Month: April 2013

Learning to Control Colloids with International Space Station Research

In today’s A Lab Aloft, guest blogger Donald Barker explains the complex world of colloids and how studying them aboard the International Space Station helps us understand and use them better here on Earth.

Colloids are fascinating. They are part of our daily lives, found in everything from our bodies to the products we purchase at the convenience store. Manufacturers use colloids and their unique structure and properties for wine making, coloring glass, and fabric softeners. You will even find them in your daily glass of milk!

So what exactly is a colloid? In our daily lives we generally think of traditional forms of matter: solids, liquids, gasses. Colloids, however, exist around and near the boundaries of these states—not quite being one or the other. Colloids generally take one of the following forms: aerosols, emulsions, gels, sols, foams or films.

Colloids form when particles disperse throughout a solvent, usually a liquid, depending on the purpose of the mixture. Colloidal particles are too small to be seen with ordinary optical microscopes. The size of the particles is somewhere between atoms and molecules, roughly 10 to 1,000 nanometer (or 1 micrometer). At such minute scales, physical interactions seem to work in mysterious and magical ways. This critical particulate size range is exactly where it needs to be in order to make it unlikely that they will settle out of their mixture; this property is why they are so useful.

For researchers interested in colloids, the International Space Station provides a unique laboratory environment to examine their properties. On Earth, gravity-induced settling or sedimentation changes or destroys the structure of a colloid over time. In microgravity, scientists have a stable setting where they can observe the particle interactions and structures while changing various environmental parameters, such as temperature and pressure.

Researchers are directly interested in the interactions occurring between the surface of the colloidal particles and their solvent. The mixture behaves in different ways, based on both the size of the colloid particles and their interactions with the solvent. Ongoing colloid investigations make use of space station facilities like the Microgravity Science Glovebox (MSG), the Fluids Integrated Rack (FIR) and the Light Microscopy Module (LMM).

 
Don Pettit, Expedition 30 Flight Engineer, working with the Microgravity Sciences Glovebox (MSG) in the U.S. Laboratory. (Credit: NASA)

Understanding the behavior of colloids allows scientists to create models and process that can be used to enhance food and chemical preservation, evenly distribute ingredients used to produce glues, jellies and gelatins or even to control the movement of light in optical devices and materials. Controlling colloidal mixtures can help global industries create better, more reliable products and processes.

 
Astronaut T.J. Creamer working at the Light Microscopy Module, or LMM, facility aboard the International Space Station. (Credit: NASA)

Colloid studies occur regularly on the space station and one of these investigations is the Advanced Colloids Experiment-1, or ACE-1. The ACE-1 containment device holds up to 20 sample disks that, in turn, each hold up to 10 wells of colloidal particles. Astronauts mix the samples in each disk and then observe them using the LMM. The crew member takes pictures for downlink to investigators for analysis on the ground, where the investigators monitor and record colloidal structural changes and particle interactions.

The goal of ACE-1 is to understand how colloids move over time in the microgravity environment. By seeing how these particles naturally aggregate or cluster without the pull of gravity, scientists can learn how to control them. Essentially, they are looking to see how nature grows at the particle level, forming order out of disorder. Researchers hope to see how well their theoretical understanding compares to the world of everyday observations.

 
Scanning electron microscope, or SEM, images of a mixture of 3.8 micron diameter “seed” particles together with the bulk colloid—0.33 micron diameter Polymethylmetachrylate, or PMMA, spheres. Recent International Space Station colloid studies show a cycle of replication, as large crystals generate smaller ones that separate and continue to grow and produce. (Credit: P.M. Chaikin and A.D. Hollingsworth, New York University)

Another set of colloid studies aboard station is the Binary Colloidal Alloy/Aggregation Test, or BCAT investigation. The BCAT-6 study is the latest in a series of related experiments run on the station. It uses a sample growth module that holds 10 couvettes—small test tubes, each with a different colloid solution mixture. Observations begin following the stirring of each sample. Manual and automated time-lapse photographs record the separation over time.

Objectives of the BCAT suite of investigations include studying the dynamics between phase separation and crystallization in the solution, as well as how order arises out of disorder in microgravity.


These images show the BCAT sample growth module (left) and a close up of a BCAT-5 sample (right) showing structural changes in the mixture aboard the International Space Station. (Credit: NASA)

Another station investigation is the Selectable Optical Diagnostics Instrument – Aggregation of Colloidal Suspensions, or SODI-Colloid. This is a series of experiments using cell chambers that hold individual samples that are measured optically using a Near-Field Scattering (NFS) technique within the MSG.

Understanding how the particles making up colloids react, move, arrange and form crystals as the temperature reaches the critical point can help with the development of materials for devices using electromagnetic waves and signals to manipulate optics, such as plasma TVs.

 
This image shows a false color NFS image during aggregation showing the distribution of particles on the smallest of scales. (Credit: S. Mazzoni, ESA)

A very different colloidal mixture—a magnetic one—is studied in Investigating the Structure of Paramagnetic Aggregates from Colloidal Emulsions-3, or InSPACE-3. This series of microgravity studies focuses on mixtures with magnetizable particles of varying shape (spheres to ellipsoids) exposed to an alternating magnetic field.

These kinds of fluids are considered to be “smart” materials, which transition into a solid-like state or gel when exposed to a magnetic field. Understanding how to control and produce colloidal materials of this kind may help in the engineering of vibration dampening systems, enhanced earthquake structural designs, robotic systems, tunable dampers, and brake and clutch systems.

 
This image shows the evolution of colloidal structure within an applied alternating magnetic field. (Credit: N. Hall, NASA)

On Earth, colloids tend to collapse, change form, or sink, depending on particle size, shape, composition, fluid solvent mixture or environmental conditions; all highly dependent on the effects of gravity. This is why the space station provides an ideal laboratory setting for researchers to tease out the underlying physical properties of colloidal solutions. As we better understand the special and fascinating properties of colloids, researchers will be able to devise better technologies and products for use back here on Earth.


Donald C. Barker (Credit: NASA)

Donald C. Barker is a scientist with the International Space Station Program Science Office. Previously Barker served as a lead systems engineer, flight controller and researcher at the Johnson Space Center. He holds a double Bachelor of Science degree in Physics and Psychology from Colorado State University, Master of Science degrees in Physics, Psychology, Mathematics and Space Architecture, and he is currently pursuing a Doctor in Philosophy in Planetary Geology at the University of Houston.

Flights of Flames for Fire Safety in Space

In today’s A Lab Aloft guest blogger, Sandra Olson, Ph.D., reveals some of the mysteries of how flames burn in microgravity, as well as how flame studies on the ground and aboard the International Space Station help with fire suppression and safety in space.

Whether dropping through a hole in the ground as part of a drop test or zipping through space aboard the International Space Station, flames behave in fascinating ways in microgravity! In the Zero Gravity Research Facility, or ZGRF, at NASA’s Glenn Research Center, I get to study solid fuel combustion behavior first hand. ZGRF is a historic landmark and the deepest drop tower in the world with a freefall of 432 feet. Drop test experiments, like the one pictured below, look at material flammability during the brief, 5.18-second period of microgravity achieved as the sample package falls.


During a Zero Gravity Research Facility tour, Facility Manager Eric Neumann (far left) shows International Space Station Program Scientist Julie Robinson (front center) and her colleagues one of the drop packages used in the facility. The top of the white vacuum drop shaft is in the background. (NASA/Marvin Smith)

The drop test was remotely run from the ZGRF control room. Controllers activated the miniature wind tunnel apparatus to establish a spacecraft ventilation flow environment, then ignited the material and dropped the experiment. Once the sample releases into freefall, the experiment is completely automated. The drop vehicle lands in the catch-bucket at the end of the 5.18 second test.

 
Experiment images (left) and catch-bucket facility images (right) appear on the ZGRF control room screen. (NASA/Marvin Smith)

We have performed many drop tests studying how materials burn in microgravity compared to how they burn in normal gravity, or 1g. What we have found is that many materials actually burn better in the spacecraft flow environment than in 1g. This is because on Earth the buoyant flow—created when less dense materials rise within greater density environments—is strong enough to blow the flame out with oxygen reduction. In low ventilation, however, the slow flow provides the oxygen at an optimum rate, so the flame can survive to lower oxygen levels than in 1g. To learn more about the concepts of microgravity and combustion in the space environment, watch this “NASA Connect” video.


A flame burning in microgravity at the end of a 5.18-second drop from the Zero Gravity Research Facility. The material for this test was cotton fabric burning in 5 centimeter per second air flow, which is the typical International Space Station atmosphere. Crew clothing is often made of cotton. (NASA)

Enhanced flammability in space was recently proven in longer duration burn experiments aboard the space station as part of the Burning and Suppression of Solids, or BASS, investigation. For this study, the crew of the space station gets to play with fire. As a co-investigator, I get to observe via video on the ground and directly talk to the crew as they ignite a flame in the controlled area of the Microgravity Science Glovebox, or MSG, filming the behavior of the burn.

After his recent return to Earth, Astronaut Don Pettit, who worked on the BASS flame study in space, testified to a Senate subcommittee about the investigation and the importance of combustion experiments in microgravity.

“If you look at fire, fire and its either discovery or learning how to tame fire is what literally brought us out of the cave and allows us to have our civilization in terms of what we know now,” said Pettit. “Fire gives us our electricity. Fire allows us to have vehicles, airplanes and cars, and machines. It literally turns the wheels of our civilization…space station now offers us the ability to dissect deeper down into what the processes are in combustion… by looking at it in an environment free from gravity, free from the gravitational-driven convection. And this allows us to look at things and figure out what’s going on at a level that you could never see without taking it to space…and what we found is that things are more flammable than what we thought.”


(Left) Astronaut Joe Acaba runs BASS in the Microgravity Science Glovebox, or MSG. (Right) Astronaut Don Pettit holds up a burned acrylic sphere to show the science team on the ground how a fine layer of soot coats the wake region of the material, while the front part of the sphere looks like a meteorite with the surface marred with many craters. (NASA)

These experiments so far have confirmed that when the air flow is turned off, the flame extinguishes rapidly as it runs out of oxygen, with no fresh air flow. The MSG provides an enclosed work area, sealed to contain fluids, gasses and equipment for the safe running of combustion experiments. The crew views the burning material through the front window. The flame can be seen through this window in the picture with Joe Acaba (above). You also can see Don Pettit working on a previous run of BASS aboard station in this video.

This finding reaffirms the space station fire alarm protocol to turn off any forced air flow in the event of a fire alarm. Surprisingly, though, when the astronauts used a small nitrogen jet built into the flow duct for fire suppression testing, the flame did not go out when the air flow was turned off, if the nitrogen jet was on. In fact, the flame appeared to get brighter. Researchers intend to continue to study this unexpected discovery in which the nitrogen jet was able to entrain air all by itself, as the finding has important implications for gaseous fire suppression systems like the
CO2 suppression system currently employed on station.


Acrylic sphere burning as part of the Burning and Suppression of Solids, or BASS, investigation aboard the International Space Station. (NASA)

BASS results also catch the attention of future spacecraft designers. One of the sample materials burned in BASS is acrylic, also called Plexiglas. This material is under consideration for spacecraft windows because of its excellent strength, mass and optical properties. However, it also burns quite well in the space station air environment. BASS payload summary reports mentioning acrylic have spurred a number of recent inquiries to the investigator team about the flammability of this material. After all, you don’t want your spacecraft windows to catch on fire!


A wax candle flame in very low air flow is nearly spherical with an inner sooty layer near the wick, and an outer blue layer. This blue is due to chemiluminescence, which is when a chemical reaction emits light. (NASA)

The BASS investigation has direct applications to spacecraft fire safety and astronaut wellbeing. A combustion experiment, BASS was jointly designed by scientists and engineers at NASA and the Universities Space Research Association, or USRA. BASS operations are scheduled to begin again aboard the space station in the spring of 2013.

The best part of my job as a researcher is the thrill of discovering new phenomena unique to microgravity. It is exciting to work with something as beautiful and powerful as fire, especially in these unique microgravity environments. The fire images have inspired me to create art images from them. 

 
2009 Art “Fire’s Ribbons and Lace”
The delicate and fractal nature of charring cellulose is amplified here in repeated magnified images of a flame spread front over ashless filter paper. (Sandra Olson)


2011 Art “Flaming Star”
Microgravity flames converging toward the center of the starburst ‘implode’ against an outflow of wind, creating a diffusion flame ‘supernova.’ (Sandra Olson)

The more we understand the behavior of flames with given materials and conditions, the better prepared we will be to harness their potential and contribute to fire safety in future space exploration. What’s next will depend on what we discover from these ongoing tests, building on the knowledge already gained from these important combustion studies.


Sandra Olson, shown here with the microgravity wind tunnel drop apparatus.

Sandra Olson, Ph.D., is a spacecraft fire safety researcher at NASA’s Glenn Research Center, as well as the project scientist and co-investigator for the BASS investigation. She has a B.S. in Chemical Engineering and a M.S. and Ph.D. in Mechanical Engineering. She has worked at NASA since 1983, most of that time studying microgravity combustion.   

 

International Space Station Engages with Education

In today’s A Lab Aloft Assistant International Space Station Program Scientist Camille Alleyne talks about a new education publication that highlights more than a decade of inspiring student opportunities with space station investigations and activities.

This October I was excited to see the publication of a book that was not only an international collaboration, but more than a decade in the making: “Inspiring the Next Generation: International Space Station Education Opportunities and Accomplishments, 2000-2012.” Readers can find the PDF version of the publication here and are encouraged to visit the space station’s opportunity site for current education activities.

From a personal perspective, the value of the space station is as a platform for promoting Science, Technology, Engineering and Math (STEM) education, and engaging and exciting students in their studies in these areas. We can really inspire and increase interest in these subjects so that our youth go on to become the next generation of scientists, engineers and explorers.

In the past 12 years of operation, there have been more than 42 million students, 2.8 million teachers and 25,000 schools from 44 countries involved in education activities aboard the space station. This is a bonus in addition to our space station research for exploration, scientific discovery and applied research.

This publication is the follow up to “Inspiring the Next Generation: Student Experiments and Educational Activities on the International Space Station, 2000–2006,” and is a product of NASA’s ISS Program Science Office. The new document is the first time we worked to share these education activities in partnership with our international partners to show the benefits of space station research and education interactions that impact our life here on Earth. There was a team of education leads from each of the international partners that contributed to this publication, which is a comprehensive documentation of all the education activities conducted on the space station since 2000. This includes activities that are ongoing and will continue for the next few years.


Cover of the education publication: “Inspiring the Next Generation: International Space Station Education Opportunities and Accomplishments, 2000-2012.” (NASA)

This book looks at education activities in several different categories. Students are able to get involved with experiments that fly on the space station. They also have opportunities to take part in competitions, with the winners getting to either fly their experiments on station or have a crew member perform some aspect of the challenge. Finally, students have the ability to participate in classroom versions of station investigations by either mimicking or outright partaking in the experiments happening aboard station.

An example of this is Tomatosphere, a Canadian Space Agency-sponsored plant investigation. Researchers fly tomato seeds aboard station, while students on the ground grow their own seeds in the classroom. The young scientists participate in the scientific process by comparing the differences in germination from seeds flown in space vs. those that never left Earth.


During a previous Tomatosphere program, students studied the growth of tomato plants in Miss Smith’s grade three class at Langley Fundamental Elementary in Vancouver, British Columbia, Canada. The students took their plants home to grow in their gardens over the summer. (Tomatosphere)

When students engage directly with researchers flying their investigations aboard station, they usually play a role in data analysis and the setting up of studies before they fly. For instance, with the International Space Station Agricultural Camera, or ISSAC, a student-based staff participated in designing, building and controlling the camera remotely during primary science operations. So there are many ways for students to engage with the space station.

There are also opportunities to learn from demonstrations by astronauts previously done aboard station. Teachers can access and play these experiments in the classroom to demonstrate different scientific concepts and theories. Taking this one step further, students can even engage in real-time crew interactions via live downlinks or ham radio contacts.

With inquiry-based activities, students get to learn how real researchers work and how the scientific process functions simply by being allowed to ask questions, develop hypothesis and analyze data. They learn to think deeply and critically about different scientific concepts, which is a true value of education engagement with the space station.


A student at Lamar Elementary School in Greenville, Texas, proudly talks to an astronaut in space. (NASA)

The opportunity to collaborate with the international partners for this project was really interesting. I was able to gain insight into their education objectives and how they compare to those here at NASA. The Japanese Space Agency, or JAXA, for instance, puts a lot of emphasis not only on STEM education, but on using cultural activities as a way of inspiring the public. That is not something we focus on at NASA, but it was fascinating to see the diverse connection between art and science in these space-related education activities. Take for example the Space Poem Chain project, which used poetry to break barriers by using space as an inspiration for contributors from ages 8 to 98 from around the globe.

It was also fantastic to have Russian Space Agency contributions and to see the ways they go about inspiring their children. Also different from the U.S., the Russians use satellite development and communications technology as their main vehicle for engaging students, while simultaneously building their sense of wonder and skill. Seeing the different cultures and what their missions are in terms of educational goals, including how they manifest into activities, was fun to learn and then to share in this collaboration.


Students from Charminade College Preparatory, West Hills, Calif., run preliminary variations of their experiment in the lab. (SSEP)

We have one story from each partner highlighted on our Benefits website in the area of education. Now we get to take all of our collective efforts and extend the benefits of these activities across the partnerships to other students in other countries around the world. Building this education book with my colleague, Susan Mayo, was a unique experience; a very rewarding one. I look forward to the continuing and growing impacts of the space station.

I feel it is important to note that the inquiry-based approach to science education, like that done with the space station, is what scholars cite as the value that excites students to pursue careers in STEM based areas. I see more of an emphasis on this type of station educational activity in the future. I would also like to see younger participants for these station activities. Consider the Kids in Micro-g project, where we had 5th graders competing to design microgravity experiments. A group of nine year-old girls won and had their investigation conducted aboard station. This led to an actual scientific discovery that nobody expected, contributing to the body of knowledge in that area of physics.


NASA astronauts Catherine (Cady) Coleman and Ronald (Ron) Garan perform the Attracting Water Drops experiment from Chabad Hebrew Academy. (NASA)

The audience for this book is primarily space station stakeholders, but the activities that make up the content have the ability to impact students everywhere, no matter what culture or language. This book has some engaging opportunities that students all over the world could participate in. The thought that 8, 9, and 10 year-olds can teach us something new about exploration and going beyond what we think we know is really exciting! I would like to see what other young minds can contribute using space station education.


Camille Alleyne is an assistant program scientist for the International Space Station Program Science Office with NASA’s Johnson Space Center where she is responsible for leading the areas of communications and education. Prior to this, she served as the Deputy Manager for the Orion Crew and Service Module Test and Verification program.  She holds a Bachelor of Science degree in Mechanical Engineering from Howard University, a Master of Science degree in Mechanical Engineering (Composite Materials) from Florida A&M University and a Master of Science degree in Aerospace Engineering (Hypersonics) from University of Maryland. She is currently working on her Doctorate in Educational Leadership at the University of Houston.

Remodeling Research for Astronaut Bone Health

In today’s A Lab Aloft blog post, guest blogger Scott M. Smith, Ph.D., reflects on the recent publication of results on human health space station research regarding the beneficial connections between bone density, diet and exercise.

This month, September 2012, marks the publication of a paper in the Journal of Bone and Mineral Research documenting how crew members that ate well, had good vitamin D status, and exercised hard maintained their bone mineral density. There are several remarkable things in and about this paper that I would like to share in this blog.

From a science perspective, this marks the first documentation of protecting bone mineral density during space flight. It’s amusing that I have already gotten several questions about whether or not we can tell if it was the exercise or the nutrition that made the beneficial difference. The answer is no, we can’t. 

I suspect some folks would like to think it is just the exercise. I am quick to point out, however, that while I am somewhat biased as a nutritionist, I believe that all aspects were critical to the success of the program. There are plenty of non-NASA studies showing that inadequate nutrition leads to bone loss. I’ve also seen online summaries of research giving vitamin D the lead role—I guess it all depends on your perspective.

Regardless, nutrition (including and beyond vitamin D) and exercise are both very important. We are not going to set up experiments to determine if limiting one of these factors has a negative effect on bone, given that would clearly be the wrong thing to do for the crew of the International Space Station.

NASA astronaut Don Pettit, Expedition 30 flight engineer, is pictured near a snack floating freely in the Unity node of the International Space Station. (Credit: NASA)

There has been immediate reaction to the Benefits for Bone from Resistance Exercise and Nutrition in Long-Duration Spaceflight: Evidence from Biochemistry and Densitometry paper. Some people feel that we can now proclaim spaceflight-induced bone loss a fixed problem and move on. This is clearly not the case, however, as what we found is that bone seems to be remodeling. In other words, bone breakdown still increases, but what happened here is that bone formation tended to increase as well, which appears to help maintain bone mineral density.

A big question remains: is the bone as strong after flight as it was before flight? Follow-on studies are underway to help answer this. Nonetheless, it is better to maintain bone mineral density with a question about strength, than to not maintain bone mineral density—which is where we’ve been up to now. We also hope to optimize both exercise protocols, for example using the Sprint Investigation aboard station, and nutritional aspects of bone health, as seen with the SOLO and Pro K studies. You can read more about these topics in my earlier blog entry on omega-3 fatty acid: Of Fish, Astronauts, and Bone Health on Earth.

NASA astronaut Mike Fossum, Expedition 29 commander, performs a SPRINT leg muscle self scan in the Columbus laboratory of the International Space Station. (Credit: NASA)

Another striking thing about this paper was the team. We had two of NASA’s bone experts, Linda Shackelford and Jean Sibonga; one of NASA’s muscle/exercise experts, Lori Ploutz-Snyder; and nutrition experts from NASA and ESA, Sara Zwart, Martina Heer, and myself. Getting all teams to come together to work on this paper required a fair amount of choreography, including agreements on presentation, interpretation and description of the data.

As an aside, in the late 1990’s Dr. Shackelford led an effort to conduct bed rest studies with resistance exercise here on the ground. She published results that mirror what we found in the flight study. Bed rest is a model of space flight, and results in a different magnitude bone loss, but nonetheless it provides evidence useful in assessing flight studies. This is a perfect example of why we test things on the ground first, but then also test them in flight. We want to be sure to know what happens in actual space flight.

Another unique aspect of this paper is the time it took to pull everything together. It was early this year, on January 26, Sara Zwart and myself were sitting in the office trying to assess what data from the Nutrition investigation we should look to try to publish next. I mentioned that we had not published any of the bone marker data, and perhaps we could look at ARED/iRED differences. ARED is the Advanced Resistive Exercise Device aboard station that the crew uses to simulate free weight exercises on orbit, while the iRED is the Interim Resistive Exercise Device used for upper body strength development.

NASA astronaut Dan Burbank, Expedition 30 flight commander, exercises, using the Advanced Resistive Exercise Device, or ARED, in the Tranquility node of the International Space Station. (Credit: NASA)

Sara and I met very early, around 1 the next morning, at the Telescience Center—one of the back rooms in Mission Control. We were waiting for the crew aboard station to awaken so astronaut Don Pettit could collect his FD30 blood sample for the Nutrition and Pro K studies. Sara mentioned that she’d looked at the blood and urine bone marker data, and there didn’t appear to be major differences between the groups: resorption (bone breakdown) wasn’t different, and formation trended up, but wasn’t overly striking. I asked Sara if she’d looked at the bone densitometry (DEXA) data to see what happened with bone and body composition, and she said no, but she would. She logged in to our lab database, and about 30 minutes later turned and said, “I take it back—there’s something there!”

I told Sara I would start working words, and she should start working tables. By 9:30 the morning of the January 27, 24-hours after we first discussed it, we had a 17-page draft of the manuscript, which included 3 tables of data and complete statistical analysis. It took us about a week (and some sleep) to clean up the draft, and we then sent it out to the coauthors to start the process of bringing in their expertise and adding in details. This was especially important regarding the exercise aspects and the bone measurement details, which elude us nutrition types, along with overall interpretations.

Essentially taking eight months from concept to publication, the journey for this paper is simply incredible! We’ve never had a scientific paper go from essentially the first look at the numbers to print this quickly before, and, well…you never count on something like this to happen again.

Cover of the September 2012 edition of the Journal of Bone and Mineral Research where the Benefits for Bone from Resistance Exercise and Nutrition in Long-Duration Spaceflight: Evidence from Biochemistry and Densitometry paper on astronaut bone health published. (Credit: JBMR)

Scott M. Smith leads NASA’s Nutritional Biochemistry Lab at Johnson Space Center. He completed his doctorate in nutrition at Penn State and conducted postdoctoral research at the U.S. Department of Agriculture’s Human Nutrition Research Center in North Dakota. Smith leads experiments, both on the ground and in space, aimed at improving astronaut nutrition. Smith’s two space station experiments include Nutritional Status Assessment and Pro K.  The Pro K study is designed to investigate the roles of animal protein and potassium in bone loss.