A medical implant for injecting into the vascular system of animals comprises a device less than 500 .mu.m in size carrying signal processing means for providing an output in response to an input signal. The input may be acoustic, electromagnetic, temperature, pH value, or chemical. The output may be acoustic, electromagnetic, or chemical. Large numbers, e.g., 10.sup.8, of the implant devices may be mixed into a saline solution and injected into a blood vessel where they are carried around the vascular system, or the devices may be mixed with a gas and inhaled. The amount of circulation depends on the device size. For the larger device e.g., above 7 .mu.m to around 250 .mu.m, circulation will be short lived. Smaller sized e.g., <3 .mu.m devices will circulate for a considerable time. Anti-bodies may be coated onto the devices to cause them to adhere to target areas such as tumors. In one device a chemical is released when illuminated externally by an acoustic beam. In another device a chemical is released when the pH of surrounding blood changes to a given level. Power to operate the device may be by an integral battery.