Students learn that engineers develop different polymers to serve various functions and are introduced to selectively permeable membranes. In a warm-up activity, they construct models of selectively permeable membranes using common household materials, and are reminded about simple diffusion and passive transport. In the main activity, student pairs test and compare the selective permeability of everyday polymer materials engineered for food storage (including plastic grocery bags, zipper sandwich bags, and plastic wrap) with various in-solution molecules (iodine, corn starch, food coloring, marker dye), assess how the polymer’s permeability relates to its function/purpose, and compare that to the permeability of dialysis tubing (which simulates a cell membrane).

Student teams make polymers using ordinary household supplies (glue, borax, water). They experiment with the semi-solid material when warm and cold to see and feel its elastic and viscous properties. Students will begin to understand how the electrical forces between particles change as temperature or the force applied to the substance changes. Is it a solid, a liquid, or something in between? How might it be used?

Students learn about the basics of molecules and how they interact with each other. They learn about the idea of polar and non-polar molecules and how they act with other fluids and surfaces. Students acquire a conceptual understanding of surfactant molecules and how they work on a molecular level. They also learn of the importance of surfactants, such as soaps, and their use in everyday life. Through associated activities, students explore how surfactant molecules are able to bring together two substances that typically do not mix, such as oil and water. This lesson and its associated activities are easily scalable for grades 3-12.

Students experience the engineering design process as they design, fabricate, test and redesign their own methods for encapsulation of a (hypothetical) new miracle drug. As if they are engineers, teams make large-size prototypes to test proof of concept. They use household materials (tape, paper towels, plastic wrap, weed-barrier fabric, glues, etc.) to attach a coating to a porous "shell" (a perforated plastic Wiffle® ball) containing the medicine (colored drink mix powder). The objective is to delay the drug release by a certain time and have a long release duration—patterned after the timed release requirements of many real-world pharmaceuticals that are released from a polymer shell via diffusion in the body. Guided by a worksheet, teams go through at least three design/test iterations, aiming to achieve a solution close to the target time release constraints.

Prior to reaching households, water is exposed to a variety of treatments designed to render it fit for human consumption and use. One of the first treatment steps is the removal of suspended solids using chemical additives called flocculants. In this activity, students learn about two commonly used flocculants and clean water collected from a local pond or river. They experiment with flocculant, stirring and pH variables.

Students reflect on their experiences making silly putty (the previous hands-on activity in the unit), especially why changing the borax concentration alters the mechanical properties of silly putty and how this pertains to tissue mechanics. Students learn why engineers must understand tissue mechanics in order to design devices that will be implanted or used inside bodies, to study pathologies of tissues and how this alters tissue function, and to design prosthetics. Finally, students learn about collagen, elastin and proteoglycans and their roles in giving body tissues their unique functions. This prepares them for the culminating design-build-test activity of the unit.

Students are introduced to the concept of viscoelasticity and some of the material behaviors of viscoelastic materials, including strain rate dependence, stress relaxation, creep, hysteresis and preconditioning. Viscoelastic material behavior is compared to elastic solids and viscous fluids. Students learn about materials that have viscoelastic behavior along with the importance of engineers understanding viscoelasticity. To best engage the students, conduct the first half of the associated Creepy Silly Putty activity before conducting this lesson.