Electric currents are mostly harmful to human bodies, and you'd be well advised not to tamper with a power outlet in the wall. However, if applied in the correct way they can be put to good use in surgery where they serve to cut away ulcers and the like.
A word of warning: much of the following deals with rather unpleasant things. Electric currents bear some similarity to poisons. They can be lethal, but at the same time they provide the basis for some very powerful medication if applied in appropriately minuscule doses. A comprehensive description needs to deal with both the deadly consequences and the applications as remedies for diseases - diseases that can be terminal if left untreated.
Currents and Living Tissues
Electric currents transport energy, and when pumped through a resistor this energy is converted into heat or light. Bodily tissue behaves like a resistor and thus gets heated. Anyone touching a 380kV power line while their feet are connected to the ground gets baked immediately.
In more detail, the mechanism is as follows: human cells mainly consist of water. Pure water is an insulator but the presence of minerals, salts, fat and proteins renders the fluids inside the cell and the cell membrane conductive. The application of an electric current leads to an increase in temperature, until at last the proteins coagulate (congeal) and the water starts boiling, producing steam which ruptures the cell.
There are other effects when exposing people to currents - the activation of muscles, transmission of signals through nerves, processing in the brain and the timing of the heartbeat are based on electric currents too. Any application of electric currents in surgery must be carefully managed to ensure they do not interfere with the currents that are essential for maintaining life itself. Experiments (and accidents) have shown that human nerves operate with signals at frequencies between zero and around 100Hz. If you touch a wire in the 50Hz1 wall plug then the muscles react by contracting and expanding at this rate. If the power supply were operating at a significantly higher frequency then, apart from a severe burn, nothing spectacular would happen.
A surgeon's electric knife, or resectoscope, consists of the following parts:
A box that holds the electronics
One or two foot pedals used to control the process
A metal tube with a handgrip. The tube is inserted into the body through a natural or artificial orifice and houses the cutting wire, a pair of optical fibres and the hoses that transport a swilling fluid. The wire ends in a loop of some 0.5cm diameter and can be moved back and forth by means of the 'trigger' part of the grip. The optical fibres serve to illuminate the scene and convey the image back to either a remote scope or a lens attached to the end of the tube.
A rubber mat with interwoven wires (called the 'neutral electrode') which is attached to the patient and closes the electric circuit.
What's the Trick?
There are several tricks involved to perform electro-cutting while avoiding electrocution:
The device operates with alternating currents (AC) at a frequency of 400kHz2. This is far higher than the frequencies that human nerves can respond to, and heat generation is the only effect that takes place.
The geometry is such that the electric current exits from the wire loop, enters the tissue and spreads out through the body until it is collected in a large surface that is represented by the grounding mat: | wire loop ________O________ / | \ } / .|. \ } / / | \ \ } tissue / / | \ \ } / / | \ \ } xxxxxxxxxxxxxxxxx neutral electrode (grounded) The local electric current density is only high enough to heat the cell water up to boiling point where the wire is actually in contact with the tissue. Thus, by pulling the loop through the tissue, small worm-like chips can be cut out. Deeper inside the tissue, the current is distributed over a much larger area so the heat generation per cell is much less.
Furthermore, the current 'concentrates itself'. As an example, let's assume there was a lump of fat cells in the way of the wire loop. Once all other cells have been vaporised, it's only these cells that make physical contact with the loop and are left for the current to pass through. The current is therefore concentrated on these cells only - a short bzzzzzz and they are gone.
This Researcher has experimented with the 'Dinosaur' of all High Frequency Surgery devices (originating from the 1930s) on slices of meat. The device didn't have a control loop and the current was set manually. Pulling the wire through a raw schnitzel was accompanied by a sizzling electric arc (prolonged spark) which was dancing around wherever the wire contacted the meat, plus a black cloud of smoke that, of course, smelt of burned flesh3. The only way to cut through concentrations of fat was to increase the power - which in turn led to even more smoke. Since creating smoke bubbles inside a patient is nothing desirable, and burns (aka necrosis) don't heal well, something had to be done to control the current and prevent an arc.
Now, enter the feedback loop. An arc is a non-linear phenomenon ie, there is no strict proportionality between the voltage and the current. This has the consequence that when the voltage is represented by a pure sine wave, the current exhibits a distorted sine wave. A distorted sine wave is equivalent to the presence of overtones - harmonics - and thus an arc can be prevented by setting up a filter and reducing the power when a signal at 800kHz (twice the frequency of the device) is detected. With the feedback loop properly set up, the wire can cut smoothly and smokeless through any mixture of fat, muscles, smaller blood vessels and mucous tissue.
As everybody can prove by cooking scrambled eggs, proteins coagulate when exposed to medium heat. A surgeon's electric knife takes advantage of this effect and leaves behind a surface that doesn't bleed. Even carcinomas in the liver (an organ which has many blood vessels) can be removed, while the surgeon is still able to see to work.
One of the major applications of high frequency surgery is transurethral resection (TUR). The prostate gland is a male organ that is located between the bladder and the penis and surrounds the urethra. Prostate glands tend to grow with age and eventually obstruct the flow of urine. The gland can be removed without cutting open the patient, by inserting the instrument through the urethra. The same technique is used for removing cancers of the bladder (cystectomy).
Resection of polyps in the large intestine (polypectomy) is also feasible without cutting through the patient's skin.
A wholly different application is the coagulation of vessels and tubes. Here the electric circuit is not set up between the instrument and a neutral electrode. Rather, the instrument is shaped like a pair of tweezers and the current is made to flow through anything that is 'grabbed' between their tips, for example a blood vessel or the oviducts (fallopian tubes) of a female when a tubal sterilisation is performed.