(645g) Fabrication of Composite Hydrogels for Tissue Engineering

Hydrogels are three dimensional (3D) polymeric networks that are able to absorb and retain large volume of water and are suitable for the delivery of cells and bioactive agents¹. Elastin-based biomaterials have great potential for use as hydrogels for regeneration of damaged tissues due to their capacity for self assembly and phase transition behaviour; they also mimic many features of the extracellular matrix and have the potential to guide the migration, growth and organization of cells during regeneration processes². However, a major limitation of elastin-based biomaterials is their low mechanical properties. The other issues associated with current elastin-based hydrogels fabrication include the extensive processing time, small pores which cause the lack of cellular growth in 3D structures, and the use of toxic solvent.

The aim of this study was to fabricate composite elastin hydrogels with mechanical properties and pore characteristics suitable for 3D cellular infiltration and proliferation. The feasibility of using dense gas CO₂ (DGO₂) for simultaneous crosslinking and generation of porosity within the structure of composite hydrogel was assessed. A DG is fluid above or close to  its critical temperature and pressure with superior properties compared to liquids and gases; higher density than gases and larger mass transfer properties than liquids³. Dense gases are excellent fluids for extraction, purification, and a media for conducting reaction³. In this study elastin-based hybrid hydrogels were fabricated by chemically cross-linking of polymer mixture solution with glutaraldehyde by using DGCO₂. The effects of processing parameters including pressure, temperature, reaction time, and depressurisation rate on the characteristics of fabricated hydrogels were also studied.

Dense gas CO₂ had a significant impact on the characteristics of the fabricated hydrogels including porosity, swelling ratio, compressive properties, and modulus of elasticity. Compared to fabrication at atmospheric pressure DGCO₂ based construction eliminated the skin-like formation on the top surfaces of composite hydrogels and generated larger pores with an average pore size of 78 ±17 µm (Figure 1a, b). The swelling ratios of composite hydrogels fabricated by DGCO₂ were lower than the gels produced at atmospheric pressure as a result of a higher degree of crosslinking. DGCO₂ substantially increased the mechanical properties of fabricated hydrogels. The compressive and tensile moduli of composite hydrogel were enhanced 2 and 2.5 fold, respectively, when the pressure was increased from 1 to 60 bar. In vitro studies showed that the presence of large pores throughout the hydrogel matrix fabricated by DGCO₂ enabled the migration of human skin fibroblast cells 300 µm into the construct (Figure 2 c). However, cells were only able to form a monolayer on the surface of hydrogels fabricated using conventional methodology (Figure 2d). This is likely due to the inability of fibroblast cells to migrate within the small and discontinuous pore networks of samples produced by the conventional methodology. The composite hydrogels fabricated by DGCO₂ serve as an elastic biomaterial for soft tissue repair applications.

Figure 1: SEM images of hydrogels produced using DGCO₂ (a), and conventional methodology (b)

Figure 2: Images of fibroblast cells attached to composite hydrogel fabricated using DGCO₂ (a, b, c), and conventional

methodology (d). Panel a, c, and d show cross-sections of hydrogel and panel d show the hydrogel surface

References

1. Hoffman A.S., Adv Drug Deliv Rev, 2002, 54, 3-12.

2. Daamen W.F. et al., Biomaterials 2007, 28, 4378-4398.

3.  Annabi N. et al., Biomaterials 2009, 30, 1-7.