Article pubs.acs.org/Langmuir
Effect of Sodium Chloride on Hydration Structures of PMEA and P(MPC‑r‑BMA) Shigeaki Morita*,† and Masaru Tanaka‡ †
Department of Engineering Science, Osaka Electro-Communication University, 18-8 Hatsucho, Neyagawa 572-8530, Japan Department of Biochemical Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-6, Yonezawa, Yamagata 992-8510, Japan
‡
ABSTRACT: The hydration structures of two different types of biomaterials, i.e., poly(2-methoxyethyl acrylate) (PMEA) and a random copolymer of 2-methacryloyloxyethyl phosphorylcholine and n-butyl methacrylate (P(MPC-r-BMA)), were investigated by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The effects of the addition of sodium chloride to liquid water in contact with the surfaces of the polymer films were examined. The neutral polymer of PMEA was easily dehydrated by NaCl addition, whereas the zwitterionic polymer of P(MPC-r-BMA) was hardly dehydrated. More specifically, nonf reezing water having a strong interaction with the PMEA chain and f reezing bound water having an intermediate interaction were hardly dehydrated by contacting with normal saline solution, whereas f reezing water having a weak interaction with the PMEA chain was readily dehydrated. In contrast, f reezing water in P(MPC-r-BMA) is exchanged for the saline solution contacting with the material surface without dehydration.
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surface are limited.12 The zwitterionic monomer of 2methacryloyloxyethyl phosphorylcholine (MPC) has a formula analogous to phospholipid compounds. Although MPC homopolymer (PMPC) shows excellent biocompatibility, it cannot be used as a film form in water because of its solubility. To form a stable surface in water, MPC is generally copolymerized with a hydrophobic monomer such as n-butyl methacrylate (BMA). In the present study, P(MPC-r-BMA) with a monomer ratio of MPC/BMA ∼ 3/7 (mol/mol) was used. Both PMPC and MPC-based copolymers have been applied to clinical use because of their excellent biocompatibilities.20−22 The equilibrium water content (EWC) in a polymer matrix is estimated from gravimetric method as
INTRODUCTION Water molecules located at the interfaces between medical polymers and biological systems play important roles for their biocompatibility.1−3 Water molecules hydrating to synthetic polymers as well as biopolymers have been characterized and classified by thermal analysis.4,5 Three different types of hydrating water of nonf reezing water (tightly bound water), f reezing bound water (loosely bound water or intermediate water), and f reezing water (scarcely bound water or free water) have been found in many biorelated molecules by the analysis.3,6 The detailed hydration structures of each classified water molecule to a polymer chain accompanied by noncovalent interactions have recently been explored at the functional group level using molecular spectroscopic techniques such as vibrational spectroscopy7,8 and nuclear magnetic resonance (NMR) spectroscopy.9−11 In the present study, the hydration structures of two different types of blood compatible polymers shown in Figure 1, i.e., poly(2-methoxyethyl acrylate) (PMEA)12 and a random copolymer of 2-methacryloyloxyethyl phosphorylcholine and n-butyl methacrylate (P(MPC-r-BMA)),13 were investigated by means of attenuated total reflection infrared (ATR-IR) spectroscopy.8,14 Water structures in those two polymers investigated using differential scanning calorimetry (DSC) have been reported to be completely different,15,16 although both exhibit excellent antithrombogenic properties. The neutral polymer of PMEA has been applied to clinical use17−19 because protein adsorption and cell adhesion onto its © 2014 American Chemical Society
ϕ=
WW WP + WW
where WP and WW are the weights of dried polymer and hydrating water, respectively. The EWC for PMEA and that for P(MPC-r-BMA) (MPC/BMA ∼ 3/7) have been reported as 9.0% and 84%, respectively.15,16 All the hydrating water in PMEA is completely dehydrated in a nitrogen atmosphere.8 However, it is difficult to obtain completely dehydrated P(MPC-r-BMA) at ambient temperature due to the strong Received: July 1, 2014 Revised: August 18, 2014 Published: August 18, 2014 10698
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were reported by Matsuda et al.29 It was concluded that the hydrodynamic radius and second virial coefficient of PMPC in aqueous solution were independent of the concentration of NaCl. In order to make clear the hydration structures of P(MPC-rBMA) in comparison with those of PMEA, the effects of the addition of sodium chloride to water contacting with the surfaces of the materials are investigated in the present study.
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EXPERIMENTAL SECTION
All the polymer samples used in the present study were obtained by radical polymerization. Detailed information about the synthesis is described elsewhere.12,23 The weight-average molecular weight for PMEA and that for P(MPC-r-BMA) were 8.5 × 104 and 6.0 × 105, respectively. Water with a resistivity of 18.2 MΩ cm was purified by a Milli-Q system. Other chemicals were purchased from Wako Pure Chemical Industries and used as received. A phosphate buffer (PB) solution containing 5.7 g L−1 Na2HPO4 and 3.6 g L−1 KH2PO4 in water was prepared. The hydrogen-ion exponent (pH) of the solution was 7.0. The polymer films were prepared on a flat surface of a hemispherical ZnSe prism by solvent casting from a chloroform solution for PMEA and an ethanol solution for P(MPC-r-BMA). The prism adhered with the polymer sample was mounted on an original flow trough cell designed for in-situ ATR-IR spectroscopy. Detailed information on the cell is described elsewhere.8 An IR beam was introduced into the prism at an incident angle of 45°, which is larger than the critical angle of ca. 34°. All the ATR-IR spectra were recorded using a Fourier transform infrared spectrometer (Varian, FTS 3000) equipped with a liquid-nitrogen-cooled HgCdTe detector. A total of 64 scans were coadded to obtain each spectrum at a spectral resolution of 2 cm−1. Before measurement, the film samples were enough dried in the cell under a flow of nitrogen, and no IR signals from the casting solvents were observed. After being enough dried the sample, neat water or PB solution was introduced into the cell, and then the wet condition was maintained for a few hours. The ATR-IR spectra of the polymer films in contact with the aqueous solutions were measured after 1 h of exchanging the solution in the cell. The thickness of the film sample prepared on the prism was controlled to be thicker than penetration depth of an evanescent wave generated at the prism/ polymer interface. As a result, the ATR-IR signals from the film sample were saturated as a function of the film thickness. It should be noted that the ATR-IR signals from the sorbed solution into the polymer matrix and those from the hydrated polymer film were selectively detected in the spectrum, excluding those from the bulk solution in contact with the polymer surface.8
Figure 1. Chemical structures of (a) PMEA and (b) P(MPC-r-BMA).
interaction among the hydrating water molecules and the ionic groups in the polymer side chain.23 Tanaka et al. demonstrated that the presence of f reezing bound water in PMEA prevents the surface of the material from cells adhesion.24 Freezing bound water is commonly found in many kinds of biological polymers, whereas rarely in artificial polymers,25 which indicates nonspecific protein adsorption on the surface. Our recent study revealed the detailed hydration structures of PMEA at the functional group level as follows:8 Each nonf reezing water molecule tightly binds to two carbonyl groups in PMEA with a CO···H−O−H···OC type of hydrogen bonding and never crystallizes even at −100 °C due to its strong interaction with the polymer chain. Freezing bound water loosely interacts with methoxy moiety in the side chain terminal, and does not freeze at 0 °C, but crystallizes in a heating process at the cold crystallization temperature of −42 °C due to its intermediate interaction. Freezing water infiltrates into the polymer matrix with a slight interaction with the polymer chains having a bulk-water-like structure with an O− H···O−H type of hydrogen bonds network among water molecules and easily crystallizes at 0 °C. Detailed hydration structures of the MPC-based copolymers have not been elucidated. Ishihara et al. reported that f reezing water reduces protein adsorption on the surfaces of PMC-based copolymers.16 Morisaku et al. reported that nonf reezing water influences the degree of protein adsorption resistance for the copolymers.26 Hatakeyama et al. found the existence of f reezing bound water in hydrated PMPC with a WW/WP range of 0.5− 0.9 (ϕ of 0.33−0.47).27 Their three results seem to be mutually inconsistent. In general, not only living cells but also artificial materials are easily dehydrated by the salt addition. Annaka et al. reported that volume phase transition temperature of a poly(Nisopropylacrylamide) gel was strongly dependent on the aqueous salt concentration (0.0−2.0 M) and was scaled by the chemical potential of the water molecules.28 The effects of NaCl addition to aqueous solutions of PMPC (0.0−0.5 M)
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RESULTS AND DISCUSSION Figure 2A shows the ATR-IR spectra in the O−H stretching region of NaCl aqueous solutions at concentrations of 0, 0.15, 0.60, 1.20, 1.71, and 2.22 mol L−1. Figure 2B displays the subtraction spectra of neat water from the obtained ATR-IR spectra, i.e., A(C) − A(0), where C is the concentration of NaCl. All the spectra were measured without any polymer film on the prism. The spectral shape of the ATR-IR spectrum of neat water (bold line in Figure 2A) is somewhat different from that of the corresponding transmission IR spectrum (not shown here) because of reflection spectrum.30 A broad feature in the O−H stretching region is an evidence for a hydrogen bonds network among water molecules.31,32 Since one water molecule has two electron donor sites and two electron acceptor sites, each water molecule can interact with at most four water molecules.33 Although there has been much discussion, two main components around 3400 and 3200 cm−1 in the O−H stretching region are assigned to three-coordinated and fourcoordinated water cluster structures, respectively.34,35 As shown 10699
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shape with bulk water having a broad feature around 3400− 3200 cm−1 is attributing to slightly interacting water with the polymer chain, i.e., f reezing water. To examine the variations in the spectral shapes shown in Figure 3, normalized subtraction spectra at each concentration are plotted in Figure 4. The spectrum at the higher
Figure 2. (A) ATR-IR spectra of NaCl aqueous solutions at different concentrations of 0, 0.15, 0.60, 1.20, 1.71, and 2.22 M and (B) subtraction spectra of neat water from the obtained spectra. Bold line in (A) corresponds to neat water.
in Figure 2B, the contribution around 3400 cm−1 increases with increasing NaCl concentration, whereas that around 3200 cm−1 decreases.36 This spectral variation represents the change in the water activity.33 That is, the water molecules are oriented around the Na+ and Cl− ions in the solution. Consequently, the total number of four-coordinated water clusters in the solution decreases with salt addition, while that of the three-coordinated water clusters increases. Figure 3 shows the ATR-IR spectra of a hydrated PMEA film in contact with NaCl aqueous solutions at different
Figure 4. Normalized subtraction spectra calculated from the spectra shown in Figure 3.
concentration was subtracted from that at the lower one. After the subtraction, spectral intensities at each wavenumber were normalized. The shapes of the normalized subtraction spectra shown in Figure 4 reflect the water structures which are dehydrated at the concentration. The subtraction of the spectrum at 0.15 M from that at 0 M (Figure 4a) shows a more or less similar spectral shape to f reezing water.8 Note that the concentration of normal saline solution, which has an osmotic pressure equal to that of bodily fluids, is 0.15 M. This result demonstrates that f reezing water in PMEA is easily dehydrated from the polymer matrix by the addition of salt at a concentration as low as 0.15 M. In contrast, the subtraction of the spectrum at 2.22 M from that at 1.71 M (Figure 4e) has two contributions of f reezing bound water around 3400 cm−1 and nonf reezing water around 3600 cm−1. However, as shown in Figure 3, the ATR-IR spectrum at 2.22 M indicates clear contributions of both f reezing bound water and nonf reezing water. These results reveal that both f reezing bound water and nonf reezing water are also dehydrated by salt addition at such a high concentration, which is 14.8 times higher than that of normal saline. However, the nonf reezing water and f reezing bound water are not completely dehydrated by the highly concentrated salt. The shape of the subtraction of the spectrum at 1.20 M from that at 0.60 M (Figure 4c) is somewhat similar to that of f reezing bound water.8 This result implies that f reezing bound water in PMEA is dehydrated around a concentration of ca. 1.2 M, which is ca. 8 times higher than the concentration of normal saline. Figure 5 shows the ATR-IR spectra of the hydrated PMEA film in contact with solutions of NaCl dissolved in PB at different concentrations. The spectral intensities in the O−H stretching region for the PB in contact with PMEA (bold line in Figure 5) are weaker than those for neat water in contact with PMEA (bold line in Figure 3), demonstrating that the PMEA film is also dehydrated by PB similarly to the NaCl solution. However, the variations in the spectral shapes induced by the addition of NaCl to PB are similar to those in pure water shown in Figure 3. That is, f reezing water is easily dehydrated, whereas
Figure 3. ATR-IR spectra of hydrated PMEA film in contact with NaCl aqueous solutions at different concentrations of 0, 0.15, 0.60, 1.20, 1.71, and 2.22 M. Bold line corresponds to PMEA in contact with neat water.
concentrations. The bands in the 3000−2800 cm−1 region assigned to the C−H stretching modes of PMEA increase with increasing NaCl concentration, whereas those in the 3700− 3000 cm−1 region assigned to the O−H stretching modes of water decrease accompanied by large spectral shape variations. These spectral variations are caused by the dehydration of the PMEA film induced by salt addition. The number of water molecules existing in the constant volume of the evanescent wave generated at the prism/polymer interface decreases due to dehydration. As a result, the segment density of the PMEA chains in this volume increases due to the shrinking of the film. Our recent study revealed that the spectral shape of the hydrated PMEA in the O−H stretching region has three contributions around 3600, 3400, and 3200 cm−1.8 The band around 3600 cm−1 is arising from water molecules hydrating to the carbonyl group, i.e., nonf reezing water. That around 3400 cm−1 is the evidence for intermediately hydrated water to the methoxy moiety, i.e., f reezing bound water. A similar spectral 10700
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Figure 7. Area intensities of the subtraction spectra shown in Figure 2B (filled circle) and Figure 6B (open circle) in the 3335−2700 cm−1 region plotted as a function of the NaCl concentration. Solid lines correspond to the fitted linear functions.
Figure 5. ATR-IR spectra of hydrated PMEA film in contact with solutions of NaCl dissolved in PB at different concentrations of 0, 0.15, 0.60, 1.20, 1.71, and 2.22 M. Bold line corresponds to PMEA in contact with PB without NaCl.
f reezing bound water and nonf reezing water are less dehydrated by the NaCl addition to PB. Figure 6A shows the ATR-IR spectra of hydrated P(MPC-rBMA) film in contact with NaCl aqueous solutions at different
gradient ratio of gMPC/g0 ∼ 0.529, which should be related to the number of water molecules perturbed by the salt addition in the P(MPC-r-BMA) matrix, will be discussed later. The ATR-IR spectra of hydrated P(MPC-r-BMA) film in contact with solutions of NaCl dissolved in PB (not shown here) were nearly the same as those in contact with aqueous NaCl solutions shown in Figure 6A. This also represents that P(MPC-r-BMA) is hardly dehydrated by change in water activity. Next, we consider the hydration structure of P(MPC-rBMA) in light of the results described above. The area intensity in the O−H stretching region (3700−3000 cm−1) for neat water hydrated to P(MPC-r-BMA) (solid line in Figure 6A) is 0.734 times lower than that for neat water without any polymer film (solid line in Figure 2A). Assuming that the area intensity is proportional to the volume fraction of water, and the density of the polymer solid is equal to that of water, the area intensity ratio of 0.734 directly corresponds to the EWC for P(MPC-rBMA), i.e., ϕ ∼ 0.734. Note that this value is more or less similar to the EWCs of MPC-based copolymers reported previously.16,26 The weight W is related to the number of moles N and the molecular weight M as W = NM. The number of hydrated water molecules per MPC unit is defined as
Figure 6. (A) ATR-IR spectra of hydrated P(MPC-r-BMA) film in contact with NaCl aqueous solutions at different concentrations of 0, 0.15, 0.60, 1.20, 1.71, and 2.22 M and (B) corresponding subtraction spectra of A(C) − A(0). Bold line in (A) corresponds to P(MPC-rBMA) in contact with neat water.
concentrations, and Figure 6B presents the corresponding subtraction spectra, A(C) − A(0). It should be noted that both the total area intensities of the C−H stretchings around 3000− 2800 cm−1 and those of the O−H stretchings around 3700− 3000 cm−1 are scarcely changed by the salt addition. Those results demonstrate that almost no water molecules are dehydrated from the P(MPC-r-BMA) matrix by salt addition, whereas the water in PMEA is easily dehydrated. However, the spectral shape of the O−H stretching region shown in Figure 6A is changed by salt addition, similarly to the case of bulk water shown in Figure 2A; i.e., the band around 3400 cm−1 increases whereas that around 3200 cm−1 decreases. This result suggests that the concentration of NaCl in the P(MPC-r-BMA) matrix is changed by the salt addition. As shown in Figures 6B and 2B, the decrease in the band intensities around 3200 cm−1 for the hydrated P(MPC-r-BMA) is approximately half of that for the bulk solution. In order to clarify the intensity variations, the area intensities of the subtraction spectra in the 3335−2700 cm−1 region for the hydrated P(MPC-r-BMA) and those for the bulk solution are plotted as a function of the NaCl concentration in Figure 7. Both plots reveal nearly linear relationships. The gradient of the fitted line given in Figure 7 for the hydrated P(MPC-r-BMA) is 0.529 times smaller than that for the bulk solution. The
Ψ=
NW ϕ MP = NP 1 − ϕ MW
where the molecular weight of one MPC unit having 7/3 BMA units in the copolymer is calculated as 7 MP = MMPC + MBMA 3 where MMPC = 264.30 and MBMA = 142.20. Consequently, Ψ ∼ 90.4 is calculated from the EWC value. Let us consider following ratio φex =
Nex Nex + Nnon
where the subscripts “ex” and “non” denote water molecules in P(MPC-r-BMA), which are completely and never exchanged for the NaCl solution after in contact with the material surface, respectively, i.e., NW = Nex + Nnon. Now the following relationship is assumed, since the gradient ratio of gMPC/g0 is related to the number of water molecules perturbed by the salt addition in the P(MPC-r-BMA) matrix. g ϕφex = MPC g0 10701
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That is, f reezing water in the polymer matrix is easily exchanged for the solution without dehydration, but nonf reezing water never exchanged due to its strong interaction with the zwitterionic groups.
This is an assumption that all the hydrated water in P(MPC-rBMA) are not exchanged for the NaCl solution as shown in Figure 7. Thus, φex = 0.721 is calculated using the values estimated from the spectroscopic analysis. That is, ca. Ψ ∼ 90 molecules are hydrating to each zwitterionic group in the MPC unit, and they are divided into following two species: The one of Ψφex ∼ 65 molecules are freely exchanged for the solution in contact with the surface of the material, and the other of the remaining Ψ(1 − φex) ∼ 25 molecules are not exchanged for the solution due to the strong interaction with the zwitterionic groups. Morisaku et al. reported the number of nonf reezable water per one MPC unit is 23−24.26 Matsuda et al. reported that the second virial coefficient of PMPC in aqueous solution is independent of the NaCl concentration.29 For those reasons, we conclude that the 65 molecules weakly bound to the chain are f reezing water, and the remaining 25 molecules tightly bound to the zwitterionic group are nonf reezing water. Note that an ionic strength of 25 water molecules hydrating to one zwitterionic group is calculated to be 2.2 M. It is likely that f reezing water in PMEA would be easily perturbed by protein adsorption and/or cell adhesion at the biointerface as in a similar way in case of contacting with the NaCl solution. However, f reezing bound water in PMEA is hardly perturbed by the biological contacting until a high ionic strength of ca. 1.20 M, which is 8 times higher than the normal saline concentration of 0.15 M. In contrast, f reezing water in P(MPC-r-BMA) is easily balanced with the biological surface and gives no perturbation to the nonf reezing water tightly bound to the polymer chain, even at an ionic strength of ca. 2.2 M. We recently found that cancer cells can attach to the PMEA surface, whereas platelets cannot.37 In contrast, these cells did not attach to the P(MPC-r-BMA) surface,37 and the MPCbased copolymers have been applied to anticancer drug delivery systems.38 These results may be interpreted as follows: The cancer cells strongly perturb the water structure at the surface of PMEA as similar to the highly concentrated solutions, which dehydrate not only freezing water but also f reezing bound water. As a result, the surface of PMEA without f reezing bound water becomes less biocompatible. On the other hand, water structure around the MPC unit hardly perturbed by the cancer cells. In the case of platelet cells, both PMEA and MPC-based copolymers protect the surfaces from the platelets attachment, though the mechanisms of the protection due to the hydrating water molecules are different.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (S.M.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge Professor Emeritus Teiji Tsuruta (The University of Tokyo) for valuable discussions.
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REFERENCES
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CONCLUSIONS The effects of NaCl addition to water in contact with two different types of biomaterials were explored by means of ATRIR spectroscopy. In the case of PMEA, f reezing water was easily dehydrated from the matrix by salt addition, but f reezing bond water and nonf reezing water were hardly dehydrated by normal saline solution. However, f reezing bound water in PMEA was more or less dehydrated by contacting with a highly concentrated solution of ca. 1.20 M, which is 8 times higher than the concentration of normal saline. Nonf reezing water in PMEA was hardly dehydrated by the salt addition until 2.22 M. In the case of P(MPC-r-BMA), in contrast, both f reezing water and nonf reezing water were hardly dehydrated by the salt addition. A total of 90 water molecules are hydrated to each zwitterionic group in the MPC unit, and 65 molecules are easily exchanged for the NaCl solution in contact with the surface of the material, but the remaining 25 molecules are not exchanged. 10702
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dx.doi.org/10.1021/la502550d | Langmuir 2014, 30, 10698−10703
