4. DESCRIPTION
STATEMENT OF INVENTION:
The present invention refers to the development of a synergistic composition of machinable bioceramic true composite of beta tricalcium phosphate (β-TCP) and lanthanum phosphate (LP) using LP below 50 wt%. The composite is drillable up to a maximum temperature of 1250°C with good strength and mechanical properties. The machinability of the composite is maintained without sacrificing its bioactivity and biocompatibility properties.
The present invention is useful for development of machinable bone graft substitutes or implants useful for biomedical applications especially for orthopedic, maxillofacial, spine surgery and dentistry where patient specific customized geometries are needed to obtain dimensional criticality and accuracy. Machinability in bioceramic materials will allow it to be dimensionally tailored and accurate which will make the implants more flexible and user friendly in biomedical applications.
BACKGROUND OF INVENTION:
The demand for β-TCP implants is due to its excellent osteoconductivity ensuring colonisation of material by new bone, bioactivity and biocompatibility with surrounding living tissues. These properties of β-TCP are mainly because of its close resemblance to the inorganic component of bone and teeth. Moreover, β-TCP has excellent bioresorbability in the human biological environment that allows progressive replacement of the natural host tissue with the gradual degradation of the implanted material. These properties made them most attractive materials for human hard tissue implants. However, replacing bones and hard tissues require specific shapes and sizes which demand machinability of the implants. Being a ceramic brittle material and having poor machinability, shaping to accurate dimensions of these β-TCP is difficult, which limits the opportunity for wider applications.
LP is a rare earth phosphate (REP) which has good chemical stability in biological environment, non-toxicity and biocompatibility when used in different bio-medical applications, mainly as bio-imaging phosphor/luminescent labeling materials for bio-imaging. It can also potentially tailor microstructure which impart and improve the machinability in a ceramic matrix composite.
The rationale for designing β-TCP based machinable bioceramics is that, in a fine-grained two-phase composite, easy material removal should occur by formation and linking of cracks at the weak interfaces between the β-TCP and LP phases. LP will act as weak interface to prevent crack growth by inducing interfacial debonding and crack deflection during machining.
Several machinable ceramic matrix composites have been developed so far by introducing weak interfacial lanthanum phosphate layer. But machinable calcium phosphates are rarely known.
Reference can be made to Davis et al. "Machinable Ceramics Containing Rare-Earth Phosphates", Journal of American ceramic society, 81. 2169-2175, [1998], where two phase composites consisting of LaPO4 or CePO4 and alumina, mullite or zirconia sintered at temperature up to

1600"C were found to be cut and drilled using conventional tungsten carbide metal working tools and the ease of machining increased with increasing volume fraction of rare earth phosphate component.
Reference may be made to Wang et al "Effect of LaPO4 content on the microstructure and machinability of AI2O3/LaPO4 composites", Materials letters, 57. 822-827 [2002]. It was reported that REP such as LP was a suitable and effective oxide interphase material which exhibited high stability at high temperature in both reducing and oxidizing environment and good chemical compatibility with AI2O3. The machinability of AI2O3/ LP composites sintered at 1600°C using cemented carbide drills was investigated. The composition maintain its composite nature up to 1600°C. Catastrophic fracture occurred during drilling of 10 wt. % and 20 wt. % LP/ AI2O3 composites. When LP content reached 30 wt. % and 40 wt. %, the composites are clearly machinable. The segregation of LP at the AI2O3 grain boundaries enhances the crack deflection and avoid the catastrophic destruction of the sample during drilling. It was found that the sinterability and hardness of AI2O3/LP composites are deeply dependent on the LP content which decreases with increasing LP addition.
Reference may be made to Gong et al, "Pressureless sintering of machinable Al203/LaPO4 composites in N2 atmosphere", Ceramic international, 32. 349-352, [2006], where machinable AI2O3/LP composites ceramics were fabricated by pressureless sintering in N2 atmosphere in a carbon furnace. LP was found to be compatible with AI2O3 up to a temperature of 1650°C, but reacted with each other to form LaA11O18 due to the carbon reduction reaction at 1700°C. The sinterability and bending strength of AI2O3/LP composites decreased with the increase of LP content. The 30 wt. % LP/AI2O3composites sintered at 1650°C could be machined using cemented carbide drills. The formation of LaA11O18 was detrimental to machinability.
Reference may be made to Morgan et al, " Ceramic composites having a weak bond material selected from monazites and xenotimes", US. Patent No. 5,514,474, [1996], and "Fibrous composites including monazites and xenotimes", US. Patent No. 5,665,463, [1997], where they patented that high temperature and damage tolerant monazite or xenomites can be used in ceramic matrix composites as an interface material where alumina fibres form as reinforcement material. The weakly bonded interphase material allows debonding and frictional sliding between the constituents of the composite and inhibits crack growth across the interface.
Reference may be made to Ergun et al, "Synthesis and characterization of machinable calcium phosphate/lanthanum phosphate composites", Journal of materials processing Technology, 199. 178-184, [2008], wherein development of machinable β-TCP-LP composite was prepared. 50 wt% LP containing composite sintered at 1100°C was reported to be drillable. But at higher temperatures formation of Ca8La2(PO4)6O2 was reported with no composite character in the compositions and no drillability. But lower sintering temperature resulted in poor mechanical properties. Also no bioactivity and biocompatibility was reported.
Therefore there is no detailed study available on compositional variations of REP, phase stability at higher temperature, machinability, mechanical properties, bio-activity and biocompatibility of

LP addition on β-TCP. Increase in the sintering temperature of the composites will also increase the densification and mechanical properties due to greater extent of sintering' at higher'' temperatures. Therefore preparation of a machinable β-TCP/LP composite at a temperature higher than 1100°C [as reported by Ergun] is desirable for higher densification and strength attainment purpose. Moreover, the composite must be stable at that higher temperature without chemically reacting with each other to remain machinable. β-TCP is stable up to 1250°C, above which it transforms into a-TCP, which has inferior bioactive property. So the need for preparation of a machinable true composite of β-TCP-LP with higher strength is important.
OBJECTIVE OF INVENTION:
β-TCP implant require strict dimensional and shape control, and machining to obtain dimensional criticality and accuracy for custom-designed implant systems. But high hardness and associated brittleness of β-TCP restrict their machinability and also their wide applicability in such applications. The only way to improve applicability in using these materials is to impart machinability. Introduction of weak interphase material like REP can impart machinability to these implants. The difficulty associated with the machinable β-TCP implant produced by Ergun et al was that it was sintered at a much low temperature and machinable only when 50% LP is present. The present invention targets to develop a well dense, high strength β-TCP-LP composite by sintering at higher temperature (upto 1250°C, above which β-TCP transforms) and to attain machinable character without compromising the bioactivity and biocompatibility.
The main objective of the present invention is to develop a sintered composite of β-TCP- LP with a synergistic composition of β-TCP and LP which will be dense, strong and maintain its composite nature even after sintering at 1250°C.
Another objective of the present invention is to attain the machinable character of the dense, strong, sintered β-TCP-LP composites using LP below 50 wt%.
Still another objective of the present invention is to obtain a machinable composite of β-TCP-LP with high bioactivity and biocompatibility.
SUMMARY OF INVENTION:
In the present invention, a synergistic composition has been developed for the production of machinable β-TCP -LP composites using synthetically prepared β-TCP and LP powders.
The machinable composites of the present invention have many advantages. Literature provides information on the development of machinable β-TCP-LP composite with 50 % LP at a much lower temperature, which will result in lower mechanical properties. Also literature does not provide any bioactivity and biocompatibility study. The advantage is imparting machinability in composites with much lower (10 wt%) LP content and sintered at high temperature will result in improved mechanical properties due to greater sintering with drillability and without compromising and biological properties.

The novelty of the present invention resides in obtaining a synergistic composition of β-TCP -LP, LP content below 50 wt%, which will maintain its composite nature even on sintering at 1250°C. As well as the composite will be drillable without causing fracture even at a much lower LP content (10wt%). The current invention develops bioceramic composites that are drillable at much lower LP content and has much higher mechanical property as well as good bioactivity and biocompatibility.
The inventive steps lie in the synthesis of β-TCP and LP calcined powders and the making of a synergistic composition for sintered composites from these two phosphate powders using LP below 50wt% sintered between 1200-1250°C. The process makes the composites mechanically strong, machinable or drillable without fracture by interfacial debonding and crack deflection due to the presence of weak interphase LP powders in between the β-TCP phase. Also the composites are highly bioactive and biocompatible.
STATEMENT OF EMBODIMENT:
To obtain the desired product, sintered, dense, strong, machinable bioceramic composite with bioactive property, the following embodiments are necessary which are detailed below.
The present invention provides a synergistic composition for the development of machinable bio-ceramicsintered composite of β-TCP LP with high strength, machinability and drillability without much altering bioactivity and biocompatibility.
In another embodiment of the present invention, the constituent components of the composites, that is synthesized β-TCP and LP powders, are used in such a way so that LP content used was below 5Cwt%.
In another embodiment of the present invention, all the synthesized materials are calcined between 700 - 900°C to obtain phase pure β-TCP and LP powders with optimum powder characteristics for preparing the composites.
In an embodiment of the present invention, up 50 wt% LP powder is mixed with rest amount of β-TCP powder in liquid alchohol medium, dried, pressed and sintered between 1200-1250°C to obtain dense and strong β-TCP-LP composite with machinability, bioactivity and biocompatibility.
In another embodiment of the present invention, the mixed compositions were pressed in a uniaxial hydraulic press at a specific pressure of 80 to 200 MPa.
DETAILED DESCRIPTION:
The present invention provides a synergistic composition for the production of machinable β-TCP - LP composites containing β-TCP (50-100wt. %) and LP (10-50wt. %) powders'/ β-TCP was prepared synthetically by different wet chemical methods, namely co-precipitation and sol gel synthesis. LP was prepared by wet chemical reaction technique. The powders were then calcined at different temperatures for phase stability between 700-900°C. The powders were mixed in particular ratio in such a way so that after sintering upto 1250"C, the composite maintains its

stability, without reacting with each other. The batch composition for the composites ( given in Table 1) is composed of β-TCP and LP calcined powders.
The sintered composite pellets were studied for densification. Bulk density and apparent porosity were measured by the conventional liquid displacement technique according to the Archimedes principle. Relative densification (percent theoretical density) was calculated as the ratio between bulk density and theoretical density of the composition. Theoretical density of each composition was calculated from the theoretical density values of each component and their weight fractions used. Flexural strength was determined by standard three-point bending method in an instrument (Tinius Olsen, USA, make). Micro-hardness of the composites was measured using a Vickers microhardness tester (LM700, LECO Corporation, Michigan, USA) by using a load of 50 gf for 15 s. Bioactivity study was done in simulated body fluid (SBF) prepared by the method followed by Tas et al, " Synthesis of biomimetic Ca-hydroxyapatite powders at 37 °C in synthetic body fluids", Biomaterials, 21. 1429-1438 [2000]. Biocompatibility was tested by MTT assay method. Drilling study was done by using Solid carbide drill bit of 4 mm dia using a conventional radial drill machine (Hindustan Machine Tools Ltd, Bangalore, India) attached with piezoelectric drill dynamometer (Type 1015, Kistler instrumente A.G., Winterthur, Switzerland).
Presence of free LP particle in between the β-TCP acts as weak interphase. While drilling the crack was deflected through the weak interphases of LP causing interfacial debonding. As a result fracture was prevented. The invention is described with the help of following experiments and examples for understanding the importance of composition in external practice. However the examples, which are given here by way of illustration, should not be construed to limit the scope of the present invention.
Example 1
β-TCP powder produced by co-precipitation method calcined at 800°C was taken for the pellet preparation. The powder was mixed in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. The mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 200 MPa pressure for 30 sec. The pressed pellets were sintered at 1210°C with a soaking period of 2hrs at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally using solid carbide drill bits. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 2
β-TCP powder produced by coprecipitation method calcined at 850°C was taken for the composite preparation. β-TCP and LP powder were mixed in the ratio 90:10 (wt.%). 45 gm of β-TCP was mixed with 5 gm of LP in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 170 MPa pressure for 30 sec. The pressed pellets were sintered at 1220°C °C with a soaking period of 2hrs

at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally using solid carbide drill bits. The maximum thrust force recorded during drilling is 113.75N when drilled at 1160 rpm speed and 0.12mm/rev feed. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 3
(β-TCP powder produced by coprecipitation method calcined at 820°C was taken for the composite preparation. β-TCP and LP powder were mixed in the ratio 80:20 (wt.%). 40 gm of β-TCP was mixed with 10 gm of LP in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 170 MPa pressure for 30 sec. The pressed pellets were sintered at 1220°C °C with a soaking period of 2hrs at.37min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally using solid carbide drill bits. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 4
β-TCP powder produced by coprecipitation method calcined at 830°C was taken for the composite preparation. β-TCP and LP powder were mixed in the ratio 70:30 (wt. %). 35 gm of β-TCP was mixed with 15 gm of LP in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were.prepared by hydraulic machine with the application of 180 MPa pressure for 30 sec. The pressed pellets were sintered at 1230°C with a soaking period of 2hrs at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally using solid carbide drill bits. The maximum thrust force recorded during drilling is 109.38N when drilled at 1160rpm speed and 0.12mm/rev feed. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 5
β-TCP powder produced by co-precipitation method calcined at 840°C was taken for the composite preparation.β-TCP and LP powder were mixed in the ratio 60:40 (wt. %). 30 gm of β-TCP was mixed with 20 gm of LP in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 140 MPa pressure for 30 sec. The pressed pellets were sintered at 1240°C with a soaking period of 2hrs at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study.was done conventionally using solid carbide drill bits. To determine the flexural strength of the composite,

bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 6
β-TCP powder produced by co-precipitation method calcined at 850°C was taken for the composite preparation β-TCP and LP powder were mixed in the ratio 50:50 (wt%). 25 gm of (β-TCP was mixed with 25 gm of LP in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 200 MPa pressure for 30 sec. The pressed pellets were sintered at 1250°C with a soaking period of 2hrs at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally using solid carbide drill bits. The maximum thrust force recorded during drilling is 82.5N when drilled at 1160rpm speed and 0.12mm/rev feed. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 7
β-TCP powder produced by sol gel method calcined at 900°C was taken for the pellet preparation. The powder was mixed in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 150 MPa pressure for 30 sec. The pressed pellets were sintered at 1250°C with a soaking period of 2hrs at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally using solid carbide drill bits. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.
Example 8
β-TCP powder produced by sol gel method calcined at 850°C was taken for the composite preparation. β-TCP and LP powder were mixed in the ratio 90:10 (wt. %). 45 gm of β-TCP was mixed with 5 gm of LP in isopropyl alcohol for 120 mins. The slurry was dried at 75°C for 12 hrs. The dried powder were again mixed with 4% (6wt %) PVA. 2 gm of the mixture were taken to make pellets of 15 mm dia. Pellets were prepared by hydraulic machine with the application of 190 MPa.pressure for 30 sec. The pressed pellets were sintered at 1240°C with a soaking period of 2hrs at 3°/min heating rate. The pressed pellets were studied for bulk density, apparent porosity, hardness, bioactivity in SBF solution and biocompatibility. The drilling study was done conventionally.using solid carbide drill bits. To determine the flexural strength of the composite, bars of dimension 60mmx6mmx6mm were made by pressing. The resultant product properties were given in table 2.


As shown in table, with the increase in LP addition bulk density decrease and apparent porosity increases but with incorporation of drillability. The more LP is added the composite becomes more easily drillable, The hardness and flexural strength also decreases with the increase in LP addition.

5. CLAIMS
We Claim:
1. A synergistic composition for the development of machinable bio-ceramic sintered composite of β-TCP LP with high strength, machinability and drillability without much altering bioactivity and biocompatibility.
2. A synergistic composition as claimed in claim 1, wherein, the constituent components of the composites, that is synthesized (β-TCP and LP powders, are used in such a way so that LP content used was below 50wt%.
3. A synergistic composition as claimed in claim 1 and 2, wherein, all the synthesized materials were calcined between 700 - 900°C to obtain phase pure β-TCP and LP powders with optimum powder characteristics for preparing the composites.
4. A synergistic composition as claimed in claim 1 to 3, wherein, up 50 wt% LP powder is mixed with rest amount of β-TCP powder in liquid alcohol medium, dried, pressed and sintered between 1200-1250°C to obtain dense and strong β-TCP-LP composite with machinability, bioactivity and biocompatibility.
5. A synergistic composition as claimed in claim 1 to 4, wherein, the mixed compositions were pressed in a uniaxial hydraulic press at a specific pressure of 80 to 200 MPa.
6. DATE AND SIGNATURE (to be given at the end of last page of specification)
7. ABSTRACT OF THE INVENTION (to be given along with complete specification on separate page)