Solvent-Free Graft-From Polymerization of Polyvinylpyrrolidone

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Solvent-Free Graft-from Polymerization of Polyvinyl Pyrrolidone Imparting Ultra-Low Bacterial Fouling and Improved Biocompatibility Min Sun, Haofeng Qiu, Cuicui Su, Xiao Shi, Zhijie Wang, Yumin Ye, and Yabin Zhu ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.9b00529 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 4, 2019

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ACS Applied Bio Materials

Solvent-Free Graft-from Polymerization of Polyvinyl Pyrrolidone Imparting Ultra-Low Bacterial Fouling and Improved Biocompatibility

Min Sun,ǂ, a Haofeng Qiu,ǂ, b Cuicui Su,a Xiao Shi,a Zhijie Wang,c Yumin Ye,*, a, d and Yabin Zhu*, b

a: Ningbo Key Laboratory of Specialty Polymers, Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China b: The Medical School of Ningbo University, Ningbo University, Ningbo 315211, P. R. China c: Key Laboratory of Semiconductor Materials Science, Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China d: State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China

*Please

address correspondences to:

Prof. Y. Ye Address: Ningbo Key Laboratory of Specialty Polymers, Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, China 1

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E-mail: [email protected] Prof. Y. Zhu Address: The Medical School of Ningbo University, Ningbo University, Ningbo 315211, P. R. China E-mail: [email protected]

KEYWORDS: initiated chemical vapor deposition, graft from, hydrophilicity, biofouling, biocompatibility

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ABSTRACT Polymer grafting has been a powerful tool in the surface modification of biomaterials. Traditional solvent-based grafting, however, often requires laborious procedures taken under harsh conditions, which seriously hinders its practical applications. Here we report a facile solvent-free graft-from method that is able to achieve superior surface functionality than most reported results from traditional solvent-based grafting. The grafting was proceeded by conformally coating a crosslinked polyvinyl pyrrolidone (PVP) prime layer in the vacuum, immediately followed by in situ grafting of PVP homopolymer chains from the propagating sites on the coating surface. The resultant coating exhibited enriched surface pyrrolidone content compared to the single-layer crosslinked counterpart and a water contact angle of 22˚, lower than most reported PVPgrafted surfaces. Medical catheters grafted with PVP achieved more than 99.9% bacterial antifouling enhancement compared to the pristine catheter, and significantly improved biocompatibility during a 4-week in vivo test in mice. The achieved surface functionality is attributed to the synergistic effect from the functional groups distributed both on the grafted chains and on the crosslinked primer. The effectiveness and simplicity of the vapor grafting method thus suggest a promising surface modification route for biomaterial and beyond.

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1. INTRODUCTION Implantation of biomaterials and biomedical devices in human body triggers foreignbody reactions that cause inflammation and impedance of the functioning of implanted devices.1-3 Recent studies suggested that the initial stage of foreign-body reaction is the biofouling of proteins, e.g. fibrinogen, on the biomaterial surface, followed by consequent adherence of macrophage and the subsequent cascade reactions that eventually leads to dense fibrous capsule formation.4,5 Fabricating surfaces that resist biofouling has thus attracted great attention recently for its applications in achieving more biocompatible materials. Highly hydrophilic surfaces with zero net charge have been reported to resist biofouling of proteins and bacteria6-8, which in turn results in reduced thrombus formation and foreign-body reactions9,10. To achieve ultra-low fouling surfaces, hydrophilic polymer coatings, such as polyethylene glycol11-14 and zwitterionic polymers,15,16 have been extensively explored. Concerning long term applications in vivo, it is essential that these coatings are stable under physiological environment. An effective strategy is to graft polymers on the surface of biomedical devices, through which polymer chains are covalently linked to the surface.17-19 The grafted chains generate a bound hydration layer that prevents the contact between foulants and surface, and induce a “cushion effect” repelling the approaching foulants.20 For example, zwitterionic coatings have been successfully grafted onto vascular catheters using a graft-from method to achieve alleviated thrombosis and infection.21 Traditional 4


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methods of polymer grafting, however, usually involve lengthy and complex procedures that require multistep treatment of substrates in different organic solvents at elevated temperatures, which seriously hindered its industrial applications. Biomaterial surfaces may suffer from pollutants in the solvent. The liquid-based methods may also not be applicable to substrates made of solvent-sensitive materials and substrates with delicate micro- and nanostructures. Over the recent years, vapor phase polymer deposition has emerged as an alternative route to functional polymer coating.22 Initiated chemical vapor deposition (iCVD) is a mild polymerization process that employs resistively heated alloy filament to generate free radicals from vaporized initiator, which subsequently polymerize monomers adsorbed on substrate surface, resulting in conformal polymer coatings.23,24 The iCVD method is especially suitable for creating functional biomedical coatings as it avoids the use of excessive energy such as plasma or high temperature; therefore perfectly capable of preserving the desired functional groups and enabling good stoichiometric control. Hydrophilic surface modification via iCVD has been reported to reduce surface fouling of proteins and bacteria.25,26 Direct grafting of linear hydrophilic polymers using iCVD, however, has not been reported, possibly due to the difficulties in creating sufficient initiation sites on the substrate prior to vapor deposition, which in turn results in rather low grafting density27. Instead, grafting of crosslinked hydrophilic polymer has been employed as an alternative to improve surface hydrophilicity and fouling 5



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resistance (Figure 1a).28,29 This strategy exploits the initiated sites on the pretreated substrate to covalently anchor a crosslinked polymer network on the surface. The functional moiety on the surface of crosslinked coating provides the desired surface functionality. The achieved surface functionality, however, is still limited, due to the rather high crosslinking degree of these coatings.30,31 Previously, we reported vapor phase grafting of crosslinked polyvinyl pyrrolidone (PVP) on glass slides to achieve enhanced surface hydrophilicity and fouling resistance compared to the pristine glass.32 The surface hydrophilicity along with fouling resistance was found to increase monotonically with the decrease of crosslinking degree. The most hydrophilic coating, hence with the lowest crosslinking degree, demonstrated a water contact angle of 33˚. Further decrease of crosslinking degree induced instability of coating, which thus limited its surface functionality. Since an ultra-low fouling surface is essential for good biocompatibility,10 here we attempt to overcome this limitation and further enhance the surface functionality using a novel solvent-free graft-from method. The grafting was implemented by first depositing a crosslinked PVP prime layer via iCVD, followed by in situ polymerization of PVP homopolymer on the top without disrupting the vacuum. The propagating chains on the prime coating surface served as the initiating sites for the subsequent grafting of PVP. The resultant coating thus consisted of a crosslinked prime layer and a top grafted PVP layer, as illustrated in Figure 1a The grafted PVP (gPVP) coating exhibited further improved surface hydrophilicity and superior fouling resistance 6


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compared to the single-layer crosslinked PVP (cPVP) coating. We then subcutaneously implanted gPVP-coated catheters in mice and examined its function through a time span of 4 weeks. Substantial alleviation of inflammatory response was observed on the surrounding tissue of gPVP coating compared to that of pristine and cPVP-coated catheters, confirming improved biocompatibility. 2. EXPERIMENTAL SECTION 2.1. Materials. Precursors of vinyl pyrrolidone (J&K, China, 99.5%), ethylene glycol diacrylate (EGDA, Acros Organics, USA, 90%) and tert-butyl peroxide (TBP, TCI, China, 98%) were used as received without further purification. 2.2. Solvent-Free Grafting. The grafting of PVP coating was carried out in a custombuilt reactor as described previously.32,33 Precursors of VP, EGDA and TBP were vaporized at 80 °C, 58 °C, and 25 °C, respectively, and fed into the reactor. The flow rates of VP, EGDA and TBP were adjusted to 5 sccm, 0.15 sccm and 0.6 sccm, respectively, using needle valves (Swagelok). During deposition, the substrate was kept at 35 °C by the circulating water located at the backside of the stage, while the filament was resistively heated to 260 °C. The pressure inside the reactor was maintained at 500 mTorr through the deposition process. Poly vinyl chloride (PVC) medical catheters (LingYang Medical Apparatus, China) were cut into pieces before coating, each with the length of 1 cm. Catheter tubes were then placed in a vacuumed container and treated with O2 plasma with the power of 500 W for 1 min to create surface hydroxyl groups. 7



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The resultant catheter tubes were subsequently placed in another vacuumed container right after, and treated with trichlorovinylsilane (TCVS, TCI, China, 98%) vapor for 30 min prior to deposition to generate surface vinyl groups. During deposition, the thickness of the coating was monitored in situ on a reference Si wafer by an interferometry system equipped with a 633 nm He–Ne laser (JDS Uniphase). After deposition, the obtained samples were immersed in deionized water for 30 s to remove ungrafted PVP chains, and subsequently dried using N2. 2.3. Characterizations. Fourier transform infrared (FTIR) spectra of coatings were obtained using Nicolet 6700 FTIR spectrometer equipped with a DTGS detector under transmission mode. The X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB 250Xi spectroscope. The surface topography of the coatings were observed using a Dimension 3100 atomic force microscope under tapping mode. The cross section of the coated catheter was observed under a Hitachi S-4800 SEM. Water contact angles (WCA) were measured using a goniometer (Kruss DSA 100, Germany) equipped with an automated liquid dispenser at room temperature. The volume of water droplets was 5 µl for each measurement, and the WCA of each sample was averaged from five measurements on different spots. 2.4. Bacterial Adhesion Test. Bacterial adhesion assay was performed on catheters using Gram-negative Escherichia coli (E. coli, ATCC 8739) as a model bacterium foulant. A single E. coli colony was cultured in Mueller-Hinton Broth (MHB) medium for 24 hours at 37 °C. The bacterial solution was re-inoculated into new MHB medium 8


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and cultured to approximately 1010 colony forming unit per ml (CFU ml-1). The MHB medium was then removed using centrifugation, and the precipitated bacterial cells were rinsed three times using PBS buffer solution. Finally, the bacterial solution was diluted to 106 CFU ml-1 with PBS, and placed in a 10 ml conical centrifuge test tube. Catheter tubes, pristine and treated, were then immersed in the bacterial solution and incubated at 37 °C for 4 h and 24 h. 10 ml bacterial solution was placed in each tube to ensure full contact of the catheters with solution during incubation. Catheters were then rinsed briefly with PBS buffer after incubation to remove the non-adhered bacteria. Subsequently, each catheter tube was placed in a 50 ml centrifuge tube containing 10 ml of PBS solution and sonicated for 10 s to remove the adhered bacteria from catheter surface. The resultant solutions were diluted to 101, 102, and 103 times in series, and 5 µl of each dilution was placed on a LB agar plate and incubated at 37 ℃ for 20 h. The number of colonies on the medium was then counted and the number of adhered bacteria was calculated. For each adhesion test, at least three catheter tubes were used for each kind. 2.5. In vivo Biocompatibility Test. All animal procedures were approved by and performed in the Animal Center of Ningbo University. C57BL mice weighing 20-30 g were used in this study. Three type of catheters, pristine, cPVP-coated, and gPVPcoated, were disinfected with 75% ethanol before implantation. For the surgical procedure, mice were anesthetized with sodium pentobarbital (0.3 wt%) by intraperitoneal injection. The surgical site on the dorsal skin was shaved and disinfected 9



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with 75% ethanol, and an incision measured approximately 1 cm in length was made on the skin. Catheters were inserted into the subcutaneous tissue subsequently. After implantation, the incision was closed using surgical suture. Each mouse received 1 sample, and at least 12 mice were assigned into one group carrying each kind of catheter. All animals were fed normally and kept in the Animal Center of Ningbo University. Daily monitoring was conducted to make sure that the wound was recovering and the implanted material was in place. At the time of 1, 2, 3, and 4 weeks after implantation, blood of three mice from each group was drawn at each time point to perform the blood routine examination, including white blood cells (WBC), lymphocytes (LYM), monocytes (MON), and granulocytes (GRA) data, which was used as an indication of body inflammation. Afterwards, three mice from each group were sacrificed to collect the implanted material with the circumferential tissue. These tissue samples were sectioned to 5 m on a microtome (CryoStar NX 50, Thermo Scientific Co., USA), stained with hematoxylin-eosin (H&E), and then observed under an optical microscope (Olympus, Germany) to evaluate the tissue response to the implants. 3. RESULTS AND DISCUSSION 3.1. Fabrication and Characterizations of gPVP Coatings. Solvent-free grafting was carried out in a two-staged iCVD process. As illustrated in Figure 1b, the initiator TBP, monomer VP, and crosslinker EGDA were simultaneously introduced into the 10


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vacuumed reactor at the first stage. TBP was thermally decomposed by the heated filament, which generated free radicals. VP and EGDA molecules were adsorbed on the substrate surface and polymerized by the free radicals, resulting in a crosslinked poly(vinyl pyrollidone-co-ethylene glycol diacrylate) (P(VP-co-EGDA)) prime layer. To provide more VP moiety on the surface, the crosslinking degree of the primer was fairly low, as to be discussed in the later sections. The flow of EGDA was then terminated at the second stage, while the flow of VP continued. As the vacuum in the reactor was not disrupted, the living surface radicals continued to propagate by consuming freshly adsorbed VP molecules, resulting in a thin layer of grafted PVP chains on the top surface (Figure 1a). Figure 1c shows the FTIR spectrum of the gPVP coating, along with iCVDsynthesized PVP and PEGDA homopolymer coatings. The vinyl bond stretching at 1630 cm-1 is absent in all spectra, confirming thorough polymerization.34 The carbonyl stretching band of EGDA moiety is observed at 1733 cm-1, while the carbonyl stretching band of VP moiety is observed at 1662 cm-1.35 The PVP spectrum also shows a broad absorption band centered around 3460 cm-1, which is attributed to the hydroxyl stretching from absorbed water.32 The spectrum of gPVP coating shows absorption bands of both carbonyl stretching and the hydroxyl stretching, indicating the incorporation of both moieties. To examine the effectiveness of grafting, we prepared a crosslinked PVP (cPVP) coating using the same deposition condition as gPVP but without the top grafted layer, 11



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and compared its XPS spectrum (Figure 2a and c) with that of the gPVP coating (Figure 2b and d). Elements of C, O, and N are observed in the two spectra, but the relative content of C and N (inset of Figure 2a and b) are clearly more abundant in the gPVP compared with that of cPVP. Indeed, the calculated surface atomic concentration of N in gPVP (11%) is much higher than that of cPVP (8.1%), and close to the theoretical N concentration in homopolymer PVP (12.5%), confirming a VP-enriched surface. Furthermore, the O 1s high resolution spectrum of gPVP coating (Figure 2d) exhibited much less –C–O*–C– content (534.5 eV)36 than that of the crosslinked counterpart. Since –C–O*–C– solely stems from the EGDA moiety, its higher concentration in cPVP proves a lower surface content of VP moiety than that of gPVP. The peak at approximately 533.2 eV likely stems from the absorbed water.37 It makes sense that the water oxygen peak from gPVP is stronger than that of cPVP, since gPVP has higher surface VP content, thus stronger hydrogen bonding with water. Figure 3 shows the 2-dimensional and 3-dimensional AFM images of the cPVP and gPVP coatings. The cPVP surface is fairly smooth with a root mean square roughness of 0.8 nm. Grafting of the PVP chains increased the surface roughness to 1.7 nm, which is due to the aggregation of grafted PVP chains on top of the crosslinked primer. The stronger interaction with water molecule resulted in higher surface hydrophilicity, which was verified by WCA measurement. The WCA on planar substrate (Si wafer) coated with cPVP is approximately 33˚ (Figure 4a), which is in 12


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accordance with our previous report.32 Grafting of the top PVP layer reduced its WCA to about 22˚, confirming further improved surface hydrophilicity. The gPVP coating was then applied onto medical catheter tubes via the same iCVD process. From SEM, a conformal coating with the thickness of approximately 100 nm can be clearly identified on the catheter surface (inset of Figure 4c). The coating conferred significant enhancement of surface hydrophilicity to the catheter tube. The pristine catheter is hydrophobic with a WCA of 104˚, and the WCA of gPVP-coated catheter reduces to 28˚ (Figure 4b). The gPVP catheter is also more hydrophilic compared to the catheter coated with cPVP, which shows a WCA of 38˚. PVP is a highly hydrophilic polymer with proved biocompatibility.38 Immobilization of PVP is expected to create biocompatible surfaces that are highly desirable in biomedical applications.39 Table 1 compares important characteristics of some typical PVP grafting methods in literature, and summarizes their representative results. Among the various methods reported, surface-initiated atom transfer radical polymerization (SI-ATRP) was reported to be effective in grafting dense PVP chains on different substrates.40-44 SI-ATRP, however, suffer from its low synthesis speed and its laborious processing procedures. The vapor phase grafting, on the other hand, is advantageous for its fast and facile processing and substrate-independent feature. Notably, the gPVP coating from this study also showed more abundant N concentration, i.e. higher VP content, and lower WCA than most reported PVP-grafted surfaces using SI-ATRP. The achieved surface functionality is attributed to the bilayer structure, 13



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where grafted polymer chains are anchored on a crosslinked polymer network instead of bare substrate (Figure 1a). Such bilayer structure is believed to yield higher surface functionality than single-layered counterparts, since both the crosslinked primer and the top grafted chains contribute functional groups. In contrast, traditional grafting of polymer generates a surface where polymer chains are directly anchored on substrate surface (Figure 1a). If the grafting density is not sufficient and/or the grafted polymer chains are not long enough, surface functionality is limited due to partial surface coverage of the polymer.45,46 3.2. In vitro Bacterial Adhesion. Bacterial adhesion on catheters poses a potential risk of infection after implantation. We assessed the bacterial adhesion on gPVP-coated catheter, and compared with that of pristine and cPVP-coated catheters. Figure 4e reveals the adhesion of E. coli on the three catheter samples after exposure to bacterial solutions for 4 h and 24 h. The pristine catheter showed adhesion of approximately 7.1×103 cells, while the cPVP catheter showed adhesion of approximately 4.1×102 cells, which was an almost 95% improvement. Remarkably, no bacterial cell was observed on gPVP catheter, demonstrating >99.98% improvement on fouling resistance compared to that of pristine catheter. Similarly, after 24 h exposure to the bacterial solution, the pristine catheters exhibited most serious bacterial adhesion with 3.5×103 cells, while the gPVP catheters showed adhesion of less than 10 cells. Such potent bacterial resistance is attributed to the increase of surface hydrophilicity of treated catheters. According to previous reports, biofouling decreases with the 14


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increase of surface hydrophilicity.20,32 For gPVP catheter, the hydrophilic VP moiety is highly-enriched on the surface. The VP moiety interacts strongly with water molecules, resulting in a hydration layer that prevents the contact of bacteria with substrate. The approaching of bacterial cells would also compress the grafted PVP chains and cause steric repulsion; therefore the adherence of bacteria would be thermodynamically unfavorable at the presence of grafted PVP.2047 The achieved fouling resistance is superior to most reported PVP-modified surfaces,36,37 and even comparable to the stateof-the-art anti-fouling and biocompatible surfaces modified with zwitterionic coatings and surface-tethered perfluorocarbon coatings.21,48 3.3. In vivo host inflammatory response. To verify if the biofouling resistance has improved biocompatibility of catheter, we subcutaneously implanted the gPVP catheter into mice, and assessed its host inflammatory response in a 4-week period. Figure 5 demonstrates the optical graphs of tissue surrounding the implanted catheters at different stages. Acute inflammatory response was observed in all three types of samples during the first week. The inflammatory response was most severe at the pristine catheter-tissue interface, where bleeding and inflammation can be clearly observed (Figure 5a). The inflammation persisted throughout the test, and a thick fibroblast capsule wrapping around the tube was generated after 3 weeks, indicating poor compatibility with tissue. Less serious inflammation occurred around the treated catheters during the first two weeks. For mice implanted with cPVP catheter, inflammation also persisted after 4 weeks. A fibroblast capsule wrapping around the 15



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catheter was still present after 4 weeks, but apparently thinner than that of the pristine catheter. In comparison, mice implanted with gPVP catheter showed least inflammation. Tissue surrounding the gPVP catheter became clear after 3 weeks, and no obvious capsule was observed, indicating good compatibility with tissue (Figure 5c). In order to further analyze the interaction between implants and tissue, we observed H&E-dyed tissue samples from the implantation site at each stage. After 1 week implantation, numerous inflammatory cells were observed on the tissue adjacent to the pristine catheter (Figure 6a). In contrast, inflammatory cells elicited by treated catheters appeared to be much less. The alleviated inflammation is likely due to the decreased adsorption of proteins and attraction of macrophages. After then, the number of inflammatory cells continued to increase around the pristine catheter, and a thick dense capsule was eventually formed. Inflammatory cells were also observed around the cPVP catheter after 2, 3, and 4 weeks (Figure 6b), but the amount of cells was much less. Tissue surrounding the gPVP catheter showed least inflammatory response, which was in agreement with the optical observation. Very few inflammatory cells were observed at all stages. The encapsulation still occurred, but the capsule was fairly thin and loosely distributed and resembled normal extracellular matrix. After implantation, proteins are adsorbed on biomaterial surface within microseconds.49 When cells reach the surface, the adhesion receptors recognize the proteins and attach themselves via ligand-receptor binding sites, which triggers the cascade of foreign-body reactions.5 The grafting of hydrophilic polymer induces the 16


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generation of a bound hydration layer in the vicinity of surface, which inhibits the initial irreversible adsorption of proteins; thus mitigates the foreign-body reactions. We have previously demonstrated that grafting crosslinked PVP coatings rendered reduced adsorption of proteins and bacteria.32 From this work, we have proved that the enhanced fouling resistance did lead to improved biocompatibility compared to the control. It is, however, apparent that the inflammatory response of cPVP-coated catheter was still substantial, possibly due to the limited VP content present on the surface, thus inadequate fouling resistance. Grafting PVP on top of the crosslinked primer further enhanced surface hydrophilicity and fouling resistance, and enabled significantly improved biocompatibility of the coating. To quantitatively examine the inflammatory response against implants, blood samples were drawn from mice implanted with pristine and gPVP catheters at each stage, and routine blood tests were performed.50,51 The concentration of blood cells, including WBC, LYM, MON, and GRA, was measured to characterize the inflammatory status of the hosts. While each individual mouse may exhibit varied host responses, the averaged value obtained from different samples implanted with the same kind of catheter can be considered as an indication of the general host response to the foreign body. The WBC counts of mice implanted with both pristine and gPVP catheters showed a rapid increase after the first week, due to the acute inflammatory response (Figure 7a). This trend decelerated after the first week for mice implanted with gPVP catheters, but not for the pristine sample. This can be explained by the weaker 17



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foreign-body response induced by the gPVP catheter, which is in agreement with the histological analysis. The WBC counts for both samples dropped after 4 weeks, but the count for the pristine sample was still higher than that of the gPVP sample. The drops in both cases were likely due to the formation of collagen capsules formed around the implants, resulting in reduced immunological responses. Although the gPVP catheter elicited a much thinner capsule, it evoked a milder immunological response than the pristine as evidenced by the lower WBC count. Other cell counts showed similar trends as WBC, and all confirmed improved biocompatibility of the gPVP coating. 4. CONCLUSIONS In summary, we report a facile solvent-free grafting method that is able to create ultralow biofouling and biocompatible surfaces. The grafting was implemented by first deposing a crosslinked PVP prime layer, followed by in situ grafting of PVP from the propagating sites on the primer. The resultant bilayer structure enabled abundant surface VP content, which imparted high surface hydrophilicity and ultra-low biofouling to medical catheters. The achieved surface functionality was comparable to the best reported results from PVP-grafted surfaces using SI-ATRP. The ultra-low biofouling was proved to be essential in achieving biocompatible surfaces, as the gPVP catheter showed significantly improved biocompatibility compared to the cPVP catheter, although latter also exhibited moderate biofouling resistance. The solvent-free grafting is suitable to arbitrary substrates disregarding their composition and geometry, and avoids potential pollutants from solvents. The vapor-based process is scalable and 18


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highly efficient. The reported method thus finds promising applications in biomaterials and biomedical devices, as well as other surface and interface engineering occasions.

ACKNOWLEDGEMENTS We are grateful for the funding support from the National Key Research and Development Program of China (2017YFA0206600), National Natural Science Foundation of China (51873093, 81471797), Technology Foundation for Selected Overseas Chinese Scholars by Ministry of Personnel of China, and Ningbo “3315” Innovation Initiative. This work was also sponsored by K. C. Wong Magna Fund in Ningbo University.

Supporting Information Additional experimental data of the XPS C1s and N1s high resolution spectra of cPVP and gPVP coatings and the stability test results of the gPVP coating are provided in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

Notes The authors declare no competing financial interest.

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Figure 1. a) Schematic illustration of traditional grafting of linear PVP chains on bare substrate surface, grafting of crosslinked PVP network, and grafting of bilayer gPVP coating; b) illustration of the two-staged iCVD process of gPVP coating and the molecular structures of VP, EGDA, and TBP; c) FTIR spectra of gPVP and iCVDsynthesized PEGDA and PVP homopolymer coatings.

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Figure 2. XPS spectra of a) cPVP and b) gPVP coatings; and O1s high resolution spectra of c) cPVP and d) gPVP coatings. Inset of a) and b) are the N1s high-resolution spectra of cPVP and gPVP coatings, respectively.

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Figure 3. 2-dimensional AFM images of a) cPVP and c) gPVP coatings; and 3dimensional AFM images of b) cPVP and d) gPVP coatings. The scanned areas were 5×5 µm2.

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Figure 4. a) Water contact angles of pristine Si wafer, Si wafer coated with cPVP and gPVP; b) water contact angles of pristine catheter, catheters coated with cPVP and gPVP; c) cross-sectional SEM images of gPVP coating on catheter; d) representative images of E. coli colonies formed on the agar plates from 4 h bacterial fouling tests of pristine, cPVP, and gPVP catheters; and e) number of bacteria adhered on pristine, cPVP, and gPVP catheters after 4 h (solid) and 24 h (striped) exposure to bacterial solutions. After 4 h exposure to bacterial solution, the gPVP coating enabled zero bacterial adhesion.

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Figure 5. Optical graphs of tissue surrounding: a) pristine, b) cPVP, and c) gPVP catheters after 1, 2, 3, and 4 weeks implantation.

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Figure 6. Micrographs of H&E stained tissues surrounding: a) pristine, b) cPVP, and c) gPVP catheters after 1, 2, 3, and 4 weeks implantation. The red arrows indicate the interface between the implanted catheter and the surrounding tissue.

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Figure. 7. a) WBC, b) LYM, c) MON, and d) GRA counts of mice implanted with pristine and gPVP catheters after 0, 1, 2, 3, and 4 weeks.

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TOC Graphic

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Table 1. Comparison between the important characteristics of PVP grafting methods in literature and their representative reported results.

Methods

SI-ATRP

Solventfree

Substrateindependence

×

×

Room temperature processing

×

Synthesis speed (nm/min)

Conformal coating

10-2

√

Representative results Substrate

Surface N%a

WCA (˚)b

PDMS41

11.37

32

Glass42

7.1

22

Si43

>6.67

24

Au44

8.74

28

UV-assisted CVD

√

√

√

~10

×

PMA52

N/A

36

iCVD vapor grafting (this work)

√

√

√

~10

√

Si/catheter

11

22/28

a).

All data were obtained from XPS analysis. Varied experimental errors may exist in XPS results due to the carbon contamination and

water adsorption from atmosphere. b).

All data were static water contact angles, except for the one in row 5, which was the receding water contact angle.

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Solvent-Free Graft-From Polymerization of Polyvinylpyrrolidone

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