Enzymatic Degradation of Supramolecular Materials Based on Partial Inclusion ComplexFormation between r-Cyclodextrin and Poly(ε-caprolactone)

Haiya Luo,†,‡ Xianwei Meng,† Cong Cheng,† Zhenqiang Dong,† Sheng Zhang,*,† andBangjing Li*,‡

State Key Laboratory of Polymer Materials Engineering (Sichuan UniVersity), Polymer Research Institute ofSichuan UniVersity, Chengdu 610065, People’s Republic of China, and Chengdu Institute of Biology, ChineseAcademy of Sciences, Chengdu 610041, People’s Republic of China

ReceiVed: January 8, 2010; ReVised Manuscript ReceiVed: February 28, 2010

Supramolecular biomaterials were prepared based on the formation of partial inclusion complexes betweenR-CD and poly(ε-caprolactone) (PCL). Enzyme-catalyzed degradation of these partial R-CD-PCL inclusioncomplexes was investigated at 37 °C in the presence of lipase from pancreas porcine. The biodegradationbehaviors were monitored by weight loss measurement, 1H NMR, XRD, GPC, DSC, and SEM. It was foundthat the formation of inclusion segments significantly accelerated the degradation of materials. A mechanismof enzymatic degradation for partial R-CD-PCL inclusion complexes was proposed. The present study providesa preliminary understanding for the biodegradation of the materials constructed by self-assembly betweencyclodextrin and polymers.

Introduction

In recent years, using host-guest inclusion in supramolecularchemistry as a new technique for construction of biomaterialshas been an attractive subject for scientists. Cyclodextrins (CDs)are one of the most popular host molecules in supramolecularchemistry and are capable of including a range of guestmolecules in their cavities with high selectivity.1,2 Since Haradaand co-workers discovered that R-CD can form an inclusioncomplex with poly(ethylene glycol) (PEG) in aqueous solution,3

increasing attention has been focused on the study of theformation of supramolecules based on self-assembly betweenCDs and polymers and their biomedical applications.4-6 Bio-degradable inclusion complexes consisting of R-CDs and PEGfor tissue engineering have been reported by Yui et al.7-10 Liet al. prepared a series of biodegradable supramolecular hydro-gels self-assembled between CDs and polymers for drug deliveryor gene delivery.11-14 Although more and more studies focuson the preparation of biodegradable polymer inclusion com-plexes and their biomedical application, the degradation proper-ties, especially the influence of inclusion structure on thedegradation of materials, have not commonly reported.

We have developed a multifunctional biodegradable materialbased on partial inclusion complex formation between R-CDand poly(ε-caprolactone) (PCL). Because it contained bothR-CD-PCL inclusion crystallites and uncovered PCL crystal-lites, this partial R-CD-PCL inclusion complex displayed goodshape memory effect.15,16 This stimuli-sensitive material haspromising potential for applications such as implants forminimally invasive surgery. For medical application, recognitionof the degradation behavior (especially the enzymatic degrada-tion) of this supramolecular material is required.

The degradation behavior of PCL homopolymers has beenstudied in detail before.17-19 In this paper, we investigate the

enzymatic degradation of a partial R-CD-PCL inclusioncomplex with the aim of identifying the effect of the inclusionintroduction on the biodegradation characteristics of PCL.

Experimental Section

Materials. PCL with an average molecular weight of 80 000(from Aldrich Chemical Co.) and R-cyclodextrin (from SigmaChemical Co.) were dried under high vacuum at 60 °C for 12 hbefore use. N,N-Dimethylformamide was a local commercialproduct and used without further purification. Phosphate buffersolutions (0.1 M, pH ) 7.4) were used for the biodegradationstudy. Lipase (type Π, from pancreas, 100-400 units/mg, solid)was purchased from Sigma Chemical Co. and chosen as themodel enzyme.

Preparation of Partial r-CD-PCL Inclusion Complexes.Films were prepared via solution casting in a rectangular glassmold. The data of the prepared samples are listed in Table 1.In the sample designation code, the number preceding “C”

* To whom correspondence should be addressed. Tel: +86-28-85400266.Fax: +86-28-85405132. E-mail: zslbj@163.com (S.Z.); libj@cib.ac.cn(B.J.L.).

† Polymer Research Institute of Sichuan University.‡ Chinese Academy of Sciences.

TABLE 1: Compositions and Characteristics of Partialr-CD-PCL Inclusion Complexes

mass content ofinclusion segment (%)

polymer

castingtemperature

(°C) feed producta

inclusioncrystal

size (nm)b �c,PCL(%)c

PCL 90 0 0 50.530C-70 70 30 52.4 25.8 70.030C-90 90 30 36.0 19.0 56.130C-120 120 30 26.6 13.1 53.520C-90 90 20 23.1 18.0 48.240C-90 90 40 46.9 23.9 41.9

a The actual mass content of inclusion segments was calculatedby 1H NMR;15,16 Mass content of inclusion segments ) n(Mw(R-CD)

+ Mw(CL in clusion))/(Mn(PCL) + nMw(R-CD)) (n ) the number of R-CD ina single PCL chain). b The inclusion crystal size was calculatedusing the Sherer formula D ) 57.3 × 0.89λ/B1/2 cos θ (λ ) 0.154nm; B1/2 denotes the full wide at half-maximum (fwhm) obtainedfrom XRD). c �c,PCL (%) ) (∆Hm/naked PCL segments masscontent)/∆H0; ∆H0 for PCL ) 139.3 J/g.19

J. Phys. Chem. B 2010, 114, 4739–4745 4739

10.1021/jp1001836 2010 American Chemical SocietyPublished on Web 03/17/2010

designates the theoretical mass content of inclusion segments(all of R-CD-PCL inclusion crystallites), and the numberfollowing “C” designates the casting temperature. For example,30C-120 contains 30 wt % inclusion segments casted at 120°C.

As a representative protocol, the preparation of 30C-90 isdescribed. A solution of R-CD in DMF (83.6 mg of R-CD in 5mL of DMF) was added dropwise to a solution of PCL in DMF(229 mg of PCL in 40 mL of DMF) at 90 °C under stirring.After stirring for 2 h, the film was prepared by casting underatmospheric conditions at 90 °C. The evaporation of the solventtook about 1 h. The resulting film was washed twice with themixture solvent of DMF/chloroform (CHCl3) (9/1, v/v) (15 mL)to remove possible free PCL and then was washed twice withdistilled water (15 mL) to remove uncomplexed R-CD. Afterdrying, the resultant films were cut into pieces having adimension of 10 × 10 × 0.1 mm3.

For comparison, the PCL homopolymer films were prepared.PCL was dissolved in DMF to yield a 20 w/v % solution whichwas allowed to evaporate at 90 °C for 2 h. Films with a thicknessof 0.1 mm were obtained after vacuum drying.

In Vitro Enzymatic Biodegradation. The enzymatic deg-radation tests in vitro were conducted in phosphate buffersolutions (0.1 M, pH ) 7.4) in the presence of lipase at 37 °C.Films with an initial weight of ∼20 mg were incubated in glassbottles containing 10 mL of phosphate buffer buffer with 2 mg/mL lipase. The buffer medium was renewed every 3 days tomaintain the original enzyme activity. Films were collectedevery 3 days, washed with distilled water, and vacuum-driedfor another 3 days at 40 °C before being subjected to analyses.

Characterization and Methods. The weights of the partialR-CD-PCL inclusion complexes films were measured with anelectrical balance. Weight loss in percentage was calculatedaccording to eq 1

where W0 is the initial weight and Wt is the weight at a giventime point. Both W0 and Wt were measured after vacuum dryingat 40 °C for 3 days. The average weight loss of threeindependent specimens was calculated and taken as the weightloss value for each sample.

The number-average molecular weight (Mn) and polydisper-sity index (PDI) of the partial R-CD-PCL inclusion complexesbefore and after degradation were determined by Waters 510gel permeation chromatography (GPC) equipped with a ShedoxKF-800 series column using tetrahydrofuran as the mobile phaseat a flow rate of 1 mL/min at 35 °C and using polystyrene asthe standard.

The composition of the partial R-CD-PCL inclusion com-plexes before and after degradation was measured by protonnuclear magnetic resonance (1H NMR, Bruker AVANCE AVΠ-600 NMR) at 600 MHz in DMSO-d6 at 30 °C. Beforemeasurements, the 1H NMR samples were heated for 20 min at100 °C to produce the samples in a fully dissociated state inthe DMSO-d6.

X-ray diffraction (XRD) spectra were conducted with PHILPX’Pert MPD (40 KV, 30 mA) using nickel-filtered Cu KRradiation. The samples were scanned from 2θ ) 10 to 35° at aspeed of 2°/min.

Thermal properties of various films were determined bydifferential scanning calorimetry (DSC, NETZSC DSC 204 F1).Each sample (5 mg) was scanned at a heat rate of 10 °C/minfrom 0 to 120 °C under a N2 atmosphere.

The surface morphology of the samples was examined byusing scanning electron micrographs (SEM) (JEOL JSM-5600LV). Micrographs were taken under reduced pressure atroom temperature.

Results and Discussion

Characterization of Partial r-CD-PCL Complexes. Fivepartial R-CD-PCL inclusion complexes were successfullyprepared by solution casting under different temperatures andwith different mass contents of inclusion segments (Table 1),30C-70, 30C-90, 30C-120, 20C-90, and 40C-90. Figure 1presents the X-ray diffraction spectra of these five partialR-CD-PCL inclusion complexes in comparison with the PCLhomopolymer before degradation. All partial R-CD-PCL inclu-sion complexes were similar, which showed both a typicalR-CD-polymer inclusion channel crystalline peak (2θ ) 20.0°)9

and pure PCL crystalline peaks (2θ ) 21.3° and 23.6°), whereasthe R-CD crystalline peak (2θ ) 21.8°) was absent. These resultsindicated that the R-CD molecules threaded on PCL chains toform a supramolecular structure containing channel-type inclu-sion crystallites and PCL crystallites simultaneously.

Table 1 shows the mass content of inclusion segments incomplexes calculated from 1H NMR spectrum. As confirmedin our previous paper, the actual composition of samples wasalmost consistent with the feed ratio when the casting temper-ature was above 90 °C. However, when the casting temperaturewas lower than 90 °C, the actual inclusion proportion of thecomplexes was more than the theoretical one because theinclusion proceeding favors low temperature.16 The castingtemperature also impacted the aggregation of R-CDs. When thecasting temperature was low, the R-CDs threaded on the polymerchains and intended to aggregate together through the hydrogenbonding between R-CDs; the resulting inclusion crystallites hadrelatively bigger size. As the casting temperature increased, theaggregation of R-CDs decreased because hydrogen bondsbetween R-CDs were attenuated significantly; the resultinginclusion crystallites had a relatively smaller size. For example,although the sample 30C-120 had more inclusion content(26.6%) than sample 20C-90 (23.1%), the size of inclusioncrystallites of 30C-120 (13.1 nm) was smaller than that of 20C-90 (18.0 nm).

Due to having PCL crystallites, all partial R-CD-PCLinclusion complexes showed a clear endothermic peak of PCL.It should be noticed that the sample 40C-90 exhibited twomelting peaks, whereas other partial R-CD-PCL complexes

Weight loss (%) ) 100(W0 - Wt)/W0 (1)

Figure 1. X-ray diffraction spectra of PCL and partial R-CD-PCLinclusion complexes before degradation.

4740 J. Phys. Chem. B, Vol. 114, No. 13, 2010 Luo et al.

showed only one similar large peak (Figure 2). These resultsmay be due to the fact that at high casting temperature, thecomplex with higher inclusion content has compositionalheterogeneity, which consists of a high naked PCL content partand low naked PCL content part.15 The crystallinity (�c) of thePCL crystallites calculated from the enthalpy of fusion (∆Hm)is summarized in Table 1. It can be seen that the �c decreasedas the casting temperature increased. This trend of �c varyingwith casting temperature could be attributed to the fact that theactual inclusion proportion of samples decreased with increasingcasting temperature. As we reported previously, the �c decreasedwith the actual inclusion proportion decreasing because theinclusion parts enhanced the nucleation and crystallization ofthe polymer. However, the �c of sample 40C-90 was much lowerthan that of other samples; even the actual inclusion proportionwas quite high. This phenomenon may have also resulted fromthe compositional heterogeneity of sample 40C-90. The com-positional heterogeneity resulted in the regular chain structureof polymer being destroyed and then restricted the crystallization.

Enzymatic Degradation. The enzymatic degradation ofpartial R-CD-PCL inclusion complexes and the PCL ho-mopolymer was investigated in phosphate buffer (pH ) 7.4)with 2 mg/mL lipase from pancreas porcine at 37 °C.

Weight Loss. Weight loss data were collected after regularinterval degradation times. As presented in Figure 3, the PCLhomopolymer showed a rather low weight loss rate. Only 20.4%of weight loss was detected after 27 days. All partial R-CD-PCLinclusion complexes degraded much faster than the PCLhomopolymer, which implies that the formation of the inclusionstructure accelerated the enzymatic degradation of materialssignificantly.

Figure 3A shows the weight loss of complexes with differentinclusion content during degradation time. It can be seen thatthe weight loss rate of complexes increases with the inclusioncontent increasing for the samples prepared at the sametemperature. After 27 days, the weight loss of 20C-90, 30C-90, and 40C-90 was 63.1, 75.7, and 82.5%, respectively.Interestingly, as the casting temperature decreased, the weightloss rate of complexes showed the trend of decrease, althoughthe actual inclusion content increased (Figure 3B). For example,the actual inclusion content of 30C-70 was 52.4%, which wasmuch higher than that of 30C-120 (actual inclusion content was26.6%), but it showed only 45.1% weight loss at 27 days, whichwas much lower compared to the 82.5% weight loss for 30C-120. These phenomena can be partially assigned to the crystal-linity of PCL. It is known that crystalline PCL chains are moreresistant to enzyme-catalyzed degradation than amorphous PCL

chains.18,19 In the present study, the crystallinity of PCL in partialinclusion complexes increased as a function of the decrease inthe casting temperature. As a consequence, the sample castingat lower temperature should be relatively hard to be degraded.However, merely PCL crystallinity explanation is not enoughsince the degradations of 30C-70, 30C-90, and 30C-120 wereall faster than that of PCL, although their PCL crystallinity washigher. Our previous work has proved that a different castingtemperature resulted in different aggregation modes and differentinternal structure in partial R-CD-PCL inclusion complexes.16

It seems that the degradation behavior of partial R-CD-PCLinclusion complexes is affected greatly by their internal structure.The detailed analysis and explanation will be discussed in alater part.

Compositional Changes during Degradation. Compositionalchanges of the partial R-CD-PCL inclusion complexes duringdegradation were examined by 1H NMR. Figure 4 shows the1H NMR spectra of 30C-120 after various enzymatic degradationtimes. It can be seen that resonance peak of R-CD C1-Hgradually disappeared over the degradation time. Similar resultswere observed in all partial R-CD-PCL inclusion complexes.The relative weight loss of R-CD content in complexes [relativeweight loss of R-CD content ) (original R-CD content - R-CDcontent at a given time point)/original R-CD content] wascalculated from 1H NMR spectra (Figure S1, SupportingInformation). It is clear that the content of R-CD in complexesdecreased significantly with the degradation for all of thesamples. The disappearing rate of R-CDs increased with theincreasing inclusion content (Figure S1A, Supporting Informa-tion). For sample 20C-90, the total disappearance of R-CDs took12 days. However, for sample 40C-90, it took 21 days. Becausethe actual inclusion content in complexes increased with thecasting temperature decrease, 30C-70 took more time for thedisappearance of R-CDs than 30C-90 and 30C-120 (Figure S1B,Supporting Information). R-CD is a cyclic oligomer composedof six D-glucopyranosidic units, which cannot be degraded bylipase.20 Therefore, it could be assumed that the reduction ofR-CDs resulted from the R-CDs sliding off of the PCL chainsas the consequence of PCL chain scission during degradation.

The reductions of R-CDs were also confirmed by XRDinvestigation. Figure 5 shows the XRD spectra of 30C-120 aftervarious enzymatic degradation times. It can be seen that thetypical R-CD-PCL inclusion channel crystalline peak (2θ )20.0°) gradually disappeared during the degradation, whichindicates the reduction of R-CDs. To gain deeper insight intothe phase structure changes, the crystal size of R-CD-PCLinclusion crystallites was calculated according to the Schererformula (Table S1, Supporting Information). Figure 6 showsthe relative crystal size of inclusion in partial R-CD-PCLinclusion complexes during enzymatic degradation. It can beseen that the crystal size of R-CD-PCL inclusion crystallitesin all partial R-CD-PCL inclusion complexes gradually de-creased to zero with the degradation time. Similar to the resultsof 1H NMR data, samples with more actual inclusion contenttook a longer time for the disappearance of R-CD-PCLinclusion.

Now the question is, does the fast weight loss of partialR-CD-PCL inclusion complexes only result from the reductionof R-CD? Figure 7 shows the weight loss of PCL in partialR-CD-PCL inclusion complexes and PCL homopolymer at 27days. Except for sample 30C-70, other complexes showed higherPCL loss than the PCL homopolymer, which indicates that theinclusion structure accelerated the enzymatic degradation of PCLindeed. For the samples casted at the same temperature, the PCL

Figure 2. DSC thermograms of PCL and partial R-CD-PCL inclusioncomplexes before degradation.

Partial Inclusion Complex Formation between R-CD and PCL J. Phys. Chem. B, Vol. 114, No. 13, 2010 4741

loss was quite close, even though the inclusion content ofsamples was different, implying that the influence of contentof R-CD on degradation was small. It is interesting that the

weight loss of PCL showed great dependence on the castingtemperature. Sample 30C-70 showed almost zero PCL loss,whereas sample 30C-120 showed about 58.9% PCL loss. Aswe have discussed, the crystallinity of PCL probably affectsdegradation. The high crystallinity of PCL in 30C-70 (�c )70.7%) probably resulted from the polymer chains withstandingthe enzymatic attack for a long time. However, it should benoticed that sample 30C-120 showed the largest PCL loss,although its crystallinity was very high (�c ) 53.5%, higherthan that of 20C-90, 40C-90, and the PCL homopolymer).Therefore there may exist other factors which also affect thedegradation. It has been confirmed that the internal structure ofpartial inclusion complexes also showed casting temperaturedependence. Because of high-temperature attenuated hydrogenbonding, samples prepared at high temperature showed relativelysmaller crystal size of inclusion (Table 1), where the R-CDsthreaded on the PCL chains in a dispersive mode. The PCLloss data of partial R-CD-PCL inclusion complexes revealedthat the sample with smaller inclusion crystallites exhibited moreweight loss of PCL.

Molecular Weight Changes. Figure 8 and Figure 9, respec-tively, show the molecular weight and polydispersity changesof the PCL homopolymer and partial R-CD-PCL inclusioncomplexes prepared at different temperatures during degradation.It can be seen that the number-average molecular weight (Mn)of PCL increased from 82 790 to 105 450 and the molecularweight distribution decreased from 1.9 to 1.6 during the first 2weeks. After this period, the molecular weight decreasedslightly, whereas the polydispersity increased. To the contrary,the partial R-CD-PCL inclusion complexes behaved in adifferent way; the molecular weight continuously decreased, andthe polydispersity continuously increased. The initial molecularweights of partial R-CD-PCL inclusion complexes were higherthan those of the PCL homopolymers because of the introductionof R-CD. The Mn values of partial R-CD-PCL inclusioncomplexes at 27 days were lower than that of the PCLhomopolymer, and the molecular weight distributions of partialR-CD-PCL inclusion complexes were much higher than that

Figure 3. Weight loss profiles of PCL and partial R-CD-PCL inclusion complexes during enzymatic degradation in the presence of lipase at 37°C. (A) Effect of inclusion content in complexes on the degradation behavior; (B) effect of casting temperature on the degradation behavior.

Figure 4. 1H NMR spectra of 30C-120 in DMSO after variousenzymatic degradation times.

Figure 5. X-ray diffraction patterns of 30C-120 after various enzymaticdegradation times.

4742 J. Phys. Chem. B, Vol. 114, No. 13, 2010 Luo et al.

of PCL, which reflected the acceleration effect of inclusionsegments on the enzymatic degradation of PCL.

It should be noted that the molecular weight of 30C-70initially decreased quickly, but over the last two weeks, themolecular weight only decreased slightly. Our analysis ofcompositional changes has shown that a large amount of R-CDmolecules slid off of the PCL chains at the early stage ofdegradation. Therefore, the degradation of 30C-70 is mainlyattributed to the loss of R-CDs, and their PCL chains withstoodthe enzyme attack because of their very high crystallinity.

However, for samples 30C-90 and 30C-120, the continuousdecrease of molecular weight and increase of polydispersityindicate that the reductions of molecular weight result from notonly the loss of R-CDs but also the PCL degradation.

Crystallinity Changes during Degradation. The crystallinitychanges of the PCL phase during degradation were measuredby DSC. As shown in Figure 10, the crystallinity of all of thesamples gradually increased to a maximum at the early stageof degradation. This finding could be assigned to two facts. (1)

Figure 6. Relative crystal size of inclusion in partial R-CD-PCL inclusion complexes during enzymatic degradation in the presence of lipase at37 °C. (A) Effect of casting temperature on the degradation behavior; (B) effect of inclusion content in complexes on the degradation behavior.

Figure 7. Weight loss of PCL in partial R-CD-PCL inclusioncomplexes during enzymatic degradation in the presence of lipase at37 °C.

Figure 8. Molecular weight (Mn) change versus degradation time forPCL and partial R-CD-PCL inclusion complexes in the presence oflipase at 37 °C.

Figure 9. Polydispersity index change versus degradation time forPCL and partial R-CD-PCL inclusion complexes in the presence oflipase at 37 °C.

Figure 10. Crystallinity of PCL change versus degradation time forPCL and partial R-CD-PCL inclusion complexes in the presence oflipase at 37 °C.

Partial Inclusion Complex Formation between R-CD and PCL J. Phys. Chem. B, Vol. 114, No. 13, 2010 4743

The degradation of PCL happened first in the amorphous area;as a result, the crystallinity of the residua increased. (2) Initialcool crystallization of the PCL phase during solvent evaporationwas incomplete due to the presence of the other phase, and asa consequence, the PCL phase continued to crystallize duringdegradation at 37 °C. After this period, the crystallinity of thesamples began to decrease, indicating that the enzymatic attackeroded the crystalline zones gradually. In the case of the PCLhomopolymer, the erosion of crystalline zones occurred in thevery late stage, and the erosion rate was low. In contrast, allpartial R-CD-PCL inclusion complexes showed erosion ofcrystalline zones only after 3 or 6 days, and the erosion rateswere relatively high. After 27 days, all of the partial R-CD-PCLinclusion complexes showed lower �c of PCL than did thehomoPCL sample, whatever their initial �c. It should be notedthat the �c of PCL in 30C-70 decreased from 70.7 to 43.2%,although the weight loss of PCL was negligible. These findingsfurther prove the acceleration effect of inclusion segments onthe enzymatic degradation of PCL.

Visual Examination by SEM. SEM was used to monitor thechanges of the film surface morphology during degradation. Asshown in Figure 11, PCL initially had a smooth and compactsurface. After 3 days, the surface was slightly degraded, with afew tiny pores appearing on the surface. After 15 days, thesurface was uniformly eroded with the formation of numerousand enlarged pores. After 27 days, the surface was eroded toleave a network-like structure. The changes could be mainlyassigned to the degradation of amorphous parts of PCL.

In the case of 30C-90 (Figure 12), the surface exhibitedinitially a rugged pattern. After 3 days, exterior erosion appearedclearly. As degradation proceeded up to 15 days, many deepand large holes appeared on the surface. After 27 days, 30C-90was strongly eroded to leave sponge-like structures. Apparently,the erosion of 30C-90 was faster than that of PCL homopoly-mers, and the enzymes penetrated deeply into the bulk ofcomplex.

Degradation Mechanism of Partial r-CD-PCL InclusionComplexes. Compared with the PCL homopolymer, partialR-CD-PCL inclusion complexes showed accelerated andenhanced enzymatic degradation behaviors. The introductionof the inclusion structure resulted in the weight of the sample,the molecular weight of samples, and the crystallinity of PCLdecreasing much faster during the degradation time than thoseof the PCL homopolymer in the presence of lipase. As controls,the weight losses of both the PCL homopolymer and partial

R-CD-PCL inclusion complexes were very low (under 5%) inthe absence of lipase, which implied that the PCL chains werehard to degrade and the R-CD molecules seldom slid off of thePCL chains without enzyme within 30 days. However, in thepresence of lipase, R-CD molecules slid off of the PCL chainsand were removed during degradation. In consequence, cracksappeared in the polymer matrix, which could facilitate enzymepenetration, and then accelerated the degradation of the materi-als. However, merely “cracks interpretation” cannot explain ourexperimental data. If the appearance of cracks was the onlyreason for the degradation acceleration, the reduction of theR-CDs content should be slight at the early stage of degradationsince only a few PCL chain splits at that time, and the

Figure 11. The surface morphology change of PCL with degradationobserved under SEM at (A) 0; (B) 3; (C) 15; and (D) 27 days. Thescale bar corresponds to 10 µm.

Figure 12. The surface morphology change of 30C-90 with degradationobserved under SEM at (A) 0; (B) 3; (C) 15; and (D) 27 days. Thescale bar corresponds to 10 µm.

Figure 13. Proposed mechanism of enzyme adsorption and enzyme-catalyzed degradation of partial R-CD-PCL inclusion complexes.

4744 J. Phys. Chem. B, Vol. 114, No. 13, 2010 Luo et al.

disappearing rate of R-CDs should increase as the degradationproceeds. However, the partial R-CD-PCL complexes lostabout 10-50% of the R-CDs only in the first 3 days. Especially,the sample 30C-70 lost all of the R-CD components at 18 days,although the scission of PCL in 30C-70 was very small.

Recently, several papers studied the relationship betweenenzyme adsorption and enzyme-catalyzed degradation of poly-ester, which suggested that adsorbed enzyme molecules maymove around to hydrolyze surrounding polymer chains, and theenzyme adsorption was a very important for degradation ofpolyester.21,22 In this work, a mechanism of the degradationoccurred in partial R-CD-PCL complexes and is proposed asFigure 13. First, the enzyme preferentially adsorbed on theinterfaces between inclusion crystallites and uncovered PCLsegments. Then, as the enzyme hydrolyzed the surrounding PCLchains, the R-CD molecules slid off from the end of cleavingchains. Therefore, at the initial stage of degradation, a largeamount of R-CDs reduction can be observed. This model alsoexplained the phenomenon where a sample with smallerinclusion crystallites exhibited more weight loss of PCL thanthe one with similar inclusion content but bigger size of theinclusion. The sample showing smaller inclusion crystallitesmeant that the R-CDs tended to disperse on the PCL chains;thus, they should have more interfaces of R-CDs and PCL chainscompared to the one where R-CDs aggregated together. Moreinterfaces and more enzyme adsorption resulted in more scissionand weight loss of PCL.

Conclusions

In this paper, the lipase-catalyzed enzymatic degradation wasstudied for supramolecular biomaterials based on partial inclu-sion complex formation between R-CD and PCL. The resultsdemonstrate that the formation of inclusion apparently acceler-ated the enzymatic degradation of materials. The enzymepreferentially adsorbed on the interfaces between inclusioncrystallites and uncovered PCL segments. Because the enzymehydrolyzed the surrounding PCL chains, the R-CD moleculesslid off from the end of cleaving chains. With the removal ofR-CDs, cracks appeared in the polymer matrix, which facilitatedboth enzyme penetration and leakage of the degradationproducts, and then accelerated the degradation of the materials.

Recently, more and more host-guest polymer inclusioncomplexes have been designed and applied in the biomedicalarea. This study provides a preliminary understanding for thebiodegradation of these materials. However, the degradation ofpartial R-CD-PCL complexes is a complex process. Furtherand detailed investigations will be performed in the future.

Acknowledgment. The research was funded by the NationalNatural Science Foundation of China (Grants 50703025 and30600148).

Supporting Information Available: Crystal size and relativeweight loss of inclusion (nm) in partial R-CD-PCL inclusioncomplexes during enzymatic degradation. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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Partial Inclusion Complex Formation between R-CD and PCL J. Phys. Chem. B, Vol. 114, No. 13, 2010 4745