Regenerated silk materials for functionalized silk orthopedic devices by mimicking natural processing

Abstract

Silk fibers spun by silkworms and spiders exhibit exceptional mechanical properties with a unique combination of strength, extensibility and toughness. In contrast, the mechanical properties of regenerated silk materials can be tuned through control of the fabrication process. Here we introduce a biomimetic, all-aqueous process, to obtain bulk regenerated silk-based materials for the fabrication of functionalized orthopedic devices. The silk materials generated in the process replicate the nano-scale structure of natural silk fibers and possess excellent mechanical properties. The biomimetic materials demonstrate excellent machinability, providing a path towards the fabrication of a new family of resorbable orthopedic devices where organic solvents are avoided, thus allowing functionalization with bioactive molecules to promote bone remodeling and integration.

Keywords: Biomimetic; Orthopedic; Self assembly; Silk.

Figure 1

Schematic illustration of the fabrication process, characterization and assembly mechanism of biomimetic silk materials. (a) Illustration of spinning duct of silkworm. (b) Solid silk materials were obtained by the biomimetic gradual dehydration of silk aqueous solution. (1) Concentrate silk solution to 25–30% (wt/wt). (2) Inject concentrated silk solution to silk blank molds. (3) Remove water by forced air flow at 10°C for 3–4 days. (4) Remove the remaining water by drying in fume hood at room temperature for 4 days and then in an oven at 45°C for 4 days. (5) Machine silk blank into orthopedic devices of desired geometry. (c) Various shapes machined from biomimetic silk materials (scale = 1 cm). (d, e) SEM and (f) AFM images of the cross section of a silk rod of 1.5 mm diameter (scale: 40 μm in d, 2 μm in e, and 200 nm in f.). (g) FTIR spectra of silk materials (A: freeze-dried concentrated silk solution; B: silk materials obtained after dehydration at 10°C and room temperature; C: silk blanks before machining.) (h) Stress-strain curve of biomimetic silk materials. (i) Comparison of specific strength and modulus of the biomimetic silk material with natural silk materials, bone, polymer, ceramic and metal/alloys. Adapted with permission from nature publishing group [34]. The yellow ellipse represents the silk and silk/HAP materials in this study. (j) Model of the assembly process of the biomimetic silk materials. The silk molecules form oligomeric aggregates in concentrated silk solution. Dehydration leads to the condensation and assembly of the silk molecules into densely packed solid materials.

Figure 2

Silk orthopedic devices. (a) Silk screw with bone screw thread. Scale = 5 mm. The major diameter of the screw was ~ 1.8 mm and pitch was 600 μm. (b) SEM image of bone screw (scale = 100 μm). (c) Silk IM rods of 1.5 mm diameter, scale = 5 mm. (d) SEM image of silk IM rods, scale = 5 mm. (e) Four-hole silk plate of 29 mm length and 2 mm thickness (scale = 5 mm). (f) SEM image of milled surface of silk plate (scale = 100 μm). (g) Representative shear strength-displacement curve of 1.5 mm diameter silk pin by double lap shear mechanical test. (h) Representative load-displacement curve of the four-hole silk plate of 29 mm length and 2 mm thickness by four-point bending test.

Figure 3

Silk-based orthopedic devices with osteoinductive functionality: in vitro differentiation of hMSCs induced by BMP2/P24 incorporated silk screws. (a) Schematic illustrating the in vitro cell culture setup. BMP2/P24 incorporated silk screws were inserted into a tubular silk sponge with pore size of 400–500 μm and hMSCs were seeded in the sponge. Osteogenic differentiation of hMSCs was assessed with OsteoImage fluorescent staining for HAP after 6-week culture in osteogenic medium. Transmitted light (b, e and h), fluorescent (c, f and i) and overlay images demonstrated the mineralization pattern in (b–d) osteogenic control, (e–g) BMP2 and (h–j) P24 groups. Scale = 200 μm.

Figure 4

Silk-based antimicrobial and biocomposite orthopedic devices. (a) Two-hole plate containing 5% (wt/wt) ciprofloxacin (scale = 5 mm). (b) A clear zone of inhibition formed around ciprofloxacin releasing silk rods against S. aureus cultures (scale = 1mm). SEM of biofilm on (c) ciprofloxacin-loaded and (d) pure silk pins, scale = 10 μm. (e) The cumulative release of ciprofloxacin from ciprofloxacin-loaded silk rod over 36 days. (f and i) Optical images, (g and j) SEM images and (h and k) FTIR spectra of the (f–h) silk/HAP (90/10 wt/wt) and (i–k) silk/SiO₂ (90/10 wt/wt) biocomposite screw with bone screw thread. Scale = 5 mm in d and g and 100 μm in e and h, respectively.