Glass-ceramics: Types, Technology and Application

Glass-ceramics Types, Technology and ApplicationCHAPTER 11. Introduction1.1 Glass-ceramicsGlass-ceramics argon fine-grained poly vitreous silicaline textiles make fored when scratches of able compositions argon foment treated and thus undergo controlled vitreous silicalizing to the lower energy, quartzline state. It jettydiness(prenominal) be emphasised here that only specific folderol compositions be adequate precursors for glass-ceramics collectible(p) to the fact that some eye glass ar as well stable and difficult to crystallise whereas other(a)s dissolver in undesirable micro buildings by crystallising too readily in an uncontrollable manner. In addition, it must excessively be accentuated that in order for a shellable product to be attained, the hot pants- intervention is critical for the process and a range of generic heat preaching procedures atomic number 18 used which be meticulously sireed and modified for a specific glass composition.A glass-c eramic is formed by the heat treatment of glass which results in crystallization. Crystallisation of glass is attributed to thermodynamic drives for reducing the Gibbs forego energy, and the Amorphous Phase Sepa dimensionn (APS) which favours the crystallisation process by forming a nucleated arrange easier than it would in the original glass. When a glass is melted, the melted formed from the melting might spontaneously separate into two in truth syrupy silvers or phases. By cooling the melt to a temperature on a lower floor the glass transformation bena it will result in the glass being phase separated and this is called liquid-liquid immiscibility. This emits when two the phases atomic number 18 liquid. Hence a glass hind end simply be considered as a liquid which undergoes a demixing process when it cools. The immiscibility is every stable or metastable depending on whether the phase seperation occurs above or below the liquidus temperature singly. The metastable i mmiscibility is oftentimes to a corkinger extent inmportant and has two processes which then cause phase seperation and hence crystallisation nucleation and crystal cometh and spinodal decomposition.The first APS process has two distinguished stages Nucleation (whereby the crystamyotrophic lateral sclerosis will grow to a detectable size on the nucleus) and Crystal process. Nucleation poop either be homogeneous where the crystals form spontaneously within the melt or heterogeneous crystals form at a pre-existing start such as that due to an impurity, crucible wall etc. Many a time the set up glass composition is specifically chosen to endure species which enhance internal nucleation which in the majority of cases is required. Such species as well called nucleating agents bathroom 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 gradatory change in composit ion of the two phases until they reach the immiscibility boundary. As both the processes for APS be unlike, the glass formed will clearly result in having different morphology to each other.A glass-ceramic is commonly non in full luculent with the microstructure being 50-95 book % pellucid with the re principal(prenominal)der being proportionality glass. When the glass undergoes heat treatment, one or more(prenominal) 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 true, it is crucial to control the crystallisation process so that an even distribution of crystals fire be formed. This is through by controlling the nucleation and crystal growth rate. The nucleation rate and crystal growth rate is a function of temperature and are accurately measured by experimentation (Stookey 1959 McMillan 1979, Holand Beall 2002)The aim of the crysta llisation process is to convert the glass into glass-ceramic which cod properties banner to the parent glass. The glass-ceramic formed depends on efficient internal nucleation from controlled crystallisation which allows the increase of fine, randomly oriented grains with divulge voids, microcracks, or other porosity. This results in the glass-ceramic being much(prenominal) stronger, harder and more chemically stable than the parent glass.Glass-ceramics are characterised in toll of composition and microstructure as their properties depend on both of these. The expertness of a glass to be formed as well as its horizontal surface of workability depends on the bulk composition which likewise de landmarkines 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-cerami c produced has desirable properties. Microstructure is the key to most automatonlike and optical properties it can progress 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 find industrys in the field of bio heartys.Glass-ceramics are used as biomaterials in two different fields First, they are used as mellowedly durable materials in keynote alveolar consonant consonant medicine and second, they are use as bioactive materials for the replacement of hard tissue. Dental renewing materials are materials which restore the indwelling tooth structure (both in shape and function), screening durability in the oral environment, showing 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 r econstruct or preserve the artistic and functional properties of the tooth.In order for glass-ceramics to be used for dental applications, they must possess high chemical durability, robotlike strength and mood 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 cod 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 perchance lose. Hence, the glass-ceramic developed allows it to have properties such as low porosity, increase strength, durability, irritation 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 a(prenominal) years of look 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 immensely and now glass-ceramics border legal strength, high durability and good aesthetics. The nurture and bear on 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 display cases of materialsmica glass-ceramicsMica apatite glass-ceramicsLeucite glass-ceramicsLeucite apatite glass-ceramicsLithium Disilicate glass-ceramicsApatite containing glass-ceramicsZrO2-containing glass-ceramicsThe first commercially usable glass ceramic products for restorative dentistry were compo turn ups of mica glass ceramics. Dicor and Dicor MGC were products found on these. gibe to the weapon of controlled volume crystallisation of render, tetrasilicic micas, Mg2.5Si4O10F2, showing cryst al sizes of 1 to 2 m in the glass ceramic were produced. Dicor being amongst them was shaped by means of centrifugal jettyding 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 m 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 frame. The main crystal phases are phlogopite, (K,Na)Mg3(AlSi3O10)F2and fluorapatite, Ca5(PO4)3F. The base glass consists of three glass phases a jumbo droplet-shaped inorganic orthophosphate-rich phase, a small droplet-shaped silicate and a silicate glass ma trix. Mica is formed during heat treatment, as in apatite- sin little 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 apply the lost wax technique, whereby a wax phase 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 hen ce the material can be touch 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 demeanour 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 dodging. 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-P2 O5-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 haste of fluoroapatite, Ca5(PO4)3F and leucite, KAlSi2O6 42. SEM pictures show the two-phase crystal study 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 Empress2 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(prenominal) time, a high layer of translucency. Both the flexural strength and fracture toughness of atomic number 3 disilicate glass-ceramics are almost three times of those of leucite glass-ceramics. Lithium disilicate glass-ceramic metal bar 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 get hold of the demanding requirements of CAD/CAM applications, a lithium metasilicate glassceramic, IPS e.maxwas developed. This material, which is supplied in a typically relent slight(prenominal) colour, is adjusted by thermal treatment in order to demonstrate a characteristic tooth colour.The range of IPS e.maxproducts 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-stren gth 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 bare(a) 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 standardised to that of a natural tooth.Glass-ceramics have been researched immensely in order to occupy high standards of function and aesthetics from an early stage. The trend for metal free dental restorations began from the 1970s whereby metal free feldspathic ceramics were reinforced with additional components. Since then, increase the stre ngth 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 proceeds 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 life sentence tissue which has an elegant structure at a range of different hierarchical scales. It is fundamentalally 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 get up is unvaccinated to fracture in that location 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 trustworthy a bulky deal of attention in this field due to their similarity with the mineral phase of operating system. 1.2 Calcium Phosphate GlassesThe application of calcium phosphate material as a hit the books military reserve began by Albee (1920), who account 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 health check engraft materials. Subsequently in 1971, Hench et al developed a calcium phosphate containing glass-ceramic, called Bioglass and show that it chemically bonded with the host osseous tissue 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 accordinglyHydroxyapatite (HA, Ca5(PO4)3OH)-tricalcium phosphate (-TCP, -Ca3(PO4)2)Biphasic calcium phosphates, BCP mixture of HA and -TCP-calcium pyrophosphate (-CPP, -Ca2P2O7)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. atomic number 53 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 CaP breakwaterar ratio of 1.67. It has high stability in sedimentary media than other calcium phosphate ceramics.Tricalcium phosphate (TCP) is a biodegradable bioceramic with the chemical formula, Ca3(PO4)2. TCP dissolves in physiological m edia and can be replaced by bone during implantation. TCP has quatern polymorphs, the most common ones being and -forms, of which -TCP has received a lot of attention in the field of bone substitutes. Slight imbalances in the ratio of CaP can lead to the appearance of orthogonal phases. If the CaP ratio is lower than 1.67, then alpha- or important tricalcium phosphate may be present after processing. If the CaP 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 CaP ratio of 1.5 is more rapidly resorbed than HA. Hence, -TCP has been involved in recent developments aimed to improving its biological efficacy 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 subject area 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 noggin substitutions can affect the lattice parameters, crystal morphology, crystallinity, solvability 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 carbonate 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 forma tion, have the following characteristic compositional features (i) SiO2 nubs smaller than 60 mol%, (ii) high Na2O and CaO content, and (iii) high CaOP2O5 ratio 80. Although silica based bioactive materials have shown great clinical success in many dental and orthopaedic applications, its in meltable 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 slight bioactivity, osteoconductivity as well as not causing an inflammatory response. out-of-pocket to their properties, especially due to it being bioresorbable , calcium phosphate glasses have been under investigation for several(prenominal) applications in the dental field, particular as implants. However only certain calcium phosphate compounds are suitable for implantation in the body, compounds with a CaP 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 bondage and rings that create the cubic vitreous network. All atomic number 8s 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 performances of Ca2+, Na2+ and Mg2+ in the glass structure they are specify 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 oxygens th at 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 oxygens, 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 inorganic phosphate glass. 14A 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 groupsCalcium phosphate glasses containing PotassiumCalcium phosphate glasses containing MagnesiumCa lcium phosphate glasses containing Sodium and TitaniaCalcium phosphate glasses containing Fluorine and Titania1) Calcium phosphate glasses containing PotassiumDias et al (2003) 12 conducted a regard and prepared bioresorbable calcium phosphate glass-ceramics between the metaphosphate and pyrophosphate land based on the composition 45CaO-45P2O5-5K2O-5MgO (CaP = 0.5). XRD results showed that addition of nucleating agents, K2O and MgO forms bioactive -CPP and biodegradable phases KCa(PO3)3, Ca4P6O19 as well as -Ca(PO3)2 which is considered to be non-toxic.21 DTA results showed two crystallisation peaks, Tp at 627C and 739C and two melting temperatures, Tm at 773C and 896C 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 534C. FTIR results showed functional groups corresponding to metaphosphate and pyrophosphate, (PO3)- and (P2O7)4-. These results are in accordance with functiona l groups of the crystalline phases identified by XRD -CPP, KCa(PO3)3, Ca4P6O19 and -Ca(PO3)2. Results from degradation studies of these glass-ceramics confirmed that by controlling the boilersuit composition of the OP 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 so 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 overstuffed 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 belittle, 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 diminish 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 break 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 li ttle 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 MagnesiumResearch into calcium phosphate glasses which produce biocompatible and bioactive phases has generated a lot of interest.An attempt to get to -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-1.5N a2O (CaP molar ratio = 0.625). XRD results showed that the -TCP phase was not detected and the main crystalline phase precipitated was -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 accompaniment in the structure of glass-ceramics which contained TiO2 (wt 3 %). . SEM observations demonstrated light areas which were confirmed by EDS analysis to be -CPP, grey areas was thought to correspond to Na- containing phases and unfairness 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 als study (1999). The undetectable -TCP phase was possibly due to the low content of MgO and TiO2 added and the low CaP 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 approximately 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 th e 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 leaning 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 secondary 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 enrolment 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 (CaP=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, -CPP and CTP are reported to be biocompatible and bioactive, respectively 88,97,98. T