Article pubs.acs.org/JPCC
Physiological Degradation Mechanisms of PLGA Membrane Films under Oxygen Plasma Treatment C. López-Santos,*,† A. Terriza,† J. Portolés,‡ F. Yubero,† and A. R. González-Elipe† †
Institute of Materials Science of Seville (CSIC-University of Seville), Americo Vespucio 49, E-41092 Seville, Spain NEXUS Nanolab, Newcastle University, G8 XPS laboratory Stephenson Building, Newcastle upon Tyne NE1 7RU, United Kingdom
‡
S Supporting Information *
ABSTRACT: Degradation under simulated physiological conditions of poly(lactic-coglycolic) (PLGA) copolymer membrane films subjected to an oxygen plasma treatment compared to its “as prepared” state has been studied by gas cluster ion beam assisted X-ray photoelectron spectroscopy for chemical depth profiling analysis. This investigation is complemented with atomic force microscopy, weight loss measurements, and visual inspection of the films at the different stages of the degradation process. The obtained results show that the carbon functional groups of the PLGA membrane films undergo a heterogeneous hydrolytic degradation to different rates depending on the plasma pretreatment. The content of glycolic groups (GA) in untreated PLGA samples immersed for 3 weeks in a phosphate-buffered saline solution decreased at the surface, whereas the ratio between glycolic and lactic units (LA) did not vary in the inner regions (∼400 nm depth) of the degraded membrane films. By contrast, oxygen plasma pretreatment enhances the degradation efficiency and causes that both lactic and glycolic functional components decreased at the surface and in the interior of the film, although with less prevalence for the lactic units that present a comparatively higher resistance to degradation.
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attention in recent “in vitro” and “in vivo” investigations aiming at determining the copolymer evolution in physiological environments.6,12,13 According to Anderson et al.,14 the hydrolytic biodegradation of PLGA copolymers involves four steps: (i) hydration; (ii) initial degradation: reduction of chain length due to the random breakage of covalent bonds; (iii) steady degradation: mass loss due to the scission of polymer chains and autocatalytic processes involving the carboxylic end groups; (iv) solubilization: release of the low molecular weight fragments in the form of soluble molecules that diffuse out of yarn and produce the shrinking of the polymer structure.1,3,4,15 PLGA degradation mechanisms have been investigated by immersion for different periods of time in a phosphate buffered saline (PBS) solution simulating the physiological conditions (37 °C and pH 7.4). For example, these in vitro studies have tracked the polymer weight loss,15 analyzed the Raman or nuclear magnetic resonance spectra,2 or used innovative fluorescence techniques such as confocal laser scanning microscopy or flow cytometry16 to follow the degradation process as a function of time. However, these bulk-sensitive methods do not differentiate between pristine and partially degraded PLGA materials or surface and bulk degradation effects. Herein, as an alternative surface-sensitive procedure, we propose the use of X-ray photoemission spectroscopy (XPS) mediated by the chemical depth profiling with a gas cluster ion
INTRODUCTION Biomedical devices, tissue engineering scaffolds, or drug delivery platforms are currently made of biocompatible and biodegradable synthetic polymers such as saturated poly(αhydroxy esters) and, particularly, poly(lactic acid), poly(glycolic acid), or poly(lactic-co-glycolic) (PLGA) copolymers.1−4 Although PLGA is a very popular biodegradable material, the absence of ionic molecular groups in its structure hinders the occurrence of processes such as osteoconductivity, biomineralization, or tissue growth which are relatively more favorable on other natural polymers.1,3 Moreover, the relative hydrophobic character of PLGA with respect to the natural extracellular matrix hinders the cell proliferation on its surface.5 To circumvent these problems, PLGA is either doped with basic additives and biologically active molecules,6 combined with ceramic materials,1 or subjected to surface modifications via plasma treatments.5,7−9 Oxygen plasma treatments induce the incorporation of negatively charged functional groups and create topographic changes that improve the cell affinity by rendering the PLGA surface hydrophilic.1 They also contribute to reduce the bacteria viability on the PLGA surfaces.8 In general, gas plasma treatments are considered to penetrate up to few millimeters through the macropores of scaffolds of PLGA or other polymers,10 although the effect is considered superficial and to do not affect the interior of the polymeric structure.11 An important feature of PLGA copolymers in relation with their use as biomedical material is their hydrolytic degradation through de-esterification, a process that has deserved much © XXXX American Chemical Society
Received: May 26, 2015 Revised: August 5, 2015
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DOI: 10.1021/acs.jpcc.5b05011 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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weight loss analysis, the samples were taken from the PBS solution, washed with ethanol and water, and finally allowed to dry. In the following description of results and discussion, the samples are named PLGA-U or PLGA-O2 (for the untreated and oxygen plasma treated samples without immersion in the PBS solution, respectively). Besides, duration of the PBS degradation tests is indicated by adding the /nW ending to the PLGA-U or PLGA-O2 names, to account for the immersion time in number (n) of weeks in the PBS solution. Argon Cluster Ion Beam Assisted XPS Depth Profiling. The GCIB-XPS study was performed using a K-Alpha XPS instrument (Thermo Scientific, East Grinstead, UK) at NEXUS. It was equipped with a Thermo MAGCIS argon cluster source located at 58° to the normal surface, operating at 4 keV of beam energy. Argon clusters with around 1000 atoms as average were used for the sputtering/etching experiments. A cluster beam of ca. 20 nA was rastered on an area of 2 mm × 2 mm. During the ion bombardment no X-rays were applied to the polymer samples. A charge neutralizer was operated during ion bombardment and photoelectron measurements to compensate for charge.17,18 Survey spectra were acquired working in the constant pass energy mode of 200 eV with 50 ms of dwell time. For high-resolution C 1s and O 1s spectra, the constant pass energy mode of 20 eV and a dwell time of 100 ms were used. Binding energy calibration was set at 284.5 eV for the C−C(H) species with the lowest binding energy of the C 1s signal. Calibration of the GCIB etching depth was done measuring the depth of the sputtered crater with an Alicona InfiniteFocus perfilometer after 4300 s sputtering time. In these conditions, the depth of the crater was about 400 nm. In this work we pay special attention to PLGA samples after 0 s (surf), 2150 s (midsurf), and 4300 s (inner-surf) sputtering times. Surface Roughness Characterization. AFM images were taken with a Cervantes AFM microscope driven with a Dulcinea control system (Nanotec, Spain) working in tapping mode and using high-frequency cantilevers. The images were processed with the Nanotec WSxM software.21
beam (GCIB) for sputtering, using a PLGA with a 75:25 lactic:glycolic acid ratio typical of this type of polymers used for biomedical applications.1 Since high lactic acid content implies more hydrophobic character due to the presence of methyl side groups that absorbs less water, it is difficult to follow the degradation of 75:25 PLGA by FTIR or Raman spectroscopies, requiring its analysis by alternative techniques. Moreover, the study of the degradation processes during the first 20 days is a challenge because no significant changes in the glycolic acid percentage of PLGA have previously been found by conventional techniques.4 Unlike conventional profiling techniques using Ar+ ions, GCIB is well suited to study the in-depth distribution of chemical groups in polymer samples because it does not induce any significant damage in the buried layers of the structure.17,18 In the present work, we attempt for the first time a GCIB-XPS analysis of the in vitro degradation process to get information about the autocatalyzed hydrolysis mechanism of pristine PLGA membrane films. An equivalent study is also carried out for oxygen plasma treated PLGA membrane films, aiming at identifying the influence of this treatment on the degradation process. This comparative study by GCIB-XPS, together with atomic force microscopy (AFM) and weight loss analysis, has shown that oxygen plasma treated PLGA films develop a high roughness and an outer chemically modified layer of several hundreds of nanometers. These changes reduce the chemical stability of PLGA films and decrease significantly their degradation time in physiological conditions. Besides this empirical evaluation of degradation efficiency, detailed information about its mechanism has been retrieved for the pristine and oxygen plasma treated samples by a thorough analysis of the XPS spectra recorded at different GCIB sputtering times.
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EXPERIMENTAL METHODS Fabrication of Pristine and Plasma-Treated PLGA Films and in Vitro Degradation Procedure. PLGA membrane foils with a thickness of the order of 50 μm were prepared from a 1.5 wt % PLGA dichloromethane solution by evaporation of the solvent on a Teflon plate, using a magnetic stirring at room temperature and relative humidity of 40% for 30 min to ensure full dissolution of the polymer. PLGA pellets with a copolymer ratio of 75:25 (lactic/glycolic acid) were purchased from Sigma-Aldrich, Inc. Samples were further dried for 48 h in air at RT. The resultant linear copolymer PLGA is composed by the lactic and glycolic acid monomers with a 75:25 ratio. It can be dissolved by several common solvents and presents an amorphous structure with ∼2 GPa of Young modulus, ∼10 kDa of molecular weight, and ∼35−55 °C of Tg reported in the literature for this PLGA composition.1,4,13 Oxygen plasma treatments were carried out for periods of 30 min in a parallel plate capacitive RF reactor working at a pressure of 0.1 mbar. A RF power of 10 W was applied to the top electrode, and a self-induced negative bias voltage of 200 V was generated on the bottom electrode acting as sample holder. Further details about the experimental plasma reactor setup can be found in previous publications.19 In vitro degradation tests were done by sample immersion in a PBS solution (Sigma-Aldrich) that reproduces a biological environment.15,16 Samples of 15 × 15 mm2 were inserted into falcon tubes containing 20 mL of PBS at 36.5 °C where they were kept at constant agitation (60 rpm). Once every week, 75% of the fluid was replaced with fresh PBS preincubated at 36.5 °C according to ref 20. Prior to their XPS, AFM, and
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RESULTS AND DISCUSSION Roughness of PLGA-U and PLGA-O2 Films and Weight Loss during Degradation. The AFM analysis of the surface topography of the untreated PLGA membranes films revealed that they were quite smooth, with a root-mean-square (RMS) roughness around 0.4 nm (Figure 1) similar to the values reported in the literature.5 When these polymeric films were exposed to the oxygen plasma, a rougher grain-like topography developed (RMS roughness of 330 nm) as shown in Figure 1. The development of this rough topography can be accounted for by the combination of surface chemical etching and polymer chain cross-linking8 mechanisms. The overall effect is a RMS roughness increase by a factor ∼900 and the evolution of hills and valleys with diameters of a few hundred nanometers as evidenced by the line profiles accompanying the topographic images in this figure. Scanning electron microscopy images have been included as Supporting Information (Figure S1) confirming the damaged PLGA surface after the plasma treatment in a larger scale of observation. In vitro degradation of PLGA-U and PLGA-O2 films after immersion in PBS solution during progressive periods of time is clearly evidenced by the weight loss reported in Figure 2. In addition, the membrane films underwent changes in their visual B
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GCIB-XPS Analysis of PLGA-U and PLGA-O2 Films without PBS Degradation. XPS depth profiling of PLGA-U and PLGA-O2 films has been carried out up to etching depths of about 400 nm (total sputtering time of ∼4300 s). Figure 3a−
Figure 1. AFM images of PLGA-U (top) and PLGA-O2 (down) before immersion in PBS solution.
Figure 3. C 1s (a, c) and O 1s (b, d) XPS peaks of PLGA-U (left) and PLGA-O2 (right) membranes before physiological degradation. Signals from the topmost surface (surf) and after GCIB sputtering of 200 nm (midsurf) and 400 nm (inner-surf) are shown.
d reports the C 1s and O 1s spectra from the carbon and oxygen atoms of the PLGA-U and PLGA-O2 films at three etching stages: surface, midsurf (∼200 nm depth), and innersurf (∼400 nm depth). The fitted C 1s spectrum of the PLGAU sample reveals the presence of three main components at 284.5, 286.6, and 288.5 eV attributed to −C−C(H), −CO, and −COO− (OC−O−) groups, respectively.22 Meanwhile, the O 1s spectrum can be fitted with two bands at 531.7 and 533.2 eV attributed to OC− and −O−C− chemical species,23 respectively. The −COO− component of C 1s and the −O−C one of O 1s spectra of the PLGA-O2 sample had a relatively higher intensity than the equivalent species in the PLGA-U sample. Besides, independently of the sputtering depth, the C 1s and O 1s components of the PLGA-O2 sample were always broader than those of the PLGA-U sample, passing from 1.5 to 1.7 eV for the C 1s and from 1.6 to 2.2 eV for the O 1s components, respectively. This broadening is basically the result of certain enrichment in oxygen of the surface of the plasma-treated samples and a randomization of their structure. In fact, the O/C atomic ratio at the surface, determined from the O 1s and C 1s intensities, was 0.63 for the pristine PLGA-U film and 0.83 for the plasma-treated PLGA-O2 material. The analysis of the GCIB etching profiles also showed that the O atomic percentage decreases slightly in the deeper layers of the PLGA-O2 samples but remained practically constant in depth in the pristine PLGA-U films (see Supporting Information S2). GCIB-XPS Analysis of PLGA-U and PLGA-O2 Films Subjected to in Vitro PBS Degradation. The considerable weight lost and breaking into pieces, characteristic according to
Figure 2. Weight loss of PLGA-U and PLGA-O2 membranes after immersion in PBS solution for increasing periods of time.
aspect (brightness decrease and color change), a slight contraction of dimensions, and ultimately breaking into pieces. These effects were exalted for the PLGA-O2 foils that underwent a considerably higher weight loss (7%/35 wt % loss after 16/22 weeks) than the PLGA-U samples (5%/22 wt % loss, respectively). As example, Figure S1 from the Supporting Information presents the SEM micrographs of PLGA-U and PLGA-O2 foils after 1 week of degradation, proving the different hydrolysis effects in function of the initial surface state: smaller polymeric aggregates appear on the PLGA- O2/1W surface whereas only few dispersed larger fragments are present on the PLGA-U/1W sample. This result confirms that the oxygen plasma treated PLGA films degrade more rapidly in PBS solution than the untreated material. In the next sections it will be proved that both chemical functionalization and enhanced roughness are responsible for the increased degradation rate of plasma treated PLGA membrane films. C
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The Journal of Physical Chemistry C Anderson14 of the solubilization stage during the hydrolytic degradation and observed here for the membrane films immersed in the physiological simulant for periods longer than 3 weeks, precludes the GCIB-XPS depth profiling analysis of such samples. Therefore, XPS results will only be reported for PLGA-U and PLGA-O2 samples degraded for 1 and 3 weeks, i.e., corresponding to the hydration and degradation steps in the Anderson model. In comparison, degradation studies from the literature4,13 have considered more than twice as long to reveal changes in the degraded polymeric material. Figures 4a and 4b show the C 1s and O 1s fitted spectra of the PLGA-U sample immersed for 1 week in the PBS solution
C(H) components present a lower intensity, both features revealing the majority loss of the oxygen groups originally formed by plasma treatment. Compared to the C 1s components of the PLGA-U/1W sample, the reduced −COO− peak intensity in sample PLGA-O2/1W suggests a notable breakage of carboxylic bonds. Degradation of the inner PLGA-O2 film regions is also supported by the preferential decrease of the oxygenated carbon groups with respect to the aliphatic ones (cf. Figure 4c). Fitting analysis of this spectrum also required a certain contribution of −C−OH species at 285.6 eV for a straightforward reproduction of the spectral envelope. In agreement with these observations at the C 1s level, the fitted O 1s spectrum (Figure 4d) is characterized by a relative increase in the OC− band and the appearance of a certain contribution at lower binding energies (small band at 531.0 eV) compatible with the presence of OH−C− groups on the surface.27 After ∼400 nm etching, the state of the partially degraded sample does not recover completely the situation of the bulk state of the sample PLGA-O2 reported in Figure 3c,d. Instead, the C 1s peak is characterized by a relatively higher C− C(H) component and two practically identical −COO− and −CO components, together with a small, but still noticeable C−OH band. Unlike the pristine PLGA-U sample, the contribution of adsorbed water in the O 1s spectrum of sample PLGA-O2/1W was clearly visible through its whole analyzed depth, which indicates that plasma treatment favors the hydration of deeper layers of the sample. In relation to the previous fitting analysis of the C 1s and O 1s spectra, particularly for that of samples PLGA-O2/1W-/3W, it is relevant to indicate that they were contaminated by phosphates and likely carbonate groups, these latter resulting from the reaction of low molecular weight PLGA fragments in the liquid medium. Survey XPS spectra effectively revealed that some residual Na, P, or Cl became incorporated in the PLGA membrane films, particularly in these two samples, where traces still remained after prolonged GCIB sputtering (see Supporting Information S3). These oxygen-containing contaminating species with contributions at 533.0 and 531.3 eV, respectively,27 would overlap with the O 1s bands of the polymer and therefore affect their relative intensities. After 3 weeks of PBS degradation, the PLGA-U/3W films exhibit clear changes in the shape of both the C 1s and O 1s spectra. The C 1s XPS spectra of sample PLGA-U/3W in Figure 5a show a loss in the intensity of the −COO− component in favor to the −CO and −C−C(H) groups and the development of a rather intense band due to −C−OH groups. In agreement with that the −O−C− component of the O 1s spectrum presents a relative increase in intensity. A relatively intense band due to H2O molecules at 535.8 eV is also visible in this spectrum. All these differential features were removed after GCIB etching for 4300 s, when the spectra became very similar to those of the sputtered PLGA-U original sample. From these results we can conclude that although the hydration process on the PLGA surface is intensified after 3 weeks in PBS, degradation process only starts to become visible at this stage. The C 1s spectra of samples PLGA-O2/3W and PLGA-O2/1W differ in that the intensity of the −CO peaks in comparison with that of −C−C(H) was smaller in the former sample as shown in Figure 5c. After GCIB etching this difference increased, which suggests that PBS degradation implies a strong loss of −CO groups in the inner regions of the plasma-treated PLGA film. An increase in the intensity of the −C−OH (and the equivalent OH−C groups in the O 1s
Figure 4. C 1s (a, c) and O 1s (b, d) XPS peaks of PLGA-U/1W (left) and PLGA-O2/1W (right) membranes after 1 week of physiological degradation. Signals from the topmost surface (surf) and after GCIB sputtering of 200 nm (midsurf) and 400 nm (inner-surf) are shown.
at physiological conditions (PLGA-U/1W). In comparison with the equivalent spectrum of the PLGA-U sample without exposure to PBS degradation, the spectrum of sample PLGAU/1W presents a new contribution in the C 1s spectrum at around 285.6 eV which can be assigned to C−OH groups.24 In the O 1s spectrum it is also worth noting the development of a small band at a very high binding energy of 535.8 eV attributed to molecular water incorporated in a partially degraded PLGA surface.25 Beyond ∼200 nm etching (midsurf), both the C 1s and O 1s signals were quite similar to the equivalent spectra of the original PLGA-U sample. This suggests that the PLGA-U/ 1W sample is only affected at its outmost surface layers where a limited hydration and hydrolysis has taken place. Other works did not find changes in PLGA weight loss or glycolic acid percentage by FTIR or Raman spectroscopies for the first degradation week,4 only some decrease of the PBS solution pH due to the release of acidic oligomers supposedly.26 The situation strongly differs in sample PLGA-O2/1W (Figure 4c,d), where the most intense C 1s component corresponds to −CO groups and the −COO− and the −C− D
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Figure 6. (a) Evolution of the [−COO−]/[−C−C(H)] ratio of the C 1s species of PLGA-U (top) and plasma-treated PLGA-O2 membranes (down) for several physiological degradation periods and depths of analysis. Dotted lines refer to glycolic/lactic content of the pristine PLGA sample. (b) PLGA degradation diagram.
6b), this ratio can be taken as a measurement of their relative concentration at the surface at given stages of the degradation process. For the original PLGA-U sample, the [−COO−]/ [−C−C(H)] ratio at the surface (i.e., for the sample not subjected to GCIB sputtering) decreases from ∼0.9 to ∼0.5 after immersion in PBS for 3 weeks. According to the structure of the PLGA copolymer in Figure 6b, this evolution suggests a preferential degradation for the glycol component of the copolymer film at its topmost layers and agrees with previous works28 showing that in PBS glycolic units degrade at a 30% faster rate than lactic units.4 Meanwhile, the ratio is rather constant in the sputtered PLGA-U/1W-3W samples, independently of whether the membrane film has been immersed or not in the PBS solution. This suggests that the degradation process does not progress in depth but remain restricted at the PLGA outer layers, at least during the 3 first weeks. According to Anderson et al.,14 this period corresponds to the PLGA surface hydration stage, just at the onset of the hydration of inner regions of the PLGA film and its initial degradation. For the PLGA-O2 film, the [−COO−]/[C−C(H)] ratio did not vary significantly when comparing the surface and the interior of the film (i.e., the GCIB sputtering time does not significantly affect this ratio). In addition, the value of this ratio (i.e., 1.20−1.35) was rather high in sample PLGA-O2 but decreased to ca. 0.6 for sample PLGA-O2/3W. Oxygen enrichment in plasma-treated polymeric membranes is a wellknown effect of this type of treatment.29 The results reported here for the PLGA membrane films suggest that the chemical functionalization processes are not restricted to their outermost surface layers as usually assumed for this type of treatments but, as suggested in previous works of our group on DLC-F films,30 may extend up to some hundreds of nanometers deep. In addition, the morphological changes and the roughness increase induced on the PLGA-O2 film surely contribute to generate pathways for the polymer functionalization in the bulk and, as evidenced by the XPS analysis of these samples, to the deep incorporation of water due to the swelling of the polymeric structure. One of the most important consequences of this increase in roughness, in the order of magnitude of the analyzed polymer depth, is the observed effects in degradation rate: the increase and extent to the whole analyzed PLGA film. In previous studies on the degradation of PLGA films, surface aspect effects (e.g., surface topography and chemical
Figure 5. C 1s (a, c) and O 1s (b, d) XPS peaks of PLGA-U/3W (left) and PLGA-O2/3W (right) membranes after 3 weeks of physiological degradation. Signals from the topmost surface (surf) and after GCIB sputtering of 200 nm (midsurf) and 400 nm (inner-surf) are shown.
signal) component was another noticeable effect in sample PLGA-O2/3W. In parallel, the O 1s spectra of this sample, mainly after ∼400 nm GCIB etching, shows a smaller prevalence of the OC− with respect to the −O−C groups if compared to the surface chemical state of sample PLGA-O2/ 1W (Figure 4d). This fact suggests that the loss rates of different oxygenated species have reached a balance after degradation for 3 weeks. Degradation Mechanism of PLGA and PlasmaTreated PLGA Surfaces. The previous results have shown that plasma treatment of PLGA promotes its degradation under simulated physiological conditions. Although no evident weight loss was found either in the PLGA-U or PLGA-O2 samples only after 3 weeks of degradation (see Figure 2), evidence shows that oxygen plasma treatment is able to reach not only the outer surface layers but also inner sample regions, beyond a few hundred nanometers (see Figure S2). We have also found that this plasma treatment contributes to enhance the film roughness and to generate new functional groups in the material. PBS immersion for times longer than 3 weeks clearly shows that this treatment contributes to the degradation of the membrane film (cf. Figure 2). The GCIB-XPS analysis has provided clear clues to understand the observed differences between samples by analyzing the initial degradation stages. From this analysis, we will also propose a mechanism to account for the hydrolysis processes occurring at the surface and that ultimately lead to the degradation of PLGA films. To get a deeper insight into the PLGA degradation and the effect of the surface plasma treatment in this process, Figure 6a presents the evolution [−COO−]/[−C−C(H)] intensity ratio between the corresponding C 1s bands as a function of both the etched depth during GCIB analysis and periods of immersion in PBS solution. Considering the structure of the glycolic and lactic components in the PLGA sample (cf. Figure E
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The Journal of Physical Chemistry C composition) were disregarded since most data referred to a bulk evaluation of the consequences of degradation.4,13,15 In this work we have shown that oxygen plasma induces topographical modifications with a high increase in surface roughness due to surface polymer etching mainly with dramatic consequences in enhancing the degradation rate. Etching rate values ∼5 × 10−5 cm/min on PLGA surfaces after similar oxygen plasma treatment8 have demonstrated that etching effect increases the roughness and generates an irregular morphology with cavities and polymeric aggregates, enabling the access of the simulant to inner zones of the sample. Meanwhile, the similar evolution of the [−COO−]/[C− C(H)] ratio for different GCIB sputtering times obtained for the three plasma-treated samples sustains that the plasma treatment has deeply affected the PLGA foil, probably far beyond the 400 nm thickness of the sputtered depths analyzed in this work. Anderson et al.14 have pointed out that the PLGA crystallinity can increase during the degradation process in PBS solution, conferring more resistance to the hydrolysis reaction. It is likely that plasma treatment not only induces the oxygen functionalization of the PLGA film but also prevents these crystallization processes and promotes the mobility and release of polymeric fragments. However, the originated cross-linking usually limits the polymer chain mobility, e.g., the fraction of mobile groups which can orientate or migrate from the surface into the polymer bulk,10 which can be interpreted as a resistance to the degradation. However, oxygen functionalization promotes an increase in the hydrophilicity, especially for the lactic acid monomers. Therefore, the limited mobility by the cross-linking can favor the hydrolysis of hydrophilic glycolic and lactic acid fragments present at the treated surface because they are preferentially affected by the physiological media and released before migrating into the bulk. Therefore, combined hydration and initial degradation have been observed in the plasma-treated PLGA film after a first week in PBS, the period during which it occurs a notable bond breaking hydrolysis contributing to separate the glycolic from the lactic units as the prominent drop of the [−COO−]/[−C−C(H)] ratio (c.f., Figure 6) indicates. The degradation rate of the plasma-treated PLGA film slows down afterward.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (C.L-S.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Junta de Andaluciá (Projects FQM6900, Pi-0047-2013, and FQM-2265), the MINECO (Projects CONSOLIDER CSD2008-00023, MAT2013-40852-R, MAT2013-42900-P, and RECUPERA2020-1.4.1), and the Framework Agreement with Abengoa Research for financial support.
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REFERENCES
(1) Gentile, P.; Chiono, V.; Carmagnola, I.; Hatton, P. An Overview of Poly(Lactic-Co-Glycolic) Acid (Plga)-Based Biomaterials for Bone Tissue Engineering. Int. J. Mol. Sci. 2014, 15, 3640−3659. (2) Ghaffar, A.; Verschuren, P. G.; Geenevasen, J. A. J.; Handels, T.; Berard, J.; Plum, B.; Dias, A. A.; Schoenmakers, P. J.; van der Wal, S. Fast in Vitro Hydrolytic Degradation of Polyester Urethane Acrylate Biomaterials: Structure Elucidation, Separation and Quantification of Degradation Products. J. Chromatogr. A 2011, 1218, 449−458. (3) Shirazi, R. N.; Aldabbagh, F.; Erxleben, A.; Rochev, Y.; McHugh, P. Nanomechanical Properties of Poly(Lactic-Co-Glycolic) Acid Film During Degradation. Acta Biomater. 2014, 10, 4695−4703. (4) Vey, E.; Rodger, C.; Booth, J.; Claybourn, M.; Miller, A. F.; Saiani, A. Degradation Kinetics of Poly(Lactic-Co-Glycolic) Acid Block Copolymer Cast Films in Phosphate Buffer Solution as Revealed by Infrared and Raman Spectroscopies. Polym. Degrad. Stab. 2011, 96, 1882−1889. (5) Croll, T. I.; O’Connor, A. J.; Stevens, G. W.; Cooper-White, J. J. Controllable Surface Modification of Poly(Lactic-Co-Glycolic Acid) (Plga) by Hydrolysis or Aminolysis I: Physical, Chemical, and Theoretical Aspects. Biomacromolecules 2004, 5, 463−473. (6) Ehrenfried, L.; Patel, M.; Cameron, R. The Effect of Tri-Calcium Phosphate (Tcp) Addition on the Degradation of Polylactide-CoGlycolide (Plga). J. Mater. Sci.: Mater. Med. 2008, 19, 459−466. (7) Esposito, A. R.; Kamikawa, C. M.; Lucchesi, C.; Ferreira, B. M. P.; Duek, E. A. d. R. Benefits of Oxygen and Nitrogen Plasma Treatment in Vero Cell Affinity to Poly(Lactide-Co-Glycolide Acid). Mater. Res. 2013, 16, 695−702. (8) Fortunati, E.; Mattioli, S.; Visai, L.; Imbriani, M.; Fierro, J. L. G.; Kenny, J. M.; Armentano, I. Combined Effects of Ag Nanoparticles and Oxygen Plasma Treatment on Plga Morphological, Chemical, and Antibacterial Properties. Biomacromolecules 2013, 14, 626−636. (9) Shen, H.; Hu, X.; Yang, F.; Bei, J.; Wang, S. Combining Oxygen Plasma Treatment with Anchorage of Cationized Gelatin for Enhancing Cell Affinity of Poly(Lactide-Co-Glycolide). Biomaterials 2007, 28, 4219−4230. (10) Morent, R.; De Geyter, N.; Desmet, T.; Dubruel, P.; Leys, C. Plasma Surface Modification of Biodegradable Polymers: A Review. Plasma Processes Polym. 2011, 8, 171. (11) López-Santos, M. C.; Yubero, F.; Espinós, J. P.; González-Elipe, A. R. Non-Destructive Depth Compositional Profiles by Xps PeakShape Analysis. Anal. Bioanal. Chem. 2010, 396, 2757−2768.
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CONCLUSION For the first time PLGA membrane film degradation under simulated physiological conditions has been studied by GCIBXPS depth profiling. The results show clear evidence of a heterogeneous hydrolysis at the surface and in-depth. Preferential glycolic unit loss occurs at the PLGA surface after 3 week in PBS solution whereas a slow decrease of other carbon functionalities occurs in the inner regions of the film. Degradation rate increases after oxygen plasma treatment which affects both the lactic and glycolic components across an extended thickness in the interior of the polymer (at least a few hundred nanometers) thanks to the enhanced roughness. The results confirm a higher resistance of the lactic units to degradation although the plasma treatment is able to stimulate it.
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Figures S1−S3, corresponding to the SEM images of PLGA-U and plasma-treated PLGA-O2 films before and after 1 week of degradation, GCIB depth profile atomic concentration of PLGA-U and plasma-treated PLGA-O2 membranes, and an example of XPS in-depth profiles for PLGA-O2/3W membranes after 3 weeks degraded in PBS solution, respectively (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05011. F
DOI: 10.1021/acs.jpcc.5b05011 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.5b05011 J. Phys. Chem. C XXXX, XXX, XXX−XXX
