A crack-free anti-corrosive coating strategy for magnesium implants under deformation
Introduction
Functional coating is a common technique for altering the surface properties of a material. Functional coatings offer enhanced mechanical protection, resistance to chemical deterioration, biocompatibility, and responsive performances to external stimuli [[1], [2], [3]]. However, the coating layers also cause intrinsic mechanical problems such as buckling, surface cracking, and delamination upon deformation. Therefore, a coating method that improves the mechanical stability of the coated product under various operation conditions is greatly desired [4]. The interfaces of different materialities, especially hard–soft interfaces, are challenging because of the large deformation discrepancy between the two materials under an external force [4,5]. To avoid hard–soft interfacial problems, the stress and strain can be gradually distributed through functionally graded layers placed between the hard and soft materials, thereby suppressing interfacial delamination [6]. The contact stability might also improve by new flexible coating materials that better match the underlying material [7,8]. Despite the proven advantages of these approaches, the limited thickness and fatigue of coating layers reduce the long-term durability of the devices in practical use. To the best of our knowledge, these issues remain unresolved [9].
The hard–soft interface problem also occurs for magnesium (Mg)-based implants when coating layers are introduced to Mg to improve corrosion resistance. Although Mg-based implants have attracted much attention based on their good biocompatibility, biodegradability, and suitable mechanical properties, they quickly corrode when exposed to water in physiological environments, producing hydrogen gas and hydroxide ions. Such corrosion often provokes acute and chronic inflammation responses in hosts [[10], [11], [12]]. The corrosion behavior of Mg implants has been moderated by protective coating methods, such as tunable coating layers, with the coating layers often improving the biological performance, providing corrosion barriers, and invoking multifunctional effects (e.g., drug delivery) in the Mg implants [[13], [14], [15]]. Common coating materials include biopolymers such as poly-l-lactic acid, poly(lactide-co-glycolide), polycaprolactone, and polyetherimide (PEI) and bioceramics such as hydroxyapatite (HA) and tricalcium phosphate [[15], [16], [17], [18], [19]]. However, similar to various coating systems in other fields, biological coatings on implants must meet the practical requirements for clinical use. For example, in orthopedic applications, the coating materials of degradable Mg implants must be mechanically stable against implant deformation during surgical operations and physiological loading conditions after fixation in the body. In addition to the load-bearing role of the implant, the substrate will be deformed post-surgery by bone regenerated through the healing process [16,20,21]. In this context, flexible biopolymer-coating materials are more mechanically stable to deformations than bioactive ceramic coatings despite their low bioactivity. To improve bioactivity while preserving mechanical stability, researchers introduced inorganic–organic coating systems [15,16,21,22]. Here, we developed a new coating strategy with nanopatterned or micropatterned surfaces that induced localized deformations associated with the pattern geometries. Hard coating (HA) and flexible coating (PEI) were introduced to regions having small and large deformations, respectively. Our results revealed that the selective coatings using surface patterns improved the Mg surface by offering corrosion resistance, mechanical stability under deformation, a maximal surface area of bioactive ceramic and good biocompatibility.
Section snippets
Computational modeling with finite element analysis (FEA) models
The micropatterned Mg was designed in FE models of Mg substrate (thickness: 10 mm) with or without PEI/HAp coating. In all FE models, the Mg surface was patterned with sinusoidal undulations using a fixed wavelength (λ = 50 μm) and various amplitudes (A = 0–50 μm), ensuring that A⁄λ was within a range of 0–1. In experimental measurements of PEI/HA-coated Mg, the HA and PEI layers were maximally thick (8 μm and 4 μm, respectively) at the peaks and valleys of the Mg substrate, respectively. The
Surface pattern designs through computational simulations
First, we compared strain contours of the FE models with the wavy geometry of the Mg surface and the flat surface under 3% global tensile strain (Fig. 1). On the flat Mg substrate, the strain contour indicated uniform deformation in accordance with 3% global strain (Fig. 1a). However, the strain contours on the wavy surface exhibited interesting features; in particular, strain in the hill regions was nearly zero, whereas a localized strain of ∼0.16 appeared in the valley region of the wavy
Conclusion
We present a novel approach for the application of a sustainable anti-corrosive coating on Mg under deformation. The method structurally modifies the substrate surface to induce a deformation gradient. The micropatterned Mg substrates were coated with corrosion-resistant polymeric and ceramic materials developed by computational and experimental methods. Guided by numerical simulations, the micropattern on Mg was designed to avoid failure caused by localized strain associated with the pattern
Conflict of interest
The authors declare no conflict of interest.
Acknowledgments
This research was supported by AcRF Tier 1 grant 2017-T1-001-246 (RG51/17) from Ministry of Education (MOE) of Singapore, the start-up funding from Nanyang Technological University and Basic Science Research Program (No. 2015R1D1A1A01057311) through the National Research Foundation (NRF) of Korea. Gao and Li also acknowledge the supports from National Science Foundation (NSF) under grant CMMI-1362893 and the start-up funding from the University of New Hampshire.
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