Available online at SciVerse ScienceDirect

J. Mater. Sci. Technol., 2013, 29(6), 582e588

Influence of Fluoride Content of Artificial Saliva on Metal Release from 17-4

PH Stainless Steel Foam for Dental Implant Applications

Ilven Mutlu*, Enver OktayDepartment of Metallurgical and Materials Engineering, Istanbul University, Istanbul, Turkey

[Manuscript received October 9, 2012, in revised form November 16, 2012, Available online 21 March 2013]

* Corres47370031005-03JournalLimited.http://dx

Highly porous 17-4 PH stainless steel foam for implant applications was produced by space holder technique.Metal release and weight loss from 17-4 PH stainless steel foams were investigated in fluoride added artificialsaliva environment by static immersion test. An inductively coupled plasma-mass spectrometer was employedto measure the concentrations of various metal ions. Effects of fluoride content of artificial saliva on metalrelease and weight loss from the steel foams were investigated. Effects of immersion time, pH value andprocess parameters on the weight loss and metal release were determined. Pore morphology, pore size andmechanical properties of the 17-4 PH stainless steel foams were also characterized.

KEY WORDS: Metal release; Dental implant; Metal foam; Fluoride; Artificial saliva

1. Introduction

Development of metallic implants for replacement of hardtissues is important in orthopedic applications. In dentistry,metals are used as implant in reconstructive surgery to replace atooth, or in production of prostheses such as plates, crowns,bridges, braces and archwires. These materials are exposed tooral cavity, which is a corrosive environment. Corrosion of im-plants is critical because it can reduce the biocompatibility andmechanical properties. Release of metals can result in adverseeffects including toxicity, carcinogenicity, and allergy[1e5].Stainless steels, Ti alloys and Co alloys are widely used as load-bearing implants. Corrosion resistance of stainless steels dependson chemical composition, microstructure, surface condition, andproduction route[5e9].Alloys that are used in dentistry are exposed to oral envi-

ronment. Saliva acts as an electrolyte, which can cause corro-sion. The pH of saliva is between 2 and 11 while temperature isbetween 0 and 70 �C[10e17]. Corrosion behavior and metalrelease of implants must be studied in saliva or artificial saliva. Inthe oral environment concentration of fluoride has an effect oncorrosion of implants. There is an increased use of dental gelsand rinses containing fluoride to prevent plaque and caries.Fluorides are harmful to metals, especially in low pH. Additionof fluoride to solution makes the metal more active and

ponding author. Ph.D.; Tel.: þ90 5365718461; Fax: þ90 212; E-mail address: imutlu@istanbul.edu.tr (I. Mutlu).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2013.03.006

accelerates corrosion. Incorporation of fluoride in oxide layerreduces the protective properties. Low pH increases corrosionrate in the presence of fluoride, due to hydrofluoric acid for-mation. Dental bleaching and fluoride treatment agents are usedfor esthetic purposes and prevention of plaque. Corrosive effectof fluoride on dental materials has not been well studied[11e17].

Metal foams are used as energy absorbers, filters, heat ex-changers, and biomedical implants. Space holder technique hasbeen used to produce foams from steels and titanium which haverelatively high melting temperatures. This process producesinterconnected porous structure with high porosity suitable forimplants. Foams exhibit a porous structure similar to cancellousbone. Use of metal foam as implant allows mechanical linking ofbone with implant by bone tissue ingrowth into pores. Addi-tionally, by adjusting the porosity, stiffness can be controlled toreduce stress-shielding effect between implant and bone. Re-quirements for implant materials are biocompatibility, openporosity, low density, corrosion resistance, wear resistance, me-chanical strength close to cancellous bone and commercializa-tion potential[18e27].Dental implants can have three types of macro designs: screw

thread design, solid body press-fit design and porous-coateddesign. While the threaded design is the most popular, porous-coated implants have also been used. They are used as abut-ments for over dentures, replacing missing teeth in narrow areas.Porous implants improve contact at implantebone interface,provide areas for bone ingrowth and improve fixation tobone[28e30].In the present study, immersion tests were carried out in

fluoride added artificial saliva solution using 17-4 PH stainlesssteel foams. Traditional AISI 316L austenitic stainless steels areused in biomedical applications. However, these steels have high

Table 1 Amount of chemical reagents for preparation of artificial saliva

Reagent Amount (g/l)

NaCl 0.40CaCl2 2H2O 0.79

KCl 0.40Na2S 9H2O 0.005

NaH2PO4 H2O 0.78UreaeCO(NH2)2 1.00

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Ni, which lead to metal sensitivity, allergy and other complica-tions when released. 17-4 PH stainless steel has lower Ni con-tent. In addition, 17-4 PH stainless steel has higher mechanicalproperties and its mechanical properties can also be improved byaging. 17-4 PH stainless steel is used in aerospace, chemical,food processing and biomedical applications. 17-4 PH stainlesssteel has combination of strength, hardenability (aging), andcorrosion resistance not found in any other steel grade. Advan-tages of steel foams are their ability to provide mechanicalanchorage for surrounding tissue, low density and sufficientstrength close to bone. In the present study, 17-4 PH stainlesssteel foams were immersed in fluoride added artificial salivasolutions and then metal release and weight loss were deter-mined. The effect of fluoride content in artificial saliva onstainless steels has not been well documented. In this study,effects of fluoride concentration, pH and immersion time onmetal release were investigated.

2. Experimental

2.1. Steel foam production

Starting material for the foam production was gas atomized17-4 PH stainless steel powder (Carpenter, Sweden) withspherical morphology. The chemical composition of the steelpowder was Fe e 4.6% Ni, 15.2% Cr, 0.7% Mo, 0.4% Nb, 4.9%Cu, 1.4% Si, 0.07% C. Mean particle size of the powder was14.6 mm. As a space holder, carbamide (Merck, Germany), infractions of 1000e1400, 710e1000, 500e710 mm with irregularshape and the fraction of 1000e1400 mm with spherical shape,was used for its high solubility in water. To enhance sintering,0.5 wt% boron (Merck, Germany) was added to the steel powderto create a liquid phase during sintering. The binder for greenstrength was polyvinylalcohol (PVA), supplied by Merck, Ger-many. PVA was preferred for its biocompatibility. 2.5 wt% PVAwas added to the steel powder. The mixture was compacted at180 MPa into cylindrical specimens of 12 mm in diameter and17 mm in height. Green specimens were immersed in water atroom temperature and w90% of the carbamide was leached outin w10 h. Thermal debinding temperature of the PVA wasdetermined to be w410 �C by using thermo gravimetric analysis(TA, SDT Q600). The PVA was thermally removed as part ofsintering cycle, which consisted of heating at a ramp rate of 5 �C/min to 410 �C (debinding) with a dwell time of 40 min, followedby heating at a rate of 10 �C/min to sintering temperature. Thefoams were sintered at 1260 �C for 40 min in H2. In addition, thefoams were aged to further enhance the mechanical properties.At the aging stage, sintered foams were austenitized at 1050 �Cin a vacuum furnace and then quenched using N2 (0.6 MPa(6 bar)) as a cooling gas. Quenched specimens were then agedfor 4 h at 480 �C in H2.

2.2. Artificial saliva preparation

Artificial saliva solution was prepared from calculatedamounts of chemicals supplied by Merck, Germany according toprocedure described in literature[10e17]. The amount of chemicalreagents for preparing artificial saliva solutions are given inTable 1. The chemical reagents were added to the solutions in theorder they are listed.In preparation of artificial saliva, firstly 750ml of distilled water

was put into a 1000 ml beaker. The temperature was kept at 37 �C.

Chemical reagents were added into the water one by one. The pHmeter (WTW inoLab 720, Germany) was calibrated with standardbuffer solutions. After addition of the chemicals, the temperatureof the solution was checked, and the electrode (WTW SenTix 81,Germany) of the pH meter was placed in the solution. After theadjustment of pH, the solution was transferred from the beaker to aflask of 1000 ml. Distilled water was added to the solution, toadjust the total volume to 1000ml. Artificial saliva with varied pHvalues were prepared to study the effect of the pH onmetal release.In order to determine the effect of fluoride on implant, artificialsaliva solutions with 0.25, 0.50, 0.75 and 1.00 wt% F concentra-tions were prepared using NaF addition. The pH was adjusted to2.30, 3.20, 5.80 and 7.40. The pH was lowered by adding lacticacid. This acidwas chosen since it is naturally released by bacteria.The pH of fluoridated odontological gels is w4.00, and after ameal, pH of buccal cavity can fall below this value.

2.3. Static immersion test

70% Porous specimens were cut along longer axes and semi-cylindrical specimens were obtained. Thus, maximum solidsurface area, which was exposured to solution, was obtained.Then, the specimens were machined, polished and washed.Porosity and surface area of each specimen were equal in im-mersion tests. Specimens were then exposed to artificial saliva.Foams with equal porosity levels (70%) were immersed in arti-ficial saliva at 37 �C for several soaking times up to 14 days.Solution volume to specimen surface area ratio was constant inall immersion tests. The inductively coupled plasma-massspectrometer, ICP-MS (Thermo Scientific Elemental X Series2) was employed to measure the concentrations of major metalions like Fe, Cr, Ni, Cu and Mo. A solution without a specimenwas used for blank test. After different soaking periods, thefoams were removed from the solutions. Then the specimenswere rinsed with water and dried. The dried specimens wereweighed and weight loss was determined.

2.4. Characterization of microstructure and mechanicalproperties

The microstructures of the foams were observed by scanningelectron microscopy (Jeol 5600), field emission gun-scanningelectron microscopy (FEI Quanta 450, FEG-SEM) and opticalmicroscopy (ME600 Nikon). Energy dispersive spectroscopy(EDS) analysis (IXRF systems 550i) was carried out to studychemical composition. The digital images of the foams wereused to determine the mean pore size and shape by using animage analyser (Clemex Vision, PE). Total porosities weredetermined from measurements of weights and dimensions.Open porosity contents of the foams were measured by using aHg porosimeter (Quantachrome Poremaster). Mechanical prop-erties were studied by compression test (Zwick-Roell Z050).

584 I. Mutlu and E. Oktay: J. Mater. Sci. Technol., 2013, 29(6), 582e588

2.5. In vitro cytotoxicity test

Biocompatibility of the foams was studied by XTT [2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboksia-nilide inner salt] in vitro cytotoxicity assay. XTT assay de-termines live cell number, which is used to assess cytotoxicity.XTT measures cell viability based on the activity of mitochon-dria enzymes in live cells that reduce XTT and are inactivatedafter death of cell. The amount of product generated from XTT isproportional to the number of living cells and can be determinedspectrophotometrically. Cytotoxicity of the sterilized foams wasevaluated in L929 mouse fibroblast cell culture. Positive (latex)and negative (high density polyethylene) control articles wereprepared to verify the system. After incubation at 37 �C in ahumidified atmosphere containing 5% CO2, XTT/PMS labelingsolution (final XTT concentration: 0.3 mg/ml) was added toculture and the culture was incubated for another 4 h. Absor-bance measurements were performed at 450 nm using a spec-trophotometer (Shimadzu BioSpec-1601, Japan).

3. Results and Discussion

3.1. Foam structure and pore properties

In this study, 17-4 PH stainless steel foams were produced byspace holder technique. Since the interconnected open porosity isimportant for tissue ingrowth, the amount of open porosity in thesteel foams was determined. Total macro-porosity of the steelfoams consists of w80%e90% open porosity and w10%e20%closed porosity. Some micro-pores are found inside the macro-pores, which suggest that the macro-pores are connected bysmall channels for the escape of gas during sintering. A certainamount of micro-pores are desired for vascularization inbiomedical implants. Fig. 1 shows the photograph of the 70%porous 17-4 PH stainless steel foams and SEM image from cracksurface of the steel foam.Fig. 2 shows SEM images from surfaces of the highly porous

17-4 PH steel foams having 50, 62, 69, and 80% porosity con-tents. SEM images show uniform distribution of pores. In spiteof sintering between steel particles, the macroporous structureremained with a small shrinkage. Morphology of the macro-pores was similar to carbamide (space holder) particles. Thissuggests that pore size and pore morphology can be designed byusing proper size, shape and content of the carbamide (spaceholder) particles.

Fig. 1 (a) Photograph of the steel foams and (b) SE

Total porosity and pore size have a critical role in tissueingrowth into the implant. The minimum pore size is consideredto be w100 mm and larger pore size results in greater tissueingrowth. In the case of pores with size less than 100 mm, cellsdid not grow into the pores because of spanning of pores by thecells. The pore size and pore morphology of the steel foamsreplicated the initial size and morphology of the carbamideparticles. In the present study, mean pore size of the steel foams,which were produced by using fraction of 710e1000 mmcarbamide, was w600 mm, which is suitable for biomedicalimplant applications. Pore morphology was also investigated interms of sphericity and found to be w0.57, while the sphericityof initial carbamide particles was w0.64. The decrease in thecarbamide particle size and sphericity was attributed to crushingof the carbamide particles during pressing (compaction) and tomoistening. The mean size and shape of carbamide particles andresulted pore size and shape in the foams are summarized inTable 2. Standard deviation for pore size and pore sphericitymeasurements was �22 mm and �0.03, respectively.

3.2. Mechanical properties

Compressive yield strength andYoung’s modulus of the 17-4 PHstainless steel foams, which have porosities between w40% and80%, were in the ranges of w50e290 MPa and w0.5e5.0 GPa,respectively. In particular, compressive yield strength and Young’smodulus of the 70% porous foams werew70MPa andw0.60 GParespectively. Themechanical properties of the foams, for biomedicalimplant applications, must fall in the range of cancellous bone.Meanwhile, compressive yield strength and Young’s modulus ofhuman cancellous bone are in the ranges of 40e150MPa and 0.09e1.5 GPa, respectively[23]. As a result, mechanical properties of thesteel foams, especially those having w70%e80% porosity, wereclose to cancellous bone. Fig. 3 shows the effects of total porosity onthe Young’s modulus and compressive yield strength of the steelfoams. It is clear that the porosity affects the Young’s modulus andcompressive yield strength.In order to achieve more control over the mechanical prop-

erties heat treatment (aging) can be carried out on the highlyporous 17-4 PH stainless steel foams. Thus, the sintered steelfoams were aged to further enhance and adjust the mechanicalproperties. Aging of the 17-4 PH stainless steel takes place inthree steps. In solution treatment (austenitizing) step, alloyingelements go into solution. In quenching step, the steel specimenis rapidly cooled to create a supersaturated solid solution. In theaging step, the quenched specimen is heated to an intermediate

M image from crack surface of the steel foam.

Fig. 2 SEM images of surfaces of the highly porous 17-4 PH steel foams having (a) 50, (b) 62, (c) 69, and (d) 80% porosity contents.

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temperature (aging temperature) and held for a determined time.At aging temperatures the alloying elements (Cu and Nb) formfine precipitate clusters. Copper rich precipitates block themovement of dislocations and the steel resists deformation andbecomes harder. At the beginning of the aging, coherent Cuclusters with body-centered cubic crystal structure nucleate andgrow in the supersaturated matrix and then lose their coherencyand become face-centered cubic after reaching a critical size.Maximum strength and hardness in the 17-4 PH stainless steelcan be obtained after aging at w480 �C for w2e3 h, duringwhich the precipitation of coherent Cu-rich clusters in optimumsize with body-centered cubic crystal structure and homogenydistribution in the microstructure occurs. Aging at higher tem-peratures results in precipitation of coarse and incoherent Cu-richprecipitates. When precipitates overgrow and lose their co-herency with matrix, precipitates lose their precipitation hard-ening effect and mechanical properties were decreased. Theresultant Young’s modulus and compressive yield strength of theaged steel foams were w1.50 GPa and w110 MPa, respectively.Compressive yield strength and Young’s modulus of as sinteredspecimens before aging were w40 MPa and w0.47 GPa,respectively. As a result, the aging (precipitation hardening) heattreatment increased the mechanical properties of the 17-4 PHstainless steel foams.

Table 2 Mean size and shape of carbamide particles and mean poressize and shape of the sintered foams

Carbamide particlesize (mm)

Poresize (mm)

Carbamidesphericity

Poresphericity

1272 920 0.64 0.57860 607 0.62 0.56580 446 0.60 0.531312 895 0.73 0.69

3.3. Metal release and weight loss behavior

Determination of the chemical interaction of biomedicalimplant materials with the human body fluids is important inorder to determine their stability in the human body environ-ment. In this study, the quantities of primary metal elementsreleased from the highly porous 17-4 PH stainless steel speci-mens into fluoride added artificial saliva solution were deter-mined. Fig. 4 shows the effect of immersion time on quantity ofmetals released from specimens into artificial saliva with 0.5%fluoride content. Fig. 5 illustrates the effect of pH on quantity ofmetals released from porous steel specimens into artificial salivawith 0.5% fluoride addition. It can be seen from Figs. 4 and 5that, increasing immersion time and decreasing pH levelincreased the quantities of released metal ions. The quantity ofFe, Ni, Cr, Cu and Mo released from stainless steel specimensgradually increased with decreasing pH level. Meanwhile, metalrelease values from low porosity dense 17-4 PH stainless steel

Fig. 3 Effect of porosity on the compressive yield strength and Young’smodulus of the steel foams.

Fig. 4 Effect of immersion time on quantity of metals released fromsteel foams into artificial saliva.

Fig. 6 Effect of fluoride concentration on quantity of metals releasedfrom steel foams into artificial saliva.

586 I. Mutlu and E. Oktay: J. Mater. Sci. Technol., 2013, 29(6), 582e588

specimens, which have the same geometrical dimensions withthe highly porous 17-4 PH stainless steel foams, are also pro-vided in Figs. 4 and 5 for comparison. As shown in these figures,the dense steel specimens show lower metal ion release quanti-ties than corresponding highly porous steel foams. This wasattributed to higher surface area of the highly porous steel foams.In order to determine the effect of fluoride on the steel foam,

artificial saliva solutions with F concentrations between 0.25 and1.00 wt% were prepared. The effect of varying fluoride contentsof the artificial saliva solution on the metal release is shown inFig. 6. As seen in Fig. 6, increasing fluoride content of theartificial saliva environment increased the metal release from the17-4 PH stainless steel foams.It can be seen from the static immersion tests results that, there

was not a higher metal release than reference levels of thesemetal ions in human body fluids. This behavior was attributed tothe resistance of strong passive film on the 17-4 PH stainlesssteel. The protective surface film on the 17-4 PH stainless steelmainly consisted of Cr2O3. This film is very thin, invisible andthe thickness of the passive oxide film is in nanometer range. Inorder to obtain a compact and continuous passive film on astainless steel, a Cr content of at least w11 wt% is required. Ingeneral, stainless steels react with oxygen and form a surfaceoxide layer which acts as a barrier against further reaction. Thisfilm acts as a block to spreading of oxygen into the bulk alloy.Passive oxide film inhibits metal ion release from the bulk ma-terial. Weakly oxidizing environments such as air and water aresufficient to passivate the stainless steels. Stainless steels maycorrode inside the human body in oxygen depleted regions. Thispassive film is self-repairing in the presence of oxygen in case of

Fig. 5 Effect of pH on quantity of metals released from steel foams intoartificial saliva for 7 days.

any damage. Biological risks of metal ions include wear debris,organometallic complexes, free metal ions, and metal oxides.Organometallic complexes are formed by binding between metalions and proteins. Proteins increase corrosion rate of a biomed-ical implant. Release of metals can result in adverse reactionsincluding toxicity, carcinogenicity, and allergy[1e5]. In stainlesssteels, Ni causes the austenitic microstructure to be maintained atroom temperature. Ni also increases ductility and corrosionresistance. However, Ni may be released into body, which leadsto sensitivity and allergy. Despite the fact that Ni is an essentialelement in the human body, the maximum daily intake shouldnot exceed w500 mg, and threshold concentration ofw30 � 10�6 is required to initiate any cytotoxic response. Inthis study, measured Ni ion levels are negligible compared todaily intake of Ni ion. In general, Ni2þ substitutes essential el-ements like Ca2þ, Mg2þ and Zn2þ in proteins and enzymes andthen modifies their structures. In addition, Cr and Ni disrupt thebody’s oxidationereduction balance and cause increase in thebody’s pH value[1e8].

Fig. 7 shows the effect of pH level of artificial saliva andimmersion time on weight loss of the 17-4 PH stainless steelfoams, revealing that the weight loss was slightly increased withincreasing immersion time in all solutions. Weight loss of thesteel foams was small. Relatively rapid increase in the weightloss was observed in the initial immersion period and furtherincrease in immersion time caused only a slight increase in theweight loss. In addition, decreasing pH value of the artificialsaliva solution slightly increased the weight loss of the foams. Itcan be seen from these results that the highly porous 17-4 PHstainless steel foams have an appropriate resistance to weight

Fig. 7 Effect of pH and immersion time on weight loss in the steelfoams.

Fig. 9 Effects of porosity and pore size on weight loss in the steelfoams. The inset graph shows the full porosity range includingthe bulk dense specimen.

I. Mutlu and E. Oktay: J. Mater. Sci. Technol., 2013, 29(6), 582e588 587

loss in a wide pH range in fluoride containing artificial salivaeven at low pH level.Fig. 8 illustrates the effect of fluoride content of artificial

saliva and immersion time on weight loss from the 17-4 PHstainless steel foams. It is seen that, increasing immersion timeand increasing fluoride content of artificial saliva solutionslightly increased the weight loss from the steel foams. Althoughweight loss was slightly increased with increasing fluoridecontent, there was not a high weight loss even at 1.00% fluorideaddition.Fig. 9 shows the effects of total porosity level and mean pore

size of the steel foam on weight loss in the fluoride containingartificial saliva environment. It can be seen that, with increasingporosity content and decreasing macro-pore size, the weight lossin the 17-4 PH stainless steel foams was increased, due to anincrease in the actual surface area, while the corrosion potentialof the stainless steel did not change. Increasing surface area in-creases total area of foam in contact with artificial saliva solu-tion. Moreover, bulk dense 17-4 PH stainless steel specimenswith same geometric dimensions were also investigated forcomparison. The inset graph in the Fig. 9 shows the full porosityrange including the bulk dense specimen.On the other hand, in the highly porous materials with inter-

connected open pores, free flow of species increases corrosionresistance. Corrosion resistance of the implants decreases withdecreasing porosity, which can be attributed to the formation ofsmall, isolated pores that traps ionic species and restricts theaccess of oxygen, which limits the available oxygen for theformation of the passive oxide layers on stainless steels. So,further experiments must be made on the effect of total porosity,pore morphology and pore size on metal release and corrosionbehavior of metal foams for biomedical implant applications.Fig. 10 shows the effect of 17-4 PH stainless steel foam

production parameters on the metal ion release. According to thestatic immersion tests, the boron-free 17-4 PH stainless steelfoams corroded more severely than boron-added steel foams. Asseen from Fig. 10, more metal ions were released from theboron-free specimen. Addition of boron slightly enhanced thecorrosion resistance of the steel foams due to enhanced sinter-ability and lower microporosity at the cell-walls. In addition, theaged steel foams release slightly more metal ions than the non-aged steel foams. Poorer corrosion resistance of the aged steelfoams could be explained by the formation of some microcracksproduced at quenching stage, and precipitate clusters. But, theprecipitation hardening (aging) did not decrease the corrosionresistance of the steel foams at excessive levels. Hence, as there

Fig. 8 Effect of fluoride concentration on weight loss in the steel foams.

is a slight increase in the metal ion release, the aging could beused to enhance the mechanical properties of the steel foams.Fig. 11 shows SEM images from the surfaces of the steel

foams before and after the static immersion tests. There was notan excessive harmful damage on the surfaces of the 17-4 PHstainless steel foams after soaking to artificial saliva solutionwith fluoride addition.General mechanism for corrosion and subsequent release of

metal ions from stainless steels involves the loss of the passiveoxide layer consisting of chromium oxide and chromium hy-droxide. Corrosion resistance of stainless steels is attributed tothe formation of a protective passive oxide film. The stronger thepassive film, the better the corrosion resistance. Passive film actsas a barrier to prevent metal ions from releasing from the bulkmetal. The quantity of released metal changes depending on thenature and strength of the metal-oxide bond, structure (va-cancies, interstitial elements, degree of ordering), alloying ele-ments, chemical composition and thickness of the passive oxidefilms. The amount of released Ni ions in the static immersionexperiments was tolerable, when compared with a human’s dailyaverage Ni intake by foods and critical Ni values of causingallergies. However, within the oral cavity, many factors mayincrease this amount. So, it is better to minimize the amount ofNi ion released from the biomedical implants.

3.4. Biocompatibility

Biocompatibility of the 17-4 PH stainless steel was studied byXTT in vitro cytotoxicity assay. It is confirmed that, the 17-4 PH

Fig. 10 Effect of process parameters on metal release in the steelfoams (BF: boron-free; BA: boron-added; BA-A: boron-added/aging; BF-A: boron-free/aging).

Fig. 11 SEM images from surfaces of the 17-4 PH stainless steel foam, (a) before immersion test and (b) after immersion test.

588 I. Mutlu and E. Oktay: J. Mater. Sci. Technol., 2013, 29(6), 582e588

stainless steel foams do not have a cytotoxic potential. Viabilityvalue of the foam (w81%) was close to viability of the negativecontrol (w91%), whichmeans the steel has no cytotoxic potential.Meanwhile, viability of the positive control specimen wasw4%.Consequently, the results indicated potential applications of the17-4 PH stainless steel foam as biomedical implant material.

4. Conclusion

In the present study, characterization of highly porous 17-4 PHstainless steel foam for biomedical implant applications wasinvestigated in fluoride added artificial saliva environment. Effectsof fluoride content and pH level of the artificial saliva solution onweight loss and metal release from 17-4 PH stainless steel foamwere determined.Highly porous 17-4 PH stainless steel foamswithtailored porosity and pore morphology were produced by spaceholder route. Pore size, total porosity and mechanical properties ofthe steel foamswere found to be similar to human cancellous bone.The quantities of Fe, Cr, Ni, Cu andMo elements released from the17-4 PH stainless steel foams were small, and there was no highermetal release than reference levels of thesemetals in humanbody. Itcan be found that 17-4 PH stainless steel foams have a suitableresistance to metal release and weight loss in fluoride containingartificial saliva environment. Therefore, experimental results revealthat the 17-4 PH stainless steel foam with low metal release leveland weight loss is considered advantageous for long-termbiomedical implant applications. It was found that the quantity ofmetal ions released from the 17-4 PH stainless steel foams wasslightly decreased with increasing pH value of the artificial saliva.In contrast, the quantity of metal ions released from the steelspecimens was increased with increasing immersion time andfluoride content of the artificial saliva. Meanwhile, in vitro cyto-toxicity assay showed that the 17-4 PH stainless steel foamdoes nothave a cytotoxic potential.

AcknowledgmentThe authors are grateful to Professor Suat Yõlmaz for pH

measurements.

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