**3.1 As-deposited films analysis**

An implant-type ideal coating should constitute a proper mechanical support while exhibiting an enhanced bioactivity.

The bioglass structure is very complex; there is only short and medium range order determined by chemical bounding and steric hindrance. The silica-based glass structure is generally viewed as a matrix composed of SiO4 tetrahedra connected at the corners to form a continuous tri-dimensional network with all bridging oxygen (BOs). The SiO4 tetrahedra network is slightly distorted due to variations in the bond angles and the torsion angles. The network modifiers (alkali and alkali-earth ions typical to bioglasses) enter the structure as singly or double charged cations and occupy interstitial sites. Their charge is compensated by non-bridging oxygen bonds (NBOs), created by breaking bridges between adjacent SiO4 tetrahedra. The increase of modifier content generates the creation of large NBOs concentrations, reducing the connectivity of the BG network, with direct effect upon electrical conduction, the thermal expansion coefficient, glass transition temperature, chemical corrosion in aqueous media and reactivity (Serra et al., 2002; Liste et al., 2004).

Magnetron Sputtered BG Thin Films: An Alternative Biofunctionalization

Approach – Peculiarities of Bioglass Sputtering and Bioactivity Behaviour 83

Si Ca P Na Mg K

Sample type Concentration (at %)

were calculated on the basis of the nominal oxides composition for the target.

soft connective tissues is essential for the clinical success.

silicate or phosphate groups.

modification.

BG1 powder target 50.28 14.68 7.73 8.85 6.81 11.65 BG1 as-deposited film 50 16 10 19 2.6 2.4 BG2 powder 36.34 21.2 4.1 38.36 - - BG2 as-deposited film 35 20 2 43 - - BG3 powder target 38.43 34.18 5.9 8.5 12.88 - BG3-1 as-deposited film 30 27 3 20 20 - BG3-2 as-deposited film 30 32 3 16 19 - BG3-3 as-deposited film 30 33 1 20 16 - BG3-4 as-deposited film 30 20 3 27 20 - Table 4. Chemical compositions in at % for the BG target powders and for the BG films deposited onto titanium substrates. The values were determined by EDS for the films, and

The *in vitro* bioactivity of these BG samples, reflected in their capability of inducing HAformation onto their surfaces, was investigated by immersion in SBF at 37ºC for various periods of time up to 30 days. The SBF had the following ionic concentrations (in mM) of 142.0 Na+, 5.0 K+, 2.5 Ca2+, 1.5 Mg2+, 147.8 Cl-, 4.2 HCO3-, 1.0 HPO42- and 0.5 SO42-, buffered at pH=7.4 with tris-hydroxymethyl-amminomethane (Tris, 50 mM) and hydrochloric acid solutions according to Kokubo (Kokubo & Takadama, 2006). A surface area to volume ratio of 0.1 cm-1 was maintained for all immersions. The biomineralization processes at the BG– SBF interface were monitored, on the surface and in the volume, by FTIR, GIXRD and SEM. Hench's theory states that the first stage of a bioglass mineralization upon immersion in SBF involves the rapid exchange of Na+ and Ca2+ ions from the glass for H+ and H3O+ ions from the solution, which will initialize the hydrolysis of the Si–O–Si bonds of the glass structure and the forming of silanol groups. The dissolution of the glass network, leading to the formation of silica-rich gel layer, the supersaturation of SBF solution with respect to hydroxyapatite and the subsequent deposition of an apatite-like layer on the glass surface, were found to be essential steps in the bioactivity evaluation both *in vivo* and *in vitro* studies (Hench, 1991). The formation of stable, mechanically strong interface with both bone and

FTIR spectroscopy is a powerful method to obtain useful information concerning the shortrange order for the as-deposited amorphous films as well as for SBF tested films, allowing the identification of specific features in the IR vibrational spectrum, such as those related to

*Figure 6* shows the FTIR spectra of the as-deposited and heat-treated BG films together with the spectra of the cathode target powder. Similar absorption envelopes for the target and as-deposited BG films have been recorded, exhibiting the same broad IR bands typical to an amorphous structure. However all the as-deposited BG films presents a shift to higher wave numbers of the maximum absorption peak with respect to the powder spectrum which indicate a certain degree of modification of the silicate tetrahedron network during sputtering, in good correlation with the compositional

The biomineralization activity of a bioglass is influenced by the concentrations of bridging and non-bridging oxygen atoms per silicon oxygen tetrahedron as a function of the alkali oxide concentration. The Qn notation expresses the concentration of bridging oxygen atoms per tetrahedron, where the value of n is equal to the number of bridging oxygen atoms (Elgayar et al., 2005). The Qn species are detected in IR spectra in the 850–1200 cm-1 region by broad bands developing in accordance with the glass alkali and alkali-earth composition. The depolymerization of silicate network is defined by bands present at lower wave numbers in the absorption envelope.

RF-MS is known as a non-equilibrium preparation method which generally produces nonstoichimetric films relative to the sputtering target composition. Therefore it is possible to prepare nanostructured BG films with stoichiometries which can not be obtained by the classical equilibrium bulk synthesis methods. However, the preliminary studies have revealed that by varying the deposition pressure and/or working atmosphere composition it is possible to obtain a stoichiometric transfer even for complex compositional systems such as BGs (G.E. Stan et al., 2010a, 2010b, 2010c, 2010d). When selecting the RF-MS deposition parameters for the BG films preparation, one should take into account the standard free energy of the oxidation reactions of the different elements involved (Alcock, 2001):


*Table 4* presents the BG films atomic concentration with respect to the cathode target original composition. As can be seen Ca is among the most reactive elements towards oxygen, and this explains why its concentration decreased in the BG3 films. The reaction between Ca and the impinging oxygen ions might account for target poisoning by binding calcium ions at the target's surface. According to this reasoning, one would expect almost similar concentration changes for Mg since its standard free energy of oxidation is close to that of Ca. However, *Table 4* shows that higher concentrations of Mg were determined for all the films in comparison to the starting bioglass powder. This suggests that the atomic weight might also play a role, with heavier elements diffusing more slowly. Na enjoys of these two characteristics, i.e., it is lighter and its standard free energy of oxidation is lower, being more easily sputtered. The formation of P4O10, and especially of PO requires higher partial pressure of oxygen. This might explain why a so small amount of P was found in the films. On the other hand, silicon also forms two possible phases, SiO2 or SiO upon oxidation with the first being favored at the temperature of the experiments. The formation of these species also requires higher partial pressure of oxygen in comparison to the formation of oxides of Na, Ca, Mg (G.E. Stan et al., 2011).

The biomineralization activity of a bioglass is influenced by the concentrations of bridging and non-bridging oxygen atoms per silicon oxygen tetrahedron as a function of the alkali oxide concentration. The Qn notation expresses the concentration of bridging oxygen atoms per tetrahedron, where the value of n is equal to the number of bridging oxygen atoms (Elgayar et al., 2005). The Qn species are detected in IR spectra in the 850–1200 cm-1 region by broad bands developing in accordance with the glass alkali and alkali-earth composition. The depolymerization of silicate network is defined by bands present at lower wave

RF-MS is known as a non-equilibrium preparation method which generally produces nonstoichimetric films relative to the sputtering target composition. Therefore it is possible to prepare nanostructured BG films with stoichiometries which can not be obtained by the classical equilibrium bulk synthesis methods. However, the preliminary studies have revealed that by varying the deposition pressure and/or working atmosphere composition it is possible to obtain a stoichiometric transfer even for complex compositional systems such as BGs (G.E. Stan et al., 2010a, 2010b, 2010c, 2010d). When selecting the RF-MS deposition parameters for the BG films preparation, one should take into account the standard free energy of the

2 Ca + O2 2 CaO G° = -1 267 600 + 206.2 T (298–1124 K), (Jmol-1)

2 Mg + O2 2 MgO G° = -1 206 300 + 273.7 T (300–900 K), (Jmol-1)

4 Na + O2 2 Na2O G° = -830 180 + 260.3 T (300–350 K), (Jmol-1)

4 K + O2 2 K2O G° = -729 100 + 287.3 T (400–370 K), (Jmol-1)

*Table 4* presents the BG films atomic concentration with respect to the cathode target original composition. As can be seen Ca is among the most reactive elements towards oxygen, and this explains why its concentration decreased in the BG3 films. The reaction between Ca and the impinging oxygen ions might account for target poisoning by binding calcium ions at the target's surface. According to this reasoning, one would expect almost similar concentration changes for Mg since its standard free energy of oxidation is close to that of Ca. However, *Table 4* shows that higher concentrations of Mg were determined for all the films in comparison to the starting bioglass powder. This suggests that the atomic weight might also play a role, with heavier elements diffusing more slowly. Na enjoys of these two characteristics, i.e., it is lighter and its standard free energy of oxidation is lower, being more easily sputtered. The formation of P4O10, and especially of PO requires higher partial pressure of oxygen. This might explain why a so small amount of P was found in the films. On the other hand, silicon also forms two possible phases, SiO2 or SiO upon oxidation with the first being favored at the temperature of the experiments. The formation of these species also requires higher partial pressure of oxygen in comparison to the formation of oxides of

Si + O2 SiO2 G° = -907 030 + 175.7 T (300–1700 K), (Jmol-1).

oxidation reactions of the different elements involved (Alcock, 2001):

numbers in the absorption envelope.

Na, Ca, Mg (G.E. Stan et al., 2011).


Table 4. Chemical compositions in at % for the BG target powders and for the BG films deposited onto titanium substrates. The values were determined by EDS for the films, and were calculated on the basis of the nominal oxides composition for the target.

The *in vitro* bioactivity of these BG samples, reflected in their capability of inducing HAformation onto their surfaces, was investigated by immersion in SBF at 37ºC for various periods of time up to 30 days. The SBF had the following ionic concentrations (in mM) of 142.0 Na+, 5.0 K+, 2.5 Ca2+, 1.5 Mg2+, 147.8 Cl- , 4.2 HCO3-, 1.0 HPO42- and 0.5 SO42-, buffered at pH=7.4 with tris-hydroxymethyl-amminomethane (Tris, 50 mM) and hydrochloric acid solutions according to Kokubo (Kokubo & Takadama, 2006). A surface area to volume ratio of 0.1 cm-1 was maintained for all immersions. The biomineralization processes at the BG– SBF interface were monitored, on the surface and in the volume, by FTIR, GIXRD and SEM. Hench's theory states that the first stage of a bioglass mineralization upon immersion in SBF involves the rapid exchange of Na+ and Ca2+ ions from the glass for H+ and H3O+ ions from the solution, which will initialize the hydrolysis of the Si–O–Si bonds of the glass structure and the forming of silanol groups. The dissolution of the glass network, leading to the formation of silica-rich gel layer, the supersaturation of SBF solution with respect to hydroxyapatite and the subsequent deposition of an apatite-like layer on the glass surface, were found to be essential steps in the bioactivity evaluation both *in vivo* and *in vitro* studies (Hench, 1991). The formation of stable, mechanically strong interface with both bone and soft connective tissues is essential for the clinical success.

FTIR spectroscopy is a powerful method to obtain useful information concerning the shortrange order for the as-deposited amorphous films as well as for SBF tested films, allowing the identification of specific features in the IR vibrational spectrum, such as those related to silicate or phosphate groups.

*Figure 6* shows the FTIR spectra of the as-deposited and heat-treated BG films together with the spectra of the cathode target powder. Similar absorption envelopes for the target and as-deposited BG films have been recorded, exhibiting the same broad IR bands typical to an amorphous structure. However all the as-deposited BG films presents a shift to higher wave numbers of the maximum absorption peak with respect to the powder spectrum which indicate a certain degree of modification of the silicate tetrahedron network during sputtering, in good correlation with the compositional modification.

Magnetron Sputtered BG Thin Films: An Alternative Biofunctionalization

agreement with the GIXRD measurements (*Figs. 1* and *4*).

**3.2 Bioactivity tests: Observations & considerations** 

time, suggesting the inert character of this material.

emphasized, indicating the resorbability of this material.

simulated body fluid up to 30 days.

2006).

2002).

Approach – Peculiarities of Bioglass Sputtering and Bioactivity Behaviour 85

splitting of the bands was noticed, pointing a strong crystallization of the BG coating, in

All the BG3 films FTIR spectra displayed as weal broad absorption bands: two dominant peak maxima at ~1030 cm-1 (Si–O stretching in Q2 and Q3 units) and at ~940 cm-1 (Si–O stretching in Q1 and Q2 units). The weak shoulder at ~725 cm-1 corresponds to Si-O-Si bending motion. Broad vibration bands centred at 1218 and 1411 cm-1 were evidenced, and are related to B–O stretching of BO3 units in borates with bridging oxygen, vibrations of metaborate triangles and B–Ø stretching of BØ4 and BØO2<sup>−</sup> units (Agathopoulos et al.,

In case of BG3 films the FTIR analysis revealed a dependence between argon deposition pressure and the short range order of the sputtered glass structure. The displacement of the asymmetric stretching vibration present at 1040 cm-1 (BG3-1) to lower wave numbers (*Fig. 6 c*) indicates the Si-O-Si linkages perturbation by a continuous formation of non-bridging oxygen type linkages with the increase of argon sputtering pressure, and the weakening of the structural bonds sustained by the bridging oxygen atoms (*Fig. 6-c*). The FTIR spectra displayed indeed an increase of the intensity ratio of the bands: 923–944 cm-1 / 1010–1040 cm-1, which denote the enrichment in Q1 and Q2 structural groups, with increasing the deposition pressure. This suggests that as the deposition pressure increases a higher concentration of alkali and alkali-earth oxides might be incorporated in the glass structure, the Q3 groups being progressively converted to Q2 groups (Socrates, 2001; Serra et al., 2002). A direct correlation between the glass thin films' composition and their structure that reflects directly in their biomineralization capability was reported in literature (Serra et al.,

*Figure 7* displays a comparison of the BG films IR spectra before and after immersion in

For the as-deposited and annealed BG1 structures, the FTIR measurements (*Fig. 7-a,b*) showed no changes in intensity and position of the original vibration bands with immersion

In case of as-deposited BG2 samples (*Fig. 7-c*) after only 24 hours of immersion, dramatic changes were observed. The spectrum revealed the disappearance of the initial BG film vibration bands and the emergence of two new and strong bands positioned at 816 and 1107 cm-1, which could be assigned to bending and stretching bands of the amorphous silica phase (Bal et al., 2001). Weaker bands at 736, 892 and 968 cm-1, were noticed, belonging most probably to the original BG layer. The intensity of these bands decreased with the immersion time suggesting a continuously leaching of BG ions into the SBF solution. After 30 days of immersion the BG films is completely dissolved, no IR bands could be

The FTIR results of the annealed BG2 samples after in-vitro testing in SBF are presented in *Figure 3-d*. After 24 hours soaking in SBF solution the amplitude of the peak at 924 cm-1 present increased values and displaced at ~ 940 cm-1. This maximum peak at 940 cm-1 is

In case of BG1 structures (*Fig. 6-a*) we noted the presence of three intense vibration bands:


Fig. 6. Comparative FTIR spectra for the target powders and the studied films: a) BG1; b) BG2; c) BG3

The peak at 1383 cm-1 might be assigned to a shifted (CO3)2- stretching band. The presence of the weaker band at 696 cm-1 corresponds to symmetric stretching bands of PØP Q3 in Q2 and Q1 units. After the heat-treatment at 700°C/2h one can observe the splitting of the envelope in two shoulders, indicating a crystallization of the BG structure, along with the appearance of new vibration band at 910 cm-1 (the stretching of the Si-O-3NBO and Si-O-2NBO) groups). A strong shift to lower wave numbers of the bands positioned at 1030 and 1120 cm-1 was also noticed.

The IR spectra of BG2 structures (*Fig. 6-b*) revealed three strong vibration bands:


The weaker band at 680 cm-1 might correspond to symmetric stretching bands of PØP Q3 in Q2 and Q1 units. Other vibrations of phosphate groups present in the bioglass are difficult to emphasize because of the superimposition of the strong bands of SiO4 units. Previous IR studies noticed also the presence of Q2, Q1, and Q0 phosphate units in the 1400–400 cm-1 IR spectra range (Socrates, 2001; Agathopoulos et al., 2006). The weak broad shoulder present at 1600 cm-1 can be assigned to water bending vibrations, indicating that BG1 material is highly hygroscopic, absorbing water vapour when in air. The presence of absorption band at 1439 cm-1 is attributed to the stretching vibrations of carbonate (CO3)2- structures incorporated during the deposition process. After the post-deposition annealing a clear

In case of BG1 structures (*Fig. 6-a*) we noted the presence of three intense vibration bands: 1001-1030 cm-1 assigned to asymmetric stretching vibrations of Si-O-Si in the Q2 and Q3

1102-1130 cm-1, more intense for the as-deposited samples could be attributed to the

794 cm-1 correspond to the bending motion of Si–O–Si links (Socrates, 2001;

Fig. 6. Comparative FTIR spectra for the target powders and the studied films: a) BG1; b)

The IR spectra of BG2 structures (*Fig. 6-b*) revealed three strong vibration bands:

The peak at 1383 cm-1 might be assigned to a shifted (CO3)2- stretching band. The presence of the weaker band at 696 cm-1 corresponds to symmetric stretching bands of PØP Q3 in Q2 and Q1 units. After the heat-treatment at 700°C/2h one can observe the splitting of the envelope in two shoulders, indicating a crystallization of the BG structure, along with the appearance of new vibration band at 910 cm-1 (the stretching of the Si-O-3NBO and Si-O-2NBO) groups). A strong shift to lower wave numbers of the bands positioned at 1030 and 1120 cm-1 was

 924-944 cm-1 - attributed to the stretching vibration of the SiO4 units with three and two non-bridging oxygen atoms, the Q1 (Si-O-3NBO), and Q2 (Si-O-2NBO) groups; 1010-1022 cm-1 assigned to the coexistence of various Q2 and Q3 Si-O-Si asymmetric

766 cm-1 correspond to the bending motion of Si–O–Si links (Socrates, 2001;

The weaker band at 680 cm-1 might correspond to symmetric stretching bands of PØP Q3 in Q2 and Q1 units. Other vibrations of phosphate groups present in the bioglass are difficult to emphasize because of the superimposition of the strong bands of SiO4 units. Previous IR studies noticed also the presence of Q2, Q1, and Q0 phosphate units in the 1400–400 cm-1 IR spectra range (Socrates, 2001; Agathopoulos et al., 2006). The weak broad shoulder present at 1600 cm-1 can be assigned to water bending vibrations, indicating that BG1 material is highly hygroscopic, absorbing water vapour when in air. The presence of absorption band at 1439 cm-1 is attributed to the stretching vibrations of carbonate (CO3)2- structures incorporated during the deposition process. After the post-deposition annealing a clear

anti-symmetric stretching mode of Si–O–Si groups;

units;

BG2; c) BG3

also noticed.

stretching vibration;

Agathopoulos et al., 2006).

Agathopoulos et al., 2006).

splitting of the bands was noticed, pointing a strong crystallization of the BG coating, in agreement with the GIXRD measurements (*Figs. 1* and *4*).

All the BG3 films FTIR spectra displayed as weal broad absorption bands: two dominant peak maxima at ~1030 cm-1 (Si–O stretching in Q2 and Q3 units) and at ~940 cm-1 (Si–O stretching in Q1 and Q2 units). The weak shoulder at ~725 cm-1 corresponds to Si-O-Si bending motion. Broad vibration bands centred at 1218 and 1411 cm-1 were evidenced, and are related to B–O stretching of BO3 units in borates with bridging oxygen, vibrations of metaborate triangles and B–Ø stretching of BØ4 and BØO2<sup>−</sup> units (Agathopoulos et al., 2006).

In case of BG3 films the FTIR analysis revealed a dependence between argon deposition pressure and the short range order of the sputtered glass structure. The displacement of the asymmetric stretching vibration present at 1040 cm-1 (BG3-1) to lower wave numbers (*Fig. 6 c*) indicates the Si-O-Si linkages perturbation by a continuous formation of non-bridging oxygen type linkages with the increase of argon sputtering pressure, and the weakening of the structural bonds sustained by the bridging oxygen atoms (*Fig. 6-c*). The FTIR spectra displayed indeed an increase of the intensity ratio of the bands: 923–944 cm-1 / 1010–1040 cm-1, which denote the enrichment in Q1 and Q2 structural groups, with increasing the deposition pressure. This suggests that as the deposition pressure increases a higher concentration of alkali and alkali-earth oxides might be incorporated in the glass structure, the Q3 groups being progressively converted to Q2 groups (Socrates, 2001; Serra et al., 2002). A direct correlation between the glass thin films' composition and their structure that reflects directly in their biomineralization capability was reported in literature (Serra et al., 2002).
