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A Single Additive Enables 3D Printing of Highly-Loaded Iron Oxide Suspensions Amin Hodaei, Omid Akhlaghi, Navid Khani, Tunahan Aytas, Dilek Sezer, Buse Tatli, Yusuf Z. Menceloglu, Bahattin Koc, and Ozge Akbulut ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00551 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Applied Materials & Interfaces

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Article type: Research Article

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‘’A Single Additive Enables 3D Printing of Highly-Loaded Iron Oxide Suspensions’’

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Amin Hodaei, Omid Akhlaghi, Navid Khani, Tunahan Aytas, Dilek Sezer, Buse Tatli, Yusuf Z.

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Menceloglu, Bahattin Koc, and Ozge Akbulut*

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Amin Hodaei, Dr. Omid Akhlaghi, Dr. Navid Khani, Tunahan Aytas, Dilek Sezer, Buse Tatli, Prof.

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Yusuf Z. Menceloglu, Prof. Bahattin Koc, Prof. Ozge Akbulut

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Faculty of Engineering and Natural Sciences, Sabanci University, Orhanli-Tuzla, Istanbul, 34956,

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Turkey.

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Dr. Navid Khani, Prof. Bahattin Koc

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3D Bioprinting Laboratory, Sabanci University Nanotechnology Research and Application

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Center, Orhanli-Tuzla, Istanbul, 34956, Turkey.

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*Correspondence and requests for materials should be addressed to Ozge Akbulut (email:

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[email protected])

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ACS Paragon Plus Environment

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Abstract

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A single additive, a grafted copolymer, is designed to ensure the stability of suspensions of highly-

3

loaded iron oxide nanoparticles (IOPs) and facilitate 3D printing of these suspensions in the filament

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form.

5

[3(dimethylamino)propyl]methacrylamide (DMAPMA) and acrylic acid (AA) harnesses both

6

electrostatic and steric repulsion to realize an optimum formulation for 3D printing. While used at 1.15

7

wt. % (by the weight of IOPs), the suspension attains ~81 wt. % solid loading—96% of the theoretical

8

limit as calculated by the Krieger-Dougherty equation. Rectangular, thick-walled toroidal, and thin-

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walled toroidal magnetic cores and a porous lattice structure are fabricated to demonstrate the

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utilization of this suspension as an ink for 3D printing. The electrical and magnetic properties of the

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magnetic cores are characterized through impedance spectroscopy (IS) and vibrating sample

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magnetometry (VSM), respectively. The IS indicates the possibility of utilizing wire-wound 3D printed

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cores as the inductive coils. The VSM verifies that the magnetic properties of IOPs before and after the

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ink formulation are kept almost unchanged due to the low dosage of the additive. This particle-targeted

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approach for the formulation of 3D printing inks allows embodiment of a fully aqueous system with

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utmost target material content.

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Keywords: 3D printing, iron oxide, colloids, magnetic nanoparticles, inductors, suspensions

This

poly

(ethylene

glycol)

(PEG)-grafted

18

19

20

21 2


copolymer

of

N-

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1. Introduction

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3D printing has manifested its potential to print functionality along with form;1-2 thus gone beyond

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prototyping and found several applications in microfluidics,3-5 energy storage/conversion,6-7

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electronics,8-11 and building hierarchical structures.12-13 A particular sub-branch of 3D printing is

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extrusion-based printing of metallic and ceramic colloidal materials, where highly concentrated

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suspensions of particles (40–55 vol. %)14 are deposited to construct near-net shaped parts with complex

7

geometries.2 For metals and ceramics, extrusion-based 3D printing requires inks with tailored

8

rheological properties to enable proper deposition through a nozzle and to maintain the shape of the

9

extrusion thereafter.15 Up to date, there are only a few number of metallic (e.g., silver16-18) and ceramic

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(e.g., silicon carbide,19 alumina,20 barium titanate21, and zirconia22) suspensions that have been printed

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through this process. To form a robust filament usually dispersants (e.g., poly(acrylic acid)23 and

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ammonium polyacrylates24), polymeric binders (e.g., poly(vinyl butyral)25), and surfactants (e.g.,

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dimethyl methyl phosphonate26 and butyric acid27) are utilized. These formulations rely mostly on the

14

electrostatic repulsion to sustain the stability and controlling the printing parameters of the

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suspensions. There is also a single report that employed a comb polymer, thus combined steric

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hindrance with electrostatic repulsion to prepare an alumina ink.20 The studies on the stability of

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colloidal systems underscore the benefit of harnessing both electrostatic and steric repulsion;28-29

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however, the translation of this knowledge to 3D printing has not yet been fully achieved. The limited

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portfolio of printable materials can be potentially expanded by employing a particle-specific design,

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which exploits both types of repulsion, to attain more robust suspensions.14

21

Realization of large-area magnets,30 transformers,31 magnets with complex shapes,32 constant-flux

22

inductors,33 and electrochemical devices34 necessitate magnetic inks. Unfortunately, until now, 3



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polymer-based composites of Neodymium Iron Boron (NdFeB) family,30, 32, 34 iron,31, 33 and NiCuZn

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ferrite33 are the only magnetic systems that are used as inks in extrusion-based 3D printing. The

3

presence of high amounts of additives (5̶–8 wt. %33 to ~69 wt. %, calculated34) intrinsically lowers the

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loading of magnetic particles and limits the performance of fabricated objects due to the reduction in

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the magnetic material filling factor.35 To be utilized as inks, polymer-based magnetic composites require

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heating to impart flowability to the system. These heat treatment steps might be detrimental for certain

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magnetic materials; for instance, iron oxide (Fe3O4) goes through a phase transition at 200 °C that

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changes its magnetic properties.36

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Here, we report the preparation and 3D printing of highly-loaded iron oxide (Fe3O4) inks through the

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minimum use of a single additive that is specifically designed for iron oxide. Iron oxide is a soft magnetic

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material with relatively high saturation magnetization, low electrical conductivity, high permeability,

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and easy magnetization/demagnetization properties which make it a material of choice for applications

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such as the cores of inductive coils,37 drug delivery systems,38 and wastewater treatment.39 We

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systematically pursued a particle-specific approach to design an additive that can cater the surface

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charge and its distribution in IOPs and can provide both electrostatic and steric repulsion to stabilize

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and control the viscosity of suspensions of IOPs. We synthesized poly(ethylene glycol) (PEG)-grafted

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copolymers of N-[3(dimethylamino)propyl]methacrylamide (DMAPMA) and acrylic acid (AA) and

18

investigated the effect of i) comonomer ratios, and ii) the density of PEG side chains on the stability of

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suspensions. The optimized ink contained ~81 wt. % of IOPs in the presence of 1.15 wt. % of a single

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additive (by weight of IOPs, hereafter referred to as wt. %) in a fully aqueous medium. To demonstrate

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the printability of various geometries, we printed three different shapes of magnetic cores (rectangular,

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thick-walled toroidal, and thin-walled toroidal cores) and a porous lattice structure. We characterized

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the electrical and magnetic properties of the magnetic cores through impedance spectroscopy (IS) and 4


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vibrating sample magnetometry (VSM), respectively. To the best of our knowledge, this fully aqueous

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ink is the first of its kind for extrusion-based 3D printing in terms of comprising a magnetic material of

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choice at its highest loading through the minimum use of a single additive.

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2. Experimental Section

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2.1 Materials:

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Iron (II, III) oxide nanoparticles (50–100 nm, 97%), acrylic acid (AA 99%), N-[3(dimethylamino)propyl]

7

methacrylamide (DMAPMA), and potassium peroxydisulfate (KPS, ≥ 99.0%) were obtained from Sigma-

8

Aldrich. Poly(ethylene glycol) (PEG, MW=1000 g mol-1) and maleic anhydride (MA, 99 %) were

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purchased from Merck. All of the chemicals were used without further purification. All of the solutions

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were prepared using distilled water with resistivity of 18.2 MΩ cm. The enameled copper wires (with

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diameters of 0.2 and 0.3 mm) were purchased from Emtel Enamel Wire and Cable Industry Co., Turkey.

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2.2 Synthesis of the Additives:

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The esterification of PEG-1000 by MA was carried out via the procedure that was proposed by Lu et

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al.40 and the product (MAPEG) was used for polymerization without any purification. In a typical

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aqueous free radical polymerization of DMAPMA:AA:MAPEG with a molar ratio of 25:25:1, a 0.05 mole

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of DMAPMA, a 0.05 mole of AA, and a 0.002 mole of MAPEG (MW≈1100 g mol-1) were dissolved in 100

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ml of distilled water. Subsequently, we charged this mixture into a 250 ml three-neck flask that is

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connected to a reflux condenser. Under nitrogen purge and magnetic stirring, the temperature of the

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mixture was raised to 50 °C. At 50 °C, 75 mL KPS (initiator) aqueous solution (1 mol. % of the total mole

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of the comonomers) was dropwise added to the reaction chamber in 10 min. The temperature of the

21

reaction was increased to 70 °C, and thereafter, the reaction continued for 12 hours. All of the other 5



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additives were also synthesized through the same procedure changing the molar ratio of the building

2

blocks (Table 1). Finally, we cooled the reaction down to room temperature, and the products were

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precipitated in either ethanol or acetone and dried under moderate vacuum at 60 °C for 24 hours.

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2.3 Preparation of the Aqueous Single Additive Ink:

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We first dissolved the copolymers in 25 g of distilled water, then suspended the IOPs in these solutions.

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A 2 g of IOPs was added slowly to this solution and the mixture was mechanically stirred for 15 min.

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Then, at 5-minute intervals, portions of 2 g of IOPs were further added to the mixture to reach the

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target solid content. The mechanical stirring of the mixture continued for another 24 hours at 400 rpm.

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We took 1 g of inks with a 5-mL syringe and immediately placed the sample into a moisture analyzer

10

(Shimadzu uniBloc MOC63u) to measure its solid content—the solid contents that are listed in this study

11

are based on these measurements.

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2.4 3D Printing of Highly-Loaded Iron Oxide Ink:

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Magnetic cores were 3D printed with the use of the ink that contains 81 wt. % of IOPs in the presence

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of 1.15 wt. % of the optimized additive through an in-house developed 3D printer (SU3D) (Figure S5,

15

Supporting Information). More details about the hardware of this equipment can be found

16

elsewhere.41-42 Magnetic cores, a porous structure, and thin-walled ‘’SU’’ letters were pneumatically

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printed onto a glass substrate through a510 μm-diameter nozzle with a speed of 10 mm s-1. The 3D

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printed objects were dried under ambient conditions without any sintering processes.

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20

6


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2.5 Zeta Potential Measurements:

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A Zeta potential analyzer (Zetasizer nanoseries, Malvern Instruments, Ltd.) equipped with a 633 nm

3

laser and scattered light detector at a constant angle of 173° was employed to record the electro-kinetic

4

behavior of IOPs in the presence of different amounts of additives applying Smoluchowski

5

approximation. The suspensions with 1 mg of nanoparticles in 100 ml of distilled water were

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ultrasonicated for 5 min in a bath sonicator. Then, the solutions of additives are added to this

7

suspension to reach a concentration of 0.2 to 2 wt. % of copolymer and these mixtures were

8

ultrasonicated for 5 min followed by mechanical stirring for 10 min. Six measurements, each with at

9

least fifteen runs, were conducted at 25 °C and the average value of these six separate measurements

10

was reported as the Zeta potential at a given concentration of the additive.

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2.6 Adsorption Behavior of the Additives:

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Adsorption behaviors of the additives were investigated via thermogravimetric analysis (TGA) (Netzsch

13

STA 449C Jupiter thermal analyzer). All suspensions were prepared by 10 min ultrasonication of 0.5 g

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of IOPs in 40 g of distilled water that contains different amounts of additives followed by 15 min of

15

mechanical stirring. Thereafter, the mixtures were centrifuged at 5000 rpm for 30 min. The sediments

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were then redispersed in distilled water and these suspensions were centrifuged for 30 min at 5000

17

rpm. Afterwards, the sediments were again collected and dried at 60 °C for 24 h. The dried samples of

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~ 40 mg were heated up under air atmosphere from room temperature to 800 °C with the heating rate

19

of 10 °C min-1. The amounts of the adsorbed additives were measured by comparing the weight loss of

20

the samples with that of the bare IOPs under the same thermal conditions. Each point of the adsorption

21

graph is the average of at least three measurements (error deviation of the measurements was ±0.05

22

wt. %). 7



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2.7 Potentiometric Titration of the Solutions of Additives:

2

We performed potentiometric titration with a HI-2211 bench top pH meter on 50 ml solution of 1 mg

3

mL-1 additives/water at 22 ± 2 °C. The pH of the solutions was adjusted by a 0.1 M HCl and these

4

solutions were then titrated with a 0.1 M NaOH.

5

2.8 Studying the Interaction of the Additives with Fe3+ Ions:

6

In order to investigate the interaction of the additives with Fe3+ ions ICP-OES (Varian, Vista-pro) was

7

used.43 For a typical sample preparation, 15 mL aqueous 1 mg mL-1 solutions of the additives that

8

contain 0.001 mol of Fe3+ ions were realized. After 30 minutes of magnetic stirring, 5 mL of 1 M NaOH

9

solution was added dropwise to these mixtures to precipitate the free Fe3+ ions. The mixtures were

10

then centrifuged at 5000 rpm for 30 min and the supernatant was separated from the brownish-colored

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precipitate. Thereafter, the Fe3+ concentration in the supernatant was measured by ICP-OES (Table S1,

12

Supporting Information).

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2.9 1H NMR and 13C NMR:

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The chemical characterization of the additives was carried out by nuclear magnetic resonance (NMR,

15

Varian Unity Inova 500 MHz spectrometer) in D2O.

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2.10 Gel Permeation Chromatography:

17

The additives were characterized by gel permeation chromatography (GPC, Agilent 1260 Infinity

18

equipped with a refractive index detector) in an aqueous solution of 0.14 mol L -1 NaCl, 0.01 mol L-1

19

Na2HPO4 and 0.01 mol L-1 NaNO3 at a flow rate of 0.7 mL min-1.

20 8


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2.11 Rheology of Iron Oxide Suspensions:

2

The rheological measurements were performed with an Anton-Paar MCR 302 rheometer with parallel

3

plates of 25 mm diameter and a gap size of 0.5 mm. After loading of each sample, a thin layer of low-

4

viscosity paraffin oil was employed to protect the samples from adsorption of humidity at the outer

5

edges of the plates. In dynamic regime, the frequency was set to 10 rad s−1 and strain was changed from

6

0.01 to 1000 % to find the range of linear viscoelastic region. In steady-state tests, the shear rate ranged

7

from 0.01 to 100 s−1.

8

2.12 Field Emission Scanning Electron Microscopy (FESEM):

9

To analyze the morphologies of the outer surface and the fractured surface of the printed structures,

10

FESEM was performed using a Leo Supra 35VP FESEM at an accelerating voltage of 4 kV and a working

11

distance of 8 mm. All of the samples were coated with platinum before measurements.

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2.13 Impedance Spectroscopy:

13

The electrical behavior of the inductors that were fabricated by enameled copper wire winding of the

14

printed cores was investigated via an impedance analyzer (Solartron analytical 1260, Impedance/Gain

15

Phase Analyzer). The inductors were connected to the impedance analyzer and their inductance and

16

resistance were recorded while sweeping the frequency from 0.1 to 10 MHz. An applied initial voltage

17

of 1V was used for all of the measurements.

18

19

20 9



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2.14 Magnetic Characterization:

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Magnetic properties of bare IOPs and the dried powder form of the ink that contained the optimized

3

additive were measured by a vibrating sample magnetometer (VSM) under a maximum applied

4

magnetic field of 11 kOe at ambient temperature.

5

3. Results and Discussion

6

3.1. Design of Chemical and Structural Properties of the Additives

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The surface of IOPs holds hydroxyl functional groups44 which can accommodate the anchoring of

8

chemical agents with certain functionalities (e.g. amine, carboxylic acid, etc.).45-46 IOPs have been

9

usually stabilized with the aid of homopolymeric additives such as dextran47, PEG,48 poly(vinyl

10

alcohol),49 poly(ethyleneimine),50 poly(vinylpyrrolidone),51 and poly(acrylic acid).52 A few systems with

11

copolymers, for instance, poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylic acid),53 poly(2-

12

vinylpyridine-grad-acrylic acid),54 polyamidoamine-graft-poly(ethylene glycol)/dodecyl amine,55 and

13

poly(vinyl alcohol-co-vinyl amine)56 have also been studied. In this work, we have chosen DMAPMA,

14

which is a polar monomer that contains a tertiary amine functional group, as one of the backbone

15

monomers. Tertiary amine groups act as proton acceptors and lone pair donors (Brönsted and Lewis

16

bases),57 and in an aqueous medium, they can be protonated by the donation of protons from water.58

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Moreover, alkyl groups that are attached to nitrogen atom stabilize its positive charge59. The amine

18

functional group adsorbs onto the surface of IOPs via electrostatic interactions60. The other

19

comonomer, AA, bears a carboxylic acid functional group that has a strong complexation ability with

20

the surface of iron oxide.61 As grafts, we have used maleic anhydride esterified PEG (MAPEG) since it is

21

water-soluble. The general chemical structure of the copolymers is shown in Figure S1 (Supporting 10


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1

Information). The molar feeding ratios of the building blocks are listed in Table 1 and the compositions

2

of the grafted copolymers were tracked by 1H NMR and 13C NMR (Supporting Information Figure S2a

3

and S2b). The amount of AA was raised to find the optimum ratio of AA/DMAPMA in additives (DMA50

4

to DMA20, hereafter referred to as DMA series). Moreover, we increased the feeding molar ratio of

5

PEG side chains to track the steric hindrance effect of the additives (P5 and P10). The ratio of

6

AA/DMAPMA and the characteristic properties of the grafted copolymers are shown Table 1 columns

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3–7.

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Table 1. The molar feed ratios and the characteristic properties of the synthesized additives.

Polymer

mmol anionic sites

Molar feed ratio DMAPMA/AAa)

acronym

9 10

a)

PEG/(DMAPMA+AA)a)

(DMAPMA/AA/PEG)

Number average PDI molecular weight

per mg of solidb)

(Mw/Mn)c) (Mn) (g/mol)c)

DMA50

50:0:1

–

0.27/50

N/A

14,596

1.49

DMA40

40:10:1

0.95

0.82/50

4.7 × 10-3

16,214

1.53

DMA30

30:20:1

0.93

0.90/50

4.9 × 10-3

20,138

1.86

DMA25

25:25:1

0.72

0.42/50

5.9 × 10-3

27,262

2.33

DMA20

20:30:1

0.98

0.41/50

4.4 × 10-3

65,635

2.99

P5

25:25:5

0.88

1.74/50

5.6 × 10-3

34,654

1.63

P10

25:25:10

0.97

2.87/50

5.3 × 10-3

27,358

1.19

Determined by 1H NMR; b) Calculated from direct addition of NaOH to the solutions of additives by

titration; c) Measured by gel permeation chromatography.

11

11



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3.2. Electro-kinetic Behavior and Adsorption Study

2

We measured Zeta potential of IOPs at different pH and in the presence of varying amounts of additives

3

to track the electro-kinetic behavior of the IOPs (Figure 1a and 1b). The hydroxyl functional groups on

4

the surface of IOPs can be protonated/deprotonated depending on the pH of the medium resulting in

5

the change of surface charge of the nanoparticles.44 The bare IOPs showed a negative Zeta potential of

6

~─29 mV at pH 7.2 (Figure S3, Supporting Information). Upon the adsorption of additives at low

7

dosages, Zeta potential of the bare IOPs increased to positive values (+20 mV +30 or +30 mV (Figure 1a), ii) showed the highest affinity of adsorption and also the highest amount

21

of adsorption to the surface of IOPs (Figure 1b), and iii) provided the IOP suspensions with the lowest

14


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viscosity response as a function of shear rate among all other additives (Figure 2a), we singled it out for

2

further studies.

3

In order to avoid inadequate or excess addition of additives, we optimized the amount of P10 by

4

investigating the change in viscosity of 55 wt. % suspensions with different amounts of this additive (1–

5

1.3 wt. %) as a function of shear rate. As shown in Figure 2b, the viscosity of suspensions decreased

6

with the additive content up to 1.15 wt. % and increased beyond this point indicating an optimum

7

dosage of 1.15 wt. % to achieve a highly-dispersed suspension.

8

We monitored the viscosity and viscoelastic properties of suspensions with varying IOP loadings and

9

optimum dosage of the additive (1.15 wt. %) to assess the printability of highly-concentrated

10

suspensions (Figure 2c and 2d). Increasing the loading of IOPs between 55–81 wt. % in the presence of

11

the optimum dosage of the additive, the viscosity of the suspensions elevated by more than three

12

orders of magnitude in the shear rate range of 0.01─100 s-1, and all of the samples exhibited shear-

13

thinning behavior (Figure 2c). Moreover, they demonstrated an elastic behavior (i.e., storage modulus

14

(G’) > loss modulus (G’’) by almost two orders of magnitude) up to the crossover point of storage and

15

loss modulus (G’=G’’) (Figure 2d). The stress at the crossover of G’ and G’’ is assigned as the yield stress

16

of the suspensions.70 The yield point of the suspensions is a measure for determining the suitability of

17

a given nozzle geometry for the formation of extrudate in 3D printing. If the yield stress exceeds the

18

maximum shear stress at the wall of the nozzle, the ink enters a plug flow regime where the flow

19

behavior of the ink and its subsequent filament formation is hard to control.70 Therefore, we

20

determined the critical condition for the most suitable shear flow during 3D printing using Equation

21

(1):71

22

τ = ( )r

∆P

(1)

2L

15



Page 16 of 28

1

where ΔP (Pa) is the maximum pressure applied at the nozzle, r (m) is the radius of the nozzle, and L

2

(m) is the length of the nozzle. We calculated the maximum shear stress at the wall of the nozzle using

3

Equation (1) as 1657.5 Pa (ΔP = 195 × 103 Pa, r = 255 × 10-6 m, and L = 15 × 10-3 m). By the increase in

4

the loading of IOPs from 55 wt. % to 81 wt. %, the yield stress of the suspensions changed from 9.7 to

5

984.5 Pa (Figure 2d)—the yield stress of the ink with 81 wt. % loading of IOPs was still below the stress

6

that will cause the plug flow (Figure 2e). This value is close to the maximum loading of IOPs in the

7

suspensions, ~84.4 wt.%, as calculated by the Krieger–Dougherty equation. Hence, we chose the ink

8

with the highest IOP loading, ~ 81 wt. %, for the 3D printing of the magnetic cores (Supporting

9

Information, Figure S7).

16


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1

2

Figure 2. a) Viscosity change in the 55 wt. % IOP suspensions in the presence of 1 wt. % of different

3

additives, b) The change in the viscosity of the suspensions of IOPs (55 wt. %) in the presence of 1─1.3 17



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wt. % P10 c) The viscosity as a function of shear rate response of IOP inks at different particle loadings

2

in the presence of the optimum amount of P10 (1.15 wt. %), d) Oscillatory rheological measurements

3

(frequency = 10 rad s-1) of IOP inks of different particle loadings ranging from 55 wt. % to 81 wt. %, and

4

e) The yield stress of the IOP inks assigned as the crossover of G’ and G’’ curves (the dotted line shows

5

the transition from shear to plug flow region).

6

3.4. 3D Printing of IOP ink and Characterization of Its Magnetic and Electrical Properties

7

We printed three different shapes of cores: i) rectangular shape of 24×5×3 mm, ii) thin-walled, and

8

thick-walled toroidal shapes of 20×18×12 and 16×8×8 mm (outer diameter×inner diameter×height),

9

respectively, and iii) a porous lattice structure (Figure 3). The printed cores were wound with 60 turns

10

of 0.2 mm thick (for thin-walled toroidal shape) and 0.3 mm thick (for rectangular and thick-walled

11

toroidal shapes) copper wires (Supporting Information, Figure S5).

18


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1

2

Figure 3. 3D printing of the magnetic cores, a) Thick-walled toroidal, b) Thin-walled toroidal, and c)

3

Rectangular shapes; d) Dried cores after printing, e) One-layer thick pattern of ‘’SU’’ letters, f) A porous

4

structure, and g) SEM of the fractured surface of the porous structure that is shown in “f”.

5

6

19



Page 20 of 28

1

To compare the magnetic properties of the bare IOPs and the ink, we measured the room temperature

2

magnetic hysteresis (M–H) loops of the as-received particles and the dried sample of the ink that

3

contains 81 wt. % IOPs and 1.15 wt. % of P10) (Figure 4). Both samples exhibited almost the same

4

magnetic behavior, constructed narrow (M–H) loops, and reached a saturation magnetization that is

5

typical for soft magnetic materials.72 This observation confirms that the ink formulation that is used in

6

this study does not affect the magnetic behavior of the nanoparticles.

7

8

Figure 4. Room temperature magnetic hysteresis (M–H) loops of the bare IOPs and the dried sample of

9

the ink that is used for printing.

10

The electrical behavior of the wire-wound cores was tracked by measuring the inductance and

11

resistance of these inductors as a function of frequency (0.1 MHz–10 MHz) by impedance spectroscopy

12

(Figure 5a and 5b). In general, soft magnetic materials are used as the materials of the cores in inductors

13

to increase the inductance.73 Expectedly, the inductance of all the inductors was enhanced due to the

14

coupling between the wire turns in the presence of the magnetic cores. However, inductance decreased

15

at higher frequencies; this behavior can be attributed to factors such as electrical currents induced in

16

the cores (eddy-current loss), displacement currents in the cores, and magnetic hysteresis losses 74. 20


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1

Moreover, the resistance of the inductors increased with frequency up to 10 MHz. This increase is

2

related to the skin effect and proximity effect as the other sources of loss.75

3

To investigate the performance of the inductors as electrical components, the quality factor (Q) of the

4

inductors, a measure of efficiency was calculated using the Equation (2): 76

5

Q=

6

where L is inductance (H), R s is series resistance (Ω), and f is the frequency of operation (Hz) (Figure

7

5c). For all inductors, Q exhibited peaks at ~31. The change of maximum values of Q depends on the

8

sources of resistance (i.e., the losses of the core and the losses of the wound wires). Since the material

9

of the cores (Fe3O4) and wires (copper) were the same for all inductors, observation of similar values

10

for the peak of Qs was expected. The thick-walled toroidal core inductor provided higher Q values

11

compared to that of other shapes at a relatively broader range of frequency; thus, possessed a wider

12

operating range of frequencies. In summary, these inductor setups indicated that there is a high

13

potential to manufacture magnetic cores in various geometries through 3D printing, which can find

14

different applications in the wide field of electronic and electromechanical systems.

2πfL

(2)

Rs

21



Page 22 of 28

1

2

Figure 5. a) The frequency response of the inductance, b) The change in the resistance as a function of

3

frequency, and c) The frequency dependence of the quality factors of the inductors with 60 turns of

4

wire.

5

4. Conclusions

6

Highly-loaded suspensions that are to be used as inks for 3D printing can only be achieved through

7

careful tailoring of additives that can cater the properties of each type of particle. The level of loading

8

that is attained in this work eliminates the need for material removal steps and offers, for the first time

9

in literature, a magnetic ink that contains highest loadings of particles with minimum amount of a single

10

additive. In addition, this fully aqueous system has the potential to pave the way for domestic printing 22


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1

of ceramics. The particle-specific approach that is described for iron oxide provides a solid route to

2

expand the limited portfolio of 3D printing inks.

3

Supporting Information

4

The chemical structure of the additives, 1H NMR and 13C NMR spectra of the optimized additive used

5

for iron oxide ink formulations, variation of the Zeta potential of iron oxide nanoparticles as a function

6

of the pH of the medium, the photo and an SEM micrograph of a 3D printed porous lattice structure,

7

the photos of the room temperature dried 3D printed cores before and after wire winding, the photo

8

of the custom-made multi-head 3D printer that was used for the extrusion-based 3D printing of iron

9

oxide inks, the ICP-OES results that show the complexation of the additives with iron ions in aqueous

10

solutions, and calculation of the maximum loading of iron oxide nanoparticles by the Krieger-Dougherty

11

Model are supplied as the Supporting information.

12

Acknowledgments

13

The funding of this project was provided by Sabanci University. The authors wish to thank Prof. Dr.

14

Nergis Arsu and Tuğçe Çinko (Yildiz Technical University, Istanbul, Turkey) for GPC; Prof. Dr. Esra

15

Alveroğlu (Istanbul Technical University, Istanbul, Turkey) for VSM, and Dr. Mustafa Atilla Yazıcı and

16

Burçin Yıldız (Sabanci University, Istanbul, Turkey) for ICP-OES and NMR, respectively.

17

Competing financial interests

18

The authors declare no competing financial interests.

19

23



1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

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