Mechanical properties and preliminary in-vitro testing of 3D printed bioactive glass combined with hydrogel

Abstract

Bioactive glass has shown promise in the field of bone tissue engineering. Bioactive glasses are biocompatible, and they have osteoconductive properties and stimulate new bone formation. Once implanted, an apatite layer forms on the material surface and allows the material to bond with bone. Hydrogels are another promising material for tissue engineering. They are polymer-based hydrophilic materials that can absorb thousands of times their dry weight in water. Their properties are often similar to the extracellular matrix, which makes them a promising material for cell culturing. In this thesis work, 3D printed bioactive glass scaffold was combined with gellan gum hydrogel to create a scaffold that could potentially become a cellularized matrix. The thesis had three aims. The first aim was to find out if it is possible to create a stable gel within the porous structure of the scaffold. The second aim was to study, how adding gel to the scaffold affects the mechanical properties and in vitro dissolution of the scaffold in simulated body fluid. The third aim was to assess if the gellan gum provide a better substrate to the cells. Firstly, 3D printed scaffolds were prepared using magnesium strontium containing borosilicate bioactive glass developed in the laboratory. After that, gellan gum was injected inside the scaffold by crosslinking Gelzan polymer solution with Spermidine polyamine. To allow for optimum gel penetration and gelation the mixing time and the mixing method were optimized. The effects of the gel on the mechanical properties and in vitro dissolution were studied. Compressive testing was performed on scaffolds with and without gellan gum in compression. The compressive strength and Young’s modulus of the material were then calculated. Regardless fof the presence or not of the gellan gum the compressive strength remain at ~ 22 MPa and the Young modulus at ~ 900 MPa. The results showed that there was no significant difference be-tween the two groups which means that the gel did not increase nor decrease the compressive strength or the Young’s modulus of the glass scaffold. The in vitro dissolution of the two scaffolds types was studied by immersion in simulated body fluid at 37 °C for two weeks. pH of the solution and the masses of the scaffolds were measured at four timepoints. pH measurements showed that pH was lower in the dissolution products of the scaffolds with gellan gum, which indicates that the gel slows down ion release. After dissolution, Fourier-transform infrared spectroscopy was performed on scaffolds that had been immersed for two-weeks and the results were compared with the results of 0-day samples. Fourier-transform infrared spectroscopy results showed that the gel promotes hydroxyapatite formation on the material surface, as evidence by the sharp and intense vibration peaks related to phosphorous vibrations. Finally, MC3T3-E1 cells were cultured onto the different scaffolds. To assess cell viability, Live/Dead experiment was performed 24 hours, 72 hours and 7 days after the start of cell culturing. At all timepoints, scaffolds with gellan gum had fewer cells than scaffolds without gellan gum. However, in both cases, the cells were confluent around the scaffolds and with higher density than the control. This suggests that gellan gum does not improve cell attachment at the materials’ surface, while the dissolution by-products promotes cell proliferation. To create a material that acts as a suitable substrate for the cells, different concentrations or other hydrogels should be tested. In conclusion, the process of adding gellan gum into a scaffold did not affect the mechanical properties, in compression, of the bioactive glass scaffold. The presence of the gel slowed down ion release but also promoted hydroxyapatite layer formation at the materials’ surface. Gellan gum did not promote cell attachment but testing other hydrogels could yield different results.