Communication Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Introducing Students to the Medical Applications of Cross-Linked Hydrogels Using Nontoxic Materials and Experiments Suitable for Many Settings Grigoriy Sereda* and Benjamin Hawkins Department of Chemistry, University of South Dakota, 414 East Clark Street, Vermillion, South Dakota 57069, United States
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S Supporting Information *
ABSTRACT: We report a series of three experiments that introduce students at different stages of their academic preparation to the medical applications of hydrogels for controlled drug delivery and wound healing. The laboratory activity bridges the fundamental concepts of chemistry (acids and bases, equilibria, solvation, and reversible esterification) with the chemistry of polymers and cross-linking, which are among the main tools of nanotechnology. The activity is customizable for students ranging from middle school to undergraduate college academic backgrounds. The two simple experiments demonstrate acid- or glucose-triggered release of a dye (model drug) from cross-linked poly(vinyl alcohol)-based gels. This mimics controlled drug delivery to more acidic cancer cells and controlled release of insulin from antidiabetic patches. The third, more technically challenging experiment demonstrates irreversible solidification of a phosphate-cross-linked chitosan gel upon heating from room temperature to body temperature. This experiment demonstrates wound healing materials that seal the wound with a nontoxic biocompatible gel. All experiments involve only nontoxic materials and can be performed under any settings. KEYWORDS: Elementary/Middle School Science, Second-Year Undergraduate, Laboratory Instruction, Polymer Chemistry, Hands-On Learning/Manipulatives, Applications of Chemistry, Materials Science, pH
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INTRODUCTION For the past decade, hydrogels have demonstrated significant potential as materials for controlled drug delivery, wound healing, and other applications such as regenerative medicine.1−3 This topic has been shown to be useful in engaging students to the fundamental concepts of chemistry and nanotechnology. In 2013, Sylman and Neeves developed a high school experiment demonstrating release of a food dye from gelatin upon its digestion by bromelain.4 Four years later, a new experiment by Hurst aimed to prepare upper-division undergraduate students to exploratory research by experimenting with pH-controlled release of genipin from PVA-crosslinked chitosan hydrogel.5 The reported experiments have inspired us to explore other chemical stimuli and mechanisms for controlled drug release and to introduce applications of hydrogels for wound healing. We have developed a laboratory activity involving three experiments customizable for students at different stages of their academic preparation and tested it in an undergraduate organic chemistry laboratory course (all three experiments) and in a summer science camp for middle school children (Lawrence Brothers Science Camp at the University of South Dakota, experiments 1 and 2). Unlike numerous slime and hydrogel demonstrations, the proposed laboratory activity introduces students to two key concepts of medical nano© XXXX American Chemical Society and Division of Chemical Education, Inc.
technology: controlled drug delivery and wound healing. The two simple experiments demonstrate acid- or glucose-triggered release of a dye (model drug) from cross-linked poly(vinyl alcohol)-based gels. This mimics controlled drug delivery to more acidic cancer cells1 and controlled release of insulin from antidiabetic patches. The third, more technically challenging, experiment demonstrates irreversible solidification of a phosphate-cross-linked chitosan gel upon heating from room temperature to body temperature. This experiment demonstrates wound healing materials that seal a wound with a nontoxic biocompatible gel.2,3
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HAZARDS
Acetic acid is an irritant. All other chemicals are nontoxic at the quantities used in the experiments. The person preparing the experimental setup must wear eye protection. All chemicals are nontoxic and biodegradable. The liquid leftovers can be disposed down the drain. The gels can be placed in nonhazardous waste. Received: February 1, 2018 Revised: August 18, 2018
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DOI: 10.1021/acs.jchemed.8b00067 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 1. Cross-linking of two PVA molecules by borate and the ball and stick representation.
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BACKGROUND OF THE EXPERIMENTS The first experiment is based on a well-known science activity of making bouncy balls from PVA-based glue and a borate linker.6 We modified this experiment by adding to the gel a food dye as a model drug and demonstrated its release under acidic conditions that cleaves the borate groups serving as cross-links (Figure 1). The figures show the final step when a boronated part of a polymer chain cross-links with a nonboronated part of another polymer chain. The second experiment demonstrates how addition of glucose breaks up a cross-linked PVA-starch gel and releases a dye trapped in the gel. This mimics glucose-induced drug release: the process currently sought for applications in antidiabetic patches.7 The PVA molecules are cross-linked by hydrogen bonds with branched molecules of starch. Figure 2 represents a fragment of a starch molecule with a branch, which does not have to be repeated regularly. The molecules of glucose form hydrogen bonds with the polymer molecules of the gel, which reduces the capability of the polymer molecules to form hydrogen bonds between themselves. This increases fluidity of the gel. However, the
results of this experiment should not be overinterpreted. Any alcohol may have a similar effect on the gel. We found that ethanol also decreased the viscosity of the gel but resulted in a lumpy mixture. Addition of NaCl increased viscosity of the gel by binding the molecules of water in the process of solvation and decreasing hydrogen bonding between the gel and water. The third experiment introduces students to the utilization of phosphate cross-linked chitosan hydrogels for wound healing.2,3 Chitosan is a biopolymer that contains amino groups (Figure 3). These amino groups can be protonated by controlling the pH, and a significant proportion of the polymer is protonated at pH 7. The positively charged molecules of chitosan can be electrostatically cross-linked by polyatomic anions such as phosphate and hydrophosphate. Heating a readily flowing solution of chitosan and phosphate from room temperature to body temperature forms a hydrogel. This demonstrates a very useful application of chitosan gels in wound dressings: the ability to seal a wound with a nontoxic biocompatible material. The cross-linking occurs upon heating because the enthropy drives the molecules of water away from the gel by breaking hydrogen bonds. The freed up -OH and -NH bonds of the chitosan molecules form hydrogen bonds with other chitosan molecules, thickening the gel (Figure 4). All three experiments have been incorporated to the laboratory activity “Polymers” for the laboratory course of sophomore organic chemistry along another experiment. In each of two laboratory sections (enrollment of 16), students worked in pairs. It has taken between 1 and 2 h for the students to complete all three experiments. To assess student learning, we incorporated six questions to the laboratory report (see the Supporting Information) and one question to the final laboratory exam (asking students to draw the structure of
Figure 2. Structure of starch. B
DOI: 10.1021/acs.jchemed.8b00067 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Figure 3. Structure of chitosan.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Grigoriy Sereda: 0000-0002-1121-8211 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the University of South Dakota for financial support of this work. We are also indebted to Brandon Karels for the adaptation of the procedure of the cross-linked chitosan gel preparation for the teaching laboratory experiment.
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Figure 4. Chitosan-phosphate gel flows down a flipped upside-down tube after 10 min at room temperature (left). Chitosan-phosphate gel stays up in a flipped upside-down tube after 10 min at body temperature (right).
chitosan at the blood pH (7.4) and determine its charge) at the end of the semester. The first two experiments have been incorporated to a laboratory activity “Shaping Materials with Light” at the Lawrence Brothers Science Camp: a traditional summer outreach event for middle school children. The activity, which also included photolithography and photochromic beads (not parts of the reported experiments), has taken 1 h to complete. In each of two laboratory sections (enrollment of 26), students worked in pairs. The children presented their experience with all activities of the 1 week long camp at a minisymposium on the last day of the camp.
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SUMMARY We have developed a simple laboratory activity suitable for introducing students at different stages of their academic preparation to two major medical applications of hydrogels, namely, controlled drug delivery and wound healing.
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REFERENCES
(1) Cinay, G.; Erkoc, P.; Alipour, M.; Hashimoto, Y.; Sasaki, Y.; Akiyoshi, K.; Kizilel, S. Nanogel-Integrated pH-Responsive Composite Hydrogels for Controlled Drug Delivery. ACS Biomater. Sci. Eng. 2017, 3 (3), 370−380. (2) Li, X.; Kong, X.; Wang, X.; Shi, S.; Guo, G.; Luo, F.; Zhao, X.; Wei, Y.; Qian, Z. Gel-sol-gel thermo-gelation behavior study of chitosan-inorganic phosphate solutions. Eur. J. Pharm. Biopharm. 2010, 75, 388−392. (3) Perale, G.; Rossi, F.; Sundstrom, E.; Bacchiega, S.; Masi, M.; Forloni, G.; Veglianese, P. Hydrogels in Spinal Cord Injury Repair Strategies. ACS Chem. Neurosci. 2011, 2 (7), 336−345. (4) Sylman, J.; Neeves, K. J. An Inquiry-Based Investigation of Controlled-Release Drug Delivery from Hydrogels: An Experiment for High School Chemistry and Biology. J. Chem. Educ. 2013, 90, 918−921. (5) Hurst, G. Green and Smart: Hydrogels To Facilitate Independent Practical Learning. J. Chem. Educ. 2017, 94, 1766−1771. (6) de Zea Bermudez, V.; de Almeida, P.; Seita, J. How To Learn and Have Fun with Poly(Vinyl Alcohol) and White Glue. J. Chem. Educ. 1998, 75, 1410−1418. (7) Gu, Z.; Aimetti, A.; Wang, Q.; Dang, T.; Zhang, Y.; Veiseh, O.; Cheng, H.; Langer, R.; Anderson, D. Injectable Nano-Network for Glucose-Mediated Insulin Delivery. ACS Nano 2013, 7 (5), 4194− 4201.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00067. Experimental procedures, student handout, and instructor notes (PDF) Experimental procedures, student handout, and instructor notes (DOCX) C
DOI: 10.1021/acs.jchemed.8b00067 J. Chem. Educ. XXXX, XXX, XXX−XXX
