Fabrication of elastomeric silk fibers

Abstract

Methods to generate fibers from hydrogels, with control over mechanical properties, fiber diameter and crystallinity, while retaining cytocompatibility and degradability, would expand options for biomaterials. Here we exploited features of silk fibroin protein for the formation of tunable silk hydrogel fibers. The biological, chemical, and morphological features inherent to silk were combined with elastomeric properties gained through enzymatic crosslinking of the protein. Post-processing via methanol and autoclaving provided tunable control of fiber features. Mechanical, optical, and chemical analyses demonstrated control of fiber properties by exploiting the physical cross-links, and generating double network hydrogels consisting of chemical and physical cross-links. Structure and chemical analysis revealed crystallinity from 30–50%, modulus from 0.5MPa to 4MPa, and ultimate strength 1 to 5 MPa depending on the processing method. Fabrication and post-processing combined provided fibers with extensibility from 100 to 400% ultimate strain. Fibers strained to 100% exhibited 4th order birefringence, revealing macroscopic orientation driven by chain mobility. The physical cross-links were influenced in part by the drying rate of fabricated materials, where bound water, packing density, and micro-structural homogeneity influenced cross-linking efficiency. The ability to generate robust and versatile hydrogel microfibers is desirable for bottom-up assembly of biological tissues and for broader biomaterial applications.

Silk Hydrogel Microfibers for Biomimetic Fibrous Material Design

“…With the hue quantification adopted here, the order of birefringence (first‐sixth) cannot be determined. In previous work, fibers with improved mechanics were capable of cycling through greater orders of birefringence as a function of strain . Therefore, we can speculate that the silk‐BSA samples possessed length‐scale hierarchy that enabled microstructural rearrangement under cyclic strain for controlled birefringent order progression, leaving silk‐BSA fibers at orange hues in a lower order (Figure A).…”

“…The fibers were embedded in OCT compound (Fisher Scientific, Waltham, MA) and cryosectioned in longitudinal 10 µm slices for polarized microscopy imaging. Birefringence was assessed similarly to our previously reported work for cyclically loaded samples to infer rearrangement and matrix organization under physiological loading regimes . Briefly, fiber cross‐sections on glass microscope slides were placed between two crossed polarizers with the fiber section long axis at 45° to the polarizer fast axis for maximum brightness under conditions of uniaxial birefringence.…”

“…Birefringence was assessed similarly to our previously reported work for cyclically loaded samples to infer rearrangement and matrix organization under physiological loading regimes. [12] Briefly, fiber cross-sections on glass microscope slides were placed between two crossed polarizers with the fiber section long axis at 45° to the polarizer fast axis for maximum brightness under conditions of uniaxial birefringence. Samples were analyzed using a Nikon Eclipse E600 Polarizing Optical Microscope (Nikon, Tokyo, Japan) connected to a CCD camera (Diagnostic Instruments, Sterling Heights, MI) and images were obtained using Spot 5.2 Advanced Image Analysis Software and a 10 or 20× objective was used to image all samples.…”

“…Further, thermodynamically favorable assembly, particularly with strain, drives micelle to nanofibril assembly . Our previous report on silk hydrogel microfibers fabricated through enzymatic cross‐linking showed that the materials exhibited strength, extensibility, and structural rearrangement with strain …”

“…[11] Our previous report on silk hydrogel microfibers fabricated through enzymatic cross-linking showed that the materials exhibited strength, extensibility, and structural rearrangement with strain. [12] The goal of this work was to develop a silk hydrogel microfiber platform suitable for fibrous scaffold applications. First, nanofibrous β-sheet toughening through inter-and intramolecular hydrogen bonding native to the silk protein was studied.…”