Coils are used across medicine for surprisingly different purposes, from sealing off dangerous bulges in blood vessels to preventing pregnancy to generating the images in an MRI scanner. The word “coil” refers to any tightly wound spiral structure, and in healthcare, that simple shape solves a wide range of problems. Here’s how coils work in their most common medical applications.
Treating Brain Aneurysms
One of the most important uses for medical coils is treating brain aneurysms, which are weak, balloon-like bulges in blood vessel walls that can rupture and cause life-threatening bleeding. Small helical platinum coils are threaded through a catheter, typically inserted at the groin, and navigated up to the brain under X-ray guidance. Once the catheter reaches the aneurysm, multiple coils are packed tightly inside it.
The first coil placed is usually a three-dimensional coil that forms a basket shape along the inner wall of the aneurysm. Additional coils are then deposited inside that basket until the aneurysm is densely packed. The coils serve two functions: they physically fill the space so blood can no longer flow into the bulge, and they trigger the body’s clotting response. A blood clot forms around the coils, effectively sealing the aneurysm off from normal circulation and dramatically reducing the risk of rupture.
This procedure, called coil embolization, was revolutionized in the early 1990s with the introduction of detachable platinum coils. Since then, the technology has continued to evolve. Some newer coils are coated with a bioabsorbable polymer that encourages the growth of connective tissue inside the aneurysm, leading to thicker tissue at the aneurysm’s neck and better long-term sealing. Another advancement pairs a synthetic hydrogel with a platinum coil. The hydrogel expands after placement, helping to fill more of the aneurysm space. Researchers have also explored adding collagen coatings, growth factors, and other biological materials to coils, though most of these remain in preclinical testing.
Blocking Abnormal Blood Vessels
The same basic principle used for brain aneurysms applies throughout the body. Coils are the most frequently used material in embolization procedures, which deliberately block blood flow in vessels that are damaged, malformed, or feeding a problem. In pelvic arteriovenous malformations (abnormal tangles of arteries and veins), coils were the preferred embolic material in more than 87% of treated patients in a systematic review of published cases. Some patients received coils alone, while others had coils combined with liquid agents or other blocking materials. The number of coils used per patient ranged from 7 to over 260, depending on the size and complexity of the malformation.
Coils are also used to treat varicoceles (enlarged veins in the scrotum) and to manage bleeding from trauma or surgical complications. In each case, the coils are delivered through a catheter and packed into the target vessel, where they trigger clotting and permanently close off blood flow.
Closing Heart Defects in Children
A patent ductus arteriosus (PDA) is a small blood vessel that normally closes shortly after birth but sometimes stays open, forcing the heart to work harder. Coil occlusion for PDA closure was introduced in 1992 and has become a standard treatment. Coils are now the most commonly used devices for closing small PDAs, while larger ones typically require a different type of plug-shaped device.
The procedure is done through a catheter rather than open-heart surgery. In a study of over 400 patients, about 74% had complete closure immediately after the procedure, and by six months, nearly all patients achieved full occlusion. Both short-term and one-year outcomes are consistently rated as excellent. The most common complication is the coil shifting out of position, which was more frequent in the early days of the technique but has become less common with improved designs and operator experience.
Contraception (The Copper Coil)
In reproductive health, “the coil” usually refers to a copper intrauterine device (IUD), a small T-shaped device wrapped in copper wire that sits inside the uterus. The copper T380A (sold as ParaGard in the U.S.) was first approved in 1984 and, as of 2024, is cleared for up to 10 years of continuous use. Hormonal IUDs, which use a different mechanism, are approved for three to eight years depending on the brand.
Copper IUDs work primarily through two mechanisms. First, the device itself triggers a local inflammatory response in the uterine lining. This creates an environment that is hostile to fertilization and implantation. Second, copper ions released from the wire dissolve into the fluids of the reproductive tract at concentrations that are directly toxic to sperm, impairing their ability to move and survive. Together, these effects make copper IUDs one of the most effective forms of reversible contraception available, with no hormones involved.
MRI Scanners
Magnetic resonance imaging relies on multiple types of coils working together inside the machine. Gradient coils produce magnetic fields that vary in strength across the scanning area. By changing how strong the magnetic field is at different positions, these coils cause hydrogen atoms in your body to resonate at slightly different frequencies depending on their location. This is what allows the scanner to map where each signal is coming from and build a detailed image, a process called spatial encoding. Gradient coils typically operate in the 0 to 3 kHz frequency range and are driven by powerful electrical currents running through precisely shaped winding patterns.
Radiofrequency (RF) coils serve a separate role. They transmit pulses of energy that tip hydrogen atoms out of alignment with the main magnetic field, then listen for the signals those atoms emit as they snap back into place. Some RF coils are built into the scanner, while others are placed directly on or around the body part being imaged (the padded “cage” placed over your head during a brain scan, for example, is an RF coil). The interplay between the main magnet, gradient coils, and RF coils is what produces the cross-sectional images that make MRI so valuable for diagnosing soft tissue injuries, tumors, and neurological conditions.
Cochlear Implants
Cochlear implants, which restore a sense of hearing to people with severe hearing loss, depend on a pair of induction coils to transmit information across the skin without any wires penetrating the body. The external processor, worn behind the ear, converts sound into a coded electrical signal. An external coil held against the scalp by a magnet transmits that signal using radiofrequency energy to a matching receiver coil implanted just beneath the skin. The internal coil picks up the signal and sends it to electrodes placed inside the inner ear, which directly stimulate the auditory nerve.
This wireless link between the two coils is what makes modern cochlear implants practical for everyday life. The concept dates back to 1957, when researchers first implanted an electrode coupled with a receiver coil and successfully stimulated it with an external coil for several months. Today’s devices use the same fundamental principle of electromagnetic induction, refined over decades into systems small and reliable enough to be worn continuously.