Effect of ZrO2 on the bioactive properties of B2O3–SiO2–P2O5–Na2O–CaO glass system

Journal of Non-Crystalline Solids 452 (2016) 23–29

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Effect of ZrO2 on the bioactive properties of B2O3–SiO2–P2O5–Na2O–CaO glass system N. Krishnamacharyulu a,b, G. Jagan Mohini a, G. Sahaya Baskaran a,⁎, V. Ravi Kumar b, N. Veeraiah b a b

Department of Physics, Andhra Loyola College, Vijayawada 520 008, Andhra Pradesh, India Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522 510, Andhra Pradesh, India

a r t i c l e

i n f o

Article history: Received 7 April 2016 Received in revised form 20 July 2016 Accepted 28 July 2016 Available online xxxx Keywords: B2O3–SiO2–P2O5–Na2O–CaO glasses Zirconium ions Hydroxyapatite layer SBF solution

a b s t r a c t Boro silica phosphate glasses with Na2O and CaO as modifiers, mixed with different concentrations of ZrO2 are developed by melt-quenching technique. Simulated body fluid (SBF) solution with pH 7.2 was prepared by the Kokobo method. The prepared glasses were immersed in simulated body fluid (SBF) solution for a period of 30 days. The weight loss and the pH of the residual solution are carried out at specific intervals during this period. The post-immersed samples are characterized by XRD and SEM techniques. These studies confirm the formation of crystalline hydroxyapatite layer (HA) on the surface of the glasses. For understanding the influence of ZrO2 on the magnitude of HA layer formation and degradability of the samples, the spectroscopic studies viz. optical absorption and IR spectral studies on post- and pre-immersed samples were undertaken. The results of spectroscopic studies, coupled with bioactive studies, are analyzed as a function of ZrO2 concentration. The analysis indicated that the glasses containing low quantities of ZrO2 exhibit better bioactive characteristics. © 2016 Elsevier B.V. All rights reserved.

1. Introduction There is a growing interest in the use of bioactive glasses in biomedical applications such as bone repair, fixation of long bone fractures and periodontal repair [1–3]. An important characteristic of these bioactive glasses is the time-dependent surface modification. Such modification leads to the formation of a biologically active hydroxyapatite (HA) layer that makes a firm bond with surrounding tissues [4–6]. The oldest bioactive material (45S5 bioglass), which was discovered by Hench et al. [7], was very well characterized and was used in a number of biomedical applications such as orthopedic implant and bone filler material [2,8,9]. However, the degradation of this silicate-based glass is very low and it was reported that it remained in the human body approximately up to one year after implantation [10]. Whereas borate-based bioactive glasses converted considerably faster to apatite through a set of dissolution and precipitation reactions similar to 45S5 bioglass. For instance, 45S5-3B bioglass (the borate equivalent of 45S5 glass) converted almost completely into HA within 3–4 days [11]. In addition, boron is expected to play an important role in many life processes that include bone growth and psychomotor skills [12]. It also appears that boron may influence the production of hormones [13]. Thus, low durable borate glasses have become a special interest that delivers boron by the degradation for bone health.

⁎ Corresponding author. E-mail address: [email protected] (G.S. Baskaran).

http://dx.doi.org/10.1016/j.jnoncrysol.2016.07.044 0022-3093/© 2016 Elsevier B.V. All rights reserved.

P2O5 is a strong glass former that participates in the glass network in the form of PO4 structural units. The PO4 tetrahedra are, in general, linked together with covalent bonding in chains or rings by bridging oxygens [14]. Both Na2O and CaO act as modifiers and introduce coordinate defects and non-bridging oxygens (NBO) into the glass network [15]. Zr is one of the common trace elements present in the environment. Metallic Zr(IV) normally exists in human bone and tissue as low traces in the range of 2–10 mg/kg body weight with an estimated average daily intake in humans of ~2.5 mg. The toxicity of Zr has been assessed to be low to moderate in animals [16]. Zr containing materials are stable, possess high mechanical strength and are biologically inert. In view of such characteristics, Zr mixed materials are found to be biocompatible and are being widely used for dental applications and also as coatings for orthopedic implants [17–20]. Li et al. [21], in their recent studies, have reported that the addition of ZrO2 decreased the pore volume and improved the bending strength of wollastinite. These authors have found that the elastic moduli of such ceramics matched with those of the human bone. The same group of authors [22] has also carried out in vitro cytotoxicity tests and demonstrated that these materials exhibited no toxicity to cells. Other previous studies also indicated that Zr implants exhibited excellent osseo integration and possess high mechanical strength and fracture toughness [17,23,24]. Recently, Lee and his coworkers [25] have carried out extensive studies on ZrO2–CaO–P2O5–Na2O–SiO2 bioactive glass ceramics. In their quantitative studies, it was proved that, ZrO2 enhanced the vitrification of bioactive glass at low temperature when compared to conventional bioglass. Further, it was confirmed that the

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N. Krishnamacharyulu et al. / Journal of Non-Crystalline Solids 452 (2016) 23–29

Table 1 Details of the chemical composition of the glasses in mol%. Sample label

B2O3

SiO2

P2O5

Na2O

CaO

ZrO2

Zr0 Zr1 Zr2 Zr3 Zr4 Zr5

43 43 43 43 43 43

5 5 5 5 5 5

2 2 2 2 2 2

20 20 20 20 20 20

30 29 28 27 26 25

0 1 2 3 4 5

mechanical properties of the glass were improved to a large extent and also promoted cell viability when ZrO2 is added at up to 10 wt%. In the glass matrices, ZrO2 participates as a network former with ZrO4 structural units and also acts as a modifier and participate with ZrO6 structural units [26–29]. In silicate glasses, its participation can be represented as  2‐ 2Si–O‐ þ ZrO2 ® Si–O–Si þ ZrO2 O2=2 Overall, ZrO2 is expected to bring interesting changes in bioactivity of alkali borophospate silicate glasses. Motivated by these studies, the present investigation is devoted to synthesizing bioactive glass with B2O3, SiO2, P2O5, Na2O, CaO and ZrO2 as the constituents, to study the degradability of the glass in SBF solution in vitro and to envisage the structural role of ZrO2 and its influence on bioactivity. This has been carried out by measuring the weight loss of the glass and by studying the variation in pH value of residual SBF solution as a function of ZrO2 concentration. The results are analyzed using mid infrared (MIR) and optical absorption spectra of the glasses. 2. Experimental methods Details of chemical composition of the glasses chosen for this study are presented in Table 1. Specified amounts of analytical grade powders of reagents H3BO3, SiO2, P2O5, Na2CO3, CaCO3 and ZrO2 are taken in mol% and thoroughly mixed in an agate mortar. Then, the charge is taken in a silica crucible and placed in a pre-heated automatic temperature controlled electrical furnace for 30 min maintained at 1050 °C. The melt is poured into a brass mould and immediately shifted to a muffle furnace maintained at 400 °C for annealing purposes. The samples are taken out of the annealing furnace after 24 h, then ground and optically polished. The density of the glasses before and after immersion in SBF solution is measured by the standard Archimedes' principle using deionized water as buoyant liquid to an accuracy of 0.0001 g/cm3 with electrical digital balance (Shimadzu make AUY 220 Model). FTIR spectra of the samples in mid infrared region was recorded. Infrared transmission spectra were recorded at ambient temperature on a JASCO-FT/IR-5300 spectrophotometer with spectral resolution 0.1 cm− 1 in the spectral range of 400–4000 cm− 1 using potassium bromide pellets (300 mg) containing pulverized sample (1.5 mg). These pellets were pressed in a vacuum die at ~680 MPa. The optical absorption spectra of the samples were recorded at room temperature in the wavelength range of 300–800 nm using JASCO make

V-670 UV–vis–NIR spectrophotometer with a scanning speed of 200 nm/min at a resolution of 0.1 nm. SEM studies were carried out on these samples using Tescan (Model VEGA3 LMU) scanning electron microscope. The X-ray diffraction analysis of the samples is performed using Bruker X D8 advance diffractometer at a temperature of 298 K. A scanning electron microscope (Tescan, Model VEGA3 LMU) was used to obtain surface images of the glass samples to explore its surface morphology. The magnitude of degradability of all the glasses is obtained by means of weight loss measurements in SBF solution. The polished glass samples are then immersed in closed poly-ethylene bottles containing pre-calculated volume of SBF solution and are incubated at 36.5 °C. The glass samples are taken out from the container at specific time intervals and then excess moisture is removed by blotting the samples dry using tissue paper. The entire measurement is carried out by keeping the surface/volume ratio of 1:10 as constant as described elsewhere [30]. By knowing the initial weight Mo of each sample and the weight Mt. at time ‘t’, the percentage of weight loss is obtained as % weight loss = [Mo − Mt./A] × 100 where ‘A’ is the surface area in cm2. All the weight measurements were carried out using Shimadzu digital balance with an accuracy of 0.0001 g, the pH value of SBF solution is measured at each time with pH meter calibrated with fresh standard buffered solution. In vitro bioactivity of the glasses, reflected in their capability for self-assembling of hydroxyapatite layer onto their surface [31], was investigated by immersion in SBF solution and incubated at 36.5 °C. The SBF solution is prepared using the Kokubo recipe [32], which is the closest to the human blood plasma with respect to ionic concentrations. The solution is buffered to pH 7.4 with Tris-buffer (hydroxyl methyl amino methane) and hydrochloric acid. Finally, after 30 days of immersion, the glass pieces were taken out and gently rinsed with distilled water and dried in air. The naked eye inspection revealed that all the soaked samples were covered with a thin white layer. The surfaces of dried samples were analyzed by MIR and optical absorption spectra to detect the formation of apatite layer on it. 3. Results 3.1. Physical parameters From the measured values of density d and calculated average molecular weight of the glasses, various physical parameters such as zirconium ion concentration Ni, mean zirconium ion separation, Ri, polaron radius, Rp and field strength Fi were evaluated using the conventional formulae mentioned below [33] and are presented in Table 2. (i) Ni (ions /cm3) = ½NA Mðmol%Þd M

From the Ni values obtained, inter-ionic distance (ri) and the polaron radius (rp) could be evaluated: (ii) Inter-ionic distance ri (Å) = ½N1i 

1=3

π 1=3 (iii) Polaron radius r p (Å) = 12 ½6N  i

Table 2 Summary of the data on various physical parameters of B2O3–SiO2–P2O5–Na2O–CaO:ZrO2 glasses. Property

Zr0

Zr1

Zr2

Zr3

Zr4

Zr5

Density (g/cm3) d

2.420 64.99

2.454 65.66

2.46 66.34

2.479 67.01

2.489 67.68

2.499 68.35

26.81 – – – –

26.75 2.251 3.541 1.427 14.73

26.96 4.467 2.818 1.1356 23.26

27.03 6.684 2.464 0.9928 30.43

27.19 8.861 2.243 0.9038 36.72

27.35 11.01 2.0863 0.841 42.4

Avg. mol. wt. M Molar volume Vm (cm3/mol) Zr ion conc. Ni (1022 ions/cm3) Inter-ionic distance of Zr ions Ri Å Polaron radius Rp Å Field strength Fi (1015/cm2)

N. Krishnamacharyulu et al. / Journal of Non-Crystalline Solids 452 (2016) 23–29

25

600

Wavelength (nm)

Fig. 1. Optical absorption plots for B2O3–SiO2–P2O5–Na2O– CaO:ZrO2 glasses.

The field strength (Fi) of Zr ion in the glass matrix was evaluated using the oxidation number (z) and the inter-ionic distance (ri) by the equation (iv) Field strength Fi (cm−2 ) = rz2 i

It is well known that density is an important parameter [34] to explore the structural compactness/softening, the change in geometrical configurations, co-ordination number and cross-link density, etc., of the glass network. In the studied glasses, B2O3, SiO2, P2O5 and Na2O contents are kept constant while CaO and ZrO2 contents are varied from 25 to 30 mol% and 0 to 5 mol%, respectively. With the addition of ZrO2 from 1.0 to 5.0 mol% to the glass matrix, the density is found to increase. The densities of all the post-immersed samples are found to decrease considerably with respect to those of corresponding preimmersed samples. The optical absorption spectra of pre-immersed B2O3–SiO2–P2O5– Na2O–CaO:ZrO2 glasses are recorded at room temperature in the wavelength region 300–800 nm. For the glass Zr0, the absorption edge is observed at 375 nm. Addition of ZrO2 (1 to 5 mol%), caused the absorption edge to shift towards shorter wavelength side as shown in Fig. 1. From the observed absorption edges, we have evaluated the optical band gaps (Eo) of these glasses by drawing Tauc plots between (α ћ ω)1/2 and ћω as per the equation:

In Eq. (1), α (ω) is the absorption coefficient, ћ ω is the incident photon energy, C is a constant, related to the extent of the band tailing. It is a temperature-independent constant, depends on the refractive index n and the electrical conductivity at absolute zero. Tauc plots drawn for all the glasses are presented in Fig. 2; in all the plots, a considerable part is observed to be linear in the high-energy region. From the extrapolation of the linear portion of these curves, the values of Eo are determined and furnished in Table 3. The optical band gap is found to increase with increase in the ZrO2 content. The mid infrared spectrum (Fig. 3) of glass Zr1 exhibited the following vibrational bands [35–39]: (i) at 1520 cm− 1 due to asymmetric stretching relaxation of the B\\O bond of trigonal BO3 units; 1080 cm−1 due to B\\O bond stretching in tetrahedral BO4 units (ii) 1018 cm− 1 due to Si–O–Si asymmetric vibrations/B\\O bond units stretching vibrations in BO4 units PO3− 4 (iii) 700 cm−1 due to bending vibrations of B\\O\\B linkages/Zr–O– Zr vibrations of ZrO4 units (iv) at 531 cm−1 due to Zn–O vibrations of ZrO4 units. The summary of various band positions of IR spectra is provided in Table 4. The close observation of MIR spectra of glasses indicated growth of the band related to tetrahedral BO4 units and ZrO4 units at the expense of triangular BO3 units and asymmetric silicate groups with the increase of ZrO2 content.

1520

1081 1018 706 532

ZrO4 units

500

Zr0

B-O-B linkages

Zr2 Zr1

Si-O-Si asym. /PO43-

400

Zr3

Cutoff wavelength (nm) 375 369 360 350 342 332

BO4 units

300

Zr4

Optical band gap E0 (eV) 2.55 2.65 2.75 2.9 3.05 3.25

B-O stretching of BO3 units

Zr5

Glass Zr0 Zr1 Zr2 Zr3 Zr4 Zr5

Stretching mode of OH groups

Absorption coefficent (Arb Units)

Table 3 Summary of data on optical absorption of glasses B2O3–SiO2–P2O5–Na2O–CaO:ZrO2.

Zr5

α ðωÞ ћ ω¼C ðћ ω–Eo Þ2 :

ð1Þ Transmittance(arb. units)

Zr4

3.8 3.3 2.8

Zr3 Zr2

Zr5 Zr4 Zr3 Zr2 Zr1 Zr0

Zr1

2.3 Zr0

1.8 1.3 0.8 0.3 1.6

2.1

2.6

3.1

3.6

4.1

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 2. Tauc plots for evaluating optical band gap of B2O3–SiO2–P2O5–Na2O–CaO:ZrO2 glasses.

Fig. 3. MIR spectra of B2O3–SiO2–P2O5–Na2O–CaO:ZrO2 glasses before immersion in SBF solution. (The spectra are Y-shifted for the sake of clarity).

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N. Krishnamacharyulu et al. / Journal of Non-Crystalline Solids 452 (2016) 23–29 6.0

Table 4 Data on MIR band positions (in cm−1) of B2O3–SiO2–P2O5–Na2O–CaO:ZrO2 glasses before immersion in SBF solution.

Asymmetric stretching relaxation of B\ \O bond in BO3 units BO4 units Si–O–Si asymmetric/PO3− 4 B\ \O\ \B linkages/Zr-O-Zr ZrO4 units

Glass Zr0

Glass Zr1

Glass Zr2

Glass Zr3

Glass Zr4

Glass Zr5

1520

1520

1520

1520

1520

1520

1081 1018 703 –

1083 1017 700 531

1083 1016 700 531

1085 1018 700 528

1080 1018 705 529

1083 1018 706 531

3.0 2.0 1.0 0.0

The percentage of weight loss of the titled glasses doped with different concentrations of ZrO2 due to immersion in SBF solution vs dissolution time is presented in Fig. 4(a). From the graph, it is clear that the rate of degradation is gradually decreased with increase of ZrO2 content. The variation of residual SBF solution (Fig. 4b) exhibited a similar trend as that of the degradation rate. Further, from a close examination of the graph, it can be visualized that there is a rapid rise in pH during the first 86 h of immersion in SBF and beyond this, the increase in pH is rather slow. Both the degradation rate and the pH of the residual solution were found to decrease with an increase in the concentration of ZrO2 (insets of Fig. 4(a) and (b), respectively). The value of optical band gap of all the studied samples before and after immersion is presented in a bar diagram (Fig. 5). These values give further insight in understanding layer formation on the surface of glasses. The layer formation on these surfaces involves several reactions such as glass dissolution, precipitation, etc., between the glass and SBF solution [40]. Such processes lead to some structural changes on the glass surface and ultimately change the optical band gap. The comparison of optical band gaps of all the soaked and pre-soaked samples exhibited remarkable differences. The diagrams indicated that band

a

2.5 Zr0

2.00

2.0

1.00

Zr1

0.00

Zr2 0

1

2

3

4

5

Zr3

1.5

Zr4 Zr5

1.0

0.5

0.0 100

200

300

400

500

600

700

b 10.5 Value of pH

10.0

Zr0

9.0

Zr1 Zr2 Zr3

8.5

Zr4 Zr5

9.5

Value of pH

8.0 7.5

9.50

0

1

2

3

4

100

200

300 400 Time (h)

Zr1

Zr2 sample code

Zr3

Zr4

Zr5

Fig. 5. The optical band gap of the B2O3–SiO2–P2O5–Na2O–CaO:ZrO2 glasses before and after immersion in SBF for 30 days.

gaps are increased significantly after dipping in SBF solution with respect to those of corresponding soaked samples (Table 5). Fig. 6 shows the MIR spectra of the layer formed on the surfaces of the glasses due to immersion in SBF solution for 30 days. These spectra exhibited additional bands due to phosphate and carbonate groups when compared with those of unsoaked samples. To obtain better inference from these studies, we have shown the spectra of glass Zr1 in Fig. 7 before and after immersion. For the sake of reference, we have also presented the standard spectrum of HA layer in the same figure. The comparison reveals that in the place of weak bands around 1020 cm−1 corresponding to BO4/PO3− 4 /Si–O–Si asymmetric vibrations, an intense broad band appeared. Besides this, a well-resolved vibrational peak at about 615 cm−1 assigned to vibrations of P\\O bonds in the PO34 − group of crystalline HA layer is also observed [41]. Further, the spectra of the post-immersed samples exhibited an intense band due to stretching vibrational mode of OH group at 3435 cm−1 [42]. A vibrational band at about 1420 cm− 1, identified as being due to C\\O stretching in carbonate groups, substituted for phosphate groups in apatite lattice [43]. The vibrational bands at 1080 cm− 1, 700 cm− 1 and 530 cm−1corresponding to BO4 units, B\\O\\B linkages/Zr–O–Zr vibrations of ZrO4 units and ZrO4 units, respectively, observed in the spectra of pre-soaked samples could not be resolved in the spectra of the post-immersed samples. The close observation of the MIR spectra of these samples further indicated a gradual decrease in the intensity of the bands corresponding to phosphate and carbonate groups with increase in the concentration of ZrO2. XRD patterns of the glass Zr1 before and after immersion are shown in Fig. 8(a) and (b), respectively. The pattern before in vitro studies confirms the amorphous nature of the glass sample. However, diffractograms of the post-immersed samples exhibited diffraction peaks related to HA layer at 2θ = 31.776° and 39.818° for crystal planes (211) and (310), respectively [JCPDS no. 09-432]. The SEM picture of the surface of the glass sample (Zr1) before immersion (Fig. 9(a)) exhibited a clear image without any crystallinity. However, after in vitro studies, the SEM image (Fig. 9(b) of the glass sample exhibited the presence of small concentration of precipitates (particle-like growths), which are presumed to be apatite crystals as identified by XRD studies. The EDS spectrum for one of the samples Zr1 is presented in Fig. 9(a). The analysis indicates the retention of all the elements in the sample.

5

Conc. of ZrO2

0

Zr0

Table 5 Showing the variation in the band gaps of soaked and unsoaked samples in SBF.

8.50

7.0

before immersion after immersion

4.0 Eo(eV)

Vib. groups

5.0

500

600

700

Fig. 4. (a) Percentage of weight loss vs dissolution time of B2O3–SiO2–P2O5–Na2O– CaO:ZrO2 glasses during degradation in SBF solution maintained at 36.5 °C; (b) variation of pH of the SBF solution measured on various time intervals for the B2O3–SiO2–P2O5– Na2O–CaO:ZrO2 glasses during immersion.

Sample

Eg before immersion (eV)

Eg after immersion (eV)

Zr0 Zr1 Zr2 Zr3 Zr4 Zr5

2.55 2.65 2.75 2.90 3.05 3.25

2.92 2.70 3.5 3.10 3.18 3.45

1561

B-O-B linkages

1024

P-O bending / ZrO4 units

PO43-/Si-O-Si asymmetric

CO32- group

BO3 units

Stretching mode of O-H

N. Krishnamacharyulu et al. / Journal of Non-Crystalline Solids 452 (2016) 23–29

705 600

1418

3435

Zr5

27

4. Discussion The composition of the titled glass system is an admixture of the glass formers, modifiers and intermediates. The addition of alkali/alkaline earth oxides such as Na2O and CaO to the main glass causes some of the borons to change from triangular to tetrahedral co-ordination in the glass network without formation of non-bridging oxygens (NBOs). This process of conversion of borons from three- to four-fold co-ordination continued until the network reached some critical concentration of tetrahedrally co-

Zr4

a

Zr3

Zr2

205

200

Zr1 195

Zr0

intensity

190

4000

3500

3000

2500 2000 -1 Wavenumber (cm )

1500

1000

185

500 180

Fig. 6. MIR spectra of glasses B2O3–SiO2–P2O5–Na2O–CaO:ZrO2 after immersion in SBF solution (spectra are Y-shifted for clarity).

175

170 0

(c)

1416

Zr1

1024

60

80

100

b

1083

(211)

(310)

Hydroxyapatite

1200

700 531 Zr1

40

613

1564

1400

(b)

20

1016

1520 1000

605 560 (a)

HA

1035

Intensity

800

600

400

200

0

3900

3400

2900

2400

1900

1400

900

400

20

30

40

50

60

70

-1

Wavenumber(cm ) Fig. 7. MIR spectra showing change in sample Zr1 (a) HA reference (b) 0 days (c) 30 days.

Fig. 8. (a) XRD patterns of glass sample Zr1 before immersion; (b) XRD patterns of glass sample Zr1 after immersion.

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N. Krishnamacharyulu et al. / Journal of Non-Crystalline Solids 452 (2016) 23–29

Fig. 9. SEM micrograph of Zr1 glass (a) before immersion (with EDS spectra) and (b) after immersion in SBF solution for 30 days.

ordinated borons; beyond that these oxides act as modifiers and induce non-bridging oxygens [44]. The increasing value of density with increasing concentration of ZrO2 reflects the increase of polymerization of the glass network [45]. The reduction in the density of SBF-treated samples is expected due to the chemical reactions that occurred at the glass/liquid interface during immersion. The molar volume indicates the spatial distribution of the oxygen in the glass network. In other words, it is connected with the compactness and the arrangement of various structural units of the glass network. SiO2 is one of the most common glass formers and participates in the glass network with tetrahedral [SiO4/2]0 units and all the four oxygens in SiO4 tetrahedron are shared. Upon the addition of modifiers like Na2O and CaO, the glass structure gets depolymerized or modified. The depolymerization results in the formation of meta-, pyro- and orthosilicates in the order: [SiO4/2]0, [SiO3/2O]−, [SiO2/2O2]2−, [SiO1/2O3]3− and [SiO4]4− which are designated as Q4, Q3, Q2, Q1 and Q0, respectively [46]. P2O5 acts as a network former and participates in the glass network with PO4 tetrahedrons [47,48]. Zr ions in general participate in the glass network with ZrO4 structural units and alternate with BO4 structural units [49]. The presence of such BO3, BO4, SiO4, PO4 and ZrO4 units is evident from MIR spectra of titled glasses. From the IR spectra, it is clear that the proportion of zirconium ions takes part in the network forming positions goes on increasing with the increase of ZrO2. The shifting of optical absorption edge towards shorter wavelength also supports this view point [50]. Weight loss measurements provide a better idea of sample dissolution or these measurements can be directly correlated to glass corrosion. The results obtained in the present study revealed that there is a gradual decrease of weight loss with increase of ZrO2. This result indicates that the addition of ZrO2 caused the strengthening of glass network by polymerization and hindered the dissolution. After immersion of the glass samples, the pH of the residual SBF solution varies due to the reactions occurring at the solid/liquid interface. The rapid rise in pH during initial immersion period (up to 86 h) indicates the rapid release of cations (Na+, Ca2+) from the glass surface during this time. Thereafter, the rate of increase in the pH value of the solution is comparatively slow and this is attributed to the compensation between the cations and anions in the solution. Further, with the gradual increase of ZrO2 content, there is a possibility for structural transformation of borate glass network. Similar to alkali ion modifiers, zirconium ions may transform BO3 units to BO4 units in the glass network and thereby the degree of polymerization of glass network increases. Such processes reduce the degradation of the glass samples in the SBF solution. The presence of vibrational band in IR spectra of all the postimmersed glasses at about 600 cm−1 identified as being due to ν4vibrations of P\\O bond clearly supports the formation of HA layer over the surface of the glass samples during in vitro study. Further, the observed enhancement in the intensity of vibrational band in the IR spectra of post-immersed samples at about 1020 cm− 1 attributed to PO3− 4 groups also confirms the formation of HA layer [51,52]. Moreover, the appearance of the band at about 1420 cm−1 due to CO2– 3 functional

group indicates the incorporation of carbonate anions from the SBF in the apatite crystal lattice [53–55]. The difference in the variation of the optical band gap of pre- and post-immersed samples is observed to decrease with an increase of ZrO2 (Fig. 5); such tendency indicates the decrease in the magnitude of degradation. Further, the gradual decrease in the intensity of phosphate and carbonate groups observed in the IR spectrum with increase in the ZrO2 content indicates the decreasing degree of bioactivity from the glass Zr1 to Zr5. According to the mechanism of HA formation, Ca ions releasing from glass in to the SBF, formation of Si–OH groups and Zr–OH groups on the glass surface facilitate the apatite formation [56,57]. In this study, Zr ions were replaced by Ca ions in the bioactive glasses, as a result, the concentration of Ca ions released from the glass decreases. Hence, there is a decrease in the HA apatite formation rate on the surface of the Zr incorporated glasses [56]. The formation of HA layer in the studied glasses in aqueous phosphate solution can be understood as follows: initially, B2O3 (in the form of (BO3)3−) and Na2O (in the form of Na+) dissolves into the phosphate solution. Additionally, Si–OH and Zr–OH groups could be formed on the surface of the glass via ion exchange mechanism and due to reaction with water molecules [58–60]. These groups act as apatite nucleation sites and facilitate the formation of HA layer. Once the apatite nuclei are formed, they grow spontaneously by combining with calcium and phosphate ions from the surrounding fluid [61]. The dissolution of Ca2 + ions from the glass might increase the activity of apatite in the surrounding body fluids. After adding ZrO2, the zirconium ions interact with BO4, SiO4 units to form a B/Si/Zr layer. Since, the electronegativity of Si is stronger than Zr, this layer could be in the form of Zr4 +/(SiO4)4 − in order to maintain the charge balance. Further, the Ca2+ ions would diffuse through this intermediate layer to react with (PO4)3 − and precipitate to form HA layer. The incorporation of (CO3)2− ions in the HA layer may occur by ion exchange mechanism, where (CO3)2− ions from the SBF partially replace the (PO4)3 − ions. Such diffusion of Ca2+ ions would be reduced in the case of polymerized glass network containing higher proportions of tetrahedral zirconium ions. In consequence, there is a decrement in the rate of formation of HA layer. The weight loss measurements of the samples and also the variation of pH of the residual solution with the concentration of ZrO2 are consistent with this argument. The increase in the value of optical band gap with ZrO2 content supports the viewpoint that the decreased rate of conversion of the glass surface into the HA layer. 5. Conclusion B2O3–SiO2–P2O5–Na2O–CaO glasses doped with different concentrations of ZrO2 were prepared. The bioactivity of these glasses was studied by immersing the glasses in synthesized SBF solution for prolonged times. FTIR and XRD results show the evidence of crystalline HA layer formation on these glasses and this is further supported by optical absorption data. SEM pictures also supports the formation of HA layer after in vitro study. Solubility of the glasses is quantitatively estimated using weight loss measurements together with the pH values of the

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residual solution. The analysis of these results with the aid of the data on optical absorption spectra, optical band gap and IR spectra has revealed that the glass with low content of ZrO2 is more appropriate for more bioactivity. In other words, these studies have indicated that the addition of small amounts of ZrO2 to the titled glass is essential to control the fast degradation due to the presence of boron. However, a higher concentration of ZrO2 is a hindrance for the bioactivity of the glasses. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

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