Carbon coated Nickel Nanoparticles in Polyacrylamide Ferrogels: Interaction with Polymeric Network and Impact on Swelling 116 D O I: 1 0. 15 82 6/ ch im te ch .2 02 0. 7. 3. 04 E. A. Mikhnevich, A. P. Safronov, I. V. Beketov, A. I. Medvedev Chimica Techno Acta. 2020. Vol. 7, no. 3. P. 116–127. ISSN 2409–5613 E. A. Mikhnevich,a* A. P. Safronov,ab I. V. Beketov,ab A. I. Medvedevab a Ural Federal University, 620002, 19 Mira St., Ekaterinburg, Russia b Institute of Electrophysics UB RAS, 620016, 106 Amundsen St., Ekaterinburg, Russia *email: emikhnevich93@gmail.com Carbon coated Nickel Nanoparticles  in Polyacrylamide Ferrogels: Interaction with Polymeric Network and Impact on Swelling Polyacrylamide ferrogels with embedded magnetic nanoparticles of metallic nickel (Ni) and nanoparticles of nickel coated with a carbon shell (Ni@C) were synthesized by radical polymerization in water. The effect of the carbon shell on the interaction of Ni and Ni@C nanoparticles with polyacrylamide matrix and on swelling ratio of the ferrogels has been studied. The deposition of carbon on the surface of Ni nanoparticles worsens their interaction with polyacrylamide but at the same time elevates the water uptake by ferrogels. Keywords: nanoparticles; nickel; composites; ferrogels; polyacrylamide; carbon coatings. Received: 26.08.2020. Accepted: 06.10.2020. Published: 07.10.2020. © E. A. Mikhnevich, A. P. Safronov, I. V. Beketov, A. I. Medvedev, 2020 1. Introduction A hydrogel is a three-dimensional polymeric network swollen in water. The in- ternal structure of a hydrogel includes flex- ible polymeric sub-chains, which are cross- linked in a certain number of points. Due to the internal cross-linking the polymeric network of  a  hydrogel might be consid- ered as a combined huge molecule. From the thermodynamic point of view hydrogel is a solution of this molecule in water. De- spite the fact that it contains large amount of water, hydrogel is not a fluid but an elastic material maintaining its shape. Due to their unique properties such as mechanical elas- ticity, softness, and biocompatibility, hydro- gels have been applied in a variety of fields, including smart devices that respond to stimuli and soft actuators in biomedicine and agriculture [1, 2]. Hydrogels are often functionalized by  incorporating various physically, chemically, and biologically active moi- eties, which endows hydrogels with new functions, such as response to specific ex- ternal stimuli and increased mechanical stability [3]. Stimulus-responsive hydro- gels can markedly change their physical and/or chemical properties when exposed to external triggers (pH, temperature, light, magnetic or electric field) [4]. For instance, the  inclusion of  magnetic nanoparticles (MNPs) in such a polymer network gives magnetic hydrogels (ferrogels) that react to  an  external magnetic field [5]. Ferro- 117 gels are usually obtained by crosslinking hydrophilic monomers in a stabilized aque- ous dispersion of magnetic nanoparticles (ferrofluid), which, in turn, provides good dispersion of nanoparticles in a polymer matrix. One of the main advantages of fer- rogels over traditional stimulus-responsive polymers is that they can be remotely ac- tivated by a non-contact force (magnetic field). This unique property makes ferro- gels prospective advanced material in vari- ous fields such as drug delivery [6, 7], soft robotics [8], tissue reconstruction [9, 10] and environmental engineering [11, 12]. Huang et al. [9] reported about a hydro- gel formed by gelatin and β-cyclodextrin with embedded magnetic Fe3O4 nano- particles for pulsed electromagnetic field therapy. Chondrogenesis of mesenchymal stem cells grown on a magnetic hydrogel was enhanced by a magnetic field. Czichy et al. [13] investigated the  mechanical properties of alginate-methylcellulose hy- drogels containing magnetite nanoparticles for the use in the additive manufacturing of implants. The study was an introduction to further research on the effect of an ex- ternal magnetic field on the  mechanical stability of composites. The most extensive study on ferrogels was carried out by Zrinyi et al. [14–16] on magnetic silicone elasto- mers filled with carbonyl iron or micron- sized magnetite. The modulus of elasticity of these magnetoelastic gels was studied in both uniform and inhomogeneous mag- netic fields. The influence of the content of magnetic particles and their distribution at various combinations of magnetic field orientation and deformation on the modu- lus has been systematically studied. Mean- while, studies of synthetic water-based fer- rogels are very limited. In the literature most of the studies ad- dress ferrogels based on polyacrylamide (PAAm) chemically cross-linked poly- meric network. Thus, the series of works were performed by  the  group of  Galicia et al [17–19] on ferrogels with embedded maghemite nanoparticles. Maghemite na- noparticles were synthesized using ferro- fluid obtained by co-precipitation of iron oxides from Fe2+/Fe3+ mixed solutions. Fer- rogels had weakly cross-linked polymeric network with monomer-to-cross-linker ratio 1000 and more and contained up to 7% (vol.) of nanoparticles. The structure of ferrogels in the swollen state and the me- chanical properties were studied. It was shown that the Young modulus of ferrogels enlarged from 4 to 16 kPa with the increase in nanoparticles content. In our previous works [20–22] we have studied PAAm ferrogels with embedded iron and nickel nanoparticles, which were synthesized by the high-power pulsed physi- cal dispersion by the electrical explosion of  metal wire (EEW) [23]. The  magne- tostriction in the uniform magnetic field and the compression modulus of ferrogels were characterized. It was shown that fer- rogels with iron nanoparticles contract by approximately 9% (vol) in the uniform magnetic field 420 mT applied for 4 hours. Shear modulus of ferrogels with embedded iron nanoparticles increased from 0.5 to 2.5 kPa with the elevation of nanoparticles con- tent up to 4% (vol.) [21]. In PAAm ferro- gels with nickel nanoparticles it was shown [22] that the elastic modulus of ferrogels linearly depended on the content of the em- bedded nickel nanoparticles. The applied magnetic field 270 Oe in the parallel direc- tion to the compression increased the elas- tic modulus by 10% while the application of the magnetic field in the transverse direc- tion decreased the modulus. Although ferrogels with embedded metal nanoparticles show a potential for 118 their use as a smart material in soft sensors and actuators their application in bioen- gineering and medicine is  doubtful due to the toxicity of metallic iron and nickel. To overcome this shortcoming the surface of  metallic particles should be covered with a biocompatible layer, which prevents the contact of an open metallic surface with biological liquids. It our earlier report [24] it was shown that the  surface of  nickel nanoparticles produced via EEW can be modified by the in-situ carbon deposition. Such a deposition is provided by adding volatile hydrocarbons to the working gas of EEW unit. In the process of the electrical explosion hydrocarbon molecules decom- pose to elements and carbon deposits on the surface of condensing metal nanopar- ticles. The modification of the surface of nick- el nanoparticles opens a question on how would it influence the properties of ferro- gels with embedded metal nanoparticles. In  the  present study we aimed to  clarify two aspects of this possible influence. First, we will focus on the interfacial interaction between polyacrylamide and modified nickel nanoparticles. Second, we will ad- dress the influence of the carbon deposi- tion on the volume swelling of ferrogels, which is a basic property for their perfor- mance in sensors and actuators. 2. Experimental Part 2.1. Synthesis of MNPs, composites, and ferrogels Nickel magnetic nanoparticles (Ni MNPs) were synthesized by  the  electri- cal explosion of wire (EEW). The essence of EEW is the evaporation of a metal wire by  a  high-voltage electrical discharge in  an  inert atmosphere of  argon and the subsequent condensation of metal va- pors into spherical nanoparticles. Nickel nanoparticles coated with a carbon shell (Ni@C MNPs) were synthesized us- ing a mixture of argon with the addition of butane as a working gas of EEW unit. Carbon content in the Ni@C was 2% (wt.). The thickness of the carbon shell was 4–6 nm. The carbon was in an amorphous state. The details of the synthetic procedure can be found in our previous reports [25]. The MNPs composites for microcalo- rimetry studies were prepared in the en- tire range of  weight fraction of  MNPs. Linear polyacrylamide (PAAm) was syn- thesized by  free radical polymerization in  1.6 M water solution. Hydrogen per- oxide in 18 mM concentration was an ini- tiator of the reaction. Polymerization was done at two steps: first at 60 °C for 30 min, second at  100 °C for 60 min. Molecular weight of linear PAAm was 7.3∙105 as de- termined by viscometry of water solution at 25 °C. The stock solution of PAAm was then vigorously stirred with weighted amounts of Ni and Ni@C MNPs in pro- portions to get resulted composites with certain MNPs/PAAm ratio. The suspension of MNPs in PAAm solution was homog- enized by ultra-sound treatment and then cast upon Teflon plate and dried to the con- stant weight at 70 oC. The obtained films of NP/PAAm composites were then used for the  microcalorimetry measurements of the enthalpy of dissolution in distilled water. Ferrogels were synthesized by radical polymerization of acrylamide (AA) (Pan- reac Quimica SA) of in 1.6 M water so- lution. Methylene diacrylamide (MDAA) (MERCK) was a cross-linker in a molar ratio to monomer of 1:100. Ammonium persulfate (APS) was used as the initiator of the polymerization. Magnetic Ni/Ni@C 119 nanoparticles were added to the reaction mixture in portions of a weighed 20% wa- ter-based suspension stabilized with Dispex A40 dispersant (R. T. Vanderbilt). The sus- pension was homogenized in an ultrasonic bath for 20 minutes. Polymerization was carried out for 60 minutes at 80 °C. Syn- thesized ferrogels were washed in distilled water for two weeks with daily water re- newal to  remove residual impurities. The equilibrium swelling ratio (maximum water uptake) was determined as the ratio of the water content in the gel to the weight of the dry gel by weighing the swollen sam- ple of the gel and the dry residue after dry- ing to a constant weight at 70 °C. 2.2. Methods Transmission electron microscopy (TEM) was performed using a  JEOL JEM2100 microscope operated at 200 kV. The  specific surface area of  MNPs was measured via low temperature adsorption of nitrogen (Brunauer-Emmett-Teller ap- proach) using a Micromeritics TriStar3000 analyzer. Phase composition of MNPs was examined using an X-ray diffractometer Bruker D8 Discover operated at  Cu Ka radiation (wavelength l = 1.5418 A) with a graphite monochromator and a scintil- lation detector. XRD results were pro- cessed using the built-in Bruker software TOPAS-3 provided the Rietveld full-pro- file refinement. Magnetic hysteresis loops were measured using a vibrating sample magnetometer (Cryogenics). Calorimet- ric measurements were done at 25 °C us- ing a  Calvet differential microcalorim- eter of laboratory design. The sensitivity was 31.5 mV/W, cell volume was 10 cm3. The  stability of  a  baseline was ±0.5 mV. Enthalpies of  dissolution of  PAAm/Ni, PAAm/Ni@C, and PAAm/C composites in  water were measured using glass am- poule cells. Water was placed in a stainless steel cell and small portion of a composite film (approximately 10–30 mg by weight) was put into a thin-walled glass ampoule. This specimen was dried to  a  constant weight in vacuum and then the ampoule was sealed. A sealed ampoule was placed in the cell with water in it. After thermal equilibration until the baseline of calorim- eter kept at a constant level the ampoule was broken inside the  cell and the  heat effect of  the  dissolution of  a  composite in water was measured. The absolute values of measured heat effects of dissolution were from 0.5 to 2 Joule depended on the com- position of a composite. The absolute er- ror of calorimetric measurements was 0.05 Joule. 3. Results and discussion 3.1. Characterization of Ni and Ni@C MNPs Fig. 1 shows the transmission electron microscopy (TEM) images of Ni and Ni@C MNPs. Both batches of  MNPs contain individual spherical particles. A  typical diameter of spherical Ni MNPs (Fig. 1a) lay within 10–100 nm range. Deposited thin carbon layers are clearly observed on the  surface of  Ni@C nanoparticles (Fig. 1b). The average diameter of MNPs was es- timated based on the value of the specific surface area of  MNPs, which was deter- mined by  low-temperature nitrogen ad- sorption. The specific surface area of Ni and Ni@C MNPs obtained from the isotherms of nitrogen adsorption by the Brunauer- Emmett-Teller (BET) treatment were equal to 12.6 and 10.8 m2/g respectively. Straightforward geometrical considera- tion of the surface of a sphere related to its 120 mass gives the following simple equation for the diameter (d) of the sphere in rela- tion to its surface (S) and the density (r) of its material: 6 d S = r (1) If applied to  the  specific surface area of the MNPs in air-dry powder equation (1) gives the average value of the diameter for the ensemble of MNPs. If S is in m2/g and r is in g/cm3 equation (1) yields the av- erage diameter in micron. Calculated val- ues for Ni and Ni@C MNPs are present- ed in Table 1. The average diameters are close to each other. The average diameter of Ni@C nanoparticles is little higher than that for Ni nanoparticles. It is consistent with the  presence of  a  deposited carbon layer on the surface of Ni@C nanoparticles. The phase composition of Ni and Ni@C MNPs was characterized by X-ray diffrac- tion (XRD). According to X-ray powder diffraction data (Fig. 2), the Ni sample con- tained 100% α-Ni phase with a cubic face- centered lattice; the  unit cell parameter а = 0.3523(2) nm. XRD pattern for Ni@C MNPs also revealed 100% α-Ni phase with Fig. 1. TEM images of Ni (a) and Ni@C (b) MNPs. Table 1 Characteristics of magnetic fillers MNPs Density, g/cm3 Shape Ssp, m 2/g dav, nm *Ms, kA/m **Hc, kA/m Ni 8.9 spherical 12.6 58 454 20.7 Ni@C 8.9 spherical 10.8 62 339 7.7 * — saturation magnetization; ** — coercive force Fig. 2. XRD pattern of Ni MNPs 121 the unit cell parameter а = 0.3533(3) nm. Carbon layer was not detected in XRD pat- tern for Ni@C because the deposited layer was too thin and its content (2%) was be- low the sensitivity of XRD. Fig. 3. Shows magnetic hysteresis loops for Ni and Ni@C MNPs. In  both cases magnetization reached saturation in high field range. The  parameters of  magnetic hysteresis loops — saturation magnetiza- tion and coercive force are given in Table 2. According to  these data, Ni and Ni@C powders were soft magnetic materials with low coercive force. Saturation magnetiza- tion of  Ni@C nanoparticles was by  ap- proximately 25% lower than saturation magnetization of Ni MNPs. It was likely due to the distortions of the crystalline lat- tice in the surface layer of Ni@C particles under the influence of deposited carbon, which could happen because of limited dis- solution of carbon in the lattice. The  structural characterization of  Ni and Ni@C MNPs showed that these na- noparticles are very much alike despite the existence of the deposited carbon layer on the surface of Ni@C MNPs. Meanwhile, this factor was of decisive importance for the  interfacial properties of  polymeric composites with Ni and Ni@C MNPs. 3.2. Interfacial adhesion of  PAAm to MNPs The  basic thermodynamic property, which stands for the interaction of a poly- meric chain of composite with an embed- ded solid particle, is the enthalpy of inter- facial adhesion. The latter is the enthalpy change during the adsorption of a macro- molecule on a solid surface. This process establishes adhesive contacts at the inter- face between a macromolecule and a par- ticle due to molecular interactions. Such enthalpy change cannot be measured di- rectly in  calorimetric experiment as  Ni, Ni@C, and PAAm are solids. The enthalpy of interfacial adhesion was determined us- ing an appropriate thermochemical cycle (Hess cycle), which included quantities measurable in calorimeter. In case of pol- ymeric composites with embedded solid particles the enthalpy of interfacial adhe- sion is equal to the enthalpy of composite mixing. Since the solid particles have not dissolved in a polymeric matrix, the only source of enthalpy change is the interface interaction. The Hess cycle for the enthalpy of mixing of PAAm/Ni (or PAAm/Ni@C) composite constitutes the  combination of the following processes. 1)  PAAm + Ni MNPs = Composite PAAm/Ni + DHadh 2) PAAm + water (excess) = PAAm so- lution + DHdis,p 3)  Ni MNPs + water (excess) = Ni MNPs suspension + DHwet 4) PAAm solution + Ni MNPs suspen- sion = Ni MNPs suspension in PAAm solu- tion + DHmix 5) Composite PAAm/Ni + water (ex- cess) = Ni MNPs suspension in  PAAm solution + DHdis,c Fig. 3. Magnetic hysteresis loops for Ni and Ni@C MNPs. Inset — hysteresis loop in low field range for Ni MNPs 122 The combination of these steps gives for the enthalpy of adhesion: DHadh = wPAAmDHdis,p + wNiDHwet + + DHmix – DHdis,c (2) In  Equation (2) wPAAm and wNi are weight fractions of PAAm and Ni in a com- posite; DHdis,p is the enthalpy of dissolution of PAAm in water; DHwet is the enthalpy of  wetting of  air-dry Ni MNPs in  water; DHmix is the enthalpy of mixing of PAAm water solution with Ni MNPs water sus- pension; DHdis,c is the enthalpy of dissolu- tion in water for a composite with wPAAm and wNi composition. Fig. 4 (a) shows dependencies of the en- thalpy of  dissolution in  water versus the  weight content of  embedded parti- cles for PAAm composites. Together with PAAm/Ni and PAAm/Ni@C composites we also took as  a  reference PAAm/Car- bon composite which was prepared us- ing commercial sample of  carbon black with specific surface area 124 m2/g. The  value at  the  left-hand vertical axis corresponds to  the  enthalpy of  dissolu- tion of  PAAm in  water, which is DHdis,p. The  value at  the  right-hand axis corre- sponds to the enthalpy of wetting of air- dry particles, which is DHwet. The symbols at the field of the plot corresponds to DHdis,c values for the composites with certain frac- tions of PAAm and a filler. One can notice that the  dependence of DHdis,c for PAAm/Ni composites is con- vex upward, while the  dependencies of DHdis,c for PAAm/Ni@C and PAAm/ Carbon are concave downwards. The data presented in Fig. 4 (a) were used to calcu- late the  enthalpy of  interfacial adhesion in  composites DHadh using equation (2). It is worth noting that the values of DHmix (step (4) of  the  Hess cycle) typically can be neglected as they lay within the experi- mental error of the other quantities of Hess cycle. The dependence of the enthalpy of in- terfacial adhesion versus the content of sol- id particles in PAAm composites is shown in Fig. 4 (b). There is a substantial differ- ence between dependencies of the enthal- py of adhesion for PAAm/Ni and PAAm/ Ni@C composites. In  case of  PAAm/Ni composite the enthalpy of adhesion is neg- ative over the entire composition range. It Fig. 4. a) — Concentration dependence of the enthalpy of dissolution of PAAm composites with embedded Ni, Ni@C, and C particles; b) — Concentration dependence of the enthalpy of interfacial adhesion in PAAm composites with embedded Ni, Ni@C, and C particles 123 means that the adhesion contact between PAAm chain and the surface of Ni MNP is energetically favorable. Such interaction promotes adsorption of PAAm chains on the surface of Ni MNPs. On the  contrary, in  case of  PAAm/ Ni@C composite the  enthalpy of  adhe- sion is positive over the entire composition range. It means that PAAm chains do not interact with the surface of Ni@C MNPs. No doubts that such interaction is ener- getically unfavorable the deposited carbon layer on the surface of MNPs. The results for the  reference system PAAm/Carbon clearly confirm this fact. In the PAAm/Car- bon composites the enthalpy of adhesion is also positive over the entire composition range similar to the case of PAAm/Ni@C composites. Thus, the  obtained thermochemical data showed that the  deposition of  car- bon on the surface of Ni MNPs worsened the adhesion of PAAm chains to modified Ni MNPs. 3.3. Swelling of PAAm ferrogels with Ni and Ni@C Fig. 5 (a) shows the  dependence of the relative degree of swelling of PAAm/ Ni and PAAm/Ni@C ferrogels versus the weight fraction of MNPs in ferrogel. The relative degree of swelling is the swell- ing ratio (water uptake) of a ferrogel divid- ed by the swelling ratio of a blank PAAm gel with the same composition. The data presented in  Fig. 7 (a) refer to  gels with a network density of 100 monomer units in linear sub-chains per one cross-link. It is worth noting that the relative swelling decreases with MNPs content in the case of ferrogels with Ni MNPs, and it increases in the case of ferrogels with Ni@C MNPs. The same trend is shown in Fig. 5 (b), which presents the  swelling ratio of  fer- rogels with different network density containing the 3.1% (wt.) of MNPs. Both in the case of ferrogels with Ni MNPs and in the case of ferrogels with Ni@C MNPs the swelling ratio increases if the network density of  a  gel decreases. It is  a  general trend in  gels. If a  number of  monomer Fig. 5. a) — Dependence of the relative degree of swelling of PAAm ferrogels with Ni and Ni@C MNPs with respect to the unfilled gel matrix (monomer to cross-link ratio 100:1); b) — Dependence of the swelling ratio of PAAm ferrogels with Ni and Ni@C MNPs on the length of the linear sub-chains between the crosslinks (MNPs content 3.1%) 124 units in  linear sub-chains enlarge and the number of cross-links diminishes, then the network loosens and it can absorb more water. Thus the water uptake of a gel in- creases. Although the basic trend in the de- pendence of the swelling ratio on the net- work density is  the  same for both types of ferrogels, there is s remarkable difference in the absolute values of the swelling ratio. It is much higher in the case of Ni@C fer- rogels. In ferrogels with the lowest network density (300 monomer units per one cross- link) the swelling ratio of PAAm ferrogel with embedded 3% Ni@C MNPs is  four times larger than that for the ferrogel with the same composition but with embedded Ni MNPs. The  reason for such a  difference in  swelling of  ferrogels with embedded Ni or Ni@C MNPs apparently stems from the  different adhesion of  PAAm chains to the surface of particles. PAAm ferrogels differ from PAAm composites, because fer- rogels contain water besides polymer and MNPs. Nevertheless, interaction between PAAm chains and the  surface of  MNPs governs the  swelling of  ferrogel. If in- teraction between polymeric chains and MNPs is strong, then the PAAm sub-chains in the network are adsorbing on the surface of the particles. Adsorption of chains ef- fectively increases the networking density and diminishes the water uptake of the gel matrix. It is the case of PAAm ferrogel with enbedded Ni MNPs. In turn, if PAAm chains do not interact with the surface of MNPs, then the MNPs will effectively repel the  sub-chains of the networks. It will increase the mobil- ity of the network, and it will promote extra swelling of ferrogel. It is the case of PAAm ferrogel with embedded Ni@C MNPs. 4. Conclusions Metallic magnetic nanoparticles (MNPs) are prospective for the  use in smart soft materials suitable for bioen- gineering and medical applications. How- ever, to provide biocompatibility they need to be coated with protective layers. Depos- ited carbon coating is among them. Mean- while, deposition of carbon affects func- tional properties of MNPs and materials, in which they are embedded. In the present study the interfacial interaction of carbon coated Ni MNPs (Ni@C) with polyacryla- mide (PAAm) has been studied in com- parison with Ni MNPs focusing on appli- cation in PAAm ferrogels. The interfacial adhesion of PAAm chains to the surface of MNPs was studied by means of thermo- chemical cycle based on calorimetric meas- urements of  the  enthalpy of  dissolution of PAAm/Ni and PAAm/Ni@C compos- ites in water, and the enthalpy of adhesion to  the  surface of  MNps was determined in  the  entire range of  MNPs content in the composite. It turned out that while interaction of Ni MNPs with PAAm was energetically favorable and led to the evo- lution of heat (negative enthalpy change) during adhesion, deposited carbon layer provided poor interaction of PAAm with the surface of Ni@C MNPs. The enthalpy of interaction between PAAm and Ni@C MNPs was positive over the entire range of MNPs content. It means that PAAm do not interact with the surface coated with carbon layer. It provided a  marked im- pact on the swelling ratio (water uptake) of PAAm ferrogels with embedded Ni@C MNPs in comparison with ferrogels with embedded Ni MNPs. The  swelling ratio of  PAAm gel matrix diminished with the embedding of Ni MNPs due to the ad- sorption of PAAm sub-chains on the sur- 125 face of MNPs. On the contrary, the embed- ding of Ni@C MNPs in PAAm gel matrix resulted in the increase of its swelling ratio owing to the effective repelling of PAAm chains by the carbon layer on the surface of MNPs. It means that poor interaction at  the  interface not necessarily worsens the functional properties of soft biocom- patible materials. The increase of the swell- ing ratio might be an  advantageous fea- ture in  certain applications of  ferrogels in  biocompatible sensors and actuators. In a whole, obtained results showed that coating of the surface of MNPs not only provides biocompatibility of  MNPs but might as well be a versatile tool to modify the functional properties of smart materials based on nanoparticles. Acknowledgements The study was financially supported by RFBR, project number 19-38-90229. References 1. 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