Bioactive Glasses for Bone Regeneration

 

Vedangi Arvind Kulkarni, Mayur Gulab Kharat, Shivani Parmeshwar Chavan,
Sarita Khushalrao Metangale, Pratiksha Purushottam Varhade

Department of Pharmaceutics, Satyajeet College of Pharmacy, Mehkar, Maharashtra, India.

 

ABSTRACT:

Bioactive glasses (BGs) are a new material for bone regeneration because they behave as a material that can bond with bone tissues to elicit a cell response. BGs were first developed by Hench in the 1960s and are silicate-based materials that have been extensively utilized for orthopedic and dental applications, due to their osteoconductive and osteoinductive properties. They achieve their bioactivity mainly through the release of ionic dissolution products (e.g., calcium, phosphate, and silicon) which precipitate a hydroxyapatite-like layer on the surfaces, supporting bone integration. These ionic dissolution products can also facilitate osteoblast formation and differentiation and, therefore, new bone formation. There have also been recent developments of BG compositions which increasingly include borate- or phosphate-based BGs that have increased degradation rates and/or improved biological interaction. Additionally, the development of nanostructured and mesoporous BGs increases bioactivity by increasing surface area or modifying ionic release. BGs are generally incorporated into polymers or ceramics and often demonstrate composites for desired mechanical properties that are more appropriate for load-bearing applications. Additional value of BGs has come from the development of therapeutic ions (e.g., strontium, copper, silver) that can be incorporated for antibacterial, angiogenic, or osteogenic action, interactions. However, the brittleness of the BGs, the degradation time of some formulations, or low mechanical strength may need further investigation for composite formulation, or 3D-printed scaffolds.

 

 

 

1. INTRODUCTION:

a) Background on problems of bone regeneration: Bone regeneration is a complex biological process that is finely influenced by investigation of cellular, molecular, and biomechanical factors¹. While bone possesses an inherent ability for self-repair, be it in critical areas such as trauma, cancers, infections, or congenital defects, there remain clinical issues with large bone defects².

 

Deficient regenerative potential for critical-sized defects, meaning the defect is too great for natural full healing with the person's healing process, represents an issue³. Poor vascularization may restrict transport of nutrients, oxygen, and ultimately delay or prevent adult bone replacement⁴. Moreover, the immune response and inflammation can function either positively or negatively as excessive perpetuation of inflammation may result in fibrous tissue rather than bone regeneration⁵. Biomaterials represent further complexity as the epoxy structures chosen for bone grafts or scaffolds have additive properties of biocompatibility, osteoinductiveness, and ability to integrate with the host tissue without adverse immune response^6. The import of biomaterials is a major contributor to the complexity of the overall process; although allografts and xenografts may be poorly integrated and have the possible risk/jurisdiction to reject immunologically, the "gold standard" for bone repair, autografts, have donor-site morbidity and availability concerns⁷. Further, to add dimensionality is the challenge for mechanical stability and load-bearing of weight–bearing bone⁸. Despite these challenges around regulatory approval, costs, and long-term efficacy, there are exciting tissue engineering methods and technologies that may allow us to engineer new tissues that possess bone strength and stability appropriate to vault and tension structures existing in adult⁹.

 

b) Limitations of traditional treatments (autografts, allografts, synthetic implants): The principal limitations in the regeneration of bone include inadequate vascularization, interactions with the immune system, biomechanical stability, and the limitations of currently-used grafting materials¹⁰.

1.     Vascularization and Oxygen Supply: One of the most significant barriers to bone regeneration is the establishment of a functional vascular supply¹¹. Bone healing relies on an adequate blood supply of oxygen, nutrition and signaling molecules, but critical-sized defects can be lacking vascularization, which may prevent any healing altogether, or postpone healing infinitely¹². Hypoxia due to the absence of blood supply can negatively impact osteoblast development and deposition of the extracellular matrix, resulting in diminished osteogenesis. Strategies to enhance vascularization, including cell-based therapies, angiogenic growth factors (e.g., VEGF, FGF), and bioengineered scaffolds with microvascular networks are being explored ¹³.

2.     Inflammation and Immune Response: Bone regeneration is supported by the immune system in two ways. Inflammatory processes of a mild magnitude are essential to triggering repair and healing. When inflammation becomes excessive or continuous, however, fibroplasia can occur, resulting in delayed healing ^14. The polarization of macrophages is an important factor in healing, given that M1 macrophages are pro-inflammatory immune cells and M2 macrophages are anti-inflammatory and assist with healing ¹⁵. When this process becomes dysregulated, poorly healed or non-union bone can result, particularly in unregulated pathways (e.g. osteoporosis or chronic infection) ^16.  Using immune-modulating biomaterials, or applying cytokines to modulate this response appropriately for ideal healing is currently being explored by researchers¹⁷.

3.     Mechanical Stability and Load Bearing: For successful bone regeneration, mechanical support that imitates the natural characteristics of bone is           necessary ¹⁸. Reduced mechanical stimulus can hinder bone regrowth and instead promote bone resorption. Conversely, excessive micromotion at the defect or fracture area can disrupt osteogenesis in load-bearing bone ¹⁹. Although existing fixation methods, such as plates, screws, and external fixators, provide stability, they must be designed to allow for progressive load transfer, as the bone grows into the defect ²⁰. Innovations in biodegradable implants and 3D-printed, patient-specific scaffolds are emerging to solve this problem²¹.

4. Limitations of Current Grafting Materials: While all options have drawbacks, bone grafting remains the gold standard treatment for critical-sized defects²².

§  An autograft sometimes known as a patient's own bone:- is a type of bone graft in which bone tissue is retrieved from one site of the patient's body, and subsequently transplanted to another site for the purpose of bone healing²³. Although they provide great osteogenic potential, donor site morbidity and availability limits their use²⁴.

§  An allograft donor bone: Is a grafted tissue or organ sourced from a donor of the same species, which has genetic differences from the recipient^25. Although they reduce the need for a second surgical site, these grafts may provide the risk of immunologic rejection and disease transmission²⁶.

§  Synthetic scaffolds ceramics, polymers, biocomposites: Are artificial structures which are biocompatible to support bone regeneration by providing a temporary scaffolding for bone growth²⁷. They are easily tunable but they usually do not have the true bone integration and bioactivity ²⁸. Newer biomaterials have been developed to improve integration and osteoinduction features with controlled drug release, nanoscale features, and bioactive coatings²⁹

 

Comparison of Bioactive Glasses and Other Bone Graft Materials:

 

5. Challenges in Translational Medicine: Despite advances in tissue engineering, the clinical translation of biomaterials into practice continues to be a challenge ³⁰. High costs, regulatory approvals, and limited improvements in patient outcomes limit the translation of new bone regeneration technologies³¹. Synthetic bone grafts will require long-term human studies to assess their longevity and safety before they can be offered as a substitute to more traditional methods³².

C)   Introduction to bioactive glasses and their discovery by Hench (1969): Bioactive glasses, a distinctive family of biocompatible materials, do interact with biological tissues to promote bone recovery and healing³³. Larry L. Hench discovered its use in 1969 when he worked with materials for military applications to promote bone healing³⁴. This work led to the development of 45S5 Bioglass, a silica-based glass composed of SiO₂, Na₂O, CaO, and P₂O₅, which showed the capability to bond directly to bone^35. Unlike inert biomaterials, bioactive glasses develop a hydroxyapatite (HA) layer when placed in contact with human fluids, this simulates natural bone mineralization allowing the bioactive glass to bond with host tissue making this discovery revolutionary³⁶. Bioactive glasses were quickly adopted as they demonstrated osteoconductive, osteoinductive, and antibacterial properties over decades for applications in tissue engineering, dental implants, and bone grafts ³⁷. Hench's pioneering work set the stage for bioactive and bioinspired bioactive materials and changed the field of biomaterials science and regenerative medicine³⁸.

D)   Objective of the review:

I.     Enhance Bone Regeneration: Function as an osteoconductive and osteoinductive material, facilitate new bone formation³⁹.

II.   Promote Bioactivity: Establish a hydroxyapatite (HA) layer with interaction with biological fluids to integrate with natural bone⁴⁰.

III.Improve Biocompatibility: Provide a material that maintains good contact with biological tissues without causing deleterious immunological reactions ⁴¹.

IV. Accelerate Healing:  Release calcium and phosphate to stimulate osteogenesis⁴².

V.   Support Vascularization:- Sustain release of calcium and phosphate to encourage osteogenesis ⁴³

VI. Reduce Infection Risk: Augment blood vessel generation to ensure nourishment and oxygen system for bone formation.Modulate pH and ion release to inhibit microbial colonization, thereby demonstrating bacteriostatic activity⁴⁴.

VII.   Provide Alternative to Bone Grafts: Provide a synthetic, infectious free option for allografts and autografts therefore minimizing donor site           morbidity⁴⁵.

VIII.          Enable Customization: The chemical composition, structure, and properties of the glass can be altered to enhance the bioactivity, mechanical strength, degradation rate, and ion release⁴⁶.

IX. Enhance Mechanical Stability: Support structure while modulating disease tissues progressively during fresh bone formation⁴⁷.

X.   Advance Tissue Engineering: Show up as an important material in regenerative medicine scaffolds like 3D-printed bone grafts⁴⁸.

 

2.     Composition and Properties of Bioactive Glasses:

·       Chemical composition (SiO₂, Na₂O, CaO, P₂O₅): Bioactive glasses are mainly composed of SiO₂, Na₂O₅, CaO, and P₂O₅, and their chemical composition greatly affects the bioactivity and properties below⁴⁹:

1.     Crucially important for bioactivity⁵⁰:^.

2.     Na₂O increases bioactivity and wettability⁵¹:

3.     Variations in the CaO/P₂O₅ molar ratio influence bioactivity^52.:

4.     Important for bioactivity and bonding with bone tissue⁵³:

 

●     Types of bioactive glasses (e.g., 45S5, S53P4): Bioactive glasses a group of biocompatible materials that are used extensively in bone regeneration and tissue engineering have garnered considerable attention because of their osteoconductive and antibacterial properties⁵⁴. The most studied forms of bioactive glasses are the 45S5 and S53P4⁵⁵. While 45S5 bioactive glass is composed of 45% SiO₂, 24.5% Na₂O₅, 24.5% CaO, and 6% P₂O₅⁵⁶, it is well-recognized for its superior osteogenic properties, including enhanced activation of BMP₂ gene expression, protein adsorption, and rapid pre-osteoblastic cell migration due to high calcium release⁵⁷. In contrast with S53P4, consisting of 53% SiO₂, 23% Na₂O₅, 20% CaO, and 4% P₂O₅⁵⁸, it is remarkably antibacterial, having activity against a variety of bacteria including MRSA and P. aeruginosa, which is better suited for bone defects at risk of infection^59,61, are also designed for specific biomedical applications, such as for soft tissue repair and load bearing implants, which further demonstrate the versatility of bioactive glass compositions toward clinical applications⁶².

 

●      S53P4: S53P4 bioactive glass is an appropriate substitute for bone grafting⁶³. Surface processes of BAG initiate this bond when in contact with extracellular fluids^64. S53P4 bioactive glass is often characterized as a well-established biomaterial for bone repair and management of infection⁶⁵. S53P4 bioactive glass contains 53% SiO₂, 23% Na₂O, 20% CaO, and 4% P₂O₅, imparting good bioactivity due to its ability to bind to bone tissue and release ions that promote osteogenesis and have antimicrobial effects^66,67. In addition, it has demonstrated immunomodulating influences, by altering macrophage metabolism, decreasing inflammation, and preventing excessive immune reactions⁶⁸. Specifically, it reduced inflammatory cytokine expression and maintained oxidative phosphorylation in macrophages challenged with inflammatory stimuli⁶⁹. Both of these effects articulated the systemic effects of the material, even in conditioned media^70,71. S53P4 bioactive glass presents a unique opportunity for future biological applications because of its ability to modulate immune responses and maintain excellent osteointegration⁷².

 

●    45S5: Composed of 45% SiO₂, 24.5% Na₂O₅, 24.5% CaO, and 6% P₂O₅, 45S5 bioactive glass, also referred to as Bioglass®, is a frequently utilized biomaterial^73. The primary function of the material is to trigger hydroxyapatite formation upon exposure to bodily fluids, and as such, to provide bond to bone, which is highly effective in bone regeneration and tissue engineering^74. It stimulates cellular activity and promotes the precipitation of calcium phosphate, thus aiding in osteogenesis^75,76. In addition, the mechanisms for calcium phosphate mineralization and dissolution are still being actively studied, with new research suggesting that these processes may involve some deviations from existing models of dissolution behavior^77. Its uses also include antimicrobial applications, as, especially in orthopaedic and dental use applications, its dissolution tends to significantly alter pH and inhibit bacterial adhesion^78,79. Expect innovations on the 45S5 formula itself in material science and more generalized applications of 45S5, i.e., for soft tissue repair and wound healing^80.

 

●      Composition of Common Bioactive Glasses

 

3.     Mechanism of Bioactivity and Hydroxyapatite Formation of Bioactive Glasses: - Bioactive glasses are acknowledged for their ability to form a hydroxyapatite (HA) layer when in contact with physiological fluids, a fundamental part of their interaction with bone tissue^81. The glass bioactivity is influenced by processing methods, glass composition, and structure^82. Each of the sections that will follow explains some of the mechanisms of bioactivity and HA formation drawing upon knowledge from pertinent studies⁸³.

i.      Ion Exchange and Silica Gel Formation: Ion exchange between the glass and physiological fluids allows bioactivity to initiate⁸⁴. Sodium ions (Na⁺) from glass exchange positions with hydrogen ions (H⁺) resulting in a gradual increase in pH near the glass interface⁸⁵. Ion exchange is key because it activates the dissolution of the glass and creates a silica gel coating⁸⁶. Calcium (Ca2 )⁸⁷ and phosphate (PO₄¹⁻) ions present in bodily fluids will find a local position on the silica gel layer^88. Formation of the gel layer is followed by the nucleation of amorphous calcium phosphate (ACP), which then transforms to crystalline H⁸⁹.

ii.    Deposition of Calcium and Phosphate Ions: Once the silica gel layer develops, calcium and phosphate ions from the solution crystallize onto the glass surface⁹⁰. The elevated pH near the surface promotes the nucleation of ACP, thereby supporting deposition⁹¹. Like the mineral phase of regular bone, the ACP phase is metastable and undergoes rapid transformations into a calcium-deficient, nanocrystalline apatitic structure⁹².

iii.  Crystallization into Hydroxyapatite: ACP converts to HA through ion organization into the crystalline        state⁹³. The glass's composition has a particular role in this transformation involving phosphorous (P) and calcium (Ca) ions⁹⁴. Studies have indicated that adding phosphorous enhances increases in monophosphate within the glass network and therefore accelerates HA's crystallization⁹⁵. Moreover, the microstructure of the glass is an even greater enhancement of crystallization. For example, tailored pore architectures of mesoporous bioactive glasses (MBGs) enhance ion release and allow for greater surface area for nucleating HA and thus increases the production of apatites⁹⁶.

iv.   Role of Glass Composition: The bioactivity of bioactive glasses stems largely from their composition⁹⁷. Intermediate oxides (e.g., P₂O₅), network formers (e.g., SiO₂), and network modifiers (e.g., CaO, Na₂O) are all crucial components⁹⁸. The ratios of these components dictate both the glass's HA forming ability and its rate of degradation⁹⁹.

Silicon (Si): Silicon ions that originate from the breakdown of glass are essential for HA nucleation and will also help to form the silica gel layer¹⁰⁰.

Calcium (Ca): The ions serve two roles: their impact on the bioactivity of the glass and the way they assist with building the HA-layer¹⁰¹.

Phosphorus (P): Phosphorus accelerates HA formation by promoting the formation of monophosphate groups, which assist with the crystallization process in the glass¹⁰².

v.     Influence of Processing Methods: The production method and glass processing method can have a significant effect on the bioactivity of glass. For example ¹⁰³:

i.      Sol-Gel Synthesis: The use of this method enables the creation of mesoporous materials that enhance ion exchange and the synthesis of HA¹⁰⁴. The addition of calcium nitrate during the making if glass is thought to promote nanocrystalline HA domains in glass networks, improving bioactivity¹⁰⁵. Sol-gel synthesis, a hydrolysis and polycondensation method commonly used to produce metal oxides, ceramics, and hybrid materials at fairly low temperatures, is an option¹⁰⁶. Metal alkoxides or inorganic salts are typically used to prepare the gel, which involves hydrolyzing and polycondensing metal precursors to form a colloidal sol, which then undergoes gelation to form the gel network¹⁰⁷. This technique is a green alternative to standard high-temperature ceramic processing because it can produce uniform, pure materials with controlled porosity and has a lower environmental impact¹⁰⁸. Recent progress in sol-gel processing is reflected in the innovation of colchicine amine organogels and in the production of mesoporous nanostructures such as TiB₂ nanoparticles with tunable stability and functionality¹⁰⁹. This research is particularly relevant to the biomedical space, where sol-gels are applied to bioactive glasses used in bone repair¹¹⁰.  The use of sol-gels as synthetic approaches to high-performing organic-inorganic hybrid materials also serves to benefit from catalysts, optical properties, and drug delivery specialty applications including quantum dots. The development of sol-gel-based materials with controlled chemical compositions and nanostructures continue to expand their utility in high-tech industries and applications¹¹¹.

ii.    Ball Milling: Mechanical methods, particularly ball milling, can modify the microstructure of the glass, creating imperfections and enhancing surface area¹¹². While excessive milling may diminish mechanical properties, it may concurrently enhance the bioactivity due to new HA formation. Ball milling is one of the key mechanical processes utilized to improve and polish bioactive glass powders for biomedical applications¹¹³. It has been demonstrated that this high-energy processing method reduces particle size and promotes a change in the structure of bioactive glasses, which increases homogeneity, surface area, and bioactivity¹¹⁴. The milling process generates fine bioactive glass particles that are appropriate for applications in bone regeneration, drug delivery, and coatings for orthopedic implants—prerequisites for bone regeneration within the human body, and when using High-energy ball milling, bioactive glass dissolving rates are improved, thereby facilitating ion exchange and HAp formation in physiological conditions¹¹⁵. More recently, studies have demonstrated that ball milling was able to modify calcium phosphate-based bioactive glasses in an efficient manner while improving both mechanical properties and bioactivity¹¹⁶. As well, this approach can also be used to produce nanoscale powders from biological materials, such as fish bones that have very similar HA composition to naturally occurring HAp. Furthermore, ball milling is also under consideration for producing bioactive compounds in a solvent-free mechanochemical manner to promote environmentally friendly processing¹¹⁷.

 

vi.   Glass-ceramics: Their function: The bioactivity of glass-ceramics, which contain glassy and crystalline phases, is improved compared to their parent glasses. In addition, the crystalline phases—for example, fluorapatite—accelerate apatite formation through nucleation¹¹⁸. Additionally, the presence of nano-sized channels and a higher surface area in glass-ceramics support apatite formation through ion release. Bioactive glass-ceramic materials utilize bioactive glasses that can be crystallized in a controlled manner to form a composite material with crystalline phases incorporated in an amorphous framework¹¹⁹. This composite material provides enhanced mechanical properties while maintaining bioactivity needed for interaction with bone. Bioactive glass-ceramics are often used in biomedical applications in which upon exposure to physiological fluids, the material will develop a layer that resembles hydroxyapatite, helping to promote bone attachment¹²⁰. Studies indicate that glass-ceramics' functional properties—including optical or luminescent properties and structural stability—can be enhanced through the judicious addition of certain elements, particularly lanthanum and europium¹²¹.

·       Role of ion release (Si, Ca, P) in osteogenesis and angiogenesis: The release of important ions such as silicon (Si), calcium (Ca), and phosphorus (P) from bioactive glasses promotes osteogenesis and angiogenesis and is therefore critical for bone regeneration¹²². In physiological conditions, bioactive glass degrades to supply those ions, which participate in biological pathways to promote bone regeneration. For example, it has been shown that silicon, in the form of silicate ions (SiO₄⁻), promotes angiogenesis and osteoblasts proliferation and differentiation through the advancement of vascular endothelial growth factor (VEGF), a key mediator of angiogenesis¹²³. Calcium ions released from bioactive glasses play an important role in cellular signaling pathways that regulate osteoblast function, process bone mineralization and respond in an angiogenic capacity. Calcium is also involved in resorption process that involves osteoclasts, thereby serving to maintain a balance during the bone remodeling process¹²⁴. Often released as phosphate ions (PO₄³⁻), phosphorus plays a structural role in the formation of hydroxyapatite (HAp), the mineral component of bone. HAp precipitation due to local supersaturation of phosphate ions within the body can also promote cell adhesion and osteogenic differentiation¹²⁵. In addition, studies indicate that phosphate ions may indirectly enhance angiogenesis by modifying the bioavailability of calcium and other signaling molecules involved in vascularization. Such ions have often been studied in bioactive glass scaffolds; the synergistic ion release associated with the dissolution of bioactive glasses can create an optimal environment for bone tissue engineering¹²⁶. Specifically, 3D-printed scaffolds for significant bone defect repair containing magnesium-doped bioactive glasses and releasing Si, Ca, and P ions have exhibited better osteogenic and angiogenic behaviors¹²⁷.

 

 

Ion Release VS Time

 

4.     Synthesis Methods of Bioactive Glasses:

A.   Melt-quenching method (traditional method, advantages, limitations):

The melt-quenching method is a common approach in fabrication of amorphous solids - especially glassy materials. This method takes advantage of a homogeneous molten liquid in a controlled high-temperature furnace; the first step is to heat the raw materials or precursors above their melting point to make a molten liquid¹²⁸. The next step is to cool the melt or "quench" it quickly, which is important in forming an amorphous or glassy state by avoiding crystallisation. The rate of cooling is critical since it strongly influences the final properties of the material, and faster quenching results in greater degrees of disorder¹²⁹. Melt-quenching is now a widely used method to fabricate many types of glasses including oxide glasses, chalcogenide glasses, and metallic glasses, and they find applications in optics, electronics, and the biomedical sciences. Furthermore, melt-quenching is especially advantageous to control properties of these materials, such as transparency, thermal stability, or electrical conductivity, because it is able to produce bulk samples of glass, with controllable glass compositions¹³⁰.

 

a.     Advantages of the Melt-Quenching Method: One of the most important benefits of the melt-quenching technique is that it generally produces homogeneous glasses of well-defined compositions. The high-temperature melting process ensures the complete mixing of elements as it occurs, resulting in uniformity of both physical and chemical properties¹³¹. Melt-quenching is also an industrially relevant, scalable and an inexpensive option for the production of bioactive glasses or coatings for metallic implants. The thermal stability of melt-quenched bioactive glasses makes them suitable for applications at high-temperatures, in addition these glasses are able to promote bioactivity due to their amorphous structure which facilitates the release of ions in physiological conditions to facilitate hydroxyapatite formation for bone bonding¹³².

 

 

i.      High Purity and Homogeneity :- The method of melting at a high temperature ensures that raw materials are exhaustively mixed and achieves chemically homogeneous glass compositions, with uniform properties resulting from homogeneity and purity¹³³.

ii.    Scalability and Industrial Feasibility:- It is very scalable and applicable for large-scale production of bioactive glasses and coatings for biomedical applications, such as orthopedic bone regeneration and dental implants¹³⁴.

iii.  Cost-Effectiveness :- Compared to other synthesis approaches, such as sol-gel processing, melt-quenching minimizes steps in the production process, which ultimately decreases production time and costs¹³⁵.

iv.   Thermal Stability :- The excellent thermal stability of melt-quenched glasses, allows for their use in high-temperature applications, such as coatings for metallic implants¹³⁶.

v.     Bioactivity under Control Ion Release: The amorphous nature of the melt-quenched bioactive glasses allows for the control of ion release in physiological environments to facilitate hydroxyapatite formation and promote bone bonding¹³⁷.

 

b.    Limitations of the Melt-Quenching Method:- The melt-quenching process has a few limitations even though it is the most common method for making bioactive glass¹³⁸

i.      High Energy Consumption:- it involves the melting of raw materials at high temperatures (1300–1500°C)¹³⁹.

ii.    Limited Control Over Porosity:- the method is costly in terms of energy consumption, unlike low-temperature methods such as sol-gel synthesis. Melt-quenched glasses are typically dense, and thus they have lower porosity compared to glasses made using the sol-gel process, making them less suitable for applications that require designed porous structures and large surface areas to enhance cell infiltration¹⁴⁰.

iii.  Crystallization Risks:. The rapid cooling process can sometimes cause unwanted crystallization, which would diminish bioactivity and alter the dissolution behavior of bioactive glasses ultimately impacting their ability to interconnect with bone¹⁴¹.

iv.   Residual Thermal Stresses :- The rapid quenching process may introduce internal thermal stresses, especially in thicker glass pieces, which could lead to mechanical failure or cracking¹⁴².

v.     Composition Limitations :- Not every composition can undergo melt-quenching because some components may evaporate at high temperatures, thus changing the final properties of the material¹⁴³

 

B.    Sol-gel method (enhanced bioactivity, nanoporosity): The sol-gel process is frequently used to make bioactive glasses and ceramics, with benefits of superior bioactivity and nanoporosity compared with typical melt-quenching¹⁴⁴. The sol-gel process begins with the hydrolysis and condensation of metal alkoxides or inorganic salts, generating a colloidal sol that subsequently gels and dries, forming a highly porous glass network¹⁴⁵.

a.     Enhanced Bioactivity: The sol-gel process is frequently used to synthesize bioactive glasses and ceramics because it results in a greater extent of bioactivity compared to more common methods such as melt-quenching. The very reactive, porous glass network formed is attributable to the hydrolysis and condensation of metal alkoxides or inorganic salts from solution¹⁴⁶. Sol-gel-derived materials have a greater surface reactivity with more ion release and increased biocompatibility, all of which would benefit methods of bone regeneration and tissue engineering¹⁴⁷.

 

Mechanism of Enhanced Bioactivity:

i.      Higher Surface Area and Porosity: The increased surface area and nanoporous structure of bioactive glasses made through sol-gel processes help achieve greater interactions with physiological fluids. This results in greater ion exchange rates, leading to the formation of hydroxyapatite (HAp)—a required component for bonding to bone¹⁴⁸.

ii.    Accelerated Ion Release: In contrast, the sol-gel technique allows for controlled incorporation of bioactive ions such as calcium (Ca²⁺), phosphorous (PO₄¹⁻), and silicon (Si⁴⁺) in the structure of bioactive glasses¹⁴⁹. These ions are essential to osteogenesis—the process of bone formation! Additionally, these ions can also promote biological responses that promote bone regeneration¹⁵⁰.

iii.  Improved Protein and Cell Adhesion: Silanol (Si-OH) groups at the bioactive glass surface, produced by sol-gel processes, offer superior protein adsorption, which improves osteoblast (bone cell) adhesion and proliferation. This has resulted in improved tissue integration and cellular behavior¹⁵¹.

iv.   Ability to Incorporate Therapeutic Elements :-The sol-gel method also allows for bioactive dopants such as magnesium (Mg), strontium (Sr), copper (Cu), and silver (Ag) to be incorporated, further enhancing angiogenic (blood vessel development), antimicrobial, and osteogenic properties of the materials¹⁵².

v.     Controlled Degradation and Resorption: The sol-gel derived bioactive glasses can thus be considered more appropriate materials for biodegradable scaffolds in tissue engineering applications than dense melt-quenched glasses, as sol-gel derived bioactive glasses degrade at controlled rates¹⁵³.

 

b.    Nanoporosity and Its advantages: A significant advantage of the sol-gel method is the formation of nanoporous structures which assist in forming large surface area and regulated degradation rates. Spores of nanoporosity enhance bioactivity via elevated ion release while maintaining mechanical integrity¹⁵⁴. As a result, sol-gel-derived glasses are ideal candidates for bone tissue engineering and drug delivery applications and the porous network¹⁵⁵. Additionally, designing sol-gel materials with engineered porosity creates bioactive multifunctional scaffolds with different degradation rates, thereby allowing for individualized resorption profiles in regenerative medicine¹⁵⁶.

·        Limitations Sol-Gels Method: Regardless of the advantages of forming highly bioactive and nanoporous materials, the sol-gel procedure has several limitations that relate to its scalability and mechanical properties ¹⁵⁷:

i.      Long Processing Time: Extended Time to Process: The sol-gel process may take days to weeks and consists of hydrolysis, condensation, gelation, aging, and drying. This is much slower than the melt-quenching method that produces glasses in a matter of hours ¹⁵⁸.

ii.    Shrinkage and Cracking: Shrinkage Upon Drying: Sol-gel materials shrink considerably during drying because solvents are removed, resulting in possible cracks or fractures. This could affect the structural integrity of both coatings and bulk        materials ¹⁵⁹.

iii.  Mechanical Fragility: Porosity: Sol-gel materials are generally quite porous, which is a good trait to have in relation to bioactivity, but it also leads to lower mechanical properties compared to dense glasses or ceramics made by melt-quenching. Less mechanical properties further limit their applicability in load-bearing applications¹⁶⁰.

iv.   Complexity in Large-Scale Production: Challenges in Up-Scaling: While the sol-gel process is ideal for laboratory scale work, the need for controlled humidity, temperature and long processing times complicate the up-scaling process for potential industrial         applications¹⁶¹.

v.     Higher Production Costs: Higher Cost of Production: In comparison to conventional glass forming techniques, the need for organic solvents, high purity precursors and specialized drying conditions raise the cost of production^162.

vi.   Limited Control Over Composition Stability: Some species (e.g. phosphorus, or volatile dopant species) may be lost due to thermal processing, affecting the material's final composition and bioactivity¹⁶³.

 

 

Figure 1. Schematic of (a) melt quench and (b) sol–gel glasses

C.   Other techniques (flame-spray, laser-assisted processing): In addition to standard methods such as melt-quenching and sol-gel synthesis, different processes such as flame-spray and laser-assisted processing have emerged for the processing of bioactive glasses and coatings^164. These methods are attractive for biomedical applications due to the rapid synthesis, high purity, and enhanced material properties¹⁶⁵.

a.     Flame-Spray Processing: Flame-spray pyrolysis (FSP) is an advanced aerosol-based manufacturing strategies to create bioactive glass nanoparticles with controlled bioactivity, high surface area, and controlled composition ^166. Actually, usually synthesizing from metal alkoxides or salts dissolved in volatile solvents, in the process, the precursor solution gets atomized into a flame at high-temperature¹⁶⁷. This process generates ultrafine bioactive glass particles with controlled properties due to rapid evaporation and combustion ¹⁶⁸.

·        Advantages of FSP in Bioactive Glasses

i.      Rapid and Scalable Production¹⁶⁹:^.

ii.    High Purity and Controlled Composition¹⁷⁰:^.

iii.  Enhanced Bioactivity and Surface Area¹⁷²:

 

7. Challenges and Future Perspectives:

Bioactive glasses (BGs) have emerged as promising candidates for bone regeneration because they can bond with bone and elicit biological responses. Nonetheless, the clinical application of BGs faces several hurdles. For example, one limitation is the brittleness and relatively low mechanical strength of BGs that limit the possibilities for load-bearing applications. Bioactive BG composites can be created and combined with polymeric, ceramic, and metal filament-based additives to improve toughness, structural integrity, and bioactivity. Another prominent challenge associated with the use of BGs is the challenge associated with controlling the degradation rates of these materials. The dissolution of BGs must be precisely modulated to correspond with the rate at which new bone is produced; this rate can vary significantly between patients. Composition-based adjustments and surface functionalization will continue to advance towards improved patient-specific degradation profiles, as well as the delivery of nutrients or the release of medication for long-term function. The contributions of nanotechnology add additional avenues for the progression of BG properties and applications. Improvements in ion release, bioactivity, and mechanical performance have been documented to take place when using formulations or biomaterials that integrate nanoscale modifications or bioactive nanoparticles. There are other considerations to adde such as better osteogenesis, and material for improved resistance to microbial contamination. In addition, additive manufacturing and 3D printing represent the potential for increased time efficiency when fabricating customized implants or larger constructs with complex architectures, improved porosity, and patient-specific modifications with respect to design.  Each of these avenues provides opportunities for improved osseointegration and mechanical performance. Moving forward, there will need to be advancements towards hybrid biomaterials, a further development of smart bioactive glasses, and advancement of individualized regenerative medicine with biomaterials in mind to improve outcomes and demonstrate an expanding role for BGs in orthopedics and dentistry.

 

8.    CONCLUSION:

Bioactive glasses (BGs) show enormous promise with respect to bone regeneration, scaffold development, and drug delivery systems with terms such as osteoconductive, osteoinductive, and even antimicrobial attributes. BGs have exhibited a role in both successful and clinical applications in orthopedic surgery, maxillofacial surgery, and periodontics; their involvement has spanned many clinical applications in regenerative medicine. BGs are expected to play a foundational role in personalized medicine in the future as scaffolds are 3D-printed based on the patient and bioactive coatings are added to implants potentially requested by the patient. Moreover, the possibilities of BGS with other advanced biomaterials and stem cell therapies is another exciting area yet to be fully exploited. Factors hindering the clinical implementation of BGs include poor strength characteristics, long-term biocompatibility, and scalability of production and processing. Future preclinical and clinical studies will help improve formulations and optimize applications for more complex tissue engineering approaches and precision healthcare. All forms of BG can potentially be integrated into the next generation of regenerative therapies that will change the way in which bone and soft tissue defects are treated as innovations continue.

 

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Received on 27.05.2025      Revised on 12.09.2025 Accepted on 18.11.2025      Published on 22.01.2026 Available online from January 29, 2026 Asian J. Pharm. Res. 2026; 16(1):61-73. DOI: 10.52711/2231-5691.2026.00008 ©Asian Pharma Press All Right Reserved
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