SEMI-CRYSTALLINE FRAMEWORK PROVIDES ELASTIC
RECOVERY IN SELF-HEALING BIOLOGICAL FIBERS
S. Krauss1,2, H. Metzger1, P. Fratzl1 and M. J. Harrington1
1
Max Planck Institute of Colloids and Interfaces, Dept. of Biomaterials, Research Campus
Golm 14424 Potsdam, Germany – e-mail: stefanie.krauss@mpikg.mpg.de,
hartmut.metzger@mpik.mpg.de, peter.fratzl@mpikg.mpg.de, matt.harrington@mpikg.mpg.de 2
BAM Federal Institute of Materials Research and Testing, Division 5.4 - Ceramic Processing and Biomaterials, Unter den Eichen 44-46 D-12203 Berlin – e-mail:stefanie.krauss@bam.de
Keywords: bio-inspiration, metal coordination, SAXS, molecular healing
ABSTRACT
Supramolecular polymers based on reversible metal coordination cross-linking have recently shown promise as self-healing materials. However, these materials are for most part entirely unstructured. In contrast, nature’s self-healing metallopolymers exploit anisotropic and hierarchical arrangements of molecular building blocks to enhance mechanical properties and increase healing efficacy. Mussel byssal threads, for example, are tough fibrous biopolymers with the ability to autonomously and intrinsically heal following damage. Healing behavior involves molecular repair of reversible metal-protein complexes embedded in extensible protein domains. Here, we present in situ X-ray diffraction data suggesting that the spatial organization of these cross-links strongly influences the mechanics and healing behavior of byssal threads.
X-ray diffraction of native threads reveals a highly organized semi-crystalline framework of protein building blocks with long-range axial and lateral order. When threads are stretched past mechanical yield, the framework deforms completely due to the rupture of sacrificial protein-metal cross-links and unfolding of hidden protein length. The semi-crystalline protein framework recovers elastically immediately following unloading; however, mechanical healing requires longer time-scales, suggesting that ruptured bonds are slower to reform. In light of these observations, we propose a two-stage molecular repair mechanism in which elastic recovery of the protein lattice is a prerequisite for reformation of reversible metal cross-links, presumably by bringing binding partners back into spatial register following damage. This mechanism allows not only precise damage localization at the molecular level, but a recoil mechanism for reuniting sacrificial bonds. Our results suggest that nanoscale order should, perhaps, be a principal design consideration for the next-generation of self-healing polymers.
