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• Surface modification plays an important role in modern materials science, biology, and chemistry [1,2]. Polydopamine (PD) coating has emerged as a versatile surface functionalization method characterized by material-independent properties and chemical reactivity [3,4]. Various surface modification techniques based on PD coatings have been developed and found application in surface functionalization across diverse fields, including biomedicine [5], energy research [6], material science [7]. However, PD coating technology encounters challenges in precisely controlling surface properties, enhancing adhesion to diverse substrates, and maintaining stability in harsh environments, which limits its application outside the laboratory [8]. Extensive research has unveiled that the inadequate stability of PD and its derivative coatings in harsh environments can primarily be attributed to the abundance of noncovalent bonds within the coating (Fig. 1A) [9–11]. Various strategies have been explored to reinforce the coating matrix, enhance its stability, strength, and resistance to external stresses. These strategies can generally be classified into two categories: in situ crosslinking strategies and post-coating treatments. The first category aims to enhance the crosslinking of PD coatings during the coating deposition process, which can be achieved by including introducing weak oxidizing agents [12,13], coordinating with metal ions [12,14], chemical crosslinkers [15,16], ultraviolet radiation [17,18], etc. The post-coating treatments include processes such as laser exposure [19], heating [20], oxidation [21], and coordination treatments [22]. These processes enhance the overall crosslinking degree of PD coatings after their deposition. Despite these active efforts, achieving stability in PD coatings, especially under extreme conditions, and simultaneously maintaining their reactivity for subsequent modifications, continue to present ongoing and critical challenges.

  Figure 1

  

  Figure 1.  The dissociation of PD coating and the in situ dynamic reassembly of PDPA coating under strong alkaline conditions. (A) Schematic illustration of the structure of the PD coating and its non-covalent interactions. (A-i) Under strongly alkaline conditions (NaOH solution, pH > 11.8), the PD coating dissociates, leading to the deconstruction of the coating and the formation of PD oligomers. (A-ii) Rapid covalent crosslinking occurs between the dissociated PD oligomers and PAH under the same alkaline condition (NaOH solution, pH > 11.8), resulting in the in situ dynamic reassembly and the reconstruction of the PDPA coating enriched with amino groups.

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  During the deposition process of PD coatings, dopamine (DA) undergo oxidative polymerization involving Michael addition, Schiff base formation, and nucleophilic substitution [23]. This polymerization process leads to the formation of PD oligomers, which play a critical role in the deposition of PD coatings [24]. However, it has limitations in terms of continuous covalent polymerization and typically forms oligomers up to hexamers [25,26]. The PD oligomers are subsequently self-assembled onto the surface of materials through non-covalent interactions, such as hydrogen bonding and π-π stacking, leading to the formation of a PD coating [9,27]. Therefore, altering the predominant non-covalent structure of PD coatings through in situ crosslinking strategies is challenging in mild alkaline environments, as it is an essential part of the PD deposition process [24]. On the other hand, once the PD oligomers are stacked, the spatial arrangement of the oligomer units becomes fixed [28], making it challenging to facilitate further covalent crosslinking within the polymer network through post-coating treatments. Moreover, the introduction of in situ crosslinking agents or post-coating treatments may potentially decrease the reactivity of PD oligomers and limit their surface functionalization potential by consuming reactive groups. Therefore, we propose the following hypothesis: To comprehensively crosslink PD oligomers while preserving their essential characteristics within the coating, one potential approach is to achieve controlled dissociation and reassembly of these oligomers simultaneously. This can be accomplished by introducing appropriate chemical stimuli or environmental conditions that disrupt non-covalent bonds. This disruption facilitates the rearrangement of the oligomers and the formation of irreversible covalent bonds between them by introducing appropriate crosslinkers with functionalization potential, resulting in a robust, versatile and highly crosslinked PD coating.

  In this study, we propose the concept of "in situ dynamic reassembly" to address the challenges encountered in PD coating, which can also be described as "reconstruction upon deconstruction". This approach involves simply immersing a pre-deposited PD coating into a strong alkaline solution containing poly(allylamine) hydrochloride (PAH). Studies have shown that when exposed to highly alkaline conditions (such as pH > 12), the amino groups in the PD coating undergo deprotonation, leading to a rapid disassembly of the coating (Fig. 1A-i). This disassembly process involves the elimination of non-covalent bonds and reversible covalent bonds in Schiff bases within the PD coatings [9,11]. In theory, the rate of the irreversible Michael addition reaction within the PD coating is significantly increased and nucleophilicity is enhanced under highly alkaline conditions. However, apparently, the internal Michael addition reaction in PD is insufficient to stabilize its coating. Therefore, we propose using alkali stable polyamines (e.g., PAH) as crosslinking agents to achieve thorough covalent crosslinking of PD oligomers. Interestingly, it was found that the irreversible Michael addition between PAH and PD oligomers occurred under such strong alkaline conditions (pH > 11.8, i.e., NaOH solution: > 0.31 mg/mL), which resulted in a PAH crosslinked PD (PDPA) coating (Fig. 1A-ii). Moreover, it seems that the “rapid disassembly” of PD by alkali was replaced by an “in situ dynamic reassembly” of PDPA coating with the participation of PAH, as the OH^- diffusion through the coating matrix, PD deconstruction and PAH mediated reconstruction happens simultaneously in theory.

  Consequently, the noncovalent instability of the PD coating gradually decreases, leading to the formation of a reconstructed PDPA coating enriched with stable amino groups and offers several advantages: (1) It has superior chemical and mechanical stability compared to PD coating; (2) It can graft significantly more molecules than PD, offering a better way to immobilize molecules on the surface. As a result, this study presents an innovative strategy for constructing a robust, material-independent, and multifunctional PD-like coating. The concept of “in situ dynamic reassembly” or “reconstruction upon deconstruction” is demonstrated, contributing to the exploration and improvement of applications involving unstable polycatecholamine materials.

  Specifically, PD coated 316L SS samples (as a representative material) were immersed in different concentrations of NaOH aqueous solutions (0, 0.04, 0.08, 0.16, 0.3, 0.6, 1.3, 2.5, 5, 10, 20, 40 mg/mL, the pH values of NaOH and corresponding NaOH + PAH solutions are nearly identical, which was ranged from about 6.4–14.0, Table S1 in Supporting information) with or without PAH (2 mg/mL) for 12 h at room temperature. The appearance of samples was shown in Fig. 2A that the less PD coating was remained on the surface as the alkalinity of environments increase, meanwhile, distinct coatings existed on the surfaces with the participation PAH, even under extremely alkaline conditions (pH 14.0, 40 mg/mL NaOH + PAH). The coating thickness were significantly increased at pH 11.8 (0.31 mg/mL NaOH) for NaOH + PAH samples, and peaked at pH 12.4 (1.25 mg/mL NaOH) for NaOH + PAH samples (Fig. 2B). Besides, and the density of surface amino groups peaked at pH 12.7 (2.5 mg/mL NaOH) for NaOH + PAH samples (Fig. 2C). If the NaOH concentration is too low (< 0.16), it cannot break the non-covalent bonds of the PD coating, and PAH will not be able to reconstruct it into a PDPA coating. If the NaOH concentration is too high (≥5 mg/mL), PD will rapidly dissociate, and PAH will be unable to covalently cross-link more PD fragments in situ, leading to a significant amount of PD fragments diffusing into the solution (Fig. 2D). At around pH 12.7 (NaOH 2.5 mg/mL), the rates of reconstruction upon deconstruction are better matched. Although a small amount of PD fragments still diffuse into the solution, a significant amount of PAH is incorporated into the coating, allowing the PDPA coating to achieve peak thickness and surface amine density.

  Figure 2

  

  Figure 2.  The influence of alkaline pH on the dissociation process of the PD coating and the formation of the PDPA coating. (A) The changes of the coating appearance, (B) thickness, (C) the surface amino group density of the PD samples, and (D) the concentration of PD oligomer in solutions after reaction with different concentrations of NaOH and NaOH + PAH solutions for 12 h. The concentration of PAH is constantly 2 mg/mL, and the concentration of NaOH is 0–40 mg/mL, the resulted pH of each solution was shown in Table S1. A pH of 12.7 was selected as an illustrative example to demonstrate (E) the FTIR spectra and (F) the high-resolution XPS spectra of C 1s, N 1s, and O 1s in PD and PDPA coatings. Data are presented as mean ± standard deviation (SD) (n = 4). One-way analysis of variance (ANOVA) with Tukey post hoc test was performed to determine the difference among various groups. n.s, not significant. P < 0.05, **P < 0.01.

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  Fourier transform infrared spectroscopy (FTIR) was employed to investigate the chemical structure of coatings (Fig. 2E). Both PD and PDPA coatings exhibited similar features. A broad band at 3600–3200 cm⁻¹ is assigned to the stretching vibration of catechol O-H and N-H. Peaks at 2940 and 2980 cm⁻¹ are assigned to the C-H stretching vibration, and the peaks at 1608 and 1520 cm⁻¹ can be attributed to the C=C stretching vibration of the aromatic ring [29]. The bending vibration of N-H peak at 1573 cm⁻¹ was greatly enhanced in PDPA, which may due to the increasing of amino groups from PAH. The bending vibration of C=O (1640 cm⁻¹) was greatly enhanced in PDPA. This may be due to the increased oxidation of the phenolic hydroxyl groups of DA molecules, leading to the formation of additional C=O in an alkaline environment. The high-resolution X-ray photoelectron spectroscopy (XPS) of C 1s, N 1s, and O 1s further confirmed the crosslinking reaction between PAH and PD oligomers (Fig. 2F). The significant increase in the C-N and C=N peaks (C 1s) at 285.9 eV in the PDPA group indicates the occurrence of a crosslinking reaction, which is most likely attributed to the Michael addition reaction. The enhanced N 1s peaks corresponding to (Ar)C-N and (Ar)C=N at 399.3 eV and -NH₃⁺ at 401.8 eV indicated the involvement of PAH molecules in the PDPA coating. Moreover, there is a significant reduction in C-O bonds and a substantial increase in C=O bonds (O 1s at 533.4 and 531.8 eV) were observed in the PDPA group, which are consistent with the FTIR results, indicating a higher degree of oxidation of phenolic hydroxyl groups, resulting in an increased formation of carbonyl groups (C=O). Despite the significant changes observed in the coating composition, the scanning electron microscope (SEM) and atomic force microscope (AFM) results revealed that the surface morphology of both PD and PDPA coatings remained similar. However, for PD coatings that were treated solely with NaOH solution, a substantial reduction in roughness was also observed (Fig. S1 in Supporting information) [9].

  As described, PAH crosslinking has completely changed the structure of the PD into PDPA. Therefore, we have conducted a systematic evaluation of the stability of PDPA coating in various extreme environments. In order to evaluate the stability of the PDPA coating in a high-salt environment, PD and PDPA-coated 316L SS discs were soaked in ultrapure water (control) and saturated NaCl solution for 24 h (Fig. 3A). SEM images revealed that the surface of the PDPA coating remained relatively intact after treatment. In contrast, the surface of PD showed evident swelling and delamination following treatment (Fig. S2 in Supporting information). To assess their acid and alkali resistance, both PD and PDPA coatings were subjected to immersion in solutions of HCl and NaOH with varying pH levels (1–4 for HCl and 11–14 for NaOH) for a duration of 24 h (Fig. 3B). The results indicated that the coatings were significantly affected by acidic conditions. At pH 1, no PD coating was observed, however, there were PDPA coating fragments remaining on the 316L SS surface. At pH 2, optical microscope images revealed a more uniform dissociation of the PD coating, whereas the PDPA coating exhibited partial peeling over a larger area. Quantified measurements of PD oligomers and coating thickness supported the superior acid resistance of the PDPA coating compared to the PD coating (Figs. 3B-i and B-ii). Resistance to alkaline environments has always been a significant challenge for PD-based coatings. Notably, the PD coating displayed gradual dissociation at pH 12 and near-complete dissociation at pH 13. Surprisingly, only partial swelling streaks were observed on the surface of PDPA even at pH 14, suggesting exceptionally high resistance to alkaline conditions (Fig. S3 in Supporting information). Oxidation also plays a crucial role in the failure of PD-based coatings. The results of immersing the samples in varying concentrations of H₂O₂ for 24 h were shown in Fig. 3C. It was observed that the PD coating endured complete dissociation in 0.35% H₂O₂, with only a few PD particles observed through SEM analysis, which was similar to the PDPA coating treated with 35% H₂O₂ (Fig. 3C and Fig. S4 in Supporting information). Additionally, the variation in coating thickness further supported the notably enhanced resistance of the PDPA coating to oxidation environments (Fig. 3C-i).

  Figure 3

  

  Figure 3.  The stability of PDPA coating versus PD coating under various conditions. The photographs of the PD and PDPA coatings after 24 h incubation under (A) high ionic strength using a saturated NaCl solution; (B) extreme pH conditions ranging of pH 1–4 and pH 11–14, depicting (B-i) the coating thickness and (B-ii) the concentration of PD oligomers in the reaction solution; and (C) oxidative conditions achieved by incubation with 0.035%–35% H₂O₂, the coating thickness was shown in (C-i). (D) The friction and wear comparison between PD and PDPA coatings conducted by testing with wet filter paper (0.01 N, 100 cm × 5 cm), performed the motion in a back-and-forth manner for 50 times. (E) The wear-resistance performance of PD and PDPA coated inferior vena cava filters evaluated through in vitro compression and placement with a sheath. Data are presented as mean ± SD (n = 4).

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  Wear resistance and mechanical strength are essential qualities in functional coatings for interventional medical devices, which are often subject to constant contact with tissues, body fluids, and other surfaces. In addition, they are often manipulated and deployed within the body, requiring functional coatings that can withstand deformation, cracking, and other form of mechanical failure, which have the potential to compromise their performance and pose risks to patient safety. Herein, the wear resistance of coatings was analyzed using a wet abrasion test with wet filter paper. It was observed that a significant portion of the PD coating exhibited noticeable wear, indicating its susceptibility to abrasion. In contrast, the PDPA coating showed more promising performance, with only partial scratching observed on its surface (Fig. 3D). Subsequently, both PD and PDPA coatings were applied to two common interventional medical devices: the inferior vena cava filter (Fig. 3E) and the cardiovascular stent (Fig. S5 in Supporting information).

  The inferior vena cava filter and sheath were rubbed three times, and the results revealed a large extent of peeling on the surface of the PD coating, however, only scratches and a small amount of peeling were observed on the surface of the PDPA coating (Fig. 3E). It has been reported that PD exhibits excellent adhesion strength and flexibility, enabling it to maintain its integrity in challenging applications, such as functional coating for cardiovascular stent [30]. The PDPA coating demonstrated an intact surface with no signs of peeling or cracking, even after undergoing compression and expansion (Fig. S5). These remarkable outcomes highlight the great potential of PDPA for the development of functional coatings for complex-shaped interventional medical devices.

  Based on the aforementioned result, we conducted a comprehensive analysis to assess the enhanced secondary surface functionalization potential of the PDPA coating in efficiently immobilizing functional molecules, in comparison to the PD coating. Figs. 4A–C illustrate typical reactions involving carbodiimide and thiol chemistry to introduce desired functional groups onto the polycatecholamine surface.

  Figure 4

  

  Figure 4.  Schematic of functional molecules grafted to the PDPA coating surfaces via sulfhydryl (A, SH-FITC), NHS-ester (B, NHS-RhB), and carboxyl (C, SeDA) groups. (A-i) PDPA coating exhibited a higher amount of SH-FITC immobilization than PD coating. (B-i) The PDPA coating demonstrated successful immobilization of NHS-RhB on the surface of the PDPA-coated stent (right), in contrast to the PD-coated stent (left). (C-i) The release flux of surface-catalyzed NO was measured for PD-SeDA and PDPA-SeDA samples. Data are presented as mean ± SD (n = 4). One-way ANOVA with Tukey post hoc test was performed to determine the difference among various groups. ***P < 0.001.

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  The immobilization of sulfhydryl molecules onto the PD surface offers a versatile and efficient means of surface functionalization, which is a widely adopted and commonly used method [3,8]. It has been observed that the amount of succinimidyl ester of fluorescein isothiocyanate (SH-FITC) immobilized on the PDPA surface is consistently twice as high as that on the PD coating (Figs. 4A and A-i), which suggested that the PDPA surface exhibits enhanced reactivity towards the immobilization of SH-molecules.

  To assess the immobilization capacity of surface amine groups for N-hydroxysuccinimide (NHS)-activated molecules, NHS Rhodamine B (NHS-RhB) was employed (Fig. 4B). Initially, PD coating was carefully applied to the intricate surface of a cardiovascular stent, with half of it converted into the PDPA coating. The stent was then incubated with NHS-RhB. The resulting distinctive color boundary indicated variations in coating thickness. Notably, a significant amount of NHS-RhB was immobilizated on the surface of the PDPA coating (Fig. 4B-i). This outcome illustrated the enhanced capability of the PDPA coating in anchoring NHS-molecules and exemplifies its potential for further functionalization and tailored utilization in medical devices applications.

  The 3,3′-diselenodipropanoic acid (COOH-SeDA), a small organic molecule catalyst for efficiently catalyze the endogenous S-nitrosothiols (RSNOs), or S-nitrosoglutathione (GSNO) (for in vitro test) into nitric oxide (NO), was selected for evaluating the immobilization efficiency of PDPA due to its proportional and highly sensitive catalytic activity (Fig. 4C) [31,32]. The PDPA-SeDA surface exhibited a significant enhancement in catalyzing the production of NO compared to the PD coating (16 times higher, Fig. 4C-i). The same study was conducted to assess the performance of the PAH-grafted PD surface compared to the PDPA coating. However, the amine groups on the PAH-grafted PD surface exhibited a threefold increase compared to the PD coating, yet this remained substantially lower than of PDPA coating (Fig. S6 in Supporting information).

  PD and other polycatecholamine-based coatings exhibit inadequate stability, particularly in strong alkali environments. This is mainly attributed to their deposition mechanisms which rely on non-covalent assembly of oligomers. To enhance their stability, we exploited the strong bases to rapidly dissociate the PD coating and accelerate the irreversible Michael addition between the amino groups of polyamine, PAH, and catechol oxide. By introducing a reaction solution with NaOH and PAH, the PD coating undergoes an in situ dynamic reassembly process, leading to the formation of a highly covalently cross-linked PDPA network. This phenomenon is referred to as "reconstruction upon deconstruction" which entails the simultaneous deconstruction of PD and the subsequent reconstruction of PDPA structures. The PDPA coating not only maintains the material-independent deposition properties commonly associated with PD-based coatings but also significantly enhances its mechanical and chemical stability. Furthermore, the PDPA coating possesses a significant number of stable amino groups, which enable various forms of common modification strategies such as nucleophilic addition, carbodiimide chemistry, diazotization-coupling reaction. Moreover, such in situ dynamic reassembly strategy offers a novel approach for enhancing other unstable polycatecholamine coatings. The innovative concept of "reconstruction upon deconstruction" not only addresses stability concerns in coatings, but also transforms these instabilities into opportunities for enhancing their potential for multi-functionalization through the use of appropriate pre-modified cross-linking agents. This method offers great potential in the fields of materials science, biomedical engineering, and nanotechnology, providing researchers with a versatile tool for surface modification and functionalization. The PDPA coating, with its robust, material-independent, and multifunctional properties, holds significant potential across various fields such as materials science, biomedical engineering, and nanotechnology. This versatile tool empowers researchers to modify and functionalize surfaces, consequently unlocking novel possibilities for exploring and innovating polycatecholamine-based materials in extreme conditions that were formerly limited.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Peng Gao: Writing – original draft, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. Hua Qiu: Formal analysis, Data curation, Conceptualization. Huan Cheng: Investigation. Zeyu Du: Methodology. Xiao Chen: Methodology, Investigation. Xing Tan: Investigation. Chenxi Cai: Data curation. Qihong Zhang: Data curation. Tong Yang: Data curation. Nan Lyu: Data curation. Qiufen Tu: Methodology. Xingyi Li: Writing – review & editing, Conceptualization. Lei Lu: Writing – review & editing, Writing – original draft, Funding acquisition, Conceptualization. Nan Huang: Writing – review & editing, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 82301156, 32371377, and 52203131), Leading Talent Project of Guangzhou Development District (No. 2020-L013), the Zhejiang Provincial Natural Science Foundation of China (No. LTGY23H140001), and the Fundamental Research Funds for Wenzhou Medical University (No. KYYW202208). We would like to thank Prof. Zhilu Yang for his help and guidance in this study.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110746.

  
  
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  Figure 1  The dissociation of PD coating and the in situ dynamic reassembly of PDPA coating under strong alkaline conditions. (A) Schematic illustration of the structure of the PD coating and its non-covalent interactions. (A-i) Under strongly alkaline conditions (NaOH solution, pH > 11.8), the PD coating dissociates, leading to the deconstruction of the coating and the formation of PD oligomers. (A-ii) Rapid covalent crosslinking occurs between the dissociated PD oligomers and PAH under the same alkaline condition (NaOH solution, pH > 11.8), resulting in the in situ dynamic reassembly and the reconstruction of the PDPA coating enriched with amino groups.

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  Figure 2  The influence of alkaline pH on the dissociation process of the PD coating and the formation of the PDPA coating. (A) The changes of the coating appearance, (B) thickness, (C) the surface amino group density of the PD samples, and (D) the concentration of PD oligomer in solutions after reaction with different concentrations of NaOH and NaOH + PAH solutions for 12 h. The concentration of PAH is constantly 2 mg/mL, and the concentration of NaOH is 0–40 mg/mL, the resulted pH of each solution was shown in Table S1. A pH of 12.7 was selected as an illustrative example to demonstrate (E) the FTIR spectra and (F) the high-resolution XPS spectra of C 1s, N 1s, and O 1s in PD and PDPA coatings. Data are presented as mean ± standard deviation (SD) (n = 4). One-way analysis of variance (ANOVA) with Tukey post hoc test was performed to determine the difference among various groups. n.s, not significant. P < 0.05, **P < 0.01.

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  Figure 3  The stability of PDPA coating versus PD coating under various conditions. The photographs of the PD and PDPA coatings after 24 h incubation under (A) high ionic strength using a saturated NaCl solution; (B) extreme pH conditions ranging of pH 1–4 and pH 11–14, depicting (B-i) the coating thickness and (B-ii) the concentration of PD oligomers in the reaction solution; and (C) oxidative conditions achieved by incubation with 0.035%–35% H₂O₂, the coating thickness was shown in (C-i). (D) The friction and wear comparison between PD and PDPA coatings conducted by testing with wet filter paper (0.01 N, 100 cm × 5 cm), performed the motion in a back-and-forth manner for 50 times. (E) The wear-resistance performance of PD and PDPA coated inferior vena cava filters evaluated through in vitro compression and placement with a sheath. Data are presented as mean ± SD (n = 4).

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  Figure 4  Schematic of functional molecules grafted to the PDPA coating surfaces via sulfhydryl (A, SH-FITC), NHS-ester (B, NHS-RhB), and carboxyl (C, SeDA) groups. (A-i) PDPA coating exhibited a higher amount of SH-FITC immobilization than PD coating. (B-i) The PDPA coating demonstrated successful immobilization of NHS-RhB on the surface of the PDPA-coated stent (right), in contrast to the PD-coated stent (left). (C-i) The release flux of surface-catalyzed NO was measured for PD-SeDA and PDPA-SeDA samples. Data are presented as mean ± SD (n = 4). One-way ANOVA with Tukey post hoc test was performed to determine the difference among various groups. ***P < 0.001.

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