Recent advances and future perspectives of sol-gel derived porous bioactive glasses: a review

Abstract

The sol-gel derived porous bioactive glasses have drawn worldwide attention by virtue of the convenience and flexibility of this versatile synthesis method. In this review, the recent advances in sol-gel processed porous bioactive glasses in biomedical fields, especially for bone tissue regeneration applications have been comprehensively reviewed. Generally, it is envisaged that the morphology and chemical compositions of sol-gel derived porous bioactive glasses significantly affect their biological properties. Therefore, the controlled synthesis of these porous glasses is critical to their effective use in the biomedical fields. With this context, the first part of the review briefly describes the fundamentals of the sol-gel technique. In the subsequent section, different approaches frequently used for the sol-gel synthesis of porous glasses such as microemulsion and acid-catalyzed based synthesis have been reviewed. In the later part of the review, different types of sol-gel derived bioactive glasses namely silica, phosphate and silica-titania based glasses along with organic-inorganic hybrids materials have been discussed. The review also discusses the chemical, surface, mechanical and biological properties and further highlights the strategies to control the pore structure, shape, size and compositions of sol-gel derived bioactive glasses. Finally, the review provides a detailed discussion about the bone tissue regeneration application of different types of sol-gel derived bioactive glasses and presents future research perspectives.

Fig. 1. Mechanism of HMBGs formation. Reproduced with permission from ref. 76. Copyright 2017, Elsevier.

Fig. 2. (a and b) SEM and (c) TEM images of synthesized HMBGs. TEM images of HMBGs with different CTAB concentrations are shown in (d–f). Reproduced with permission from ref. 76. Copyright 2017, Elsevier.

Fig. 3. TEM images of synthesized HMBGs nanoparticles. Adapted from ref. 79. Copyright 2019, Wang, Pan and Chen.

Fig. 4. Mechanism of HMBGs microsphere formation. Reproduced with permission from ref. 81. Copyright 2016, Elsevier.

Fig. 5. Schematic illustration showing the fabrication process of HMBGs nanofiber based scaffold. Reproduced with permission from ref. 82. Copyright 2019, Elsevier.

Fig. 6. Photographs of (a) dried silica gel and (b) silica glass obtained by sintering at 1150 °C for one hour. Reproduced with permission from ref. 92. Copyright 2012, Elsevier.

Fig. 7. Schematic representation of Si tetrahedral sites of silicate glasses.

Fig. 8. Schematic representation of glass network formers and modifiers.

Fig. 9. Schematic representation of PO₄³⁻ tetrahedral sites of phosphate glasses.

Fig. 10. Releasing of various biotherapeutic ions from CaP glasses and their respective roles in the bone tissue regeneration. Reproduced with permission from ref. 191. Copyright 2018, Elsevier.

Fig. 11. SEM micrographs of dried gels prepared using (a and b) TIP–Acac and (c and d) TBOT. The figure inset shows the photograph of the synthesized Si–Ti based monolith. Adapted from ref. 213. Copyright 2011, IOP Publishing Ltd.

Fig. 12. SEM micrographs of dried gels prepared using (a and b) TiCl₄ and (c and d) Ti(SO₄)₂. Adapted from ref. 213. Copyright 2011, IOP Publishing Ltd.

Fig. 13. SEM micrographs of fractured surface of Ti–Si based samples prepared with (a) impregnation (b) co-gelation, (c) two step hydrolysis with 0.6 g PEG and 11 g water. Adapted from ref. 205. Copyright 2012, Springer Nature. (d) SEM micrograph of Si–Ti sol–gel glasses prepared via similar method using PEG as phase separating agent and HNO₃ as catalyst.

Fig. 14. Photographs of sol–gel derived (a) Si (b) Si–Ti based aerogel monoliths. Adapted from ref. 229. Copyright 2016, IEEE.

Fig. 15. Schematic representation of polymer–silica Class II hybrids. Adapted from ref. 235. Copyright 2011, the Royal Society of Chemistry.

Fig. 16. Molecular assembly and different levels of pores created to design scaffolds for bone tissue repair. Adapted from ref. 1. Copyright 2016, Elsevier.

Fig. 17. Various parameters controlling the loading capacity and the release rate of biomolecules in mesoporous materials. Adapted from ref. 1. Copyright 2016, Elsevier.

Fig. 18. (a) SEM micrographs of coated scaffolds with three immersions in fresh sol–gel solution. (b) Coated scaffolds with two immersions during 8 h of condensation. (c) Coated scaffolds after 2 days of cultivation. Reproduced with permission from ref. 281. Copyright 2019, Elsevier. (d–f) Micrographs showing surface structure of (d) PAA (e) CaO–SiO₂/PAA and (f) CaO–SiO₂–Ag₂O/PAA. Reproduced with permission from ref. 282. Copyright 2016, Elsevier. (g–i) SEM micrographs of gelatin/nanosilver/bioactive glass scaffolds with different concentrations (g) BGA0% (h) BGA20% and (i) BGA40%. Adapted from ref. 283. Copyright 2014, American Scientific Publishers.

Fig. 19. (a) Photograph showing 70S30C cotton wool like bioactive glass fibers. (b) Demonstration of ease of packing in a tooth extraction socket. (c) Insertion of implant after bone regeneration. (d) Crown placement. Adapted from ref. 290. Copyright 2014, Elsevier.

Fig. 20. (a) Procedure for preparation of APTES and MA functionalized bioactive glass. (b and c) Preparation of APTES and MA functionalized bioactive glass with cysteamine and 5-aminofluorescein conjugate. (d–f) Models for cell binding and protein absorption. Adapted from ref. 320. Copyright 2017, Springer.

Fig. 21. Schematic outline of biological response to bioactive glass based ionic dissolution products.

Fig. 22. Demonstration of the apatite layer formation on the surface of traditional sol–gel glass (left) and MBG (right) after contacting the biological fluids. Adapted from ref. 381. Copyright 2012, CRC Press.

Fig. 23. Different stages of surface reactions and the development of Si-rich and CaP layer at the bioactive glass surface. (I) Initially, a glass layer with few micron thickness is dissolved. (II) Later, Na, Ca, P and Si are leached out from the glass surface. (III) Si-rich layer is developed via repolymerization. (IV) Ca and P from the solution are partly leached out from the glass precipitate on the Si-rich layer. Reproduced with permission from ref. 382. Copyright 2018, Elsevier.

Fig. 24. A schematic representation of the fabrication process of PLAGA–bioactive glass composite scaffolds. Reproduced with permission from ref. 445. Copyright 2003, John Wiley & Sons.

Fig. 25. Schematic presentation of ordered MBGs for bone tissue regeneration. Reproduced with permission from ref. 382. Copyright 2018, Elsevier.

Fig. 26. Schematic representation of sol–gel foaming process. Adapted from ref. 465. Copyright 2012, the Royal Society of Chemistry.

Fig. 27. (a) Photograph of bioactive glass foam scaffold and (b) X-ray micro CT image. Reproduced with permission from ref. 467. Copyright 2015, John Wiley & Sons.

Fig. 28. (a) Representation of synthesized sol–gel glass foam. (b) SEM microstructure. Adapted from ref. 473. Copyright 2005, Springer Nature.

Fig. 29. μCT images of (a) 3D porous scaffold prepared from sol–gel derived bioactive silicate glass and (b) human trabecular bone. Reproduced with permission from ref. 474. Copyright 2007, Elsevier.