BME Polymer Lab

Transcription

1 BME Polymer Lab Background Information Hydrogels, which are water-swollen, cross-linked polymer networks, have emerged as an important class of materials in biomedical engineering. Methods of preparation typically involve radical polymerization to create the polymer network, and swelling is achieved by soaking in either water or biological fluid[1]. To date hydrogels have been used in a variety of applications (e.g. drug delivery, tissue engineering, contact lenses) and a variety of polymeric formulations have been developed. However, choosing the right hydrogel for a specific application requires evaluation and thoughtful consideration of the hydrogel s properties. Two important properties of hydrogels are swelling ratio and elastic modulus. Swelling ratio is important because it influences the solute diffusion coefficient in the hydrogel[1]. Obviously, this parameter is important for pharmaceutical applications, as it will affect the drug release kinetics. A common method for evaluating the swelling properties of hydrogels is to compare dry and wet weights. By this method, the swelling ratio is calculated using Equation 1, where q is the swelling ratio, W t is the wet weight, and W 0 is the dry weight. q = Wt W W 0 0 (1) Elastic modulus is an important characteristic because it is a measure of the stiffness of the hydrogel. Thus, this parameter provides information about the mechanical performance of the hydrogel. Unlike traditional materials though, the elastic modulus of hydrogels cannot simply be determined from a stress-strain curve. Rather, Equation 2 must be utilized, where τ is the applied force per unit area, F is the measured force, S 0 is the cross-sectional area of the undeformed sample, E is the elastic modulus, and λ is the extension ratio (see Figure 1 for definition)[2, 3]. When τ is plotted as a function of -(λ- λ -2 ), the slope of the linear portion of this curve represents E. τ = F S 0 = E(λ λ 2 ) (2) l 0 l Undeformed hydrogel Deformed hydrogel 1 of 8

2 λ l' l displacment 0 = = (3) l 0 l0 Figure 1. Determining of the extension ratio, λ. 2 of 8

3 Laboratory Objectives The objective of this lab is to evaluate the effect of polymer concentration on hydrogel swelling ratio and compressive modulus. The specific polymer we will be using is poly(ethylene glycol) (PEG, MW 600), and we will make hydrogels that are 20, 25, and 30% PEG by weight. Laboratory Schedule Week Tasks 1 make 20% PEG specimens 2 (Monday) come to lab and measure the diameter and weight of the 20% PEG dry gels place the 20% PEG dry gels into water for swelling 2 make 25% PEG specimens measure the weight of the swollen 20% PEG specimens trim the ends of the samples flat, measure the diameter and height of the 20% PEG specimens test the 20% PEG specimens in compression 3 (Monday) come to lab and measure the diameter and weight of the 25% PEG dry gels place the 25% PEG dry gels into water for swelling 3 make 30% PEG specimens measure the weight of the swollen 25% PEG specimens trim the ends of the samples flat, measure the diameter and height of the 25% PEG specimens test the 25% PEG specimens in compression 4 (Monday) come to lab and measure the diameter and weight of the 30% PEG dry gels place the 30% PEG dry gels into water for swelling 4 measure the weight of the swollen 30% PEG specimens trim the ends of the samples flat, measure the diameter and height of the 30% PEG specimens test the 30% PEG specimens in compression Experimental Methods Preparation of Macromer Solutions 1. Calculate the amounts of PEGDA 600, Irgacure 2959, and distilled H 2 O you will need to make 5 g of a solution that is either 20, 25, or 30% PEGDA (to the weight of total macromer solution) and 1% Irgacure 2959 (to the weight of monomers) by weight. Confirm your calculation with Dr. Chu or Danny before making samples. 2. Add these amounts to an empty glass vial (label the vial w/ group number and composition) and vortex thoroughly. Suggestions: Place the glass vial on the balance and use a pipette to drip the PEGDA 600 into the vial. Weigh out the appropriate amount of Irgacure 2959 before you add it to the vial. Vortex the mixture until you see Iragure 2959 dissolve completely in the solution. 3 of 8

4 Composition PEGDA 600 Irgacure 2959 dh 2 O 20% PEGDA, 1% Irgacure % PEGDA, 1% Irgacure % PEGDA, 1% Irgacure 2959 Sample Preparation 1. Obtain five cylindrical molds, and seal one end of each with parafilm. Then, use a pipette to add 175µl of macromer solution to each mold (check to make sure that there are no leaks). Place the molds horizontally on the UV box and cure them for 15 minutes and rotate them and cure the other side for another 15 minutes. 2. Remove the parafilm, and use a syringe plunger to slowly push the hydrogels out of the molds. Then, place the hydrogels in a weight boat to dry over the weekend (be sure to label the weigh boat with your group number and the hydrogel composition). Danny will collect your samples after 24 hours and vacuum dry the samples for another 48 hours over the weekend. 3. You have to come back the following Monday to measure your samples and put them in vial for the swelling test. After drying, measure and record the masses of the hydrogels. Suggestions: Be sure to keep track of your samples. If you mix them up, then you will not be able to accurately analyze your data from subsequent tests. Swelling Test 1. Place the dried hydrogels in individual glass vials and add 4 ml of DI water. Label the glass vials with your group number, hydrogel composition, and sample number. Then, place the vials in the 37 C incubator and leave them for four days. 2. On day four, remove the vials from the incubator and take the hydrogels out of the vials. After blotting the hydrogels dry, measure and record their dimensions (i.e. diameter and height) and masses. Compressive Testing 1. Compressive testing will be performed on the Chatillon Vitrodyne materials testing system. To open the testing program, double click the icon for V Check the setup. The motion should be set for compression at a speed of mm/s, a distance of 2 mm, and the data point interval should be set to s. The units should be set so that force and displacement are measured in N and mm respectively. Finally, the display for the output should be set so that the x-axis goes from -2.0 mm to 0 mm (the V1000 software measures compression as negative displacement), and the y-axis goes from 0 to 10 N. 3. Prior to testing, carefully us a scalpel to cut the edges/lips off of the hydrogels so that both ends are flat. Then, use a digital caliper to measure the dimensions (i.e. diameter and height) of the specimens. 4. Place your hydrogel sample on the testing platform so that it will be loaded axially, and make sure that it is well centered. 5. Open the Run screen and select the radio button for compression testing. Then, click the Engage Actuator button to activate the up and down buttons on the Vitrodyne (they are 4 of 8

5 labeled Jog ) and click the Tare Force button (the force values tend to fluctuate at very low values). Next, slowly lower the testing device and pre-load your hydrogel to N, and then click the Zero Position button. Finally, click the Start button. *** Be careful when pre-loading your sample! If you lower the testing device too quickly, you may damage your sample *** 6. After the test is complete, save your data in the BME 383 folder. When naming your file, please use the following convention: Group#_%PEG(Sample No.). For example, if you are in Group 1 and you just tested your first sample of the 20% PEG hydrogel, you should save your data as 1_20(1). 7. Use the Jog buttons to raise the testing device. Then remove your hydrogel sample from the testing platform; samples can then be discarded. 8. Repeat steps 3 through 6 for all of your remaining samples. Data analysis Polymer concentration and swelling ratio 1. Calculate the swelling ratio of all samples. 2. Calculate the average and standard deviation of each group. 3. Plot averaged swelling ratio on y axis against polymer concentration on x axis. Put standard deviation in each concentration group. 4. Obtain regression line and regression equation. - Polymer concentration and compressive modulus 1. Import the txt file into Excel file. 2. Calculate loading stress (τ ) by dividing the load at each point with cross sectional area. 3. Calculate λ from equation 3 and then calculate -(λ- λ -2 ). 4. Plot τ versus -(λ- λ -2 ). 5. Identify the linear portion of the curve. 6. Find compressive modulus from the slope of the linear portion of the curve. 7. Obtain compressive modulus for each sample. 8. Calculate the average and standard deviation of compressive modulus of each group. 9. Plot average compressive modulus in y axis against polymer concentration in x axis. Put standard deviation in each concentration group. 10. Obtain regression line and regression equation. 5 of 8

6 11. Datasheet % PEG Sample No. Initial Mass (g) Final Mass (g) Diameter (mm) Height (mm) of 8

7 Format/Suggestions for Your Written Report Abstract - Give a brief summary of what you did, the results you obtained, and what you learned. Introduction - What are hydrogels and why are they of interest to biomedical engineers? - Describe what PEG is and why it is an important biomaterial. Then, briefly cover the mechanism that we used for making our PEG hydrogels (hint we used a difunctional PEG, a photoinitiator, and UV light). - What was the objective of this lab exercise? Materials and Methods - Preparation of Macromer Solutions - Sample Preparation - Measurement of Swelling Ratio - Measurement of Compressive Modulus Results & Discussion - In this lab, we used PEGDA with a molecular weight of 600 Da. Provide the structure of PEGDA and then calculate n (i.e. the number of repeat units in the polymer chain) for this molecular weight. - Present your swelling ratio and polymer concentration plot. Paste the graph from Excel into Word using Edit Paste Special Picture option to reduce file size. - Provide an example τ versus -(λ- λ -2 ) curve (only the linear portion) and include the trendline. - Present your compressive modulus and polymer concentration plot. - Discuss the trends you observed? Do they make sense? Engineering Design Problem - Suppose you are working on a design team to develop a medical device for the replacement of intervertebral disks. You think that a PEG hydrogel may be a good material, but you need to determine the optimum composition. Assuming that intervertebral bodies have a compressive modulus of roughly MPa, which of the three compositions tested in this lab would you choose[4]? Additionally, assuming that you want to minimize swelling in order to prevent spinal cord impingement, do you think this composition is appropriate? If not, what changes in the PEG hydrogel composition would you suggest? General Comments: Your report should be around 5 pages in length (double spaced, 1 margins). I encourage you to use references, and you may use whatever citation format you like. Just be sure to cite them. 7 of 8

8 References 1. Peppas, N.A., Hydrogels, in Biomaterials Science: an introduction to materials in medicine, B.D. Ratner, et al., Editors. 2004, Elsevier Academic Press: San Diego, CA. p Park, K., W.S.W. Shalaby, and H. Park, Biodegradable Hydrogels for Drug Delivery. 1993, Boca Raton, FL: CRC Press. 3. Peppas, N.A. and B.D. Barr-Howell, Characterization of the cross-linked structure of hydrogels, in Hydrogels in Medicine and Pharmacy, N.A. Peppas, Editor. 1986, CRC Press: Boca Raton, FL. p Perie, D., D. Korda, and J.C. Iatridis, Confined compression experiments on bovine nucleus pulposus and annulus fibrosus: sensitivity of the experiment in the determination of compressive modulus and hydraulic permeability. Journal of Biomechanics, : p of 8