Geopolymer Materials for Bone Tissue Applications

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This entry is adapted from the peer-reviewed paper 10.3390/polym15051087

Geopolymers (alkali-activated materials) are amorphous inorganic polymeric systems with aluminosilicate bases produced by alkalinising natural or waste substances, such as metallurgical, industrial, urban, and agricultural wastes. There is increasing academic interest in geopolymer materials for biomedical applications.

geopolymer bone tissue biocompatibility hydroxyapatite biomaterials

1. Geopolymer Materials: Synthesis and Applications

A class of aluminosilicate materials referred to as geopolymers are produced by an inorganic polycondensation reaction (also known as “geopolymerization”) between solid aluminosilicate precursors and alkaline solutions such as sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium silicate (Na₂SiO₃), or potassium silicate (K₂SiO₃) or highly concentrated aqueous alkali hydroxide.

Usually, active silicon and aluminium are abundant in the raw materials used to make geopolymers. Generally, they consist of metakaolin and/or the byproducts of various industrial processes, such as fly ash, bottom ash, red mud, biomass ash, steel slag, volcanic ash, waste glass, coal gangue, diatomite, bauxite, high-magnesium nickel slag, etc.

The most significant addition to the understanding and scientific study of geopolymer materials was made by Davidovits [73]. By reacting natural minerals containing silicon (Si) and aluminium (Al), such as slag, clay, fly ash, pozzolan, and an alkaline activator under mild conditions (below 160 °C), he created the first inorganic geopolymer material in the 1980s [74].

The geopolymerisation reaction process may be broken down into three main steps [8,75,76,77,78,79,80,81]:

1.: The dissolution of aluminosilicate materials in the concentrated alkali solution forms free silica and alumina tetrahedron units.
2.: The condensation process between alumina and silica hydroxyl results in an inorganic geopolymer gel phase. This process causes water to leave the structure.
3.: The developed three-dimensional silicoaluminate network hardens and condenses.

In aluminosilicate materials with a high degree of geopolymerisation, a high dissolution rate of Si⁴⁺ and Al³⁺ ions is found at high pH values (NaOH concentration > 10 M) [81,82].

The curing temperature is also crucial for the geopolymerisation process. The temperature accelerates the dissolution response of raw materials, and temperatures above room temperature (60–80 °C) are ideal for geopolymerisation [82].

Moreover, to create novel materials for cutting-edge technological applications, geopolymers can be functionalised, created as organic–inorganic hybrids, or combined with other materials to form composites [13,14,15,16,17,18,19,20,21,31,32,40,].

Geopolymer paste can add the organic phase in a liquid or solid form, such as powder, fibres, or particles [83,84,85,86,87]. Due to the chemical incompatibility between strongly polar aqueous and apolar organic phases, adding a second liquid to a geopolymer that is non-miscible with water is particularly challenging. In particular, the organic component can be added using several methods and at various phases of the production of the composite: (i) Direct method. The solid precursors first dissolve in the alkaline aqueous solution to create the paste slurry. Before the system hardens, the organic phase is immediately absorbed into the slurry while being vigorously mechanically mixed. (ii) Method of pre-emulsification. First, the organic component emulsifies while the activating solution still lacks solid precursors. The solid precursor is added to the stable emulsion of the organic phase in the aqueous activating solution to start the paste-hardening process. (iii) Process of solid impregnation. Before being added to the geopolymer slurry, the organic phase is impregnated on a solid powder (either the aluminosilicate precursor or a specific adsorbing powder) and added to the alkaline activating solution.

The two primary geopolymer applications are those with traditional physical and mechanical qualities and those for functional and advanced applications.

Geopolymers in the first category can find applications in building, construction, repair, restoration, marine construction, pavement base materials, 3D printing, high-temperature and fire-resistant materials, and thermal and acoustic insulation. Special applications include heavy-metal pollution immobilisation, pH regulator materials, catalysts, conductive materials for moisture sensor applications, and thermal storage [88,89,90,91,92,93,94].

Functional applications can be employed for buildings in specific industries, such as fire prevention structures, insulation walls, and nuclear power plants. These include fire prevention, isolation, heat preservation, and adsorption of hazardous ions [95,96,97,98,99,100,101,102].

2. Applications in the Field of Biomaterials

Due to their capacity to adhere to the bone structure, geopolymer materials have attracted more interest in recent years in the field of hard tissue regeneration [102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120].

To minimise toxicity towards the tissues, using geopolymers in biomaterials calls for some fundamental characteristics, such as an aluminium silicate source free of heavy metals and/or other contaminants and biocompatibility that depends on the pH levels and the amount of aluminium released [110,120].

A high-mechanical-strength geopolymer matrix was studied by Catauro et al. (compressive strength of 50 MPa) [103]. Geopolymer samples were immersed in a simulated bodily fluid (SBF) to assess their bioactivity, and the layering of hydroxyapatite (hydroxyapatite formation is considered a critical index for scaffold biocompatibility) on their surface was examined using SEM characterisations. However, only modest bioactivity was found.

Pangdaeng et al. [104] studied calcined kaolin–white Portland cement geopolymer as a possible biomaterial. In this instance, the mixture had a 28-day compressive strength of 59.0 MPa and contained 50% white Portland cement and 50% calcined kaolin. Still, only a small number of hydroxyapatite particles were formed on the surface of the geopolymer–Portland cement composite, indicating low bioactivity.

It is also important to note that high-temperature treatments have been shown to encourage the release of water molecules (the removal of absorbed water molecules, up to 100 °C, or differentially coupled, free water molecules inside the pores). When structural water and water bonded in nanopores are removed at high temperatures from the silicate matrix, the porosity of the final geopolymer material increases [105].

The porosity of the geopolymer matrix is an essential key factor in increasing the biocompatibility of aluminosilicate systems by promoting bone tissue growth. The microstructure of the porous geopolymer appears very similar to bone tissue.

The bone graft’s porous nature is crucial to biofactors’ distribution and tissue volume maintenance [107]. Because the microarchitecture of three-dimensional scaffolds regulates cell migration behaviour via junction interactions, the pores must have interconnected structures to allow for cell growth and migration. Finally, it is recognised that the size of the pores affects the vascularisation, infiltration, and cell attachment of the bone transplant [108].

As was already indicated, in addition to porosity, the pH value, which is well known to be relatively high in the aluminosilicate matrix, is a crucial factor in the biocompatibility of geopolymers. Thus, managing its high pH value is one of the critical factors needing careful attention for an alkaline geopolymer’s biological application.

Furthermore, a fundamental restriction on using geopolymers in biomaterials is the proportion of “free” aluminium, which may cause significant toxicity [109]. Lowering the quantity of aluminium used in the geopolymerisation reaction is an effective strategy to reduce toxicity.

As possible biomaterials, Oudanesse et al. [110] investigated amorphous geopolymers of the potassium-poly(sialate)-nanopolymer type with a mole ratio of Si:Al = 3:1. After, a heat treatment at 500 °C was carried out to lower the geopolymer matrix’s alkalinity from pH 11.5 to pH 7.1, excellent porosity for biological compatibility was attained.

The authors report that the high-temperature treatment made it possible to positively impact the mechanical strength and the geopolymer network, with the possible stabilisation of the free alkali present in the aluminosilicate matrix.

In this context, it is essential to note that porous geopolymers can be created at high temperatures or by utilising specific foaming agents, such as hydrogen peroxide, metallic Al or Si powders, or both [111,112]. Porous materials with pore sizes ranging from nanometres to a few millimetres and a total porosity of up to 90% [111,112] can be obtained without using high-temperature treatments (such as burnout of organics and sintering).

This is particularly true if foaming agents are included in the geopolymer paste before it is condensed. Due to its simplicity and low cost, the direct foaming approach in additive manufacturing has recently been tested to produce high-strength geopolymer foams [113,114,115,116,117,118].

Other foaming techniques have also been developed, including gel casting, saponification, and foaming agents that include different oils. These enable the creation of extended and cellular structures with pores ranging from the meso- to the macro-range for use in wastewater treatment, catalysis, and thermal and acoustic insulation, among other fields [15,95,102].

In this regard, Faza et al. [119] studied metakaolin geopolymer-based foams obtained as potential scaffolds for bone substitutes. The authors developed metakaolin-based porous geopolymers using aluminium powder as a foaming agent. Aluminium powder was added to a combination of metakaolin, sodium silicate, and sodium hydroxide in the following ratios: 1:1, 1:1,5, 1:2,5, and 1:3. The samples were hardened in an oven at 80 °C for four hours. SEM micrographs show a sample morphology like human spongy bone (with a size void of 80–400 µm).

By combining varying concentrations of hydrogen peroxide (H₂O₂) as a foaming agent from 0 to 6 vol% and heat treating it at 500 °C for 1 h, Sayed et al. [120] reported the synthesis of foamed geopolymer structures. The formulation with 4.5 vol% H₂O₂ was the best regarding the examined samples’ open porosity and mechanical characteristics.

Geopolymer samples showed an open porosity of 71 vol% and compressive strength of 3.56 MPa, which are suitable for 3D scaffolds used in biomaterial applications.

It is important to note that the foamed geopolymer’s internal microstructure and bone tissue’s morphology are highly comparable. Finally, after 28 days of immersion in simulated bodily fluid solutions, the pH level of the geopolymers remained near the physiological value. The geopolymer foams showed bioactivity in an in vitro investigation, as shown by apatite particles appearing on their surface after 28 days of immersion in a solution simulating bodily fluids.

Using two different preparation techniques, Catauro et al. [121] studied the application of geopolymers with the composition H₂₄AlK₇Si₃1O₇₉9 and a ratio of Si/Al = 31. In the first case, the alkaline activating solution was made using KOH in the form of pellets in a potassium silicate solution. In the second case, a solution of KOH 8M was added to the potassium silicate solution. Varied water contents were employed, and only some manufactured samples were heated.

The study demonstrated that the mechanical characteristics of geopolymer materials are influenced mainly by the changing experimental conditions (chemical composition and curing temperature). It was reported that using KOH 8 M to generate the alkaline activating solution and heating it to 65 °C was the best method to improve mechanical properties (i.e., compressive strength up to 1.95 MPa). According to the same authors, heat-curing eliminates the water generated during the condensation stage and enhances the geopolymerisation reactions. However, increasing the aqueous phase could lead to uncontrolled void nucleation during the geopolymerisation stage. The geopolymer bioactivity was evaluated by monitoring the apatite-forming ability on the aluminosilicate matrix surface after being soaked in SBF for 21 days.

The authors concluded that the hydroxyl-apataversaite [Ca₁₀(PO₄₎₆(OH)₂] formation was detected on the surfaces of all geopolymers after their immersion in an SBF solution for 21 days and thus, all samples with both alkaline activating solutions and treatments at room temperature and high temperature (hydroxyapatite crystal growth was found to be independent of the type of alkaline activating solution and heat treatment).

According to the literature [122,123], hydroxyl groups on the surfaces of silica glasses and ceramics encourage the formation and nucleation of hydroxyl-apatite. This process is enhanced when the materials include cations, such as Na⁺ or K⁺ ions. These systems can release cations via a cation-exchange mechanism with H₃O⁺ ions in SBF to form Al-OH and Si-OH groups on their surface; a rise in pH is observed during this reaction in the SBF solution, and this results in the dissociation of Al-OH and Si-OH groups into the negatively charged units Al-O and Si-O. These anions boost the positive charge on the surface when they connect with the Ca²⁺ ions already present in the fluid. The negative charge of the phosphate ions and the simultaneous binding of Ca²⁺ cations create an amorphous phosphate, which results in the synthesis of hydroxyl-apatite [Ca₁₀(PO₄₎₆(OH)₂].

An interesting study published by Pangdaeng et al. [124] focused on improving geopolymer bioactivity through CaCl₂.

Specifically, calcined kaolin (metakaolin), a sodium hydroxide (NaOH) solution, a sodium silicate solution (as an alkaline activating solution), and heat curing (60 °C for 24 h) were used to create the geopolymer material. The geopolymer samples were treated using the soaked-treatment approach, which accelerated apatite precipitation and slowed the rise in pH. A CaCl₂ solution was used as an ion-exchange agent. After preparation, the samples were treated in the CaCl₂ solution for 24 h at 23 °C.

The contact of the calcium chloride solution with the geopolymer increased the calcium ion absorption. It improved the chemical interaction between aluminosilicate components via a mechanism like that observed for cementitious materials [125,126].

The surface of the geopolymer is subjected to an increase in its hardness due to a cation-exchange mechanism between Ca²⁺ and Na⁺ ions. This phenomenon causes calcium precipitation on the surface because of the improvement in surface hardness.

The geopolymer material surface’s microstructure showed a dense hydroxyapatite structure with a thickness of around 15 µm.

In research on the effects of the curing time and temperature on geopolymer systems made from hydroxyapatite and calcined kaolin powders as raw materials, Sutthi et al. [127] presented their findings. In particular, the effects of curing hydroxyapatite and calcined kaolin at different temperatures (40 °C, 60 °C, and 80 °C) and for different times (2, 7, 14, 21, and 28 days) were examined.

A statistical analysis was conducted to determine the extent of each variable’s effect. The authors concluded that as the curing time and temperature increased, the compressive strength of the geopolymer materials also dramatically increased.

The best compressive strength measurement (37.8 MPa) was attained after 28 days of curing at 80 °C. The best experimental conditions that led to high compressive strength were also optimal for bioactivity and, thus, for applying geopolymer systems as bone substitute materials.

Tippayasam et al. [128] investigated the effect of calcium hydroxide addition to geopolymer material formulations. Moreover, their mechanical properties were investigated, and chemical–physical and bioactivity characterisations were carried out. The potential bioactivity of the aluminosilicate systems was studied after soaking them in SBF solution for 28 days.

Radhi et al. [129] produced a foamed geopolymer material and evaluated its potential application as a bone substitute. A metakaolin-based system employing various ratios of olive oil and hydrogen peroxide as foaming agents was prepared, resulting in different porosity percentages and sizes of geopolymers.

For in vivo testing on rabbits and biocompatibility analysis, the aluminosilicate material with the greatest porosity percentage and size range was chosen. In detail, geopolymers were implanted in femur bones (right femur as the positive control). Biopsies were carried out for histological analysis two and four weeks after implantation.

Histological tests revealed that the implanted geopolymer material allowed the development of bone trabeculae with minimal inflammation. The authors concluded that foamed geopolymer systems improved bone formation compared to commercial bone substitutes. Thus, geopolymers could be considered promising scaffolds for bone substitutes, thanks to their availability and cost-effective characteristics.

Mejíaet al. [130] prepared metakaolin- and CaCO₃-based geopolymer materials and investigated their physical, mechanical, and biological (in vitro) properties. After seven days of curing, the ceramic materials displayed a percentage porosity between 50 and 30 and compressive strength between 18 and 29 MPa. The geopolymer sample with the highest compressive strength underwent a reactivity test for 28 days while exposed to simulated bodily fluid (SBF). This enables the identification of components such as Ca and P.

A geopolymer material that can develop a calcium phosphate layer on its surface was used for biological and antibacterial testing. In this instance, a 25 MPa compressive strength value and a rise in the pH of the SBF solution were found.

According to the findings of biological experiments, the high pH of the cellular environment damages red blood cells by causing them to burst. The authors concluded that the chemical composition of the specified geopolymer materials needs to be improved to reduce the pH value and the phenomenon of alkaline components seeping into bodily fluids. Finally, antibacterial tests have revalued the inhibitory activity towards Pseudomonas aeruginosa, indicating potential applications of geopolymer materials in external environments.

To investigate a Ti6Al4V alloy’s possible use as a composite material for prosthetic devices, Rondinella et al. [131] reported the creation of a thin and homogenous geopolymer covering it. Different geopolymer formulations (acidic and alkaline activation) were tested to maximise adhesion between the geopolymer coating and Ti6Al4V alloy, and multilayered coatings were added using the dip-coating technique. Scratch tests determined how well the geopolymer adhered to the metal substrate.

After analysing its surface’s morphological and chemical features, a bacterial growth test confirmed the coating’s antibacterial properties.

According to the geopolymer coating’s microstructural, mechanical, and antibacterial characterisations, alkaline and acidic geopolymer coatings look structurally compact and seem to be suitable for biomedical applications.

Moreover, the fact that metakaolin was the most unreacted in the acid-activated geopolymer material indicates that these formulations have lower reactivity than alkaline ones. The acidic geopolymer coating was easily removed under light loads in scratch tests, while the alkaline formulation presented superior adhesion to the metal substrate. Both activation procedures’ coatings demonstrated comparable antibacterial properties, referring to the micro-organism’s growth.

3. Main Achieved Outcomes and Future Outlook for Geopolymer Materials

The analysis of literature data shows that significant research advancements in geopolymer materials such as bioscaffolds have been made in recent years.

The main results achieved are as follows:

1.: The starting aluminosilicate-based raw materials must be subject to careful characterisations to exclude the presence of toxic substances and harmful heavy metals.
2.: A critical issue associated with geopolymer materials for biomedical applications is their high pH values, which could severely limit their biocompatibility. Geopolymers are generally obtained from the reaction of aluminosilicates with alkaline activating solutions (such as NaOH or sodium silicate). A helpful strategy to apply in this case is a high-temperature treatment of the hardened geopolymer matrix, which significantly reduces the geopolymer pH from 11.5 to 7.1 (physiological value).
3.: The presence of “free” aluminium ions, which could cause significant toxicity, is a limiting factor for using geopolymers in bone tissue regeneration. A possible strategy is reducing the amount of aluminium involved in the reaction of geopolymerisation. This can be achieved with high-temperature treatments that involve the possible stabilisation of the free alkali present in the aluminosilicate matrix.
4.: Geopolymers for biomedical applications should present a high porosity. It has been shown that highly porous geopolymers with a microstructure similar to bone tissue have increased biocompatibility while promoting more bone tissue growth.
5.: A soaked-treatment method with a CaCl₂ solution as an ion-exchange agent (after preparation, the samples were treated by soaking them in a CaCl₂ solution for 24 h at 23 °C) enhances compressive strength and bioactivity by accelerating hydroxyapatite formation and slowing down the rise in pH.
6.: The growth and nucleation of hydroxyl-apatite groups promote the biocompatibility of geopolymer materials. This phenomenon is improved when the materials contain cations, e.g., Na⁺, K⁺, or Ca²⁺ ions. Thanks to a cation-exchange mechanism, it is possible to observe the formation of hydroxyl-apatite [Ca₁₀(PO₄₎₆(OH)₂], which is very important for improving the osteoconductive properties of geopolymer materials.

Future studies may aim to optimise formulations to achieve trabecular geopolymer structures with controlled porosity and high strength using an additive manufacturing process. This technique allows for obtaining several complex geometries and the high reproducibility of experimental conditions.

Durability studies of geopolymer scaffolds should also be conducted to understand the potential average lifetime of the geopolymer implant in the bone.

In addition, to improve the biocompatibility and osteoconductive properties of the geopolymer matrix, the research could be directed towards developing organic geopolymer-based composite materials with biopolymers.

The combination of a geopolymer matrix, which allows the obtaining of highly porous and mechanically stable systems, with biopolymers, which significantly increase biocompatibility and osteoconduction processes, could create the ideal material for bone tissue engineering.

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1. Geopolymer Materials: Synthesis and Applications

2. Applications in the Field of Biomaterials

3. Main Achieved Outcomes and Future Outlook for Geopolymer Materials

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