Effect of clay content on electrostimulus deformation
and volume recovery behavior of a clay–chitosan hybrid composite
Kun-Ho Liu, Ting-Yu Liu, San-Yuan Chen
*, Dean-Mo Liu
Department of Materials Science and Engineering, National Chiao Tung University, 1001 Ta-hsueh Road, Hsinchu, Taiwan, China
Received 27 February 2007; received in revised form 30 May 2007; accepted 11 June 2007 Available online 24 June 2007
Abstract
Electrostimulus-responsive hybrid composites composed of chitosan (CS) and clay were successfully developed and systematically characterized. The addition of negatively charged clay as an ionic cross-linker strongly affect the cross-linking density as well as the mechanical property, swelling–deswelling behavior and fatigue property of the hybrids. With lower clay content, the crystallinity of the CS was slightly reduced, resulting in a decrease in the mechanical properties and an increase in the swelling ratio of the hybrid. How-ever, the swelling kinetics were accelerated due to a reduction in CS crystallinity. On the other hand, with increasing clay concentration, the increased cross-linked bonding mechanically reinforced the hybrid beyond the aforementioned adverse effect, to show improved ten-sile strength and a decrease in the swelling ratio. The voltage-induced deformation of hybrids became more pronounced with increasing applied voltage, but became less pronounced with increasing clay content under an applied electric field. After repeatedly switching the
electric field on and off, the higher clay concentration (Cclay> 0.5 wt.%) of the hybrid composites maintained the same capability of
deswelling and swelling after more than 10 cycles, compared with both the pure CS film and the hybrid composites with lower clay con-tent (e.g., 0.5 wt.%). Compared with pure CS, a significant improvement in the anti-fatigue property against cyclic electric stimulations of the hybrid was found, which encourages the use of such a new class of hybrid composite in medical and pharmaceutical applications. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Chitosan; Clay; Electric stimuli; Hybrid film; Anti-fatigue
1. Introduction
‘‘Intelligent’’ or ‘‘smart’’ hydrogels which can control drug release by changing the gel structure in response to environmental stimuli have been used in diverse applica-tions, such as artificial muscles [1], bioseparation [2] and drug delivery systems [3,4]. Environmental stimuli, such as pH, pressure, temperature, light, magnetic fields and electric fields, cause smart hydrogels to undergo macro-scopic deformation and produce contractile force. Of those stimuli, the application of an electric field is one of the most frequently employed methods of triggering the desirable mechanical deformation of smart hydrogels for specific
engineering purposes. A typical example is artificial muscle, which can be applied directly as a medical device with a simple mechanical movement or employed as an electri-cally controllable matrix or switch for drug delivery in vitro and in vivo[5]. Using an electric field as an external stimulus has advantages, with the increased availability of equipment in the market that allows the precise control of a number of parameters, including the magnitude of the cur-rent, the duration of pulses and the intervals between pulses, which can be directly translated into precise defor-mation behavior of the hydrogels.
A significant drawback of most environmentally stim-ulus-sensitive hydrogels is that the responsivity and revers-ibility often decrease with both time and number of on–off operation cycles as the gel fatigues considerably. Generally, the deformation under a stimulus is increased with greater
1742-7061/$ - see front matter Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2007.06.002
*
Corresponding author. Tel.: +886 3 5731818; fax: +886 3 5725490. E-mail address:sanyuanchen@mail.nctu.edu.tw(S.-Y. Chen).
molecular mobility in a gel of smaller cross-linking density. It is well known that the extent of gel deswelling increases with the magnitude of the electric voltage, but this increase is not linear. Gong et al. [6] reported that the extent of deswelling depended on the amount of charge transported through the gel, rather than on the voltage applied. Fur-thermore, after removal of the stimulation, volume recov-ery occurs because the gel absorbs fluid and swells, and this is also increased with low cross-linking density. Sutani et al.[7]reported that the differences, including cross-link-ing density, the mobility and flexibility of the network and the viscosity properties of the gel, might affect the deforma-tion and relaxadeforma-tion properties of gel with cyclic on–off operations. At the same time, it was also proved that there is a most suitable composition and viscoelasticity at a cer-tain cross-linking density for optimal electroresponsive-ness. Shiga et al. found that an acrylacid–acrylamide gel swelled or deswelled under electrical stimulation, depend-ing on the concentration of ions in the gel[8]. In order to further understand the dynamics of the ion species in ionic gels, Doi et al. [9]proposed a semi-quantitative theory to explain the swelling and shrinking (deswelling) behaviors of electroresponsive gels. However, the actual mechanism that causes the fatigue of the gel under stimulation is still not clear. Moreover, the fatigue problem of these hydrogels has to be circumvented to achieve the reliable performance required in medicine.
During the last decade, considerable attention was paid to inorganic–organic hybrid materials because their solid-state properties could be tailored in relation to the nature and relative content of their constitutive components. Low-volume additions (1–5 wt.%) of highly anisotropic nanoparticles, such as layered silicates, provide property enhancement with respect to the neat polymer that are comparable to those achieved by conventional loadings (15–40 wt.%) of traditional fillers. Besides, unique value-added properties not normally possible with traditional fillers are also observed, such as enhanced strength, elec-trical conductivity, electrostatic discharge, remote-actu-ated shape recovery and ablation resistance [10,11]. Wang et al. [12] successfully synthesized chitosan/mont-morillonite nanocomposites and reported that nano-dispersed clay improved the thermal stability and enhanced the hardness and elastic modulus of the matrix systematically with increased clay loading, up to a loading of 10 wt.%. However, higher clay loading in the matrix could increase the possibility of inhomogeneous distribu-tion of the clay. In this case, this increase in both hardness and elastic modulus of the chitosan/clay nanocomposite imparts sufficient rigidity to the nanocomposite. This will then deteriorate desirable flexibility of the composite upon controlled contraction–expansion deformation under envi-ronmental stimuli. Therefore, it is more technically inter-esting to prepare such a nanocomposite system which is flexible and mechanically strong enough to be operated reliably under cyclic environmental stimuli, such as electri-cal stimulation.
By taking advantage of the electrochemical properties, i.e., the surface charge, and nanostructural properties, i.e., the layer of the clay particles, a new class of electrically charged hybrid composites based on chitosan (CS) and clay was studied. The inorganic material, clay, used in this inves-tigation is polysilicate magadiite (Na2Si14O29Æ nH2O),
which is composed of one or multiple negatively charged sheets of SiO4tetrahedra with an abundant
silanol-termi-nated surface compensated by either Na+or H+in the inter-layer spacing. The cationic biopolymer chitosan, composed mainly of b-(1,4)-linked 2-deoxy-2-amino-D-glucopyranose
units, is a deacetylated product of chitin. In the present study, with its incorporation into CS matrix, it is expected to enhance the thermal stability and mechanical properties of the resulting hybrid composites compared with neat CS polymer. Moreover, the electrically stimulated swelling– deswelling behavior, mechanical deformation and clay con-tent of the hybrid composites will be elucidated.
2. Materials and methods 2.1. Materials
The chitosan used in this study to prepare the CS–clay nanocomposites was supplied by Aldrich–Sigma and used without purification. The same type of chitosan was used by Darder et al. [13], who found it to have an average molecular weight of 34 2500 g mol1 and a deacetylation degree (DD) of75%. Sodium phosphate for the prepara-tion of buffers and acetic acid were purchased from Aldrich Chemicals. As inorganic clay, Na–magadiite was pur-chased from Chang-Chun Petrochemical Co. (Hsinchu, Taiwan). The mean particle size and size distribution of the clay were measured by an ultrafine particle size ana-lyzer (Honeywell). The mean particle size of the clay (Na–magadiite) was estimated about 3 lm, and a narrow particle size distribution of 2–4 lm can be observed. 2.2. Preparation of CS–clay films
To prepare the CS–clay films, 1 g of CS was first dis-solved in 40 ml of 1% acetic acid solution followed by cen-trifugation to remove the insoluble material. This ensures that the chitosan used to prepare the hybrid films can be completely dissolved without detectable insoluble frac-tions. Then, a small amount of clay (0.005, 0.01, 0.015, 0.02 and 0.03 g) was added to 10 ml of distilled water to form a suspension. Next, the mixed suspension added into the prepared CS solution with a clay content of 0.5, 1, 1.5, 2 and 3 wt.%, followed by stirring at 60°C until a uni-formly distributed CS–clay suspension was obtained. This solution was then cast into Petri dishes (radius 1.5 cm) and dried at 30°C for 24 h. The dried films were then immersed into an aqueous solution of 1 M NaOH to remove residual acetic acid. The obtained products were washed with distilled water and dried for 1 week at 40°C in vacuum.
2.3. Characterization
The crystal structures of hybrid films were determined using an X-ray diffraction (XRD) diffractometer (Siemens D5000) equipped with a Cu Ka radiation source (k = 0.154 nm). The diffraction data were collected from 2h = 2 25°. Fourier transform infrared (FTIR) spectra were recorded using a Bomem, DA8.3 FTIR spectrometer with attenuated total reflectance (Harrick). A Malvern Zreasizer HS3000 photon correlation spectrometer with an applied voltage of 100 V and a 5 cm quartz cell was used to deter-mine the zeta potential values of clay (Na–magadiite) in phosphate-buffered solution (PBS) with different pH values (4, 6, 7 and 10). The concentration of clay in aqueous sus-pension was fixed at 0.1 wt.%. The tensile mechanical prop-erties of the hybrid composites were measured using a complete MTS Tytron 250 and TestStar IIs system in the following conditions: crosshead speed, 10 mm min1; test temperature, 25°C. The initial cross-section (10 mm2) was used to calculate the tensile strengths and the tensile modulus. All results reflect the average of three measure-ments. The difference between each measurement is <5%. 2.4. Swelling behavior of CS–clay films
The film was first cut into a circular plate (radius 1 cm). The average thickness and weight of the obtained dried films were measured to be 0.20 mm and 0.12 g, respectively. In order to measure the swelling ratio, each sample was weighed before and after immersion in PBS. After the excessive surface water had been removed with filter paper, the weight of swollen samples was measured at various time intervals. The procedure was repeated five times, until no further weight gain was detected. The swell-ing ratio was determined accordswell-ing to the followswell-ing equation:
Swelling ratioð%Þ ¼ ½ðWs WdÞ=Wd 100
where Wsand Wdare the weights of the swollen and dried
samples, respectively. All results reflect the average of five measurements. The difference between each measurement is <5%.
2.5. Electrostimulus response behavior of CS–clay films The film, pre-equilibrated and swollen in PBS, was cut into a circular plate (radius1 cm) and weighed. Two plat-inum electrodes (radius1.5 cm) were kept in contact with opposite surface of the film. The released water from the hybrid films was continually removed using filter paper and the weight change of the swollen hybrid films was checked periodically under an electric field. The deswelling water ratio was evaluated as Wt/Wt0 where Wt0 and Wt
were the initial weight of the fully swollen films and the weight of films at deswelling time t, respectively. All results reflect the average of five measurements. The difference between each measurement is <5%.
3. Results and discussion
3.1. Chemical interaction between the CS and the clay in the hybrid
Magadiite has a layered structure with negatively charged silicate layers compensated by interlayer sodium ions. The zeta potential profile inFig. 1shows the change of the electrical charge of the clay in buffer solution with different pH values. The isoelectric point of the clay was determined to be about 5.3, which indicates that above pH 5.3 the net charge of the clay is negative. The polycat-ionic nature of CS makes it an excellent candidate for inter-action with the negatively charged Na–magadiite clay by means of electrostatic attraction [14]. Thus, the clay in the CS matrix can act as an effective multi-functional cross-linker [15]. For this purpose, aqueous medium with a low pH value is employed to generate ionized NHþ3
groups in the CS structure. An electrostatic attraction is expected to take place as a result of Coulombic interactions between the positively charged NHþ3 groups of the CS
and the negatively charged sites in the clay structure, which mainly controls the adsorption process.Fig. 2a shows the low-angle XRD patterns of the Na–magadiite, neat CS and CS–clay hybrid films. The d001 spacing is obtained
using the first rational orders corresponding to the (0 0 1) reflection. From the XRD pattern of Na–magadiite, three reflection peaks can be observed at about 2h = 5.63°, 11.3° and 17.4°, corresponding to the (0 0 1), (0 0 2) and (0 0 3) reflection planes. Herein, the peak at 5.63° corre-sponds to a basal spacing of 1.55 nm. After incorporating a small amount of the clay (1 and 2 wt.%), the basal plane of the clay at 2h = 5.63° disappeared, substituted by a new weakened broad peak at around 2h = 8.1°. The movement of the basal reflection of clay from 2h = 5.63° to 8.1° (the basal spacing = 1.1 nm) is believed to be a result of the replacement of Na+by H+. When the clay is placed in con-tact with acids, even weak and diluted acids, an exchange reaction of the interlayer sodium ions by protons takes
3 10 11 -50 -40 -30 -20 -10 0 10 20 30 40 50 IEP=5.3 Zeta potential (mV) pH values 4 5 6 7 8 9
Fig. 1. Zeta potential profiles of nanoclay (magadiite) at various pH values.
place. It is believed that during the preparation of chitosan-based film, due to the presence of acetic acid, sodium ions are exchanged by protons, yielding the layered silicic acid called H-magadiite[16]. On the other hand, it is found that the intensity of the peak at 2h 20°, which was identified as semi-crystalline CS, decreased with the incorporation of the clay. From the above results, it is believed that the dispersion of layer-type H-magadiite in the CS matrix may slightly deteriorate the crystallinity of CS. A further examination using IR spectroscopy reveals a strong absorption peak at k = 1560 cm1, which corresponds to the vibration of the protonated amine group (dNH3Þ in CS
and is broadened with the increase of clay addition (2 wt.%), as shown in Fig. 2b. This correlation strongly suggests that the NHþ
3 groups in the CS interacted
elec-trostatically with the negatively charged sites of clay sur-face. This electrostatic interaction between the CS and the clay particles ensures the formation of bonding in between, which further generates a strong cross-linking structure in the final hybrid.
3.2. Degree of cross-linking in the hybrid
As negatively charged clay acts as an ionic cross-linker, the addition of clay will strongly affect the cross-linking density as well as the mechanical properties of the hybrids.
From mechanical properties, Fig. 3 shows that both the breaking tensile strength and the tensile modulus decreased slightly as 0.5 wt.% clay was added. However, both param-eters improved with a further increase of clay concentra-tion. The improvement in the mechanical properties was believed to be attributed to the formation of a higher population of CS–clay bonds in the hybrid and a more effective reduction in the molecular relaxation of the CS matrix. Therefore, it is reasonable to believe from the mechanical behavior of the hybrid composites that the neg-atively charged clay incorporated is able to form a physi-cally strong network structure associated with the positively charged CS matrix. Furthermore, the link-ing density, which is equivalent to the number of cross-linked chains per unit volume, is mainly determined by the concentration of CS polymer and the initiator at a fixed amount of clay. The number of cross-linked polymer chains per unit volume of the hybrid, N*, can be estimated
according to Eq. (1) [17] by using the stress at 100% elongation (a = 2): F ¼ UNkT a 1 a ð1Þ
here, F is the force per unit original cross-sectional area of the swollen network, U is a front factor (=1), a is the elon-gation ratio, and k and T are Boltzmann’s constant and the absolute temperature. The data, N*, measured from Eq.(1)
with different concentrations of clay (Cclay) are illustrated
inFig. 4. At lower clay content (e.g. 0.5–1 wt.%), it was ob-served that the value of N* of the hybrids was less than that of the pure CS. This lower cross-linking density of the hybrids is most probably due to its lower crystallinity. As evidenced from the XRD results (Fig. 2a), it was
2000 1750 1500 1250 1000 750 500 2 wt % Clay 1 wt % Clay pure CS 1560 cm-1 Intensity (a.u.) Wavelength (cm-1) 6 10 12 14 16 18 20 22 pure CS 2 wt % Clay 1 wt % Clay Na-magadiite 17.4o(003) 11.3o(002) 5.63o(001) 2 (degrees) Intensity (a.u.) 8
Fig. 2. (a) Low-angle powder XRD patterns and (b) FTIR spectra of hybrid films with various clay loadings.
2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 -1 0 1 2 3 Strength Strength (MPa) Clay content (%) Modulus Modulus (Mpa)
Fig. 3. Mechanical properties of hybrid film. The initial cross-sectional area (10 mm2) was used for calculating the modulus and strength.
demonstrated that the incorporation of clay would slightly deteriorate the crystallinity of CS. However, at the same time, increasing the clay in the CS matrix will also provide more chemically cross-linked bonds between the CS and clay, resulting in a more pronounced improvement in the mechanical properties. As clay is incorporated, competi-tion arises between the deterioracompeti-tion of CS crystallinity and the increase in cross-linking density that dominates the performance of the resulting hybrid composites. With further increasing clay concentration, the increased cross-linked bonds reinforce the hybrid over the adverse effect with only a small addition of clay. Thus, the number of cross-linked polymer chains per unit volume (N*) is
in-creased with increasing Cclay, wherein improved physical
properties can then be expected.
Accordingly, it is of great interest to explore the swelling and deswelling behavior of this new class of hybrids, and in particular the improved anti-fatigue properties under cyclic electrostimulations.
3.3. 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 Cclayare displayed in Fig. 5a. It should be noted that the
swelling equilibrium was reached rapidly (within 20 min) at pH 7.4. Reaching an equilibrium state in such a short time period was helpful for further cyclic swelling and deswelling tests.
In order to gain a better understanding of the effect of clay on the swelling behavior of the composites, plots of ln(W0/Wt) against time after the initial 10 min are
illus-trated in Fig. 5b. All plots show straight lines, indicating that the process of the swelling may be illustrated by
apparent first-order kinetics as described by the pseudo-first-order kinetic equation [18]:
ln W0 Wt
¼ nt þ k
ð2Þ
where W0and Wtare the weights of the hybrid composites
at t = 0 and any time t, respectively, n is the first-order rate constant (1 s1) and k is a constant. Evidently, the linear fitting showed a reasonably good correlation (with a corre-lation coefficient, r2= 0.9862). It can be seen that the value of the first-order rate constant n showed a small increase,
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Clay content (%) N * (n o ./u m 3 )
Fig. 4. Clay content dependencies of the number of cross-linked chains (N*). 0 10 20 30 40 50 60 0 20 40 60 80 100 120 Time (Min.) Sw el lin g rati o (W s -W d )/ Wd (% ) pure CS 0.5wt% Clay 1wt% Clay 2wt% Clay 3wt% Clay 0.0 0.5 1.0 1.5 2.0 2.5 3.0 85 90 95 100 105 110 115 120 Clay Content (%)
Equilibrium Swelling Ratio (%)
0 2 8 10 -0.2 0.0 0.2 0.4 0.6 pure CS n=0.063 0.5wt% Clay n=0.065 1wt% Clay n=0.066 2wt% Clay n=0.062 3wt% Clay n=0.055 Time (min.) ln (W t /W 0 ) 4 6
Fig. 5. (a) Swelling kinetics, (b) plots of ln(W0/Wt) against time and
compared with the pure CS, 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.%, the high cross-linking degree decreased the swelling rate, resulting in a greater decrease in n value (n = 0.054). In addition, the equilibrium swelling ratios of the composites are also affected by the incorporation of the clay. As seen from
Fig. 5c, the equilibrium swelling ratio of the composites re-duced 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 result can be attributed to either a reduced number of functional groups or a proportional reduction in ionizable functional groups in the CS as a re-sult of extensive cross-linked bonding formed between the positively charged CS and the negatively charged clay. From the above results, 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 amount of clay was added (0.5–2 wt.%), the value of n and equilibrium swelling ratio was calculated to lie be-tween 0.063 and 0.062 (106% and 102%), respectively. However, the hybrid film with 3 wt.% addition of clay does show a larger difference in the swelling kinetics and equilib-rium swelling ratio compared with 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 the chitosan molecular chain can be effi-ciently developed. Hence, high cross-linking density caused by a higher Cclay(3 wt.%) decreased both the swelling rate
and the equilibrium swelling ratio.
CS is a cationic biopolymer and has been proposed for electrically modulated drug delivery [19]. Its deswelling behavior is critical for the reliable performance of a num-ber of biomedical applications, such as drug delivery, wherein a release rate can be controlled externally or inter-nally from slow to pulsatile release profiles according to practical needs. The mechanism of the deswelling behavior is generally thought to be the macroscopic contractile deformation of a polymer hydrogel under an electric field. This is due to the 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 poly-mer[8].Fig. 6a exhibits the deswelling ratio as a function of time with different applied voltages and clay concentra-tions. The deswelling rate of the composites was enhanced in proportion to the applied voltage, which suggests that the increased potential gradient in the electric field caused an increase in the rate of movement of those counterions to different electrolytes. At the same time, the incorpora-tion of the clay was found to influence strongly the defor-mation of the hybrid films under an electrical field. As a consequence, the electrostimulus deformation reached equilibrium after 1.5 h of operation. Concerning the deswelling ratio at a time of 1.5 h, as shown in Fig. 6b, the hybrids with lower Cclayare subjected to smaller
restric-tions of molecule relaxation than is pure CS. In other words, with increasing Cclay, the mobility of the hybrid
composites was gradually restricted by the formation of an increasing amount of cross-linking bonds. Therefore, the deswelling ratio of the resulting hybrid decreases with increasing Cclay. As shown inFig. 6, 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.
3.4. Cyclic deformation and recovery of hybrid films Above a threshold value of a given electrical field, the hybrid film showed 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.Fig. 7a shows the swelling ratio of the CS film after cyclic on–off
0 10 20 30 40 50 60 70 80 90 50 60 70 80 90 100 pure CS V=1V pure CS V=5V 1 wt % Clay V=5V 1.5 wt % Clay V=5V pure CS V=10V
Deswelling water ratio W
t /W o (%) Time (min.) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 50 60 70 80 90 100
Deswelling water ratio W
t /W o (%) Clay content (%) 1V 5V 10V
Fig. 6. (a) Deswelling behavior and (b) deswelling water ratio of hybrid film with different Cclayin a 1.5 h interval under applied voltages of 1, 5
operation of the electrical field in a 20 min interval relative to the original equilibrium swelling ratio of the hybrid film with different clay concentrations under an applied voltage of 10 V. Initially, the equilibrium swelling of pure CS film was increased slightly for several cycles of the on–off oper-ation. This may be due to the collapse of the CS structure under applied voltage and the production of more and more porous structure on the CS film, as was observed visually. The enhanced porosity would improve the degree of swelling to about 1.15 times after six cycles. The degree of swelling would subsequently start to reduce, indicating a fatigue behavior of the CS film. This observation suggests that the CS film should lose its structural integrity after six cyclic operations, thereby causing the CS film to deteri-orate 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, allow-ing the hybrid to undergo as many as 15 operation cycles, with a higher relative ratio of swelling achievable at about 1.3. The increase in the degree of swelling may be associ-ated with the decrease in the degree of cross-linking caused by the 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 stimulations exceeded six. In fact, the swelling ratio would not decrease
enormously with increased cyclic stimulation as in the case of pure CS and the hybrid with 0.5 wt.% clay. It is believed that the greater cross-linking density of the hybrid compos-ites is capable of bearing a larger applied voltage and more on–off switching operations. Similarly, the hybrid compos-ites with higher Cclay(>0.5 wt.%) could maintain the same
degree of deswelling after more than 10 operation cycles, compared with the pure CS film and hybrid composites with lower clay content, as shown in Fig. 7b. Hence, the incorporation of clay particles can structurally adjust the cross-linking density of the hybrid and improve the anti-fatigue properties of the hybrids under cyclic electrostimulation.
Fig. 8shows the weight changes of the pure CS film and the hybrid film (1 wt.% clay) under an applied voltage of 10 V in PBS and consecutive on–off operations in a 20 min interval. For pure CS, the degree of reversibility (i.e., the ability for the hybrid to structurally return to the initial swelling state) is apparently decreased when the on–off operation exceeds seven cycles. Furthermore, the deswelling ratio of the pure CS film could not be restored to the 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 com-position of 1 wt.% clay showed the best anti-fatigue prop-erty where the swelling and deswelling behavior remained identical even after 20 on–off cycles. Since the contractile deformation under stimulation and volume recovery of a given polyelectrolyte hydrogel are associated with the opti-mal viscoelasticity at a certain cross-linking density, it is suggested that the incorporation of negatively charged clay as a cross-linker provides a more effective anti-fatigue property for pure positively charged chitosan under cyclic electrostimulation. 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 10 15 20 0.5 1.0 1.5 2.0 pure CS 0.5 wt % Clay 1 wt % Clay 1.5 wt % Clay
Relative ratio of deswelling
Cycle number 0 10 15 20 0.9 1.0 1.1 1.2 1.3 pure CS 0.5 wt % Clay 1 wt % Clay 1.5 wt % Clay
Relative ratio of swelling
Cycle number 5
5
Fig. 7. Relative ratio of (a) swelling and (b) deswelling after cyclic on–off switching of 5 V electrostimuli in a 20 min interval of hybrid films with different Cclay. 0 150 300 450 600 750 0.30 0.35 0.40 0.45 0.50 pure CS 1% clay Weight (g) Time (min.)
Fig. 8. Weight changes of pure chitosan and hybrid film with 1 wt.% clay addition under cyclic on–off switching of 5 V electrostimuli.
4. Conclusion
An electrostimulus-responsive hybrid composed of chito-san (CS) and clay was successfully prepared and systemati-cally characterized. It was found that the incorporated clay is able to form cross-links with the CS matrix because of the electrostatic interaction between the positively charged NHþ3 group of CS and the negatively charged clay sheets.
With considerably increased clay content, this formation of cross-linking reinforces the hybrids and overcomes the adverse effect caused by the small addition of clay. At the same time, the increased cross-linking density improved the mechanical properties of the hybrids, but restricted the swelling–deswelling kinetics. After the repeated switching on and off of a given electric field, a relatively constant swell-ing–deswelling behavior was achieved for more than 10 cycles for the hybrids with higher Cclay(>0.5 wt.%),
com-pared with that of the pure CS. The incorporation of the clay not only improved the mechanical strength and anti-fatigue properties under cyclic electrical stimulation of the hybrids, but also imparted the hybrid with a sufficient flexible charac-ter to deform in a technically desirable manner under cyclic contraction–expansion. Further investigation of the con-trolled release profile for a number of clinically valuable drugs using this new class of hybrid composite is underway and will be reported separately.
Acknowledgement
This work was financially supported by the National Science Council of the Republic of China, Taiwan under Contract No. NSC-95-2216-E-009-026.
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