Abstract
Temperature sensitive hydrogels are probably the most commonly studied class of environmentally sensitive polymer systems in drug delivery research. In this study, polyvinyl alcohol (PVA) matrix with nanosized pores was obtained by treatment with silica and glutaraldehyde. Then, the internal pores of the dry PVA matrix were filled with poly(N-isopropylacrylamide) (PNIPAAm) hydrogels to form a thermoresponsive hybrid PVA/PNIPAAm hydrogel for controlled drug release. The morphology and functional groups of the hybrid hydrogel were characterised by SEM and Fourier transform infrared spectroscopy. The swelling kinetics and the temperature dependence of equilibrium swelling ratio in distilled water were also investigated. Rhodamine B was loaded to the hybrid hydrogels for release study, and the release rate was fast when the temperature was above the lower critical solution temperature of PNIPAAm. The release data suggest that an excellent controlled release can be achieved by the hybrid hydrogel without losing the intelligent properties of the PNIPAAm hydrogel.
Introduction
Controlled drug delivery systems, which are intended to deliver drugs at predetermined rates for predefined periods of time, have been used to overcome the shortcomings of conventional drug formulations. This is because the controlled drug delivery system can provide a sustained therapeutic level of drug concentration without toxicity. It will be more beneficial and ideal if the drug can be delivered by a device that responds to physiopathological signals from an underling disease. The correct amounts of drugs will be released under the stimulation of these physiopathological signals.1–10
Hydrogels have been used extensively in the development of smart drug delivery systems. Hydrogels are cross-linked polymer networks swollen in a liquid medium. The liquid inside the hydrogel allows the diffusion of some solute molecules, while the polymer network serves as a matrix to hold the liquid together. Poly(N-isopropylacrylamide) (PNIPAAm) is a temperature sensitive hydrogel exhibiting volume phase transition at ∼33°C. Below this temperature, the hydrogel swells, and it shrinks as the temperature is raised. On the other hand, uncross-linked PNIPAAm polymer chains in the water exhibit low critical solution temperature (LCST) behaviour. The temperature sensitivity of the PNIPAAm hydrogel has attracted considerable attention in recent years due to both fundamental and technological interests. The PNIPAAm hydrogel and its derivatives have potential applications for controlled drug delivery, chemical separation, sensors and actuators.11–14
However, during some applications, there are obvious limitations of the normal PNIPAAm hydrogel, such as poor mechanical property and fast release of the drug. These limitations can be attributed to its lower polymer mass per unit volume, which leads to poor mechanical property and many open spaces or channels for the impregnated drug to diffuse out quickly. The relatively weak intermolecular interactions among this swollen PNIPAAm drug delivery system also leads to a fast drug release rate from the drug reserve system. 15 , 16
This paper suggests some modifications, e.g. the co-polymerisation of PNIPAAm with hydrophobic co-monomers to overcome the deficiencies of PNIPAAm based drug carrier. In this study, we prepared a thermoresponsive hybrid polyvinyl alcohol (PVA)/PNIPAAm hydrogel using PVA and silica as matrix and porogenic agent respectively. The morphology and functional groups of the hybrid hydrogel were characterised by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The swelling kinetics and the temperature dependence of equilibrium swelling ratio (SR) in distilled water were also investigated. Finally, Rhodamine B (RB) was chosen as the model drug for release study.
Experimental
Materials and methods
N-Isopropylacrylamide (NIPAAm) from Aldrich Chemical (St Louis, MO, USA) was further purified by dissolution and recrystallisation in benzene/cyclohexane. N, N-Methylenebisacrylamide, ammonium persulphate, sodium bisulphite and glutaraldehyde (25%) were analytical reagents and used as supplied. Silica (diameter, 30–40 nm) was purchased from Zhoushan Company (Zhejiang, China), and PVA (polymerisation degree, 1750; alcoholysis degree, 97%) was from CNPC Lanzhou Chemical Company. The model drug RB was a gift from China National Medicines Corporation Ltd.
Synthesis of hybrid PVA/PNIPAAm hydrogel
The PVA (0·2 g) was dissolved in 20 mL deionised water at 80°C, and the solution was poured into tetrafluoroethylene dish and cooled to 30°C for gelation overnight. The gelatinised PVA was immersed in glutaraldehyde solution (2%) for 24 h in the presence of a few dilute sulphuric acid. Finally, the PVA hydrogel was washed consecutively with deionised water for 1 week, dried in an oven at room temperature and labelled as H0.
The PVA (0·2 g) was dissolved in 20 mL of deionised water at 80°C, and silica was added under ultrasonic dispersion for 30 min. The composite solution was treated as described above to gelate. The hydrogel was washed with hydrofluoric acid to remove silica and other impurities. The hydrogel was then immersed in sodium bisulphite solution (0·15%) for 24 h and dried in an oven at room temperature. Finally, the porous PVA hydrogel W1 (g) was immersed in NIPAAm (0·500 g) solution containing N, N-methylenebisacrylamide (0·010 g) and ammonium persulphate (0·005 g) for 24 h under nitrogen atmosphere. After the reaction completed, the hybrid hydrogel was dried in an oven at room temperature and weighted W2 (g). The relative content C of PNIPAAm was calculated from the following equation: (W2−W1)/W1. The results are summarised in Table 1.
Microsphere PNIPAAm hydrogel dispersed in PVA matrix
The symbol ‘C’ was the relative content of PNIPAAm in the hybrid PVA/PNIPAAm hydrogel.
Morphology
Scanning electron microscopy observations of the hybrid hydrogel were performed on a scanning electron microscope (Hitachi S-450). The hydrogels were gold coated before analysis, and the energy of the electron beam was 20 kV.
Fourier transform infrared spectroscopy
The FTIR spectrum was performed using KBr pellet in an Alpha Centauri FTIR spectrophotometer.
Measurement of swelling kinetics
Swelling kinetics was measured by the method below. Dried hydrogel was immersed in distilled water at 20°C. The sample was removed at specified intervals and frequently weighted after trapped with a filter paper to remove excess water on the surface. Thus, the weight of the swollen sample was recorded at regular time intervals. The SR was calculated using the following equation: (W4−W3)/W3, where W3 and W4 are the weight of the samples before immersing and after swelling with water in different times respectively.
Measurement of temperature sensitive behaviour
The temperature sensitive behaviour was determined by equilibrium SR in the temperature range from 20 to 45°C. Dried hydrogel was immersed in distilled water at a desired temperature for 2 days to reach equilibrium state. The sample was removed from distilled water and frequently weighted after trapping with a filter paper to remove excess water on its surface. Equilibrium SR was calculated by the following equation: (W5−W3)/W3, where W5 is the weight of the swollen sample at a predetermined temperature, which has reached its equilibrium state, and W3 is the same as defined earlier.
In vitro drug release study
Rhodamine B, which has an absorption maximum at 548 nm, was used as the model drug. The swollen PVA/PNIPAAm hydrogel was dried in vacuum overnight until its weight remained unchanged. The vacuum dried hydrogel was immersed in 2·0% aqueous RB solution at 20°C for at least 24 h to reach the equilibrated state. During this period, the drug diffused into the hydrogel network with the water.
The RB release experiment was conducted by immersing the above drug loaded hydrogel into 2000 mL distilled water equipped with an external stirrer (100 rev min−1) at 20°C (below LCST) and 38°C (above LCST) respectively. During the drug release experiment, 4 mL aliquots of the release media was taken out with reconstitution of 4 mL fresh distilled water at every predetermined time interval, and the concentration of the RB released was monitored at 548 nm using a UV spectrophotometer.
Results and discussion
Morphology
The pure PVA hydrogel has a smooth, homogeneous and compact surface (H0, Fig. 1a). As far as hydrogel H1 and H2 are concerned, we can observe that there is PNIPAAm microgel in the PVA matrix, and the size is much larger than silica. This is attributed to the aggregation of silica in the PVA polymer solution. Therefore, as the silica particle is eroded, there is a large space left for the polymerisation of the NIPAAm monomer. When the amount of silica is higher, the PNIPAAm microgel distributes densely (H2, Fig. 1c). As the amount of silica is lower, the PNIPAAm microgel distributes homogeneously (H1, Fig. 1b).
Images (SEM) of hydrogels a H0, b H1 and c H2
Fourier transform infrared spectra
Figure 2 shows the FTIR spectra of PVA hydrogel, PNIPAAm hydrogel and PVA/PNIPAAm hybrid hydrogel. The characteristic absorption peak of the PVA and PNIPAAm hydrogels also appeared in the hybrid hydrogel. The peak of 3438 cm−1 is the stretch vibration of N–H of PNIPAAm, and 1635 and 1542 cm−1 are the absorption peaks of the amide bands. The absorption peak at 1083 cm−1 can be attributed to hemiacetal, which is produced by the cross-linking reaction between the hydroxyl group of PVA and glutaraldehyde. As a result, it can be concluded that monomer NIPAAm polymerises successfully in the PVA matrix.
Fourier transform infrared spectra
Swelling kinetics
Figure 3 shows the swelling kinetics of the hydrogels (H0, H1 and H2) in distilled water at 20°C. As shown in Fig. 3, the equilibrium SR values of H0, H1 and H2 show little difference. This may due to the PVA matrix of the three hydrogels. Moreover, hydrogels H1 and H2 have a higher SR, which could reach equilibrium within 100 min. This phenomenon is obviously attributed to the distribution of the PNIPAAm hydrogel in the PVA matrix. As far as hydrogel H0 is concerned, it has a dense surface without pore structure because of the absence of the PNIPAAm hydrogel. As a result, H0 has a lower SR, and this is in agreement with the SEM analysis.
Swelling kinetics of hydrogels in distilled water at 20°C: (Δ) hydrogel H0, (▪) hydrogel H1 and (○) hydrogel H2
Temperature dependence of equilibrium SR
As shown in Fig. 4, the pure PVA hydrogel (H0) has no temperature sensitivity. Regardless of the different concentration of PNIPAAm hydrogel, H1 and H2 have the same volume phase transition phenomenon (31–35°C). As the temperature reaches below the LCST, H1 and H2 are at swelling state and, above the LCST, are at the shrinking state respectively. It can also be concluded that the PVA matrix has no effect on the temperature sensitivity and phase transition temperature of the PNIPAAm hydrogel.
Effect of temperature sensitivity on equilibrium SR of hydrogels: (Δ) hydrogel H0, (▪) hydrogel H1 and (○) hydrogel H2
In vitro drug release
Figure 5 shows the cumulative drug release from each hydrogel in distilled water at two temperatures (20 and 38°C). As shown in Fig. 5a, no significant difference in release behaviour is observed for the hydrogel H0 at the two temperatures. The release behaviours of RB from hydrogels H1 and H2 in response to predetermined temperatures (20 and 38°C) are shown in Fig. 5b and c. Significant changes in the release behaviour are observed, whereas RB in hydrogels H1 and H2 are released slowly at 20°C and then released faster at 38°C. Data from Fig. 5b and c suggest that the PNIPAAm hydrogel swells below the LCST, and thus, RB molecules are absorbed, and the diffusion resistance is large. Above the LCST, the PNIPAAm hydrogel shrinks so that RB is released with the water molecules. The release curves in this figure directly support that the PVA/PNIPAAm hybrid hydrogel is effective in giving a positive drug controlled release, i.e. the drug release rate is accelerated at an increased temperature.
Release behaviour of RB from a hydrogel H0 at 20°C (□) and 38°C (▪), b hydrogel H1 at 20°C (Δ) and 38°C (▴) and c hydrogel H2 at 20°C (○) and 38°C (•)
Conclusions
A thermoresponsive PVA/PNIPAAm hybrid hydrogel was prepared by the gelation of a NIPAAm monomer in the presence of cross-linkers into the pores of the PVA matrix. The properties, such as morphology, swelling kinetics, temperature dependence of equilibrium SR and functional groups, were characterised. This hybrid hydrogel shows a thermoresponsive release of RB. The results show that the cumulative release of RB belongs to on–off switches, which will achieve ‘on’ at high temperature (above LCST) and ‘off’ at low temperature (below LCST).17 The hybrid hydrogel reported here has to be improved or optimised, and further studies are underway. This novel hybrid hydrogel would have potential and promising applications in cases where positive drug controlled release is need.