Loading Gentamicin and Zn2+ on TiO2 Nanotubes to Improve Anticoagulation, Endothelial Cell Growth, and Antibacterial Activities

Abstract

Titanium and its alloys are widely used in blood-contacting implantable and interventional medical devices; however, their biocompatibility is still facing great challenges. In the present study, in order to improve the biocompatibility and antibacterial activities of titanium, TiO₂ nanotubes were firstly in situ prepared on the titanium surface by anodization, followed by the introduction of polyacrylic acid (PAA) and gentamicin (GS) on the nanotube surface by layer-by-layer assembly, and finally, zinc ions were loaded on the surface to further improve the bioactivities. The nanotubes displayed excellent hydrophilicity and special nanotube-like structure, which can selectively promote the albumin adsorption, enhance the blood compatibility, and promote the growth of endothelial cells to some degree. After the introduction of PAA and GS, although the superhydrophilicity cannot be achieved, the results of platelet adhesion, cyclic guanosine monophosphate (cGMP) activity, hemolysis rate, and activated partial thromboplastin time (APTT) showed that the blood compatibility was improved, and the blood compatibility was further enhanced after zinc ion loading. On the other hand, the modified surface showed good cytocompatibility to endothelial cells. The introduction of PAA and zinc ions not only promoted the adhesion and proliferation of endothelial cells but also upregulated expression of vascular endothelial growth factor (VEGF) and nitric oxide (NO). The slow and continuous release of GS and Zn²⁺ over 14 days can significantly improve the antibacterial properties. Therefore, the present study provides an effective method for the surface modification of titanium-based blood-contacting materials to simultaneously endow with good blood compatibility, endothelial growth behaviors, and antibacterial properties.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Blood-contacting implantable and interventional medical devices, such as artificial heart valves and vascular stents, have saved thousands of lives [1, 2]. Titanium and its alloys have been widely used in blood-contacting medical devices, but they still face great challenges in clinical applications, such as inflammation, thrombosis, and infection. Generally speaking, ideal implants should have the abilities to integrate and communicate with surrounding tissues or cells, trigger specific cell responses and maintain the function of tissues and organs, and prevent infections caused by microorganisms after the implantation [3, 4]. In this regard, surface functionalization represents one of the straightforward and effective methods to endow biomaterials with excellent properties and functions [5–7]. According to the mechanism of the interactions between the implant and the surrounding physiological microenvironment, the introduction of bioactive substances on the surface by physical or chemical conjugation can endow the inert biomaterial with good biological activities, so as to regulate the cell-material interaction behaviors, induce specific cell responses, and prevent the infection caused by implantation [8–11].

Although great progress has been made in the surface functionalization of titanium-based biomaterials, there are still many issues to be solved, including the delayed surface endothelialization, thrombosis, and infection after implantation [12, 13]. Studies have shown that the surface properties of the implant are related not only to the surface bioactivities but also to the surface topographies. In recent years, the nanomaterials with special tubular structure have attracted great attention in the blood-contacting biomaterials [14]. Anodization is a surface modification technology that can in situ prepare nanotubes on the titanium surface [15, 16]. The as-prepared nanotubes not only do not change the mechanical properties of bulk materials but also provide an excellent platform for loading bioactive molecules to enhance the surface bioactivities [17–19]. Our previous results showed that the anodized TiO₂ nanotubes with the different dimensions have different effects on hemocompatibility and endothelial cell behaviors, demonstrating that the interfacial biological behaviors between the implanted materials and tissues can be regulated by the surface morphologies [20]. Therefore, loading the bioactive factors into the nanotubes can further enhance the biocompatibility from two aspects of surface bioactivities and surface morphologies.

Zinc is the second most abundant trace element in the human body, which participates in a large number of physiological reactions and is an important substance involved in cell growth behavior and cell function expression [21]. Zinc ions also play an important role in the cardiovascular system, and it can prevent local vascular ischemia and vascular infarction [22]. Zinc deficiency is closely related to atherosclerosis, and zinc can protect the integrity of the vascular endothelium by preventing nuclear factor apoptosis and inflammation-related genes [23]. Moreover, zinc ions can induce bacterial apoptosis by changing the charge balance of bacteria [24]. Therefore, the loading of zinc ions into the anodized TiO₂ nanotubes can not only improve the anticoagulant and antibacterial activities but also promote the growth of endothelial cells.

In addition, intravascular devices should have good antibacterial properties because the implant-centered infection is often one of the important reasons of the implantation failure [25]. Loading or immobilization of antibacterial substances on the surface is an important approach to endow devices with antibacterial properties [26]. Polyacrylic acid (PAA) is a cheap and environmentally friendly water-soluble organic polymer. The carboxylic acid groups of PAA can absorb a large number of metal ions, drugs, and other positively charged substances, so it is widely used in the field of biomaterials [27]. Gentamicin (GS), a widely used antibacterial substance in the clinic, is a kind of aminoglycoside, and it has a broad spectrum of antimicrobial activity and especially has excellent antibacterial activity for Gram-negative bacteria [28]. Therefore, in the present study, we first prepared TiO₂ nanotubes on the titanium surface by anodization, and then, PAA was further introduced on the surface followed by loading GS through layer-by-layer (LBL) and zinc ions with the help of carboxylic acid groups of PAA. The results indicated that PAA and the continuous released zinc ions can significantly improve the anticoagulant and endothelial cell growth, and the excellent and long-lasting antibacterial properties can be achieved through the release of GS and zinc ions.

2. Materials and Methods

2.1. Preparation of TiO₂ Nanotubes on Titanium Surface

Titanium plates (TA2) with a diameter of 15 mm and a thickness of 2 mm were successively polished with sandpapers of 400#, 800#, 1200#, 1500#, and 2000# and then polished to the mirror with a polishing machine. After being ultrasonically cleaned for 10 min with acetone, ethanol, and deionized water, the titanium plates were immersed in 50 mL electrolyte (ethylene glycol solution containing 0.25%wt NH₄F and 6 mL deionized water) to anodize 3 h at 30 V using the plate as the anode. After anodization, the plates were cleaned ultrasonically for 30 min in ethylene glycol solution and for 5 min in ethanol. In order to change the synthesized TiO₂ nanotubes into an anatase-type structure, the sample was dried and heat-treated at 500°C in air for 3 h and named TNT.

2.2. Loading GS and Zn²⁺ on TiO₂ Nanotubes

The TNT samples were firstly immersed in 2 mg/mL dopamine solution (pH 8.0) for 12 h and then washed with the deionized water. The process was repeated three times, and the as-prepared samples were named as TNT-Dopa. The TNT-Dopa samples were immersed in a polyethyleneimine (PEI) solution (5 mg/mL, pH 10.0) for 30 min. After cleaning, the sample was immersed in 1 mg/mL PAA solution (pH 7.4) for 10 min, and the obtained sample was labelled as TNT-PAA. For loading gentamicin (GS), the TNT-PAA sample was alternately immersed in GS (1 mg/mL) and PAA solutions (1 mg/mL), and the process was repeated 10 times; the obtained sample was named as TNT-PAA/GS, and the outermost layer was PAA. Finally, the sample was immersed in 1 M ZnSO₄ for 2 h to load Zn²⁺, and the final sample was labelled as TNT-PAA/GS-Zn.

2.3. Sample Characterization

The surface morphologies of the samples were observed by scanning electron microscopy (SEM, FEI Quanta 250). The changes of chemical groups on the surface were examined by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, TENSOR 27, Bruker of Germany); the measurements were carried out at room temperature, and the scanning range was from 650 cm^-1 to 4000 cm^-1. The surface atomic concentrations of the different samples were measured by X-ray photoelectron spectroscopy (XPS, VG Science, East Grinstead, UK). Water contact angle measurement (DSA25, Krüss GmbH, Germany) was used to characterize the surface hydrophilicity, and five parallel samples were measured and averaged.

2.4. Protein Adsorption

The adsorption behaviors of fibrinogen (FIB) and bovine serum albumin (BSA) were measured by the BCA method. The samples were firstly immersed in ethanol for 30 min and then in phosphate buffer (PBS) for 10 h. After that, the samples were placed into 1 mg/mL BSA solution and 1 mg/mL FIB solution for 2 h at 37°C, respectively. After washing twice by PBS, the sample was put into 2 mL SDS (sodium dodecyl sulfate, 1%wt) solution for ultrasonic desorbing for 30 min. We take 150 μL eluent and 150 μL BCA working solution ($\text{reagent}\text{A}:\text{reagent}\text{B}:\text{reagent}\text{C}=25:24:1$) to react 1 h at 60°C, and then 200 μL mixing solution was transferred into a 96-well plate to measure the absorbance at 562 nm by the microplate reader (BioTek, Eons), and the adsorption amount of protein was calculated according to the standard curve.

2.5. Release Profiles of GS and Zn²⁺

The TNT-PAA/GS-Zn sample was immersed in 5 mL 37°C PBS solution for 1 h, 3 h, 5 h, 7 h, 1 d, 3 d, 7 d, and 14 d, and then, 200 μL solution was transferred into the 96-well plate. The absorbance at 562 nm was measured by a microplate reader (BioTek, Eons), and the release amount of GS was calculated according to the standard curve. The concentration of zinc ions was measured by an inductively coupled plasma emission spectrometer (Optima 7000 DV), and the release concentration was calculated according to the standard curve. Three parallel samples were measured and averaged, and the release profiles were further plotted.

2.6. Blood Compatibility

2.6.1. Hemolysis Assay

The hemolysis rate was measured according to the ISO10993-4 standard. Fresh human blood from a healthy volunteer was centrifuged at 1500 rpm for 10 min to obtain the red blood cells. The red blood cells were prepared into 2% suspension with physiological saline. The samples were incubated 1 h in the suspension solution at 37°C. The solution was then centrifuged 5 min at 3000 rpm. We take 100 μL supernatant into a 96-well plate, and the absorbance ($A$) was measured at 450 nm by the microplate reader. Under the same conditions, the absorbance value ($B$) of the mixed solution of 2% red blood cells and 98% normal saline was measured as the negative control, and the absorbance value ($C$) of the solution of 2% red blood cells and 98% deionized water was recorded as the positive controls. Three parallel samples were measured, and the values were averaged. The hemolysis rate was calculated according to the following formula. $$\begin{array}{}\text{(1)}& \text{Hemolysis}\text{rate}\%=\frac{A-B}{C-B}\times 100\%.\end{array}$$

2.6.2. Adhesion and Activation

Fresh healthy human whole blood was centrifuged at 1500 rpm for 15 min to obtain platelet-rich plasma (PRP). 200 μL PRP was fully covered on each sample surface to incubate 2 h at 37°C, and then, the samples were rinsed thrice with PBS. The adherent platelets were fixed with 2.5% glutaraldehyde (in PBS buffer) for 24 h and then rinsed with PBS. The samples were successively dehydrated with 30%, 50%, 75%, 90%, and 100% ethanol solutions for 10 min each, and the samples were dried in the air. After spraying gold on the surface, the morphologies of the platelets were observed by SEM (FEI Quanta 250). Five SEM images with small magnification (×1000) were randomly selected for calculating the number of platelets, and the values were averaged and expressed as platelets per mm².

For the platelet activation assay, an enzyme-linked immunosorbent assay (ELISA kit, Beyotime Biotechnology, Shanghai, China) was used to measure the activity of cGMP (cyclic guanosine monophosphate) secreted by platelets. In brief, 200 μL PRP was dropped on each sample surface to cover the whole surface. After incubating at 37°C for 2 h, the plasma on the surface was diluted 5 times; subsequently, the plasma was transferred to the enzyme plate for culturing 30 min at 37°C. 50 μL of enzyme-labeled reagent was added to each well, cultured at 37°C for 30 min. Then, 50 μL chromogenic agent A and 50 μL chromogenic agent B were added to each well and kept away from light for 10 min at 37°C. Finally, the terminating solution was added to stop the reaction. The absorbance at 450 nm was measured, and the concentration of cGMP was calculated according to the standard curve.

2.6.3. APTT

Fresh human anticoagulant whole blood was centrifuged 15 min at 3000 rpm to obtain platelet-poor plasma (PPP). 100 μL PPP was covered onto the sample surface and cultured 15 min at 37°C. Subsequently, 50 μL PPP and 50 μL APTT reagent (Sysmex, Japan) were added into the test tube and cultured at 37°C for 3 min. 50 μL 0.025 M CaCl₂ solution was finally added. The clotting time was measured by an automatic coagulation meter (CA-1500, Sysmex, Japan), and the average values of three parallel samples were measured.

2.7. Endothelial Cell Growth Behaviors

2.7.1. Cell Adhesion and Proliferation

The samples were firstly placed in a 24-well culture plate and sterilized overnight with ultraviolet light on the superclean table, and then, 0.5 mL endothelial cell suspension ($5\times {10}^4$ cells/mL) and 1.5 mL cell culture medium (DMEM/F-12 supplemented with 10% fetal bovine serum and 1% penicillin-streptomycin, HyClone) were added to each sample surface. After incubating at 37°C and 5% CO₂ for 1 and 3 days, respectively, the samples were washed with PBS for 3 times. Each sample surface was stained with 200 μL rhodamine (in PBS, 1 : 400) for 20 min and then washed with PBS for 3 times. Finally, 200 μL of DAPI (PBS, 1 : 400) was added to the surface for 3 min. After being washed 3 times with PBS, the cells were observed by fluorescence microscopy (Zeiss, inverted A2).

For cell proliferation, endothelial cells were cultured in the same way as mentioned above. After 1 and 3 days, the samples were washed with PBS for 3 times. 0.5 mL CCK-8 (Sigma-Aldrich, Shanghai, China) solution (10% in cell culture medium) was added and incubated in a 37°C incubator for 3.5 h. After that, 200 μL medium was transferred into a 96-well plate, and the absorbance at 450 nm was measured by a microplate reader (BioTek, Eons) to determine the proliferation activity of endothelial cells.

2.7.2. NO and VEGF Expression

The NO release from endothelial cells was measured by the Griess method. The endothelial cells were cultured on the sample surface for 1 day and 3 days, the supernatant was added to the 96-well plate, and then, Griess Reagents I and II (Beyotime Biotechnology, Shanghai, China) were added successively. The absorbance at 540 nm was determined by a microplate reader, and the NO concentration was calculated according to the standard curve.

For the VEGF assay, according to the instructions of the enzyme-linked immunosorbent assay kit (Jiangsu Enzymatic Immunity Industry Co., Ltd.), endothelial cells were cultured on the sample surface for 1 day and 3 days, and then, the supernatant was absorbed to dilute 5 times and finally added to the enzyme plate. After being incubated at 37°C for 30 min, 50 μL of the enzyme labeled reagent was added and incubated at 37°C for another 30 min, and then, 50 μL chromogenic agent A and 50 μL chromogenic agent B were added to each well to react 10 min at 37°C in the dark. Finally, the terminating solution was added to stop the reaction, and the OD value at 450 nm was determined by a microplate reader. Three parallel samples were measured and averaged. The VEGF concentration was determined according to the standard curve.

2.8. Antibacterial Activities

2.8.1. Bacterial Adhesion

Escherichia coli (ATCC 25922; HuanKai Microbial, Guangzhou, China) was cultured in the mixture of 0.2% liquid medium (5 g/L yeast extract and 10 g/L tryptone) and 99.8% 10 g/L NaCl solution for 20 h; 10 mL solution was centrifuged at 2000 rpm for 3 min and then dispersed evenly. The bacterial solution was diluted 10 times, and 50 μL bacterial suspension was dropped on the sample surface to culture 2 h. The sample was then washed 3 times with PBS and fixed 2.5 h with 2.5% glutaraldehyde solution, followed by washing the sample with PBS, and finally dehydrated with 50%, 70%, 90%, and 100% ethanol solutions for 10 min each time. After spraying gold on the sample surface, the adhered bacteria were observed by SEM (FEI Quanta 250).

2.8.2. Antibacterial Activities

Escherichia coli (ATCC 25922) was cultured overnight as mentioned above; 10 mL bacteria solution was taken and centrifuged at 1300 rpm for 5 min to determine the survival and number of bacteria. The sample was placed into a 12-well plate, and then, 500 μL bacterial solution was added. After being cultured at 37°C for 30 min, 1500 mL sterilized deionized water was added and continued to culture at 37°C for 24 h. 50 μL of bacterial liquid was evenly covered on the surface of the solid medium. After being cultured at 37°C overnight, the bacteria was observed by taking pictures using a Huawei Mobile (Nova 6).

2.9. Statistical Analysis

Statistical analysis was performed using the SPSS software. All data were expressed as $\text{mean}\pm \text{standard}\text{derivation}$ (SD) and statistically analyzed using one-way analysis of variance (ANOVA). $p<0.05$ is considered to be statistically significant. All of the tests were conducted with no less than three parallel samples.

3. Results and Discussion

3.1. Surface Characterization

Figure 1 shows the representative SEM images of the titanium dioxide nanotubes modified by the different bioactive factors. It is obvious that after different surface modification processes, the surface nanotube structure remains intact. Compared with TNT-Dopa, the surface immobilization of PAA and the loading of GS and Zn²⁺ gradually reduced the diameter of the nanotube and increased the thickness of the tube wall. Furthermore, the chemical group changes on the surface were examined by ATR-FTIR. It can be seen from Figure 2(a) that there was almost no infrared absorption on the unmodified titanium surface. Our previous work showed that the anodized TiO₂ nanotubes have a small amount of hydroxyl on the surface [20], but it cannot be directly used to immobilize the bioactive molecules. Dopamine can self-polymerize on the surfaces of almost all solid materials to form polydopamine, which is widely used in the surface modification of biomaterials. The polydopamine coating can easily react with amine or thiol through Michael addition reaction and Schiff base reaction to introduce bioactive substances on the surface. PEI contains rich amine groups and has a large number of positive charges, so it can easily react with polydopamine to form a positively charged surface. PAA is a kind of polymer with rich negative charges and good biocompatibility, which can combine with GS or zinc ions. Therefore, GS and zinc ions can be loaded on the surface by the electrostatic interaction between PEI and PAA. The results of Figure 2(a) show that the stretching vibration and in-plane bending vibration of the -NH bond and -OH bond appeared on the TNT-Dopa surface at 1590 cm^-1 and 3300 cm^-1, respectively, and the stretching vibration of the -OH bond occurred at around 3700 cm^-1, indicating that the polydopamine coating had been successfully prepared on the surface. After grafting of PEI, the in-plane bending vibration and stretching vibration of the -CH₂ bond can be observed at 1462 cm^-1 and 2832 cm^-1, and the bending vibration of the -NH bond of the primary amine and secondary amine and the stretching vibration of the C-N bond can be detected at 1200 cm^-1 and 1656 cm^-1, indicating that PEI had successfully covalently linked with the polydopamine coating. For TNT-PAA/GS, the stretching vibrations of the C=O bond and the -COOH group appeared at 1519 cm^-1 and 1694 cm^-1, respectively, suggesting that PAA and GS were successfully self-assembled onto the PEI-modified surface. In order to further clarify the surface element composition, the surface element composition of the modified sample was further analyzed by XPS. Figure 2(b) is the XPS diagram of the different samples, and the element compositions are shown in Table 1; it can be seen that the characteristic peaks of C1s (285.2 eV) and N1s (400.3 eV) appeared on the TNT-Dopa. After the immobilization of PAA, the characteristic peak of O1s (531.8 eV) increased obviously; concurrently, the characteristic peak of C1s (285.2 eV) and carbon content on the surface was reduced, indicating that PAA was successfully grafted onto the surface. For the TNT-PAA/GS, the increased nitrogen content indicated that GS was successfully loaded on the surface. The occurrence of the Zn2p peak (1020.9 eV) on TNT-PAA/GS-Zn proved that the Zn ions were successfully chelated to the surface.

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Table 1

The surface element concentration of the different samples measured by XPS.

Sample	Atomic concentration (at.%)
	Ti	O	C	N	Zn
Ti	61.12	29.86	9.02	0	0
TNT-Dopa	9.28	14.30	71.83	4.59	0
TNT-PAA	9.97	30.42	56.41	3.20	—
TNT-PAA/GS	3.27	31.77	56.20	8.76	—
TNT-PAA/GS-Zn	1.86	28.78	59.73	6.26	3.37

3.2. GS and Zn²⁺ Release Profiles, Surface Hydrophilicity, and Protein Adsorption

In order to characterize the release profiles of gentamicin and Zn²⁺, TNT-PAA/GS-Zn was immersed in the PBS solution for different times, and gentamicin and Zn ions were collected to measure the release kinetics curves. As can be seen from the release curve of Figure 3(a), both gentamicin and zinc ions were released over 14 days. After being immersed in PBS solution for 4 h, the release concentration of gentamicin reached 3.9 μg/mL, and the total release concentration was 13.54 μg/mL at the 14th day. Previous studies have shown that the working concentration of gentamicin is 4-20 μg/mL [29]; therefore, the continuous antibacterial activities can last at least 14 days. At the same time, it can also be seen from Figure 3(a) that there was an obvious burst release period of one day for gentamicin and Zn²⁺. The release rate was relatively large before 1 day, and gentamicin reached 5.5 μg/mL, more than 40% of the total release in 14 days. After one day, the release rates of gentamicin and zinc ions became stable. After 14 days, the total concentration of zinc ion was 0.63 mg/L. It was reported that zinc ion concentration of 0.49-5.2 mg/L can promote cell viability, proliferation, adhesion, and migration; therefore, the released Zn²⁺ content was within the range of cell physiological concentration.

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Biomaterials should have good surface properties to avoid adverse host reactions after contact with organisms [30], in which wettability is an important factor affecting interface biological reactions [31]. The surface hydrophilicity/hydrophobicity is closely related to the protein adsorption and biocompatibility. Generally speaking, the good wettability is helpful to prevent the nonspecific protein adhesion and promote the cell adhesion and proliferation [32]. As can be seen from Figure 3(b), the water contact angle of the blank titanium decreased obviously after anodizing; it was considered that the introduction of a large amount of oxygen elements and the special nanoporous structure can contribute to the excellent hydrophilicity. Due to the introduction of hydrophilic amine groups after the immobilization of dopamine, TNT-Dopa still had excellent hydrophilicity. However, after the immobilization of PAA, the water contact angle increased significantly, which was mainly because the porous structure on the surface was filled to some extent after the immobilization of PEI and PAA (as shown in Figure 1), which partially changed the surface morphology and made it difficult for water molecules to enter the interior of the nanotubes, so the contact angle increased. After GS loading, the surface porous structure was further filled, and thus, the water contact angle also increased although the hydrophilic carboxyl groups were introduced. Finally, because the positively charged zinc ions can chelate with the hydrophilic -COOH groups on the surface, combining with the further porous filling, the water contact angle increased further.

It is well known that protein adsorption is the first event when biomaterials contact blood, and it plays a decisive role in the blood compatibility [33]. Albumin and fibrinogen are the two main proteins in the blood. In general, albumin adsorption can reduce platelet adhesion. On the contrary, the adsorption of fibrinogen could increase the platelet adhesion and activation. Figures 3(c) and 3(d) show the adsorption concentrations of bovine serum albumin (BSA) and fibrinogen (Fib) on the different surfaces. As compared to the pristine titanium, the anodized titanium surface can enhance albumin adsorption, while the fibrinogen adsorption decreased to some degree, indicating that the nanotube array can selectively adsorb albumin. It was considered that the behaviors of protein adsorption were related to the surface wettability, surface morphologies, and surface charges [34]. The study showed that when the water contact angle is less than 110°, fibrinogen would preferentially be adsorbed on the hydrophobic surface [35]. At the same time, the superhydrophilic surface is obtained by anodization, which is beneficial to the adsorption of hydrophilic albumin; moreover, the anodized surface has negative charges due to the introduction of a large amount of hydroxyl groups, which is also beneficial to the adsorption of positively charged albumin. The polydopamine has very strong stickiness to lysine-rich proteins [36], so the content of BSA and fibrinogen adsorbed on TNT-Dopa surface increased significantly. It has been shown that the immobilization of polyacrylic acid on the surface can repel the nonspecific protein adsorption because of the hydration layer formed by PAA and the negatively charged character of PAA [37]. Therefore, after the immobilization of PAA on TNT-Dopa, both BSA and fibrinogen adsorption decreased significantly. However, it was worth noting that BSA adsorption returned to the level of titanium oxide nanotubes, while fibrinogen adsorption was slightly higher than that of TNT. Furthermore, after loading GS by the layer-by-layer technique, due to the further increase of hydrophobicity, the adsorption content of BSA did not change significantly, but the adsorption amount of fibrinogen decreased, indicating that with the increase of self-assembly layers, the introduction of a large number of PAA increased the content of negative charges on the surface, so that the negatively charged fibrinogen was not easily adsorbed on the PAA surface, resulting in the decreased fibrinogen adsorption. The zinc ions can chelate with the carboxyl group of PAA to reduce the surface hydrophilicity; at the same time, zinc ion loading reduced the content of negative charges on the surface, so BSA adsorption increased slightly, while the adsorption capacity of fibrinogen further decreased.

3.3. Blood Compatibility

Blood compatibility refers to the required response of blood to exogenous substances or materials, which generally refers to the compatibility between materials and various components of blood [38]. Generally speaking, blood compatibility includes three aspects: the interaction between materials and plasma proteins, the interaction between materials and blood cells, and the interaction between materials and coagulation factors.

Platelets are one of the main components of the human blood. Its main functions are clotting and hemostasis as well as repairing damaged blood vessels. The platelet adhesion to the biomaterial surface is a key event of coagulation. Platelet adhesion, aggregation, and activation will promote blood coagulation. Therefore, the biomaterials with good blood compatibility should have the role of maintaining normal platelet physiological function and can effectively prevent platelet adhesion, aggregation, and activation [39]. At the same time, the increase of cGMP released from platelets can inhibit platelet activation [40]. In this paper, the adhesion and aggregation of platelets were observed by SEM, and the platelet activation was evaluated by measuring the cGMP activities. The results are shown in Figures 4(a)–4(c). There were a large number of platelets adhered to the blank titanium surface, and the adhered platelets displayed the spread state and extended pseudopodia, indicating that the platelets on the titanium surface may have been activated; the results of cGMP further proved this point. After anodization, the number of platelets adhered to the surface was significantly reduced. On the one hand, the anodized TiO₂ nanotube arrays had excellent hydrophilic properties, which can reduce platelet adhesion and aggregation; on the other hand, the selective adsorption of albumin by the TiO₂ nanotube contributed to not only reduce platelet adhesion and aggregation but also promote the expression of cGMP (Figure 4(c)), leading to the improved blood compatibility. For TNT-Dopa, although its hydrophilicity was still excellent, compared with TNT, the number of platelets adhered to the surface was still significantly increased, while the expression of cGMP was also decreased, which could be due to the fact that the polydopamine coating had the ability of nonspecific protein adsorption which can significantly increase the fibrinogen adhesion to the surface so as to enhance platelet adhesion and activation. Compared with the blank titanium and TNT, the number of platelets on the TNT-PAA/GS surface decreased sharply, and the expression of cGMP also increased, indicating that the anticoagulation was enhanced. It was considered that PAA itself is a substance with good blood compatibility and can effectively reduce fibrinogen adsorption [41]. When zinc ions were loaded on the surface, the adsorption amount of fibrinogen further decreased, while the albumin adsorption increased, which not only reduced the adhesion and aggregation of platelets but also increased the cGMP release and inhibited platelet activation. On the other hand, zinc ions can make platelets produce more NO signals which can prevent platelet adhesion and aggregation by inhibiting the thromboxane A2 (TXA-2) receptor [42]. Therefore, the loading of zinc ions on the surface further improved the blood compatibility.

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The effects of the different materials on red blood cells were further studied. Hemolysis rate is one of the important methods to characterize the interaction between materials and red blood cells. According to the international ISO10993-4 standard, the hemolysis rate (HR) above 5% is not suitable to be used as a blood-contacting material. Figure 4(d) shows the hemolysis rates of the different samples. The hemolysis rates of all samples were less than 5%, indicating that none of them could cause severe hemolysis. The hemolysis rate of TiO₂ nanotubes was lower than that of pure titanium, but the hemolysis rate increased slightly after the preparation of polydopamine coating. When the composite film of PAA and GS was prepared and the zinc ion was loaded, the hemolysis rate further decreased significantly, indicating that PAA and zinc ions can improve the blood compatibility.

Generally speaking, when the biomaterial interacts with human blood, blood coagulation may happen. Blood coagulation is a complex chain process involving a series of stimulus responses in conjunction with coagulation factors and enzymes, whose intent is to stop blood fluxes when a vascular tissue injury occurs [43]. According to the difference of the initial pathway and participating factors, blood coagulation can be divided into two pathways: endogenous coagulation and exogenous coagulation. Among them, the endogenous coagulation pathway is initiated by the activation of factor XII [44], and the activated partial thromboplastin time (APTT) is an important method reflecting the endogenous coagulation pathway, especially the activity of coagulation factor XII [45]. Figure 4(e) shows the APTTs of the different samples. It can be seen that the clotting time of blank titanium was shorter than that of normal plasma, indicating that it may promote blood coagulation to a certain extent. However, the clotting time of the anodized titanium was longer than that of normal plasma, suggesting that the surface of the TiO₂ nanotube had good anticoagulation performance. Although the clotting time decreased after dopamine surface modification, the clotting time was significantly prolonged after the immobilization of PAA and the loading of GS and Zn²⁺, indicating that the subsequent surface modification improved the anticoagulant properties of the materials.

3.4. Endothelial Cell Growth Behaviors

Figures 5(a) and 5(b) show the fluorescent images, and the CCK-8 values of endothelial cells adhered to the different surfaces. It can be clearly seen that the number of cells that adhered to the surface of the blank titanium was less than that of other modified samples. After anodizing, the number of adherent cells on the surface increased, and the morphologies of adherent cells displayed the spread state; its proliferation was also improved (Figure 5(b)). It can be concluded that the surface became more hydrophilic after anodizing, which can improve cell adhesion and proliferation on the surface through the exchange and adsorption of extracellular matrix proteins [46]; moreover, the special nanostructure also contributed to cell adhesion and spreading. The polydopamine coating on the surface can promote the adhesion and proliferation of endothelial cells [47]. Therefore, the number of endothelial cells adhered to TNT-Dopa increased significantly, and the proliferation performance was further improved. After the preparation of the first PAA layer on the PEI-modified TNT surface, due to the cytotoxicity of PEI and the hydrophobicity of PAA, the adhesion of cells to the surface was reduced, and the morphologies of cells did not spread well, so the proliferation of endothelial cells decreased slightly. When PAA and GS were deposited alternately for 10 times, the thickness of the coating increased, which eliminated the effect of PEI on cells. Therefore, the number of endothelial cells attached on the surface increased again, and the spreading and proliferation of endothelial cells were better than those of TNT-Dopa. Zinc is an essential micronutrient for human health, and Zn²⁺ homeostasis in cells is essential for cell function and survival [48]. Zn²⁺ acts as the first or second messenger and is the signal pathway that triggers physiological functions. In our previous work, zinc ions were doped into TiO₂ nanotubes by the hydrothermal method. The results showed that the release of zinc ions not only increased the anticoagulant properties but also promoted the adhesion and proliferation of endothelial cells [49]. In this study, the zinc ions were loaded on the TNT surface by chelation with PAA. The results of Figure 5 showed that the release of zinc ions from the surface can significantly promote the adhesion and proliferation of the endothelial cells.

[figures omitted; refer to PDF]

Furthermore, the function expression of endothelial cells was studied. Figure 5(c) shows the results of VEGF secretion of endothelial cells on the different samples. It can be seen that, compared with the blank titanium, anodization can promote the expression of VEGF in endothelial cells. When the polydopamine coating was prepared on the surface, due to the enhancement on cell adhesion and proliferation, endothelial cells could express more VEGF. For TNT-PAA and TNT-PAA/GS, the amount of VEGF decreased slightly as compared to TNT-Dopa, but it was not significant. After zinc ion loading, the content of VEGF increased significantly, indicating that the release of zinc ions can upregulate the VEGF expression of the endothelial cells, which was conducive to maintaining the growth of endothelial cells and promoting the surface endothelialization of the implanted materials. Figure 5(d) shows the NO secretion of endothelial cells grown on the different samples. The NO content released by endothelial cells on the surface increased after anodizing. The NO content for TNT-Dopa was larger because the polydopamine coating can promote the growth of endothelial cells. After further preparation of the PAA film and alternating deposition of PAA and GS, the growth state of cells was slightly worse (Figure 5(a)). At the same time, the content of VEGF release could also indirectly affect the production of eNOS in endothelial cells, thus affecting the release of NO [50]; therefore, the NO content decreased slightly. However, it was worth noting that the loading zinc ions can significantly promote the activity of intracellular NO enzyme and promote the release of NO and thus enhance the NO expression of endothelial cells.

3.5. Antibacterial Activities

Bacterial adhesion is widespread in nature, and there are usually two antibacterial strategies: killing bacteria and reducing bacterial adhesion [51]. In this study, gentamicin and zinc ions were loaded into titanium oxide nanotubes to prevent infection. Gram-negative bacteria-Escherichia coli was used as a test strain to evaluate the antibacterial properties of the modified material surface. Figure 6(a) shows the SEM images of bacterial adhesion on the different sample surfaces. It can be seen that although titanium had good biocompatibility to endothelial cells, its antibacterial adhesion property was poor, and there were a large number of Escherichia coli adhering to the surface. In contrast, the property of nonspecific protein adsorption caused by the excellent hydrophilicity of the nanotubes and a small number of negative charges on the surface could contribute to inhibit the adhesion of negatively charged Escherichia coli. The same with the cell results, polydopamine coating could make Escherichia coli adhere to the surface easily; therefore, compared with the TNT sample, the adsorption of bacteria on TNT-Dopa increased. When PAA was prepared on the surface, the number of negative charges increased, which reduced the adhesion of Escherichia coli to the surface. Moreover, after the multilayer film of PAA and GS was prepared, the increased negative charges on the surface and the continuous GS release could further prevent the negatively charged Escherichia coli to stay on the surface. Due to the antibacterial and bactericidal activity of releasing zinc ions [52], there was almost no bacterial adhesion on TNT-PAA/GS-Zn. Figure 6(b) shows the antibacterial properties of the different samples, from which it can be seen that pure titanium, TiO₂ nanotubes, and dopamine coating had poor bactericidal properties. After further immobilization of PAA, the number of Escherichia coli colonies decreased significantly, indicating that the PAA coating had certain antibacterial properties. When the PAA and GS composite coating was prepared, the release of GS could effectively kill bacteria, so the number of observed colonies decreased significantly. Finally, the zinc ion can also inactivate the protein needed by bacteria and cause the condensation of DNA [53], so the antibacterial activity was further improved after Zn²⁺ loading.

[figures omitted; refer to PDF]

4. Conclusion

In this paper, PAA/GS-Zn multifunctional bioactive coating was successfully prepared on the titanium surface with a nanostructure. The specific nanotube structure and the following functionalization significantly influenced the surface hydrophilicity, protein adsorption, blood compatibility, and endothelial cell growth behaviors of the materials. TiO₂ nanotubes had excellent hydrophilicity, but after the immobilization of PAA and loading of GS and zinc ions, the hydrophilicity became worse due to the decrease of the nanotube diameter. The superhydrophilic TiO₂ can selectively promote albumin adsorption, while the immobilization of PAA and the loading of GS and zinc ions can prevent the nonspecific protein adsorption. At the same time, combining the good blood compatibility and negative charged characteristics of PAA with the physiological activity of zinc ions, PAA/GS-Zn coating can not only significantly prevent platelet adhesion, aggregation, and activation but also reduce the hemolysis rate and increase the activated partial thromboplastin time, thus significantly improving the blood compatibility. In addition, the anodized nanotube array can promote endothelial cell adhesion and proliferation and upregulate expression of VEGF and NO. Although PAA/GS coating can promote cell adhesion and proliferation and upregulate NO expression, it cannot significantly promote the VEGF expression. Loading Zn²⁺ can not only significantly promote endothelial cell adhesion and proliferation but also upregulate the NO and VEGF expression. Finally, due to the continuous and slow release of GS and zinc ions, the surface-modified materials showed good antibacterial and germicidal efficacy to Escherichia coli.

Disclosure

This paper can be found as the presentation of the manuscript in Research Square (https://www.researchsquare.com/article/rs-50126/v1). Yuebin Lin and Li Zhang should be considered as co-first authors.

Authors’ Contributions

Yuebin Lin and Li Zhang contributed to the methodology, investigation, and writing—original draft. Ya Yang, Minhui Yang, and Qingxiang Hong contributed to the investigation and writing—original draft. Keming Chang and Juan Dai contributed to the cell experiments. Changjiang Pan contributed to the conceptualization, resources, supervision, and writing—review and editing. Yanchun Wei and Li Quan contributed to the blood compatibility experiments. Zhongmei Yang and Sen Liu contributed to the writing—review and editing. Yuebin Lin and Li Zhang contributed equally to this work.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (31870952), the Natural Science Foundation of Jiangsu Province of China (BK20181480), and the Natural Science Foundation of Jiangsu Higher Education Institution of China (19KJB430013).

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