Spider silk inspires a new route to organic magnets

Spider dragline silk is one of the most versatile natural materials ever known, with several incredible mechanical, optical, thermal, piezoelectric, and biological properties. However, its fundamental magnetic nature remains unknown. In the present study, we report the observation of room-temperature ferromagnetism in metal-free pristine spider dragline silks upon induction of defects in its beta-sheet nanocrystals. The magnetism originates in spider silks due to ferromagnetic coupling among carbon radicals (dangling bonds) generated in beta-sheet nanocrystals. Direct control over silk's magnetic properties can be achieved by controlling its microstructure. This was achieved by changing the spinning speed of dragline silks from the spider and observing a direct effect on its magnetism. Owing to the high-temperature stability of silk, their ferromagnetism survives up to 400 K and remains unaffected by high humidity or contact with water. This makes silk-based magnets suitable for medical and technological applications. Spider silk can thus act as a multifunctional nontoxic biomagnet with incredible mechanical properties. Our work demonstrates a new paradigm of magnetic proteins and opens a route toward the bioinspired discovery of iron-free magnetic proteins. Biomimicking its structure is of great importance for designing future medical sensors and actuators, including advancements in tissue engineering and artificial muscles.Impact statementIt is well known that densely bound beta-sheet nanocrystals within silk biopolymers are responsible for their incredible mechanical strength and stiffness. In the present study, we show that these beta-sheet nanocrystals also create an ideal environment for stable carbon radicals within the silk structures. A magnetic exchange interaction among these radicals results in a stable and robust carbon-based ferromagnetism at room temperature in these polymers. These are the first ever reports of observation of room-temperature ferromagnetism in pristine spider silks. Inducing defects in these nanocrystals by applying strain on dragline silk samples leads to an enhanced saturation magnetization. A direct effect of nanocrystallite size on the ferromagnetic properties of silk was also observed. Blending magnetism in a bioinspired and metal-free protein-based biomaterial can tremendously impact biomedical applications such as nanoscale drug delivery systems, magnetic resonance imaging contrast agents, magnetic scaffolds, and artificial muscles. Our work will stimulate a new theoretical understanding of the origin of magnetism in peptide-based biomaterials with consequences in quantum biology and spintronics. Our work establishes a novel method to control the magnetic responsivity of proteins by engineering atomic defects.