Polyamine-Polymeric Micelle Hybrid Hydrogel: Microscopic Properties of Crosslinkers Affecting Macroscopic Rheological Properties of Hydrogel
Author(s)
Chikara Yoshida,
Tomoki Ito,
Ryosuke Anzai,
Kengo Fukuda,
Keita Kinoshita,
Seiichi Sonotaki,
Taku Takami,
Yoshihiko Murakami
ABSTRACT
We have developed a hybrid hydrogel that is formed from a crosslinkable polymeric micelle and a polyamine. Under optimal conditions, the hydrogel rapidly formed in one second after a crosslinkable polymeric micelle solution was mixed with a polyamine solution. We could change the hydrogel’s gelation properties, such as the storage modulus and gelation time by tuning the molecular weights of block copolymers and by tuning the pH of the dissolving-solvent of the hydrogel’s constituent components. Furthermore, we have clarified here that the structural difference among the micelles acting as crosslinkers can affect the gelation properties of the hydrogel. According to our findings, the hydrogel that was formed from the polymeric micelles possessing a highly packed (i.e., well-entangled or crosslinked) inner core exhibited a higher storage modulus than the hydrogel that was formed from the polymeric micelles possessing a lowly packed structure. Our results demonstrate that a microscopic structural difference among crosslinkers can induce a macroscopic change in the properties of the resulting hydrogels. For medical applications, the hydrogel proposed in the present paper can encapsulate the hydrophobic compounds in crosslinkers (polymeric micelles) so that the hydrogel can be available as the biomaterial for their sustained release.
We have developed a hybrid hydrogel that is formed from a crosslinkable polymeric micelle and a polyamine. Under optimal conditions, the hydrogel rapidly formed in one second after a crosslinkable polymeric micelle solution was mixed with a polyamine solution. We could change the hydrogel’s gelation properties, such as the storage modulus and gelation time by tuning the molecular weights of block copolymers and by tuning the pH of the dissolving-solvent of the hydrogel’s constituent components. Furthermore, we have clarified here that the structural difference among the micelles acting as crosslinkers can affect the gelation properties of the hydrogel. According to our findings, the hydrogel that was formed from the polymeric micelles possessing a highly packed (i.e., well-entangled or crosslinked) inner core exhibited a higher storage modulus than the hydrogel that was formed from the polymeric micelles possessing a lowly packed structure. Our results demonstrate that a microscopic structural difference among crosslinkers can induce a macroscopic change in the properties of the resulting hydrogels. For medical applications, the hydrogel proposed in the present paper can encapsulate the hydrophobic compounds in crosslinkers (polymeric micelles) so that the hydrogel can be available as the biomaterial for their sustained release.
Cite this paper
Yoshida, C. , Ito, T. , Anzai, R. , Fukuda, K. , Kinoshita, K. , Sonotaki, S. , Takami, T. and Murakami, Y. (2015) Polyamine-Polymeric Micelle Hybrid Hydrogel: Microscopic Properties of Crosslinkers Affecting Macroscopic Rheological Properties of Hydrogel. Journal of Biomaterials and Nanobiotechnology, 6, 36-44. doi: 10.4236/jbnb.2015.61004.
Yoshida, C. , Ito, T. , Anzai, R. , Fukuda, K. , Kinoshita, K. , Sonotaki, S. , Takami, T. and Murakami, Y. (2015) Polyamine-Polymeric Micelle Hybrid Hydrogel: Microscopic Properties of Crosslinkers Affecting Macroscopic Rheological Properties of Hydrogel. Journal of Biomaterials and Nanobiotechnology, 6, 36-44. doi: 10.4236/jbnb.2015.61004.
References
[1] Morikawa, T. (2001) Tissue Sealing. The American Journal of Surgery, 182, 29S-35S.
http://dx.doi.org/10.1016/S0002-9610(01)00774-7 [2] MacGillivray, T.E. (2003) Fibrin Sealants and Glues. Journal of Cardiac Surgery, 18, 480-485.
http://dx.doi.org/10.1046/j.0886-0440.2003.02073.x [3] Czerny, M., Verrel, F., Weber, H., Muller, N., Kircheis, L., Lang, W., Steckmeier, B. and Trubel, W. (2000) Collagen Patch Coated with Fibrin Glue Components. Treatment of Suture Hole Bleedings in Vascular Reconstruction. The Journal of Cardiovascular Surgery, 41, 553-557. [4] Turner, A.S., Parker, D., Egbert, B., Maroney, M., Armstrong, R. and Powers, N. (2002) Evaluation of a Novel Hemostatic Device in an Ovine Parenchymal Organ Bleeding Model of Normal and Impaired Hemostasis. Journal of Biomedical Materials Research (Applied Biomaterials), 63, 37-47.
http://dx.doi.org/10.1002/jbm.10081 [5] Taguchi, T., Saito, H., Uchida, Y., Sakane, M., Kobayashi, H., Kataoka, K. and Tanaka, J. (2004) Bonding of Soft Tissues Using a Novel Tissue Adhesive Consisting of a Citric Acid Derivative and Collagen. Materials Science and Engineering: C, 24, 775-780.
http://dx.doi.org/10.1016/j.msec.2004.08.037 [6] Fukunaga, S., Karck, M., Harringer, W., Cremer, J., Rhein, C. and Haverich, A. (1999) The Use of Gelatin-Resorcin- Formalin Glue in Acute Aortic Dissection Type A. European Journal of Cardio-Thoracic Surgery, 15, 564-570.
http://dx.doi.org/10.1016/S1010-7940(99)00084-6 [7] Kumar, A., Maartens, N.F. and Kaye, A.H. (2003) Evaluation of the Use of BioGlue in Neurosurgical Procedures. Journal of Clinical Neuroscience, 10, 661-664. http://dx.doi.org/10.1016/S0967-5868(03)00163-2 [8] Singer, A.J. and Thode, H.C. (2004) A Review of the Literature on Octyl Cyanoacrylate Tissue Adhesive. The American Journal of Surgery, 187, 238-248.
http://dx.doi.org/10.1016/j.amjsurg.2003.11.017 [9] Ramakumar, S., Roberts, W.W., Fugita, O.E., Colegrove, P., Nicol, T.M., Jarrett, T.W., Kavoussi, L.R. and Slepian, M.J. (2002) Local Hemostasis during Laparoscopic Partial Nephrectomy Using Biodegradable Hydrogels: Initial Porcine Results. Journal of Endourology, 16, 489-494. http://dx.doi.org/10.1089/089277902760367458 [10] Nakayama, Y. and Matsuda, T. (1999) Photocurable Surgical Tissue Adhesive Glues Composed of Photoreactive Gelatin and Poly(Ethylene Glycol) Diacrylate. Journal of Biomedical Materials Research (Applied Biomaterials), 48, 511-521.
http://dx.doi.org/10.1002/(SICI)1097-4636(1999)48:4%3C511::AID-JBM17%3E3.3.CO;2-M [11] Ferland, R., Mulani, D. and Campbell, P.K. (2001) Evaluation of Sprayable Polyethylene Glycol Adhesion Barrier in a Porcine Efficacy Model. Human Reproduction, 16, 2718-2723.
http://dx.doi.org/10.1093/humrep/16.12.2718 [12] Wallace, D.G., Cruise, G.M., Rhee, W.M., Schroeder, J.A., Prior, J.J., Ju, J., Maroney, M., Duronio, J., Ngo, M.H., Estridge, T. and Coker, G.C. (2001) A Tissue Sealant Based on Reactive Multifunctional Polyethylene Glycol. Journal of Biomedical Materials Research (Applied Biomaterials), 58, 545-555. http://dx.doi.org/10.1002/jbm.1053 [13] Buchta, C., Hedrich, H.C., Macher, M., Hocker, P. and Redl, H. (2005) Biochemical Characterization of Autologous Fibrin Sealants Produced by CryoSeal® and Vivostat® in Comparison to the Homologous Fibrin Sealant Product Tissucol/Tisseel®. Biomaterials, 26, 6233-6241.
http://dx.doi.org/10.1016/j.biomaterials.2005.04.014 [14] Canonico, S. (2003) The Use of Human Fibrin Glue in the Surgical Operations. Acta Biomedica, 74, 21-25. [15] Siedentop, K.H., Park, J.J., Shah, A.N., Bhattacharyya, T.K. and O’Grady, K.M. (2001) Safety and Efficacy of Currently Available Fibrin Tissue Adhesives. American Journal of Otolaryngology, 22, 230-235.
http://dx.doi.org/10.1053/ajot.2001.24817 [16] Silver, F.H., Wang, M.C. and Pins, G.D. (1995) Preparation and Use of Fibrin Glue in Surgery. Biomaterials, 16, 891- 903. http://dx.doi.org/10.1016/0142-9612(95)93113-R [17] Ciapetti, G., Stea, S., Cenni, E., Sudanese, A., Marraro, D., Toni, A. and Pizzoferrato, A. (1994) Cytotoxicity Testing of Cyanoacrylates Using Direct Contact Assay on Cell Cultures. Biomaterials, 15, 63-67.
http://dx.doi.org/10.1016/0142-9612(94)90199-6 [18] Kaplan, M. and Baysal, K. (2005) In Vitro Toxicity Test of Ethyl 2-Cyanoacrylate, a Tissue Adhesive Used in Cardiovascular Surgery, by Fibroblast Cell Culture Method. The Heart Surgery Forum, 8, E169-E172.
http://dx.doi.org/10.1532/HSF98.20041126 [19] Murakami, Y., Yokoyama, M., Okano, T., Nishida, H., Tomizawa, Y., Endo, M. and Kurosawa, H. (2007) A Novel Synthetic Tissue-Adhesive Hydrogel Using a Crosslinkable Polymeric Micelle. Journal of Biomedical Materials Research Part A, 80, 421-427. http://dx.doi.org/10.1002/jbm.a.30911 [20] Murakami, Y., Yokoyama, M., Nishida, H., Tomizawa, Y. and Kurosawa, H. (2008) A Simple Hemostasis Model for the Quantitative Evaluation of Hydrogel-Based Local Hemostatic Biomaterials on Tissue Surface. Colloids and Surfaces B: Biointerfaces, 65, 186-189.
http://dx.doi.org/10.1016/j.colsurfb.2008.04.005 [21] Murakami, Y., Yokoyama, M., Nishida, H., Tomizawa, Y. and Kurosawa, H. (2009) In Vivo and in Vitro Evaluation of Gelation and Hemostatic Properties of a Novel Tissue-Adhesive Hydrogel Containing a Cross-Linkable Polymeric Micelle. Journal of Biomedical Materials Research (Applied Biomaterials), 91, 102-108.
http://dx.doi.org/10.1002/jbm.b.31378 [22] Uchida, Y., Fukuda, K. and Murakami, Y. (2013) The Hydrogel Containing a Novel Vesicle-Like Soft Crosslinker, a “Trilayered” Polymeric Micelle, Shows Characteristic Rheological Properties. Journal of Polymer Science Part B: Polymer Physics, 51, 124-131. http://dx.doi.org/10.1002/polb.23187 [23] Moroishi, H., Yoshida. C. and Murakami, Y. (2013) A Free-Standing, Sheet-Shaped, “Hydrophobic” Biomaterial Containing Polymeric Micelles Formed from Poly(ethylene glycol)-Poly(lactic acid) Block Copolymer for Possible Incorporation/Release of “Hydrophilic” Compounds. Colloids and Surfaces B: Biointerfaces, 102, 597-603.
http://dx.doi.org/10.1016/j.colsurfb.2012.08.050 [24] Ito, T., Yoshida. C. and Murakami, Y. (2013) Design of Novel Sheet-Shaped Chitosan Hydrogel for Wound Healing: A Hybrid Biomaterial Consisting of Both PEG-Grafted Chitosan and Crosslinkable Polymeric Micelles Acting as Drug Containers. Materials Science and Engineering: C, 33, 3697-3703. http://dx.doi.org/10.1016/j.msec.2013.04.056 [25] Zhang, Q., Wang, C.R., Babukutty, Y., Ohyama, T., Kogoma, M. and Kodama, M. (2002) Biocompatibility Evaluation of ePTFE Membrane Modified with PEG in Atmospheric Pressure Glow Discharge. Journal of Biomedical Materials Research, 60, 502-509.
http://dx.doi.org/10.1002/jbm.1294 [26] Lee, H.J., Lee, J.S., Chansakul, T., Yu, C., Elisseeff, J.H. and Yu, S.M. (2006) Collagen Mimetic Peptide-Conjugated Photopolymerizable PEG Hydrogel. Biomaterials, 27, 5268-5276. [27] Murakami, Y. and Hirata, A. (1999) Complex between α-Chymotrypsin and Poly(ethylene glycol) Catalytically Active in Organic Media. Biotechnology Techniques, 13, 545-548.
http://dx.doi.org/10.1023/A:1008987431361 [28] Murakami, Y. and Hirata, A. (1999) Poly(ethylene glycol)-α-Chymotrypsin Complex Catalytically Active in Anhydrous Isooctane. Journal of Bioscience and Bioengineering, 88, 441-443.
http://dx.doi.org/10.1016/S1389-1723(99)80224-2 [29] Murakami, Y., Hoshi, R. and Hirata, A. (2001) Borate Buffer Dramatically Enhances the Activity of Poly(ethylene glycol)-α-Chymotrypsin Complex Catalytically Active in Anhydrous Isooctane Than Conventional Phosphate Buffer Even at Low Concentration. Biotechnology Letters, 23, 125-129. http://dx.doi.org/10.1023/A:1010345318275 [30] Murakami, Y., Hoshi, R. and Hirata, A. (2003) Characterization of Polymer-Enzyme Complex as a Novel Biocatalyst for Nonaqueous Enzymology. Journal of Molecular Catalysis B: Enzymatic, 22, 79-88.
http://dx.doi.org/10.1016/S1381-1177(03)00009-2 [31] Murakami, Y. and Hirata, A. (1998) Enzymatic Synthesis of Peptides Review. Seibutsu-Kogaku Kaishi, 76, 238-254. [32] Ferry, J.D., Fitzgerald, E.R., Grandine, L.D. and Williams, M.L. (1952) Temperature Dependence of Dynamic Properties of Elastomers: Relaxation Distributions. Rubber Chemistry and Technology, 25, 720-729.
http://dx.doi.org/10.5254/1.3543437 [33] Dohno, C., Okamoto, A. and Saito, I. (2005) Stable, Specific, and Reversible Base Pairing via Schiff Base. Journal of the American Chemical Society, 127, 16681-16684.
http://dx.doi.org/10.1021/ja054618q
[1] Morikawa, T. (2001) Tissue Sealing. The American Journal of Surgery, 182, 29S-35S.
http://dx.doi.org/10.1016/S0002-9610(01)00774-7 [2] MacGillivray, T.E. (2003) Fibrin Sealants and Glues. Journal of Cardiac Surgery, 18, 480-485.
http://dx.doi.org/10.1046/j.0886-0440.2003.02073.x [3] Czerny, M., Verrel, F., Weber, H., Muller, N., Kircheis, L., Lang, W., Steckmeier, B. and Trubel, W. (2000) Collagen Patch Coated with Fibrin Glue Components. Treatment of Suture Hole Bleedings in Vascular Reconstruction. The Journal of Cardiovascular Surgery, 41, 553-557. [4] Turner, A.S., Parker, D., Egbert, B., Maroney, M., Armstrong, R. and Powers, N. (2002) Evaluation of a Novel Hemostatic Device in an Ovine Parenchymal Organ Bleeding Model of Normal and Impaired Hemostasis. Journal of Biomedical Materials Research (Applied Biomaterials), 63, 37-47.
http://dx.doi.org/10.1002/jbm.10081 [5] Taguchi, T., Saito, H., Uchida, Y., Sakane, M., Kobayashi, H., Kataoka, K. and Tanaka, J. (2004) Bonding of Soft Tissues Using a Novel Tissue Adhesive Consisting of a Citric Acid Derivative and Collagen. Materials Science and Engineering: C, 24, 775-780.
http://dx.doi.org/10.1016/j.msec.2004.08.037 [6] Fukunaga, S., Karck, M., Harringer, W., Cremer, J., Rhein, C. and Haverich, A. (1999) The Use of Gelatin-Resorcin- Formalin Glue in Acute Aortic Dissection Type A. European Journal of Cardio-Thoracic Surgery, 15, 564-570.
http://dx.doi.org/10.1016/S1010-7940(99)00084-6 [7] Kumar, A., Maartens, N.F. and Kaye, A.H. (2003) Evaluation of the Use of BioGlue in Neurosurgical Procedures. Journal of Clinical Neuroscience, 10, 661-664. http://dx.doi.org/10.1016/S0967-5868(03)00163-2 [8] Singer, A.J. and Thode, H.C. (2004) A Review of the Literature on Octyl Cyanoacrylate Tissue Adhesive. The American Journal of Surgery, 187, 238-248.
http://dx.doi.org/10.1016/j.amjsurg.2003.11.017 [9] Ramakumar, S., Roberts, W.W., Fugita, O.E., Colegrove, P., Nicol, T.M., Jarrett, T.W., Kavoussi, L.R. and Slepian, M.J. (2002) Local Hemostasis during Laparoscopic Partial Nephrectomy Using Biodegradable Hydrogels: Initial Porcine Results. Journal of Endourology, 16, 489-494. http://dx.doi.org/10.1089/089277902760367458 [10] Nakayama, Y. and Matsuda, T. (1999) Photocurable Surgical Tissue Adhesive Glues Composed of Photoreactive Gelatin and Poly(Ethylene Glycol) Diacrylate. Journal of Biomedical Materials Research (Applied Biomaterials), 48, 511-521.
http://dx.doi.org/10.1002/(SICI)1097-4636(1999)48:4%3C511::AID-JBM17%3E3.3.CO;2-M [11] Ferland, R., Mulani, D. and Campbell, P.K. (2001) Evaluation of Sprayable Polyethylene Glycol Adhesion Barrier in a Porcine Efficacy Model. Human Reproduction, 16, 2718-2723.
http://dx.doi.org/10.1093/humrep/16.12.2718 [12] Wallace, D.G., Cruise, G.M., Rhee, W.M., Schroeder, J.A., Prior, J.J., Ju, J., Maroney, M., Duronio, J., Ngo, M.H., Estridge, T. and Coker, G.C. (2001) A Tissue Sealant Based on Reactive Multifunctional Polyethylene Glycol. Journal of Biomedical Materials Research (Applied Biomaterials), 58, 545-555. http://dx.doi.org/10.1002/jbm.1053 [13] Buchta, C., Hedrich, H.C., Macher, M., Hocker, P. and Redl, H. (2005) Biochemical Characterization of Autologous Fibrin Sealants Produced by CryoSeal® and Vivostat® in Comparison to the Homologous Fibrin Sealant Product Tissucol/Tisseel®. Biomaterials, 26, 6233-6241.
http://dx.doi.org/10.1016/j.biomaterials.2005.04.014 [14] Canonico, S. (2003) The Use of Human Fibrin Glue in the Surgical Operations. Acta Biomedica, 74, 21-25. [15] Siedentop, K.H., Park, J.J., Shah, A.N., Bhattacharyya, T.K. and O’Grady, K.M. (2001) Safety and Efficacy of Currently Available Fibrin Tissue Adhesives. American Journal of Otolaryngology, 22, 230-235.
http://dx.doi.org/10.1053/ajot.2001.24817 [16] Silver, F.H., Wang, M.C. and Pins, G.D. (1995) Preparation and Use of Fibrin Glue in Surgery. Biomaterials, 16, 891- 903. http://dx.doi.org/10.1016/0142-9612(95)93113-R [17] Ciapetti, G., Stea, S., Cenni, E., Sudanese, A., Marraro, D., Toni, A. and Pizzoferrato, A. (1994) Cytotoxicity Testing of Cyanoacrylates Using Direct Contact Assay on Cell Cultures. Biomaterials, 15, 63-67.
http://dx.doi.org/10.1016/0142-9612(94)90199-6 [18] Kaplan, M. and Baysal, K. (2005) In Vitro Toxicity Test of Ethyl 2-Cyanoacrylate, a Tissue Adhesive Used in Cardiovascular Surgery, by Fibroblast Cell Culture Method. The Heart Surgery Forum, 8, E169-E172.
http://dx.doi.org/10.1532/HSF98.20041126 [19] Murakami, Y., Yokoyama, M., Okano, T., Nishida, H., Tomizawa, Y., Endo, M. and Kurosawa, H. (2007) A Novel Synthetic Tissue-Adhesive Hydrogel Using a Crosslinkable Polymeric Micelle. Journal of Biomedical Materials Research Part A, 80, 421-427. http://dx.doi.org/10.1002/jbm.a.30911 [20] Murakami, Y., Yokoyama, M., Nishida, H., Tomizawa, Y. and Kurosawa, H. (2008) A Simple Hemostasis Model for the Quantitative Evaluation of Hydrogel-Based Local Hemostatic Biomaterials on Tissue Surface. Colloids and Surfaces B: Biointerfaces, 65, 186-189.
http://dx.doi.org/10.1016/j.colsurfb.2008.04.005 [21] Murakami, Y., Yokoyama, M., Nishida, H., Tomizawa, Y. and Kurosawa, H. (2009) In Vivo and in Vitro Evaluation of Gelation and Hemostatic Properties of a Novel Tissue-Adhesive Hydrogel Containing a Cross-Linkable Polymeric Micelle. Journal of Biomedical Materials Research (Applied Biomaterials), 91, 102-108.
http://dx.doi.org/10.1002/jbm.b.31378 [22] Uchida, Y., Fukuda, K. and Murakami, Y. (2013) The Hydrogel Containing a Novel Vesicle-Like Soft Crosslinker, a “Trilayered” Polymeric Micelle, Shows Characteristic Rheological Properties. Journal of Polymer Science Part B: Polymer Physics, 51, 124-131. http://dx.doi.org/10.1002/polb.23187 [23] Moroishi, H., Yoshida. C. and Murakami, Y. (2013) A Free-Standing, Sheet-Shaped, “Hydrophobic” Biomaterial Containing Polymeric Micelles Formed from Poly(ethylene glycol)-Poly(lactic acid) Block Copolymer for Possible Incorporation/Release of “Hydrophilic” Compounds. Colloids and Surfaces B: Biointerfaces, 102, 597-603.
http://dx.doi.org/10.1016/j.colsurfb.2012.08.050 [24] Ito, T., Yoshida. C. and Murakami, Y. (2013) Design of Novel Sheet-Shaped Chitosan Hydrogel for Wound Healing: A Hybrid Biomaterial Consisting of Both PEG-Grafted Chitosan and Crosslinkable Polymeric Micelles Acting as Drug Containers. Materials Science and Engineering: C, 33, 3697-3703. http://dx.doi.org/10.1016/j.msec.2013.04.056 [25] Zhang, Q., Wang, C.R., Babukutty, Y., Ohyama, T., Kogoma, M. and Kodama, M. (2002) Biocompatibility Evaluation of ePTFE Membrane Modified with PEG in Atmospheric Pressure Glow Discharge. Journal of Biomedical Materials Research, 60, 502-509.
http://dx.doi.org/10.1002/jbm.1294 [26] Lee, H.J., Lee, J.S., Chansakul, T., Yu, C., Elisseeff, J.H. and Yu, S.M. (2006) Collagen Mimetic Peptide-Conjugated Photopolymerizable PEG Hydrogel. Biomaterials, 27, 5268-5276. [27] Murakami, Y. and Hirata, A. (1999) Complex between α-Chymotrypsin and Poly(ethylene glycol) Catalytically Active in Organic Media. Biotechnology Techniques, 13, 545-548.
http://dx.doi.org/10.1023/A:1008987431361 [28] Murakami, Y. and Hirata, A. (1999) Poly(ethylene glycol)-α-Chymotrypsin Complex Catalytically Active in Anhydrous Isooctane. Journal of Bioscience and Bioengineering, 88, 441-443.
http://dx.doi.org/10.1016/S1389-1723(99)80224-2 [29] Murakami, Y., Hoshi, R. and Hirata, A. (2001) Borate Buffer Dramatically Enhances the Activity of Poly(ethylene glycol)-α-Chymotrypsin Complex Catalytically Active in Anhydrous Isooctane Than Conventional Phosphate Buffer Even at Low Concentration. Biotechnology Letters, 23, 125-129. http://dx.doi.org/10.1023/A:1010345318275 [30] Murakami, Y., Hoshi, R. and Hirata, A. (2003) Characterization of Polymer-Enzyme Complex as a Novel Biocatalyst for Nonaqueous Enzymology. Journal of Molecular Catalysis B: Enzymatic, 22, 79-88.
http://dx.doi.org/10.1016/S1381-1177(03)00009-2 [31] Murakami, Y. and Hirata, A. (1998) Enzymatic Synthesis of Peptides Review. Seibutsu-Kogaku Kaishi, 76, 238-254. [32] Ferry, J.D., Fitzgerald, E.R., Grandine, L.D. and Williams, M.L. (1952) Temperature Dependence of Dynamic Properties of Elastomers: Relaxation Distributions. Rubber Chemistry and Technology, 25, 720-729.
http://dx.doi.org/10.5254/1.3543437 [33] Dohno, C., Okamoto, A. and Saito, I. (2005) Stable, Specific, and Reversible Base Pairing via Schiff Base. Journal of the American Chemical Society, 127, 16681-16684.
http://dx.doi.org/10.1021/ja054618q