Evaluating Current Public Health Practices in Villa El Salvador, Peru

Catalyst Volume 6

Abstract

Initiatives in public health have gained more popularity in recent years due to their simplicity and effectiveness. The success rates of such projects depend heavily upon their adaptability to the community’s needs, which in turn depend on pre-existing health data. The purpose of this study was to formally quantify and evaluate the current health practices in Villa El Salvador, Peru. The study formally verifies the need for clean water and preventative care education public health projects to address the growing health concerns in this specific community.

Introduction

Public health is defined as the science and art of preventing disease, prolonging life, and promoting health and well-being through organized community effort for the sanitation of the environment.¹ While such practices can enhance health in many settings, recently the trend has been to study and apply these principles to smaller communities. The motivation in targeting smaller communities lies in enacting grassroots health movements, spreading awareness of basic, yet essential health measures to a specific population. By tailoring these efforts, specific areas of health salient to the community are emphasized. While their level of success has varied, the inception of such projects has drawn awareness to the field of public health and basic health issues worldwide.

Successful and sustainable public health programs must be well adapted to the unique needs of their target community.² However, a component frequently overlooked is feedback from the community. Prior data describing community needs is essential when planning and piloting person-specific initiatives. Despite the correlation between the availability of current public health data and the success of public health initiatives, many small communities do not have the resources to enact widespread studies.

One such example is Villa El Salvador, a community of over 400,000 people in Lima, Peru.³ Founded on May 11, 1971 by a group of nearly 200 families, Villa El Salvador continues to remain as a “self-managed” community with both commercial and residential areas. After many organized protests, most of Villa El Salvador today now has electricity and water. However, poverty is a major issue in the community. An estimated 21.9% and 0.8% of the population falls into the categories of “poverty” and “extreme poverty” respectively, according to official Peruvian standards: these levels correspond to a family of four members living with $2/$1 daily, respectively.⁴

As a result, nonessential “luxuries” are often spared from the budget. Healthcare is often one such example. Most people cannot afford the medical services offered by the four major hospitals in the area.⁵ In response, smaller community health clinics including “San Martín de Porres Centro de Salud” have attempted to bridge the socioeconomic gap of attaining quality care. People attend these clinics to receive affordable, and sometimes even free, medical attention. While such establishments have continued to serve the people of Villa El Salvador, many are unable to periodically seek medical assistance. A heightened awareness of preventative care is severely lacking in the community, which can be addressed through targeted public health initiatives. Unfortunately, accurate and current health data for Villa El Salvador does not exist.

The purpose of this study is to formally evaluate the health practices of people in Villa El Salvador. Through this initiative, I aim to provide basic, yet meaningful data through the use of surveys for future campaigns in public health and preventative care. Through the information attained from this study, I aspire to provide insight into valid points of focus for the overall improvement in community health. By attaining specific, quantifiable data firsthand from the citizens, future public health projects will be able to mold their initiatives based off of specific community needs and therefore enact consequential and sustainable change.

Experimental

I designed a public health survey to study potential factors contributing to the health issues in Villa El Salvador. After researching prior literature and assessing community needs I targeted several factors: exercise, nutrition, sources and amount of water, hindrances for medical attention, time spent washing hands, and vaccinations. The final version of the survey featured seven questions targeting the areas mentioned. All seven questions featured multiple-choice responses to minimize time spent completing the survey and maximize regularity to yield meaningful results.

I first distributed surveys on June 19, 2012 during the San Martin de Porres Centro de Salud Health Campaign, which offered free healthcare at a local park in Villa El Salvador. This event was specifically chosen as a starting point of the study to collect an accurate sample of the population, minimizing socioeconomic inequalities. The surveys were then distributed in San Martin de Porres Centro de Salud in the mornings for the following week to collect more responses. Respondents were randomly chosen as they waited for medical services offered at the center. After giving informed consent, subjects were told to mark the best response for each question with the exception of the final vaccine question, where all pertinent answer choices were selected.

A total of 98 responses were attained in the two-week span. Thirty-six respondents were between the ages of 15-30, 53 from the ages of 31-50, six from the ages of 51-65, and three from 65 years and above. Since most of the patients of the clinic are females, 19.4% of males were surveyed. Besides the differences in gender, the sample population accurately reflects the demographics of Villa El Salvador.

Results

From the population sampled, 19.4% of participants reported consuming more than two servings of fruits and vegetables combined (Figure 1). The majority of the population reported consuming 1/2 or one serving (35.7% for both categories, respectively). Furthermore, only 12.5% of the population above the age of 50 reported consuming more than one serving of fruits and vegetables daily. Finally, two percent of the respondents reported consuming no fruits and vegetables.

Forty percent of the sample population reported consuming eight or more glasses of liquid daily (Figure 2). According to the results attained, 33.7% of the people consume less than two servings of liquid. The most common source of water for the population sampled was tap by an overwhelming percentage (53%, Figure 3). Both bottle and cistern options yielded 23.5% respectively.

Cost served as the biggest obstacle to periodically visit a doctor for 38.8% of survey participants (Figure 4). However, many 17.3% of the respondents (17.3%) reported distance from a medical facility as the most significant hindrance, while fear for seeing a medical professional was the next most selected response (11.2%). It is important to note that when presented with this question, 9.2% of the respondents reported “trabajo” or work as their answer even though it was not an answer choice.

The majority of the population (52%) reported spending 10 seconds or less washing their hands per attempt, while the second most common response (30.6%) reported was up to twenty seconds (Figure 5). Only 11.2% reported spending up to 30 seconds per attempt, while more than 30 seconds was the least common response (6.2%).

Discussion

A majority of the respondents reported consuming either ½ or 1 serving of fruits and vegetables together. According to the United States Department of Agriculture, individuals should consume at least five to seven daily servings of fruits and vegetables combined, depending on factors such as gender and age.⁶ This survey finding contrasts the steady decrease in malnutrition Peru experienced nationwide from 2005-2010 and most significantly in small, semi-urban areas such as Villa El Salvador.⁷ It is clear that the majority of Peruvians are getting something to eat, at least from the perspective of the Peruvian government.

The issue then arises of what is being consumed. According to the World Health Organization, Peru is expected to have about two million people with diabetes by 2030, triple of what it had in 2010.⁸ The increasing prevalence of heart disease has also been documented⁹ An unhealthy diet may point to the rise in noncommunicable diseases in this community. The data I acquired from the study points to the reduced consumption of fruits and vegetables could serve as major reason for why this disturbing trend is present.

The third question on the survey originally asked for asked for a respondent’s daily water consumption, but much of the Peruvian diet involves juice, soup, and other milk-based products that contain water. Hence, to get an accurate tabulation of water intake, I included juice and milk in the survey. Experts recommend drinking seven to eight glasses of water daily. The majority of people consume two to five glasses of water-based liquids according to the data I attained. In addition, most people cited “tap” as their major source of water. While initiatives promoting the healthy benefits of drinking water would prove to be helpful by emphasizing the importance of increased water consumption daily, the issue of attaining clean water sources must also be addressed.

The principle of preventative care is often deemphasized in many small communities worldwide, regardless of socioeconomic status. For a community such as Villa El Salvador, the importance of this concept multiplies. Realistically, the majority of people in Villa El Salvador cannot financially afford to see a specialized healthcare professional. Hence, regular checkups with a physician to help monitor physical well-being serve as paramount health checkpoints for patients. The real issue is when even these checkups become too expensive. As discussed in the results section, this is unfortunately the case; the majority of people reported cost as their biggest obstacle to seeing the doctor periodically. Keeping in mind that they surveyed population is from a clinic that already provides relatively inexpensive medical services compared to those provided “on the street”, or outside of the clinic, the results are quite discouraging.

The health clinic cannot do much to reduce the cost; most of the employees are volunteers that work for little or no money, making layoffs and reductions in salaries imprudent. Paperwork and other administrative tasks could be streamlined via computers to help improve efficiency, but such a change would not occur overnight. Furthermore, there is always the issue of funds. While places such as the health clinic could redistribute their prices towards their more popular revenue streams and incentivize those that come often, simple public health outreach solutions could prove to be quite effective. Demonstrations in the community focusing on self-check and self-evaluations would increase accountability while upholding the idea of preventative care. In addition, other healthcare professionals besides doctors could make periodic home visits to “high-risk” patients as part of the care they receive from the Centro de Salud. While the latter would require more human resources, it could potentially give students from nearby universities the opportunity to engage in basic physical examination practices. This would be a unique outreach initiative the Centro de Salud could pilot to reduce its own patient inflow.

Hand washing is one of the most popular public health topics in terms of universality and applicability.¹⁰ Preventing the spread of infections and illnesses is key for a preventative care approach. The Centers of Disease Control and Prevention recommends washing hands for at least 20 seconds, and up to 40 seconds depending on the drying mechanism. Over 80% of the sample population reported washing hands for less than 20 seconds. This helps to explain the spread of sicknesses and parasites in Villa El Salvador. The frequency of hand washing could also play a role, though this was not evaluated in this study. There have been initiatives involving hand washing in Villa El Salvador (Centro de Salud has one once a year), but these projects are targeted towards children. While it is important for children to learn the proper technique, it is just as important (if not more) for adults to learn as well. The adults usually prepare the food, the latter serving as a major source of illness. Furthermore, they serve as role models for their children; if they engage in proper hand washing, their children are more likely to as well¹¹ In essence, while the community has shown its support for hand washing, the older generation must take the issue more seriously.

While access to care has improved significantly in Villa El Salvador with the emergence of smaller clinics, there is still room for much improvement for the overall health of the community. The aim of this study was to quantify the current health practices of the people of Villa El Salvador to provide community-specific data. The effectiveness of follow-up studies would increase if more people were surveyed in different areas of Villa El Salvador, particularly people over the age of 50 and males. Furthermore, delving into one specific topic, such as nutrition and hand washing, would provide more depth for the respective facet of health than this study presented. Regardless, the study was successfully completed and conveys tangible information concerning the health practices the target community. It is the hope that the investigation served as a solid starting point for prospective public health initiatives in Villa El Salvador and Peru at large.

Acknowledgements

I would like to thank Enrique Bossio Montellanos, Director of Cross Cultural Solutions in Lima, Peru and Carol Soto, Head Coordinator of the San Martin de Porres Centro de Salud and the entire San Martin de Porres Centro de Salud staff for all of their support. Also, I would like to thank The Rice University Loewenstern Fellowship, and the Rice University Community Involvement Center for funding my trip. Finally, a special thanks to Sarah Hodgkinson and Mac Griswold for all of their guidance.

References

1. Clinton County Health Department website. http://www.clintoncounty      gov.com/departments/health/aboutus.html (Accessed Jul. 7, 2012).
2. Trust for America’s Health: Examples of Successful Community-Based Public Health Interventions (State-by-State). http://www.cahpf.org/GoDocUserFiles/601.TFAH_Examplesbystate1009.pdf (Accessed Jul. 7, 2012).
3. Participant Handbook: Lima, Peru. New Rochelle: Cross Cultural Solutions, 2012.
4. Perspectivas Socioeconómicas para Villa El Salvador, Observatorio Socio       Económico Laboral, Lima Sur, Lima, Peru, Jul. 2009.
5. Portal de la Muncipalidad de Villa El Salvador. http://www.munives.gob.pe/index.php (Accessed Jul. 7, 2012).
6. Vegetables: Choose My Plate. USDA. http://www.choosemyplate.gov/food-groups/vegetables.html (Accessed Jul. 22, 2012).
7. Acosta, A. M. Working Papers at IDS. 2011, 367.
8. WHO Country and Regional Data on Diabetes. http://www.who.int/diabetes/facts/world_figures/en/ (Accessed Jul. 7, 2012).
9. Fraser, B. The Lancet. 2006, 367, 2049-50.
10. Vessel Sanitation Program. Centers for Disease Control and Prevention, http://www.cdc.gov/nceh/vsp/cruiselines/hand_hygiene_general.htm (Accessed Jul. 20, 2012).
11. Stephens, K. Parenting Exchange. 2004. 19, 1-2.

Engineered Piezoelectricity of Graphene

Catalyst Volume 6

Abstract

To determine whether piezoelectric properties can be engineered selectively into graphene by doping one side of the two-dimensional sheet with compatible adatoms. Density Functional Theory (DFT) calculations compare piezoelectric stress (d₃₁) and strain (e₃₁) coefficients for several adatom combinations to other piezoelectric materials, indicating possible applications in the dynamic control of motion and deformation within nanoelectromechanical systems and structures.

Introduction

The transition from the use of microelectromechanical systems (MEMS) to that of nanoelectromechanical systems (NEMS) poses many problems, notably that many of the methods employed to engineer properties such as transduction and strain response into MEMS begin to fail when carried into the nanoscale.^1,2 One of the greatest challenges faced with implementing NEMS in consumer technology is generating the ability to exert dynamic control over the motion and deformation of nano-structures. Piezoelectric materials are already featured prominently as strain sensors for vibration detectors, electromechanical transducers, and probes for atomic force microscopes to achieve this kind of control.^1,3 However, no promising analogue among known materials exists to afford this at the nanoscale.

Piezoelectricity is a remarkable property of some materials in which mechanical deformation results in the production of an electric field.⁴ The reverse piezoelectric effect is also true, and exposure to an electric field will cause a change in the dimensions of the piezoelectric material. In the modeling of the piezoelectric behavior of a material, d31 and e₃₁ are among several of the coefficients used. The d₃₁ coefficient relate in-plane strain to the electric field and the electrical polarization perpendicular to the plane, while e₃₁ describes the magnitude of the piezoelectric effect displayed in the material for a small deformation or applied electric field.^5,6

The ability to produce piezoelectric nanomaterials efficient at creating desired functionalities in a nano-device is an attractive prospect. To this end, it is thought possible to make nano-scale materials piezoelectric, even if they do not intrinsically possess this quality, by introducing nanoscale inhomogeneities into the material. In such systems, provided that they are non-centrosymmetric (i.e. they lack inversion symmetry), even uniform stress would induce polarization of the material which would indicate piezoelectric behaviour.²

Theoretical demonstrations have calculated that by inducing strain in piezoelectric materials it is possible to bring about a reversal in the magnetic fields of other materials.⁷ This effect is studied in a new field known as straintronics. In applications such as computation and signal processing, ambient energy alone is enough to generate this level of strain, resulting in devices requiring ultralow energy inputs to function as well as a minimization of energy dissipation. This effect has been computationally displayed in lead-zirconate-titanate (PZT) nanomagnets shown to have extraordinarily low energy dependence.⁸ The novelty of nanoscale films with these properties would allow for versatile and energy efficient control of nanodevices. Specific control over the dimensionality of thin films allows for precise control of the magnitude of their resultant properties. This is well understood for many materials through the transition from their bulk to their nanoscale analogs.

A two-dimensional sheet would constitute the idealized thin film, but it was initially believed that two-dimensional materials would be thermodynamically unstable and could not exist. This was based primarily on the observation that the melting temperature of thin film materials decreases rapidly with decreased thickness.⁹ The surprising discovery of graphene provided a material stronger than any other, that was still lightweight and flexible.¹⁰ Graphene exists as a monolayer of sp² hybridized carbon atoms one atomic layer thick, making it a two-dimensional material.⁹ Graphene is traditionally isolated from graphite by a mechanical exfoliation process, but has also been formed using chemical vapor deposition and arc discharge techniques, which allow for the production of graphene with high electrical conductivities.^11,12,13

Because graphene is well within the nanoregime, it shows much promise in aiding the transition from MEMS to NEMS and there has been a concerted effort to uncover the extent of its potential in technology. Chemical doping is one way to tailor graphene’s properties to suit various needs within device applications. Studies demonstrate that doping graphene with potassium changes its molecular symmetry and modifies its electronic properties.¹⁴ Understanding how dopants affect graphene could provide piezoelectric nanofilms for NEMS device control. Pertinent to this is the distribution density of adatoms, and the sites they reside in. The specific binding site on the graphene sheet may affect graphene’s symmetry and subsequently its properties, as this is seen in other molecules.¹⁵ This may be above a carbon atom (top site), in the center of the hexagon (hollow site) or over a bond joining carbon atoms (bond site) (see Fig. 4A).

Computer modeling techniques such as Density Functional Theory (DFT) are useful in this regard because they enable the use of theory to predict the behavior of matter under different conditions.¹⁶ The results of these calculations can then inform of the optimal synthesis routes to pursue in generating the desired reaction or products. DFT allows us to do this through the observation that a given wave function contains much more information about a system than is necessary to describe its behavior. Ground state molecular energy, the wave function, and all other molecular electronic properties are uniquely determined by the electron probability density ρ(x,y,z), which as a three-variable function is less computationally intensive. In principle, given the ground state electron density, the Hohenberg-Kohn theorem propounds that all ground state molecular properties can be calculated from ρ.¹⁷

Methods

Density functional theory implemented into the Quantum-ESPRESSO ab initio software package was used to carry out the calculations in this study. Ion cores were treated using Vanderbilt pseudopotentials in all cases except that of potassium, in which a norm-conserving pseudopotential treatment was utilized. A nonlinear core correction was included for potassium and fluorine. Electron exchange and correlation effects were described using the spin-polarized generalized-gradient corrected Perdew-Burke-Ernzerhof (PBE) approximation. All calculations were done using periodic boundary conditions and a primitive cell with one atom for every two carbon atoms except when indicated otherwise (Fig. 1C). The electronic wave function is expanded in a plane wave basis set with an energy cut-off of 60 Ry.

Single atom adsorption on one side gives rise to an asymmetric surface with a net electric dipole moment. A dipole correction was used to cancel out the artificial electrical field that arises from this dipole moment. Focus was placed upon calculating the d₃₁ and e₃₁ piezoelectric coefficients, where d₃₁ is the transverse piezoelectric coefficient that describes the deflection normal to the direction of polarization. The e₃₁ term signifies the intrinsic piezoelectric coefficient, where a large e₃₁ value corresponds to a large electric charge induced at a small cost of mechanical strain, or conversely a large mechanical force generated in the presence of a small electric field.

Cases examined graphene doped with uniform coverages of lithium, potassium, hydrogen, and fluorine atoms. Consideration was given to situations involving two different atom dopants on opposite sides of the graphene sheet, such as fluorine and hydrogen, or fluorine and lithium. Several atom coverage densities were modeled using lithium by placing a single lithium atom in 1 X 1, 2 X 2, 3 X 3, and 4 X 4 graphene periodic supercells. A Löwdin analysis was used to calculate the partial charges on the lithium atoms at each of concentrations. Furthermore, the effect of adatom position on piezoelectric response was examined, in addition to the effect of crystallographic patterning of adatoms on the graphene surface for a fixed concentration of C₃₂Li₂.

Results

DFT calculations showed that Li and K preferentially bound to the central hollow sites of the graphene sheets, resulting in hexagonal (6mm) point group symmetry (Fig. 1B). H and F were found however, to bind at the top site which resulted in trigonal (3m) symmetry (Fig. 1B). In the cases involving two atoms, at least one of these atoms must bind to the top site, producing trigonal (3m) symmetry (Fig. 1B).

The 6mm, 32, and 3m point groups all result from the destruction of inversion symmetry, hence these materials will be non-centrosymmetric and display piezoelectric behavior. Point group symmetry enables the determination of those materials having non-zero d₃₁ and e₃₁ coefficients, which are common to all configurations in Fig. 1B. Upon application of a sawtooth potential (ensuring forces acting on the system are asymmetric, allowing the system center to experience force) with a width of 10 Å, an electric field is applied to the material. A roughly linear relationship is found between the electric field and the strain induced in the material when the field amplitude lies between -0.5 and 0.5 V/Å for many graphene-adatom combinations examined.

These behaviours have been experimentally achieved in devices containing graphene.¹⁸ The d₃₁ coefficient is equal in magnitude to the gradient of the trend-lines (Fig. 2A & 2B) and Table 1 shows the d₃₁ coefficients extracted from these lines, which display variability within three orders of magnitude. It was found that the binding of F to the graphene sheets resulted in only minor changes to the piezoelectric coefficient. This occurs similarly when H and F bind in an alternating manner and transverse to one another. The alkali metals Li and K however, produced much larger effects on the d₃₁ coefficient. The greatest effect is achieved when F is added to the three top sites, with Li residing in the hollow on the opposite side of the sheet, yielding a d₃₁ value of 3 X 10^-1 pm/V. This is comparable to the theoretical value for the 3D piezoelectric boron nitride (BN), which is 3.3 X 10^-1 pm/V.

The e₃₁ coefficients were obtained by calculating the change in polarization normal to the surface as a function of the equibiaxial strain in the plane (from Fig. 2B). This gives a linear relationship in all cases between low strain values of -1% to 1%. A consequence of employing the equibiaxial in-plain strain is that the gradient of the trend-line has a value of twice the e₃₁ coefficient for each atom. It was found that both alkali metals Li and K possessed the largest values for e₃₁, indicating that they will deform the most in the presence of a small electric field, and they will generate the greatest charge under a given strain. Unlike the results for the d₃₁ coefficient, e₃₁ does not undergo a significant change when F is placed in the top sites with Li in the opposite hollow site.

To compare values for 2D graphene to those of traditional and well understood 3D materials, the e₃₁ and d₃₁ coefficients were divided by the 3.35 Å interlayer spacing yielding an e₃₁,3D of 0.17 C/m² and a similar d₃₁ value of 0.19 C/m². When compared to the e₁₁,3D value of 0.731 C/m² for two dimensional BN, it is smaller by a factor of 4, but it must be noted that e₁₁ coefficients are generally much larger than those of e₃₁. When the e₃₁,3D is calculated this difference becomes much smaller, specifically as 0.31 C/m² for wurtzite BN and -0.55 C/m² for gallium nitride. It is important to note that graphene has the potential to have much larger polarization magnitudes than either of these materials, since graphene can undergo much more elastic strain before plastically deforming. This demonstrates the possibility of engineering piezoelectric graphene comparable to known materials.

It was found that varying Li coverage (Fig. 2C) causes deviation in the relationship between the electric field and the induced strain (Fig. 3A). By plotting the d₃₁ coefficient as a function of Li coverage density, a maximum value is obtained when n = 8, corresponding to the unit cell C₈Li. When this density is decreased the d₃₁ value shows a steep decline (Fig. 3A inset). In contrast, the static dipole moment of Li on graphene increases as the Li coverage density decreases, producing a stronger interaction with the electric field (Fig. 3B bottom inset).

However, at low coverage densities this interaction is diminished, and a maximum is found between coverage density and d¬31 as a result of their competitive effects (Fig. 3A inset). The e₃₁ coefficient varies in a similar manner to that of d₃₁, also showing a maximum value for the C₈Li unit cell, with decreasing values as coverage density decreases. This shows that the magnitude of the piezoelectric response doped graphene will display can be varied as a function of the adatom coverage density.

Considering only the C₃₂Li system, there is nonlinear piezoelectric behavior in d31 when the electric field is less than -0.1 V/Å. When the field strength becomes less than this, nonlinear behavior is sharply displayed, and the strain exhibits a rapid linear decrease, with a trend-line gradient of 0.19 pm/V. This is because at approximately -0.1 V/Å, Li and graphene begin to undergo a charge transfer process. It should be noted that this charge transfer process could be used to very efficiently power the opening and closing of gates in transistor technologies.

Again considering all tested values of n, for electric field strengths greater than -0.1 V/Å, Li maintains a constant charge, but when this number is lesser, the charge transfer process comes into effect and this value decreases. Furthermore, when varying the height of Li above the graphene sheet as a function of the electric field, there is a minimum value obtained for -0.1 V/Å for the C₃₂Li system, while the other coverage densities still display linear behavior. When the electric field value is larger, varying the distance between Li and the graphene sheet produces no change in charge. When the value is smaller, charge transfer occurs. The implication is that the charge transfer from graphene to Li is theoretically responsible for the large piezoelectric response to fields lesser than -0.1 V/Å.

We also investigated the effects of adatom position on the piezoelectric response of the doped graphene. While K and Li appear to diffuse moderately across the graphene sheet, it is not expected that this will affect the materials piezoelectric behaviour. This is because the doped graphene will be non-centrosymmetric regardless of the adatom location. The d₃₁ coefficient was calculated when Li was placed at the hollow, top, and bond sites of the unit cell and the strain varies upon subjection to an electric field normal to the surface of the adatom positions (Fig. 4A). Again, the gradient of the trend-line gives the value of the d₃₁ coefficient, and these are all found to be within 5% of one another, indicating that the piezoelectric response is independent of the position for the adatoms.

Crystallographic patterning of adatoms on graphene sheets with an adatom coverage density of C₃₂Li has shown that by varying the relative position of the Li atoms in the unit cell, a 20% change is produced in the d₃₁ coefficient, showing closer resemblance to a coverage density of C₈Li (Fig. 4B). Despite this, it is unlikely that the pattern of adatoms on the sheet has a significant impact on the piezoelectric response strength of the graphene-adatom system based on the calculations carried out by the authors.

Discussion

Possessing the ability to calculate ground state molecular properties allows for an accurate determination of the behavior of materials under different theoretical conditions and permits the dynamic alteration of variables affecting the system. Widespread difficulty has been associated with producing specific desired behaviors in devices at the nanoscale (such as challenges of self-assembly).^19,20 As such, being able to rapidly vary parameters in a given system and observe how the theoretical model responds to those changes is a very powerful tool. Undoubtedly, modeling techniques will play a large role in the creation of next generation devices and materials, allowing us to push the limits of technology beyond the bulk, and to finally enter the realm of nanoscience with devices such as sensors/actuators and transistors, among others.^21,22

The authors’ work has done much to further this goal. This study paints graphene in an entirely new light as a potential host to a myriad of applications in nanoscience and nanotechnology previously inaccessible. The novelty of demonstrating for the first time that a material which is not intrinsically piezoelectric can be made so through careful chemical processes broadens our understanding of how to tailor nanomaterials so we can make them work for us. Given this, there are many topics that can be discussed in hindsight of this theoretical study and which the authors may consider.

The ability of graphene to display piezoelectric behavior could be exploited extensively in the fledgling field of straintronics.^7,8 In the previously discussed theoretical study, PZT nanomagnets were shown to be viable power sources for low-energy devices. One of the limitations of using this kind of material is its rigid nature and relatively large thickness (relative to graphene sheets) on the order of ~50 nm.⁸ By partially hydrogenating a graphene sheet, a compound known as graphone, a ferromagnetic and hence magnetostrictive material is obtained.²³ A small number of these could be stacked with a layer of piezoelectric graphene surrounding, within, or in between these, resulting in a setup comparable to the PZT nanomagnet studied. An advantage of this type of material stems from the enormous tensile strength and high flexibility of graphene films.^24,25,26

Additionally, the excellent ability of graphene to conduct thermal energy only increases its appeal for use in molecular electronics.²⁷ Nanodevices are currently expensive to manufacture, and their oftentimes integrated nature and small sizes impossible to manually fix necessitate that if a defect occurs, the device as a whole would need to replaced. With increasing power densities as devices are scaled down, thermal effects on devices must be given increasing importance. The high thermal conductance displayed by graphene may play a key role in maintaining the functionality of nanodevices under thermal stress by facilitating heat flow away from the delicate electronics within.²⁸ Thermal expansion necessarily resulting from such applications as thermal conductance produces strain in the graphene that could further assist in power generation in straintronics applications.

Graphene has also found uses in supercapacitor technology as electrodes. A graphene nanofiber composite with conducting polymer polyaniline has shown capacitance values as high as 480 F/g at a current density of 0.1 A/g. This nanocomposite also shows good cycling stability during the charge-discharge process.²⁹ Traditional piezoelectric materials display problems such as brittleness (as with PZT), among others.³⁰ Graphene based piezoelectric devices would not have issues of brittleness as a result of their high flexibility. Furthermore, piezoelectric graphene would allow for efficient energy harvesting for its use in supercapacitors. Such technologies discussed here allude to the integration of graphene as various components of the same devices based on its treatment and subsequent properties. This theme may represent a future where carbon based materials completely replace silicon in fields such as molecular electronics. Graphene has the potential to revolutionize the development of all nanodevices as a result of its ability to take on so many chemical and physical roles as a material because of its chemical versatility and its exceptional mechanical and electrical properties.

Despite all of this, graphene is not centrosymmetric and as such intrinsically is not a piezoelectric material. If the calculations in this work lead to synthetic realization of the properties modeled then it will require processing to make it such. Since BN and GaN, among others, are well established piezoelectric materials that are clearly capable of being produced as nanotubes or monolayer sheets, why should graphene be a more attractive alternative?^31,32,33 This appeal is outlined by the authors and it is exactly because graphene is not intrinsically piezoelectric that it holds such potential. Since piezoelectricity arises from adatom doping, graphene could be selectively doped to produce areas or patterns that are piezoelectric, and ones that are not. Furthermore, graphene has a zero band gap, but this can easily be varied by adding dopants to the sheet. Choosing BN nanosheets necessitates that the device has a band gap of 4.60 eV, and for GaN sheets of 3.4 eV, without significant evidence for altering these values appreciably.^31,33 Graphene therefore possesses much more versatility in its applications, since it can be altered in various manners to serve a multitude of different tasks in the same device.

In future studies towards this goal, it would be valuable to examine this response at higher strain. The authors model the piezoelectric effect between -1% and 1% strain, noting that a rough conversion indicates graphene may be comparable to BN and GaN. They also mention that it may be able to surpass these materials since it is capable of enduring higher strain before failing. It would be interesting to probe the extent to which graphene can be deformed as a piezoelectric material and how this affects the magnitude of the piezoelectric response.

However, the authors’ model may oversimplify the assumptions leading to the proposed unit cells. They model every carbon as associated with an adatom for top site binding, or every hollow site as associated with an adatom for hollow site binding. Hydrogen is one of the adatoms calculated where all of the top sites are occupied on the graphene sheet. Contrary to this, there have been experimental studies that have demonstrated that at high hydrogen concentrations there is a problem with the clustering of hydrogen adatoms rather than their maintained homogeneous dispersion (Fig. 5).³⁴ It has also been demonstrated that the binding energy for fluorine with this pattern is extremely low compared to more dispersed patterning and that if this were to occur it is likely to be a highly unstable compound.³⁵ While this may be the case for low dosages of adatoms, it may be necessary to synthesize such piezoelectric graphene to say with any great conviction how adatoms will behave at high concentrations on the sheets surface.

The authors’ work has opened up many avenues of pursuit in efforts to shift to carbon based electronics. Of particular note, in the event of its synthesis, are the applications for straintronic devices and similar molecular electronics. There are undoubtedly a host of other applications in which piezoelectric graphene could serve as an invaluable material. Research in graphene will almost certainly provide the materials necessary to transition from current technologies to smaller, more efficient, and less energy intensive ones. Graphene possesses an apparent wealth of properties through manipulation of its structure and interactions, its insurmountable thinness, and its extraordinary strength. These are just some of the properties of graphene known to us that suggest it will play a major role in the implementation of nanotechnologies as the way of the future.

References

1. Ekinci, K. L. “Electromechanical Transducers at the Nanoscale:Actuation and Sensing of Motion in Nanoelectromechanical Systems (NEMS).” Small 8.9 (2005): 786-797.
2. Sharma, N. D., R. Maraganti and P. Sharma. “On the Possibility of Piezoelectric Nanocomposites Without Using Piezoelectric Materials.” Journal of the Mechanics and Physics of Solids 55.11 (2007): 2338-2350.
3. Kon, Stanley, Kenn Oldham and Roberto Horowitz. “Piezoresistive and Piezoelectric MEMS Strain Sensors for Vibration Detection.” Tomizuka, M., Chung-Bang Yun and Victor Giurguitui. Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems 2007. SPIE, 2007. 65292V-1 - 65292V-11.
4. Callister, William D. Jr. and David G. Rethwisch. Materials Science and Engineering: An Introduction. 8e. Hoboken: John Wiley & Sons Inc., 2010.
5. Do, Dal-Hyun. Investigation of Ferroelectricity and Piezoelectricity in Ferroelectric Thin Film Capacitors Using Synchotron X-Ray Microdiffration. PhD Thesis. University of Wisconsin-Madison. Madison: ProQuest Dissertations and Theses, 2006.
6. Ouyang, Jun, R. Ramesh and A. L. Roytburd. “Intrinsic Effective Piezoelectric Coefficient e31,f for Ferroelectric Thin Films.” Applied Physics Letters 86 (2005): (152901)1-3.
7. Iwasaka, Yoh. “Stress-Driven Magnetization Reversal in Magnetostrictive Films With In-Plane Magnetocrystalline Anisotropy.” Journal of Magnetism and Magnetic Materials 240 (2002): 395-397.
8. Roy, Kuntal, Supriyo Bandyopadhyay and Jayasimha Atulasimha. “Hybrid Spintronics and Straintronics: A Magnetic Technology for Ultra Low Energy Computing and Signal Processing.” Applied Physics Letters 99.6 (2011): (063108)1-3.
9. Geim, A. K. and K. S. Novoselov. “The Rise of Graphene.” Nature Materials 6 (2007): 183-191.
10. Lee, Changgu, et al. “Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene.” Science (2008): 385-388.
11. Novoselov, K. S., et al. “Electric Field Effect in Atomically Thin Carbon Films.” Science 306 (2004): 666-669.
12. Lee, Youngbin, et al. “Wafer-Scale Synthesis and Transfer of Graphene Films.” Nano Letters 10 (2010): 490-493.
13. Wu, Zhong-Shaui, et al. “Synthesis of Graphene Sheets with High Electrical Conductivity and Good Thermal Stability by Hydrogen Arc Discharge Exfoliation.” ACS Nano 3.2 (2009): 411-417.
14. Ohta, Taisuke, et al. “Controlling the Electronic Structure of Bilayer Graphene.” Science 313 (2006): 951-954.
15. Sonnleitner, Tobias, et al. “Molecular Symmetry Governs Surface Diffusion.” Physical Review Letters 107 (2011): (186103)1-4.
16. Argaman, N. and G. Makov. “Density Functional Theory: An Introduction.” American Journal of Physics 68.1 (2000): 69-79.
17. Levine, Ira N. Quantum Chemistry. 4th Edition. Englewood Cliffs: Prentice-Hall, 1991.
18. Zhang, Tuanbo, et al. “Direct Observation of a Widely Tunable Bandgap in Bilayer Graphene.” Nature 459 (2009): 820-823.
19. Katsuhiko, A., et al. “Challenges and Breakthroughs in Recent Research on Self-Assembly.” Science and Technology of Advanced Materials 9 (2008).
20. Cui, Z. and C. Gu. “Nanofrabrication Challenges for NEMS.” 1st IEEE International Conference on Nano/Micro Engineered and Molecular Systems. 2006.
21. Ekinci, K. L. “Electromechanical Transducers at the Nanoscale:Actuation and Sensing of Motion in Nanoelectromechanical Systems (NEMS).” Small 8.9 (2005): 786-797.
22. Timp, G., et al. “The Ballistic Nano-Transistor.” Electron Devices Meeting. 1999. 55-58.
23. Zhou, J., et al. “Ferromagnetism in Semihydrogenated Graphene Sheet.” Nano Letters 9.11 (2009): 3867-3870.
24. Frank, I. and D. Tanenbaum. “Mechanical Properties of Suspended Graphene Sheets.” Journal of Vacuum Science and Technology B 25.6 (2007): 2558-2561.
25. Bunch, Joseph. “Mechanical and Electrical Properties of Graphene Sheets.” Dissertation. 2008.
26. Kim, B., et al. “High-Performance Flexible Graphene Field Effect Transistors with Ion Gel Gate Dielectrics.” Nano Letters 10 (2010): 3464-3466.
27. Balandin, Alexander, et al. “Superior Thermal Conductivity of Single-Layer Graphene.” Nano Letters 8.3 (2008): 902-907.
28. Shahil, Khan and Alexander Balandin. “Graphene-Multilayer Graphene Nanocomposites as Highly Effiicient Thermal Interface Materials.” Nano Letters 12 (2012): 861-867.
29. Zhang, K., et al. “Graphene/Polyaniline Nanofiber Composites as Supercapacitor Electrodes.” Chemistry of Materials 22.4 (2010): 1392-1401.
30. Wang, Lei. “Vibration Energy Harvesting By Magnetostrictive Material for Powering Wireless Sensors.” PhD Thesis. 2007.
31. Golberg, D., et al. “Boron Nitride Nanotubes and Nanosheets.” ACS Nano 4.6 (2010): 2979-2993.
32. Goldberger, J., et al. “Single-Crystal Gallium Nitride Nanotubes.” Letters to Nature 422 (2003): 599-602.
33. Haiming, L., et al. “Electronic Structures and Magnetic Properties of GaN Sheets and Nanoribbons.” Journal of Physical Chemistry C 114.26 (2010): 11390-11394.
34. Balog, R., et al. “Atomic Hydrogen Adsorbate Structures on Graphene.” Journal of the American Chemical Society 131 (2009): 8744-8745.
35. Robinson, J., et al. “Properties of Fluorinated Graphene Films.” Nano Letters 10 (2010): 3001-3005.

Eating Wheat: Avoiding the Bad and Getting the Good

Catalyst Volume 6

A bagel or bowl of cereal is common for breakfast, followed by a sandwich or burger for lunch. Dinner often stars pasta, pizza, or a casserole as the main dish. There is one ingredient that lurks in nearly every American meal.

Wheat. It’s the main ingredient in bread, the most purchased packaged food in the United States.¹ It plays an integral role in many diets, but if not correctly consumed, it can damage the human body. The harmful effects include increased risk of weight gain, cardiovascular disease, and even cancer.² To avoid adverse effects while reaping the benefits wheat offers, three factors should be considered: wheat type (whole-grain or refined), the portion size, and the accompanying ingredients.

Whole Grains Instead of Refined Grains

Guidelines by the United States Department of Agriculture (USDA) recommend Americans consume whole-grain rather than refined wheat. Currently, the average consumption of whole-grain foods is approximately one serving a day, falling short of the recommended three servings.³ Wheat grains are divided into three parts: endosperm, germ, and bran (Figure 1). Whole-grain wheat grains have the germ and bran intact. In contrast, refined grains that have the bran and germ separated from the starchy endosperm comprises 80% of the grain. Unfortunately, this processing robs wheat of the majority of its nutrients, which are concentrated in the bran and germ.

Whole grain wheat has nearly ten times more dietary fiber, five times as many vitamins and cancer-preventing phenolic compounds, and three times as many essential minerals including zinc, iron, and selenium (Table 1).

The extra dietary fiber of whole-grain wheat itself is a compelling reason to choose it over refined wheat. Increased consumption of dietary fiber has been observed to improve cholesterol concentrations, lower blood pressure, and aid in weight loss. These effects all reduce the risk for coronary heart disease, the leading cause of adult deaths in the United States.⁴ High-fiber foods facilitate metabolic effects and control caloric intake by increasing satiety. Dietary fiber, consisting of insoluble and soluble components, promotes gastrointestinal health as a probiotic for beneficial bacteria in the colon. Both fibers also provide cardiovascular benefits by lowering “bad” cholesterol, or LDL.

In the broader context of a person’s entire diet, high-fiber foods often have lower energy density and take longer to eat. These two traits promote satiety, curbing consumption of potentially unhealthy foods and lowering total caloric intake. Eating refined wheat, such as white bread and pasta, causes one to not only forego nutrients, but also consume more calories before feeling full. Overconsumption of calories coupled with physical inactivity are major risk factors leading to heart disease and obesity.⁵

Control Portion Size

In addition to considering what type of wheat one eats (e.g., wholewheat instead of white bread for toast in the morning), an equally important factor is quantity. Feasting upon large portions of wholegrain wheat regularly results in damaging spikes in blood sugar that can lead to an chronic state of Type 2 diabetes.⁶ Since diabetes is the leading cause of kidney failure in the United States and doubles the risk of stroke, its correlation with consumption of refined wheat is important to understand.⁷

The biochemical phenomenon underlying this link is called insulin resistance. Insulin is a hormone stimulating various tissues to store glucose from the blood as glycogen. When carbohydrates are digested, they are broken down into glucose, which is transported into the bloodstream, consequently increasing blood sugar levels. This causes pancreatic beta cells to synthesize insulin to convert the increased glucose into glycogen. When the body does not perform these functions well, the resulting condition is Type 2 diabetes.

Even though the USDA advises adding whole-grain wheat to one’s diet, USDA guidelines do not account for the spiking effect on blood sugar when a large portion is eaten in a short time frame. Their guidelines use a rating system called the glycemic index (GI) that is widely utilized in nutrition studies as a quality standard of carbohydrate foods.⁸ Wonder®, fully enriched white bread, has a GI of 71 while bread made of 80% whole-grain and 20% refined wheat flour has a GI of 52.⁸ In practical terms, these GI values indicate a 70% increase in blood sugar compared to the blood sugar increase caused by a comparable amount of pure glucose. Likewise, whole wheat bread causes an increase in blood sugar 52% of that caused by glucose. Based on the aforementioned pathogenic contribution of blood sugar spikes, the lower GI of whole-wheat bread quantitatively demonstrates its superiority over white bread.

However, consider the following: the Twix candy bar has an even lower GI of 44. Watermelon has a GI of 72. How does this make sense? The glycemic index fails to account for realistic portion sizes. When the foods are empirically tested on people for their effects on blood sugar, the quantities eaten are equivalent to 50 grams of carbohydrates. Three-quarters of a king-sized Twix bar constitutes 50 grams of carbohydrate, but so do 5 cups of diced watermelon. This difference in volume is due to the fiber and water content of watermelon.

Realistically, a person is likely to eat a whole king-sized Twix bar or one cup of diced watermelon in one sitting. Adjusting for actual serving sizes and assuming linearity, the Twix bar has what is now called a glycemic load (GL) of 58.7 and watermelon a GL of 14.4. As a more relevant implementation of GI values, glycemic load emphasizes control of portion sizes in eating carbohydrates. The GI value of whole-grain wheat is always lower than refined wheat vary, but the difference is small enough that one cup of refined flour pasta might be better than 2 cups of whole-wheat flour pasta in preventing Type 2 diabetes.

Watch Out for Accompanying Ingredients

The final factor to consider is that wheat is rarely eaten alone. In the processing and cooking to make it edible, wheat is nearly always mixed with other ingredients that are potentially harmful. Most breads, pastas, pancakes, cereals, and other wheat products have at least five ingredients trailing behind the primary wheat ingredient, which are broadly classified as preservatives, sweeteners, emulsifiers, leavening agents, flavor enhancers, and dough conditioners. All of these additives to wheat affect short-term feelings after consumption as well as long-term effects on the body. In particular, one should avoid partially hydrogenated oils and moderate high-fructose corn syrup.

Added as dough conditioners and preservatives, partially hydrogenated oils are considerable factors in coronary artery disease, which causes at least 30,000 premature American deaths per year.⁹ They contain trans fats, which have been unequivocally linked to lowering “good” high-density lipoprotein (HDL) cholesterol and raising “bad” low-density lipoprotein (LDL) cholesterol. Although large companies have removed trans fats, including partially hydrogenated oils, from foods such as Kraft’s Oreos in response to mounting criticism beginning in 2005, numerous food companies still include partially hydrogenated oils in their wheat products. For example, cake mixes, packaged baked goods, and peanut butter are commercially made with partially hydrogenated oils on a regular basis because they simplify manufacturing and reduce costs while increasing the final product’s shelf life. Manufacturers obfuscate this addition by stating the trans fat content of foods as “0g” on nutrition labels. This is allowed because 0.5 grams of trans fat is one serving. However, less than 0.5 grams of trans fat per serving can accumulate when consuming multiple servings of foods such as chips or crackers. Instead, check for the words “partially hydrogenated” or “shortening” in the ingredients list.

While partially hydrogenated oils are conclusively life-threatening, high-fructose corn syrup (HFCS) is a controversial additive. Manufacturers favor the use of HFCS as a sweetener in wheat products due to lower cost, sweeter taste, and higher miscibility. Scientists hypothesize that corn-derived sugar has endocrine effects that lead to obesity, Type 2 diabetes, and metabolic syndrome.⁸ Insulin and leptin are key hormone signals that regulate a person’s sense of hunger, but consumption of high-fructose corn syrup depresses these internal signals from controlling calorie intake. Another consequence of foods sweetened with HFCS is plaque buildup inside the arteries.¹⁰ Nearly any sweet good made from wheat will likely contain HFCS. Although data about its health effects are still inconclusive, HFCS should be avoided.

Being a health-conscious consumer of wheat can mean significant changes in daily choices of which foods to eat and how to eat them. Whole grains provide more fiber and life-boosting nutrients than refined grains, but accompanying ingredients in available food choices need to be considered as well. More importantly, the impacts of wheat on blood sugar need to be controlled by consuming a commensurate amount of fruits and vegetables. Awareness and application of these principles are the main steps to avoiding the bad and getting the good of wheat.

References

1. Nielsen Homescan Facts, The Nielsen Company. http://www.marketingcharts.com/television/nielsen-issues-us-top-10-lists-for-2007-2700/nielsen-2007-top-10-cpg-purchased-us-homes.jpg/(Accessed Jan. 15, 2013).
2. Slavin, J. L. Amer. J. Clin Nutr. 1999, 70, 459S-63S.
3. Cleveland, L. E. J. Amer. Coll. Nutr. 2000, 19, 331–8.
4. Anderson, J. W. Nutr. Rev. 2009, 67, 188–205.
5. Swinburn, B. Public Health Nutr. 2007, 7, 123–46.
6. Liu, S. J. Amer. Coll. Nutr. 2002, 21, 298–306.
7. World Health Organization: Diabetes Fact Sheet, Media Centre. 2012 http://www.who.int/mediacentre/factsheets/fs312/en/index.html (Accessed Jan. 15, 2013).
8. Foster-Powell, K. Amer. J. Clin Nutr. 2002, 76, 5–56.
9. Ascherio, A. Amer. J. Clin Nutr. 1997, 66, 1006S–10S.
10. Stanhope, K. Amer. J. Clin Nutr. 2008, 88, 1733S-7S.
11. General Mills. What is Whole Grain, Anyway? Demystifying Whole Grains. http://wholegrainnation.eatbetteramerica.com/images/content/facts_seed.jpg (Accessed Jan. 15, 2013).
12. Thompson, L. U. Contemp. Nutr. 1992, 17.