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PAAm hydrogels
Journal of Membrane Science 275 (2006) 187–194
Thermo-responsive sandwiched-like membranes of IPN-PNIPAAm/PAAm hydrogels Marcos R. Guilherme b , Gilsinei M. Campese a , Eduardo Radovanovic b , Adley F. Rubira b , Elias B. Tambourgi a , Edvani C. Muniz b,∗ a
Universidade Estadual de Campinas, Faculdade de Engenharia Qu´ımica, Departamento de Sistemas Qu´ımicos e Inform´atica DESQ, Zeferino Vaz 13081-970, Campinas, SP, Brazil b Universidade Estadual de Maring´ a, Departamento de Qu´ımica, Grupo de Materiais Polim´ericos e Comp´ositos, Av. Colombo 5790, 87020-970 Maring´a, Paran´a, Brazil Received 2 March 2005; received in revised form 23 August 2005; accepted 10 September 2005 Available online 17 October 2005
Abstract The synthesis of sandwiched-like IPN hydrogels having cross-linked PNIPAAm interpenetrated into PAAm networks was carried out by UVinduced polymerization using potassium periodate as a sensitizer. NIPAAm monomers and MBAAm cross-linker were co-polymerized inside previously synthesized cross-linked PAAm. Through SEM images, we observed that the hydrogel membranes were formed by three layers; the internal layer was fully involved by the two others. Significant morphological difference between the internal and external layers was also observed. Thus, sandwiched-like interpenetrating polymer networks were obtained. The internal layer shrank significantly after warming the swollen hydrogel above LCST of PNIPAAm in water, 30–35 ◦ C while the external layers remained swollen and highly porous due to the hydrophilicity of PAAm. The experiments performed at 40 ◦ C revealed that the hydrogels shrank considerably. Collapsed PNIPAAm chains induced a substantial contraction of the internal hydrogel layer. The hydrogel contraction was accompanied by an increase in gel strength and elasticity modulus. The presence of the PNIPAAm network in the internal layer reinforced the hydrogel and this effect was more pronounced above LCST. The permeability of sandwich-like membrane significantly rather decreases as the temperature increases. There was a decrease in the permeability of 52% as the temperature was increased from 25 to 40 ◦ C. It was suggested that the structural changes in the sandwich membrane induce to significant flux control. © 2005 Elsevier B.V. All rights reserved. Keywords: Sandwich-like membrane; NIPAAm; Acrylamide; Morphology; Mechanical properties
1. Introduction Over recent years, applications of thermo-responsive hydrogels as drug delivery devices, drug permeation membranes and cell growth and detachment scaffolding [1–6] have been widely investigated. Moreover, there is an increased interest in developing polymer matrixes that present this property in common with soft biological materials [7–9]. For these reasons, particular attention has been directed to polymers that undergo phase transition under thermal stimuli in an attempt to adjust them to near human body temperature operation [2,4]. Poly(Nisopropylacrylamide) (PNIPAAm) is among the polymers that
∗
Corresponding author. Fax: +55 44 3261 4125. E-mail address: [email protected] (E.C. Muniz).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.09.019
respond to changes in temperature and therefore has been widely applied as a biomaterial [4,9,10]. Since the discovery of PNIPAAm by Tanaka in 1970s [11], a large number of publications on hydrogels based on PNIPAAm has already been reported in the literature. In an aqueous environment, PNIPAAm shows reversible phase transition at low critical solution temperature (LCST) with resulting dramatic shrinkage [4,12,13]. Experimentally, it occurs at temperatures between ca. 30 and ca. 35 ◦ C [4]. However, the low mechanical strength of swollen PNIPAAm hydrogel limits its use in many applications [14,15]. In the collapsed state, PNIPAAm hydrogels have an elasticity modulus ca. 17-fold higher than that in the swollen state [16]. Interpenetrating polymer networks (IPNs) have been used to improve the mechanical properties of hydrogel [17,18]. IPNs are composed of two or more chemically distinct networks linked solely by their permanent entanglements [19–21]. On the
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Fig. 1. Mechanism of UV-induced polymerization in aqueous environment using potassium periodate as a sensitizer.
other hand, the use of semi-interpenetrating polymer networks (semi-IPNs) with hydrogels have also been subject of many investigations [15,22–24]. A system is semi-IPN if one of the assembly components is cross-linked and leaves the other in its linear form [25,26]. Semi-IPN hydrogels formed by cross-linked polyacrylamide having entangled PNIPAAm have already been reported in the literature [14,27,28]. These hydrogels have good mechanical properties at temperatures below and above LCST although PNIPAAm chains are not chemically linked to PAAm polymer chains [15]. In this work, NIPAAm monomers were reacted inside cross-linked PAAm in the presence of a crosslinker to form an interpenetrating polymer network that resulted in a sandwiched-like hydrogel. The mechanical stability of these hydrogels was studied by compressive load measurements in dependence of temperature. UV irradiation-induced photo-polymerization using potassium periodate as a sensitizer has been found to be a feasible route to cross-link PNIPAAm inside previously cross-linked PAAm. UV-induced polymerization using a periodate salt as a sensitizer has been described in the literature [29–31]. Fig. 1 shows the mechanism for radical formation through UV irradiation and the subsequent polymerization reaction. The methodology developed in this work to obtain sandwiched-like IPN hydrogel membranes has been described in a Brazilian Patent [32] recently. 2. Experimental 2.1. Synthesis of PAAm hydrogels PAAm hydrogels were prepared from aqueous solution of acrylamide (AAm) (Aldrich, 14,866-0) with methylene-bisacrylamide (MBAAm) (Plusone, 17-1304-02) as a cross-linker and periodate potassium (KIO4 ) (Cod 700, Vetec, Brazil) as a sensitizer. After homogenization, the solution was inserted between two transparent glass plates with 0.12 m in size and separated by a rubber gasket spacer 1.5 mm thick. The solution was exposed to the light of a low-pressure Hg lamp (λ = 254–580 nm, 215 W) for 40 min. Thereafter, all hydrogels were cut in square pieces of 200 mm2 area and completely dried in vacuum. 2.2. Synthesis of PNIPAAm inside PAAm hydrogel In this stage, the dried PAAm hydrogels were keep in 50 ml of an aqueous solution prepared by adding the desired amount of N-
isopropylacrylamide (NIPAAm) (Aldrich, 41,532-4), MBAAm and KIO4 at 25 ◦ C to swell. After 48 h, the PAAm hydrogel reached swelling equilibrium and the swollen hydrogel was placed between two transparent glass plates and then exposed to the light of the low-pressure Hg lamp for 40 min. Thereafter, co-polymerization of PNIPAAm occurred as the gel became completely opalescent. The hydrogel was dipped into distilled water for 1 week to remove non-reactants and water was renewed everyday. The swollen hydrogel was stored in distilled water at room temperature. Different NIPAAm concentrations were used in the feed aqueous solutions. The IPN hydrogels were labeled as (AAm–P) to indicate the compositions of PAAm and PNIPAAm. In this sense, the first term expresses the molar concentration of AAm (fixed at 2.5), while the second, P, represents the concentrations of the NIPAAm monomer used in the feed solution to swell the PAAm hydrogel. The cross-linker concentration was fixed at 1 mol% relative to the amounts of AAm and NIPAAm. The compositions of all reagents used to prepare the hydrogels are summarized in Table 1. 2.3. Morphological analysis of hydrogels by SEM Hydrogel morphology was analyzed by scanning electron microscopy (SEM) (Shimadzu, model SS 550) operating at 12 keV. The hydrogels were immersed into distilled water at either 25 or 40 ◦ C until swelling equilibrium was reached. After that, the samples were removed and immediately frozen by immersion in liquid nitrogen. Thereafter, the frozen hydrogels were fractured and lyophilized by a freeze-dryer (Christ gefriertrocknungsanlagen) at −60 ◦ C for 24 h. Before observation by SEM, the dried-lyophilized hydrogels were gold-coated by sputtering.
Table 1 Amounts of AAm, NIPAAm and MBAAm used in hydrogels synthesis, in mol ml−1 Hydrogel
AAma
MBAAma
NIPAAmb
MBAAmb
2.5–0 2.5–1.3 2.5–2.5 2.5–5
2500 2500 2500 2500
25 25 25 25
0 111 221 442
0 1.1 2.2 4.4
a b
Amount used in feed solution in the synthesis of PAAm. Amount used to swell dried PAAm hydrogels.
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2.4. Water uptake measurements
2.7. Measures of permeability to orange II dye
The swelling capability of the hydrogels was investigated through their water uptake (WU) using the following equation:
The permeability experiments were determined by the measures of orange II diffused through the sandwich-like membrane in dependence of temperature. Before measuring, the membranes used in the permeability tests were brought in fresh water for more than 2 days. After that, a circular membrane was used to separate two compartments of permeation cell. The downstream compartment was filled up with 50 ml of distillate water and the upstream one was filled with aqueous solution of orange II (0.1 mmol l−1 ), the concentration of which changes by less than 5% during the experiment. The increase of the orange II concentration in the downstream was evaluated in dependence of time using a spectrophotometer at 486 nm. The measurements were run at temperatures 25, 30, 35 and 40 ◦ C. The same experiments were repeated at least three times to check reproducibility.
WU =
Ww Wd
(1)
where Ww and Wd are the weight of the swollen hydrogel at equilibrium and the weight of the dried hydrogel, respectively. 2.5. Gel strength obtained from compressive tests Gel strength was measured by compressing the hydrogels to 1 mm deformation by using a texture analyzer (TATX2i Stable Micro System, Haslemere, Surrey, UK) equipped with a 5 kg load cell. The apparatus was equipped with a circular probe of 12.7 mm diameter, which descended to the gel surface at a constant speed of 0.2 mm s−1 . After adjusting the experimental parameters, each measurement was performed in less than 1 min to avoid water loss by the hydrogel during the experiment. All tests were run four times for each gel. Before measurement, the gels were maintained in water for 48 h at the desired temperature and the force necessary to compress the hydrogel to 1 mm deformation was recorded at the temperatures of 25, 30, 35 and 40 ◦ C. 2.6. Stress–strain measurements All mechanical tests were carried out on fully swollen hydrogels free from air bubbles or physical imperfections. This was determined visually. The data collected by the texture analyzer (as described in the previous section) are given as force. Stress, σ, data were determined using the equation [33]: σ=
F A
(2)
where F is the force and A is the cross-sectional area of the strained specimen. This information was then converted to modulus of elasticity, E, from the slope of linear dependence [14,15] σ = E(λ − λ2 )
(3)
where λ is the relative deformation of the specimen calculated from equation [14]: λ=
L L0
(4)
where L is the deformation of the sample and L0 is the initial sample length. The apparent cross-linking density, νe , was obtained from slope of linear dependence [14,15] σ = RT
φg,0 φg
2/3
φg νe (λ − λ−2 )
(5)
where φg,0 and φg are the polymer volume fractions of the gel in the relaxed state and swollen state, respectively.
3. Results and discussion 3.1. Hydrogel morphology Fig. 2a–c illustrates SEM images of fractured frozen hydrogels lyophilized at 25 and 40 ◦ C, respectively. In all figures, it can be observed that the hydrogel membranes are formed by three layers. The internal layer (mid-layer) is fully involved by the two external layers, which have similar morphology, but clearly different from that of the internal layer. After being heated above LCST, the internal layer shrank significantly. It could be suggested that the internal layer is richer in PNIPAAm network than the external ones. This was attributed to the following: the copolymerized NIPAAm monomers reacted inside the fully closed glass plates. By exposing the plates to the low-pressure Hg lamp light the temperature of system reached 60 ◦ C. At this temperature, PNIPAAm is hydrophobic and tends to expel water in both external directions, resulting in a more concentrated PNIPAAm in the internal membrane. It should be highlighted that the hydrogel became opalescence gradually, showing that polymerization did not occur immediately. As a result, the diffusion of PNIPAAm to the internal layer could take place. The argument that the internal layer is formed mainly by PNIPAAm is strengthened by morphological analysis of the hydrogel frozen after swelling at 40 ◦ C (Fig. 2b). The comparison of Fig. 2a and b shows that the internal layer shrank significantly. At 25 ◦ C, the measured thickness of internal layer was 1.66 ± 0.04 mm, but at 40 ◦ C this value decreased to 0.23 ± 0.02 mm. This effect was attributed to the collapse of the PNIPAAm polymer networks, which induced a significant contraction of the internal layer and pushed water out. On the other hand, the expansion of the external layers was clearly observed. From these results, it could be pointed out that these hydrogels can be considered as sandwich-like. Regarding the total shrinkage, the membrane thickness changed from 2.40 ± 0.02 to 0.90 ± 0.03 mm with a total contraction of ca. 37.5%. The contraction of the internal layer occurred simultaneously to the expansion of the external ones. Regarding the contraction of the internal layer, thickness changed from 1.63 to 0.25 mm, a contraction of ca. 85%. Regarding the expansion of the left external layer, thickness changed from 0.31 to 0.45 mm,
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Fig. 2. SEM images of (2.5–2.5) sandwich-like IPN hydrogel having PNIPAAm inside PAAm networks. Micrographs of fractured hydrogel freeze-dried after swelling at (a) 25 ◦ C and (b and c) 40 ◦ C. The last one corresponds to another hydrogel synthesis batch.
which represented an expansion of 45%. The membranes synthesis experiments were repeated to check reproducibility of morphology formation. Fig. 2c shows the SEM image of another hydrogel synthesis batch. In all synthesis experiments, it was observed the same morphology formation for all the IPN hydrogels. Semi-IPN hydrogels constituted of PAAm and PNIPAAm have already been reported in the literature [14,27]. They were formed by co-polymerizing AAm and MBAAm in the presence of PNIPAAm. In these hydrogels, PNIPAAm chains are randomly distributed and trapped at temperatures below PNIPAAm LCST. As the temperature is raised above LCST, collapse of
PNIPAAm chains does not result in significant shrinking because they are mechanically supported by PAAm networks [15]. Fig. 3a and b shows the micrographs of the external and internal layers, respectively, of lyophilized hydrogels frozen after reaching the swelling equilibrium at 40 ◦ C. The external layers remained highly porous and this characteristic was attributed to the hydrophilicity of the PAAm hydrogel, as already mentioned above. This porous feature resulted from dried-lyophilizing the hydrogels previously swollen at 40 ◦ C. At this temperature, PAAm hydrogels tend to be highly swollen and their network becomes more expanded than at 25 ◦ C (Fig. 3a). On the other hand, when heated to the same temperature, the PNIPAAm net-
Fig. 3. SEM images of (2.5–2.5) sandwich-like IPN hydrogel having PNIPAAm inside PAAm networks. Micrographs of fractured hydrogel freeze-dried after swelling at 40 ◦ C (a) external layer and (b) internal layer.
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Fig. 4. Dependence of water uptake on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
work in the internal layer collapses and a more compact or tighter structure is reached (Fig. 3b). 3.2. Water uptake Fig. 4 displays the water uptake capacity of hydrogels (2.5–P) plotted against the temperature. The membrane constituted only by PAAm network (2.5–0) absorbs more water than the IPN hydrogels. The inclusion of PNIPAAm networks inside PAAm networks creates a hydrophobic environment and increases the degree of cross-linking of the system. It is known that the increase in the cross-linking degree of a matrix polymer results in a gel with a tighter structure, and therefore, it swells less [34]. Another important feature is related to the changes in WU values, which were proportional to the PNIPAAm content included in the gel. Temperature affected most the WU of the hydrogel with a higher amount of PNIPAAm. It exhibited a characteristic sigmoid-like curve with a transition at ca. 32–35 ◦ C, a typical curve obtained when aqueous PNIPAAm undergoes the phase transition around LCST [4]. Although the sharp transition is not observed for (2.5–1.3) and it is only slightly present for (2.5–2.5) hydrogels, WU decreased as IPN hydrogels were warmed. The sharper change in WU observed for (2.5–5) hydrogel indicates that the collapse of its PNIPAAm network was more pronounced and it became more hydrophobic when warmed. A higher PNIPAAm content is required to visualize this effect on IPN hydrogels composed of PNIPAAm networks and PAAm network. 3.3. Mechanical behavior of hydrogels Fig. 5 shows the force necessary to compress (2.5–P) IPN hydrogels at 1 mm deformation (gel strength) as a function of temperature. It was observed that the values of compressive force for IPN hydrogels are larger than the respective force for PAAm hydrogel at the same temperature. This was attributed to two main contributions: (i) the fact that IPN hydrogels are denser in polymer chains, which results in a positive contribution to the compressive force and (ii) the larger swelling of PAAm hydro-
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Fig. 5. Dependence of compressive force (gel strength) on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
gel (which results in a negative contribution). Therefore, a lower force, 1 mm depth, is required to deform PAAm (2.5–0) hydrogel than IPN (2.5–P) hydrogels. On the other hand, the force necessary to compress IPN hydrogels leading to same deformation (1 mm depth) is greatly increased by heating the hydrogel above LCST. The curves are sigmoid-like and exhibit a transition close to 32–35 ◦ C, which is indicative of the collapse transition of PNIPAAm. The collapse of PNIPAAm chains induced the large contraction which pulled the PAAm networks back and yielded a hydrogel with a tighter structure. Consequently, an additional force is required to compress this hydrogel. This finding is indicative that the gel strength of the hydrogel was improved by the collapse of PNIPAAm. Moreover, their gel strength became more evident with the increase in the amount of PNIPAAm in the PAAm hydrogel. It should be highlighted that a 1-mm deformation is also enough to deform the internal layer, because the thickness of external layers is smaller than 1 mm even at 40 ◦ C (Fig. 3b). Fig. 6a and b displays the stress–strain curves for the hydrogels at 25 and 40 ◦ C, respectively. It is observed that the plot of stress versus strain resulted in a linear relationship, which is indicative that these hydrogels experienced elastic deformation when subjected to compressive load as used in this work [35,36]. This means that when the gel is compressed by an external force, the deformation is accommodated by the rearrangement of the polymer chains within the gel. During this process, a retracting elastic force develops within the polymer network due to its tendency to recover its original configuration [2]. The shape and characteristic of the stress–strain curves are similar for all hydrogels in spite of the curves of pure PAAm hydrogel being smaller when compared to those of IPN hydrogels at 25 and 40 ◦ C. The reason for this is that the PAAm hydrogel is more swollen and its polymer network is more expanded. This difference becomes more pronounced at temperatures above LCST due to the collapse of the PNIPAAm chains to lead the gel structure to a shrunken state. Consequently, the polymer network of IPN becomes tighter, which increases the strength of the IPN hydrogel to elastic deformation.
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Fig. 8. Dependence of apparent cross-linking density on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
Fig. 6. Typical stress–strain curves of sandwich-like IPN hydrogel for four different amount of PNIPAAm included, and pure PAAm hydrogel at (a) 25 ◦ C and (b) 40 ◦ C.
The dependence of the elastic modulus (E) of PAAm and IPN hydrogels on temperature from 25 to 40 ◦ C is shown in Fig. 7. The E values range from ca. 50 to ca. 62 kPa for IPN hydrogels, and from ca. 41 to ca. 45 kPa for PAAm hydrogel. The higher elastic modulus of IPN hydrogels indicates that these materials are more rigid, mainly because of the presence of PNIPAAm in the internal layer. Furthermore, it was observed a significant decrease in the modulus of elasticity of PAAm hydrogel after
Fig. 7. Dependence of modulus of elasticity (compressive load) on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
heating as it becomes softer at high temperatures. In contrast, for IPN hydrogels the modulus is enhanced as the temperature is raised above LCST. From these results, it could be suggested that the collapse of the PNIPAAm chains inside the polymer matrix leads to a stronger hydrogel. The dependence of apparent cross-linking density, νe , on the temperature from 25 to 40 ◦ C for PAAm and IPN hydrogels is shown in Fig. 8. As expected, νe increased with the increase in the amount of PNIPAAm in the hydrogel; below LCST the increase is due to entanglements; above LCST, the collapse of PNIPAAm gives an additional contribution to νe . 3.4. Permeability to orange II dye through sandwich-like membrane The concentration of orange II diffused through the membrane was obtained by the calibration curve of photometric signal, which correlates the absorbance to the concentration of dye. It was found that the concentration of orange II diffused to downstream compartment increases linearly as a function of time (data not shown here). Eq. (6), based on the first Fick’s law, was used to calculate the permeability (P): dC Vd P= (6) dt CA where dC/dt is the slope of straight line given by the concentration of diffused orange II versus time, C the initial concentration of orange II in the upstream compartment, V the volume of each compartment and A is the area of membrane in contact with upstream compartment. The parameter d is the thickness of swollen membrane at equilibrium in water, at desired temperature. It was measured using a micrometer. Fig. 9 shows permeability curves of (2.5–2.5) sandwich-like membrane to orange II and (2.5–0) pure PAAm hydrogel, in dependence of temperature. The pure (2.5–0) hydrogel (without PNIPAAm), used as ‘blank’ in this work, presented a linear increase in permeability as the temperature increases because there is no shrinking of the PAAm hydrogel. It was found that the permeability of (2.5–2.5) IPN membrane significantly
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Fig. 9. Permeability to orange II dye for the (2.5–2.5) sandwich-like IPN hydrogel and (2.5–0) pure PAAm hydrogel (without PNIPAAm) as a function of temperature.
rather decreases as the temperature increases. The permeability changes of 6.8 × 10−10 m2 s−1 at 25 ◦ C to 3.6 × 10−10 m2 s−1 at 40 ◦ C and this represents a decreases of 52%. This effect was attributed to the collapse of the PNIPAAm polymer networks, which induced a significant contraction of the mid-layer of membrane. In this condition, the mid-layer acts as a ‘barrier’ that minimizes the permeation of orange II through the sandwich membrane. The results shown here (Fig. 9) suggest that the structural changes in the sandwich membrane (shown in Fig. 2) induce to significant flux control. The methodology used in this work produces thermoresponsive hydrogels with specific configuration that can be considered a novelty in the membrane synthesis. Sandwichlike membranes with a temperature responsive mid-layer surrounded by non-responsive surface layers can act as thermoresponsive flux controlling membranes in separation process of protein. Other motivation is the potential use of sandwichlike membranes as devices for controlling release of drugs with hydrophobic nature. For instance, a hydrophobic drug (e.g. corticosteroids) may be entrapped in the mid-layer of membrane during its synthesis. Corticosteroids have been used to treat ulcer diseases in colon but need to be ingested in significant rather amount to reach the colon because the GI tract is one of route the most complex for therapy [34]. The authors have suggested that the sandwich-like membrane could act as a colon-specific delivery system to transport the needed amount of drug for effective therapy. Reaching the colon, the drug could be released by the erosion mechanism, or any other degradation mechanism that will be evaluated forward. 4. Conclusions Sandwiched-like thermo-sensitive membranes of IPN hydrogels have been synthesized. NIPAAm monomers and MBAAm cross-linker were co-polymerized inside previously synthesized cross-linked PAAm. SEM, water uptake capacity and compressive load tests were used to determine hydrogel properties. Through SEM images, we observed that the hydrogel mem-
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branes are formed by three layers, an internal layer fully involved by two others. Significant difference in morphological aspects was also observed between internal and external layers. By SEM, it was observed that the PNIPAAm network is present mainly in the internal layer, because as the hydrogel is warmed above the LCST, the internal layer contracted, while the external layers expanded and remained highly porous. This characteristic was attributed to the hydrophilicity of the PAAm hydrogel. On heating the IPN hydrogel, it was verified an increase in either gel strength or modulus of elasticity, indicating that the material became more rigid. The presence of PNIPAAm networks in the internal layer reinforced the hydrogel significantly and this effect was more evident above LCST. This was attributed to the large shrinking, which resulted in a stronger hydrogel. The permeability of sandwich membrane significantly decreases as the temperature increases above the LCST. There was a decrease in the permeability of 52% as the temperature was increased from 25 to 40 ◦ C. It was suggested that the structural changes in the sandwich membrane induce to significant flux control. According to these results, these hydrogels displayed excellent properties for further tests as thermo-responsive flux controlling membranes in separation process. Furthermore, this hydrogels have attributes that make them an attractive to be tested forward as a drug delivery system. Acknowledgments G.M.C. thanks to Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo (FAPESP) for his post-doctoral fellowship (Proc 03/10897-6). MRG is grateful to Conselho Nacional de Desenvolvimento e Tecnol´ogico—CNPp (Brazil) for his master fellowship. The authors wish to thank CNPq for the financial support (Process no. 479221/2001-4) and to Univ. Est. LondrinaUEL, for using TAXT2i Texture Analyzers. References [1] J.S. Turner, Y.-L. Cheng, pH dependence of PDMS–PMAA IPN morphology and transport properties, J. Membr. Sci. 240 (2004) 19. [2] C.L. Bell, N.A. Peppas, Biomedical membranes from hydrogels and interpolymer complexes, Biopolym. Adv. Polym. Sci. 22 (1995) 125. [3] L. Shapiro, S. Cohen, Novel alginate sponges for cell culture and transplantation, Biomaterials 18 (1997) 583. [4] H.G. Schild, Poly(n-isopropylacrylamide)—experiment, theory and application, Prog. Polym. Sci. 17 (1992) 163. [5] A. Okamura, M. Itayagoshi, T. Hagiwara, M. Yamaguchi, T. Kanamori, T. Shinbo, P.-C. Wang, Poly(N-isopropylacrylamide)-graft-polypropylene membranes containing adsorbed antibody for cell separation, Biomaterials 26 (2005) 1287. [6] L. Ying, E.T. Kang, K.G. Neoh, K. Kand, H. Iwata, Drug permeation through temperature-sensitive membranes prepared from poly(vinylidene fluoride) with grafted poly(N-isopropylacrylamide) chains, J. Membr. Sci. 243 (2004) 253. [7] M. Solari, Evaluation of the mechanical-properties of a hydrogel fiber in the development of a polymeric actuator, J. Intel. Mater. Syst. Struct. 5 (1994) 295. [8] Y. Osada, S.B. Rossmurphy, Intelligent gels, Sci. Am. 268 (1993) 82. [9] L. Ying, E.T. Kang, K.G. Neoh, Characterization of membranes prepared from blends of poly(acrylic acid)-graft-poly(vinylidene fluoride) with poly(N-isopropylacrylamide) and their temperature- and pH-sensitive microfiltration, J. Membr. Sci. 224 (2003) 93.
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Thermo-responsive sandwiched-like membranes of IPN-PNIPAAm/PAAm hydrogels Marcos R. Guilherme b , Gilsinei M. Campese a , Eduardo Radovanovic b , Adley F. Rubira b , Elias B. Tambourgi a , Edvani C. Muniz b,∗ a
Universidade Estadual de Campinas, Faculdade de Engenharia Qu´ımica, Departamento de Sistemas Qu´ımicos e Inform´atica DESQ, Zeferino Vaz 13081-970, Campinas, SP, Brazil b Universidade Estadual de Maring´ a, Departamento de Qu´ımica, Grupo de Materiais Polim´ericos e Comp´ositos, Av. Colombo 5790, 87020-970 Maring´a, Paran´a, Brazil Received 2 March 2005; received in revised form 23 August 2005; accepted 10 September 2005 Available online 17 October 2005
Abstract The synthesis of sandwiched-like IPN hydrogels having cross-linked PNIPAAm interpenetrated into PAAm networks was carried out by UVinduced polymerization using potassium periodate as a sensitizer. NIPAAm monomers and MBAAm cross-linker were co-polymerized inside previously synthesized cross-linked PAAm. Through SEM images, we observed that the hydrogel membranes were formed by three layers; the internal layer was fully involved by the two others. Significant morphological difference between the internal and external layers was also observed. Thus, sandwiched-like interpenetrating polymer networks were obtained. The internal layer shrank significantly after warming the swollen hydrogel above LCST of PNIPAAm in water, 30–35 ◦ C while the external layers remained swollen and highly porous due to the hydrophilicity of PAAm. The experiments performed at 40 ◦ C revealed that the hydrogels shrank considerably. Collapsed PNIPAAm chains induced a substantial contraction of the internal hydrogel layer. The hydrogel contraction was accompanied by an increase in gel strength and elasticity modulus. The presence of the PNIPAAm network in the internal layer reinforced the hydrogel and this effect was more pronounced above LCST. The permeability of sandwich-like membrane significantly rather decreases as the temperature increases. There was a decrease in the permeability of 52% as the temperature was increased from 25 to 40 ◦ C. It was suggested that the structural changes in the sandwich membrane induce to significant flux control. © 2005 Elsevier B.V. All rights reserved. Keywords: Sandwich-like membrane; NIPAAm; Acrylamide; Morphology; Mechanical properties
1. Introduction Over recent years, applications of thermo-responsive hydrogels as drug delivery devices, drug permeation membranes and cell growth and detachment scaffolding [1–6] have been widely investigated. Moreover, there is an increased interest in developing polymer matrixes that present this property in common with soft biological materials [7–9]. For these reasons, particular attention has been directed to polymers that undergo phase transition under thermal stimuli in an attempt to adjust them to near human body temperature operation [2,4]. Poly(Nisopropylacrylamide) (PNIPAAm) is among the polymers that
∗
Corresponding author. Fax: +55 44 3261 4125. E-mail address: [email protected] (E.C. Muniz).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.09.019
respond to changes in temperature and therefore has been widely applied as a biomaterial [4,9,10]. Since the discovery of PNIPAAm by Tanaka in 1970s [11], a large number of publications on hydrogels based on PNIPAAm has already been reported in the literature. In an aqueous environment, PNIPAAm shows reversible phase transition at low critical solution temperature (LCST) with resulting dramatic shrinkage [4,12,13]. Experimentally, it occurs at temperatures between ca. 30 and ca. 35 ◦ C [4]. However, the low mechanical strength of swollen PNIPAAm hydrogel limits its use in many applications [14,15]. In the collapsed state, PNIPAAm hydrogels have an elasticity modulus ca. 17-fold higher than that in the swollen state [16]. Interpenetrating polymer networks (IPNs) have been used to improve the mechanical properties of hydrogel [17,18]. IPNs are composed of two or more chemically distinct networks linked solely by their permanent entanglements [19–21]. On the
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Fig. 1. Mechanism of UV-induced polymerization in aqueous environment using potassium periodate as a sensitizer.
other hand, the use of semi-interpenetrating polymer networks (semi-IPNs) with hydrogels have also been subject of many investigations [15,22–24]. A system is semi-IPN if one of the assembly components is cross-linked and leaves the other in its linear form [25,26]. Semi-IPN hydrogels formed by cross-linked polyacrylamide having entangled PNIPAAm have already been reported in the literature [14,27,28]. These hydrogels have good mechanical properties at temperatures below and above LCST although PNIPAAm chains are not chemically linked to PAAm polymer chains [15]. In this work, NIPAAm monomers were reacted inside cross-linked PAAm in the presence of a crosslinker to form an interpenetrating polymer network that resulted in a sandwiched-like hydrogel. The mechanical stability of these hydrogels was studied by compressive load measurements in dependence of temperature. UV irradiation-induced photo-polymerization using potassium periodate as a sensitizer has been found to be a feasible route to cross-link PNIPAAm inside previously cross-linked PAAm. UV-induced polymerization using a periodate salt as a sensitizer has been described in the literature [29–31]. Fig. 1 shows the mechanism for radical formation through UV irradiation and the subsequent polymerization reaction. The methodology developed in this work to obtain sandwiched-like IPN hydrogel membranes has been described in a Brazilian Patent [32] recently. 2. Experimental 2.1. Synthesis of PAAm hydrogels PAAm hydrogels were prepared from aqueous solution of acrylamide (AAm) (Aldrich, 14,866-0) with methylene-bisacrylamide (MBAAm) (Plusone, 17-1304-02) as a cross-linker and periodate potassium (KIO4 ) (Cod 700, Vetec, Brazil) as a sensitizer. After homogenization, the solution was inserted between two transparent glass plates with 0.12 m in size and separated by a rubber gasket spacer 1.5 mm thick. The solution was exposed to the light of a low-pressure Hg lamp (λ = 254–580 nm, 215 W) for 40 min. Thereafter, all hydrogels were cut in square pieces of 200 mm2 area and completely dried in vacuum. 2.2. Synthesis of PNIPAAm inside PAAm hydrogel In this stage, the dried PAAm hydrogels were keep in 50 ml of an aqueous solution prepared by adding the desired amount of N-
isopropylacrylamide (NIPAAm) (Aldrich, 41,532-4), MBAAm and KIO4 at 25 ◦ C to swell. After 48 h, the PAAm hydrogel reached swelling equilibrium and the swollen hydrogel was placed between two transparent glass plates and then exposed to the light of the low-pressure Hg lamp for 40 min. Thereafter, co-polymerization of PNIPAAm occurred as the gel became completely opalescent. The hydrogel was dipped into distilled water for 1 week to remove non-reactants and water was renewed everyday. The swollen hydrogel was stored in distilled water at room temperature. Different NIPAAm concentrations were used in the feed aqueous solutions. The IPN hydrogels were labeled as (AAm–P) to indicate the compositions of PAAm and PNIPAAm. In this sense, the first term expresses the molar concentration of AAm (fixed at 2.5), while the second, P, represents the concentrations of the NIPAAm monomer used in the feed solution to swell the PAAm hydrogel. The cross-linker concentration was fixed at 1 mol% relative to the amounts of AAm and NIPAAm. The compositions of all reagents used to prepare the hydrogels are summarized in Table 1. 2.3. Morphological analysis of hydrogels by SEM Hydrogel morphology was analyzed by scanning electron microscopy (SEM) (Shimadzu, model SS 550) operating at 12 keV. The hydrogels were immersed into distilled water at either 25 or 40 ◦ C until swelling equilibrium was reached. After that, the samples were removed and immediately frozen by immersion in liquid nitrogen. Thereafter, the frozen hydrogels were fractured and lyophilized by a freeze-dryer (Christ gefriertrocknungsanlagen) at −60 ◦ C for 24 h. Before observation by SEM, the dried-lyophilized hydrogels were gold-coated by sputtering.
Table 1 Amounts of AAm, NIPAAm and MBAAm used in hydrogels synthesis, in mol ml−1 Hydrogel
AAma
MBAAma
NIPAAmb
MBAAmb
2.5–0 2.5–1.3 2.5–2.5 2.5–5
2500 2500 2500 2500
25 25 25 25
0 111 221 442
0 1.1 2.2 4.4
a b
Amount used in feed solution in the synthesis of PAAm. Amount used to swell dried PAAm hydrogels.
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2.4. Water uptake measurements
2.7. Measures of permeability to orange II dye
The swelling capability of the hydrogels was investigated through their water uptake (WU) using the following equation:
The permeability experiments were determined by the measures of orange II diffused through the sandwich-like membrane in dependence of temperature. Before measuring, the membranes used in the permeability tests were brought in fresh water for more than 2 days. After that, a circular membrane was used to separate two compartments of permeation cell. The downstream compartment was filled up with 50 ml of distillate water and the upstream one was filled with aqueous solution of orange II (0.1 mmol l−1 ), the concentration of which changes by less than 5% during the experiment. The increase of the orange II concentration in the downstream was evaluated in dependence of time using a spectrophotometer at 486 nm. The measurements were run at temperatures 25, 30, 35 and 40 ◦ C. The same experiments were repeated at least three times to check reproducibility.
WU =
Ww Wd
(1)
where Ww and Wd are the weight of the swollen hydrogel at equilibrium and the weight of the dried hydrogel, respectively. 2.5. Gel strength obtained from compressive tests Gel strength was measured by compressing the hydrogels to 1 mm deformation by using a texture analyzer (TATX2i Stable Micro System, Haslemere, Surrey, UK) equipped with a 5 kg load cell. The apparatus was equipped with a circular probe of 12.7 mm diameter, which descended to the gel surface at a constant speed of 0.2 mm s−1 . After adjusting the experimental parameters, each measurement was performed in less than 1 min to avoid water loss by the hydrogel during the experiment. All tests were run four times for each gel. Before measurement, the gels were maintained in water for 48 h at the desired temperature and the force necessary to compress the hydrogel to 1 mm deformation was recorded at the temperatures of 25, 30, 35 and 40 ◦ C. 2.6. Stress–strain measurements All mechanical tests were carried out on fully swollen hydrogels free from air bubbles or physical imperfections. This was determined visually. The data collected by the texture analyzer (as described in the previous section) are given as force. Stress, σ, data were determined using the equation [33]: σ=
F A
(2)
where F is the force and A is the cross-sectional area of the strained specimen. This information was then converted to modulus of elasticity, E, from the slope of linear dependence [14,15] σ = E(λ − λ2 )
(3)
where λ is the relative deformation of the specimen calculated from equation [14]: λ=
L L0
(4)
where L is the deformation of the sample and L0 is the initial sample length. The apparent cross-linking density, νe , was obtained from slope of linear dependence [14,15] σ = RT
φg,0 φg
2/3
φg νe (λ − λ−2 )
(5)
where φg,0 and φg are the polymer volume fractions of the gel in the relaxed state and swollen state, respectively.
3. Results and discussion 3.1. Hydrogel morphology Fig. 2a–c illustrates SEM images of fractured frozen hydrogels lyophilized at 25 and 40 ◦ C, respectively. In all figures, it can be observed that the hydrogel membranes are formed by three layers. The internal layer (mid-layer) is fully involved by the two external layers, which have similar morphology, but clearly different from that of the internal layer. After being heated above LCST, the internal layer shrank significantly. It could be suggested that the internal layer is richer in PNIPAAm network than the external ones. This was attributed to the following: the copolymerized NIPAAm monomers reacted inside the fully closed glass plates. By exposing the plates to the low-pressure Hg lamp light the temperature of system reached 60 ◦ C. At this temperature, PNIPAAm is hydrophobic and tends to expel water in both external directions, resulting in a more concentrated PNIPAAm in the internal membrane. It should be highlighted that the hydrogel became opalescence gradually, showing that polymerization did not occur immediately. As a result, the diffusion of PNIPAAm to the internal layer could take place. The argument that the internal layer is formed mainly by PNIPAAm is strengthened by morphological analysis of the hydrogel frozen after swelling at 40 ◦ C (Fig. 2b). The comparison of Fig. 2a and b shows that the internal layer shrank significantly. At 25 ◦ C, the measured thickness of internal layer was 1.66 ± 0.04 mm, but at 40 ◦ C this value decreased to 0.23 ± 0.02 mm. This effect was attributed to the collapse of the PNIPAAm polymer networks, which induced a significant contraction of the internal layer and pushed water out. On the other hand, the expansion of the external layers was clearly observed. From these results, it could be pointed out that these hydrogels can be considered as sandwich-like. Regarding the total shrinkage, the membrane thickness changed from 2.40 ± 0.02 to 0.90 ± 0.03 mm with a total contraction of ca. 37.5%. The contraction of the internal layer occurred simultaneously to the expansion of the external ones. Regarding the contraction of the internal layer, thickness changed from 1.63 to 0.25 mm, a contraction of ca. 85%. Regarding the expansion of the left external layer, thickness changed from 0.31 to 0.45 mm,
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Fig. 2. SEM images of (2.5–2.5) sandwich-like IPN hydrogel having PNIPAAm inside PAAm networks. Micrographs of fractured hydrogel freeze-dried after swelling at (a) 25 ◦ C and (b and c) 40 ◦ C. The last one corresponds to another hydrogel synthesis batch.
which represented an expansion of 45%. The membranes synthesis experiments were repeated to check reproducibility of morphology formation. Fig. 2c shows the SEM image of another hydrogel synthesis batch. In all synthesis experiments, it was observed the same morphology formation for all the IPN hydrogels. Semi-IPN hydrogels constituted of PAAm and PNIPAAm have already been reported in the literature [14,27]. They were formed by co-polymerizing AAm and MBAAm in the presence of PNIPAAm. In these hydrogels, PNIPAAm chains are randomly distributed and trapped at temperatures below PNIPAAm LCST. As the temperature is raised above LCST, collapse of
PNIPAAm chains does not result in significant shrinking because they are mechanically supported by PAAm networks [15]. Fig. 3a and b shows the micrographs of the external and internal layers, respectively, of lyophilized hydrogels frozen after reaching the swelling equilibrium at 40 ◦ C. The external layers remained highly porous and this characteristic was attributed to the hydrophilicity of the PAAm hydrogel, as already mentioned above. This porous feature resulted from dried-lyophilizing the hydrogels previously swollen at 40 ◦ C. At this temperature, PAAm hydrogels tend to be highly swollen and their network becomes more expanded than at 25 ◦ C (Fig. 3a). On the other hand, when heated to the same temperature, the PNIPAAm net-
Fig. 3. SEM images of (2.5–2.5) sandwich-like IPN hydrogel having PNIPAAm inside PAAm networks. Micrographs of fractured hydrogel freeze-dried after swelling at 40 ◦ C (a) external layer and (b) internal layer.
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Fig. 4. Dependence of water uptake on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
work in the internal layer collapses and a more compact or tighter structure is reached (Fig. 3b). 3.2. Water uptake Fig. 4 displays the water uptake capacity of hydrogels (2.5–P) plotted against the temperature. The membrane constituted only by PAAm network (2.5–0) absorbs more water than the IPN hydrogels. The inclusion of PNIPAAm networks inside PAAm networks creates a hydrophobic environment and increases the degree of cross-linking of the system. It is known that the increase in the cross-linking degree of a matrix polymer results in a gel with a tighter structure, and therefore, it swells less [34]. Another important feature is related to the changes in WU values, which were proportional to the PNIPAAm content included in the gel. Temperature affected most the WU of the hydrogel with a higher amount of PNIPAAm. It exhibited a characteristic sigmoid-like curve with a transition at ca. 32–35 ◦ C, a typical curve obtained when aqueous PNIPAAm undergoes the phase transition around LCST [4]. Although the sharp transition is not observed for (2.5–1.3) and it is only slightly present for (2.5–2.5) hydrogels, WU decreased as IPN hydrogels were warmed. The sharper change in WU observed for (2.5–5) hydrogel indicates that the collapse of its PNIPAAm network was more pronounced and it became more hydrophobic when warmed. A higher PNIPAAm content is required to visualize this effect on IPN hydrogels composed of PNIPAAm networks and PAAm network. 3.3. Mechanical behavior of hydrogels Fig. 5 shows the force necessary to compress (2.5–P) IPN hydrogels at 1 mm deformation (gel strength) as a function of temperature. It was observed that the values of compressive force for IPN hydrogels are larger than the respective force for PAAm hydrogel at the same temperature. This was attributed to two main contributions: (i) the fact that IPN hydrogels are denser in polymer chains, which results in a positive contribution to the compressive force and (ii) the larger swelling of PAAm hydro-
191
Fig. 5. Dependence of compressive force (gel strength) on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
gel (which results in a negative contribution). Therefore, a lower force, 1 mm depth, is required to deform PAAm (2.5–0) hydrogel than IPN (2.5–P) hydrogels. On the other hand, the force necessary to compress IPN hydrogels leading to same deformation (1 mm depth) is greatly increased by heating the hydrogel above LCST. The curves are sigmoid-like and exhibit a transition close to 32–35 ◦ C, which is indicative of the collapse transition of PNIPAAm. The collapse of PNIPAAm chains induced the large contraction which pulled the PAAm networks back and yielded a hydrogel with a tighter structure. Consequently, an additional force is required to compress this hydrogel. This finding is indicative that the gel strength of the hydrogel was improved by the collapse of PNIPAAm. Moreover, their gel strength became more evident with the increase in the amount of PNIPAAm in the PAAm hydrogel. It should be highlighted that a 1-mm deformation is also enough to deform the internal layer, because the thickness of external layers is smaller than 1 mm even at 40 ◦ C (Fig. 3b). Fig. 6a and b displays the stress–strain curves for the hydrogels at 25 and 40 ◦ C, respectively. It is observed that the plot of stress versus strain resulted in a linear relationship, which is indicative that these hydrogels experienced elastic deformation when subjected to compressive load as used in this work [35,36]. This means that when the gel is compressed by an external force, the deformation is accommodated by the rearrangement of the polymer chains within the gel. During this process, a retracting elastic force develops within the polymer network due to its tendency to recover its original configuration [2]. The shape and characteristic of the stress–strain curves are similar for all hydrogels in spite of the curves of pure PAAm hydrogel being smaller when compared to those of IPN hydrogels at 25 and 40 ◦ C. The reason for this is that the PAAm hydrogel is more swollen and its polymer network is more expanded. This difference becomes more pronounced at temperatures above LCST due to the collapse of the PNIPAAm chains to lead the gel structure to a shrunken state. Consequently, the polymer network of IPN becomes tighter, which increases the strength of the IPN hydrogel to elastic deformation.
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Fig. 8. Dependence of apparent cross-linking density on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
Fig. 6. Typical stress–strain curves of sandwich-like IPN hydrogel for four different amount of PNIPAAm included, and pure PAAm hydrogel at (a) 25 ◦ C and (b) 40 ◦ C.
The dependence of the elastic modulus (E) of PAAm and IPN hydrogels on temperature from 25 to 40 ◦ C is shown in Fig. 7. The E values range from ca. 50 to ca. 62 kPa for IPN hydrogels, and from ca. 41 to ca. 45 kPa for PAAm hydrogel. The higher elastic modulus of IPN hydrogels indicates that these materials are more rigid, mainly because of the presence of PNIPAAm in the internal layer. Furthermore, it was observed a significant decrease in the modulus of elasticity of PAAm hydrogel after
Fig. 7. Dependence of modulus of elasticity (compressive load) on temperature for PAAm hydrogel and sandwich-like IPN hydrogels for different amounts of PNIPAAm.
heating as it becomes softer at high temperatures. In contrast, for IPN hydrogels the modulus is enhanced as the temperature is raised above LCST. From these results, it could be suggested that the collapse of the PNIPAAm chains inside the polymer matrix leads to a stronger hydrogel. The dependence of apparent cross-linking density, νe , on the temperature from 25 to 40 ◦ C for PAAm and IPN hydrogels is shown in Fig. 8. As expected, νe increased with the increase in the amount of PNIPAAm in the hydrogel; below LCST the increase is due to entanglements; above LCST, the collapse of PNIPAAm gives an additional contribution to νe . 3.4. Permeability to orange II dye through sandwich-like membrane The concentration of orange II diffused through the membrane was obtained by the calibration curve of photometric signal, which correlates the absorbance to the concentration of dye. It was found that the concentration of orange II diffused to downstream compartment increases linearly as a function of time (data not shown here). Eq. (6), based on the first Fick’s law, was used to calculate the permeability (P): dC Vd P= (6) dt CA where dC/dt is the slope of straight line given by the concentration of diffused orange II versus time, C the initial concentration of orange II in the upstream compartment, V the volume of each compartment and A is the area of membrane in contact with upstream compartment. The parameter d is the thickness of swollen membrane at equilibrium in water, at desired temperature. It was measured using a micrometer. Fig. 9 shows permeability curves of (2.5–2.5) sandwich-like membrane to orange II and (2.5–0) pure PAAm hydrogel, in dependence of temperature. The pure (2.5–0) hydrogel (without PNIPAAm), used as ‘blank’ in this work, presented a linear increase in permeability as the temperature increases because there is no shrinking of the PAAm hydrogel. It was found that the permeability of (2.5–2.5) IPN membrane significantly
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Fig. 9. Permeability to orange II dye for the (2.5–2.5) sandwich-like IPN hydrogel and (2.5–0) pure PAAm hydrogel (without PNIPAAm) as a function of temperature.
rather decreases as the temperature increases. The permeability changes of 6.8 × 10−10 m2 s−1 at 25 ◦ C to 3.6 × 10−10 m2 s−1 at 40 ◦ C and this represents a decreases of 52%. This effect was attributed to the collapse of the PNIPAAm polymer networks, which induced a significant contraction of the mid-layer of membrane. In this condition, the mid-layer acts as a ‘barrier’ that minimizes the permeation of orange II through the sandwich membrane. The results shown here (Fig. 9) suggest that the structural changes in the sandwich membrane (shown in Fig. 2) induce to significant flux control. The methodology used in this work produces thermoresponsive hydrogels with specific configuration that can be considered a novelty in the membrane synthesis. Sandwichlike membranes with a temperature responsive mid-layer surrounded by non-responsive surface layers can act as thermoresponsive flux controlling membranes in separation process of protein. Other motivation is the potential use of sandwichlike membranes as devices for controlling release of drugs with hydrophobic nature. For instance, a hydrophobic drug (e.g. corticosteroids) may be entrapped in the mid-layer of membrane during its synthesis. Corticosteroids have been used to treat ulcer diseases in colon but need to be ingested in significant rather amount to reach the colon because the GI tract is one of route the most complex for therapy [34]. The authors have suggested that the sandwich-like membrane could act as a colon-specific delivery system to transport the needed amount of drug for effective therapy. Reaching the colon, the drug could be released by the erosion mechanism, or any other degradation mechanism that will be evaluated forward. 4. Conclusions Sandwiched-like thermo-sensitive membranes of IPN hydrogels have been synthesized. NIPAAm monomers and MBAAm cross-linker were co-polymerized inside previously synthesized cross-linked PAAm. SEM, water uptake capacity and compressive load tests were used to determine hydrogel properties. Through SEM images, we observed that the hydrogel mem-
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branes are formed by three layers, an internal layer fully involved by two others. Significant difference in morphological aspects was also observed between internal and external layers. By SEM, it was observed that the PNIPAAm network is present mainly in the internal layer, because as the hydrogel is warmed above the LCST, the internal layer contracted, while the external layers expanded and remained highly porous. This characteristic was attributed to the hydrophilicity of the PAAm hydrogel. On heating the IPN hydrogel, it was verified an increase in either gel strength or modulus of elasticity, indicating that the material became more rigid. The presence of PNIPAAm networks in the internal layer reinforced the hydrogel significantly and this effect was more evident above LCST. This was attributed to the large shrinking, which resulted in a stronger hydrogel. The permeability of sandwich membrane significantly decreases as the temperature increases above the LCST. There was a decrease in the permeability of 52% as the temperature was increased from 25 to 40 ◦ C. It was suggested that the structural changes in the sandwich membrane induce to significant flux control. According to these results, these hydrogels displayed excellent properties for further tests as thermo-responsive flux controlling membranes in separation process. Furthermore, this hydrogels have attributes that make them an attractive to be tested forward as a drug delivery system. Acknowledgments G.M.C. thanks to Fundac¸a˜ o de Amparo a` Pesquisa do Estado de S˜ao Paulo (FAPESP) for his post-doctoral fellowship (Proc 03/10897-6). MRG is grateful to Conselho Nacional de Desenvolvimento e Tecnol´ogico—CNPp (Brazil) for his master fellowship. The authors wish to thank CNPq for the financial support (Process no. 479221/2001-4) and to Univ. Est. LondrinaUEL, for using TAXT2i Texture Analyzers. References [1] J.S. Turner, Y.-L. Cheng, pH dependence of PDMS–PMAA IPN morphology and transport properties, J. Membr. Sci. 240 (2004) 19. [2] C.L. Bell, N.A. Peppas, Biomedical membranes from hydrogels and interpolymer complexes, Biopolym. Adv. Polym. Sci. 22 (1995) 125. [3] L. Shapiro, S. Cohen, Novel alginate sponges for cell culture and transplantation, Biomaterials 18 (1997) 583. [4] H.G. Schild, Poly(n-isopropylacrylamide)—experiment, theory and application, Prog. Polym. Sci. 17 (1992) 163. [5] A. Okamura, M. Itayagoshi, T. Hagiwara, M. Yamaguchi, T. Kanamori, T. Shinbo, P.-C. Wang, Poly(N-isopropylacrylamide)-graft-polypropylene membranes containing adsorbed antibody for cell separation, Biomaterials 26 (2005) 1287. [6] L. Ying, E.T. Kang, K.G. Neoh, K. Kand, H. Iwata, Drug permeation through temperature-sensitive membranes prepared from poly(vinylidene fluoride) with grafted poly(N-isopropylacrylamide) chains, J. Membr. Sci. 243 (2004) 253. [7] M. Solari, Evaluation of the mechanical-properties of a hydrogel fiber in the development of a polymeric actuator, J. Intel. Mater. Syst. Struct. 5 (1994) 295. [8] Y. Osada, S.B. Rossmurphy, Intelligent gels, Sci. Am. 268 (1993) 82. [9] L. Ying, E.T. Kang, K.G. Neoh, Characterization of membranes prepared from blends of poly(acrylic acid)-graft-poly(vinylidene fluoride) with poly(N-isopropylacrylamide) and their temperature- and pH-sensitive microfiltration, J. Membr. Sci. 224 (2003) 93.
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