Now, engineers at Princeton and the Georgia Institute of Technology have developed a technique that can be used to maintain structural integrity by essentially hiding the opening from the surrounding forces. Rather than reinforcing the opening to protect against a few select forces, the new approach reorganizes nearly any set of forces that could affect the surrounding material to avoid the opening.In a May 5 <a href="https://www.pnas.org/doi/10.1073/pnas.2415056122">article</a> in the Proceedings of the National Academy of Sciences, the researchers reported that they surrounded openings with microstructures designed to protect against many loads — external forces that cause stress, movement or deformation. The microstructures’ shapes and orientations are calibrated to work with the most challenging loads facing the structure, allowing designers to counter multiple stresses at once.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1747834729405-16x9-for-web-UTF-8Bunny_Defect_result_v9-1_Page_1_Image_0016-v2-1536x846.jpg" style="width:100%px" class="fr-fic fr-dib" />Microstructures surround an opening and minimize the stress the opening applies to the surrounding structure.“Think about a plate with a hole in it. If you put it under stress, if you pull on it, you are going to get a concentration of stress where the plate fails sooner than it would without the hole,” said <a href="https://www.me.gatech.edu/faculty/sanders">Emily D. Sanders</a>, an assistant professor of mechanical engineering at Georgia Tech and one of the authors. “We want to design something around this hole, or defect, so it seems like the hole does not exist.”<a href="https://engineering.princeton.edu/faculty/glaucio-paulino">Glaucio Paulino</a>, the study’s senior author and the Margareta Engman Augustine Professor of Engineering at Princeton, said designers typically reinforce the structure at openings like windows or tunnels. But he said that by increasing structural strength in one direction, reinforcement can introduce other problems by creating new stress in a different direction. The goal of the cloaking technique is to protect the structure by redirecting the force without creating new or undesirable stress levels.The researchers were inspired by knots in trees, where it seems like microstructures in the tree knots direct force around the site of intrusions like branches or roots and maintain structural strength. The researchers wanted to know if they could engineer structures to do the same thing in manufactured materials.Paulino said the technique relies on two optimization problems, which are designed to select the best solutions from a range of choices. The first problem uncovers the loads that will produce the greatest challenge to the object’s structure. This is more challenging than it sounds because loads on a structure or a machine can change with circumstances.“Any structure can potentially have an infinite number of loads. Every time you drive your car, the loads are different, the wind may blow in different directions, or the temperature may fluctuate,” Paulino said.The researchers found that calculating six to 10 of the worst-case loads for a structure yields the most effective results. With that information, they run a second optimization problem to find the most effective way to create and deploy microstructures surrounding the window or conduit.“The optimization technique introduced by the authors represents a breakthrough methodology for achieving the invisibility of a defect, irrespective of the direction of any externally applied force,” said <a href="https://bigoni.dicam.unitn.it/">Davide Bigoni</a>, a professor of solid and structural mechanics at the Universita’ di Trento in Italy, who was not involved in the study. “This results in omnidirectional cloaking, a property with broad applications. These include ensuring mechanical stress neutrality in organ tissue replacement, modifying structural elements to facilitate the passage of installations in machinery or civil infrastructure, and enhancing the restoration of artwork.”The idea is similar to cloaking techniques that have been developed to hide objects on the electromagnetic spectrum such as stealth aircraft. Paulino explained that the equations for solid materials can be more challenging than those for electromagnetism. But he said the goal is the same.“Any elastic disturbance is hidden by the cloak,” he said. “It is like it does not exist.”<hr />The article, “<a href="https://www.pnas.org/doi/10.1073/pnas.2415056122">Unbiased Mechanical Cloaks</a>,” was published in the journal PNAS on May 5. Besides Paulino and Sanders, it was co-authored by Fernando Vasconcelos Senhora of the Georgia Institute of Technology. Support for the project was provided in part by the National Science Foundation and the Princeton Materials Institute.

The researchers used microstructures to shield a defect shaped like a rabbit. Illustrations by the researchers

Whether designing a window in an airliner or a cable conduit for an engine, manufacturers devote a lot of effort to reinforcing openings for structural integrity. But the reinforcement is rarely perfect and often creates structural weaknesses elsewhere.

Physical cloaking works like a disappearing act for structural defects

The Harvard RoboBee has long shown it can <a href="https://seas.harvard.edu/news/2017/10/new-robobee-flies-dives-swims-and-explodes-out-water">fly, dive, and hover</a> like a real insect. But what good is the miracle of flight without a safe way to land?    <iframe width="640" height="360" src="https://www.youtube.com/embed/IIuBagiw17U?&amp;wmode=opaque&amp;rel=0&amp;enablejsapi=1&amp;origin=https://www.wevolver.com" frameborder="0" class="fr-draggable"></iframe> A storied engineering achievement by the <a href="https://www.micro.seas.harvard.edu/?_gl=1*l83if0*_gcl_au*MTEzODE3NDM1MS4xNzM3NDY1ODIx*_ga*MTQ2OTg4MDQ1My4xNzMzMjM4MDMz*_ga_FNNHYP076Y*MTc0MzYxODM1OC4yNTcuMS4xNzQzNjE4NDg1LjAuMC4w*_ga_Q97GE6JKJ9*MTc0MzYxODM1OC4yNjQuMS4xNzQzNjE4NDg0LjAuMC4w" target="_blank">Harvard Microrobotics Laboratory</a>, the RoboBee is now outfitted with its most reliable landing gear to date, inspired by one of nature’s most graceful landers: the crane fly.   <img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1745596861838-scirobotics.2025.10.issue-101.largecover.jpg" style="width:100%px" class="fr-fic fr-dib" />Publishing in <a href="https://www.science.org/doi/10.1126/scirobotics.adq3059" target="_blank">Science Robotics</a> and <a href="https://www.science.org/toc/scirobotics/10/101" target="_blank">featured</a> on the cover, the team led by <a href="https://seas.harvard.edu/person/robert-wood">Robert Wood</a>, the Harry Lewis and Marlyn McGrath Professor of Engineering and Applied Sciences in the <a href="https://seas.harvard.edu/">John A. Paulson School of Engineering and Applied Sciences</a> (SEAS), has given their flying robot a set of long, jointed legs that help ease its transition from air to ground. The robot has also received an updated controller that helps it decelerate on approach, resulting in a gentle plop-down. These improvements protect the robot’s delicate piezoelectric actuators – energy-dense “muscles” deployed for flight that are easily fractured by external forces from rough landings and collisions.Landing has been problematic for the RoboBee due in part to how small and light it is – weighing just a tenth of a gram, with a wingspan of 3 centimeters. Previous iterations suffered from significant ground effect, or instability as a result of air vortices from its flapping wings – much like the groundward-facing full-force gales generated by helicopter propellers.“Previously, if we were to go in for a landing, we’d turn off the vehicle a little bit above the ground and just drop it, and pray that it will land upright and safely,” explained co-first author Christian Chan, a graduate student who led the mechanical redesign of the robot.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1745596876606-Leaf_Landing_Composite.png" style="width:100%px" class="fr-fic fr-dib" />A composite image of the Harvard RoboBee landing on a leaf.Their paper describes improvement of the robot’s controller, or brain, to adapt to the ground effects as it approaches, an effort led by co-first author and former postdoctoral researcher Nak-seung Patrick Hyun. Hyun led controlled landing tests on a leaf as well as rigid surfaces.“The successful landing of any flying vehicle relies on minimizing the velocity as it approaches the surface before impact and dissipating energy quickly after the impact,” said Hyun, now an assistant professor at Purdue University. “Even with the tiny wing flaps of RoboBee, the ground effect is non-negligible when flying close to the surface, and things can get worse after the impact as it bounces and tumbles.”The lab looked to nature to inspire mechanical upgrades for skillful flight and graceful landing on a variety of terrains. They chose the crane fly, the relatively slow-moving, harmless insect that emerges from spring to fall and is often mistaken for a giant mosquito. “The size and scale of our platform’s wingspan and body size was fairly similar to crane flies,” Chan said.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1745596894791-comparison.jpeg" style="width:100%px" class="fr-fic fr-dib" />A comparison shot shows the relative size of the current RoboBee platform with a penny, a previous iteration of the RoboBee, and a crane fly. They noted crane flies’ long, jointed appendages that most likely give the insects the ability to dampen their landings. Crane flies are further characterized by their short-duration flights – much of their brief adult lifespan (days to a couple weeks) is spent landing and taking off.Considering specimen records from Harvard's Museum of Comparative Zoology database, the team created prototypes of different leg architectures, settling on designs similar to a crane fly’s leg segmentation and joint location. The lab used manufacturing methods pioneered in the Harvard Microrobotics Lab for adapting the stiffness and damping of each joint.Postdoctoral researcher and co-author Alyssa Hernandez brought her biology expertise to the project, having received her Ph.D. from Harvard’s Department of Organismic and Evolutionary Biology, where she studied insect locomotion. “RoboBee is an excellent platform to explore the interface of biology and robotics,” Hernandez said. “Seeking bioinspiration within the amazing diversity of insects offers us countless avenues to continue improving the robot. Reciprocally, we can use these robotic platforms as tools for biological research, producing studies that test biomechanical hypotheses.”<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1745596912994-Robobee_christian_Alyssa.jpg" style="width:100%px" class="fr-fic fr-dib" />Paper co-authors Christian Chan and Alyssa Hernandez in the Harvard Microrobotics Laboratory. Currently the RoboBee stays tethered to off-board control systems. The team will continue to focus on scaling up the vehicle and incorporating onboard electronics to give the robot sensor, power, and control autonomy – a three-pronged holy grail that would allow the RoboBee platform to truly take off. “The longer-term goal is full autonomy, but in the interim we have been working through challenges for electrical and mechanical components using tethered devices,” said Wood. “The safety tethers were, unsurprisingly, getting in the way of our experiments, and so safe landing is one critical step to remove those tethers.”The RoboBee’s diminutive size and insect-like flight prowess offer intriguing possibilities for future applications, including environmental monitoring and disaster surveillance. Among Chan’s favorite potential applications is artificial pollination – picture swarms of RoboBees buzzing around vertical farms and gardens of the future.The National Science Foundation Graduate Research Fellowship Program under Grant No. DGE 2140743 supported this research.

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Physical cloaking works like a disappearing act for structural defects

RoboBee comes in for a landing