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Periodic Surface-Ring Pattern Formation for Hydroxyapatite Thin Films Formed by Biomineralization-Inspired Processes Yulai Han, Tatsuya Nishimura, Misato Iimura, Takeshi Sakamoto, Chikara Ohtsuki, and Takashi Kato Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02126 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 3, 2017

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Periodic Surface-Ring Pattern Formation for Hydroxyapatite Thin Films Formed by Biomineralization-Inspired Processes Yulai Han,†,§ Tatsuya Nishimura,*,†,|| Misato Iimura,† Takeshi Sakamoto,† Chikara Ohtsuki,‡ and Takashi Kato*,†

†

Department of Chemistry and Biotechnology, School of Engineering, The University

of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. ‡

Department of Materials Chemistry, Graduate School of Engineering, Nagoya

University, Chikusa-ku, Nagoya 464-8603, Japan. §

Current address: School of New Materials and New Energies, Shenzhen Technology

University, Shenzhen 518118, P.R. China. ||

Current address: School of Chemistry, College of Science and Engineering, Kanazawa

University, Kakuma-machi, Kanazawa 920-1192, Japan. *E-mail: [email protected] *E-mail: [email protected] Supporting Information

1

ACS Paragon Plus Environment

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ABSTRACT

Surface morphology is a key factor that might significantly influence the properties of biomaterials. In this study, periodic surface-ring structures have been constructed for calcium phosphate thin films via biomineralization-inspired crystallization process. The patterned octacalcium phosphate crystals have been obtained on poly(2-hydroxyethyl methacrylate) matrix in the presence of poly(acrylic acid)(PAA). The patterned surface morphologies of the crystal thin films could be tuned by the amount of PAA additives. In addition, the rapid and topotactic transformation to hydroxyapatite (HAP) thin films with surface-ring structures has also been achieved. This study may provide new strategy towards the design of functional calcium phosphate-based thin-film hybrids.

INTRODUCTION

In nature, many kinds of self-organized patterns are formed. Some of these periodic relief morphologies exhibit important functions for the living organisms, such as insect

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bite avoidance, social interactions, or camouflage against predator through background matching.1 Great efforts have been directed toward the understanding of how nature controls these pattern formation. For example, the reaction-diffusion models proposed by Alan Turing2 are widely recognized to explain the spontaneous formation mechanism of relief patterns.3,4 For synthetic materials, organic/inorganic hybrid structures with patterned

morphologies,

which

are

formed

by

biomineralization-inspired

crystallization5-7 are also quite attractive, because these morphologies in micro- and nano-meter scale might be significantly useful for optical and biological devices.8-12 In view of the importance of surface patterned morphologies, intensive studies on the preparation of calcium carbonate based thin films with various surface morphologies has been carried out. For instance, optical lithography has been used for the pattern formation of calcium carbonate thin films.13,14 This method led to the formation of precisely patterned structures with several hundred micrometers in width, which are depended on the employed photomask. These pattern morphologies of calcium carbonate films were demonstrated as a 2D model substrate for bone-cell cultures.14 However, it was difficult to prepare fully covered calcium carbonate films with

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patterned morphologies with this process, because the development process removed polymer resin that was important to calcium carbonate crystallization. In our previous studies, CaCO3 crystal thin films with three-dimensional concentric relief structures have been synthesized via biomineralization-inspired crystallization, which exhibited structural color due to their diffraction grating structure.15-17 Moreover, a study carried out by S. F. Wang et al. shows that the adhesion, spreading and proliferation of the mouse pre-osteoblastic MC3T3-E1 cells could be remarkably enhanced by the relief structures on CaCO3 crystal thin films.18 These works reveal that the surface morphologies of calcium-containing crystal thin films are of critical importance, and might greatly influence the behavior of cells on the thin films. However, synthesis of ordered HAP thin films with controlled and homogeneously distributed surface ring pattern in vitro is still a challenging target due to the diversity of calcium phosphate polymorphs and their sensitivity to pH and ion concentrations. Our approach here is to develop pattern formation of hydroxyapatite (HAP) using functionalized polymer. HAP, the least soluble and the most thermodynamically stable calcium orthophosphate crystal phase under physiological conditions,19 has been viewed

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as the model mineral compound of bones and teeth.20,21 If a new process to introduce controllable surface morphologies on this biocompatible material could be achieved, this will have a major impact on the areas of optical and bio functional materials. The preparation of HAP thin coatings with various surface morphologies have been carried out for several decades.11,12,22-32 However, synthesis of ordered HAP thin films with controlled and homogeneously distributed surface topography in vitro is still a challenging task due to the diversity of calcium phosphate polymorphs and their sensitivity to pH and ion concentrations.33 Previously, we have reported synthesis of HAP crystal thin films with flat surface.34 The essential points were the use of amorphous calcium phosphate to tune the crystallization of calcium phosphate35,36 and the crystallization of octacalcium phosphate (OCP) as a precursor for HAP.37,38 We found that bioinspired crystallization of calcium phosphate through amorphous calcium phosphate led to the formation of thin-film structure with oriented OCP nanorod crystals. The HAP/PVA hybrid thin film was formed after the transformation without the change in the original OCP/PVA nanostructures.34

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Herein, we report the synthesis and surface morphological control of calcium phosphate crystal thin film for developing new functional materials. To functionalize the thin film as opt- or biomaterials, we focus on the combination of poly(acrylic acid) (PAA)

additives

and

poly(2-hydroxyethyl

methacrylate)

matrices

for

the

biomineralization-inspired synthesis of HAP thin films with surface-ring structures via OCP phase (Figure 1).

EXPERIMENTAL SECTION

Materials

Poly(2-hydroxyethyl methacrylate) (Mv =300,000) and Poly(acrylic acid) (PAA, Mw = 2.0 × 103) were purchased from Aldrich (USA). Dimethyl sulfoxide (DMSO) was obtained from Kanto chemical (Tokyo, Japan). Dipotassium phosphate (K2HPO4), calcium chloride, calcium ion and phosphate ion standard solution (analytical grade) were purchased from Wako (Tokyo, Japan). All reagents were used as received without further purification.

Preparation of Poly(2-hydroxyethyl methacrylate) Matrices 6


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poly(2-hydroxyethyl methacrylate) in DMSO solution (12 wt%) was spin-coated on oxygen-plasma treated glasses at a spinning rate of 2000 rpm for 30 s, followed by subsequent thermal curing on a hot-stage at 180 °C. poly(2-hydroxyethyl methacrylate) matrices thermally annealed for 30 min were employed as crystallization matrices for calcium phosphate.

Synthesis

of

OCP/Poly(2-hydroxyethyl

methacrylate)

and

HAP/poly(2-hydroxyethyl methacrylate) Hybrid Thin Films

Purified water obtained from an Auto pure WT100 purification system (Yamato, relative resistivity: maximum 1.8×107 Ω cm) was employed as the solvent for calcium phosphate crystallization. A typical crystallization solution is as follows: 20 mM of CaCl2, 20 mM of K2HPO4 and PAA additives with a concentration of 7.2 × 10-2 wt%. Solutions with different concentrations of PAA were prepared to perform calcium phosphate crystallization. Crystallization was allowed to proceed at 25 °C for a desired period, typically 72 h. Before characterization, all the poly(2-hydroxyethyl methacrylate)/ calcium phosphate crystal thin film samples were washed with purified water to remove the loosely attached precipitations from crystallization solution. 7


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HAP/poly(2-hydroxyethyl methacrylate) hybrid thin films were prepared by immersing the obtained OCP/poly(2-hydroxyethyl methacrylate) hybrid thin films to hot water (80 °C) for 40 min. Characterization

Fourier transform infrared (FTIR) spectra were measured on a JASCO FT/IR-6100 spectrometer (JASCO, Tokyo). Scanning electron microscopic (SEM) images were obtained using a Hitachi S-4700 field-emission SEM operated at 3-5 kV, and all samples were coated with a layer of platinum by using a Hitachi E-1030 ion sputter (Hitachi, Tokyo) prior to SEM measurement. Transmission electron microscopic (TEM) image were obtained using a JEOL JEM-2010HC (JEOL, Tokyo), operated at 200 kV, and no conductive treatments were performed during observation. Polarizing optical microscopy images were taken with an Olympus BX51 polarizing optical microscope (Olympus, Tokyo). X-ray diffraction (XRD) measurements were performed using a Rigaku SmartLab Intelligent X-ray Diffraction System (Rigaku, Tokyo) with filtered Cu Kα radiation (λ = 1.5406 Å, operating at 40 kV and 40 mA). Change of pH value of the crystallization solution with the transition of time was recorded by using a TOADKK

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IM-55G pH meter (Tokyo, Japan). The composition of thin film crystals formed in the poly(2-hydroxyethyl methacrylate) matrices were determined with inductively-coupled plasma atomic emission spectrometer (ICP/AES) (iCAP DUO-6300, Thermo scientific) (Massachusetts, USA).

RESULTS AND DISSCUSION

Surface-Ring Structure Formation for HAP Crystal Thin Films Obtained via OCP Phase

Spin-coated and thermally annealed poly(2-hydroxyethyl methacrylate) matrices were employed as crystallization matrices according to previous studies.16 OCP crystal thin films with regular surface were prepared by soaking poly(2-hydroxyethyl methacrylate) matrices into a crystallization solution containing CaCl2, K2HPO4, and PAA for 72 h at room temperature. For transformation to HAP thin-films, the precursor hybrid thin films were soaked into 80 °C water for 40 min to obtain HAP/polymer hybrid thin films.

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Concentric mosaic patterns due to the regular orientation of the crystal assemblies are observed under polarizing optical microscopy observation (Figure 2a). The size of each spherical crystal domain is approximately 200-300 µm in diameter. The SEM image (Figure 2b) of the hybrid thin films reveals the formation of three-dimensional concentric ring structures. These structures are homogeneously distributed on the matrices, with ridge height of about 1 µm and the distance between the grooves approximately 1 µm (Figure 2c). The values of ridge height and groove distance in the ring structures are almost constant across the entire surface. To obtain the statistical information on the preferential orientation of the crystal thin films, the X-ray diffraction (XRD) patterns were detected along the in-plane and the out-of-plane of the substrate of poly(2-hydroxyethyl methacrylate) matrix. Herein the direction of the in-plane and out-of-plane were defined by the substrate set in sample folder and the detector in X-ray diffractometer. The diffraction patterns provide information on the differences of orientation of the polycrystalline phase between the in-plane and out-of-plane detection. The in-plane and out-of-plane X-ray diffraction patterns of the thin composite films show characteristic peaks of OCP crystals (Figure

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3), with diffraction peaks around 9.4°, 26.0°, 31.6° attributable to the reflections of 020, 002 and 260 planes in OCP crystals respectively.39 In comparison with the standard powder XRD pattern of OCP (No. 26-1056), intensified peaks assignable to the 020 plane in the out-of-plane XRD pattern and 002 in the in-plane XRD pattern are observed. Since 020 and 002 planes are parallel and perpendicular to the c axis respectively, these observations suggest that the c axis in the hybrid thin films preferentially adopt a parallel alignment to the substrate. Composition analysis of the hybrid thin films using inductively-coupled plasma atomic emission spectrometer (ICP/AES) technique indicates that the molar ratio of calcium and phosphate ions (Ca/P) is approximately 1.50, a value that is higher than that of the theoretical value (1.33) for OCP crystals.40 This deviation from the theoretical value probably arise from two aspects, the imperfect structures41 in the resultant OCP crystal thin films and the possibly adsorbed calcium ions into the poly(2-hydroxyethyl methacrylate) gel matrix during the crystallization process. It is known that TEM measurement provides information on the nano-meter-sized structures of the samples. Therefore, the structure of the obtained OCP thin films were

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also examined by TEM with selected area electron diffraction (SAED) analysis (Figure 4). Crystals with rod-like structures approximately 10 nm in width were observed, and they are preferentially oriented parallel to the thin film surface (Figure 4a). The observation of the 002 reflection patterns (Figure 4b) confirmed the orientation of the thin films as well, indicating that the orientation of the c axis is generally parallel to the surface. Crystal pattern formation in hydrogels were previously reported.42-45 It was suggested that the formation mechanism of patterned morphologies is a self-organization process in the reaction-diffusion crystallization system, which is exemplified by Liesegang rings. We have also reported on the spontaneous formation of patterned morphologies of CaCO3 thin-films by using polymer templates.15-17,46,47 Although it is difficult to fully understand the mechanism for the formation of concentric ring patterns, it is assumed that these patterns are induced by a self-organization process in the reaction (crystallization)-diffusion crystallization system (Figure S1). In the system, the fluctuation of local concentration of the amorphous precursor stabilized by PAA near the gel matrix plays an important role for the pattern formation. A high local

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concentration of amorphous precursors induced the formation of the ridge domains, which in turn cause a consumption of local concentration (step1). The decrease of the local concentration lead to the formation of the valley domains. We assumed that the repetition of low and high concentration of the local amorphous phases ultimately induced the formation of OCP crystal thin films with three-dimensional concentric ring structures. Moreover, we found that the time lapse observation was an effective way to reveal the mechanisms for the formation of patterned surface.15,17 In the present study, the observation of crystallization with the passage of time showed that the OCP crystal thin films with crystal domains approximately 10 µm in diameter were formed after crystallization for 8 h (Figure S2 a-c), and the domain size of the crystal thin films gradually increased with the prolongation of crystallization time. It is noteworthy that the groove distances were almost maintained constant during the growth process, with a value of about 1 µm (Figure S2). Immersion of the OCP/poly(2-hydroxyethyl methacrylate) hybrid thin films in hot water (80°C) for 40 min led to the formation of HAP/poly(2-hydroxyethyl methacrylate) thin films. Figure 5 shows the hybrid thin films of calcium phosphates after the

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conversion. It is noteworthy that the resultant HAP/poly(2-hydroxyethyl methacrylate) hybrid thin films possess concentric ring structures as well, as have been observed for OCP/poly(2-hydroxyethyl methacrylate) hybrid thin films (Figure 5a and 5b). For samples before and after hydrothermal transformation, no obvious difference between the groove distances of the hybrid thin films was observed, both of which showing approximate values of 1 µm (Figure 5 parts b and c). However, cross-sectional SEM observation clearly demonstrates a decrease in the height of the ring structures, from approximately 1 µm for OCP/poly(2-hydroxyethyl methacrylate) thin films (Figure 2c) to about 700 nm for HAP/poly(2-hydroxyethyl methacrylate) hybrid thin films (Figure 5c), indicating that a dissolution process of OCP crystals occurs during the transformation to HAP. The successful transformation was confirmed by XRD measurement (Figure 6). The peak around 9.8°, which is the characteristic diffraction peak of the 110 plane in OCP crystals (Figure 3), shifts to 10.8° after soaking the sample in hot water for 40 min (Figure 6). It is known from the HAP JCPDS (PDF No. 09-432) that the peak around 10.8° corresponds to the 110 reflection in HAP crystals. Meanwhile, the characteristic peaks that can be assigned to OCP crystal planes around

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31.6° (Figure 3) diminishes, and peaks around 31.8°, 32.2° and 32.9° that can be assigned to the reflections of the planes 211, 112 and 300 in HAP crystals are observed in the diffraction patterns (Figure 6). Composition analysis of the hybrid thin films after soaking in hot water for 40 min by using ICP/AES technique shows that the molar ratio of Ca/P is approximately 1.66, a value that is very close to that of the theoretical value (1.67) for HAP. In the in-plane and out-of-plane XRD spectra of HAP/poly(2-hydroxyethyl methacrylate) hybrids thin films (Figure 6), diffraction peaks corresponding to planes of 100, 300 and 310, which are parallel to the c axis are observed in the out-of-plane XRD pattern. Meanwhile, diffraction peak attributable to 002 plane that is perpendicular to the c axis of the thin films is only observed in the in-plane XRD pattern. These results suggest that the c axis in the resultant HAP/poly(2-hydroxyethyl methacrylate) hybrid thin films is preferentially aligned parallel to the substrate surface. The surface morphology as well as the preferential orientation in the OCP/poly(2-hydroxyethyl methacrylate)

thin

films

were

preserved

during

HAP/poly(2-hydroxyethyl methacrylate) composites.

15


the

transformation

to

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Structure

analysis

of

the

obtained

HAP/poly(2-hydroxyethyl

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methacrylate)

composites by TEM (Figure 7) shows that the hybrids are composed of oriented nano-rod crystals approximately 4-5 nm in width, a value larger than the OCP/poly(2-hydroxyethyl methacrylate) precursor. TEM observation confirmed that the c axis of the crystal thin films preferentially aligns parallel to the matrices as well, because of the observation of 002 diffraction pattern. However, the change of the width of nanorod crystals in HAP/poly(2-hydroxyethyl methacrylate) composite films indicates that a re-crystallization process occurs during the transformation from OCP to HAP. Effects of PAA on the Morphologies of OCP/Poly(2-hydroxyethyl methacrylate) Hybrid Thin Films The amount of PAA additives has significant influence on the morphologies of the obtained OCP/poly(2-hydroxyethyl methacrylate) hybrid thin films. Without PAA additives, no observable crystals were developed on poly(2-hydroxyethyl methacrylate) matrices (Figure S3). SEM observations show that PAA with a concentration ranging from 1.8×10-2 wt% to 1.44×10-1 wt% favors the formation of OCP thin films on

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poly(2-hydroxyethyl methacrylate) matrices (Figure S3). No crystal thin films were observed in the presence of PAA with a concentration higher than 1.44×10-1 wt%. This is probably due to the strong interaction between PAA and calcium ions that favors the stabilization of ACP and suppresses its transformation into crystals.48 The use of PAA with a concentration of 1.8×10-2 wt% results in the formation of OCP crystal thin films with flower-like structures (Figure 8a-c), with a domain size of approximately 20 µm in diameter. The transformation of flower-like OCP thin films into HAP thin films by immersing in hot water for 40 min was confirmed by XRD measurement (Figure S4). Interestingly, the flower-like surface morphologies were preserved in the resultant HAP thin films as well (Figure 8d-f). Water-soluble macromolecular additives play an important role in the calcium phosphate-based biomineralization during the formation process of bone and teeth for vertebrata.49-52 Studies on the biomineralization of CaCO3 indicates that calcium carbonate in biological tissues are formed via amorphous calcium carbonate (ACC) transitional phase.53-55 Similarly, the existence of ACP as a transitional precursor phase in the formation of biological HAP has been recently reported by several groups.35,36,56

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In addition, previous research on the biomineralization of collagen indicates that the addition of poly (aspartic acid) (PAsp) significantly slows the kinetics of amorphous-crystalline transformation, and the delay effect is probably due to the interaction between the PAsp additive and ACP, which in turn results in the retardation of the dissolution of ACP.57 In addition, the presence of PAsp was also shown to increase the amount of HAP crystals inside assembling collagen fibrils.58 In the present system, it is assumed that in the formation process of the OCP thin films with concentric ring and flower-like patterns on poly(2-hydroxyethyl methacrylate) matrices, the ACP precursor phase with an enhanced stability by PAA additives plays an important role.

CONCLUSIONS

Periodic surface-ring pattern morphologies have been successfully formed on the OCP and HAP crystal thin films by employing poly(2-hydroxyethyl methacrylate) matrices and acidic polymer additives. The role of the acidic polymer additives to stabilize ACP precursor, was essential in the formation of patterned calcium phosphate crystal thin films. From the time lapse observation, we assumed that the formation of

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patterned surface might undergo the mechanism of reaction-diffusion crystallization systems. These organic/inorganic hybrid materials with periodic surface-ring morphologies are expected to contribute to the development of optical or bioactive HAP-based coating materials.

ASSOCIATED CONTENTS

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION

Corresponding Authors *E-mail [email protected] *E-mail [email protected] ORCID Yulai Han: 0000-0002-8696-5529 19


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Tatsuya Nishimura: 0000-0002-8416-4007 Chikara Ohtsuki: 0000-0002-6474-1540 Takeshi Sakamoto: 0000-0001-6312-2249 Takashi Kato: 0000-0002-0571-0883 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This study was partially supported by KAKENHI 15H02179, and 15K17864 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). Partial financial support from CREST, JST (JPMJCR15Q3) is also acknowledged. The authors would also like to thank Nanotechnology Platform from the University of Tokyo for the TEM observation. We thank Dr. Satoshi Kajiyama for helpful discussions.

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Calcium 2016,

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Figure

1.

Schematic

illustration

of

our

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strategy

for

the

synthesis

of

HAP/poly(2-hydroxyethyl methacrylate) hybrid thin-film with surface ring structures via biomimically obtained OCP/poly(2-hydroxyethyl methacrylate)hybrids. Surface morphology of hybrid thin-film is preserved after transformation.

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Figure 2. OCP/ poly(2-hydroxyethyl methacrylate) thin-film hybrids with surface-ring structures: (a) polarizing optical microscopy images with the magnified image (inside: scale bar indicates 10 µm) (b) SEM image and (c) cross-sectional SEM image of the hybrids.

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Figure 3. In-plane and out-of-plane XRD patterns of the OCP/poly(2-hydroxyethyl methacrylate) hybrids formed on poly(2-hydroxyethyl methacrylate) matrices in the presence of PAA with a concentration of 7.2×10-2 wt%.

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Figure 4. (a) TEM image of the OCP crystals developed on poly(2-hydroxyethyl methacrylate) matrix, and (b) corresponding SAED patterns of the crystal assemblies. Arrow in (a) shows the orientational direction of the nano-rod crystals.

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Figure 5. HAP/ poly(2-hydroxyethyl methacrylate) thin-film hybrids with surface-ring structures: (a) polarizing optical microscopy images with the magnified image (inside: scale bar indicates 10 µm), (b) SEM images and (c) cross-sectional SEM images of the hybrids.

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Figure 6. In-plane and out-of-plane XRD patterns of the HAP/poly(2-hydroxyethyl methacrylate) hybrids transformed from OCP/poly(2-hydroxyethyl methacrylate) hybrid thin films after immersion in hot water for 40 min.

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Figure 7. (a) TEM image of the HAP crystals on poly(2-hydroxyethyl methacrylate) matrix, and (b) corresponding SAED patterns of the crystal assemblies. Arrow in (a) shows the orientational direction of the nano-rod crystals.

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Figure 8. (a) Polarizing optical microscopy image, (b) SEM image showing the convergence of several crystal domains and (c) magnified SEM image of the OCP/poly(2-hydroxyethyl methacrylate) hybrid thin films after crystallization for 72 h in the presence of PAA with a concentration of 1.8×10-2 wt%. After immersion in hot water for 40 min, the polarizing optical microscopy image, SEM image and magnified SEM image of the thin films are shown in (d), (e) and (f) respectively.

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For Table of Contents Only

Periodic surface-ring structures constructed in hydroxyapatite thin films are obtained via biomimetic approaches by using poly(2-hydroxyethyl methacrylate) matrix and poly(acrylic acid) additive.

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Periodic Surface-Ring Pattern Formation for ... - ACS Publications

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