimizing dimensions of the architecture. [PERSON] et al. [17] have presented a systematic unit cell design methodology to generate micro-architected structures and a 12% higher thermal conductivity was achieved by optimizing the connectivity of struts while maintaining the periodicity. [PERSON] et al. [18] have proposed an optimization method with the combination of structural topology and material chemistry for the Octet structure, and found the proper unit cell size and relative density for attaining optimal strength of Cu micro-lattice. Although highly unusual thermal or mechanical properties can be obtained by appropriate design and optimization of the unit cell architecture [17], marginal improvement has been made on the connectivity among the unit cells. In other word, research work has been limited to the architected metamaterials with periodically-distributed lattices. Various base materials are applied to prepare such architected metamaterials, such as polymers [19], metals [3; 18] or ceramics [20]. The mechanical response is to a large extent influenced by the base material since different materials exhibits different strength or work-hardening behavior [21].