Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 64

medical applications. In one recent published study [47], the biocompatibil-
ity and biofouling of the materials used in a MEMS-based implantable drug
delivery device was studied to determine the vivo inflammatory and wound
healing response of the materials used in the device, including gold, silicon
nitride, silicon dioxide, silicon, and a formation of SU-8. These tests were per-
formed using the cage implant system, with the materials being placed into
stainless steel cages and implanted into a rodent model. At 4, 7, 14, and 21 days,
the leukocyte concentrations were measured. The researchers reported that
the inflammatory responses elicited by the MEMS materials examined were
not significantly higher than those of the empty cage control tests over the
duration period of the experiments. They also examined fouling of the mate-
rials by inspecting samples that had been implanted for 4, 7, 14, and 21 days
into rodents. The extracted sample materials were inspected using a scanning
electron microscope to measure the amount of surface cell attachment on the
samples, including macrophages and foreign body giant cells, two of the prin-
cipal ways of determining biofouling. Significantly, the researchers reported
that the materials gold, silicon nitride, silicon dioxide, the formation of SU-8
examined, and silicon were all found to be biocompatible, with gold, silicon
nitride, silicon dioxide and SU-8 all exhibiting reduced biofouling.
As demonstrated by this research and others, the issue of biocompatibility
of MEMS materials is a difficult subject that requires careful and extensive
study. Readers are referred to [48] for more information.
Applications of MEMS in Medicine
There is a wide variety of applications for MEMS in medicine. The first and
by far the most successful application of MEMS in medicine (at least in terms
of number of devices and market size) are MEMS pressure sensors, which
have been in use for several decades [49, 50]. The market for these pressure
sensors is extremely diverse and highly fragmented, with a few large mar-
kets and many smaller ones. Nevertheless, the contribution to patient care
for all of these applications has been enormous. More recently, MEMS iner-
tial sensors, specifically accelerometers and rate sensors, are being used as
activity sensors. Perhaps the foremost application of inertial sensors in medi-
cine is in cardiac pacemakers, in which they are used to help determine the
optimum pacing rate for patients based on their activity level. MEMS devices
are also starting to be employed in drug delivery devices, for both ambu-
latory and implantable applications. MEMS electrodes are also being used
in neuro-signal detection and neuro-stimulation applications. A variety of
biological and chemical MEMS sensors for invasive and non-invasive uses
are beginning to be marketed. Lab-on-a-chip and miniaturized biochemical
analytical instruments are being marketed as well.