Composites: Part B 41 (2010) 537–542

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Composites: Part B

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Crosslinked poly(e-caprolactone)/poly(sebacic anhydride) compositescombining biodegradation, controlled drug release and shape memory effect

Yu Xiao, Shaobing Zhou *, Lin Wang, Xiaotong Zheng, Tao GongSchool of Materials Science and Engineering, Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu 610031, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 March 2010Received in revised form 7 June 2010Accepted 10 July 2010Available online 16 July 2010

Keywords:A. Polymer–matrix composites (PMCs)A. Smart materialsB. Stress relaxationD. Mechanical testing

1359-8368/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.compositesb.2010.07.001

* Corresponding author. Tel.: +86 28 87634023; faxE-mail addresses: shaobingzhou@swjtu.cn, zhoush

In this study, we investigated the shape memory effect and drug release behavior of a biodegradablepolymeric composite consisted of crosslinked poly(e-caprolactone) (cPCL) and poly(sebacic anhydride)(PSA). This composite was prepared by a solution-casting method. The drug delivery system was appliedto cooperate with the shape memory property in the biodegradable polymeric composites for the firsttime. The effect of PSA addition on the mechanical, shape memory, in vitro degradation and drug releasebehavior was studied by static tensile test, dynamic mechanical analysis (DMA), FT-IR and degradationevaluation, etc. In vitro degradation and drug release results showed that the degradation speed of cPCLand the release accumulation of drug could be enhanced by adding PSA into cPCL matrix. The multifunc-tional polymer composite has great potential as drug eluting stents in biomedical field.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Shape memory polymers (SMP) are drawing more and moreattention due to their fantastic properties and potential applica-tions in recent years, especially in the biomedical field [1,2]. Whatmakes SMP superior to shape memory alloys and ceramics is largerecoverable strain, low energy consumption, excellent manufactu-rability and bio-degradability [3–5]. However, with the increasingand much more complex requirements, the single-functional SMPhave not fulfilled their technological use [1,2,6]. Therefore, weneed some multifunctional SMP [7].

In fact, the multifunctional SMP that combine two functionssuch as shape memory effect and bio-degradability or shape mem-ory effect and drug release have been already realized [8–14].However, the SMP that combine three above functions have notyet been demonstrated. In this study, we firstly added thepoly(sebacic anhydride) (PSA) into the poly(e-caprolactone) (cPCL)matrix to prepared a new kind of multifunctional SMP. In view ofthe wonderful shape memory effect, excellent biocompatibility,non-toxicity, bio-degradability and drug permeability, we chosecPCL as the drug carrier [15,16]. Then, considering the long degra-dation time of cPCL (more than 14 months) we added PSA to adjustits degradation rate. PSA can be used as controlled release devicesfor short-lived drugs by the surface erosion phenomenon and con-sequently provides a sustained release effect for the drug over anextended period of time. Furthermore, the degradation rate of

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: +86 28 87634649.aobing@gmail.com (S. Zhou).

PSA can be well adjusted by changing its molecular weight[17,18]. Because of the above excellent properties, we believePSA could become a good reinforcement to lower the cPCL’s degra-dation time and the device based on biodegradable polymer candegrade after a defined time period, thus eliminating the needfor a second surgery for removal.

On the other hand, SMP were rapidly developed in biomedicalfields for their potential applications in recent years, includingthe seam of the minimally invasive surgery, the stent of boneand tissue repair [1,14,15]. Unfortunately, some side effects couldbe observed after these surgeries or repairs [16,19]. The commonsolution to this problem was to take medicine by oral or injection,which turns out not effective enough [20]. In order to fulfill thecomplex demands as medical devices during biomaterials-assistedtherapies, the biomaterials with several functions such as shapememory effect and controlled drug release have been realized[7]. Considering this situation, we have pulled toward a betterway to design a drug-loaded shape memory polymer. We expectedthat it has excellent shape memory property, what’s more impor-tant is that through sustained release, the loaded drug can resist lo-cal inflammation. In summary, the shape memory effect enablesthe minimally invasive implantation of bulky devices. The functionof controlled drug release can be used to treat infections and re-duce inflammatory response.

In this study, a series of cPCL/PSA composites were prepared inorder to colligate their own advantages and overcome their draw-backs. Then, the drug paracetamol was incorporated in the com-posites to investigate its drug delivery properties. We preparedthree kinds of samples: cPCL, paracetamol-loaded cPCL and

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paracetamol-loaded cPCL/PSA composites, and further investigatedtheir properties, including shape memory effect, mechanical per-formance and in vitro bio-degradability.

Fig. 1. The FT-IR spectrum of (a) paracetamol drug, (b) cPCL, and (c) cPCL/paracetamol.

2. Materials and methods

2.1. Materials

Linear PCL was synthesized in our lab as the previous report[21]. The molecular weight (Mw) determined by gel permeationchromatograph (GPC) is 112,000. Benzoyl peroxide (BPO) was pur-chased from Chengdu Kelong Chemical Reagent Company (Sichu-an, Chengdu, China). PSA (Mw: 20,000) was polymerized by amelt polycondensation process without adding any catalyst inour lab. Paracetamol was obtained from Kangquan Pharmaceuti-cals Inc., China. All the other chemicals and solvents were of re-agent grade or better.

2.2. Preparing of cPCL, paracetamol-loaded cPCL and cPCL/PSAcomposites

Pre-weighted linear PCL with 1.5 wt.% BPO (BPO as the cross-linking agent) [22] was dissolved in CH2Cl2 under stirring, and or-ganic solvent was volatilized by stirring and then dried undervacuum. Later, these completely dried composites were press-molded at 135 �C for 10 min in a mold (because BPO enable cross-linking reaction to occur at 130 �C [22]. As a result, the requiredslices made up of cPCL with thickness of about 0.2 mm wereobtained.

Paracetamol (5.0 wt.%) was dissolved in 5 mL acetone, and thesolution was transferred to 20 mL CH2Cl2 under stirring. Then,2 h later after cPCL was immersed in mixed solution, paraceta-mol-loaded cPCL gel (cPCL/drug) was obtained due to paracetamolsolution penetrating into its cross-linking structure. Finally, the re-sulted cPCL gel was dried by volatilizing.

We can derive the paracetamol-loaded cPCL/PSA composite(cPCL/PSA/drug) in a similar way as above. The only differencewas in the first stage linear PCL as well as 1.5 wt.% BPO and5.0 wt.% PSA were dissolved in CH2Cl2 under stirring with the fol-lowing steps the same as the fabrication of cPCL. Paracetamol-loaded cPCL/PSA composite with thickness of about 0.2 mm canbe fabricated, which was prepared as mentioned above.

2.3. Characterization

Nicolet 5700 Fourier Transform Infrared Spectroscopy (FT-IR,Thermo Electron, USA) was performed to identify the changes ofsome functional groups. All specimens were made into particlesand mixed with KBr grains at a weight ratio of 0.5–1%. Pure KBrwas used as IR spectral reference and each sample was recordedfrom 4000 to 400 cm�1 by 64 scans.

Static tensile test was accomplished at the crosshead speed of5 mm/min at room temperature using a universal testing machineInstron 5567, Instron Co., Massachusetts. Prior to the test the spec-imens should be of dumbbell shape cut from pressed composites.Of all the mechanical properties, Young’s modulus E and tensilestrength rb were tested.

Dynamic mechanical analysis (DMA) was carried out on aDMA983 analyzer (Du Pont, USA), using a tensile resonant modeat a heating rate of 5 �C/min from 30 to 90 �C and at a frequencyof 1 Hz. The storage modulus E0 for specimen size50 � 10 � 2 mm (length �width � thickness) was tested.

Gel fraction estimate can be done by the following method: allthe pre-weighted specimens, m0, are subjected to swell in CHCl3 inan attempt to gather gel, which needs 24 h to insure steady gel

fraction values, and then a high speed centrifuge was employedto detach gel from sol. During the process, an observable phenom-enon may be noted that some agglomerate of gel floats on the sur-face of transparent gel solution in the centrifuge tube. Afterward adried gel mass, m1 is noted, the gel fraction can be calculated asfollows:

Gel fraction ð%Þ ¼ m1m0 � 100%:

In vitro degradation of all samples was carried out as follows.Pre-weighed samples were placed individually in test tubes con-taining 10 mL of 0.1 M phosphate buffered saline (PBS) at pH 7.4.The tubes were kept in a thermo-stated incubator (Haerbin Dong-ming Medical Equipment Company) which was maintained at37 �C and 107 cycles per minute. The degradation process wasevaluated from the weight loss, the pH change, the gel fraction,shape memory properties and mechanical properties at predeter-mined intervals.

In vitro drug release was carried out as follows. Predeterminedsamples were suspended in a test tube containing PBS with pH 7.4.The test tubes were placed in a same incubator and continuouslyagitated with the same condition as mentioned above. At predeter-mined intervals, 1.0 mL of supernatant was collected and 1.0 mL offresh PBS was added to the test tube. Amount of released paracet-amol was determined with an UV–visible spectrumphotometer atabsorbance of 247 nm.

3. Results and discussions

3.1. Characteristic analysis

Fig. 1 shows FT-IR images of cPCL, paracetamol and paraceta-mol-loaded cPCL. In view of strong oxidizing property of BPO andweak reducibility of paracetamol, the drug may be oxidized if thesetwo substances meet together [23,24]. Therefore, during our exper-iment the drug was added into polymer matrix after the crosslink-ing process was finished in order to avoid the pollution. FT-IR testwas used to inspect whether the added paracetamol was oxidizedby BPO. From Fig. 1a, we can see that the IR absorption peaks of par-acetamol mainly consist of the –NH– groups characteristic spectralline at 3325 cm�1, the C@O flex vibrate characteristic spectral lineat 1653 cm�1, and phenyl-hydroxyl characteristic spectral line at1245 cm�1. In Fig. 1b, we can observe that the IR absorption peaksof cPCL mainly consist of carbonyl characteristic spectral line at1171 cm�1, ester functional groups at 1722 cm�1 and methyleneat 2935 cm�1. As shown in Fig. 1c, we could find the FT-IR bands

Table 1Mechanical properties of cPCL, cPCL/paracetamol and cPCL/PSA/paracetamol samples.

Samples E (MPa) ds (MPa) db (MPa) Le (%)

cPCL 411 ± 20.6 18.84 ± 2.61 20.12 ± 2.38 800 ± 54.3cPCL/drug 398 ± 38.7 15.05 ± 1.32 17.05 ± 1.59 657 ± 36.8cPCL/PSA/drug 280 ± 35.9 11.04 ± 1.36 13.20 ± 2.21 402 ± 40.1

E: modulus of elasticity; ds: yield strength; db: tensile strength; Le: elongation atbreak.

Table 2Shape memory properties of cPCL, cPCL/paracetamol and cPCL/PSA/paracetamolsamples.

Samples Recovery ratio (%) Gel fraction (%)

cPCL 95.6 ± 1.05 42.8 ± 0.95cPCL/drug 94.5 ± 0.92 45.2 ± 0.99cPCL/PSA/drug 95.2 ± 1.07 39.6 ± 0.89

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of paracetamol took a slight 1red-shift and comparatively the bandsof cPCL took blue-shift. This result indicated that there is a smallinteraction between cPCL matrix and paracetamol. According to someprevious literatures, we speculate this interaction is the hydrogenbonding which attributed by the –OH from the paracetamol andthe C@O from the cPCL [25]. However, from the curves of Fig. 1a–c,we could observe that this red or blue-shift is too small and cannotmake great influence to the structure of paracetamol and cPCL. Onthe other hand, from the Fig. 1c, we could not find distinct C@C bandsof quinone which was generated by the oxidized hydroxybenzene at1690 cm�1. So, we could draw a conclusion that the paracetamol isnot polluted during the crosslinking process.

3.2. Mechanical and shape memory properties tests

Table 1 shows the mechanical properties of cPCL, cPCL/drug andcPCL/PSA/drug samples. From this table, we can definitely find thatthe pure cPCL possess the best mechanical properties. With drug orPSA added into cPCL matrix, the mechanical properties such aselasticity modulus, yields strength, broken strength and elongationat break decreased gradually. The reason mainly could be summa-rized as follows: There will be a few lacunas such as cracks, hol-lows after the addition of PSA or drug. Those lacunas couldlargely decrease the interfacial tension [26]. On the other hand,the addition PSA may bring a phase separation between PSA mol-ecules and cPCL’s crosslinked structure. Noteworthily, according tothe report by Wu et al. the basic requirement of mechanical prop-erties for biomedical application is relatively low [27]. Therefore,although the accession PSA or drugs to polymer matrix led poorermechanical properties, it could not distinctly influence the applica-tion of the materials in biomedical field.

Table 2 summarizes the recovery ratio and gel fraction of cPCL,cPCL/drug and cPCL/PSA/drug. From this table, the shape recoveryratio of the three samples is nearly similar. The results illuminatethat the added PSA and drug have almost no impact to the shapememory properties. Simultaneously, the measure of gel fractionis carried out to reflect the crosslinking degree of these compositesand the relationship between the crosslinking degree and theshape memory properties. As reported by a few researchers, thegel content has a close contact to the crosslinking degree andmoreover, the crosslinking degree is the main factor to influencethe shape memory properties of polymer [28,29]. Therefore, toinvestigate the relationships between shape memory propertyand the gel content is necessary. Our previous report indicated thatthe shape memory property of cPCL was mainly dependent on itscrosslinking degree, or in other words, dependent on the BPO con-tent [28]. It can be explained that crosslinking process will producethe crosslinked points acted as the fixed phase for shape memoryin the PCL matrix. Therefore, the shape memory properties werenaturally enhanced with the increasing of the fixed phase [29]. Fur-thermore, cPCL with higher crosslinked degree held more cross-linking structures, i.e. chemical crosslinked network, which can

1 For interpretation of color in Figs. 1–6, the reader is referred to the web version ofthis article.

store more elastic deformation energy. Thus, the nearly same gelfraction also determines the close recovery ratio of the three sam-ples. In addition, the reversible strain of cPCL decreased withincreasing gel content. So we can conclude that the more gel con-tent in polymer matrix, the better shape memory properties couldbe obtained.

Fig. 2 shows the change of storage modulus (E0) among purecPCL, cPCL/drug, cPCL/PSA/drug from DMA behavior. All the threespecimens have a phase-transition temperature range of about40 �C where E0 suddenly decreases with the increasing tempera-ture. This is necessary for shape memory polymers. The peak ofmodulus curve is often employed to define the glass transitiontemperature (Tg) [26], but in our test, the decrease of the E0 is cor-responding to the melting temperature (Tm) area of PCL. Therefore,our DMA results illustrate that the storage modulus of the speci-mens is almost constant at a temperature area of the ordinarystate. For example, Tm for the cPCL is about 55.5 �C [16], the storagemodulus is almost constant below 35 �C (Tm �20 �C) at 330 MPaand we also can observe a lower modulus plain emerges whereabout 0.7 MPa at 80 �C (Tm +20 �C). As reported by Zhou et al. thestorage modulus of their composites at 22.8 �C (E0 = 3220 MPa) isabout two orders of magnitude larger than that at 82.8 �C(E = 29.6 MPa) [26]. Thus, we could find great shape memory prop-erties from their composites (the recovery ratio is nearly 95%). Afall up to three orders of magnitude can be obtained in our DMAimages. Hence, these composites can provide novel shape memoryproperties.

To approve the great shape memory properties of our samples,we also took a series of photos with digital camera to show thespecimens’ shape recovery progress. As shown in Fig. 3, the initialshape of our materials was made to a strip (the angle was 180�),and then these two strips were completely fold up (the anglewas 0�). After that, the deformed samples were heated and started

Fig. 2. Storage modulus vs. temperature of cPCL, cPCL/paracetamol and cPCL/PSA/paracetamol by DMA.

Fig. 3. The photo showing the shape memory recovery process of (a) cPCL/paracetamol and (b) cPCL/PSA/paracetamol.

540 Y. Xiao et al. / Composites: Part B 41 (2010) 537–542

to recover immediately. The samples were recovered to their halfshape (the angle was 90�) at 15 s and recovered basically to the ori-ginal shape only at 30 s. The results indicated that our compositeshad excellent shape memory properties.

3.3. In vitro degradation and drug release test

From the Fig. 4 we can observe that the addition of PSA indeedspeeded up the biodegradation of cPCL. Fig. 4a shows the weightloss of the three samples vs. degradation time, and we can clearlysee that the weight of pristine cPCL decreases quite smoothly, andchanges a little on the whole, and the other two samples possess aquick-drop process in the first 2 weeks. This is because the lacunasengendered from the paracetamol introduced into cPCL can im-prove water penetrating into polymer matrix, which results in a

Fig. 4. Biodegradation properties vs. biodegradation time of cPCL, cPCL/paracetamol and

faster degradation of cPCL. Furthermore, compared to the cPCL/drug sample, the weight of cPCL/PSA/drug decreased much faster.It means that the addition of PSA will further speed up the biodeg-radation of cPCL. The reason may be analyzed from two aspectsthat PSA can accelerate the degradation rate of cPCL matrix. Oneis that the phase separation between PSA and cPCL could increasewater penetrating into cPCL matrix, the other is as a result of theacidity of the degradation products of PSA. The nature of cPCL deg-radation is the hydrolysis of ester bonds in cPCL chains. It is wellknown that the hydrolysis can be triggered by water and catalyzedby the acidic medium. Fig. 4b displays the media pH decrease vs.degradation time. The result is almost consistent with Fig. 4a.

To evaluate the effect of polymer degradation on its shape mem-ory property, we investigated the change of recovery ratio and gelcontent vs. degradation time. As shown in Fig. 4c and d, the trend

cPCL/PSA/paracetamol: (a) loss weight, (b) pH, (c) recovery ratio and (d) gel content.

Fig. 6. In vitro paracetamol release from cPCL/paracetamol and cPCL/PSA/paracet-amol composite matrix.

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profiles almost correspond to the Fig. 4a and b. We can see that withthe decrease of gel content the samples’ shape recovery ratio got anobvious fall, too. This phenomenon is similar to some previous re-ports and the reason can be described as follows: the inner cross-linked structure of the cPCL is impaired companying with thedegradation process, and the number of the crosslinked point,namely the shape memory fixed phase decreased [29]. It also canbe proved by the gel content as shown in Fig. 4d. Along with the bio-degradation experiment, the gel content of these three samples de-creases, which inevitably leads to recede the shape memoryproperties. Perhaps a better question now is why there is no recoveryratio data of cPCL/PSA/drug after 6 weeks. This is because 6 weeks’degradation makes these kinds of samples more and more brittle,the material would be broken immediately if we changed the sam-ple’s shape, so we cannot get the proper data in the following period.

The mechanical properties must be changed due to the highmolecular weight polymer degradation into low molecular weightpolymer. So we estimated the degradation behaviors by testing themechanical strength of samples. Fig. 5 shows the mechanical prop-erties of the three samples, including elasticity modulus and ten-sile strength. The results and the reasons are similar as we got inFig. 4. Here, we need to emphasize the phenomenon of the biodeg-radation test. For the pristine cPCL, in shape these samples have al-most no changes after 14 weeks biodegradation except a littlewhite floccus can be found in the biodegradation medium. Forthe cPCL/drug samples, some apertures appear on their surfacewith the degradation time increasing, and the changes of the sam-ples’ shape became more obvious, and the white floccus in themedium was much more than that of pristine cPCL. For the last

Fig. 5. Mechanical properties vs. biodegradation time of cPCL, cPCL/paracetamoland cPCL/PSA/paracetamol: (a) elasticity modulus; (b) tensile strength.

samples, the shape kept well, but the material got quite brittle.The reason is that the added PSA improves the biodegradation rateof the cPCL.

Fig. 6 shows the drug release profiles of cPCL/PSA/drug andcPCL/drug samples. From this image, we can see that initial bursteffect happened during the first 4 days, and within the periodnearly 45% paracetamol was released, however, the value hasplummeted to only 15% during the following 16 days. It is wellknown that drug release from biodegradable polymer is mainlydue to polymer degradation. So here the release result is in accordwith our conclusions from Figs. 4 and 5. In the other words, theaddition of PSA accelerating the polymer matrix degradation re-sulted in a faster drug release.

4. Conclusions

In this paper, we successfully prepared three kinds of samplescombining biodegradation, controlled drug release and shapememory effect by a simple method. All the drug-loaded sampleshave satisfactory shape memory properties, mechanical propertiesand drug release behavior. The degradation rate of the PCL/PSA/drug is significantly faster than the other two samples, and simul-taneously leads a faster drug release, which means that the intro-ducing of PSA into cPCL matrix can adjust its degradation. Althoughthe addition of PSA and drug will depress cPCL’s mechanical prop-erty, it has little influence on its shape memory effect, and thus itcould not bring an adverse impact on its biomedical application. Ina word, the multifunctional polymer composite has great potentialin minimally invasive surgery such as drug eluting stents in bio-medical field.

Acknowledgements

This work was partially supported by National Natural ScienceFoundation of China (50773065, 30970723), Programs for NewCentury Excellent Talents in university, Ministry of Education ofChina (NCET-07-0719) and Sichuan Prominent Young Talent Pro-gram (08ZQ026-040).

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