Chemical Education Today
Reports from Other Journals
Research Advances by Angela G. King
DRPs: Let the Blood Flow! Frictional pressure drop, or drag, is generated by flowing fluid coming into contact with a solid surface, such as a pipe wall, and generating resistance. There are two main types of flow: laminar and turbulent. The friction of laminar flow is determined solely by the physical properties of the fluid. However, turbulence, which causes the energy applied to the fluid to be dissipated as eddy currents and indiscriminate motion, can be reduced by adding small amounts of certain polymers to the flowing fluid. These drag-reducing polymers (DRPs) have an average molecular weight greater than 1 million Daltons, a fairly linear structure, and a flexible backbone. While the phenomenon generated by adding minute quantities of DRPs to a turbulent system is not yet fully understood, it is a demonstration of the Toms effect, the de-
Figure 1. Schematic of system used for turbulent flow studies. Reprinted with permission from Biomacromolecules, 2006, 7, 1597– 1603. Copyright 2006 American Chemical Society.
Figure 2. RBCs on a slide before (upper left) and after (upper right) PNVF addition and slowly flowing in a capillary tube before (lower left) and after (lower right) PNVF addition. PNVF did not affect the RBC morphology. Reprinted with permission from Biomacromolecules, 2006, 7, 1597–1603. Copyright 2006 American Chemical Society.
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crease in resistance to flow without affecting viscosity. This decrease in resistance to flow decreases energy dissipation through turbulence and allows increased flow using the same amount of energy. The Toms phenomenon may be utilized in applications ranging from delivery of crude oil via pipelines to firefighting and reducing drag on submarines. While vascular systems do not display much turbulence, nanomolar concentrations of DRPs do increase blood flow and reduce vascular resistance and have been shown through animal models to be beneficial in improving impaired blood circulation, including delaying atherosclerosis development and increasing the number of functioning capillaries in diabetes. Water-soluble DRPs afford the most promise for biocompatibility. Polymers previously considered for watersoluble DRP applications include poly(ethylene oxides), poly(acrylamides) and polysaccharides from plants. These candidates are less than optimal due to rapid degradation, toxicity and difficulty in manufacturing large quantities, respectively. Recent work by a team from the University of Pittsburgh’s McGowan Institute for Regenerative Medicine demonstrated the potential for clinical use of the DRP poly(N-vinylformamide), or PNVF. Advances in synthetic chemistry have made the monomer, N-vinylformamide, more available, and PNVF has been studied for use in applications ranging from water treatment to adhesives. With a molecular weight of up to 6.7 ⫻ 106 Da, high water solubility and low toxicity of the monomer, PNVF was identified as a lead candidate for a biocompatible DRP and subjected to physiochemical, drag-reducing, viscoelastic and mechanical characterization. Gel permeation chromatography was used to measure molecular weight, intrinsic viscosity, radius of gyration and polydispersity of PNVF that was synthesized by a published procedure. An in vitro circulating system was used to evaluate both the PNVF’s ability to reduce the resistance to turbulent flow of a circulating saline solution and the mechanical degradation of the polymer. Rheological properties, including viscosity, elasticity and relaxation time have been previously shown to correlate with the drag-reducing effectiveness of a polymer and were measured for concentrated solutions of PNVF using a viscoelasticity analyzer. High molecular weight PNVF was shown in this study to significantly reduce resistance to turbulent flow in a pipe and to enhance blood flow in animal models. The polymer is known to possess low toxicity and was found to have acceptable biocompatibility. It withstands mechanical stresses much more readily than poly(ethylene oxide) and as a result, might be an excellent candidate for future clinical trials.
More Information 1. Marhefka, Joie N.; Marascalco, Philip J.; Chapman, Toby M.; Russell, Alan J.; Kameneva, Marina V. Poly(N-vinyl-
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Chemical Education Today
Reports from Other Journals formamide)—A Drag-Reducing Polymer for Biomedical Applications. Biomacromolecules 2006, 7, 1597–1603. 2. This Journal has published a demonstration on polyacrylamide. See Silversmith, Ernest F. Free-radical Polymerization of Acrylamide. J. Chem. Educ. 1992, 69, 763. 3. Coverage of related research can be found online at http:// www.post-gazette.com/pg/04215/355296.stm and http:// www.mirm.pitt.edu/people/bios/KamenevaP1.html (both sites accessed Oct 2006).
Smart Cellulose May Mean Paper Airplanes That Fly Like Butterflies Old-fashioned paper airplanes may be just plane fun for kids, but the next generation of paper flying objects could flap their wings like butterflies and pack micro-cameras and sensors for battlefield surveillance or security monitoring. Those and a range of other new applications for cellulose— the stuff of paper—may be on the horizon, scientists report, after their discovery that cellulose is a “smart” material. Shear piezoelectricity is a linear coupling between electrical and mechanical properties displayed by crystal structures that lack a center of symmetry. Most biopolymers, including DNA, hair, bone, and wood, exhibit shear piezoelectricity due to the rotation of polar atomic groups associated with asymmetric carbon atoms. As far back as 1950, a piezoelectric response in wood was noted. Cellulose, the primary component of wood, does not exist as a single chain, but rather a crystalline array of microfibrils, many parallel oriented chains that contain both crystalline and amorphous domains. In 1955, the shear piezoelectricity of wood was attributed to oriented cellulose crystallites. Until recently no one exploited the piezoelectric potential of cellulose, the world’s most abundant natural polymer, for use in sensors and actuators, devices that transform electrical signals into motion. Given that the annual biomass production of cellulose is 1.5 trillion tons, it would be an inexhaustible green source of raw material for such applications. In the same manner as biological muscles, cellulose is a biopolymer that responds to an electrical stimulus. Such materials flap, bend and move in other ways when given a tiny jolt of electricity. Classified as an electroactive polymer (EAP), cellulose was investigated for an ability to respond in a predictable manner to electrical impulses. Now a team of researchers from Inha University in Korea and Texas A & M University terms smart cellulose “electroactive paper” (EAPap). EAPap is prepared using cellophane, a film of cellulose, with extremely thin gold electrodes deposited on both sides by vapor deposition. When electrical voltage is applied on the electrodes, the EAPap bends. To measure the tip displacement of the EAPap actuators, a high-precision laser Doppler vibrometer was mounted on an optical table and the signal converted to displacement through Labview software. At the same time, the input current from the function generator was measured. Additionally, researchers measured the output force of the EAPap actuator utilizing a microbalance mounted on an optical table while an electrical stimulus was provided by a function generator. 12
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No wires or batteries are needed for sensors developed from EAPap, because a special microstrip antenna and other lightweight electronic components can be integrated into the EAPap. Radio waves beamed to the antenna then would be converted into electricity that moves the EAPap. “This means that EAPap actuators can be remotely driven using microwaves, making them attractive candidates for ultra-lightweight multifunctional applications, such as micro-insect robots, flapping wings for flying objects, smart wallpaper, micro electro-mechanical systems and so on,” the authors state.
More Information 1. Kim, Jaehwan; Yun, Sungryul; Ounaies, Zoubeida. Discovery of cellulose as a smart material. Macromolecules 2006, 39, 4202–4206. 2. In addition to “smart paper”, wood cellulose is a source of other starting materials. See J. Chem. Educ. 1986, 63, 49–53. 3. The chemistry of a conducting polymer actuator has been published in this Journal. See Cortés, María T.; Moreno, Juan C. Electropolymerized Conducting Polymer as Actuator and Sensor Device: An Undergraduate Electrochemical Laboratory Experiment. 2005, 82, 1372–1373. 4. NASA’s interest in this research is described at http:// www.technologyreview.com/read_article.aspx?id=17127&ch=infotech (accessed Oct 2006). 5. General background on EAPs is available online at http:// www.eapap.com/html/intro.html (accessed Oct 2006).
Converting Biomass Directly into Electricity Sticker-shock gasoline prices have sharpened public awareness that switch grass, cornstalks and other plant matter is the most economical raw material for making ethanol, which can be mixed with gasoline. Now a study by scientists in the Department of Civil and Environmental Engineering at Penn State University reveals that cornstalks can also be an alternative source of electricity. Bruce E. Logan and colleagues focused research on processed corn stalks, or corn stover, which is the biggest waste biomass resource in the United States. An estimated 250 million tons of corn stover is produced annually. While a small amount of corn stover is reused as animal bedding or feed, about 90 percent of it is left unused in farm fields after the harvest. Corn stover is typically 70% cellulose and hemicellulose and 15–20% lignin. Microbial fuel cells (MFCs) afford new methods to both reduce waste and generate energy. Electrochemically active bacteria in MFCs generate electricity by oxidizing organic molecules at the anode surface of the MFC. The electrons released through the oxidation travel through an external circuit to the cathode, where they are accepted by chemicals such as oxygen or ferricyanide. The MFCs can produce electricity from a wide range of carbohydrate sources, including sucrose, starch, molasses and wastewater from cereal processing. The Coulombic efficiency, which gives the percentage of electrons recovered from the organic matter, of MFCs has a wide range that is affected by the design of the fuel cell and the carbohydrate source.
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Reports from Other Journals
More Information 1. Zuo, Yi; Maness, Pin-Ching; Logan, Bruce E. Electricity Production from Steam-Exploded Corn Stover Biomass. Energy & Fuels 2006, 20, 1716–1721.
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photo: Bruce Logan.
Logan’s team investigated the first use of corn stover in MFCs. They found that a process termed “steam explosion” converts the cellulose in the corn stalks into fuel suitable for MFCs. The corn stalks were soaked in water or dilute sulfuric acid before being subjected to high-pressure steam. Filtering out solids left a solution of hemicellulose-derived hydrolysates, which were then treated with calcium hydroxide to remove lignin-derived phenolics. The samples were diluted with a buffered nutrient medium and conductivity, which has been shown to affect the maximum power output, was adjusted to 10 mS/cm with sodium chloride. MFCs that had been generating power from standard glucose solutions were then switched to a medium of corn stover hydrolysate and a repeatable cycle of power generation was observed. After a complete cycle of power generation, both monomeric and oligomeric sugars were completely degraded and no volatile organic acids could be detected in solution. The maximum power densities observed for neutral and acidic hydrolysates were 475 and 422 mW/m2, respectively. A pure glucose sample produced 494 mW/m2 from the same system. All values increased if a diffusion layer was added to the cathode of the MFC. “Most people have suddenly learned that ethanol can be made from corn stalks,” said Logan, who headed the study. “We show here that you can use bacteria to make electricity from this material [corn stalks], which is an alternative to ethanol. Coupled with our other work, it should also be possible to make hydrogen from this material as well in a slightly modified process.”
Figure 3. Researcher Bruce Logan sits in front of a collection of microbial fuel cells (MFCs) capable of generting electricity from corn stover.
2. Logan’s research is described at http://www.engr.psu.edu/ce/enve/ logan.htm (accessed Oct 2006). You will also find MFC-cam there, an online demonstration of an MFC. 3. Details concerning related work on using MFCs to obtain hydrogen is available online at http://www.sciencedaily.com/releases/ 2005/04/050422165917.htm (accessed Oct 2006).
Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109;
[email protected].
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