Specialty & fine chemicals
Silk fibroin biomaterials
Sericin as the actual immunogen, the silk I to silk II transition and how processing sets beta-sheet content, why crystallinity controls both mechanics and degradation time, and what regenerated fibroin can be formed into.
Silk fibroin used as a biomaterial is a different subject from silk used as a fibre. Here the protein is dissolved and re-formed into films, sponges, gels and coatings, and the properties that matter are set almost entirely by one structural variable.
The immunogen is the glue
A silk thread from Bombyx mori is two fibroin filaments cemented together by sericin, a glue-like glycoprotein coating.
Historical reports of inflammatory and allergic reactions to silk sutures are attributable largely to sericin rather than to fibroin. Degumming — removing sericin, conventionally by boiling in alkaline carbonate solution — leaves fibroin that is well tolerated, and the completeness of degumming is therefore a determinant of biocompatibility rather than a cosmetic processing detail. A material specification that does not state residual sericin is leaving out the variable that matters most.
Recombinant fibroin produced microbially avoids the issue entirely, since sericin is a separate gene product that is simply not expressed.
One structural variable governs everything
Fibroin’s heavy chain is dominated by long repeats of glycine–alanine and the motif GAGAGS. These repeats have essentially no side-chain bulk, so the chains pack into tightly hydrogen-bonded beta-sheet crystallites, embedded in a less ordered amorphous phase.
Regenerated fibroin exists in two states. Silk I is the metastable, largely amorphous form obtained when a fibroin solution is simply dried — water-soluble and mechanically weak. Silk II is the beta-sheet-rich crystalline form.
The transition is induced deliberately, by treatment with methanol or ethanol, by water annealing under controlled humidity, or by applied shear. And because the transition is the material’s only major structural degree of freedom, beta-sheet content is the dial that sets every property at once:
- Mechanics. More crystallinity gives higher stiffness and strength and lower extensibility.
- Water stability. Silk I dissolves; silk II does not, because the hydrogen-bonded sheets exclude water.
- Degradation rate. This is the useful part. Fibroin is degraded by proteases in vivo, and proteolysis requires access to an extended chain. Crystalline regions are inaccessible, so degradation proceeds through the amorphous phase. Raising beta-sheet content slows it — and the achievable range is very wide, from weeks to a year or more.
A single protein can therefore be processed into a rapidly resorbing scaffold or a long-lived implant coating without changing its chemistry at all. Few biomaterials offer that range from one input.
What it can be made into
Because the starting point is a solution, the format is a processing choice: cast films, freeze-dried porous sponges whose pore size follows the ice crystals, electrospun mats, hydrogels formed by sonication or shear-induced gelation, microspheres, and coatings.
Two further properties are worth knowing. Fibroin can be processed entirely in water, so heat- and solvent-sensitive molecules — including enzymes and vaccines — can be embedded during forming and are stabilised by the surrounding matrix. And its degradation products are peptides and amino acids rather than acidic monomers, so it does not produce the local pH drop that accompanies degradation of polyesters such as polylactide.