in recent years , a variety of materials are chosen as drug carriers for their excellent properties . hydrotalcites ( 1 ) have attracted attention as controlled release system which was intercalated kinds of drug anions in the interlayer gallery as well as montmorillonite ( 2 ) . in addition , polymer - based sustained release system has been studied as means of delivery drugs . such as polymeric matrix ( 3 , 4 ) , which is one of the useful devices for controlled delivery and stabilization of a drug , and hydrogel ( 5 ) , which has played an important role in regional delivery in order to provide high local concentration and to decrease the incidence of side effects commonly observed with systemic therapy ( 6 ) . carbon nanotubes ( cnts ) have attracted considerable attention on application of drug carrier system since their discovery by iijima in 1991 for the excellent physicochemical properties ( 7 ) as well as the unique hollow structure and nano diameter ( 8) . however , the chemical inertness limits its vast application ( 9 ) . over the last decade , lots of effects have been made to optimize conditions for functionalization of cnts using polymers ( 10 , 11 ) . chitosan is well - known as nontoxic and biologically degradable ( 12 , 13 ) and increase its use in a suitable functional material for biomedical utilization for its bioactivity and biocompatibility . in this work , multi - walled carbon nanotubes / chitosan hydrogel served as carrier and naproxen was used as model drugs . the in - vitro releases of the drug were study in different ph conditions . release mechanisms of the model drugs from hydrogel were investigated via four kinetics equations . the comparison studies on drug delivery behavior and swelling ability of cnts - gel with hydrogel synthesized without cnts ( gel ) were conducted . shenzhen , china . according to the specification of the manufacturer , the samples of nanotubes are 20 - 40 nm in diameter and about 5 - 15 m in length , with approximately less than 3% amorphous carbon . synthesis of hydrogels 
  hydrogels containing cnts ( cnts - gel ) and the pure hydrogel without cnts ( gel ) were both prepared in this experiment in order to compare the effectiveness of cnts on the sustained release of this kind of hydrogel . the commercial mwcnts were treated with concentrated nitric acid at 100c for 8 h , washed with deionized water several times and dried at 80c . acid - treated mwcnts were placed into reactor containing n , n-2-dimethyl formamide ( dmf ) and acetic acid solution of chitosan in scheduled proportion . after dispersing in sonicator with ultrasonic power : 200 w and frequency : 40 khz for 30 min under room temperature , the reaction was performed at 95c for 24 h. the products , named cs - mwcnts , obtained by twice adjusting ph value of the mixture and centrifuging with 3000 rpm in succession to remove un - reacted materials , were vacuum filtered , and the filter cakes were washed with ethanol for three times , and then dried under vacuum at 80c for 24 h. scanning electron microscope ( sem ) and fourier transform infrared ( ftir ) spectroscopy were both used to observe the products . cnts - gel was prepared by cs - cnts gelling with chitosan ( mass ratio of cs / cs - mwcnts=1:3 ) , taking glutaraldehyde ( 2.5% about 2 ml ) as gelling agent . in addition , a cleaning process was needed to remove all the impurities and unreacted chemicals in the method of immersing hydrogels in deionized water and changing water at regular time . gel was prepared in a similar way , but only by chitosan with help of glutaraldehyde serving as gelling agent and with a cleaning process as well . both kinds of hydrogels should be dried in an oven under vacuum at about 40c for 7 h. 
 synthesis of naproxen - loaded cnts - gel and gel 
  naproxen was entrapped into the cnts - gel and gel by soak method . a certain amount of cnts - gel ( m01 ) and gel ( m02 ) was soaked in naproxen solution for 4 h respectively . drug - loaded cnts - gel and gel were vacuum dried to constant weight ( m1 ) . loading efficiency of naproxen was determined as follows :  
 e%=m1-m0m0100  ( 1 ) swelling studies of cnts - gel and gel were carried out in two aqueous media : 0.1 mol / l hcl solution ( ph 1.2 ) as simulated gastric fluid and phosphate buffer solutions ( ph 7.4 ) as simulated intestinal fluid . the sample was placed at 37c in the vessel containing the two kinds of media , respectively . at regular intervals , the swelling ratio of the hydrogel was determined as follows : 
 s = ws - wowo100  ( 2 )   where w0 represents weights of initial hydrogel , ws is weight of wet hydrogel and s is swelling ration of hydrogel . in - vitro release experiments 
  the in - vitro drug release tests were performed in a vessel containing 100 ml medium of simulated gastric fluid ( ph 1.2 ) or simulated intestinal fluid ( ph 7.4 ) , respectively . the temperature was maintained at 37  0.5 c , and the stirring speed was 100 rpm . after each sample collection , the medium was replenished with equal amount of fresh buffer to maintain a constant volume . samples were analyzed by uv - vis spectrophotometer at maximum absorption wavelength based on the established uv standard absorbance curves for drugs . sem of cnts before and after modification 
  scanning electron microscopy ( sem ) studies were performed for characterizing the structure of the acid - treated mwcnts and cs - mwcnts . it is found that the mwcnts treated by concentrated nitric acid ( figure 1a ) presents slender with bulge on partial surface , it may be attributed to that nitric acid treatment produces carboxylic groups and the functionalized amorphous carbon on the mwcnts according to theory from the previous works ( 14 , 15 ) . after surface modification of mwcnts with biopolymer cs by direct heating approach ( figure 1b ) , the surface topography of mwcnts changed significantly , which proves that surface of mwcnts is covered with chitosan , simultaneous with wrapping of the polymers to the surface of mwcnts . sem images of acid - treated cnts ( a ) and cs - cnts ( b ) . ftir analysis of cnts before and after modification 
  fourier transform infrared ( ftir ) spectra of acid - treated mwcnts ( a ) and cs - mwcnts ( b ) are shown in figure 2 . in figure 2(a ) , the wide and intense band at 3220 cm is attributed to the vibration of -oh in carboxyl group , the characteristic peaks at 1513 cm and 1040 cm are due to the vibration of c = o and c - o , respectively . in figure 2(b ) , the intense bands at 3480 cm and 1040 cm are attributed to the stretching vibration of n - h and c = o in -conh , respectively , which confirm that the polymer was grafted onto mw cnts via the amide linkage . infrared spectra of acid - treated cnts ( a ) and cs - cnts ( b ) . swelling studies on hydrogel 
  in order to simulate the possible effect of ph on drug release rate , a swelling studies of cnts- gel and gel were conducted in simulated gastric fluid ( ph 1.2 ) and simulated intestinal fluid ( ph 7.4 ) at physiological temperature of 37c0.5c . hydrogel is a cross - linked hydrophilic polymer , so it can interact with the aqueous medium and swell in water to an equilibrium volume , but preserve its shape . as shown in figure 3 , swelling ratios of the two kinds of hydrogel in artificial intestinal juice is much higher than in artificial gastric juice . side - chain amino of chitosan is ph - sensitive . in alkaline medium , amino group exists in the free state , and the hydrogen bonding interaction is enhanced , so the swelling ratio is relative small . it is found that the swelling rate for the cnts - gel is slower than that for hydrophilic gel . the significant difference of final ratio is probably due to the change of the osmotic pressure in two artificial juices . swelling ratio profiles of the cnts - gel and gel in simulative gastrointestinal fluids . in - vitro drug release studies 
 
 table1 shows that naproxen - loaded efficiency of cnts - gel is quite different from naproxen- loaded gel . naproxen - loaded efficiency of cnts - gel is much higher than that of gel . it can be inferred from that gel of cross - linked chitosan can not interact effectively with hydrophobic naproxen . figure 4 showed the release behaviors of drugs in buffer solutions . in this study , naproxen was taken as model drug to examine the release behavior from the cnts - gel and gel in simulative gastrointestinal fluid ( ph 7.4 and ph 1.2 ) , respectively . release of naproxen from the hydrogel was shown in figure 4 , which indicated a burst effect in both cnts - gel and gel mediums in first 30 min , which should be attributed to the concentration gradient of the drugs between the gel and the media . the release behavior of naproxen is identical with the swelling of both kinds of gels in two different fluids . accumulative release profiles of naproxen in simulative gastrointestinal fluid drug - loading ratios of cnts - gel or gel e% , naproxen loading efficiency ; rsd , e relative standard deviation it is noted that the dissolution rate of naproxen is somehow ph dependent . ph value of solution also influence the characteristics of molecular . in the acidic solution , repulsive forces between naproxen molecular and cnts , collaborating with swelling effect force naproxen release to solution . because of naproxen hydrophobicity , naproxen release performance will decrease in relative high concentrations of naproxen solution . with the purpose of getting insight into the release mechanism of carbon nanotubes hydrogel , the release data of two types of medicine carrying hydrogel in simulative gastrointestinal fluid were fitted to classic drug - release kinetics models . bhaskar model ( eq.(3 ) ) , higuchi model ( eq.(4 ) ) , first order model ( eq . ( 5 ) ) and ritger - peppas empirical model ( eq.(6 ) ) ( 16 , 17 ) . ln(1r)= ct  ( 3 ) 
 r = c  t 
  ( 4 ) 
 ln(1r ) = ct   ( 5 ) 
 lg r = k lg(ct )   ( 6 ) where r represents release percentage , t is the time , c is release rate constant and k , release exponent . release data of naproxen is better fitted ritger - peppas empirical model in ph=1.2 , indicating that it was fick - diffusion ( k  0.43 ) while in ph=7.4 it was non - fick diffusion involving surface diffusion and corrosion diffusion processes ( 0.45 k 0.89 ) . kinetics data of carbon nanotubes hydrogel in different conditions c , release rate constant ; r , correlation coefficients ; k , release exponent a new kind of drug carrier , carbon nanotubes hydrogel ( cnts - gel ) was synthesized by chitosan modified multi - walled carbon nanotubes ( cs - mwcnts ) . the comparison of in - vitro drug release behaviors between cnts - gel and the hydrogel without mwcnts was conducted , so as to corroborate that mwcnts contributed to improve the effect of sustained release of hydrogel . the release mechanism in ph=1.2 was found as fick - diffusion but a non - fick diffusion process involving surface diffusion and corrosion diffusion in ph = 7.4 .
in this paper , carbon nanotubes ( cnts ) were added into chitosan ( cs ) hydrogels in the form of chitosan modified cnts ( cs - cnts ) composites to prepare carbon nanotubes hydrogels ( cnts - gel ) . 
 the products , named cs - mwcnts , were characterized by scanning electron microscope ( sem ) and fourier transform infrared ( ftir ) spectroscopy . 
 swelling properties and effect of ph on controlled release performance of the two kinds of hydrogels , cnts- gel and pure chitosan hydrogels without cnts ( gel ) , were investigated respectively . 
 the results showed that cnts - gel possess better controlled release performance than gel . 
 the releasing equilibrium time of cnts - gel was longer than that of gel in both ph = w7.4 and ph=1.2 conditions , although the release ratios of the model drug are similar in the same ph buffer solutions . 
 it is found that release kinetics is better fitted ritger - peppas empirical model indicating a fick - diffusion process in ph = 1.2 , while in ph = 7.4 it was non - fick diffusion involving surface diffusion and corrosion diffusion processes .