Organ Development in Microgravity: Growing Tissues Beyond Earth

In April 2024, a SpaceX Dragon capsule brought home a sample of living human heart tissue that had been bioprinted on the International Space Station. Redwire, the company that operates the printer, described it as the first live human heart tissue sample it had bioprinted aboard the station, and trade press called it the first living cardiac tissue bioprinted in space. The company's argument for why the work belongs in orbit is simple: without gravity, soft biological material can hold a three-dimensional shape without scaffolds. This article explains why gravity is such a problem for tissue engineering, what has actually flown, what the results do and do not show, and why nobody has grown a transplantable organ in space.

Why gravity is the enemy of tissue engineering

Organs are not just collections of cells. A working liver, heart, or kidney depends on cells arranged in a specific three-dimensional pattern, with blood vessels threading through supporting tissue and different cell types layered in precise ratios. Reproducing that architecture in a petri dish or bioreactor on Earth runs into a stubborn physical problem: gravity.

Left alone in a liquid culture, heavier cells and denser biomaterials sink. Lighter components float. Bioprinted structures made from soft material sag under their own weight before the cells have time to bond into cohesive tissue. Engineers get around this on Earth by adding scaffolds made of collagen, hydrogel, or synthetic polymers, but a scaffold is a physical object sitting between cells, and it can get in the way of the direct cell-to-cell contact that lets tissue organize itself.

Sedimentation causes a second problem: uneven delivery of nutrients and oxygen. Cells that settle on the bottom of a vessel compete for what diffuses in from above, while cells higher up may get more than they need. The result can be tissue with dead zones and inconsistent cell behavior.

In orbit, an object and its container fall around Earth together, so there is no persistent "down" for particles to settle toward. The ISS is not truly free of gravity (residual accelerations are on the order of 10⁻⁴ to 10⁻⁶ g, according to a 2025 review in the International Journal of Molecular Sciences), which is why researchers say "microgravity" rather than "zero gravity" (Wehland et al., 2025). That is low enough for cells and printed bioinks to stay suspended and for forces to act roughly equally in all directions.

Redwire describes the consequence this way: microgravity lets bioprinted materials hold a three-dimensional shape "without the use of scaffolds or additional chemical support" (Redwire, May 8, 2024). That is the company's own framing, and it is a claim about the printing process rather than a measured comparison against an Earth-printed control. It is still the clearest statement of why a printer would be worth launching.

Redwire's BioFabrication Facility returned the first living human cardiac tissue bioprinted in space to Earth in April 2024. The company says microgravity lets printed material hold its shape without scaffolds, and its stated next target is bioprinting human blood vessels on the station.

Think of stacking wet sand on a tilted plate

An analogy helps here. Building a soft, tall structure on Earth is like trying to stack wet sand on a plate that is tilted, with a fan blowing: gravity pulls every grain in one direction, and you spend your effort on supports. In microgravity, the plate is level in every direction at once. You still need the sand to be the right wetness (the bioink chemistry), but the structure no longer slumps. The analogy has a limit, though. Gravity is not the only thing cells care about, and removing it does not tell the cells what to become.

How tissue engineers learned to fake weightlessness on Earth

Long before anyone printed anything on the ISS, NASA engineers were trying to get microgravity's benefits without leaving the ground. In 1986, after the Challenger disaster grounded the shuttle fleet, researchers at Johnson Space Center (David Wolf, Tinh Trinh, and Ray Schwarz) developed the rotating wall bioreactor, a horizontal rotating vessel that keeps cells suspended in a state of continuous falling (NASA Spinoff). Cells that would flatten into a film on a petri dish instead grew in three dimensions.

NASA Spinoff reports that experiments led by Johnson scientist Thomas Goodwin showed the device could produce tissues that more closely resembled those in the body, and that shuttle missions and work on the Mir space station suggested the effects were stronger in real orbit. Those early results are the reason cell biologists took spaceflight seriously as a tool rather than a curiosity.

What simulators can and cannot do

Three ground devices dominate the field. A clinostat rotates samples around a single horizontal axis. A rotating wall vessel does the same at lower speed with larger chambers, mostly for suspended samples. A random positioning machine rotates around two perpendicular axes so the average direction of gravity cancels out, and it can hold larger specimens (Wehland et al., 2025).

None of these creates real weightlessness. They average gravity over time, and they introduce their own shear forces and fluid motion. The 2025 review stresses that simulated and real microgravity can give different results, which is why the field keeps returning to orbit despite the cost. Real flights add their own stresses too: samples endure launch loads of roughly 4 to 6 g and vibration, face launch delays, and share a limited amount of crew time.

Method	How it works	Approximate duration	Main drawback
Drop tower	Free fall in a tower	Seconds	Far too short for tissue growth
Parabolic flight	Aircraft arcs	About 22 seconds per arc	Repeated brief windows only
Clinostat / rotating wall vessel	Slow rotation averages out gravity	Days to weeks	Shear and fluid motion; not true weightlessness
Random positioning machine	Two-axis rotation	Days to weeks	Complex fluid flow adds shear
ISS	Continuous orbital free fall	Days to months	Radiation, launch stress, cost, limited crew time

Sources: durations and device descriptions from Wehland et al., International Journal of Molecular Sciences, 2025.

How organ tissue actually gets built in orbit

Space-based tissue engineering on the station runs through two main technical routes, and they solve different parts of the problem.

Bioprinting. Redwire's BioFabrication Facility (BFF) is a 3D printer that dispenses bioinks, mixtures of living cells suspended in a supportive gel, layer by layer to build a tissue structure. The ISS National Laboratory lists the facility as owned and operated by Redwire, and says it has printed and returned both a human knee meniscus and human heart tissue (ISS National Lab, BFF). The ISS National Lab's page describes the eventual goal as the manufacture of whole human organs.

Cell and organoid culture. The second route skips printing and lets cells self-assemble. Researchers load induced pluripotent stem cells (iPSCs, adult cells reprogrammed into a stem-like state that can become almost any tissue type) or engineered tissue constructs into small sealed containers, launch them to the station, and let them grow for weeks before returning them for analysis. An organoid is a small clump of cells that organizes itself into a miniature, simplified version of an organ.

Bioprinting is like using a mold to place each layer of wet sand exactly where you want it, so you control the structure directly. Organoid culture is more like shaking wet sand in a bucket until the grains settle into a stable shape guided by physics rather than a blueprint. In microgravity, both approaches have the same hope: that the "settling" becomes uniform instead of downward.

What is the BioFabrication Facility on the ISS?

The BioFabrication Facility is a 3D bioprinter built by Techshot (now part of Redwire) with printer maker nScrypt. It launched to the station in July 2019 aboard SpaceX CRS-18 (ISS National Lab, CRS-18). Its first test prints used bioink containing human heart muscle, nerve, and vascular cells and returned on the CRS-19 Dragon (Techshot, January 2020).

A short timeline of space bioprinting

The 2024 headline was the culmination of a sequence of steps rather than a sudden jump. The record is easier to follow in order.

Date	Milestone	Source
1986	NASA develops the rotating wall bioreactor to simulate weightlessness on the ground	NASA Spinoff
July 2019	BioFabrication Facility launches on SpaceX CRS-18	ISS National Lab
January 2020	First test prints (heart muscle, nerve, and vascular cells) announced after returning on CRS-19; the constructs measured 30 mm by 20 mm by 12.6 mm	Techshot
March to April 2020	Partial human meniscus test prints, launched on CRS-20	ISS National Lab
April 30, 2024	Dragon returns the first living human cardiac tissue bioprinted in space	Payload; Redwire

The 2020 test prints are the reason it matters to read the claim carefully. The first cardiac-type constructs flew years before 2024; what the 2024 release added was living cardiac tissue that returned for further testing at Redwire's Greenville, Indiana, facility. Redwire has not described the outcome of that testing in detail in the sources reviewed for this article.

Heart tissue: the ISS's most advanced organ-tissue program

Cardiac research is the most mature organ-tissue application running on the station, and it exists for a practical reason. Spaceflight changes the heart. NASA describes microgravity's effect on heart function as resembling normal aging, and its Engineered Heart Tissue investigation used 3D-cultured cardiac muscle tissue, grown on scaffolds from human cells, in an automated heart-on-a-chip platform. The tissues showed "impairments that led to increased arrhythmias and loss of muscle strength," changes similar to cardiac aging (NASA, Station Science Top News, October 4, 2024).

An earlier Stanford study set the template. A team led by Joseph Wu and Arun Sharma cultured human iPSC-derived cardiomyocytes (heart muscle cells) on the station for 5.5 weeks. RNA sequencing found 2,635 differentially expressed genes across in-orbit, postflight, and ground samples, and the flown cells showed decreased calcium recycling and irregular beating. About 10 days after landing, gene expression had moved back toward the ground pattern (ISS National Lab summary of the Stem Cell Reports paper). That matters: the genetic response looks at least partly reversible after return to normal gravity.

Mount Sinai's Cardiovascular Research Institute sent heart muscle cells and micro-tissues to the station on SpaceX CRS-29, launched November 9, 2023. The team grew them for about 30 days in sealed vials, as 2D layers, 3D spheroids, and 3D elongated tissues, and compared them with matching samples kept on the ground (Mount Sinai, November 2023). One of the stated questions was whether 3D cultures offer a biological advantage over 2D cultures in orbit.

Other groups pair that physiology with regenerative goals. An Emory University team led by Chunhui Xu is testing whether heart cells, like some cancer cells, proliferate and survive better in microgravity, which could ease two bottlenecks in cell-based heart therapies (ISS National Lab, Emory). A collaboration between Oregon State and Texas Tech flew 3D-bioprinted cardiac organoids on SpaceX CRS-31 in November 2024 to study heart muscle atrophy (ISS National Lab, CRS-31).

This dual framing matters. The same experiments can protect astronauts on a future Mars-class mission and generate disease models for the ground. For the cardiovascular side of that story, see Cardiovascular Changes in Astronauts; for the muscle side, see Muscle Atrophy Research: How Weightlessness Weakens Astronauts.

What did the 2024 heart tissue experiment actually prove?

It showed that a bioprinter can run, and living human cardiac tissue can be produced and returned, in orbit. Redwire names heart patches for damaged cardiac tissue as a possible eventual use. A single returned sample that is still under testing does not yet show that the tissue beats in a coordinated way, integrates with host tissue, or outperforms ground-printed controls.

Is microgravity good for the heart cells themselves?

The evidence is mixed. A 2025 review of tissue engineering under microgravity found that some ISS and simulator studies of stem-cell-derived cardiomyocytes showed improved maturation markers, while others showed reduced contractility, arrhythmia, or senescence-like stress (Wehland et al., 2025). That split is why researchers use the station both to protect astronauts and to test whether the environment helps tissue.

Retinas, cartilage, and the other tissues already flying

Cardiac tissue gets most of the headlines, but it isn't the only organ-relevant program on the station.

Artificial retinas. LambdaVision, a Connecticut biotech company, is developing a light-activated artificial retina made from 200 layers of protein thin film, aimed at restoring vision in people with retinal degeneration. The company says Earth manufacturing is not impossible but hard to scale because gravity-driven sedimentation and convection disturb the layering, and that nine ISS missions showed microgravity improves the films' homogeneity, stability, and performance. It describes itself as a pre-clinical-stage company and reports no human trial results. On June 24, 2026, it announced a memorandum of understanding with Vast, a commercial space station developer, for future work on Vast's Haven-1 and later stations (LambdaVision, June 24, 2026). Every performance claim here comes from the company, not from an independent clinical test.

Cartilage and connective tissue. Techshot's BioFabrication Facility printed partial human meniscus, the C-shaped cartilage that cushions the knee, in test prints announced in April 2020 using four print heads and gel materials (ISS National Lab). The announcement raised the possibility that printing in microgravity could remove the need for scaffolding. Separately, ground-based simulators have produced scaffold-free cartilage-like tissue, with constructs about 5 mm across after 14 days in a rotating wall vessel, according to the 2025 review.

Brain organoids. A study led by researchers at the New York Stem Cell Foundation Research Institute flew cortical and dopaminergic neural organoids from four iPSC lines to the station for 30 days on SpaceX CRS-19. Organoids from low Earth orbit showed lower expression of proliferation and progenitor markers and higher expression of neuronal maturity markers than ground controls, with 1,183 differentially expressed genes in the dopaminergic organoids and 926 in the cortical ones (Marotta et al., Stem Cells Translational Medicine, 2024). The authors describe this as accelerated maturation, and note that they did not test accelerated aging.

Their own limitations are worth reading closely. It was a single flight at a single 30-day timepoint, with four donor lines, so it was not powered to detect disease-specific differences. The organoids also experienced launch stress. Radiation over the 30 days was estimated at about 12 mGy, which the authors argue is unlikely to explain the differences, but that is an inference, not a direct test.

The clearest through-line in these programs is one physical mechanism: remove gravity-driven sedimentation and convection, and cells and printed layers tend to organize more uniformly. The clearest caveat is just as consistent: most results come from small, single-flight studies, and several of the performance claims come from the companies that run them.

A snapshot of what's actually flying on the ISS

The table below summarizes the major organ- and tissue-relevant programs discussed above, based on public program descriptions from NASA, the ISS National Laboratory, and the companies and universities running the work.

Table: Organ and tissue-engineering programs on the ISS

Program / Organization	Tissue type	Method	Primary goal	Status per cited source
Redwire BioFabrication Facility	Cardiac tissue	3D bioprinting, no scaffold	Heart patches; organ printing is a long-term aim	First living cardiac tissue returned April 2024; blood vessel printing planned next
NASA Engineered Heart Tissue	Cardiac muscle	Scaffold-based 3D culture in a heart-on-a-chip platform	Countermeasures for astronaut cardiovascular decline	Results reported October 2024: more arrhythmias, less muscle strength
Stanford (Wu lab)	iPSC-derived heart cells	Cell culture, about 5.5 weeks	Understand cardiac effects of spaceflight	Published 2019; gene expression moved back toward ground pattern about 10 days after return
Mount Sinai	Heart muscle cells and micro-tissues	Sealed-vial culture, 2D and 3D	Cardiac cell adaptation; 3D versus 2D	Launched on CRS-29 in November 2023
Emory University	Heart cells	Cell culture	Cell-based therapies for heart disease	Two flight investigations
Oregon State / Texas Tech	Cardiac organoids	Bioprinted organoids	Heart muscle atrophy	Flown on CRS-31, November 2024
LambdaVision	Retinal implant (protein film)	Layer-by-layer protein deposition	Treat retinal degeneration	Nine missions; pre-clinical
BFF meniscus prints	Cartilage	3D bioprinting	Test structural tissue printing	Test prints announced April 2020
NYSCF neural organoids	Cortical and dopaminergic organoids	iPSC organoid culture, 30 days	Study neurodevelopment	Published 2024; single flight

Sources: NASA, ISS National Laboratory, Redwire, Mount Sinai, LambdaVision, and Stem Cells Translational Medicine, as cited throughout this article.

Why this matters beyond the space station

None of this research exists purely for astronauts' benefit, even though astronaut health is a real and immediate driver. The practical payoff on Earth comes through three channels.

Better disease models. Pharmaceutical researchers rely on lab-grown tissue to predict how a drug will behave in a human body before clinical trials. A model with more realistic three-dimensional structure and gene expression could make those predictions more reliable. NASA frames the Engineered Heart Tissue work in exactly this way: the findings could inform both crew countermeasures and drug development for heart disease on Earth.

Aging as a model. Mount Sinai researcher David Sachs described the aim as to "use microgravity as a model of human aging." The idea is that fast, reversible, aging-like changes in cells offer a convenient way to test interventions that would take decades to show up in people.

A path toward transplant tissue. This is the long-term prize and the most distant one. The ISS National Lab lists the BioFabrication Facility's eventual goal as the manufacture of whole human organs (ISS National Lab). The 2025 review reports that the roughly 170,000 solid organ transplants performed worldwide in 2023 covered only about 10 percent of global need, which is the pressure behind the whole field, but nobody in it has published a credible near-term timeline for a full organ. Techshot's own January 2020 release said printing organs for transplant was likely about a decade away. That forecast is a company statement, and nothing in the 2024 results shows the field is on that schedule.

The vascularization problem

The hardest obstacle is not gravity. It is plumbing. Cells in a thick tissue need oxygen and nutrients, and diffusion alone only works over a short distance. A 2024 review in Frontiers in Cardiovascular Medicine puts the oxygen and nutrient diffusion limit at approximately 100 to 200 micrometers, beyond which tissue viability drops sharply (Nwokoye and Abilez, 2024). That is a fraction of a millimeter.

A heart is many centimeters thick. A patch of printed muscle that thin can survive on diffusion, but a full organ needs a branching network of blood vessels that connects to the body's own circulation. Redwire's stated plan to bioprint human blood vessels on its next mission is therefore a meaningful step, and also an illustration of how far the field has left to go. Microgravity may help the printer hold a delicate tube open, but it does not teach those vessels to carry blood, avoid clotting, or fuse to a recipient's arteries.

For a sense of how cells decide what to become once they are in place, see How Stem Cells Know What to Become and How Stem Cells Work and What They Can Actually Treat.

What people get wrong about space-grown organs

The gap between "growing tissue in space" and "printing transplantable organs" gets collapsed constantly in casual coverage, so it helps to be precise about where the science stands.

Myth: full organs are close to ready. What has flown and returned are tissue patches and miniature structures: a piece of cardiac tissue, a layer of retinal protein film, test prints of meniscus, and small organoids. None are functioning, vascularized, whole organs, and the organizations involved do not claim otherwise in their technical communications.

Myth: this is only about astronaut health. Astronaut cardiovascular and muscular decline is a real research driver, since NASA needs countermeasures before crews attempt multi-year missions. But Emory's therapy-oriented work, LambdaVision's retina program, and Redwire's patch goals are all built around Earth-bound medical applications as well.

Myth: microgravity does all the work by itself. Removing gravity addresses sedimentation and slumping. Cells still need the right growth factors, gas exchange, temperature control, and time to differentiate and mature. Microgravity removes one obstacle; it does not replace the rest of tissue engineering. In addition, spaceflight adds stressors of its own. The 2025 review notes that radiation on the station is far higher than on the ground, and that launch subjects samples to high loads.

Myth: tissue always grows better in space. The record is genuinely mixed. Neural organoids matured faster in the NYSCF study, but cardiac cells have shown both improved maturation markers and reduced contractility depending on the study. Tumor cells often form spheroids more readily, with effects that vary widely by cell line (Wehland et al., 2025).

Myth: a successful print means a successful therapy. Printing is the first step. A tissue must then survive, mature, connect to a blood supply, escape immune rejection, and be made safely at scale. Ground-based regenerative medicine is still working through those steps for even the simplest tissues.

Open questions the field still has to answer

Several unresolved issues limit how much anyone should read into the early results.

• Controls and sample size. Most flights carry a small number of samples from one to a few donors, flown once. Without repeated flights and matched ground controls, differences can reflect launch stress, temperature drift, or batch effects as easily as gravity.
• Real versus simulated. Because ground simulators add shear and fluid motion, results from clinostats and random positioning machines do not always match orbit. Tumor data in particular come mostly from simulators, and the 2025 review calls for more real-microgravity work.
• Radiation. The station sits within Earth's magnetic field but still exposes tissue to far more radiation than the ground does. Isolating microgravity from radiation effects is difficult when both are always present. The NYSCF authors treated radiation as unlikely to explain their results, but could not test it directly.
• Scale and cost. Launch capacity, crew time, and the station's planned retirement all limit throughput. This is part of why LambdaVision has signed an agreement about commercial stations, and why Redwire's aim of reliable, large-scale commercial biotech services in orbit is still an aim.
• Independent validation. Several performance claims, such as those about retinal film quality, come from the companies doing the work and have not yet been confirmed in independent clinical testing.

For related station research on how the body itself responds to orbit, see Immune System Changes in Space, and for how crews live in the environment where this work happens, see The ISS Explained. The same weightlessness that helps film layers settle uniformly is also behind Protein Crystal Growth in Microgravity.

Frequently asked questions

Q: Has a full organ ever been grown in space? A: No. What has been produced and returned to Earth are tissue patches and structures: cardiac tissue samples, protein films for retinal implants, cartilage test prints, and neural organoids. A complete, vascularized, transplant-ready organ has not been demonstrated by any program.

Q: Why does microgravity help tissue grow more realistically? A: On Earth, gravity makes denser cells and materials sink and lighter ones float, which creates uneven nutrient distribution and forces soft bioprinted structures to rely on scaffolds. In microgravity, cells and bioinks stay suspended, so shapes can hold without scaffolding, according to Redwire (Redwire, 2024). Results still vary by cell type.

Q: What was the first tissue bioprinted in space? A: Test prints made on the station with bioink containing human heart muscle, nerve, and vascular cells came home in January 2020. Redwire's May 2024 release separately describes the first living human cardiac tissue bioprinted in orbit, returned in April 2024 (Techshot, 2020).

Q: Do space-made retinas actually work better than Earth-made ones? A: According to LambdaVision, films made across nine ISS missions show better homogeneity, stability, and performance than Earth-made ones. These are company-reported pre-clinical claims, not independent clinical results, and no human trial results have been reported (LambdaVision, 2026).

Q: Why do astronauts' hearts matter to this research? A: NASA describes the effect of microgravity on heart function as resembling normal aging. In its Engineered Heart Tissue study, space-exposed tissue showed more arrhythmias and less muscle strength, which could inform countermeasures for long missions and drug development on Earth (NASA, October 2024).

Q: Does tissue actually mature faster in microgravity? A: Sometimes. Neural organoids on the ISS showed lower proliferation markers and higher maturity markers than ground controls in one 30-day study, but the authors note it was a single flight with four donor lines. Cardiac results are mixed (Marotta et al., 2024).

Q: What is a bioink? A: A bioink is a printable mixture of living cells in a supportive gel or biomaterial. It works like ink in a paper printer, except the ink itself is alive and has to stay functional through the printing process. Alginate and hyaluronic acid hydrogels, for example, are often favored for soft tissues such as heart and liver tissue.

Q: Can simulators on Earth reproduce this work? A: Partly. Clinostats, rotating wall vessels, and random positioning machines average out gravity and have produced scaffold-free cartilage-like and bone-like tissue. They do not create real weightlessness and add shear forces, so their results do not always match orbital experiments (Wehland et al., 2025).

Q: Could this move to commercial space stations? A: That is the plan for some programs. LambdaVision announced a memorandum of understanding with Vast in June 2026 covering future work on Haven-1 and later stations. It is a framework for collaboration, not a binding production contract, and Vast's materials say Haven-1 is expected to launch in 2027.

The takeaway

The honest state of organ engineering in space is that nobody has grown a transplantable organ. What multiple teams have shown is narrower and still useful: removing gravity-driven sedimentation changes how soft tissue and thin films form, and some tissues, such as neural organoids, seem to mature differently in orbit. Redwire's returned cardiac tissue and LambdaVision's retinal films are real engineering milestones, but the field's decisive tests, independent replication and blood vessels that carry blood, have not yet been passed. The realistic near-term payoff is better disease models, drug testing, and a clearer picture of how the heart ages, while whole-organ manufacturing remains a goal rather than a result.

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Read next:

• What Is Gene Therapy — and How Is It Curing Genetic Diseases?
• Stem Cells and Regenerative Medicine Explained
• Cardiovascular Changes in Astronauts: ISS Long-Term Health Studies
• The International Space Station: How It Works and Why It Matters

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Sources:

• Redwire, "Redwire Pioneering Biopharma Production in Space by Successfully Bioprinting Live Human Heart Tissue" (May 8, 2024)
• Techshot, "Success: 3D Bioprinter in Space Prints With Human Heart Cells" (January 7, 2020)
• Payload, "Redwire returns first bioprinted cardiac tissue from ISS" (May 2024)
• ISS National Laboratory, BioFabrication Facility
• ISS National Laboratory, Techshot's BioFabrication Facility Bioprints Meniscus Tissue
• NASA Spinoff, Health and Medicine: rotating wall bioreactor
• NASA, Station Science Top News, October 4, 2024
• Mount Sinai, "Mount Sinai Bioengineers Send Cardiac Muscle Samples Into Space" (November 14, 2023)
• ISS National Laboratory, SpaceX CRS-31 Research
• ISS National Laboratory, Emory University Cardiac Cells Research
• ISS National Laboratory, Stem Cell Reports Publishes Stanford Cardiac Research Results
• Marotta et al., "Effects of microgravity on human iPSC-derived neural organoids on the ISS," Stem Cells Translational Medicine (2024)
• Wehland et al., "New Knowledge About Tissue Engineering Under Microgravity Conditions in Space and on Earth," International Journal of Molecular Sciences (2025)
• Nwokoye and Abilez, "Blood vessels in a dish," Frontiers in Cardiovascular Medicine (2024)
• LambdaVision, agreement with Vast (June 24, 2026)