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Published in final edited form as: Chem Eng J. 2023 May 24;468:143716. doi: 10.1016/j.cej.2023.143716

Acrylate monomer polymerization triggered by iron oxide magnetic nanoparticles and catechol containing microgels

Bo Liu a,b, Zhongtian Zhang b, Bingqian Li a, Qingping Liu a, Bruce P Lee b
PMCID: PMC10275414  NIHMSID: NIHMS1906492  PMID: 37334100

Abstract

Phenol and its derivatives are the most used polymerization inhibitors for vinyl-based monomers. Here, we reported a novel catalytic system composed of mussel inspired adhesive moiety, catechol, in combination with iron oxide nanoparticles (IONPs) to generate hydroxyl radical (•OH) at pH 7.4. Catechol-containing microgel (DHM) was prepared by copolymerizing dopamine methacrylamide (DMA) and N-hydroxyethyl acrylamide (HEAA), which generated superoxide (•O2−) and hydrogen peroxide (H2O2) as a result of catechol oxidation. In the presence of IONPs, the generated reactive oxygen species were further converted to •OH, which initiated free radical polymerization of various water-soluble acrylate-based monomers including neutral (acrylamide, methyl acrylamide, etc.), anionic (2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), cationic ([2-(methacryloyloxy)ethyl]trimethylammonium chloride), and zwitterionic (2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide) monomers. Compared with the typical free radical initiating systems, the reported system does not require the addition of extra initiators for polymerization. During the process of polymerization, a bilayer hydrogel was formed in situ and exhibited the ability to bend during the process of swelling. The incorporation of IONPs significantly enhanced magnetic property of the hydrogel and the combination of DHM and IONPs also improved the mechanical properties of these hydrogels.

Keywords: catechol, iron oxide, free radical polymerization, bilayer hydrogels, in-situ

Graphical Abstract

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1. Introduction

Phenolic compounds are the most used inhibitors for free radical polymerization of vinyl- and acrylate-based monomers.[1–3] These phenolic compounds usually work together with molecular oxygen to react with polymer alkyl radicals produced by the initiator and forms peroxy radicals.[4–6] The phenolic hydroxyl groups are susceptible to integrate with peroxy radicals to form the more stable radicals that terminate peroxy radicals.[7] This inhibition property is affected by the number of phenolic hydroxyl groups and electronic effect of aromatic substituents.[8] Catechol, an adhesive moiety found in mussel adhesive proteins, is a phenolic compound consisting of two hydroxyl groups in the ortho position.[9–12] Catechol is a well-known inhibitor and can react with the initiating radical through the formation of aryloxy free radicals.[13–15] Hence, the protecting groups like methoxy, borax, triethylsilyl, acetal etc. are often employed to chemically modify catechol to during the process of free-radical initiated polymerization.[16, 17]

Catechol can generate various types of reactive oxygen species (ROS) during different oxidation conditions such as autoxidation, chemical-mediated oxidation, and metal ion-mediated oxidation.[18–20] ROS are highly reactive radicals and non-radical derivatives formed from molecular oxygen. Common ROS are superoxide (•O2−), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (•OH).[21] Catechol generates •O2− during autoxidation, which can be further converted into H2O2 under basic condition.[22, 23] During iron oxide nanoparticles (IONPs) mediated catechol oxidation, •O2− can be converted into 1O2 at both acidic and basic pH ranges.[20] •OH is a highly reactive and potent oxidant. •OH can be produced by converting catechol-generated H2O2 in the presence of iron containing compound such as hematin via a Fenton-like reaction.[24] These ROS can be used for organic compound degradation, cancer therapy, and antimicrobial applications.[25]

The main objective of this study is to determine the feasibility of utilizing catechol-generated ROS to initiate free radical polymerization of a wide range of acrylate-based monomers. To this end, we exploited a metal-catechol two-component catalytic polymerization system that can produce the highly active •OH (Figure 1). In this design, catechol-containing microgel is used as the source of ROS generation. Catechol converts molecular O2 in an aqueous solution into •O2− and H2O2, successively, through autoxidation when the microgels were hydrated. The generated ROS can be further converted to •OH by IONPs through reacting with •O2− via the Haber–Weiss reaction or H2O2 via the Fenton reaction. As such, catechol and IONPs can form the redox polymerization initiators, which can catalyze the polymerization of various acrylate monomers into mechanically strong and highly stretchable hydrogels. More interestingly, the formed hydrogels were divided into two layers due to the settling of the microgel and IONPs. The layer thicknesses, actuation, and mechanical property of these hydrogels could be tuned with the content of IONPs. Finally, magnetic properties of the bilayer hydrogels were also investigated.

Figure 1.

Figure 1.

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(a) Schematic of hydrogels formation by IONPs and DMA/HEAA microgels, (b) overhead, (c) cross-section and (d) magnetic attraction of the resulting hydrogel, vial inversing testing for (e) AM, HEMA, NIPPAM and (f) AA, VBS and AMPS, (g) Applicable monomers for this catalytic system.

2. Materials and methods

2.1. Materials

IONPs, acrylamide (AM), methyl acrylamide (MAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (METAC), [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA), 1-vinyl-2-pyrrolidone (N-VP), N-hydroxyethyl acrylamide (HEAA), 2-hydroxyethyl methacrylate (HEMA), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt solution (AMPS-Na), acrylic acid (AA), N-isopropylacrylamide (NIPPAM), 4-acryloylmorpholine (AMP), 4-vinylbenzenesulfonic acid (VBS), 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS), and MQuant® Peroxide Test Strips were purchased from Sigma-Aldrich. 2,2’-Azobis(2-methyl-N-(2-hydroxyethyl)propionamide) were purchased from FUJIFILM Wako Pure Chemical Corporation. Polyethylene glycol dimethacrylate-8K (PEGDMA) was purchased from polymer science. Phosphate buffer saline (PBS), ferrous oxidation–xylenol orange (FOX) Assay Kit, hydroxyphenyl fluorescein (HPF) and dihydroethidium (DHE) were purchased from Thermo Fisher Scientific. Catechol-containing microgels were prepared by copolymerizing dopamine methacrylamide (DMA) with HEAA following published protocol.[23]

2.2. Hydrogel preparation by DHM and IONPs

Described below is the procedure for the synthesis of AP-1 hydrogel with AM and PEGDMA. Other monomer-based hydrogels were prepared using a similar procedure. AM (0.355 g, 5 mmol), PEGDMA (20.0 mg, 2.5 × 10−3 mmol) were first dissolved by 1 mL PBS (pH = 7.4) in a 5 mL vial. After that, DHM (20.0 mg) and IONPs (20.0 mg) were added into the solution and vortexed for disperse the solution for 1 min. The mixture was put into the oven, which was set as 70 °C. The solution fully changed to gel after 120 min. The hydrogel was flushed with deionized water 3–5 times. The mass of DHM was 20.0 mg, and the mass of IONPs were 5 and 10 mg for AP-0.25 and AP-0.5, respectively.

2.3. AP-0 Hydrogel preparation by 2,2’-azobis(2-methyl-N-(2-hydroxyethyl)propionamide)

AM (0.355 g, 5 mmol), PEGDMA (20.0 mg, 2.5 × 10−3 mmol) were first dissolved by 1 mL PBS (pH = 7.4) in a 5 mL vial. After that, 2,2’-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] (20 mg, 0.069 mmol) were added into the solution and the vortex was used for disperse the solution for 1 min. The mixture was put into the oven, which was set as 70 °C. The solution fully changed to gel after 120 min. The hydrogel was flushed with deionized water 3–5 times.

2.4. Methods

H2O2 characterization:

Twenty-five milligrams of DHMs (containing 10 mol % DMA) were added to 1 mL pH 7.4 PBS with and without 25 mg IONPs to determine the generation of H2O2. H2O2 generation was determined using FOX Assay Kit by following a published protocol using a Synergy Mx microplate reader (BioTek, VT). To determine if the catalytic system still generate H2O2 after hydrogel solidification, a MQuant® Peroxide Test Strip was brought into contact with the surface of the hydrogel.

•OH characterization:

Twenty-five milligrams of DHMs were hydrated in 1 mL pH 7.4 PBS with and without 25 mg IONPs. Three microliters of HPF (10 μM) were added to the microgel suspension while protected from ambient light. The microgel suspension was incubated for up to 24 h at 37 °C (in the dark) with gentle agitation on a shaking plate. Microgels were separated by centrifugation at 8000 rpm for 15 min and the fluorescence intensity was measured in the supernatant solution using the same microplate reader above with an excitation/emission wavelength of 490/528 nm.

•O2− characterization:

All the procedures were kept the same as those used in hydroxyl radical characterization besides two modifications. One modification was that HPF was replaced by DHE without changing the concentration. The other modification was using an excitation/emission wavelength of 518/606 nm for DHE detection.

FTIR:

The transparent layers were cut off after the hydrogels were fully swollen. FTIR of the lyophilized the transparent layers were tested on Shimadzu IRTracer-100 spectrometer.

FE-SEM and EDX:

To image the network morphology, hydrogels were freeze-dried, coated with platinum, and imaged using a FE-SEM in conjunction with EDX spectroscopy. (FE-SEM, HITACHI S-4700).

Swelling ratio:

Lyophilized hydrogels (diameter × height = 10 mm × 2.5 mm) were incubated in pH 3, pH 7.4 and pH 9 at 25 °C for up to 24 h. The mass of hydrogels in the swollen (Ws) and dried (Wd) states were used to determine the swelling ratio (SR) as calculated by: SR = (Ws-Wd)/Wd × 100%.

Finite Element Simulations:

Finite element simulations were conducted to predict and verify the shape evolution of the double-layered structures by using the commercially available finite element software ABAQUS (Dassault Systems). The geometrical configurations of simulation models were identical to that of the samples in the experiment. For both the black layer and the transparent layer, elastic modulus is kept constant, and the swelling ratio is the only variable and it changes over time. The shape morphing is predicted by the principle of thermal expansion, and the predefined thermal field could be utilized to tune the deforming curvature.

Stimuli-responsive shape transformations of bilayer hydrogels:

The bilayer hydrogel was cut into strips (length × height × width = 30 mm × 2.5 mm × 2.5 mm) and equilibrated in DI water at 25 °C for 24 h. The shape transformation of the hydrogel in these environments was recorded using iPhone 8. The bending angle of the bilayer hydrogel in the digital photo was measured, and the curve of bending angle versus time was plotted.

Magnetic property of hydrogels:

The magnetic properties of the hydrogels were observed visually with a magnet, and their magnetization was measured accurately using a vibrating sample magnetometer (VSM) (PPMS-9, Quantum Design, USA) at 300 K. The range of the applied magnetic field strengths was from −60 kOe to 60 kOe. The saturation magnetization and the coercivity were determined.

Rheometry:

The rheological properties of the hydrogels were examined using a TA Discovery Hybrid Rheometer-2. A cone-and-plate geometry with a 20 mm diameter plate, and 3 mm gap was used for all experiments on hydrogels. Hydrogels (diameter = 20 mm, thickness = 2.5 mm) were equilibrated in DI water with nutation for 48 h. The modulus was determined at an oscillatory strain range of 0.1% to 500% at a constant frequency (f) of 0.1 Hz. To evaluate the effect of molecular oxygen on the mechanical property of the solidified hydrogels, reaction mixture was frozen by liquid nitrogen, subjected to vacuum and backfilled with nitrogen gas 3 times. The reaction mixture was allow to cure under nitrogen at 70 °C for 120 min. Hydrogels were cut in to disc shape (diameter = 8 mm, thickness = 3 mm) equilibrated in DI water with nutation for 48 h, and subjected to oscillatory strain (0.1% to 500 %) at a constant frequency (f) of 0.1 Hz using a HAAKE MARS 60 rheometer.

Tensile:

The tensile test was performed on MTS-1025638 (MTS Systems Corporation). The long strip samples (length × height × width = 30 mm × 2~2.5 mm × 6 mm) with a gap length of 5 mm were stretched at a crosshead speed of 0.41667 mm/s. Tensile strain = (l - l0/l0) × 100%, where l is the stretch length and l0 is the gap length. Tensile stress = F/S (Pa), where F is the tensile force and S is the square of original cross-sectional area.

Compression:

Compressive stress-strain of hydrogels (diameter = 4 mm, thickness = 5 mm) were obtained by a mini compression test machine (Bose ELF3200, USA). The compression test was operated with a load cell of 10 N, the compression velocity was 0.1 mm/s. The stress and strain values presented were taken from 0% to 80% compressive strain.

3. Results and discussions

To initiate free-radical polymerization, DHM and IONPs were added to a precursor solution containing a monomer, AM, and a crosslinker, PEGDMA, in PBS (pH 7.4) (Figure 1(a)). The precursor solution solidified to form black and compact hydrogel after 120 min (Figures 1(b) and 1(c)). Interestingly, the whole polymerization process did not require removal of molecular oxygen from the reaction mixture. Due to the magnetic nature of IONPs, the obtained hydrogel could be easily abstracted by a magnet (Figure 1(d)). FTIR spectra of hydrogels exhibited the characteristic peaks at 3320 and 1620 cm−1 for AM that are associated with –NH2 and phenol –C=O stretching groups, respectively (Figure S1).

Various water-soluble acrylate monomers including neutral monomers (AM, MAM, AMP, NIPPAm, N-VP, HEMA, HEAA), anionic monomer (AMPS-Na), cationic monomer (METAC), and zwitterionic monomer (SBMA) were polymerized in the presence of DHM and IONPs, which confirmed the applicability of this catalytic system. However, anionic monomers without counter ions such as AA with a carboxylic acid group and 4-vinylbenzenesulfonic acid (VBS) or 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) with a sulfonic acid group did not polymerize using the same reaction condition (Figure 1(f)). This is mainly due to the pH change of the precursor solution to an acidic condition. Similarly, when the precursor solution pH was adjusted to pH 3, all the monomers (Figure 1(g)) did not polymerize. This indicated that the polymerization only occurred in the basic solution, which is necessary to promote catechol autoxidation and ROS generation.[26] Using a catechol-free HEAA microgel in combination with IONPs or DHM without INOP did not initiate polymerization. Therefore, the combination of catechol, iron, and pH were directly responsible for hydrogel formation.

The amount of •O2−, H2O2 and •OH generated by DHM with and without IONPs were determined by using DHE, FOX assay, and HPF assay, respectively (Figure 2). Upon hydration, DHM started to generate •O2− and the DHE fluorescence intensity was around 150 after 15 min and remained unchanged over 180 min. O2 oxidizes catechol to semiquinone and quinone, while generating •O2− by one-electron oxidation of catechol during the hydration.[27] Meanwhile, H2O2 was also detected from DHM. Concentration of H2O2 increased over time and reached a maximum concentration of around 2100 μM after 180 min. The H2O2 release profile was consistent with previously published report.[26] •O2− is generated during catechol oxidation, which further reacted with protons in the solution to form H2O2. However, •OH was not detected from DHM. When both DHM and IONPs were incubated together, there was a marked reduction in the amount of •O2− and H2O2 that were detected when compared to DHM alone. However, the HPF fluorescence intensity increased over 7 folds with the addition of IONPs. This clearly indicated that the generated •O2− and H2O2 were converted to •OH in the presence of the iron metal.

Figure 2.

Figure 2.

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(a) •O2− detection using DHE probe, (b) H2O2 generation, (c) •OH detection using HPF probe from microgels or microgels with IONPs hydrated in PBS with different time. (d) proposed mechanism of polymerization. * p < 0.05 when compared to DHM with different time.

Based on the characterization of different types of ROS generated, both catechol and IONPs are required to generate the potent •OH needed to initiate free radical polymerization. In the absence of IONPs, catechol autoxidize to generate •O2−, which can be further converted into H2O2 through further oxidation of catechol or reaction with proton ions.[22, 23] However, •OH was not detected from DHM alone. When IONPs was added to DHM, iron catalyzed the conversion of the catechol-generated H2O2 to •OH.[24] Additionally, catechol can be oxidized by Fe3+ to form a semiquinone radical intermediate. •O2− was obtained by the O2 reduction of semiquinone and Fe2+. Subsequently, •O2− was further reduced to H2O2, which is further reduced by Fe2+ to produce •OH (Figure 2(d)). •OH was the main initiator that induced the polymerization of the various monomers. The catalytic system continued to generate H2O2 even after the hydrogel had solidified after 120 min and over 3 mM of H2O2 was detected on the surface of the hydrogel (Figure S2). It was previously demonstrated that autoxidation of catechol results in sustained generation of H2O2 will cease to generate H2O2 after oxidation and crosslinking.[22, 28, 29] However, it is not clear whether the catalytic system can continue to generate •OH. Additional work is require to determine the duration the catalytic system could continue to generate •OH and initiate free radical polymer.

The effect of the mass ratio between IONPs and DHM (0.25:1, 0.5:1, and 1:1 denoted as AP-0.25, AP-0.5, and AP-1, respectively) on the formation of the cured hydrogel was further investigated using AM as the monomer and PEGDMA as the crosslinker (Table S1). All the three hydrogels were formed after 120 min of polymerization. For comparison purposes, the control hydrogel (AP-0) was formed through polymerization using 2,2’-azobis(2-methyl-N-(2-hydroxyethyl) propionamide) at 70 °C without IONP and DHM. Interestingly, AP-0.25, AP-0.5, and AP-1 formed bilayer structures consisting of a transparent upper layer and a black bottom layer, while the control hydrogel AP-0 was the homogenous transparent layer (Figure 3(a)). The bilayer hydrogels formed due to settling of IONPs and DHM due to the hydrophobic nature of IONPs and non-water solubility of microgels when the polymerization began. The monomers were polymerized immediately adjacent to the settled DHM and IONPs given the proximity to the catalyzing moieties. The transparent top layer also formed a crosslinked hydrogel, indicating that the chain propagation did not need to occur near the catalytic DHM and IONPs.

Figure 3.

Figure 3.

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(a) Optical images of hydrogels and FE-SEM images of (b) AP-0, (c) AP-0.25, (d) AP-0.5, and (e) AP-1 at (i) 30×, (ii) 250×, and (iii) 1000× magnification. (ii) and (iii) images for column (c), (d), and (e) are images of the black layer for DHM and IONPs initiated hydrogels. The double-sided arrows indicate the bottom, black layer of the hydrogel and the red circles indicate DHM.

The microstructures of these hydrogels were also confirmed by FE-SEM) (Figure 3. The control AP-0 exhibited a homogenous network structure typical of a crosslinked hydrogel. Hydrogels initiated by DHM and IONPs exhibited a bottom black layer and the thickness of this black layer increased with increasing INOP content. The black layer thickness in AP-1 was about 2.2 times and 1.3 times thicker when compared to those of AP-0.25 and AP-0.5 due to the higher IONPs content in AP-1 (Figure 3(c) (i), Figure 3(d) (i) and, Figure 3(e) (i)). Moreover, the spherical microgels were embedded into the black layers (Figure 3(c) (ii), Figure 3(d) (ii) and, Figure 3(e) (ii)). The transparent upper layers did not contain any microgels or IONPs and resembled porous network structure of a hydrogel. Energy-dispersive X-ray spectroscopy (EDX) analysis of AP-1 demonstrated that Fe elements from IONPs were distributed around the microgels in the black layer (Figure S3). No Fe element was detected in the transparent layer. This indicated that IONPs were attached to the surface of the microgels because of catechol’s affinity for Fe.

The magnetic properties of hydrogels were measured by VSM with a magnetic field range of −60 kOe to 60 kOe at 300 K (Figure 4). The saturation magnetization values for AP-0.25, AP-0.5, and AP-1 were 2.47, 1.28 and 0.52 emu/g, respectively (Figure 4). As expected, the magnetic intensity of the hydrogels increased with increasing content of IONPs. The hysteresis loop was from −0.1 kOe to 0.1 kOe, indicating that very low energy is required to reverse the magnetization.

Figure 4.

Figure 4.

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(a) Saturation magnetization values and (b) magnetic hysteresis loops of AP-0.25, AP-0.5 and AP-1 at 300 K.

The swelling ratios of hydrogels were around 13 to 18 after incubating the deionized water for 24 h. AP-0 exhibited the largest swelling ratio (18.2) since the hydrogel consisted of hydrophilic AM and PEGDMA. Swelling ratios for AP-0.25, AP-0.5, and AP-1 were 12.5, 13.2 and 14.1, respectively; because the more hydrophobic DHM and IONPs were embedded within the black layers (Figure 5(a)). Due to the swelling ratio differences within the bilayers, the dry AP hydrogels had an outward bend, arching the black layers of the materials after equilibrated in the deionized water (Figure 5(b)). Especially for AP-1, the curvature angel can reach up to 300° after 10 min of incubation. Water entered the porous structures in the black layers more easily, which was also consistent with 1 h swelling ratio results. As the swelling continued, the transparent layer absorbed more water than the black layer. As such, the curvature angel gradually decreased, and was only 45° after 24 h incubation (Figure 5(c)). Abaqus was used to simulate the deformation of AP-0 and AP-1. The results indicated that AP-0 remained mostly undeformed, while AP-1 deformed in the similar manner with the experiment results (Figure 5(d)–5(i)).

Figure 5.

Figure 5.

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(a) The swelling ratios; (b) images; and (c) the curvature angels of hydrogels equilibrated in deionized water for different time. Deformation simulation of (d-e) AP-0 and (f-i) AP-1 using Abaqus.

The mechanical property of AP hydrogels was also investigated. The storage modulus (G’) value was 1 order of magnitude higher than the loss modulus (G”) in all cases, indicating that all the hydrogels were fully crosslinked. IONPs-containing hydrogels (AP-0.25, AP-0.5, and AP-1) exhibited higher G’ values in the linear viscoelasticity regions when compared to that of the IONPs-free hydrogel AP-0 (Figure 6(a)). Similarly, the compression modulus of AP-0.25, AP-0.5, and AP-1 were 300.0, 295.7, and 265.4 kPa, respectively. These values were all much higher than 88.9 kPa of AP-0 (Figure 6(b)). Interestingly, AP-1 recovered automatically and rapidly to its original cylindrical shape over the multiple compressive cycles (Movie S1 and Figure S4).

Figure 6.

Figure 6.

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Mechanical properties of AP-X hydrogels. (a) Storage (G′, filled symbol) and loss (G″, open symbol) moduli, (b) compressive stress–strain curves, (c), (d) tensile stress and tensile strain curves of AP-X hydrogels, (e) fracture mode of AP-0 AP-0.25, AP-0.5 and AP-1.

Figures 6(c) and 6(d) show the typical tensile stress–strain curves of the hydrogels in uniaxial tensile tests at a speed of 120 mm min−1. IONPs free AP-0 exhibited a maximum tensile strain of 1094%, which was about 150% higher than IONPs containing hydrogels (Figure S5). The maximum tensile stress increased significantly with increasing IONPs content. Maximum tensile stress of AP-1 was 260 kPa, which was more than three times higher than that of AP-0 (85 kPa). It indicated the incorporation of DHM and IONPs improved the mechanical strength of these hydrogels only with a little sacrifice of elasticity because of the chelation effect between iron ions, catechol and amide moieties. The decreasing of elasticity was also agreed with the rheometry results. Notably, the bilayer hydrogel, AP-1 was fractured at the upper, transparent layer rather than the black layer potentially due to the higher strength of this layer (Figure 6(e)).

Overall, hydrogels prepared using DHM and IONPs demonstrated significantly higher mechanical properties when compared to those of the control hydrogel. Both DHM and IONPs functioned as fillers and chemical crosslinkers to improve the mechanical property of hydrogels. When comparing hydrogels of different compositions, increasing IONPs concentration resulted in a minor reduction in the compression modulus of the material (~35 kPa reduction when comparing AP-0.25 and AP-1). The modulus or stiffness of a hydrogel is directly correlated to its crosslinking density.[30] AP-1 demonstrated the highest swelling ratio and the lowest crosslinking densities among the 3 bilayer gels, which corresponded to a lower stiffness. The number of IONPs likely corresponded to the number of propagating polymer chains, and higher IONPs likely resulted in reduced chain length. Interestingly, the maximum tensile strength increased extensively with increasing IONPs content (~125 kPa increase when comparing AP-0.25 and AP-1). Additional investigation is required to further elucidate the cause of this observed increase in the strength of the bilayer hydrogel. This observed increase in tensile strength did not result in a large reduction of maximum tensile strain.

To investigate the effect of molecular oxygen on the quality of hydrogel formed, we attempted to remove oxygen from the reaction mixture by performing 3 pump-freeze-though cycles and backfilled the solution with nitrogen gas each time. The reaction mixture was kept under nitrogen atmosphere for 120 min at 70°C. However, the reaction mixture was still able to form a hydrogel. It is likely that we were not able to completely remove molecular oxygen from the reaction mixture. However, this gel exhibited a G’ value that was more than 60% lower when compared to a hydrogel that were solidified without similar oxygen-removal process (Figure S6). The reduced oxygen content resulted in a reduction in the stiffness and crosslinking density of the hydrogel. This indicated that molecular oxygen is needed to generate the ROS, that is required to initiate free radical polymerization.

Taken together, we combined the polymerization inhibitors, catechol and O2, with IONPs, to create a new redox initiator that can generate a highly reactive ROS, •OH. •OH was generated by IONPs-catalyzed oxidation of catechol and effectively initiate various acrylate monomers polymerization via a Fenton-like reaction. Compared with the typical free radical initiating systems, no extra initiators such as photo- or thermal initiators were required for polymerization. Unlike the other existing metal-catechol polymerization system[31–34], this presented polymerization occurred in a mild aqueous condition and without requiring additional source to generate free radicals. This differs from recent reports that combined metal ions (Fe3+ and Ag+) and catechol while using ammonium persulfate (APS) as added source of free radicals to initiate polymerization.[31] During the process of polymerization, DHM and IONPs settled to the bottom of the reaction mixture to generate bilayer hydrogel actuator. Comparing with the existing bilayer hydrogels which were usually fabricated in a two-step method[35, 36] or with some other supplementary device[37, 38], the recipe presented in this work was much simpler and tunable. The thickness of the two layers can be easily adjusted by altering the content of IONPs. In addition, the incorporation of DHM and IONPs into AM-based hydrogels exhibited an increase of mechanical properties. The bilayer hydrogels reported here can potentially function as programmable and self-bending actuators. Additionally, the magnetic property of the hydrogel will enable it to function as stimuli-responsive biomaterials and contrast agent for medical imaging [39, 40].

4. Conclusion

In conclusion, a novel catalytic polymerization system composed of IONPs and catechol containing microgels was reported. This combination initiated the polymerization of a wide range of monomers to form a hydrogel network in an oxygenated aqueous solution. Oxidation of catechol generated •O2− and H2O2, which was subsequently converted into •OH by IONPs in the presence of O2. Bilayer hydrogels were formed in situ and exhibited the ability to bend during the process of swelling. The incorporation of IONPs significantly enhanced magnetic property of the hydrogel and the combination of DHM and IONPs also improved the mechanical properties of these hydrogels. Finally, this manuscript reported an interesting phenomenon where catechol, a common free-radical inhibitor and retardant, is utilized to initiate polymerization in this newly reported catalytic system.

Supplementary Material

1
2
Download video file (12.2MB, mov)

Highlights.

  1. Novel catalytic system consisted of iron oxide magnetic nanoparticle and catechol

  2. Catechol-generated ROS further converted to hydroxyl radical by iron

  3. Initiates polymerization of wide ranges of monomers in an oxygenated solution

  4. Magnetic, bilayer hydrogels were formed with improved mechanical property

Acknowledgment

This project was supported by the Office of Naval Research under award numbers N00014-20-1-2230 and N00014-21-1-2877, the National Science Foundation under award number DMR 2001076, and the National Institutes of Health under award number R15GM135875.

Footnotes

Appendix A. Supplementary data

Supplementary data to this article can be found online at

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