We're changing the shape to make it work better.
Each year, a small but significant number of Americans who undergo weight-loss surgery face a complication their surgeons must treat with tools never designed for the task. Researchers at New York University have responded to this quiet suffering with a question that is both simple and profound: what if the shape of a medical device, not its material, is what determines how well it heals? Their answer—a mathematically optimized stent called Lily—challenges a long-held assumption about drainage and offers a glimpse of a future where medical instruments are engineered from first principles rather than inherited convention.
- Between 1 and 3 percent of sleeve gastrectomy patients develop gastric leaks—pooling fluid in irregular cavities that standard bile-duct stents were never built to handle.
- Patients caught in this gap endure repeated procedures stretched across weeks or months, their recoveries held hostage by a tool mismatched to their condition.
- NYU researchers upended the intuitive fix—a wider tube—by proving mathematically that enlarging the interior actually slows drainage by collapsing the exterior gap where fluid truly moves.
- Their PETALS framework translated this insight into Lily, a six-lobed stent whose exterior geometry accelerates drainage without requiring new materials or specialized manufacturing.
- Early results show flexibility, biocompatibility, and conventional manufacturability—but animal studies and clinical trials still stand between the lab bench and the operating room.
A quarter-million Americans undergo sleeve gastrectomy each year, and for most the surgery delivers a straightforward recovery. But between one and three percent develop a gastric leak—a rupture in the stomach wall that lets fluid pool in surrounding tissue. In revision surgeries, that risk rises to one in ten. When a leak occurs, doctors thread a double-pigtail stent through the stomach wall to drain the fluid. The device was designed for bile ducts, not the irregular, viscous cavities that gastric leaks create. Stents slip. Drainage is slow. Patients return for procedure after procedure.
Researchers at New York University spent years asking what would happen if the stent were simply shaped differently. Using computer simulations and mathematical modeling, they found something counterintuitive: a wider interior diameter actually drains more slowly. Enlarging the inside of the tube shrinks the gap between the stent's outer surface and the cavity wall—and that exterior gap is where most fluid movement happens. Interior volume, it turns out, barely matters. Exterior shape is everything.
This discovery became the foundation of PETALS, a mathematical framework for optimizing drain geometry based on the specific properties of the fluid being moved. From it, the team designed Lily—a six-part stent whose cross-section creates more efficient pathways for fluid to flow around and through the device. Senior author Khalil Ramadi put it plainly: the geometry of the cross-section fundamentally determines flow speed. The team is changing the shape, not the material.
Lily also proved more flexible than conventional polyethylene stents—a quality associated with better tissue tolerance—and early animal studies showed no significant inflammatory response. Because the design maintains a constant cross-section, it can be manufactured through standard extrusion, requiring no new hospital infrastructure.
Roughly 2,500 Americans need gastric leak treatment annually after bariatric surgery. If Lily performs as the lab work suggests, faster drainage could mean shorter hospital stays, fewer repeat procedures, and patients returning to their lives sooner. Animal studies and clinical trials still lie ahead, and the researchers are careful not to overstate what has been shown. But the work points toward a future where medical tools are built not by convention, but by understanding the precise physics of the problem they must solve.
A quarter-million Americans choose sleeve gastrectomy each year, seeking to reshape their bodies and their lives. For most, the surgery delivers what it promises—a straightforward recovery, a new beginning. But for a small fraction, something goes wrong. Between one and three percent of routine cases develop a gastric leak, a rupture in the stomach wall that allows fluid to pool and fester in the surrounding tissue. In revision surgeries, the risk climbs to one in ten.
When a leak occurs, doctors reach for a tool that was never quite designed for the job. They thread a thin plastic tube called a double-pigtail stent through the stomach wall, hoping to drain the accumulated fluid. The device works, mostly, but it was engineered for bile ducts—narrow, predictable passages. A gastric leak creates something altogether different: an irregular cavity filled with thick, viscous fluid that resists the geometry of a standard tube. The stents slip. They drain slowly. Patients return for procedure after procedure, their recovery stretched across weeks or months.
Researchers at New York University have spent the last several years asking a deceptively simple question: what if the stent itself were different? Not made of different material, but shaped differently. Working with computer simulations and mathematical models, they discovered something counterintuitive. A wider interior diameter does not drain faster. In fact, it drains slower. When you enlarge the inside of the tube, you shrink the gap between the stent's outer surface and the cavity wall—and that gap is where most of the fluid actually moves. The interior volume barely matters. The exterior shape is everything.
This insight became the foundation of what the team calls PETALS, an acronym for Personalized Endoscopic Transmural Abscess Leak Solution. It is a mathematical framework for optimizing the geometry of a drain based on the specific properties of the fluid it must move. Using this approach, the researchers designed a new stent they named Lily, a six-part structure whose cross-section creates more efficient pathways for fluid to flow around and through the device. The work, published in Advanced Healthcare Materials, represents a shift in how engineers think about drainage: not as a problem of interior volume, but as a problem of exterior topology.
Khalil Ramadi, the study's senior author and an assistant professor at NYU Abu Dhabi and NYU Tandon School of Engineering, frames it plainly: the geometry of the tube's cross-section fundamentally determines how fast fluid moves. "We're not just making it out of a different material," he says. "We're changing the shape to make it work better."
The Lily stent also proved more flexible than the polyethylene devices it would replace—a quality surgeons associate with better tolerance and less tissue damage. Early animal studies showed no significant inflammatory response around the implanted material, a promising sign for biocompatibility. And because the design maintains a constant cross-section, it can be manufactured using conventional extrusion methods. Hospitals would not need to invest in 3-D printing infrastructure.
The stakes are concrete. Roughly 2,500 Americans require treatment for gastric leaks each year after bariatric surgery. If the Lily stent performs as the lab work suggests, faster drainage could shorten recovery and eliminate the need for repeated procedures. That means less time in the hospital, fewer interventions, lower costs, and patients returning to their lives sooner.
But the device remains early in its journey. It has been tested only in simulations and benchtop models so far. Animal studies lie ahead, and then the longer road toward clinical trials and eventual approval. The researchers are careful not to overstate what they have shown. Still, the work points toward a future in which the tools we use to treat complications are engineered not by accident or convention, but by understanding the precise physics of the problem they must solve.
Notable Quotes
We're not just making it out of a different material. We're changing the shape to make it work better.— Khalil Ramadi, assistant professor at NYU Abu Dhabi and NYU Tandon School of Engineering
Our work shifts the focus from just placing a stent to engineering its function at a structural level.— Parima Phowarasoontorn, research assistant and first author of the study
The Hearth Conversation Another angle on the story
Why does the shape of the outside matter more than the size of the hole?
Because the fluid doesn't flow through the middle of the tube the way you'd expect. It flows around the outside, in the narrow space between the stent and the cavity wall. Make the hole bigger and you squeeze that space smaller. You're actually making drainage worse.
That seems backwards.
It does. That's why they had to model it mathematically to prove it. Intuition fails here because we're dealing with thick, viscous fluid under pressure in an irregular space. The standard stents were designed for bile ducts, which are different animals entirely.
How many people are we talking about?
About 2,500 Americans a year need treatment for gastric leaks after weight-loss surgery. Most recover, but it takes time and often requires multiple procedures. If this stent works, it could cut that recovery window significantly.
Is it ready to use?
Not yet. It's been tested in computer models and on benches. They need animal studies next, then human trials. Years away, probably. But the math is solid, and the early signs are good.
What makes this different from just using a better material?
Everything. They kept the material mostly the same but completely reimagined the geometry. The cross-section of the tube is now optimized for the specific properties of gastric fluid. It's engineering from first principles instead of adapting something designed for a different purpose.
And it's flexible?
More flexible than what's used now, which matters because rigid devices can damage tissue. The animal studies showed no inflammation around it, which is what you want to see.