Evaluating Potential of Sr-Doped Bioactive Borosilicate Glasses in Improving Bioactivity and Corrosion Resistance of SS-316L Implants

doi:10.21203/rs.3.rs-4840227/v1

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Evaluating Potential of Sr-Doped Bioactive Borosilicate Glasses in Improving Bioactivity and Corrosion Resistance of SS-316L Implants

https://doi.org/10.21203/rs.3.rs-4840227/v1

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The pursuit of improved implant materials for biomedical applications has prompted the development of hybrid inorganic-organic sol-gel methods to coat SS-316L implants. This study focuses on enhancing corrosion resistance and bioactivity through the integration of bioactive particles, particularly investigating the versatility of silicate-based bioactive glasses. By manipulating the chemical composition, especially through the incorporation of strontium (Sr), this research aims to control dissolution rates and bioactivity properties.

A series of bioactive borosilicate glasses with increasing Sr concentrations were synthesized using a quick alkali sol-gel method. These glasses underwent in vitro testing with simulated body fluid (SBF) to evaluate their bioactive behavior concerning escalating Sr content. X-ray diffraction (XRD) analysis revealed the emergence of hydroxyapatite (HA) alongside a presence of calcite phase (CaCO3) after 7 days. Notably, scanning electron microscopy (SEM) showcased the presence of HA and calcite crystals. However, at higher Sr concentrations (Sr = 6, 8 mol %), the formation of calcite was significantly suppressed, potentially due to the larger ionic size of Sr hindering the movement and release of other ions required for calcite formation.

Based on these outcomes, the Sr-6 glass was selected for coating SS-316L substrates using a sol-gel dip coating method, successfully achieving a 1µm thick bioactive layer. Fourier-transform infrared (FTIR) studies confirmed the formation of a bioactive layer on the surface, demonstrating its capability to form HA. Electrochemical tests conducted illustrated the protective behavior of the coating, markedly enhancing the corrosion resistance of SS-316L plates.

This research highlights the potential of Sr-doped bioactive glass coatings to not only improve the corrosion resistance of SS-316L implants but also promote bioactivity, offering a promising avenue for advanced implant materials in biomedical applications. The tailored chemical composition of these glasses holds significant promise in fine-tuning properties critical for implant success, presenting avenues for further exploration and development in the field of biomaterials for medical devices.

Metallic materials, integral to engineering, possess unique properties crucial for medical applications. SS-316L, Co-Cr, and Ti6Al4V are extensively used in orthopaedic and dental implants due to their high mechanical strength, toughness, and resistance to corrosion and wear [1]. Among these, stainless steel stands out for its affordability, ease of fabrication, and widespread availability. However, its inadequate anticorrosive and bioactive properties pose challenges to its biocompatibility and mechanical integrity when exposed to physiological fluids. The resultant release of harmful metal ions can lead to mechanical failures and health hazards [2]–[5]

Addressing these challenges necessitates surface engineering. Bioactive glasses, pioneered by Prof L. L. Hench, have exhibited remarkable potential. They chemically interact with surrounding tissue, fostering a strong bond between bone and implant. Upon exposure to biological environments, these glasses develop a layer of carbonated hydroxyapatite akin to bone composition and structure. Their rapid dissolution and ion release enhance biological responses, fostering osteointegration, angiogenesis, and antibacterial behavior. Despite their clinical use in regenerative dentistry and orthopaedics, the brittleness and susceptibility to crack propagation limit the direct use of bioactive glasses in load-bearing applications. Utilizing them as coatings on implants appears to be the optimal strategy, leveraging their excellent bioactivity alongside the mechanical prowess of metals [6] .

Researchers have increasingly explored bioactive borosilicate glasses as implant coatings. These glasses dissolve faster due to lower chemical durability in water, with released boron participating in metabolic processes crucial for bone growth and maintenance. Their higher thermal expansion coefficient ensures superior adhesion compared to silicate glasses [7]–[9]. The ability to modify glass composition by substituting calcium for other cations allows for tailored characteristics or dissolution mechanism. Notably, strontium inclusion in bioactive glasses has gained significant attention for its dual effects on bone—antiresorptive and anabolic. Its use in osteoporosis treatment underscores its impact on bone health [10]. Recent studies have highlighted its biocompatibility and osteogenic potential, mitigating cytotoxicity caused by rapid glass dissolution [11], [12].

Although there are various methods to apply bioactive glasses as a coating including plasma spray and electrophoretic deposition, the sol gel offers number of advantages such as homogeneity, application of multiple layers, control of morphology and chemistry etc [13]. In light of this inorganic organic sol gel-based coating method has been developed which involves application of multiple layers. In this process the upper layers are incorporated with bioactive particles to provide bioactivity while the lower layers prevent corrosion of underlying metal. As it involves introduction of alkoxysilanes, the coating produced by this method are tougher and more resilient against catastrophic fracture [14].

This work aims to synthesize Sr-doped borosilicate glass and assess the Sr influence on bioactivity. Hybrid double-layer coatings integrating suitable glass samples aim to enhance adhesion and corrosion resistance of SS-316 implants.

TEOS (tetraethyl orthosilicate), MTES (Methyltriethoxysilane), H₃BO₃ (Boric acid), Ca (NO₃)₂. 4H₂O (Calcium nitrate tetrahydrate), TEP (Triethyl phosphate), Sr(NO₃)₂ (Strontium nitrate anhydrous), 2M HNO₃, 1M NH₄OH, SS-316L Plates (1x1 cm), Distilled water.

Glass synthesis

Given composition 40Si0₂-15B₂O₃-40CaO-5P₂O₅ was prepared using quick alkali sol-gel process. To start with, take water in bottle and adjust its pH with the help of 2M HNO₃. Add TEOS to the solution in a 1:4 ratio. Stir the mixture for 1 hour using a magnetic stirrer. After this duration, introduce H₃BO₃ and continue stirring for 30 minutes. Then, add TEP followed by Ca(NO₃)₂.4H₂O at 30-minute intervals while maintaining stirring. After this step move bottle from magnetic stirrer to ultrasonicator and sonicate it for 15 mins. During sonication add 1M NH₄OH drop wise while stirring with glass rod until gel is formed. Once the gel is formed dry the sample at 75°C for 2 days. After two days crush the dried sample using mortar pestle and carry out calcination in muffle furnace at 600°C for 2h with heating rate of 5°C/min.

For Strontium doped bioactive glass, Sr(NO₃)₂ (strontium nitrate anhydrous) was added after dissolution of Ca(NO₃)₂.4H₂O and keep it stirring for another 30 minutes till Sr(NO₃)₂ is completely dissolved. Various compositions of bioactive glass prepared in this study are listed in Table 1.

Table 1

Nominal composition of Sr doped bioactive glasses.
Sample	SiO₂	B₂O₃	CaO	P₂0₅	SrO
40S-B15	40 mol%	15 mol%	40 mol%	5 mol%	0 mol%
40S-Sr2	40 mol%	15 mol%	38 mol%	5 mol%	2 mol%
40S-Sr4	40 mol%	15 mol%	36 mol%	5 mol%	4 mol%
40S-Sr6	40 mol%	15 mol%	34 mol%	5 mol%	6 mol%
40S-Sr8	40 mol%	15 mol%	32 mol%	5 mol%	8 mol%

Table 2

Reagents required for preparation of 50ml SBF.
Chemicals	Quantity
NaCl	0.4017g
NaHCO₃	0.177 g
KCl	0.112 g
K₂HPO₄	0.008 g
MgCl₂. 6H2O	0.0155 g
CaCl₂	0.0146 g
Na₂SO₄	0.0036 g
Tris (without HCl)	0.3059 g
1M HCl	Added drop wise to adjust pH

In-Vitro bioactivity test:

Glass particles were immersed in simulated body fluid (SBF) at 37°C for 7, 14, and 30 days under static conditions. The SBF solution, following Prof. Kokubo's method [15], was prepared in 50 ml batches. Distilled water (30 ml) was added to a plastic container, followed by the sequential addition of compounds listed in table no. 2, adjusting the pH to 7.46 using 1M HCl or Tris as needed.

The concentration of glass particles in SBF was maintained at 1.5g/L. Post-incubation, each sample underwent filtration using Whatman filter paper grade 1. The pH of the filtrate before washing with distilled water was recorded. Samples were then air-dried in a desiccator for 24 hours. Surface changes were investigated using XRD and SEM analysis.

Substrate Preparation and Coating Deposition

a) Metal Substrate Preparation: SS-316 plates (1.5 x 1.5 mm) were acquired from SMT (Surface Modification Technology Pvt. Ltd). These plates underwent initial sandpaper polishing (P1200 grit), followed by ultrasonic washing sequences with ethanol (15 mins), distilled water (15 mins), and acetone (15 mins). They were then rinsed with distilled water and oven-dried.

b) Coating Solution Preparation: A hybrid inorganic-organic sol was formulated using tetraethoxysilane (TEOS, 98% Sigma Aldrich) and methyltriethoxysilane (MTES, 98% Sigma-Aldrich). The silanes were mixed in a constant molar ratio (TEOS/MTES = 40/60) in an acidic medium with nitric acid as a catalyst and absolute ethanol as the solvent. The mixture was stirred for 1 hour to complete hydrolysis .

c) Deposition Process: Once the sol was established, glass powder was introduced. After stirring for 10 minutes to achieve suitable dispersion, dip coating commenced at a withdrawal rate of 1cm/min. Coated samples were dried in a desiccator for 24 hours and subsequently sintered at 450°C for 30 minutes in a muffle furnace.

Corrosion Test:

The corrosion resistance was assessed via potentio-dynamic polarization tests using a DY2300 SERIES POTENTIOSTAT at Surface Modification Technology PVT. LTD. This setup employed a three-electrode configuration: the sample served as the working electrode, a saturated calomel electrode as the reference electrode, and graphite as the counter electrode. The electrolyte solution used was SBF. Tests were conducted pre and post 14-day incubation treatment with SBF. Corrosion polarization curves were generated with a scan rate potential of 5mV/s, covering the cathodic to anodic range. Parameters like corrosion potential, current density, and charge transfer resistance were determined using the Tafel extrapolation method. Additionally, the polarization resistance (Rp) was calculated employing the Stern-Geary equation [16]:

Rp = [(βa.βb)/2.31(βa + βb)]

Here, Rp signifies the polarization resistance, where βa represents the anodic slope and βb represents the cathodic slope.

Bioactive Glass powder

X-ray Diffraction:

The glass’s amorphous nature was determined through XRD analysis of the base glass B-15 and Sr-doped borosilicate glasses. The XRD pattern of the base glass (B-15) showed no presence of crystalline phases, confirming its amorphous structure.

In contrast, the XRD patterns of Sr-doped glasses revealed a distinctive diffused peak around 31.6° with increasing Sr content. This observed broad peak aligns with the (002) plane of the β-Sr₂SiO₄ phase according to JCPDS card no: 038–0271 [11]. The emergence of this peak suggests an alteration in the glass structure due to the larger ionic size of Sr ions compared to Ca [17]. This concise analysis confirmed the amorphous nature of the base glass and highlighted the appearance of the β-Sr₂SiO₄ phase in Sr-doped glasses, crucial findings in understanding structural modifications induced by Sr incorporation.

Invitro bioactivity Test

pH Study:

The pH dynamics during incubation in simulated body fluid (SBF) revealed an intriguing trend, depicting a noteworthy journey mirroring the underlying mechanisms of bioactive glass behaviour. Initially, a rapid surge in pH characterized the early days of incubation, followed by a gradual and steady increase, culminating in a peak value of 8.27 after 14 days. This dynamic shift finds its roots in the intricate interplay between the glass composition and the surrounding physiological environment.

This observed pH trajectory remarkably aligns with the foundational principles of apatite formation, as theorized by Prof. Hench. This mechanism commences with the hydrolysis-driven breakdown of silica and cationic bonds within the glass structure, initiating the formation of a silanol layer. This layer serves as a platform where alkali/alkaline earth cations migrate, orchestrating the development of a thin CaO-P₂O₅ film. It is within this space that interactions with CO₃²⁻ and OH⁻ ions from the surrounding media transpire, leading to the crystallization of amorphous calcium phosphate into hydroxyapatite [18].

This transformative process not only fosters the crystalline transition but also crucially reduces the exposed surface area of the glass, resulting in a subsequent decline in the dissolution rate and eventually achieving a state of saturation.

XRD Study:

In-vitro apatite forming ability of bioactive glasses was assessed using simulated body fluid (SBF). Synthesized glass powders were immersed in SBF for different time intervals (7, 14 and 30 days) and then assessed using XRD to identify formation of HA layer. Generally, bioactive glasses show two diffuse reflections centered at 2theta values of 26° and 32° that correspond to Hydroxyapatite (002) and (211) reflections respectively (standard JCPDS file no. (82-1943)) [19]. After 7 days all the samples showed presence of characteristic HA peak around 25.8° and 31.8° corresponding to (002) and (211) planes respectively (Fig. (a)). Along with HA, calcite peak is also observed around 29.4° corresponding to (104) plane. Similarly, Fig. 17 (b) and (c) shows the XRD pattern of samples after 14 and 30 days respectively. In these graphs the characteristics HA peaks which are seen in Fig. (a) became more intense thus confirming crystallization of HA peaks. After 14 days additional calcite peaks appeared at 43.1° and 48.4° corresponding to (202) and (116) planes respectively. In addition to this, from Fig. (a) and (c) it is seen that with increasing Sr content calcite formation is reduced. This could be due to larger ionic size of Sr which could potentially inhibit the movement and release of other ions [12]. Overall, all the glasses were appreciably bioactive, and presence of Sr did not affect the bioactivity to larger extent which is contradictory to Hesaraki et al who suggested that Sr retards the apatite nucleation rate in sol-gel derived glasses [20].

Coating Characterization

Thickness measurement:

The thickness of a coating is a crucial surface property, serving not only for cell attachment but also providing corrosion protection through adhesion to the substrate. In this section, we elucidate the application of a coating in a 6-layered system. This multilayer approach is chosen to ensure the development of a sufficiently thick film, crucial for effective corrosion resistance. Among all the samples, the Sr-6 glass sample is selected due to its absence of calcite formation during incubation with simulated body fluid (SBF). The thickness of coating is found to be 1.08 µm which is less than the reported values [21], [22]

FTIR:

To analyze the chemical composition and bonding on the substrate, Fourier Transform Infrared Spectroscopy (FTIR) was employed. Fig. (a) and (b) depict the FTIR spectra of borosilicate glass-coated SS-316L substrate before and after exposure to simulated body fluid (SBF).

In the FTIR spectra of the coated sample (Fig. a), characteristic bands confirm the presence of a silicate network, possibly due to SiO₂ as a major component. The primary band at 1025 cm^− 1 corresponds to the asymmetric stretching of Si-O-Si, while the symmetric stretching of Si-O-Si non-bridging oxygen (NBO) atoms between tetrahedra is responsible for the bands at 438 cm^− 1 and 774 cm^− 1. Additionally, two peaks related to C-O and C = O stretching modes are observed at 1416 cm^− 1 and 1575 cm^− 1, respectively [21], [23].

The FTIR spectrum in Fig. (b), obtained after 14 days of immersion in SBF, reveals a new band at 527 cm^− 1, associated with P-O bending (crystalline) vibrations. Moreover, a weak band around 1633 cm^− 1 is attributed to C-O resonance in CO₃^2−, indicating the development of hydroxycarbonated apatite (HCA) [23], [24]. Notably, a Si-O-Si-related band, initially located at 1025 cm^− 1, has shifted to 1044 cm^− 1.

Corrosion Test:

As discussed, bioactive glass coatings have demonstrated to improve corrosion resistance of SS316L implants. To measure corrosion resistance non-destructive test such as potentio-dynamic test was conducted. Tests were done on uncoated sample along with Sr coated sample before and after incubation in SBF for 14 days. Such samples were chosen so that immediate release of alkali and alkali earth ions do not affect the measurement. Fig. shows the potentio-dynamic curves of uncoated, Sr-coated substrate along with Sr-coated substrate with SBF treatment, while the Fig. shows their comparative graph.

From the graphs and Table 8, it is observed that after coating, the corrosion current density decreases (from 6.766 x 10 − 6 Am/Cm2 to 4.540 x 10-6Am/Cm2 ), while corrosion potential increases (-0.415 V to -0.142 V). Furthermore, after 14 days, there is a further decrease in corrosion current density, and the corrosion potential move towards more positive side reaching − 0.117 V. The polarization resistance which is directly related to corrosion rate calculated from Eq. 4 shows that it tends to increase after 14 days. As the polarization resistance increases sample becomes more resistant against oxidation and thus corrosion current density decreases, and corrosion potential becomes more positive. Such behaviour demonstrates the protective nature of coating. This process is associated to the nucleation of HAp on the surface. During the first few days, alkali and alkali earth ions from bioactive glass diffuse more than ion precipitation on the surface. As a result of the increased exposed surface area, pores, and defects in the coating, the corrosion rate increases in the early days. After the HAp layer forms on the surface, the effective area of exposed surface decreases, and pores and defects in the coating may be blocked by particles in the coating [25]. Such blocking may result in a higher resistance to electro-active species diffusion to the metallic substrate, increasing the system's apparent resistance over time and decreasing the rate of dissolution [26], [27].

In this work, series of bioactive borosilicate glasses doped with Sr were prepared and coated on SS-316L plates to study their potential as the bio-implant coating. Borosilicate glasses with different concentrations of Sr were prepared via quick alkali sol-gel process and effect of Sr on its dissolution as well as bioactive behaviour was studied. XRD pattern of base glass confirms its amorphous nature. With the addition of Sr, a broad peak of SrSiO2 was observed due to its larger ionic size increasing the average spacing. In-vitro bioactivity tests conducted after 7, 14 and 30 days showed the appearance of HA peaks which confirmed bioactive nature of prepared samples. Along with HA, calcite peaks are also observed in samples, but with increase in Sr content such calcite phase is suppressed which could be due to larger Sr ions inhibiting the movements of other ions and thus the dissolution. SEM images of samples further confirms the formation of HA layer and calcite crystals after 30 days. Based on results from XRD and SEM, Sr-6 sample was selected for coating. Coating is done through hybrid inorganic-organic sol-gel based method. 6-layer coating was done to achieve thickness of around 1 µm. FTIR spectra confirm the layer of bioactive glass particles formed on surface. In regards to their bioactive behaviour samples should be kept in SBF for much longer period to get more resolved bands. Potentio-dynamic polarization test carried out to measure corrosion resistance showed increase in polarization resistance along with decrease in corrosion current density for coated samples. This confirms that corrosion resistance is increased for coated samples and thus making this coating a very promising option for implant protection as well as bone regeneration.

Author Contribution

We thank the National Center for Nanoscience and Nanotechnology (NCNN), University of Mumbai, forfunding, and the Indian Institute of Technology Bombay , Sophisticated Analytical Instrumentation Facility(IITB SAIF) for FTIR characterization assistance. Special thanks to SMT.PVT.LTD for their support withcorrosion testing.

Acknowledgement

We thank the National Center for Nanoscience and Nanotechnology (NCNN), University of Mumbai, forfunding, and the Indian Institute of Technology Bombay & Sophisticated Analytical Instrumentation Facility(IITB SAIF) for FTIR characterization assistance. Special thanks to SMT.PVT.LTD for their support withcorrosion testing.

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No competing interests reported.

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You are reading this latest preprint version

Evaluating Potential of Sr-Doped Bioactive Borosilicate Glasses in Improving Bioactivity and Corrosion Resistance of SS-316L Implants

Status:

Version 1

Abstract

Figures

Introduction

Methods and Materials

Glass synthesis

In-Vitro bioactivity test:

Corrosion Test:

Result and Discussion

Bioactive Glass powder

X-ray Diffraction:

Invitro bioactivity Test

pH Study:

XRD Study:

Coating Characterization

Thickness measurement:

FTIR:

Corrosion Test:

Conclusion

Declarations

Author Contribution

Acknowledgement

References

Additional Declarations

Status:

Version 1