In medicine, a catheter is a thin tube made from medical grade materials serving a broad range of functions. Catheters are medical devices that can be inserted in the body to treat diseases or perform a surgical procedure. By modifying the material or adjusting the way catheters are manufactured, it is possible to tailor catheters for cardiovascular, urological, gastrointestinal, neurovascular, and ophthalmic applications. The process of inserting a catheter is "catheterization".
In most uses, a catheter is a thin, flexible tube ("soft" catheter) though catheters are available in varying levels of stiffness depending on the application. A catheter left inside the body, either temporarily or permanently, may be referred to as an "indwelling catheter" (for example, a peripherally inserted central catheter). A permanently inserted catheter may be referred to as a "permcath" (originally a trademark).
Catheters can be inserted into a body cavity, duct, or vessel, brain, skin or adipose tissue. Functionally, they allow drainage, administration of fluids or gases, access by surgical instruments, and also perform a wide variety of other tasks depending on the type of catheter. Special types of catheters, also called probes, are used in preclinical or clinical research for sampling of lipophilic and hydrophilic compounds, protein-bound and unbound drugs, neurotransmitters, peptides and proteins, antibodies, nanoparticles and nanocarriers, enzymes and vesicles.
"Catheter" (from Greek ? kathet?r) comes from the Greek verb kathíemai, meaning "to thrust into" or "to send down" because the catheter allowed fluid to be "sent down" from the body. 
Single-use urinary catheter, 40 cm
Placement of a catheter into a particular part of the body may allow:
The earliest invention of the flexible catheter was during the 18th century. Extending his inventiveness to his family's medical problems, Benjamin Franklin invented the flexible catheter in 1752 when his brother John suffered from bladder stones. Franklin's catheter was made of metal with segments hinged together with a wire enclosed to provide rigidity during insertion.
According to a footnote in his letter in Volume 4 of the Papers of Benjamin Franklin (1959), Franklin credits Francesco Roncelli-Pardino from 1720 as the inventor of a flexible catheter. In fact, Franklin claims the flexible catheter may have been designed even earlier.
An early modern application of the catheter was employed by Claude Bernard for the purpose of cardiac catheterization in 1844. The procedure involved entering a horse's ventricles via the jugular vein and carotid artery. This appears to be an earlier and modern application of the catheter because this catheter approach technique is still performed by neurosurgeons, cardiologists, and cardiothoracic surgeons.
David S. Sheridan invented the modern disposable catheter in the 1940s. Prior to this, some reusable catheters consisted of braided cotton tubes, which were varnished, heat-treated and polished. As these were primarily produced in France, the advent of World War II threatened the supply chain. Other reusable catheters consisted of red rubber tubes. Although sterilized prior to reuse, they still posed a high risk of infection and often led to the spread of disease.:142 Sheridan was dubbed the "Catheter King" by Forbes magazine in 1988. He also invented the modern "disposable" plastic endotracheal tube now used routinely in surgery.
A range of polymers are used for the construction of catheters, including silicone rubber, nylon, polyurethane, polyethylene terephthalate (PET), latex, and thermoplastic elastomers. Silicone is one of the most common implantable choice because it is inert and unreactive to body fluids and a range of medical fluids with which it might come into contact. On the other hand, the polymer is weak mechanically, and a number of serious fractures have occurred in catheters. For example, silicone is used in Foley catheters where fractures have been reported, often requiring surgery to remove the tip left in the bladder.
Polyimides are used to manufacture vascular catheters for insertion into small vessels in the neck, head and brain.
There are many different types of catheters for bladder problems. A typical modern intermittent catheter is made from polyurethane and comes in different lengths and sizes for men, women and children. The most advanced catheters have a thin hydrophilic surface coating. When immersed in water this coating swells to a smooth, slippery film making the catheter safer and more comfortable to insert. Some catheters are packed in a sterile saline solution.
Various settings of a 6 French pigtail catheter with locking string, obturator (also called stiffening cannula) and puncture needle. A. Overview B. Both puncture needle and obturator engaged, allowing for direct insertion. C. Puncture needle retracted. Obturator engaged. Used for example in steady advancement of the catheter on a guidewire. D. Both obturator and puncture needle retracted, when the catheter is in place. E. Locking string is pulled (bottom center) and then wrapped and attached to the superficial end of the catheter.
Hemodialysis uses a specialized "tunneled" catheter, placed under the skin. It features two pathways, one to draw blood from an artery, and into the dialysis device, and a second pathway, to return cleansed blood into the body, through a vein. 
"Any foreign object in the body carries an infection risk, and a catheter can serve as a superhighway for bacteria to enter the bloodstream or body", according to Milisa Manojlovich, a professor at the University of Michigan School of Nursing. 
Catheters can be difficult to clean, and therefore harbor antibiotic resistant or otherwise pathogenic bacteria.