New opportunities for an ancient material

Abstract

Spiders and silkworms generate silk protein fibers that embody strength and beauty. Orb webs are fascinating feats of bioengineering in nature, displaying magnificent architectures while providing essential survival utility for spiders. The unusual combination of high strength and extensibility is a characteristic unavailable to date in synthetic materials yet is attained in nature with a relatively simple protein processed from water. This biological template suggests new directions to emulate in the pursuit of new high-performance, multifunctional materials generated with a green chemistry and processing approach. These bio-inspired and high-technology materials can lead to multifunctional material platforms that integrate with living systems for medical materials and a host of other applications.

Fig. 1

Mechanical properties of silks. (A) Impressive toughness and relative strength of reeled spider dragline silk. The area under the curve shown indicates fiber toughness or the energy taken up by the material before breaking. In terms of strength-to-weight ratio, the spider silk strength (1.1 GPa) is about equivalent to high-tensile engineering steel (1.3 GPa), yet spider silk has a relative density of 1.3 compared with that of steel at 7.8, when reeled at 20 mm s⁻¹ at 25°C for Nephila edulis. In terms of toughness, spider silk is 165 ± 30 kJ kg⁻¹, which is substantially higher than that of Kevlar 81 (33 kJ kg⁻¹) (1, 5). (B) Spinning silk from B. mori silkworms at different speeds illustrates control of fiber mechanical properties resulting from processing inputs to complement the importance of chemistry. The data show that the properties can match those of spider silks when spun from the worms at higher rates than native processes. The speeds shown for the lines reflect the rate at which the silk was drawn from the silkworm under controlled conditions at 25°C and are compared with standard degummed silk from cocoons, which are spun from the glands at a natural speed of 4 to 15 mm s⁻¹ at 20°C (3). (C) Stress strain curves for major ampullate (MA) gland silk (red line) and viscid silk (blue line) from the spider A. diadematus. E_init = initial stiffness (4). (D) Compilation of data from multiple sources and based on data from the spider A. diadematus (4). RH indicates relative humidity.

Fig. 2

Modular designs of silk proteins. Silks are fibrous proteins and are characterized by modular units linked together to form high molecular weight, highly repetitive proteins. These modular units or domains, each with specific amino acid sequences and chemistries, provide specific functions. In particular, sequence motifs such as polyalanine (polyA) and poly alanine-glycine (polyAG) (β sheet–forming), GXX (31-helix), GXG (stiffness), and GPGXX (β spiral) are key components in different silks whose relative positioning and arrangement are intimately tied with the end material properties. These domains are linked together to generate high molecular weights and also include characteristic and highly conserved N and C termini. Charged amino acids are strategically located at the chain ends and in spacers to optimize water interactions related to processing and assembly. Current modes of expression of silks in heterologous hosts are generally confined to a limited number of thesemodular units and lack the inclusion of both N- C-terminal domains. The incorporation of all of these key domains, along with issues of size (number of modular units), will facilitate improved material properties from silks generated by recombinant DNA techniques.

Fig. 3

Generating new materials from silks. Native silk processing on the left illustrates the transition from synthesis, storage, and spinning related to protein concentration, pH, salts, and physical phenomena. On the right, analogous processes with source materials from reconstituted native silk proteins or genetically engineered silks lead to new materials and technology platforms with control of mechanical, morphological, and structural features. A range of materials can be generated from silks through processing into hydrogels, fibers, sponges, films, and other forms. The properties of these systems can bemodified (e.g., mechanical, degradation profile, and optical clarity) depending on the processing modes used and then generated into functional devices and technology platforms.