Isolated brain

An isolated brain is a brain kept alive in vitro, either by perfusion or by a blood substitute, often an oxygenated solution of various salts, or by submerging the brain in oxygenated artificial cerebrospinal fluid (CSF). It is the biological counterpart of brain in a vat. A related concept, attaching the brain or head to the circulatory system of another organism, is called a head transplant. An isolated brain, however, is more typically attached to an artificial perfusion device rather than a biological body.
The brains of many different organisms have been kept alive in vitro for hours, or in some cases days. The central nervous system of invertebrate animals is often easily maintained as they need less oxygen and to a larger extent get their oxygen from CSF; for this reason their brains are more easily maintained without perfusion. Mammalian brains, on the other hand, have a much lesser degree of survival without perfusion and an artificial blood perfusate is usually used.
For methodological reasons, most research on isolated mammalian brains has been done with guinea pigs. These animals have a significantly larger basilar artery (a major artery of the brain) compared to rats and mice, which makes cannulation (to supply CSF) much easier.
In philosophy.
In philosophy, the brain in a vat is any of a variety of thought experiments intended to draw out certain features of our ideas about knowledge, reality, truth, mind, and meaning. A contemporary version of the argument originally given by Descartes in "Meditations on First Philosophy" (i.e., that he could not trust his perceptions on the grounds that an evil demon might, conceivably, be controlling his every experience), the "brain in a vat" is the idea that a brain can be fooled into anything when fed appropriate stimuli.
The inherently philosophical idea has also become a staple of many science fiction stories, with many such stories involving a mad scientist who might remove a person's brain from the body, suspend it in a vat of life-sustaining liquid, and connect its neurons by wires to a supercomputer which would provide it with electrical impulses identical to those the brain normally receives. According to such science fiction stories, the computer would then be simulating a virtual reality (including appropriate responses to the brain's own output) and the person with the "disembodied" brain would continue to have perfectly normal conscious experiences without these being related to objects or events in the real world.
No such procedure in humans has ever been reported by a research paper in a scholarly journal, or other reliable source. Also, the ability to send external electric signals to the brain of a sort that the brain can interpret, and the ability to communicate thoughts or perceptions to any external entity by wire, is, except for very basic commands, well beyond current (2021) technology.
Grown.
In 2004 Thomas DeMarse and Karl Dockendorf made an "adaptive flight control with living neuronal networks on microelectrode arrays".
Teams at the Georgia Institute of Technology and the University of Reading have created neurological entities integrated with a robot body. The brain receives input from sensors on the robot body and the resultant output from the brain provides the robot's only motor signals.
In fiction.
The concept of a brain in a jar (or brain in a vat) is a common theme in science fiction.