Hands-on Activity Just Like Kidneys:
Semipermeable Membrane Prototypes

Learning Objectives

After this activity, students should be able to:

• Define diffusion, osmosis and semipermeable.
• Explain the importance of a semipermeable membrane.
• Follow the steps of the engineering design process as they create prototypes.
• Discuss and compare design successes and failures to come up with an optimal design.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• Semipermeable Membranes Pre/Post-Quiz, two per student
• Semipermeable Membranes Worksheet, one per student
• Semipermeable Membranes Project Rubric, one per group
• 3 materials from which to build and refine its semipermeable membrane prototype/model, groups choose from teacher-provided supplies, such as those suggested below
• items for testing permeability: ring stand, pipette 3-4 clothes pins
• timer, to count seconds

To share with the entire class:

• an assortment of possible prototype-building materials, from which each group chooses three; suggested materials: sugar cubes, newspaper, copy paper, magazine paper, cardboard, paper plates, paper towel squares, cotton fabric squares (~1-foot square); consider sourcing many items from recycling bins (paper, newspaper, magazine paper, cardboard); consider getting fabric scraps donated from quilters, sewers, parents, or buy from fabric store bargain bin
• assorted tools for construction/fabrication: scissors, craft knives, tape, cotton string (2-foot lengths)
• items for testing permeability: tap water, liquid food coloring

Worksheets and Attachments

Pre-Req Knowledge

Students must know about the mechanisms cells use to maintain homeostasis. One component of homeostasis is the diffusion of materials across a semipermeable membrane. It is helpful if students are familiar with the steps of the engineering design process.

Introduction/Motivation

Do you know of anyone who has had a medical treatment called dialysis? What do you know about it? (See what students know.) During dialysis, waste is filtered out of the blood of people whose kidneys no longer perform this function. How does dialysis work? Blood cells are too large to pass through a semipermeable membrane, while waste in the blood passes through easily. So, semipermeable membranes are used as the filter in dialysis treatments.

(Present to the class the following hypothetical scenario as the context for the design project.) For this engineering challenge, your biomedical engineering team designs and creates a model that demonstrates semipermeability for use as a teaching tool for medical students who need to understand the concept as it applies to the human body. Your team may choose three of the materials that I have provided for you at the front table to develop your prototype. Use those materials to make a prototype that is composed of two layers of a medium separated by material acting as the membrane, so that water droplets pass through the first layer, through the membrane, and into the second layer.

You will compete against other companies (student teams) to create and refine the best semipermeable membrane demonstration prototype, which will be tested by both the number of drops of water it takes to reach the second layer of medium and how fast it permeates the membrane.

Then we’ll pool our best ideas to make an even better prototype. The winning prototype will be shared at a national medical conference and be developed as a marketable product with the potential for a good future income for you—its engineering inventors.

Procedure

Background

In the human body, semipermeable membranes are thin tissue sheets that separate two areas of the body. They permit some particles to pass freely though while holding others back, which means they serve as filters. Hemodialysis—or kidney dialysis—is a medical treatment that mimics the kidneys by using semipermeable membranes to filter out waste. In the dialysis process, waste-containing blood is pumped from the patient and through a dialyzer that contains semipermeable membranes. Waste in the blood passes through the membranes into the dialysate—which is a solution that contains sodium, potassium, calcium, magnesium, and other electrolytes—that flows in the opposite direction of the blood. Then, the “clean” blood is returned to the body.

Diffusion is when particles spread out evenly throughout a solution; the particles move from areas of high concentration to fill in areas of low concentration until homeostasis is achieved. Osmosis is similar. In osmosis, water moves from areas where particles are at low concentration to areas where particles are at high concentration. When diffusion and osmosis are combined with a semipermeable membrane, dialysis is possible. Waste passes through the semipermeable membrane into the dialysate, which has a low concentration of waste.

Engineers follow the steps of the engineering design process to guide them as they solve problems. The basic steps are:

• Identify the needs and constraints of the problem.
• Research the problem.
• Develop possible solutions.
• Choose one promising solution.
• Build a prototype.
• Test and evaluate the prototype.
• Redesign the prototype to improve its results.

The design process is iterative, meaning that steps are repeated multiple times as engineers evolve their ideas and improve their designs. To explore and demonstrate different ideas, many prototypes may be created, revised and discarded as engineers work towards developing the best solution. Often, engineers design and create prototypes to test a solution on a small scale, which enables revisions to be made quickly and less expensively than working at full scale.

Teacher Notes: During this open-ended design project, encourage students to freely brainstorm, create and experiment with possible solutions. The aim is to make a semipermeable membrane prototype that does not immediately let water pass through, but also does not hold back the water for a long period of time, which would not meet the challenge of demonstrating permeability.

As an example, one possible “optimal” solution, given the suggested materials and tools, might be making a prototype of newspaper, fabric and paper that permits water to pass through (demonstrating permeability), but not immediately upon placing a drop on the membrane.

For a testing setup, have students use clothes pins to place their membrane prototypes on top of a ring held up by the stand, and then use a pipette to drop colored water onto their membranes—counting drops and timing the permeability.

Expect most teams to avoid choosing sugar cubes, although they could potentially be useful by making a saturated solution in the membrane that prevented water from going through. While sugar might be a less-desirable material, a team might need to consider it if its members did not come up with an idea quick enough to get the “good stuff” from the first-come, first-served table of available materials.

Before the Activity

• Make copies of the Semipermeable Membranes Pre/Post-Quiz, two per student; Semipermeable Membranes Worksheet, one per student; and the Semipermeable Membranes Project Rubric, one per group.
• Gather tools and materials, as suggested in the Materials List. For the activity, give teams the opportunity to choose their construction/fabrication materials—three per group—to create their prototypes. Place the materials at the front of the classroom or in a central location that is accessible for all students. As a project constraint, make the materials available in a first-come, first served basis. The worksheet also lists the available materials so make it match what you supply before making copies.

With the Students

1. Administer the pre-quiz, as described in the Assessment section.
2. Present the Introduction/Motivation content to the class, which includes the design challenge. Discuss semipermeability and permeability as it relates to membranes and medicine.
3. Divide the class into groups—such as pairs. Hand out the worksheets, which includes a recap of the engineering design challenge.
4. Give teams time to read through the worksheet and ask any questions. Provide clarifying instructions and have students get to work.
5. Have students apply their knowledge of semipermeability and the list of available materials to brainstorm which three materials their groups choose to use to create semipermeable membrane demonstration prototypes.
6. After brainstorming, expect groups to have made a plan and decided which three materials they will use. Have groups obtain their materials on a first come, first served basis. Remind students that the limitation on materials serves as one of the project constraints and that real-world engineers work within constraints as they go through the engineering design process.
7. After groups obtain their materials, have them build their prototypes. Remind students to include a drawing of the finished prototype on the worksheet. As students work, ask them: Why do engineers create prototypes? Remind students that this is part of the engineering design process and that engineers learn a lot from the successes and failures of their prototypes. That’s how they end up with final solutions that work.
8. Have each group test its prototype against two criteria: 1) how many drops of water it takes to permeate (go through) the membrane and 2) how long it takes for the water to reach the second layer of material. Record results in the worksheet test table.
9. After at least two trials, have teams discuss their results with their group members, deciding as a group how to redesign their membranes so they work better. This is the time to troubleshoot and experiment. Remind students to complete the worksheet by listing what they want to keep and what they want to change in the next prototype. Also make sure they make a drawing of the revamped design.
10. Have teams test their redesigned membranes and discuss any changes in the results (such as more/less water drops, longer/shorter time).
11. Have groups make any desired changes to their prototypes one more time (another “design iteration”) before final presentations. Inform students that this is the last cycle to make improvements; they will not get another chance to test their final designs before presenting them to the class.
12. Have students present their designs and test the final prototype designs in front of the class. To determine the best prototype, drop water from above each model, counting and recording the number of drops it takes to permeate the membrane, as well timing and recording as how long it takes to permeate the membrane. Which design is the winner?
13. After all designs have been tested, direct all groups to work together as a class to answer the worksheet analysis questions. This is the time to discuss and compare design successes and failures towards coming up with the optimal design.
14. As a class, have students create a new prototype and test it. Have students make drawings and record test results on their worksheets. Was an improved design achieved?
15. Direct students to clean all workspaces and put away all materials.
16. Administer the post-quiz.

Vocabulary/Definitions

concentration gradient: The direction in which molecules move from high to low concentration.

diffusion : The movement of molecules from high to low concentration.

homeostasis : Maintenance of a stable internal environment.

osmosis: The diffusion of water molecules.

semipermeable membrane: A membrane that only permits certain materials to pass through.

Assessment

Pre-Activity Assessment

Pre-Quiz: Prior to the activity, have students complete the three-question, short answer Semipermeable Membranes Pre/Post-Quiz to assess students’ base knowledge of semipermeable membranes, diffusion, osmosis, and the steps of the engineering design process. Administer the same quiz at activity end to gauge student knowledge gains.

Activity Embedded Assessment

Worksheet: Have students use the Semipermeable Membranes Worksheet to guide them through the activity. The worksheet prompts each step of the design/build/test process for creating the prototypes.

Teacher Observations: While students are working on the activity, observe their progress and grade them according to the Semipermeable Membranes Project Rubric.

Team Presentations: Have each group demonstrate its final prototype design to the rest of the class for testing. Students’ final prototypes reveal their depth of effort and comprehension of the scientific concepts and project objectives.

Post-Activity Assessment

Post-Quiz: After students have built, tested, and redesigned their prototypes, as well as worked as a class to create an optimal design, administer the Semipermeable Membranes Pre/Post-Quiz again. Compare students’ pre/post-quiz scores to gauge knowledge gains.

Safety Issues

Use caution when working with craft knives and scissors.

Activity Extensions

• Consider making available some other membrane examples such as dialysis tubing (Visking tubing) and/or various types of sausage casings. Then have students compare their own semipermeable membranes to these real-world semipermeable membranes by conducting the water drops and timing tests and discussing how they are similar/different from their own.
• Consider adding a budget requirement. Assign costs to building materials—such as $2 per five sugar cubes, $5 per cardboard piece—and give each team a maximum of $20 to spend. Then eliminate the three-material limit and permit students to “purchase” any mix of materials. Remind students that incorporating cost considerations reflects the real-world of engineering in which engineers are often constrained by a budget. 

Activity Scaling

• For lower grades, show students a few prototype designs and have them test and discuss how well each works.
• For higher grades, have students brainstorm and draw prototype designs to build without being given materials.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This curriculum was developed in the College of Engineering’s Research Experience for Teachers: Engineering in Precision Agriculture for Rural STEM Educators program supported by the National Science Foundation under grant no. EEC 1542370. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.