Signal processing: How did we get to where we’re going?

On May 24, Ford Professor of Engineering Al Oppenheim addressed a standing-room-only audience at MIT to give the talk of a lifetime. Entitled “Signal Processing: How Did We Get to Where We’re Going?”, Oppenheim’s personal account of his involvement in the early years of the digital signal processing field included a photo retrospective — and some handheld historical artifacts — that showed just how far the field has come since its birth at MIT and Lincoln Laboratory. Hosted by Anantha Chandrakasan, chief innovation and strategy officer, dean of engineering, and the Vannevar Bush Professor of Electrical Engineering and Computer Science, the event included a lively Q & A, giving students the chance to gain Oppenheim’s insight about the trajectory of this ever-growing field.

Al Oppenheim received a ScD degree in 1964 at MIT and is also the recipient of an honorary doctorate from Tel Aviv University. During his career, he has been a member of the Research Laboratory of Electronics and closely affiliated with MIT Lincoln Laboratory and with the Woods Hole Oceanographic Institution. His research interests are in the general area of signal processing algorithms, systems, and applications. He is co-author of the widely used textbooks “Digital Signal Processing,” “Discrete-Time Signal Processing” (currently in its third edition), “Signals and Systems” (currently in its second edition), and most recently “Signals, Systems & Interference,” published in 2016. He is also the author of several video courses available online. He is editor of several advanced books on signal processing. Throughout his career he has published extensively in research journals and conference proceedings.

Oppenheim is a member of the National Academy of Engineering, an IEEE Life Fellow, and has been a  Guggenheim Fellow in France and a Sackler Fellow in Israel. He has received a number of IEEE awards for outstanding research, teaching, and mentoring, including the IEEE Kilby Medal; the IEEE Education Medal; the IEEE Centennial Award; the IEEE Third Millennium Medal; the Norbert Wiener Society award; and the Society, Technical Achievement, and Senior Awards of the IEEE Society on Acoustics, Speech and Signal Processing; as well as a number of research, teaching, and mentoring awards at MIT.

 

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How to increase the rate of plastics recycling

While recycling systems and bottle deposits have become increasingly widespread in the U.S., actual rates of recycling are “abysmal,” according to a team of MIT researchers who studied the rates for recycling of PET, the plastic commonly used in beverage bottles. However, their findings suggest some ways to change this.

The present rate of recycling for PET, or polyethylene terephthalate, bottles nationwide is about 24 percent and has remained stagnant for a decade, the researchers say. But their study indicates that with a nationwide bottle deposit program, the rates could increase to 82 percent, with nearly two-thirds of all PET bottles being recycled into new bottles, at a net cost of just a penny a bottle when demand is robust. At the same time, they say, policies would be needed to ensure a sufficient demand for the recycled material.

The findings are being published today in the Journal of Industrial Ecology, in a paper by MIT professor of materials science and engineering Elsa Olivetti, graduate students Basuhi Ravi and Karan Bhuwalka, and research scientist Richard Roth.

The team looked at PET bottle collection and recycling rates in different states as well as other nations with and without bottle deposit policies, and with or without curbside recycling programs, as well as the inputs and outputs of various recycling companies and methods. The researchers say this study is the first to look in detail at the interplay between public policies and the end-to-end realities of the packaging production and recycling market.

They found that bottle deposit programs are highly effective in the areas where they are in place, but at present there is not nearly enough collection of used bottles to meet the targets set by the packaging industry. Their analysis suggests that a uniform nationwide bottle deposit policy could achieve the levels of recycling that have been mandated by proposed legislation and corporate commitments.

The recycling of PET is highly successful in terms of quality, with new products made from all-recycled material virtually matching the qualities of virgin material. And brands have shown that new bottles can be safely made with 100 percent postconsumer waste. But the team found that collection of the material is a crucial bottleneck that leaves processing plants unable to meet their needs. However, with the right policies in place, “one can be optimistic,” says Olivetti, who is the Jerry McAfee Professor in Engineering and the associate dean of the School of Engineering.

“A message that we have found in a number of cases in the recycling space is that if you do the right work to support policies that think about both the demand but also the supply,” then significant improvements are possible, she says. “You have to think about the response and the behavior of multiple actors in the system holistically to be viable,” she says. “We are optimistic, but there are many ways to be pessimistic if we’re not thinking about that in a holistic way.”

For example, the study found that it is important to consider the needs of existing municipal waste-recovery facilities. While expanded bottle deposit programs are essential to increase recycling rates and provide the feedstock to companies recycling PET into new products, the current facilities that process material from curbside recycling programs will lose revenue from PET bottles, which are a relatively high-value product compared to the other materials in the recycled waste stream. These companies would lose a source of their income if the bottles are collected through deposit programs, leaving them with only the lower-value mixed plastics.

The researchers developed economic models based on rates of collection found in the states with deposit programs, recycled-content requirements, and other policies, and used these models to extrapolate to the nation as a whole. Overall, they found that the supply needs of packaging producers could be met through a nationwide bottle deposit system with a 10-cent deposit per bottle — at a net cost of about 1 cent per bottle produced when demand is strong. This need not be a federal program, but rather one where the implementation would be left up to the individual states, Olivetti says.

Other countries have been much more successful in implementing deposit systems that result in very high participation rates. Several European countries manage to collect more than 90 percent of PET bottles for recycling, for example. But in the U.S., less than 29 percent are collected, and after losses in the recycling chain about 24 percent actually get recycled, the researchers found. Whereas 73 percent of Americans have access to curbside recycling, presently only 10 states have bottle deposit systems in place.

Yet the demand is there so far. “There is a market for this material,” says Olivetti. While bottles collected through mixed-waste collection can still be recycled to some extent, those collected through deposit systems tend to be much cleaner and require less processing, and so are more economical to recycle into new bottles, or into textiles.

To be effective, policies need to not just focus on increasing rates of recycling, but on the whole cycle of supply and demand and the different players involved, Olivetti says. Safeguards would need to be in place to protect existing recycling facilities from the lost revenues they would suffer as a result of bottle deposits, perhaps in the form of subsidies funded by fees on the bottle producers, to avoid putting these essential parts of the processing chain out of business. And other policies may be needed to ensure the continued market for the material that gets collected, including recycled content requirements and extended producer responsibility regulations, the team found.

At this stage, it’s important to focus on the specific waste streams that can most effectively be recycled, and PET, along with many metals, clearly fit that category. “When we start to think about mixed plastic streams, that’s much more challenging from an environmental perspective,” she says. “Recycling systems need to be pursuing extended producers’ responsibility, or specifically thinking about materials designed more effectively toward recycled content,” she says.

It's also important to address “what the right metrics are to design for sustainably managed materials streams,” she says. “It could be energy use, could be circularity [for example, making old bottles into new bottles], could be around waste reduction, and making sure those are all aligned. That’s another kind of policy coordination that’s needed.”

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The rules of the game

At the core of Raymond Wang’s work lies a seemingly simple question: Can’t we just get along?

Wang, a fifth-year political science graduate student, is a native of Hong Kong who witnessed firsthand the shakeup and conflict engendered by China’s takeover of the former British colony. “That type of experience makes you wonder why things are so complicated,” he says. “Why is it so hard to live with your neighbors?”

Today, Wang is focused on ways of managing a rapidly intensifying U.S.-China competition, and more broadly, on identifying how China — and other emerging global powers — bend, break, or creatively accommodate international rules in trade, finance, maritime, and arms control matters to achieve their ends.

The current game for global dominance between the United States and China continually threatens to erupt into dangerous confrontation. Wang’s research aims to construct a more nuanced take on China’s behaviors in this game.

“U.S. policy towards China should be informed by a better understanding of China’s behaviors if we are to avoid the worst-case scenario,” Wang believes.

“Selective and smart”

One of Wang’s major research thrusts is the ongoing trade war between the two nations. “The U.S. views China as rewriting the rules, creating an alternative world order — and accuses China of violating World Trade Organization (WTO) rules,” says Wang. “But in fact, China has been very selective and smart about responding to these rules.”

One critical, and controversial, WTO matter involves determining whether state-owned enterprises are, in the arcane vocabulary of the group, “public bodies,” which are subject to sometimes punitive WTO rules. The United States asserts that if a government owns 51 percent of a company, it is a public body. This means that many essential Chinese state-owned enterprises (SOEs) — manufacturers of electric vehicles, steel, or chemicals, for example — would fall under WTO provisions, and potentially face punitive discipline.

But China isn’t the only nation with SOEs. Many European countries, including stalwart U.S. partners France and Norway, subsidize companies that qualify as public bodies according to the U.S. definition. They, too, could be subject to tough WTO regulations.

“This could harm a swathe of the E.U. economy,” says Wang. “So China intelligently made the case to the international community that the U.S. position is extreme, and has pushed for a more favorable interpretation through litigation at the WTO.”

For Wang, this example highlights a key insight of his research: “Rising powers such as China exhibit cautious opportunism,” he says. “China will try to work with the existing rules as much as possible, including bending them in creative ways.”

But when it comes down to it, Wang argues, China would rather avoid the costs of building something completely new.

“If you can repurpose an old tool, why would you buy a new one?” he asks. “The vast majority of actions China is taking involves reshaping the existing order, not introducing new rules or blowing up institutions and building new ones.”

Interviewing key players

To bolster his theory of “cautious opportunism,” Wang’s doctoral project sets out a suite of rule-shaping strategies adopted by rising powers in international organizations. His analysis is driven by case studies of disputes recently concluded, or ongoing, in the WTO, the World Bank, and other bodies responsible for defining and policing rules that govern all manner of international relations and commerce.

Gathering evidence for his argument, Wang has been interviewing people critical to the disputes on all sides.

“My approach is to figure out who was in the room when certain decisions were made and talk to every single person there,” he says. “For the WTO and World Bank, I’ve interviewed close to 50 relevant personnel, including front-line lawyers, senior leadership, and former government officials.” These interviews took place in Geneva, Singapore, Tokyo, and Washington.

But writing about disputes that involve China poses a unique set of problems. “It’s difficult to talk to actively serving Chinese officials, and in general, nobody wants to go on the record because all the content is sensitive.” 

As Wang moves on to cases in maritime governance, he will be reaching out to the key players involved in managing sensitive conflicts in the South China Sea, an Indo-Pacific region dotted with shoals and offering desirable fisheries as well as oil and gas resources.

Even here, Wang suggests, China may find reason to be cautious rather than opportunistic, preferring to carve out exemptions for itself or shift interpretations, rather than overturning the existing rules wholesale.

Indeed, Wang believes China and other rising powers introduce new rules only when conditions open up a window of opportunity: “It may be worth doing so when using traditional tools doesn’t get you what you want, if your competitors are unable or unwilling to counter mobilize against you, and you see that the costs of establishing these new rules are worth it,” he says.

Beyond Wang’s dissertation, he has also been part of a research team led by M. Taylor Fravel, Arthur and Ruth Sloan Professor of Political Science, that has published papers on China’s Belt and Road Initiative.

From friends to enemies

Wang left Hong Kong and its political ferment behind at age 15, but the challenge of dealing with a powerful neighbor and the potential crisis it represented stayed with him. In Italy, he attended a United World College — part of a network of schools bringing together young people from different nations and cultures for the purpose of training leaders and peacemakers.

“It’s a utopian idea, where you force teenagers from all around the world to live and study together and get along for two years,” says Wang. “There were people from countries in the Balkans that were actively at war with each other, who grew up with the memory of air raid sirens and family members who fought each other, but these kids would just hang out together.”  

Coexistence was possible on the individual level, Wang realized, but he wondered, “What systemic thing happens that makes people do messed-up stuff to each other when they are in a group?”

With this question in mind, he went to the University of St. Andrews for his undergraduate and master’s degrees in international relations and modern history. As China continued its economic and military march onto the world stage, and Iran generated international tensions over its nuclear ambitions, Wang became interested in nuclear disarmament. He drilled down into the subject at the Middlebury Institute of International Studies at Monterey, where he earned a second master’s degree in nonproliferation and terrorism studies.

Leaning into a career revolving around policy, he applied to MIT’s security studies doctoral program, hoping to focus on the impact of emerging technologies on strategic nuclear stability. But events in the world led him to pivot. “When I started in the fall of 2019, the U.S.-China relationship was going off the rails with the trade war,” he says. “It was clear that managing the relationship would be one of the biggest foreign policy challenges for the foreseeable future, and I wanted to do research that would help ensure that the relationship wouldn’t tip into a nuclear war.”

Cooling tensions

Wang has no illusions about the difficulty of containing tensions between a superpower eager to assert its role in the world order, and one determined to hold onto its primacy. His goal is to make the competition more transparent, and if possible, less overtly threatening. He is preparing a paper, “Guns and Butter: Measuring Spillover and Implications for Technological Competition,” that outlines the different paths taken by the United States and China in developing defense-related technology that also benefits the civilian economy.

As he wades into the final phase of his thesis and contemplates his next steps, Wang hopes that his research insights might inform policymakers, especially in the United States, in their approach to China. While there is a fiercely competitive relationship, “there is still room for diplomacy,” he believes. “If you accept my theory that a rising power will try and use, or even abuse,  existing rules as much as possible, then you need non-military — State Department — boots on the ground to monitor what is going on at all the international institutions,” he says. The more information and understanding the United States has of China’s behavior, the more likely it will be able “to cool down some of the tensions,” says Wang. “We need to develop a strategic empathy.”
 

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MIT researchers identify routes to stronger titanium alloys

Titanium alloys are essential structural materials for a wide variety of applications, from aerospace and energy infrastructure to biomedical equipment. But like most metals, optimizing their properties tends to involve a tradeoff between two key characteristics: strength and ductility. Stronger materials tend to be less deformable, and deformable materials tend to be mechanically weak.

Now, researchers at MIT, collaborating with researchers at ATI Specialty Materials, have discovered an approach for creating new titanium alloys that can exceed this historical tradeoff, leading to new alloys with exceptional combinations of strength and ductility, which might lead to new applications.

The findings are described in the journal Advanced Materials, in a paper by Shaolou Wei ScD ’22, Professor C. Cem Tasan, postdoc Kyung-Shik Kim, and John Foltz from ATI Inc. The improvements, the team says, arise from tailoring the chemical composition and the lattice structure of the alloy, while also adjusting the processing techniques used to produce the material at industrial scale.

Titanium alloys have been important because of their exceptional mechanical properties, corrosion resistance, and light weight when compared to steels for example. Through careful selection of the alloying elements and their relative proportions, and of the way the material is processed, “you can create various different structures, and this creates a big playground for you to get good property combinations, both for cryogenic and elevated temperatures,” Tasan says.

But that big assortment of possibilities in turn requires a way to guide the selections to produce a material that meets the specific needs of a particular application. The analysis and experimental results described in the new study provide that guidance.

The structure of titanium alloys, all the way down to atomic scale, governs their properties, Tasan explains. And in some titanium alloys, this structure is even more complex, made up of two different intermixed phases, known as the alpha and beta phases.

“The key strategy in this design approach is to take considerations of different scales,” he says. “One scale is the structure of individual crystal. For example, by choosing the alloying elements carefully, you can have a more ideal crystal structure of the alpha phase that enables particular deformation mechanisms. The other scale is the polycrystal scale, that involves interactions of the alpha and beta phases. So, the approach that’s followed here involves design considerations for both.”

In addition to choosing the right alloying materials and proportions, steps in the processing turned out to play an important role. A technique called cross-rolling is another key to achieving the exceptional combination of strength and ductility, the team found.

Working together with ATI researchers, the team tested a variety of alloys under a scanning electron microscope as they were being deformed, revealing details of how their microstructures respond to external mechanical load. They found that there was a particular set of parameters — of composition, proportions, and processing method — that yielded a structure where the alpha and beta phases shared the deformation uniformly, mitigating the cracking tendency that is likely to occur between the phases when they respond differently. “The phases deform in harmony,” Tasan says. This cooperative response to deformation can yield a superior material, they found.

“We looked at the structure of the material to understand these two phases and their morphologies, and we looked at their chemistries by carrying out local chemical analysis at the atomic scale. We adopted a wide variety of techniques to quantify various properties of the material across multiple length scales, says Tasan, who is the POSCO Professor of Materials Science and Engineering and an associate professor of metallurgy. “When we look at the overall properties” of the titanium alloys produced according to their system, “the properties are really much better than comparable alloys.”

This was industry-supported academic research aimed at proving design principles for alloys that can be commercially produced at scale, according to Tasan. “What we do in this collaboration is really toward a fundamental understanding of crystal plasticity,” he says. “We show that this design strategy is validated, and we show scientifically how it works,” he adds, noting that there remains significant room for further improvement.

As for potential applications of these findings, he says, “for any aerospace application where an improved combination of strength and ductility are useful, this kind of invention is providing new opportunities.”

The work was supported by ATI Specialty Rolled Products and used facilities of MIT.nano and the Center for Nanoscale Systems at Harvard University.

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Implantable microphone could lead to fully internal cochlear implants

Cochlear implants, tiny electronic devices that can provide a sense of sound to people who are deaf or hard of hearing, have helped improve hearing for more than a million people worldwide, according to the National Institutes of Health.

However, cochlear implants today are only partially implanted, and they rely on external hardware that typically sits on the side of the head. These components restrict users, who can’t, for instance, swim, exercise, or sleep while wearing the external unit, and they may cause others to forgo the implant altogether.

On the way to creating a fully internal cochlear implant, a multidisciplinary team of researchers at MIT, Massachusetts Eye and Ear, Harvard Medical School, and Columbia University has produced an implantable microphone that performs as well as commercial external hearing aid microphones. The microphone remains one of the largest roadblocks to adopting a fully internalized cochlear implant.

This tiny microphone, a sensor produced from a biocompatible piezoelectric material, measures miniscule movements on the underside of the ear drum. Piezoelectric materials generate an electric charge when compressed or stretched. To maximize the device’s performance, the team also developed a low-noise amplifier that enhances the signal while minimizing noise from the electronics.

While many challenges must be overcome before such a microphone could be used with a cochlear implant, the collaborative team looks forward to further refining and testing this prototype, which builds off work begun at MIT and Mass Eye and Ear more than a decade ago.

“It starts with the ear doctors who are with this every day of the week, trying to improve people’s hearing, recognizing a need, and bringing that need to us. If it weren’t for this team collaboration, we wouldn’t be where we are today,” says Jeffrey Lang, the Vitesse Professor of Electrical Engineering, a member of the Research Laboratory of Electronics (RLE), and co-senior author of a paper on the microphone.

Lang’s coauthors include co-lead authors Emma Wawrzynek, an electrical engineering and computer science (EECS) graduate student, and Aaron Yeiser SM ’21; as well as mechanical engineering graduate student John Zhang; Lukas Graf and Christopher McHugh of Mass Eye and Ear; Ioannis Kymissis, the Kenneth Brayer Professor of Electrical Engineering at Columbia; Elizabeth S. Olson, a professor of biomedical engineering and auditory biophysics at Columbia; and co-senior author Hideko Heidi Nakajima, an associate professor of otolaryngology-head and neck surgery at Harvard Medical School and Mass Eye and Ear. The research is published today in the Journal of Micromechanics and Microengineering.

Overcoming an implant impasse

Cochlear implant microphones are usually placed on the side of the head, which means that users can’t take advantage of noise filtering and sound localization cues provided by the structure of the outer ear.

Fully implantable microphones offer many advantages. But most devices currently in development, which sense sound under the skin or motion of middle ear bones, can struggle to capture soft sounds and wide frequencies.

For the new microphone, the team targeted a part of the middle ear called the umbo. The umbo vibrates unidirectionally (inward and outward), making it easier to sense these simple movements.

Although the umbo has the largest range of movement of the middle-ear bones, it only moves by a few nanometers. Developing a device to measure such diminutive vibrations presents its own challenges.

On top of that, any implantable sensor must be biocompatible and able to withstand the body’s humid, dynamic environment without causing harm, which limits the materials that can be used.

“Our goal is that a surgeon implants this device at the same time as the cochlear implant and internalized processor, which means optimizing the surgery while working around the internal structures of the ear without disrupting any of the processes that go on in there,” Wawrzynek says.

With careful engineering, the team overcame these challenges.

They created the UmboMic, a triangular, 3-millimeter by 3-millimeter motion sensor composed of two layers of a biocompatible piezoelectric material called polyvinylidene difluoride (PVDF). These PVDF layers are sandwiched on either side of a flexible printed circuit board (PCB), forming a microphone that is about the size of a grain of rice and 200 micrometers thick. (An average human hair is about 100 micrometers thick.)

The narrow tip of the UmboMic would be placed against the umbo. When the umbo vibrates and pushes against the piezoelectric material, the PVDF layers bend and generate electric charges, which are measured by electrodes in the PCB layer.

Amplifying performance

The team used a “PVDF sandwich” design to reduce noise. When the sensor is bent, one layer of PVDF produces a positive charge and the other produces a negative charge. Electrical interference adds to both equally, so taking the difference between the charges cancels out the noise.

Using PVDF provides many advantages, but the material made fabrication especially difficult. PVDF loses its piezoelectric properties when exposed to temperatures above around 80 degrees Celsius, yet very high temperatures are needed to vaporize and deposit titanium, another biocompatible material, onto the sensor. Wawrzynek worked around this problem by depositing the titanium gradually and employing a heat sink to cool the PVDF.

But developing the sensor was only half the battle — umbo vibrations are so tiny that the team needed to amplify the signal without introducing too much noise. When they couldn’t find a suitable low-noise amplifier that also used very little power, they built their own.

With both prototypes in place, the researchers tested the UmboMic in human ear bones from cadavers and found that it had robust performance within the intensity and frequency range of human speech. The microphone and amplifier together also have a low noise floor, which means they could distinguish very quiet sounds from the overall noise level.

“One thing we saw that was really interesting is that the frequency response of the sensor is influenced by the anatomy of the ear we are experimenting on, because the umbo moves slightly differently in different people’s ears,” Wawrzynek says.

The researchers are preparing to launch live animal studies to further explore this finding. These experiments will also help them determine how the UmboMic responds to being implanted.

In addition, they are studying ways to encapsulate the sensor so it can remain in the body safely for up to 10 years but still be flexible enough to capture vibrations. Implants are often packaged in titanium, which would be too rigid for the UmboMic. They also plan to explore methods for mounting the UmboMic that won’t introduce vibrations.

“The results in this paper show the necessary broad-band response and low noise needed to act as an acoustic sensor. This result is surprising, because the bandwidth and noise floor are so competitive with the commercial hearing aid microphone. This performance shows the promise of the approach, which should inspire others to adopt this concept. I would expect that smaller size sensing elements and lower power electronics would be needed for next generation devices to enhance ease of implantation and battery life issues,” says Karl Grosh, professor of mechanical engineering at the University of Michigan, who was not involved with this work.

This research was funded, in part, by the National Institutes of Health, the National Science Foundation, the Cloetta Foundation in Zurich, Switzerland, and the Research Fund of the University of Basel, Switzerland.

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Detachable cardiac pacing lead may improve safety for cardiac patients

In 2012, Neil Armstrong, the first man to walk on the moon, died of post-surgery complications at the age of 82 following what should have been a routine heart surgery. Armstrong had undergone bypass surgery, the most common open-heart operation in the United States, and a surgery where the overall chance of death has dropped to almost zero.

Armstrong’s death was caused by heart damage that occurred during the removal of temporary cardiac pacing leads. Pacing leads are routinely used to monitor patients and protect against the risk of postoperative arrhythmias, including complete blockages, during the recovery period after cardiac surgery. However, because current methods rely on surgical suturing or direct insertion of electrodes to the heart tissue, trauma can occur during implantation and removal, increasing the potential for damage, bleeding, and device failure.

A coffee chat in 2019 about Armstrong’s untimely death helped inspire new research, published in the journal Science Translational Medicine. The research demonstrates findings that may offer a promising new platform for adhesive bioelectronic devices for cardiac monitoring, diagnosis, and treatment, and offer inspiration for the future development of bioadhesive electronics.

“While discussing the story, our team had a eureka moment that we probably could do something to prevent such complications by realizing a completely atraumatic version of it based on our bioadhesive technologies,” says Hyunwoo Yuk SM ’16, PhD ’21, a former MIT research scientist who is now the chief technology officer at SanaHeal. “It was such an exciting idea, and the rest was just making it happen.”

The team, comprising researchers affiliated with the lab of Xuanhe Zhao, professor of mechanical engineering and of civil and environmental engineering, has introduced a 3D-printable bioadhesive pacing lead that can directly interface with cardiac tissue, supporting minimally invasive adhesive implantation and providing a detachment solution that allows for gentle removal. Yuk and Zhao are the corresponding authors of the study; former MIT researcher Jue Deng is the paper’s first author.

“This work introduces the first on-demand detachable bioadhesive version of temporary cardiac pacing lead that offers atraumatic application and removal of the device with enhanced safety while offering improved bioelectronic performance,” says Zhao.

The development of the bioadhesive pacing lead is a combination of technologies that the team has developed over the last several years in the field of bioadhesive, bioelectronics, and 3D printing. SanaHeal, a company born from the team’s ongoing work, is commercializing bioadhesive technologies for various clinical applications.

“We hope that our ongoing effort on commercialization of our bioadhesive technology might help faster clinical translation of our bioadhesive pacing lead as well,” says Yuk.

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“Rollerama” roller rink opens in Kendall Square

The former U.S. Department of Transportation (DOT) Volpe Center site — now named “Kendall Common” in anticipation of its transformation into a vibrant mixed-use development — is now activated and open to all this summer. “Rollerama at Kendall Common” offers free roller-skating and roller skate rentals, community programming, and family-friendly events through September.

“We are extremely excited to bring Kendall Common to life in a way that is inviting and authentically Cambridge, while channeling MIT’s spirit of innovation throughout the project,” says Patrick Rowe, senior vice president, MIT Investment Management Co. “This parcel of land — right in the heart of Kendall Square — has been closed off to local residents and visitors for far too long, and we look forward to opening it up and making it accessible for all to utilize and enjoy.”

Located at the corner of Broadway and Third Street, Rollerama offers specialty themed skating nights and live entertainment, as well as food and beverage from local restaurants for purchase. Optional skate rental donations will be directed to local nonprofits. A highlight of the space is a new 7,000 square foot mural by Boston-based artist Massiel Grullón featuring retro-inspired shapes.

The first of two opening weekends took place June 28-30; the next one will be July 5-7 from 2-8 p.m. on Fridays, and 11 a.m. to 8 p.m. on Saturdays and Sundays. From July 10 through Sept. 29, Rollerama will be open Wednesdays, Thursdays, and Fridays from 2-8 p.m., and on Saturdays and Sundays from 11 a.m. to 8 p.m.

“We’re delighted to see this underutilized space activated with vibrant and playful programming,” says Jess Smith, director of MIT Open Space Programming. “Rollerama will add to the energy of Kendall Square and provide yet another compelling reason for employees, residents, students, and visitors to mix and mingle here. With food and drink available from Cambridge partners and voluntary donations going to Cambridge nonprofits, these activities in Kendall Common will contribute significantly to the sense of community in Kendall.”

The activation of Kendall Common will complement other new additions MIT has recently brought to the Kendall Square neighborhood, including Ripple Café, Row 34, Life Alive Café, Locke Bar, and Flat Top Johnny’s, along with the MIT Museum and MIT Press Book Store.

MIT assumed ownership of 10 acres of the former U.S. DOT Volpe Center site in Kendall Square earlier this year, and will commence infrastructure and site preparation for the redevelopment this fall. Over the coming years, MIT aims to transform Kendall Common into a vibrant, mixed-use development that will strengthen connections in the Cambridge community through new open green spaces, housing, retail offerings, restaurants, a community center, and science and innovation facilities.

Kendall Common will eventually include four residential buildings, four commercial buildings, four parks and a community center. Designed to be an inclusive and equitable urban environment with a focus on sustainability, the development is intended to nurture and inspire the local community.

For more information visit the Kendall Common website, Instagram page, and Facebook page.

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A prosthesis driven by the nervous system helps people with amputation walk naturally

State-of-the-art prosthetic limbs can help people with amputations achieve a natural walking gait, but they don’t give the user full neural control over the limb. Instead, they rely on robotic sensors and controllers that move the limb using predefined gait algorithms.

Using a new type of surgical intervention and neuroprosthetic interface, MIT researchers, in collaboration with colleagues from Brigham and Women’s Hospital, have shown that a natural walking gait is achievable using a prosthetic leg fully driven by the body’s own nervous system. The surgical amputation procedure reconnects muscles in the residual limb, which allows patients to receive “proprioceptive” feedback about where their prosthetic limb is in space.

In a study of seven patients who had this surgery, the MIT team found that they were able to walk faster, avoid obstacles, and climb stairs much more naturally than people with a traditional amputation.

“This is the first prosthetic study in history that shows a leg prosthesis under full neural modulation, where a biomimetic gait emerges. No one has been able to show this level of brain control that produces a natural gait, where the human’s nervous system is controlling the movement, not a robotic control algorithm,” says Hugh Herr, a professor of media arts and sciences, co-director of the K. Lisa Yang Center for Bionics at MIT, an associate member of MIT’s McGovern Institute for Brain Research, and the senior author of the new study.

Patients also experienced less pain and less muscle atrophy following this surgery, which is known as the agonist-antagonist myoneural interface (AMI). So far, about 60 patients around the world have received this type of surgery, which can also be done for people with arm amputations.

Hyungeun Song, a postdoc in MIT’s Media Lab, is the lead author of the paper, which appears today in Nature Medicine.

Sensory feedback

Most limb movement is controlled by pairs of muscles that take turns stretching and contracting. During a traditional below-the-knee amputation, the interactions of these paired muscles are disrupted. This makes it very difficult for the nervous system to sense the position of a muscle and how fast it’s contracting — sensory information that is critical for the brain to decide how to move the limb.

People with this kind of amputation may have trouble controlling their prosthetic limb because they can’t accurately sense where the limb is in space. Instead, they rely on robotic controllers built into the prosthetic limb. These limbs also include sensors that can detect and adjust to slopes and obstacles.

To try to help people achieve a natural gait under full nervous system control, Herr and his colleagues began developing the AMI surgery several years ago. Instead of severing natural agonist-antagonist muscle interactions, they connect the two ends of the muscles so that they still dynamically communicate with each other within the residual limb. This surgery can be done during a primary amputation, or the muscles can be reconnected after the initial amputation as part of a revision procedure.

“With the AMI amputation procedure, to the greatest extent possible, we attempt to connect native agonists to native antagonists in a physiological way so that after amputation, a person can move their full phantom limb with physiologic levels of proprioception and range of movement,” Herr says.

In a 2021 study, Herr’s lab found that patients who had this surgery were able to more precisely control the muscles of their amputated limb, and that those muscles produced electrical signals similar to those from their intact limb.

After those encouraging results, the researchers set out to explore whether those electrical signals could generate commands for a prosthetic limb and at the same time give the user feedback about the limb’s position in space. The person wearing the prosthetic limb could then use that proprioceptive feedback to volitionally adjust their gait as needed.

In the new Nature Medicine study, the MIT team found this sensory feedback did indeed translate into a smooth, near-natural ability to walk and navigate obstacles.

“Because of the AMI neuroprosthetic interface, we were able to boost that neural signaling, preserving as much as we could. This was able to restore a person's neural capability to continuously and directly control the full gait, across different walking speeds, stairs, slopes, even going over obstacles,” Song says.

A natural gait

For this study, the researchers compared seven people who had the AMI surgery with seven who had traditional below-the-knee amputations. All of the subjects used the same type of bionic limb: a prosthesis with a powered ankle as well as electrodes that can sense electromyography (EMG) signals from the tibialis anterior the gastrocnemius muscles. These signals are fed into a robotic controller that helps the prosthesis calculate how much to bend the ankle, how much torque to apply, or how much power to deliver.

The researchers tested the subjects in several different situations: level-ground walking across a 10-meter pathway, walking up a slope, walking down a ramp, walking up and down stairs, and walking on a level surface while avoiding obstacles.

In all of these tasks, the people with the AMI neuroprosthetic interface were able to walk faster — at about the same rate as people without amputations — and navigate around obstacles more easily. They also showed more natural movements, such as pointing the toes of the prosthesis upward while going up stairs or stepping over an obstacle, and they were better able to coordinate the movements of their prosthetic limb and their intact limb. They were also able to push off the ground with the same amount of force as someone without an amputation.

“With the AMI cohort, we saw natural biomimetic behaviors emerge,” Herr says. “The cohort that didn’t have the AMI, they were able to walk, but the prosthetic movements weren’t natural, and their movements were generally slower.”

These natural behaviors emerged even though the amount of sensory feedback provided by the AMI was less than 20 percent of what would normally be received in people without an amputation.

“One of the main findings here is that a small increase in neural feedback from your amputated limb can restore significant bionic neural controllability, to a point where you allow people to directly neurally control the speed of walking, adapt to different terrain, and avoid obstacles,” Song says.

“This work represents yet another step in us demonstrating what is possible in terms of restoring function in patients who suffer from severe limb injury. It is through collaborative efforts such as this that we are able to make transformational progress in patient care,” says Matthew Carty, a surgeon at Brigham and Women’s Hospital and associate professor at Harvard Medical School, who is also an author of the paper.

Enabling neural control by the person using the limb is a step toward Herr’s lab’s goal of “rebuilding human bodies,” rather than having people rely on ever more sophisticated robotic controllers and sensors — tools that are powerful but do not feel like part of the user’s body.

“The problem with that long-term approach is that the user would never feel embodied with their prosthesis. They would never view the prosthesis as part of their body, part of self,” Herr says. “The approach we’re taking is trying to comprehensively connect the brain of the human to the electromechanics.”

The research was funded by the MIT K. Lisa Yang Center for Bionics and the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

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Scientists observe record-setting electron mobility in a new crystal film

A material with a high electron mobility is like a highway without traffic. Any electrons that flow into the material experience a commuter’s dream, breezing through without any obstacles or congestion to slow or scatter them off their path.

The higher a material’s electron mobility, the more efficient its electrical conductivity, and the less energy is lost or wasted as electrons zip through. Advanced materials that exhibit high electron mobility will be essential for more efficient and sustainable electronic devices that can do more work with less power.

Now, physicists at MIT, the Army Research Lab, and elsewhere have achieved a record-setting level of electron mobility in a thin film of ternary tetradymite — a class of mineral that is naturally found in deep hydrothermal deposits of gold and quartz.

For this study, the scientists grew pure, ultrathin films of the material, in a way that minimized defects in its crystalline structure. They found that this nearly perfect film — much thinner than a human hair — exhibits the highest electron mobility in its class.

The team was able to estimate the material’s electron mobility by detecting quantum oscillations when electric current passes through. These oscillations are a signature of the quantum mechanical behavior of electrons in a material. The researchers detected a particular rhythm of oscillations that is characteristic of high electron mobility — higher than any ternary thin films of this class to date.

“Before, what people had achieved in terms of electron mobility in these systems was like traffic on a road under construction — you’re backed up, you can’t drive, it’s dusty, and it’s a mess,” says Jagadeesh Moodera, a senior research scientist in MIT’s Department of Physics. “In this newly optimized material, it’s like driving on the Mass Pike with no traffic.”

The team’s results, which appear today in the journal Materials Today Physics, point to ternary tetradymite thin films as a promising material for future electronics, such as wearable thermoelectric devices that efficiently convert waste heat into electricity. (Tetradymites are the active materials that cause the cooling effect in commercial thermoelectric coolers.) The material could also be the basis for spintronic devices, which process information using an electron’s spin, using far less power than conventional silicon-based devices.

The study also uses quantum oscillations as a highly effective tool for measuring a material’s electronic performance.

“We are using this oscillation as a rapid test kit,” says study author Hang Chi, a former research scientist at MIT who is now at the University of Ottawa. “By studying this delicate quantum dance of electrons, scientists can start to understand and identify new materials for the next generation of technologies that will power our world.”

Chi and Moodera’s co-authors include Patrick Taylor, formerly of MIT Lincoln Laboratory, along with Owen Vail and Harry Hier of the Army Research Lab, and Brandi Wooten and Joseph Heremans of Ohio State University.

Beam down

The name “tetradymite” derives from the Greek “tetra” for “four,” and “dymite,” meaning “twin.” Both terms describe the mineral’s crystal structure, which consists of rhombohedral crystals that are “twinned” in groups of four — i.e. they have identical crystal structures that share a side.

Tetradymites comprise combinations of bismuth, antimony tellurium, sulfur, and selenium. In the 1950s, scientists found that tetradymites exhibit semiconducting properties that could be ideal for thermoelectric applications: The mineral in its bulk crystal form was able to passively convert heat into electricity.

Then, in the 1990s, the late Institute Professor Mildred Dresselhaus proposed that the mineral’s thermoelectric properties might be significantly enhanced, not in its bulk form but within its microscopic, nanometer-scale surface, where the interactions of electrons is more pronounced. (Heremans happened to work in Dresselhaus’ group at the time.)

“It became clear that when you look at this material long enough and close enough, new things will happen,” Chi says. “This material was identified as a topological insulator, where scientists could see very interesting phenomena on their surface. But to keep uncovering new things, we have to master the material growth.”

To grow thin films of pure crystal, the researchers employed molecular beam epitaxy — a method by which a beam of molecules is fired at a substrate, typically in a vacuum, and with precisely controlled temperatures. When the molecules deposit on the substrate, they condense and build up slowly, one atomic layer at a time. By controlling the timing and type of molecules deposited, scientists can grow ultrathin crystal films in exact configurations, with few if any defects.

“Normally, bismuth and tellurium can interchange their position, which creates defects in the crystal,” co-author Taylor explains. “The system we used to grow these films came down with me from MIT Lincoln Laboratory, where we use high purity materials to minimize impurities to undetectable limits. It is the perfect tool to explore this research.”

Free flow

The team grew thin films of ternary tetradymite, each about 100 nanometers thin. They then tested the film’s electronic properties by looking for Shubnikov-de Haas quantum oscillations — a phenomenon that was discovered by physicists Lev Shubnikov and Wander de Haas, who found that a material’s electrical conductivity can oscillate when exposed to a strong magnetic field at low temperatures. This effect occurs because the material’s electrons fill up specific energy levels that shift as the magnetic field changes.

Such quantum oscillations could serve as a signature of a material’s electronic structure, and the ways in which electrons behave and interact. Most notably for the MIT team, the oscillations could determine a material’s electron mobility: If oscillations exist, it must mean that the material’s electrical resistance is able to change, and by inference, electrons can be mobile, and made to easily flow.

The team looked for signs of quantum oscillations in their new films, by first exposing them to ultracold temperatures and a strong magnetic field, then running an electric current through the film and measuring the voltage along its path, as they tuned the magnetic field up and down.

“It turns out, to our great joy and excitement, that the material’s electrical resistance oscillates,” Chi says. “Immediately, that tells you that this has very high electron mobility.”

Specifically, the team estimates that the ternary tetradymite thin film exhibits an electron mobility of 10,000 cm²/V-s — the highest mobility of any ternary tetradymite film yet measured. The team suspects that the film’s record mobility has something to do with its low defects and impurities, which they were able to minimize with their precise growth strategies. The fewer a material’s defects, the fewer obstacles an electron encounters, and the more freely it can flow.

“This is showing it’s possible to go a giant step further, when properly controlling these complex systems,” Moodera says. “This tells us we’re in the right direction, and we have the right system to proceed further, to keep perfecting this material down to even much thinner films and proximity coupling for use in future spintronics and wearable thermoelectric devices.”

This research was supported in part by the Army Research Office, National Science Foundation, Office of Naval Research, Canada Research Chairs Program and Natural Sciences and Engineering Research Council of Canada.

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