Bioactive Glasses: From Parent 45S5 Composition to Scaffold-Assisted Tissue-Healing Therapies

Abstract

Nowadays, bioactive glasses (BGs) are mainly used to improve and support the healing process of osseous defects deriving from traumatic events, tumor removal, congenital pathologies, implant revisions, or infections. In the past, several approaches have been proposed in the replacement of extensive bone defects, each one with its own advantages and drawbacks. As a result, the need for synthetic bone grafts is still a remarkable clinical challenge since more than 1 million bone-graft surgical operations are annually performed worldwide. Moreover, recent studies show the effectiveness of BGs in the regeneration of soft tissues, too. Often, surgical criteria do not match the engineering ones and, thus, a compromise is required for getting closer to an ideal outcome in terms of good regeneration, mechanical support, and biocompatibility in contact with living tissues. The aim of the present review is providing a general overview of BGs, with particular reference to their use in clinics over the last decades and the latest synthesis/processing methods. Recent advances in the use of BGs in tissue engineering are outlined, where the use of porous scaffolds is gaining growing importance thanks to the new possibilities given by technological progress extended to both manufacturing processes and functionalization techniques.

Keywords: Bioglass; borate glass; drug release; glass-ceramic; mesoporous bioactive glass; phosphate glass; scaffold; silicate glass; sol–gel; tissue engineering.

Figure 1

Compositional diagram for bone-bonding. Note regions A, B, C, D. Region S is a region of class A bioactivity where bioactive glasses (BGs) bond to bone and soft tissues and are gene activating. Reproduced with permission from [2].

Figure 2

Quantitative comparison of the percentage of bone growth into the bone defect from 1 to 24 weeks owing to 45S5 Bioglass, A–W glass-ceramics and sintered hydroxyapatite (HA) particles of 100–300 µm. Reproduced with permission from [40].

Figure 3

The cell cycle. Reproduced with permission from [2].

Figure 4

Schematization of the sol–gel process for the production of BGs.

Figure 5

Schematization of a tissue engineering (TE) approach starting from the biopsy and culminating in the implantation of the tissue substitute.

Figure 6

SEM micrograph of a HCA layer precipitated on the surface of a BG sample after 14 days of immersion in SBF. The HCA observed shows a typical cauliflower-like morphology. Reproduced with permission from [101].

Figure 7

Compressive strength–porosity curve for glass-ceramic scaffolds. The negative slope indicates that an increase in porosity percentage reduces mechanical compressive strength following a linear relationship. However, for very high values of porosity percentage (≈85–95%) the relation cannot be described by a linear curve and the mechanical performances of the scaffold become inconsistent. Reproduced with permission from [100].

Figure 8

Linear correlation between Young’s modulus and porosity percentage in glass-ceramic scaffolds and comparison with bone. Reproduced with permission from [100].

Figure 9

Fabrication process of a composite (PLAGA-BG) of PLAGA and BG. Composites were prepared both in thin film form and as 3D porous scaffolds. Reproduced with permission from [113].

Figure 10

Fractional weight loss (a) and pH values (b) versus reaction time for particles of the four glasses during immersion in 0.02 M K₂HPO₄ solution at 37 °C. Reproduced with permission from [128].

Figure 11

3D borate BG scaffolds with trabecular (a), oriented (b) and fibrous (c) microstructure. Reproduced with permission from [131].

Figure 12

SEM images showing the tubular structures formed from glass fibers after 18 months of degradation; (a) 3 and (b) 5 mol % Fe₂O₃ containing glass fibers respectively. Reproduced with permission from [44].