2.5 Manufacturing Techniques for the Production of BGs
2.5.2 Sol-Gel Technique
The first sol–gel bioactive glasses were developed in the early 1990s by Li et al. [58] and Pereira et al. [59]. Their composition was called 58S (60 mol% SiO2, 36 mol% CaO, 4 mol% P2O5) and was very similar to the melt-derived compositions developed previously [10].
The sol–gel technique is a chemistry-based synthesis method in which the polymerization reaction of a solution including oxides precursors carries out the gelation of the sol at room temperature [60].
Beforeexplaining the characteristic reaction steps, it is necessary to provide some definitions:
• a sol is a colloidal suspension of solid particles with a diameter in a range of 1–100 nm in a liquid, where the colloids move randomly guided by momentum imparted by collisions with molecules of the suspending medium. This movement is called Brownian motion [13].
• A gel is a rigid network of covalently bonded silica formed of interconnected pores and polymeric chains [13],[54]. Gels can be classified into three groups: alcogels, xerogels and aerogels [61].
Alcogels are characterized by the presence of an alcohol-based pore-liquid, while xerogels are created from thermal remove of pore liquid. Finally, aerogels are low-density gels (80 kg/m3) with large pore volumes (up to 98%), which are produced by removing pore liquid from the rigid network without collapsing it [13], [54].
According to the classification given by Flory [62], gels can be categorized into four different classes:
(i) well-ordered lamellar structures, (ii) disordered covalent polymeric networks, (iii) disordered and physically aggregated polymeric network and (iv) particular disordered structures [62].
There are three different routes necessary to obtain sol-gel monoliths: (i) gelation of solution of colloidal powders; (ii) reactions of hydrolysis and poly-condensation of alkoxide or nitrate precursors, followed by hypercritical drying of the gel; (iii) reaction of hydrolysis and polycondensation of alkoxide precursors to which follows the aging and drying under ambient atmosphere [61].
In order to produce biomedical sol-gel glasses, the third route is usually adopted, which consist of seven steps [63]:
1. Mixing the reagents at room temperatures and formation of strong covalent bonds between the elements [60]. Hydrolysis and poly-condensation reactions are competitive during this step and happen at the same time, until the solution is completely homogeneous under mild reaction conditions [63].
2. Casting the sol into moulds of different geometries to obtain the final product of the desired shape.
Actually, this step is not indispensable if the bowl used for the mixing possess adequate characteristics in terms of shape and material [61].
3. Gelation, which leads to the formation of a 3D network, and consequently an increase in viscosity.
During this step, it is possible to draw the gel into fibres by operating a strict control on the rheological properties of the material [61].
4. Syneresis/aging of a gel in which the continuous process of polycondensation and re-precipitation of the gel network causes the decrease in porosity, hence the increase in strength. Moreover, this step also includes the phase transformation. The aging process can be accelerated by hydrothermal treatment. The aging process largely influences the physical properties such as pore volume, gel surface and density and thus the resulting glass structure [1], [54].
5. Drying, by removing the liquid in the pores of the 3D network [61]. As concern the fabrication of monoliths, the main problem is the cracking and the shrinkage which take place during this stage, most often causing the fracture of the material [64].
6. Dehydration or chemical stabilization, which entails the removal of silanol bonds from the pore network, allowing to obtain a chemically stable solid [61].
7. Densification of the gel through a high-temperature thermal treatment that removes the pores [61].
The schematization of the sol-gel process is shown in Figure 5.
Figure 5: Schematization of sol-gel glass synthesis. Figure adapted from Fiume et al. [56].
2.5.2.1 Hydrolysis and Polycondensation
Silica gels are commonly formed by hydrolysing alkoxide precursors by means of catalysts action, which could be either an acid or a base [65].
The most frequently used agents to form the network are tetra-functional monomeric alkoxides with general chemical formula Si(OR)n, where R is the alkyl group, such as ethyl group (C2H5) in tetraethyl orthosilicate (TEOS, Si(OC2H5)4), and methyl group CH3 in tetramethoxysilane (TMOS, Si(OCH3)4) [61].
The chemical reaction below describes the production of TEOS, which starts from anhydrous ethanol and tetra-chlorosilane, and produces HCl as a by-product [65].
SiCl4 + 4EtOH ® Si(OEt)4 + 4HCl (4) Subsequently, chemical reactions which describe the processes of hydrolysis and poly-condensation involved in sol-gel synthesis are given:
1. Hydrolysis involves nucleophilic attack during which the OH group substitutes the OR group according to the reaction:
≡ Si – OR + H2O ⟶
⟶⇄ ≡ Si – OH + ROH (5) Hydrolysis
2. Condensation causes the formation of siloxane bonds and water or alcohols, according to the reactions shown below:
≡ Si – OR + OH – Si ≡
⟶
⟶⇄
≡ Si – O – Si ≡ + ROH (6)
≡ Si – OH + OH – Si ≡
⟶
⟶⇄
≡ Si – O – Si ≡ + H2O (7)
where R represents the alkyl functional group in the form: CxH2x+1 [65].
Sol-gel process tends to be particularly sensitive to the following parameters:
• Catalysts: catalysts, both acidic and basic, lower the activation energy causing a faster evolution of hydrolysis and poly-condensation reactions [65].
For the hydrolysis reaction, mineral acids and ammonia are the most used catalysts [65].
As concern the condensation reaction, the use of catalysts is beneficial, but not indispensable. In this case, the most used catalysts are mineral acids, ammonia, alkali metal hydroxides, and fluoride anions [65].
The type of catalysts, for both hydrolysis and condensation reaction, plays a crucial role in setting the gelling time of the system [66], as shown in Table 3.
Table 3: Gelation times and solution pH for TEOS systems employing different catalysts. Table reproduced from Pope et al. [66].
Catalysts TEOS
(mol/L)
Initial pH of the solution
Gelation time (h)
HF 0.05 1.90 12
HCL 0.05 0.05 92
HNO3 0.05 0.05 100
H2SO4 0.05 0.05 106
HOAc 0.05 3.70 72
NH4OH 0.05 9.95 107
None - 5.00 1000
Condensation with alcohol
elimination
Alcolysis
Condensation with water elimination
Hydrolysis
• pH: particularly, low pH values induce the protonation of the leaving groups favouring the hydrolysis reaction, while high pH values cause the deprotonation of the -OH groups favouring the condensation reaction [60].
The isoelectric point of silica (pH=2.5) defines the acidic and basic conditions, in fact if the pH is
< 2.5, the reaction advances by acid catalysis, while if the pH is > 2.5, the conditions of basic catalysis are ensured [60].
Under acidic conditions, mesopores are organized in well-ordered hexagonal structures because hydrolysis occurs faster than condensation. On the contrary, under basic or neutral conditions, a gel-like structure is formed without any mesopores [60].
• Solvent: during the primary stages of reactions, solvents are commonly used to increase the solubility of TEOS in aqueous solutions as well as to control H2O and silicates concentrations, which define the kinetics of the gelling process. However, it is often preferable to avoid the addition of mutual solvents, because the alcohols deriving from TEOS hydrolysis have the same ability by taking part into the esterification and alcoholysis reaction (reverse reaction) [65].
Four kinds of solvents can be adopted in sol-gel synthesis: polar, nonpolar, protic, and aprotic [65].
The quantity of the solvent concentration affects the gelation kinetics [61].
• H2O:Si ratio: the H2O:Si ratio (r) can vary from 1 to 25 [65].
Higher values of r-ratio support the reaching of a complete hydrolysis before achieving advanced phases of the condensation reaction. Anyway, high values of r-ratio combine with a constant solvent:silicate ratio, leading to a decrease in the concentration of silicate, which in turn reduces the silicates’ concentration, causing an increasing in gelling time [65].
• Temperature: the chemical kinetics of the several reactions involved in the formation of nanoparticles and the aggregation of them in a gel network is accelerated with temperature, which influences the gelation time. At very low temperatures, gelation slowly occurs, over weeks or months. Contra, at high temperatures, the reactions that bind the nanoparticles to the gel network happens so fast that lumps form. The gelation temperature must be controlled to optimize the reaction time [67].
2.5.2.2 Physical, Chemical and Biological Properties of Sol-Gel Derived Materials Mesoporous Structure
Sol-gel materials are characterized by porous structure, which is due to the synthesis process [68].
According to IUPAC, based on their pore size, porous materials can be classified into three classes:
1. Microporous materials, with pores size < 2 nm [68].
2. Mesoporous materials, with pores size between 2 and 50 nm [68].
3. Macroporous material, with pores size > 50 nm [68].
The mesopores average size of silicate materials obtained by “conventional” sol-gel method is typically 10-40 nm [69].
In order to obtain smaller mesopores, it is sufficient to use a surfactant as a mesopore template [70].
This approach allows controlling the mesopore size, which is essential when the material is designed as a drug delivery system [71], and is usually applied for the synthesis of the so-called mesoporous bioactive glasses (MBGs) [72].
For the porosity characterization, several aspects must be considered, such as shape of the pores, size distribution and interconnectivity [63].
The control of porosity is fundamental for the manufacture of substitutes of living tissues, since porosity features affect the entity of the interactions with cells and the surrounding environment, and the drug release kinetics in drug delivery systems [63].
Chemical Properties
Chemical composition affects biological activities of BGs in physiological environment [73].
In order to have suitable bioactivity, glasses should have:
• Si-OH (silanol) or R-OH groups on the surface to form nucleation sites for HA/ HCA [73].
• Ca2+ and phosphate ions near the material surface to allow HCA crystallization, even though this might be not necessary due to the presence of both Ca and P in body fluids [73].
Due to high specific surface area, sol-gel glasses expose a high density of silanol groups which allow bonding of functional groups on the surface of the glass [60].
Functional groups are generally grafted using two different routes, called stepwise synthesis and one-pot synthesis [60]. These groups enhance adsorption of biomolecules, binding affinity, biocompatibility and load capability for non-polar drugs in controlled and targeted drug delivery devices [63].
Mechanical Properties
Due to their inherent porous structure, sol-gel materials are characterized by poor mechanical performances [60]. This problem is more evident when sol-gel materials are used to develop scaffolds with multiscale porosity [74].
It has been proven that the change of some processing parameters affects mechanical properties of the materials. For example, higher amount of water causes an accelerated hydrolysis reaction, resulting in lower density values [75]. Murtagh et al. [75] showed that the elastic, shear, and bulk moduli of the silica gel decrease as the water content increases from 4:1 to 24:1 [76].
The shrinkage, occurring during the drying process, develops internal stresses, which can cause the catastrophic fracture of sol-gel monoliths. The shrinkage is deeply related to the pH of the sol and lower shrinkages were detected for higher values of solution pH [76].
The precursors utilized during synthesis can also influence the mechanical behaviour of the final material. Therefore, the sintering process could be appropriately optimized in order to improve mechanical properties by diminishing pore size [77]. Nonetheless, there are two types of limitations:
first, sintering at high temperature could cause disappearance of the mesopores because of densification, which affects bioactivity and the release of ionic dissolution products, as well as makes the final product unsuitable for drug delivery applications; secondly, if biomolecules are embedded into the material before sintering, the temperature must be kept below a certain threshold, in order to safeguard biological molecules from degradation and prevent undesired crystallization process [60].
Biological Properties
Jones et al. [78] have observed that, since the dissolution of the glass occurs as of the material surface, a higher surface area is generally responsible for faster dissolution rates [78].
In addition, current studies have proved the beneficial effect of high surface area and pore volume in accelerating the apatite-forming ability of sol-gel BGs. On the other hand, the significant pH changes produced by the fast dissolution kinetics of the glass may be a problem for biomedical applications because it can lead to toxicity [63].
Summary and Differences between Melt-derived and Sol-gel derived BGs The most interesting characteristics of sol-gel glasses are shown below:
1. Intrinsic nanoporosity, which is reflected in a higher surface area, which in turn leads to enhanced cellular response as well as higher solubility and reactivity in aqueous environments [79].
2. Basic composition, because addition oxides are not necessary to facilitate the processing of the material [80].
3. Possibility to obtain nano-porous powders, monoliths and nanoparticles by changing pH conditions [81].
4. Possibility to control the pore size distribution by adjusting precursors and pH conditions. Usually pores between 2 and 30 nm in diameters (mesoporous range) are obtained [63].
5. Possibility to adapt the oxide composition by adding appropriate precursors during the sol’s mixing [18].
The main difference between sol-gel and melt-derived bioactive glasses is that sol-gel glasses are intrinsically nano-porous, whereas melt-derived glasses are dense [18]. This means that the specific surface area of the sol-gel glasses is two orders of magnitude higher than for similar compositions of melt-derived glass, which results in greater dissolution rate, and hence higher cellular response [18], [13].
From a chemical point of view, sol-gel glasses with composition containing up to 90 mol% SiO2 can still be bioactive [82]. This can be explained considering the reduction in network connectivity because of the presence of hydroxyl groups along with the inherent glass nanoporosity. On the contrary, melt-derived glasses require <60 wt% of SiO2 to maintain their bioactivity [58].
The composition of melt-derived glasses is more complex than that of sol-gel glasses as additional components are necessary to foster the processing of the material. For example, Na2O is used to lower the melting point in melt-derived glass, improving its workability; while, it is not required in sol-gel glasses, due to the absence of melting. However, sol-gel glasses with a composition similar to that of 45S5 were produced [18].
Contra, melt-derive glasses are more suitable to produce scaffolds for hard tissue engineering because they have higher mechanical performances than sol-gel glasses [13].