ABSTRACT
Optical techniques provide a useful and powerful means for monitoring biological tissue and fluids such as human skin and blood because they are convenient to apply and often involve only a minimal invasion of the tissue or fluid when used in vivo. However, such tissues and fluids provide stern challenges for optical monitoring because of their inhomogeneous structures, complex chemistry, and variable nature. For example, cutaneous tissue consists of skin layers-epidermis and derma-and fatty tissue, muscle, cartilage, blood-filled capillaries, sweat glands, sebaceous glands, hair follicles, etc. A simplified diagram of the basic structure is given in Figure 8.1.1a. (West 1993). This shows the location of veins and arteries in the dermis, which in turn is separated from the skin surface by the stratum corneum. Thus, even for in vitro monitoring (e.g., identification of different cells in pathological samples, etc.), the conditions to be monitored are complex. For in vivo monitoring, there are additional complexities such as regular pulsations by which blood flows through the veins and arteries, carrying with them a range of hemoglobin derivatives (West 1993). Optical monitoring of such a complex structure with its different functionalities is therefore technologically demanding.
