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Laboratory Experiment pubs.acs.org/jchemeduc
Polymeric Medical Sutures: An Exploration of Polymers and Green Chemistry Cassandra M. Knutson,‡ Deborah K. Schneiderman,† Ming Yu,† Cassidy H. Javner,§ Mark D. Distefano,† and Jane E. Wissinger*,† †
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States White Bear Lake High School, White Bear Lake, Minnesota 55110, United States § Shakopee High School, Shakopee, Minnesota 55379, United States ‡
S Supporting Information *
ABSTRACT: With new K−12 national science standards emerging, there is an increased need for experiments that integrate engineering into the context of society. Here we describe a chemistry experiment that combines science and engineering principles while introducing basic polymer and green chemistry concepts. Using medical sutures as a platform for investigating polymers, students explore the physical and mechanical properties of threads drawn from poly(ε-caprolactone) samples of different molecular masses and actual purchased absorbable and nonabsorbable medical sutures. An inquiry-based part of the experiment tasks students with designing their own experiment to probe the potential of melt blending poly(ε-caprolactone) with commercially available polylactide products in order to modify the properties of the “sutures” drawn. Through these lessons students gain an appreciation for the importance of plastics in our society and how scientists are working to develop more sustainable alternatives. Overall, this laboratory experiment provides a feasible, versatile, sophisticated laboratory experience that engages students in a relatable topic and meets many of the Next Generation Science Standards. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Laboratory Instruction, Polymer Chemistry, Organic Chemistry, Collaborative/Cooperative Learning, Inquiry-Based/Discovery Learning, Chemical Engineering, Green Chemistry, Industrial Chemistry, Materials Science
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INTRODUCTION Polymers are omnipresent in our daily lives and a topic easily introduced to young adults when identified as the plastics they encounter in their water bottles, cell phones, and cars each day. Today’s high school and college students recognize the importance of plastics in society along with recent concerns of their negative side effects, including the potential toxicity of plastic additives [they may, for example, use bisphenol A (BPA)-free bottles] and the accumulation of postconsumer plastic waste in the oceans. Most students are familiar with recycling of plastics as a means to reduce their environmental impact, but few are aware of current efforts to discover new polymers derived from renewable resources and designed for degradation. Additionally, students generally do not understand polymers at a molecular level and what gives these materials their unique and varied properties that allow them to be transformed into consumer products that can be hard, flexible, sticky, stretchy, moldable, and more. In short, we envisioned that science, technology, engineering, and mathematics (STEM) goals of interdisciplinary and applied content coupled with connection to society could be uniformly addressed with a well-designed polymer experiment. © 2017 American Chemical Society and Division of Chemical Education, Inc.
The Polymer Ambassadors program, which is supported by the American Chemical Society Polymer Division, the Intersociety Polymer Education Council (IPEC), and other polymer-affiliated organizations, has been instrumental in promoting and publishing experiments suitable for the K−12 classroom.1 For grades 9−12, sample experiments include the use of latex bands, balls, and paints, neoprene, polyethylene, poly(vinyl alcohol), and other noncompostable/nondegradable materials.1,2 In recent years, experiments emphasizing environmentally friendly polymer experiments with a target audience of middle and high school students have been published in this Journal in an effort to connect polymers and societal sustainability issues.3−7 For example, Mc Ilrath et al.8 published an inquiry-based experiment performed in an Advanced Placement (AP) chemistry high school classroom wherein students polymerize ε-caprolactone to poly(ε-caprolactone), a biodegradable polyester. In another example, Hudson et al.9 Special Issue: Polymer Concepts across the Curriculum Received: October 30, 2016 Revised: April 18, 2017 Published: May 26, 2017 1761
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placed in an aluminum dish and melted at 60 °C on a hot plate to obtain a clear molten polymer. It was discovered that if a glass stirring rod is dipped into the melted polymers of the 45 and 80 kg/mol samples, twisted a few times, and then slowly lifted, a strand of polymer can be pulled from the melt (Figure 1B). As the threads cool and crystallize, they solidify to form opaque thin strands that can be stretched to great lengths. Indeed, as these cooled samples are pulled, the strands can be observed to “cold-draw”, where a thinner thread appears to be pulled out of the thicker thread. This fun activity was the starting point for our experiment. Next, real medical sutures were explored. A variety of sutures were purchased from AD surgical, including Premium Nylon as a nonabsorbable example, Premium PGA as an absorbable example, and Premium RPGA (rapid absorbable PGA), each in different gauge sizes.16 RPGA is no longer available from this particular vendor and has recently been substituted with Premium Chromic Gut (PMC). This natural collagen-based suture also works well as a replacement for RPGA. By convention, larger gauge sizes of sutures correspond to thinner thread diameters. The needles were removed from the sutures, and tensile testing was performed on these samples by tying the threads to a ring stand or other stationary object and pulling to breakage using either an Ohaus pull-type spring scale (11.25 lb type)17 or an RCBS 8 oz to 8 lb premium trigger scale.18 The advantage of the trigger scale is that the poundage is automatically recorded, whereas the spring scales must be carefully watched until the point of break. Through these trials it was concluded that the RCBS trigger scale is appropriate for sutures of nylon, PGA, and RPGA with gauges of 5-0 or 4-0, whereas the Ohaus spring scale is required for 3-0 and larger sutures. When this experiment was implemented in high school classrooms, it was discovered that many schools also have Vernier force probes with LabQuest interface software available to use for this testing.19,20 These systems not only give breakage data but also graphing during and after the test. It was recognized that purchasing enough medical sutures (see the Supporting Information for prices) for each group of students to test all of the samples would be prohibitively expensive for many high school chemistry class budgets. However, classroom testing where each group was responsible for the evaluation of one type of thread and then the data from all of the groups were pooled for comparison worked efficiently and effectively, adding to a cooperative learning environment. Part II of the experiment therefore included evaluating the tieability (the ability of the sutures to be tied to form a strong knot, termed knotability in the medical field) and tensile strength of both the pulled PCL threads from part I and the actual medical suture(s). Conversion calculations from pounds to newtons were required when the pull scales or trigger scales were used. To demonstrate degradation, commercial medical sutures were placed in phosphate-buffered saline solutions with and without lipase and protease enzymes to approximate physiological conditions. The Premium PGA sutures are reported to hold tensile strength for 28−35 days and absorb within 55−70 days, whereas the Premium Chromic Gut sutures are less resilient, with tensile strength retained for 14−21 days and absorption expected in 56−72 days, depending on the gauge.16 Not surprisingly, several weeks to months were required for observable degradation effects (e.g., strength loss or mass loss of the dried thread) in the buffered solutions under all of the conditions studied. This time frame was unattractive for use in
reported an activity in which students use chitin, the major structural component of arthropods and fungi, to make degradable plastic objects.
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BACKGROUND In line with the educational objectives of the National Science Foundation (NSF)-funded Center of Sustainable Polymers,10 a novel experiment was sought for the high school/introductory chemistry classroom with the following objectives: • Introduce basic polymer structures and properties • Use nontoxic and safe chemicals • Exploit the mechanical properties of polymeric materials to include engineering concepts of design and testing • Use a guided-inquiry approach to instruction • Provide context for discussion of green chemistry principles and sustainability issues In the search for a topic engaging for young adults, it was recognized that medical/surgical sutures, often called “stitches”, are available in different designs depending on the applications and could serve as a platform for meeting many of the goals targeted. Some medical sutures are nonabsorbable (composed of nondegradable polymeric materials) and require removal by hand, whereas others are absorbable (composed of degradable polymeric materials) and will dissolve naturally in the body over time.11,12 We foresaw that these characteristics could serve as a natural starting point for discussing the fate of plastics in the environment, both persistent and degradable. Our original idea was to first have students synthesize polyester homopolymers or copolymers, e.g., polylactide (PLA), polyglycolide (PGA), poly(ε-caprolactone) (PCL), poly(lactide-co-glycolide) (PLGA), and poly(lactide-co-ε-caprolactone) (PLCL) and then draw strands of threads from the polymer melt. PLGA and PLCL were particularly attractive, as methods to synthesize these copolymers have been reported and high-molar-mass PLGA is already widely used in commercially available absorbable sutures.13,14 However, the need to purify the monomers rigorously in order to obtain high-molar-mass polymers precludes the reproducible synthesis of these polymers in most high school chemistry laboratories. Because of this limitation, we next investigated what polymers or copolymers could be readily purchased. In this search, we found that PCLs with molar masses of ∼14, ∼45, and 80 kg/ mol are available from Sigma-Aldrich (Figure 1A) and are relatively inexpensive.15 These white opaque pellets were
Figure 1. (A) Structure of poly(ε-caprolactone) (PCL). The value of n for each molar mass is shown. (B) Pulling a thread from melted 45 kg/ mol PCL. 1762
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Again, the concept of molecular size and entanglement is highlighted. Students are tasked with pulling and saving at least three threads of approximately the same thickness and length. However, time should be allotted for exploration of the stretchy properties of particularly the 80K polymer, and a competition generally ensues with who can pull the longest thread. While doing so, students observe the phenomenon of “cold-drawing”. Students keep their 45K and 80K PCL threads for parts II and III of the experiment. Part II: Tieability and Tensile Strength Testing. In part II of the experiment, students test the tieability and tensile strength of the PCL threads (45K and 80K) created in part I and compare these properties to those of actual medical sutures. Students first tie a knot with a small portion of each suture and record qualitative observations about how easy or difficult it is to tie the suture to make a knot that does not slip. Then, using an improvised tensile strength tester, students measure the maximum amount of force, in newtons, required to break the suture. We note here that if the diameter of the suture is known, an additional calculation can be performed to convert this force to tensile strength by dividing the force by the area squared. After testing the actual medical sutures, students observe that they are strong enough to hold an incision together yet flexible enough to tie. When comparing the different types of samples, students find that the poly(εcaprolactone) “sutures” lack these important properties. Part III: Degradability Testing. Part III of the experiment introduces the 12 principles of green chemistry,24 which provide a framework for scientists to use when designing new products that have minimal effect on human health and the environment. Specifically, students are directed to Principle 10, which promotes design for degradation. To test the degradability of the sutures and PCL threads, each sample is added to a basic (NaOH or KOH) water/ethanol solution heated to 50 °C. The time required to dissolve the suture is recorded. Students find that their hand-drawn PCL threads and all of the actual medical sutures except nylon degrade in the solution. Part IV: Guided Inquiry with Polylactide. The introduction to part IV discusses Principle 7, which encourages the use of renewable raw materials instead of depleting resources such as fossil fuels.24 PLA (Figure 2) is presented
most classroom settings, and therefore, other solution media were explored. Ultimately, a 1:1 (v/v) mixture of 10% aqueous NaOH (or KOH) and ethanol heated to 50 °C was found to accelerate the degradation rates for both the PCL and absorbable medical sutures. Degradation times of 1−7 min were observed, with distinguishable differences between the PCL, PGA, RPGA, and PMC threads. As expected, the nylon threads did not dissolve noticeably under these conditions. Finally, a guided-inquiry version of the experiment was developed to build upon the observations and results of parts I−III. When comparing the PCL threads and the actual medical sutures, students noted that their PCL strands were too stretchy and weak to be used for real medical sutures. Students were then provided with a commercial PLA product such as Greenware plastic cups21 or World Centric plastic cups,22 which they could observe to be hard and somewhat brittle. Students were then asked to design a series of experiments to systematically explore how combining the mechanical properties of PCL and PLA could lead to a suture thread with improved characteristics. It should be noted that pieces of PLA from commercially available compostable cups can also be melted and threads drawn from the melt, but the cooled fibers are quite brittle and break. This exploratory exercise meets many of the “12 indicators” published by Moore et al.23 that characterize a “quality engineering education” in the K−12 classroom. These include investigating the process of design through planning and implementing a hypothesis and follow through with testing and evaluation.
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EXPERIMENTAL DESIGN
Materials and Overview
A description of all of the materials and where items can be purchased is provided in the Supporting Information. This four-part experiment can be performed in three or four onehour class periods or as individual exercises. Part I can be completed in one 50-minute class period, including 15 minutes for an introduction to the experiment. Parts II and III can be completed together in a 50-minute class period (see the Supporting Information for a detailed breakdown). Part IV can be completed in one 50-minute class period, though we found that students were eager for more time to investigate their selfdesigned projects. Part I: Drawing Threads Using Poly(ε-caprolactone). Part I of the experiment handout introduces students to medical sutures and polymer structures and terminology. A simple demonstration is suggested in which two piles of yarn, one containing short pieces of yarn and the other containing long pieces of yarn, are used to demonstrate entanglement by attempting to remove one thread from the pile. The first activity involves having students make observations of actual medical sutures to become familiar with their properties. Students then melt poly(ε-caprolactone) of varying molar masses [14 kg/mol (14K), 45 kg/mol (45K), and 80 kg/mol (80K)] and observe the time required for melting. Notably, even though the melting points of the three polymers are reported by the supplier to be the same (60 °C), the students observe that the 14K PCL melts fastest and the 80K PCL melts slowest because of the difference in the sizes of the molecular chains and their entanglement. Students then attempt to draw the melted PCL samples into suturelike threads. Students discover that the 14K PCL cannot be drawn and that the 45K and 80K PCL can be drawn into threads with increasing ease.
Figure 2. Structure of polylactide (PLA).
as a success story where scientists discovered how to convert corn starch or sugar cane into a polymer that is now used in commercial products. Additionally, PLA is degradable through industrial composting. After examining the properties of their PLA product, students hypothesize how the properties of the PCL threads could be improved by melt-blending with PLA and design an experimental procedure to test this hypothesis. The level of inquiry can be adjusted on the basis of time and student level. For example, for open inquiry, students can develop their own testable question regarding PLA, PCL, and 1763
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suture properties, or for more guided inquiry, student groups can be assigned a question to address.
Table 1. Polymer and Green Chemistry Concepts Survey of High School Students
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Students Answering Correctly, %a
HAZARDS Students should wear goggles during the experiment and add gloves when performing the degradation tests. Caution should be used with the hot plate and when handling the hot aluminum dishes and hot poly(ε-caprolactone). Ethanol is a highly flammable liquid, and its vapors may cause respiratory irritation. Breathing the fumes should be avoided, and it should be used in a well-ventilated area. Sodium hydroxide is corrosive and a skin and eye irritant. Breathing the fumes should be avoided, and hands and any exposed skin should be washed thoroughly after handling. Needles should be removed from all purchased medical sutures to avoid puncture wounds.
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Question Prompt Polymers are Large linear polymer chains generally Green chemistry seeks to Modern medical sutures, “stitches”, are made from Polylactide (PLA) is
RESULTS AND DISCUSSION
a
Correct Answer
Preactivity
Postactivity
Made of small repeating units called monomers Have more entanglement
93
100
50
86
Design materials from renewable resources that are degradable Both degradable and nondegradable materials
57
76
76
86
A plastic used to make compostable cups and dinnerware
14
64
N = 14.
Classroom Testing
Parts I−III of the medical sutures experiment were successfully performed in these settings: • Six chemistry classes (non-AP) with approximately 200 students at St. Paul Central High School • Two University of Minnesota summer outreach programs (Exploring Careers in Engineering and the Physical Sciences) for 11th-grade girls involving a total of 40 students • A college environmental science class for nonscience majors (22 students) at Augsburg College. Parts I−IV were performed in two AP high school chemistry courses (22 students total).
Supporting Information for the complete data set). Most notable was an improved understanding of entanglement related to the size of polymer chains and in the question of what is PLA. When asked for suggestions and comments, many students noted that they liked the lab and desired more time for the inquiry portion of the experiment. Examples of student responses include the following: • “I really liked the lab. I think that it was nice that the lab started with a simpler experiment, and slowly got more complex until we were able to design our own experiments.” • “I liked the experiment, but I think it would be even more fun if there was more of the guided-inquiry experiments.” • “This was a really interesting experiment because I didn’t know much going in so I had no idea what would end up happening in the experiment.”
Student and Teacher Feedback
In all trials of the experiment, positive feedback was received from students, who enjoyed both the experimental aspects and learning about new sustainable plastics. Two Minnesota high school teachers confirmed that the experiment met the Minnesota Science Education Standards in the following categories: 9.1.3.3.3 (Strand) The Nature of Science and Engineering, (Substrand) Interactions among Science, Technology, Engineering, Mathematics, and Society; and 9.1.2.1.3 (Strand) The Nature of Science and Engineering (Substrand) The Practice of Engineering.25 We propose that many of the national Next Generation Science Standards (NGGS), including but not limited to standards in the categories of Matter and its Interactions, Earth and Human Activity, and Engineering Design, also are addressed by incorporation of polymer science, green chemistry principles, and discussion of plastics in the environment.26 For example, HS-ETS1-3 is “Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.”27 To gain more insight into the learning outcomes, 14 AP chemistry students from White Bear Lake High School completed pre- and postactivity survey questionnaires that contained five identical questions. For each of the questions, the number of students that answered correctly increased from the pre- to postactivity survey (Table 1). Ten of the participating students answered more questions correctly on the postactivity survey than on the preactivity survey, and the other four participating students answered the same number of questions correctly on the pre- and postactivity surveys (see the
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SUMMARY A unique and versatile experiment based on exploring the properties of purchased medical sutures and hand-drawn threads was developed for the high school classroom and is also suitable for an introductory college chemistry or science laboratory course. Implementation in various classroom settings resulted in positive feedback from teachers and students, who appreciated learning more about the nature of polymers, their pervasiveness in society, and how scientists are applying green chemistry principles to design new materials. Engineering principles were invoked through mechanical testing of the threads, degradation studies, and an opportunity to design new materials in combination with PLA products.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00835. Student handouts, teaching notes, lists of materials and suppliers, and pre- and postactivity survey data for a selected class (PDF, DOCX) 1764
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0&mode=match%20partialmax&lang=en®ion=US&focus=product (accessed April 2017). (16) AD Surgical. UNIFY Surgical Sutures. http://www.ad-surgical. com/medical_unify-sutures/ (accessed April 2017). (17) Ohaus. Mechanical Scales & Balances: Spring Scales. http://us. ohaus.com/en-us/springscales (accessed April 2017). (18) Amazon Sports & Outdoors. RCBS Premium Trigger Pull Scale. https://www.amazon.com/RCBS-Premium-Trigger-Pull-Scale/dp/ B000G738XI (accessed April 2017). (19) Vernier. Dual-Range Force Sensor. http://www.vernier.com/ products/sensors/force-sensors/dfs-bta/ (accessed April 2017). (20) Vernier. LabQuest 2. https://www.vernier.com/products/ interfaces/labq2/ (accessed April 2017). (21) Fabri-Kal. Greenware cold drink cups. http://www.fabri-kal. com/product/greenware-cold-drink-cups/ (accessed April 2017). (22) World Centric. Ingeo Cold Cups & Lids. http://worldcentric. org/biocompostables/cups/pla-cold-cups (accessed Apr 2017). (23) Moore, T. J.; Glancy, A. W.; Tank, K. M.; Kersten, J. A.; Smith, K. A.; Stohlmann, M. S. A Framework for Quality K-12 Engineering Education: Research and Development. J. Pre-Coll. Eng. Educ. Res. 2014, 4 (1), 2. (24) Anastas, P. J.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (25) Minnesota Department of Education. Academic Standards, Science K−12, 2009. http://education.state.mn.us/MDE/dse/stds/ stem/index.htm (accessed April 2017). (26) Next Generation Science Standards: For States, By States. Standards by Topic. http://www.nextgenscience.org/overview-topics (accessed April 2017). (27) Next Generation Science Standards: For States, By States. HSETS1-3 Engineering Design. http://www.nextgenscience.org/pe/hsets1-3-engineering-design (accessed April 2017).
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jane E. Wissinger: 0000-0002-9240-3629 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge St. Paul Central High School teachers Craig Karlen and Alka Goyal for early testing and feedback for this experiment, Michael Wentzel from Augsburg College, and students of Cassandra Knutson’s AP chemistry classes in spring 2016 and fall 2016. This work was funded through grants from the Minnesota Pollution Control Agency (Swift 52526), the University of Minnesota Center for Sustainable Polymers (CHE-1413862 and CHE-1136607), and the Research Experiences for Teachers (RET) Program of the National Science Foundation (DMR-1559833).
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