Enhanced Corrosion Resistance and Biofilm Inhibition of Functionally Graded TC4/TC4-5Cu Fabricated by Selective Laser Melting against Streptococcus mutans

doi:10.1007/s40195-023-01622-8

Abstract

Abstract:

Ti-6Al-4V (TC4) used in dentistry and orthopedics as implant biomaterial faces the risk of microbiologically influenced corrosion (MIC) owing to the residence of diverse oral microorganisms. Hereinto, Streptococcus mutans is a critical pathogenic microorganism that causes dental caries. This work investigated the corrosive effects of S. mutans on TC4 and functional gradient TC4/TC4-5Cu coupons fabricated by selective laser melting (SLM) through various electrochemical measurements, surface examination, observation of biofilm and corrosion analysis. The results indicated that the Cu-bearing alloy showed an inhibitory effect on the biofilms due to the release of Cu element, thereby reducing the corrosion rate of MIC. The corrosion current density (i_corr) of TC4 (11.7 ± 0.8) nA cm⁻² is higher than that of TC4/TC4-5Cu (7.4 ± 0.4) nA cm⁻² in the presence of S. mutans, while the maximum pit depth of TC4 is 1.6 times that of TC4/TC4-5Cu. Therefore, metal modification through Cu alloying is an effective strategy to improve the MIC resistance.

Key words: Microbiologically influenced corrosion, Streptococcus mutans, Selective laser melting, TC4/TC4-5Cu, Biofilm

Xing Zhou, Qiyue Zhang, Jiarui Lu, Ying Zheng, Lin Wu, Dake Xu, Xue Zhang, Qiang Wang. Enhanced Corrosion Resistance and Biofilm Inhibition of Functionally Graded TC4/TC4-5Cu Fabricated by Selective Laser Melting against Streptococcus mutans[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(12): 1961-1978.

Figures/Tables 18

Fig. 1 a Morphology of the TC4 metal powder, b energy spectrum of the powders, c physical image of the samples, d design schematic diagram of the TC4/TC4-5Cu gradient composites

Table 1 Main parameters of the SLM process

Laser power (W)	Scanning speed (mm/s)	Layer thickness (μm)	Spot size (μm)	Scanning interval (μm)	Overlap rate (%)
249	1000	40	50	40	40

Fig. 2 a Metallographic structure of TC4, b metallographic structure of TC4/TC4-5Cu, c, d microstructures of TC4, e microstructure of layer B, f XRD spectrum

Fig. 3 Morphology and line energy spectrum of TC4/TC4-5Cu samples

Table 2 Elemental contents (wt%) of Points 1 and 2 on the side and Points 3 and 4 on the surface of TC4 and TC4/TC4-5Cu samples

Elements	Ti	Al	V	Cu
Point 1	90.32	5.9	3.78
Point 2	86.63	5.37	3.4	4.6
Point 3	90.28	5.734	4.27
Point 4	87.54	4.51	3.43	4.52

Fig. 4 EBSD results of TC4 and TC4/TC4-5Cu: orientation map of TC4/TC4-5Cu samples a, the distribution of the thickness and length of grain for TC4 b and TC4/TC4-5Cu c samples, the pole figures for TC4 d and TC4/TC4-5Cu e

Fig. 5 Antibacterial results of TC4 a, b and TC4/TC4-5Cu c, d samples co-cultured with S. mutans for 24 h

Fig. 6 SEM morphologies of S. mutans biofilms on the surfaces of TC4 and TC4/TC4-5Cu samples under 6 h a, 12 h b, 18 h c and 24 h d co-culture

Fig. 7 Morphologies of biofilms for TC4 a, c and TC4/TC4-5Cu b, d samples co-cultured with S. mutans for 7 days and 14 days: a, b for 7 days c, d for 14 days

Fig. 8 Live and dead state of bacteria on the surface of TC4 a, c and TC4/TC4-5Cu b, d samples co-cultured with S. mutans for 7 a, b days and 14 c, d days

Fig. 9 Electrochemical results of TC4 and TC4/TC4-5Cu samples immersed with and without S. mutans for 14 days: a OCP, b LPR, c potentiodynamic polarization curves

Table 3 Potentiodynamic polarization parameters of TC4 and TC4/TC4-5Cu samples co-cultured with and without S. mutans for 14 days

	i_corr (nA cm⁻²)	E_corr (mV vs SCE)
SLM TC4 in the sterile medium	2.6 ± 0.6	− 67.1 ± 8.7
SLM TC4 in the presence of S. mutans	11.7 ± 0.8	− 498.4 ± 13.2
SLM TC4/TC4-5Cu in the sterile medium	4.3 ± 0.6	− 129.5 ± 14.6
SLM TC4/TC4-5Cu in the presence of S. mutans	7.4 ± 0.4	− 338.1 ± 22.0

Fig. 10 EIS results of TC4 a, b and TC4/TC4-5Cu c, d samples co-cultured without a, c and with b, d S. mutans for 1, 7 and 14 days

Table 4 Results of EIS electrochemical parameter for TC4 and TC4/TC4-5Cu samples co-cultured with and without S. mutans for 1, 7 and 14 days

Days (d)	R_s (Ω cm²)	C_b × 10⁻⁶ (F cm⁻²)	R_b × 10⁴ (Ω cm²)	C_dl × 10⁻⁶ (F cm⁻²)	R_ct × 10⁶ (Ω cm²)
SLM TC4 in the sterile medium
1	14.6 ± 1.4			1.1 ± 0.3	2.3 ± 0.5
7	16.6 ± 1.6			1.1 ± 0.2	2.1 ± 0.3
14	19.9 ± 1.8			1.1 ± 0.4	1.8 ± 0.6
SLM TC4 in the presence of S. mutans
1	15.9 ± 2.1	1.1 ± 0.3	16 ± 4.3	8.5 ± 2.5	1.4 ± 0.3
7	17.4 ± 2.6	1.4 ± 0.4	1 ± 0.2	14 ± 2.4	0.5 ± 0.1
14	18.8 ± 2.8	1.2 ± 0.1	7.8 ± 1.3	13 ± 3.1	0.5 ± 0.2
SLM TC4/TC4-5Cu in the sterile medium
1	29.1 ± 3.2			1.2 ± 0.3	1.5 ± 0.4
7	23.5 ± 2.8			1.2 ± 0.2	1.6 ± 0.2
14	24.9 ± 3.3			1.3 ± 0.3	1.5 ± 0.2
SLM TC4/TC4-5Cu in the presence of S. mutans
1	24.1 ± 1.5	1.2 ± 0.2	4.8 ± 0.9	5.3 ± 1.9	1.8 ± 0.5
7	17.4 ± 2.3	2.5 ± 0.3	2.9 ± 0.6	7.0 ± 2.3	0.8 ± 0.2
14	14.4 ± 3.1	2.4 ± 0.4	1.5 ± 0.5	8.0 ± 2.7	0.8 ± 0.1

Fig. 11 CLSM morphologies of corrosion pits of SLM TC4 a, b and TC4/TC4-5Cu c, d samples co-cultured without a, c and with b, d S. mutans for 14 days

Fig. 12 XPS results of corrosion products for TC4 a, b and TC4/TC4-5Cu c, d samples after immersion under sterile a, c and aseptic b, d conditions for 14 days

Fig. 13 a pH changes of TC4 and TC4/TC4-5Cu samples immersed for different days and b concentrations of Ti and Cu ions in T1-T4 groups after immersion for 14 days

Fig. 14 Schematic diagram for improving MIC resistance of Cu-containing gradient TC4/TC4-5Cu alloy fabricated by SLM

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