Swelling - Deswelling behavior of the hybrid films

It is well-known that positively charged CS at low pH exhibits a high swelling ratio due to the repulsive force between the same positively-charged molecules, which would result in longer intermolecular distance and more hydrophilic property. In other words, increasing the pH of the solution will reduce the repulsion force, thus further limiting the hydration of the CS. The swelling kinetics at PBS (pH=7.4) of pure CS and hybrid film with different Cclay

34

are displayed in Figure 4-5(a). It should be noticed that the swelling equilibrium was reached rapidly at pH 7.4 within 20 minutes. The gained equilibrium state in a short time period was helpful for further cyclic swelling and de-swelling test. Swelling ratio (W s-W d)/W d (%)

pure CS

Figure 4-5(a) Swelling kinetics of hybrid film with different Cclay at pH 7.4.

In order to gain better understanding of the effect of clay on the swelling behavior of the composites, the plots of ln(W0/Wt) against time at initial ten minutes was illustrated in Figure 4-5(b). All plots show straight-like lines, indicating that the process of the swelling may be illustrated with apparent first-order kinetics as described by the pseudo-first-order kinetic equation [79]:

k t, respectively, n is the first-order rate constant (1/s), and k is a constant.

Evidently, the linear fitting showed a reasonably good correlation (with a correlation coefficient, r²=0.9862). It can be found that the value of the first-order rate constant n showed a small increase, compared to the pure

chitosan, from 0.063 to 0.066 as a small amount of clay was added (0.5-1 wt

%). However, when the amount of the clay increased above 3 wt%, high cross-linking degree decreased the swelling rate, resulting in a greater decrease in n value (n=0.054) was observed. In addition, the equilibrium swelling ratios of the composites are also affected by the incorporation of the clay.

Figure 4-5(b) Plots of ln(W0/Wt) against time of hybrid film with different Cclay at pH 7.4.

36

As seen from Figure 4-5(c), the equilibrium swelling ratio of the composites reduced from 106% to 91.0% with the increase of clay from 0 to 3 wt %. The decrease in the equilibrium swelling ratio with increasing Cclay was also observed at pH 4 and 10 (not showed here). This can be attributed as a result of either a reduced amount of functional groups or a proportional reduction in ionizable functional groups in the CS as a result of extensive cross-linking bonding formed between the positively-charged CS and the negatively-charged clay. From the results above, the swelling behavior of chitosan film does not differ substantially from that of the hybrid films with 0-2

wt % clay contents. For example, when a small amounts of clay was added (0.5-2 wt %), the value of n and equilibrium swelling ratio was calculated to lying between 0.063~0.062 and 106%-102%, respectively. However, the hybrid film with 3 wt% addition of clay does show a larger difference in the swelling kinetics and equilibrium swelling ratio, compared to other samples, i.e., n=

0.054 and 91.0%, respectively. This suggests that as the clay concentration exceeds a critical concentration, an effective cross-linked interaction between the surface of clay particles and chitosan molecular chain can be efficiently developed. Hence, high cross-linking density caused by higher addition of the Cclay (3 wt %) decreased both the swelling rate and the equilibrium swelling ratio as well.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 85

90 95 100 105 110 115 120

Clay Content (%)

Equi libr iu m Sw el ling Rat io ( % )

(c)

Figure 4-5(c) Equilibrium swelling ratio of hybrid film with different Cclay

at pH 7.4.

Since CS is a cationic biopolymer and has been proposed for electrically-modulated drug delivery [21], therefore, the deswelling behavior is becoming so critical for a reliable performance for a number of biomedical

37

applications, such as drug delivery, wherein a release rate can be externally or internally controlled from slow to pulsatile release profiles according to practical needs. The mechanism of deswelling behavior is generally thought to be a macroscopic contractile deformation of a polymer hydrogel under an electric field. This is due to a voltage-induced movement of ions across the entire polymeric matrix and the concomitant expansion on one side and contraction on the other side of the polymer [8]. Figure 4-6(a) exhibits the deswelling ratio as a function of time with different applied voltages and clay concentrations. Deswelling water ratio W t/W o (%)

Time (min.)

(a)

Figure 4-6(a) Deswelling behavior in 1.5h of hybrid film with different Cclay under applied voltage of 1, 5, and 10V.

The deswelling rate of the composites was enhanced in proportion to the applied voltage, which suggests that the increased potential gradient in electric field caused an increase in the rate of movement of those counter ions to different electrolytes.

38

At the same time, the incorporation of the clay was found to influence strongly the deformation of the hybrid films under an electrical field. As a

consequence, the electro-stimuli deformation reached equilibrium after 1.5h of operation. Concerning the deswelling ratio at the time of 1.5h, as shown in Figure 4-6(b), the hybrids with lower Cclay are subjected to smaller restriction of molecule relaxation than that of pure CS. In other words, with increasing Cclay, the mobility of the hybrid composites was gradually restricted by the formation of increasing amount of the cross-linking bonds.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 50

60 70 80 90 100

D es w el lin g w at er ra tio W

t

/W

o

(% )

Clay content (%)

1V 5V 10V

(b)

Figure 4-6(b) Deswelling water ratio in 1.5h of hybrid film with different

Cclay under applied voltage of 1, 5, and 10V.

Therefore, the deswelling ratio of the resulting hybrid is decreased with increase of Cclay. As shown in Fig. 4-6(b), the hybrid composites containing the lower clay concentration (0.5-1 wt %) exhibited more rapid responsivity and larger deformation to a given electrical voltage. However, the rate and extent of the deswelling kinetics are both decreased as Cclay is increased.

4.5 Cyclic deformation and recovery of hybrid films

Above a threshold value of a given electrical field, the hybrid film showed

39

a deswelling behavior wherein the water is gradually expelled from the hybrid.

When the electric field was removed, the matrix swelled by absorbing surrounding water. A repeated on-off operation of the electrical field stimulates the hybrid films with a cyclic swelling-deswelling mechanical deformation.

Figure 4-7(a) shows that the relative swelling ratio of the CS film after cyclic on-off operation of the electrical field in a 20-minute interval to that of original equilibrium swelling ratio of the hybrid film with different clay concentrations under applied voltage of 10V.

0 5 10 15 20

Figure 4-7(a) Relative ratio of swelling after cyclic on-off switching of electric-stimuli of 5V in a 20-minute interval of hybrid films with different Cclay.

40

Initially, the equilibrium swelling of pure CS film was slightly increased at first several times of the on-off operation. It may be due to a collapse of the CS structure under applied voltage and then produce more and more porous structure on the CS film as was visually observed. The enhanced porosity will improve the degree of swelling to about 1.15 times after six cycles. After then, the degree of swelling started to reduce, indicating a fatigue behavior of the CS

film. This observation suggests that the CS film should lose its structural integrity after 6 cyclic operations and this deteriorates the CS film by losing its capability of absorbing water. On the contrary, the hybrid composite with lower clay content (0.5 wt %) showed much better structural integrity allowing the hybrid to be operated by as high as 15 times of the cyclic operation and a higher relative ratio of swelling achievable at about 1.3.

41

Figure 4-7(b). Relative ratio of deswelling after cyclic on-off switching of electric-stimuli of 5V in a 20-minute interval of hybrid films

with different Cclay.

The increase of swelling degree may be associated with the decrease of cross-linking degree caused by lower Cclay. However, as the higher concentration of the clay was loaded (>0.5 wt %), the swelling ratio remained the same when the number of cyclic stimulation exceeded six times. Actually, the swelling ratio would also not decrease enormously with increased cyclic stimulation like the case of pure CS and hybrid of 0.5 wt % clay. It is believed that greater cross-linking density of the hybrid composites is capable of

42

bearing larger applied voltage and more on-off switching operations. Similarly, the higher Cclay (>0.5 wt %) of the hybrid composites can maintain the same degree of deswelling after over 10 times of cyclic operation, compared to the pure CS film and hybrid composites with lower clay content , as shown in Figure 4-7(b). Hence, the incorporation of the clay particles can structurally adjust the cross-linking density of the hybrid with improved anti-fatigue property of the hybrids under cyclic electric-stimuli operation.

Figure 4-8 shows the weight changes of the pure CS film and the hybrid film (1 wt % clay) under an applied voltage of 10V in PBS and consecutive on-off operation in a 20-minute interval. For pure CS, the degree of reversibility (i.e., the ability for the hybrid to structurally return back to initial swelling state as first prepared) is decreased apparently with the on-off operation more than 7 cycles. Furthermore, the deswelling ratio of the pure CS film could not restore to original level after several cyclic operations. The decrease of the swelling and deswelling ratio of the hybrid film can be translated directly as an indication of fatigue behavior of the hybrid. In this study, the films with a composition of 1 wt % clay showed the best anti-fatigue property where the swelling and deswelling behavior remained identical even after 20 cycles of on-off operation. Since the contractile deformation under stimulus and volume recovery of a given polyelectrolyte hydrogel is associated with the optimal viscoelasticity at a certain cross-linking density, it is suggested that the incorporation of the negatively-charged clay as cross-linkers provides more effective anti-fatigue property for pure positively-charged chitosan under cyclic electro-stimuli operation. On this base, it is believed that this hybrid is considered in biomedical applications with improved and reliable performance, especially used as drug delivery system which is currently under investigation

and will be reported shortly.

0 150 300 450 600 750 0.30

0.35 0.40 0.45

0.50 pure CS 1% clay

We ig ht ( g )

Time (min.)

Figure 4-8. Weight changes of pure chitosan and hybrid film with 1 wt % clay addition under cyclic on-off switching of electric-stimuli of 5V.

43

44

Chapter 5

Drug release behavior of chitosan–montmorillonite nanocomposite hydrogels following electrostimulation

5.1 Introduction

Smart polymer hydrogels have been studied with a particular emphasis on their reversible volume changes in response to external stimuli, such as pH, solvent composition, temperature, ionic concentration, and electric field [80-82].

These hydrogels have been developed and studied with regard to the application to several biomedical fields; for example, separation techniques, soft-actuators, and controlled drug delivery systems [83, 84]. Among them, electrically controlled drug delivery may particularly offer unique advantages for providing on-demand release of drug molecules from implantable reservoirs. In addition, electrical control is advantageous for coupling to sensors and microelectronics in feedback controlled systems [85].

For electro-sensitive hydrogels as controlled drug delivery systems, the drug release rate can be easily controlled simply by modulating the electric field. Generally, the extent of drug release increases with the magnitude of electric field and time, but is not linearly proportional to them [86]. Hence, it becomes more difficult to precisely control the release of drug by electric-stimulation. In particular, an important goal of drug delivery is to obtain a constant release rate for a prolonged time. However, as common problems of all hydrogels, the responsivity and reversibility will be decreased after several switching on-off operations. For commercial application, this fatigue property has to be improved to reach stable pulsatile release under repeatedly

45

operations. Unfortunately, as we have learned so far, little study had addressed this important issue and this is then one of the research objectives of this investigation. Hence, in order to overcome the fatigue problem to a certain extent of the conventional hydrogels, incorporation of inorganic nanophase becomes an attractive alternative, i.e., inorganic-organic nanocomposite hydrogel (hereinafter, we named it as nanohydrogel), where the properties of polymer matrix would be improved with a large effect on the electric-deformation and relaxation behaviors [70]. For example, Gong et al.

[87] had reported that organically modified clay can enhance temperature response of clay-poly(N-isopropylacrylamide) (PNIPAAm) nanocomposites.

Based on hydration theory, the organically modified clay introduces a hydrophobic environment at the interface that would enhance the efficiency of the thermal transition, narrow the transition range, and increase the transition rate. However, to the best of our knowledge, little research work had been reported on the electric-stimuli drug release behavior of polymer-(nano)clay nanohydrogel.

Polymer-clay nanohydrogels are expected to have novel properties because of the nanometric scale on which the nanoclay particles (i.e., plate-like shape) would alter the physical and chemical properties of the polymeric materials and improved their mechanical properties and thermal stability [35]. Chitosan (CS) used as polymeric matrix in this work is a cationic biopolymer and has been proposed for electrically-modulated drug delivery [21]. In chapter 4, it was demonstrated that the addition of clay in the CS matrix could strongly affect the cross-linking density as well as the mechanical property, swelling–deswelling behavior and fatigue property of the nanohybrids.

Hence, the incorporation of negatively-charged delaminated (exfoliated)

46

montmorillonite (MMT) is expected to electrostatically interact with the positively-charged –NH3+ group of CS, which generates a strong cross-linking structure in the nanohydrogel [76] and then, strongly affects the macroscopic property of the nanohydrogel and the drug diffusion through the bulk entity. In present work, the release kinetics and mechanism of the vitamin B12 in terms of MMT contents were investigated under a given electric-field stimulus.

Furthermore, the anti-fatigue behavior with respect to the repeated field stimuli of the resulting nanohydrogel in terms of MMT addition was also elucidated.

5.2 Preparation of CS-MMT nanohydrogels

To prepare the CS-MMT nanohydrogels, the preparation procedure is separated into two stages, the first stage is first to prepare a suspension containing MMT and CS with a weight ratio of 1:2 (where the CS solution was prepared by dissolving the pre-determined amounts of CS in 1 wt % acetic acid solution and stirring for about 4h till the CS completely dissolved). The CS-MMT suspensions were obtained by adding CS, to an aqueous solution containing 2 wt % MMT (i.e., 0.5 g of Na⁺-MMT dispersed in 25 ml of double-distilled water), stirred at 50^oC for 24h. To enhance the formation of exfoliation of the MMT in the final nanohydrogel, the suspension with CS to MMT ratio of 2:1 was then subject to ball-milling for 24h, after then, the as-prepared final CS-MMT suspension was used to form nanohydrogel.

In the second stage of the CS-MMT nanohydrogel preparation, 2 wt % CS solution was obtained by dissolving CS into 1 wt % acetic acid solution.

Then, a small amount of the ball-milled CS-MMT suspension was added into the prepared CS solution to form CS-rich suspension with the MMT content controlled in the range of 1 wt %, 2 wt %, 3 wt %, and 4 wt %, relative to the

47

total weight of CS in the suspensions, under continuous stirring at 60^oC for 1h.

This final suspension was then cast onto petri-dishes and dried at 30^oC for 24 h, to form a final dried nanohydrogels. The dried nanohydrogels were then rinsed with an aqueous solution of 1M NaOH to remove residual acetic acid, followed by washing with distilled water and dried for a week at 40^oC in vacuum till use. The compositions of the nanohydrogels are expressed using the value of n to define the content of MMT (1-4 wt %) in the CS-MMTn, where n=CMMT, the content of the incorporated MMT, for the nanohydrogels, ranging from 1 to 4 %.

5.3 Structural Characterization

The hydrophilic and polycationic nature of CS in acidic media permit a good miscibility with MMT and can easily intercalate into the interlayers of the MMT by means of cationic exchange processes [33]. For this purpose, an acidic pH value is used to ionize the formation of –NH3+ groups in the CS structure. Given that the pKa of the primary amine groups in the CS structure is 6.3, 95% of the amine groups will be protonated at pH 5 of the CS-MMT suspensions. Figure 5-1 illustrates the XRD patterns of neat MMT and CS-MMT suspensions with different ratios of CS-to-MMT. The XRD pattern of the neat MMT (Figure 5-1a) shows a reflection peak at about 2θ=6.5^o, corresponding to a basal spacing of 1.35 nm.

2 4 6 8 10 12

d c b a

4.6

^o

5.6

^o

6.5

^o

2 θ (degrees)

In te n sity (a .u .)

Figure 5-1 Low-angle powder XRD patterns of (a) neat MMT and the CS/MMT nanocomposites with the CS and MMT ratios of (b) 0.5:1 (c) 2:1 and (d) ball milling for 24 h.

48

The MMT in the suspension with lower CS-to-MMT ratio (0.5:1) shows a decrease of 2θ value to about 5.6^o suggesting the intercalation of CS in a monolayer disposition (Figure 5-1b). The lower 2θ value obtained for the MMT

49

in the suspension with the highest ratio of CS-to-MMT (2:1) is related to the intercalation of the CS as a bi-layer (Figure 5-1c). After ball-milling for 24h, the XRD pattern of the MMT in the suspension shows a faint, broad peak at around 2θ=5-6^o (Figure 5-1d). The broad peak with decreased intensity most likely indicated a disordered, exfoliated and little intercalated structures of the MMT. In addition, it is hard to give a definitive conclusion regarding the nanostructural evolution without more convincing information but through XRD analysis only. Recording the optical absorbance with time gives a suitable indication of the aggregation and sedimentation of the MMT in the suspensions.

This method allows us to distinguish accurately the difference in surface structures acquired by particle aggregates.

Figure 5-2 shows the time dependence of the optical absorbance A (relative to its initial value, A0) of the exfoliated MMT, original MMT and monolayer or bilayer intercalated-MMT suspensions. In theory [88], sedimentation velocity U of interacting colloidal particles depends both on the hydrodynamic interactions mediated by the suspending solvent, and on the microstructure of the suspension. In equilibrium, the latter is determined by direct potential forces arising, for example, from the steric repulsion between the particles and from the electrostatic repulsion of overlapping double layers.

On the other hand, the long-ranged electrostatic repulsion occurring in suspensions of charged particles can give rise to a reduction in U, as compared to hard spheres dispersion at the same volume fraction. Watzlawek and Nägele [18] proposed a model that the reduced sedimentation velocity of dilute deionized suspensions of weakly charged particles scaled like U/U0=1-pψ1/2 where ψ is the particle volume fraction and U0 is the sedimentation velocity at finite dilution, with a parameter p depending on the

macroion charge Z (p ～ ｜ Z ｜³). Hence, it is believed from the time dependence of optical absorbance A, Fig. 5-2a, that the exfoliated MMT keeps a greater value of A/A0 (~1) for a period of 180 minutes which is due to that it carried more negatively-charged sites on the surface layer resulting in stronger electrical repulsion than that of non-exfoliated MMT.

0 30 60 90 120 150 180

0.4 0.6 0.8 1.0

c b a

Time(min)

A /A

0

Figure 5-2 Time dependence of optical absorbance A (relative to its initial value, A0) of the (a) exfoliated MMT (b) original MMT and (c) monolayer or bilayer intercalated-MMT suspensions.

Nevertheless, the electrostatic attraction between ionized –NH3+ groups in the CS structure and negatively charged silicate layers neutralizes this repulsion force, it is then expected that monolayer or bilayer intercalated-MMT (Figure 5-2c) exhibited faster sedimentation ( lower values of A/A0 after 180 minutes) than non-exfoliated MMT (Figure 5-2b).

50

As shown in Figure 5-3, it was found that the intensity of the diffraction peaks at 2θ~20^o which was identified as semi-crystalline CS decreased with the incorporation of MMT. Furthermore, the XRD patterns do not show any diffraction peak at 2θ=2-10^o as opposed to the diffraction peak at 2θ= 6.5^o (d spacing = 1.35 nm) for original MMT, indicating development of exfoliated

在文檔中 幾丁聚醣複合物奈米結構與可控制藥物釋放行為之研究 (頁 51-0)