AAWSAP DIRD, Biosensors and BioMEMS: A Survey of the Present Field, March 2010

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have a significant problem in controlling their blood sugar and sometimes insulin doses
are not adjusted finely enough or frequently enough to maintain the blood glucose at
healthy levels. This form of diabetes is responsible for fainting (ketoacidosis) and other
serious symptoms such as poor blood circulation to the limbs. Poor circulation can
result in diabetic ulcers and may sometimes necessitate amputation of the limb.
Insulin pumps are often worn by brittle diabetics because a slow infusion of insulin
works better in stabilizing blood glucose levels rather than periodic injections. A belt­
worn insulin-delivery system looks something like an old-style audio player. These
systems are typically controlled by a small screw-type pump powered by batteries. The
reservoir carries several milliliters of insulin for dispensing over an extended period of
time. The injection needle is connected (underneath the clothes) to a catheter and then
to the pump. The systems are reasonably effective but are cumbersome and require the
needle to continuously reside subcutaneously in the abdomen.
A MEMS implanted insulin pump is a less cumbersome and perhaps more convenient
means of slowly infusing insulin at a programmed rate. The implanted device is refilled
periodically by introducing a needle through the skin and tissue to a septum in the
device.
The MEMS insulin pump shown in Figure 6 is surgically placed under the skin. It has a
rubber septum on the top for filling with insulin. This implementation has a piezoelectric
element that moves in response to electrical charge. When actuated by a timer the
element oscillates, creating a pressure inside the device that dispenses insulin. The
device is programmable for dispensing at various rates.
Although the implantable MEMS pump was developed for diabetes, the pump has
application to the slow measured delivery of many other drugs including 5-fluoruracil
used for cancer therapy and theophyllne for treatment of asthma.
What is a Biosensor?
A primary application of BioMEMS is in the creation of sensors for blood chemistry and
other biophysical parameters of the human body.
We can define a Biosensor as:
•
A sensor whose application is primarily in the measurement of quantities within a
biological system, such as chemical, electrical, and physical parameters.
•
A sensor incorporating a biological component (enzymes, living cells, antibodies)
typically used to measure chemical concentration. This definition does not require
that the sensor be deployed within a biological system.
The two definitions which are somewhat different have their origin with different
influential investigators who wrote textbooks in the early days of this field. The latter
definition is prevalent in Europe.
A biosensor is normally constructed by immobilizing a biologically active material (such
as an enzyme) onto an electrical sensor that measures a fundamental physical quantity
like electrical current, voltage, mechanical strain, temperature, or frequency. The
specific sensor material is chosen because it reacts to a desired measurand (such as
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