Controlled drug release of levo loxacin from poly (acrylamide) hydrogel

Hydrogels are 3D polymer networks capable to absorb and release water or biological luids. They are stimuli-responsivematerials, which can show rapid volume changeswith response to small changes in environmental parameters such as ionic strength, pH, and temperature. In this work, we performed a synthesis of Poly(acrylamide) hydrogel and tested for controlled release of levo loxacin hemihydrate as a model drug. We used sodium metabisul ite and potassium persulphate as free radical initiators to prepare hydrogel with methylenebisacrylamide as a crosslinker. Characterization of hydrogel was performed by TGA, SEM, and FT-IR. Swelling study and drug release were performed at pH 1.2 and 7.4 solutions, identical to the gastrointestinal luid at 37◦C (human body temperature) to examine possible site-speci ic drug delivery. UV-Visible spectrophotometer was used to measure the concentration of drug release. Results exhibited the pH and temperature-dependent drug release. The amount of drug release was found to be 17% and 99% in acidic and alkaline pH of 1.2 and 7.4, respectively, after 6 hours.

Hydrogel, free radical polymerization, pH, levo loxacin, drug delivery A Hydrogels are 3D polymer networks capable to absorb and release water or biological luids. They are stimuli-responsive materials, which can show rapid volume changes with response to small changes in environmental parameters such as ionic strength, pH, and temperature. In this work, we performed a synthesis of Poly(acrylamide) hydrogel and tested for controlled release of levo loxacin hemihydrate as a model drug. We used sodium metabisul ite and potassium persulphate as free radical initiators to prepare hydrogel with methylenebisacrylamide as a crosslinker. Characterization of hydrogel was performed by TGA, SEM, and FT-IR. Swelling study and drug release were performed at pH 1.2 and 7.4 solutions, identical to the gastrointestinal luid at 37 • C (human body temperature) to examine possible site-speci ic drug delivery. UV-Visible spectrophotometer was used to measure the concentration of drug release. Results exhibited the pH and temperature-dependent drug release. The amount of drug release was found to be 17% and 99% in acidic and alkaline pH of 1.2 and 7.4, respectively, after 6 hours.

INTRODUCTION
The polymers are continuous repeating monomer units. The general biomedical uses of polymers are in drug delivery systems, pharmaceutical adhesives, coating material, and emulsifying agents for dosage forms in site-speci ic and controlled drug delivery systems. Polymer molecules are linear or branched or may be crosslinked. The chemical response of polymers depends on the monomer unit present in the polymer chain. The homopolymers are having identical monomeric units and copolymers are formed from more than one monomer (Kamboj and Verma, 2015;Florence and Attwood, 1998;Moghimi and Hunter, 2000).
The irst polymer gel was prepared in 1949 by Katchalsky. This gel responds to the surrounding environment solution by swelling or gathering from a network of water-soluble polyelectrolytes (Katchalsky and Gillis, 1949). In 1950 medical applications and its importance of hydrogels were revealed and using 2-hydroxyethyl methacrylate gel, soft contact lenses are manufactured. The smart hydrogel, like temperature-sensitive hydrogels, is focused till the mid-1980 (Kim et al., 2013). In the drug delivery systems, pH-sensitive hydrogels are the best materials for drug release to the target site of the body (Hamidi et al., 2008;Huynh et al., 2011).
Poly(acrylamide) hydrogel polymer backbone containing functional group like amine is sensitive to charge by either release or accept protons in the aqueous media. The electrostatic repulsion in the polymer backbone network promotes swelling and then water diffusion. These hydrogels are more sensitive to slight changes in environmental factors. Those are called smart hydrogels (Wan et al., 2016;Wu et al., 2016;Katchalsky and Gillis, 1949).  Figure 1 show its molecular structure. Levo loxacin is a luoroquinolone antibacterial drug with an active L-isomer of o loxacin (Hurst et al., 2002;Chen et al., 2003). Levo loxacin is used to cure the disease of gram-negative and gram-positive bacteria like keratitis, bacterial conjunctivitis, and other eye infections by inhibiting topoisomerase IV and DNA gyrase enzymes. These enzymes are important to DNA replication, recombination, transcription, and repair (Noel, 2009;Hooper, 1999).
In the present work, we evaluate the drug release from poly(acrylamide) hydrogel using different temperature and pH solutions. Crosslinker methylenebisacrylamide used to control the network characteristic and model drug levo loxacin hemihydrate was used for drug release studies.

Hydrogel synthesis
Poly(acrylamide) hydrogel was synthesized by a free radical mechanism. Primarily, redox initiators of potassium persulphate (45 mg) and sodium metabisul ite (32 mg) were shifted into a vial containing 10 ml of deionized water. Then, add acrylamide (600 mg), allowed to stir for 10 minutes at room temperature after this crosslinker methylenebisacrylamide (06 mg) were added. Then, this composite was kept in a water bath until the gel was formed. The synthesized gel was washed with water to remove unreacted components. Then, the hydrogel was dried at 50 o C in the oven for 24 hours.

FT-IR analysis
Levo loxacin and levo loxacin loaded poly(acrylamide) hydrogel spectra were recorded using an FT-IR spectrometer (Shimadzu ATR) in the range of 400 to 4000 cm −1 to determining their intermolecular interactions and structure.

TGA analysis
To determine the thermal stability of poly(acrylamide) hydrogel was performed using a thermogravimetric analyzer (Perkin Elmer STA 600) by increasing the heating rate to 20 o C.

Morphological examination
The morphology of poly(acrylamide) hydrogel structures was determined using SEM (scanning electron microscope). Hydrogel composites were cut to expose their structure and imaged in an (SEM Zeiss, LS15) scanning electron microscope.

Swelling study
The swelling study of synthesized hydrogels was determined using dry samples in acidic buffer pH 1.2 and phosphate buffer pH 7.4 solutions. The preweighed hydrogel samples were immersed in solutions at 37 o C for swelling. At periodic intervals, the swollen samples were taken out from the solution and excess droplets on the surface of the hydrogel were withdrawn by wiping with ilter paper then weighed. The swelling ratio of hydrogel was determined from Equation (1). Similarly, the swelling ratio was observed at 27 o C in pH 7.4 solution with time intervals.
Where, W a and W b is the dry and swollen gel weight.

Preparation of calibration curves
To construct a calibration curve, a stock solution 1000 mg/l of Levo loxacin drug solution was prepared using water as a solvent, then 2, 4, 6, 8, and 10 mg/l solutions were prepared by dilution of the stock solution. Using a UV-9000A spectrophotometer (Shanghai Metash), scan the solutions between 200 to 400 nm and absorption maximum was recorded to construct a calibration curve.

Drug loading and drug release studies
Levo loxacin hemihydrate was selected as a model drug. The loading of the drug was conducted by 1 mg/ml concentration solution using water as a solvent. Place 0.1 g dry hydrogel to 100 ml levo loxacin solution. The loaded hydrogel was dried and note down the loaded hydrogel weight. The in vitro release study was conducted by placing drugloaded hydrogel in 100 ml of acidic buffer pH 1.2 and phosphate buffer pH 7.4 solution at 37 • C and withdrawn 1 ml dissolution medium sample at regular time intervals (30 minutes) with stirring and replace fresh solution to maintain constant dissolution media. Using a UV-Visible spectrophotometer, scan the solutions between 200 to 400 nm with suitable dilution, note down the λ max absorbance. The percentage of released levo loxacin was calculated and its corresponding drug release graph was plotted. Similarly, temperature effects on drug release will study by setting temperatures at 27 and 37 • C in pH 7.4 solution.

RESULTS AND DISCUSSION
Poly(acrylamide) hydrogel was synthesized by a radical polymerization method and its swelling study was performed. Moreover, drug loading and drug release also performed using levo loxacin as a model drug and the effect of pH, temperature, and time of the drug release will also be studied.

TGA analysis
The thermogram of hydrogel was shown in Figure 4. The irst stage of weight loss, consider as loss of moisture present in the hydrogel, was observed at 169ºC with mass loss of 3.4%, then degradation occurred at 179ºC with weight loss of 9.6%, and maximum weight loss occurred at 381ºC with mass loss of 24.4% due to cleavage of the polymer chain in hydrogel (Ebrahimi and Salavaty, 2018).

Morphological examination
The surface morphology of synthesized hydrogel was studied by SEM. The micrograph Figure 5(a) and Figure 5(b) reveal that the surface is uniform and smooth in nature (Chen et al., 2009;Aouada et al., 2009).

Swelling study
The swelling study of the synthesized hydrogel was performed in buffer solutions. The swelling study of poly(acrylamide) hydrogels in pH 1.2 and 7.4 solutions similar to gastrointestinal luids at 37 • C are shown in Figure 6. From these results, the higher swelling rate observed at pH 7.4 when compared to pH 1.2 solution. In an acidic medium of pH 1.2, the ammonium groups (NH 3+ ) are formed by protonation but due to the presence of chloride (Cl − ), counterions drastically decreased its swelling (Wu et al., 2001;Pourjavadi and Mahdavinia, 2006). However, at pH 7.4, the (-CONH 2 ) and (-CONH-) groups are deprotonated and the presence of sodium (Na + ) ions in the solution will produce high osmotic swelling pressure hence shows maximum swelling. Similarly, the swelling was observed at temperature 27 and 37 • C (Figure 7) for temperature sensitivity.
The results show when the temperature increases swelling ratio also increases.

Drug selection
Drug selection for the loading and release is most important because it should not react with hydrogel and solvents. This helps to avoid the λ max shift. Levo loxacin drug has good material because no change was observed in the λ max over time. Using a UV-Visible spectrophotometer scan, the solutions   Hence, we conclude that drug release depends on the pH of the solution because the swelling ratio is more in pH 7.4 than in the pH 1.2 solution. The drug release from hydrogel into solution depends on swelling and the controlled release of levo loxacin was observed up to 6 hours (Figure 11).

Drug release with temperature
The temperature effect on drug release was evaluated and studied at temperatures 27 and 37 • C with time ( Figure 12). When the temperature increases, drug release also increased, and at 37 • C we observed the maximum drug release. When the temperature increases, the hydrogel network lexibility also increases. Hence, the more amount of buffer solution enters into hydrogel promotes more amount of drug is released.

Kinetic model drug release
The kinetic model of drug release will study by using various mathematical models. The obtained results are given in Table 1. It helps to promote an ideal kinetic model to illustrate in vitro drug release data in terms of relevant parameters. From the obtained results, the best correlation coef icient (R 2 ) is 0.996 in Zero-order kinetics; hence synthe-sized poly(acrylamide) hydrogel follows the zeroorder kinetic model.

CONCLUSION
Poly(acrylamide) hydrogel cross-linked with methylenebisacrylamide was synthesized and studied their swelling and drug release properties. The swelling study of synthesized hydrogel was examined at pH 1.2 and pH 7.4 solutions at 37 o C and levo loxacin drug release studies were carried out in the same conditions. The drug released amount from the hydrogel was more at alkaline pH 7.4 than in the acidic pH 1.2 solution. Because at pH 7.4 solution, the amide groups are de-protonated and the presence of sodium (Na + ) ions in the solution will produce high osmotic swelling pressure hence it will swell more and the amount of drug release is more. The temperature and pH effects on drug release were also studied and the hydrogel follows a zero-order kinetic model; hence these hydrogels can be used in controlled drug release and biomedical applications due to good swelling properties.