Synthetic Biology Prospective Synthetic biology for fibers, adhesives, and active camouflage materials in protection and aerospace

Synthetic biology has a huge potential to produce the next generation of advanced materials by accessing previously unreachable (bio)chemical space. In this prospective review, we take a snapshot of current activity in this rapidly developing area, focusing on prominent examples for high-performance applications such as those required for protective materials and the aerospace sector. The continued growth of this emerging field will be facilitated by the convergence of expertise from a range of diverse disciplines, including molecular biology, polymer chemistry, materials science, and process engineering. This review highlights the most significant recent advances and addresses the cross-disciplinary challenges currently being faced. Introduction The synthetic biology revolution Synthetic biology is the application of engineering principles to design and construct new biologic entities. Drawing from disciplines including molecular and systems biology, synthetic biology is set to revolutionize how we utilize biology, just as chemical synthesis transformed chemistry, and the integrated circuit transformed computing. Specifically, synthetic biology allows biologic systems to be engineered for manufacturing chemicals, foods, or fabricating materials, producing healthcare products, processing information, and producing energy. One of the most prominent examples of synthetic biology is the production of the anti-malaria drug artemisinin through an engineered yeast host. Other healthcare examples include the accelerated production of vaccines, or mining biosynthetic gene clusters for novel antibiotic production. Synthetic biology has also catalyzed exciting developments for the production of sustainable food alternatives, including the production of grapefruit flavoring nootkatone, yeast-produced milk or egg substitutes, and laboratory-grown meat. The production of many commodity chemicals has also been demonstrated, some of which are entering commercial scale production, moving us away from reliance on petrochemical sources. Recently, significant funding has also been raised by a number of synthetic biology companies seeking to produce novel materials at commercial scale. The rapidly falling costs for both DNA sequencing and DNA synthesis, a trend often referred to as the Carlson curve, have stimulated progress in synthetic biology. This has led to phenomenal growth in available DNA sequence data, providing inspiration for “biobricks”which can be cheaply synthesized for recombinant expression. Furthermore, developments in DNA assembly techniques, the characterization and standardization of DNA parts, and the implementation of engineering design– build–test cycles into automated work flows is revolutionizing our ability to engineer biology for our own ends. Synthetic biology for the fabrication of novel materials In essence, nature is a molecular assembler. It is a nanotechnology which we are able to control through synthetic biology techniques, allowing us to access nature’s hard-won advanced materials, and to even engineer them toward our own needs and applications. In the context of material fabrication, it is helpful to categorize materials derived through synthetic biology into two classes: direct and indirect materials. Direct synthetic biologic materials include structural proteins and carbohydrates, such * Equal author contribution MRS Communications (2019), 9, 486–504 © Materials Research Society, 2019. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. doi:10.1557/mrc.2019. 35 486▪ MRS COMMUNICATIONS • VOLUME 9 • ISSUE 2 • www.mrs.org/mrc https://doi.org/10.1557/mrc.2019. 35 Downloaded from https://www.cambridge. org/core. IP address: 207.241. 231.108, on 17 Feb 2020 at 00:41:10, subject to the Cambridge Core terms of use, available at https://www.cambridge. org/core/terms. as spider silk, cellulose, and collagen. These are structural materials produced by cells. Indirect synthetic biologic materials, on the other hand, include the production of material precursors (such as monomeric small molecules for polymer or adhesive production) through engineered enzymes. Prominent examples of each are presented in this review. Biologic systems achieve impressive feats of materials engineering through near-flawless control over the synthesis and assembly of the constituent parts, primarily proteins. From a materials science perspective, proteins can be considered highly multifunctional polyamide hetero-polymers with unparalleled complexity compared with synthetic analogs. Moreover, proteins can offer perfect mono-dispersity and can selfassemble through protein folding to attain complete control over microstructure, which in turn governs macroscopic mechanical properties. Custom-designed, recombinant proteins can now be produced relatively easily in biological host organisms (as well as cell-free systems) through the tools of synthetic biology. The direct biomanufacturing of monomers and other compounds from engineered organisms is another way in which synthetic biology could contribute to the next generation of materials. Within cells, cascades of enzyme-catalyzed chemical reactions occur to produce all manner of biologically derived molecules. By engineering cells to express a multitude of bespoke enzymes, tailored molecules––including the monomers to advanced polymeric materials––can be produced. Furthermore, synthetic biology offers us the potential to reach a novel chemical space, inaccessible through traditional methods. Finally, whilst the synthetic chemist is highly dependent on petroleum-derived feedstocks, solvents, and relatively harsh, energy-intensive processing conditions, the synthetic biologist by definition employs benign conditions (conditions that are compatible with life) and renewable, naturally-occurring feedstocks and nutrients as precursors. The natural world has produced an array of materials with outstanding strength, toughness, resilience, and optical properties, despite being hugely constrained by processing conditions (ambient temperature and pressure, and minimal pH swings), limited access to raw materials and comparatively few building blocks (20 amino acids and 32 glycans, compared to the vast number of possible building blocks of synthetic chemistry). The synthetic biology toolkit Synthetic biologists have a growing repertoire of tools and methods at their disposal for the direct and indirect production of synthetic biological materials. The decrease in DNA sequencing costs provides biologists with a rapidly expanding set of natural templates for the production of a diverse range of biomaterials. For example, reading the genome of a spider allows the genes which code for spider silk proteins to be identified, copied, and engineered to be expressed in organisms that are much easier to propagate, such as bacteria, as shown in Fig. 1(a). However, and very importantly, the decreasing cost and enhanced power of DNA synthesis, editing and assembly methods, mean that synthetic biologists are not limited to molecular building blocks that are found in nature. They can synthesize new DNA sequences, allowing them to design novel proteins, specified precisely down to the amino acid level. A key advantage of this bioengineering approach is the fact that rapid optimization of functional proteins and biopolymers can be achieved through several rounds of random DNA mutagenesis, informed by sequence–activity relationships, followed by rapid screening and selection of desirable properties. Furthermore, protein domains, or enzymes in metabolic pathways, can be assembled in innovative combinations, for the production of protein chimeras or unique pathways toward desired metabolic products, as shown in Fig. 1(b). Increasingly predictive computational tools ensure that the initial designs are reliably translated into engineered organisms. The scale and ambition of these biologic engineering projects have increased by several orders of magnitude in the last years, with the refactoring and de novo synthesis of entire microbial genomes now realistically within reach. A number of reviews offer a more detailed overview of synthetic biology techniques and recent developments. Challenges for applying synthetic biology in the production of materials Synthetic biology seeks to apply the engineering design–build– test cycle to biology, testing many samples in parallel and iterating over a number of cycles to find an optimum solution (Fig. 2). Automation, liquid handling robots, and highthroughput screening are currently being employed in established synthetic biology areas, to allow large numbers of designs to be built and tested quickly and efficiently. Crucially, this approach requires the “test” to be executed on a small scale, for example, in a 96or 384-well plate format, somewhat at odds with traditional material testing approaches. The large data-sets this approach provides can be used to feed machine learning algorithms or empirical models, which can in turn be used to inform the next round of designs. One route toward applying this methodology may be the use of “surrogate” factors, which correlate with a desired property but are more easily measured in a high-throughput setting. A small number of promising leads might then be selected from a pool of hundreds of thousands, for more traditional, larger-scale material tests. Synthetic biology for advanced materials in protection and aerospace Whilst the focus of research into synthetic biological materials has primarily been on biomedical materials (materials designed to interact with biologic tissue) for applications such as regenerative medicine, the delivery of therapeutic agents, and other “soft” material applications, there exists great po