Jingwei
Fan
,
Jiong
Zou
*,
Xun
He
,
Fuwu
Zhang
,
Shiyi
Zhang
,
Jeffery E.
Raymond
and
Karen L.
Wooley
*
Departments of Chemistry and Chemical Engineering, Laboratory for Synthetic-Biologic Interactions, Texas A&M University, P.O. BOX 30012, 3255 TAMU, College Station, TX 77842, USA. E-mail: jiong.zou@chem.tamu.edu; wooley@chem.tamu.edu; Tel: +1-979-845-4077
First published on 18th October 2013
The simple copolymerization of N-carboxyanhydride (NCA) monomers is utilized to generate copolypeptides having a combination of α-helix and β-sheet sub-structures that, when grown from a solvophilic synthetic polymer block segment, are capable of driving mechano-responsive supramolecular sol-to-gel-to-sol and sol-to-gel-to-gel transitions reversibly, which allow also for injection-based processing and self-healing behaviors. A new type of polypeptide-based organogelator, methoxy poly(ethylene glycol)-block-poly(γ-benzyl-L-glutamate-co-glycine) (mPEG-b-P(BLG-co-Gly)), is facilely synthesized by statistical ring-opening copolymerizations (ROPs) of γ-benzyl-L-glutamate (BLG) and glycine (Gly) NCAs initiated by mPEG-amine. These systems exhibit tunable secondary structures and result in sonication stimulus responsiveness of the organogels with the polypeptide segment variation, controlled by varying the ratio of BLG NCA to Gly NCA during the copolymerizations. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) studies indicate the α-helical component decreases while the β-sheet content increases systematically with a higher mole fraction of Gly in the polypeptide segment. The supramolecular assembly of β-sheet nanofibrils, having a tunable width over the range of 10.4–14.5 nm with varied BLG to Gly ratio, are characterized by transmission electron microscopy (TEM). The further self-assembly of these nanostructures into 3-D gel networks within N,N-dimethylformamide (DMF) occurs at low critical gelation concentrations (CGC) (lowest ca. 0.6 wt%). Increased BLG to Gly ratios lead to an increase of the α-helical component in the secondary structures of the polypeptide segments, resulting in wider and more flexible nanofibrils. The presence of α-helical component in the polymers enhances the stability of the organogels against sonication, and instantaneous gel-to-gel transitions are observed as in situ reconstruction of networks occurs within the gelled materials after sonication. In marked contrast, the β-sheet-rich gel, prepared from mPEG-b-PGly, exhibits an instant gel-to-sol transition after sonication is applied. The CGC concentration and stiffness of this mPEG-b-P(BLG-co-Gly) organogel system can be tuned by simply varying the percentages of α-helix and β-sheet in the secondary structures through control of the BLG to Gly ratio during synthesis. The mechanical properties of these organogels are studied by dynamic mechanical analyses (DMA), having storage moduli of ca. 12.1 kPa at room temperature. The injectability and self-healing capabilities are demonstrated by direct observation of the macroscopic self-healing behavior experiment.
Polypeptide-based materials have been widely used in supramolecularly-assembled hydrogels12 and organogels.13 Their broad applicability is due to the precisely-defined nano- and microstructures derived from supramolecular hierarchical assemblies of polypeptides and the variety of stimuli responses available to natural and synthetic polypeptides, which are often unobtainable from non-polypeptide-based materials. Secondary structures from the self-assembly of polypeptides, especially α-helix and β-sheet conformations, have been proposed as driving forces for the construction of organogels. For example, Kim et al. reported poly(ferrocenysilane)-block-poly(γ-benzyl-L-glutamate) (PFS-b-PBLG) as an organogelator in toluene with thermoreversible gel-to-sol behavior, originating from the monolayer 1-D stacking of the α-helical polypeptide segments (PBLG) into nanoribbons.13a In 2008, Cameron and co-worker reported thermoreversible organogelators, poly(O-benzyl-L-threonine)-block-poly(γ-benzyl-L-glutamate) (PBnT-b-BnE) and poly(O-benzyl-L-threonine)-block-poly(ε-N-Boc-L-lysine) (PBnT-b-BocK); both of which contained one α-helix- and one β-sheet-producing polypeptide segment.13c The gelation was found to be driven by β-sheet formation and stacking into nanotapes by the PBnT segments. At the same time the α-helical structure from the PBnE or PBocK block, in an energetically-unfavorable parallel configuration, triggered twisting of the nanotapes in order to minimize these interactions. At present, tunable sonication-responsive organogels from polypeptides have not been explored, and we were interested in investigating such systems, including the self-assembly and gelation behavior of hybrid statistical copolypeptides.
Recently, our group reported a chemically-reactive polypeptide-based organogelator, poly(ethylene glycol)-block-poly(DL-allylglycine) (PEG-b-PDLAG), containing a racemic PDLAG homopolypeptide block.14 Gel formation was driven by supramolecular assembly of β-sheet secondary structures into nano-fibrils in the nanodomain, and which underwent gel-to-sol transition when subjected to sonication stimulus. To explore the effect of constituency control of the secondary structures formed, particularly α-helix and β-sheet configurations, on the physical and mechanical properties, this current work involves a new type of organogelator, methoxy poly(ethylene glycol)-block-poly(γ-benzyl-L-glutamate-co-glycine) (mPEG-b-P(BLG-co-Gly)). These organogel systems exhibited tunable sonication-responsive properties by variation of the secondary structures from the hybrid copolypeptide without the need of elaborate design and multi-step syntheses. Secondary structures were modified by systematically altering the ratios of BLG to Gly, which, respectively, have preferences for α-helix and β-sheet configurations.15 The organogels from α-helix-rich diblock copolymers showed higher stability against sonication and instantaneous gel-to-gel transitions were observed. However, the β-sheet-rich organogel formulations exhibited immediate gel-to-sol transition when sonication was applied. Further, the injectability and self-healing abilities of these organogel systems were directly observed through assessment of the bulk macroscopic self-healing responses.
Thermogravimetric analysis (TGA) was performed under argon atmosphere using a Mettler-Toledo model TGA/SDTA851e (Mettler-Toledo, Inc., Columbus, OH), with a heating rate of 10 °C min−1. Measurements were analyzed using Mettler-Toledo STARe v. 7.01 software. Glass transition temperatures (Tg) were measured by differential scanning calorimetry (DSC) on a Mettler-Toredo DSC822®, with a heating rate of 10 °C min−1. Measurements were analyzed using Mettler-Toledo STARe v. 7.01 software. The Tg was taken as the midpoint of the inflection tangent, upon the second heating scan.
Wide-angle X-ray scattering (WAXS) was performed on a Bruker D8 Bragg–Brentano X-ray powder diffractometer. The sample was placed in the sample holder of a two circle goniometer, enclosed in a radiation safety enclosure. The X-ray source was a 2.2 kW Cu X-ray tube, maintained at an operating current of 40 kV and 40 mA. The X-ray optics was the standard Bragg–Brentano para-focusing mode with the X-ray diverging from a DS slit (1 mm) at the tube to strike the sample and then converging at a position sensitive X-ray Detector (Lynx-Eye, Bruker-AXS). The two-circle 250 mm diameter goniometer was computer controlled with independent stepper motors and optical encoders for the θ and 2θ circles with the smallest angular step size of 0.0001° 2θ. The software suit for data collection and evaluation was window based. Data collection was automated COMMANDER program by employing a DQL file and analysed by the program EVA.
Transmission electron microscopy (TEM) images were collected on a JEOL 1200 EX operating at 100 kV and micrographs were recorded at calibrated magnifications using a SLA-15C CCD camera. The final pixel size was 0.42 nm per pixel. Samples for TEM measurements were prepared as follows: 10 μL of the dilute solution (with a polymer concentration of 1 mg mL−1) was deposited onto a carbon-coated copper grid, and after 2 min, the excess of the solution was quickly wicked away by a piece of filter paper. The samples were then negatively stained with 1 wt% phosphotungstic acid (PTA) aqueous solution. After 30 s, the excess staining solution was quickly wicked away by a piece of filter paper and the samples were left to dry under vacuum overnight.
Sonication responsive experiments were performed in an ultrasonic homogenizer (maximum power, 150 W, 20 kHz, Model 150 V/T, Biologics, Inc.) equipped with a micro tip with a diameter of 3.81 mm, employing the power outlet of 45 W in the frequency of 20 kHz at room temperature.
Dynamic mechanical analysis (DMA) was performed on a Mettler Toledo TT-DMA system. DMA of 5 wt% samples in DMF were performed over 3 h in compression on a 3.2 mm thick, 10 mm diameter cylinder. Dynamic measurements were recorded over a range of 0.1 to 10 Hz at room temperature with static stress modulated to 2% compression and a dynamic force applied to provide ±1% deformation. Kinetic data presented were obtained as a single exponential decay using Origin Pro 8.1 software. The gel stiffness was quantified by the evaluation of the compression storage modulus (E′).16
Confocal images were taken using a FV 1000 system (Olympus) with a 10× objective (Olympus) and a 543 nm He–Ne laser excitation source. Emission was collected using a monochrometer set to a 100 nm band pass (560–660 nm) and a photomultiplier tube. Image analysis was performed in Fluoviow software (Olympus) and a 1.1 gamma correction was used to enhance feature contrast after scaling image contrast intensity to minimum and maximum threshold set points of 1% above dark count and 1% below maximum counts for both fluorescence and DIC images. Slides were generated by lightly compressing the gels between a cleaned glass microscope slide and a cleaned glass cover slip after dye addition.
Polymer |
1![]() ![]() |
DPnb | Number of repeat unitb | χ Gly b (%) | M n b (kDa) | CGCc (wt%) | |
---|---|---|---|---|---|---|---|
1, BLG | 2, Gly | ||||||
a The mPEG45-NH2 (3) were used as macroinitiators in all polymerizations with monomer![]() ![]() ![]() ![]() |
|||||||
4 | 1![]() ![]() |
33 | 33 | 0 | 0 | 9.2 | —d |
5 | 6![]() ![]() |
33 | 29 | 4 | 12 | 8.6 | —d |
6 | 3![]() ![]() |
31 | 25 | 6 | 19 | 7.8 | 2.5 |
7 | 2![]() ![]() |
31 | 22 | 9 | 29 | 7.3 | 1.5 |
8 | 1.5![]() ![]() |
30 | 19 | 11 | 37 | 6.8 | 1.2 |
9 | 1.2![]() ![]() |
30 | 17 | 13 | 43 | 6.5 | 0.9 |
10 | 1![]() ![]() |
32 | 16 | 16 | 50 | 6.4 | 0.6 |
11 | 1![]() ![]() |
32 | 6 | 26 | 81 | 4.8 | 0.8 |
12 | 0![]() ![]() |
33 | 0 | 33 | 100 | 3.9 | 2.0 |
To investigate the incorporated ratios of the two monomers in the copolymers under different feed ratios, 1H NMR spectroscopy characterization was employed using deuterated trifluoroacetic acid (TFA-D) as the solvent, which was capable of breaking the strong hydrogen-bonding in these systems and maintaining the polymers in the solution state (Fig. 1a). The average degrees of polymerization, numbers of repeat units for BLG and Gly, mole fractions of Gly in the polypeptide segments, and number-average molecular weights of the block copolymers were determined by comparison of the methylene proton intensities of the mPEG chain resonating at ca. 3.83 ppm (b in Fig. 1a) with the intensities of the PBLG benzyl methylene protons resonating at ca. 5.07 ppm (j in Fig. 1a), the PBLG methine proton at ca. 4.65 ppm (f in Fig. 1a), and the PBLG phenyl protons at ca. 7.22 ppm (k in Fig. 1a), and the integration of the Gly methylene protons at ca. 4.15 ppm (d in Fig. 1a), and were found to be in agreement with the stoichiometry used in the polymer syntheses (Table 1). For both BLG (Fig. 1b) and Gly (Fig. 1c), the calculated numbers of repeat units from 1H NMR spectroscopy showed linear correlations with the theoretical numbers of repeat units, which were obtained according to the feed ratios of the two monomers and the ratios of monomers to macroinitiator, indicating that BLG and Gly NCA monomers had similar reactivities and were fully consumed during the polymerizations.20
![]() | ||
Fig. 1 (a) 1H NMR spectrum of polymer 10 dissolved in TFA-D. Calculated numbers of (b) BLG and (c) Gly repeat units as a function of theoretical numbers of repeat units in the polymers 4–12. |
The ability of mPEG-b-P(BLG-co-Gly) diblock copolymer to form organogels in different solvents was investigated and the properties of DMF gels were studied comprehensively. To prepare the organogel systems, the denoted weight fractions of polymer and DMF were mixed and stable organogels were generated after standing at room temperature for up to 3 days. To investigate the solvent effect in the gel formation, several organic solvents were screened. The polymers could not be dissolved into non-polar or low-polar organic solvents, such as hexane, dichloromethane (DCM), chloroform and tetrahydrofuran (THF), while gelation was observed in relatively high-polarity organic solvents, such as dioxane and dimethyl sulfoxide (DMSO). The critical gel concentrations (CGCs, Table 1) were obtained by preparing different polymer concentrations in DMF and measured by the test tube inversion method at room temperature.21 By comparing polymer 11 (0.8 wt%) with polymer 10 (0.6 wt%), it could be concluded that the CGC decreased with increased Gly content in the polypeptide segment when the Gly content was less than 50 mol%, while this trend reversed with Gly at greater than 50 mol%. The turbidities of the organogels at 2.5 wt% also changed from being transparent to opaque with increased Gly content in the polypeptide segments (Fig. S1 in ESI†).
In order to further understand the properties of these organogel systems, the supramolecular structures of the dried gels from polymers 4–12 were investigated by ATR-FTIR (Fig. 2a). The characteristic absorbances of secondary structures were clearly observed in the FTIR spectra, especially in the region of amide I band (1700–1600 cm−1). The absorbances at 1650 cm−1 (amide I) and at 1547 cm−1 (amide II) were attributed to α-helical conformations.22 The β-sheet secondary structure was observed at 1675 cm−1 and 1626 cm−1 (both in amide I), in combination with a peak at 1520 cm−1 (amide II).23 Other secondary structure populations, such as random coils or turns, showed absorbances at 1600–1620 cm−1, 1640–1650 cm−1, 1660–1670 cm−1 and 1680–1700 cm−1.24 The peak at 1728 cm−1 corresponded to the CO stretch of the benzyl ester group in the side chain of BLG.16 Qualitatively, the absorption intensity assigned to the peaks of β-sheet secondary structures increased with higher mole fraction of Gly in the polypeptide segment, while the intensities of peaks of α-helices decreased correspondingly.
![]() | ||
Fig. 2 (a) ATR-FTIR spectra and (b) secondary structure populations of polymers 4–12 in the solid state. |
Further quantitative analyses of secondary structures were conducted by employing second derivative and deconvolution strategies to extract relative peak intensities of individual bands in the ATR-FTIR spectra. These analyses provided extraction of secondary structural components in the heavily populated amide I regions, followed by curve fitting to determine signal amplitude for each band (Fig. S2 in ESI†), as summarized in Fig. 2b.16,25 For each polymer, the supramolecular structures were composed of α-helix, β-sheet and other secondary structures, regardless of the difference in the chemical compositions. As the mole fraction of Gly in the polypeptide segment increased from 0% to 100%, the α-helical content decreased systematically from 65% to 6% of total band intensity and the β-sheet component raised gradually from 14% to 66%. The β-sheet contribution to secondary structure became dominant over the α-helix contribution with Gly mole fraction greater than 19% in the polypeptide segment. Other secondary structure band intensities remained relatively constant for all formulations.
The higher-order self-assembled structures of the dried gels from mPEG-b-P(BLG-co-Gly) diblock copolymers were also investigated by WAXS. The WAXS pattern for gel from polymer 12 (Fig. S3 in ESI†) gave a d spacing of 4.4 Å with stronger signals for PEG crystallization reflection, which was also demonstrated by the melting and crystallizing peaks in the DSC trace of polymer 12 (Fig. S4b in ESI†). This d spacing of 4.4 Å has been observed and assigned to antiparallel β-sheet formation in poly(L-alanylglycine)26 and the self-assembled PEG-b-β-strand-peptides.27 The WAXS patterns for gels from polymers 7, 10 and 11 had similar d spacings at 4.6 Å, 4.6 Å and 4.5 Å, respectively. These slightly increased d spacings suggested higher α-helix content in the secondary structures, which could enlarge the distance between adjacent polypeptide chains. The presence of α-helical content also appeared to have disrupted PEG chain order, observed as a lack of PEG crystallization peaks in the WAXS patterns of the gels from polymers 7, 10 and 11 and a lack of melting transition in the DSC trace of polymer 11 (Fig. S4a in ESI†).
TEM was employed to study the nanostructures of the organogels as related to the copolypeptide composition. The TEM samples were prepared with a polymer concentration of 1.0 mg mL−1 in DMF and stained by PTA aqueous solution. Bright-field TEM images of organogels from polymers 7, 10, 11 and 12 (Fig. 3a, c, e and g, respectively) showed fibrillar nanostructures with average widths of 14.5, 12.1, 11.5 and 10.4 nm. The narrow width distributions of nanofibrils from polymers 7, 10, 11 and 12 (Fig. 3b, d, f and h, respectively), obtained by counting 100 nanofibrils in the TEM images, indicated that all of the organogels had well-defined nanofibrillar morphologies. The average diameters of nanofibrils increased with a larger α-helical content in the secondary structures, which might be attributed to the formation of twisted fibrils (inset images at the upper right corners of Fig. 3a, c, e and g, respectively), originating from a combination of increased steric hindrance and chirality change with the introduced BLG component.13c,28 Furthermore, the β-sheet-rich polymer 12 showed more rigid nanofibrils when compared with the other fibril nanostructures from polymers with increased α-helical components.
Additional studies of the thermal properties and external stimuli-responsive features of the organogel systems from polymers 7, 10, 11 and 12 in DMF were also performed. Gels from all four polymers, with a concentration of 5 wt% in DMF, could withstand temperatures of 90 °C over 1 h, however, the four gels behaved quite differently after 30 s sonication at room temperature. The gels from polymers 7, 10 and 11 remained in the gel state, even with longer sonication time (ca. 30 min), while the gel from polymer 12 was transformed to a solution state after 30 s sonication (Fig. 4). This transition was reversible, with the gel reforming after being placed at room temperature for 3 h. TEM was also used to characterize the changes of nanostructural morphologies within the gels before and after sonication was applied. The gels from polymers 11 and 12, with an initial concentration of 5 wt% in DMF, were diluted to polymer solutions of 1.0 mg mL−1 after 30 s sonication, drop deposited, and stained with PTA aqueous solution to prepare the TEM samples. Alterations in the nanostructures for gels from polymers 11 and 12 were observed by comparison of TEM images before (Fig. 3e and g) and after (Fig. 5a and b) sonication. In the gel from polymer 11, longer nanofibrils disappeared after sonication and, instead, short nano-rods were observed. The ability of the gel states to reform and for the gel assemblies to undergo toughening was also demonstrated by dynamic mechanical analysis (DMA), in which a 20% increase in storage modulus was detected for the gel after sonication (Fig. S5 in ESI†). This sonication-promoted gel-to-gel transition with increased modulus implied that a temporary disruption of nanostructures occurred for polymer 11 during sonication and resulted in ordering that was even more stable, akin to an annealing process. Immediate reassembly of short nano-rods, in situ after sonication, facilitated the rebuilding of 3-D networks and resulted in a sonication-triggered gel-to-gel transition. However, in the gel produced from polymer 12, the longer nanofibrils were expressly converted into short nano-rods, which were well-aligned in nanodomains, but lacked longer range connectivity between clusters, ultimately resulting in a gel-to-sol transition. When the solution was maintained at room temperature for 3 h, the one-dimensional growth of nano-rods to longer nanofibrils resulted in reconstruction of the gel.
![]() | ||
Fig. 4 Images of organogels from polymers 7, 10, 11 and 12 in DMF (5.0 wt%) before (left) and after (right) sonication. |
While the nano- and supramolecular-scale properties could be correlated to the macroscale gel-to-gel and gel-to-sol transitions, it is also important to study the micro- and meso-scale structures of the gels in order to fully understand the gel behavior through all scales of self-interaction. To this effect, dye incubation (1 wt% 0.1 mM Alizarin Red S in DMF) in 5 wt% gels from polymers 11 and 12 were studied by laser scanning confocal microscopy (Fig. 6). It was observed that the gel homogeneity on the micro- and meso-scales was much higher in the organogel from polymer 12, both before and after sonication (Fig. 6.3 and 4) when compared to the gel of polymer 11 (Fig. 6.1 and 2). Observing the organogel from polymer 11 in the bulk (Fig. 6.1a), as well as in isolated regions (Fig. 6.1b–e), it was evident that both larger mesoscale domains (ca. 50 μm) and microscale (ca. 1–10 μm) structures could be observed. On sonication of the organogel from polymer 11 (Fig. 6.2), both types of structures remained on these scales and longer range interactions did not appear to be overtly changed in the gel, correlating well to the tube inversion and DMA findings. Pre-sonication, organogel from polymer 12 possessed a homogeneous background emission with a collection of proximate microdomains (ca. <1 to 5 μm). Also of note, the fluorescent image showed diffused (Fig. 6.3b) structure edges coincident with the differential imaging contrast (DIC) micrograph, but significantly broader than those in the gels from polymer 11, indicating extradomain interactions beyond that observed in DIC. Considering organogel from polymer 12 post-sonication (Fig. 6.4), it was observed that the higher emission regions were smaller and more numerous in the fluorescence channel, while in DIC the majority of microstructures had been eliminated. These findings indicated the generation of isolated, sub-diffraction scale domains, which lacked intrinsic long-range interactions.
The mechanical properties of the organogel systems were characterized by DMA on polymers 7, 10, 11 and 12 in the gel state at a concentration of 5 wt%, which was sufficiently high to produce a robust gel that could withstand high frequency operation. Frequency modulation showed systematic changes in the storage (E′) and loss (E′′) moduli of the organogels for polymers 7, 10, 11 and 12 (Fig. 7a). For each polymer, the loss modulus remained smaller than the storage modulus within the frequencies observed (0.1 to 10 Hz), demonstrating no gel-to-sol transition in this frequency range, which was also observed in the tan(δ) data (Fig. S6 in ESI†). The storage moduli at 1 Hz of the four gels were 3.0, 8.4, 11.6 and 3.9 kPa for the samples 7, 10, 11 and 12. With the mole fraction of Gly changing from 19% to 81%, the gel moduli increased by a factor of 4, demonstrating an enhancement of the strength of the gel systems. However, the storage modulus dropped significantly for the 100% Gly (polymer 12) system (Fig. 7b), in comparison to those copolypeptide block copolymer materials containing proportions of BLG.
Taking together the results of NMR, FTIR, WAXS, DSC, TEM, confocal and DMA analyses, a mechanism for organogel formation from the mPEG-b-P(BLG-co-Gly) diblock copolymers and the relative stabilities of the matrices to sonication is proposed (Scheme 2). The supramolecular self-assembly of the polypeptide segments into well-defined nanofibrils was driven by the formation of β-sheet secondary structure, as seen spectrally and microscopically, resulting in the construction of organogels. The presence of α-helical content in the secondary structures broadened the distance between the polypeptide chains and the width of nanofibrils. In the gel state, after sonication, the nanofibrillar structures were deconstructed into short nano-rods by cleaving the supramolecular interactions between polypeptide segments. In the α-helix-rich organogels, the reassembly of relatively less locally ordered and more flexible nano-rods, in combination with the maintenance of long-range interactions, resulted in the immediate reconstruction of organogels in situ as a sonication-triggered gel-to-gel transition. However, the failure to reconstruct 3-D network from more locally aligned and rigid nano-rods, and the disappearance of micro- and mesoscale interactions for the β-sheet-rich polymer, led to a sonication-triggered gel-to-sol transition. Ultimately, the effect on the external sonication-responsive property due to secondary structure control of statistical copolypeptide-based organogels provides a novel and facile method to modify the properties of stimuli-responsive materials by tuning the self-assembled nano- or microstructures without the need of precise control on the molecular level.
With the construction of organogels from mPEG-b-P(BLG-co-Gly) diblock copolymers being driven by β-sheet formation in the polypeptide segments, a self-healing property of the gel system was expected and demonstrated by a macroscopic self-healing experiment conducted on polymer 11 in DMF at a concentration of 20 wt%.29 Half of the organogel was blended with 0.01 mL of Alizarin Red S DMF solution for visibility (Fig. 8a). Both of the gel fractions were then loaded into syringes (Fig. 8b), extruded into a glass Petri dish and cut into pieces (Fig. 8c). Self-healing behavior began immediately after two pieces of organogel were placed into contact on the cut faces (Fig. 8d). After 3 min, the combined gel was sufficiently strong to be picked up by tweezers (Fig. 8e). Multiple pieces of cut gel could be reconnected together by attaching them one-by-one (Fig. 8f). The diffusion of Alizarin Red S was observed in the self-healed gel at room temperature, and afforded a single gel matrix with a homogeneous color after 3 days (Fig. 8g).
Footnote |
† Electronic supplementary information (ESI) available: Fig. S1–S6. See DOI: 10.1039/c3sc52504j |
This journal is © The Royal Society of Chemistry 2014 |