treatment of disease by multiple dosing strategies and using conventional drug formulation poses many drawbacks . among them are fluctuation of drug plasma concentration , the various side effects due to toxic plasma level of drug , or ineffectiveness treatment because of the lower concentration of drug than its therapeutic index [ 2 , 3 ] . another important limitation of multiple dosing regimens is poor patient compliance especially for diseases for which patient must take drugs for a long time such as epilepsy [ 4 , 5 ] or for some specific drugs such as naltrexone hydrochloride which is used for maintenance abstinence in detoxified addicted persons after withdrawal phase of opioids and alcohol [ 6 , 7 ] . nowadays scientists are interested in novel drug delivery systems which are used to direct drugs to the specific site of action and to achieve a controlled release of drug with a desirable release kinetics [ 1 , 8 ] . among the most studied of these systems are hydrogels which are insoluble matrices of hydrophilic block copolymers which swell in presence of fluids [ 9 , 10 ] . smart hydrogels are the polymeric networks which release drugs in response to the environmental parameters such as temperature [ 1113 ] , ph [ 14 , 15 ] , light [ 16 , 17 ] , magnetisms [ 18 , 19 ] . thermoresponsive hydrogels are extensively investigated smart hydrogels because of their simple application , low adverse effect on tissues compared with some other stimuli , and the difference between room and body temperature [ 12 , 20 ] . amphiphilic triblock copolymers , pcl - peg - pcl , composed of polyethylene glycol ( peg ) and poly ( -caprolactone ) ( pcl ) , form a thermoresponsive , in situ forming , and biodegradable hydrogel [ 21 , 22 ] which have been studied as drug delivery systems [ 2325 ] . mw irradiation for the high - speed , reproducible , and scalable preparation of polymeric materials has been utilized to reduce reaction times from days and hours to minutes and seconds and facilitate scale - up procedure and technology transfer from bench to industry [ 26 , 27 ] . in this study , pcl - peg - pcl with various weight ratios of each block of copolymer ( pcl : peg : pcl ) , 1000  :  1000  :  1000 and 500  :  1000  :  500 , was synthesized by three different methods : using stainless steel reactor , two - necked round - bottomed flask , and under microwave irradiation . structure , molecular weight ( mw ) , and sol - gel transition of copolymers were investigated . also in vitro drug release behavior was evaluated in this study using naltrexone hydrochloride and vitamin b12 . poly ( ethylene glycol ) ( peg , mn = 1000 , merck , germany ) , -caprolactone ( sigma aldrich , usa ) , stannous octoate ( sn(oct)2 , sigma aldrich , usa ) , naltrexone hydrochloride was kindly donated by razak co. , iran , and vitamin b12 were kindly donated by iran hormone co. , iran . synthesis of pcl - peg - pcl copolymers was carried out with two different block lengths of ( pcl : peg : pcl ) : 1000  :  1000  :  1000 ( copolymer 1 ) and 500  :  1000  :  500 ( copolymer 2 ) by a ring - opening polymerization technique . the synthesis in stainless steel reactor and two - necked round - bottomed flask was performed as described before by gong et al . the copolymer was also synthesized using microwave irradiation . briefly to synthesize 1000  :  1000  :  1000 , 10  g dried peg1000 , 20  g -caprolactone , and 2  g sn(oct)2 were added to a glassy flask fitted with condenser and was placed in a microwave . the mixture was irradiated and stirred at 800  watt while temperature was kept at 130c . the irradiation time was different in each synthesis ( 15 , 20 , 25  min ) to choose an optimum time for rapid preparation of the copolymer .   in order to purify the product , at first , obtained copolymers were dissolved in 20  ml dichloromethane . by gradually adding 800  ml of petroleum ether , the viscous copolymer precipitants appeared . at the final step , the purification process was done 3 times and the final product was dried by freeze drying method . the yield of reaction by each method was calculated by dividing the weight of the purified pcl - peg - pcl to the weight of initial materials used in the polymer preparation . the copolymer structure and number - average molecular weights ( mn ) were determined using nuclear magnetic resonance analysis ( h - nmr , ac 80 brucker , germany ) at 80  mhz and 25c in cdcl3 . gel permeation chromatography ( gpc , agilent gpc - addon apparatus and rid - a refractive index signal detector coupled to plgel columns ) was utilized to determine the weight - average molecular weight ( mw ) , mn , and polydispersity of the block copolymers using tetrahydrofuran as the solvent and eluent with a rate of 1  ml / min and polystyrene as the calibration standard .   to determine the sol to gel and gel to precipitant transition temperatures , 3  ml of the copolymers solution in distilled water ( 15 , 20 , 25 , and 30% ( w / v ) ) was poured in to 5  ml vials kept in a refrigerated bath circulator ( p-22 wisd , south korea ) . the temperature increased with a rate of 1c/3  min from 10  0.1 to 60  0.1c . after each 3  min , the vials were inverted to evaluate the viscosity of the content . the temperature which the content did not flow by inverting vials was assumed as the sol - gel transition temperature . this study was done before and after drug loading to determine the effect of drug on the phase transition temperatures . the reversibility of the gel and the precipitant forming was also investigated by decreasing the temperature . to determine the time of the formation of the gels , all samples were transferred to a 37c water bath ( n - biotek-304 , south korea ) and the time of sol - gel transition was recorded . all experiments were performed 3 times . according to the result of the polymer characterizations and the phase transition evaluations , two appropriate concentrations of copolymer 1 ( 1000 - 1000 - 1000 ) , which was synthesized by a microwave irradiation in 15  min , were chosen as a drug delivery system for the in vitro drug release experiment . for the purpose of preparing drug loaded hydrogels , adequate amount of the copolymer 2  ml of each formulation was placed in tubes and was transferred to the reciprocal water bath at 37. after the formation of the hydrogels , 4  ml of the phosphate buffer solution ( pbs , ph = 7.4 ) was added to each sample tube as a release media and samples were shaken at a rate of 60 times / min . in each sampling time , 0.5  ml of the medium was withdrawn and replaced by 0.5  ml fresh buffer . drug analysis in samples was performed using high performance liquid chromatography ( hplc , acme 9000 young lin , south korea , c18 ( 4.6    250  mm , 52  m ) column ) . mobile phase for naltrexone hydrochloride was composed of 14% acetonitrile , 85.5% deionized water , and 0.5% acetic acid ( v / v ) and for b12 was a mixture of 30% methanol and 70% deionized water . the flow rate in both analyses was 1  ml / min and the uv detection wave length was set at 281 and 305  nm for naltrexone hydrochloride and b12 , respectively . in order to compare different copolymer synthesis methods , the copolymerization was done by three different procedures that two of them , using two - necked flask and the stainless steel reactor , were conventional methods and take longer time compared to the other method which was a high - speed microwave - assisted way of the copolymer synthesis . since the microwave heating process provided a reaction condition with a higher temperature and pressure , the reaction took relatively short times and also the side reaction was limited and consequently greater yields were usually obtained . according to the yield of each method ( table 2 ) , the microwave - assisted synthesis was the best method in which the reaction just took a few minutes . by increasing the time of the reaction from 15  min to 25  min , no significant difference was found in the yield of the reaction or the quality of the products . thus we considered 15  min as an adequate time for the microwave - assisted synthesis of the pcl - peg - pcl copolymers . yields of the copolymers synthesized by the stainless steel reactor method were a little higher than the two - necked flask method . the h - nmr spectrum of the pcl - peg - pcl ( 1000 - 1000 - 1000 ) synthesized by the stainless steel reactor is shown in figure 1 . the signals appeared in 3.6 ( a ) , 4.1 ( h ) , 2.2 ( d ) , and 1.5 ( e , f , g ) ppm are related to the protons of peg ( a ) and the ch2 groups of caprolactone ( h , d , e , f , g ) . the mn of the copolymer was calculated by the integration of the signals pertaining to each monomer . mn of the copolymer ( 1000 - 1000 - 1000 ) synthesized by the reactor and the microwave ( 15  min ) were 2590 and 3412 , respectively . as the mn of the copolymers indicated , the one which was prepared by the reactor , in contrast to the one which prepared by the microwave - assisted method , has mn lower than the theoretical amount ( 3000 ) . mw and mn obtained from h - nmr and gpc , the polydispersity of the copolymer which was estimated by mw / mn obtained from gpc , and the caprolactone ( cl ) to peg ratio obtained from h - nmr are presented in table 3 . the data in table 3 belongs to copolymer 1 synthesized by the microwave - assisted method . the pcl - peg - pcl tri - block copolymers are in situ gel - forming thermo - responsive copolymers composed of the hydrophobic pcl and the hydrophilic peg blocks . when hydrophobic blocks of pcl are surrounded by water molecules , they aggregate to reduce their exposed surface area in an aqueous environment and form micelles . but this phenomenon is temperature dependent . in temperatures below ( lower critical solution temperature ) lcst , since there is hydrogen bonding between hydrophilic peg blocks and water molecules , the system is in sol condition and contains micelles . by increasing the temperature above lcst , weakening of the hydrogen bonds and the formation of the hydrophobic bridge between micelles through pcl segments the sol - gel transition temperatures of the copolymers with various concentrations are presented in table 4 . copolymer 1 ( 1000 - 1000 - 1000 ) could form gel in all concentrations , but copolymer 2 ( 500 - 1000 - 500 ) prepared by both conventional synthesis method could not form gel at the concentration of 15% and not even at the concentration of 20% for the two - necked flask method ( w / v ) due to its shorter hydrophobic segments ( cl ) compared to copolymer 1 and an incomplete copolymer synthesis by these methods . although the copolymer 1 ( 1000 - 1000 - 1000 ) formed gel in all concentrations , the sol - gel transition occurred in lower temperatures when it was synthesized using the microwave irradiation ( p < 0.05 ) . in all methods , as the copolymer concentrations increased , the phase transition temperatures shifted to the lower temperatures due to an increase in hydrophobic interactions . there was no significant difference between the gel forming temperatures of the copolymers synthesized by the microwave method in different reaction times ( p > 0.05 ) . the phase transition temperatures in all concentrations of copolymer 1 synthesized by the microwave irradiation were below the room temperature . this causes copolymer 1 synthesized by the microwave to become a good candidate for an injectable carrier which converts to the gel in the body and releases the drug loaded inside in a sustained and controlled manner . the phase transition temperatures of all copolymers synthesized by the conventional methods were above the body temperature and therefore could not gel in the body and were not suitable as a drug carrier . another limitation of the conventional synthesized copolymers was the delay in gel formation that may cause a dose dumping effect in the body . for the microwave - assisted synthesized copolymers , the sol - gel transition occurred in about 20 seconds , whereas this amount was 20  min for some of the copolymers synthesized by the conventional methods .   the effect of the copolymer concentration on the phase transition temperature was shown in figure 3 . as it is indicated , by increasing copolymer 1 concentration ( microwave synthesized ) , the sol - gel transition temperatures decreased and the precipitation temperatures increased due to a larger number of hydrophobic interactions . we also investigated the effect of the drug content on the sol - gel transition temperature . as it was indicated in table 5 , loading vitamin b12 , a large molecule having 9 positions on it that can form hydrogen bonds with water molecules , lowered the activity of water and favored hydrophobic interactions between the pcl blocks of the copolymers . however , loading a fairly small molecule such as naltrexone hydrochloride in copolymers does not have a significant effect on the phase transition temperatures . in vitro drug release through the pcl - peg - pcl ( 1000 - 1000 - 1000 ) synthesized by the microwave - assisted method the rate of drug release through the hydrogels with the same concentration of the copolymer but a different concentration of the drug was not significant ; just a negligible increase in the rate of drug release after an increase in naltrexone hydrochloride concentration was observed . this could be due to the fact that naltrexone hydrochloride , which is a small molecule in contrast to b12 , was released by diffusion rather than the hydrogel degradation . according to the fick 's law of diffusion , increasing drug concentration causes the rate of diffusion and the consequent drug release increases . as it is shown in figures 4 and 5 , the release of naltrexone hydrochloride and vitamin b12 through the hydrogels with higher copolymer concentration was slower . this could be attributed to the fact that higher copolymer concentration made the hydrogels low porous with a big tortuosity and viscosity . here , the effect of the molecular weight on the drug release through the hydrogels was also investigated by comparing the naltrexone hydrochloride and vitamin b12 loaded systems . as indicated in figure 6 , vitamin b12 with a higher molecular weight ( mw = 1355 ) compared to naltrexone hydrochloride the release kinetics of both drugs was determined by fitting data according to zero order ( release by hydrogel erosion ) and higuchi kinetic models [ 31 , 32 ] . since r of both models were approximately similar in all formulations , it was assumed that both mechanisms of diffusion and the hydrogel erosion played an important role in the drug release through the hydrogels ( table 6 ) . synthesis of pcl - peg - pcl using microwave irradiation is the fast , simple , and cost effective method which causes higher yield of reaction . pcl 1000-peg 1000-pcl 1000 tri - block copolymer solution which forms gel in body temperature even with low concentration of polymer appears to be a suitable in situ forming system as the pharmaceutical and clinical purpose . different parameters such as polymer concentration and drug structure have influence on profile of drug release from systems .
traditional drug delivery systems which are based on multiple dosing regimens usually pose many disadvantages such as poor compliance of patients and drug plasma level variation . to overcome the obstacles of traditional drug formulations , novel 
 drug delivery system pcl - peg - pcl hydrogels have been purposed in this study . 
 copolymers were synthesized by rapid microwave - assisted and conventional synthesis methods . 
 polymer characterizations were done using gel permeation chromatography and 1h - nmr . 
 phase transition behavior was evaluated by inverting tube method and in vitro drug release profile was determined using naltrexone hydrochloride and vitamin b12 as drug models . 
 the results indicated that loaded drug structure and copolymer concentration play critical roles in release profile of drugs from these hydrogels . 
 this study also confirmed that synthesis of copolymer using microwave is the most effective method for synthesis of this kind of copolymer .