Iron lung
An iron lung is a large machine that enables a person to breathe when normal muscle control has been lost or the work of breathing exceeds the person's ability. It is a form of medical ventilator. Properly, it is called a negative pressure ventilator.
The person using the iron lung is placed into the central chamber, a cylindrical steel drum. A door allowing the head and neck to remain free is then closed, forming a sealed, air-tight compartment enclosing the rest of the person's body. Pumps that control airflow periodically decrease and increase the air pressure within the chamber, and particularly, on the chest. When the pressure falls below that within the lungs, the lungs expand and air from outside the chamber is sucked in via the person's nose and airways to keep the lungs filled; when the pressure rises above that within the lungs, the reverse occurs, and air is expelled. In this manner, the iron lung mimics the physiologic action of breathing: by periodically altering intrathoracic pressure, it  causes air to flow in and out of the lungs.  The iron lung is a form of non-invasive therapy.
The machine was invented by Philip Drinker and Louis Agassiz Shaw, of the Harvard School of Public Health, originally for treatment of coal gas poisoning. But it found its most famous use in the mid-1900s when victims of poliomyelitis (more commonly known as polio), stricken with paralysis (including of the diaphragm, the cone shaped muscle at the bottom of the rib-cage whose action controls intrathoracic pressure), became unable to breathe, and were placed in these steel chambers to survive. The first iron lung was used on October 12, 1928 at Children's Hospital, Boston, in a child unconscious from respiratory failure; her dramatic recovery, within seconds of being placed within the chamber, did much to popularize the "Drinker Respirator."
In 1931, inveterate tinkerer John Haven "Jack" Emerson unveiled an improved iron lung, which was smaller, cheaper, lighter, quieter, and much more reliable than Drinker's. Drinker and Harvard promptly sued Emerson for patent violations, which proved unwise.  In the subsequent legal battles Emerson demonstrated that every aspect of Drinker's patents had been patented by others at earlier times.  Emerson won the case, and Drinker's patents were declared invalid.
Entire hospital wards were filled with rows of Emerson iron lungs at the height of the polio outbreaks of the 1940s and 50s. With the success of the worldwide polio vaccination programs which have virtually eradicated the disease, and the advent of modern ventilators that control breathing via the direct intubation of the airway, the use of the iron lung has sharply declined.
The positive pressure ventilator, which instead blows air into the patient's lungs by intubation through the airway, was used for the first time in Blegdams Hospital, Copenhagen, Denmark during a polio outbreak in 1952. It proved a success and soon superseded the iron lung all over Europe.
The iron lung now has a marginal place in modern respiratory therapy. Most patients with paralysis of the breathing muscles use modern mechanical ventilators that push air into the airway with positive pressure.  These are generally efficacious and have the advantage of not restricting patients' movements or caregivers' ability to examine the patients as significantly as an iron lung does. However, negative pressure ventilation is a truer approximation of normal physiological breathing and results in more normal distribution of air in the lungs.  It may also be preferable in certain rare conditions, such as Ondine's curse, in which failure of the medullary respiratory centers at the base of the brain result in patients having no autonomic control of breathing.  Thus, there are patients who still today use the older machines, often in their homes, despite the occasional difficulty in finding replacement parts.
Biphasic Cuirass Ventilation is a modern development of the iron lung, consisting of a wearable rigid upper-body shell (a cuirass) which functions as a negative pressure respirator.