Biomedical Engineering Reference
In-Depth Information
analyte) of interest, and a supporting structure that also acts as a transducer and is in intimate
contact with the biological sensing sensed by the biological recognition element into a quan-
tifiable measurement, typically in the element. The purpose of the transducer is to convert the
biochemical reaction into the form of an optical, electrical, or physical signal that is propor-
tional to the concentration of a specific chemical. Thus, a blood pH sensor is not considered
a biosensor according to this classification, although it measures a biologically important var-
iable. It is simply a chemical sensor that can be used to measure a biological quantity.
10.1.2 Sensor Packaging
Packaging of certain biomedical sensors, primarily sensors for in vivo applications, is an
important consideration during the design, fabrication, and use of the device. Obviously,
the sensor must be safe and remain functionally reliable. In the development of implantable
biosensors, an additional key issue is the long operational lifetime and biocompatibility of
the sensor. Whenever a sensor comes into contact with body fluids, the host itself may affect
the function of the sensor, or the sensor may affect the site in which it is implanted. For exam-
ple, protein absorption and cellular deposits can alter the permeability of the sensor packag-
ing that is designed to both protect the sensor and allow free chemical diffusion of certain
analytes between the body fluids and the biosensor. Improper packaging of implantable bio-
medical sensors could lead to drift and a gradual loss of sensor sensitivity and stability over
time. Furthermore, inflammation of tissue, infection, or clotting in a vascular site may pro-
duce harmful adverse effects. Hence, the materials used in the construction of the sensor's
outer body must be biocompatible, since they play a critical role in determining the overall
performance and longevity of an implantable sensor. One convenient strategy is to utilize
various polymeric covering materials and barrier layers to minimize leaching of potentially
toxic sensor components into the body. It is also important to keep in mind that once the sen-
sor is manufactured, common sterilization practices by steam, ethylene oxide, or gamma
radiation must not alter the chemical diffusion properties of the sensor packaging material.
10.1.3 Sensor Specifications
The need for accurate medical diagnostic procedures places stringent requirements on
the design and use of biomedical sensors. Depending on the intended application, the per-
formance specifications of a biomedical sensor may be evaluated in vitro and in vivo to
ensure that the measurement meets the design specifications.
To understand sensor performance characteristics, it is important first to understand
some of the common terminology associated with sensor specifications. The following defi-
nitions are commonly used to describe sensor characteristics and selecting sensors for par-
ticular applications.
Sensitivity
Sensitivity
is typically defined as the ratio of output change for a given change in input.
Another way to define sensitivity is by finding the slope of the calibration line relating the
input to the output (i.e.,
Input), as illustrated in Figure 10.1. A high sensi-
tivity implies that a small change in input quantity causes a large change in its output.
D
Output/
D
