Microcrimped Collagen Fiber-Elastin Composites

Emerging biomaterials based upon analogues of native extracellular matrix proteins provide an opportunity to create protein scaffolds that mimic tissue mechanical behavior and guide cellular responses. However, in order to reproduce macroscale tissue properties, protein analogues must be endowed with appropriate microstructural features. In particular, the crimped or wavy microstructure of native collagen fibers, with a periodicity of 10 – 200 μm, contributes in a significant manner to the compliance, strength, and durability of soft tissues. In this report, we describe a templating strategy based upon the application of micropatterned elastomeric substrates, which yields dense, aligned arrays of synthetic collagen microfibers that display a well-defined microcrimped pattern. Following crosslinking with glutaraldehyde vapor, fiber arrays were embedded in a recombinant elastin protein polymer,[1] which contributes to the resilience of the composite structure by bearing tensile loads at low strains, analogous to a native elastin fiber network.[2, 3] We demonstrate the preservation of fiber crimp after repetitive cyclic loading, as well as the assembly of hierarchical microcrimped multilamellar composites with mechanical responses similar to native tissues. The periodic waviness of fibrous collagen is observed in nearly all human tissues, including blood vessels, valve leaflets, intestine, tendon, and intervertebral discs.[4-6] The morphological features of crimp structure has been characterized as planar zig-zag,[7] sinusoid,[8] or helical[9, 10] with wavelengths between 10 to 200 μm. Crimp ensures that at low levels of tensile strain, loads are sustained both by the surrounding matrix and the fiber network. Typically, fibers straighten as a load is imposed with an observed transition from low to high tissue stiffness.[4, 5, 11, 12] These mechanisms serve to enhance compliance at low strain while generating greater strength as load increases. Since physiologic strains are imposed at levels of stress where fibers are often not fully extended, the propensity for fatigue-related fiber damage is minimized. All told, fiber crimp has evolved as an important bioengineering principle that affords a favorable combination of compliance, strength, and durability. A set of techniques using soft, contracting substrates to shape thin coatings of high modulus materials into crimped, wrinkled, and wavy structures has recently emerged.[13-19] For example, Bowden and colleagues deposited metal films on heated PDMS and noted that, upon cooling, the contraction of the PDMS buckled the metal layer into sophisticated patterns of wrinkles.[13] However, in these cases the extent of waviness is limited by the extent of inducible thermal shrinkage. Alternatively, an elastomeric substrate may be mechanically stretched prior to the application of a thin film,[15] array of nanoribbons,[14, 16] integrated circuit,[17] or carbon nanotubes,[18] with relaxation of stretch producing defined wavy structures. Collectively, these studies have lead to the fabrication of controlled micro- and nano-scale waveforms. However, microcrimping techniques have not been developed that are suitable for biological materials, such as collagen fibers.