Recombinant spider silk

Spider dragline silk is a benchmark material: tough, extensible, spun at ambient temperature and pressure from a water-based feedstock. It cannot be farmed, because orb-weavers are territorial and cannibalistic, so producing it means producing the protein in a microbe. The persistent lesson of the field is that the protein alone is not the material.

What the protein is

Dragline silk is built from spidroins — very large proteins, typically well above 200 kDa, with a highly repetitive core flanked by short non-repetitive N- and C-terminal domains. The core alternates two motif types: polyalanine blocks, which stack into β-sheet nanocrystals, and glycine-rich blocks, which stay disordered. The mechanical result is a composite at nanometre scale — stiff crystallites carrying load inside a compliant matrix that supplies extensibility. Strength tracks crystallite alignment and, importantly, chain length: shorter chains mean fewer entanglements bridging crystallites, and the fibre fails earlier.

The terminal domains are not decoration. They are the switch. The C-terminal domain holds the protein soluble at high concentration and dimerises; the N-terminal domain is pH-sensitive and triggers assembly as conditions change along the duct.

The spinning duct is the reactor

In the gland, spidroin is stored at very high concentration as a liquid-crystalline dope without aggregating. Passing down the duct, three things change together: pH falls as CO₂ is exchanged and protons are pumped in, ions are exchanged — sodium and chloride out, potassium and phosphate in — and the tapering geometry imposes elongational shear. Acidification triggers the N-terminal domains to lock the chains together, ion exchange salts out the glycine-rich regions, and shear aligns the whole assembly along the fibre axis. Water is then withdrawn. Assembly is thus staged and directional, not a precipitation event.

Industrial routes reproduce this only partially: protein is dissolved into a dope, extruded through a spinneret into a coagulation bath, and drawn in several stages to force alignment. Post-spin drawing does much of the work the duct would otherwise do.

Why microbes resist

The spidroin gene is the problem. Long, highly repetitive, GC-rich sequences recombine in a bacterial host, so constructs truncate; and translating a protein that is mostly alanine and glycine depletes those charged tRNAs, stalling the ribosome. The practical consequence is that recombinant spidroins are usually much shorter than native ones — and since strength depends on chain length, that shortfall is mechanical, not cosmetic. Codon design, repeat diversification, host tRNA supplementation and alternative hosts including yeasts and transgenic silkworms all attack this same bottleneck.

Reading the numbers honestly

Published tensile values for recombinant fibres vary widely, and comparisons need care: measured strength and toughness for any silk depend on gauge length, strain rate and humidity, and there is no single conditioning convention across the literature. A recombinant fibre reported as matching native dragline may have been tested under conditions the native comparison was not. The safe statement is that engineered fibres have approached native mechanical behaviour in specific tests, and that reproducing it as a bulk, consistent, high-throughput product remains unresolved.

Silk is also supercontracting: dragline shrinks and softens when wet, a behaviour any application has to design around.