Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King
Erasable Graffiti? Imagine writing “Happy Birthday Mom!” in flamingo pink letters three-feet high on the front lawn or the side of the house. Then imagine washing them away easily when the party is over. This is one scenario that may soon be possible with a recent breakthrough by scientists at Cornell University. Alginates are a family of chemical compounds naturally produced by brown algae, seaweed and kelp, making up 35% dry weight of some giant kelps. Their structures vary, but all are large molecules, copolymers of manuronic (M) and guluronic (G) acids. Depending on the species of algae, the ratio of G to M and the order can vary in alginates. Blocks of consecutive G, consecutive M, and alternating G and M building blocks are commonly occurring motifs. D. Tyler McQuade and Muris Kobaslija recently employed alginates to develop a new family of removable colored coatings that could make temporary messages like the one described earlier feasible (Figure 1). The nontoxic, biodegradable coating is made using calcium alginate and dyes that are widely used as food colorants. Calcium alginate forms when calcium ions chelate carboxyl groups and form crosslinks between the polymer chains of alginate forming a hydrogel. Calcium alginate is a firm, clear, quick-setting gel used in food thickeners and coatings, microcapsules for drug delivery, and other products. Researchers demonstrated that both leaching of dye from the coatings and the degradation rate of the coating depend on the amount of crosslinks formed. The new material adheres to surfaces easily and firmly, remaining intact when sprayed with a jet of water. However, it comes right off when treated with a non-toxic solution of ethylene diamine tetraacetic acid (EDTA). EDTA is a chelating agent that strongly binds calcium ions and solubilizes the hydrogel. “Temporary field lines and logos for sports complexes and roadway markings, as well as coatings for plants, fruit, and the body are just a few examples that underscore where removable coatings could be applied,” the researchers said. The amount of EDTA applied also affects the rates of coating degradation and dye leaching.
More Information 1. Kobaslija, Muris; McQuade, D. Tyler. Removable Colored Coatings Based on Calcium Alginate Hydrogels. Biomacromolecules 2006, 7, 2357–2361. 2. An outreach activity involving calcium alginate has been published in this Journal. See Waldman, Amy Sue; Schechinger, Linda; Govindarajoo, Geeta; Nowick, James S.; Pignolet, Louis H. The Alginate Demonstration: Polymers, Food Science, and Ion Exchange. J. Chem. Educ. 1998, 75, 1430–1431. 3. A laboratory activity using alginate (found in Gaviscon) has been published in this Journal. See Criswell, Brett. A Diaper a Day and What’s Going on with Gaviscon?: Two Lab Activities Focusing on Chemical Bonding Concepts. J. Chem. Educ. 2006, 83, 574–576. 746
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Figure 1. Application of the colored coating onto an artificial turf surface. (A) An aqueous solution of CaCl2 is sprayed onto the surface; (B) a mask (template) is placed on the turf; (C) a colored alginate solution (red food color, 1% v/v) is sprayed on the template; and (D) a logotype is revealed upon mask removal. Reprinted with permission from Biomacromolecules 2006, 7, 2357–2361. Copyright 2006 American Chemical Society.
4. Three overhead projector demonstrations involving encapsulation and release with alginates are available. See Friedli, Andrienne C.; Schlager, Inge R. Demonstrating Encapsulation and Release: A New Take on Alginate Complexation and the Nylon Rope Trick. J. Chem. Educ. 2005, 82, 1017–1020. 5. More information on McQuade’s research can be found online at http://www.chem.cornell.edu/dtm25/pages/home.html (accessed Feb 2007). 6. Additional information on alginates can be found online at http://www.seaweed.ie and http://www.lsbu.ac.uk/water/hyalg.html (both sites accessed Feb 2007). 7. Safety information on EDTA has been previously published in this Journal. See J. Chem. Educ. 2002, 79, 426.
Consumer’s Delight: A Practical Way To Make Round Salt In what may be a boon to consumers and industry, a group of chemists in India is reporting the first practical method for making round salt. Round salt may sound like a candidate for science’s list of wacky and absurd inventions. However, its debut represents a dream come true for researchers who have strived for years to smooth the shape of common salt. Likewise for anyone who knows the frustration of coping with a saltshaker in humid summer weather. Table salt normally exists in those familiar cube-shaped crystals but for years researchers have tried to alter the geometry of its crystals. Common approaches to altering the morphology of crystalline materials include using additives, changing the crystallization solvent, altering the rate of evaporation, and inducing non-equilibrium behavior such as supersaturation of the solution. Pushpito K. Ghosh, P. Dastidar,
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Image by P. Dastidar
Chemical Education Today
Figure 2. At left, a single rhombic dodecahedron crystal of NaCl mounted on a glass fiber used to obtain the single crystal XRD. At right, transformations of NaCl crystals from cube to rhombic dodecahedron via octadecahedron in the presence of glycine as a crystal habitat modifier. Graphic on the right reprinted with permission from Crystal Growth & Design 2006, 6, 1591–1594. Copyright 2006 American Chemical Society.
and colleagues developed a method of producing large quantities of salt in a nearly round, or spherical, form by employing glycine as a morphology modifier. Three different crystal faces ([100], [111], and [110]) are involved in NaCl crystallization, with the latter two involved in octahedron and rhombic dodecahedron forms of the crystalline compound. If glycine is added to the crystallization solution, the [100] faces grow faster than the [110] faces, which leads to the formation of a regular rhombic dodecahedron form, which resembles a sphere (Figure 2). However this change in morphology requires a high concentration of glycine, and due to its poor solubility in organic solvents, the glycine cannot be recovered for reuse from the crystallization solution using extraction. Ghosh, Dastidar, and colleagues overcame this barrier by developing a procedure in which the glycine that crystallizes with the round salt is washed away with a fresh batch of brine. The glycine can be recycled and used to form multiple batches of round salt. The procedure works under conditions employed in solar salt production and using natural brines such as sea water. Round salt’s advantage is the ability to flow more freely because of its shape, although it is not immune to caking. Its optimum performance would be realized when anti-caking agents are used in combination. Humid summer weather often frustrates diners, as table salt cakes and sticks inside saltshakers. Such round salt may ease that frustration. A bigger market may be industries that store and use sodium chloride by the ton to make everything from bulk chemicals to dyes, fertilizers, paper, and pharmaceuticals. For these companies, non-caking salt can be a boon that keeps the sodium chloride flowing freely onto the production line. The research group, from the Central Salt & Marine Chemicals Research Institute in Bhavnagar, India, has developed the new free-flowing table salt in collaboration with a major food company in India.
More Information 1. Ballabh, Amar; Trivedi, Darshak R.; Dastidar, Parthasarathi; Ghosh, Pushpito K.; Pramanik, Amitava; Kumar, V. G. A Practical Approach To Produce Near-Spherical Common Salt Crystals with
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Better Flow Characteristics. Crystal Growth & Design 2006, 6, 1591–1594. 2. This research was described in the New York Times. See http://www.nytimes.com/2006/06/13/science/13find.html?ex= 1307851200&en=05a2f204fb73d33a&ei=5088&partner=rssnyt&emc=rss (accessed Feb 2007). 3. An excellent discussion of growing NaCl crystals is available in this Journal. See Davidson, Charles F.; Slabaugh, Michael R. Salt Crystals—Science behind the Magic. J. Chem. Educ. 2003, 80, 155–156. 4. Crystal growing activities can be found online at http:// chemistry.about.com/od/growingcrystals/ht/saltcrystals.htm (accessed Feb 2007). 5. Photographs of salt crystals taken through a microscope appeared in this Journal. See Ramette, R. W. J. Chem. Educ. 2007, 84, 16–18.
Spinning a New Yarn: Silicone Fibers with Living Organisms In a feat once as unlikely as the miller’s daughter of fairytale fame spinning straw into gold, scientists in the United Kingdom have spun fine threads of biocompatible silicone that contain living human brain cells. The cells remained alive and capable of growth afterward, they say. “This has far-reaching implications and will enable significant advances to be made in technologies ranging from tissue engineering to regenerative medicine,” researchers Suwan N. Jayasinghe and Andrea Townsend-Nicholson state. “The ability to electrospin biologically active threads and scaffolds of living organisms will be tremendously useful for the development of a whole host of novel bioengineering and medical applications.” Electrospinning and electrospraying are related techniques. In electrospraying, a potential difference is applied between the jetting needle and a ground electrode to create an electric field that generates droplets. The ionic nature of buffers needed for viable biosuspensions make electrospraying difficult but this has been overcome (1). Electrospinning is a well-established process for drawing fibers,
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Reports from Other Journals not droplets, out of a thick polymer by use of an electric field (Figure 3). Previously electrospinning has been utilized in the preparation of nanoscale mats and scaffolds for cell proliferation studies. Now the research team from University College London have shown that living organisms can be electrospun and remain viable, a breakthrough that will be used in regenerative medicine, bioengineering, and other medical applications. The scientists used an electrospinning approach in which a concentrated suspension of living cells flowed through a tiny inner needle while thick poly(dimethylsiloxane) (PDMS) flowed from an outer needle. The silicone material formed a fiber around the cells. Jayasinghe and Townsend-Nicholson say that one of the many topics awaiting study is whether the process has any effects on the biological make-up of the threaded cells in the long term (Figure 4). They are also working to have precise control over the number of cells within the threads and exploring the use of polymers other than PDMS.
More Information 1. Jayasinghe, Suwan N.; Townsend-Nicholson, Andrea. Stable Electric-Field Driven Cone-Jetting of Concentrated Biosuspensions. Lab Chip 2006, 6, 1086–1099.
Figure 3. (A) Coaxial cell electrospinning device setup, showing the flow inlets for the biosuspension and PDMS media. (B) A schematic representation of the generated thread. Reprinted with permission from Biomacromolecules 2006, 7, 3364–3369. Copyright 2006 American Chemical Society.
2. Townsend-Nicholson, Andrea; Jayasinghe, Suwan N. Cell Electrospinning: a Unique Biotechnique for Encapsulating Living Organisms for Generating Active Biological Microthreads/Scaffolds. Biomacromolecules 2006, 7, 3364–3369. 3. Background on electrospinning is available at http:// www.people.vcu.edu/~glbowlin/electrospinning.htm (accessed Feb 2007). 4. More information on this project can be found online at http:/ /www.iom3.org/materialsworld/jan07/news2.htm (accessed Feb 2007).
Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P. O. Box 7486, Winston-Salem, NC 27109;
[email protected].
Figure 4. Characteristic photomicrographs of collected cells cultured over an incubation period of nine days. Control cells are shown in panels A, B, and C. Electrospun cells are shown in panels D, E, and F. Photomicrographs shown were taken at 55h (A, D), 80 h (B, E) and at 6 days (postcell electrospinning). The scale bar shown in F represents 200 m. Reprinted with permission from Biomacromolecules, 2006, 7, 3364– 3369. Copyright 2006 American Chemical Society.
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