The Watchman device is made of two primary materials: a nitinol (nickel-titanium alloy) metal frame and a fabric membrane made of polyethylene terephthalate, commonly known as PET. These materials work together to seal off a small pouch in the heart called the left atrial appendage, where blood clots can form in people with atrial fibrillation.
The Nitinol Frame
The core structure of the Watchman is a self-expanding cage made from nitinol, a metal alloy of nickel and titanium. Nitinol is widely used in medical implants because of a unique property: shape memory. The metal can be compressed into a narrow tube for delivery through a catheter, then spring back to its original shape once released inside the heart. The current version, the Watchman FLX, uses an 18-strut nitinol frame that radially expands to fit snugly inside the left atrial appendage.
Nitinol is also highly flexible and fatigue-resistant, which matters for a device that sits inside a beating heart for the rest of your life. It maintains its shape through millions of cardiac cycles without cracking or deforming.
The PET Fabric Membrane
Stretched across the face of the nitinol frame is a thin, porous membrane made of polyethylene terephthalate (PET). This is the same type of synthetic fabric used in surgical mesh and vascular grafts for decades. The membrane acts as a cap, blocking blood from pooling in the appendage while still allowing your body’s own tissue to gradually grow over it.
That tissue growth, called endothelialization, is the long-term goal. After implantation, heart tissue slowly covers the PET membrane, essentially incorporating the device into the heart wall. Current practice calls for blood thinners for about 45 days after the procedure, during which the tissue covering is expected to form. Animal studies have shown complete coverage within about three months.
Anchors That Hold It in Place
Small metal anchors, also made of nitinol, secure the device to the walls of the left atrial appendage and prevent it from shifting. The original Watchman 2.5 used barb-like tines that pointed outward. The newer Watchman FLX redesigned these anchors significantly: the tips are folded back (called J-hooks) to reduce the risk of puncturing the heart wall. The FLX also features dual-row anchors for a more secure hold, which helps minimize both device movement and gaps around the edges where blood could leak through.
Differences Between Watchman Generations
Both the original Watchman 2.5 and the newer Watchman FLX use the same core materials: nitinol and PET fabric. The differences are in the engineering. The FLX has more struts, a wider range of available sizes, and a shorter overall profile, which makes it easier to fit into different appendage shapes. Its redesigned frame also exposes less bare metal to the bloodstream, which may lower the risk of blood clots forming on the device surface during the healing period.
Nickel Sensitivity and the Watchman
Because nitinol is roughly half nickel by composition, patients with known nickel allergies should be aware. The FDA lists hypersensitivity to any component of the device as a contraindication, meaning the Watchman should not be used if you have a confirmed nickel allergy. If you’ve had skin reactions to nickel jewelry, that doesn’t automatically rule you out, but it’s something to discuss with your cardiologist before the procedure. Patch testing or blood testing for nickel sensitivity can help clarify the risk.
MRI Safety With the Watchman
The nitinol frame is non-ferromagnetic, meaning it won’t be pulled by a magnet, which makes the Watchman conditionally safe for MRI scans. According to FDA-cleared labeling, patients with a Watchman implant can undergo MRI immediately after the procedure under specific conditions: the scanner must operate at 1.5 or 3 Tesla (the two most common strengths in clinical use), and scan time is limited to 15 minutes per session. These parameters cover the vast majority of routine MRI exams, so having a Watchman implant rarely prevents you from getting scans you need down the road.