Patent Application: US-201314075754-A

Abstract:
the present invention provides a highly - sintered fluorapatite glass - ceramic comprising a high ca / al or sr / al mole - ratio , that possesses a microstructure that induces apatite / bone deposition .

Description:
fluorapatite glass - ceramics have been shown to be excellent candidates as scaffold materials for bone grafts , however , scaffold production by sintering is hindered by concurrent crystallization of the glass . the present invention is based on the effect of ca / al ratio on the sintering behavior of nb - doped fluorapatite - based glasses in the sio 2 — al 2 o 3 — p 2 o 5 — mgo — na 2 o — k 2 o — cao — caf 2 system . glass compositions with ca / al mole - ratio of 1 ( a ), 2 ( b ), 4 ( c ) and 19 ( d ) were prepared by twice melting the oxide / fluoride powders at 1525 ° c . for 3 h . the resulting glasses were either cast as cylindrical ingots or ground into powders . disc - shaped specimens were prepared by either sectioning from the ingots or powder - compacting in a mold , followed by heat treatment at temperatures ranging between 700 and 1050 ° c . for 1 h . the density was measured on both sintered specimens and heat treated discs as controls . the degree of sintering was determined from these measurements . x - ray diffraction ( xrd ) showed that fluorapatite crystallized in all of the glass - ceramics . a high degree of sintering was achieved at 775 ° c . for glass - ceramic d ( 98 . 99 ± 0 . 04 %), and 900 ° c . for glass - ceramic c ( 91 . 31 ± 0 . 10 ). glass - ceramics a or b were only partially sintered at 1000 ° c . ( 63 . 6 ± 0 . 8 % and 74 . 1 ± 1 . 5 %, respectively ). sem revealed a unique microstructure of micron - sized spherulitic fluorapatite crystals in glass - ceramics c and d . increasing the ca / al ratio promoted low temperature sintering of fluorapatite glass - ceramics , which are traditionally difficult to sinter . increasing the amount of calcium oxide at the expense of aluminum oxide increases sinterability by extending the processing range prior to crystallization of fluorapatite . replacing a portion , or all , of the cao with sro can also be advantageous . the invention will be further described by references to the following detailed examples . four glass compositions with increasing calcium to aluminum mole ratios were prepared by mixing reagent grade oxides and caf 2 ( table 1 ). glass compositions with ca / al mole ratios of 1 ( a ), 2 ( b ), 4 ( c ) and 19 ( d ) were prepared . fluoride was introduced as calcium fluoride and was added in excess , as 50 % losses by volatilization were anticipated . the compositions a - d were melted at 1525 ° c . for 3 h in platinum crucibles . after quenching in deionized water , the resulting glass frits were powdered and melted again at 1525 ° c . for 3 h . the molten glasses were cast to form cylindrical rods ( 10 mm in diameter ; 60 mm in length ). the rods were furnace - cooled from 685 ° c . to room temperature and sectioned into disks ( 10 mm in diameter ; 1 mm thick ) using a low speed diamond saw . specimens were then heat treated in the temperature range 700 - 1050 ° c . for one hour at a heating rate of 2 ° c . per minute . glass frits from the same compositions were prepared as described earlier . frits were powdered and sieved to a particle size of 45 micrometers or less . disk - shaped specimens ( 10 mm in diameter ; 1 mm thick ) were produced by uniaxial pressure using a polyvinylsiloxane mold . specimens were sintered in air at temperatures ranging between 700 and 1050 ° c . for one hour . the crystallization behavior was investigated by differential scanning calorimetry ( q600 dsc / tga , ta instruments ). analyses were performed on glass frit particles placed in matched platinum - rhodium crucibles under nitrogen gas flow . aluminum oxide powder ( 99 . 99 %) served as a reference standard . first , the optimum nucleation temperature ( ont ) was determined for each glass composition . this temperature is defined as the “ temperature at which the most number of stable nuclei form per volume element ” [ 18 ]. ont was investigated by dsc after one hour nucleation holds at various temperatures , followed by full crystallization , according to the method proposed by marotta et al . [ 41 ]. the ont was determined as the hold temperature leading to the lowest temperature for the first crystallization exotherm . further dsc experiments were conducted at heating rates of 10 , 20 , 30 and 40 ° c ./ min . on samples nucleated at the ont . the activation energy of crystallization ( e c ) for fluorapatite was calculated from the slope of the classical kissinger plot , using the kissinger equation [ 42 ]: where t p is the temperature of the crystallization exotherm and v is the heating rate . the coefficient of thermal expansion and softening point were determined by dilatometry on glass bars sectioned from the cast ingots ( orton dilatometer , model 1600d ). crystalline phases were characterized by x - ray diffraction on powdered and bulk specimens . scans were performed in the two - theta range 10 - 60 °, at a scanning rate of 1 ° c ./ min . ( miniflex ii diffractometer , rigaku americas ). additionally , specimens sintered at 800 ° c ./ 1 h were analyzed at a scanning rate of 0 . 2 degrees per minute in the two theta range 3 - 90 degrees at 40 kv and 44 ma in bragg - brentano configuration ( smartlab , rigaku americas ). peak positions were determined using pdxl - 2 analysis software ( rigaku corporation ) after calibration using silicon powder standard ( nist , 640d ). the microstructure was investigated by scanning electron microscopy under secondary electron imaging ( hitachi s - 4800 field emission sem ). specimens were polished to a 0 . 5 μm finish using as series of abrasives ending with diamond polishing pastes . they were etched with diluted hydrofluoric acid and gold coated prior to sem examination . a variable pressure scanning electron microscope ( vp - sem , model 3400 , hitachi high technologies america , inc . ), coupled with a back - scattered electron detector , was used to perform semi - quantitative energy dispersive spectroscopic analyses on uncoated specimens . x - rays were detected at an accelerating voltage of 15 kv and a working distance of 10 mm ( bruker axs microanalyzer ). zaf matrix correction factors were applied in instrumental software calculations . the density of bulk glass - ceramic specimens as well as sintered specimens was measured by archimedes &# 39 ; method . the degree of sintering was calculated assuming 100 % theoretical density for bulk heat treated glass - ceramic specimens . the sintering behavior was also assessed by scanning electron microscopy on both as - sintered and sintered and polished specimens . results were analyzed by anova and tukey &# 39 ; s test to detect statistically significant differences . a p - value of less than 0 . 05 was considered statistically significant . characteristic x - ray powder diffraction patterns of specimens sintered at 775 , 800 or 900 ° c . for 1 h are displayed in fig1 ( a - d ). fluorapatite ( ca 5 ( po 4 ) 3 f , pdf 15 - 876 ) was present for all compositions from 775 ° c . and up . the figure of merit for fluorapatite was determined using pdxl 2 data analysis software on precision scans performed at 0 . 2 degree per minute ( fig1 d ) and was less than one for all compositions , indicating an excellent match . akermanite ( ca 2 mgsi 2 o 7 , pdf 35 - 592 ) started to appear at 775 ° c . for composition d and 900 ° c . for composition c . it became the predominant phase at 800 ° c . for composition d . a small amount of forsterite ( mg 2 sio 4 , pdf 34 - 189 ) was found in composition d after heat treatment at 800 ° c . and in all compositions at 900 ° c . x - ray powder diffraction patterns obtained on disks sectioned form bulk glass ingots and further heat treated matched those of sintered specimens . results from differential thermal analyses are summarized in table 2 and graphically displayed in fig2 . for each glass composition , two exothermic peaks were observed . for all compositions , the first exotherm , occurring between 783 and 864 ° c ., is attributed to the crystallization of fluorapatite . according to xrd analyses , the second exotherm , occurring between 981 and 1074 ° c ., is attributed to the crystallization of forsterite in compositions a and b and concurrent crystallization of akermanite and forsterite in compositions c and d . the optimum nucleation temperature varied between 670 and 700 ° c ., depending on the composition . the plots obtained from the kissinger equation ( 1 ), supra , are shown in fig2 . the activation energy for the crystallization of fluorapatite slightly increased with increasing the calcium to aluminum ratio and ranged between 343 kjmol − 1 for composition a to 353 kjmol − 1 for composition d . the results from dilatometric analyses , including the softening point and coefficient of thermal expansion are listed in table 2 . increasing the calcium to aluminum ratio led to a decrease of the softening point from 725 ° c . for composition a to 663 ° c . for composition d , while the coefficient of thermal expansion increased from 9 . 5 to 11 . 5 × 10 − 6 /° c . the microstructure of the glass - ceramics after heat treatment at 800 ° c . is shown in fig4 , panels a - d . a fine coast - and - island relict microstructure , characteristic of glass - in - glass phase separation , followed by crystallization of island - shaped fluorapatite crystals can be seen in composition a . spherulitic fluorapatite crystals are present in compositions b and c , while well developed , flower - shaped fluorapatite crystals are clearly seen in composition d . the evolution of the microstructure with heat treatment temperature for composition d is displayed in fig5 ( panels a - d ). darker island - shaped akermanite crystals started to appear at 800 ° c ., became more defined at 850 ° c . and dominated the microstructure at 900 ° c . results from eds analyses are summarized in table 3 . the calculated degree of sintering as a function of temperature is graphically displayed in fig6 . the lack of mechanical integrity prevented accurate measurement for compositions a and b after sintering at 700 ° c . for 1 h . the degree of sintering for composition a only remained around 60 %, even after sintering up to 1050 ° c . for one hour . the degree of sintering for composition b increased slowly from 63 . 5 % at 750 ° c . up to a maximum of 74 . 4 % at 950 ° c . for 1 h . concurrently , significant changes in the degree of sintering were observed for compositions c and d over the same temperature range . the largest amount of sintering occurred between 700 and 775 ° c . for both compositions . composition c reached a maximum of 91 % after sintering at 900 ° c ., followed by a linear decrease to 75 % as the sintering temperature was raised to 1050 ° c . the degree of sintering of composition d rapidly reached 99 % at 775 ° c . and remained in this range up to 950 ° c ., while a slight decrease in density was observed to 93 . 7 % at 1050 ° c . above 700 ° c ., the overall degree of sintering was significantly different between all compositions ( p & lt ; 0 . 0001 ). composition d exhibited the greatest degree of sintering followed by c , b and a . within groups c and d , sintering at 1050 ° c . led to a significantly lower degree of sintering than other temperatures ( p & lt ; 0 . 0001 ). scanning electron micrographs of specimens sintered at 775 ° c . for 1 h are shown in fig7 ( panels a - d ). sharp - angled particles , indicating a lack of sintering , were present for composition a ( fig7 a ). rounding of the powder particles occurred for composition b , together with some particle necking ( fig7 b ), while interconnected pores remained . a noticeably greater degree of necking , with progressive disappearance of open pores is evident for composition c ( fig7 c ). only a few pores remain and individual particles are no longer discernable for composition d , indicating that the sintering process is complete ( fig7 d ). the microstructure after sintering at 800 ° c . for 1 h for composition d is shown in fig8 . flower - shaped fluorapatite crystals similar in shape and size to those observed after heat treatment of bulk specimens at 800 ° c . for 1 h are present . xrd and dsc results revealed that increasing the calcium to aluminum ratio led to a significant increase from 783 to 864 ° c . in the crystallization temperature of fluorapatite , accompanied by a slight increase in the activation energy from 343 to 353 kjmol − 1 . meanwhile , the softening temperature decreased from 725 ° c . to 663 ° c . it was previously reported that glasses in the compositional range corresponding to composition a underwent amorphous phase separation , with formation of a droplet phase that later crystallized into small spherical fluorapatite crystals [ 15 , 16 ]. this particular glass composition has a measured ca : p ratio of 1 . 8 , with phosphorous being the limiting element to the amount of fluorapatite crystallization . the remaining glassy matrix after fap crystallization is likely to contain a small amount of fluorine , but very little calcium . concurrently , composition d has a measured ca : p mole ratio of 4 . 1 , phosphorous being again the limiting factor to fap crystallization , the remaining glassy matrix will contain a substantial amount of calcium . this explains the crystallization of akermanite ( ca 2 mgsi 2 o 7 ), immediately following that of fluorapatite . the activation energy for fluorapatite crystallization falls within the range of 163 to 682 kj mol − 1 reported for other fap glass - ceramic systems [ 43 ]. the slight increase in activation energy for composition d in bulk form was not expected due to the increase in the amount of calcium in the composition . calcium is a network modifier , expected to lower glass network connectivity and increase viscous flow , thereby inducing a decrease in the activation energy for fap crystallization . o &# 39 ; flynn et al . used a similar experimental design for dsc analyses , with determination of the optimal nucleation temperature and a nucleation hold , to study the crystallization behavior of apatite - mullite glass - ceramics [ 18 ]. they first determined the optimal nucleation temperature , and then applied a one hour nucleation hold at that temperature prior to investigating the effect of heating rate on the crystallization exotherms temperature . they observed an increase in the activation energy for fluorapatite crystallization with increasing amounts of calcium fluoride in the glass , with values in the range 292 to 471 kjmol − 1 . it was proposed that higher amounts of fluorine led to a higher nucleation density during the isothermal hold . the creation of these nuclei consuming fluorine could have made crystal growth more difficult , hence increasing the activation energy . the apatite nucleation rate is also likely to increase with increasing the amount of calcium in the glass [ 44 ]. it is believed that , in this example , a similar mechanism involving a higher nucleation rate and / or phase separation of a calcium - rich phase , in relation with the subsequent crystallization of akermanite be responsible for a slightly more difficult crystal growth in composition d , ultimately leading to an increase in the activation energy and crystallization temperature of fluorapatite . although semi - quantitative , eds analyses confirmed that glass melting was associated with significant fluorine losses by volatilization , they also showed losses in other species such as phosphorous and potassium . however , fluorine losses were anticipated and excess fluorine remained in all compositions with regard to the stoichiometry of fluorapatite , most certainly playing an important role on the behavior of the glasses , by lowering network connectivity and further promoting phase separation [ 45 , 46 ]. as the glasses phase - separated into ca — p - rich droplets [ 16 ], modifier cations were no longer needed to charge - balance phosphorous in the glass network , allowing a decrease in connectivity . meanwhile , increasing the amount of charge - balancing calcium available and decreasing the amount of aluminum is likely to have prevented al 3 + ions from reaching a four - fold coordination state , also leading to a decrease in connectivity . therefore , it is believed that both calcium and fluorine led to a decrease in network connectivity , permitting better sintering by viscous flow prior to fluorapatite full crystallization . the observed increase in fap crystallization temperature , together with a decrease in softening temperature considerably extended the processing window for compositions c and d , which explains the large increase observed in the degree of sintering as a function of calcium to aluminum ratio . this result is also in line with the overall decrease in the glass transition temperature and increase in coefficient of thermal expansion , reported to occur with an increase in cao / sio 2 ratio in diopside - fluorapatite - wollastonite - based glass - ceramics [ 31 ]. sintering started and nearly reached completion prior to crystallization of fluorapatite in compositions c and d . the poor sintering behavior observed for compositions a and b is likely due to earlier crystallization of fluorapatite , together with lower amounts of calcium in the glassy matrix , as compared to compositions c and d , thereby hindering the sintering process by increasing the viscosity . the crystallization of forsterite ( mg 2 sio 4 ) at higher temperatures further prevented sintering . spherulitic crystallization of fluorapatite was observed in glass - ceramics b , c and d after heat treatment at 800 ° c . according to granasy : “ spatial heterogeneities due to phase separation can provide a source of static disorder giving rise to spherulitic growth ” [ 47 ]. as mentioned earlier , glass - in - glass phase separation occurs in our compositional range . moreover , this type of apatite growth was first reported by carpenter et al ., in phase - separated calcium alumino - silicate glasses nucleated with p 2 o 5 [ 48 ], and later described by shyu and wu in mgo — cao — sio 2 — p 2 o 5 glasses [ 49 ]. more recently , stanton and hill reported spherulitic heterogeneous nucleation in apatite - mullite glass - ceramics , with an interesting mechanism : phase separation first occurred into amorphous droplets rich in ca , p and f , surrounded by regions rich in si and al [ 50 ]. the ensuing crystallization of fluorapatite left the surrounding glass depleted in ca , p and f , and close to the composition of mullite . as mullite crystallized , the surrounding glass in turn became enriched in ca . p and f and fluorapatite formed again . a similar route could be happening in composition d , with fluorapatite spherulites crystallizing first , followed by akermanite . this could explain the interpenetrating microstructure obtained after heat treatment at 850 ° c ., temperature at which both phases co - exist . as shown on fig4 , it is also possible that the phase - separated area grew larger as the calcium to aluminum ratio increased from composition a to d , leading to progressively larger spherulites . compositions c and d are excellent candidates for the production of bioactive ceramic scaffolds by sintering at temperatures as low as 775 ° c . the unique microstructure conveyed by spherulitic crystallization of fluorapatite may provide an excellent basis for epitaxial growth of apatite crystals . further crystallization of akermanite could impart additional attractive properties such as enhanced stimulation of cell differentiation into osteoblastic lineage , as suggested by recent studies on sintered akermanite ceramics [ 51 - 54 ]. increasing the calcium to aluminum ratio led to both a decrease in softening temperature and an increase in crystallization temperature and activation energy for fluorapatite in this compositional range . this led to a widening of the processing window and made possible sintering to near theoretical density at temperatures between 775 and 800 ° c . for compositions c and d , corresponding to the highest amounts of calcium . fluorapatite crystallized following a spherulitic pattern , with increasingly larger spherulites as the calcium to aluminum ratio increased . this microstructure may provide excellent conditions for epitaxial apatite deposition in vivo , as it mimics carbonated apatite crystal habits . the crystallization of akermanite could lead to enhanced stimulation of cell differentiation . four fluorapatite glass compositions in the sio 2 — al 2 o 3 — p 2 o 5 — mgo — na 2 o — k 2 o — cao — caf 2 system , with increasing amounts of strontium oxide ( d : 0 , e : 12 , f : 18 and g : 24 mol . %) in replacement for calcium oxide ( d : 23 . 9 ( table 1 ), e : 12 , f : 18 , g : 0 ( mol . %) ( table 5 ), were prepared by twice melting mixtures d - f at 1525 ° c . for 3 h ( table 5 ). after the second melt , glasses were cast into cylindrical ingots . differential thermal analyses were performed on powdered glasses to determine glass transition and crystallization temperature . disc - shaped specimens ( 1 . 2 mm thick , 10 mm in diameter ) were sectioned from the ingots . specimens were heat treated at various temperatures ranging from 775 to 900 ° c . for 1 h , and analyzed by x - ray powder diffraction ( rigaku smartlab ). the density of the glass - ceramics was measured by archimedes &# 39 ; method . the chemical solubility was determined on powders after incubation for 120 h at 37 ° c . in either tris - hcl buffer at ph 7 . 4 or citric acid buffer at ph 3 . 0 , to create either moderate or extreme aging conditions , according to iso standard 10993 - 14 . the ph of the buffer solutions was measured before and after incubation . specimens were dried at 80 ° c . until no further weight loss was observed . the percent weight loss was then calculated for each composition and each buffer . a variable pressure sem ( vp - sem , hitachi 3400 ) coupled with a back - scattered electron detector , was used to perform semi - quantitative eds analyses of the powders before and after incubation ( bruker axs microanalyzer ). xrd analyses revealed the formation of both strontium - substituted fluorapatite and strontium - substituted akermanite in the strontium - containing glasses . differential thermal analyses of powdered glasses showed that the crystallization temperature of sr - substituted fluorapatite varied only slightly among the compositions . meanwhile the crystallization temperature for sr - substituted akermanite increased linearly with strontium content . the density of the glass - ceramics after heat treatment at 775 ° c ./ 1 h increased linearly ( r 2 = 0 . 985 ) with sr content from 2 . 951 ± 0 . 005 for composition d to 3 . 353 ± 0 . 021 for composition g . the ph after incubation in tris - hcl buffer increased slightly for all compositions . the ph after incubation in citric acid buffer ( ph 3 ) increased markedly for composition g ( 7 . 11 ± 0 . 42 ) and was highest for composition d ( 8 . 58 ± 0 . 42 ). the chemical solubility increased as the amount of strontium in the composition increased , reaching a maximum of 2 . 6 ± 1 . 2 wt . % in tris - hcl buffer and 10 . 4 ± 0 . 6 wt . % in citric acid buffer , both for composition g ( fig1 ). the weight loss in citric acid buffer for composition d ( 4 . 2 ± 1 . 2 ) was not significantly different from that of a synthetic fluorapatite control ( 4 . 6 ± 0 . 6 ; clarkson chromatography , inc .). eds analyses after incubation in citric acid buffer revealed that the sr - free composition ( d ) underwent losses in fluorine , sodium and magnesium . composition e exhibited losses ( 0 . 2 to 1 . 4 at . %) in fluorine , sodium , calcium , strontium , magnesium and potassium . composition f showed losses ( 0 . 3 to 1 . 8 at . %) in all above species except fluorine . moderate losses in fluorine and sodium ( 0 . 1 to 0 . 2 at . %) were recorded for composition g . all compositions , with the exception of d , exhibited very large variations in the amount of silicon after incubation . strontium additions in this system led to crystallization of sr - substituted fluorapatite and sr - akermanite (( ca , sr ) 2 mgsi 2 o 7 ) the chemical solubility increased with the amount of strontium in the composition in both tris - hcl buffer and citric acid buffer . various species were released during incubation including strontium and calcium . the large variations in silicon content for e , f and g could indicate that the glass network was at various stages of dissolution . the following references are incorporated by reference herein , as though fully set forth : clifford a , hill r . apatite - mullite glass - ceramics . journal of non - crystalline solids , 1996 ; 196 : 346 - 351 . 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