Knowing When to Fold 'Em

Caltech engineers and an origami expert are joining forces to build a retinal implant to treat blindness

Electrical engineer Azita Emami is an expert in the 21st century technology of analog and digital circuits for computers, sensors, and other applications, so when she came to Caltech in 2007, she never imagined that she would be incorporating in her research an art form that originated centuries ago. But origami—the Japanese art of paper folding—could play a critical role in her project to design an artificial retina, which may one day help thousands of blind and visually impaired people regain their vision.

Retinal implants are designed to bypass the photoreceptors in the retina that have been damaged by diseases such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD). About four years before Emami arrived at the Institute, Caltech investigators began working on a retinal implant through USC's Biomimetic Microelectronic Systems–Engineering Research Center, funded by the National Science Foundation (NSF). The basic idea is to use a miniature camera mounted on a pair of eyeglasses to capture images, then process the images and send the digital information wirelessly to an implantable microchip. The microchip generates electrical currents for stimulation, and a tiny cable carries the currents to an electrode array attached to the patient's retina. The electrodes stimulate cells in the eye, which transmit signals through the optic nerve to the part of the brain that creates a picture.

The center's director, Mark Humayun, an ophthalmologist at USC's Doheny Eye Institute and a pioneer in artificial-retina surgery, has implanted such a device in several completely blind patients suffering from end-stage RP, restoring some of their vision. The 60-electrode array allows these patients to see light as well as low-resolution representations of objects and enlarged letters.

Hundreds of thousands of people who suffer from AMD, however, are able to see at least that much on their own and thus would derive no benefit from the array. To create an artificial retina that could help these people, Humayun needed a better chip and an array that had more electrodes to stimulate more cells in the eye. At the suggestion of Caltech professor of electrical engineering and mechanical engineering Yu-Chong Tai, who had worked with Humayun on packaging and integration of the retinal implants, Emami, an assistant professor of electrical engineering and an expert in building ultralow-power circuits, joined the team to focus on the next generation retinal implants.

Emami's lab recently developed just such a chip, which supports 512 electrodes and is extendable to 1024 electrodes if two chips are used. The chip has wireless capabilities for power and data telemetry and can fit inside the eyeball, eliminating the need for the infection-prone cable used in the earlier system. The design also features many novel techniques for reducing size and power. Reducing the power consumption is critical for wireless power delivery and to avoid tissue damage due to the heat generated by the chip. Humayun will soon test the chip on subjects to see exactly how much of their vision is restored.

But even that electrode-rich array won't solve two of the biggest challenges of the technology: creating a device that requires only a minimally invasive incision to implant, and one that also conforms to the shape of the eye. The original electrode array was mounted on a relatively flat substrate that required a large surgical incision for implantation. It could only be tacked onto one spot on the retina to avoid damaging the neurons—which meant that it pulled away at the loose end. And that also meant that some of the electrodes would be completely ineffective while others needed a greater current from the chip to properly stimulate retinal cells, leading to high power consumption. 

Emami, Humayun, and Tai realized that a flexible substrate that could be folded up, origami-style, before implantation and then opened up to a curved shape once inside would need only a minimally invasive incision to be slid into place. Instead of one large chip, many smaller chips distributed over the substrate and between the folds would remove the need for the cable and lead to better reliability and lower cost, Emami says. With a system that conformed to the curve of the eye, the location of the chips and the electrodes could be optimized through the design of the origami structure, precisely matching the parts of the eye to be stimulated.

To create such a design, Emami recruited Caltech alum Robert Lang (BS '82, PhD '86), one of the world's leading origami experts. Lang, who has practiced origami for more than 40 years, is known for developing mathematical equations to enable the construction of highly complex origami designs. Over the summer, Emami received an NSF grant to build the first prototype of an origami implant that will fit inside the eye and match the contour of the retina. 

"I'm used to working with paper that starts out as no smaller than two inches square," Lang says. This new creation, however, will be less than one quarter that size, will be made out of plastic, and will have to deploy perfectly after surgical implantation.

Assisting Lang in the design is Sergio Pellegrino, the Joyce and Kent Kresa Professor of Aeronautics and professor of civil engineering and a senior research scientist at JPL. Pellegrino is an expert at developing origami-like structures, but on a giant scale: he devises lightweight expandable structures for use on spacecraft—such as foldable booms that serve as antenna and deployable masts.

The ability to translate these sorts of very large designs to something that can be unobtrusively inserted and then unfolded in the eye "is a matter of scaling, and that's an engineering principle. It is what engineers do," says Ares Rosakis, chair of the division of engineering and applied science. "The difference is that at Caltech we also invent and scale our own inventions: we invent something for X and we use it for Y. So someone like Pellegrino can invent something for space and then have fantastic successes by scaling it for use in medical engineering."

While Pellegrino and Lang work on the origami, Emami will continue working on the chip. By the end of next year they hope to show in animal models that an origami substrate can be inserted inside the eye, unfolded, and held in place by either retinal tacks or a less invasive method, also using origami. Soon after, they hope to have a foldable artificial retina that can be tested on a patient.

Once perfected, Emami thinks that the new retinal implant technology could be applied to other medical applications, such as neural implants that are being developed to help paralyzed people regain movement. "Our origami approach is fundamentally different and can lead to a new area in engineering with a great impact for neuroscience and biomedical devices," Emami says. "We may be able to benefit many people."

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Nano Insights Could Lead to Improved Nuclear Reactors

Caltech researchers examine self-healing abilities of some materials

PASADENA, Calif.—In order to build the next generation of nuclear reactors, materials scientists are trying to unlock the secrets of certain materials that are radiation-damage tolerant. Now researchers at the California Institute of Technology (Caltech) have brought new understanding to one of those secrets—how the interfaces between two carefully selected metals can absorb, or heal, radiation damage.

"When it comes to selecting proper structural materials for advanced nuclear reactors, it is crucial that we understand radiation damage and its effects on materials properties. And we need to study these effects on isolated small-scale features," says Julia R. Greer, an assistant professor of materials science and mechanics at Caltech. With that in mind, Greer and colleagues from Caltech, Sandia National Laboratories, UC Berkeley, and Los Alamos National Laboratory have taken a closer look at radiation-induced damage, zooming in all the way to the nanoscale—where lengths are measured in billionths of meters. Their results appear online in the journals Advanced Functional Materials and Small.

During nuclear irradiation, energetic particles like neutrons and ions displace atoms from their regular lattice sites within the metals that make up a reactor, setting off cascades of collisions that ultimately damage materials such as steel. One of the byproducts of this process is the formation of helium bubbles. Since helium does not dissolve within solid materials, it forms pressurized gas bubbles that can coalesce, making the material porous, brittle, and therefore susceptible to breakage.  

Some nano-engineered materials are able to resist such damage and may, for example, prevent helium bubbles from coalescing into larger voids. For instance, some metallic nanolaminates—materials made up of extremely thin alternating layers of different metals—are able to absorb various types of radiation-induced defects at the interfaces between the layers because of the mismatch that exists between their crystal structures.

"People have an idea, from computations, of what the interfaces as a whole may be doing, and they know from experiments what their combined global effect is. What they don't know is what exactly one individual interface is doing and what specific role the nanoscale dimensions play," says Greer. "And that's what we were able to investigate."

Peri Landau and Guo Qiang, both postdoctoral scholars in Greer's lab at the time of this study, used a chemical procedure called electroplating to either grow miniature pillars of pure copper or pillars containing exactly one interface—in which an iron crystal sits atop a copper crystal. Then, working with partners at Sandia and Los Alamos, in order to replicate the effect of helium irradiation, they implanted those nanopillars with helium ions, both directly at the interface and, in separate experiments, throughout the pillar.

The researchers then used a one-of-a-kind nanomechanical testing instrument, called the SEMentor, which is located in the subbasement of the W. M. Keck Engineering Laboratories building at Caltech, to both compress the tiny pillars and pull on them as a way to learn about the mechanical properties of the pillars—how their length changed when a certain stress was applied, and where they broke, for example. 

"These experiments are very, very delicate," Landau says. "If you think about it, each one of the pillars—which are only 100 nanometers wide and about 700 nanometers long—is a thousand times thinner than a single strand of hair. We can only see them with high-resolution microscopes."

The team found that once they inserted a small amount of helium into a pillar at the interface between the iron and copper crystals, the pillar's strength increased by more than 60 percent compared to a pillar without helium. That much was expected, Landau explains, because "irradiation hardening is a well-known phenomenon in bulk materials." However, she notes, such hardening is typically linked with embrittlement, "and we do not want materials to be brittle."

Surprisingly, the researchers found that in their nanopillars, the increase in strength did not come along with embrittlement, either when the helium was implanted at the interface, or when it was distributed more broadly. Indeed, Greer and her team found, the material was able to maintain its ductility because the interface itself was able to deform gradually under stress.

This means that in a metallic nanolaminate material, small helium bubbles are able to migrate to an interface, which is never more than a few tens of nanometers away, essentially healing the material. "What we're showing is that it doesn't matter if the bubble is within the interface or uniformly distributed—the pillars don't ever fail in a catastrophic, abrupt fashion," Greer says. She notes that the implanted helium bubbles—which are described in the Advanced Functional Materials paper—were one to two nanometers in diameter; in future studies, the group will repeat the experiment with larger bubbles at higher temperatures in order to represent additional conditions related to radiation damage.

In the Small paper, the researchers showed that even nanopillars made entirely of copper, with no layering of metals, exhibited irradiation-induced hardening. That stands in stark contrast to the results from previous work by other researchers on proton-irradiated copper nanopillars, which exhibited the same strengths as those that had not been irradiated. Greer says that this points to the need to evaluate different types of irradiation-induced defects at the nanoscale, because they may not all have the same effects on materials.

While no one is likely to be building nuclear reactors out of nanopillars anytime soon, Greer argues that it is important to understand how individual interfaces and nanostructures behave. "This work is basically teaching us what gives materials the ability to heal radiation damage—what tolerances they have and how to design them," she says. That information can be incorporated into future models of material behavior that can help with the design of new materials.

Along with Greer, Landau, and Qiang, Khalid Hattar of Sandia National Laboratories is also a coauthor on the paper "The Effect of He Implantation on the Tensile Properties and Microstructure of Cu/Fe Nano-bicrystals," which appears online in Advanced Functional Materials. Peter Hosemann of UC Berkeley and Yongqiang Wang of Los Alamos National Laboratory are coauthors on the paper "Helium Implantation Effects on the Compressive Response of Cu Nanopillars," which appears online in the journal Small. The work was supported by the U.S. Department of Energy and carried out, in part, in the Kavli Nanoscience Institute at Caltech.

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Diving Into the Unknown: An Interview with Andrei Faraon

This fall, Andrei Faraon (BS '04) returned to his alma mater to take a position as an assistant professor of applied physics and materials science. Faraon, originally from Falticeni, Romania, came to the United States in 2001 to study at Caltech and earned his BS in physics in 2004, then moved to Stanford University, where he received a master's degree in electrical engineering and a PhD in applied physics. Faraon recently answered some questions about his work and returning to Caltech. 

First of all, how does it feel to be back at Caltech?

It feels great—something like a homecoming.

What is the focus of your research?

I build devices that are based on the fundamentals of light–matter interaction. What we're trying to do is manipulate single quantum systems in solids—systems like single atoms or single quantum dots—using light. Light is great for this purpose because it allows us to address these systems without destroying their fragile quantum states, and because it can easily interconnect quantum systems over large distances.

This work has applications in quantum and classical information technologies, and also for the development of sensors with very high spatial resolution and very high sensitivity. These sensors are used to probe other quantum systems and also have applications in biotechnology.

Does this have anything to do with the development of quantum computers?

The quantum computer is something of a Holy Grail, but down the road there is hope that other applications, like quantum repeaters that could enable very secure communications, will come out of these new technologies. In general, the field is just trying to get an understanding of how to better control and manipulate quantum systems in order to develop devices based on these quantum concepts.

What has been your most recent development in this line of work?

In my postdoctoral work, I was able to combine some impurities in diamond, called nitrogen vacancy centers, with optical structures known as nanoscale optical resonators. Nitrogen vacancy centers are defects in the atomic lattice that makes up diamond in which nitrogen atoms have basically replaced carbon atoms. They are interesting because they have very good quantum-coherence properties—meaning that you can actually store information in the quantum state of the impurities and keep it preserved for a relatively long time. These impurities can be used for the sensing of electromagnetic fields with high resolution, or to store and process quantum information. By combining the impurities with optical structures we are actually able to better control and modify their properties.

In general, light interacts weakly with these impurities. By embedding the nitrogen-vacancy centers in resonators, we can create a stronger interaction between them and the light field. The resonators can be further integrated in an on-chip optical network. Since the impurities are coupled to the resonators, we actually interconnect multiple nitrogen-vacancy centers on a chip, thus creating a quantum network that forms the basis of future devices for quantum information processing.

What do you find most exciting about your research?

It is really at the forefront of experimental research and it allows me to really dive into the unknown. I love the fact that we often discover unexpected things and that there is also great potential that this work will result in revolutionary technologies.

What brought you back to Caltech?

Caltech provides the best environment in which to do my research in terms of facilities, the quality of students, and the faculty that I can interact with. Caltech has a very strong effort both in photonics, which is my field of study, and also in quantum information. I think that the people are actually the greatest resource that Caltech has. 

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Caltech Mourns the Passing of David G. Goodwin

1957–2012

David G. Goodwin, professor of mechanical engineering and applied physics, emeritus, passed away at his home in Pasadena on Sunday, November 11, after a five-year battle with brain cancer and a struggle with Parkinson's disease that began in 1998. He was 55 years old. Born on October 15, 1957, Goodwin grew up in Rancho Cordova, a suburb of Sacramento, near the Aerojet plant where his father worked as an engineer. He came to Caltech in 1988 as an assistant professor of mechanical engineering, was promoted to associate professor of mechanical engineering and applied physics in 1993 and professor in 2000, and retired in 2011.

Goodwin was best known for developing ways to grow thin films of high-purity diamond. Diamond films—transparent, scratch-resistant, and efficient dissipaters of the heat generated by high-powered computer chips—are now routinely used to protect electronic and optical components, and diamond-coated drill bits can be found at any hardware store.

But the diamond work was just one facet of Goodwin's research. According to longtime collaborator David Boyd, once a postdoc of Goodwin's and now a Caltech staff member, "Dave's real passion was modeling. He felt that he never fully understood something unless he could model it. He had a keen insight into how things work. He would proffer an oftentimes very simple explanation that captured the essential physics, and was able to see how that applied in engineering terms. It's really unusual for an engineer to know that much physics, or a physicist to have that much engineering."

The Mideast oil crises of Goodwin's teenage years sparked a lifelong interest in energy issues, and much of his work revolved around the intricacies of combustion. He fluently translated the complex interplay of heat flow and atomic behavior within swirling mixtures of turbulent gases into detailed mathematical models that accurately predicted how real-world, industrial-scale chemical processors would operate.

After earning his BS in engineering from Harvey Mudd College in 1979, Goodwin joined the Stanford University High Temperature Gasdynamics Laboratory, which was working on an ultraefficient method for generating electricity by burning coal at very high temperatures to create an electrically charged plasma. The process proved too expensive to be practical, but the mastery Goodwin acquired of chemical kinetics—the mathematical descriptions of how reactions proceed—set the course of his career. He earned his MS and PhD in 1980 and 1986 respectively, both in mechanical engineering.

Goodwin arrived at Caltech amid an explosion of interest in growing diamond coatings via chemical vapor deposition. The process is high-tech, but the basic idea is simple. Playing a methane flame over an object deposits carbon atoms on its surface, and under the right conditions these atoms will organize themselves into a sheen of high-purity diamond instead of the usual smudge of soot. "People had found a process that worked," says Boyd, "but really did not know how or why it did." Goodwin's models explained it all, and the set of papers he published beginning in 1990 "really turned artificial diamond into an engineering material," says Harry Atwater, the Hughes Professor and professor of applied physics and materials science, and director of the Resnick Sustainability Institute.

But far beyond that, "Dave was one of these people whose impact you measure by the codes he wrote for others to use," Atwater says. Goodwin began writing code in earnest in the 1990s, when he led the Virtual Integrated Prototyping project for the Defense Advanced Research Projects Agency. This sprawling endeavor, on which Atwater was a collaborator, created a set of simulations that began at the atomic level and went up to encompass an entire chemical reactor in order to figure out how to grow superconducting metal oxides and other thin films with demanding atomic arrangements. Atwater and Goodwin then built the reactor, which is still in use at Caltech and whose design has been widely copied.

Along the way, Goodwin wrote an extensive overhaul of CHEMKIN (for "chemical kinetics"), a collection of programs that had been developed at Sandia National Laboratories in the 1970s and had quickly gone into worldwide use. He then wrote—from scratch—his own software toolkit for modeling basic thermodynamics and chemical kinetics, which he dubbed Cantera. Breaking with the usual practice of creating a convoluted descriptor to yield a clever acronym, Cantera doesn't stand for anything, says Professor of Mechanical Engineering Tim Colonius. "He just wanted to give it a nice soothing, relaxing name, like pharmaceutical companies do. That was typical of his sense of humor." The open-source code is available pro bono and has been downloaded 120,000 times since 2004, according to Sandia's Harry Moffat, one of Cantera's current developers and the manager of the website. Says Moffat, "We have ventured into areas that CHEMKIN cannot go, including liquid-solid interactions and electrochemical applications such as batteries."

Goodwin also found time to court Frances Teng, an obstetrician-gynecologist at nearby Huntington Hospital, whose own parents had gotten married while postdocs at Caltech in the 1960s. Dave and Frances were married at the Athenaeum, Caltech's faculty club, in April 1993.

Goodwin eventually returned to the energy issues that had motivated him to become an engineer in the first place. "He really pushed us to start teaching some energy-related courses in the early 2000s," says Vice Provost Melany Hunt, the Kenan Professor of Mechanical Engineering, and the executive officer for mechanical engineering at the time. This led to ME 122, Sustainable Energy Engineering, which Goodwin inaugurated in 2008. ME 122 lives on as the centerpiece of the Energy Science and Technology option, now renumbered EST/EE/ME 109 and renamed Energy: Supply and Demand.

During that time Goodwin also collaborated on three major fuel-cell projects with Sossina Haile, professor of materials science and chemical engineering, in which he modeled the processes by which fuel molecules reacted with oxygen ions to produce electricity. "Dave was looking at it from a computational perspective, and we were looking at it from an experimental perspective," says Haile. "He pulled together all that we know from fundamental physics and chemistry to say, 'This is how the fuel cell works, and this is how to configure it so that it will actually deliver the power that you want.' Most people do a lot of parameter fitting and approximations, but he treated the problem in a very physics-based, solid way."

Goodwin was as active in the greater life of the Institute as he was in his lab. He served on the faculty board from 1996 to 1999 and from 2001 to 2005, the last two years as faculty chair. During that time, he successfully lobbied to extend the timetable for granting junior faculty tenure in cases of childbirth or adoption, Hunt says. "Dave was always concerned about diversity issues. He would say, 'Are there women coming in? Are there minority students coming in? We should make sure that we are doing things to ensure that we have a diverse group coming in to Caltech.'" Hunt recalls that when two young women wanted to take a class that wasn't offered that year, "Dave met with them in his office three times a week. He wanted to be helpful. He just felt a responsibility to do it."

"The thing that was remarkable about David Goodwin," says Haile, "was that when he was diagnosed with this rare form of cancer for which there is no rhyme or reason, he said, 'I'm so glad that I lived my life in a healthy way and that I didn't do anything that caused this,' not 'I can't believe I lived my life in such a healthy way, and it's so unfair that I got struck by this.' It was stunning. He had an incredibly optimistic view."

"Dave made you happy whenever you ran into him," says Kaushik Bhattacharya, the Howell N. Tyson, Sr., Professor of Mechanics and professor of materials science, and executive officer for mechanical and civil engineering. "You could go into his office and have a wonderful conversation about any topic in the world. He had an easy smile and a wicked sense of humor."

Goodwin's honors include five years as a National Science Foundation Presidential Young Investigator and two NASA Certificates of Recognition for his diamond-film work. He was a member of the Electrochemical Society, the American Chemical Society, the Combustion Institute, the American Physical Society, the American Society of Mechanical Engineers, and the Materials Research Society. He wrote or coauthored more than 60 papers.

In his spare time, Goodwin was an accomplished guitarist, a skilled woodworker who made several pieces of furniture for the family's Craftsman house, and a prolific painter in oils.

Goodwin is survived by his parents, George and Verma Goodwin, of Cameron Park, California; his sisters, Ellen Goodwin Levy of Sacramento and Jennifer Goodwin Smith of Elk Grove; his wife, Frances Teng; and his children, Tim, 18, and Erica, 15.

A memorial service will be held on January 12, 2013, at 1:00 p.m. at the Caltech Athenaeum, and an annual speakership in mechanical engineering is being established in his honor; contributions to the David Goodwin Memorial Lectureship can be made here

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Julia Greer Receives NASA Research Grant and Early Career Awards

Julia Greer is going to need to find space in her office for all of the awards, medals, and grant acceptance letters she has been receiving lately.

While celebrating her recent Breakthrough Award from Popular Mechanics magazine, Greer—a Caltech assistant professor of materials science and mechanics—received notice from NASA that she is among a select group of 10 recipients of the agency's inaugural Space Technology Research Opportunities Early Career Faculty grants.

Greer also recently learned that she has received the 2013 Early Career Faculty Fellow award from the Minerals, Metals & Materials Society (TMS) and the Young Investigator's Medal from the Society of Engineering Science (SES).

NASA's Space Technology Research Opportunities (STRO) program was established to identify and develop technologies that advance the agency's space-exploration priorities. For future space missions, NASA needs spacecraft and equipment to be made of strong, light, and durable materials. Through the STRO Early Career Faculty program, Greer's materials-science laboratory at Caltech—which recently helped develop the world's lightest solid material, a lattice composed of tiny, metallic tubes with a density of just 0.9 milligrams per cubic centimeter—will have access to as much as $200,000 per year for three years to develop structures made of lightweight, radiation- and damage-tolerant materials.

"In addition to being proud at having been chosen by NASA for this grant, I speak for my whole lab group when I say we are excited to be working to create the next generation of materials to be used in space exploration," says Greer.

"I am also very pleased to have been recognized by TMS as an early career fellow and the SES as a young investigator medal winner," Greer adds. "It is nice to see that the innovations that are happening here at Caltech are getting recognition from the broader community of materials scientists."

According to TMS, the Early Career Faculty Award recognizes assistant professors for accomplishments that have advanced their academic institutions and that broaden the technological profile of TMS. Formal presentation of Greer's TMS early career award will take place at the 142nd TMS annual meeting in San Antonio, Texas, on March 5.

The SES Young Investigator Medal is awarded each year to researchers within 10 years of earning their doctoral degree and is given for work that has made a significant impact on their field within engineering science. The medal comes with a $1,000 prize and an invitation to give an address at the annual meeting of the SES.

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Progress for Paraplegics

Caltech investigators expand project to restore functions to people with spinal-cord injuries

In May 2011, a new therapy created in part by Caltech engineers enabled a paraplegic man to stand and move his legs voluntarily. Now those same researchers are working on a way to automate their system, which provides epidural electrical stimulation to the lower spinal cord. Their goal is for the system to soon be made available to rehab clinics—and thousands of patients—worldwide.

That first patient—former athlete Rob Summers, who had been completely paralyzed below the chest following a 2006 accident—performed remarkably well with the electromechanical system. Although it wasn't initially part of the testing protocol established by the Food and Drug Administration, the FDA allowed Summers to take the entire system with him when he left the Frazier Rehab Institute in Louisville—where his postsurgical physical therapy was done—provided he returns every three months for a checkup.

Joel Burdick, the Richard L. and Dorothy M. Hayman Professor of Mechanical Engineering and Bioengineering at Caltech, and Yu-Chong Tai, a Caltech professor of electrical engineering and mechanical engineering, helped create the therapy, which involves the use of a sheetlike array of electrodes that stimulate Summers' neurons and thus activate the circuits in his lower spinal cord that control standing and stepping. The approach has subsequently been successfully tested on a second paraplegic, and therapists are about to finish testing a third subject, who has shown positive results.

But Tai and Burdick want to keep the technology, as well as the subjects, moving forward. To that end, Tai is developing new versions of the electrode array currently approved for human implantation; these will improve patients' stepping motions, among other advances, and they will be easier to implant. Burdick is also working on a way to let a computer control the pattern of electrical stimulation applied to the spinal cord.

"We need to go further," Burdick says. "And for that, we need new technology."

Because spinal-cord injuries vary from patient to patient, deploying the system has required constant individualized adjustments by clinicians and researchers at the Frazier Institute, a leading center for spinal-cord rehabilitation. "Right now there are 16 electrodes in the array, and for each individual electrode, we send a pulse, which can be varied for amplitude and frequency to cause a response in the patient," Burdick says. Using the current method, he notes, "it takes substantial effort to test all the variables to find the optimum setting for a patient for each of the different functions we want to activate."

The team of investigators, which also includes researchers from UCLA and the University of Louisville, has until now used intelligent guesswork to determine which stimuli might work best. But soon, using a new algorithm developed by Burdick, they will be able to rely on a computer to determine the optimum stimulation levels, based on the patient's response to previous stimuli. This would allow patients to go home after the extensive rehab process with a system that could be continually adjusted by computer—saving Summers and the other patients many of those inconvenient trips back to Louisville. Doctors and technicians could monitor patients' progress remotely.

In addition to providing the subjects with continued benefits from the use of the device, there are other practical reasons for wanting to automate the system. An automated system would be easier to share with other hospitals and clinics around the world, Burdick says, and without a need for intensive training, it could lower the cost.

The FDA has approved testing the system in five spinal-cord injury patients, including the three already enrolled in the trial; Burdick is planning to test the new computerized version in the fourth patient, as well as in Rob Summers during 2013. Once the investigators have completed testing on all five patients, Burdick says, the team will spend time analyzing the data before deciding how to improve the system and expand its use.

The strategy is not a cure for paraplegics, but a tool that can be used to help improve the quality of their health, Burdick says. The technology could also complement stem-cell therapies or other methods that biologists are working on to repair or circumvent the damage to nervous systems that results from spinal-cord injury.

"There's not going to be one silver bullet for treating spinal-cord injuries," Burdick says. "We think that our technique will play a role in the rehabilitation of spinal-cord injury patients, but a more permanent cure will likely come from biological solutions."

Even with the limitations of the current system, Burdick says, the results have exceeded his expectations.

"All three subjects stood up within 48 hours of turning on the array," Burdick says. "This shows that the first patient wasn't a fluke, and that many aspects of the process are repeatable." In some ways, the second and third patients are performing even better than Summers, though it will be some time before the team can fully analyze those results. "We were expecting variations because of the distinct differences in the patients' injuries. Rob gave us a starting point, and now we've learned how to tune the array for each patient and to make adjustments as each patient changes over time.

"I do this work because I love it," Burdick says. "When you work with these people and get to know them and see how they are improving, it's personally inspiring."

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Reconsidering the Global Thermostat

Caltech researcher and colleagues show outcome of geoengineering can be tunable

PASADENA, Calif.—From making clouds whiter and injecting aerosols into the stratosphere, to building enormous sunshades in space, people have floated many ideas about how the planet's climate could be manipulated to counteract the effects of global warming—a concept known as geoengineering. Many of the ideas involve deflecting incoming sunlight before it has a chance to further warm the earth. Because this could affect areas of the planet inequitably, geoengineering has often raised an ethical question: Whose hand would control the global thermostat?

Now a team of researchers from the California Institute of Technology (Caltech), Harvard University, and the Carnegie Institution says there doesn't have to be just a single global control. Using computer modeling, they have shown that varying the amount of sunlight deflected away from the earth by season and by region can significantly improve the parity of the situation. The results appear in an advance online publication of the journal Nature Climate Change.

Previous geoengineering studies have typically assumed uniform deflection of sunlight everywhere on the planet. But the pattern of temperature and precipitation effects that would result from such efforts would never compensate perfectly for the complex pattern of changes that have resulted from global warming. Some areas would end up better off than others, and the climate effects are complex. For example, as the planet warms, the poles are heating up more than the tropics. However, in models where sunlight is deflected uniformly, when enough sunlight is redirected to compensate for this polar warming, the tropics end up colder than they were before man-made activities pumped excess carbon dioxide into the atmosphere.

In the new study, the researchers worked with a climate model of relatively coarse resolution. Rather than selecting one geoengineering strategy, they mimicked the desired effect of many projects by simply "turning down the sun"—decreasing the amount of sunlight reaching the planet. Instead of turning down the sun uniformly, they tailored when and where they reduced incoming sunlight, looking at 15 different combinations. In one, for example, they turned down the sun between January and March while also turning it down more at the poles than at the tropics.

"That essentially gives us 15 knobs that we can tune in order to try to minimize effects at the worst-off regions on the planet," says Doug MacMartin, a senior research associate at Caltech and lead author of the new paper. "In our model, we were able to reduce the residual climate changes (after geoengineering) in the worst-off regions by about 30 percent relative to what could be achieved using a uniform reduction in sunlight."

The group also found that by varying where and when sunlight was reduced, they needed to turn down the sun just 70 percent as much as they would in uniform reflectance to get a similar result. "Based on this work, it's at least plausible that there are ways that you could implement a geoengineering solution that would have less severe consequences, such as a reduced impact on ozone," MacMartin says.

The researchers also used the tuning approach to focus on recovering Arctic sea ice. In their model, it took five times less solar reduction than in the uniform reflectance models to recover the Arctic sea ice to the extent typical of pre-Industrial years.

"These results indicate that varying geoengineering efforts by region and over different periods of time could potentially improve the effectiveness of solar geoengineering and reduce climate impacts in at-risk areas," says Ken Caldeira of the Carnegie Institution. "For example, these approaches may be able to reverse long-term changes in the Arctic sea ice."

The group acknowledges that geoengineering ideas are untested and could come with serious consequences, such as making the skies whiter and depleting the ozone layer, not to mention the unintended consequences that tend to arise when dealing with such a complicated system as the planet. They also say that the best solution would be to reduce greenhouse gas emissions. "I'm approaching it as an engineering problem," MacMartin says. "I'm interested in whether we can come up with a better way of doing the geoengineering that minimizes the negative consequences."  

In addition to MacMartin and Caldeira, David Keith of Harvard University and Ben Kravitz, formerly of the Carnegie Institution but now at the DOE's Pacific Northwest National Lab, are also coauthors on the paper, "Management of trade-offs in geoengineering through optimal choice of non-uniform radiative forcing."

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Engineering with Impact

Watson Lecture Preview

Guruswami (Ravi) Ravichandran is an expert on breakups—of ceramics and metals, not relationships. The John E. Goode, Jr., Professor of Aerospace and professor of mechanical engineering and the director of the Graduate Aerospace Laboratories at Caltech, Ravichandran will talk about his work at the leading edge of impact mechanics on Wednesday, October 24, at 8:00 p.m. in Caltech's Beckman Auditorium. Admission is free.

 

Q: What do you do?

A: I study the dynamic behavior of materials. How they deform on impact. How they break up, how they absorb energy and disperse momentum. We subject materials to millions of atmospheres of pressure by shooting projectiles at them, or dropping weights on them, or jabbing them with a bar like a medieval battering ram. You cannot simulate the pressure at the core of the earth, for example, in a normal laboratory. But impact provides a means for creating those conditions, and lets you explore properties of these materials at these extreme conditions.

 

Q: Why is this cool?

A: It is cool because we look at things that you cannot ordinarily look at—not only extremes of temperature and pressure, but time. We have very high-speed optical and thermal infrared cameras that can go up to one hundred million frames per second. That's one frame every 10 nanoseconds, and it lets us visualize phenomena that you normally won't be able to see. We can watch a crack tear through a metal plate in real time, for example, and we can map the shear stresses and temperature fields across the plate as it stretches before giving way.

We are motivated by the big questions. Where did we come from? The extinction of the dinosaurs, how did that occur? And what's our relationship to that period of the earth's history? But we also explore real-life problems, such as how to build protective structures such as armor. If you think of the crash-worthiness of a car, how do you make a better cage to strengthen the passenger compartment?

And we can even create new materials. A few years back, for example, we made some titanium / silicon carbide composites that could not have been made otherwise. They're more ductile than usual, which could be a useful property for aerospace applications.

 

Q: How did you get into this line of work?

A: When I started graduate school at Brown I was already working on impact-related problems. I enjoyed them, so I've kept up with the thing ever since. It just happened to fall into place. But I did always wonder about those Road Runner cartoons—you see a rock or something break apart after an impact, and you think, "Are those crack patterns for real?"

 

Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.

 

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Watson Lecture: "Engineering with Impact"
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Developing the Next Generation of Microsensors

Caltech researchers engineer microscale optical accelerometer

PASADENA, Calif.—Imagine navigating through a grocery store with your cell phone. As you turn down the bread aisle, ads and coupons for hot dog buns and English muffins pop up on your screen. The electronics industry would like to make such personal navigators a reality, but, to do so, they need the next generation of microsensors.

Thanks to an ultrasensitive accelerometer—a type of motion detector—developed by researchers at the California Institute of Technology (Caltech) and the University of Rochester, this new class of microsensors is a step closer to reality. Beyond consumer electronics, such sensors could help with oil and gas exploration deep within the earth, could improve the stabilization systems of fighter jets, and could even be used in some biomedical applications where more traditional sensors cannot operate.

Caltech professor of applied physics Oskar Painter and his team describe the new device and its capabilities in an advance online publication of the journal Nature Photonics.

Rather than using an electrical circuit to gauge movements, their accelerometer uses laser light. And despite the device's tiny size, it is an extremely sensitive probe of motion. Thanks to its low mass, it can also operate at a large range of frequencies, meaning that it is sensitive to motions that occur in tens of microseconds, thousands of times faster than the motions that the most sensitive sensors used today can detect.

"The new engineered structures we made show that optical sensors of very high performance are possible, and one can miniaturize them and integrate them so that they could one day be commercialized," says Painter, who is also codirector of Caltech's Kavli Nanoscience Institute.

Although the average person may not notice them, microchip accelerometers are quite common in our daily lives. They are used in vehicle airbag deployment systems, in navigation systems, and in conjunction with other types of sensors in cameras and cell phones. They have successfully moved into commercial use because they can be made very small and at low cost.

Accelerometers work by using a sensitive displacement detector to measure the motion of a flexibly mounted mass, called a proof mass. Most commonly, that detector is an electrical circuit. But because laser light is one of the most sensitive ways to measure position, there has been interest in making such a device with an optical readout. For example, projects such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) rely on optical interferometers, which use laser light reflecting off mirrors separated by kilometers of distance to sensitively measure relative motion of the end mirrors. Lasers can have very little intrinsic noise—meaning that their intensity fluctuates little—and are typically limited by the quantum properties of light itself, so they make it much easier to detect very small movements.

People have tried, with limited success, to make miniature versions of these large-scale interferometers. One stumbling block for miniaturization has been that, in general, the larger the proof mass, the larger the resulting motion when the sensor is accelerated. So it is typically easier to detect accelerations with larger sensors. Also, when dealing with light rather than electrons—as in optical accelerometers—it is a challenge to integrate all the components (the lasers, detectors, and interferometer) into a micropackage.

"What our work really shows is that we can take a silicon microchip and scale this concept of a large-scale optical interferometer all the way down to the nanoscale," Painter says. "The key is this little optical cavity we engineered to read out the motion."

The optical cavity is only about 20 microns (millionths of a meter) long, a single micron wide, and a few tenths of a micron thick. It consists of two silicon nanobeams, situated like the two sides of a zipper, with one side attached to the proof mass. When laser light enters the system, the nanobeams act like a "light pipe," guiding the light into an area where it bounces back and forth between holes in the nanobeams. When the tethered proof mass moves, it changes the gap between the two nanobeams, resulting in a change in the intensity of the laser light being reflected out of the system. The reflected laser signal is in fact tremendously sensitive to the motion of the proof mass, with displacements as small as a few femtometers (roughly the diameter of a proton) being probed on the timescale of a second.

It turns out that because the cavity and proof mass are so small, the light bouncing back and forth in the system pushes the proof mass—and in a special way: when the proof mass moves away, the light helps push it further, and when the proof mass moves closer, the light pulls it in. In short, the laser light softens and damps the proof mass's motion.

"Most sensors are completely limited by thermal noise, or mechanical vibrations—they jiggle around at room temperature, and applied accelerations get lost in that noise," Painter says. "In our device, the light applies a force that tends to reduce the thermal motion, cooling the system." This cooling—down to a temperature of three kelvins (about –270°C) in the current devices—increases the range of accelerations that the device can measure, making it capable of measuring both extremely small and extremely large accelerations.

"We made a very sensitive sensor that, at the same time, can also measure very large accelerations, which is valuable in many applications," Painter says.

The team envisions its optical accelerometers becoming integrated with lasers and detectors in silicon microchips. Microelectronics companies have been working for the past 10 or 15 years to try to integrate lasers and optics into their silicon microelectronics. Painter says that a lot of engineering work still needs to be done to make this happen, but adds that "because of the technological advancements that have been made by these companies, it looks like one can actually start making microversions of these very sensitive optical interferometers."

"Professor Painter's research in this area nicely illustrates how the Engineering and Applied Science faculty at Caltech are working at the edges of fundamental science to invent the technologies of the future," says Ares Rosakis, chair of Caltech's Division of Engineering and Applied Science.  "It is very exciting to envision the ways this research might transform the microelectronics industry and our daily lives."

The lead authors on the paper, titled "A high-resolution microchip optomechanical accelerometer," have all worked in Painter's lab. Alexander Krause and Tim Blasius are currently graduate students at Caltech, while Martin Winger is a former postdoctoral scholar who now works for a sensor company called Sensirion in Zurich, Switzerland. This work was performed in collaboration with Qiang Lin, a former postdoctoral scholar of the Painter group, who now leads his own research group at the University of Rochester. The work is supported by the Defense Advanced Research Projects Administration QuASaR program, the National Science Foundation Graduate Research Fellowship Program, and Intellectual Ventures.

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