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Glass-ceramics: Types, Technology and Application

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Published: Tue, 20 Feb 2018


1. Introduction

1.1 Glass-ceramics

Glass-ceramics are fine-grained polycrystalline materials formed when glasses of suitable compositions are heat treated and thus undergo controlled crystallisation to the lower energy, crystalline state. It must be emphasised here that only specific glass compositions are suitable precursors for glass-ceramics due to the fact that some glasses are too stable and difficult to crystallise whereas others result in undesirable microstructures by crystallising too readily in an uncontrollable manner. In addition, it must also be accentuated that in order for a suitable product to be attained, the heat-treatment is critical for the process and a range of generic heat treatment procedures are used which are meticulously developed and modified for a specific glass composition.

A glass-ceramic is formed by the heat treatment of glass which results in crystallisation. Crystallisation of glasses is attributed to thermodynamic drives for reducing the Gibbs’ free energy, and the Amorphous Phase Separation (APS) which favours the crystallisation process by forming a nucleated phase easier than it would in the original glass. When a glass is melted, the liquid formed from the melting might spontaneously separate into two very viscous liquids or phases. By cooling the melt to a temperature below the glass transformation region it will result in the glass being phase separated and this is called liquid-liquid immiscibility. This occurs when both the phases are liquid. Hence a glass can simply be considered as a liquid which undergoes a demixing process when it cools. The immiscibility is either stable or metastable depending on whether the phase seperation occurs above or below the liquidus temperature respectively. The metastable immiscibility is much more inmportant and has two processes which then cause phase seperation and hence crystallisation; nucleation and crystal growth and spinodal decomposition.

The first APS process has two distinguished stages; Nucleation (whereby the crystals will grow to a detectable size on the nucleus) and Crystal growth. Nucleation can either be homogeneous; where the crystals form spontaneously within the melt or heterogeneous; crystals form at a pre-existing surface such as that due to an impurity, crucible wall etc. Many a time the parent glass composition is specifically chosen to contain species which enhance internal nucleation which in the majority of cases is required. Such species also called nucleating agents can include metallic agents such as Ag, Pt and Pd or non-metallic agents such as TiO­2, P2O5 and fluorides. The second process is spinodal decomposition which involves a gradual change in composition of the two phases until they reach the immiscibility boundary. As both the processes for APS are different, the glass formed will clearly result in having different morphology to each other.

A glass-ceramic is usually not fully crystalline; with the microstructure being 50-95 volume % crystalline with the remainder being residual glass. When the glass undergoes heat treatment, one or more crystalline phases may form. Both the compositions of the crystalline and residual glass are different to the parent glass. In order for glass-ceramics having desirable properties to be developed, it is crucial to control the crystallisation process so that an even distribution of crystals can be formed. This is done by controlling the nucleation and crystal growth rate. The nucleation rate and crystal growth rate is a function of temperature and are accurately measured experimentally (Stookey 1959; McMillan 1979, Holand & Beall 2002)

The aim of the crystallisation process is to convert the glass into glass-ceramic which have properties superior to the parent glass. The glass-ceramic formed depends on efficient internal nucleation from controlled crystallisation which allows the development of fine, randomly oriented grains without voids, microcracks, or other porosity. This results in the glass-ceramic being much stronger, harder and more chemically stable than the parent glass.

Glass-ceramics are characterised in terms of composition and microstructure as their properties depend on both of these. The ability of a glass to be formed as well as its degree of workability depends on the bulk composition which also determines the grouping of crystalline phases which consecutively govern the general physical and chemical characteristics, e.g. hardness, density, acid resistance, etc. As mentioned earlier, nucleating agents are used in order for internal nucleation to occur so that the glass-ceramic produced has desirable properties. Microstructure is the key to most mechanical and optical properties; it can promote or diminish the role of the key crystals in the glass-ceramic. The desirable properties obtained from glass-ceramics are crucial in order for them to have applications in the field of biomaterials.

Glass-ceramics are used as biomaterials in two different fields: First, they are used as highly durable materials in restorative dentistry and second, they are applied as bioactive materials for the replacement of hard tissue. Dental restorative materials are materials which restore the natural tooth structure (both in shape and function), exhibit durability in the oral environment, exhibit high strength and are wear resistance. In order for dental restorative materials to restore the natural tooth structure, it is crucial to maintain the vitality of the tooth. . However non-vital teeth may also be treated with restorative materials to reconstruct or preserve the aesthetic and functional properties of the tooth.

In order for glass-ceramics to be used for dental applications, they must possess high chemical durability, mechanical strength and toughness and should exhibit properties which mimic the natural tooth microstructure in order for it to be successful as an aesthetic. Glass-ceramics allow all these properties to be united within one material. As mentioned previously, for a glass-ceramic to have the desired properties, the glass is converted into a glass-ceramic via controlled crystallisation to achieve the crystal phase wanted and hence the desired properties it could possibly have. Hence, the glass-ceramic developed allows it to have properties such as low porosity, increased strength, durability, toughness etc which are crucial in the field of dental restorations as it prevents restorative failures which are mainly due to stress and porosity which causes cracks and hence failures.

It took many years of research in order to get a material strong enough to be initially used as a dental reconstructive material. However over the past 10-15 years, research has progressed vastly and now glass-ceramics demonstrate good strength, high durability and good aesthetics. The development and processing of glass-ceramics has been focused on particular clinical applications, such as dental inlays, crowns, veneers, bridges and dental posts with abutments.

Glass-ceramics are divided into seven types of materials:

  1. Mica glass-ceramics
  2. Mica apatite glass-ceramics
  3. Leucite glass-ceramics
  4. Leucite apatite glass-ceramics
  5. Lithium Disilicate glass-ceramics
  6. Apatite containing glass-ceramics
  7. ZrO2-containing glass-ceramics

The first commercially usable glass ceramic products for restorative dentistry were composites of mica glass ceramics. Dicor® and Dicor® MGC were products based on these. According to the mechanism of controlled volume crystallisation of glasses, tetrasilicic micas, Mg2.5Si4O10F2, showing crystal sizes of 1 to 2 μm in the glass ceramic were produced. Dicor® being amongst them was shaped by means of centrifugal casting methods to produce dental restorations such as dental crowns and inlays. Depending on the different crystal sizes and the corresponding microstructure of the glass ceramic, it was also possible to manufacture glass ceramics for machining applications. [53], Dicor® MGC being amongst them. This resulted in the characteristic of good machinability in this type of glass-ceramic to be exploited and results concluded that crystals upto only 2 μm in length in the material improved mechanical strength over other materials.

Mica-apatite glass-ceramics have been produced in the SiO2-Al2O3-Na2O-K2O-MgO-CaO-P2O5-F system. The main crystal phases are phlogopite, (K,Na)Mg3(AlSi3O10)F2 and fluorapatite, Ca5(PO4)3F. The base glass consists of three glass phases: a large droplet-shaped phosphate-rich phase, a small droplet-shaped silicate and a silicate glass matrix. Mica is formed during heat treatment, as in apatite-free glass-ceramics, by in-situ crystallization via the mechanism of volume crystallization. Apatite is formed within the phosphate-rich droplet phase. Astonishingly, every single apatite crystal possesses its own nucleation site in the form of a single phosphate drop. The glass-ceramic is biocompatible and suitable for applications in head and neck surgery as well as in the field of orthopaedics.

Leucite glass-ceramics can be formed by applying the advantage of the viscous flow mechanism. IPS Empress® is of this type of glass-ceramic. The material is processed by using the lost wax technique, whereby a wax pattern of the dental restoration such as an inlay, onlay, veneer or crown is produced and then put in a refractory die material. Then the wax is burnt out to create space to be filled by the glass-ceramic. As the glass-ceramic has a certain volume of glass phase, the principle of viscous flow can be applied and hence the material can be pressed into a mould. Surface crystallisation and surface nucleation mechanisms were controlled in order for this type of glass-ceramic to be formed. [42, 54] Consequently, the manufacturing of inlays and crowns developed due to the application of viscous flow mechanism of glass-ceramics in different shapes. The resulting leucite glass-ceramic restorations transluceny, colour and wear resistance behaviour can then be adjusted to those of natural tooth.[55] Additionally, the leucite glass-ceramic restorations can be produced by machining with CAD/CAM. IPS ProCAD® and IPS Empress® CAD are glass ceramics produced via this method. All leucite glass-ceramic restorations are bonded to the tooth structure with a luting material, preferably an adhesive bonding system. The retentive pattern produced on the glass-ceramic surface is particularly advantageous in this respect.

It was possible to develop a leucite apatite glass-ceramic derived from the SiO2-Al2O3-Na2O-K2O-CaO-P2O5-F system by combining two different mechanisms, i.e. controlled surface nucleation and controlled bulk nucleation. IPS d.SIGN® is amongst these. The glass-ceramic was prepared according to the classic method of glass-ceramic formation: melting, casting to prepare a glass frit, controlled nucleation and crystallization. A two-fold reaction mechanism leads to the precipitation of fluoroapatite, Ca5(PO4)3F and leucite, KAlSi2O6 [42]. SEM pictures show the two-phase crystal content of apatite and leucite in this type of glass-ceramic. Fluoroapatite phase takes the form of needle-shaped crystals whereas the oval areas are the leucite crystals. The clinical application of this glass-ceramic has been proven to be suitable for clinical application as veneering material on metal frameworks for single units as well as for large dental bridges involving more than three units.

The first glass-ceramic to be developed was by Stookey et al (1959) which contained Lithium disilicate. [37]. Further research into this field allowed for IPS Empress®2 to be developed. This glass-ceramic was developed in order to extend the range of indications of glass-ceramics from inlay and crowns to three-unit bridges, by offering high strength, high fracture toughness and at the same time, a high degree of translucency. Both the flexural strength and fracture toughness of lithium disilicate glass-ceramics are almost three times of those of leucite glass-ceramics. Lithium disilicate glass-ceramic ingot are utilizied to produce the crown or bridge framework in combination with the viscous flow process. To further improve the aesthetic properties, i.e. translucency and shade match, and to optimally adjust the wear behaviour to that of the natural tooth, the lithium disilicate glass ceramic is veneered with an apatite-containing glass-ceramic using a sintering process.

In order to meet the demanding requirements of CAD/CAM applications, a lithium metasilicate glassceramic, IPS e.max®was developed. This material, which is supplied in a typically blue colour, is adjusted by thermal treatment in order to demonstrate a characteristic tooth colour.

The range of IPS e.max®products also encompasses various apatite-containing glass ceramics that are suitable for both layering material on lithium disilicate glass-ceramic and veneering material on ZrO2 sintered ceramic. The apatite crystal phase of the Ca5(PO4)3F type acts as a component that adjusts the optical properties of the restoration to natural tooth. For this reason, the crystallites are of nanoscale dimension.

ZrO2 containing glass-ceramics was the first glass-ceramic developed to be fused to high strength ZrO2 ceramic dental posts. The glass-ceramic contains Li12ZrSi6O15 crystals as the main phase; however different types of crystals are also precipitated in the glassy matrix. ZrO2 has become very interesting not only in the field of medicine but also in dental applications. High-strength and high toughness dental posts, crowns and bridges can be prepared from this material.

In order for a dental restorative material to be of clinical success, their most important properties include; high strength, high toughness, abrasion behaviour comparable to natural teeth, translucency, colour, durability) and the processing technologies (moulding, machining, sintering). [56] Furthermore, the material should have good marginal fit with the tooth, biocompatibility, good mechanical properties and low porosity. In addition to the aforementioned properties, the recent requirement for dental restorative materials is for its appearance to be similar to that of a natural tooth.

Glass-ceramics have been researched immensely in order to fulfil high standards of function and aesthetics from an early stage. The trend for metal free dental restorations began from the 1970’s whereby metal free feldspathic ceramics were reinforced with additional components. Since then, increasing the strength of these materials progressed rapidly by controlling the nucleation and crystallisation of glasses, as discussed earlier. These developments have now led to the introduction of a trend which is focused on achieving exceptional aesthetic results with glass ceramics as metal free dental restorations.

Although glass-ceramics exhibit the desired properties for dental restoration, their main drawback is that they are brittle which the main cause of failure is. This is due to either fabrication defects; which are created during production of the glass-ceramic or secondly, surface cracks; which are due to machining or grinding. Therefore when processing the glass-ceramic, care needs to be taken in addition to choosing the suitable method for production for specific compositions of the glass-ceramic in order to improve their mechanical properties.

Apart from the use of glass-ceramics for dental restorations, they can also be applied as bioactive materials for the replacement of hard tissue. Bone is a complex living tissue which has an elegant structure at a range of different hierarchical scales. It is basically a composite comprising collagen, calcium phosphate (being in the form of crystallised hydroxyapatite, HA or amorphous calcium phosphate, ACP) and water. Additionally, other organic materials, such as proteins, polysaccharides, and lipids are also present in small quantities. Because bone is susceptible to fracture; there has always been a need, since the earliest time, for the repair of damaged hard tissue.

Many years of research has attempted to use biomaterials to replace hard tissue, ranging from using bioinert materials, to bioactive materials such as ‘Bioglass’ (Hench et al) to ‘Apatite-wollastonite (A-W) glass-ceramics (Kokubo et al) and to calcium phosphate materials. Calcium phosphate based materials have received a great deal of attention in this field due to their similarity with the mineral phase of bone.

1.2 Calcium Phosphate Glasses

The application of calcium phosphate material as a bone substitute began by Albee (1920), who reported that a tricalcium phosphate compound used in a bony defect promoted osteogenesis. Many years later, Levitt et al (1969) [65] and Monroe et al (1971) were the first to suggest the use of calcium phosphate ceramics for dental and medical implant materials. Subsequently in 1971, Hench et al developed a calcium phosphate containing glass-ceramic, called Bioglass® and demonstrated that it chemically bonded with the host bone through a calcium phosphate rich layer. Furthermore the advantageous properties of calcium phosphate ceramics arose when Nery et al (1975) used a calcium phosphate ceramic for implants in surgically produced infrabony defects in dogs. This demonstrated that the calcium phosphate ceramic was nontoxic, biocompatible, and caused no significant haematological changes in the calcium and phosphorus levels. Since then, a great deal of research into calcium phosphate glass-ceramics has been conducted as potentially bone substitutes in dentistry.

Calcium phosphate based ceramics can be characterised accordingly;

  1. Hydroxyapatite (HA, Ca5(PO4)3OH)
  2. β-tricalcium phosphate (β-TCP, β-Ca3(PO4)2)
  3. Biphasic calcium phosphates, BCP; mixture of HA and β-TCP
  4. β-calcium pyrophosphate (β-CPP, β-Ca2P2O7)
  5. Fluorapatite (FAP, Ca5(PO4)3F)

Calcium phosphate based ceramics and their properties have been characterised according to the proportion of calcium to phosphorus ions in the structure. One of the most widely used synthetic calcium phosphate ceramics is hydroxyapatite, Ca5(PO4)3OH, HA and this is due to its chemical similarities to the inorganic component of hard tissues. HA, has a Ca:P molar ratio of 1.67. It has higher stability in aqueous media than other calcium phosphate ceramics.

Tricalcium phosphate (TCP) is a biodegradable bioceramic with the chemical formula, Ca3(PO4)2. TCP dissolves in physiological media and can be replaced by bone during implantation. TCP has four polymorphs, the most common ones being α and β-forms, of which β-TCP has received a lot of attention in the field of bone substitutes. Slight imbalances in the ratio of Ca:P can lead to the appearance of extraneous phases. If the Ca:P ratio is lower than 1.67, then alpha- or beta tricalcium phosphate may be present after processing. If the Ca:P is higher than 1.67, calcium oxide (CaO) may be present along with the HA phase. These extraneous phases may adversely affect the biological response to the implant in-vivo. A TCP with a Ca:P ratio of 1.5 is more rapidly resorbed than HA. Hence, β-TCP has been involved in recent developments aimed to improving its biological efficiency and its mechanical properties in order for it to be successful as bone substitutes.

Mixtures of HA and TCP, known as biphasic calcium phosphate (BCP), have also been investigated as bone substitutes and the higher the TCP content in BCP, the higher the dissolution rate.

The crystal structure of HA can accommodate substitutions by various other ions for the Ca2+, PO43− and OH− groups. The ionic substitutions can affect the lattice parameters, crystal morphology, crystallinity, solubility and thermal stability of HA. Anionic substitutions can either occur in the phosphate- or hydroxyl positions. Fluorapatite and chlorapatite are common examples of anionically substituted HA. They display a similar structure to HA, but the F− and Cl− ions substitute for OH−. A lot of research has gone into carbonate substituted HA and it has shown to have increased bioactivity compared to pure HA, which is attributed to the greater solubility of the carbonated substituted HA. Thus, recent work has been in progress in order to optimise the production and sintering behaviour of carbonated substituted HA in order for use in biomedical applications.

Materials which are bioactive i.e. the ability to bond to living tissue and enhance bone formation, have the following characteristic compositional features: (i) SiO2 contents smaller than 60 mol%, (ii) high Na2O and CaO content, and (iii) high CaO:P2O5 ratio [80]. Although silica based bioactive materials have shown great clinical success in many dental and orthopaedic applications, its insoluble properties has resulted in it as a potential for a long term device and the long term reaction to silica, both locally and systematically is still unknown. [81] Therefore, silica free, calcium phosphate glasses have attracted much interest due to their chemical and physical properties. They offer a more controlled rate of dissolution compared to silica containing glasses, they are simple, easy to produce, biodegradable, biocompatible, bioresorbable due to their ability to completely dissolve in an aqueous environment and have excellent bioactivity, osteoconductivity as well as not causing an inflammatory response. Due to their properties, especially due to it being bioresorbable, calcium phosphate glasses have been under investigation for several applications in the dental field, particular as implants. However only certain calcium phosphate compounds are suitable for implantation in the body, compounds with a Ca:P ratio less than 1 are not suitable for biological implantation due to their high solubility.

The structural unit of phosphate glasses is a PO4­ tetrahedron. The basic phosphate tetrahedra form long chains and rings that create the three-dimensional vitreous network. All oxygens in the glass structure are bridging oxygens (BO), and the non-bridging oxygens (NBO) can be formed by including other species such as CaO and Na2O or MgO. Do to the effects of Ca2+, Na2+­ and Mg2+ in the glass structure; they are defined as glass network modifiers, which form the glassy state and are called ‘invert glasses.’ Hence the structure of phosphate glasses can be described using the Qn terminology, where n represents the number of bridging oxygen’s that a PO4 tetrahedron has in a P2O5 glass, every tetrahedron can bond at three corners producing layers of oxygen polyhedra which are connected together with Van der Waals bonds. When the PO4 tetrahedron bonds with three bridging oxygens, giving the Q3 species, it is referred to as an ultraphosphate glass, which usually consists of a 2D network. When it bonds to two bridging oxygen’s, usually in a 3D-network it gives the Q2 species, it is referred to as metaphosphate glass. Further addition gives Q1 species, also called pyrophosphate glass, which bonds only to one bridging oxygen. Finally, the Q0 species do not bond to any bridging oxygen and hence is known as an orthophosphate glass. [14]

A large number of calcium phosphate glass compositions have been studied in order to exhibit suitable properties for use in biomedical applications until now, and they can be categorised into four groups:

  1. Calcium phosphate glasses containing Potassium
  2. Calcium phosphate glasses containing Magnesium
  3. Calcium phosphate glasses containing Sodium and Titania
  4. Calcium phosphate glasses containing Fluorine and Titania

1) Calcium phosphate glasses containing Potassium:

Dias et al (2003) [12] conducted a study and prepared bioresorbable calcium phosphate glass-ceramics between the metaphosphate and pyrophosphate region based on the composition 45CaO-45P2O5-5K­2­O-5MgO (Ca:P = 0.5). XRD results showed that addition of nucleating agents, K2O and MgO forms bioactive: β-CPP and biodegradable phases: KCa(PO3)3, Ca4P6O19 as well as β-Ca(PO3)2 which is considered to be non-toxic.[21] DTA results showed two crystallisation peaks, Tp at 627°C and 739°C and two melting temperatures, Tm at 773°C and 896°C which was thought to be due to the partial melting of the crystalline phases or residual glass matrix. The glass transition temperature, Tg was observed at 534°C. FTIR results showed functional groups corresponding to metaphosphate and pyrophosphate, (PO3)- and (P2O7)4-. These results are in accordance with functional groups of the crystalline phases identified by XRD: β-CPP, KCa(PO3)3, Ca4P6O19 and β-Ca(PO3)2. Results from degradation studies of these glass-ceramics confirmed that by controlling the overall composition of the O:P in the glass, glass ceramics with high degree of degradability can be obtained. The level of chemical degradation observed for these materials is well-above that reported in literature for bioactive ceramics that are clinically used, namely HA and TCP. It was therefore concluded that the incorporation of K2O in glass ceramics increases the solubility and also these calcium phosphate glass ceramics makes them potentially clinically helpful for promoting the regeneration of soft as well as hard connective tissue by allowing the degradability to be controlled.

A study by Knowles et al (2001) [92] investigated the solubility and the effect of K2O in the glass-system based on the general composition: K­2­O-Na2O-CaO-P2O5. The exchange of a mono or divalent ion with another of a similar charge was therefore investigated. The P2O5 and CaO content were fixed, at 45 mol% and the CaO content at 20, 24 or 28 mol% and the ratio of K2O to Na2O was varied from 0 to 25mol %. Results showed, firstly an increase in CaO content caused the solubility to decrease, as expected and confirmed from previous studies. [81,94] Secondly, for all CaO contents there was an increase in solubility, when K2O content was increased. [92] In a recent study by Marikani et al (2008), based on the same general composition, they demonstrated that the addition of K2O caused a decrease in both density (from 2.635 g cm-3 to 2.715 g cm -3 and microhardness measurements (from 257 to 335 HV) and hence weakens the structure. These findings are attributed to the replacement of lighter cation (Na2O) by a heavier one (K2O). The ionic radius of potassium is larger than the ionic radius of sodium so, the addition of K2O has a larger disrupting effect on the structure and hence weakens the glass-network. The decrease of melting point with the addition of K2O content indicates that K2O increases network disruption by producing non-bridging oxygens. And the low value of Tg indicates that the glass samples are thermally unstable. Additionally, the elastic modulus, decreases when the concentration of K2O is increased, which implies the weakening of the overall bonding strength, as more cross linking is degraded. The increase of the internal friction and the decrease of the thermal expansion coefficient with the addition of the K2O content are due to the formation of non-bridging oxygen ions. The SEM micrographs of the glass samples recorded before immersion in SBF indicates the amorphous nature of the materials and when glasses were immersed in SBF solutions for 10 days, the glass-samples showed bioactivity.

Although the addition of K2O to the ternary Na2O-CaO-P2O5 based system offers greater flexibility in terms of tailoring the solubility to suit potential biomedical applications, only little research has been conducted in using K2O in calcium phosphate glasses, probably because it has shown to increase network disruption which was confirmed by decrease in Tm, addition of K2O causes a decrease in density and microhardness measurements, it weakens overall bonding strength confirmed by a decrease in the elastic modulus, causing it to be less rigid as well as producing thermally unstable glasses which was confirmed by the low values of Tg. These mechanical properties are not desirable in the long run and due to it being less rigid, it would not withstand stress in biomedical applications and consequently result in failure.

2) Calcium phosphate glasses containing Magnesium:

Research into calcium phosphate glasses which produce biocompatible and bioactive phases has generated a lot of interest.

–An attempt to induce β-TCP was undertaken by Zhang et al (2000) on calcium phosphate glass-ceramics in the pyrophosphate region based on the composition 50CaO-40P2O5-7TiO2-1.5MgO-1.5Na2O (Ca:P molar ratio = 0.625). XRD results showed that the β-TCP phase was not detected and the main crystalline phase precipitated was β-CPP with smaller amounts of soluble Calcium titanophosphate, CaTi4(PO4)6 CTP, and Sodium titanophosphate, NaTi2(PO4)3. Kasuga et al (1998) reported a similar occurrence in the structure of glass-ceramics which contained TiO2 (wt 3 %). . SEM observations demonstrated light areas which were confirmed by EDS analysis to be β-CPP, grey areas was thought to correspond to Na- containing phases and dark areas were composed of lower CaO contents compared to the other two areas and contained MgO and Na2O. These results were identical to Kasuga et al’s study (1999). The undetectable β-TCP phase was possibly due to the low content of MgO and TiO2 added and the low Ca:P ratio of the glass. Although bioactive and biosoluble phases were precipitated in the glass-ceramic, no continuous apatite layer was formed even after 8 weeks of immersion in SBF solution.

–A study by Brauer et al (2007) observed the solubility of several phosphate glasses in the system P2O5-CaO-MgO-Na2O-TiO2. The glass compositions ranged from ultraphosphate glasses (with phosphate contents over 50 mol %) to polyphosphate glasses (containing 50 mol% P2O5 or less which are formed by phosphate chains or rings possessing different chain lengths) to invert glasses (pyrophosphate glasses- P2O5 concentrations of around 34 mol %.). Results showed that the phosphate glasses showed a uniform dissolution. No selective alkali leaching, which is known from silica based glasses, was observed. Also that the solubility of the glasses strongly depend on the glass-composition. The higher the phosphate content resulted in an increase in solubility; According to Vogel et al [104], this is due to the polymerisation of the phosphate chains and the Q1 end units being more susceptible to hydration and subsequent hydrolysis than Q2 middle groups. Also it was observed that the higher the concentration of Na2O resulted in an increase in solubility too due to the effect Na+ has on the glass structure. Addition of titanium oxide resulted in a decrease in both the solubility and the tendency of the glasses to crystallise by forming cross links between phosphate groups and titanium ions. Invert glasses showed a considerably smaller solubility than polyphosphate glasses and offer an alternative to polyphosphate glasses, since they are more stable to moisture attack. However, decreasing the P2O5 content makes glasses not only more stable to hydrolysis but also restricts the glass forming area. Hence, glasses in the pyrophosphate region show a larger tendency to crystallize than polyphosphate glasses [96]. However invert glasses in the system P2O5-CaO-MgO-Na2O showed that properties such as solubility and crystallization tendency can be controlled by adding small amounts of metal oxides [95]. Results of solubility experiments showed that the glass system investigated enabled adjustment of solubility with only minor chemical changes. This ability to control the solubility is very promising for medical application where the coordination of implant degradation and bone formation are a key issue.

A study by Dias et al (2005) studied the crystallisation of the glass-system: 37P2O5-45CaO-5MgO-13TiO2 (Ca:P=0.6)in the pyrophosphate and orthophosphate region, by using TiO2 as a nucleating agent and MgO as a network modifier. Results showed that they contained four different crystalline phases; two of them, β-CPP and CTP are reported to be biocompatible and bioactive, respectively [88,97,98]. T

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