Making Slime and Glue in the Classroom

Fig. 3 - 'Gunge'

Polymers are long molecules made up of repeating units called monomers that are joined together by covalent bonds. Natural, or "biopolymers" include proteins, DNA and cellulose, while rayon, polystyrene, Teflon and PVC are among the many synthetic polymers currently available. The predominant element in most of these polymers is carbon.

The physical properties of all polymers are dependent on the length of the molecule and the presence or absence of side chains and cross linkages between polymer chains. As molecule length increases, so, too, do melting and boiling points, impact resistance and viscosity. In addition, as branching and cross linking increase, the flexibility and durability of the polymer is also increased.

Making Slime – Materials and Teaching Method

Slime, Silly Putty and similar preparations owe their properties to the cross links made between molecules of the polymer poly vinyl acetate (PVA) and borax particles. PVA is readily available as a craft or wood glue, and consists of repeating vinyl acetate monomers (see figure 1). Borax, a common ingredient in laundry detergents, can be obtained from supermarkets in the form of sodium tetraborate (see figure 2).

Fig.1 - PVA Glue

The following materials and equipment are required per group of around four students:

•                30 ml PVA glue

•                7 ml borax solution (this can be prepared by dissolving a tablespoon of sodium tetraborate in warm water)

•                ziplock bag

•                30 ml water

•                food colouring

Students should be instructed to copy down the following directions. A teacher-led explanation could follow this.

•                Combine the PVA glue with 30 ml water in a ziplock bag.

•                Add a drop of food colouring, close the bag and mix the ingredients around.

•                Add 7 ml borax solution.

Fig. 2 - Borax Soap

•                Close the bag again and squash it around. As the borax and glue combine they form "cross linked" polymers and the mixture begins to form a semi-solid (see figure 3).

The following questions could be written on the board after students write up the experiment and their observations:

1.             How does adding the borax to the PVA glue change the properties of the mixture?

2.             Does your slime bounce when you drop it on the floor?

3.             Pull the slime apart slowly and compare the results with when you pull it apart quickly.

4.             Viscosity is a measure of a substance’s ability to "spread out." What do you think will happen to the viscosity of your slime if more borax is added to it?

5.             Adding acid to the slime will remove some of the borax cross links. Explain how this will affect the properties of the slime.

Note that slime is regarded as a "Non Newtonian" fluid because its viscosity does not obey predictable laws. It can display both elastic and inelastic properties as a result of the borax cross links between the PVA chains

Making Casein Glue – Materials and Teaching Method

Many glues are made from either natural or synthetic polymers. One of the properties these polymers have in common is that they can form hydrogen bonds with other surfaces such as wood and paper.

Casein glue is made from the milk protein, casein, which consists of multiple amino acid molecules linked together.In this procedure, vinegar is added to allow the casein molecules to clump together, while the sodium bicarbonate acts to neutralise the vinegar and allow the casein to become sticky.

The following materials and equipment are required per group of around four students:

Fig.4 - Casein Protein

•                100 ml milk

•                10 ml white vinegar

•                ½ teaspoon sodium bicarbonate (baking soda)

•                beaker

•                hotplate

•                thermometer

•                "chux" wiping cloth

•                plastic cup

Students should be instructed to copy down the following directions, which could be followed by a teacher-led explanation.

•                Heat 100 ml milk to about 55°C in a beaker. Use a thermometer to monitor the temperature.

•                Stir in 10ml white vinegar to the milk.

•                Remove from heat and stir until the solid curds are separated from the yellowish whey.

•                Filter the mixture through the dishcloth.

•                Squeeze out any remaining liquid from the curds (see figure 4) and place the curds in a plastic cup.

•                Add two tablespoons of water and 1/2 teaspoon of baking soda and stir to form a thick, opaque liquid. Keep adding baking soda until no more bubbles appear.

Students can test their glue on wood, paper and small pieces of fabric. The following questions could be written on the board after students write up the experiment and their observations.

1.             What type of molecule is casein?

2.             Why are the vinegar and sodium carbonate added in this procedure?

3.             What type of bonds form between the casein glue and the surfaces it sticks to?

Polymers Follow-Up Activities

Casein plastic, another derivative of the milk protein, casein, can be made using the method described above, but without adding the sodium carbonate to the milk curds. The curds will form a plastic if allowed to harden overnight. Different plastics such as polystyrene, bakelite and polythene can be classified as "thermosetting" or "thermosoftening" by touching them with a red hot piece of wire or other type of metal. Thermosoftening plastics will melt, whereas thermosetting plastics will burn or char.

References

"Designer Polymers." BBC. 2010.

"What Are Polymers?" Wisedude, 2006

Making Natural Acid-Base Indicators in the Science Classroom

Fig.2 - Red Cabbage Natural Indicator

Acids are substances that are sour to the taste and corrosive. Weak acids found around the home include lemon juice, vinegar and soft drink, while car battery acid and rust remover (phosphoric acid) are of a much stronger acidity. All acids are contain hydrogen ions (positively charged hydrogen atoms), and act to donate these ions in reactions.

Bases act in the opposite way to acids and can accept hydrogen ions in reactions. Bases are slippery to the touch, have a bitter taste and can also be corrosive in some cases. Common bases found around the house include soaps, detergents, toothpaste, baking powder and drain cleaner. Most bases are the oxides or hydroxides of metals – examples include sodium hydroxide and magnesium oxide. Metal carbonates such as sodium carbonate may also behave as bases.

Anthocyanin – A Natural Indicator Found in Many Plants

Interestingly, the juices of plants containing anthocyanin pigments have been found to change colour in the presence of acids or bases. Red cabbages, grapes, blackcurrants, beetroot, eggplant and turnips are included in this group. Solutions of these plant materials turn pink when acid is added to them and blue/purple when in the presence of bases.

This is because the anthocyanin molecule has both acidic and basic properties and can itself donate or accept hydrogen ions (see figure 1). When it accepts a hydrogen ion it turns pink and when it loses a hydrogen ion it turns blue .A whole spectrum of colours within this range can be achieved depending upon the strength of the added acid or base (see figure 2).

Making a Natural Indicator – Materials and Teaching Method

This indicator can be prepared in a science laboratory or, alternatively, in a home kitchen using a saucepan, hotplate and several glasses. Many of the substances that are tested are readily available in most homes.

The following materials and equipment are required per group of around 4 students:

•                one small handful of chopped red cabbage or beetroot

•                beaker or small saucepan.

•                hotplate

•                enough water to cover chopped plant material

•                7 test tubes or small glasses

•                1 teaspoon of each of lemon juice, vinegar, lemonade, detergent, shampoo, baking powder and liquid soap

•                eyedropper

Students should be instructed to copy down the following directions:

Fig.1 - Anthocyanin Equilibrium

•                Add about 10 ml of each of the household products listed above to the 7 test tubes.

•                Place the chopped cabbage or beetroot in a saucepan or beaker and cover with water.

•                Boil the mixture for 5 minutes, then strain the resulting purple liquid into a beaker or cup.

•                Add a few drops of the purple solution to each of the household products in the test tubes.

•                Record the colour change that occurs in each of the 7 solutions.

A suitable results table could have the following headings at the top of each column: 'Solution Tested', 'Colour in Red Cabbage/Beetroot Indicator' and 'Is the Solution an Acid or a Base?'. Students should complete this table as they carry out their tests.

The following questions could be written on the board after students write up their experiment;

1.             Name the pigment present in cabbages and beetroots that acts as an acid-base indicator.

2.             Which of the household solutions tested was the strongest acid? Explain your answer.

3.             Which of the household solutions tested was the strongest base? Explain your answer.

4.             Name some other fruits and vegetables that contain anthocyanins.

Natural Indicators Follow -Up Activities

The double arrows in figure 1 indicate that in aqueous solution, anthocyanin exists in an equilibrium between its corresponding (conjugate) acid and base, as indicated below:

HIn = H+ + In-

More capable students could research how this equilibrium shifts from one side to the other, and thus from one colour to another, depending on the hydrogen ion concentration of the solution. Students could also experiment with other natural indicators such as tea, turmeric powder, the petals of petunia, impatiens, primrose, rose, marigold and hydrangea flowers, grapes and blackberries.

References

Carboni, G. "Experiments With Acids and Bases." Funscience,funsci.com, 2004.

Sonawane, L.V. "Natural pH Indicators." Pharmainfo, pharmainfo.net, 2007.

Withers, G. "Natural Indicators: How Do They Work?" cmu.edu, 2001.

Tissue Engineering

Fig. 1 - Cell Culturing of Cytoskeleton

The need to replace or repair damaged skin, bone, cartilage, organ and even nervous tissue is driving the biomedical discipline of tissue engineering, an expanding area of research that draws from many areas of biological and engineering expertise. Current studies are exploring tissue replacement using both cellular therapy and artificial tissue constructs.

Cell Therapies Associated With Tissue Engineering

The reconstruction of functional tissue using living cells is widely considered to hold the most promise in terms of creating effective implants that mimic the metabolic processes of their surrounding cells (Fodor, 2003). Cell therapies currently being investigated include those using mature cells and stem cells from the actual patient (autologous cells), those involving cells from other humans (allogeneic cells), and even transgenic cells from other species (xenogeneic cells).

Cell Therapy Using Autologous Cells

The advantage of culturing and then implanting tissue from the patient’s own body is that rejection by the person’s immune system will not occur, thereby obviating the need for inconvenient and potentially hazardous immunosuppressive drugs. Some of the cell types in this category include chondrocytes, which have the ability to repair cartilage, myocytes, capable of repairing myocardial tissue, and keratinocytes, cells used to repair wounds and burn affected tissue. Other autologous cells under investigation include retinal epithelial cells, which have the potential to treat macular degeneration and Schwann cells, which can repair the myelin that coats nerve cells.

Clinical trials in this area have involved removing the desired cells from a patient, culturing them in vitro, and implanting them in another part of the body. This procedure can be fraught with difficulties such as the pain and discomfort of surgery and problems associated with culturing enough of the cells on nutrient media. Researchers need to supply cell cultures with optimum conditions of temperature, oxygenation, pH, nutrients and humidity, and monitor diffusion rates across the cell mass as it grows. If the culture does become too large and complex other methods are required to transport materials to and from the cells.

Despite these drawbacks, two procedures have gained clinical approval by the FDA. The Biotech company Genozyme supplies a therapy that repairs damaged cartilage using autologous chondrocytes and also markets ‘Epicel’, an autologous keratinocyte treatment for burns.

Cell Therapy Using Autologous and Allogeneic Stem Cells

Autologous stem cells from bone marrow have been shown to have the ability to differentiate into nerve, myocardial, liver and cartilage tissue. Such stem cells are said to be ‘multipotent’, as opposed to ‘pluripotent’ embryonic stem cells, which have even more potential to diversify. Autologous stem cell research is still in the experimental stage.

Allogeneic stem cells, including those of blood and bone marrow, have also displayed the potential to develop into various cell types, including neural cells with the potential to repair spinal injuries. Any future treatments would need to be accompanied by immunosuppressive medication, or the cells could possibly be genetically engineered to reduce tissue rejection. A similar scenario exists with embryonic stem cells, but ethical issues and the need for further research have delayed progress in this area.

Cell Therapy Using Allogeneic Cells

Mature allogeneic cells, coupled with immunosuppressive drugs such as cyclosporine, also have the potential to form tissue cultures suitable for implantation. More successful trials, as with those using autologous cells, have been in the area of connective tissue and skin repair. Two clinically approved products, 'Apligraft' (Organogenesis) and 'Dermagraft' (Smith and Nephew), are presently used to replace skin in ulcerated wounds. Both employ implants that have been constructed from cultured neonatal foreskin cells.

Cell Therapy Using Xenogeneic Cells

An interesting development in tissue engineering research has involved the use of aortic valves from pigs in the replacement of faulty valves in humans. These porcine valves are first ‘acellularised’, which involves removing their cells with the enzyme trypsin, and then used as a scaffold for the ‘reseeding’ and growth of autologous cells from a patient with faulty heart valves.

Difficulties associated with this technique include the fact that these valves are of limited durability and their functioning is often impaired by the chemical acellularising treatment they are exposed to.

Tissue Engineering Using Artificial Constructs

In addition to aortic valves from pigs, synthetic polymers such as polylactic acid, polyglycolic acid and polyglactin are being investigated as possible scaffolds for the re-seeding of human cells and subsequent implantation as replacement valves. Most of the polymers used in this situation are biodegradable, which means they can be reabsorbed by the body after they have served their purpose as a matrix for the replacement tissue.

Such artificial constructs may also serve as matrices for replacement skin, bone and cartilage tissue. Millenium Biologix, in conjunction with Biodyn and the Marshall Space Centre, is in fact conducting experiments in which synthetic bone is seeded with human bone cells. These tissue constructs are grown in space (see fig. 1), as cells grown in this 'microgravity' environment tend to grow in a similar manner to the way they grow in the human body.

The potential of these synthetic scaffolds for encapsulating implants as a means of reducing the immune response is also being examined (National Institute of Standards and Technology, 2005). Indeed, it may become possible to introduce insulin producing pancreatic cells into diabetic patients by surrounding these cells with a synthetic ‘cage’ to minimise rejection.

Tissue Engineering and the Future

Successful outcomes in the area of organ replacement research, in particular, could result in major economic and health benefits globally. Moreover, further breakthroughs in research aimed at reducing tissue rejection (including the production of transgenic animals as a source of xenografts) could help to save or prolong human life on a scale previously considered unattainable.

References

Biotissue Technologies, 2004, 'NIH Definition of Tissue Engineering', tissue-engineering.net

Fodor, W., 2003, ‘Tissue Engineering and Cell based Therapies, From the Bench to the Clinic: The Potential to Replace, Repair and Regenerate’, Reproductive Biology and endocrinology, nih.gov

Marshall Space Flight Centre, 2008, 'Cellular Biotechnology Operations Support System (CBOSS) - Expedition Three', nasa.gov

National Institute of Standards and Technology, 2005, 'ATP Focused Program: Tissue Engineering', nist.gov

Ethical Issues in Biotechnology

Fig. 2 - Bollworm Larva

Today, the concept of biotechnology also covers a range of sophisticated processes that raise ethical, social, environmental, moral and legal issues. Some of these involve the effects on human embryos, experimental animals, the environment, farmers, consumers and various religious groups.

Stem Cell Research and Human Embryos

Much of the controversy around the use of human embryonic stem cells arises from current ‘therapeutic cloning’ research. This process involves the creation of a human embryo using the techniques of somatic cell nuclear transfer (SCNT). In this procedure, the nucleus from an adult body cell is removed and inserted into an enucleated egg cell (see fig. 1). The resulting diploid cell is then cultivated in the laboratory until it reaches the blastocyst stage (a mass of around 200 cells).

Stem cells are then extracted from the blastocyst and stimulated to grow into the specific cells required in patients suffering from diseases such as diabetes, Parkinson’s disease and Alzheimer’s disease. Other areas of research include the use of these stem cells to grow replacement bone, cartilage, muscle and neural tissue in humans.

Some human life groups regard the blastocysts produced for therapeutic cloning as human beings that consequently should not be used for experimental purposes. In 2006, George Bush vetoed a bill to extend the funding for stem cell research in the U.S., with the result that only cells derived from existing embryos could be used. President Obama reversed this decision in 2009 to include newer stem cell lines but funding for cells created in the future is limited to those produced privately or at a state level.

Alternatives to the use of embryonic stem cells are currently being investigated, and include the potential use of adult stem cells from bone marrow and umbilical cords to replace skin, nerve or muscle cells and the reprogramming of adult skin cells to emulate stem cells by introducing artificial viruses. This research has the approval of many Church groups as it does not involve the destruction of embryos.

Fig. 1 - Somatic Cell Nuclear Transfer

Genetically Modified Foods

Genetically modified food is produced from organisms containing genes from another species. Examples include Bt cotton , which contains a bacterial gene for bollworm resistance (see fig. 2) , 'Roundup Ready' GM Canola, which is resistant to herbicides and virus-resistant sweet potatoes. Other examples include Golden rice, high in beta carotene because it contains genes from daffodils and bacteria and ‘Sumo Salmon’, which contains a gene for bovine growth hormone. The obvious benefits of GM foods include improved yield and nutrition, resistance to pesticides, herbicides and weather extremes, longer shelf life, and increased food security in poorer nations.

Concerns with GM crops include the fact that some of them contain patented genes, which has several ramifications, including the risk of horizontal gene transfer via pollen to neighbouring crops. Farmers owning these crops then become potentially liable if found to have plants containing the gene. This was in fact the case with the Percy Schmeiser family of Saskatchewan, whose crop was contaminated with a gene from Monsanto’s GM Canola.

In addition, many GM crops are sterile, which means that farmers need to keep buying the seeds from the affluent multinationals that produce them. General issues of access also apply here; third world countries, for instance, may have less chance of obtaining the much promoted food security offered by these crops if large companies have a monopoly over them.

Moreover, since some GM companies have prevented peer review of their research, some doubt inevitably remains over the safety of GM foods. Some of these concerns include the risk of contamination by antibiotic resistant marker genes, which could enter the human digestive system, and allergic reactions to unforeseen proteins and toxins produced by introduced genes.

Environmental concerns include horizontal gene transfer, which could, for instance, confer herbicide resistance to weeds, and a general reduction in biodiversity. Genetically modified animals or plants may also overrun native species if released into the wild and harmless insect species may be affected by insecticide resistant plants such as Bt cotton. In addition, religious groups and vegetarians may object to eating plants containing animal genes.

Monoclonal Antibodies and the Treatment of Animals

The production of monoclonal antibodies (Mabs) is a recent development in the field of medical biotechnology. These antibodies are produced by injecting an antigen into mice, which stimulates the production of B lymphocytes specific to the antigen in question. The B lymphocytes are then removed from the mouse and fused to cancerous cells (forming hybridoma’ cells) so that they will multiply rapidly and produce large amounts of the desired antibody. Mabs can be used to treat diseases such as breast cancer and leukaemia or to diagnose the presence of cancers or hormones in humans.

Grave concerns, however, have been raised regarding the treatment of mice in this procedure. This includes the fact that the spleen of the mouse is often removed to obtain the required B lymphocytes. Moreover, the use of adjuvants (chemicals which over-stimulate the immune system) has been shown to cause great distress in the animals, as has the ascites method for propagating hybridoma cells. In this method, used when in vitro methods are not feasible, the hybridoma cells are re-injected into the abdominal cavity of the mouse, cultivated and then removed.

Other areas of biotechnology raise similar issues. The benefits we are gaining or may gain in the future from these techniques should be balanced against such concerns.

References

Australian Government NHMRC,2008, 'Guidelines for Monoclonal Antibody Production', nhmrc.gov.au

Human Genome Program, 2008 ‘Genetically Modified Foods and Organisms’,ornl.gov

Monsanto v. Schmeiser, 2010, percyschmeiser.com

National Institutes of Health, 2010, 'Stem Cell Information', nih.gov