Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 63

FIGuRE 3.26
SEM of an array of thin film polysilicon beams that are clamped at both ends and that have a
compressive residual stress. As can be seen, the beams are buckled by the compressive residual
stress in the polysilicon layer. Fortunately, the residual stress can be reduced or eliminated by
a high temperature anneal. For example, it is very common to perform a high-temperature
anneal on polysilicon thin films that have been deposited using LPCVD to reduce the high
compressive residual stress in these layers in a surface micromachining process.
involved implantation, the device must withstand long-term exposure to the
physiological environment into which it is placed. This includes enduring
the influence of the surrounding tissues on the device's function over its life-
time of operation.
Despite these very demanding requirements, MEMS devices have been
used in several medical applications, primarily as sensor devices. In most
of these applications the devices are not used in implanted applications, and
the devices are packaged in such a way that the sensor is not in direct con-
tact with tissue. Instead the device is packaged within an enclosure that is
made of approved and commonly used suitable materials, usually synthetic
polymers such as polyethylene and polyurethanes. Importantly, the medi-
cal industry tries to use already-approved and “off-the-shelf” materials since
the cost of the experimental work for new materials for new applications is
frequently prohibitive.
Nevertheless, recently several research groups have been examining the
biocompatibility of MEMS devices and materials, particularly for implantable