Bioactive materials for biomedical applications using sol–gel technology This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 Biomed. Mater. 3 034005 (http://iopscience.iop.org/1748-605X/3/3/034005) Download details: IP Address: 171.67.34.69 The article was downloaded on 23/08/2012 at 08:35 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Bioactive materials for biomedical applications using sol–gel technology
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
2008 Biomed. Mater. 3 034005
(http://iopscience.iop.org/1748-605X/3/3/034005)
Download details:
IP Address: 171.67.34.69
The article was downloaded on 23/08/2012 at 08:35
Please note that terms and conditions apply.
View the table of contents for this issue, or go to the journal homepage for more
Home Search Collections Journals About Contact us My IOPscience
Received 2 December 2007Accepted for publication 18 April 2008Published 8 August 2008Online at stacks.iop.org/BMM/3/034005
AbstractThis review paper focuses on the sol–gel technology that has been applied in many of thepotential research areas and highlights the importance of sol–gel technology for preparingbioactive materials for biomedical applications. The versatility of sol–gel chemistry enables usto manipulate the characteristics of material required for particular applications. Sol–gelderived materials have proved to be good biomaterials for coating films and for theconstruction of super-paramagnetic nanoparticles, bioactive glasses and fiberoptic applicatorsfor various biomedical applications. The introduction of the sol–gel route in a conventionalmethod of preparing implants improves the mechanical strength, biocompatibility andbioactivity of scaffolds and prevents corrosion of metallic implants. The use of organicallymodified silanes (ORMOSILS) yields flexible and bioactive materials for soft and hard tissuereplacement. A novel approach of nitric-oxide-releasing sol–gels as antibacterial coatings forreducing the infection around orthopedic implants has also been discussed.
(Some figures in this article are in colour only in the electronic version)
1. Sol–gel technology
Sol–gel technology is a wonderful advancement in scienceand requires a multidisciplinary approach for its variousapplications. It is the process of making ceramic andglass materials at a relatively low temperature that allowsdoping of various inorganic, organic and biomoleculesduring the formation of a glassy matrix. The sol–gel process was known as early as the 1800s, but inthe last two decades sol–gel applications have increasedmanyfold. It has been used for the fabrication of opticalfibers, optical coatings, electrooptic materials, nanocrystallinesemiconductor-doped xerogels, colloidal silica powders forchromatographic stationary phase and as catalytic support,nanoporous carbon xerogels and aerogels as hydrogen storagematerials, luminescence concentrators, tunable lasers, activewave guides, semiconducting devices, sunscreen formulations(sol–gel pearls) and chemical sensors for detecting gases,heavy metals and pH, and for many biosensor applications.Potential applications of sol–gel technology in the areasof defense, nanotechnology, environmental monitoring andbiomedical devices are now continuously emerging [1, 2].
In this review, we have focused on bioactive materialsfor biomedical applications using sol–gel technology. Thelow processing temperature of sol–gel technology combinedwith the intrinsic bio-compatibility (the ability of materialsnot to produce a significant rejection or immune responsewhen they are inserted into the body) and environmentalfriendliness makes it an ideal technology for the fabricationof bioactive materials. A bioactive material is defined as amaterial that elicits a specific response at the interface of thematerial, which results in the formation of a bond betweenthe tissue and that material [3]. In addition, the ability ofsol–gel technology to manipulate the structure of materials atthe molecular level as well as its ability to precisely controlthe nature of interfaces make it an interesting approach for awide range of practical applications. The sol–gel technologybased on various alkoxides allows production of conventionalsilica glasses as well as multi-component materials, mergingsilicates with titanates, borates and a variety of other oxides.The alkoxide gel method can also be used for the production ofcertain nonsilicate oxide glass-like materials (e.g. ZrO−
2 etc).Using sol–gel technology, organic–inorganic hybrid materialscan be prepared either by dissolution of organic molecules
Biomed. Mater. 3 (2008) 034005 R Gupta and A Kumar
Figure 1. Schematic diagram showing the sol–gel process and its various products.
in a liquid sol–gel or by impregnation of a porous gel in theorganic solution. Another way is to use inorganic precursorcontaining an organic group or carry out sol–gel reactions in aliquid solution to form chemical bonds in the hybrid gel [4].
2. Sol–gel-derived biomaterials
The broad range of possible applications of sol–gel-derivedmaterials and biomaterials can be seen from the increasingnumber of publications in the literature. The sol–gel processhas been known since the late 1800s. The versatility of thetechnique was rediscovered in the early 1970s when glasseswere produced without high-temperature melting processes[5]. In general, the sol–gel process involves the transition of asystem from a liquid ‘sol’ (mostly colloidal) into a solid ‘gel’phase. The schematic of sol–gel process and various productsand sol–gel reactions are shown in figures 1 and 2, respectively.The starting materials used in the preparation of the sol areusually inorganic metal salts or metal organic compounds suchas metal alkoxides [M(OR)n] where M represents a network-forming element such as Si, Ti, Zr, Al, B, etc, and R is typicallyan alkyl group. The most commonly used precursors aretetramethyl-orthosilicate (TMOS) and tetraethyl-orthosilicate(TEOS) in the sol–gel process. The basic sol–gel reactionbegins when metal alkoxide is mixed with water and a mutualsolvent (mostly alcohol) in the presence of acid or basecatalyst. Generally, both the hydrolysis and condensationreactions occur simultaneously once the hydrolysis reaction
has been initiated. Hydrolysis leads to the formation ofsilanol groups (≡Si–OH) and condensation reactions producesiloxane bonds (≡Si–O–Si≡), resulting in the production ofalcohol and water as by-products. The chemical reactionsoccurring during the sol–gel process strongly influence theproperties and composition of the final material. Furtherprocessing of the sol enables one to make sol–gel materialsin different configurations. Thin films can be produced on apiece of substrate by dip, spin and spray coating. During thesol–gel transformation, the viscosity of the solution graduallyincreases as the sol becomes interconnected to form a rigid,porous network of gel. With further drying and heat treatment,the gel can be converted into dense ceramic or glass particles.During the drying process (at ambient pressure), the solventliquid is removed and substantial shrinkage occurs. Theresulting material is known as a xerogel. When solventremoval occurs under hypercritical (supercritical) conditions,the network does not shrink and a highly porous, low-densitymaterial known as an aerogel is produced. Heat treatment ofa xerogel at elevated temperature produces viscous sintering(shrinkage of the xerogel due to a small amount of viscousflow) and effectively transforms the porous gel into a denseglass. As the viscosity of the sol is adjusted into a properviscosity range, ceramic fibers can be drawn from the sol.Ultra-fine and uniform ceramic powders are formed byprecipitation, spray pyrolysis or emulsion techniques [6–8].
The sol–gel-derived materials provide excellent matricesfor entrapping a variety of organic and inorganic compounds
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Figure 2. Chemical reactions in sol–gel synthesis.
and biologically important molecules. The conventional sol–gel processing methods that involve either low pH or highalcohol levels destabilize biological molecules and causesubstantial loss of biological function upon entrapment, andsignificant structural changes in biomolecules occur uponentrapment and as the materials age. In recent years, anumber of sol–gel-derived materials have been designed withthe purpose of making the matrix more compatible withentrapped biological molecules for developing biosensors.New biocompatible silane precursors and processing methodshave recently been reported based on glycerated silanes,sodium silicate, or aqueous processing methods that involveremoval of alcohol by-products by evaporation before theaddition of biomolecules [9]. Typical applications of sol–gel biomaterials include selective coatings for optical andelectrochemical sensors and biosensors, stationary phases foraffinity chromatography, immunoadsorbent and solid-phaseextraction materials, controlled release agents, solid-phasebiosynthesis, and unique matrices for biophysical studies [4].
3. Bioactive sol–gel materials for biomedicalapplications
Applications utilizing sol–gel as a porous material toencapsulate sensor molecules, enzymes and many othercompounds are most common; however, some potentialapplications of sol–gel-derived materials in biomedicalapplications are fast emerging. Biomedical applicationsrequire the design of new biomaterials and this can be achievedby merging sol–gel chemistry and biochemistry. The gel-derived materials are excellent model systems for studyingand controlling biochemical interactions within constrainedmatrices with enhanced bioactivity because of their residualhydroxyl ions, micro-pores and large specific surface [10]. Inall biomedical applications, the coating of the medical devicesis an important issue. Materials used in medical devices
should have appropriate structural and mechanical propertiesand ideally promote a healing response without causing severebodily reactions. Medical device designers use varioussurface treatments such as coating that enhance or modifyproperties such as lubricity, hydrophilicity/hydrophobicity,functionality and biocompatibility. Sol–gel technology offersan alternative technique for producing bioactive surfaces forvarious biomedical applications. Sol–gel thin film processingoffers a number of advantages including low-temperatureprocessing, ease of fabrication and precise microstructuraland chemical control. The sol–gel-derived film or layer notonly provides a good degree of biocompatibility, but also ahigh specific surface area (which can be used as a carrier ofadsorbed drugs) and an external surface whose rich chemistryallows easy functionalization by suitable biomolecules. Also,controlling the thickness and pore-size distribution of thesilica coating provides a direct method to tailor the rate andduration of drug release. The rate of diffusion and releaseof a drug are correlated with the thickness and porosity ofcoated films that can be controlled via the withdrawal speedand sol compositions using the sol–gel dip-coating technique.The advantages of using the sol–gel dip-coating technique arethat it is independent of substrate shape and that control oversurface properties and a high degree of thickness uniformityare achievable [11–13]. Gao et al [14] evaluated the effects ofthe amount of channeling agents, the addition of colloidalsilica and the pH of the dissolution media on the releaseof the drug hydrochlorothiazide from compressed tablets.These tablets were spray coated using poly(dimethylsilaxone)(PDMS) lattices with various polyethylene glycol (PEG)loadings as channeling agents. The rate of drug release wasfound to be constant in coated tablets containing up to 25%(w/w) PEG. Higher amounts of PEG resulted in nonlinearrelease patterns. The addition of colloidal silica decreased therates of drug release. The pH of dissolution media affectedthe structures of the exposed PDMS films. Scanning electron
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Figure 3. Energy filtered transmission electron microscopy(EFTEM) of silica-coated iron nanoparticles. Reproduced withpermission from Fernandez-Pacheco et al [16].
microscopy (SEM) and density measurements showed thatthe films obtained after soaking in higher-pH media weremore condensed, with corresponding changes in drug-releaserates. Radin and Ducheyne [15] described the synthesisof thin, resorbable, controlled release bactericidal sol–gelfilms on a Ti-alloy substrate and determined the effect ofprocessing parameters on its degradation and vancomycinrelease. Vancomycin is a potent antibiotic used in treatingosteomyelitis. A close correlation between release anddegradation rates suggested that film degradation is the mainmechanism underlying the control of release and depends uponsol–gel processing parameters. The bactericidal propertiesof released vancomycin and the biocompatibility of the sol–gel films suggest great potential to prevent and treat boneinfections in a clinical setting.
The development of magnetic nanoparticles that can beused as drug delivery vectors remains a significant challengefor material scientists. Fernandez-Pacheco et al [16] describeda simple and inexpensive method for the preparation ofencapsulated magnetic nanoparticles consisting of a metalliciron core and an amorphous silica shell by using a modificationof the arc-discharge method. The nanoparticles thus obtainedpresent a much stronger magnetic response than any compositematerial produced up to now involving magnetic nanoparticlesencapsulated in inorganic matrices, and the rich chemistryand easy functionalization of the silica outer surface makethem promising materials for their application as magneticcarriers and can allow the binding of antibodies, proteins,medical drugs or other biomolecules to the system. Energy-filtered transmission electron microscopy (EFTEM) of silica-coated iron nanoparticles is shown in figure 3. Silica coatinghelps to make the particles biocompatible, preventing theiraggregation and the degradation of the metallic core, andreducing the extent of clearance by the reticuloendothelialsystem. Beganskiene et al [17] prepared and characterized
the modified sol–gel-derived silica coatings in which aminoand methyl groups were introduced onto the colloidal silica.The coatings of colloidal silica (water contact angle 17◦),polysiloxane sol (61◦) methyl-modified sols (158◦ and 46◦)with various wettability properties were tested for cellproliferation. Methyl-modified coating has proved to be thebest substrate for cell proliferation.
There is an increasing interest in the use of opticaltechniques for applications such as local treatment oftumors. Recently, much effort has been directed towardthe development of novel methods including photodynamictherapy (PDT), which means the nonthermal destructionof tumors by combined action of a chemical compound(photosensitizer) and low- or medium-energy laser radiationand interstitial laser-induced thermotherapy with laser energywhich is far lower than that used in ordinary laser surgery.Interstitial laser-induced thermotherapy is a quite newtreatment modality designed for minimal invasive destructionof pathologic tissues in difficult-to-access environments (e.g.brain, liver). Fiber-optic laser applicators are used to performinterstitial therapy with laser light, where the applicator isinserted into the pathologic lesion and curing laser lightis guided through the fiber [18–20]. Silica-based sol–gelcoatings are also used for production of fiber-optic applicatorsfor laser therapies as sol–gel-derived materials are opticallytransparent, frequently used for the construction of fiber-opticsensors and allow optical measurements. These are alsorelatively safe and biocompatible for use within the humanbody. Sol–gel coatings on fiber cores, depending on the waterto alkoxide (R) ratio, influence the light distribution and soit is possible to obtain various shapes of light beam emittedfrom the applicator [21–23]. In the following subsectionsvarious sol–gel-derived bioactive materials for biomedicalapplications are discussed.
3.1. Sol–gel-generated mixed-oxide layers
The adsorption of proteins of the body fluid to implant surfacesdepends on the properties of the surface oxide layer, especiallythe electronic structure. Therefore, tailoring of the oxide layeris a method for influencing protein adsorption. Schenk-Meuseret al [24] coated titanium plates by the sol–gel process withmixed oxides containing the biocompatible elements Ti, Nb,Zr and Ta and tailored the produced oxide layer by usingmixed precursors. The design of a flexible technologicalprocess for high-performance ceramics production beginswith the nanocrystalline powder synthesis. Manufacturingnanocrystalline powders based on ZrO2 requires a high degreeof process control to achieve the desired microstructuralcharacteristics, e.g., small defect size, high dispersity of otherphases and homogeneity of the grain-boundary composition.These powders can be used in manufacturing scalpels forpediatric, eye and neuro-microsurgery as well as in ceramicpassive bioimplants and solid electrolytes for fuel elements[25]. The complex doping of ZrO2 with yttria and ceria makesit possible to produce fine-grained materials of high strengthand resistance to low-temperature ageing. Introduction ofcorundum in the matrix of ZrO2 (Y2O3, CeO2) solid solutions
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increases the fracture toughness and modulus of elasticity ofthese materials. Materials in the ZrO2–Y2O3–CeO2–Al2O3
system will allow one to provide the necessary propertiesof ceramic implants and surgical cutting tools [26–28].Shevchenko et al [29] produced and characterized such a typeof powder using a complex method, which integrates sol–geltechnology and hydrothermal synthesis.
Superparamagnetic nanoparticles have been found to bevery useful in biomedical applications such as in magneticresonance imaging (MRI), targeted drug delivery and magneticseparation [30]. In bioanalysis, luminescence has beenextensively utilized for detection and sensing. However,organic dyes have widely been used as signaling sourcesbased on their emission properties but suffer from severephotobleaching during the detection process. In some cases,their low signaling intensity limits the achievable detectionsensitivity. To solve these limitations, it is important todevelop highly sensitive and photostable signaling materials.Great efforts have been made toward designing new labelingmaterials such as quantum dots, resonance light scatteringparticles and dye-doped nanoparticles. Of these materials,the last type appears most promising. By encapsulatingthousands of dye molecules into one protective nanoparticle,excellent photostability and enhanced signal density are tobe expected. Silica turns out to be a very good material fora protective matrix on account of its proven biocompatibilityand stability in most biosystems. Moreover, silica chemistry iswell known, and standard chemistry protocols can be followedto conjugate various biomolecules to the silica surface, thusenabling silica-based particles to couple and label biotargetswith selectivity and specificity [31]. Tan and co-workers[32–35] have synthesized dye-doped silica nanoparticles witha reverse microemulsion technique and demonstrated theirpotential in biodetection and shown an increase of signalintensity by four orders of magnitude. Ma et al [31]combined the two useful functions, superparamagnetism andluminescence, along with an easily conjugated silica surfaceinto multifunctional nanoarchitecture. The direct attachmentof dye molecules to magnetic nanoparticles resulted inluminescence quenching. To avoid this problem, the magneticcore was surrounded by the first silica shell, and then thedye molecules were doped inside a second silica shell toconcentrate the emission signal and enhance the photostability.The authors presented the sol–gel synthesis and detailedstructural, magnetic and optical characterizations of double-shell structure. In recent years, the magnetic propertiesof nanometer-sized iron oxide powders such as α-Fe2O3,γ -Fe2O3 and Fe3O4 have widely been studied for biomedicalapplications. These applications require small particle size,discrete and superparamagnetic iron oxide nanoparticles thatcan be successfully prepared by the sol–gel method due tolower annealing temperature [36]. An et al [36] reporteda novel synthesis of γ -Fe2O3 magnetic nanoparticles by asol–gel method and characterized their superparamagneticproperty for biomedical applications.
3.3. Sol–gel-based hydroxyapatite (HA) biomaterialscaffolds for hard tissue regeneration
Engineering new bone tissue with cells and a syntheticextracellular matrix represents a new approach for theregeneration of mineralized tissues compared with thetransplantation of bone. This require a scaffold materialupon which cells can attach, proliferate and differentiate intoa functionally and structurally appropriate tissue for the bodylocation into which it will be placed. Bone tissue is consideredas minerals and proteins and the minerals are mostly apatitessuch as hydroxyapatite (HA) [Ca10(PO4)6(OH)2], fluorapatiteand carbonate-apatite. In general, HA is a main componentof bone mineral and in some cases carbonate-apatite is amain hard tissue component as in dental enamel. HA iswidely accepted as a bioactive material and has excellentbiocompatibility with hard tissues and high osteoconductivitydespite its low degradation rate, mechanical strength andosteoinductive potential. HA has widely been used in tissueengineering especially in bone and cartilage regeneration.Porosity in the HA structure is important since it allowsfor the growth of tissue and bone around a supportingframework and allows passage of nutrients. The porosity inthe human bone structure varies widely with the type andfunction. Sol–gel technology offers an alternative techniquefor producing tailored porous bioactive and osteoconductiveSi-substituted HA, which is a highly promising material in thesense of bioactivity improvement [37–41]. Blanch et al [42]developed a methodology for improving HA biocompatibilityand bioactivity of HA scaffolds appropriate for any specificbiomedical application through controlling composition,impurity concentration, crystal size and morphology usingsol–gel technology. Ravikrishna et al [41] described thenovel application of polyaphron templates for the generationof three-dimensional porous hydroxyapatite structures usingthe sol–gel chemistry. Polyaphrons are biliquid foams wheremicrometer size oil droplets are encapsulated within a waterfilm and regarded as a special class of high internal phaseratio emulsions (HIPRE) that can potentially be used in thecontinuous processing of long-range-ordered macroporoussolids [43, 44]. The advantages that polyaphrons offer in thesynthesis of porous material are (a) the stability of the templateduring processing, (b) the relative ease in the removal of thetemplate and (c) the capability of controlling the pore sizes byvarying several operational parameters. If one of the reactantsin the sol–gel reaction is aqueous, the process can be initiatedin the aqueous phase of the polyaphrons that surround sphericaloil cores, thus forming the basis for a porous solid. The porousstructure of the bulk HA under different magnifications isshown in figure 4. These studies demonstrated the potential ofsol–gel technology for the synthesis of porous hydroxyapatitestructures using a low-temperature sol–gel process which canlead to several biomedical applications that require porous HAwith controlled porosities using a low-energy process.
3.4. Sol–gel-derived hydroxyapatite-coated films on metalimplants
The mechanical strength of HA is fairly poor and therefore,for many purposes, bulk materials cannot be used as implants.
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(a) (b)
(c) (d )
Figure 4. Scanning electron microscopic (SEM) images of porous HA at various magnifications: (a) 5000× image showing details of thepore structure, (b) 1000× magnification showing the three-dimensional pore structure, (c) 300× magnification illustrating the long-rangeporous structures synthesis and (d) HA synthesized without polyaphron templates clearly showing no pores, thus confirming the effect ofpolyaphrons on the pore formation. Reproduced with permission from Ravikrishna et al [41].
In order to obtain bioactive materials with high mechanicalstrength, usually metal implants (titanium and stainless steelalloys) are coated with a thin layer of HA using plasma spraytechniques [45–47]. The main problem associated with thistechnique is the lack of exact stoichiometry and the occurrenceof glassy phases in the ceramic layer. Some of these additionalphases do not show a bioactive behavior or dissolve in thebiological environment [48]. Sol–gel processing is classifiedas an intermediate temperature technique, which like othertechniques, can produce either an amorphous or crystallinecoating, but more importantly results in stoichiometricallyhomogeneous coating. The morphology and the compositionof the thin films can be relatively easy to control by sol–gel processing parameters such as starting pH of the sol,annealing for crystallization temperature and addition ofauxiliary chemical reagents [49–52]. A major processing stageinvolves solution chemistry, whereby a sol is produced fromsuitable alkoxides or salts to yield a HA composition uponheating. The phase purity of the coating is thus determinedearly in the process and is dictated by the purity of the precursormaterials and the ageing time of the solution; firing of thecoating then involves the removal of the organic constituentto produce HA [53]. Sol–gel dip coating of substratesusing viscous precursor solutions and subsequent drying andsintering is a method of choice for the formation of thin andfully dense HA coatings. Balamurugan et al [54] preparedthin-film HA deposits through dip coating onto prefinished lowcarbon 316L stainless steel (SS) substrates using the alkoxide-
based sol–gel technique. FT-IR, energy dispersive analysis(EDXA) and SEM analysis revealed the stoichiometric andmicroporous nature of the HA coatings. Shear bond strengthanalysis showed that the 15 µm HA coating adheres verywell to 316L SS substrate when compared to other coatingthicknesses. The potentiodynamic polarization measurementand inductively coupled plasma-accelerated leaching studies(ICP-AES) analysis showed the protective nature of the HAcoatings on low carbon 316L SS and also it acts as a barrier tocorrosion attack in a simulated body fluid (SBF) environment.Thus sol–gel is a potential and practical method for preparingHA bioceramic films on metallic implants to improve both thecorrosion resistance and biocompatibility.
The solubility of apatite films, which determines theirlongevity, is strongly influenced by both the chemicalcomposition and the crystallinity of the apatite. Recently,fluorapatite (FA) [Ca10(PO4)6F2] coatings on metallicsubstrates have attracted a great deal of attention in areasrequiring long-term chemical and mechanical stability. Themineral phase of hard tissues contains a significant amountof fluoride. Fluoride ions present in saliva and blood plasmaare required for normal dental and skeletal development. Theincorporation of fluoride ions into the structure of HA canstimulate bone cell proliferation and increase new mineraldeposition in cancellous bone. Besides, the better stabilityof fluor-hydroxyapatite (FHA) in biological environmentsas compared to HA can also be considered as a positivefactor for a bone drug delivery system. Pure FA is known
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Table 1. Sequence of interfacial reactions involved in forming a bond between tissue and bioactive glass.
Stages Reactions
1 Rapid exchange of Na+ or Ca+2 with H+ or H3O+ from solution: ≡Si–O–Na+ + H+ + OH− → ≡Si–OH + Na+ + OH−
2 Loss of soluble silica in the form of Si(OH)4 to the solution resulting from breakage of Si–O–Si bonds and formation of Si–OHat the glass/solution interface: ≡Si–O–Si≡ + H2O → ≡Si–OH + OH–Si≡
3 Condensation and repolymerization of a SiO2-rich gel layer on the surface depleted in alkalis and alkaline-earth cations:(RO)3–Si–OH + HO–Si–(OR)3 → (RO)3–Si–O–(OR)3 + H2O
4 Migration of Ca+2 and PO−34 groups to the surface through the SiO2-rich layer forming a CaO–P2O5-rich film on top of the
SiO2-rich layer, followed by growth of the amorphous CaO–P2O5-rich film by incorporation of soluble calcium andphosphates from solution.
5 Crystallization of the amorphous CaO–P2O5 film by incorporation of OH− or CO−3 anions from solution to form a mixedhydroxyl-carbonate apatite layer.
6 Agglomeration and chemical bonding of biological moities in the HCA layer7 Action of macrophages8 Attachment of mesenchymal stem cells9 Proliferation and differentiation of stem cells
10 Generation of matrix11 Crystallization of matrix12 Proliferation of bones
to have a lower bio-resorption rate than HA and retainscomparable biocompatibility to HA in terms of its fixationto bone and bone in growth. The FA forms a full rangesolid solution with HA to form FHA [Ca10(PO4)6(OH,F)2] byreplacing OH− with F−. Hence, it is possible to improve theintegrity and stability of the HA-coating layer by incorporatingF ions [55]. Gineste et al [56] investigated the degradationrate of dental implants with 50 and 100 µm coatings of HA,FA, or FHA in dog jaws and retrieved them for histologicalanalysis after 3, 6 and 12 months. The image analysis of thethickness of calcium phosphate coatings and resorption indexstudies indicated that HA and FA coatings were almost totallydegraded within the implantation period. In contrast, the FHAcoatings did not show significant degradation during the sameperiod. Kim et al [57] coated FHA films on a zirconia (ZrO2)substrate by a sol–gel method using the spin-coating technique.The dissolution rate of the FHA coating layer varied accordingto the heat treatment temperature, which was closely related tothe film crystallinity. The dissolution rate of the FHA film waslower than that of the HA film, suggesting the possibility of afunctional gradient coating of HA and FHA. The MG63 cellsseeded onto the FHA films were found to be proliferated in asimilar manner to those seeded onto pure HA ceramic and aplastic control. The MG63 cells on the FHA coating exhibitedsimilar proliferation behaviors to the culture dish and the pureHA ceramic, confirming the comparable cell viability of theFHA coating. Stan and Ferreira [52] prepared bioactive FHAfilms on a Ti6Al4V medical grade alloy using three sol–gelchemical routes by mixing various Ca/P precursors and Fpromoting reagents. Their bioactive potential was tested byusing the in vitro Kokubo test. In conclusion, the FHA coatingsshowed good integration in the bone tissue and lasted muchlonger than conventional HA.
3.5. Sol–gel-derived bioactive glasses for tissue engineering
of such glass compositions. The relatively low silicon andhigh alkaline content lead to a rapid ion exchange in aqueousenvironments. This exchange generally leads to an increasein solution pH, which can be substantial for finely grainedpowders having high surface to volume ratios. The initiallyrapid release of sodium is accompanied by a somewhat slowerrelease of other ion species, predominantly calcium and silica.Under certain conditions in solution, these ion species willprecipitate onto the glass and onto other nearby surfaces toform calcium-containing mineral layers, or sometimes theouter glass surface itself can transform to hydroxy-carbono-apatite (HCA). The ability to build such a surface is sometimesreferred to as a measure of the ‘bioactivity’ of the glass.When implanted into the body, repair cells will colonizethe bioactive surface, laying down new tissue on and inthe glass. The reaction stages of bioglass are shown intable 1. The activity is strongly dependent on the particlesize, increasing as particle size goes down. In a finelygrained powder form, bioactive glasses have additionallydemonstrated anti-microbial and anti-inflammatory properties.Other bioactive glass compositions, as well as other bioactivematerials such as glass ceramics and calcium phosphate-based ceramics (referred to generally as bioactive ceramics),have also been developed. Among the bioactive ceramics,bioactive glasses have the highest levels of bioactivity basedon their rate of reaction and bone bonding [58–62]. Bioactiveglasses have many applications but these are primarily inthe areas of bone repair and bone regeneration via tissueengineering. These glasses have been used successfully asbone-filling materials in orthopedic and dental surgery, buttheir poor mechanical strength limits their applications in load-bearing positions. However, many methods are reported toimprove the mechanical strength of these bioactive glassessuch as transformation of bioactive glasses into glass-ceramics,fiber/particulate reinforced bioglass, using bioactive glass asa coating on a substrate, etc but approaches to strengthen thesematerials decrease their bioactivity. Thermal treatments alsoaffect the microstructure of bioactive glasses and hence theirbioactivity [63]. Yurong and Lian [64] observed that thermal
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(a) (b)
(c) (d )
(e) (f )
Figure 5. Scanning electron microscopic (SEM) images of osteoblasts cultured on Bioglass 45S5. (a) Two days after seeding, sphericalosteoblasts contacted the substrate by means of numerous filopodia and fiber-like processes. (b) After six days of culture, cells wereanchored to the substrate by multiple lamellipodia. (c) After 12 days of culture, cells appeared to be well spread and grew in multilayerfashion. (d) SEM image of a bone nodule present on Bioglass 45S5 on day 12. (e) Higher magnification of the central domed region of thebone nodule shown in (d). (f) After six days of culture, osteoblasts seeded on an inert control substrate exhibited a smooth dorsal surface,adopted a flattened configuration, and formed confluent monolayers. Reproduced with permission from Xynos et al [67].
treatment of gel-derived bioglasses results in phase separationand minor crystallization which leads to some changes in themicrostructure of samples, such as the density and porosity,that increase the bending strength and fracture toughness withsome extent of decrease in bioactivity. The reason underlyingthis is the separation of silica-rich and phosphate-rich phaseswhich causes a dramatic change in the bioactivity reactionsat the material–biological fluid interface in consequence ofthe increase of viscosity of the phase-separated glass and thedecrease of Si and Ca ions in solution [35, 65].
Bioactive glasses and glass–ceramics have been attractivefor several biomedical applications because of the natureof the spontaneous bond to living bone when implantedin a bony defect. Their flexibility can be increasedby designing organic–inorganic hybrid materials throughincorporating essential constituents for bioactivity (Si–O andcalcium ions) with organic polymers. Organic–inorganichybrid materials prepared by the sol–gel approach haverapidly become a fascinating new field of research inmaterials science. Organically modified silanes (ORMOSILS)have several attractive features as compared to inorganicsol–gel and provide a versatile way to prepare modifiedbiocompatible sol–gel materials [68–70]. Ohtsuki etal [69] synthesized an organic–inorganic hybrid from3-methacryloxypropyltrimethoxysilane (MTMOS) and 2-hydroxyethylmethacrylate (HEMA), which formed the apatitelayer when mixed with calcium chloride. Such a type ofhybrid is expected to be a novel bone-repairing materialwith bioactivity as well as mechanical properties closeto conventional poly(methyl methacrylate) (PMMA) bonecement. Kros et al [71] prepared different hybrid silane
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materials and discussed their possible use in biomedicalapplications. However, several studies have been reportedin the literature for coating of glucose sensors withvarious polymeric materials such as cellulose acetate,polyethyleneglycol, poly(vinyl chloride), polyurethane andnafion [72–79], but their in vivo studies were not verypromising. Bioactivity and compatibility of sol–gel-derivedhybrids makes them attractive for the coating of implantablebiosensing devices in vivo, since they have been demonstratedto be highly compatible with proteins, enzymes and otherbiomolecules [80–87]. Kros et al [88] prepared a silica-basedhybrid biocompatible coating, which can be used for futureimplantable glucose sensors by mixing TEOS as the maininorganic precursor with different organic molecules such asPEG, heparin, dextran sulfate, nafion or polystyrene sulfonate.The toxicity of the coatings was examined in vitro using humandermal fibroblasts. All materials were found to be non-toxicand the cell proliferation rate of fibroblasts was found tobe dependent on the additive. Glucose measurements usingglucose oxidase-based sensors coated with the different hybridfilms were performed both in buffered solutions containingbovine serum albumin and in serum. Stable glucose responseswere obtained for the coated sensors in both media. Thedextran sulfate-derived sol–gel coating appeared to be mostpromising for future in vivo glucose measurements. Theencapsulation of pancreatic islets for secreting insulin in sol–gel has also been reported. Pope et al [89] demonstratedthe potential of silica-gel-encapsulated pancreatic islets ofLangerhans by the measurement of insulin secretory responsein vitro and blood sugar levels of diabetic mice in vivo.Peterson et al [90] developed a sol–gel-derived biocompositematerial in the form of a capsule for encapsulation of insulin-secreting murine islet cells as the first mammalian material insol–gel using the drop-tower sphere generation and emulsiontechnique. Average pore sizes were 161 A for drop-towerspheres and 105 A for emulsion spheres. These capsulesallowed the passage of insulin and cytokines but not thepassage of antibodies. Implantation of encapsulated isletsdid not result in fibrosis of the capsule in vivo, and retrieval ofcapsules after one month in vivo documented continued insulinsecretory capacity. Thus silica-based hybrid encapsulationprovides a potentially useful alternative for encapsulation ofcells for transplantation or drug delivery.
3.7. Sol–gel-derived surface modification of metallicimplants
Metallic materials are an important class of implant materialsbecause of their combination of strength and ductility ascompared to polymers and ceramics. On the other hand,metallic materials are less corrosion resistant compared withthe other two classes of implant materials. Corrosionreduces strength and causes premature failure of implantsand may also impose harmful effects on the surroundingtissues. Stainless steels, titanium alloys and cobalt alloysare commonly used as biomaterials. Shape memory alloys(SMA) are a relatively new group of metallic biomaterials.The extraordinary properties of NiTi shape memory alloys are
widely used in medical applications such as in orthodontics,cardiovascular, orthopedics, urology, etc due to their uniqueshape memory effects, superelasticity and good corrosionresistance. However, the high nickel content (about 50%)in NiTi has caused some concern about its safe use in vivobecause Ni is allergenic and toxic when its concentrationin the human body exceeds a certain level. In viewof this, surface treatment of NiTi implants for improvingcorrosion resistance and hence reducing the amount of Nireleased is necessary for biomedical implants. A numberof surface treatment methods, namely chemical passivation,electropolishing, anodization, thermal oxidation, laser surfacemelting, nitriding, plasma ion implantation and sol–gel-derived coating, have been reported. Out of these, the sol–gelroute for surface modification of NiTi implants is of particularinterest because of simple and inexpensive methodology, low-temperature processing and suitability for coating substratesof irregular shapes, such as implants. Conventional heattreatment for sol–gel-derived titania coatings is not desirabledue to the higher temperature (400–500 ◦C) employed becausethermomechanical properties of NiTi implants are highlysensitive to heat treatment [91–99]. The sol–gel hydrothermalprocess is a potential low-temperature route for depositingoxide coating on NiTi implants and has been attempted byseveral authors [100–102]. Cheng et al [100] improved thecorrosion resistance of NiTi implants via sol–gel dip coatingwith TiO2, employing hydrothermal treatment to crystallizeand densify the amorphous film, and to remove the organicresidue. Electrochemical impedance spectroscopy (EIS) andpolarization studies indicated a significantly larger increasein corrosion resistance compared with the coated samplesdry heated at 500 ◦C. Chiu et al [103] showed that steamcrystallization is a feasible low-temperature treatment methodfor sol–gel-derived titania coating on NiTi in biomedicalapplications. The authors prepared dip-coated titania films viathe sol–gel route using titanium butoxide [Ti(OC4H9)4] as aprecursor and further crystallized by treatment in steam at105 ◦C. Liu et al [104] prepared TiO2 thin films on an NiTisurgical alloy by the sol–gel method. The electrochemicalcorrosion measurement indicated that the TiO2 thin film waseffective for improving the corrosion resistance of the NiTialloy. In vitro blood compatibility of the film and the NiTialloy was also evaluated by dynamic clotting time and bloodplatelet adhesion tests. The results showed that the NiTi alloycoated with the TiO2 film had improved blood compatibility.
3.8. Biocidal sol–gel coatings
Bacterial infection due to an implanted medical device suchas prosthetic hip implants, central venous catheters andurinary catheters etc is a potentially serious complication,typically leading to premature implant removal, whichis costly, traumatic to the patient and might be lethal.Despite various preventative methods such as sterilization,meticulous surgical procedure and following proper infectioncontrol guidelines, invasive bacteria can be found at ∼90%of implantation sites immediately after surgery. TheStaphylococci species including Staphylococcus aureus and
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Biomed. Mater. 3 (2008) 034005 R Gupta and A Kumar
Figure 6. Antibacterial activity of Ag-doped PhTEOS coatings against planktonic S. epidermidis (CSF 41498) after 24 h. Left: undopedcoating. Right: silver-doped coating. Reproduced with permission from Stobie et al [120].
Staphylococcus epidermidis are responsible for the majorityof biofilms found on explanted orthopedic devices. Recently,new strategies are immerging for developing materials withantimicrobial activity that can be used for controlling andpreventing microbial contamination of medical devices. Theimmobilization of an antimicrobial agent in a matrix capableof binding to different surfaces is an interesting way todevelop such antibacterial materials. The sol–gel dip-coating process is an effective procedure for developingantimicrobial coatings as compared to other immobilizationmethods [105–111]. Several studies have shown thatsilver or silver ions have broad-spectrum antibacterialactivity against Gram-positive and Gram-negative strains,including antibiotic resistant strains [112–117]. Jeon et al[118] prepared silver-doped TEOS-derived silica thin filmsby the sol–gel method, showing an antibacterial effectagainst Escherichia coli and Staphylococcus aureus. Dıaz-Flores et al [119] prepared antibacterial TEOS-derived sol–gel thin films and microcrystalline powders doped withAg and Cu. Stobie et al [120] reported a potentialsolution to the problem of biofilm growth on short-termindwelling surfaces using low-temperature-processed silver-doped phenyltriethoxysilane (PhTEOS) sol–gel coating whichreduced the formation of Staphylococcus epidermidis biofilmover a ten day period. However, high temperature causes anincrease in crystallinity of sol–gel coating and consequently areduction in silver release kinetics. The antibacterial activity ofAg-doped PhTEOS coatings against planktonic S. epidermidisis shown in figure 6, where the release of silver ions caused anapproximate 100% kill rate. Copello et al [111] immobilizedthe antimicrobial compound dodecyl-di(aminoethyl)-glycinein TEOS-derived xerogel films coated on a glass surfacevia the dip-coating technique. When antimicrobial-coatedglasses were compared with antimicrobial-free coated glasses,the former showed greater than 99% reduction of colonyforming units for Escherichia coli, Pseudomonas aeruginosaand Staphylococcus aureus.
Recently, a localized persistent concentration of nitricoxide (NO) in the vicinity of an invasive medical devicemay have proven to be a novel approach for reducing the
implant-associated infection due to its short half-life (rangingfrom 1 s to a few minutes depending on the concentrationof oxygen and the presence of NO scavengers such asoxyhemoglobin) in biological milieu. In the literature, it hasbeen reported that NO gas destroys plated colonies of bacteria.A number of synthetic NO donors including nitrosothiols,nitrosamines, diazeniumdiolates, metal complexes andorganic nitrates/nitrites have been used to design polymercoatings capable of slowly releasing therapeutic levels of NOthat are effective in reducing biofouling. Of these NO donorspecies, N-diazeniumdiolates have emerged as attractivecandidates for designing more biocompatible coatingsdue to their ability to generate NO spontaneously underphysiological conditions. When NO reacts with amines, azwitterionic stabilized structure known as N-diazeniumdiolateis produced which decomposes spontaneously in aqueousmedia to NO (figure 7(a)). The rate of release of NO dependsupon pH, temperature and/or the structure of the aminemoiety [121–124]. Sol–gel coatings capable of NOrelease have recently been shown to decrease bacterialadhesion. Schoenfisch and co-workers [125–130] havereported significant work on the synthesis and charac-terization of sol–gel-derived materials (xerogels) whereN-diazeniumdiolate NO donors were covalently bound to thexerogel backbone (figure 7(b)). A range of inorganic–organichybrid xerogels have been functionalized to release NOby incorporating diamine-containing organosilanes intoa sol–gel matrix and their properties were tailored byvarying the type and amount of alkyl- and aminosilaneprecursors and processing conditions (e.g., pH, catalyst,water content, and drying time and temperature). Severalalkyl- and aminosilane precursors were evaluated for thispurpose, including methyl-, ethyl- and butyltrimethoxysilanes(MTMOS, ETMOS and BTMOS, respectively), (aminoethyl-aminomethyl) phenethyltrimethoxysilane (AEMP3), N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAP3),N-(6-aminohexyl)aminopropyltrimethoxysilane (AHAP3)and N-[3-(trimethoxysilyl)-propyl]diethylenetriamine(DET3). Upon exposure to high pressures of NO (g), thediamine coordinates two molecules of NO to form a NO donor
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Biomed. Mater. 3 (2008) 034005 R Gupta and A Kumar
(a)
(b)
Figure 7. (a) Reaction of NO with amines to produce N-diazeniumdiolate NO donors followed by the subsequent generation of NO in thepresence of water. (b) Schematic of NO generation from N-diazeniumdiolate-modified xerogel network occurring upon exposure to aqueousconditions. Reproduced with permission from Shin and Schoenfisch [123].
molecule known as a diazeniumdiolate. When the sol–gel isintroduced into an aqueous environment, the diazeniumdiolatedecomposes to NO and the diamine precursor. The localsurface flux of NO generated from these xerogels significantlyreduces the adhesion of Pseudomonas aeruginosa by up to95%, demonstrating that NO release may represent a newclass of antibacterial biomaterials.
Nablo et al [131] coated medical grade SS with a sol–gel film of 40% AHAP3 and 60% BTMOS. The bacterialadhesion resistance of NO-releasing coatings was evaluatedin vitro by exposing bare steel, sol–gel and NO-releasingsol–gel-coated steel to cell suspensions of Pseudomonasaeruginosa, Staphylococcus aureus and Staphylococcusepidermidis at 25 ◦C and 37 ◦C. Cell adhesion to bare andsol–gel-coated steel was similar, while NO-releasing surfaceshad significantly less bacterial adhesion for all species andtemperatures investigated.
The NO-releasing sol–gel materials for in vivo biosensorapplications are also emerging and are in development. NOis a highly reactive radical and affects enzymatic activity[132–134]. Shin et al [135] developed an NO-releasinghybrid sol–gel/polyurethane glucose biosensor by dopingdiazeniumdiolate-modified sol–gel particles in a polyurethanemembrane on platinum electrodes which was sandwiched withadditional polyurethane membranes to reduce both enzymeinactivation by NO and sol–gel particle leaching. The responsecharacteristics of the hybrid-NO-releasing glucose biosensorremained stable through 18 days, after which the linear rangedecreased from 0 to 60 to 0–20 mM glucose, and the responsetime increased from less than 20 s to over 65 s. Oh et al[136] demonstrated the fabrication of a miniaturized needle-type glucose biosensor patterned with an N-diazeniumdiolate-modified xerogel microarray for overcoming the reducedanalyte permeability observed with xerogel films. Further
studies are in progress for identifying methods to improvethe duration of NO release and comprehensive in vivobiocompatibility testing of NO-releasing sol–gels.
In conclusion, the sol–gel technology has great potentialfor biomedical applications. Sol–gel-derived materialspossess many interesting features that have been utilized invarious practical applications ranging from ceramics, glassmaterials and optical devices to optical sensors and biosensors.This review is an attempt to highlight the usefulness of thesol–gel route for preparing bioactive nanoparticles, powders,coatings, glasses and hybrids for biomedical applications aswell as for designing more biocompatible in vivo sensors. Itis envisaged that many other possibilities can exist and newapplications can arise in future where bioactive porous sol–gel-derived materials can be utilized successfully.
Acknowledgments
The authors would like to acknowledge all those researcherswhose works are being cited here. Radha Gupta would liketo thank Department of Science and Technology (DST), Indiafor providing financial support through Young Scientist Awardunder SERC-Fast track scheme. The financial support fromDepartment of Biotechnology (DBT), India and DST, India isalso acknowledged.
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