The audience was not ready to leave when Prof. HUANG Yonggang, Jan and Marcia Achenbach Professor of Mechanical Engineering and Civil and Environmental Engineering at Northwestern University, United States, finished his presentation at the PAIR Distinguished Lecture. A group of young researchers, deeply inspired by the professor’s sharing on shape programming, approached the front stage of the hall. They lined up one after another, waiting their turn to ask the speaker for his answers to puzzling scientific questions. A number of the researchers pulled out scholarly books written by the reputable scientist, feeling excited as they eagerly awaited an autograph from the pioneer in stretchable and flexible electronics.
The fans left the lecture hall with smiles on their faces after a good talk with their science idol, holding precious books freshly signed by the author. It was now time for PAIR to interview Prof. Huang about his scientific discoveries and insights into achieving success in research and teaching. The scientist’s works in the areas of “mechanics of stretchable electronics” and “mechanically guided, deterministic 3D assembly” are seminal, having attracted extensive media coverage and award recognitions. In 2024, the Society of Engineering Science (SES) renamed its Engineering Science Medal after Prof. Huang, recognising his singularly important contribution to engineering science.
Prof. HUANG Yonggang
At the microscale level: Building 3D meso-structures from components a thousand times smaller than a millimetre
In 2022, Prof. Huang and his university colleague, Prof. John ROGERS, along with Prof. ZHANG Yihui from Tsinghua University, published a paper in the journal Science Robotics on their successful development of the smallest-ever remote-controlled walking robot. This robot, which can bend, twist, expand, crawl, walk, turn and even jump, is just half a millimetre wide and resembles a peekytoe crab.
The minuscule robot is produced using a pop-up assembly method that was inspired by the art of origami and geometry in children’s pop-up books—folding, cutting and unfolding a piece of paper in certain patterns to form 3D movable parts and pop-ups. However, unlike pop-ups created from paper at the macroscale level, the works by Prof. Huang and his colleagues involve silicon, or metals, or their heterogeneous integration of any pattern. These can be as small as a few micrometres, i.e., about 1,000 times smaller than a millimetre.
Silicon and metals are both intrinsically hard substances because of their atomic arrangements. Theoretically speaking, deforming and folding these materials without breaking them is very difficult. With the unique design developed by Prof. Huang and his colleagues, such hardness is no longer a barrier to folding. The walking crab is fabricated from a 2D precursor made of silicon or metals. The precursor lays out the flat, planar geometrical structure of the walking crab, and is bonded to a slightly stretched rubber substrate. As a result, when the stretched substrate is relaxed, a process called “controlled mechanical buckling” occurs, enabling the 2D precursor to “pop” into a 3D crab shape.
These micro-robot crabs, unlike typical robots, do not require electricity. Their movement is thermally driven—and the underlying process is mesmerising. When a laser is directed at the robot in such a way that only half of the robot is exposed to the laser beam, the heated side expands more. Such asymmetrical heating causes asymmetrical thermal expansion and friction force, thus driving the robot to move. The controllable mobility of these micro-robots, together with their small size, gives them the necessary capacity to carry out important, practical tasks within confined spaces. This function benefits the medical field and industries greatly. The robots can be used to perform a range of critical tasks, from helping surgeons to conduct minimally invasive procedures such as clearing clogged arteries, stopping internal bleeding or eliminating cancerous tumours, to facilitating the monitoring of the structural health of machines.
Source: Submillimeter-scale multimaterial terrestrial robots (https://www.science.org/doi/10.1126/scirobotics.abn0602)
Propelling soft electronics development: Making folding possible for hard materials
A core focus of Prof. Huang’s scientific works has been the use of “hard” materials such as silicon and metals. Using these materials, the team has not only successfully built 3D meso-structures like the walking crabs, but also “soft” electronics. Silicon and metals are widely used in the semiconductor industry. The team has developed innovative methods so that these “hard” materials can be made “soft” enough to conform to any curvilinear surface.
To the lay audience, building something “soft” from something “hard” may be a very puzzling concept. During the interview, Prof. Huang picked up a piece of paper and explained further. “If you take a piece of paper and wrap it around your arm, it stays in place fairly well,” Prof. Huang said. “But when you use this paper to wrap a cup from the bottom, the paper crumples, and you can see wrinkles in the paper. We have the same experience with an adhesive bandage. When we apply bandages to our arms, they stick well, and the surface is smooth. However, when we try to apply them to our finger tips, winkles are formed. There is a similar problem in the electronics field. The wrinkles are places where electronics fail because they are not stretchable. This is why we need “stretchable electronics” at the places where wrinkles form. We need electronics that can adopt and fit into a variety of shapes, so that they can be seamlessly introduced into human bodies.”
Stretchable silicon for wearable sensors and biomedical applications
Prof. Huang, his collaborator Prof. Rogers, and their teams have been developing methods to produce stretchable silicon for high-performance electronics. During the mid 2000s and 2010s, they attained successful outcomes, such as a hemispherical electronic eye camera based on compressible silicon optoelectronics, stretchable silicon integrated circuits, and implantable, multifunctional silicon sensors for the brain. Their works have been published in Science and Nature journals. “We are developing tools which can be adopted by others. Our work is not really for conventional consumer electronics, but rather for bioelectronics,” he explained.
The Electronic Tattoo, a microelectronic that can be attached to human skin for monitoring health, is one successful example of their impactful collaboration. Silicon is a natural semiconductor that has the electrical properties of a conductor and an insulator, enabling it to be used as a material to turn electricity on or off. To make these electronics as flexible as possible, so that they can conform to curvilinear surfaces such as the morphology of human skin, the two scientists and their teams have combined their expertise in materials science and mechanics and embarked on a collaborative research journey.
The collaboration has yielded a series of significant breakthroughs, including a new technique for slicing off the surface layer of a silicon wafer, resulting in a thin layer of silicon that can bend like a piece of paper; a method for making silicon “elastic”, which allows silicon to bend, stretch, deform, coat and conform to curvilinear shapes; a bio-inspired technique for increasing silicon’s adhesiveness to the skin without the need for glue; and many more.
To date, the results of their research have been applied successfully to sweat sensors used in sports, as well as to the measurement of electrophysiological signals (e.g., electrocardiograms, electromyography, electroencephalograms, and electrooculograms) outside the hospital environment.
“In the past, health monitoring and sensing could only be performed in hospitals, but now they can be done anywhere, away from hospitals. The bioelectronics we developed are very soft, so that they can be integrated with the human body. A small bioelectronic patch can measure all the electrophysiological signals, which can then be downloaded to our smartphones and sent to clinics or hospitals. The sweat sensors we developed can analyse the content of the sweat on our body and provide useful information spontaneously, such as what specific nutrients we need and what kinds of minerals we have lost during sweating,” Prof. Huang further explained.
He sees bioelectronics as a field that can save many lives, and soft electronics has a critical role to play in achieving this goal. “Lifestyle is a major factor in non-communicable diseases,” he said. “Very often, it is already too late by the time individuals receive their diagnoses. If people have access to these flexible, wearable sensors, which pick up early warning signs of diseases, then early detection and treatment is possible.”
Source: Chemical sensing systems that utilize soft electronics on thin elastomeric substrates with open cellular designs (https://pubmed.ncbi.nlm.nih.gov/28989338/)
Source: Stretchable and foldable silicon integrated circuits (https://www.science.org/doi/full/10.1126/science.1154367)
The university as a hub for impactful research
Universities are now looking for ways to maximise the impact of their research. One way is to provide seed funding support to specific research areas. “Northwestern University provides support in the form of seed funding to help researchers set up their programmes in important, emerging areas. This enables faculty members to obtain preliminary results, so that they can then apply for large grants from industries or government agencies. It’s very helpful. With seed money, ideas are much easier to get off the ground,” said Prof. Huang.
Another way to drive impactful research is collaboration. Interdisciplinary research integrates perspectives, information and tools from two or more disciplines. PAIR utilises interdisciplinary research as a strategic approach to bring impactful solutions to a range of complex challenges in society. When asked for his advice on making interdisciplinary research successful, Prof. Huang stressed the importance of collaboration.
“It is best to collaborate, rather than for each researcher to learn all the new developments outside his/her area of expertise and try to be an expert in a new field. That would take too much time. We should encourage experts from different fields to collaborate,” said Prof. Huang. “From my experience as well as from the many successful collaborations at Northwestern University, an expert in existing field A does not have to become an expert in another existing field, B. Instead, he or she can collaborate with an expert in B to create a new field, C,” he further explained.
He also pointed out the importance of effective communication for successful collaboration, “We have to express ideas using layman’s language—common basic terms in the sciences. In this way, scientists are able to make themselves easily understandable to others, enabling ideas and concepts to be passed from one field to another. This is somewhat like teaching. Teachers need to pass knowledge on to other people, and they need to deliver the content in a way that university students can understand.”
Scientific storytelling in university teaching and advising: The art of communication
Prof. Huang’s language awareness (i.e., the awareness of how words should be used so as to help others learn) is evident, not only in the interview, but also in the distinguished lecture which he gave before the interview. He explained the complex science of his works using demonstrations, videos and images, employing expressions that were lively and easily understandable. At the other universities where he previously taught, Prof. Huang’s teaching was ranked as excellent by his students in consecutive years. He has also received awards for teaching and undergraduate advising at all the universities at which he has taught.
At Northwestern University, he has been selected twice for the Cole-Higgins Award for Excellence in Teaching, an annual award that is given to only one tenured or tenure track faculty member in engineering. Prof. Huang is the first at the university to win this award twice. He received the 2016 award in recognition of his “engaging teaching style that combines the big picture and technical details, while encouraging students to develop their own problem-solving style.” In 2024, he was selected for “combining ‘brilliant’ presentation with deep care for student learning and success in Engineering Analysis.” Engineering Analysis is a required course for all freshmen engineering students.
Do research because you are curious
Prof. Huang’s research achievements are remarkable. He has published two books and more than 700 journal papers, including 15 in Science and 8 in Nature. When asked for his advice to young researchers on how to get papers published in high-impact journals, the humble scientist responded, “I am not a good person to answer that because my first paper in Science was published in my 40s.”
He then talked openly about what has driven his high research productivity through the years. “We have to select problems which we are interested in and feel curious about,” said Prof. Huang. “Passion is the fundamental force that drives research.”