Huh’s organ-on-a-chip devices contain human cells, allowing for experiments that could not otherwise be practically or ethically performed.
Chlorine gas is a commonly used industrial chemical. It is also highly toxic and potentially deadly; it was used as a chemical weapon in both World War I and the Syrian Civil War and has led to multiple deaths from industrial accidents. Mixing certain household cleaners can also produce the toxic gas, leading to lasting lung injuries for which there are currently no effective treatments.
Now, researchers at Penn Engineering and Penn’s Perelman School of Medicine are collaborating with BARDA, the U.S. Office of Health and Human Services’ Biomedical Advanced Research and Development Authority, to address this need using their lung-on-a-chip technology.
The laboratory of Dan Huh, associate professor in the Department of Bioengineering, has developed a series of organ-on-a-chip platforms. These devices incorporate human cells into precisely engineered microfluidic channels that mimic an organ’s natural environment, providing a way to conduct experiments that would not otherwise be feasible.
Dan Huh, PhD
Huh’s previous research has involved using a placenta-on-a-chip to study which drugs are able to reach a developing fetus; investigating microgravity’s effect on the immune system by sending one of his chips to the International Space Station; and testing treatments for dry eye disease using an eye-on-a-chip, complete with a mechanical blinking eyelid.
The Chan Zuckerberg Initiative (CZI) has announced $14 million in funding to support 29 interdisciplinary teams who are investigating the role of inflammation in disease. Among these recipients is Dan Huh, Associate Professor in Bioengineering, whose placenta-on-a-chip research will “explore how maternal and fetal cells respond to specific inflammatory signals and analyze the network of placental cells and immune cells that impact pregnancy outcomes in chronic inflammatory diseases.”
Kellie Ann Jurado, Presidential Assistant Professor in the Perelman School of Medicine’s Department of Microbiology, will lead the research team. She and Huh will collaborate with Monica Mainigi, William Shippen, Jr. Assistant Professor of Human Reproduction in Penn Medicine.
A version of the Huh Lab’s placenta-on-a-chip from 2018
Huh’s placenta-on-a-chip consists of a small block of silicone containing microfluidic channels separated by a membrane of human cells. Variations in designs and cell types allow researchers to study how different molecules cross that barrier, allowing for experiments that would be otherwise impossible or unethical. For example, Huh and his group previously used a placenta-on-a-chip designed to model the placental barrier to research the effect of maternally-administered medications on the fetal bloodstream.
In this new study, Huh, Jurando and Mainigi were motivated by even more fundamental questions of pregnancy.
“It has been known for quite some time that women with chronic inflammatory diseases are at increased risk of developing various complications during pregnancy,” Huh says. “Despite accumulating clinical evidence, we understand little about how inflammation contributes to adverse pregnancy outcomes.”
Each week, the National Science Foundation highlights “4 Awesome Discoveries You Probably Didn’t Hear About” — a kid-friendly YouTube series that highlights particularly eye-popping NSF-supported research.
This week, one of those stories was literally about an eye, or rather, a synthetic model of one.
Dan Huh, associate professor in the Department of Bioengineering, and graduate student Jeongyun Seo, recently published a paper that outlined their new blinking eye-on-a-chip. Containing human cells and mechanical parts designed to mimic natural biological functions, including a motorized eyelid, the device was developed as platform for modeling dry eye disease and testing drugs to treat it.
See more of the series at the NSF’s Science360 site, and read more about Huh’s blinking-eye-on-a-chip research here.
Rachel Young, a graduate student in Huh’s lab, holds up the new eye-on-a-chip device. The latest iteration of the lab’s eye-on-a-chip has a mechanical eyelid to simulate blinking, and was used to test an experimental drug for dry eye disease. By incorporating human cells into an engineered scaffolding, the eye-on-a-chip has many of the benefits of testing on living subjects, while minimizing risks and ethical concerns.
People who spend eight or more hours a day staring at a computer screen may notice their eyes becoming tired or dry, and, if those conditions are severe enough, they may eventually develop dry eye disease (DED). DED is a common disease with shockingly few FDA-approved drug options, partially because of the difficulties of modeling the complex pathophysiology in human eyes. Enter the blinking eye-on-a-chip: an artificial human eye replica constructed in the laboratory of Penn Engineering researchers.
This eye-on-a-chip, complete with a blinking eyelid, is helping scientists and drug developers to improve their understanding and treatment of DED, among other potential uses. The research, published in Nature Medicine, outlines the accuracy of the eye-on-a-chip as an organ stand-in and demonstrates its utility as a drug testing platform.
They collaborated with Vivian Lee, Vatinee Bunya and Mina Massaro-Giordano from the Department of Ophthalmology in Penn’s Perelman School of Medicine, as well as with Vivek Shenoy, Eduardo D. Glandt President’s Distinguished Professor in Penn Engineering’s Department of Materials Science and Engineering. Other collaborators included Woo Byun, Andrei Georgescu and Yoon-suk Yi, members of Huh’s lab, and Farid Alisafaei, a member of Shenoy’s lab.
Huh’s lab specializes in creating organs-on-a-chip that provide microengineered in vitro platforms to mimic their in vivo counterparts, including lung and bone marrow proxies launched into space this May to study astronaut illness. The lab has spent years fine-tuning its eye-on-a-chip, which earned them the 2018 Lush Prize for its promise in animal-free testing of drugs, chemicals, and cosmetics.
In this study, Huh and Seo focused on engineering an eye model that could imitate a healthy eye and an eye with DED, allowing them to test an experimental drug without risk of human harm.
The Huh lab’s eye-on-a-chip attached to a motorized, gelatin-based eyelid. Blinking spreads tears over the corneal surface, and so was a critical aspect to replicate in the researchers’ model of dry eye disease. cells. The cells of the cornea grow on the inner circle of scaffolding, dyed yellow, and the cells of the conjunctiva grow on the surrounding red circle. Artificial tears are supplied by a tear duct, dyed blue.
To construct their eye-on-a-chip, Huh’s team starts with a porous scaffold engineered with 3D printing, about the size of a dime and the shape of a contact lens, on which they grow human eye cells. The cells of the cornea grow on the inner circle of scaffolding, dyed yellow, and the cells of the conjunctiva, the specialized tissue covering the white part of human eyes, grow on the surrounding red circle. A slab of gelatin acts as the eyelid, mechanically sliding over the eye at the same rate as human blinking. Fed by a tear duct, dyed blue, the eyelid spreads artificial tear secretions over the eye to form what is called a tear film.
“From an engineering standpoint, we found it interesting to think about the possibility of mimicking the dynamic environment of a blinking human eye. Blinking serves to spread tears and generate a thin film that keeps the ocular surface hydrated. It also helps form a smooth refractive surface for light transmission. This was a key feature of the ocular surface that we wanted to recapitulate in our device,” says Huh.
For people with DED, that tear film evaporates faster than it’s replenished, resulting in inflammation and irritation. A common cause of DED is the reduced blinking that occurs during excessive computer usage, but people can develop the disease for other reasons as well. DED affects about 14 percent of the world’s population but has been notably difficult to develop new treatments for, with 200 failed clinical drug trials since 2010 and only two currently available FDA-approved drugs for treatment.
Huh’s lab has been considering the drug-testing potential of organs-on-a-chip since their initial conceptualization, and, because of its surface-level area of impact, DED seemed the perfect place to start putting their eye model to the test. But before they started a drug trial, the team had to ensure their model could really imitate the conditions of DED.
“Initially, we thought modeling DED would be as simple as just keeping the culture environment dry. But as it turns out, it’s an incredibly complex multifactorial disease with a variety of sub-types,” Huh says. “Regardless of type, however, there are two core mechanisms that underlie the development and progression of DED. First, as water evaporates from the tear film, salt concentration increases dramatically, resulting in hyperosmolarity of tears. And second, with increased tear evaporation, the tear film becomes thinner more rapidly and often ruptures prematurely, which is referred to as tear film instability. The question was: Is our model capable of modeling these core mechanisms of dry eye?”
The answer, after much experimentation, was yes. The team evoked DED conditions in their eye-on-a-chip by cutting their device’s artificial blinking in half and carefully creating an enclosed environment that simulated the humidity of real-life conditions. When put to the test against real human eyes, both healthy and with DED, the corresponding eye-on-a-chip models proved their similarity to the actual organ on multiple clinical measures. The eyes-on-a-chip mimicked actual eyes’ performance in a Schirmer strip, which tests liquid production; in an osmolarity test, which looks at tear film salt content; and in a keratography test, which evaluates the time it takes for a tear film to break up.
Having confirmed their eye-on-a-chip’s ability to mirror the performance of a human eye in normal and DED-inducing settings, Huh’s team turned to the pharmaceutical industry to find a promising DED drug candidate to test-drive their model. They landed on an upcoming drug based on lubricin, a protein primarily found in the lubricating fluid that protects joints.
“When people think of DED, they normally treat it as a chronic disease driven by inflammation,” says Huh, “but there’s now increasing evidence suggesting that mechanical forces are important for understanding the pathophysiology of DED. As the tear film becomes thinner and more unstable, friction between the eyelids and the ocular surface increases, and this can damage the epithelial surface and also trigger adverse biological responses such as inflammation. Based on these observations, there is emerging interest in developing ophthalmic lubricants as a topical treatment for dry eye. In our study, we used an lubricin-based drug that is currently undergoing clinical trials. When we tested this drug in our device, we were able to demonstrate its friction-lowering effects, but, more importantly, using this model we discovered its previously unknown capacity to suppress inflammation of the ocular surface.”
By comparing the testing results of their models of a healthy eye, an eye with DED, and an eye with DED plus lubricin, Huh and Seo were able to further scientists’ understanding of how lubricin works and show the drug’s promise as a DED treatment.
Similarly, the process of building a blinking eye-on-a-chip pushed forward scientists’ understanding of the eye itself, providing insights into the role of mechanics in biology. Collaborating with Shenoy, director of the Center for Engineering MechanoBiology, the team’s attention was drawn to how the physical blinking action was affecting the cells they cultivated to engineer an artificial eye on top of their scaffolding.
“Initially, the corneal cells start off as a single layer, but they become stratified and form multiple layers as a result of differentiation, which happens when these cells are cultured at the air-liquid interface. They also form tight cell-cell junctions and express a set of markers during differentiation,” Huh says. “Interestingly, we found out that mechanical forces due to blinking actually help the cells differentiate more rapidly and more efficiently. When the corneal cells were cultured under air in the presence of blinking, the rate and extent of differentiation increased significantly in comparison to static models without blinking. Based on this result, we speculate that blink-induced physiological forces may contribute to differentiation and maintenance of the cornea.”
In other words, human cornea cells growing on the scientists’ scaffold more quickly became specialized and efficient at their particular jobs when the artificial eyelid was blinking on top of them, suggesting that mechanical forces like blinking contribute significantly to how cells function. These types of conceptual advances, coupled with drug discovery applications, highlight the multifaceted value that engineered organs-on-a-chip can contribute to science.
Huh and Seo’s eye-on-a-chip is still just dipping its toes into the field of drug testing, but this first step is a victory that represents years of work refining their artificial eye to reach this level of accuracy and utility.
“Although we have just demonstrated proof-of-concept,” says Seo, “I hope our eye-on-a-chip platform is further advanced and used for a variety of applications besides drug screening, such as testing of contact lenses and eye surgeries in the future.”
“We are particularly proud of the fact that our work offers a great and rare example of interdisciplinary efforts encompassing a broad spectrum of research activities from design and fabrication of novel bioengineering systems to in vitro modeling of complex human disease to drug testing,” says Huh. “I think this is what makes our study unique and representative of innovation that can be brought about by organ-on-a-chip technology.”
This work was supported by the National Institutes of Health through grants 1DP2HL127720–0, R01EY026972 and K08EY025742–01, the National Science Foundation through grants CMMI:15–48571, and Research to Prevent Blindness.
Brian Chow, David Issadore, Dongeun (Dan) Huh, Linh Thi Xuan Phan, Amish Patel and Aleksandra Vojvodic
The School of Engineering and Applied Science has granted tenure to six faculty members, including three from the Department of Bioengineering.
Tenured faculty at Penn Engineering demonstrate teaching excellence and international leadership in their fields of study and research collaborations.
Brian Chow
Associate Professor in Bioengineering Chow’s research focuses on the discovery and engineering of photoreceptors and sensory proteins for manipulating and monitoring the physiology of genetically targeted cells, and the application of these tools to reveal principles of cellular dynamics. His work has advanced the rational design of light activated proteins and the use of optogenetic reagents to study cell signaling.
David Issadore
Associate Professor in Bioengineering Issadore’s research combines microelectronics, microfluidics, and nanomaterials to create miniaturized platforms for the diagnosis of disease. His work has the potential to radically change the way we diagnose and treat diseases by bringing the technologies out of the lab and directly to the point of care.
Dongeun (Dan) Huh
Associate Professor in Bioengineering Huh’s research aims to develop innovative bioengineering tools and technologies using biologically inspired design principles and micro- and nano-scale engineering techniques to create systems that mimic the structure and function of human physiological systems.
Linh Thi Xuan Phan
Associate Professor in Computer and Information Science Phan’s work focuses on making cyber-physical systems (CPS) safer, faster, and more secure, both by strengthening the theoretical foundations and by developing practical solutions. Her recent projects include a cloud platform with real-time capabilities, a new diagnosis technique for timing-related faults, and new ways to defend CPS against attacks from insiders and/or external attackers.
Amish Patel
Associate Professor in Chemical and Biomolecular Engineering Patel’s research strives to achieve a molecular-level understanding of solvation and transport in aqueous and polymeric systems, with applications ranging from the prediction of protein interactions to the design of advanced materials for water purification and energy storage. His group combines principles of statistical mechanics and liquid state theory with state-of-the-art molecular modeling and atomistic simulation techniques to study these biological, nanoscopic and polymeric systems.
Aleksandra Vojvodic
Associate Professor in Chemical and Biomolecular Engineering Vojvodic’s research focuses on theory and computation-driven materials design. Her lab uses computational frameworks to obtain fundamental understanding of surface and interface properties of complex materials that can be used to develop theoretical models for chemical transformations and energy conversion. These models have been used to predict new catalyst materials for several chemical reactions which have been experimentally synthesized and tested, validating the desired properties of the computationally predicted catalyst material.
We would like to congratulate Penn Bioengineering faculty members Brian Chow, Ph.D., Dongeun (Dan) Huh, Ph.D., and David Issadore, Ph.D., on all of their recent promotions to tenured positions as Associate Professors. Both Chow and Issadore taught the second half of the foundational course in the Penn Bioengineering undergraduate curriculum, Bioengineering Modeling, Analysis, and Design Laboratory, in which students form lab groups to complete modules in microfluidics, synthetic biology, bioelectrical signal analysis, and bioanalytical spectroscopy.
Brian Chow, Ph.D.
Outside of the classroom, Chow’s research focuses on the creation of dynamic input and output interfaces for cells through the use of optogenetics, synthetic biology, genomics, and device engineering. The Chow lab’s current projects include the exploration of functional diversity of photoreception, engineering optically active genetically encoded tools, and their applications in neuroscience and mammalian synthetic biology. His research is supported by the NIH and he is the recipient of a 2017 NSF CAREER Award. Chow also supports undergraduate innovations in research by hosting the annual Penn team for the International Genetically Engineered Machine (iGEM) competition, a program which he helped to create during his time as a graduate student at MIT. One group of Bioengineering students under Chow’s mentorship used the iGEM project as a springboard to create an accessible, open-source plate reader.
David Issadore, Ph.D.
The Issadore lab at Penn focuses on the use of microelectronics and microfluidics for medical diagnostics. In projects that combine elements of bioengineering, electrical engineering, chemical engineering, and applied physics, Issadore and his team use an interdisciplinary approach to create miniaturized low-cost platforms for disease diagnosis. His company Chip Diagnostics received the JPOD @ Philadelphia QuickFire Challenge Award last month. Earlier this year, Issadore taught the Penn Engineering course Appropriate Point of Care Diagnostics (APOC), which culminated in a service trip to Ghana (read blog posts written by participating students here). This fall, he will take over the core Bioengineering undergraduate course in Bioengineering Signals and Systems, which focuses on applications in ECG signaling, cochlear implants, and biomedical imaging.
Dan Huh, Ph.D.
Dr. Huh is the principal investigator of the BIOLines Lab at Penn, which is best known for its work on bioinspired engineering systems that Huh calls “organs-on-a-chip.” Using design and engineering principles based on microfluidics and biomimicry, the Huh lab creates microengineered systems that can reconstitute the structural and functional complexity of healthy and diseased human physiological systems in ways not possible using traditional cell culture techniques. His research has been featured in TEDx, and he has won several prestigious honors and awards including the Bernard Langer Distinguished Lectureship, Lush Prize, the McPherson Distinguished Lectureship, CRI Technology Impact Award, John J. Ryan Medal, Design of the Year Award and Best Product of the Year Award from London Design Museum, NIH Director’s New Innovator Award, and Analytical Chemistry Young Innovator Award. This fall, Huh will teach a graduate level course in biomicrofluidics that will cover the use of microfluidics for biomedical application.
Dan Huh, the Wilf Family Term Assistant Professor in the Department of Bioengineering, focuses his research on creating organs-on-chips: specially manufactured micro-devices with human cells that mimic the natural cellular processes of organs. Huh’s lab has engineered chips that approximate the functioning of placentas and lung disease, some of which were launched into space in May. Most recently, Huh published a review of organ-on-a-chip technology in the journal Science with graduate students Sunghee Estelle Park and Andrei Georgescu.
The June 2019 issue of Science is a special issue centered around the science of growing human organ models in the laboratory. Such in vitro organs are known as organoids; they grow and develop much like organs do in the body, as opposed to Huh’s organs-on-chips, in which cells from the relevant organs are grown within a fabricated device that imitates some of the organ’s functions and natural environment.
In a video accompanying the review article, Huh explains how organoid and organ-on-a-chip technologies differ and the advantages that accompany each approach:
Unlike Organ-on-a-Chip, which are heavily engineered man-made systems, organoids allow us to mimic the complex of the human body in a more natural way. So organoids represent a more realistic model, but they have problems because they develop in a highly variable fashion and it’s not very easy to control their environment. So we think that Organ-on-a-Chip Technology is a promising solution to many of these problems.
Read Huh, Park, and Georgescu’s review article at Science.
SpaceX launched its 17th resupply mission to the International Space Station on May 4, with bioengineering professor Dan Huh’s organ-on-a-chip experiments in tow.
Dan Huh, the Wilf Family Term Assistant Professor in the Department of Bioengineering, researches human organs and the diseases that infect them by engineering devices made of living cells that act as stand-ins for organs. Huh’s lab has developed imitations of many organs, including the placenta and the eye, but it’s his lung-on-a-chip and his bone-marrow-on-a-chip that are reaching unprecedented heights as part of a new experiment taking place at the International Space Station (ISS).
On May 4, SpaceX launched a ISS-bound cargo capsule carrying Huh’s organ-on-a-chip experiments, which will remain in space for a month. Once back on Earth, the chips that spent time in space will be compared to control chips from Huh’s lab that are being monitored in parallel. Huh’s team is looking to see how being in space affects bacterial infections in lungs and white blood cell behavior in bone marrow. The researchers’ hope is that their studies will reveal important information about how human organs function both in space and on Earth.
Daniel Oberhaus of WIRED wrote an article describing the multiple organs-on-a-chip experiments being conducted at the ISS, including the two experiments headed by Huh:
Dan Huh is a bioengineer at the University of Pennsylvania and the lead researcher on the lung tissue chip headed to the ISS. This lung chip models a human airway and will be infected with Pseudomonas aeruginosa, a species of bacteria that had previously been found on the ISS. On Earth this bacteria is usually associated with respiratory infections, which are one of the leading types of illness on long-duration missions to the ISS.
Huh says scientists still know very little about why astronauts’ immune response seems to become suppressed in orbit, and the tissue chips are aimed at building a better understanding of the phenomenon.
Graduate student Andrei Georgescu and Assistant Professor Dan Huh in Huh’s lab. Adapting the organ-on-a-chip technology for a trip to the International Space Station presented Huh’s team with a number of engineering challenges. (Photo: Kevin Monko)
Throughout the 60-year history of the U.S. space program—from the Mercury capsules of the 1960s to today’s International Space Station—astronauts have been getting sick. Researchers know being in orbit seems to suppress their immune systems, creating a more fertile ground for infections to grow. But nobody really understands why.
Early on the morning of April 26, a SpaceX Falcon 9 rocket will launch a cargo mission to the ISS from Cape Canaveral Air Force Station. Along with fresh water, food, and other necessities for the crew, the craft will be carrying two experiments designed by Penn scientists that could help shed light on why bugs have bedeviled space travelers.
For more than a decade, Dan Huh, the Wilf Family Term Assistant Professor of Bioengineering in the School of Engineering and Applied Science, has been developing super-small devices that use living cells to stand in for larger organs. These organs-on-a-chip hold great promise for all kinds of research, from diagnosing disease to curing them. They’re also a way to test things, including drugs and cosmetics, in a way that mimics real life without relying on animal subjects.
Online Tool for 3D Visualization of Gene Mutations
DNA inside cell nuclei undergoing the process of mitosis.
Fifteen years ago marked a major milestone in the Human Genome Project: scientists successfully sequenced all of the base pairs in our 23 sets of chromosomes. Following this accomplishment, researchers assembled generations of mathematical models to understand how gene mutations result in disease. A key barrier in developing these models is the size of genome itself: a single human genome requires approximately 2 GB of storage, and many studies examine thousands of genomes to detect changes in a small number of patients. Both processing these large datasets and efficiently storing them create challenges. Making these model predictions accurate and complete is another challenge.
Scientists collaborating among several universities on three continents developed an online computational tool to help overcome these barriers. The scientists, who include Bernhard Palsson, Ph.D., Galletti Professor of Bioengineering at University of California, San Diego, as one of the lead authors, report on the resource in a recent issue of Nature Biotechnology.
Called Recon3D, the new resource provides a metabolic network model using approximately 17% of known human genes. The model combines data on the genes, metabolites, proteins, and metabolic reactions for human metabolism. In addition, as the model’s name implies, Recon3D accounts for the physical structure of model components, imporving significantly on past models that relied on linear, two-dimensional models. Although the model still has 83% of genes left to incorporate, it could ultimately unravel some of the mysteries underlying virtually any disease with a genetic cause, from inborn errors of metabolism to cancer.
Bioengineering for Refugees
As the war in Syria enters its seventh year, at least five million refugees have left the country to seek asylum elsewhere. Roughly 20% of the refugees are now in Lebanon, where many reside in refugee camps. Although these refugees are now much safer than before, even in the best of circumstances, the conditions in refugee camps can compromise health and wellness.
Engineering can offer relief for some of these conditions. A three-week course offered in January at the American University of Beirut, co-designed and taught by Muhammad Zaman, Ph.D., Professor of Biomedical Engineering at Boston University, and entitled “Humanitarian Engineering: Designing Solutions for Health Challenges in Crises,” had students devising solutions to the issues facing these refugees.
Among the ideas generated by the students was “3D Safe Water” – a device designed to detect the contamination of water, decontaminate it, and deploy the technology in low resource settings. The device uses sensors to detect contamination and chlorine to decontaminate. With water-borne diseases taking an especially hard toll on camps like these, the device could significantly improve living conditions for refugees.
Placenta on a Chip
Organ-on-chip technologies use microfluidics to model organs or organ systems. So far, engineers have developed chip-based models of the lungs, heart, and kidneys, as well as the circulatory system.
The most recent addition to the organ-on-chip family is the placenta-on-a-chip, developed by Dan Huh, Ph.D., Wilf Family Term Assistant Professor of Bioengineering at the University of Pennsylvania. Modeling the organ that mediates and communicates between a pregnant woman and the fetus, Dr. Huh created a chip to study how drugs move from the bloodstream of the mother to the fetus. With this knowledge, one could determine more safely and more accurately how drugs taken by the mother can affect a pregnancy.
People and Places
Two colleges have announced new biomedical engineering programs. George Fox University, a Christian college in Oregon, will offer a BME concentration for engineering majors starting this fall. On the other side of the country, Springfield Technical Community College in western Massachusetts will offer a two-year associate’s degree in BME technology.
The University of Arizona, in cooperation with the City of Phoenix, will launch a new medical technology accelerator program, to be called InnoVention. It will be located on UofA’s Phoenix Biomedical Campus. Frederic Zenhausern, PhD, MBA, Professor of Basic Medical Sciences and Director of the Center for Applied NanoBioscience and Medicine at Arizona, is among the people leading the effort.
Finally, Distinguished Professor Craig Simmons of the University of Toronto’s Institute of Biomaterials and Biomedical Engineering is among 10 awardees sharing a $3.5 million grant (approximately $2.7 million in U.S. currency) for the development of medical devices and technologies. Dr. Simmons, a former postdoc at Penn, will use his funding to investigate the use of stem cells to repair congenital heart defects in infants.
