Silk: a versatile biomaterial for advanced optics and photonics

Silk-protein-based optical devices and multifunctional devices.

The growing developments of optical technologies in energy, environmental, information, and biomedical applications are creating a demand for optical materials and devices which are not only sustainable but also implantable and bioresorbable. Within this context, naturally-derived biomaterials, such as silk, cellulose, chitin, melanin, and DNA, provide a unique opportunity by being simultaneously "technological" (e.g., optically active, micro- and nanoscale processable), "structural" (e.g., rich surface chemistry, mechanical flexibility), "sustainable" (e.g., renewable, eco-friendly), and "biological" (e.g., biocompatible, biodegradable) making them ideally suited for applications at the interface between optical technologies and environment within human or around human.

Silks belong to a large family of structural proteins and are mainly synthesized by several arthropods, including silkworms and spiders. Among the different sources, domestic Bombyx mori silks come from abundant sources and have been the main focus for multi-field applications. Recent decades have witnessed the transition of applications of such an ancient material from traditional textile fields to advanced technical fields including optics and photonics, energy, electronics, and optoelectronics. More recently, great achievements have been made in the development of silk-based optical systems due to the advancements in multiscale manufacturing technologies.

In the article titled "Silk: a versatile biomaterial for advanced optics and photonics" published in Chinese Optics Letters, Vol. 18, Issue 8, 2020 (Yushu Wang, Meng Li, Yu Wang. Silk: a versatile biomaterial for advanced optics and photonics [Invited][J]. Chinese Optics Letters, 2020, 18(8):80004), the authors demonstrate the recent progress in advanced optical devices constructed from silk protein with a particular emphasis on the structural designs responsible for the final optical functionalities.

First, the justifications of using silk for optical applications are studied. The excellent balance of "structure-process-property-function" relationship of silk protein makes it distinct from other biopolymers. Then, the state-of-the-art silk-based optical devices are summarized in detail. The seamless match between silk protein and the advanced micro-/nanomanufacturing technologies, together with the versatility of silk protein, allows for the creation of silk optical devices with different forms, scales, and functions, including structural color materials, optical fibers and waveguides, diffractive optical elements, bio-lasers, plasmonic devices, metamaterials, and broadband reflection materials. Moreover, recent advances in silk-protein-based multifunctional optical devices are also demonstrated. The combination of silk self-assembly, top-down manufacturing techniques, and ease of functionalization enables the formation of silk optical devices with function diversity, such as the integration of different optical functions, the incorporation of optical function into structural function, and the coupling of optical function, information encoding, and thermal regulation. Finally, the challenges and directions for future devising silk-based optical materials and devices are discussed.

"Silk fibroin provides new opportunities towards the replacement of existing non-renewable optical platforms with environmentally friendly, biocompatible systems that match the high performance of their synthetic counterparts, while minimizing waste, environmental degradation, and energy-intensive input.", says the corresponding author Dr. Yu Wang, "silk will play a pivotal role in the future exploitation of sustainable, intelligent and adaptive, wearable/implantable, and multifunctional optical devices."

Although silk has shown great potential for applications in optics and photonics, further efforts still need to be devoted to developing silk-based optical materials and devices with an optimized "structure-property-function" relationship through sustainable, scalable, and facile manufacturing techniques.

综述：丝蛋白制备先进光学和光子学器件

基于丝蛋白的光学器件及多功能器件

光学技术在能源、环境、信息和生物医学领域中应用的不断发展正在对光学材料及器件提出全新的要求，材料需要可持续性而且具有可植入、可被生物吸收的能力。在此大背景下，天然衍生的生物材料（例如丝蛋白、纤维素、几丁质、黑色素以及DNA）体现出了独特的价值。该类材料同时具有 “技术的”（如光学活性、多尺度可加工性）、“结构的”（如丰富的表面化学、机械柔性）、“可持续的”（如可再生、环境友好）以及“生物的”（如生物相容性、可生物降解性）的特性，这使其非常适合应用于人体内/外环境之间的界面。

天然丝纤维属于一种结构蛋白，它可由多种节肢动物“纺制”而成，包括蚕和蜘蛛。其中，家养桑蚕丝来源丰富、应用范围广。近十几年里，这种古老材料的应用领域正在从传统的纺织领域向先进的技术领域（包括光学和光子学、能源、电子学和光电学）拓展。最近，微纳制造技术的不断创新进一步推动了丝蛋白在光学及光子学领域中的发展。

在Chinese Optics Letters 2020年第8期的封面文章中，美国塔夫茨大学的研究人员综述了利用丝蛋白构建先进光学器件的最新进展，作者重点关注了结构设计与最终光学功能之间的关系（Yushu Wang, Meng Li, Yu Wang. Silk: a versatile biomaterial for advanced optics and photonics [Invited][J]. Chinese Optics Letters, 2020, 18(8):80004）。

首先，综述文章总结了以丝蛋白为载体发展光学器件的优势。丝蛋白所具有的良好的“结构-加工-性质-功能”关系使其从所有备选生物聚合物中脱颖而出。

然后，文章详细总结了最先进的丝蛋白基光学器件。丝蛋白与先进的微纳加工技术之间无缝匹配，结合丝蛋白的多功能性，制造出一系列不同形式、规模和功能的丝蛋白光学材料和器件，包括结构色材料、光导纤维、衍射光学元件、生物激光器、等离子器件、超材料以及宽带反射材料等。

此外，文章还介绍了基于丝蛋白的多功能光学器件的最新进展。通过整合丝蛋白自组装、“自上而下”的制造技术以及丝蛋白易于功能化的特性，可以实现功能多样化的丝蛋白基光学器件，如不同光学功能、光学功能与结构功能以及光学功能与信息编码、热调节功能的整合。

最后，文章讨论了未来设计丝蛋白基光学材料和器件的挑战和方向。

文章通讯作者王瑜博士认为：“丝蛋白为利用环保、生物相容的系统替代现有的不可再生光学平台提供了新的机遇。这些新型的光学系统在维持高性能的同时也避免了光学垃圾、环境降解和能源密集型输入。未来，丝蛋白必将在开发可持续、智能和自适应、可穿戴/可植入以及多功能光学器件中发挥关键作用。”

丝蛋白在光学及光子学领域已展示出很大的应用潜力，然而，如何通过可持续、可规模化的生产以及便捷的制造技术来开发“结构-性质-功能”关系优化的丝蛋白基光学材料及器件仍面临挑战，未来仍需在此方面做出更多的努力。