Lesson Plan. Hydrogels: Synthesis and Applications

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1 Lesson Plan Hydrogels: Synthesis and Applications Objectives: Materials: 1. Learn how certain drugs or biomolecules can be encapsulated inside a calcium alginate hydrogel bead 2. Study the release of various food coloring dyes from calcium alginate hydrogel beads 3. Use this to understand diffusion processes 4. Using a UV-Vis spectrometer, perform a semi-quantitative time-based study of the release of the dye that demonstrates how the pore size in the alginate hydrogel bead affects the rate of dye release. (Optional) Sodium alginate Distilled water CaCl 2, anhydrous powder Commercial food coloring, various colors (avoid yellow) 3 ml disposable syringe Small magnetic stir bar Stirrer Filtration apparatus Preparatory Work 1. Assignment: Research diffusion 2. Assignment: review Wikipedia hydrogel material From Wikipedia: gels are apparently solid, jelly-like material formed from a colloidal solution. By weight, gels are mostly liquid, yet they behave like solids due to the addition of a gelling agent. Development Sequence (detailed steps to perform classroom activity together with explanations and exercises) 1. Explain diffusion and illustrate by use of applets B. Stein

2 Applet: This applet demonstrates diffusion dramatically. You can set it up so that no particles are on the right and show how they diffuse through the opening in the wall from left to right (from high concentration to low concentration). The graph below shows the percentage on either side changing with time. Eventually, the particles will equilibrate with ~50% on either side. Settings: Clear Change Wall Set hole size to 10 Add particles to left; 4 clicks will do it Click Go & observe the particles moving to the right and the distribution of particles between sections on the graph. Question, why do we move toward 50-50, and why doesn t the number stay at exactly 50-50? 2. Teacher will illustrate diffusion in a liquid by the warmup exercise. 3. Introduce hydrogels, def. a colloidal gel in which water is the dispersion medium 4. Explain how hydrogels are formed and examine their structure (c.f. figures in Appendix I). Relate this to the beads produced in this experiment. 5. Relate the color change of the water in which the beads are placed to the diffusion process previously described. 6. Explain how the pore size of the beads depends on the concentration of Calcium Chloride 7. Question: How could we measure the pore size? Teacher will lead class into a discussion of nano measurement techniques. 8. Teacher will describe the function and principles of operation of the UV-Vis spectrometer 9. Teacher will perform a remote demonstration using the UV-Vis spectrometer to measure the diffusion rate of dye out of hydrogel beads B. Stein

3 10. Describe unique features of hydrogels, encapsulation of materials, and relate this to a method of drug delivery Warm-up exercise Illustrate diffusion: Drop food coloring into water & observe color spreading throughout. Relate this to the effect that we ll study in this lesson. Experimental Procedure: A. Preparation of solutions 1. Several days in advance, prepare 10 ml or more of a 2% (weight/volume) (2 grams in 100mL water) aqueous sodium alginate solution a. The bulk amount of this solution can be varied based on class size b. We recommend that this solution be prepared 2-3 days in advance c. Place the sodium alginate in a small beaker or flask; add the water and 2-3 drops of food coloring. DO OT STIR!! Allow the material to dissolve slowly over 2-3 days. Be patient; all of it will dissolve. d. OTE: In order to increase student interest, student groups should use different color dye. They may think that there s a competition. 2. Prepare two strengths of CaCl 2 solution. (Note: we may choose to supply the CaCl 2 solution depending on the teacher s assessment of the class s ability to do so) a. 3.7g/500mL water (50mM)-S1 b. 37g/500mL water (500mM)-S2 B. Preparation of Alginate Gel Beads OTE: Two sets of beads will be formed in this experiment. The teacher has the option of dividing the class into groups of 4 students and having one set of groups use 50mM CaCl 2 while the other set uses 500mM CaCl 2. Each section will get 40mL of CaCl 2 solution. Procedure 1. Place the CaCl 2 solution (either 50mM or 500mM) and a magnetic stir bar in a 100 ml beaker. 2. Place the beaker on a stirring plate and stir at the highest speed. B. Stein

4 3. Remove the piston from the 3 ml syringe. 4. SLOWLY pour 1mL of the colored sodium alginate solution into the syringe. a. The sodium alginate solution is quite viscous! b. The farther in advance the solution is prepared, the less viscous it becomes 5. Carefully put the piston back on the syringe. 6. Slowly drip the colored sodium alginate solution into the CaCl 2 solution. Maintain a steady drip rate to ensure the production of uniform beads. 7. As soon as these drops fall into the CaCl 2 solution, they form calcium alginate hydrogel beads. 8. Now the food coloring (model drug) is encapsulated inside the hydrogel bead. 9. The CaCl 2 solution will turn a faint shade of the color of the dye which is a result of some of the dye being released from the surface of the hydrogel beads. The dye released into the CaCl 2 solution is only a small portion of that which is encapsulated; to avoid complications, this amount is assumed to be negligible. 10. Once all the solution inside the syringe has been dispensed, wait for approximately 2 minutes and stop the stirrer. 11. Filter out the beads from the beaker using a filtration apparatus or filter paper. Wash once with distilled water. 12. Allow the beads to air dry for a few minutes. The beads can then be blotted dry with a paper towel. C. Diffusion from Beads OTE: This is a qualitative measure of the diffusion rate. Questions for students: What do you think will happen when the beads are placed in water? Do you think that the rate of color change should depend on the color of the bead? Procedure B. Stein

5 1. Place the freshly prepared and dry beads in a 25 or 50 ml beaker. 2. Add 15 ml of distilled water to the beaker. 3. As soon as the distilled water is added note the time. 4. Periodically observe the water above the beads and look for the appearance of the color of the dye. Note appearance of the water and the time of the observation. 5. Repeat the observations until the color in the water above the beads does not appear to get any more intense. 6. Note the time taken to reach the final color for he two types of beads (prepared with S1 and S2 calcium chloride) Teacher s note: Time to completion: S1 ~15 minutes; S2~40 minutes Explanation: The more concentrated the CaCl 2, the smaller the pore size of the bead and the slower the dye diffuses out. The reason that the pore size becomes smaller with increased concentration is as follows: Make the analogy with knitting a sweater. Ask the students what a sweater looks like as you make more knots. The sweater with more knots has a finer mesh and smaller holes. The CaCl 2 acts to produce knots in the alginate gel (actually sites at which the gel can nucleate). The more chloride present, the more knots (nucleation sites) formed, and the finer the mesh or pore size. D. Determination of Diffusion Rate from the Beads using the Spectrophotometer In this phase of the experiment, we use the magnitude of the optical absorption of the dye in the liquid to investigate the time for the dye to diffuse out of the beads into water. We will access the spectrometer through the NTEN Internet portal (see separate protocol) There are two ways of doing this. 1. Observe the intensity (height) of the dye s absorption peak (see figure 4) as the dye is released. Question for students-how do you think that the peak will change with time? NOTE: Use the color wheel (figure 5) to explain the location and meaning of peaks in the absorption. The observed color is the opposite to the color absorbed. So that the line labeled red dye shows that the light is absorbed in the green (~ ). The ** represents the green dye, B. Stein

6 E. Questions: which is a combination of blue and red dyes, so that we observe two peaks. Procedure: Set the spectrometer to view absorption (y axis) vs. wavelength of light (x axis). a. The assistant at the university inserts beads into the spectrometer cuvette (container). b. The class should record the time at which the peak reaches its maximum value. This corresponds to the maximum absorption of light and the most dye released 2. Observe the absorption change with time. Question for students-what do you think that the curve will look like? Procedure: Set the spectrometer to view absorption (y axis) vs. time (x axis). The shape of the curve will reflect the diffusion of dye into the water. Advanced classes can calculate the rate and determine the diffusion coefficient. 1. Why does the color change at a different rate for the beads formed from different CaCl 2 solutions? 2. Why is the curve of absorption vs. time non-linear (optional for advanced classes) 3. For teachers: relate this experiment to standards F. Application: Infection prevention, Grade PA Academic Standard Description A Structure & prop. of matter, solutions, mixtures B2 Analysis of recently invented items D Scale as relating to concepts Correlation to anotechnology Experiment Metals, insulators, semiconductors, colloids Atomic Force Microscope (AFM) The nano scale & its consequences A scale Use of the Powers of Ten B. Stein

to illustrate nano scale 10-12 3.2.10C Scientific inquiry to Tele-experimentation. solve problems 11-12 3.4.12A Molecular forces AFM 12 3.1.10C Waves, sound, light, Fiber optics colors, optics 12 3.2.10D Pendulum Atomic forces, vibrations 12 3.

7 to illustrate nano scale C Scientific inquiry to Tele-experimentation. solve problems A Molecular forces AFM C Waves, sound, light, Fiber optics colors, optics D Pendulum Atomic forces, vibrations A Atomic & molecular bonds Dipoles-atomic force microscope, semiconductors, QDots Formative Assessment 1. Give a short quiz in class or a take-home quiz 2. Have students write for one/two minutes on what they thought was the most important information that they learned Practice/Reinforcement 1. Investigate applications of hydrogels that involve diffusion (e.g., controlled drug delivery) 2. Identify other hydrogel applications 3. What tools can be used to observe directly at the atomic level? Hydrogel SEM image CC Chu, Cornell University B. Stein

8 Appendix I: Figures 1. Dendrimer example of encapsulation of a drug. (Dendritic Nanotechnologies, Inc) B. Stein

9 2. Hydrogel Molecular Structure B. Stein

10 3. SEM photo of a gel with encapsulated molecules at 22KX magnification 500 nm B. Stein

11 4. Representative Food Coloring Scan (Absorption vs. Wavelength) Red dye Blue dye Green dye** B. Stein

12 5. Color Wheel nm nm nm nm nm nm B. Stein

13 Appendix II: Spectrometer Operation A diagram of the components of a typical spectrometer is shown in the following diagram. The functioning of this instrument is relatively straightforward. A beam of light from a visible and/or UV light source (colored red) is separated into its component wavelengths by a prism or diffraction grating. Each monochromatic (single wavelength) beam in turn is split into two equal intensity beams by a half-mirrored device. One beam, the sample beam (colored magenta), passes through a small transparent container (cuvette) containing a solution of the compound being studied in a transparent solvent. The other beam, the reference (colored blue), passes through an identical cuvette containing only the solvent. The intensities of these light beams are then measured by electronic detectors and compared. The intensity of the reference beam, which should have suffered little or no light absorption, is defined as I 0. The intensity of the sample beam is defined as I. Over a short period of time, the spectrometer automatically scans all the component wavelengths in the manner described. The ultraviolet (UV) region scanned is normally from 200 to 400 nm, and the visible portion is from 400 to 800 nm. Note to remote demonstration: The spectrometer absorption scale is logarithmic: 0 represents 1, 1 represents 10, & 2 represents 100, & 3 represents B. Stein

14 In spectroscopy, the absorbance A is defined as, where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and I 0 is the intensity of the light before it enters the sample or incident light intensity. References: 1. Siddharth B. Gadkari, Optimal hydrogels for fast and safe delivery of bioactive Compounds: Master of Science in Biomedical Engineering Thesis, Drexel University, December Elisabeth Papazoglou, Chetana Sunkari, Drexel University, 2008 (private communication) B. Stein

AP Chemistry Cell Phone Spectroscopy Lab Adopted from Alexander Scheeline Department of Chemistry University of Illinois at Urbana-Champaign

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