Patent Abstract:
tissue engineering is a growing field where new materials are being developed for implantation into the body . one important area involves bone graft materials to replace areas of bone lost to trauma or disease . traditionally , graft material may be harvested from the bone of the individual receiving the graft material . however , this requires an additional surgery and additional recovery . bone also may be taken from others , or even cadavers , but this introduces biocompatibility problems as well as the risk of disease transfer . ideally , a biocompatible material is sought that will act as a filler with appropriate mechanical strength , encourage bone healing , and degrade to allow new bone ingrowth without the risk of disease transfer . the present invention is a new composite bone graft material made from biocompatible poly and nano - sized hydroxyapatite particles exposed on its surface using a gas foaming particle leaching method . a further embodiment of this invention involves coating this plga / hydroxyapatite biomaterial with an adherent , fast , uniform coating of a mineral such as apatite . the plga polymer portion of the composite provides sufficient mechanical strength to replace bone and is degradable over time to allow new bone tissue ingrowth . the incorporated hydroxyapatite particles increase the composite material &# 39 ; s osteogenic properties by providing sites for tissue attachment and propagation . finally , a uniform coating of mineral apatite on the surface of this novel biomaterial composite further enhances its osteogenic qualities .

Detailed Description:
the present invention is a novel biomaterial with special characteristics that allow it to perform well as a bone graft material . it is comprised of a degradable poly ( d , llactic - co - glycolic acid ) polymer scaffolding with incorporated , nano - sized hydroxyapatite particles made by a gas foaming and particle leaching ( gf / pl ) method . a further embodiment of this invention describes the same biomaterial with an adherent , highly uniform apatite coating . the method of constructing a plga polymer scaffold using gf / pl is described thoroughly in the journal article titled , “ open pore biodegradable matrices formed with gas foaming ” ( harris l d , kim b s , and mooney d j ; j biomed mater res , 42 , 396 - 402 , 1998 ). this entire article is hereby incorporated by reference . the research reported in this article found that the porosity and pore size of the plga scaffold can be controlled by the salt / plga ratio and respective particle sizes . also , the pores of the matrix are interconnected and highly uniform . in this manner , a useful scaffold can be created without the use of organic solvents or high temperatures . although the method for constructing a polymer scaffold using gf / pl is already known , the addition of nano - sized hydroxyapatite particles to this specific polymer scaffolding has not been taught in the prior art . an article has recently been published by the inventor of this application . it describes the addition of hydroxyapatite particles to the plga scaffold . it is called , poly ( lactide - co - glycolide )/ hydroxyapatite composite scaffolds for bone tissue engineering ( kim s s , park m s , jeon o , choi c y , and kim b s ; biomaterial , 27 , 1399 - 1409 , available online oct . 5 , 2005 ). this article is also hereby incorporated by reference . it is important to note that this article is not by another . students of dr . byung - soo kim , the sole inventor of the present invention , conducted the research for the article , but the ideas for the invention are uniquely those of dr . byung - soo kim the research describes how porous plga / ha composite scaffolds were fabricated by the modification of the previously described gf / pl method ( harris l d , kim b s , and mooney d j ; j biomed mater res , 42 , 396 - 402 , 1998 ). the most significant modification of the previous method is the non - obvious addition of nano - sized hydroxyapatite particles that end up being highly exposed on the polymer surface . it is not obvious to one of ordinary skill in the art to add nano - sized hydroxyapatite particles to the polymer matrix and to expose the hydroxyapatite particles on the polymer surface . plga / ha composites were prepared with 75 : 25 plga particles ( diameter = 100 - 200 pm , molecular weight = 100 , 000 da , birmingham polymers , birmingham , ala . ), ha nanoparticles ( diameter = approximately 100 nm , berkeley advanced biomaterials inc ., berkeley , calif . ), and sodium chloride particles ( diameter = 100 - 200 pm , sigma , st . louis , mo .). the plga pellets were ground using a tekmar grinder ( bel - art products , pequannock , n . j .) and sieved to obtain particles ranging from 100 to 200 pm . the salt particles were sieved to yield a range of sizes from 100 to 200 pm . the polymer particles were mixed with the ha and nacl particles . the plga / ha / nacl mass ratio was 1 : 1 : 9 . the mixture was loaded into a disk mold ( diameter = 1 . 35 cm ; aldrich chemical co ., milwaukee , wis .) and compressed at 2000 psi for 1 min using a carver laboratory press ( fred s . carver , inc ., menominee falls , wis .) to yield solid disks with a thickness of about 1 . 7 mm . the samples were then exposed to high pressure co2 gas ( 800 psi ) for 48 hours to saturate the polymer with the gas . then , decreasing the gas pressure to ambient pressure created a thermodynamic instability . this led to the nucleation and growth of co2 pores within the polymer scaffolds . the sodium chloride particles were subsequently removed from the scaffolds by leaching the scaffolds in distilled water for 48 hours . while the above materials are preferred , it is believed that the present invention will work with polymers of diameters from 50 - 400 pm and with ha particles having diameters of 50 - 200 pm and salt particles having diameters from 50 - 300 pm . it is believed that the ratios of polymer to hydroxy apatite to nacl can vary by as much as 50 % without deviating from the spirit of this invention . the process for creating these plga / ha composite biomaterials can be summarized by the steps of : ( 1 ) grinding plga to small particles , ( 2 ) sieving the plga and sodium chloride particles to yield particles with a 100 - 200 pm diameter , ( 3 ) mixing the particles plga / ha / nacl in a mass ratio of 1 : 1 : 9 , ( 4 ) loading the mixture of particles into a disk mold , ( 5 ) compressing the mixture with a very high pressure for a certain amount of time , ( 6 ) exposing the newly formed disk to high pressure co2 gas long enough to saturate the disk , ( 7 ) decreasing the pressure on the disk until it returns to ambient pressure , ( 8 ) soaking the disk in distilled water to dissolve and leach out the sodium chloride particles . a further embodiment of the invention involves forming a uniform mineral coating of apatite on the surface of the plga / hydroxyapatite biomaterial . this apatite layer enhances the osteogenic potential of the biomaterial scaffold . the apatite layer is created by incubating the scaffolds in an ion rich simulated body fluid ( sbf ) solution . the solution is prepared by dissolving reagent grade nacl , nahco3 , na2so4 , kcl , k2hpo4 , mgcl2 . 6h2o , and cacl2 . 2h2o in distilled deionized water . 1 × sbf has the same ion concentrations as blood plasma while 5 × sbf has ion concentrations five times greater than blood plasma . the ph is adjusted to 6 . 4 with tris ( hydroxymethyl ) aminomethane . the described plga / hydroxyapatite biomaterial can be coated with apatite relatively quickly because the exposed hydroxyapatite particles act as nucleation sites for the growth of the mineral apatite layer in sbf solution . although the method for coating of polymeric biomaterial with apatite by incubating the biomaterial in sbf solution is already known , accelerated coating by incubating polymeric biomaterial with nano - hydroxyapatites exposed on the biomaterial surface has not been taught in the prior art . a study was conducted to compare the formation of an apatite layer on both plga and plga / nanohydroxyapatite scaffolds created by the gf / pl method . each was incubated in 1 × and 5 × sbf for up to five days . a series of brief evacuation - repressurization cycles were performed to force the solution into the pores of the scaffold . the sbf was refreshed every day . after various incubation times the samples were removed , rinsed , and dried in vacuum before being characterized . the plga and plga / nano - hydroxyapatite specimens were characterized . the morphologies of the scaffolds were examined by scanning electron microscopy ( sem ; jsm6330f , jeol , tokyo , japan ) after platinum coating . x - ray diffraction ( xrd ) spectra were obtained using an x - ray diffractometer ( d / max - 2500 , rigaku co ., tokyo , japan ) with a mixed incidence of 1 ° and a 2θ scanning rate of 2 . 5 °/ min in the range of 10 - 60 °. cu kα radiation , with a voltage of 40 kv and a current of 100 ma , was used for the diffraction . the xrd results are shown in fig1 . it is apparent that the plga / hydroxyapatite results show the higher intensity peaks expected from greater apatite formation on the plga / hydroxyapatite composite biomaterial . fourier transformed infrared spectroscopy ( ft - ir ) spectra were obtained using a ft - ir spectrometer ( avatar 360 , nicolet instrument corp ., madison , wis ., usa ) with a resolution of 8 cmil . for ft - ir analysis , the scaffolds were cut into fine particles , milled with potassium bromide , and pressed into transparent thin discs . the scaffold mass increase during apatite formation in sbf was measured using an analytical balance accurate to 10 - 4 g ( epg214c , ohaus corp ., pine brook , n . j ., usa ), and the data is shown in fig2 . finally , the scaffolds were air - dried and then vacuum dried . the mass increase from apatite formation was expressed as a percent increase compared to the scaffold mass when incubated in a tris - buffer at the same ph value , at the same temperature , and for the same time intervals . the data in fig3 shows that the plga / hydroxyapatite scaffolds ( circles ) gained the greatest mass due to apatite formation . furthermore , it is also evident that sbf solutions with higher ion concentrations lead to greater apatite deposition . sem micrographs of the apatite coated plga samples revealed that the apatite layer was not uniform . there were areas of bare plga , which shows that there was poor apatite deposition . however , the plga / hydroxyapatite scaffolds showed a more desirable , uniform apatite layer . more significantly , the apatite - coated plga / hydroxyapatite scaffolds exhibited noticeably improved cell growth and mineralization , when seeded with osteoblast cells , compared with apatite - coated plga scaffolds in vitro . this result supports the hypothesis that the uniform apatite layer is favorable for osteogenic properties . the biomimetic apatite coating process is enhanced by introducing nano - sized hydroxyapatite nucleation sites and by using concentrated sbf solution . this coating is advantageous because it conveys better osteogenic properties to the plga / hydroxyapatite biomaterial . a further embodiment of this invention involves the coating of plga / nanohydroxyapatite particles ( rather than scaffolds ) with a biomimetic , adherent , and uniform apatite coating . the particles may be the product of a reaction process or be ground down from bulk plga / nano - hydroxyapatite composite to a size of 30 - 2000 pm . the particles will be sieved to isolate particles with a more narrow size distribution depending on the desired application . these particles will then be soaked in sbf solution to coat them with a uniform layer of biomimetic apatite . most of the previous methods for fabricating polymer / bioceramic composite scaffolds , such as the solvent casting and particulate leaching ( sc / pl ) method or the phase separation method , use organic solvents . however , residual solvents in the scaffolds may be harmful to transplanted cells or host tissues . furthermore , the polymer coating on the ceramics created by polymer solutions may hinder the exposure of the ceramics to the scaffold surfaces ( fig4 a ), which could decrease the chance that osteogenic cells make contact with the bioactive ceramics . the preferred embodiment of the present invention relies on gas forming and particulate leaching ( gf / pl ) methods to fabricate plga / ha composite scaffolds for bone tissue engineering . this method efficiently exposes the bioceramic on the scaffold surfaces and avoids the use of organic solvents . to reduce the amount of ha ( which degrades extremely slowly in vivo ) required , and to increase the ha exposure to the scaffold surface , ha particles approximately 100 nm in size rather than micro - sized particles , are used to fabricate the composite scaffolds . the ha exposure at the scaffold surface in gf / pl scaffolds was compared to that in sc / pl scaffolds see fig4 a - c . porous plga / ha composite scaffolds were fabricated by the modification of a previously described gf / pl method of 24 . harris l d , kim b s , mooney d j . open pore biodegradable matrices formed with gas foaming . j biomed mater res 1998 ; 42 : 396 - 402 . plga / ha composites were prepared with 75 : 25 plga particles ( diameter = 100 - 200 mm , molecular weight = 100 , 000 da , birmingham polymers , birmingham , ala . ), ha nanoparticles ( diameter = approximately 100 nm , berkeley advanced biomaterials inc ., berkeley , calif . ), and sodium chloride particles ( diameter = 100 - 200 mm , sigma , st . louis , mo .). the plga pellets were ground using a tekmar grinder ( bel - art products , pequannock , n . j .) and sieved to obtain particles ranging from 100 to 200 mm . the salt particles were sieved to yield a range of sizes from 100 to 200 mm . the polymer particles were mixed with the ha and nacl particles . the plga / ha / nacl mass ratio was 1 : 1 : 9 . the mixture was loaded into a disk mold ( diameter = 1 . 35 cm ; aldrich chemical co ., milwaukee , wis .) and compressed at 2000 psi for 1 min using a carver laboratory press ( fred s . carver , inc ., menominee falls , wis .) to yield solid disks with a thickness of 1 . 7 mm . the samples were then exposed to high pressure co2 gas ( 800 psi ) for 48 h to saturate the polymer with the gas . then , decreasing the gas pressure to ambient pressure created a thermodynamic instability . this led to the nucleation and growth of co2 pores within the polymer scaffolds . the nacl particles were subsequently removed from the scaffolds by leaching the scaffolds in distilled water for 48 h . plga scaffolds without ha were also fabricated by the gf / pl method and used as a control ( gf / pl - no ha ). porous plga / ha scaffolds were also fabricated by the modification of a previously described sc / pl methods of wei g , ma p x . structure and properties of nano - hydroxyapatite / polymer composite scaffolds for bone tissue engineering . biomaterials 2004 ; 25 : 4749 - 57 ; cho s w , kim i k , lim s h , kim d i , kang s w , kim s h , et al . smooth muscle - like tissues engineered with bone marrow stromal cells . biomaterials 2004 ; 25 : 2979 - 86 ; cho s w , kim s s , rhie j w , cho h m , choi c y , kim b s . engineering of volume - stable adipose tissues . biomaterials 2005 ; 26 : 3577 - 85 ; kim b s , jeong s i , cho s w , nikolovski j , mooney d j , lee s h , et al . tissue engineering of smooth muscle under a mechanically dynamic condition . j microbiol biotech 2003 ; 13 : 841 - 5 , and were used as another control . in this process , plga was dissolved in methylene chloride ( j . t . baker , phillipsburg , n . j .) at a 10 % ( w / v ) concentration , and ha and nacl were added to the plga solution at the same sizes and ratios as for the gf / pl scaffolds . this mixture was loaded into teflon cylinders ( diameter = 21 . 5 mm , height = 25 mm ; cole - parmer instrument company , vernon hills , ill .). following solvent evaporation , the polymer disks with entrapped salt particles were removed from the molds . the salt was removed by immersing disks in distilled water for 48 h . the porosity of fabricated scaffolds was measured using mercury intrusion porosimetry ( autopore iv 9500 , micromeritics instrument corporation , norcross , ga .). a contact angle of 1301 for mercury on the scaffold was used for this analysis . the pore structures of the scaffolds were examined using a scanning electron microscope ( sem , jeol , tokyo , japan ). compression and tensile tests were performed with an instron mechanical tester ( instron 4201 , instrons , canton , mass .). the scaffold samples were cut into 1 × 1 cm2 for compression testing . for tensile testing , the samples ( 1 × 1 cm2 ) were attached to cardboard using epoxy glue . the sample was centered in a 7 mm slot in the center of the cardboard and then glued to standardize the gauge length . compression and tensile tests were performed with a constant strain rate of 1 mm / min . the moduli were determined from the slopes in the initial elastic portion of the stress - strain diagram . to examine the distribution and extent of surface exposure of ha in the scaffolds , the ha exposed to the scaffold surface was visualized with a hydrophilic dye ( trypan blue , sigma ) staining . the residual dye was removed by sonication in 100 % ethanol . afterwards , the surface of the plga / ha scaffolds was examined with a microscope ( camscope , samtech , seoul , korea ). to examine the chemical composition of the scaffold surface , we carried out x - ray photoelectron spectroscopic ( xps ; sigma probe , thermovg scientific , west sussex , uk ) analyses , evaluating the o 1s , c 1s , ca 2p , and p 2p peaks . the residual pressure in the spectrometer was 1 . 1 × 10 − 8 pa , and a mg anode ( 1 . 25 kev ) powered at 250 w was used as an x - ray source . the constant pass energy was 23 ev . all xps data were acquired at a nominal photoelectron takeoff angle of 551 . the area of the xps peaks was determined after background subtraction , and the atomic percentage was determined by normalizing the peak area of each element by the total peak areas of all elements . osteoblasts were isolated from the calvaria of neonatal ( less than one day old ) sprague - dawley rats ( slc , tokyo , japan ) by an enzymatic digestive process . the calvaria were isolated , and all connective tissues were carefully removed . the parietal bones were minced into pieces measuring about 1 × 1 mm 2 using sterile surgical scissors . osteoblasts were isolated by an enzyme solution containing 1 . 37 mg / ml collagenase type i ( sigma ) and 0 . 5 mg / ml trypsin ( sigma ). following 30 min of incubation , the released cells were discarded to prevent contamination with other cell types . the minced bones were redigested with the enzyme solution for 30 min , and the supernatant was transferred to the culture medium , dulbecco &# 39 ; s modified eagles medium ( dmem , gibco brl , gaithersburg , md .) containing 10 % ( v / v ) fetal bovine serum ( gibco brl ), 1 % ( v / v ) penicillin - streptomycin ( gibco brl ), 10 mm b - glycerophosphate ( sigma ), 50 mg / ml l - ascorbic acid ( sigma ), and 100 nm dexamethasone ( sigma ). this process was repeated three times , and then finally the collected solution was centrifuged for 10 min at 1500 rpm . cells were plated into tissue culture flasks and cultured in a humidified incubator at 37 ° c . with 5 % ( v / v ) co 2 . the fabricated scaffolds were sterilized by ethylene oxide gas and pre - wetted in the culture medium for 12 h . aliquots of 50 ml of the cell suspension ( 4 . 0 × 10 7 cells / ml , 2 . 0 × 10 6 cells / scaffold ) were seeded onto the tops of the pre - wetted scaffolds . the scaffolds were left undisturbed in an incubator for 3 h to allow the cells to attach to the scaffolds . an additional 1 and 10 ml of culture medium were added to each scaffold at 6 and 8 h , respectively . the cell / scaffold constructs were cultured in a humidified incubator at 37 ° c . with 5 % ( v / v ) co 2 for eight weeks . the medium was changed everyday . analytical assays were performed at 7 , 14 , 28 , and 56 days . to determine the seeding efficiency and cell growth on the scaffolds , cell numbers were determined by quantitative dna assays ( n = 3 ). dna was isolated using a wizard genomic dna purification kit ( promega , madison , wis .). for dna isolation , the cell / scaffold constructs were washed twice with phosphate - buffered saline . the specimens were placed in a 1 . 5 - ml tube and crushed with a homogenizer ( powergen 125 , fisher scientific , germany ). dna was isolated according to the kit protocol , and dna content was measured with an ultraviolet absorbance spectrophotometer ( jasco v - 530 , tokyo , japan ) at 260 nm . the cell numbers were calculated from a dna standard curve of identical cells . the alkaline phosphatase ( alp ) production of osteoblasts cultured on scaffolds was measured spectroscopically ( n = 3 ) using the methods of ekholm m , hietanen j , tulamo r m , muhonen j , lindqvist c , kellomaki m , et al . tissue reactions of subcutaneously implanted mixture of epsilon - caprolactone - lactide copolymer and tricalcium phosphate . an electron microscopic evaluation in sheep . j mater sci mater med 2003 ; 14 : 913 - 8 . the osteoblast / scaffold constructs were washed with pbs , homogenized with 1 ml tris buffer ( 1 m , ph 8 . 0 , sigma ), and sonicated for 4 min on ice . aliquots of 20 ml were incubated with 1 ml of a p - nitrophenyl phosphate solution ( 16 mm , sigma ) at 30 1 c . for up to 5 min . the production of p - nitrophenol in the presence of alp was measured by monitoring light absorbance at 405 nm . the amount of calcium deposited in the cell - scaffold constructs was measured using a previously reported method ( n = 3 ) of jaiswal n , haynesworth s e , caplan a i , bruder s p . osteogenic differentiation of purified , culture - expanded human mesenchymal stem cells in vitro . j cell biochem 1997 ; 64 : 295 - 312 . after the cell - scaffold constructs were rinsed twice with pbs and homogenized with 0 . 6 n hcl , calcium was extracted by shaking for 4 h at 4 ° c . the lysate was then centrifuged at 1000 g for 5 min , and the supernatant was used to determine calcium content . to measure the amount of calcium produced by the seeded osteoblasts , the calcium content of the plga / ha scaffold itself was also measured , and the calcium content of the scaffold itself was subtracted from the total calcium content of the lysate . the calcium concentration in the cell lysates was quantified spectrophotometrically with cresolphthalein complexone ( sigma ). three minutes after the addition of reagents , the absorbance of the samples was read at 575 nm using a microplate reader ( multiskan spectrum , thermo electron co ., vantaa , finland ). the calcium concentration was calculated from a standard curve generated from a serial dilution of a calcium standard solution ( sigma ). the surface and cross - sectional morphologies of the scaffolds and cell - scaffold constructs were examined using a sem . the samples were washed twice with pbs , prefixed in 1 % ( v / v ) buffered glutaraldehyde for 1 h , and fixed in 0 . 1 % ( v / v ) buffered formaldehyde for 24 h . the fixed samples were dehydrated in ascending grades of ethanol , dried , and mounted on aluminum stubs using double - sided carbon tape . the specimens were coated with gold using a sputter coater ( cressington 108 , cressington scientific instruments , cranberry , pa .) and examined with sem at an acceleration voltage of 10 kv . in addition to the culture of cell - scaffold constructs in vitro , cell scaffold constructs were implanted into the subcutaneous space of athymic mice ( balb / c - nu , 7 weeks old , female , slc , tokyo , japan ). after the mice were anesthetized with an intramuscular administration of ketamine hydrochloride ( 50 mg / kg , yuhan co ., seoul , korea ) and xylazine hydrochloride ( 5 mg / kg , bayer korea ltd ., seoul , korea ), small incisions were made on the dorsal skins of six mice . four pouches per animal were made by blunt dissection in subcutaneous sites , and cell - seeded scaffolds were immediately implanted into the pouches ( n = 4 ). subsequently , the skin was closed with 5 - 0 vicryl sutures ( ethicon , lenneke marelaan , belgium ). the mice were housed singly after surgery and received humane care in compliance with the hanyang university guidelines for the care and use of laboratory animals . the implants were retrieved for analysis at five and eight weeks after implantation . cell - scaffold constructs were retrieved from athymic mice at five and eight weeks after implantation ( fig8 b ), fixed in 10 % ( v / v ) buffered formaldehyde , dehydrated in ascending grades of ethanol , and embedded in paraffin . the tissue blocks were sectioned at a 4 - mm thicknesses and stained with hematoxylin and eosin ( h & amp ; e ) and masson &# 39 ; s trichrome . the masson &# 39 ; s trichrome - stained mid - portion sections were examined with a microscope for histomorphometry . the percentage of bone occupying space within the constructs was measured using an image analysis system ( ks400 , zeiss , munich , germany ) coupled to a light microscope . the bone formation area was expressed as the percentage of bone area in the available pore space ( bone area / pore area × 100 %). quantitative data were expressed as the mean standard deviation . statistical comparisons were carried out using analysis of variance ( anova , sas institute inc ., cary , n . c .). a value of p & lt ; 0 : 05 was considered to be statistically significant . gas foaming and the subsequent salt leaching of scaffolds containing a high percentage ( 90 %) of nacl particles ( diameter range 100 - 200 mm ) led to the formation of highly porous , open pore structures with no evidence of an external , nonporous skin layer ( fig4 c and d ). the pore structure observed in the cross - sections of the gf / pl scaffolds was similar to that of the scaffolds fabricated by the sc / pl method ( fig4 a and b ). the sc / pl method produced scaffolds with pore sizes of approximately 100 - 200 mm ( fig5 a ). in contrast , the gf / pl process resulted in scaffolds with two levels of porosity : interconnected macropores ( 100 - 200 mm ) were created by the leaching of the nacl particles , and smaller , closed pores ( 10 - 45 mm ) were created by the nucleation and growth of gas pores within the polymer particles ( fig5 b ). the average porosities of the gf / pl and sc / pl scaffolds were 91 ± 3 % and 85 ± 3 % respectively . the mechanical properties of the scaffolds were assessed using compressive and tensile mechanical tests . the gf / pl scaffolds exhibited enhanced mechanical properties as compared to the sc / pl scaffolds . the average compressive moduli were 2 . 3 ± 0 . 4 and 4 . 5 ± 0 . 3 mpa ( p & lt ; 0 : 05 ) for the sc / pl and gf / pl scaffolds , respectively . the average tensile moduli were 2 . 0 ± 0 . 1 and 26 . 9 ± 0 . 2 mpa ( p & lt ; 0 : 05 ) for the sc / pl and gf / pl scaffolds , respectively . these data represent a 99 % increase in the compression modulus and a 1331 % increase in tensile modulus , demonstrating the positive effects of the gf / pl fabrication process in enhancing the mechanical properties of the scaffolds . to determine whether the scaffold fabrication process affects the extent of ha exposure at the scaffold surface , the exposed ha was stained with a hydrophilic dye . ha was stained more abundantly in the gf / pl scaffolds than in the sc / pl scaffolds ( fig6 a , c and d ). the surface composition of the plga / ha composite scaffolds was also analyzed with xps . the amount of ca in the gf / pl scaffold surface was greater than in the sc / pl scaffold surface ( fig6 e and f ). the atomic ratio of the ca exposed on the scaffold surface was 156 % higher on the gf / pl scaffold surface compared with the sc / pl scaffold surface ( fig6 g ). both types of the plga / ha composite scaffolds allowed for the adhesion and proliferation ( fig7 a ) of the seeded rat calvarial osteoblasts over the 56 - day in vitro culture period . the initial cell seeding density of 2 . 00 × 106 cells / scaffold resulted in 1 . 33 × 106 cells / scaffold remaining attached to the gf / pl scaffold after one day in culture , giving an adhesion percentage of 66 . 5 %. for the sc / pl scaffold , the cell adhesion efficiency was 62 . 0 %. osteoblasts grew more rapidly in the gf / pl scaffolds than in the sc / pl scaffolds ( fig7 a ). the average cell density of the gf / pl scaffolds was 2 . 48 × 106 cells / scaffold after four weeks in culture , while that of the sc / pl scaffolds was 2 . 19 × 106 cells / scaffold , corresponding to 86 . 5 % and 69 . 7 % increases in cell density for the gf / pl and sc / pl scaffolds , respectively ( fig8 a and fig8 b ). the alp activity of the osteoblasts cultured on both types of plga / ha composite scaffolds increased during the four - week culture period and decreased at eight weeks ( fig7 b ). in contrast , the alp activity of the osteoblasts grown on the plga scaffolds without ha was low and did not show significant changes during the culture period . the osteoblasts on the gf / pl scaffolds showed significantly higher ( p & lt ; 0 : 05 ) levels of alp activity compared to the osteoblasts on the sc / pl scaffolds during the first four weeks of culturing , but showed no significant differences at eight weeks . the calcium deposition by cultured osteoblasts was significantly higher ( p & lt ; 0 : 05 ) on the gf / pl scaffolds than on the sc / pl scaffolds during the 8 - week culture period ( fig7 c ). the deposition on both types of the plga / ha scaffolds gradually increased during the culture period . on the plga scaffolds without ha , calcium deposition was significantly lower than on both types of the plga / ha scaffolds . the calcium deposition on the plga scaffolds remained constant at low levels for the first four weeks , and increased slightly at eight weeks . the implantation of both types of the osteoblast - seeded plga / ha composite scaffolds resulted in new bone formation in vivo in ectopic sites at five and eight weeks after implantation . five weeks after implantation , a small amount of woven bone was detected in both the sc / pl ( fig9 a and b ) and the gf / pl ( fig9 c and d ) scaffolds . eight weeks after implantation , osteogenesis had progressed , and more bone with lamellar structures appeared ( fig1 c - f ). histomorphometric analyses of the mid - portion sections of the regenerated tissues showed enhanced bone formation in the gf / pl scaffolds , compared with the sc / pl scaffolds and the plga scaffolds with no ha , at five and eight weeks after implantation ( fig1 a ). in contrast , the cell - seeded plga scaffolds with no ha had produced nearly no new bone in vivo for eight weeks . most of the pores of the plga scaffolds with no ha were filled with loose fibrous connective tissues without evidence of bone formation at five and eight weeks after implantation ( fig1 a and b ). the calcium deposition in the regenerated tissues was much more extensive in the gf / pl scaffold group than in the sc / pl scaffold group at five and eight weeks after implantation ( fig1 b ), although the calcium deposition in both groups increased with the implantation period . the calcium deposition in the plga scaffold group with no ha was far less than that in both ha - containing scaffold groups . the plga / ha scaffolds fabricated by the gf / pl method exhibited a higher exposure of ha at the scaffold surface and much better bone formation in vitro and in vivo than those fabricated by the conventional sc / pl method . as compared with other methods for fabricating biodegradable polymer / ceramic composite scaffolds , the gf / pl method has a number of advantages . first , the gf / pl process avoids the use of organic solvents . residual organic solvents remaining in scaffolds may damage transplanted cells and surrounding tissues . furthermore , exposure to organic solvents may inactivate biologically active factors . therefore , the gf / pl process may cause less denaturation of the growth factors incorporated within the scaffolds . second , the gf / pl method can efficiently expose bioceramics at the surface of the polymer / bioceramic composite scaffolds . staining with a hydrophilic dye and xps analysis showed that the gf / pl method exposed a significantly higher extent of ha at the scaffold surface than did the conventional sc / pl method in this study ( fig6 ). the sc / pl method causes the polymer coating on the bioceramics by polymer solutions , which hinders the exposure of bioceramics on the scaffold surfaces , while the 5 weeks 8 weeks gf / pl method , which does not use a polymer solution , efficiently exposes the bioceramics on the scaffold surface . therefore , a gf / pl scaffold can increase the chances of osteogenic cells to make contact with the bioactive ceramics , which enhances osteoblast differentiation and growth . third , the gf / pl scaffolds exhibit enhanced mechanical properties as compared to the sc / pl scaffolds . the gf / pl scaffolds had significantly higher compressive and tensile moduli than the sc / pl scaffolds . this might be due to a closer packing of the polymer chains under the high pressure in the gf / pl process and to be tensile alignment of the polymer chains by the polymer elongation that occurs during the foaming . in addition , the residual solvent in the sc / pl scaffolds may function as a plasticizer and make the polymer more ductile . although the gf / pl composite scaffolds showed greatly enhanced mechanical properties , as compared with the sc / pl composite scaffolds , the measured compressive moduli of the prepared scaffolds is rather low compared to that of human bone . this might be due to the highly porous structure of the fabricated scaffolds and the poor mechanical properties the use of gf / pl scaffolds resulted in enhanced osteogenic potentials in both in vitro and in vivo experiments . since the sc / pl and gf / pl scaffolds have similar physical properties such as porosity , pore size , and interconnectivity , the difference in osteogenic ability between the two scaffold types might be due to their different surface chemistries . enhanced bone formation in vitro and in vivo on the gf / pl scaffolds may result from the direct contact of seeded cells with the ha particles exposed on the scaffold surface , which stimulate the cell proliferation and osteogenic differentiation . in contrast , the ha particles would be coated with the polymer , which would hinder interaction with cells in the sc / pl scaffolds . since the sc / pl scaffold has a large portion of ha particles buried in plga polymer , the degradation of plga will expose ha particles on its surface . however , the plga degradation requires a long time and there will be no acceleration of bone formation by ha during this period . the alp activity on the plga scaffold without ha did not show any significant changes during the culture period in vitro , but the calcium concentration increased at 56 - days in this group . this disparity could be due to the fact that alp is an early marker for osteogenic differentiation and usually peaks early , while mineralization occurs continuously over the in vitro culture period . in this study , we used nano - sized ha particles to fabricate plga / ha composite scaffolds . since the highly crystalline ha degrades over long periods of time in vivo , the incompletely degrading residual ha may hinder or slow the complete bone healing . therefore , to reduce the total amount of ha while enhancing the ha distribution on the scaffold surface , we used nano - sized ha particles , which have a high surface area , to fabricate the composite scaffolds , instead of micro - sized ha particles . furthermore , the nano - sized ha particles showed improved bioactivity and osteointegration when implanted in the bone defect sites , as compared with the micro - sized ha particles . it has been also reported that protein adsorption and cell adhesion can be enhanced by using nano - sized ha particles instead of micro - sized ha particles . increasing interest has currently been focused on polymer / ceramic composite materials as bone substitutes because these materials have advantages over ceramic scaffolds and polymer scaffolds for bone tissue engineering . calcium phosphate - based ceramics , such as hydroxyapatite ( ha ) and tricalcium phosphate , have been used as bone substitutes , but these materials have poor mechanical performance . most synthetic polymer biomaterials have low surface wettability due to their composition of noncharged elements . such hydrophobic surfaces are unfavorable to osteogenic cells as they show a lower proliferative and a higher apoptotic rate on hydrophobic surfaces than on hydrophilic surfaces . in addition , these polymeric biomaterials have a bioinert surface that lacks bioactive functions for bone formation , therefore evoking minimal tissue responses . an essential requirement for bone grafts is the ability to create a bond with the living host bone through the formation of a biologically active bonelike apatite layer on the surface of the bone grafts . bioinert bone substitutes often become encapsulated with fibrous tissues , thereby resulting in disturbed bone formation . therefore , the addition of calcium phosphate ceramics to biodegradable polymers , such as poly ( glycolic acid ), poly ( l - lactic acid ), and poly ( d , l - lactic - co - glycolic acid ) ( plga ), would allow for better surface wettability as well as enhanced osteoconductivity . most of the previously available methods for the fabrication of polymer / ceramic composite scaffolds , such as the solvent casting and particulate leaching ( sc / pl ) method and the phase separation method , use organic solvents . however , residual solvents in the scaffolds may be harmful to transplanted cells or host tissues . furthermore , polymer coating on the ceramic particles by polymer solutions may hinder the exposure of the ceramics to the scaffold surfaces [ fig1 ( a ) ], which decreases the chance of contact between the osteogenic cells and the bioactive ceramics . we thought the gas foaming and particulate leaching ( gf / pl ) method would efficiently expose bioactive ceramics on the scaffold surfaces and that these efficiently exposed ceramics would thus enhance the osteoconductivity and wetability of the scaffolds . in the present study , we tested this hypothesis by transplanting scaffolds to rat skull critical size defects and examining the bone formation . the ha exposure to the scaffold surface was compared between gf / pl scaffolds and sc / pl scaffolds . bone regeneration using gf / pl scaffolds was evaluated in vivo and compared with that using sc / pl scaffolds . porous plga / ha composite scaffolds were fabricated by the modification of a previously described gf / pl method [ fig1 ( b ) ] of harris l d , kim b s , mooney d j . open pore biodegradable matrices formed with gas foaming , j biomed mater res 1998 ; 42 : 396 - 402 ; plga / ha composites were prepared with 75 : 25 plga particles ( diameter = 100 - 200 mm , molecular weight = 100 , 000 da ; birmingham polymers , birmingham , ala . ), ha nanoparticles ( diameter = approximately 100 nm ; berkeley advanced biomaterials , berkeley , calif . ), and sodium chloride particles ( diameter = 100 - 200 mm ; sigma , st . louis , mo .). the mixed plga / ha / nacl mass ratio was 1 : 1 : 9 . the mixture was loaded into a disk mold ( diameter = 13 . 5 mm ; aldrich chemical , milwaukee , wis .) and compressed at 2000 psi for 1 min using a carver laboratory press ( fred s . carver , menominee falls , wis .) to yield solid disks with a thickness of 1 mm . the samples were exposed to high pressure co2 gas ( 800 psi ) for 48 h to saturate the polymer with the gas . next , a thermodynamic instability was created by decreasing the gas pressure to ambient pressure . the nacl particles were subsequently removed from the scaffolds by leaching the scaffolds in distilled water for 48 h . plga scaffolds without ha were also fabricated by the gf / pl method ( gf / pl - no ha ). porous plga / ha scaffolds were also fabricated by the modification of a previously described sc / pl method set forth in lu h h , el - amin s f , scott k d , laurencin c t , three - dimensional , bioactive , biodegradable polymer - bioactive glass composite scaffolds with improved mechanical properties support collagen synthesis and mineralization of human osteoblast - like cells in vitro , j biomed mater res a 2003 ; 64 : 465 - 474 ; cho s w , kim i k , lim s h , kim d i , kang s w , kim s h , et al . smooth muscle - like tissues engineered with bone marrow stromal cells , biomaterials 2004 ; 25 : 2979 - 86 ; and kim b s , jeong s i , cho s w , nikolovski j , mooney d j , lee s h , et al ., tissue engineering of smooth muscle under a mechanically dynamic condition , j microbiol biotech 2003 ; 13 : 841 - 5 and used as another control . in this process , plga was dissolved in methylene chloride ( j . t . baker , phillipsburg , n . j .) at a 10 % ( w / v ) concentration , and ha and nacl were added to the plga solution at the same size and ratio as those of the gf / pl scaffolds . this mixture was loaded into teflon cylinders ( diameter = 21 . 5 mm , height = 25 mm ; cole - parmer instrument company , vernon hills , ill .). following solvent evaporation , polymer disks with entrapped salt particles were removed from the molds . the salt was removed by immersing disks in distilled water for 48 h . the dimension of fabricated sc / pl scaffolds was same with that of gf / pl scaffolds . the morphologies of the scaffolds were examined using a scanning electron microscope ( sem ; jsm - 6330f , jeol , tokyo , japan ). samples were dehydrated in ascending grades of ethanol , dried , and mounted on an aluminum stub using a double - sided carbon tape . the specimens were coated with platinum using a sputter coater ( cressington 108 , cressington scientific instruments , cranberry , pa .) and examined with sem at an acceleration voltage of 10 kv . the porosity of fabricated scaffolds was measured using mercury intrusion porosimetry ( autopore iv 9500 , micromeritics instrument corporation , norcross , ga .). a contact angle of 1308 for mercury on the scaffold was used for this analysis . compression and tensile tests were performed with an instron mechanical tester ( instron 4201 , instron ®, canton , mass .). the scaffold samples were cut into 1 × 1 cm 2 for compression testing . for tensile testing , the samples ( 1 × 1 cm 2 ) were attached to cardboard using epoxy glue . the sample was centered in a 7 mm slot in the centre of the card board and then glued to standardize the gauge length . compression and tensile tests were performed with a constant strain rate of 1 mm / min . the moduli were determined from the slopes in the initial elastic portion of the stress - strain diagram . to examine the distribution and the extent of surface exposure of ha in the scaffolds , ha exposed to the scaffold surface was visualized with a hydrophilic dye ( trypan blue ) staining or von kossa &# 39 ; s silver staining . for trypan blue ( sigma ) staining , the residual dye was removed by sonication in 100 % ethanol . for the von kossa staining , scaffolds were immersed in 2 % ( w / v ) silver nitrate ( sigma ) solution and placed directly in front of a bright lamp for 30 min . scaffolds were then rinsed in distilled water . the surface of the plga / ha scaffolds was then examined with a microscope ( camscope , samtech , seoul , south korea ). the scaffolds were then tested for their wettability . for this , trypan blue solution was dropped on top of the scaffold and the time required for complete absorption of the solution into the scaffold was measured . sprague - dawley rats ( 8 - week - old males , n = 24 ; slc , tokyo , japan ) were anesthetized with an intraperitoneal injection ( 5 mg / kg body wt .) of a 4 : 1 solution of ketamine hydrochloride ( ketara , yuhan , seoul , south korea ) and xylazine ( rompun , bayer korea , seoul , south korea ). after shaving the scalp hair , a longitudinal incision was made in the midline of cranium from the nasal bone to the posterior nuchal line , and the periosteum was elevated to expose the surface of the parietal bones . using a surgical trephine bur ( ace surgical supply , brockton , mass .) and a low - speed micromotor , a circular and transosseous defect ( 8 mm in diameter ) was produced in the parietal bone . the drilling site was irrigated with saline and the bleeding point was electrocauterized . the defect was filled with the fabricated scaffolds . the periosteum and skin were then closed in layers with resorbable 4 - 0 vicryl ® ( ethicon , edinburgh , uk ) sutures . the rats were housed singly after surgery and received humane care in compliance with seoul national university guidelines for the care and use of laboratory animals . the implants were retrieved 8 weeks after implantation for analyses . following euthanasia , the craniums including implanted scaffolds were retrieved at 8 weeks after surgery . samples were fixed immediately in a 10 % ( v / v ) neutral buffered formalin solution . one specimen from each condition was scanned using a desktop x - ray 3d micro computed tomography ( ct ; skyscan 1072 ®, skyscan bvba , aartselaar , belgium ) to analyze bone formation . a microfocus x - ray tube with a focal spot of 8 mm was used as a source and a 1024 × 1024 12 - bit digital ccd x - ray detector were used . the coronal view was imaged at 8 mm slices . each coronal ct image was reconstructed by v - works ™( cybermed , seoul , south korea ) to visualize the three - dimensional volume image of new bone and to examine the microarchitecture of the regenerated bone tissue . after collection of the micro ct scans , all samples ( n = 24 ) were decalcified in edta ( ph 6 . 0 ) for 7 days and embedded in paraffin . the tissue blocks were sectioned at a 4 mm thickness and stained with hematoxylin and eosin ( h & amp ; e ) and masson &# 39 ; s trichrome . the masson &# 39 ; s trichrome - stained mid - portion sections were examined with a microscope for histomorphometry . the percentage of bone - occupying space within the constructs was measured using an image analysis system ( ks400 , zeiss , munich , germany ) coupled to a light microscope . the bone formation area was expressed as percent bone area in available pore space ( bone area / pore area = 100 %). quantitative data were expressed as the mean 6 standard deviation . statistical comparisons were carried out using analysis of variance ( anova , sas institute , cary , n . c .). a value of p & lt ; 0 . 05 was considered to be statistically significant . gas foaming and subsequent salt leaching of scaffolds containing a high percentage of nacl particles led to the formation of highly porous structures with no evidence of an external , non porous skin layer [ fig1 ( b ) ]. the gf / pl scaffolds exhibited highly porous and open - pore structures . the pore structure observed in the cross - sections of the gf / pl scaffolds was similar to that of the sc / pl scaffolds [ fig1 ( a ) ]. the average porosities of the gf / pl and sc / pl scaffolds were ( 91 ± 3 )% and ( 86 ± 3 )%, respectively . the mechanical properties of the scaffolds were assessed using compressive and tensile mechanical tests . the gf / pl scaffolds exhibited enhanced mechanical properties as compared to the sc / pl scaffolds . the average compression modulus was 2 . 3 ± 0 . 4 and 4 . 5 ± 0 . 3 mpa ( p & lt ; 0 . 05 ) for the sc / pl and gf / pl scaffolds , f3 respectively [ fig1 ( a ) ]. the average tensile modulus was 2 . 0 6 0 . 1 and 26 . 9 6 0 . 2 mpa ( p & lt ; 0 . 05 ) for the sc / pl and gf / pl scaffolds , respectively [ fig1 ( b , c )]. these data represent a 99 % increase in compression modulus and a 1331 % increase in tensile modulus . to determine whether the scaffold fabrication process affected the extent of ha exposure to the scaffold surface , the exposed ha was stained with von kossa &# 39 ; s silver nitrate [ fig1 ( a ) ] and a hydrophilic trypan blue dye [ fig4 ( b ) ]. ha was stained more abundantly in the gf / pl scaffolds than in the sc / pl scaffolds with both staining methods ( fig1 ). in contrast , gf / pl scaffolds without ha ( gf / pl - no ha scaffolds ) showed no positive staining with either staining method [ fig1 ( a - c ) ]. to evaluate whether the hydrophilicity of the plga scaffold and plga / ha composite scaffolds could be improved by the addition of ha and by the application of different fabrication processes , respectively , the wettability of gf / pl , sc / pl , and gf / pl - no ha scaffolds was measured . when a trypan blue dye solution was dropped to the scaffolds , the solution was completely absorbed into the gf / pl scaffold within 5 s [ fig1 ( c ) ]. however , it was not absorbed at all into f5 the gf / pl - no ha scaffold even after 60 min because of the hydrophobic character of the scaffold [ fig1 ( a ) ]. the sc / pl scaffolds absorbed the dye solution slowly within 15 min [ fig1 ( b ) ]. the faster wetting of the gf / pl scaffold compared to that of the sc / pl scaffold correlated with the amount of ha exposed onto the scaffold surface . the implantation of both types of the plga / ha composite scaffolds into critical size defects in rat skulls resulted in enhanced bone formation in vivo compared with the plga scaffold ( fig1 ). eight weeks after implantation , new bone with lamellar structures and osteoid formation was appreciated in the sc / pl [ fig1 ( b , e ) and fig1 ( b , e )] and gf / pl [ fig1 ( c , f ) and 19 ( c , f )] scaffolds at f8 the defect edges and midsites of the grafts . the bone formation area in the gf / pl scaffold was higher than that in the sc / pl scaffold and gf / pl - no ha scaffold at 8 weeks after implantation ( fig2 ). the plga polymer with no ha produced very little new bone in vivo in the 8 weeks following implantation [ fig1 ( a , d ) and 19 ( a , d )]. most of the pores of the plga scaffolds were filled with loose fibrous connective tissues without evidence of bone formation [ fig1 ( d ) and 19 ( d ) ]. over time , the plga seemed to continuously degrade without adverse reactions , and had not completely resorbed during the 8 weeks following implantation . micro ct evaluation allowed the mineralized tissues to distinguish from the remaining soft tissues present inside the defects ( fig2 ). the reconstructed three - dimensional images showed the formation of new bone inside both types of the plga / ha composite scaffolds . within the scaffolds , dispersed irregular - shaped mineralized tissues were found throughout the implant . mineralized tissue areas were significantly larger in the gf / pl scaffolds than in the sc / pl and gf / pl - no ha scaffolds . as compared to the conventional methods for the fabrication of biodegradable polymer / ceramic composite scaffolds , the gf / pl method has several advantages . first , the gf / pl method avoids the use of organic solvents . residual organic solvents remaining in the scaffold may damage transplanted cells and surrounding tissues . furthermore , exposure to organic solvents may inactivate biologically active factors . therefore , the gf / pl method may cause less denaturation of growth factors incorporated within the scaffold . second , the gf / pl scaffold exhibits a higher exposure of ha to the scaffold surface than the sc / pl scaffold . the sc / pl method causes the coating of the ha by the polymer solution , which hinders the exposure of ha to the scaffold surfaces , while the gf / pl method efficiently exposes ha to the scaffold surface due to the lack of a requirement of a polymer solution . therefore , the gf / pl scaffold can increase the chances of contact between the osteogenic cells and the bioactive ceramics that are exposed on the scaffold surface . in addition , the enhanced exposure of hydrophilic ha nanoparticles to the scaffold surface affected the hydrophilicity of the scaffold . the hydrophobic surfaces of most synthetic polymer biomaterials are unfavorable to osteogenic cells , because they show a lower proliferative rate and a higher apoptotic rate on hydrophobic surface than on hydrophilic surfaces there fore the addition of ha to the plga scaffold may enhance the scaffolds hydrophilicity as well as its osteoconductivity . the scaffold fabrication process also affected the hydrophilicity of the plga / ha scaffolds . the faster wetting of the gf / pl scaffold compared with the sc / pl scaffold was likely due to higher amount of ha exposed to the surface of the gf / pl scaffold . the gf / pl scaffold exhibited enhanced bone regeneration in vivo when compared to the sc / pl scaffold . the plga in the plga / ha scaffolds is bioinert for bone formation . however , the ha exposed to the scaffold surface stimulates bone formation . enhanced bone formation on the gf / pl scaffolds may result from the direct contact of migrating osteogenic cells from the surrounding tissues with the ha particles exposed to the scaffold surface , which then stimulates cell proliferation and osteogenic differentiation . in contrast , the sc / pl scaffold had ha particles coated with the polymer , which hindered interaction of ha with the osteogenic cells and thus hindered the bone formation process . in this study , we used nanosized ha particles to fabricate plga / ha composite scaffolds . since the highly crystalline ha degrades over a long period in vivo , incompletely degraded residual ha may hinder or slow the complete bone healing . therefore , to reduce the total amount of ha while enhancing the ha exposure to the scaffold surface , nanosized ha particles , which have a high surface area , were used to fabricate the composite scaffolds instead of microsized ha particles . furthermore , nanosized ha particles showed improved bioactivity and osteointegration when implanted to the bone defect site compared to the microsized ha particles . protein adsorption and cell adhesion have also been reported to be enhanced by the use of nanosized ha particles compared to microsized ha particles . the plga / ha composite scaffolds of the present invention show enhanced hydrophilicity and osteoconductivity compared with the sc / pl scaffolds . this enhancement was most likely due to a higher extent of exposure of ha particles to the scaffold surface . the biodegradable polymer / bioceramic composite scaffolds fabricated by the gf / pl method could enhance bone regeneration efficacy for the treatment of bone defects compared with conventional composite scaffolds . one of skill in the art will be readily aware that while polyglycolic acid polymers are most preferred due to their osteoconductive and osteoinductive properties , other polymers can be used to achieve similar results to the present invention . such other polymers include but are not limited to a bioabsorbable , or biodegradable , synthetic polymer such as a polyanhydride , polyorthoester , polylactic acid , and copolymers or blends thereof . non - degradable materials can also be used to form the matrix . examples of suitable materials include ethylene vinyl acetate , derivatives of polyvinyl alcohol , teflon , and nylon . the preferred non - degradable materials are a polyvinyl alcohol sponge , or alkylation , and acylation derivatives thereof , including esters . collagen can be used , but is not as controllable and is not preferred . these materials are all commercially available . non - biodegradable polymer materials can be used , depending on the ultimate disposition of the growing cells , including polymethacrylate and silicon polymers . those of skill in the art are familiar with the use of bone graft materials . the present invention can be used in the same manner as any other bone graft material . in a preferred delivery system , the bone graft material is mixed with polyethylene glycol to form a paste . the material can be premixed and sold in a syringe for easy application or can be mixed at the point of use and delivered via any convenient means . when provided in a dry state , any suitable biocompatible fluid can be used to wet the material and create a paste for administration to the patient . examples of such biocompatible fluid wetting agents include , but are not limited to : dextrose , glucose , maltose or sodium chloride solutions , blood , serum , platelet concentrate , bone marrow aspirate , and synovial fluid . a biological fluid can be used in the form obtained from the biological source , or it can be processed by application of one or more desired useful techniques , examples of which include , separation techniques , such as filtration ( macro -, micro -, or ultra - filtration ); purification techniques , such as dialysis ; concentration techniques ; and sterilization techniques . one of skill in the 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