Processing Techniques and Applications of Silk Hydrogels in Bioengineering

Abstract

Hydrogels are an attractive class of tunable material platforms that, combined with their structural and functional likeness to biological environments, have a diversity of applications in bioengineering. Several polymers, natural and synthetic, can be used, the material selection being based on the required functional characteristics of the prepared hydrogels. Silk fibroin (SF) is an attractive natural polymer for its excellent processability, biocompatibility, controlled degradation, mechanical properties and tunable formats and a good candidate for the fabrication of hydrogels. Tremendous effort has been made to control the structural and functional characteristic of silk hydrogels, integrating novel biological features with advanced processing techniques, to develop the next generation of functional SF hydrogels. Here, we review the several processing methods developed to prepare advanced SF hydrogel formats, emphasizing a bottom-up approach beginning with critical structural characteristics of silk proteins and their behavior under specific gelation environments. Additionally, the preparation of SF hydrogel blends and other advanced formats will also be discussed. We conclude with a brief description of the attractive utility of SF hydrogels in relevant bioengineering applications.

Keywords: bioengineering; hydrogel; silk fibroin.

Figure 1

Frequency of publications related to silk fibroin (SF) (grey) and SF hydrogels (blue) by year. Data obtained by searching for SF and SF hydrogel in the Web of Science.

Figure 2

SF hydrogels can be prepared from several methods. Chemical methods: precipitants, pH, HP CO₂, chemical crosslinking, chemical modification. Physical methods: temperature, shear force, ultrasound, electric fields.

Figure 3

Mechanically-diverse SF hydrogels prepared by different processing techniques. (A) Blending SF with hydroxypropyl methyl cellulose (HPMC) produces robust mechanical prosperities highlighted visually by bending, knotting and compressing. Tensile curves prepared from SF-HPMC blend hydrogels of different concentrations (E). (B–D) Highly elastic hydrogels of SF have been prepared by chemically crosslinking tyrosine residues, dityrosine bonds, within SF via a horseradish peroxidase (HRP) reaction resulting in robust hydrogel networks displaying excellent elasticity and resilience. (D) Strain response of elastic SF hydrogel after compression with 50 g (2) and 100 g (3) brass weights and exhibiting complete recovery after removal (4). (F) Cyclic compression of elastic SF hydrogels reveal excellent recovery below 70% strain; the inset displays complete recovery below 40% strain. (A,D) Reproduced with permission from [64]. (B–D,F) Reproduced with permission from [41].

Figure 4

Applications of SF hydrogels in bioengineering. (A) hMSCs cultured on SF hydrogels of different stiffness with 10 ng/mL TGF-β1 for 72 h for selected vascular SMC markers: calponin (green), myosin heavy chain (MYH11) (orange). Scale bar: 200 μm. (B) hNSCs encapsulated in unmodified and IKVAV-modified SF hydrogels after seven days. Cells were stained with Nestin (green), βIII-tubulin (red); bars = 100 μm. (C) Image of self-assembling SF hydrogels loaded with different amounts of the anticancer drug doxorubicin (red). (D) SF-doxorubicin hydrogel loaded syringe displaying injectability for clinical use. € Influence of SF hydrogel processing and cocoon degumming parameters on the cumulative doxorubicin release into PBS. Statistical analysis was performed by comparison with SF 6 wt % hydrogel; ** p < 0.001, *** p < 0.0001. (A) Reproduced with permission from [15]. (B) Reproduced with permission from [82]. (C–E) Reproduced with permission from [83].