- Email: [email protected]
hydroxyapatite composite hydrogel
Materials Science and Engineering C 59 (2016) 948–957
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
The physical and chemical properties of the polyvinylalcohol/ polyvinylpyrrolidone/hydroxyapatite composite hydrogel Yahui Ma, Tongchun Bai ⁎, Fei Wang College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 31 March 2015 Received in revised form 25 September 2015 Accepted 26 October 2015 Available online xxxx Keywords: Polyvinylalcohol Polyvinylpyrrolidone Hydroxyapatite Hydrogel Storage modulus Water content Swelling kinetics Dehydration kinetics
a b s t r a c t A hydrogel of polyvinylalcohol (PVA)/polyvinylpyrrolidone (PVP)/hydroxyapatite (HA) was prepared by a repeated freezing and thawing technique. The effect of HA on the hydrogel was evaluated by comparing the physical and chemical properties of PVA/PVP/HA and PVA/PVP hydrogels. By using theoretical models, the information about the swelling kinetics and the dehydration kinetics have been obtained. From the analysis of structure, mechanical properties, and molecular interaction, the application of PVA/PVP/HA hydrogel as a biomaterial has been evaluated. Relative to PVA/PVP, the PVA/PVP/HA hydrogel is of denser network structure, lower water content, larger storage modulus, and higher dehydration activation energy. These results reveal that, as HA fills in the hydrogel, the molecular interaction is enhanced, the free space of network is compressed, and the diffusion activation energy of water is increased. In spite of its water content being decreased, it is still in the range of meeting the requirement of bio-application. When the hydrogel is subjected to external forces, the matrix will transfer the load to the HA powder, thus enhance the strength of the hydrogel. For application in bio-materials, HA will still have osteoinductivity because its crystalline structure is not interrupted in PVA/PVP/HA hydrogel environment. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polyvinylalcohol (PVA) hydrogels are three-dimensional network polymers that are physically or chemically cross-linked and are able to absorb large amounts of water [1,2]. Due to its higher water content [3], microporous structure [4], and favorable mechanical and lubricating properties [5–7], the PVA hydrogels have been extensively recognized as a potential material for a variety of applications in the pharmaceutical, biomedical industry, [8,9] especially in artificial biomaterials for cartilage repair [2,3,6,7]. Even so, some physical–chemistry properties, mechanical properties and biocompatibilities of PVA hydrogels do not seem to be sufficient to serve as a biomaterial for artificial articular cartilage [10,11]. In clinical application, some problems such as fixation method and the durability remain to be solved [3]. In biomedical application, the adherence of cell on PVA hydrogel is inhibited by its highly hydrophilic nature [12, 13]. Therefore, there are still tremendous works to be done. Some researchers focus on blending PVA with other bioactive macromolecules, for example gelatin [2], poly(vinyl pyrrolidone) (PVP) [14], and chitosan [15] to form composite hydrogels, and so to improve their biocompatibility. Polyvinylpyrrolidone (PVP) is a biocompatible and hydrophilic polymer which has been used for a number of biomedical applications [14]. ⁎ Corresponding author. E-mail address: [email protected] (T. Bai).
http://dx.doi.org/10.1016/j.msec.2015.10.081 0928-4931/© 2015 Elsevier B.V. All rights reserved.
Over the last decades, the miscibility and mechanical strength of the hydrogel by blending PVA and PVP have been intensively investigated [4,14,16–18]. Nishio et al. studied the miscibility and orientation behavior of PVP/PVA blends by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) [18]. Ma et al. prepared a novel PVP/PVA hydrogel through a repeated freezing and thawing method. Their results indicated that the mechanical and tribological properties of PVP/PVA hydrogels were significantly dependent on PVP content [14]. Recently, calcium phosphate (CaP), especially the hydroxyapatite [Ca10(PO4)6(OH)2, acronym HA] is known for its biocompatible, bioactive (i.e. ability of forming a direct chemical bond with surrounding tissues), osteoconductive, non-toxic, non-inflammatory, and non-immunogenic properties [19,20]. Thus, HA is generally used as an osteogenic inducer for bone substitution [21]. Mostly, the synthesized hydroxyapatite with bone-bonding properties is widely used in hard tissue replacement [22– 27]. Additionally, there are also some literatures reporting polymer/ hydroxyapatite hydrogels for articular cartilage repairing [28–31], and confirming that the addition of HA nanocrystals in cartilage scaffolds can improve cellular adhesion and proliferation significantly, thus contributing to the repair and regeneration of cartilage [20,32]. For example, M. Du et al. have used Ca2+ as a crosslinker to fabricate nanohydroxyapatite (nHA)/alginate (ALG) hydrogel, the culture experiments of human embryo skin fibroblast (ESF) exhibit that nHA can effectively improve cellular adhesion on the hydrogel surface [32]. Wu et al. have incorporated HA to PVA hydrogel to form a bioactive composite for
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
applications as artificial cartilage [33]. In the same field, Gonzalez et al. have successfully produced and characterized PVA/HA composite hydrogels, with different nanofiller contents, with potential application as articular cartilage replacement [31]. To utilize the advantages of PVA, PVP, and HA, the composite hydrogels of PVA/PVP/HA with different mass percent fractions of HA were prepared in this work. In the view of literatures cited above, the chemical composition of this composite hydrogel will be in favor of the application in articular cartilage repairing or replacement, and will improve the biocompatibility and the mechanical property. However, this suppose should be checked by experiments, and the physical properties of the composite hydrogel should meet the requirement of biomaterials. But, report on the physical chemical properties is less in detail, especially in the effect of HA on the molecular interactions in the hydrogel network environment. And theoretically, the physical properties should be explained reasonably. Therefore, in this work, some physical and chemical properties about structure, molecular interaction, and mechanical properties were determined in detail. By comparing the properties of PVA/PVP/HA and PVA/PVP, the effect of HA on the hydrogel was evaluated. And moreover, some theoretical models were introduced to explain the experimental results and to provide theoretical parameters to evaluate the application of the hydrogel in biomaterials. 2. Experimental 2.1. Materials Polyethanol, commonly known as poly(vinyl alcohol), given the acronym PVA, with mass fraction N0.99, weight average molar mass = 68.8 kg/mol and determined by the intrinsic viscosity method, was supplied by the Shanghai Chemical Reagent Inc., and used in experiments without further purification. Polyvinylpyrrolidone (PVP) K30 (CAS. no.: 9003-39-8, with IUPAC name: 1-ethenylpyrrolidin-2-one and a 0.985 mass fraction purity) was received from Sinopharm Chemical Reagents Co. Ltd. To prepare the hydroxyapatite particles, calcium nitrate [Ca (NO3)2 ⋅ 4H2O] and ammonium hydrogen phosphate [(NH4)2HPO4], were also supplied by Sinopharm Chemical Reagent Co. Ltd., China. 2.2. Methods 2.2.1. Preparation of PVA/PVP/HA composite hydrogel Aqueous solutions of PVA/PVP were prepared by dissolving 10 g PVA and 4 g PVP in water at 90 °C and slowly stirred for 4 h by a magnetic stirrer. To prepare hydroxyapatite particles, aqueous solution of (NH4)2HPO4 was added into the PVA/PVP stirred solution drop-by-drop at temperature of 80 °C, followed by the addition of Ca(NO3)2 aqueous solution. The quantities of Ca(NO3)2 ∙4H2O and (NH4)2HPO4 were controlled to provide a Ca/P molar ratio of 1.67. Meanwhile, the ammonia was added into the solution drop-by-drop to maintain pH within 9–10 by adjusting the water content to control the total mass of above mixture to be 100 g. The obtained suspension mixture was stirred at about 1200 rpm for further 4 h, and then cooled to room temperature. The suspension mixture was then poured into a mold, and temperature was controlled to −15 °C for 10 h, and then thawed at room temperature for another 2 h. This freezing and thawing cycle was repeated for 7 times. The obtained PVA/PVP/HA composite hydrogels with different HA mass percent fractions (0.0, 0.5, 1.5, 3.0, 4.5) % represented by symbols 0.0HA, 0.5HA, 1.5HA, 3.0HA, and 4.5HA, respectively, were stored in water for further characterization. The mass fraction of HA was calculated by w (HA) = m (HA) / [m (HA) + m (PVA) + m (PVP) + m (H2O)], where m is the mass of component. In the process of preparation, the
949
concentration of PVA and PVP is 10% and 4% (mass percent), respectively. 2.2.2. X-ray diffraction (XRD) measurement The X-ray diffraction spectra of HA, PVA/PVP and PVA/PVP/HA composite hydrogels with different mass percent fractions of HA were obtained by X-ray diffractometer (X'pert, PRO PAN Alytical, Cu − Kα, λ = 0.15406 nm) at a scanning 2θ ranging from 10 to 70 with a step width of 0.003. 2.2.3. Fourier-transform infrared spectroscopy (FT-IR) analysis The FT-IR spectra of HA, PVA/PVP and PVA/PVP/HA composite hydrogels were recorded by infrared spectrometer (Nicolet 6700, American), with a wave number range from 400 to 4000 cm−1. 2.2.4. Morphology characterization Phase morphology characterization of the gel samples (vacuum dried after freeze drying) were performed by scanning electron microscopy (SEM) (S-4700, Hitachi Limited, Japan) using 15 kV for secondary electron imaging after coating with Au using a sputter coater. 2.2.5. Rheological analysis Rheological characterization of hydrogel samples was performed on a Haake Rheostress 6000 rheometer (Thermo Electron Co., Karslruhe, Germany) with oscillation frequency sweep. The tests were operated at 37 °C, a shear stress of 1 Pa, and the angular frequency ω range from 0.1 to 100 rad/s, and the measuring gap size is about 0.2 mm. 2.2.6. Water content determination To determine the water content of PVA/PVP/HA composite hydrogels with different mass percent fractions of HA, the samples were dried to a constant weight in a vacuum oven at 70 °C for at least 36 h. The water content W was calculated by the following equation: W ¼ ðm−m0 Þ=m
ð1Þ
where, m and m0 represent the mass of the PVA/PVP/HA composite hydrogels before and after drying respectively. All samples were determined three times. The average value is obtained with a standard deviation of 0.0003. 2.2.7. Swelling behavior characterization The dried hydrogels (in vacuum at 70 °C for 36 h) were immersed in water, aqueous electrolyte solution (NaCl, c = 0.15 mol/dm3) and aqueous Dextran solution (0.1 g/dm3) at 37 °C. The swollen weight was determined by weighting gel block samples after wiping off the surface water at time t in a regular time interval. The swelling ratio was defined by the equation as follows: W ðt Þ ¼ ðmt −m0 Þ=mt
ð2Þ
where, m0 and mt are the weight of the dried gel sample and the sample after immersed in solution at time t, respectively. The swelling kinetics was analyzed from the experimental data of W(t)–t. 2.2.8. Melting enthalpy Differential scanning calorimetric (DSC) measurements of dried samples were performed using a Netzsch DSC 204F1 instrument (Germany) under nitrogen gas flow rate of 70 mL/min and at a heating rate of 5 °C/min from room temperature to 350 °C. Melting enthalpy and melting temperature (Tm) were obtained from the thermograms. 2.2.9. Dehydration kinetics measurements Differential scanning calorimetric (DSC) measurements were carried out on a Netzsch DSC 204F1 instrument (Germany). The dehydration curves of hydrogel samples were recorded from room temperature to
950
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
150 °C at heating rates βi = 1, 2, 3, 4, 5 K/min, respectively, in a nitrogen atmosphere. The dehydration kinetics was analyzed from these curves. 3. Results and discussion 3.1. XRD analysis The XRD spectrum of HA and PVA/PVP/HA dried gel samples is displayed in Fig. 1. The symbols of 0.0HA, 0.5HA, 1.5HA, 3.0HA, and 4.5HA represent HA mass percent fraction to be 0.0, 0.5, 1.5, 3.0, and 4.5%, respectively. In Fig. 1(a), the peaks at 2θ = 19.61 and 40.82 show the characteristic peaks of PVA/PVP gel, which indicates the gel is an amorphous solid. In Fig. 1(f), the peaks centered at 2θ = 25.92; 28.15, 28.87; 31.85, 32.26, 32.88, 34.22; 39.79; 46.79; and 49.47 etc. represent the characteristic peaks of the HA crystalline structure [34]. For PVA/PVP/HA dried gel samples, the observed spectra of Fig. 1(b), (c), (d), (e) can be considered as the combination of both PVA/PVP gel and HA crystal. The intensity of the amorphous peak of PVA/PVP at 2θ = 19.61 apparently decreases with the increasing in HA percentage, while the typical crystalline peaks of HA are enhanced. These indicate that there is no apparent crystalline interruption about the HA crystalline in the PVA/PVP/HA gel samples. Therefore, a conclusion can be obtained from the above observation that HA was filled in the gap of PVA/PVP network, and there is no apparent interruption on both the HA crystalline and the PVA/PVP amorphous state. In Elashmawi's work [35], they inferred that there was a significant motion of polymer blend chains in the amorphous region or there were some defects existing at an interface between the polymer chain and the HA filler. For application in bio-materials, our results suggest that the osteoinductivity of HA was not interrupted in PVA/PVP/HA hydrogel network environment. 3.2. FT-IR analysis Fig. 2 shows the FT-IR spectrum of (a), HA powder, (b), PVA/PVP dried gel, and (c), PVA/PVP/HA dried gel (due to the fact that all the PVA/PVP/HA samples have similar FT-IR spectrum, only the spectrum of 1.5HA sample is shown for a clear comparison and symbol marking). The characteristic peaks of HA appearing at 1626 cm− 1 and 1377 cm−1 belong to the hydroxyl vibrations, while peaks located at 1052 cm−1, 621.3 cm− 1 and 583.1 cm− 1 belong to the bending and
Fig. 2. The FT-IR spectrum of (a), HA powder, (b), PVA/PVP dried gel and (c), PVA/PVP/HA dried gel with mass percent fraction 1.5% of HA.
stretching of phosphate group. The broad characteristic band around 3413 cm−1 is caused by the absorption of H2O. The peaks at 2921 cm−1, 1434 cm−1, 1281 cm−1, 1090 cm−1 and 834.2 cm−1 in Fig. 2(b) of PVA/PVP can be assigned to the vibrations of –CH2–, –C_C–, and –CO– respectively; the peaks around 3308 cm−1 and 1637 cm−1 are attributed to the absorption of H2O and the hydroxyl vibrations. For PVA/PVP/HA, the FT-IR spectra seem to be a superposition of the spectra of HA and PVA/PVP. A slight shift of the peaks of PVA/PVP/HA from PVA/PVP is observed. However, there are two peaks that differ from the PVA/PVP and the HA. At 1718 and 1647 cm− 1, it can be thought as the splitting of hydroxyl vibrations, and at 1453 cm− 1, it can be thought as the shift of the peak of PVA/PVP at 1434 cm−1. While, the OH vibration peak of HA at 1377 cm−1 disappeared. These may be an evidence of intermolecular interaction between HA and PVA/PVP matrices, especially in the form of hydrogen bond interaction, just as Gonzalez indicated in literature [31]. But no matter what, FT-IR spectrum does not indicate chemical reactions having taken place.
3.3. Morphology
Fig. 1. The X-ray diffraction spectra of HA and PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e), 4.5% respectively and (f), HA powder.
The SEM images of the cross-section of PVA/PVP hydrogels after freeze–drying are shown in Fig. 3. A porous structure can be observed from it. Apparently, from Fig. 3(a)–(e), the pore size decreases with the increase in the content of HA. This phenomenon can be attributed to the fact that the network space filled with HA, and the interaction between HA and PVA/PVP lead the gel space to be compressed. Fig. 4 shows the dispersion state of HA on the gel surface. Apparently, it is relative to the amount of HA to be blended. For the sample of 1.5HA in Fig. 4(c), the mass percent fraction is 1.5%; HA disperses homogeneously in the gel. With more HA mixed in, as in the cases of samples of 3.0HA and 4.5HA in Fig. 4(d) and (e), agglomeration is observed. This may be due to the saturation of HA in PVA/PVP solution in the preparation process, which leads the gel to be macroscopically heterogeneous distribution. A similar result was reported by Gonzalez et al., but their sample composition is of HA 7.5% (mass percent fraction) which consisted only of PVA and HA [31]. In Elashmawi's work [35], a blend of PVP/PVP showed a uniform surface morphology and revealed a rather smooth surface, and after adding HA, an aggregation of the calcium phosphate particles or chunks that randomly distributed on the top surface, their results revealed that the polymer and the HA were compatible.
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
951
Fig. 3. SEM micrographs show the cross-section morphologies of PVA/PVP/HA composite hydrogels after freeze–drying. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e) 4.5% respectively.
3.4. Rheological analysis The mechanical properties of the hydrogels were examined by oscillatory rheological experiments as a function of frequency. Fig. 5 shows the changes in the storage modulus (G′, Fig. 5a), and the loss modulus (G″, Fig. 5b) for the PVA/PVP/HA hydrogels with different HA mass percent fractions. The G′ reflects the elasticity of hydrogel samples, which is defined as the stored energy due to the elastic deformation, and the G″ reflects the viscosity of hydrogel samples [34]. It can be seen from Fig. 5(a) that the G′ of all the samples is independent on the frequency, but increases with the increase in the HA mass fraction until w(HA) = 3.0%, and then decrease at w(HA) = 4.5%. So is the situation of G″ in Fig. 5(b). A similar result was reported by Pan et al.; their result indicated that the G′ and G″ reached the maximum value while nano-HA content is 6%, then decreased to 9% [29]. This phenomenon is relative to the relationship between the chain segment oscillation and the angular frequency of a rheometer. The independence of G′ on frequency indicates that the macromolecular chains can keep up with the change of angular
frequency, and the hysteresis effect is very low. The phenomenon of the increase of G′ with the increase in HA amount indicates that HA can accommodate hydrogel to the higher movement frequency, which can be applied to intense conditions. When the hydrogel is subjected to external forces, the matrix would transfer the load to the HA powder, thus enhancing the strength of hydrogel [36]. 3.5. Water content Water content is the essential quantity for biomaterials. For different biological applications, the amount of requirement is different. For the application in articular cartilage repairing, the water content required is 79.2% [37]. And moreover, water content reflects the free space of molecule motion, and it is relative to the network structure of hydrogel. Fig. 6 shows the dependence of water content (W, mass fraction) on the mass fraction of HA (WHA, mass fraction) for PVA/PVP/HA hydrogels. Apparently, a liner decrease relationship between the W and the WHA is observed. For hydrogels, the network space is occupied by water. The decrease of water content with the amount of HA increasing is caused
952
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 4. SEM micrographs show the dispersion state of HA in PVA/PVP/HA composite hydrogels after vacuum drying. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e) 4.5%, respectively.
by two factors; the interaction between Ca2 + ion and the matrix of PVA/PVP gel leading the matrix become denser, and the hydroxyapatite particles occupy the space to squeeze out water from the hydrogel. Therefore, the more the HA content, the smaller the
space to support water. But, even if the water content of the PVA/ PVP/HA hydrogel decreases to a minimum value of 83.67%, it still is a suitable amount to satisfy the requirements of many biomaterial applications.
Fig. 5. The dependence of storage modulus G′/MPa (a) and loss modulus G″/MPa (b) on the angular frequency for the PVA/PVP/HA composite hydrogels with different HA mass fractions. The mass percent fraction of HA is (■), 0.0%; (●), 0.5%; (▲), 1.5%; (▼), 3.0%; and (◆), 4.5%, respectively.
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
953
Fig. 6. The dependence of the water content (100W, mass percent fraction) on the mass percent fraction of HA (100WHA); the standard deviation is shown in bars. The average standard deviation is 0.03%.
3.6. Swelling behavior Swelling is a continuous transition process from the solvent-free glassy state or partially rubbery state to a relaxed rubbery solvent containing state [38]. Two opposite forces, the solvent infiltration and the elastic shrinkage from the network strain, form a confrontation in the process of swelling. When they reach a dynamic balance, swelling reaches the equilibrium. The transport of molecules from the medium into the gel's network is the primary process of swelling. The swelling trail records the kinetics character of a dried gel to absorb water, and the historical trail to reach the swelling equilibrium. The swelling behavior of polymeric hydrogel will provide information about the conformational change of the polymer chain and the volume change of the hydrogel network. The swelling kinetics will give parameters of the equilibrium water content and the swelling rate constant. The equilibrium water content reflects the free volume of hydrogel to accommodate water, and the swelling rate constant reflects the rate to absorb water. Obviously, they are relative to the interaction between water and the molecule groups in the hydrogel chain. Usually, the swelling curve can be fitted by a second order kinetics equation, the swelling rate constant and the equilibrium water content can be obtained. The swelling rate at time t may be expressed as: dW=dt ¼ ks ðW e −W Þ2
ð3Þ
where, the W is the water content (mass fraction) of the gel in the swollen state at time t, the We is the maximum or equilibrium water content, and the ks is the rate constant of swelling. By integrating and rearranging the equation, the following expression can be obtained: W ¼ W e ½1−1=ð1 þ ks tW e Þ
ð4Þ
Applying the non-linear curve fitting method to fit the experimental swelling data using this equation, the swelling kinetic parameters (We and ks) can be obtained. The swelling isotherms of PVA/PVP/HA composite hydrogels in water, in aqueous NaCl solution (c = 0.15 mol/dm3), and in aqueous dextran solution (0.1 g/dm3) are shown in Fig. 7. In the beginning region of swelling, the water content W(t) increases sharply with time t, and then in the following region, W(t) gradually reaches to an equilibrium value as time goes on. With increasing in the amount of HA, the equilibrium water content We of hydrogels decreases; that is because more
Fig. 7. The swelling kinetics curves of PVA/PVP/HA hydrogels in (a), water; (b), 0.15 mol/dm3 NaCl solution; and (c), 0.1 g/dm3 aqueous dextran solution, respectively. The mass percent fraction of HA is (■), 0.0%; (●), 0.5%; (▲), 1.5%; (▼), 3.0%; and (◆), 4.5%, respectively.
network space is occupied by HA. And the interaction between the Ca2+ ion and the matrix of PVA/PVP gel leads the matrix be compressed. By fitting the experimental swelling data using Eq. (4), the swelling kinetics parameters (We and ks) are obtained. The plots of We and ks as a function of mass fraction of HA are shown in Figs. 8 and 9, respectively. From the result in Fig. 8, it can be observed that the water content decreases with the increasing in WHA; that is HA leads the free volume of the hydrogel to be compressed. For NaCl and dextran aqueous
954
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 8. The dependence of the swelling equilibrium water content We on the mass percent fraction of HA WHA, for the PVA/PVP/HA hydrogels swelling in (■), water; (●), 0.15 mol/dm3 aqueous NaCl solution; and (▲), 0.1 g/dm3 aqueous dextran solution, respectively where: W HA ¼
mHA
∑mi
; W e;H2 O ¼
mH2 O
∑mi
; W e;aq:NaCI ¼
maq:NaCI;solution
∑mi
; W e;aq:Dex ¼
maq:Dex;solution
∑mi
Fig. 10. The DSC thermograms of vacuum dried PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e) 4.5%, respectively.
:
solution, their water content is lower than the value of pure water. This may be due to the salting out effect of electrolytes, and due to the volume effect of macromolecules when it infiltrates into the network space. From the result in Fig. 9, it can be observed that the variations of ks with WHA in aqueous NaCl solution and in aqueous Dextran solution are obviously lower than the values in water. This result indicates that (1), in pure water, the swelling rate increases with the HA content, and a maximum is existed; that is HA has a limited capacity to absorb water; (2), in electrolyte solution, the capacity of HA to absorb water is resisted by electrolyte; and (3), in dextran solution, the capacity of HA to absorb water is blocked by macromolecule. Above all, the salting out effect of electrolytes and the volume effect of macromolecules will depress the equilibrium water content and swelling rate when they infiltrate the network space of hydrogel. That is to say, the 3D cellular structure of PVA/PVP/HA composite hydrogel has selectivity on the permeability of electrolyte solution and macromolecular solution. Furthermore, by comparing the values of the equilibrium water content We obtained from swelling in Fig. 8 and the value of the water
content W from vacuum drying in Fig. 6, it is found that the value of We is far less than the value of W. This phenomenon is due to the irreversible structure destroyed during the process of vacuum drying. 3.7. Melting enthalpy and degree of crystallinity The crystallinity of PVA is proportional to its melting enthalpy, and the crystallinity of a vacuum dried gel is defined as the ratio of melting enthalpy of the sample (ΔfusH) to the theoretical heat needed for melting a 100% crystalline PVA (ΔfusH°). If the mass fraction of PVA is WPVA, the theoretical contribution from PVA will be (ΔfusH° × WPVA). Therefore, the crystallinity degree of a sample is estimated by equation: f c ðDSC Þ ¼ Δfus H= Δfus H W PVA 100%
ð5Þ
where, ΔfusH (J/g) is the enthalpy of melting per unit weight of the semicrystalline polymer, which can be measured by DSC, and ΔfusH°(J/g) is the enthalpy of melting per unit weight of the 100% perfect crystalline polymer PVA. For PVA, ΔfusH° =138.6 J/g, which is taken from literature [39]. Fig. 10 shows the DSC thermograms (dH/dT vs. T) of vacuum dried PVA/PVP/HA composite hydrogels with different mass percent fractions of HA. The sharp endothermic peak at about 504 K is corresponding to the melting and the wide one appearing at about 540 K is attributed to the decomposition of sample as literature reports the transition region from 280 °C (553 K) to 435 °C (708 K) [35]. The melting temperature (Tm), decomposition temperature (Td), melting enthalpy (ΔfusH) and degree of crystallinity by DSC, f (DSC), are summarized in Table 1. It reveals that Tm, ΔfusH and f (DSC) are decreased with the increase in the amount of HA. HA occupies a part of the cross-linkage point in the network, breaks down the primary Table 1 The thermal parameters and the degree of crystallinity for PVA/PVP/HA composite hydrogel.
Fig. 9. The dependence of the kinetic rate constant (ks) on the mass percent fraction of HA (WHA) for the swelling process (■), in water, (●), in 0.15 mol/dm3 aqueous NaCl solution and (▲), in 0.1 g/dm3 aqueous dextran solution, respectively.
100W(HA)
Tm (K)
T d (K)
ΔfusH (J g−1)
WPVA
f (DSC)
0.0 0.5 1.5 3.0 4.5
505.1 503.8 503.1 503.2 503.0
525.4 543.6 546.6 541.4 537.2
53.59 50.29 45.42 44.83 43.83
0.960 0.955 0.945 0.930 0.915
40.28 37.99 34.68 34.55 34.09
Where: T m: melting temperature, Td: decomposition temperature, ΔfusH: melting enthalpy, and WPVA: weight fraction of PVA in the composite hydrogel.
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 11. The DSC curves (dH/dT vs. T) of different heating rates (β = 1 to 5 K/min) for the dehydration of PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0% and (b), 1.5% respectively.
inter-linkage chains of polymer, and results in the decrease of crystallinity degree. This result reveals that with the filling of HA into the PVA/PVP hydrogels, the linkage between PVA chains was interrupted, which leads to the crystallinity degree decrease. In spite of this, the interaction between ion and matrix, and the occupation of HA in network space, will lead the space to be compressed and the mechanical properties (G′) to be strengthened.
955
Fig. 13. The linear relationship between ln(βi/T2α,i)and 1000/T obtained with α changes from 0.05 to 0.95 (from right to left) for PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0% and (b), 1.5%, respectively.
3.8. Dehydration kinetics analysis The dehydration process of hydrogel is composed of two steps, the diffusion of water in the hydrogel network, and then evaporation. The dehydration kinetic parameters, especially the activation energy, will help us to recognize the energy needed in diffusion and evaporation. Apparently, diffusion is relative to the interaction between water and hydrogel chains.
Fig. 12. The relationship between the conversion (α) and the temperature T for the heating rates (βi) changes from 1 K/min−1 to 5 K/min−1 (from left to right). The mass percent fraction of HA is (a), 0.0% and (b), 1.5%, respectively.
956
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 14. The dependence of the dehydration activation energy (Ea) on the conversion (α) obtained by KAS method. The mass percent fraction of HA is (■), 1.5%; (●), 0.0%; respectively, and (▲) indicates the water evaporation.
Nonisothermal dehydration kinetics for PVA/PVP/HA hydrogel samples with mass percent fraction of 0.0 and 1.5% of HA were obtained by DSC iso-conversional method, which needs a series of DSC curves (dH/ dT vs. T) with different heating rates (β) from 1 to 5 K/min as shown in Fig. 11. The iso-conversional method is usually divided into differential method and integral method. For the integral method, many approximation methods were developed based on the following linear equation [40]:
ln βi =T B α;i ¼ const−C Ea =RT α;i
ð6Þ
where, the α is the reaction conversion, β is the heating rate of DSC operation, Ea is the activation energy, and the value of parameters B and C depends on the integral approximation type of temperature. This equation is used for the DSC curves with different heating rates (βi) at identical conversion α. Among the various approximation methods, the Kissinger–Akahira–Sunose (KAS) iso-conversional method is more extensively used, because of its practicality and accuracy. The KAS equation is expressed with approximation of B = 2 and C = 1, that is,
ln β i =T 2 α;i ¼ const− Ea =RT α;i
ð7Þ
Fig. 15. The dependence of ΔEa on α for PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (■), 1.5% and (●), 0.0%, respectively.
To estimate the activation energy using Eq. (7), we need to do the following works: (1), for all the DSC curves with different heating rates (βi), in the region of α = 0.05–0.95, convert the DSC curve to the (α vs. T) curve; and then (2), to construct the curves of the [ln(βi/T2α, i) vs. 1/T] with several constant α. The curves of (α vs. T) are shown in Fig. 12, and the curves of [ln(βi/T2α,i) vs. 1000/T] are shown in Fig. 13. The data of ln(βi/T2α, i) can be calculated from the data in Fig. 12. In Fig. 13, the linear relationship at different α is constructed by Eq. (7). From the slope of these linear curves, the activation energy of dehydration Ea at different α is obtained, apparently, Ea varying with conversion α. The dependence of the dehydration activation energy (Ea) on the conversion (α) by KAS method is showed in Fig. 14. The dehydration process of hydrogel can be regarded as the vaporization process of liquid water. For a comparison, the evaporation of liquid water is also measured by DSC under the same operation conditions. The activation energy is shown in Fig. 14 for comparison. Apparently, the activation energies of hydrogel dehydration are higher than the value of liquid water evaporation. Actually, there are two significant factors affecting the process of dehydration, the interaction between water and gels and the diffusion of water molecules in network medium [41]. Fig. 14 reveals that the addition of HA leads the dehydration activation energy to be increased. Apparently, this result indicates that HA improves the interaction between water and network, and impedes the diffusion of water. The difference between the activation energy of hydrogel and liquid water at the same conversion is used to estimate the diffusion activation energy of water in the network space [42]. ΔEa ¼ Ea ðhydrogelÞ−Ea ðliq; waterÞ
ð8Þ
The variation of ΔEa with α is shown in Fig. 15. It reveals that the diffusion activation energy of water is higher in PVA/PVP/HA, (approximately 13 kJ/mol), than in PVA/PVP (approximately 6 kJ/mol). This result tells us that the diffusion of water in hydrogel is different from the diffusion in pure liquid state. The interaction between water and hydrogel chain has impact on the diffusion. By comparing the value of ΔEa, it reveals that PVA/PVP/HA has higher ΔEa value, and so has higher capacity to absorb water; this capacity comes from the interaction between HA and water. 4. Conclusion In this work, a hydrogel of PVA/PVP/HA was prepared by a repeated freezing and thawing technique. By using theoretical models, the information about the swelling kinetics and the dehydration kinetics has been obtained. The effect of HA on the hydrogel was evaluated by comparing the physical and chemical properties of PVA/PVP/HA and PVA/ PVP hydrogels. By analyzing the structure, mechanical properties, and molecular interaction, the application of PVA/PVP/HA hydrogel to serve as a biomaterial has been evaluated. The X-ray result shows that, for PVA/PVP/HA network, there is no apparent interruption on both the HA crystalline and PVA/PVP amorphous state. For application in bio-materials, our results suggest that the osteoinductivity of HA was not interrupted in the PVA/PVP/HA hydrogel network environment. FT-IR spectrum indicates that the intermolecular interaction is the main form of the interaction in the hydrogel environment. The hydrogel structure, mechanical properties, water content, and dehydration kinetics all depends on this interaction. SEM images, water content test, and swelling test all reflect that the free space is compressed with more HA filled in. Additionally, the results of mechanical properties and the dehydration kinetics were considered; it can be found that, with the increase in the HA mass fraction, the water content is reduced, the G′ and G″ is enlarged, and the diffusion activation energy of water is increased. And moreover, in electrolyte solution and
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
polymer solution, HA leads the swelling rate constant to be lower than the value in pure water. Above all, in the PVA/PVP/HA hydrogel, HA will enhance the molecular interaction, and lead the network space to be compressed. Relative to PVA/PVP, in spite of its water content being decreased, it still meets the requirement of bio-materials. When the hydrogel is subjected to external forces, the matrix would transfer the load to the HA powder, thus enhancing the strength of hydrogel. Acknowledgments This project was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] S. Van Vlierberghe, P. Dubruel, E. Schacht, Biomacromolecules 12 (2011) 1387. [2] Y. Liu, L.M. Geever, J.E. Kennedy, C.L. Higginbotham, P.A. Cahill, G.B. Mcguinness, J. Mech. Behav. Biomed. 3 (2010) 203. [3] M. Kobayashi, J. Toguchida, M. Oka, Biomaterials 24 (2003) 639. [4] K.L. Spiller, S.J. Laurencin, D. Charlton, S.A. Maher, A.M. Lowman, Acta Biomater. 4 (2008) 17. [5] B.R. Rawal, R. Ribeiro, M. Chouksey, K. Tripathi, J. Med. Sci. 13 (8) (2013) 615. [6] M.H. Alves, B.E.B. Jensen, A.A.A. Smith, A.N. Zelikin, Macromol. Biosci. 11 (2011) 1293. [7] M.I. Baker, S.P. Walsh, Z. Schwartz, B.D. Boyan, J. Biomed. Mater. Res. B 100B (2012) 1451. [8] K.L. Spiller, S.A. Maher, A.M. Lowman, Tissue. Eng. B 17 (2011) 281. [9] B. Balakrishnan, R. Banerjee, Chem. Rev. 111 (2011) 4453–4474. [10] J.A. Stammen, S. Williams, D.N. Ku, R.E. Guldberg, Biomaterials 22 (2001) 799. [11] M. Oka, J. Orthop. Sci. 6 (2001) 448. [12] C.R. Nuttelman, D.J. Mortisen, S.M. Henry, K.S. Anseth, J. Biomed. Mater. Res. 57 (2001) 217. [13] D.R. Pereira, J. Silva-Correia, J.M. Oliveira, R.L. Reis, J. Tissue Eng. Regen. Med. 7 (2013) 85.
957
[14] R. Ma, D. Xiong, F. Miao, J. Zhang, Y. Peng, Mater. Sci. Eng. C 29 (2009) 1979. [15] T. Koyano, N. Minoura, M. Nagura, K. Kobayashi, J. Biomed. Mater. Res. 39 (1998) 486. [16] J.K. Katta, M. Marcolongo, A. Lowman, K.A. Mansmann, J. Biomed. Mater. Res. A 83A (2007) 471. [17] Y. Shi, D. Xiong, Wear 305 (2013) 280. [18] Y. Nishio, T. Haratani, T. Takahashi, J. Polym. Sci. B 28 (3) (1989) 355. [19] R. Murugan, S. Ramakrishna, Biomaterials 25 (2004) 3829. [20] C. Zhou, Y. Hong, X. Zhang, Biomater. Sci. 1 (2013) 1012. [21] K.M. Woo, J. Seo, R. Zhang, P.X. Ma, Biomaterials 28 (2007) 2622. [22] F. Sun, H. Zhou, J. Lee, Acta Biomater. 7 (2011) 3813. [23] G. Turco, E. Marsich, F. Bellomo, S. Semeraro, I. Donati, F. Brun, M. Grandolfo, A. Accardo, S. Paoletti, Biomacromolecules 10 (2009) 1575. [24] J. Han, Z. Zhou, R. Yin, D. Yang, J. Nie, Int. J. Biol. Macromol. 46 (2010) 199. [25] M. Swethaa, K. Sahithi, A. Moorthi, N. Srinivasan, K. Ramasamy, N. Selvamurugan, Int. J. Biol. Macromol. 47 (2010) 1. [26] S. Wu, H. Hsu, S. Hsu, W. Wang, W. Ho, Mater. Charact. 62 (2011) 526. [27] H. Zhou, J. Lee, Acta Biomater. 7 (2011) 2769. [28] Y. Pan, D. Xiong, Wear 266 (2009) 699. [29] Y. Pan, D. Xiong, F. Gao, J. Mater. Sci. Mater. Med. 19 (2008) 1963. [30] A.S. Maiolo, M.N. Amado, J.S. Gonzalez, V.A. Alvarez, Mater. Sci. Eng. C 32 (2012) 1490. [31] J.S. Gonzalez, V.A. Alvarez, J. Mech. Behav. Biomed. 34 (2014) 47. [32] M. Du, W. Song, Y. Cui, Y. Yang, J. Li, J. Mater. Chem. 21 (2011) 2228. [33] G. Wu, B. Su, W. Zhang, C. Wang, Mater. Chem. Phys. 107 (2008) 364. [34] X. Huang, Y. Zuo, J.D. Li, Y.B. Li, Mater. Res. Innov. 13 (2009) 98. [35] I.S. Elashmawi, H.E. Abdel Baieth, Curr. Appl. Phys. 12 (2012) 141. [36] S.S. Silva, A. Motta, M.T. Rodrigues, A.F. Pinheiro, M.E. Gomes, J.F. Mano, R.L. Reis, C. Miqliaresi, Biomacromolecules 9 (2008) 2764. [37] K. Toth, Z. Aigner, K. Wellinger, P. Szabo-Revesz, G. Sohar, Thermochim. Acta 506 (2010) 94. [38] F. Ganji, S. Vashehani-Farahani, E. Vasheghani-Farahani, Iran. Polym. J. l19 (5) (2010) 375. [39] N.A. Peppas, E.W. Merrill, J. Appl. Polym. Sci. 20 (1976) 1457. [40] S. Vyazovkin, A.K. Burnham, J.M. Criado, L.A. Pérez-Maqueda, C. Popescud, N. Sbirrazzuolie, Thermochim. Acta 520 (2011) 1. [41] L. Liu, B. Wang, Y. Gao, T. Bai, Carbohydr. Polym. 97 (2013) 152–158. [42] J. Kučerík, A. PrűŠová, A. Rotaru, K. Flimel, J. Janeček, P. Conte, Thermochim. Acta 523 (2011) 245.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
The physical and chemical properties of the polyvinylalcohol/ polyvinylpyrrolidone/hydroxyapatite composite hydrogel Yahui Ma, Tongchun Bai ⁎, Fei Wang College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
a r t i c l e
i n f o
Article history: Received 31 March 2015 Received in revised form 25 September 2015 Accepted 26 October 2015 Available online xxxx Keywords: Polyvinylalcohol Polyvinylpyrrolidone Hydroxyapatite Hydrogel Storage modulus Water content Swelling kinetics Dehydration kinetics
a b s t r a c t A hydrogel of polyvinylalcohol (PVA)/polyvinylpyrrolidone (PVP)/hydroxyapatite (HA) was prepared by a repeated freezing and thawing technique. The effect of HA on the hydrogel was evaluated by comparing the physical and chemical properties of PVA/PVP/HA and PVA/PVP hydrogels. By using theoretical models, the information about the swelling kinetics and the dehydration kinetics have been obtained. From the analysis of structure, mechanical properties, and molecular interaction, the application of PVA/PVP/HA hydrogel as a biomaterial has been evaluated. Relative to PVA/PVP, the PVA/PVP/HA hydrogel is of denser network structure, lower water content, larger storage modulus, and higher dehydration activation energy. These results reveal that, as HA fills in the hydrogel, the molecular interaction is enhanced, the free space of network is compressed, and the diffusion activation energy of water is increased. In spite of its water content being decreased, it is still in the range of meeting the requirement of bio-application. When the hydrogel is subjected to external forces, the matrix will transfer the load to the HA powder, thus enhance the strength of the hydrogel. For application in bio-materials, HA will still have osteoinductivity because its crystalline structure is not interrupted in PVA/PVP/HA hydrogel environment. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polyvinylalcohol (PVA) hydrogels are three-dimensional network polymers that are physically or chemically cross-linked and are able to absorb large amounts of water [1,2]. Due to its higher water content [3], microporous structure [4], and favorable mechanical and lubricating properties [5–7], the PVA hydrogels have been extensively recognized as a potential material for a variety of applications in the pharmaceutical, biomedical industry, [8,9] especially in artificial biomaterials for cartilage repair [2,3,6,7]. Even so, some physical–chemistry properties, mechanical properties and biocompatibilities of PVA hydrogels do not seem to be sufficient to serve as a biomaterial for artificial articular cartilage [10,11]. In clinical application, some problems such as fixation method and the durability remain to be solved [3]. In biomedical application, the adherence of cell on PVA hydrogel is inhibited by its highly hydrophilic nature [12, 13]. Therefore, there are still tremendous works to be done. Some researchers focus on blending PVA with other bioactive macromolecules, for example gelatin [2], poly(vinyl pyrrolidone) (PVP) [14], and chitosan [15] to form composite hydrogels, and so to improve their biocompatibility. Polyvinylpyrrolidone (PVP) is a biocompatible and hydrophilic polymer which has been used for a number of biomedical applications [14]. ⁎ Corresponding author. E-mail address: [email protected] (T. Bai).
http://dx.doi.org/10.1016/j.msec.2015.10.081 0928-4931/© 2015 Elsevier B.V. All rights reserved.
Over the last decades, the miscibility and mechanical strength of the hydrogel by blending PVA and PVP have been intensively investigated [4,14,16–18]. Nishio et al. studied the miscibility and orientation behavior of PVP/PVA blends by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) [18]. Ma et al. prepared a novel PVP/PVA hydrogel through a repeated freezing and thawing method. Their results indicated that the mechanical and tribological properties of PVP/PVA hydrogels were significantly dependent on PVP content [14]. Recently, calcium phosphate (CaP), especially the hydroxyapatite [Ca10(PO4)6(OH)2, acronym HA] is known for its biocompatible, bioactive (i.e. ability of forming a direct chemical bond with surrounding tissues), osteoconductive, non-toxic, non-inflammatory, and non-immunogenic properties [19,20]. Thus, HA is generally used as an osteogenic inducer for bone substitution [21]. Mostly, the synthesized hydroxyapatite with bone-bonding properties is widely used in hard tissue replacement [22– 27]. Additionally, there are also some literatures reporting polymer/ hydroxyapatite hydrogels for articular cartilage repairing [28–31], and confirming that the addition of HA nanocrystals in cartilage scaffolds can improve cellular adhesion and proliferation significantly, thus contributing to the repair and regeneration of cartilage [20,32]. For example, M. Du et al. have used Ca2+ as a crosslinker to fabricate nanohydroxyapatite (nHA)/alginate (ALG) hydrogel, the culture experiments of human embryo skin fibroblast (ESF) exhibit that nHA can effectively improve cellular adhesion on the hydrogel surface [32]. Wu et al. have incorporated HA to PVA hydrogel to form a bioactive composite for
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
applications as artificial cartilage [33]. In the same field, Gonzalez et al. have successfully produced and characterized PVA/HA composite hydrogels, with different nanofiller contents, with potential application as articular cartilage replacement [31]. To utilize the advantages of PVA, PVP, and HA, the composite hydrogels of PVA/PVP/HA with different mass percent fractions of HA were prepared in this work. In the view of literatures cited above, the chemical composition of this composite hydrogel will be in favor of the application in articular cartilage repairing or replacement, and will improve the biocompatibility and the mechanical property. However, this suppose should be checked by experiments, and the physical properties of the composite hydrogel should meet the requirement of biomaterials. But, report on the physical chemical properties is less in detail, especially in the effect of HA on the molecular interactions in the hydrogel network environment. And theoretically, the physical properties should be explained reasonably. Therefore, in this work, some physical and chemical properties about structure, molecular interaction, and mechanical properties were determined in detail. By comparing the properties of PVA/PVP/HA and PVA/PVP, the effect of HA on the hydrogel was evaluated. And moreover, some theoretical models were introduced to explain the experimental results and to provide theoretical parameters to evaluate the application of the hydrogel in biomaterials. 2. Experimental 2.1. Materials Polyethanol, commonly known as poly(vinyl alcohol), given the acronym PVA, with mass fraction N0.99, weight average molar mass = 68.8 kg/mol and determined by the intrinsic viscosity method, was supplied by the Shanghai Chemical Reagent Inc., and used in experiments without further purification. Polyvinylpyrrolidone (PVP) K30 (CAS. no.: 9003-39-8, with IUPAC name: 1-ethenylpyrrolidin-2-one and a 0.985 mass fraction purity) was received from Sinopharm Chemical Reagents Co. Ltd. To prepare the hydroxyapatite particles, calcium nitrate [Ca (NO3)2 ⋅ 4H2O] and ammonium hydrogen phosphate [(NH4)2HPO4], were also supplied by Sinopharm Chemical Reagent Co. Ltd., China. 2.2. Methods 2.2.1. Preparation of PVA/PVP/HA composite hydrogel Aqueous solutions of PVA/PVP were prepared by dissolving 10 g PVA and 4 g PVP in water at 90 °C and slowly stirred for 4 h by a magnetic stirrer. To prepare hydroxyapatite particles, aqueous solution of (NH4)2HPO4 was added into the PVA/PVP stirred solution drop-by-drop at temperature of 80 °C, followed by the addition of Ca(NO3)2 aqueous solution. The quantities of Ca(NO3)2 ∙4H2O and (NH4)2HPO4 were controlled to provide a Ca/P molar ratio of 1.67. Meanwhile, the ammonia was added into the solution drop-by-drop to maintain pH within 9–10 by adjusting the water content to control the total mass of above mixture to be 100 g. The obtained suspension mixture was stirred at about 1200 rpm for further 4 h, and then cooled to room temperature. The suspension mixture was then poured into a mold, and temperature was controlled to −15 °C for 10 h, and then thawed at room temperature for another 2 h. This freezing and thawing cycle was repeated for 7 times. The obtained PVA/PVP/HA composite hydrogels with different HA mass percent fractions (0.0, 0.5, 1.5, 3.0, 4.5) % represented by symbols 0.0HA, 0.5HA, 1.5HA, 3.0HA, and 4.5HA, respectively, were stored in water for further characterization. The mass fraction of HA was calculated by w (HA) = m (HA) / [m (HA) + m (PVA) + m (PVP) + m (H2O)], where m is the mass of component. In the process of preparation, the
949
concentration of PVA and PVP is 10% and 4% (mass percent), respectively. 2.2.2. X-ray diffraction (XRD) measurement The X-ray diffraction spectra of HA, PVA/PVP and PVA/PVP/HA composite hydrogels with different mass percent fractions of HA were obtained by X-ray diffractometer (X'pert, PRO PAN Alytical, Cu − Kα, λ = 0.15406 nm) at a scanning 2θ ranging from 10 to 70 with a step width of 0.003. 2.2.3. Fourier-transform infrared spectroscopy (FT-IR) analysis The FT-IR spectra of HA, PVA/PVP and PVA/PVP/HA composite hydrogels were recorded by infrared spectrometer (Nicolet 6700, American), with a wave number range from 400 to 4000 cm−1. 2.2.4. Morphology characterization Phase morphology characterization of the gel samples (vacuum dried after freeze drying) were performed by scanning electron microscopy (SEM) (S-4700, Hitachi Limited, Japan) using 15 kV for secondary electron imaging after coating with Au using a sputter coater. 2.2.5. Rheological analysis Rheological characterization of hydrogel samples was performed on a Haake Rheostress 6000 rheometer (Thermo Electron Co., Karslruhe, Germany) with oscillation frequency sweep. The tests were operated at 37 °C, a shear stress of 1 Pa, and the angular frequency ω range from 0.1 to 100 rad/s, and the measuring gap size is about 0.2 mm. 2.2.6. Water content determination To determine the water content of PVA/PVP/HA composite hydrogels with different mass percent fractions of HA, the samples were dried to a constant weight in a vacuum oven at 70 °C for at least 36 h. The water content W was calculated by the following equation: W ¼ ðm−m0 Þ=m
ð1Þ
where, m and m0 represent the mass of the PVA/PVP/HA composite hydrogels before and after drying respectively. All samples were determined three times. The average value is obtained with a standard deviation of 0.0003. 2.2.7. Swelling behavior characterization The dried hydrogels (in vacuum at 70 °C for 36 h) were immersed in water, aqueous electrolyte solution (NaCl, c = 0.15 mol/dm3) and aqueous Dextran solution (0.1 g/dm3) at 37 °C. The swollen weight was determined by weighting gel block samples after wiping off the surface water at time t in a regular time interval. The swelling ratio was defined by the equation as follows: W ðt Þ ¼ ðmt −m0 Þ=mt
ð2Þ
where, m0 and mt are the weight of the dried gel sample and the sample after immersed in solution at time t, respectively. The swelling kinetics was analyzed from the experimental data of W(t)–t. 2.2.8. Melting enthalpy Differential scanning calorimetric (DSC) measurements of dried samples were performed using a Netzsch DSC 204F1 instrument (Germany) under nitrogen gas flow rate of 70 mL/min and at a heating rate of 5 °C/min from room temperature to 350 °C. Melting enthalpy and melting temperature (Tm) were obtained from the thermograms. 2.2.9. Dehydration kinetics measurements Differential scanning calorimetric (DSC) measurements were carried out on a Netzsch DSC 204F1 instrument (Germany). The dehydration curves of hydrogel samples were recorded from room temperature to
950
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
150 °C at heating rates βi = 1, 2, 3, 4, 5 K/min, respectively, in a nitrogen atmosphere. The dehydration kinetics was analyzed from these curves. 3. Results and discussion 3.1. XRD analysis The XRD spectrum of HA and PVA/PVP/HA dried gel samples is displayed in Fig. 1. The symbols of 0.0HA, 0.5HA, 1.5HA, 3.0HA, and 4.5HA represent HA mass percent fraction to be 0.0, 0.5, 1.5, 3.0, and 4.5%, respectively. In Fig. 1(a), the peaks at 2θ = 19.61 and 40.82 show the characteristic peaks of PVA/PVP gel, which indicates the gel is an amorphous solid. In Fig. 1(f), the peaks centered at 2θ = 25.92; 28.15, 28.87; 31.85, 32.26, 32.88, 34.22; 39.79; 46.79; and 49.47 etc. represent the characteristic peaks of the HA crystalline structure [34]. For PVA/PVP/HA dried gel samples, the observed spectra of Fig. 1(b), (c), (d), (e) can be considered as the combination of both PVA/PVP gel and HA crystal. The intensity of the amorphous peak of PVA/PVP at 2θ = 19.61 apparently decreases with the increasing in HA percentage, while the typical crystalline peaks of HA are enhanced. These indicate that there is no apparent crystalline interruption about the HA crystalline in the PVA/PVP/HA gel samples. Therefore, a conclusion can be obtained from the above observation that HA was filled in the gap of PVA/PVP network, and there is no apparent interruption on both the HA crystalline and the PVA/PVP amorphous state. In Elashmawi's work [35], they inferred that there was a significant motion of polymer blend chains in the amorphous region or there were some defects existing at an interface between the polymer chain and the HA filler. For application in bio-materials, our results suggest that the osteoinductivity of HA was not interrupted in PVA/PVP/HA hydrogel network environment. 3.2. FT-IR analysis Fig. 2 shows the FT-IR spectrum of (a), HA powder, (b), PVA/PVP dried gel, and (c), PVA/PVP/HA dried gel (due to the fact that all the PVA/PVP/HA samples have similar FT-IR spectrum, only the spectrum of 1.5HA sample is shown for a clear comparison and symbol marking). The characteristic peaks of HA appearing at 1626 cm− 1 and 1377 cm−1 belong to the hydroxyl vibrations, while peaks located at 1052 cm−1, 621.3 cm− 1 and 583.1 cm− 1 belong to the bending and
Fig. 2. The FT-IR spectrum of (a), HA powder, (b), PVA/PVP dried gel and (c), PVA/PVP/HA dried gel with mass percent fraction 1.5% of HA.
stretching of phosphate group. The broad characteristic band around 3413 cm−1 is caused by the absorption of H2O. The peaks at 2921 cm−1, 1434 cm−1, 1281 cm−1, 1090 cm−1 and 834.2 cm−1 in Fig. 2(b) of PVA/PVP can be assigned to the vibrations of –CH2–, –C_C–, and –CO– respectively; the peaks around 3308 cm−1 and 1637 cm−1 are attributed to the absorption of H2O and the hydroxyl vibrations. For PVA/PVP/HA, the FT-IR spectra seem to be a superposition of the spectra of HA and PVA/PVP. A slight shift of the peaks of PVA/PVP/HA from PVA/PVP is observed. However, there are two peaks that differ from the PVA/PVP and the HA. At 1718 and 1647 cm− 1, it can be thought as the splitting of hydroxyl vibrations, and at 1453 cm− 1, it can be thought as the shift of the peak of PVA/PVP at 1434 cm−1. While, the OH vibration peak of HA at 1377 cm−1 disappeared. These may be an evidence of intermolecular interaction between HA and PVA/PVP matrices, especially in the form of hydrogen bond interaction, just as Gonzalez indicated in literature [31]. But no matter what, FT-IR spectrum does not indicate chemical reactions having taken place.
3.3. Morphology
Fig. 1. The X-ray diffraction spectra of HA and PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e), 4.5% respectively and (f), HA powder.
The SEM images of the cross-section of PVA/PVP hydrogels after freeze–drying are shown in Fig. 3. A porous structure can be observed from it. Apparently, from Fig. 3(a)–(e), the pore size decreases with the increase in the content of HA. This phenomenon can be attributed to the fact that the network space filled with HA, and the interaction between HA and PVA/PVP lead the gel space to be compressed. Fig. 4 shows the dispersion state of HA on the gel surface. Apparently, it is relative to the amount of HA to be blended. For the sample of 1.5HA in Fig. 4(c), the mass percent fraction is 1.5%; HA disperses homogeneously in the gel. With more HA mixed in, as in the cases of samples of 3.0HA and 4.5HA in Fig. 4(d) and (e), agglomeration is observed. This may be due to the saturation of HA in PVA/PVP solution in the preparation process, which leads the gel to be macroscopically heterogeneous distribution. A similar result was reported by Gonzalez et al., but their sample composition is of HA 7.5% (mass percent fraction) which consisted only of PVA and HA [31]. In Elashmawi's work [35], a blend of PVP/PVP showed a uniform surface morphology and revealed a rather smooth surface, and after adding HA, an aggregation of the calcium phosphate particles or chunks that randomly distributed on the top surface, their results revealed that the polymer and the HA were compatible.
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
951
Fig. 3. SEM micrographs show the cross-section morphologies of PVA/PVP/HA composite hydrogels after freeze–drying. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e) 4.5% respectively.
3.4. Rheological analysis The mechanical properties of the hydrogels were examined by oscillatory rheological experiments as a function of frequency. Fig. 5 shows the changes in the storage modulus (G′, Fig. 5a), and the loss modulus (G″, Fig. 5b) for the PVA/PVP/HA hydrogels with different HA mass percent fractions. The G′ reflects the elasticity of hydrogel samples, which is defined as the stored energy due to the elastic deformation, and the G″ reflects the viscosity of hydrogel samples [34]. It can be seen from Fig. 5(a) that the G′ of all the samples is independent on the frequency, but increases with the increase in the HA mass fraction until w(HA) = 3.0%, and then decrease at w(HA) = 4.5%. So is the situation of G″ in Fig. 5(b). A similar result was reported by Pan et al.; their result indicated that the G′ and G″ reached the maximum value while nano-HA content is 6%, then decreased to 9% [29]. This phenomenon is relative to the relationship between the chain segment oscillation and the angular frequency of a rheometer. The independence of G′ on frequency indicates that the macromolecular chains can keep up with the change of angular
frequency, and the hysteresis effect is very low. The phenomenon of the increase of G′ with the increase in HA amount indicates that HA can accommodate hydrogel to the higher movement frequency, which can be applied to intense conditions. When the hydrogel is subjected to external forces, the matrix would transfer the load to the HA powder, thus enhancing the strength of hydrogel [36]. 3.5. Water content Water content is the essential quantity for biomaterials. For different biological applications, the amount of requirement is different. For the application in articular cartilage repairing, the water content required is 79.2% [37]. And moreover, water content reflects the free space of molecule motion, and it is relative to the network structure of hydrogel. Fig. 6 shows the dependence of water content (W, mass fraction) on the mass fraction of HA (WHA, mass fraction) for PVA/PVP/HA hydrogels. Apparently, a liner decrease relationship between the W and the WHA is observed. For hydrogels, the network space is occupied by water. The decrease of water content with the amount of HA increasing is caused
952
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 4. SEM micrographs show the dispersion state of HA in PVA/PVP/HA composite hydrogels after vacuum drying. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e) 4.5%, respectively.
by two factors; the interaction between Ca2 + ion and the matrix of PVA/PVP gel leading the matrix become denser, and the hydroxyapatite particles occupy the space to squeeze out water from the hydrogel. Therefore, the more the HA content, the smaller the
space to support water. But, even if the water content of the PVA/ PVP/HA hydrogel decreases to a minimum value of 83.67%, it still is a suitable amount to satisfy the requirements of many biomaterial applications.
Fig. 5. The dependence of storage modulus G′/MPa (a) and loss modulus G″/MPa (b) on the angular frequency for the PVA/PVP/HA composite hydrogels with different HA mass fractions. The mass percent fraction of HA is (■), 0.0%; (●), 0.5%; (▲), 1.5%; (▼), 3.0%; and (◆), 4.5%, respectively.
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
953
Fig. 6. The dependence of the water content (100W, mass percent fraction) on the mass percent fraction of HA (100WHA); the standard deviation is shown in bars. The average standard deviation is 0.03%.
3.6. Swelling behavior Swelling is a continuous transition process from the solvent-free glassy state or partially rubbery state to a relaxed rubbery solvent containing state [38]. Two opposite forces, the solvent infiltration and the elastic shrinkage from the network strain, form a confrontation in the process of swelling. When they reach a dynamic balance, swelling reaches the equilibrium. The transport of molecules from the medium into the gel's network is the primary process of swelling. The swelling trail records the kinetics character of a dried gel to absorb water, and the historical trail to reach the swelling equilibrium. The swelling behavior of polymeric hydrogel will provide information about the conformational change of the polymer chain and the volume change of the hydrogel network. The swelling kinetics will give parameters of the equilibrium water content and the swelling rate constant. The equilibrium water content reflects the free volume of hydrogel to accommodate water, and the swelling rate constant reflects the rate to absorb water. Obviously, they are relative to the interaction between water and the molecule groups in the hydrogel chain. Usually, the swelling curve can be fitted by a second order kinetics equation, the swelling rate constant and the equilibrium water content can be obtained. The swelling rate at time t may be expressed as: dW=dt ¼ ks ðW e −W Þ2
ð3Þ
where, the W is the water content (mass fraction) of the gel in the swollen state at time t, the We is the maximum or equilibrium water content, and the ks is the rate constant of swelling. By integrating and rearranging the equation, the following expression can be obtained: W ¼ W e ½1−1=ð1 þ ks tW e Þ
ð4Þ
Applying the non-linear curve fitting method to fit the experimental swelling data using this equation, the swelling kinetic parameters (We and ks) can be obtained. The swelling isotherms of PVA/PVP/HA composite hydrogels in water, in aqueous NaCl solution (c = 0.15 mol/dm3), and in aqueous dextran solution (0.1 g/dm3) are shown in Fig. 7. In the beginning region of swelling, the water content W(t) increases sharply with time t, and then in the following region, W(t) gradually reaches to an equilibrium value as time goes on. With increasing in the amount of HA, the equilibrium water content We of hydrogels decreases; that is because more
Fig. 7. The swelling kinetics curves of PVA/PVP/HA hydrogels in (a), water; (b), 0.15 mol/dm3 NaCl solution; and (c), 0.1 g/dm3 aqueous dextran solution, respectively. The mass percent fraction of HA is (■), 0.0%; (●), 0.5%; (▲), 1.5%; (▼), 3.0%; and (◆), 4.5%, respectively.
network space is occupied by HA. And the interaction between the Ca2+ ion and the matrix of PVA/PVP gel leads the matrix be compressed. By fitting the experimental swelling data using Eq. (4), the swelling kinetics parameters (We and ks) are obtained. The plots of We and ks as a function of mass fraction of HA are shown in Figs. 8 and 9, respectively. From the result in Fig. 8, it can be observed that the water content decreases with the increasing in WHA; that is HA leads the free volume of the hydrogel to be compressed. For NaCl and dextran aqueous
954
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 8. The dependence of the swelling equilibrium water content We on the mass percent fraction of HA WHA, for the PVA/PVP/HA hydrogels swelling in (■), water; (●), 0.15 mol/dm3 aqueous NaCl solution; and (▲), 0.1 g/dm3 aqueous dextran solution, respectively where: W HA ¼
mHA
∑mi
; W e;H2 O ¼
mH2 O
∑mi
; W e;aq:NaCI ¼
maq:NaCI;solution
∑mi
; W e;aq:Dex ¼
maq:Dex;solution
∑mi
Fig. 10. The DSC thermograms of vacuum dried PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0%; (b), 0.5%; (c), 1.5%; (d), 3.0%; and (e) 4.5%, respectively.
:
solution, their water content is lower than the value of pure water. This may be due to the salting out effect of electrolytes, and due to the volume effect of macromolecules when it infiltrates into the network space. From the result in Fig. 9, it can be observed that the variations of ks with WHA in aqueous NaCl solution and in aqueous Dextran solution are obviously lower than the values in water. This result indicates that (1), in pure water, the swelling rate increases with the HA content, and a maximum is existed; that is HA has a limited capacity to absorb water; (2), in electrolyte solution, the capacity of HA to absorb water is resisted by electrolyte; and (3), in dextran solution, the capacity of HA to absorb water is blocked by macromolecule. Above all, the salting out effect of electrolytes and the volume effect of macromolecules will depress the equilibrium water content and swelling rate when they infiltrate the network space of hydrogel. That is to say, the 3D cellular structure of PVA/PVP/HA composite hydrogel has selectivity on the permeability of electrolyte solution and macromolecular solution. Furthermore, by comparing the values of the equilibrium water content We obtained from swelling in Fig. 8 and the value of the water
content W from vacuum drying in Fig. 6, it is found that the value of We is far less than the value of W. This phenomenon is due to the irreversible structure destroyed during the process of vacuum drying. 3.7. Melting enthalpy and degree of crystallinity The crystallinity of PVA is proportional to its melting enthalpy, and the crystallinity of a vacuum dried gel is defined as the ratio of melting enthalpy of the sample (ΔfusH) to the theoretical heat needed for melting a 100% crystalline PVA (ΔfusH°). If the mass fraction of PVA is WPVA, the theoretical contribution from PVA will be (ΔfusH° × WPVA). Therefore, the crystallinity degree of a sample is estimated by equation: f c ðDSC Þ ¼ Δfus H= Δfus H W PVA 100%
ð5Þ
where, ΔfusH (J/g) is the enthalpy of melting per unit weight of the semicrystalline polymer, which can be measured by DSC, and ΔfusH°(J/g) is the enthalpy of melting per unit weight of the 100% perfect crystalline polymer PVA. For PVA, ΔfusH° =138.6 J/g, which is taken from literature [39]. Fig. 10 shows the DSC thermograms (dH/dT vs. T) of vacuum dried PVA/PVP/HA composite hydrogels with different mass percent fractions of HA. The sharp endothermic peak at about 504 K is corresponding to the melting and the wide one appearing at about 540 K is attributed to the decomposition of sample as literature reports the transition region from 280 °C (553 K) to 435 °C (708 K) [35]. The melting temperature (Tm), decomposition temperature (Td), melting enthalpy (ΔfusH) and degree of crystallinity by DSC, f (DSC), are summarized in Table 1. It reveals that Tm, ΔfusH and f (DSC) are decreased with the increase in the amount of HA. HA occupies a part of the cross-linkage point in the network, breaks down the primary Table 1 The thermal parameters and the degree of crystallinity for PVA/PVP/HA composite hydrogel.
Fig. 9. The dependence of the kinetic rate constant (ks) on the mass percent fraction of HA (WHA) for the swelling process (■), in water, (●), in 0.15 mol/dm3 aqueous NaCl solution and (▲), in 0.1 g/dm3 aqueous dextran solution, respectively.
100W(HA)
Tm (K)
T d (K)
ΔfusH (J g−1)
WPVA
f (DSC)
0.0 0.5 1.5 3.0 4.5
505.1 503.8 503.1 503.2 503.0
525.4 543.6 546.6 541.4 537.2
53.59 50.29 45.42 44.83 43.83
0.960 0.955 0.945 0.930 0.915
40.28 37.99 34.68 34.55 34.09
Where: T m: melting temperature, Td: decomposition temperature, ΔfusH: melting enthalpy, and WPVA: weight fraction of PVA in the composite hydrogel.
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 11. The DSC curves (dH/dT vs. T) of different heating rates (β = 1 to 5 K/min) for the dehydration of PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0% and (b), 1.5% respectively.
inter-linkage chains of polymer, and results in the decrease of crystallinity degree. This result reveals that with the filling of HA into the PVA/PVP hydrogels, the linkage between PVA chains was interrupted, which leads to the crystallinity degree decrease. In spite of this, the interaction between ion and matrix, and the occupation of HA in network space, will lead the space to be compressed and the mechanical properties (G′) to be strengthened.
955
Fig. 13. The linear relationship between ln(βi/T2α,i)and 1000/T obtained with α changes from 0.05 to 0.95 (from right to left) for PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (a), 0.0% and (b), 1.5%, respectively.
3.8. Dehydration kinetics analysis The dehydration process of hydrogel is composed of two steps, the diffusion of water in the hydrogel network, and then evaporation. The dehydration kinetic parameters, especially the activation energy, will help us to recognize the energy needed in diffusion and evaporation. Apparently, diffusion is relative to the interaction between water and hydrogel chains.
Fig. 12. The relationship between the conversion (α) and the temperature T for the heating rates (βi) changes from 1 K/min−1 to 5 K/min−1 (from left to right). The mass percent fraction of HA is (a), 0.0% and (b), 1.5%, respectively.
956
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
Fig. 14. The dependence of the dehydration activation energy (Ea) on the conversion (α) obtained by KAS method. The mass percent fraction of HA is (■), 1.5%; (●), 0.0%; respectively, and (▲) indicates the water evaporation.
Nonisothermal dehydration kinetics for PVA/PVP/HA hydrogel samples with mass percent fraction of 0.0 and 1.5% of HA were obtained by DSC iso-conversional method, which needs a series of DSC curves (dH/ dT vs. T) with different heating rates (β) from 1 to 5 K/min as shown in Fig. 11. The iso-conversional method is usually divided into differential method and integral method. For the integral method, many approximation methods were developed based on the following linear equation [40]:
ln βi =T B α;i ¼ const−C Ea =RT α;i
ð6Þ
where, the α is the reaction conversion, β is the heating rate of DSC operation, Ea is the activation energy, and the value of parameters B and C depends on the integral approximation type of temperature. This equation is used for the DSC curves with different heating rates (βi) at identical conversion α. Among the various approximation methods, the Kissinger–Akahira–Sunose (KAS) iso-conversional method is more extensively used, because of its practicality and accuracy. The KAS equation is expressed with approximation of B = 2 and C = 1, that is,
ln β i =T 2 α;i ¼ const− Ea =RT α;i
ð7Þ
Fig. 15. The dependence of ΔEa on α for PVA/PVP/HA composite hydrogels. The mass percent fraction of HA is (■), 1.5% and (●), 0.0%, respectively.
To estimate the activation energy using Eq. (7), we need to do the following works: (1), for all the DSC curves with different heating rates (βi), in the region of α = 0.05–0.95, convert the DSC curve to the (α vs. T) curve; and then (2), to construct the curves of the [ln(βi/T2α, i) vs. 1/T] with several constant α. The curves of (α vs. T) are shown in Fig. 12, and the curves of [ln(βi/T2α,i) vs. 1000/T] are shown in Fig. 13. The data of ln(βi/T2α, i) can be calculated from the data in Fig. 12. In Fig. 13, the linear relationship at different α is constructed by Eq. (7). From the slope of these linear curves, the activation energy of dehydration Ea at different α is obtained, apparently, Ea varying with conversion α. The dependence of the dehydration activation energy (Ea) on the conversion (α) by KAS method is showed in Fig. 14. The dehydration process of hydrogel can be regarded as the vaporization process of liquid water. For a comparison, the evaporation of liquid water is also measured by DSC under the same operation conditions. The activation energy is shown in Fig. 14 for comparison. Apparently, the activation energies of hydrogel dehydration are higher than the value of liquid water evaporation. Actually, there are two significant factors affecting the process of dehydration, the interaction between water and gels and the diffusion of water molecules in network medium [41]. Fig. 14 reveals that the addition of HA leads the dehydration activation energy to be increased. Apparently, this result indicates that HA improves the interaction between water and network, and impedes the diffusion of water. The difference between the activation energy of hydrogel and liquid water at the same conversion is used to estimate the diffusion activation energy of water in the network space [42]. ΔEa ¼ Ea ðhydrogelÞ−Ea ðliq; waterÞ
ð8Þ
The variation of ΔEa with α is shown in Fig. 15. It reveals that the diffusion activation energy of water is higher in PVA/PVP/HA, (approximately 13 kJ/mol), than in PVA/PVP (approximately 6 kJ/mol). This result tells us that the diffusion of water in hydrogel is different from the diffusion in pure liquid state. The interaction between water and hydrogel chain has impact on the diffusion. By comparing the value of ΔEa, it reveals that PVA/PVP/HA has higher ΔEa value, and so has higher capacity to absorb water; this capacity comes from the interaction between HA and water. 4. Conclusion In this work, a hydrogel of PVA/PVP/HA was prepared by a repeated freezing and thawing technique. By using theoretical models, the information about the swelling kinetics and the dehydration kinetics has been obtained. The effect of HA on the hydrogel was evaluated by comparing the physical and chemical properties of PVA/PVP/HA and PVA/ PVP hydrogels. By analyzing the structure, mechanical properties, and molecular interaction, the application of PVA/PVP/HA hydrogel to serve as a biomaterial has been evaluated. The X-ray result shows that, for PVA/PVP/HA network, there is no apparent interruption on both the HA crystalline and PVA/PVP amorphous state. For application in bio-materials, our results suggest that the osteoinductivity of HA was not interrupted in the PVA/PVP/HA hydrogel network environment. FT-IR spectrum indicates that the intermolecular interaction is the main form of the interaction in the hydrogel environment. The hydrogel structure, mechanical properties, water content, and dehydration kinetics all depends on this interaction. SEM images, water content test, and swelling test all reflect that the free space is compressed with more HA filled in. Additionally, the results of mechanical properties and the dehydration kinetics were considered; it can be found that, with the increase in the HA mass fraction, the water content is reduced, the G′ and G″ is enlarged, and the diffusion activation energy of water is increased. And moreover, in electrolyte solution and
Y. Ma et al. / Materials Science and Engineering C 59 (2016) 948–957
polymer solution, HA leads the swelling rate constant to be lower than the value in pure water. Above all, in the PVA/PVP/HA hydrogel, HA will enhance the molecular interaction, and lead the network space to be compressed. Relative to PVA/PVP, in spite of its water content being decreased, it still meets the requirement of bio-materials. When the hydrogel is subjected to external forces, the matrix would transfer the load to the HA powder, thus enhancing the strength of hydrogel. Acknowledgments This project was funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] S. Van Vlierberghe, P. Dubruel, E. Schacht, Biomacromolecules 12 (2011) 1387. [2] Y. Liu, L.M. Geever, J.E. Kennedy, C.L. Higginbotham, P.A. Cahill, G.B. Mcguinness, J. Mech. Behav. Biomed. 3 (2010) 203. [3] M. Kobayashi, J. Toguchida, M. Oka, Biomaterials 24 (2003) 639. [4] K.L. Spiller, S.J. Laurencin, D. Charlton, S.A. Maher, A.M. Lowman, Acta Biomater. 4 (2008) 17. [5] B.R. Rawal, R. Ribeiro, M. Chouksey, K. Tripathi, J. Med. Sci. 13 (8) (2013) 615. [6] M.H. Alves, B.E.B. Jensen, A.A.A. Smith, A.N. Zelikin, Macromol. Biosci. 11 (2011) 1293. [7] M.I. Baker, S.P. Walsh, Z. Schwartz, B.D. Boyan, J. Biomed. Mater. Res. B 100B (2012) 1451. [8] K.L. Spiller, S.A. Maher, A.M. Lowman, Tissue. Eng. B 17 (2011) 281. [9] B. Balakrishnan, R. Banerjee, Chem. Rev. 111 (2011) 4453–4474. [10] J.A. Stammen, S. Williams, D.N. Ku, R.E. Guldberg, Biomaterials 22 (2001) 799. [11] M. Oka, J. Orthop. Sci. 6 (2001) 448. [12] C.R. Nuttelman, D.J. Mortisen, S.M. Henry, K.S. Anseth, J. Biomed. Mater. Res. 57 (2001) 217. [13] D.R. Pereira, J. Silva-Correia, J.M. Oliveira, R.L. Reis, J. Tissue Eng. Regen. Med. 7 (2013) 85.
957
[14] R. Ma, D. Xiong, F. Miao, J. Zhang, Y. Peng, Mater. Sci. Eng. C 29 (2009) 1979. [15] T. Koyano, N. Minoura, M. Nagura, K. Kobayashi, J. Biomed. Mater. Res. 39 (1998) 486. [16] J.K. Katta, M. Marcolongo, A. Lowman, K.A. Mansmann, J. Biomed. Mater. Res. A 83A (2007) 471. [17] Y. Shi, D. Xiong, Wear 305 (2013) 280. [18] Y. Nishio, T. Haratani, T. Takahashi, J. Polym. Sci. B 28 (3) (1989) 355. [19] R. Murugan, S. Ramakrishna, Biomaterials 25 (2004) 3829. [20] C. Zhou, Y. Hong, X. Zhang, Biomater. Sci. 1 (2013) 1012. [21] K.M. Woo, J. Seo, R. Zhang, P.X. Ma, Biomaterials 28 (2007) 2622. [22] F. Sun, H. Zhou, J. Lee, Acta Biomater. 7 (2011) 3813. [23] G. Turco, E. Marsich, F. Bellomo, S. Semeraro, I. Donati, F. Brun, M. Grandolfo, A. Accardo, S. Paoletti, Biomacromolecules 10 (2009) 1575. [24] J. Han, Z. Zhou, R. Yin, D. Yang, J. Nie, Int. J. Biol. Macromol. 46 (2010) 199. [25] M. Swethaa, K. Sahithi, A. Moorthi, N. Srinivasan, K. Ramasamy, N. Selvamurugan, Int. J. Biol. Macromol. 47 (2010) 1. [26] S. Wu, H. Hsu, S. Hsu, W. Wang, W. Ho, Mater. Charact. 62 (2011) 526. [27] H. Zhou, J. Lee, Acta Biomater. 7 (2011) 2769. [28] Y. Pan, D. Xiong, Wear 266 (2009) 699. [29] Y. Pan, D. Xiong, F. Gao, J. Mater. Sci. Mater. Med. 19 (2008) 1963. [30] A.S. Maiolo, M.N. Amado, J.S. Gonzalez, V.A. Alvarez, Mater. Sci. Eng. C 32 (2012) 1490. [31] J.S. Gonzalez, V.A. Alvarez, J. Mech. Behav. Biomed. 34 (2014) 47. [32] M. Du, W. Song, Y. Cui, Y. Yang, J. Li, J. Mater. Chem. 21 (2011) 2228. [33] G. Wu, B. Su, W. Zhang, C. Wang, Mater. Chem. Phys. 107 (2008) 364. [34] X. Huang, Y. Zuo, J.D. Li, Y.B. Li, Mater. Res. Innov. 13 (2009) 98. [35] I.S. Elashmawi, H.E. Abdel Baieth, Curr. Appl. Phys. 12 (2012) 141. [36] S.S. Silva, A. Motta, M.T. Rodrigues, A.F. Pinheiro, M.E. Gomes, J.F. Mano, R.L. Reis, C. Miqliaresi, Biomacromolecules 9 (2008) 2764. [37] K. Toth, Z. Aigner, K. Wellinger, P. Szabo-Revesz, G. Sohar, Thermochim. Acta 506 (2010) 94. [38] F. Ganji, S. Vashehani-Farahani, E. Vasheghani-Farahani, Iran. Polym. J. l19 (5) (2010) 375. [39] N.A. Peppas, E.W. Merrill, J. Appl. Polym. Sci. 20 (1976) 1457. [40] S. Vyazovkin, A.K. Burnham, J.M. Criado, L.A. Pérez-Maqueda, C. Popescud, N. Sbirrazzuolie, Thermochim. Acta 520 (2011) 1. [41] L. Liu, B. Wang, Y. Gao, T. Bai, Carbohydr. Polym. 97 (2013) 152–158. [42] J. Kučerík, A. PrűŠová, A. Rotaru, K. Flimel, J. Janeček, P. Conte, Thermochim. Acta 523 (2011) 245.