Conceptual illustration of the Seebeck effect

The Seebeck effect converts a temperature difference across a conductor or junction of two materials into a voltage. This article reviews the physics, Seebeck coefficients, thermoelectric materials, figure of merit, thermocouples, and generator design.

The Seebeck Effect: Principles, Materials and Applications

<section id="isPasted">Ultrasounds are a critical part of modern health care, helping to image soft tissue and organs, measure blood flow, and monitor fetal development. But the technique has constraints, including a limited field of view and the potential for operator error. To address current shortfalls and push the technology toward new applications, Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, and a team of Caltech researchers have developed a system that can perform ultrasound tomography (UST) imaging on whole cross-sections of the body."Much like a standard whole-body X-ray, MRI, or PET scan, our system is operator independent. It offers a large field of view because you see the entire cross-section and it doesn't compress the tissue, which can cause distortion," says Wang, who is also the Andrew and Peggy Cherng Medical Engineering Leadership Chair and executive officer for medical engineering at Caltech. "Plus, ultrasound is entirely harmless to patients, which is a big advantage over techniques that use ionizing radiation."The team's novel whole cross-section UST system is explained in a paper published April 24, 2026, in Nature Biomedical Engineering.Traditional ultrasounds work by using a device (called an ultrasonic transducer) that emits pulses of high-frequency sound into the body and measures how those sound vibrations echo throughout tissues and organs to generate two-dimensional images of those structures.Sound waves move through water faster than through air, which is why a hydrogel is applied to the skin during conventional ultrasound imaging to eliminate any air gaps. To scan an organ from all sides, UST requires the entire body part to be in contact with water. As a way of demonstrating the feasibility of their new, larger system, Wang and the team drew inspiration from an early ultrasound system from the 1950s, in which patients were submerged in a tank of water."To propagate ultrasound efficiently, we need to have water between the patient and our ultrasound transducers, so we had participants sit in an immersion tank with their heads out of the water," explains Wang, the corresponding author of the paper. "There is a lab-made ring-like structure around the tank made up of 512 transducers that we use to scan up and down the body and image different cross-sections."In addition to measuring echoes, Wang's system can measure the transmission signal, quantifying the speed of sound to distinguish between different tissue types, and record the attenuation, or reduction of the signal's energy as it is absorbed or deflected in the body, of each sound wave."When tissue varies, it can be used as an indicator of certain diseases such as chronic inflammation and cancer. For example, if there's solidification of tissue due to a tumor, it becomes stiffer, and the echogenicity, speed of sound, or attenuation can change as it encounters that tissue," Wang explains. "The more physical parameters we can quantify, the better the chance that we can correlate those measurements with physiologically meaningful parameters to help us diagnose different diseases."Wang and the team tested their system on five healthy volunteers, scanning their abdomens for 10 seconds at a time. Unlike traditional ultrasound, the UST device was able to image deep in the body and produce images similar to those made by MRI and other standard whole-body imaging modalities.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1782309301428-Wang_Lihong-FullBodyUltrasound-Figure-WEB.original.jpg" style="width:100%px" class="fr-fic fr-dib" />Images of a healthy female's abdomen taken by the Wang lab's ultrasound tomography system (left) are compared to clinical MRI images of a corresponding region (right). Credit: Caltech Optical Imaging Laboratory"Our work shows that whole cross-sectional UST is a potential low-cost, safe, fast, and convenient tool for screening and monitoring health conditions," says Wang says, noting that while the team focused on the abdomen for this study, they believe the technique will be successful at imaging other areas of the body, including the extremities, the neck, and ultimately the brain.Eventually, he would like to convert the system from a vertical tank of liquid to a horizontal bed that uses a thin pouch of water paired with a conductive gel under the patient's body instead. In theory, this would enable real-time imaging during surgical procedures to help guide physicians—and even robotic surgical technologies—in a way that no other modality could perform. It could also be used for image-guided biopsies, among other applications, especially if combined with another technology in Wang's lab, called photoacoustic imaging, to offer molecular contrasts, he says.For now, the next step is to test the vertical system in patients with liposarcoma, a rare cancer that originates in fat cells and is hard to image with current technologies due to tumors that have difficult-to-define features and are often located deep within tissue. The researchers are collaborating with City of Hope Medical Center, a cancer hospital in the Los Angeles area, to evaluate the effectiveness of whole cross-section UST in identifying and monitoring liposarcomas."You want to quantify the fat-layer thickness to assess and track tumors, and we can provide much more accurate quantification using this technology," explains Wang, who notes that the team has a patent application pending for its UST system. "Because the system is safe and pain-free, we can monitor the same subject as frequently as we need."<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1782309251893-1782309251893.png" class="fr-fic fr-dib" />At left, an illustration of the ultrasound tomography (UST) system developed by Lihong Wang and members of his lab. While a patient is submerged in a water tank, a lab-made ring-like structure made up of 512 transducers is used to scan up and down the body and image different cross-sections. At right, UST images of a human abdomen show how information is gathered from different types of measurements to produce images of organs and tissues inside the body. Credit: Caltech Optical Imaging LaboratoryIn addition to liposarcoma, Wang says the technology could aid in the early detection of tumors in other organs, such as the liver or pancreas. Additionally, it can potentially assist in musculoskeletal imaging by identifying soft tissue tears or degenerative changes."Frequent imaging can detect subtle changes in the body over time, meaning this longitudinal capability offers deeper pathological insights than a single snapshot, enabling physicians to track progression or treatment response with greater precision," he says.The Nature Biomedical Engineer paper is titled "Whole Cross-Sectional Human Ultrasound Tomography." Additional authors from Caltech's Andrew and Peggy Cherng Department of Medical Engineering are graduate students David C. Garrett and Jinhua Xu (MS '23), postdoctoral scholar Donghyeon Oh and former postdoctoral scholar Yousuf Aborahama, and research faculty members Geng Ku and Konstantin Maslov. William Tseng, a surgical oncologist at City of Hope is also a co-author. The study was supported with funding from the National Institutes of Health, the Chan Zuckerberg Initiative, and by the National Research Foundation of Korea. The City of Hope collaboration is funded by the Merkin Institute of Translational Research at Caltech.</section><section> </section>

A team of Caltech researchers has developed a system that can perform ultrasound tomography (UST) imaging on whole cross-sections of the body.

Scanning the Body with Sound

Credit cards embedded chips, national mints printed watermarks, and high-profile locations installed retina scanners all for the same reason—to protect information. As attackers grow smarter, so must defense.<a href="https://engineering.cmu.edu/directory/bios/shen-sheng.html" target="_blank">Sheng Shen</a>, along with collaborators at Penn State University, have developed a pixel-by-pixel approach to visible camouflage with hopes of scaling it for enhanced infrared surveillance, optical security, and anti-forgery protections.“Our collaborators came to us with brochosomes—a ‘magic’ structure leafhoppers produce to create a cloak effect to hide from predators,” said Shen, a professor of <a href="https://www.meche.engineering.cmu.edu/index.html" rel="noopener" target="_blank">mechanical engineeringOpens in new window</a>. “We wanted to understand brochosomes’ optical limitations to see what more we could do with them.”Brochosomes are 3D soccer ball-like objects with nanoscale cavities that internally absorb light rather than reflect it onto outside structures. In nature, biologists suspect that this allows leafhoppers to blend in with their background.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1710755534602-0301-em-mirco-mirage-3.png" style="width: 100%px;" class="fr-fic fr-dib">To test functionality, the team simulated two different versions of the structure, one with cavities for light absorption and one without.“There is a fundamental law in physics that if a structure is a good absorber of energy, it can emit an equal amount of energy,” explained Zhuo Li, Ph.D. candidate at Carnegie Mellon. “We quickly realized that if we put both structures together, one would emit more energy than the other. That would make one appear brighter to an infrared camera than the other.”And so began the development of the world’s smallest QR code.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1710755551949-0301-em-micro-mirage-1.png" style="width: 100%px;" class="fr-fic fr-dib">Using an advanced 3D printing technique, developed by the Penn State collaborators, the team controlled whether each pixel was printed as a structure with or without holes, allowing them to fabricate a QR code readable by infrared camera alone.Sizing up at less than two percent of an inch, the code is only visible under a microscope but both teams plan to explore ways to scale it for commercial use too.<iframe width="640" height="360" src="https://www.youtube.com/embed/x54k-rT5OAA?&amp;wmode=opaque&amp;rel=0&amp;enablejsapi=1&amp;origin=https://www.wevolver.com" frameborder="0" allowfullscreen="" class="fr-draggable" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true">&nbsp;&nbsp;</iframe>“With this technology we are ultimately distorting an objects’ thermal signature,” said Li. “We have the power to disguise how objects are displayed on an infrared camera. Hypothetically, if we laid the brochosome pixels accordingly, we could paint a patrol car to appear as a delivery van to infrared security.”The team’s unique combination of visible camouflage and infrared display creates new opportunities for data encryption and optical security.<blockquote>We’ve taken infrared light and turned it from an energy carrier to an information carrier.<cite>Shen Sheng, Professor, Mechanical Engineering</cite></blockquote>“This is just the start of a new research area my team can explore,” said Sheng. “We’ve taken infrared light and turned it from an energy carrier to an information carrier.”This research was done in collaboration with The Pennsylvania State University researchers Dr. Lin Wang and Professor Tak-Sing Wong. It was first published in <a href="https://www.science.org/doi/10.1126/sciadv.adl4027" rel="noopener" target="_blank">Science AdvancesOpens in new window</a>. Zhuo Li and Lin Wang contributed equally to this work. The authors acknowledge support from the Department of Defense Office of Naval Research.

Researchers fabricate the world’s smallest QR code that, invisible to the naked eye, encrypts information to only be visible by an infrared camera lens.

Micro mirage: the infrared information carrier

At Caracol, we constantly work to improve and perfect our technology&rsquo;s <a href="https://www.instagram.com/explore/tags/quality/" id="isPasted" tabindex="0" target="_blank">#quality</a> and&nbsp;<a href="https://www.instagram.com/explore/tags/accuracy/" tabindex="0" target="_blank">#accuracy</a> throughout the production process. Thanks to the continuous improvement work conducted by our R&amp;D engineers, we&rsquo;ve recently integrated thermal monitoring systems and Infrared (IR) cameras on our&nbsp;<a href="https://www.instagram.com/explore/tags/lfam/" tabindex="0" target="_blank">#LFAM</a> process. The top layer IR video lets us analyze thermal evolution in time and space and identify hot and cold areas. This allows us to guarantee optimal surface adhesion between printed layers, monitor the progress and precision throughout the job, and automatically detect defects to ultimately deliver zero-defect production and the best properties on our final printed parts.

Caracol has introduced thermal monitoring and Infrared (IR) cameras on our #LFAM technology, to deliver zero-defect production and the best properties on final printed parts.

thermal scanner

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