One interesting form of prosthesis is the prosthetic cell. At first, this may seem like a silly idea. Who cares about one measly little cell? The idea, however, is not to give someone one cell, but a whole bunch of cells which help produce a missing or needed biologically active molecules.
There are many diseases which result in the lack of production certain, necessary organic molecules.
In healthy individuals, cells maintain normal physiological function by secreting or metabolizing substances, such as sugars, amino acids, neurotransmitters and hormones, which are essential to life. When cells are damaged or destroyed, they no longer produce, metabolize or accurately regulate critical molecular substances required by the body. For example, the progressive decline common to many neuro-degenerative diseases, such as Parkinson's Disease and ALS, is associated with impaired cellular function.
(from Cytotheraputics* home page)
Another example is that of the diabetic. A patient with diabetes must regularly inject himself with insulin because he is not producing/using enough. If we could somehow give that patient a constant source of this product, then he could live a life without regular external administration of medicines.
One might suggest that a way to solve this type of problem would be to take "normal" cells from someone else and transplant them into the sick patient, as is done with bone marrow transplants. We know, however, that you can't just take any person's cells and put them into someone else because the immune system will attack and destroy any foreign cells. One must either get cells from someone with a very close genetic make-up (siblings) or one must take powerful drugs which suppress the immune system (a dangerous process, as all types of infections can occur more easily).
A proposed solution to this problem is the idea of "Encapsulated Cell Therapy" (Galletti, 1995). It basically works as follows: The cells used can be either human or other animal cells. These cells are genetically altered to produce whatever it is that is needed in the body (insulin, other needed proteins, hormones, etc.). The cells are then protected from the immune system in a very clever "capsule" system made of a polymer material. This polymer has specially designed pores that are large enough to allow the desired excretion of therapeutic substances, but small enough to keep out the body's immune system. In effect, we have a cybernetic system here, in which an isolated "colony" of foreign cells lives, protected, inside the human's system. This colony feeds off of the human's blood, and excretes its wastes and its specially designed products into the human's circulatory system.
There are other applications for this type of technology such as pain control systems and treatment of otherwise untreatable Central Nervous System disorders. There are many diseases of the Central Nervous System (Parkinson's disease, and amyotrophic lateral sclerosis ("ALS"), Huntington's disease) which could be treated with pharmaceuticals, if only there were an easy and effective way to get the therapeutic substances across the highly protected blood-brain barrier. The encapsulated, genetically engineered cells could be designed to deliver badly needed proteins within the central nervous system itself. This ability could also be used to deliver powerful pain suppressors for terminal cancer patients or others who live with long-term pain. This type of drug delivery system is in fact currently being used experimentally on patients.