Opportunities at the skin interface for continuous patient monitoring: a development model based on lactate and glucose

In the search for devices for continuous monitoring of patients or tissue a great deal of effort has centred on implantable devices. A good example of this is the development of implantable glucose sensors and subcutaneous sensors are currently available with lifetimes of several days. Other implantation systems have been used in the development of artificial retina. Furthermore, there is a large body of microdevice know-how for in vitro measurements such as monitoring of cells on microelectrodes or lab-on-a-chip diagnostics. However, long term implantation of biosensors remains an elusive goal. Unless the active components of a biosensor can be regenerated in situ in the human body, they will always have a limited lifetime. Good possibilities exist for micro and nano technology to contribute to this area through the development of non-invasive, wearable sensors and multi-sensor arrays. The very dimensions of the transmission paths through skin could lend themselves to direct study by miniaturised devices. However, a complete understanding of transmission mechanisms (electroosmosis, diffusion, ion and molecule drift currents) must be incorporated in sensor and device development and calibration for skin use. For example, uncharged molecules may be delivered through skin by electroosmosis due to convective solvent flow, but once though the skin the molecules arrive at the sensor surface by diffusion This diffusion must take place in a suitable conducting hydrogel which will provide both the skin-hydrating contact and the current path from the iontophoresis electrodes.