Sol-gel production of bioactive nanocoatings for medical applications. Part II: current research and development

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Sol-gel production of bioactive nanocoatings for medical applications. Part II: current research and development
Andy H Choi & Besim Ben-Nissan†
†Author for correspondenceDepartment of Chemistry, Materials and Forensic Science, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, AustraliaTel.: +61 295 141 784;Fax: +61 295 141 460;E-mail: [email protected]

part of

Keywords: bone nanocoating, hydroxyapatite, implant

10.2217/17435889.2.1.51 © 20

Over the years, the use of hydroxyapatite as coatings for medical devices and drug-delivery systems has gone through a revolution – from being a rarity to being an absolute necessity. Without these coatings, many medical implants and devices would never reach their true potential in their intended applications, such as in the dental and orthopedic fields. Coatings of hydroxyapatite are often applied to metallic implants to alter their surface properties. The aim of this article is to present an evaluation of the published work regarding current research and applications and to review the methods used for the production of hydroxyapatite nanocoatings.

Numerous studies have found hydroxyapatite(HAp) to be a favorable material for orthopediccoatings. HAp-coated implants have demon-strated extensive bone apposition in animalmodels. Development of good implant–boneinterfacial strength is thought to be a result ofthe biological interactions of released calciumand phosphate ions. Quality HAp-coatedimplants heal faster and attach more completelyto the bone. The long-term performance of a cal-cium phosphate-coated implant depends oncoating properties (thickness, porosity, phasesand crystallinity), implant surface roughness andoverall design.

Three different properties determine the suc-cess rate of an implant: bioactive, bioinert andbiodegradable. Bioactive coatings are materials(natural or synthetic) that are built up to the sur-face of an implant to promote and enhance bio-logical interactions and hence accelerate fixation.The goal of calcium phosphate, specificallyHAp, as a bioactive coating is to achieve rapidbiological fixation to bone. Biological fixation isdefined as the process by which prosthetic com-ponents become firmly bonded to the host boneby bone in-growth, without the use of adhesiveor mechanical fixation.

Synthetic HAp is chemically similar to themineral component of bones and hard tissues inmammals. Biological apatites are the inorganicphases of calcified tissues (teeth and bones) andhave been idealized as calcium HAp. However,they are different in lattice parameters and in theassociation of other ions (Mg and CO3) and areusually calcium-deficient [1].

Since porous HAp has unfavorable mechani-cal properties, it cannot be used for load-bearingpurposes. This generated an impetus for the

production and use of a thin and thick filmcoating on metallic alloys. Because HAp is bio-active, using it as a thin film on biometallicalloys, such as titanium, enables the coupling ofthe two materials’ primary properties to form afunctional single component [2].

All metallic orthopedic and dental implantsare bioinert and do not bond chemically to bone.Thus, the only means of fixation is mechanicalinterlock, whereby the implant must be manu-factured in such a way that it possesses suitablesurface roughness by micro- or macro-texturing.By increasing the surface roughness, the surfacearea increases, which in turn increases the area offixation. Other methods available at present tofix implants firmly in place are the use of screwsor bone cement, which are both used currentlyin dental and orthopedic implants.

An understanding of the mechanisms by whichbiofixation occurs with the use of HAp coatings ispertinent. It has been suggested that the apatitemust first partially dissolve, thereby increasing theconcentration of calcium and phosphate in themicroenvironment. Carbonate apatite microcrys-tals then form and associate with the organicmatrix of bone, causing biological growth of bonetissue [3].

Nanostructured HAp via thesol-gel methodSynthetic production methods for HAp over thepast 30 years have been extensive, especiallyonce it was discovered that it has nearly the samemineral component as bone and that it can beimplemented as a bone substitute material.

Most published information on HAp is classi-fied under calcium phosphates, to which HApbelongs. Therefore, the chemical properties will

07 Future Medicine Ltd ISSN 1743-5889 Nanomedicine (2007) 2(1), 51–61 51

PERSPECTIVE – Choi & Ben-Nissan

52

Table 1. Chemical fophosphate compou

Chemical compound

Amorphous calcium pho

Dicalcium phosphate

Tricalcium phosphate

Tricalcium phosphate

Tetracalcium phosphate

be viewed from the standpoint that HAp is a cal-cium phosphate, although it will have differentsolubility and reactivities to other phosphateswithin the physiological environment.

Calcium phosphates are characterized byparticular solubilities, such as when bondingto the surrounding tissue, and their ability todegrade and be replaced by advancing bonegrowth. The solubilities of various calciumphosphate compounds are given in Table 1 andcan be shown as: amorphous calcium phos-phate (ACP) > dibasic calcium phosphate(DCP) > tetracalcium phosphate (TTCP) >α-tricalcium phosphate (α-TCP) > β-TCP> HAp.

As the calcium phosphate or HAp comesinto contact with an aqueous solution, its sur-face ions can be exchanged with those of theaqueous solution or various ions and moleculescan be adsorbed on the surface, such as collagenand proteins.

Another pertinent issue is the stability of HApat elevated temperatures. This has been coveredin plasma- or spray-coated materials [4].

Sol-gel technology offers an improved alter-native technique for producing bioactive sur-faces for better bone attachment owing to itsnanocrystalline characteristics, homogeneityand reactivity.

Various sol-gel routes have been used for theproduction of synthetic HAp. A number ofgood studies have been carried out on a range ofprecursors to produce pure nanocrystalline apa-tites for medical applications. The major onesare calcium acetate, calcium alkoxide, calciumchloride, calcium hydroxide, calcium nitrateand dicalcium phosphate dihydrate (Table 2).The coating thickness in the range of70–9000 nm has been reported by some of theinvestigators. Table 3 describes the thickness ofthe sol-gel-derived HAp coatings produced bysome of the investigators, as well as the substratematerials used in their studies.

Calcium acetateAqueous sol-gel chemistry routes based onammonium–hydrogen phosphate as the phos-phorus precursor and calcium acetate monohy-drate as the source of calcium ions have beendeveloped by Bogdanoviciene and colleagues toprepare HAp powder samples with differentmorphological properties [5]. In the sol-gel proc-esses, an aqueous solution of ethylene diaminetetra-acetic acid (EDTA) or tartaric acid (TA) ascomplexing agents was added to the reactionmixture. The monophasic Ca10(PO4)6(OH)2samples were obtained by calcination of precur-sor gels for 5 h at 1000°C. They have demon-strated that the proposed aqueous sol-gelmethods are very simple, inexpensive and thusappropriate for the large-scale production ofcalcium HAp ceramics.

Syntheses of calcium phosphates frommethyl, ethyl, i-propyl and n-butyl phosphatesand calcium acetate in acidic and basicwater–alcoholic solutions were studied by Cihlarand Castkova [6]. A mixture of HAp and β-TCPwas achieved through the reaction of all alkyl-phosphates with calcium acetate in an acidicsolution. Calcium pyrophosphate was achievedwhen the reaction proceeded in the presence ofammonium hydroxide. HAp with an admixtureof calcium pyrophosphate or β-TCP was pre-pared by means of monoethanolamine ordiethanolamine catalysts.

Calcium alkoxideMasuda and colleagues investigated the alkoxide-based system containing calcium diethoxide, tri-ethyl phosphite, ethanediol and ethanol, modi-fied with water and acetic acid [7]. Within thissystem, they produced powders and found thatthe determining factor for the composition ofthe resultant powder was the solution’s pH.

Layrolle and Lebugle developed a synthesisroute of different calcium phosphates, usingcalcium diethoxide (Ca(OEt)2) and orthophos-phoric acid (H3PO4) as reagents and anhydrousethanol as a solvent [8]. By a simple variance ofthe ratio of reagents, calcium phosphates of var-ious chemical compositions Cax(HPO4)y(PO4)zare precipitated in the ethanol. The solids thatformed were characterized by different physico-chemical and thermal analyses. The resultsindicated that the different solid calcium phos-phates are amorphous and nanosized and havelarge specific surface areas and high reactivi-ties. Layrolle and colleagues also described theproduction of an amorphous, nanosized and

rmula of various calcium nds.

Abbreviation Chemical formula

sphate ACP

DCP CaHPO4

α-TCP Ca3(PO4)2

β-TCP Ca3(PO4)2

TTCP Ca4O(PO4)2

Nanomedicine (2007) 2(1) future science groupfuture science group

Sol-gel production of bioactive nanocoatings for medical applications – PERSPECTIVE

future science groupfuture science group

Table 2. Various calcnanocrystalline hyd

Calcium and phosph

Calcium acetate

Calcium alkoxide

Calcium chloride

Calcium hydroxide

Calcium nitrate

Calcium phosphate

carbonate-containing calcium phosphate powdersynthesized from calcium diethoxide and phos-phoric acid in ethanol via a sol-gel method [9].They concluded that, after sintering, thedecomposition of carbonated HAp generated amicroporous ceramic with an average pore sizeof 0.2 µm and an open porosity of 15.5% andthat this microporous bioceramic can be used asbone filler.

Chai and colleagues [10], Ben-Nissan and col-leagues [11] and Green [12] all employed an alkox-ide process to synthesize HAp powders andcoatings via the sol-gel technique. Milev andcolleagues used multinuclear nuclear magneticresonance (NMR) spectroscopy to monitor thesynthesis of HAp for powder and coating pro-ductions [13]. HAp was prepared by dissolvingcalcium diethoxide in a solvent containingethylene glycol and acetic acid.

Zreiqat and colleagues investigated theeffect of surface chemistry modification of tita-nium alloy (Ti–6Al–4V) with zinc, magnesiumor alkoxide-derived hydroxy carbonate apatite(CHAP) on the regulation of key intracellularsignaling proteins in human bone-derived cells(HBDCs) cultured on these modifiedTi–6Al–4V surfaces [14]. They have concludedthat the surface modification with CHAP orMg may contribute to successful osteoblastfunction and differentiation at the skeletaltissue–device interface.

Calcium chlorideAndersson and colleagues developed a degrada-ble, hierarchically porous apatite compositematerial from a simple low-temperature synthe-sis [15]. HAp is synthesized through a sol-gelmethod at near-room temperature conditions.

ium and phosphorous precursors used in the synthesis of roxyapatite.

orus precursor Authors Chemical formula Ref.

Bogdanoviciene et al. Ca(CH3COO)2·H2O / (NH4)2HPO4 [5]

Cihlar and Castkova Ca(CH3COO)2 [6]

Masuda et al. Ca(OEt)2 / P(OC2H5)3 [7]

Layrolle et al. Ca(OEt)2 / (H3PO4) [8,9]

Chai et al. Ben-Nissan et al. Green Milev et al.

Ca(OEt)2 / (C2H5O)2P(O)H [10]

[11]

[12]

[13]

Andersson et al. CaCl2·2H2O / Na2HPO4·2H2O [15]

Sarig and Kahana CaCl2 / NaH2PO4 [16]

Wang et al. Ca(OH)2 – PVA / H3PO4 [17]

Tkalcec et al. Ca(O2C8H15)2 / C16H35O4P [18]

Han et al. Ca(NO3)·4H2O / (NH4)2HPO4 / citric acid [19]

Weng et al. Ca(NO3)·4H2O / PO(OH)3-x(OEt)x / citric acid [20]

Balamurugan et al. Ca(NO3) / P(OC2H5)3 [21]

Kim et al. Ca(NO3)2·4H2O / P(OC2H5)3 [22]

Stoch et al. Ca(NO3)2·4H2O / (NH4)3PO4·3H2O [23]

Yang et al. Ca(NO3)2·4H2O / P2O5 [24]

Gan and Pilliar Ca(NO3)2·4H2O / P(OC2H5)3 and Ca(NO3)2·4H2O / (NH4)3PO4·3H2O

[25]

Cheng et al. Ca(NO3)2·4H2O / P4O10 and NH4PF6 and

Ca(NO3)2·4H2O / P2O5 / CF3COOH / N(CH2CH2OH)3

[26,27]

Bose and Saha Ca(NO3)2 / (NH4)3HPO4 [28]

Hsieh et al. Ca(NO3)2·4H2O / (C2H5O)3PO [29]

Lim et al. Ca(NO3)2·4H2O / H3PO4 [30]

Shin et al. CaHPO4·2H2O / CaCO3 [36]

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Starting materials used in this investigationwere CaCl2·2H2O, Na2HPO4·2H2O, cetyltri-methylammonium bromide (C16TAB), ammo-nia solution and tetraethoxysilane (TEOS).Two stock solutions were prepared fromNa2HPO4·2H2O and CaCl2·2H2O, using dis-tilled and deionized water. C16TAB was dis-solved in phosphate-containing stock solution,after which ammonia was added. The mixturewas kept under constant stirring in a closedbeaker. Gel-formation was induced upon addi-tion of the calcium chloride stock solution tothe mixture. The resulting gel was reported toconsist of HAp and sodium calcium phosphate.The hybrid material is shown to induce calciumphosphate formation efficiently under in vitroconditions and also works simultaneously as acarrier system for drugs.

Sarig and Kahana produced nanocrystallineplate-shaped particles of HAp precipitateddirectly from dilute calcium chloride andsodium phosphate solutions [16]. The directprecipitation of HAp was achieved by submit-ting the aqueous solutions of calcium and phos-phate to microwave irradiation immediatelyafter mixing.

Calcium hydroxideSol-gel inorganic–organic hybrid materialcoated on glassy carbon electrode for use in theimmobilization and study of double-strandedDNA with redox-active molecules was devel-oped by Wang and colleagues [17]. They pro-duced a hybrid material coating consisting ofnano-HAp-polyvinyl alcohol (PVA). Thehybrid was prepared from phosphoric acidsolution and saturated calcium hydroxide solu-tion, while the PVA was dissolved in distilledwater. Mixing a suitable quantity of PVA inHAp-sol can prevent the HAp coating from

cracking and enhance its stability, and theproperties of the nano-HAp–PVA hybrid areaffected by the concentration of PVA.

HAp and TCP powders and coatings wereprepared by Tkalcec and colleagues via the sol-gel technique to study the formation mecha-nism of the crystalline phases in the firing proc-esses of coatings with different Ca:P molarratios [18]. Calcium hydroxide was suspended inethanol and ethylhexanoic acid (EHA) wasadded drop-wise to this suspension. The solu-tion was then filtered by pressure filtering toobtain a clear solution of calcium 2-ethylhex-anoate (Ca(O2C8H15)2). Ca(O2C8H15)2 and2-ethyl-hexyl-phosphate were used as calciumand phosphorus precursors, respectively. Coat-ings were performed on Si-wafer and Ti-alloysubstrates by dipping the substrates into sols atroom temperature. They have concluded thatdip-coating and sintering in two cycles yielded ahomogeneous and dense coated film with athickness of 250 nm.

Calcium nitrateCalcium nitrate (Ca(NO3)2) is a popular calciumprecursor for the synthesis of HAp.

The citric acid sol-gel combustion methodwas used by Han and colleagues for the synthe-sis of nanocrystalline HAp powder from cal-cium nitrate, diammonium hydrogenphosphate and citric acid [19]. HAp powder hasbeen sintered into a ceramic in order to illus-trate its sinterability, open porosity, flexuralstrength and structural property. The scanningelectron microscope (SEM) micrographsrevealed that there were many micropore sizesranging between 1 and 5 µm with irregularshape. Although the open porosity of theresulting ceramic is approximately 19%, thepore size is not good for bone in-growth. After

Table 3. The substrate material and thickness of the sol-gel-derived hydroxyapatite coating reported by some investigators.

Authors Substrate material Thickness of coating Ref.

Chai et al. Al2O3, Vycor glass, PSZ, Ti-6Al-4V and single crystal MgO

70 nm [10]

Zreiqat et al. Ti-6Al-4V ~70 nm [14]

Tkalcec et al. Single-crystal silicon wafers, Ti-30Nb-3Al and Ti-5Al-2.5Fe

250 nm [18]

Kim et al. Commercially pure Ti 800–900 nm [22]

Gan and Pilliar Ti-6Al-4V 1–2 µm [25]

Hsieh et al. Ti-6Al-4V 3–9 µm [29]

Lim et al. Ti-6Al-4V 0.12–0.13 µm [30]




sintering at 1200°C, the grain size is approxi-mately 3 µm, which is approximately 20-timesthat of HAp powder calcined at 750°C andwith a flexural strength of 37 MPa.

Weng and colleagues also investigated theeffect of the addition of citric acid on the for-mation of sol-gel-derived HAp [20]. Toimprove the gelation in the sol-gel preparationof HAp by using Ca(NO3)2 and PO(OH)3-

x(OEt)x as precursors, citric acid was selectedas an enhancing gelation additive. HApderived from the mixed precursor solutionswith citric acid showed a different reactionpath from that without citric acid. They havesuggested that citric acid plays a role inenhancing gelation through the strong coordi-nation ability of calcium ions with citrategroups. They also concluded that the additionof citric acid can also provide a way to synthe-size a pure oxy-HAp or an apatite with carbon-ated HAp, HAp and minor β-TCP, whichmight have a good bioactivity.

Balamurugan and colleagues developed anovel sol-gel technique for the synthesis phaseof pure HAp powder [21]. Triethyl phosphiteand calcium nitrate were used as phosphorusand calcium precursors. The powders obtainedwere dried and calcined at different tempera-tures up to 900°C. X-ray diffraction analysisand Raman spectra were reported to show thepresence of phase pure HAp.

HAp composites with titania (TiO2) werecoated on titanium substrate by a sol-gel routeconducted by Kim and colleagues and themechanical and biological properties of thecoating systems were evaluated [22]. Calciumnitrate tetrahydrate (Ca(NO3)2·4H2O) and tri-ethyl phosphite (P(OC2H5)3) were hydrolyzedtogether with ethanol and distilled water.Ammonium hydroxide (NH4OH) was addedstepwise to the mixture. To produce a TiO2 sol,titanium propoxide (Ti(OCH2CH2CH3)4) washydrolyzed within an ethanol-based solutioncontaining diethanolamine((HOCH2CH2)2NH) and distilled water. Theprepared HAp and TiO2 sols were mixedtogether and stirred vigorously to obtainHAp–TiO2 composite sols. Coatings were pro-duced under a controlled spinning and heattreatment process. The HAp–TiO2 compositecoating layers were homogeneous and highlydense, with a thickness of approximately800–900 nm. The adhesion strength of thecoating layers with regard to the titanium sub-strate increased as the TiO2 addition was

increased. The highest strength obtained was ashigh as 56 MPa, with an improvement ofapproximately 50% when compared with pureHAp (37 MPa). The osteoblast-like cells grewand spread actively on all the composite coat-ings. They concluded that the sol-gel-derivedHAp–TiO2 composite coatings possess excel-lent properties for hard-tissue applications fromthe mechanical and biological perspective.

Stoch and colleagues produced HAp coat-ings on titanium and its alloy for facilitatingand shortening the processes towardsosseointegration [23]. HAp coatings wereobtained by the sol-gel method with sol solu-tions prepared from calcium nitrate tetrahy-drate and triammonium phosphate trihydrateas the calcium and phosphorous sources. Twotypes of gelatine were added to the sol:agar–agar or animal gelatine. Both enhancedthe formation and stability of amorphous HApusing soluble salts as the sources of calcium andphosphate. The biological activity of phosphatecoatings was observed in the simulated bodyfluid (SBF). The chemical composition andstructure of HAp coatings depends on the pHand final thermal treatment of the layer.

Sol-gel synthesis and template preparationof nanomaterials to yield a new general routefor fabricating highly ordered HAp nanowirearrays were synthesized by Yang and colleaguesusing a porous anodic aluminum oxide (AAO)template from a sol-gel solution containingP2O5 and Ca(NO3)2 [24]. The AAO templatemembrane was immersed into this sol for thedesired amount of time and allowed to dry inair. Excess sol on the membrane surface waswiped off carefully and then heat-treated in anopen furnace. HAp nanowire arrays wereformed inside the pores of the AAO template.Various characterization techniques, such astransmission electron microscope (TEM),SEM, x-ray diffraction (XRD) and x-ray pho-toelectron spectroscopy (XPS), were applied toexamine the structure of HAp nanowires. HApnanowires have a uniform length and diameterand form highly ordered arrays, which aredetermined by the pore diameter and thethickness of the applied AAO template. Theauthors believe that their novel method of pre-paring highly ordered HAp nanowires with alarge area might be very important in manybiomedical applications.

Thin sol-gel derived calcium phosphateCa–P films formed on sintered porous-surfacedimplants as an approach to increasing the rate



56

Figure 1. Proceduresample production

DTA: Differential thermalNMR: Nuclear magnetic TGA: Thermogravimetric

EtOH Ca(OEt)2

Solution 31P NMR

TGA/DTA

SEM

of bone in-growth were investigated by Ganand Pilliar [25]. Porous-surfaced dental implants(Endopores implants) were used for the sol-gelcoating of sintered porous surface structures.The porous surface was created by sintering Ti-6Al-4V particles of 45–150-µm diameter ontoa machined Ti-6Al-4V substrate. The filmswere prepared using both an inorganic precur-sor solution (with calcium nitrate tetrahydrateand ammonium dihydrogen phosphate) and anorganic precursor solution (with calciumnitrate tetrahydrate and triethyl phosphite).They reported that both approaches resulted inthe formation of nanocrystalline carbonatedHAp films but with different Ca:P ratios andstructures. They commented that, while boththe inorganic and organic methods resulted infilms with nanocrystalline or submicron crys-talline carbonated HAp films, the inorganicmethod formed films that differed significantlyin structure, displaying a more irregular surfacetexture and being less dense. Cross-sectionalTEM studies revealed an interfacial reactionproduct phase, when using the inorganicmethod, and calcium titanium oxidewas developed.

A sol-gel method was utilized by Cheng andcolleagues to synthesize a fluoridated HAp(FHA) phase [26]. Calcium nitrate tetrahydrate,phosphoric pentoxide and ammonium hexa-fluorophosphate were used as precursors. TheCa, P and F precursors were mixed under desig-nated proportions to form solutions with a Ca:Pratio of 1.67. To obtain an FHA phase withvarious fluorine contents, different amounts ofammonium hexafluorophosphate were added inthe Ca–P mixed solutions.

In 2001, Cheng and colleagues also devel-oped a sol-gel method to synthesize aHAp/fluoroapatite (FA) solid solution [27]. Cal-cium nitrate-4 hydrate, phosphoric pentoxideand trifluoroacetic acid (TFA) were used as theprecursors. Triethanolamine (TEA) was used asa promoter for incorporating fluorine into Caphosphates. Mixed ethanol solutions of the Caand P precursors in the Ca:P ratio of 1.67 withdifferent amounts of TFA and TEA were pre-pared; the mixed solutions were dried on a hotplate to convert them to the as-prepared pow-ders. After the powders were calcined at tem-peratures up to 900°C, HA/FA solid solutionswere obtained.

Bose and Saha described the synthesis of HApnanopowders using a sol-gel route with calciumnitrate and ammonium hydrogen phosphate ascalcium and phosphorous precursors, respec-tively [28]. Sucrose was used as the templatematerial and alumina was added as a dopant tostudy its effects on particle size and surface area.The average particle size of mesoporous HApsamples was 30–50 nm.

A simple rapid-heating method was devel-oped successfully by Hsieh and colleagues forcalcium phosphate coatings on Ti-6Al-4V sub-strates deposited by using a sol-gel-derived pre-cursor [29]. The preparation of the precursorwas carried out by mixing Ca(NO3)2·4H2Oand (C2H5O)3PO in 2-methoxy ethanol.Upon aging, the as-prepared solution wasclosely capped and placed in an oil bath for16 h. Upon gelation (drying), the solvent wasevaporated in the same oil bath so that a vis-cous precursor was obtained. Adhesive strengthtests were conducted and the results indicatedthat, at the first coating layer, using either spinor dip coating, the breakages occur at theglue–coating interface, representing an adhe-sive strength higher than 90 MPa. Thus, thefirst layer is adhered firmly to the substrate.Hsieh and colleagues also discovered that aporous structure, with a pore size 10–20 µm, is

used for the synthesis of the solutions, and analysis techniques.

analysis; FT-IR: Fourier transform infrared; resonance; SEM: Scanning electron microscope; analysis; XRD: X-ray diffraction.

Et(OH)2HOP(OEt)2 EtOH

Mixing

Dry N2 atmosphere

Powders, platelets or coatings

Hydrolyze

Heat-treated

FT-IR

XRD




formed on the outermost coating surface bythe sol-gel method. It was reported that thisstructure is a result of the fast decompositionduring rapid heating of the precursor depositedon the substrate and is very suitable forin-growth of living cells.

Lim and colleagues studied the bioactivities ofthe coating by examining the variation of ionconcentrations of Ca and P in simulated bodyfluid after soaking, using an inductively coupledplasma-atomic emission spectrometer [30].Ti/HAp-coating solutions with variable HApconcentrations were derived from calciumnitrate (Ca(NO3)2·4H2O) and phosphoric acid(H3PO4) dissolved in ethylene glycol mono-methyl ether (CH3OCH2CH2OH). Coatingsurfaces, after soaking in simulated body fluid,indicated significant morphological changes byfield emission-SEM.

Sol-gel HAp-nanocoated coralline apatiteCoral mineral has had considerable successconsidering its porous structure, which rangesfrom 150 to 500 µm. It is similar to thehuman cancellous bone and is one of a limitednumber of materials that will form chemicalbonds with bone and soft tissue in vivo. How-ever, coralline calcium carbonate is unsuitablefor most implant purposes, owing to its highdissolution rate.

Current commercially available materialsusing hydrothermal conversion only achievepartial conversion of coralline calcium carbon-ate to HAp. Unfortunately, being limited tothe outer surface, the inner core remains asunconverted calcium carbonate of the originalcoral [31]. This material has the advantage ofretaining a favorable pore size and bioactivity,with improved biodegradation properties anddurability, compared with native coral. Unfor-tunately, this also generates an unknown factorof biodegradation owing to the differing solu-bility rates of these two structures. As a result,this material is subject to dissolution in thephysiological environment, compromisingdurability, tissue integration and, ultimately,the longevity.

The application of a HAp sol-gel coating ontothe monophasic HAp derived from the hydro-thermal method increased its mechanical proper-ties [32,33]. This nanocoating was reported toincrease the biaxial strength by twofold of the con-verted coralline HAp. Coral, in addition to themacro-pores, also contains meso- and nano-pores

ranging between 5 and 50 nm within a some-how fibrous structure that makes the solidmatter between the macropores.

The method involves a unique two-step conver-sion procedure. First, corals possessing a suitablemacropore structure are converted to pure HAp,with complete replacement of calcium carbonateby phosphatic material throughout the specimen.The HAp can be prepared from the washed coralby reaction in a hydrothermal reactor. The reac-tion can be carried out at 250°C with an ammo-nium monohydrogen phosphate solution for apredetermined period of time. After conversion,the product is washed with water to extensivelyremove excess ammonium monohydrogen phos-phate. The second stage involves the sol-gel-derived HAp of the converted coralline apatite.

In general, the alkoxide method used involvesthe formulation of a homogeneous solutioncontaining all of the component metals in the

Figure 2. Scanning electron microscope images of the coral structure before and after the convertion and coating process.

(A) Coral structure. (B) Converted hydroxyapatite coral. (C) Converted and nanocoated coralline apatite.

A

B

C300 nm



58

Figure 3. Bone regemesenchymal cells f

(A) Bone marrow cells fro(HAp). (B) The marrow cmedium. (C) After 2–3 winserted into a bone defeModified from [35].

A

correct stoichiometry. Mixtures of metal alkox-ides and/or metal alkanoates in organic sol-vents, which have been stabilized againstprecipitation by chemical additives (e.g.,amines, glycols or acetylacetone), have proventhe most successful [2]. The procedures used forthe preparation of HAp samples and theanalysis techniques are shown in Figure 1.

All synthetic procedures for the preparationof sols were carried out in a dry nitrogenatmosphere owing to the hygroscopic nature ofthe starting alkoxide precursors. The calciumalkoxide precursor solution was prepared bydispersing calcium diethoxide (Ca(OEt)2) inabsolute ethanol (Et(OH)), followed by theaddition of ethylene glycol (C2H4(OH)2) todissolve the Ca(OEt)2, maintaining vigorousstirring throughout.

Phosphorous precursor solution was thenprepared by diluting triethyl phosphite(P(OEt)3) in absolute Et(OH) whilemaintaining stirring.

After complete dissolution of the calciumdiethoxide, the phosphorous precursor solu-tion can be added drop-wise to the calciumprecursor solution. It is necessary to maintainslow addition of the solutions to avoid anyconcentration gradients within the sol and toprevent any possible precipitation. Vigorous stir-ring was maintained throughout the addition.The clear solutions were aged at ambient temper-ature before being used. Coatings were formedusing these solutions, followed by subsequentheat treatment for further densification.

Detailed analyses using SEM revealed that thecoating step effectively obliterates the surfacemeso- and nano-pore systems, while leaving themacropore system intact (Figure 2).

This has significant effects: the improvementof the mechanical properties compared withtraditional coral materials. Biaxial mechanicaltesting indicated that the products haveincreased the mechanical strength by 120% [33].It is anticipated that this new patented processand material can be applied to bone graft appli-cations where high strength requirements andlongevity are pertinent.

Future perspectiveRecently, tissue engineering has been directedtoward taking advantage of the combined use ofliving cells and 3D ceramic scaffolds to delivervital cells to damaged sites in the body.

The process of bone regeneration is commonto the repair of fractures. The incorporation ofbone grafts, the skeletal homeostasis and the cas-cading sequence of biological events are oftendescribed as the remodeling cycle. These eventsrequire a blood supply as a system of humoralfactors (cytokines) that integrate and regulatethese events. Cytokine families include membersof the transforming growth factor (TGF)-βsuperfamily. The TGF-β superfamily causes therecruitment of mesenchymal stem cells and theirdifferentiation into chondrogenic and osteogenicpopulations (Figure 3) [34].

Stem cells have been incorporated into a rangeof bioceramics. Cultured bone marrow cellsderived from adult stem cells can be regarded asmesenchymal precursor cell populations and aresimilar to stem cells in that they can also differ-entiate into different lineages, which are osteo-blasts, chondrocytes, adipocytes and myocytes.When implanted, these cells can combine withmineralized 3D scaffolds to form highly vascu-larized bone tissue. These nanoscale culturedcell–bioceramic composites can be used to treatfull-thickness gaps in lone bone shafts, providingexcellent integration of the ceramic scaffold withbone and good functional recovery.

Nanoscale coatings and surface modificationmethods are being used currently to producebody-interactive materials that help the bodyto heal and promote regeneration of tissues,thus restoring physiological function. Thisapproach is being explored in the developmentof a new generation of nanobioceramics with awidened range of applications in maxillofacialand orthopedic surgery.

neration therapy using marrow or bone-graft application.

m a syringe are combined with porous hydroxyapatite ell/HAp composite is cultured in an osteogenic eeks of culture, the cultured bone graft/HAp hybrid is ct.

B C




Executive summary

Introduction

• The performance of ainfluence the properti

• Synthetic HAp has deand bioactivity.

Synthetic HAp via the

• Calcium phosphates ato degrade and be rep

• There have been variophosphorous precurso

• The major methods aphosphate dihydrate.

• Calcium nitrate has b

Sol-gel HAp-coated co

• Current commerciallycarbonate to HAp.

• The sol-gel coating on• This nanocoating was

in comparison to the • Ultrastructural analysi

meso- and nano-pore

Future perspective

• Nanostructured mate• Stem cells have been

scaffolds to form high• Nanoscale coatings an

the body to heal and • The current research a

broad spectrum of medrug-delivery systems

• The high surface areastandards, risk assessm

• Currently, standards sexposure risks to nano

• More thorough resear

There has been a major increase in interest innanostructured material in advanced technologies,such as medical technology, during the last decades.

Nanostructured materials are associated with avariety of uses within the medical field, for exam-ple, nanoparticles in drug-delivery systems, in bio-materials science and diagnostic systems and inregenerative medicine.

Although the impact of nanotechnology iswidely considered to be very beneficial, considera-tion has to be given to the potential risk associatedwith such structures. Situations can arise thatcause diseases, for example, in the workplace,related to toxic dust nanoparticles.

Currently, standards seem absent for the use ofnanomaterials in medicine, with no effective

methods to measure and assess exposure risks tonanoparticles in patients. Nanoparticle exposurelimits do not exist and manufacturers have noobligation currently to publish details of thesafety checks imposed on their nanoproducts.

Problems may also arise from incomplete nano-medicine safety and toxicity profiles, which in turnmay influence socio-economic issues in addition toa large range of long-term medical problems. Thefocal point of these discussions is the size of thesenanomaterials – typically one billionth of a meter;that is to say, approximately 70-times smaller thana red blood cell in size and close to a DNA mole-cule in diameter. There is concern that thesedimensions might allow them to penetrate the skinand possibly even elude the immune system to

hydroxyapatite (HAp)-coated implant depends on the coating chemistry and synthesis methods that es (thickness, porosity, HAp content and crystallinity), implant surface roughness and overall design.monstrated an excellent means of fixation for biomedical implants, achieving good biocompatibility

sol-gel method

re characterized by particular solubilities, such as when bonding to the surrounding tissue and their ability laced by advancing bone growth.us sol-gel methods used for the production of synthetic HAp using different calcium and rs.

re calcium acetate, calcium alkoxide, calcium chloride, calcium hydroxide, calcium nitrate and calcium

een a popular calcium precursor for the synthesis of HAp.

ralline apatite

available materials using hydrothermal conversion only achieve partial conversion of coralline calcium

to the monophasic coralline HAp has increased its mechanical properties. reported to increase the biaxial strength by twofold of the converted and then nanocoated coralline HAp, converted-only coralline.s using scanning electron microscope has revealed that the coating step effectively obliterates the surface systems, while leaving the macro-pore system intact.

rials are associated with a variety of uses within the medical field.incorporated into a range of bioceramics. When implanted, these cells can combine with mineralized 3D ly vascularized bone tissue.d surface-modification methods are being used currently to produce body-interactive materials that help

promote regeneration of tissues, thus restoring physiological function.nd development in the field of new and unique nanoceramics are encouraging. They can be used for a dical applications, such as implantable medical devices, in tissue engineering and in slow and targeted

. and very small size of nanoparticles means that traditional worker safety regulations regarding exposure

ents, measurement methods and equipment will need modifications.eem absent for the use of nanomaterials in medicine, with no effective methods to measure and assess particles in patients or healthcare workers.ch is required to produce more stable nanostructured material by emulating nature.



60

reach the brain. For now, however, the lack ofgenuine scientific data on the potential hazardsof nanotechnology on human health and theenvironment has misled the discussions: debateabout the risks of nanotechnology today trulyamounts to the perceived risks of nanotechnol-ogy – since the technical, scientifically estimatedrisks remain elusive.

The current research and development in thefield of new and unique nanocoatings are encour-aging. They can be used for a broad spectrum ofmedical applications, such as implantable medi-cal devices, in tissue engineering and in drug-delivery systems. Nanobioceramics are funda-mental to the design and development of a widevariety of medical devices and implants.

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Sol-gel production of bioactive nanocoatings for medical applications. Part II: current research and development