Electrochemical impedance spectroscopy study on silver coated metallic implants

Electrochimica Acta 56 (2011) 7787–7795

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Electrochemical impedance spectroscopy study on silver coated metallic implants Magda Lakatos-Varsányi ∗,1 , Monika Furko, Tamás Pozman Bay Zoltan Foundation, Institute for Materials Science and Technology, Budapest H-1116, Fehérvári út 130, Hungary

a r t i c l e

i n f o

Article history: Received 17 July 2010 Received in revised form 19 January 2011 Accepted 20 January 2011 Available online 27 January 2011 Keywords: Biomedical implants Silver nanocoatings Pulse plating technique Antimicrobial activity Galvanic corrosion

a b s t r a c t This work presents the preparation of nano-structured silver coatings on TiAl6 V4 and CoCrMo alloys by a pulse current technique and the study of time dependent electrochemical behaviour of silver coated metallic implants. EIS data as a function of immersion time in 0.9 wt% NaCl solution have been obtained to clarify the electrochemical processes occurring in the system. During early stage immersion (1–2 days), the impedance response shows near capacitive behaviour. As the time passes, the electrolyte gradually penetrates the silver coating and the sandblasted metallic implants. The silver comes into contact with the electrolyte and the conditions for galvanic corrosion are fulfilled. Due to the potential difference between silver coating and the metallic alloy, discrete anodic and cathodic areas are formed, which result in the release of silver, since the silver acts as an anode in galvanic cells. The cathode process is the reduction of the dissolved oxygen at the surface of the substrate. For antimicrobial applications of nanosilver coated TiAl6 V4 and CoCrMo alloys, it is very important to maintain the continuous release silver ions. Degradation of silver coatings have been traced and confirmed by different methods such as SEM micrographs, EDX analysis, EIS measurements and solution analysis by ICP-MS methods. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction TiAl6 V4 and CoCrMo are widely used as orthopaedic prostheses. This is due to their excellent mechanical strength, chemical stability and biocompatibility [1–6]. However, bacterial adhesion and biofilm formation on these alloys can cause various human diseases [7]. Removing bacteria in a biofilm is impossible and a local or systemic antibiotic treatment is not effective. Therefore, the inhibition of bacterial adhesion is the most critical step in preventing implantassociated infections [8]. In view of the problem of bacterial resistance to antibiotics and antiseptics, nanosilver coatings may be an effective strategy to prevent device related infections, because its high and permanent antimicrobial activity combines with a remarkably low human toxicity [9,10]. Silver, in particular, free silver ions, is well known for its broadspectrum antimicrobial activity and its low toxicity to mammalian

∗ Corresponding author. Tel.: +36 14630500; fax: +36 14630529. E-mail address: [email protected] (M. Lakatos-Varsányi). 1 ISE active member. 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.072

cells, but still allows for the independent use of therapeutic antibiotics. Today the silver ions are used to control the bacterial growth in a wide range of medical applications, such as dental work, catheters, implant materials wound healing and electrical appliances [8,11–15]. The antimicrobial success of silver come from the fact that the silver cations (Ag+ ) bind strongly to electron donor groups in biological molecules containing thiol (sulfydril, –SH) groups, oxygen and nitrogen [8,14]. The binding of silver ions to bacterial DNA may inhibit a number of important transport processes, such as phosphate and succinate uptake and can interact with cellular oxidation processes as well as the respiratory chain. Silver coated medical devices can only be clinically effective when the concentration of free silver ions can be increased [9]. Metallic silver has only slight antibacterial effects because it is fairly chemically stable. Some literature data report (mainly in animal studies) the application of electrically activated silver coated implant, the use of applied potential or periodically applied anodic current to inhibit the bacterial activities [16]. Therefore, it is very important to develop biocompatible coatings that exhibit a controlled silver release and to sustain this silver release from nanosilver coatings.

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Table 1 Pulse current parameters for silver deposition on TiAl6 V4 and CoCrMo alloys. Substrate

Duty cycle/%

Frequency/Hz

ton /ms

TiAl6 V4 and CoCrMo

50 25 10

100 50 100

5 5 1

Clinical studies of silver coated medical devices in patientgroups are rare and have involved very small numbers of patients. Some laboratory and animal studies stated strong antimicrobial activities of silver coated medical devices, but convincing clinical studies are still incomplete [16–18]. The aim of this study was to optimise the electrochemical deposition of a nano-structured silver coating and to study the release of silver ions from the nanosilver coated metallic implants by different electrochemical and surface analytical methods.

toff /ms 5 15 9

jp /mA cm−2

jav. /mA cm−2

10 40 100

5 10 10

2. Experimental 2.1. Preparation of silver coated implant materials Titanium alloy (TiAl6 V4 ) and CoCrMo alloy discs (17 mm diameter × 1 mm) were used as substrates. One side of each disc was roughened using a sandblasting procedure with a 180-grit aluminium oxide media. After roughening, the surfaces were ultrasonically washed in ethanol for 3 min to remove any residual aluminium oxide media and grease. The electrochemical bath was Glanzsilberbad Elfit 73 (Schlötter), a commercially available cyanide bath. The coatings were electrodeposited onto the sandblasted disc substrate using IGTV-4i/6t type pulse current generator. The electrodeposition was carried out in a two-electrode cell under normal atmospheric conditions. The anode was a silver sheet and the metallic implant disc was used as a cathode. During the deposition process the electrolyte was stirred. The thickness of the deposited silver layer changed between 2 and 15 ␮m. The morphologies of nanosilver deposits were studied by scanning electron microscopy (SEM). 2.2. Electrochemical tests The potentiodynamic and electrochemical impedance spectra studies were performed using a Zahner IM6e electrochemical workstation (Zahner, Germany). In the electrochemical measurements, the working electrode was a silver coated TiAl6 V4 or CoCrMo disc electrode with a surface area of 1.37 cm2 . Platinum net and Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. Electrochemical impedance spectra measurements were carried out at open circuit potential of the working electrode in 0.9 wt% NaCl solution at room temperature, with and without magnetic stirring. During the EIS measurements the applied frequency range was changed from 100 kHz (initial frequency) to 10 mHz (final frequency), AC sine wave amplitude of 10 mV. Impedance spectra measurements were recorded several times during all measurements in order to trace the degradation of the silver coatings. In tables and figures the immersion time is always indicated, which reflects

100 99

η/%

98 97 96

TiAl6V4

95

CoCrMo

94 5

10

15

20

j av. / mA cm-2 Fig. 1. Surface roughness on sandblasted TiAl6 V4 alloy (a) and silver coated TiAl6 V4 (b). (Optical microscopic photos.)

Fig. 2. Current efficiency vs average current density.

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Fig. 3. SEM images on silver coated TiAl6 V4 alloy (a) and CoCrMo alloy (b) for determination the grain size.

Fig. 4. SEM images on silver coated TiAl6 V4 as deposited (a) and exposured to 0.9 wt% chloride solution for a month (b).

the duration of the actual measurement. The recorded spectra were analysed as Nyquist and Bode diagrams. All impedance data were fitted and analysed using Zview program. We have also investigated the silver-ion release by measuring the silver concentration of the electrolyte after the EIS measurements by ICP-MS analysis.

3. Results and discussion 3.1. Silver deposition In our studies, nano-crystalline silver layers were deposited on pre-treated TiAl6 V4 and CoCrMo alloy substrates from a standard

Fig. 5. EDX analysis of Ag/TiAl6 V4 samples as deposited (a) and exposured to 0.9 wt% chloride solution for a month (b).

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Pt Au Sandblasted TiAl6V4 alloy Silver wire

0,3

0,06 0,05

E vs SCE / V

E vs SCE / V

0,2

0,1

0,0

0,04 0,03 CrCoMo alloy Silver coated CrCoMo alloy Silver wire

0,02 0,01

-0,1 0

100

200

300

400

500

600

0

700

50

100

t/h Fig. 6. Long term corrosion potential measurements on Pt, Au, sandblasted TiAl6 V4 alloy and silver wire, in 0.9 wt% NaCl solution at room temperature and air condition.

0,18 0,16 TiAl6V4 alloy Silver coated TiAl6V4 alloy Silver wire

E vs SCE / V

0,14 0,12 0,10 0,03 0,02 0,01 0

50

100

150

200

250

t/h Fig. 7. Long term corrosion potential measurements on TiAl6 V4 alloy, silver coated TiAl6 V4 alloy and silver wire, in 0.9 wt% NaCl solution at room temperature and air condition.

150

200

t/h Fig. 8. Long term corrosion potential measurements on CoCrMo alloy, silver coated CoCrMo alloy and silver wire, in 0.9 wt% NaCl solution at room temperature.

In order to determine the effect of different pulse parameters on the morphology of the layers we changed the pulse parameters within the ranges shown in Table 1. The average current densities were either 5 or 10 mA cm−2 . The current efficiency () increases with an average current up to 99%, but after it the current efficiency hardly changes as a function of average current densities (Fig. 2). It was found that the coating deposited by the parameters of ton : 5 ms; toff : 15 ms and jp : 40 mA cm−2 has the smallest grain size. Regarding the grain size distribution of the silver deposits, it is hard to determine the medium grain size by analysis of the SEM images since the silver grains form groups of larger structures. Some efforts have been done by SEM measurements to determine at least the region within the size of independent grains varies. In the SEM images shown in Fig. 3, the independent grains are indicated. It is visible that the silver grain size changes about between 35.7 and 123 nm for Ag/TiAl6 V4 , while it is typical about between 91.4 and 199 nm for Ag/CoCrMo system (see in Fig. 3(a) and (b)). 3.2. In vitro corrosion measurements

cyanide-plating bath using pulse-plating technique. There is an oxide layer on the surface of the substrate alloy, therefore adhesion of the silver would be very poor if a pre-treatment is not applied before plating. The solution is to deposit the silver on sandblasted alloy substrates. The optical microscopic photo in Fig. 1 (a) shows the roughness along the indicated line. The change between the maximum and minimum deepness is 24 ␮m (as it can be seen on the graph below the picture). As Fig. 1(b) illustrates, the surface has become smoother after silver deposition; the maximum roughness decreased to 12 ␮m. -2

(a)

-4

Exposing samples to the 0.9 wt% chloride solution for long periods of time (one-month), corrosion spots can be seen on SEM images shown in Fig. 4(b), while Fig. 4(a) shows the silver coated TiAl6 V4 alloy as deposited. Comparing the results of EDX analysis of the sample as plated to that exposed to the chloride solution it is visible that only the silver signal can be identified in the first case, while in the second case, (after one month immersion) besides the silver signal Ti was also identified (Fig. 5(a) and (b)). -2

-2

jcorr: 30 nA cm

(b) -2

εcorr: -117 mV

jcorr: 39 nA cm

Ag/ TiAl6V4 alloy

-4

εcorr: -70.7 mV

TiAl6V4 alloy -8

-6 CoCrMo alloy -8

-10

-10

-12 -0,3

log (|i| / A)

log (|i| / A)

Ag/ CoCrMo alloy -6

-0,2

-0,1

0,0

0,1

E vs (Ag/AgCl) / V

0,2

-12 -0,3

-0,2

-0,1

0,0

0,1

0,2

E vs (Ag/AgCl) / V

Fig. 9. Polarization measurements on bare and silver coated TiAl6 V4 (a) as well as CoCrMo alloys (b) after 125 h immersion time in 0.9 wt% NaCl solution at 0.1 mV s−1 scan rate.

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0,0

150

-0,1

mixed potential

100

mixed current

-0,2

j / nA cm

3.3. Tafel analysis

-2

E vs SCE / V

means that in each case the silver acts as anode in a galvanic corrosion pairs. Long-term corrosion potential measurements on bare and silver coated CoCrMo alloy as well as on silver wire are shown in Fig. 8. In this case the same trend is valid than that for silver coated and bare TiAl6 V4 alloy substrate.

200

0,1

Quantitative assessment of degradation of the silver coatings has been analysed by potentiodynamic polarization measurements. The polarization curves were detected on TiAl6 V4 and CoCrMo alloy substrates and on both silver coated substrates after 5 days of immersion time. The surface areas of the specimens were almost the same and the polarization scan rate was 0.1 mV s−1 . In order to find the galvanic potentials and currents, the polarization curves on substrates and on silver coated substrates were superimposed on each other as it is shown in Fig. 9(a) and (b). The intersection of cathodic polarization curve of more electropositive metal with the anodic polarization curve of more electronegative metal gives the galvanic corrosion potential and the galvanic corrosion current. The galvanic potentials are found to be −117 mV and −70.7 mV, while the galvanic current densities are respectively 30 and 39 nA cm−2 for the Ag/TiAl6 V4 alloy and for the Ag/CoCrMo alloy galvanic cells.

50 -0,3

0

-0,4

0

20

40

60

7791

80

t/h Fig. 10. Galvanic current density and galvanic potential of coupled silver–Ti alloy electrodes in 0.9 wt% NaCl solution.

It is thought that due to the potential difference between the coating and the substrate in chloride solution, discrete anodic and cathodic areas are formed, which results in a continuous release of silver and the silver acts as an anode in the galvanic cell system. The cathode process is the oxygen reduction of the dissolved oxygen on the surface of the Ti alloy substrate. In order to confirm this assumption long-term corrosion potential measurements were carried out on the pure silver and different substrate materials such as Pt, Au and sandblasted Ti alloys, in a chloride solution. The time dependent open circuit potential (ocp) data are presented in Fig. 6. Fig. 7 shows similar measurements where the potential of pure silver wire, the silver coated Ti alloys and the pure substrate material were compared. The results revealed that the potential of substrate materials have a greater positive value than that of pure silver. It

3.4. Galvanic current measurements by ZRA The electrochemical behaviour was also assessed using zero resistance amperometry (ZRA), which is a nonperturbative, time dependent galvanic corrosion measurement. It is well known that the zero resistance amperometry is widely used to measure the corrosion current density of metallic electrodes [19,20]. Simultane-

(a)

4,0

(b)

80

4000

2000

log (IZI / Ω cm2)

1h 454h 1013h

0

3,5

60

1h 454h 1013h

3,0

40

2,5 20

2,0

0

1,5 0

2000

4000

-3

6000

- Φ / degree

-Z (im) / Ω cm 2

6000

-2

-1

0

Z (re) / Ω cm2

1

2

3

4

5

6

log (f / Hz)

Fig. 11. Nyquist (a) and Bode (b) plots of silver coated TiAl6 V4 alloy as a function of immersion time.

4,5

(a) log (|Z| / Ω cm 2)

10000

-6mV (ocp) +5mV +15mV

5000

-6mV (ocp)

3,5

60

+5mV +15mV

3,0

40

2,5 20

2,0 1,5

0 0

5000

10000

Z (re) / Ω cm2

15000

1,0 -3

- Φ / degree

-Z (im) / Ω cm2

80

(b) 4,0

15000

0 -2

-1

0

1

2

3

4

5

6

log (f / Hz)

Fig. 12. Nyquist (a) and Bode (b) plots of silver coated TiAl6 V4 alloy at three different electrode potential after 190 h of immersion time.

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Table 2 Equivalent circuit parameters for EIS spectra as a function of immersion time. Samples

Day

Rs/ cm2

CPE1/10−6 F cm−2

n1

Ag/TiAl6 V4

1 3 5 10

49.4 50.3 51.0 51.3

98 126 158 151

1.00 0.96 0.93 0.94

Ag/CoCrMo

1 5 15 29

38.1 39.2 38.7 36.0

135 124 122 117

0.80 0.80 0.80 0.81

4.0 4.0 4.1 4.6 280 279 284 241

CPE2/10−6 F cm−2

n2

316 279 244 243

0.85 0.86 0.86 0.86

51,199 34,896 31,468 28,282

0.86 0.59 1.92 0.75

134 133 134 140

0.72 0.70 0.68 0.68

89,817 78,561 48,258 28,016

10.10 9.26 12.80 4.59

5

(a) 2

log (|Z| / Ω cm )

(b)

20000 15000 10000

without stirring with stirring

5000

80

without stirring with stirring

4

60 40

3

20

2 0

0 0

5000

2

4

6

log (f / Hz) 5

(c)

(d) 2

20000

log (|Z|/ Ω cm )

2

0

cm2

15000 10000

without stirring with stirring

5000

without stirring with stirring

80

4

60 40

3

20

2

- Φ / degree

-Z (im) / Ω cm

-2

10000 15000 20000 25000

Z (re) / Ω 25000

Chi-squared/10−5

R2/ cm2

- Φ / degree

-Z (im) / Ω cm2

25000

R1/ cm2

0

0

1 0

5000

10000 15000 20000 25000

-2

0

2

4

6

log (f / Hz)

Z (re) / Ω cm 2

Fig. 13. Impedance data recorded on silver coated TiAl6 V4 and CoCrMo alloys with and without stirring after 48 h of immersion time in 0.9 wt% NaCl solution. Nyquist and Bode plot of Ag/TiAl6 V4 (a and b); Nyquist and Bode plot of Ag/CoCrMo alloy (c and d).

ously with the current measurement, the galvanic potential of the electrode pair was also registered against a saturated calomel electrode (SCE) as a reference electrode. The galvanic current and the potential were measured at sampling rate of 12 point min−1 . The data were collected by an Agilent 34970A data Acquisition/Switch Unit. In the examined system, the anode was silver deposited silver plate and the cathode was a sandblasted TiAl6 V4 alloy. The deposition of silver was carried out with pulse plating technique with same parameters. The surface area ratio of cathode to anode was 1:1. Electrochemical measurements were performed

CPE 1 Rs

in aerated 0.9 wt% NaCl solutions with neutral pH and room temperature. In Fig. 10, it is clearly seen that the galvanic current density strongly decreases to a local minimum at about 22 h then slightly increases with time and during 90 h reaches the value of about 33 nA cm−2 . This is consistent with the results of the Tafel measurements. This galvanic current ensures the spontaneous and continuously silver dissolution. The mixed potential of the silver/silver-sandblasted TiAl6 V4 electrode pair in the first few hours is about −120 mV. After this initial stage the potential increases and reaches the steady state value of −22 mV. 3.5. EIS studies

CPE 2 R1 R2

Fig. 14. Equivalent electric circuit in Zview notation used to simulate the impedance spectra of silver coated metallic implants.

We also studied the local dissolution of the silver coated alloy samples in chloride solutions by EIS measurements. Fig. 11 presents the Nyquist and Bode plots of silver coated TiAl6 V4 alloy as a function of immersion time. During the early stage of immersion, the impulse response is characterized by a near capacitive behaviour. As time passes, the electrolyte gradually penetrates into the silver

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500

(a)

(b)

Ag / TiAl6V4 Ag / CoCrMo

Ag / CoCrMo

400

80000

R2 / Ω cm2

R1 / Ω cm2

100000

Ag / TiAl6V4

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300 200

60000 40000

100 20000

0 -100

0 0

200

400

600

800

0

200

400

600

800

t/h

t/h

Fig. 15. R1 (a) and R2 (b) values as a function of immersion time for Ag/TiAl6 V4 and Ag/CoCrMo.

coating and the substrate comes into contact with the electrolyte. The conditions for bimetallic corrosion are established and the result is a decrease in the diameter of the Nyquist plot indicating that electrode reaction, charge transfer step took place. Variation of different parameters (electrode potential and stirring) was studied on the silver release from the silver coated metallic implants. The electrode potential relative to the ocp effects the degradation of silver coating as it is presented in Fig. 12. The spectra were recorded at three different potentials after 190 h of immersion time. As it is visible, an increase in the potential to anodic direction enhances the degradation of the silver coating. Fig. 13 shows the effects of stirring. As it can be expected stirring increases the rate of dissolution, so that the rate of the corrosion of the silver coating. The presented parameters exert the same influ-

25000

ence for the degradation of the silver coated CoCrMo alloy, however in much less extent. The impedance spectra for Ag-coated alloys exposed to a 0.9 wt% chloride solution were analysed using electrical equivalent circuit model is shown in Fig. 14. This model is generally proposed to describe the localised corrosion of coated stainless steel. RS represents solution resistance between the silver coating and the reference electrode. Due to the surface pre-treatment the substrate surface is rough, therefore the capacity in the circuit model is substituted by constant phase elements (CPE). The impedance of the constant phase elements is expressed as: ZCPE = [Q(jω)n ]−1 , where Q is the magnitude and n is the exponent of the CPE. In the equivalent circuit the parallel-connected elements R2 Q2 are used to describe the charge transfer process at the substrate/coating inter-

(a)

7

(b)

80 70

6

20000

log (|Z| / Ω cm2)

without stirring 15000

with stirring

10000 5000

measured and fitted data

60 5

50 40

4

30 20

3

- Φ / degree

-Z (im) / Ω cm2

measured data with stirring measured data without stirring fitted data

10 2

0

0 -10

0

5000

10000

15000

20000

-2

25000

0

(c)

7

without stirring

log (|Z| / Ω cm2 )

10000 5000

(d)

measured data with stirring measured data without stirring fitted data

6

with stirring

15000

6

measured and fitted data

80 70 60

5

50 40

4

30 3

20 10

2

0

0 1 0

5000

10000

15000

Z (re) / Ω cm2

20000

- Φ / degree

-Z (im) / Ω cm2

20000

4

log (f / Hz)

Z (re) / Ω cm2 25000

2

25000

-10 -2

0

2

4

6

log (f / Hz)

Fig. 16. Experimental and fitted EIS data of silver coated TiAl6 V4 alloy (a and b) and Ag/CoCrMo alloy (c and d) in 0.9 wt% NaCl solution with and without stirring after 96 h of immersion time.

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Fig. 17. Cross-sectional SEM images and analysis of elements on silver coated TiAl6 V4 alloy.

face through the pores, where R2 represents the charge transfer resistance and Q2 is the CPE of the substrate. In series connection with (R2 ; Q2 ), the R1 is electrical resistance of the pore forming during the dissolution process. Q1 is the coating capacity represents the area, where the coating remains intact during immersion. Difference in value of R1 for silver coated TiAl6 V4 and CoCrMo alloys is related to different size and shape of grains in nanostructured silver deposits. Pal et al. [21] investigated the antibacterial properties of differently shaped silver nanoparticles against Escherichia coli in liquid system and found that the silver nanoparticles undergo shape dependent interaction with the bacterium E. coli. Based on their result we also assume that the differences in size and shape of grains in the silver deposits on two different substrates (Fig. 3) causes that the R1 is different for Ag/TiAl6 V4 and Ag/CoCrMo systems. The value of R2 is indicative of electron transfer across the interface and reflects the extent of corrosion degradation at continuous immersion. 0,50

(a)

0,50

Ag / TiAl6V4

0,40 0,35 0,30

0,35 0,30 0,25

0,20

0,20 200

300

400

500

t/h

600

Ag / CoCrMo

0,40

0,25

0,15 100

(b)

0,45

cAg+ / mg dm-3

0,45

cAg+ / mg dm-3

Fig. 15 reveals that the values of R1 and R2 for Ag/CoCrMo are higher than that for Ag/TiAl6 V4 in each case. R1 almost constant in time whereas the decrease of R2 is more notable. This deviation might be in connection also with the different morphology and structures of silver layer deposits and the higher galvanic potential difference between silver deposits and TiAl6 V4 as well as CoCrMo alloy. The measured and the fitted electrochemical impedance data can be seen in Fig. 16, presented in Nyquist and Bode plots. The fitting quality is evaluated by the 2 , which are between 10−4 and 10−5 , indicating good agreement between the measured and simulated values (Table 2). As it was shown on the previous SEM pictures (Fig. 3), silver coating on TiAl6 V4 or CoCrMo alloy is nano-structured and it consists of small grains that form larger structures. This structure provides direct path for corrosive electrolyte and for oxygen to reach the silver/coating interface, where the localized galvanic corrosion can be initiated due to the potential difference between the silver coating and metallic implants (either TiAl6 V4 or CoCrMo alloys). The cross-sectional microstructure of Ag-coated TiAl6 V4 and CoCrMo substrates is studied by SEM images combining with EDX elemental analysis. In Fig. 17 the dark field represents the resin, while the white section part corresponds to the silver coating, the upper part indicates the substrate on the TiAl6 V4 alloy. It is visible that the oxygen concentration gradually decreases from the electrolyte/coating interface to the surface of the substrate, proving the oxygen transport from the environment to the metallic surfaces to sustain the spontaneous silver dissolution in the silver/metallic implant galvanic cells. In 0.9 wt% NaCl solutions the silver dissolution was also checked by ICP-MS analysis. The concentration of released silver ions from the silver coated TiAl6 V4 and CoCrMo substrates as a function of immersion time is presented in Fig. 18. All measurements have been performed for approximately 700–1400 h of immersion time. In case of the silver deposited TiAl6 V4 alloy using same experimental conditions (pulse plating parameters and layer thickness) the amount of released silver vs. time shows a little increasing tendency or remains constant in time (Fig. 18(a)). As for the silver coated CoCrMo system (Fig. 18(b)) the silver ion concentration increases distinctly in early stage of immersion time, afterward a slight increase occurs over time. Comparing these solution analysis data to the charge transfer resistance R2 values (that are inversely proportional to the silver corrosion) we can draw the conclusion that both methods show the increase of corrosion of silver deposits with the immersion time and there is a little difference in trend of the silver release from the silver coated substrates.

700

800

0,15 600

800

1000

t/h

Fig. 18. Solution analysis after EIS measurements Ag/TiAl6 V4 (a) Ag/CoCrMo (b).

1200

1400

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4. Conclusions

References

This work was focused on production of nanosilver coatings on TiAl6 V4 - and CoCrMo alloys with controlled and continuous Ag+ ion release. Nano-structured silver coatings on the metallic implants were performed by pulse plating electrochemical deposition from commonly used cyanide bath. Long-term dissolution of silver coating due to the forming galvanic cells between the silver coating and the metallic substrate was proved by five independent test methods: polarization measurements (Tafel curves), corrosion potential measurements, ZRA method, long term EIS measurements, solution analysis after EIS measurements. These results may be the basis for further clinical research and application of silver coated metallic implants. Comparing the corrosion behaviour of the silver coated TiAl6 V4 and the silver coated CoCrMo alloys it was found, that the galvanic potential difference is greater for silver coated TiAl6 V4 alloys as well as the rate of silver dissolution is slightly more in the case of silver coated TiAl6 V4 metallic implants. Further examinations are needed to clarify the antimicrobial ability of silver coatings and it is also necessary to examine if complete or patterned silver coatings ensure better osteointegrative properties for practical applications.

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Acknowledgements The authors gratefully acknowledge the financial support of Nanobact project. (Project code: No. OM-00018-00022/2008).