Letter pubs.acs.org/macroletters
2‑Hydroxyethylcellulose and Amphiphilic Block Polymer Conjugates Form Mechanically Tunable and Nonswellable Hydrogels Jukuan Zheng,† Seyoung Jung,‡ Peter W. Schmidt,‡ Timothy P. Lodge,†,‡ and Theresa M. Reineke*,† †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Herein, we report a family of mechanically tunable, nonswellable hydrogels that are based on a 2hydroxyethylcellulose (HEC) scaffold grafted with amphiphilic diblock copolymers. Poly[(oligo(ethylene glycol)methyl ether methacrylate]-b-poly(methyl methacrylate) (POEGMA-bPMMA) diblock copolymers of different compositions were created via RAFT polymerization using an alkyne terminated macro chain transfer agent (CTA). 2-Hydroxyethylcellulose (HEC) was modified with azide groups and the diblock copolymers were attached to the backbone via the coppercatalyzed click reaction to yield HEC-g-(POEGMA-b-PMMA) graft terpolymers. The resulting conjugates were soluble in DMF and able to form hydrogels upon simple solvent exchange in water. By increasing the concentration of the conjugates in DMF, the storage moduli of the hydrogels increased and the pore size in the gel decreased. After hydrogel formation, the structures were also found to be nonswellable (no macroscopic volume change upon incubation in water), which is an important feature for retaining size and mechanical integrity of the gels over time. Moreover, these materials were able to be electrospun into fibers that, upon hydration, formed fibrous hydrogel structures. The nonswellable and tunable mechanical properties of these materials imply great potential for a variety of applications such as personal care, active delivery, and tissue engineering.
H
ponsive poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) cross-linker.26 Moreover, Dai et al. have also presented a nonswellable hydrogel system by multivalent hydrogen-bonding with poly(N-acryloyl glycinamide).24 Cellulose is the most abundant, sustainable polymeric material on earth.27,28 Due to its low cost and benign impact on the environment and biological systems, cellulose and its derivatives have been widely applied to fabricate numerous materials including hydrogels with differing composite combinations, solvent systems, and physical/chemical crosslinking methods.28−36 However, it is well-known that cellulosebased hydrogels usually absorb large quantities of water, causing significant swelling.37,38 We sought to further take advantage of the benign and low-cost nature of cellulosic building blocks to construct robust and tunable hydrogel structures, while simultaneously limiting the swelling of these materials. Herein, 2-hydroxyethylcellulose (HEC) was grafted with a family of diblock amphiphilic copolymers, yielding four graft polymer conjugates. This study presents the first example of a cellulosebased, nonswellable hydrogel by exploiting strong noncovalent interactions through simple grafting of block polymers to the backbone. The modulus of these materials was found to be
ydrogels are three-dimensional networks of hydrophilic polymers cross-linked by covalent bonding or physical interactions.1−6 These materials have broad utility in numerous applications ranging from drug delivery, tissue engineering, and medical devices/implants to adhesives, contact lenses, and personal care products.7,8 In general, hydrogel structures are developed to possess a range of physical attributes such as elastomeric,9−11 self-healing,12−15 double-network,16,17 and stimuli-responsive18−21 properties. A major limiting factor for these systems is swelling (an increase in macroscopic volume while reaching equilibrium in solution), which alters the original size/shape of the hydrogel structure and can simultaneously compromise mechanical integrity.16 Indeed, swelling limits the viability of the hydrogel application, especially when a fixed volume and threshold of a particular mechanical property are prerequisites for practical use (i.e., medical devices, adhesives, personal care, etc.).22 New hydrogels that are designed to deter swelling are highly desirable. However, only a few examples of nonswellable hydrogels exist.22−24 For example, Kamata et al. have demonstrated a nonswellable, highly stretchable hydrogel, where careful incorporation of a thermoresponsive poly(ethyl glycidyl ether-co-methyl glycidyl ether) block in the polymeric network controls swelling behavior with temperature-induced shrinkage.25 Alternatively, Truong et al. have created a double network hydrogel system that exhibits nonswellable behavior by incorporating a second network that contains a thermores© XXXX American Chemical Society
Received: December 15, 2016 Accepted: January 12, 2017
145
DOI: 10.1021/acsmacrolett.6b00954 ACS Macro Lett. 2017, 6, 145−149
Letter
ACS Macro Letters
Table 1. Characterization Data Including Molecular Weights and Dispersities of the Four Alkyne-POEGMA-b-PMMA Diblock Copolymers (Performed by SEC Equipped with a Multi-Angle Light Scattering Detectors) and Subsequent HEC-P# Conjugates (Performed by SLS) Resultsa block polymer P1 P2 P3 P4
Mw (g/mol)
Đ
graft terpolymer
× × × ×
1.32 1.31 1.36 1 38
HEC-P1 HEC-P2 HEC-P3 HEC-P4
6.6 7.7 1.2 1.4
104 104 105 105
Mw (g/mol)
Rg (nm)
grafts per chain
× × × ×
71 ± 7 46 ± 4 45 ± 3 104 ± 6
41 21 9 59b
2.8 1.7 1.3 8.4
106 106 106 106
a
The grafts per chain is the average number of diblock copolymer grafted to each cellulose backbone. bThis grafts per chain is likely due to aggregation and, thus, does not reflect the true value.
Scheme 1. (A) Structure of the Alkyne-POEGMA-b-PMMA Block Polymers That Are Conjugated to HEC-(N3)n; (B) Demonstration of Hydrogel Formation with the Chemically Conjugated Structure after Dissolving in DMF and Exchange with Water; (C) Physical Mixing of the HEC-(N3)n and POEGMA-b-PMMA Diblock Copolymers at the Same Concentration Results in a Solution That Does Not Gel
These four different diblock copolymers (P#; # ranging from 1 to 4) were conjugated to HEC-(N3)n using the coppercatalyzed click reaction.39 Successful coupling was demonstrated by the disappearance of the azide peak around 2100 cm−1 by FTIR (Figure S4, Supporting Information). The molar mass and size (radius of gyration, Rg) of the graft terpolymers were examined in detail with static light scattering using a Berry-modified Zimm analysis.40 Table 1 shows the size (Rg), weight-averaged molar mass (Mw), and calculated number of grafts per chain of the HEC-P# (# ranges from 1 to 4, corresponding to the product from the reaction between HEC(N3)n and P#) conjugates in dimethylformamide (DMF). It was found that, as the molecular weight of the conjugated diblock copolymer increased from P1 to P3, the final HEC-P# Mw and Rg decreased. HEC-P4 was the exception, which exhibited the highest Mw and Rg. Graft efficiencies were calculated using the Mw of P# and HEC-P#. Considering the graft efficiency calculated from the average molar masses of the cellulose, polymer P#, and HEC-P# (Table 1) data, this result implies that, as the molecular weight of P# increases, a decrease in grafting efficiency results. This is likely due to steric hindrance of the longer block polymer, inducing a lower molar mass of the final HEC-P# conjugate. HEC-P4 appeared not to be molecularly dissolved in DMF (aggregation was noticed based on the observation that it could not pass through a 450 nm syringe filter). Thus, the reported Mw and calculated
tunable based on molecular weight and concentration of the grafted terpolymers in solution. Hydrogel fibers were also generated by electrospinning, leading to potential application as fibrous hydrogel scaffolds. HEC was selected as a platform for functionalization due to its better solubility in organic solvents and thus ease in solubility and chemical modification. Hydroxyl groups on the backbone of HEC were converted to azide groups by reacting with thionyl chloride and sodium azide, resulting in HEC(N3)n. The HEC-(N3)n was characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Successful conversion of hydroxyl groups was confirmed with the observation of an azide stretch at 2100 cm−1 (Figure S4, Supporting Information). Next, a series of four poly[oligo(ethylene glycol) methyl ether methacrylate]-b-poly(methyl methacrylate) (POEGMA-b-PMMA) diblock copolymers were synthesized by reversible addition−fragmentation chain transfer (RAFT) polymerization using an alkyne-terminated chain transfer agent. The obtained alkyne-functionalized POEGMAb-PMMA structures were characterized by 1H NMR spectroscopy and size-exclusion chromatography (SEC; see Supporting Information, Figures S2 and S3). Four diblock copolymers with different POEGMA and PMMA block lengths were obtained: alkyne-POEGMA75-b-PMMA116 (P1), alkyne-POEGMA75-bPMMA204 (P2), alkyne-POEGMA139-b-PMMA212 (P3), and alkyne-POEGMA139-b-PMMA324 (P4) (Table 1). 146
DOI: 10.1021/acsmacrolett.6b00954 ACS Macro Lett. 2017, 6, 145−149
Letter
ACS Macro Letters number of grafts per chain of HEC-P4 in Table 1 likely represent aggregates rather than isolated molecularly dissolved conjugates. It should be noted that we were unable to utilize further characterization techniques to support final product molecular weights. In the 1H NMR spectra, resonances from the HEC-(N3)n backbone were not resolved from the grafted diblock copolymer peaks (Supporting Information, Figure S5). Additionally, SEC characterization of the HEC-P1−4 terpolymer conjugates was also attempted, but the materials were found to strongly interact with the SEC columns in a number of mobile phase trials, including DMF. To fabricate the hydrogels, each HEC-P# terpolymer conjugate was dissolved in DMF. Precursor solutions with concentrations of 5, 10, and 20% w/v (mg/μL) were investigated with each of the HEC-P1−4 terpolymer conjugates to obtain different hydrogel samples. The viscous solutions were added into an O-ring as a structural template in a glass dish. The gel was formed by complete coverage of the material in the O-ring with 5 mL of water, promoting solvent exchange (10 min, Scheme 1) and gelation. Translucent hydrogels were obtained as shown in Scheme 1. As a control, samples of the HEC-(N3)n precursor and block copolymer were physically mixed in DMF, added to an O-ring, and subjected to water exchange. These control solutions did not lead to hydrogels by visual inspection (Scheme 1). This result demonstrates that, as a chemically conjugated structure, the self-assembly of grafted diblock copolymers POEGMA-b-PMMA (1−4 conjugates) promote noncovalent interactions via physical cross-linking of the PMMA blocks. This also points to the establishment of a continuous three-dimensional network that contains POEGMA blocks facilitating hydrophilicity and, thus, water retention. Small angle X-ray scattering (SAXS) experiments were performed to characterize the microstructure of the HEC-P# hydrogels. For all the hydrogel samples investigated, a broad shoulder was present between a q range of 9 × 10−3 to 2 × 10−2 Å−1 (q = 2π/d). This corresponds to features with an average size ranging from 30 to 80 nm (Figure S6, Supporting Information), which we assign to microphase separation of the PMMA-rich domains from the hydrophilic blocks plus water. There is also a power law relationship at low q for all samples investigated that indicates the sample is heterogeneous beyond the lowest length scale measured. To characterize the structure of the hydrogels, cryogenic scanning electron microscopy (cryo-SEM) was also utilized (Figure 1A−C), which clearly shows nanoporous networks of HEC-P2 hydrogels at three different concentrations. The pore sizes of these three different hydrogels were calculated using ImageJ. As the concentration was increased from 5 to 10 to 20%, the average pore size in the hydrogels systematically decreased (260 ± 130 nm, 220 ± 100 nm, and 100 ± 30 nm, respectively). Although we do not observe a substantial change in pore size when the concentration increases from 5 to 10%, we do observe an increase in the fiber size and the flattening of the fibers (compare Figure 1A and B). After successfully preparing the hydrogels from each of the HEC-P# conjugates with varying block lengths, the mechanical properties of each gel were studied and compared. The storage moduli of each hydrogel sample (HEC-P1−4 each created at 5, 10, and 20% w/v) were measured and the data are summarized in Figure 1D (see Supporting Information, Table S1, for all tabulated data). The modulus of the hydrogels increased with increasing precursor solution concentration. When comparing the gels formed from graft terpolymers of different molecular
Figure 1. Cryo-scanning electron microscopy (cryo-SEM) images of HEC-P2 hydrogels at (A) 5, (B) 10, and (C) 20% (w/v). The scale bar represents 500 nm. The average pore size of each image was calculated using ImageJ with data from 50 pores, the longest axis length of each pore was used as its pore size. (D) Storage moduli of obtained hydrogels from the four HEC-P1−4 conjugates at three different concentrations. A dynamic oscillatory strain sweep was performed at 25 °C and 1 rad/s to determine the linear viscoelastic region (LVR). Then a dynamic frequency sweep was conducted at 0.4% strain over the range of 0.1 to 10 rad/s at 25 °C to get the storage moduli values. The HEC-P1 at the 5% concentration was too soft to form a disk-shaped hydrogel needed for measurement. Error bars denote the standard deviation of four runs for each sample. Experimental methods and details of all data are available in the Supporting Information.
weights at equivalent concentrations, it was clearly observed that longer hydrophobic PMMA chain lengths lead to stiffer hydrogels. Importantly, the mechanical properties can be tuned with these variables; a three orders of magnitude increase in the storage modulus (∼101−104 Pa) can be obtained simply by increasing the concentration of the original terpolymer solution or changing the graft length conjugated to HEC. Considering the influence of hydrogel modulus on a number of different applications, this class of materials may be promising as tunable scaffolds to induce cell infiltration, growth, and differentiation.41 Furthermore, the swelling behavior of these as-prepared hydrogels was further examined. Each of the four different HEC-polymer conjugates were fabricated into hydrogels using the same method mentioned above by soaking the precursor DMF solution containing each terpolymer at three concentrations (5, 10, and 20%). Upon submersion in water and during the solvent exchange process, no volume change was observed in any sample (Supporting Information, Figure S7). The prepared hydrogels were then removed from the O-ring and were further incubated at room temperature completely submerged in water to observe their swelling behavior over time. The swelling ratio (Q = Vt/V0 × 100%) was calculated based on the macroscopic hydrogel volume change from the initial state (V0) to the volume at time t (Vt).25 We defined the initial time as the time point when a hydrogel sample was prepared. As shown in Figure 2, the HEC-P2, HEC-P3, and 147
DOI: 10.1021/acsmacrolett.6b00954 ACS Macro Lett. 2017, 6, 145−149
Letter
ACS Macro Letters
Figure 3. Optical microscope images of (A) pristine electrospun fibers and (B) the same hydrogel fibers after soaking in water. Slight swelling is visually observed immediately upon addition of water. The black dots in (B), indicated with arrows, are water residue on the glass slide. Scale bars represent 1000 μm.
3B) upon submersion in water, denoting promising application as a stable fibrous hydrogel scaffolding material. In conclusion, we have demonstrated the first example of a cellulose-based nonswellable hydrogel through grafting of POEGMA-b-PMMA polymers to HEC. The nonswellable hydrogel structures were formed through noncovalent crosslinking of the hydrophobic PMMA blocks, which are presented on the exterior of the graft terpolymer. The modulus and nanoporous structure of the hydrogels can be tuned by altering the cellulose−polymer conjugate solution concentration and the molecular weight of the diblock copolymer graft. To expand the potential utility of these structures, the material was electrospun into fibers and through hydration, fibrous hydrogels were formed. More broadly, the cellulose-based hydrogel scaffolds and materials design principal presented herein can be used to create numerous nonswellable gel platforms for a host of next-generation applications.
Figure 2. Representative hydrogel swelling test images of the (A) asprepared hydrogel, (B) 1, (C) 2, and (D) 20 days after incubation in water at room temperature. The scale bar represents 1 cm for all images. (E) Plot of the macroscopic volume change as a function of time for the HEC-P1−4 gels formed from 10% w/v solutions (other concentrations exhibited the same behavior as the 10% w/v samples.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00954. The synthesis and characterization of polymers and the conjugates, fabrication, and characterization of the hydrogel and fibrous hydrogel (PDF).
HEC-P4 hydrogels maintained their original sizes even after 20 days of incubation in water. The HEC-P1 hydrogel was found to swell to about 130% after incubation in water overnight, which was attributed to the presence of the shortest PMMA block length (and likely weaker hydrophobic noncovalent interactions). This phenomenon also suggests that the hydrogels are not in an equilibrium state upon fabrication. In comparison, the HEC-P2, HEC-P3, and HEC-P4 hydrogels all maintained their original volume over 20 days in water, even for the 5% concentration, even though they formed very soft materials. This nonswelling behavior departs from previous observations with cellulosic hydrogels, which tend to absorb large quantities of water and significantly swell compared to their as-prepared states.37,38 We speculate that the nonswellable nature of these HEC-P2−4 hydrogels results from hydrophobic interactions facilitated by the glassy PMMA blocks along the graft terpolymer structure. The PMMA blocks likely create noncovalent cross-linking points that restrict the network from further swelling. Previous work has also shown that fibrous hydrogels are promising biomimetic scaffolds to mimic the fibrous structure of extracellular matrix for biomedical applications.42,43 As a representative example, a 17% (w/v) HEC-P3 solution in DMF was electrospun under a voltage of 10 kV and a 12 cm receiving distance. As shown in Figure 3A, fibers with diameters around 6 μm were obtained. The fibers were then submerged in deionized water and minimal swelling was observed (Figure
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Timothy P. Lodge: 0000-0001-5916-8834 Theresa M. Reineke: 0000-0001-7020-3450 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors gratefully acknowledge support from the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013. We thank Goeun Heo and Professor Alptekin Aksan in the Department of Mechanical Engineering at the University of Minnesota for helping with the electrospinning procedure. The SAXS data shown in this work was performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is 148
DOI: 10.1021/acsmacrolett.6b00954 ACS Macro Lett. 2017, 6, 145−149
Letter
ACS Macro Letters
(31) Yang, J.; Zhang, X.-M.; Xu, F. Macromolecules 2015, 48, 1231− 1239. (32) Gandini, A.; Lacerda, T. M.; Carvalho, A. J. F.; Trovatti, E. Chem. Rev. 2016, 116, 1637−1669. (33) Yang, X.; Bakaic, E.; Hoare, T.; Cranston, E. D. Biomacromolecules 2013, 14, 4447−4455. (34) Torres-Rendon, J. G.; Köpf, M.; Gehlen, D.; Blaeser, A.; Fischer, H.; Laporte, L. D.; Walther, A. Biomacromolecules 2016, 17, 905−913. (35) Shi, Z.; Huang, J.; Liu, C.; Ding, B.; Kuga, S.; Cai, J.; Zhang, L. ACS Appl. Mater. Interfaces 2015, 7, 22990−22998. (36) Tang, H.; Butchosa, N.; Zhou, Q. Adv. Mater. 2015, 27, 2070− 2076. (37) Chang, C.; He, M.; Zhou, J.; Zhang, L. Macromolecules 2011, 44, 1642−1648. (38) Chang, C.; Duan, B.; Cai, J.; Zhang, L. Eur. Polym. J. 2010, 46, 92−100. (39) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596−2599. (40) Berry, G. C. J. Chem. Phys. 1966, 44, 4550−4564. (41) Engler, A. J.; Sen, S.; Sweeney, H. L.; Discher, D. E. Cell 2006, 126, 677−689. (42) Wade, R. J.; Bassin, E. J.; Rodell, C. B.; Burdick, J. A. Nat. Commun. 2015, 6, 6639. (43) Wade, R. J.; Bassin, E. J.; Gramlich, W. M.; Burdick, J. A. Adv. Mater. 2015, 27, 1356−1362.
supported by Northwestern University, E.I. DuPont de Nemours & Co., and The Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
■
REFERENCES
(1) Zheng, J.; Smith Callahan, L. A.; Hao, J.; Guo, K.; Wesdemiotis, C.; Weiss, R. A.; Becker, M. L. ACS Macro Lett. 2012, 1, 1071−1073. (2) Hunt, J. N.; Feldman, K. E.; Lynd, N. A.; Deek, J.; Campos, L. M.; Spruell, J. M.; Hernandez, B. M.; Kramer, E. J.; Hawker, C. J. Adv. Mater. 2011, 23, 2327−2331. (3) Deng, G.; Ma, Q.; Yu, H.; Zhang, Y.; Yan, Z.; Liu, F.; Liu, C.; Jiang, H.; Chen, Y. ACS Macro Lett. 2015, 4, 467−471. (4) Peppas, N. A.; Hilt, J. Z.; Khademhosseini, A.; Langer, R. Adv. Mater. 2006, 18, 1345−1360. (5) Peppas, N. A. Curr. Opin. Colloid Interface Sci. 1997, 2, 531−537. (6) Hodgson, S. M.; Bakaic, E.; Stewart, S. A.; Hoare, T.; Adronov, A. Biomacromolecules 2016, 17, 1093−1100. (7) Deng, X.; Korogiannaki, M.; Rastegari, B.; Zhang, J.; Chen, M.; Fu, Q.; Sheardown, H.; Filipe, C. D. M.; Hoare, T. ACS Appl. Mater. Interfaces 2016, 8, 22064−22073. (8) Du, X.; Zhou, J.; Shi, J.; Xu, B. Chem. Rev. 2015, 115, 13165− 13307. (9) Hong, S.; Sycks, D.; Chan, H. F.; Lin, S.; Lopez, G. P.; Guilak, F.; Leong, K. W.; Zhao, X. Adv. Mater. 2015, 27, 4035−4040. (10) Sun, J.-Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K. H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Nature 2012, 489, 133−136. (11) Cipriano, B. H.; Banik, S. J.; Sharma, R.; Rumore, D.; Hwang, W.; Briber, R. M.; Raghavan, S. R. Macromolecules 2014, 47, 4445− 4452. (12) Deng, C. C.; Brooks, W. L. A.; Abboud, K. A.; Sumerlin, B. S. ACS Macro Lett. 2015, 4, 220−224. (13) Luo, F.; Sun, T. L.; Nakajima, T.; Kurokawa, T.; Ihsan, A. B.; Li, X.; Guo, H.; Gong, J. P. ACS Macro Lett. 2015, 4, 961−964. (14) Li, G.; Wu, J.; Wang, B.; Yan, S.; Zhang, K.; Ding, J.; Yin, J. Biomacromolecules 2015, 16, 3508−3518. (15) He, L.; Szopinski, D.; Wu, Y.; Luinstra, G. A.; Theato, P. ACS Macro Lett. 2015, 4, 673−678. (16) Nakayama, A.; Kakugo, A.; Gong, J. P.; Osada, Y.; Takai, M.; Erata, T.; Kawano, S. Adv. Funct. Mater. 2004, 14, 1124−1128. (17) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Adv. Mater. 2003, 15, 1155−1158. (18) Azagarsamy, M. A.; Marozas, I. A.; Spaans, S.; Anseth, K. S. ACS Macro Lett. 2016, 5, 19−23. (19) Liang, Y.; Kiick, K. L. Biomacromolecules 2016, 17, 601−614. (20) Gupta, M. K.; Martin, J. R.; Werfel, T. A.; Shen, T.; Page, J. M.; Duvall, C. L. J. Am. Chem. Soc. 2014, 136, 14896−14902. (21) Yan, B.; Boyer, J.-C.; Habault, D.; Branda, N. R.; Zhao, Y. J. Am. Chem. Soc. 2012, 134, 16558−16561. (22) O’Shea, T. M.; Aimetti, A. A.; Kim, E.; Yesilyurt, V.; Langer, R. Adv. Mater. 2015, 27, 65−72. (23) Cai, L.; Wang, K.; Wang, S. Biomaterials 2010, 31, 4457−4466. (24) Dai, X.; Zhang, Y.; Gao, L.; Bai, T.; Wang, W.; Cui, Y.; Liu, W. Adv. Mater. 2015, 27, 3566−3571. (25) Kamata, H.; Akagi, Y.; Kayasuga-Kariya, Y.; Chung, U.-i.; Sakai, T. Science 2014, 343, 873−875. (26) Truong, V. X.; Ablett, M. P.; Richardson, S. M.; Hoyland, J. A.; Dove, A. P. J. Am. Chem. Soc. 2015, 137, 1618−1622. (27) Eichhorn, S. J.; Young, R. J.; Davies, G. R. Biomacromolecules 2005, 6, 507−513. (28) Chang, C.; Zhang, L. Carbohydr. Polym. 2011, 84, 40−53. (29) He, M.; Zhao, Y.; Duan, J.; Wang, Z.; Chen, Y.; Zhang, L. ACS Appl. Mater. Interfaces 2014, 6, 1872−1878. (30) Lott, J. R.; McAllister, J. W.; Arvidson, S. A.; Bates, F. S.; Lodge, T. P. Biomacromolecules 2013, 14, 2484−2488. 149
DOI: 10.1021/acsmacrolett.6b00954 ACS Macro Lett. 2017, 6, 145−149
