Powering a Pacemaker with the Heart it Keeps in Rhythm

The implant contains a flexible piezoelectric film and a tiny rechargeable battery. (Credit: John Rogers, University of Illinois)

The implant contains a flexible piezoelectric film and a tiny rechargeable battery. (Credit: John Rogers, University of Illinois)

It’s beginning to become pretty apparent to me that Yonggang Huang of Northwestern University and John Rogers of the University of Illinois are two of the biggest science rock stars in the Big Ten. And when you put the two of them together in collaboration, amazing things will soon follow. The duo have a longstanding collaboration that researchers flexible electronics.

Take, for example, my post from 2011 about stick-on “tattoos” that are basically fully functioning, wireless computer chips just 40 micrometers thick that stick to human skin without any sort of adhesive. These could act as an EEG or EMG sensor to monitor nerve and muscle activity, or a sensor to monitor brain waves, sleeping patterns and other brain functions. Current methods require sticky gel pads to be placed all over the body with long, clunky wires attached to computers and power sources. But the new electronic tattoos have all of that built in. You could walk around all day, sleep on your sofa and go for a run without even noticing that you’re wearing medical devices.

Or there’s my post from 2012 on similarly thin electronic strips with all the wiring embedded that dissolve away completely in the presence of water. The possibilities are endless. Medical implants – sensors or thermal therapeutic devices, for example – could simply vanish once they are no longer needed. Environmental sensors could be deployed without the need to retrieve them ever. Hell, cell phones could disintegrate away into thin air once the next model gets released.

Or the post from 2013 on radically new camera lenses that integrate many cameras on a spherical surface much like a bug’s eye to create a wide field of vision. Again, flexible electronics that can conform to the shape of a sphere were integral to the project.

Well, they’re at it again, folks, this time with some piezoelectric magic.

Piezo comes from the Greek word for pressure, while electric means exactly what you think it is. So these materials are able to generate electricity through the pressures exerted on them and the deformations those pressures cause. There are a lot of projects out there trying to harness energy from wearable electronics embedded in your shoes or clothing, so that natural everyday movements could charge electronic devices.

But for their new project, Huang and Rogers went for something even more intrinsic to your everyday life – the beating of a human heart.

The duo have now demonstrated a thin-film, all-in-one electronic device that can adhere to the surface of a heart and generate electricity from its constant beating. Such a device could power pacemakers, defibrillators and heart-rate monitors naturally and reliably and reduce or eliminate the need for batteries.

You could see this sort of advance coming by looking at their previous work. Thin film electronics, biocompatible materials, adhesive films—it all adds up to implantable microdevices. In their study, the team attached their prototype to the hearts, lungs, and diaphragms of living animals and produced enough electricity to charge a 3.8-volt battery.

“This work is a great demonstration of engineers working with doctors and taking advantage of the natural properties of a beating heart,” Huang said. “We envision this device being used to power a pacemaker with the energy coming right from the heart.”

With a proof-of-concept in the bag, the pair are now working to optimize different design layouts of the stretchable mechanical energy harvester to facilitate its easy use. This is step one toward having everything mentioned above actually available in humans.

So stay tuned.

The paper, “Conformal piezoelectric energy harvesting and storage from motions of the heart, lung, and diaphragm,” was published in the Proceedings of the National Academy of Sciences by Huang and Rogers, and, of course, an entire slew of brilliant team members.

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Seeing the Forrest for the Batshit

?????????????????????????????????????????????????Just time for a quickie this morning, partly because I’m busy and partly because there’s not a ton to say about this particular interesting tidbit.

What do you do when fruit bats start spreading a deadly virus in rural populations of developing nations that kills in more than 70 percent of cases and has no cure nor vaccine? The immediate answer might be to kill all the fruit bats. No bats equals no viral spread.

But you might also consider planting some trees.

After going door-to-door in affected areas of Bangladesh, using GPS-assisted data collection and satellite remote sensing, researchers noticed a glaring trend; people who lived in deforested areas were the only ones getting sick.

The reason?

In areas with a lot of trees and thus a lot of fruit, the fruit bats didn’t need to find their food in human villages. Thus if they’re not flying around the villages, they’re not defecating into common sap collection jars and other food-gathering tools.

First of all, gross. Aren’t you glad you live in a place where you don’t have to worry about bats shitting in your food? And second of all, goddamnitthatsgross.

“The immediate reaction may be to get rid of the bats,” said Micah Hahn, who received her joint PhD in epidemiology and environment and resources from the University of Wisconsin, and is now working on a joint post-doctoral fellowship with the Centers for Disease Control in Fort Collins, Colo. and the National Center for Atmospheric Research in Boulder, Colo., “so we won’t have Nipah virus, but bats are important as seed spreaders for forest regeneration, they eat a lot of different fruits and fly long distances. The presence of bats alone is not a risk factor; there are villages with bats but that haven’t had Nipah virus cases. This is not just about having bats, the disease risk is a result of humans changing the landscape in ways that create opportunities for human-wildlife interactions.”

The study, “The Role of Landscape Composition and Configuration on Pteropus giganteus Roosting Ecology and Nipah Virus Spillover Risk in Bangladesh,” was published in the American Journal of Tropical Medicine and Hygiene by first author Hahn.

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Messing with a Spud’s Glycemic Index

chipsAnybody who has worried about cutting carbs or sticking to a South Beach diet has probably heard of the term glycemic index. In short, it’s a scale that indicates how quickly eating a specific type of food will raise your blood sugar levels.

If you’re an athlete, you want that blood sugar spike to fuel your competition. If you’re dieting, you want to avoid that rush because excess sugar will quickly be stored as fat if you don’t use it immediately and you’ll be hungry again very quickly. And if you’re a diabetic, the sugar rush can kill you.

There’s not much left to do but avoid certain foods like highly processed carbohydrates and simple starches. That is, unless you’re a kick-ass food scientist.

Take Srinivas Janaswamy from Purdue University, for example. He’s figured out a way to embed different kinds of molecules within a simple starch lattice.

A simple starch is little more than a handful of glucose (sugar) molecules linked together in a certain pattern—a pattern that creates holes. Janaswamy has shown that he can insert different molecules inside of these natural holes, changing the way the starch is digested without actually changing the chemistry of the starch itself.

Sort of like sticking a bunch of fruit inside of a Jello mold; you don’t change the Jello at all but the bits get trapped and go along for the ride.

So far he’s shown that adding certain molecules can slow down the digestion of a potato starch, which in theory could stop the common food from causing blood sugar spikes. But he’s also going further and showing that you can embed just about anything from antioxidants to vitamins to flavor molecules. Sounds like the first step to a healthy potato chip that tastes like chocolate to me!

There is one catch, of course, and that is that the process has only been shown to work in vitro. For those of you lacking in your Latin, that phrase means that it hasn’t been tested inside of an actual stomach yet, only outside of an organism in a dish or beaker somewhere. Only time will tell if the method works inside of an actual stomach.

The paper, “Encapsulation altered starch digestion: Toward developing starch-based delivery systems,” was published in the journal Carbohydrate Polymers by Janaswarmy and Janaswarmy alone.

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Maybe Pot Needs a Minimum Legal Age, Too

exploredti-tractsAfter being mostly legalized in Washington and Colorado, marijuana is becoming a trendier news topic with every passing day. And when a topic is trending in the 24-hour news cycle, you can be sure that scientific studies are soon to follow. What greater influence on those who award the grants is there?

One common argument for the legalization of pot is that it isn’t harmful to human health; or at the very least, not as harmful as alcohol, a substance that was only illegal for a brief, dark period in human history in a very backward corner of the globe.

Whether or not the substance is addictive is, to me at least, an open question. A website from the NIH reports that marijuana is addictive to about 9 percent of users, raising to 17 percent in teenage users and 25-50 percent in daily users. Then again, they also report the “withdrawal” symptoms of irritability, sleeplessness, anxiety and drug craving, all of which my wife exhibits after a couple of days without chocolate and seem less of a problem than someone going through caffeine withdrawal.

But I’m not here to make judgments or assumptions; I’m just here to report the facts. According to a recent study from Northwestern University, there is at least one health concern that might be worth taking into account when decriminalizing this controlled substance, and that’s age.White_Matter_Fiber_Architecture_Slideshow_full

As many studies have shown, at least some of the wiring in the human brain continues to develop well into the early 20s. We are born with a massive number of jumbled connections that slowly get either strengthened or pruned as we experience the world around us. Sufficed to say, those experiences have an impact on how our wiring takes shape and the ability for it to change years later down the line. So introducing any type of chemical that might affect how these wires are crossed is an issue well worth considering.

In the new study, researchers looked at the brains of 97 individuals who partook in daily THC doses in their late teens or early twenties for several years, but had since stopped the habit. They used fMRI machines to trace the connections made deep within the brain’s white matter.

A thin layer of neurons at the surface of the brain called the cortex is where most of the actual computation is going on at any given time. Called grey matter, these brain cells are responsible for seeing, hearing, memory, emotions, speech, decision making, and self-control, to name a few, and consume 95 percent of the oxygen that is sent to your noggin.

By contrast, white matter mostly makes up the interior of your brain and acts more or less as connections between the different areas on the surface. They make crucial communication pathways between your emotional center, decision-making center and muscle control functions, for example. How else are you going to binge eat after watching the Red Wedding for a tenth time?

It is these wires or connections that the researchers mapped out. And what they found was that those who had previously used marijuana on a daily basis had distinctly different white matter patterns than those who did not. What’s more, those changes bore a striking resemblance to disparities found in those suffering from schizophrenia—a condition whose strength of affliction has been linked to marijuana use in previous studies. It has also been linked to a poor working memory, which predicts poor academic performance and everyday functioning.

Now, of course, this study is not definitive. What if the marijuana users had these subtle differences in brain wiring before they started smoking? What if these differences are actually part of the reason they started in the first place? Or maybe there’s some other confounding factor that links the dozens of subjects together?

Some sort of long-term longitudinal study would be need to tease out the exact implications, meaning they’d have to map a lot of people’s brain before they started smoking as well as years afterward and compare them to similar controls. And who knows? Maybe that might be possible if more states legalize the practice.

But for now, if I were a neuroscience graduate student studying in Colorado or Washington, I know exactly what study I’d be proposing for my thesis.

The study, “Cannabis-Related Working Memory Deficits and Associated Subcortical Morphological Differences in Healthy Individuals and Schizophrenia Subjects,” was published in the journal Schizophrenia Bulletin by senior co-author John G. Csernansky, chair of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and Northwestern Memorial Hospital.

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Self-Healing Engineered Muscle Grown in the Laboratory

Long, colorful strands of engineered muscle fiber have been stained to observe growth after implantation into a mouse.

Long, colorful strands of engineered muscle fiber have been stained to observe growth after implantation into a mouse.

Biomedical engineers have grown living skeletal muscle that looks a lot like the real thing. It contracts powerfully and rapidly, integrates into mice quickly, and for the first time, demonstrates the ability to heal itself both inside the laboratory and inside an animal.

The study conducted at Duke University tested the bioengineered muscle by literally watching it through a window on the back of living mouse. The novel technique allowed for real-time monitoring of the muscle’s integration and maturation inside a living, walking animal.

Both the lab-grown muscle and experimental techniques are important steps toward growing viable muscle for studying diseases and treating injuries, said Nenad Bursac, associate professor of biomedical engineering at Duke.

This series of images shows the destruction and subsequent recovery of engineered muscle fibers that had been exposed to a toxin found in snake venom. This marks the first time engineered muscle has been shown to repair itself after implantation into a living animal.

This series of images shows the destruction and subsequent recovery of engineered muscle fibers that had been exposed to a toxin found in snake venom. This marks the first time engineered muscle has been shown to repair itself after implantation into a living animal.

The results appear the week of March 25 in the Proceedings of the National Academy of Sciences Early Edition.

“The muscle we have made represents an important advance for the field,” Bursac said. “It’s the first time engineered muscle has been created that contracts as strongly as native neonatal skeletal muscle.”

Through years of perfecting their techniques, a team led by Bursac and graduate student Mark Juhas discovered that preparing better muscle requires two things — well-developed contractile muscle fibers and a pool of muscle stem cells, known as satellite cells.

Every muscle has satellite cells on reserve, ready to activate upon injury and begin the regeneration process. The key to the team’s success was successfully creating the microenvironments — called niches — where these stem cells await their call to duty.

This series of images shows the progress  of veins slowly growing into implanted engineered muscle fibers.

This series of images shows the progress of veins slowly growing into implanted engineered muscle fibers.

“Simply implanting satellite cells or less-developed muscle doesn’t work as well,” said Juhas. “The well-developed muscle we made provides niches for satellite cells to live in, and, when needed, to restore the robust musculature and its function.”

To put their muscle to the test, the engineers ran it through a gauntlet of trials in the laboratory. By stimulating it with electric pulses, they measured its contractile strength, showing that it was more than 10 times stronger than any previous engineered muscles. They damaged it with a toxin found in snake venom to prove that the satellite cells could activate, multiply and successfully heal the injured muscle fibers.

Then they moved it out of a dish and into a mouse.

With the help of Greg Palmer, an assistant professor of radiation oncology in the Duke University School of Medicine, the team inserted their lab-grown muscle into a small chamber placed on the backs of live mice. The chamber was then covered by a glass panel. Every two days for two weeks, Juhas imaged the implanted muscles through the window to check on their progress.

By genetically modifying the muscle fibers to produce fluorescent flashes during calcium spikes — which cause muscle to contract — the researchers could watch the flashes become brighter as the muscle grew stronger.

“We could see and measure in real time how blood vessels grew into the implanted muscle fibers, maturing toward equaling the strength of its native counterpart,” said Juhas.

The engineers are now beginning work to see if their biomimetic muscle can be used to repair actual muscle injuries and disease.

“Can it vascularize, innervate and repair the damaged muscle’s function?” asked Bursac. “That is what we will be working on for the next several years.”

This work was supported by a National Science Foundation Graduate Research Fellowship and the National Institute of Arthritis and Musculoskeletal and Skin Diseases (AR055226).

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“Biomimetic engineered muscle with capacity for vascular integration and functional maturation in vivo.” Juhas, M., Engelmayr, Jr., G.C., Fontanella, A.N., Palmer, G.M., Bursac, N. PNAS Early Edition, March, 2014. DOI: 10.1073/pnas.1402723111

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Catheter Innovation Destroys Dangerous Biofilms

For the millions of people forced to rely on a plastic tube to eliminate their urine, developing an infection is nearly a 100 percent guarantee after just four weeks. But with the help of a little bubble-blowing, biomedical engineers hope to bring relief to urethras everywhere.

About half of the time, the interior of long-term urinary catheters become plagued by biofilms—structures formed by colonies of bacteria that are extremely difficult to kill. Once established, it is only a matter of time before the biofilm becomes a welcoming host for other, more dangerous bacteria or begins to choke urine drainage, causing leakage—or even trauma to the patient’s body.

An artistic configuration of catheters in various stages of being clogged by biofilm.

An artistic configuration of catheters in various stages of being clogged by biofilm.

Duke University engineers have developed a new urinary catheter design that can eliminate nearly all of the hard-to-kill biofilm from the catheter’s walls. Instead of focusing on expensive antibacterial coatings, the researchers use physical deformation to knock the infectious film from its moorings.

“A biofilm is like a city that protects and harbors harmful bacteria,” said Vrad Levering, a PhD student in biomedical engineering. “Our solution is like an earthquake that demolishes the infrastructure, leaving the rubble to be easily washed away by a flood of urine.”

The study appears online March 25 in Advanced Healthcare Materials.

One in five people admitted to the hospital requires a urinary catheter, contributing to the more than 30 million used each year in the United States. These catheters are the number-one cause of hospital-acquired infections in the United States.

This series of photos demonstrates how an innovative new catheter can be kept free of biofilms. When biofilm builds inside the catheter (denoted by the pink substance in the left image), pressure is forced through the neighboring channel (the dotted line in the left image and the expanded “P” space in the central graphic) causing it to rapidly expand into the main shaft. The result is the dislodging of the biofilm, as evident in the final image.

This series of photos demonstrates how an innovative new catheter can be kept free of biofilms. When biofilm builds inside the catheter (denoted by the pink substance in the left image), pressure is forced through the neighboring channel (the dotted line in the left image and the expanded “P” space in the central graphic) causing it to rapidly expand into the main shaft. The result is the dislodging of the biofilm, as evident in the final image.

Outside of the hospital, catheters are also commonly required by paralysis victims and the immobile elderly. And with an aging population, the use of catheters is likely to increase in coming decades.

For years, researchers have focused on developing antimicrobial treatments to stop the formation of biofilms, but as the statistics indicate, they have yet to find an affordable, effective solution. Besides the costs and technical challenges, many doctors fear antimicrobial solutions would promote the evolution of antibiotic-resistant superbugs.

So Duke engineers decided to think outside of the cylinder.

“We ran experiments showing that if you stretch an elastic piece of rubber at a proper rate, you can pop various types of sticky biofilms right off of its surface,” said Xuanhe Zhao, professor of mechanical engineering and materials science, whose team partnered with that of Gabriel Lopez, professor of biomedical engineering.

“Those tests were initially aimed at cleaning submerged surfaces in marine environments, but the principle has many possible applications,” continued Lopez. “So we thought, why not catheters?”

Their first model features a single channel that can be inflated with liquid or air running parallel to the main urinary tract, with nothing but a thin, flexible barrier between the two. Pushing liquid through the small inflation channel forces the thin wall into the urinary tract while leaving the outer dimensions of the catheter intact.

A prototype molded from a 3D printed form worked beautifully, Levering said. The sudden deformation unseated more than 90 percent of the biofilm, which was washed away by a flow matching the slow movement of urine. Biofilm on the wall opposite the inflation channel was mostly unharmed, but the collaborative team has plans to produce a new prototype with inflation channels running along both sides of the main channel.

The yellow cast of an innovative new catheter design created by a 3D printer is shown on the left along with the finished prototype on the right. The markings indicate the urinary duct (U), the flexible inner wall (i.w.) between the urinary duct and the inflation channel (I) and the stiff exterior wall (e.w.). Pressurizing the narrow chamber deforms the main channel, dislodging biofilm so that it can be flushed from the tube.

The yellow cast of an innovative new catheter design created by a 3D printer is shown on the left along with the finished prototype on the right. The markings indicate the urinary duct (U), the flexible inner wall (i.w.) between the urinary duct and the inflation channel (I) and the stiff exterior wall (e.w.). Pressurizing the narrow chamber deforms the main channel, dislodging biofilm so that it can be flushed from the tube.

Lopez believes the demonstration is a clear proof-of-principle that their simple mechanical solution could revolutionize the catheter industry. Because the design would have low implementation costs, closely adheres to the dimensions of current catheters and would be easy for medical clinicians to operate, the team hopes to bring it to market and is currently searching for partners.

“There are more than 30 million of these used every year,” said Levering. “And for a technology that has changed very little in 50 years, the problem is kind of atrocious. We hope we have found a solution.”

Levering said the general concept has potential applications for a wide range of industries currently plagued by biofilms, such as dairy processing, petroleum transport, city drinking water and heat exchangers.

“We don’t want to get in over our heads, but there are lots of other places where biofilms are severe problems,” said Levering. “It’s a multi-billion-dollar-per-year problem for sea water filtration alone. There are definitely other potential markets out there.”

The research was a collaboration between the Lopez and Zhao groups that, besides Levering, includes Qiming Wang, a PhD student in mechanical engineering and materials science, and Phanindhar Shivapooja, a PhD student in biomedical engineering.

This work was supported by the National Science Foundation’s Research Triangle Materials Research Science and Engineering Center (DMR-1121107), the Office of Naval Research (N0014-13-1-0828) and National Institutes of Health Training Grant (#5T32GM008555-18).

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“Soft Robotic Concepts in Catheter Design: an On-demand Fouling-release Urinary Catheter,” Levering, V., Wang, Q., Shivapooja P., Zhao, X., Lopez, G.P. Advanced Healthcare Materials, 2014. DOI: 10.1002/ ((ADHM.201400035))

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Smashing (Kidney) Stones

Pei Zhong, the Anderson-Rupp Professor of Mechanical Engineering and Materials Science at Duke University, with a modern lithotripsy machine.

Pei Zhong, the Anderson-Rupp Professor of Mechanical Engineering and Materials Science at Duke University, with a modern lithotripsy machine.

Duke engineers have devised a way to improve the efficiency of lithotripsy—the demolition of kidney stones using focused shock waves. After decades of research, all it took was cutting a groove near the perimeter of the shock wave-focusing lens and changing its curvature.

“I’ve spent more than 20 years investigating the physics and engineering aspects of shock wave lithotripsy,” said Pei Zhong, the Anderson-Rupp Professor of Mechanical Engineering and Materials Science at Duke University. “And now, thanks to the willingness of Siemens (a leading lithotripter manufacturer) to collaborate, we’ve developed a solution that is simple, cost-effective and reliable that can be quickly implemented on their machines.”

The study appears online the week of March 17, 2014, in the Proceedings of the National Academy of Sciences.

The incidence of kidney stones in the United States has more than doubled during the past two decades, due at least in part to the expanding waistlines of its citizens. The condition has also been linked to hot, humid climates and high levels of stress—a combination of living environments that seems to have led to a rise in kidney stone rates of veterans returning home from Iraq and Afghanistan.

During the past two decades, lithotripter manufacturers introduced multiple changes to their machines. Rather than having patients submerged in a bath of lukewarm water, newer machines feature a water-filled pouch that transfers the shock wave into the flesh. An electrohydraulic shock wave generator used in the past was replaced by an electromagnetic model that is more powerful, more reliable and more consistent. lithotripsy_lens

The new designs made the devices more convenient and comfortable to use, but reduced the effectiveness of the treatment. After years of research, Zhong and his colleagues have determined why.

The increased power in some third-generation shock wave lithotripters narrowed the wave’s focal width to reduce damage to surrounding tissues. But this power jump also shifted the shock wave’s focal point as much as 20 millimeters toward the device, ironically contributing to efficiency loss and raising the potential for tissue damage. The new electromagnetic shock wave generators also produced a secondary compressive wave that disrupted one of the primary stone-smashing mechanisms, cavitation bubbles.

“We were presented with the challenge of engineering a design solution that mitigated these drawbacks without being too expensive,” said Zhong. “It had to be something that was effective and reliable, but also something that the manufacturer was willing to adopt. So we decided to focus on a new lens design while keeping everything else in their system intact.”

The solution was to cut a groove near the perimeter of the backside of the lens and change its geometry. This realigned the device’s focal point and optimized the pressure distribution with a broad focal width and lower peak pressure. It also allowed more cavitation bubbles to form around the targeted stone instead of in the surrounding tissue.

Modern lithotripsy machines used to break apart kidney stones have been declining in efficiency for decades. Duke engineers have just modified the shock wave lens to improve treatment. (Image courtesy of Siemens Healthcare)

Modern lithotripsy machines used to break apart kidney stones have been declining in efficiency for decades. Duke engineers have just modified the shock wave lens to improve treatment. (Image courtesy of Siemens Healthcare)

In laboratory tests, the researchers sent shock waves through a tank of water and used a fiber optic pressure sensor to ensure the shock wave was focusing on target. They broke apart synthetic stones in a model human kidney and in anesthesized pigs and used a high-speed camera to watch the distribution of cavitation bubbles forming and collapsing—a process that happens too fast for the human eye to see.

The results showed that while the current commercial version reduced 54 percent of the stones into fragments less than two millimeters in diameter, the new version pulverized 89 percent of the stones while also reducing the amount of damage to surrounding tissue. Smaller fragments are more easily passed out of the body and less likely to recur.

“We feel we have exceeded expectations in our evaluation of this new lens design, which is based on solid physics and engineering principles,” said Zhong, who expects the new lens to enter clinical trials in Germany this summer.

“My hope is that this will be a breaking point demonstrating that effective, interactive collaboration between academia and industry can really improve the design of lithotripters that will benefit millions of stone patients worldwide who suffer from this painful disease,” Zhong said. “Our design, in principle, can be adapted by other manufacturers to improve their machines as well. I would like to see all lithotripsy machines improved so that urologists can treat stones more effectively and patients can receive better treatment and feel more comfortable with the procedure.”

This research is a collaborative effort between Zhong’s group in the Pratt School of Engineering and Dr. Glenn Preminger’s group in urologic surgery at Duke University Medical Center, together with Sorin Mitran’s group in the department of mathematics at UNC – Chapel Hill.  Andreas Neisius, a urology fellow from Mainz, Germany, and Nathan Smith, a postdoctoral Fellow in Zhong’s group, made equal contributions as first authors.

This work was supported by the National Institutes of Health (grant R37-DK052985-17) and a Ferdinand Eisenberger grant of the German Society of Urology (ID NeA1/FE-11).

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“Improving the lens design and performance of a contemporary electromagnetic shock wave lithotripter,” Neisius, A., Smith, N., Sankin, G., Kuntz, N.J., Madden, J.F., Fovargue, D.E., Mitran, S., Lipkin, M.E., Simmons, W.N., Preminger, G.M., Zhong, P.  PNAS, March 17, 2014. DOI: 10.1073/pnas.1319203111

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