Hybrid enrichment of Ti13Nb13Zr alloy with zinc ions and silver nanoparticles using a combination of micro-arc oxidation and electrophoretic deposition

Abstract

A double-coated Ti13Nb13Zr alloy was created by arc micro-oxidation (MAO) and subsequent electrophoretic deposition (EPD). The MAO ceramic was enriched with zinc ions, while the EPD-deposited chitosan coating contained silver nanoparticles. Both elements are important for the potential medical applications of the carved base material. To investigate the effects of the modification, morphological imaging by SEM, and spectroscopy analysis like EDS, XPS, and FTIR were performed. In addition, surface roughness, immersion, and wettability tests were carried out. Fabricated samples were also subjected to porosity, microhardness, and corrosion examination. The tests indicated that the MAO modifications carried out yielded a ceramic layer free of defects, while the most porous layer was found to be the one with a processing time of 10 min (MAO10). EPD coatings allowed the MAO pores to be covered, which did not reduce the surface roughness. Chemical bonding and composition studies confirmed the presence of Zn and Ag ions for each of the MAO and MAO_EPD modifications, with the XPS study showing these elements to be in oxide form. Roughness tests showed that the presence of the EPD coating generates increased standard deviations, which may be indicative of an irregular modified area, while a wetting test showed that each modified surface is hydrophilic and the MAO_EPD samples exhibit higher contact angle values than the single MAO modification. Corrosion resistance tests did not reveal a significant improvement compared to the native material; however, the obtained values are satisfactory for medical applications. All E_corr values remained in the nanoampere per square centimeter range—for example, Ref. Ecorr: -0.263 ± 0.01 V; MAO10_EPD Ecorr: -0.012 ± 0.001 V. No substantial differences were observed between the various surface modification groups. The proposed modifications are promising in terms of apatite crystallization capacity. The primary biomedical application is expected to be in orthopedic implants

Introduction

Currently, the spectrum of applications for biomaterials is very wide, pointing to the replacement of bones, hip, knee, and shoulder joints as well as veins or heart valves¹ The most commonly used biomaterials come from the group of metallic materials², ceramics³, and also polymers⁴. Titanium and its alloys are a group of metals of great interest to the medical sector due to their high biocompatibility. Titanium and its alloys do not naturally promote hydroxyapatite formation. Surface modifications are used to create sites for calcium phosphate precipitation⁵, among other things, it is suggested that the surface be enriched with sodium ions and OH groups⁶. Increased bioactivity of apatite formation can promote osteoblast growth⁷. The most commonly used surface modification methods are electrophoretic deposition^8–10, sol–gel^11–13, chemical vapor deposition^12,13, and micro-arc oxidation^14,15 and modifications using a laser beam^16–18. As surface modifications evolve, one method is often not enough. Therefore, hierarchical modifications combine different methods to meet biomaterial requirements^19,20.

As surface modifications evolve, one method is often not enough. Therefore, hierarchical modifications combine different methods to meet biomaterial requirements²¹. This process allows incorporation of ions from the exothermic reaction environment^22,23. The MAO ceramic layer is characterized by significant porosity, and high hardness and, with a well-chosen electrolyte, is biocompatible and bioactive^24–28. Scientific articles point to the use of electrolytes rich in calcium ions^15,29, sodium³⁰ zinc^31,32 and phosphate groups²⁷, hydroxyl groups¹⁵, which are part of the chemical composition of bone. One of the elements studied that affects the osseointegration process is zinc. It is the element that is the sixth most important in the human body³³. This element contributes 30% to bone formation³⁴, in addition, it is involved in the mineralization process and also activates osteoblast proliferation³⁵ and is present during the synthesis of the collagen matrix³⁶. Studies indicate that people with osteoporosis have a lower zinc content in their bones than healthy people, as confirmed by physicochemical tests^35,36

Electrophoretic deposition (EPD) of coatings is a widely developed technique with great potential for producing coatings with different properties and chemical compositions^9,37–40. The advantage of this technique is the ability to control many process parameters (time, voltage, temperature, pH, suspension composition) that can affect the quality of the coatings produced^9,41. EPD allows the manufacture of polymeric coatings containing chitosan (CS), which is used in tissue regeneration, over and above this biopolymer is used in controlled-release systems as a carrier for therapeutic substances^8,42. The element that can be linked to chitosan is silver. Silver nanoparticles (AgNPs) have potential antimicrobial properties, but at the same time, too high a dose can cause a cytotoxic effect, which is why embedding these ions in the polymer coating allows for a controlled, slow release of the silver^43,44.

Both micro-arc oxidation and electrophoretic deposition are methods that allow modification of the material surface of even very complex components^45–47, this property of the techniques determines the potentially possible use in the medical industry, where bone and dental implants take different, personalized forms. The most common combination is made of magnesium alloys^48,49. However, the material currently under extensive discussion is titanium and its alloys. Nie et al.⁵⁰ presented a hybrid modification on titanium alloy Ti6Al4V by producing a ceramic layer with hydroxyapatite-coated with EPD containing titanium oxide, resulting in a corrosion-resistant surface modification and improved mechanical stability concerning the parent material. The improved corrosion resistance of such a combination of modifications was also demonstrated in works^49,50. In addition, Bai et al.^51,52 producing a hydroxyapatite-TiO₂ and HA-NaOH coating by a combination of MAO-EPD, proved that the modification stimulates the formation of more apatite than on the raw titanium material. The work⁵³ confirmed that the formation of an MAO coating before the EPD process supported the formation of a homogeneous, more densely packed magnesium layer. The fabricated coatings by hierarchical MAO-EPD modification were demonstrated in the work⁵⁴ showing amorphous, less rough, and lower wettability properties^19,20.

The aim of the following work was the hierarchical modification of Ti13Nb13Zr alloy by producing an oxide layer using the micro-arc oxidation process and then applying a composite coating (CS/AgNPs) using the electrophoretic deposition technique. Ti13Nb13Zr alloy was used because of its low Young’s modulus close to that of human bone⁵⁵. Improved biocompatibility, and potentially reduced risk of implant rejection⁵⁶ and neurological diseases⁵⁷. Despite these favorable properties, the alloy exhibits limited bioactivity and does not naturally promote apatite formation, which is crucial for implant integration with bone tissue. Additionally, its unmodified surface lacks antimicrobial properties and may be susceptible to localized corrosion in physiological environments.

Therefore, the goal of this study was to develop a novel hierarchical surface modification that combines micro-arc oxidation (MAO)—to produce a porous, hard, bioactive ceramic layer enriched with zinc ions—with electrophoretic deposition (EPD) of a composite coating containing chitosan and silver nanoparticles. This approach is designed to simultaneously enhance bioactivity, improve corrosion resistance, and introduce controlled antimicrobial properties. The modification was subjected to mechanical tests, structural morphology studies, chemical composition, corrosion resistance, and in vitro acellular bioactivity test. The work presents an innovative combination of a ceramic layer with embedded zinc ions coated with a composite CS/AgNPs coating that can be released in a controlled manner. The novelty of this work lies in the unique combination of a zinc-ion enriched ceramic MAO layer with a chitosan/silver nanoparticle composite coating deposited by EPD, which enables a controlled release of antimicrobial agents while maintaining mechanical integrity and bioactivity.The term hierarchical structure refers to the functional combination of two complementary layers with distinct physicochemical and biological properties. Together, they address the key challenges associated with titanium-based implants, offering a multifunctional surface tailored for demanding biomedical applications. The combination of these two techniques offers promising opportunities to increase bioactivity, corrosion resistance, and mechanical properties and to enrich the Ti13Nb13Zr alloy with bioactive elements.

Materials and methodology

Substrate preparation

Ti13Nb13Zr alloy was used in the study; the chemical composition of the alloy is listed in Table 1. The titanium alloy specimens were cut from the rod into discs with a diameter of 14 mm and a height of 4 mm. Each sample was ground with #120/400/800/1200/2000 grade sandpaper. After grinding, each sample was washed in an ultrasonic cleaner for 15 min in isopropanol and then placed in a washer for 15 min in distilled water. After cleaning, the samples were air-dried.

Table 1.

Samples and modifications.

Sample	Material and modification
Reference (Ref.)	Ti13Nb13Zr
	Ti	Nb	Zr	Fe	C	N	H	O
	Rest	13.02	13.22	0.008	0.007	0.021	0.006	0.07
MAO5	Ti13Nb13Zr				MAO, V = 300; t = 5 min, I = 180 mA
MAO7					MAO, V = 300; t = 7 min, I = 180 mA
MAO10					MAO, V = 300; t = 10 min, I = 180 mA
MAO5_EPD					MAO, V = 300; t = 5 min, I = 180 mA EPD, V = 10; t = 1 min
MAO7_EPD					MAO , V = 300; t = 7 min, I = 180 mA EPD, V = 10; t = 1 min
MAO10_EPD					MAO , V = 300; t = 10 min, I = 180 mA EPD, V = 10; t = 1 min

Micro-arc oxidation process

The MAO process was performed under normal conditions in an aqueous electrolyte of 0.17 mol NaOH (WARCHEM, Warsaw, Poland) and 0.016 mol ZnC4H6O4, × 2 hydrate (97%, WARCHEM, Warsaw, Poland), which was stirred for 2 h before starting the MAO process, stirrer speed at 600. The MAO process station consisted of a power supply (DC Programmable DC Power Supply, BK PRECISION, MR 100020, 1000 V/20A 5 kW, Taiwan), a bath of water to keep the process temperature constant, and two electrodes (Fig. 1). The cathode, which was a platinum electrode, and the anode which was a sample of Ti13Nb13Zr alloy. The process was carried out at a constant voltage of 300 V and a current intensity of 180 mA, with time indicated as the variable the temperature during the process was abou 23–25 °C. The process was carried out for 5, 7 or 10 min. The process parameters were chosen based on previous work¹⁴

Fig. 1.

Scheme of hierarchical modification combined with micro-arc oxidation process station and electrophoretic deposition.

Electrophoretic deposition

Suspensions for electrophoretic deposition of composite coatings were prepared by introducing 0.1 g of chitosan (HMW, high purity > 99%, Sigma-Aldrich, St. Louis, MO, USA) into 20 mL of 1% acetic acid (99.9%, Stanlab, Lublin, Poland) and subjected to magnetic stirring (Dragon Lab MS-H-Pro+, Schiltigheim, France) for 24 h. Then, a suspension of 80 mL of ethanol (purity 99.8%, Alchem, Poland) containing 0.005 g of AgNPs (delivered by Hongwu International Group Ltd., Guangzhou, China, mean size 30 nm) was prepared and exposed to ultrasonication for 15 min. Subsequently, the two suspensions were combined by slowly pouring the alcohol with metallic nanoparticles into the chitosan suspension stirred at 800 rpm. After mixing, the system was left on a magnetic stirrer to mix for another 0.5 h.

Electrophoretic deposition of coatings was carried out in a two-electrode system, one electrode being a platinum mesh and the other being a Ti13Nb13Zr alloy sample after appropriate modification by MAO. Both electrodes were connected to a DC power supply (MCP/SPN110-01C, Shanghai MCP Corp., Shanghai, China). The deposition was carried out for 1 min using a voltage of 10 V. The deposition parameters were chosen based on previous work^8,58. After deposition, the samples were left to air dry for 24 h. A summary of the modifications of Ti13Nb13Zr is presented in Table 1.

Testing of the microstructure and morphology of the modified surfaces

A high-resolution scanning electron microscope (SEM, JEOL JSM-7800 F, JEOL Ltd., Tokyo, Japan) was used to study the microstructure using an acceleration voltage of 5 kV and magnifications of 1,000, 10,000 times. For the samples that were coated with the polymer coating, magnetron sputtering (DC table, EM SCD 500, Leica, Wetzlar, Austria) of a 10 nm thick gold coating was performed before testing, the process taking place in an argon atmosphere (Argon, Air Products, Warsaw). Chemical composition studies were performed using energy-dispersive X-ray spectroscopy (EDS, Edax Inc., Pleasanton, CA, USA).

The surface porosity of the formed ceramic layers was also determined using scanning electron microscopy (SEM) images with the ImageJ software.

Taking advantage of Fourier-transform infrared spectroscopy (FTIR) with a spectrophotometer (Perkin Elmer Frontier, Waltham, MA, USA) in the range of 400–4000 cm^-1 and at a 2 cm^-1 resolution, a study of the chemical bonds formed in the deposited layers and coatings on Ti13Nb13Zr alloy was performed for all of the samples once.

Detailed X-ray photoelectron spectroscopy (XPS) analyses were carried out to probe the surface chemistry of the ceramic films and polymer coatings being formed by studying characteristic core levels, such as Na1s, Zn2p, O1s, N1s, Ag3d, C1s, over a wide range of binding energies for once for all of the samples. For this purpose, an advanced Escalab 250Xi from ThermoFisher Scientific was used, using AlKα X-rays with a precise spot diameter of 250 μm and a transition energy of 20 eV. Low-energy electron and Ar⁺ ion bombardment techniques were used to effectively negate potential surface charges. To accurately analyze the data obtained from the measurements, the advanced Avantage v5.9921 software (ThermoFisher Scientific) was used, enabling peak deconvolution and calibration, using the characteristic binding energy for C1s carbon (284.6 eV).

Wettability

The wetting angle was measured using a goniometer together with AttentionOne software. The wetting angle was tested using the falling drop method. The wetting angle was studied using water, for 10 s. Three measurements were taken for each sample.

Roughness

The surface topography of the samples was analyzed using a Sensofar S Neox 3D optical profilometer (Sensofar Metrology, Terrassa, Spain) with the confocal microscope method. Measurements were performed following ISO 25178 and ISO 21920 standards, utilizing a Nikon –EPI 20 × magnification objective and SensoSCAN S neox 7.7 software. The pixel size was set to 0.69 µm. For each sample, three randomly chosen areas of 1.60 × 1.33 mm were observed to calculate average values with standard deviations.

Corrosion study

The corrosion resistance of all modified Ti13Nb13Zr titanium alloy samples was determined using the potentiodynamic polarization method three times for each sample. A three-electrode system was used for the test—a sample, a saturated calomel reference electrode, and a platinum counter-electrode, connected to a potentiostat (Atlas Sollich, Poland). During the first hour of the test, the open circuit potential (OCP) was measured. Once the OCP was determined, a test was performed to measure the change in applied potential from -1 V to 1 V at a scan rate of 1 mV/s. The test was conducted in PBS solution at 37 °C, which corresponded to the physiological conditions of the human body. Using available software, a Tafel extrapolation was carried out and the values of the corrosion parameters—corrosion potential (E_corr), and corrosion current density (j_corr)—were determined.

Microhardness test

The hardness tests were carried out using an FM-800 analog microhardness tester (TSI Systems, Kawasaki, Japan) using the Vickers method, the load used during the test was 1 N, and the duration of the indentation of a diamond quadrilateral pyramid was 10 s. Three measurements were taken on each of the MAO ceramic-coated specimen.

Immersion test

The immersion test was carried out for 14 days in a PBS environment at 37 °C for one samples for each condition. After the immersion test, the samples were observed under a scanning electron microscope (SEM).

Results and discussion

SEM observation and EDS and porosity analysis

Based on the SEM results, images were generated to visualize the porosity of the micro-arc oxidation-modified individual samples. Due to the potential use of this type of modification (MAO) for the manufacture of implants with developed surface morphology, it is indicated that the porosity of the layers is important, as it improves the bonding of the implant to the bone^59,60. The results show that the sample that was modified with MAO10 for the longest time has the highest porosity (Fig. 2B) and the highest number of pores in the surface area range of 0.01–0.1 µm². In the paper⁶¹, it was also observed that increasing the duration of the MAO process on titanium increased the porosity of the ceramic layer and also increased the irregularity. At the same time, the pore sizes obtained are lower than the values reported in the literature^62,63. It is indicated that two types of pores can be observed on the surface of the MAO ceramic layer, through pores and non-through pores⁶⁴. From Figs. 2 and 3, it can be concluded that the pores obtained are through pores.

Fig. 2.

The porosity of the single modified samples by MAO on the left, (A) A histogram of pore sizes of ceramic layer by MAO for single modified samples on Ti13Nn13Zr, (B) Percentage porosity for ceramic layer by MAO for single modified samples on Ti13Nn13Zr.

Fig. 3.

SEM images of the surface topography of single modified Ti13Nb13Zr at different magnifications × 1000 (left), 10,000 (right), and EDS report of these samples.

The SEM images presented (Figs. 3 and 4) allow the topography and morphology features of the modified single (MAO) and double (MAO + EPD) to be determined. The EDS results presented also allowed the chemical composition determination of the fabricated coatings and layers. Figure 3 shows the topography of the microstructure of the MAO5, MAO7, and MAO10 single-modified samples; no cracks, spalls, or other defects were observed in the topography of the resulting ceramics, and it was assumed that the flawless topography was indicative of proper micro-arc oxidation at 5, 7 and 10 min. The formation of pores characteristic of the MAO process is observed for each of the durations, which are caused by the formation of oxygen bubbles during the high-energy electrolytic process. In the MAO process, electric gaps are formed that raise the temperature and produce bubbles of O₂⁶⁵. In addition, for the MAO10 sample, the hierarchical build-up of the ceramic layer comprising a pancake structure is very evident⁶⁶. The discharge channels in the ceramics are complete, and not many elongations are observed disrupting the shape of the craters. It is indicated that the time of the process creation influenced the pore size distribution of the ceramic and the pore size as shown in Fig. 1. The micro-arc oxidation process was carried out in an electrolyte environment consisting of NaOH and ZnC₄H₆O₄, thus the incorporation of Zn ions into the structure of the ceramic layer was crucial for the results. The EDS results indicated that for each of the MAO5, MAO7, and MAO10 samples, Zn²⁺ ions were observed, qualitatively indicating that the greatest number of ions were incorporated into the ceramic layer at 7 min. It is also key that for each of the three samples high maxima are observed for oxygen and titanium forming oxides of this metal characteristic of ceramic formation in the micro-arc oxidation process.

Fig. 4.

SEM images of the surface topography of double modified Ti13Nb13Zr at different magnifications × 1000 (left), 10,000 (right), and EDS report of these samples.

Figure 4 shows the SEM and EDS topography results for Ti13Nb13Zr samples that have been double-coated with a ceramic layer and a polymer coating. No damage to the microstructure of the Ti13Nb13Zr surface is observed on the post-coated MAO5_EPD, MAO7_EPD, and MAO10_EPD samples as in Fig. 3. The applied polymeric coating by electrophoretic deposition resulted in the replenishment of the craters formed on the MAO ceramics, the surface became smoother, however, bulges are observable, which were formed as a result of the coverage of the volcanic MAO structure. In the paper⁶⁷ it has been indicated that the incomplete coverage of craters and the MAO structure may be due to a too-low voltage set in the EPD process. Crucial to the redeposited study is the fact that silver agglomerates were observed, as indicated in Fig. 4 (MAO5_EPD), and confirmed by EDS studies. The gold (Au) present in the EDS images for MAO5_EPD, MAO7_EPD, and MAO10_EPD comes from the necessary sputtering process of the sample before entering the SEM microscope chamber. Significantly, the presence of carbon on the EDS reports confirms the deposition of the polymeric coating made of chitosan. In the results of the polymer coating, no elements from the ceramic layer are observed in the EDS report.

FTIR analysis

Fourier transform infrared spectroscopy FTIR spectra were observed in the range from 400 to 4000 cm^-1 and are shown in Fig. 5. The range of maxima from 400 to 700 cm^-1 is characteristic of oxidized titanium and, more specifically, O–Ti–O bonds^68,69. This is particularly evident for sample MAO10 for which the prolonged duration of the micro-arc oxidation process is responsible. For samples MAO5_EPD-MAO10_EPD, characteristic outcrops are evident for the value of 1001 cm^-1 for bonds –C–O–C⁷⁰, 1152, 1060, and 1024 cm^-1 for bonds –CO^70,71 indicates that the range of elevations from 896 to 1154 cm^-1 reveals the saccharide structure of chitosan⁷². In the range of bands from 450 to 533 cm^-1, a characteristic absorption is observed for zinc oxide Zn–O bonds^73–75, as can be seen in Fig. 5. For samples MAO5, MAO7, MAO10 there are also variations for Zn in the region of 900 cm^-174. Bonds in the range of 1900 to 2350 cm^-1 characteristic are for triple bonds, in the case of the material studied, the presence of carbon–carbon or carbon–nitrogen bonds is indicated. It was possible to indicate small excursions at 4343 cm^-1 and 3437 cm^-1 indicating the presence of Ag⁷⁰ which are bands originating from groups that bond to Ag molecules⁷⁶. The small outcrops are due to the small thickness of the polymeric coating, where silver was the additive.

Fig. 5.

FTIR spectra of all of the modified samples of Ti13Nb13Zr.

XPS analyse

The binding energies for the MAO5_Na (1 s) signal pair 1–1074.5 eV and 2–1072.8 eV are indicated to be assigned for Na⁺ ions⁷⁷ derived from sodium hydroxide present in the electrolyte environment for the micro-arc oxidation process. For MAO5_Zn, 6 maxima were observed for zinc Zn 2p. Peak 4 corresponds to zinc in the second oxidation state, which is present in zinc oxide ZnO. Peak 5 corresponds to Zn2⁺, while peak 6 is derived from the association of zinc with the hydroxyl group OH⁷⁸ (Fig. 6). For masks 1 to 3 assuming energies of ~ 1047 eV, ~ 1045 eV, and ~ 1044 eV respectively, the presence of zinc metal is indicated^79–81. As a result of the MAO process, an oxide layer of ceramic is formed on the surface of the materials, in the case of Ti13Nb13Zr this is titanium, the XPS study shows 5 outcrops in the image for the modification by the MAO technique only (Fig. 6), in Fig. 6 we can observe those maxima from titanium are not detectable for each of the samples, which has undergone the double modification of layering and coating (MAO5_EPD, MAO7_EPD, MAO10_EPD). The XPS image for titanium is in Fig. 6 shows maxima 1, 2, and 5, which take on energy levels of 466 eV, ~ 464 eV, and 458.7 eV, respectively, which corresponds to titanium at oxidation stage IV, where it is in the form of TiO₂^82–84. The binding energy of 462.5 eV corresponds to titanium in the second oxidation state, where it also takes the form of titanium oxide, but in the form of TiO, the maximum for energy and 460.2 eV may also correspond to titanium in oxide, but may also indicate binding to carbon⁸⁵. For each of the modified samples, energy changes in the XPS spectrum for telne O 1 s were recorded. The samples MOA5_EPD, MAO7_EPD, and MAO10_EPD were characterized by the expected higher gradients than the samples after a single modification. For O on the example of the MAO5 sample (Fig. 6), three maxima characterized by the following bond energies were shown: 1–533.5 eV, 2 to 531. 3 eV to ~ 530 eV. Maxima described by 2 and 3 were characterized as oxygen coming from the bond with titanium O–Ti–O^86,87. The peak indicated as No. 1 is characterized by the binding of oxygen to the carbon C=O⁸⁸, as evidenced by the introduction of zinc acetate into the ceramics.

Fig. 6.

(A)Survey XPS scpectra examples for modified Ti13Nb13Zr (B) Example of XPS high-resolution spectra for single modified sample by MAO in 5 min and example for the carbon and silver particles from EPD in MAO5_EPD sample of Ti13Nb13Zr.

For carbon bonds, the highest deflections were recorded for samples where double deposition was performed because the polymer layer consisted of chitosan, where its constituent is, among other things, carbon. For the samples with a single modification, a small amount of carbon bonds in the layer was determined, with zinc bonds being much more prominent (Fig. 6, MAO5/MAO7/MAO10), indicating that this element was better incorporated into the ceramic during the MAO process. The binding energies for C 1 s for the MAO5_EPD sample were for the three maxima, respectively: 288.5 eV, 286.5 eV, and 285 eV, these deflections are seen in Fig. 7 (MAO5_EPD_C) were identified as 1-the double bond of carbon with oxygen C=O, 2-a bond of carbon with hydroxyl group C–OH, 3-a single bond of carbon C–C, and a single bond of carbon and hydrogen C–H⁸⁹. In the XPS study, characteristic excursions were observed for the N 1 s nitrogen bond energies, and analysis of the results allowed the identification of two maxima (Fig. 7, MAO5_EPD_N) MAO5_EPD, MAO7_EPD, MAO10_EPD. Peak 1, with a binding energy of ~ 402 eV, was scraped as a C-NH bond, while peak 2–400 eV corresponds to a C=N double bond, originating from chitosan^89,90. XPS spectra for Ag3d were determined for each of the three samples MAO5_EPD, MAO7_EPD, and MAO10_EPD, two intense peaks were determined at energy levels 373.7 and 367.7 and indicate the dominant presence of silver oxide AgO⁵⁸.

Fig. 7.

Surface topographies with the histogram of peak and valley distributions for the produced coatings.

Roughness analyse

Figure 7 shows the images acquired with the confocal microscopy technique of the obtained coatings. Selected roughness parameters resulting from the observations are summarized in Table 2.

Table 2.

Selected roughness parameters for coatings obtained by confocal microscopy.

Parameter	MAO5	MAO7	MAO10	MAO5_EPD	MAO7_EPD	MAO10_EPD
Sq [µm]	0.53 ± 0.05	0.45 ± 0.09	0.64 ± 0.04	0.72 ± 0.24	0.59 ± 0.14	0.34 ± 0.02
Sp [µm]	14.40 ± 6.35	15.57 ± 1.72	18.28 ± 1.90	25.47 ± 17.28	20.41 ± 3.22	9.59 ± 2.43
Sv[µm]	9.25 ± 1.03	9.24 ± 4.43	8.25 ± 1.07	16.06 ± 8.52	10.90 ± 2.53	9.01 ± 3.16
Sz [µm]	23.65 ± 7.36	24.81 ± 3.93	26.54 ± 0.96	41.52 ± 14.84	31.31 + 4.02	18.61 ± 5.52
Sa [µm]	0.40 ± 0.03	0.28 ± 0.02	0.42 ± 0.2	0.43 ± 0.03	0.36 ± 0.01	0.25 ± 0.02
Rq [µm]	0.46 ± 0.06	0.26 ± 0.01	0.45 ± 0.02	0.40 ± 0.01	0.41 ± 0.06	0.24 ± 0.04
Rz [µm]	4.12 ± 1.17	1.65 ± 0.08	2.70 ± 0.27	3.04 ± 0.47	2.23 ± 0.3	1.60 ± 0.30
Ra [µm]	0.34 ± 0.03	0.20 ± 0.008	0.34 ± 0.02	0.31 ± 0.01	0.33 ± 0.05	0.19 ± 0.03

The roughness parameters presented in Table 2 include both surface parameters determined according to ISO 25,178 and linear parameters based on ISO 21,920. Surface roughness enhances osseointegration by providing a larger surface area for bone cells (osteoblasts) to adhere to. The rough texture facilitates the migration, proliferation, and differentiation of these cells, leading to stronger and more stable implant integration. Sa is one of the most commonly used parameters to describe surface roughness. It provides a general sense of the surface texture and is often correlated with cell attachment and proliferation. The highest value of 0.43 ± 0.03 µm was observed for the MAO5_EPD sample, while electrophoretic deposition of the coating treatment did not significantly change the value of this parameter, with a value of 0.40 ± 0.03 µm for the MAO5 sample. Comparing the Sp parameter analyzing the height of the highest elevations from the average plane, it is noted that as the time of the MAO process increases, the value of the Sp parameter increases, meaning that areas with higher height appear on the profile. Peak-related parameters are critical for providing anchor points for osteoblasts, the cells responsible for bone formation. Also, higher and more numerous peaks can increase the initial mechanical interlocking between the implant and the bone, providing better primary stability. The same observation concerns the Sz and Rz parameters. Sz and Rz parameters are useful for understanding the maximum peak-to-valley height differences, which can be critical for mechanical interlocking and the overall roughness profile of the implant. Based on the obtained results it should be noticed that for the samples subjected only to the arc micro-oxidation process, the highest roughness parameters were obtained for the surface for which the process time equaled 10 min. The deposition of the coating on this sample, in contrast, resulted in a significant reduction in all analyzed parameters. The stability of the process and the control of the EPD parameters, such as voltage, time, and particle concentration, can lead to a more uniform and smoother coating. An inverse relationship can be observed for the sample after the MAO process carried out for 5 min. MAO treatment results in the surface of relatively low roughness parameters. After electrophoretic deposition of the coating, an increase in almost all analyzed roughness parameters was observed. It should be also noticed that compared to the MAO5_EPD sample, the MAO10_EPD sample had lower standard deviations, implying high homogeneity and reproducibility of the obtained structure. However, electrophoretic deposition of the coating on the MAO substrate increases the standard deviations, which means the surfaces have lower repeatability and homogeneity.

The influence of surface roughness on corrosion behavior should also be considered. As shown in Table 2, samples subjected to longer MAO treatment (e.g., MAO10) exhibited increased roughness parameters such as Sa, Sz, and Rz, which may contribute to a more porous and reactive surface, thereby intensifying corrosion processes. In contrast, the EPD coating applied after MAO led to a notable decrease in these parameters, suggesting a smoothing effect that could limit the penetration of corrosive agents. These findings support the hypothesis that higher surface roughness correlates with increased corrosion activity, particularly in coatings with higher porosity or surface development.

Wettability analyse

Wettability is defined as a property of materials to determine the phase tension relationship between a liquid and a solid⁹¹. When titanium materials are used in medical applications, wettability analysis is necessary, as it affects protein binding, adhesion, and cell proliferation⁹².

Figure 8 shows the wetting angle results for each of the modifications on the Ti13Nb13Zr titanium alloy. Each modified surface shows a hydrophilic character (CA < 90°⁹²) and the contact angle values for each sample are in the range of 35 to 80°, which is indicated as optimal for tissue regeneration^93,94.

Fig. 8.

Wettability results for single (MAO) and double coated (MAO_EPD) Ti13Nb13Zr samples.

The collected wetting angle results allow the conclusion that a modification time of 7 min during the MAO process on the Ti13Nb13Zr surface resulted in a structure with the highest contact angle. At the same time, the shortest time for the formation of the ceramic layer characterizes the surface as the most hydrophilic for both the single modification (MAO5) and the double modification with a polymer coating (MAO5_EPD). The application of the double modification with MAO_EPD resulted in an increase in the contact angle for these samples, the highest difference being observed for MAO5 and MAO5_EPD. The paper⁹⁴, indicated that the optimum surface contact angle is 55° for the results presented, the closest contact angle was obtained for the MAO10 single-modified sample and the MAO5_EPD double-modified sample. The increased hydrophilicity observed in these coatings is primarily related to the chemical composition and porosity of the surface layers, which promote spreading of water. Although no direct linear correlation between roughness and wettability was confirmed, the results suggest that greater surface heterogeneity (e.g., for MAO5_EPD) may contribute to more complex wetting behavior due to capillary effects within pores and microstructural valleys.

The study does not show a direct relationship between the contact angle to the duration of the micro-arc oxidation process on Ti13Nb13Zr alloy, nor does it show a dependency between the contact angle to the surface roughness, which may be due to the high variability of the surface morphology and the different porosity values of these surfaces, by which water is distributed differently on the surface of the material during the exponentiation. Based on the article⁹⁵ concludes that it is not possible to identify a single correct and optimal correlation of results.

Microhardness analyse

The results of microhardness tests (Fig. 9) carried out on the original material and on samples modified using only the micro-arc oxidation technique indicate that the formation of a ceramic layer increases microhardness. The conclusions of the study are in line with the following publications^15,24,96, where the microhardness of titanium and its alloys is reported to increase significantly after the application of ceramic layers. In addition, a change in the micro-arc oxidation process time parameter influences the hardness of the material. The longer the micro-arc oxidation process time specified in this study, the greater the microhardness of the coating. Analogous conclusions were drawn in the following studies^97,98, where the microhardness of the coating was investigated after different process times on pure titanium. The increase in the material’s hardness after the formation of the MAO layer is due to the thermal transformation processes of the anatase phase into the rutile phase¹⁴, with longer process times, the proportion of this phase in the material to be modified increases⁹⁸.

Fig. 9.

Microhardness of single modified samples by micro-arc oxidation on Ti13Nb13Zr.

Corrosion results analysis

The results of the performed corrosion tests are presented in Fig. 10 and Table 3. The obtained courses of open circuit potential change over time showed stabilization of all tested systems after one hour. All modified samples demonstrated a higher OCP value after this time compared to the substrate after grinding. The OCP values of the modified samples were directed more toward positive values. Considering the corrosion curves and the E_corr, j_corr and efficiency values determined by Tafel extrapolation, the corrosion potentials were found to shift towards positive values. Based on efficiency noted that the most protective modification is hierarchical modification of MAO5_EPD, the weakest modification is MAO5. The lowest j_corr value was recorded for the unmodified substrate after grinding, but all measured values were of the order of nA/cm², hence it can be concluded that all tested samples show satisfactory corrosion resistance. Concerning the samples after the MAO process, the high development of these surfaces and porosity may contribute to the intensification of corrosion processes, hence the recorded higher j_corr values. As for the samples with CS/AgNPs coatings, here partial swelling of the coating under the influence of the SBF solution used for the test may have occurred, which also translated into higher j_corr values. In addition to chitosan swelling, the presence of AgNPs may induce local galvanic interactions due to the electrochemical potential difference between silver and the titanium substrate. This effect, combined with the porosity of the MAO layer, may partly explain the increased j_corr values observed for MAO5_EPD. No clear trend was noted to determine the effect of the used MAO process parameters on the corrosion resistance of the tested samples. Also, the deposition of the composite coating clearly did not contribute to the corrosion resistance of these systems. However, it can be concluded that all tested systems meet the relatively high corrosion resistance requirement. A similar effect of surface modification of titanium and titanium alloy was noted in the literature. The use of various surface modification methods (such as MAO, EPD, laser treatment, hydrothermal oxidation) of metallic biomaterials for the implant aimed at improving osteointegration properties or providing antimicrobial activity negatively affects corrosion resistance. It should also be mentioned that the obtained results of corrosion tests depend on the parameters of the test, and the other hand on the thickness and chemical composition of the modified layers and applied coatings, but also on the uniformity, strength, appearance of cracks or their absence^99,100.

Fig. 10.

(A) Open circuit potential for single (MAO) and double (MAO + EPD) modified samples of Ti13Nb13Zr, (B) Potentiodynamic polarization curves of single (MAO) and double (MAO + EPD) modified samples of Ti13Nb13Zr.

Table 3.

Determined values of corrosion potentials, corrosion density and slopes of the cathodic and anodic curves and efficiency for all tested samples.

Sample	Corrosion results
	Ecorr (V)	jcorr (nA/cm2)	βa	βc	Efficiency
Ref	-0.263 ± 0.001	125 ± 6	5.25 × 101 ± 0.001	1.49 × 10–1 ± 0.001	-
MAO5	-0.208 ± 0.001	126 ± 11	1.03 × 101 ± 0.001	8.65 × 10–2 ± 0.001	0.79
MAO7	-0.216 ± 0.001	451 ± 47	2.27 × 10–1 ± 0.001	4.99 × 10–2 ± 0.001	72.28%
MAO10	-0.117 ± 0.001	241 ± 38	7.30 × 10–1 ± 0.001	6.78 × 10–2 ± 0.001	48.13%
MAO5_EPD	-0.135 ± 0.001	594 ± 73	4.88 × 10–1 ± 0.001	3.18 × 10–1 ± 0.001	78.96%
MAO7_EPD	-0.168 ± 0.001	147 ± 9	4.29 × 10–1 ± 0.001	1.43 × 10–1 ± 0.001	14.97%
MAO10_EPD	-0.012 ± 0.001	204 ± 29	2.62 × 10–1 ± 0.001	2.94 × 10–1 ± 0.001	38.73%

Based on the analysis of corrosion potential (E_corr) and corrosion current density (j_corr), it can be observed that the samples with MAO layers formed for longer durations (e.g., MAO10) show higher E_corr and lower j_corr values compared to the reference sample, indicating improved corrosion resistance. At the same time, the increased porosity and the developed ceramic structure observed in SEM images (e.g., in MAO10 – pancake-like structure and through-pores) may promote electrolyte retention, which can locally reduce corrosion resistance. Therefore, higher porosity enhances biointegration but may have an ambivalent effect on corrosion resistance, as confirmed by the higher j_corr values for MAO10 and MAO10_EPD compared to MAO7_EPD.

Immersion test analysis

After a 14-day immersion test, SEM and EDS images were taken of the single (MAO) and double (MAO_EPD) modified samples. For each of the samples, the presence of salt crystals was observed at magnifications of 1 000 and 10 000 (Figs. 11 and 12), which was confirmed by the chemical composition test. The presence of calcium and sodium salts was demonstrated. After the test of immersion in PBS solution, the increase of oxygen on the surface was determined by EDS analysis, a result that is valid given previous work by other researchers, where the same effect was demonstrated¹⁰¹.

Fig. 11.

SEM images (two pictures on the left) of topography and EDX result (picture on the right) after immersion test for single modified samples by micro-arc oxidation on Ti13Nb13Zr.

Fig. 12.

SEM images (two pictures on the left) of topography and EDX result (picture on the right) after immersion test for double modified samples by micro-arc oxidation and electrophoretic deposition on Ti13Nb13Zr.

It can be seen that the salts have penetrated the pores of the MAO5, MAO7, and MAO10 ceramics. In Fig. 12, where the results for the post-dually modified samples are shown, there is a higher number of salt crystals formed in comparison to the samples with a single modification (MAO), moreover, the distribution of the role is more uniform and large spherical NaCl crystals are observed (Fig. 12, MAO7_EPD EDS spectrum). Considering the chemical composition of the SBF solution used, the expected effect was obtained. It is noteworthy that the modified surfaces promote the crystallization of inorganic salts, so when using solutions rich in calcium and phosphorus compounds, high bioactivity of these surfaces and crystallization of calcium phosphates should be expected.

Conclusion

The presented study allowed for a comprehensive analysis of the hierarchical modification of Ti13Nb13Zr alloy using the processes of micro-arc oxidation and electrophoretic deposition (EPD). The dual modification led to significant improvement of the alloy towards meeting the requirements for biomaterials.

The micro-arc oxidation process was carried out for three different periods: 5 min (MAO5), 7 min (MAO7), and 10 min (MAO10). Subsequently, electrophoretic deposition was applied to these samples, resulting in the modifications MAO5_EPD, MAO7_EPD, and MAO10_EPD.

The SEM analysis revealed that the MAO10 samples exhibit the highest surface porosity, with the largest pores among all modifications, indicating a well-developed structure morphology conducive to improved implant binding. Chemical composition analyses using EDS, XPS, and chemical bonds FTIR demonstrated the presence of zinc ions for each modification, as well as the presence of silver for the MAO_EPD modification, with both elements present in the coatings in the form of oxides. The presence of these compounds has the potential to enhance antibacterial properties. Wetting tests showed hydrophilic properties for each conducted modification on the Ti13Nb13Zr alloy, with each value falling within the literature’s range of optimal contact angle values, indicating surface bioactivity. The roughness tests did not clearly show the changes in roughness after the hierarchical modification. However, analysis of the SEM images revealed a smoothing of the surface for both MAO10 and MAO10_EPD. The Sa parameter decreased from 0.41 µm to 0.25 µm. The immersion test demonstrated bioactivity for each individually and hierarchically modified surface, as salt crystals were observed through SEM and EDS studies. The most homogeneous salt distribution was seen on the MAO10_EPD sample. Corrosion tests did not show significant differences and improvements in corrosion resistance for samples subjected to hierarchical modification with Ti13Nb13Zr. Corrosion current densities were at the nA/cm² level, and the lowest j_corr value for the hierarchically modified samples was recorded for the MAO7_EPD sample. The microhardness test indicated that the application of the MAO ceramic layer increased the hardness of the parent material, with the hardness of the layer increasing with a longer process duration. However, it was not possible to carry out this test for ceramic/polymer combinations.

The hierarchical modification mentioned above holds significant promise for biomedical applications, owing to its diverse capabilities for introducing supplementary elements into base materials. The combination of chitosan polymer with silver, coupled with a porous MAO layer containing zinc ions, affords antibacterial properties alongside enhanced adhesion of bone cells to the surface. These modifications enable a surface with meticulously controlled morphological and chemical characteristics, which is highly pertinent given the demands for surface biocompatibility of the implant with bone tissue.

Author contributions

J.S.; Ł.P.;—conceptualization; J.S.; Ł.P; A.M.—wrote the main manuscript file; J.S.; Ł.P.; A.M.—examination of the samples; J.S.; Ł.P.; A.M.; prepared figures; M.S.—supervising.

Data availability

The datasets used and/or analysed during the current availiable from the corresponding author on resonable request.

Declarations

Competing interests

The authors declare no competing interests.