Filamin-Inspired Protein Fibers Beat Spider Silk on Humidity Resistance

Spider silk has dominated protein fiber research for two decades, and for good reason: it combines high tensile strength with remarkable toughness in a lightweight package. But it has a persistent flaw. Expose spider silk fiber to humidity and it shrinks - sometimes dramatically - which limits its use in clothing, biomedical implants, and any application where stable dimensions matter.

A team at Washington University in St. Louis has produced protein fibers from a different biological template - muscle proteins - and found that at least one, derived from the filamin protein, sidesteps that weakness while adding properties spider silk cannot match. The results appear in Advanced Functional Materials.

Why muscle proteins make promising fiber templates

Natural muscle is made of proteins that stretch, contract, absorb mechanical shocks, and return to their original shape - repeatedly, over a lifetime of use. Many of these proteins share a structural feature: immunoglobulin-like domains arranged in series along a protein chain, creating spring-like behavior at the molecular level. Evolution has refined these structures over hundreds of millions of years for mechanical reliability under load.

"Many muscle proteins share similar immunoglobulin-like structures while bearing diverse amino acid sequences. These natural materials provide great inspiration for designing the next generation of protein-based materials," said Fuzhong Zhang, Francis F. Ahmann Professor in energy, environmental and chemical engineering at WashU's McKelvey School of Engineering.

Zhang's lab takes a synthetic biology route. Rather than extracting proteins from animal tissue - a process too slow and low-yield for materials production - the team genetically programs microbes to produce specific proteins in bioreactors. PhD student Shri Venkatesh Subramani, first author of the study, oversaw fiber fabrication and testing.

The performance hierarchy across muscle proteins

The team produced fibers from multiple muscle-derived proteins and tested them systematically. Not all performed equally. Fibers derived from filamin stood out across every performance dimension the researchers measured: tensile strength, toughness, energy damping capacity, shape recovery, and - critically - dimensional stability under both high humidity and elevated temperature.

The key variable, the team found through collaboration with Sinan Keten's group at Northwestern University, was hydrophobicity. Filamin's immunoglobulin domains are relatively water-repellent compared with those of other muscle proteins. "The more hydrophobic the structure is, the better fiber properties you get," Subramani said. That hydrophobicity reduces the fiber's tendency to absorb moisture and swell or contract, directly addressing spider silk's main limitation.

The production process also proved more robust than competing protein fiber systems. Because filamin contains a wider variety of amino acids than repetitive sequences like those in spider silk, the producing microbes are less prone to misreading or dropping portions of the genetic code. "That's one limitation of existing materials that we've solved," Subramani said.

Potential applications

Active wear is an obvious target. Garments worn during exercise must handle repeated mechanical stress, high humidity from perspiration, and frequent washing - conditions that would degrade humidity-sensitive fibers. Biomedical implants, including tissue scaffolds that need to maintain shape in a moist physiological environment, represent another application the team highlights.

Perhaps the most unexpected proposed use is structured plant-based or cultivated meat. Because the fibers are made from the same class of proteins that form animal muscle, they can in principle be processed into structures with muscle-like texture. "These are just regular muscle proteins that have the same processes as animal muscle. It can be processed into a meat-like structure," Subramani said.

What remains to be solved

The current work is at lab scale. Scaling bioreactor protein production to commercial volumes is a well-known challenge in the synthetic biology field; costs per gram remain high compared with petroleum-derived synthetic fibers, and achieving consistent fiber properties at scale requires tight process control. The team identifies scale-up and multi-market evaluation as the immediate next steps.

The study was funded by the National Science Foundation (awards DMR-2207879 and OIA-2219142). An independent assessment of real-world durability under extended use conditions, across different humidity and temperature ranges, would strengthen the case for specific applications.