“An Extra-Uterine System to Physiologically Support the Extreme Premature Lamb” (2017), by Emily Partridge, Marcus Davey, Matthew Hornick, Patrick McGovern, Ali Mejaddam, Jesse Vrecenak, Carmen Mesas-Burgos, Aliza Olive, Robert Caskey, et al.

In April 2017, Alan Flake and colleagues published “An Extra-Uterine System to Physiologically Support the Extreme Premature Lamb,” hereafter “An Extra-Uterine System,” in the journal Nature Communications. “An Extra-Uterine System” reports on the development and testing of an artificial uterus system to keep alive prematurely born animals. Prematurity, or birth prior to thirty-seven weeks of gestational development, is the global leading cause of death in children under the age of five years. The artificial uterus technology, which the authors call the Biobag, is a flexible, sealable container. It’s filled with fluid and nutrients, which replicate the environment of a uterus, the organ where a fetus typically develops. Flake and colleagues showed that their Biobag technology could keep eight premature fetal lambs alive for up to four weeks. “An Extra-Uterine System” provoked discussion among scientists and the public about the possible use of the technology to improve survival rates for premature infants and the ethics of possible future uses of the technology.

1. Background and Context
2. Article Contents
3. Impacts

Background and Context

At the time of publication, the authors were affiliated with the Children’s Hospital of Philadelphia, or CHOP, in Philadelphia, Pennsylvania. Alan Flake, who studies pediatric and fetal surgery at CHOP, is the corresponding author of the paper. Emily Partridge, a pediatric and fetal surgeon, and Marcus Davey, a professor of surgery, are the first and second authors, respectively. Sixteen of the additional authors also worked in the Center for Fetal Research in the Department of Surgery at CHOP. Two of the authors were from the Division of Cardiology in the Department of Pediatrics at CHOP.

The research described in “An Extra-Uterine System” builds on a prior history of scientists who attempted to create an artificial uterus to address the needs of premature infants. In the 1950s, researchers began experimenting with artificial uteruses to treat lung failure in premature infants. Many of those devices consisted of a machine connected to the human fetal bloodstream that maintains blood flow and an oxygenator, which is a medical device that permits gas exchange. In 1965, a research team in Canada connected an oxygenator to a blood flow system in nine lambs, which resulted in limited success. In 1969, researchers from the National Heart Institute in Bethesda, Maryland, successfully kept fetal sheep alive for fifty-five hours in their artificial uterus design.

In the 1980s and 1990s, researchers in Japan created another artificial uterus model that demonstrated fetuses could be sustained in an artificial environment. In the early 1980s, a research team at the Juntendo University in Tokyo, Japan, began an extensive research project on artificial uteruses. In 1996, they successfully nurtured fetal goats in their system design. It featured a tank containing an artificially created amniotic fluid, or the fluid that surrounds a fetus in a natural uterus during development and provides nutrients.

In the twenty-first century, researchers worldwide continued to develop artificial uterus models. In the early 2000s, Helen Hung-Ching Liu, who studies reproductive medicine and fetal development at Weill Medical College of Cornell University in New York City, New York, which, as of 2015, is called Weill Cornell Medicine, experimented with growing mouse and human embryos in artificial uteruses. In 2014, researchers at the University of Michigan in Ann Arbor, Michigan successfully sustained fetal lambs for up to a week in their extracorporeal life support system, or ECMO. In 2013, scientists at Western Australia’s Women and Infants Research Foundation in Subiaco, Australia, began working on an artificial uterus called the ex-vivo uterine environment, or EVE, which achieved success at incubating fetal lambs for one week in 2017. None of the artificial uteruses discussed above reached approval from the Food and Drug Administration, or FDA, or became widely used.

Flake and colleagues began developing their own artificial uterus, in 2014, to address the ongoing need of caring for premature infants. According to the Pan American Health Organization, approximately one in ten babies worldwide are born premature. In 2020, nearly one million infants died from complications associated with extreme prematurity.

Article Contents

“An Extra-Uterine System” is a research report that describes how the CHOP team developed the Biobag technology and used it to keep extremely premature lambs alive. Flake and colleagues placed eight fetal lambs in the Biobag for a maximum of twenty-eight days and monitored their growth and development over that period. The fetal lambs were between 105 to 120 gestational days old, which is equivalent to twenty-three or twenty-four weeks of gestation of a human fetus in terms of stage of embryological development. The fetal lambs successfully lived up to twenty-eight days in the Biobag, at which point the researchers euthanized them as called for by ethical guidelines.

“An Extra-Uterine System” begins with a brief abstract and is divided into four sections. In the “Introduction,” the authors explain that extreme prematurity is the leading cause of infant morbidity and propose that an artificial uterus system may provide a solution. The following section, “Results,” is divided into six subsections. The authors state they conducted initial pilot studies to determine the ideal design of the Biobag. They describe its main features, including a pumpless cardiac circuit, closed-fluid environment, and access to the bloodstream via the umbilical cord, that led to successful development in experimental lambs. In “Discussion,” the researchers describe that the Biobag’s features enabled successful growth and development in lambs. They also argue that future research could eventually lead to application in humans, for premature infants born between twenty-three to twenty-five weeks gestation. The final section, “Methods,” is divided into eleven subsections. There, the authors describe that they allowed eight lamb fetuses in eight Biobags to mature and then monitored their blood flow, nutrition, growth rate, and lung development.

In the “Introduction,” the CHOP research team states that they created the Biobag to address the need for advanced care to support premature infants. The authors note that extreme prematurity is one of the leading causes of infant mortality, with one-third of all infant deaths and one-half of all cases of cerebral palsy in humans attributed to prematurity. Many premature infants delivered at twenty-two to twenty-three weeks gestation survive. Still, those infants are at an increased risk for respiratory failure, chronic lung disease, and organ immaturity. The researchers highlight that respiratory failure is one of the most challenging and common problems for premature infants. Their lungs are structurally and functionally immature, which impairs gas exchange. They state that those problems indicate the need for technology to support the healthy development of a premature infant, such as an extra-uterine system like the Biobag.

Also, in the “Introduction,” Flake and colleagues note that research into artificial uteruses goes back more than fifty years, but had demonstrated limited success at that time. They argue that previous artificial uteruses had challenges supplying adequate amounts of blood to the fetal body and combatting infection. The authors state that they the Biobag to improve on previous efforts and overcome those challenges.

Lastly, in the “Introduction,” the authors write that the Biobag has three main features they claim are central to its success. First, it consists of a pumpless arteriovenous system, or circulatory system, which means the fetal heart exclusively powers the flow of blood. Second, it provides a fluid environment enclosed by a flexible bag. That bag enhances sterility and allows the Biobag to adapt to the fetus’s body shape. Third, a tube from the Biobag connects to the fetal bloodstream through the umbilical cord. That tube allows the researchers to monitor and administer nutrients to the fetus’s bloodstream. The umbilical cord is a tube that connects the fetus to the mother during pregnancy. It allows for the delivery of food and oxygen and the removal of waste to the fetus. The authors state that their Biobag supported premature fetal lamb growth and development for up to four weeks. They claim that those results are better than all previous attempts at developing an artificial uterus.

In the first of six subsections of “Results,” titled “Pilot Studies,” the researchers describe that they performed three pilot studies prior to reporting the results of the Biobag in “An Extra-Uterine System.” They describe how those studies influenced the final design. Prior to the publication of “An Extra-Uterine System,” Flake and colleagues developed two prototype artificial uterus devices and then conducted initial pilot studies of their final Biobag model. Three years prior to the publication of their article, the CHOP team created their first prototype artificial uterus. The researchers created that prototype from a glass incubator tank using an open fluid bath, meaning there was exposure of the amniotic fluid solution to air. The premature lambs they placed in the initial prototype did not survive due to infection, likely because of the open design. The second prototype included a semi-closed design with continuous amniotic fluid exchange to improve gas and nutrient exchange. Three of the five premature lambs in the second design died from infection. Next, the researchers created the Biobag, which incorporated a closed amniotic fluid system, in which the fluid remained entirely enclosed. They created the sealed bag system from a clear polyethylene film material to conform to the shape of the fetus inside of it. The research group connected the Biobag to the carotid artery, which is a blood vessel in the neck that carries blood to the brain, and umbilical vein in an effort to improve blood and oxygen flow and ensure adequate blood circulation in the fetus. However, experimental fetal lambs from that study died due to decreased blood oxygen flow, which indicated a need for improved vascular access. They resolved that problem in their final design by connecting the Biobag to the umbilical artery and vein.

In the next three subsections in “Results,” the CHOP team describes the changes they made to earlier prototypes to produce better outcomes. Following those initial pilot studies, the researchers describe their final Biobag design. In “A Pumpless Arteriovenous Circuit,” the researchers discuss that they retained a pumpless circuit and oxygenator used in their pilot studies. That closely replicated the typical, stable fetal blood flow of a fetus inside its mother’s uterus. In “A Closed Sterile Environment,” the authors state that they created the Biobag from a translucent, flexible polyethylene film material to allow for easy monitoring of the fetus. They write that the design has one sealable side to allow for the insertion of the fetal lamb. However, once sealed, it is a single-use, sterile, and closed system. In “Umbilical Vascular Access,” the research group discusses that they connected a cannula, which is a thin tube, from the Biobag to blood vessels in the umbilical cord to facilitate nutrient and blood flow.

In the final two subsections of “Results,” the CHOP research team discusses the Biobag’s success at keeping fetal lambs alive and healthy. In the fifth subsection, “Physiologic Extracorporeal Support of the Fetus,” the authors discuss that the eight lambs exhibited normal blood, cardiac, and oxygen output. They explain that they terminated the trials at twenty-eight days due to animal experimentation protocols. Still, some lambs demonstrated stability, meaning the Biobags could have supported the lambs for longer than four weeks. In the sixth subsection, “Growth and Organ Maturation,” the CHOP team states that the development of the eight lambs in the Biobags paralleled the typical growth of a lamb in a natural uterus with their size, breathing patterns, and movement. The researchers also discuss that magnetic resonance imaging, or MRI, scans of the lambs’ brains demonstrated normal brain development without any structural defects or tissue damage. Finally, the researchers acknowledge that the brain development of fetal lambs is very different from human brain development. Therefore, the experimental results regarding brain development cannot predict human fetal brain development in a Biobag.

In the “Discussion,” the CHOP research team claims the Biobag is successful at eliminating the risks of cardiac failure, respiratory failure, and brain damage in premature fetal lambs. The authors discuss that their previous artificial uterus experiments implemented a pump-supported system to facilitate blood flow from the fetal heart during development. However, that type of system led to cardiac failure in many animals. The researchers state that they chose a pumpless cardiac circuit for their Biobag, and none of the lambs experienced cardiac failure. The researchers also discuss that one of physicians’ main concerns regarding premature human infants is intracranial hemorrhage, or brain bleeding. Previous research demonstrates that hemorrhage is a result of pressure ventilation in systems that support an extremely premature infant. Thus, they hypothesize that removing a ventilation system from the Biobag could reduce the risk of bleeding.

Also, in “Discussion,” the CHOP research team highlights that a closed fluid environment with access to the bloodstream via the umbilical cord is a critical feature of the Biobag that allows for its success. They state that a closed fluid environment has many advantages, such as preserving the lungs, protecting the fetus from contaminants outside the Biobag, and maintaining amniotic fluid levels to provide the fetus with proper nutrition. The authors acknowledge that the closed fluid system limits access to the fetus for physical examination. They argue that the Biobag addresses that criticism with sterile access ports for examination and a translucent film that allows for ultrasound imaging.

The research team also discusses the implication that the concept of growing a human fetus in a similar sort of bag could discourage some parents. They offer solutions, such as camera and sound technology, to enhance parental interaction with the fetus while in the Biobag. The researchers argue that the umbilical cord is the only viable access point to connect the Biobag to the fetus to facilitate normal blood and nutrient flows. They state that previous researchers connected their artificial uterus devices directly in the center of the umbilical cord’s blood vessels to avoid umbilical vascular spasm, or the narrowing of blood vessels that results in reduced blood flow. The CHOP research team argues that those previous methods would compromise the human fetus. Therefore, they chose to connect the end of the umbilical cord to a cannula on the Biobag. They argue that their method eliminates the risk of vascular spasms and results in stable fetal blood flow.

At the end of “Discussion,” the authors state that further development and evaluation of the Biobag is necessary before clinical application to humans. They note that a fetal lamb is significantly larger than a premature infant and discuss that it is difficult to determine the exact modifications to apply the Biobag to a human fetus. Based on the results of the study, the researchers argue that the current Biobag model can support human infants after Cesarean section, or C-section, delivery since the umbilical cord can directly connect to the Biobag. A C-section is the surgical delivery of an infant through an incision on a female’s abdomen. They state that they do not know whether they can place an infant in a Biobag following vaginal delivery. The CHOP research team details that the target population for their Biobag is extreme premature infants between twenty-three to twenty-five weeks gestation and future research could extend that age limit. They argue that future applications of the Biobag could allow researchers the opportunity to deliver infants before term to receive stem cell therapies or treatment for congenital defects. The research team explains that the Biobag will allow for further research in analyzing the role of the uterus and placenta. The placenta is an organ in the uterus that provides nutrients to the fetus during development.

In the final section, “Methods,” the CHOP research team details the protocols for removing and placing the fetus into the Biobag, monitoring the fetus, and analyzing experimental results after death. In “Surgical Procedure,” the authors state that they first anesthetized pregnant lambs carrying the experimental subjects. Then, they surgically removed the fetuses and inserted cannulas into their umbilical cord or neck blood vessels, depending on their prototype group. In the following subsection, “Circuit,” the researchers describe the connections of the arteriovenous circuit. They state that the arteries connect to the oxygenator, which connects to the vein to facilitate blood flow. In the next subsection, “Fluid Incubation,” Flake and colleagues write that the first prototype was a glass tank. The prototypes evolved into individual polyethylene film bags. Next, in “Fetal Lamb Maintenance Circuit,” the authors discuss that following the transfer of the experimental fetuses, they intravenously administered maternal blood and nutrition to stabilize the lambs as needed. A technician performed a heart ultrasound one to two times daily.

In the subsection “Data Acquisition and Formulas,” the researchers provide the formulas they used to calculate the circuit flow, oxygen content, and blood flow for all fetuses. Next, the research team discusses that they removed the fetuses from the Biobags following the incubation period and monitored their breathing patterns in “Decannulation and Mechanical Ventilation.” In the last five subsections, Flake and colleagues describe that they weighed the body, brain, and lungs following death. Using that data, they estimated the lung volume and performed histological and statistical analyses to quantify their results.

Impacts

Following the publication of “An Extra-Uterine System” in 2017, Flake and colleagues received mixed feedback regarding the ethics and utility of the Biobag. In a review article titled “Premature Lambs Grown in a Bag” published in 2017, Claire Roberts, who studies placental biology at the University of Adelaide in Adelaide, Australia, outlines researchers’ main concerns about the experiment. Roberts points out that physicians typically prescribe women at risk for premature labor with glucocorticoid. Glucocorticoid is a hormone that enhances fetal development in the uterus. However, the Biobag study failed to take the effects of that common medication and did not expose the fetuses to the hormone. Roberts also highlights that premature infants typically develop an infection in the uterus. Flake and colleagues do not address how the Biobag accommodates treatments for premature infants who develop an infection. Incubator methods for monitoring premature infants typically put off parents. Roberts notes that the idea of storing a fetus in a Biobag may receive significant parental disapproval. The application of the Biobag might improve premature infant mortality rates in theory. Still, researchers question the translatability of animal studies to human clinical trials and the ethics of experimenting with human fetuses.

Flake and colleagues also received popular attention from various news sources and magazines that informed the public of the Biobag’s development. According to an article by Sara Talpos published in The Atlantic in 2017 right after the publication of “An Extra-Uterine System,” Flake and colleagues received an elaborative media response and predictions of a dystopian nature. In that article, Talpos states that the public and bioethicists expressed concern that artificial uteruses could lead to complete ectogenesis. Ectogenesis is the growth of an organism entirely in an artificial environment outside of the body, not requiring a female body at all. Many of those individuals speculated on the effect of ectogenesis on various topics, such as whether artificial uteruses could threaten abortion rights, make women obsolete, or instill fear in women.

In 2020, Jenny Kleeman from The Guardian reported on the CHOP research team’s publication. In an article, Kleeman criticizes the team for canceling an interview with her and limiting public statements. She states that the researchers want their device to be seen as ethically unremarkable to avoid ethical criticism. Kleeman further criticizes their failure to discuss the implications the Biobag could bring for women if they successfully applied it in human models. Kleeman also speculates on the potential of ectogenesis, arguing that society believes in natural reproduction. According to Kleeman, the implementation of artificial uteruses for ectogenesis could lead to ethical debates regarding female reproductive rights and bioethics. In 2021, Katarina Zimmer from IEEE Spectrum, a global engineering magazine, published an article on technological developments in artificial uteruses. Zimmer states that the publication of “An Extra-Uterine System” led to intense media coverage and public speculation that the Biobag could potentially replace pregnancy altogether.

Despite criticism from researchers and the public regarding the Biobag and artificial uterus research, many scientists worldwide continue to attempt to create artificial uterus models following the CHOP’s 2017 study. In 2019, researchers at the Eindhoven University of Technology in Eindhoven, Netherlands, received a grant of €3 million for their Perinatal Life Support Project. That research project aims to develop a liquid-based uterus environment to facilitate fetal development. In 2021, the Weizmann Institute of Science in Rehovot, Israel, grew mouse embryos for up to six days in an artificial uterus. A few of the researchers from the CHOP group created a company, Vitara Biomedical, based in Philadelphia. As of 2025, Vitara Biomedical is developing an artificial uterus in preparation for clinical trials.

“An Extra-Uterine System” demonstrated the feasibility of an artificial uterus to support animal fetuses and maintain normal growth and development. The article raised questions as to the future applications of artificial womb technology. “An Extra-Uterine System” also encouraged additional researchers worldwide to improve upon and advance the Biobag model. As of 2025, various research articles have cited the publication over 430 times. According to a 2017 article by Helen Sedgwick for the Guardian, artificial uteruses like the Biobag have the potential for many medical benefits, such as decreasing mortality rates for premature infants, assisting infertile and homosexual couples with having a child, and offering a safer alternative method to traditional pregnancy for women. However, as of 2025, despite the wishes of many for such a technology, the Biobag has not yet been adapted for human use.

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