Application of synthetic biology in living functional materials

The fast development of synthetic biology has greatly promoted the understanding of biology and broadened the application of engineered biological systems. Numerous cell-based circuits are assembled using modular biological building blocks. These circuits are further utilized to program the living organisms to attain multiple well-defined functions. Especially, the field of biological engineered living materials (ELMs) emerged at the intersection between synthetic biology and material science. Actually, living organisms themselves are made of materials with remarkable properties including self-organization and adaption to the stimulus. In nature, various patterns form autonomously during the development of the animal, and the bacteria can secrete biofilm made of polysaccharides and proteins to protect themselves when treated with antibiotics. The existing material systems typically lack these "smart" properties, which are urgently requested by the next generation materials. A key difference between the ELMs and classical material is that ELMs are composed of living cells that form or assemble the material itself. The living cells act as building blocks, modulate and direct the formation and function of the desired material. Therefore, the ELMs exhibit living properties, such as evolvability, self-repair and responsiveness to environment. Synthetic biological circuits can further program the cells spatially and temporally, and brought various functionalities into the ELMs. The most pioneering efforts in ELMs development have focused on engineering functional amyloids, which are secreted and assembled into nanofibrous structure on the cell surface during biofilm formation. Among them, the curli produced by E. coli is the most well-studied system. During the curli formation, the membrane curli-specific gene B (CsgB) proteins functioned as nucleators and guided the polymerization of the major protein subunit CsgA into curli fibrous networks extracellularly. The system is highly engineerable by fusing heterologous protein domains with specific functions to CsgA, including the curli that can be designed to bind inorganic materials and become electrically conductive by fusing functional tags. Besides E. coli, other organisms spanning from prokaryotic systems to eukaryotic cells are also engineered and adopted for ELMs assembly. By integrating with the tools and technologies (3D printing, microfluidics, etc) from the material engineering and other disciplines, these rewired cells can be further engineered into materials with complex functions such as disease detection and bioremediation. Here we review the efforts on ELMs development, with an emphasis on its implementation and production using engineered bacterial systems, and also discuss the current limitation and future challenges of the ELMs.