Activity pubs.acs.org/jchemeduc
Exploring pH-Sensitive Hydrogels Using an Ionic Soft Contact Lens: An Activity Using Common Household Materials Yueh-Huey Chen* and Yu-Chi He Department of Chemistry, National Dong Hwa University, Hualien 97401, Taiwan, Republic of China
Jing-Fun Yaung Department of Food Science, National Quemoy University, Kinmen 893, Taiwan, Republic of China S Supporting Information *
ABSTRACT: Hydrogels of the so-called smart polymers or environment-sensitive polymers are important modern biomaterials. Herein, we describe a hands-on activity to explore the pHresponsive characteristics of hydrogels using a commercially available ionic soft contact lens that is a hydrogel of poly(2-hydroxyethyl methacrylate-co-methacrylic acid), poly(HEMA-co-MAA). The observed oscillatory swelling and contracting behaviors of the ionic soft contact lens in response to the changes in solution pH exhibit the reversible pH-responsive property of the hydrogel. Chemical structures of functional groups and the associated intermolecular forces in connection with the physical states of the hydrogel are demonstrated. Solutions of household materials such as vinegar, baking soda, and table salt were used to provide solutions with various conditions. This activity covers the chemistry of polymers and the topics relevant for general chemistry such as molecular interactions, acid−base reaction, solution concentration, and osmosis. The swelling and contracting of the ionic soft contact lens in solutions with various tonicities also make a unique class demonstration for teaching the topic of osmosis due to the quick response of the lens to the environmental solutions. KEYWORDS: General Public, High School/Introductory Chemistry, First-Year Undergraduate/General, Polymer Chemistry, Hands-On Learning/Manipulatives, Acids/Bases, Aqueous Solution Chemistry, Hydrogen Bonding
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ydrogels are cross-linked hydrophilic polymeric networks that have the ability to absorb and retain a significant volume of water but not to dissolve.1 Hydrogels of smart polymers have found a variety of applications in biomedicine and biotechnology, such as materials for contact lenses, artificial cartilage and organs, tissue reconstruction scaffold, and protein separation, because they are biocompatible and environmentsensitive.2−5 The swelling properties of hydrogels as a function of temperature, pH, composition, and ionic strength of the surrounding solutions have also been widely used in the designs of biosensors or controlled release devices for drug delivery.6 Although a number of experiments and demonstrations based on the hydrogel materials have appeared in this Journal,7−12 there are only a few laboratory experiments using real-world objects. A hands-on activity using a commercially available ionic soft contact lens as the pH-sensitive hydrogel material is presented. The dynamic swelling properties of the pH-sensitive hydrogel in response to the solutions of baking soda and vinegar are explored. Students gain the exposure to this intriguing class of materials, the so-called smart polymer or intelligent polymer, from the activity.
(HEMA-co-MAA)13−16 (Figure 1). The water imbibed in the hydrogel contributes to the lens softness and flexibility. However, the water content of the hydrogel is influenced by external stimuli. Exposures to solutions with various conditions result in hydrogel lenses with different sizes (Figure 2).
COMMERCIAL HYDROGEL: SOFT CONTACT LENS The hydrogel used in this activity is the soft contact lens, an ionic hydrogel lens that is made of hydrated cross-linked poly(2-hydroxyethyl methacrylate-co-methacrylic acid), poly-
Figure 1. Chemical structures of HEMA, MAA, and cross-linked poly(HEMA-co-MAA) copolymer.
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© XXXX American Chemical Society and Division of Chemical Education, Inc.
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a hand-held LED flashlight (1−5 W white light), table salt (NaCl), baking soda (NaHCO3), and vinegar (acidity 4.5−5%) were all purchased from local stores. The solution of 0.5% (w/ v) baking soda was prepared by dissolving 5.0 g of solid baking soda in 1 L of deionized water. The solution of 10% vinegar (0.45−0.50% acetic acid) was prepared by diluting 100 mL of vinegar with deionized water to 1 L. Because the contact lens is pale blue and transparent, it is much easier to be visualized against a white background. A ruler image printed on the center of a piece of white paper (A4 or letter size) is used to measure the lens diameters. A Petri dish loaded with the ionic soft contact lens and the solution is placed on the white paper. Ten minutes are required for the lens to reach the equilibrium. Under the white light shining perpendicularly from a small hand-held LED flashlight, the diameter of the lens positioned with concave face downward is measured using the ruler image. After the activity, the contact lenses are collected and stored in the lens buffer solution for students of the next session.
Figure 2. The ionic hydrogel contact lenses exhibit different swelling levels in (A) isotonic contact lens buffer solution, (B) 10% vinegar solution, and (C) 0.5% (w/v) baking soda solution.
Moreover, the methacrylic acid molecules (MAA) integrated in the polymer chains make the hydrogel pH-sensitive.17,18 The interconversion between the groups of −COOH and −COO− associated with pH adjustments affects the intermolecular forces between the polymer chains in the hydrogel and further gives rise to different swelling behaviors of the hydrogel19,20 (Scheme I). In solutions at low pH, −COOH groups cannot be
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ACTIVITY Prelab discussion begins with the question of what could happen if the ionic soft contact lenses were stored in the solutions other than the special buffer solution. Students are encouraged to make predictions on what the changes of the solution pH could do to the lens. Follow-up is the introductions to the hydrogel materials used for the ionic soft contact lenses focusing on the water absorbing properties in connection with the hydrophilic functional groups in the hydrogel materials. A short review is also given on the reversible pH-dependent exchange between the groups of −COOH and −COO− in a molecule as well as the intermolecular interactions involved. Students then further predict what the graph of the data would look like regarding the reversible pH impacts on the lens. They carefully examine the sizes of the hydrogel lens in the solutions provided. The followup questions help them to understand the rationale of the physical state changes of the ionic hydrogel lens. This activity has been performed by a group of 12 college students. The activity can be accomplished within 65 min. Most of the materials used are common, nontoxic, inexpensive, and readily available in the stores. None of special laboratory facilities and skills is required. Photos referring to the activity, typical student data, and tips for teaching are included in the online Supporting Information.
Scheme I. Interconversion between −COOH Groups and −COO− Groups Associated with the pH Adjustments
easily ionized. Additional attraction forces resulted from Hbond interactions initiated by the −COOH groups between the polymer chains restrain the hydrogel from swelling. Consequently, a swollen hydrogel contracts in acidic solutions. On the other hand, when exposed to alkaline solutions, the −COOH groups in the hydrogel are ionized to −COO− groups. The effect of electrostatic repulsions between the negatively charged −COO− groups plays a dominant role, leading to the expansion of the polymer chains. Thus, the hydrogel swells to a higher extent. However, the pH-responsive characteristics of the hydrogel are reversible.19,21−23 The hydrogel swells and contracts repeatedly with gel shape retained when cycled between solutions with low pH and high pH.
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HAZARDS The materials involved in this activity pose little risk to the students. However, as the safety precaution, students should not wear the soft contact lenses or taste any of the materials in the laboratory. They should follow general safety guidelines and wear eye protection throughout the activity. After the activity, the contact lens should be collected and saved in the buffer solution for further use, and all the solutions can be disposed of down the sink.
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HOUSEHOLD MATERIALS Hydrogel contact lenses, 1-DAY ACUVUE MOIST from Johnson & Johnson, were used in the activity. However, other commercially available ionic soft contact lenses manufactured from HEMA-MAA copolymers are also appropriate.19 The cost of one box 1-DAY ACUVUE MOIST containing 30 lenses is about $20, but it is unnecessary to use new lenses each time to obtain the expected results. The lenses can be reused in cycles unless they are broken. Lenses of various powers exhibit similar oscillatory swelling and contracting properties. Rulers (15 cm),
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RESULTS In this activity, two household materials, vinegar and baking soda, were used to prepare low pH solution and high pH solution, respectively. The changes in the physical state of the hydrogel were monitored by examining the diameters of the ionic soft contact lens in the solutions. The hydrogel lens had a diameter of ∼1.42 cm in the original packaging solution. In B
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0.5% (w/v) baking soda solution (pH = ∼8.3), the hydrogel swelled so that the diameter of the lens extended to ∼1.58 cm. When the external solution was switched to 10% (v/v) vinegar solution (pH = ∼2.9), the hydrogel contracted and the diameter of the lens reduced to ∼1.26 cm. Upon repeatedly switching between the two solutions, the hydrogel contact lens exhibited good reversible oscillatory swelling and contracting behaviors (Figure 3).
AUTHOR INFORMATION
Corresponding Author
*Y.-H. Chen. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors have no commercial interests in any of the contact lenses or the materials used in this activity. We would like to express gratitude to the editor and the reviewers for the constructive suggestions regarding the manuscript. The development of this activity was supported by National Science Council of Taiwan under grant NSC-100-2511-S-259-007.
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REFERENCES
(1) Gupta, P.; Vermani, K.; Gary, S. Hydrogels: From Controlled Release to pH-Responsive Drug Delivery. Drug Discovery Today 2002, 7 (10), 569−579. (2) Kumar, A.; Srivastava, A.; Galaev, I. Y.; Mattiasson, B. Smart Polymers: Physical Forms and Bioengineering Applications. Prog. Polym. Sci. 2007, 32, 1205−1237. (3) Klouda, L.; Mikos, A. Thermoresponsive Hydrogels in Biomedical Applications. Eur. J. Pharm. Biopharm. 2008, 68, 34−45. (4) Peppas, N. A.; Bures, P.; Leobandung, W.; Ichikawa, H. Hydrogels in Pharmaceutical Formulations. Eur. J. Pharm. Biopharm. 2000, 50, 27−46. (5) Lewis, A. L.; Driver, M. Blending in with the Body. J. Chem. Educ. 2002, 79 (3), 321−326. (6) Hoffman, A. S. Hydrogels for Biomedical Applications. Adv. Drug Delivery Rev. 2002, 43, 3−12. (7) Sylman, J. L.; Neeves, K. B. An Inquiry-Based Investigation of Controlled-Release Drug Delivery from Hydrogels: An Experiment for High School Chemistry and Biology. J. Chem. Educ. 2013, 90 (7), 918−921. (8) Bowles, R. D.; Saroka, J. M.; Archer, S. D.; Bonassar, L. J. Novel Model-Based Inquiry of Ionic Bonding in Alginate Hydrogels Used in Tissue Engineering for High School Students. J. Chem. Educ. 2012, 89 (9), 1308−1311. (9) Chen, Y.-H.; Yaung, J.-F; Lin, J.-Y.; Wang, Y.-C. JCE Classroom Activity Connections: NaCl or CaCl2, Smart Polymer Gel Tells More. J. Chem. Educ. 2010, 87 (12), 1329−1331. (10) Fischer, F.; Freitag, R. Microwave-Induced Chain Transfer Polymerization of a Stimuli-Responsive Polymer and Determination of Its Critical Solution Temperature. J. Chem. Educ. 2006, 83 (3), 447− 450. (11) Friedli, A. C.; Schlager, R.; Wright, S. W. Demonstrating Encapsulation and Release: A New Take on Alginate Complexation and the Nylon Rope Trick. J. Chem. Educ. 2005, 82 (7), 1017−1020. (12) Schueneman, S. M.; Chen, W. Environmentally Responsive Hydrogels. J. Chem. Educ. 2002, 79 (7), 860−862. (13) Nicolson, P. C.; Vogt, J. Soft Contact Lens Polymers: An evolution. Biomaterials 2001, 22, 3273−3283. (14) Watanabe, J.; Kiritoshi, Y.; Nam, K. W.; Ishihara, K. Hydrogels. In Encyclopedia of Biomaterials and Biomedical Engineering; Marcel Dekker Inc.: New York, 2004; pp 790−801. (15) Bennette, Ed.; Henry V. University of Missouri-St. Louis School of Optometry Contact Lens Division Hydrogel Lens Materials. http:// www.umsl.edu/∼bennette/hydromaterial.html (accessed February 2014). (16) Contact Lens Materials. http://www.opticianonline.net/assets/ getAsset.aspx?ItemID=2724 (accessed February 2014). (17) Luensmann, D.; Jones, L. Protein Deposition on Contact Lenses: The Past, the Present, and the Future. Contact Lens & Anterior Eye 2012, 35, 53−64.
Figure 3. Oscillatory swelling and contracting behaviors of the hydrogel lens in solutions alternating between 0.5% baking soda (sequence step number 1, 3, and 5) and 10% vinegar (sequence step number 2 and 4).
In addition to the pH of the solution, the swelling level of the hydrogel lens was also a function of salt concentration. The lens contracted to a diameter of ∼1.24 cm in 10% NaCl solution and swelled to a diameter of ∼1.50 cm in 0.5% NaCl solution. The hydrogel lens also exhibited oscillatory swelling and contracting characteristics when cycled between the two NaCl solutions.
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INTEGRATING THE ACTIVITY INTO THE CURRICULUM This activity uses a real-world object to explore the realm of modern polymeric materials. The oscillatory swelling and contracting property of a pH-sensitive hydrogel in response to external pH stimuli is revealed. Students observe how the hydrogel materials perceive the prevailing stimuli and respond by exhibiting changes in their physical states. They learn how the physical states of a hydrogel material are influenced by the chemical properties of the material. Chemical concepts involved in this activity include the properties of aqueous solutions, osmosis, acid−base, ionic compound, concentration of solution, chemical structures of functional groups, and intermolecular forces. This activity is designed for introductory college students or high school students. Prior knowledge of solution properties, osmosis, acid−base, and ionic compounds is required. The activity can be used as an introduction to the topics of polymer chemistry. Because everyday-life materials are used and the expanding and contracting of the lens is fun and thought provoking, students are encouraged to take the activity home and share the learning with their families.
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Activity
ASSOCIATED CONTENT
S Supporting Information *
Student activity worksheet, notes for instructor; photographs of the lens in each solution; typical student data; answers to questions. This material is available via the Internet at http:// pubs.acs.org. C
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(18) Kim, B.; Shin, Y. pH-Sensitive Swelling and Release Behaviors of Anionic Hydrogels for Intelligent Drug Delivery System. J. Appl. Polym. Sci. 2007, 105, 3656−3661. (19) Liu, Y.-Y.; Fan, X.-D. Synthesis and Characterization of pH- and Temperature-Sensitive Hydrogel of N-Isopropylacrylamide/cyclodextrin Based Polymer. Polymer 2002, 43, 4997−5003. (20) Qiu, Y.; Park, K. Environment-Sensitive Hydrogels for Drug Delivery. Adv. Drug Delivery Rev. 2001, 53, 321−339. (21) Zhang, Y. X.; Wu, F. P.; Li, M. Z.; Wang, E. J. pH-Switching OnOff Semi-IPN Hydrogel Based on Cross-Linked Poly(acrylamide-coacrylic acid) and Linear Polyallylamine. Polymer 2005, 46, 7695−7700. (22) Jianqi, F.; Lixia, G. PVA/PAA Thermo-Crosslinking Hydrogel Fiber: Preparation and pH-Sensitive Properties in Electrolyte Solution. Eur. Polym. J. 2002, 38, 1653−1658. (23) Şen, M.; Kantoğlu, Ö .; Güven, O. The Effect of External Stimuli on the Equilibrium Swelling Properties of Poly(N-vinyl 2-pyrrolidone/ itaconic acid) Poly-electrolyte Hydrogels. Polymer 1999, 40, 913−917.
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