Biodegradation, hemocompatibility and covalent bonding mechanism of electrografting polyethylacrylate coating on Mg alloy for cardiovascular stent

doi:10.1016/j.jmst.2019.12.011

Abstract

Abstract:

Organic coatings are the most widely employed approach for the promotion of corrosion resistance of magnesium (Mg) alloys. Unfortunately, traditional organic coatings are weakly bonded to Mg substrates due to physical adsorption. Herein, a polyethylacrylate (PEA) coating was fabricated on Mg-Zn-Y-Nd alloy via electro-grafting. The surface structure and chemical composition were characterized by means of scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscope (AFM) and Fourier transform infrared (FTIR) as well as time of flight-secondary ion mass spectrometer (ToF-SIMS). The results showed that the surface roughness of PEA coating was dominated by scan rate; while the coverage and integrity of PEA coating were mainly affected by the monomer concentration and sweep circles. ToF-SIMS results indicated that PEA coating was wholly covered on Mg alloy, and the presence of C₂H₃Mg^- fragment confirmed the covalent bond between PEA coating and Mg alloy. In addition, DFT calculation results of the adsorption of EA molecules with Mg substrate agree well with the experimental phenomena and observation, suggesting that the electrons in 3 s orbit of Mg atoms and 2p_z orbit of C₁ atom participated in the formation of covalent bond between PEA coating and Mg substrate. Potentiodynamic polarization curves and immersion test demonstrated that the PEA coatings could effectively enhance the corrosion resistance of Mg alloy. The platelet adhesion results designated that platelets were barely visible on PEA coating, which implied that PEA coating could effectively prevent the thrombosis and coagulation of platelets. PEA coating might be a promising candidate coating of Mg alloy for cardiovascular stent.

Key words: Magnesium alloy, Organic coating, Electro-grafting, Covalent bond, Density functional theory, Biomaterial

Yong-Xin Yang, Zhe Fang, Yi-Hao Liu, Ya-Chen Hou, Li-Guo Wang, Yi-Fan Zhou, Shi-Jie Zhu, Rong-Chang Zeng, Yu-Feng Zheng, Shao-Kang Guan. Biodegradation, hemocompatibility and covalent bonding mechanism of electrografting polyethylacrylate coating on Mg alloy for cardiovascular stent[J]. J. Mater. Sci. Technol., 2020, 46: 114-126.

Figures/Tables 21

Table 1 Experimental parameters of the prepared coatings on Mg-Zn-Y-Nd alloy.

Specimen	Concentration (mol/L)	Cycles	Scan rate (mV/s)
#1	0.5	7	20
#2	1.0	7	20
#3	1.5	4	20
#4	1.5	7	20
#5	1.5	10	20
#6	1.5	7	40
#7	1.5	7	60

Fig. 1. ATR-FTIR spectra of grafted coatings on Mg-Zn-Y-Nd alloy.

Fig. 2. SEM morphologies (top) and 3D topographies (bottom) of PEA coatings on Mg-Zn-Y-Nd alloy with different cycles: (a, d) #3; (b, e) #4; (c, f) #5.

Fig. 3. SEM morphologies (top) and 3D topographies (bottom) of PEA coatings on Mg-Zn-Y-Nd alloy with different monomer concentrations: (a, d) #1; (b, e) #2; (c, f) #4.

Fig. 4. SEM morphologies (top) and 3D topographies (bottom) of PEA coatings on Mg-Zn-Y-Nd alloy with different scan rates: (a, d) #4; (b, e) #6; (c, f) #7.

Table 2 Surface element content of PEA coatings on Mg-Zn-Y-Nd alloy (wt% (at.%)).

Specimen	C	O	Mg
#1	48.70	6.14	45.16
#1	(64.72)	(6.04)	(29.24)
#2	75.81	10.95	13.24
#2	(83.90)	(8.96)	(7.14)
#3	60.86	8.34	30.80
#3	(74.12)	(7.54)	(18.34)
#4	84.33	15.46	0.21
#4	(87.65)	(12.25)	(0.10)
#5	85.02	14.53	0.45
#5	(88.27)	(11.50)	(0.23)
#6	78.62	21.15	0.23
#6	(82.88)	(16.98)	(0.14)
#7	58.40	8.02	33.58
#7	(72.72)	(7.27)	(20.01)

Fig. 5. Relevant ToF-SIMS spectra of samples before (up) and after (bottom) electro-grafting of PEA coating: (a) Mg+; (b) C2H5O-; (c) C2H3Mg-; (d) C3H3O-.

Fig. 6. The optimized structure (a) and adsorption configuration (b) of EA on Mg (0001) surface.

Table 3 Bond lengths of EA molecules before and after the adsorption (?).

State	C1-C2	C2-C3	C3-O1	C3-O2	C4-O2	C4-C5
Before adsorption	1.34	1.48	1.22	1.36	1.45	1.51
After adsorption	1.49	1.39	1.29	1.37	1.45	1.51

Fig. 7. PDOS of C1 (a, b), Mg (c, d), O1 (e, f) atoms before (left) and after (right) adsorption.

Fig. 8. Electron density difference of EA adsorption on Mg (0001). The yellow and light blue iso-surfaces indicate the gain and loss of electron density.

Fig. 9. Potentiodynamic polarization curves of Mg-Zn-Y-Nd alloy with and without PEA coatings in SBF solution.

Table 4 Fitting results of Mg alloy and PEA coated samples.

Specimen	E_corr (V_SCE)	i_corr (μA/ cm²)	E_b (V_SCE)	β_a (mV/dec)	β_c (mV/dec)	R_p(kΩ cm²)
Substrate	-1.79	577	-1.51	322.3	237.6	0.10
#1-0.5-7-20	-1.70	33.4	-1.56	163.1	190.3	1.14
#2-1.0-7-20	-1.67	30.2	-1.55	159.4	243.6	1.39
#3-1.5-4-20	-1.60	21.3	-1.53	88.1	199.9	1.25
#4-1.5-7-20	-1.56	8.17	-	47.5	193.5	2.03
#5-1.5-10-20	-1.55	8.24	-1.49	91.1	167.1	2.99
#6-1.5-7-40	-1.65	24.7	-1.52	185.8	212.4	1.74
#7-1.5-7-60	-1.69	75.3	-1.58	161.6	301.4	0.61

Fig. 10. Surface morphologies of the specimens after immersion in SBF for 7 d: (a) Mg alloy; (b) #1; (c) #2; (d) #3; (e) #4; (f) #5; (g) #6; (h) #7.

Table 5 Surface compositions of specimens after immersion in SBF (wt% (at.%)).

Specimen	C	O	Mg	Na	P	Ca	Zn	Nd
Substrate	16.94	30.28	5.86	3.36	14.34	15.43	8.25	5.54
Substrate	(30.33)	(40.05)	(5.03)	(3.05)	(10.04)	(8.07)	(2.63)	(0.8)
#1	37.82	24.71	5.93	3.04	9.00	7.69	7.20	4.61
#1	(55.85)	(27.47)	(4.36)	(1.00)	(5.37)	(3.50)	(1.90)	(0.55)
#2	47.31	27.21	18.77	0.87	2.79	3.05	-	-
#2	(59.87)	(25.50)	(11.57)	(0.56)	(1.35)	(1.15)	-	-
#3	65.07	20.21	4.43	0.59	4.83	4.87	-	-
#3	(75.48)	(17.49)	(2.52)	(0.36)	(2.16)	(1.99)	-	-
#4	76.40	22.85	0.09	0.66	-	-	-	-
#4	(81.46)	(18.12)	(0.05)	(0.37)	-	-	-	-
5#	73.72	22.00	1.01	0.22	1.30	1.75	-	-
5#	(80.35)	(17.88)	(0.54)	(0.12)	(0.54)	(0.57)	-	-
#6	72.23	22.03	1.78	0.40	1.59	1.97	-	-
#6	(79.28)	(18.08)	(0.96)	(0.23)	(0.66)	(0.79)	-	-
#7	26.79	36.51	21.17	1.86	7.33	6.34	-	-
#7	(38.27)	(38.99)	(14.76)	(1.32)	(3.84)	(2.82)	-	-

Fig. 11. Mass losses of the specimens with different parameters after immersion in SBF for 7 d after removing degradation products with chromic acid.

Fig. 12. Results of hemolysis ratio of Mg alloy and PEA coatings.

Fig. 13. SEM images of specimens after incubation in PRP for 1 h: (a) Mg alloy; (b) #3; (c) #4; (d) #5.

Fig. 14. CV curves of PEA coating electro-grafted on Mg alloy (a) and the cross-section image of #4 (b).

Fig. 15. Schematic of the formation mechanism of PEA coating on Mg alloy (a), SEM images of PEA coatings at step 1 (b), step 2 (c) and step 3 (d).

Table 6 Comparison of degradation rate of specimens between immersion and polarization (mm/a).

Methods	Substrate	#1	#2	#3	#4	#5	#6	#7
P_w	6.97	4.37	3.32	2.23	1.20	1.32	2.25	4.57
P_i	13.18	0.76	0.69	0.49	0.19	0.19	0.56	1.72

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