A sheath material is any substance that forms a protective outer layer around a structure, whether that structure is a nerve fiber, a tendon, an electrical cable, or a medical catheter. The term spans biology, medicine, and engineering, and the specific material varies dramatically depending on the application. Here’s what sheath materials look like across the most common contexts.
Myelin: The Sheath Around Your Nerves
In the human body, the most well-known sheath is the myelin sheath, a fatty insulating layer that wraps around nerve fibers and speeds up electrical signals traveling through your nervous system. Unlike most cell membranes, myelin is unusually rich in fat: 70% to 85% lipids, with only 15% to 30% protein. That high fat content is what makes it such an effective insulator.
The lipid makeup of myelin breaks down into three major classes: cholesterol (the single largest component, making up 41% to 46% of myelin lipids), phospholipids (26% to 29%), and glycolipids (11% to 20%). The exact proportions shift depending on location. Myelin in the brain contains more cholesterol and glycolipids, while myelin around peripheral nerves in the arms and legs has more of a phospholipid called lecithin and a higher proportion of sphingomyelin. Diseases that damage this fatty sheath, like multiple sclerosis, disrupt signal transmission and cause neurological symptoms ranging from numbness to difficulty walking.
Tendon Sheaths: Synovial Tissue
Tendons in areas that require smooth, repetitive motion (especially the hands and feet) are enclosed in synovial sheaths. These aren’t made of a single material in the engineering sense. Instead, they consist of two thin layers of serous tissue: a parietal sheet that lines the outer tunnel wall and a visceral sheet that directly covers the tendon’s surface. The narrow space between these two layers is filled with peritendinous fluid, which acts as a lubricant so the tendon can glide back and forth without friction.
When these sheaths become inflamed, the condition is called tenosynovitis. It commonly affects the wrist and fingers, causing pain, swelling, and difficulty moving the joint. The sheath material itself is living tissue that can heal, but chronic irritation or infection can permanently thicken it and restrict tendon movement.
Electrical Cable Sheathing
In electrical engineering, a cable’s sheath (also called its jacket) is the outermost protective layer that shields internal conductors from physical damage, moisture, chemicals, and fire. Three materials dominate the industry, each with distinct tradeoffs.
- PVC (polyvinyl chloride) is the most common and affordable option. It provides solid abrasion and chemical resistance but has a significant drawback: when it burns, it releases chlorine gas, which is toxic to breathe and can form hydrochloric acid on contact with water, damaging both people and nearby equipment.
- CPE (chlorinated polyethylene) offers better chemical, fire, and abrasion resistance than PVC. It also handles cold temperatures better, resisting the brittleness that can crack PVC jackets in freezing conditions.
- LSZH (low smoke zero halogen) is designed for enclosed spaces like tunnels, aircraft, and data centers where toxic smoke would be especially dangerous. LSZH sheaths resist burning for extended periods and release minimal smoke and no chlorine when they do burn, making them the safest option in fire scenarios but typically more expensive.
The choice between these materials depends almost entirely on the installation environment. An outdoor industrial cable might use CPE for its toughness, while a cable running through a hospital ceiling would likely require LSZH to meet fire safety codes.
Medical Catheter and Introducer Sheaths
In medicine, sheaths are thin, flexible tubes inserted into blood vessels to create a temporary pathway for catheters, guidewires, and other devices. The material used for these sheaths matters enormously because it contacts blood directly and must be both flexible enough to navigate vessels and stiff enough to hold its shape.
Common Polymers
Medical sheaths are typically made from specialized polymers. Polyurethane is widely used for its combination of flexibility and strength. PEBAX, a polyether block amide, is a high-performance alternative available in a range of hardness levels (Shore D 35 to 72), which lets manufacturers tune stiffness for different procedures. Softer grades work well for sheaths that need to navigate curved vessels, while harder grades provide the rigidity needed for pushability in straight-line access.
PTFE (the same material as Teflon) is valued for its extremely low friction and chemical inertness, making it useful as a liner or coating inside sheaths to help devices slide through smoothly.
Hydrophilic Coatings
Many modern sheaths are coated with a hydrophilic (water-attracting) layer that becomes slippery when wet. This coating reduces friction dramatically. In vitro testing has shown hydrophilic-coated sheaths reduce friction resistance by about 70% compared to uncoated versions, and clinicians report significantly easier removal after procedures. This reduction in friction also means less irritation to the blood vessel wall during insertion and removal.
Sizing and the French Scale
Medical sheaths are measured using the French (Fr) scale, where each French unit equals one-third of a millimeter in outer diameter. A 6 Fr sheath has a 2 mm diameter, a 9 Fr sheath is 3 mm, and so on. The formula is simply the French number divided by 3. Common sizes for cardiac procedures range from 5 Fr to 8 Fr, while larger interventional procedures may require sheaths up to 24 Fr (8 mm).
Biocompatibility Requirements
Because medical sheaths contact blood, they must pass rigorous biocompatibility testing. The FDA categorizes testing requirements by how long a device stays in the body. A sheath used for less than 24 hours needs testing for cell toxicity, allergic sensitization, and tissue irritation. Sheaths that remain in place for 24 hours to 30 days require additional testing for systemic toxicity and fever-causing reactions. Devices that stay longer than 30 days face the most extensive evaluation, including testing for cancer-causing potential, genetic damage, and effects on reproduction. Materials that dissolve in the body must be tested at multiple stages of breakdown to ensure the intermediate byproducts are also safe.
How Context Determines the Material
What all these sheaths share is a common purpose: protecting something inside from the environment outside (or vice versa) while allowing the enclosed structure to function. The material choice always reflects the specific demands of the situation. Nerve sheaths need high fat content to insulate electrical signals. Tendon sheaths need fluid-filled layers for frictionless gliding. Electrical cable sheaths need chemical and flame resistance. Medical sheaths need body-safe polymers that balance flexibility with strength.
If you’re researching sheath materials for a specific project, the key variables to consider are the environment (heat, chemicals, moisture, biological tissue), the mechanical demands (flexibility versus rigidity, friction tolerance), and any regulatory standards that apply. For electrical applications, local fire codes often dictate the jacket material. For medical devices, biocompatibility testing requirements set a hard floor on which polymers are acceptable.