05-04-2012, 03:33 PM
Applications of MEMS in Surgery
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I. INTRODUCTION
Microelectromechanical systems (MEMS) is a technology
developed from the integrated circuit (IC) industry
to create miniature sensors and actuators. Originally these
semiconductor processes and materials were used to build
electrical and mechanical systems, but have now expanded
to include biological, optical, fluidic, magnetic, and other
systems as well. The term “MEMS” originated in the United
States and typically contains a moving or deformable object.
In Europe this technology goes by the name “microsystems
technology” or “microstructures technology” (MST) and
also encompasses the method of making these devices,
which is referred to as micromachining. In Japan and Asia,
MEMS are called micromachines when mechanisms and
motion are involved.
MEMS devices first were used in medical applications in
the early 1970s with the advent of the silicon micromachined
disposable blood pressure sensors [1]. Currently MEMS
devices own this market and are rapidly expanding into
other medical areas. Medical applications of MEMS devices
are growing at a compounded growth rate of 11.4% from
Manuscript received March 9, 2003; revised July 1, 2003.
K. J. Rebello is with the Research Technology Development Center, Applied
Physics Lab., The Johns Hopkins University, Laurel, MD 20723 USA
(e-mail: keith.rebello[at]jhuapl.edu).
Digital Object Identifier 10.1109/JPROC.2003.820536
Fig. 1. Worldwide forecast for MEMS in medicine.
$850 million in 2003 to over $1 billion in 2006 [2] (Fig. 1).
The incorporation of MEMS devices on surgical tools
represents one of the greatest growth areas. In the surgical
field there is an increased need by doctors and surgeons for
real-time feedback during operations. MEMS technology
can improve surgical outcomes, lower risk, and help control
costs by providing the surgeon with real-time data about
instrument force, performance, tissue density, temperature,
or chemistry, as well as provide better and faster methods of
tissue/fluid preparation, cutting, and extraction.
Recently there has been an increase in research activity
in surgical applications of MEMS. While not all-inclusive,
relevant research is listed by group in Table 1. Despite the
large number of research activities, few surgical MEMS
devices have made it to mass market. Surgical MEMS
devices must deal with all of the challenges that conventional
MEMS devices must overcome including manufacturability,
integration with electronics and signal processing, reliability,
calibration, testing, and packaging. Surgical applications also
have additional unique concerns that must be addressed in
order for MEMS surgical tools to evolve away from basic
research and toward commercialization.
II. PRODUCT DEVELOPMENT
When developing a MEMS-based product for the surgical
market, it is important to keep the end in mind. The greatest
whiz-bang sensor design and fabrication technology will not
produce a marketable product if the right application is not
chosen. As when building any MEMS product, it is important
to evaluate the market for the device, and in this regard
the surgical market is a good one. Minimally invasive
surgery (MIS) clearly cuts costs and has many other benefits
for the surgeon and patient alike. Minimally invasive procedures
are growing rapidly, with 40% of surgeries performed
in this manner [35]. The world market share for minimally
invasive products is more than $5 billion and in the next 15
years 80% of all surgeries will be done via MIS [36] (Fig. 2).
Targeting a disease for which there are a large number of
surgical procedures performed, such as heart, lung, cancer,
etc., will ensure that the device will receive the required attention
from funding sources, researchers, and surgeons. For
example, coronary artery disease has a 120-billion-dollar
economic impact [15], which has fueled research and development
of catheter devices.
While it is always important to know your target audience,
this is especially important when developing surgical
tools. Partnering with surgeons and doctors early in the design
process yields a deeper understanding of the problems
and issues faced in the operating room. This can shorten the
development cycle, as well as result in a tool which better
matches the surgeons’ needs. These surgeons and doctors
will be the end users and clinical champions of the surgical
devices and can not only help MEMS engineers to understand
what the real problem to be solved is, but also ensure
that the device is accepted in the medical community. Interfacing
early with the medical community will help to determine
if the surgical tool is really needed and if it will be used
by surgeons. MEMS engineers should take an honest look
to determine if MEMS really is the best choice to solve the
problem, or if competing technologies will perform better.
Not only must the surgical tool compete with other devices
technically, but it must also compete on a cost basis.
This has become more important now that medical providers
are under great pressure to reduce costs. Before developing a
product it is important to do the math. The device must make
a significant impact on a medical procedure to justify any
additional cost. In order to do this, MEMS engineers need to
44 PROCEEDINGS OF THE IEEE, VOL. 92, NO. 1, JANUARY 2004
focus on disruptive technologies which will reduce the skill
needed to perform complex procedures and allow them to be
performed in more convenient and lower cost settings. The
leading medical device companies are often reluctant to incorporate
disruptive technology in their products, and tend to
favor low-risk incremental improvements. To convince them
otherwise takes time, which many young MEMS companies
often do not have. Market followers are more likely to
take risks and incorporate new technologies in their products
in the hope of gaining market share. By teaming with the
market followers at an early stage in the development process
MEMS companies can overcome the inertia inherent in the
medical device industry.
The surgical device market has both low-volume/high
value products and high-volume/low-cost devices. While
MEMS devices are able to offer competitive advantages
due to their batch fabrication, small size, and improved
functionality to both these segments, they also have a
reputation for being low cost due to their IC roots. The
high-volume market is very price sensitive and has low
margins but high volumes, which are attractive to MEMS
fabrication facilities. The high-value market has high margins
and is a good fit for research institutions, research
firms, or the medical device companies themselves. The
complexities and development of the MEMS fabrication
process and other nonrecurring engineering costs as well as
fabrication equipment costs need to be factored into the cost
of the product. MEMS companies typically cannot sustain
themselves in this segment due to the low volumes involved.
A. Biocompatibility and Packaging
MEMS devices which come into contact with the body
must be biocompatible. This adds complexity to the already
challenging issue of MEMS packaging. The toxicity and
hemocompatibility of the materials used in MEMS are still
not understood and more rigorous research studies need to
be done. Traditional biocompatibility studies have looked at
some MEMS materials in bulk form, but research needs to
be carried out on the effect of thin films, as their properties
can be different due to their deposition processes. Although
preliminary results indicate that there are no cellular toxicity
effects and slight increases in clotting with conventional
MEMS materials [15], the current approach in using MEMS
in vivo relies on isolating MEMS devices from the body by
packaging them in biocompatible polymers. These polymers
can add to the size of MEMS devices as well as reduce their
accuracy. To overcome these issues, nanoparticle coatings
and biocompatible polymer micromachining need to be
investigated further.
Packaged MEMS devices must be able to survive the sterilization
procedures used in the surgical environment. They
must withstand exposure to high temperatures and moisture
in autoclaves and steam sterilizers. Alternative sterilization
methods include ethylene oxide and irradiation. Ethylene
oxide is a harsh organic solvent and packages must be made
of a compatible material. MEMS devices are inherently
radiation hardened, but their associated electronics are not.
They must be specially designed using radiation-hardened
IC processes and packages.
B. Regulatory Challenges
Medical products, of which surgical tools are a subset,
are subject to many regulatory controls. The Food and Drug
Administration (FDA) and European Community (EC) determine
whether a product is fit for sale in the United States
and Europe, respectively. Any MEMS devices which have
biomedical applications (bioMEMS) such as DNA chips
[37], pumps [38], blood glucose detectors [39], catheters
[26]–[28], cochlear implants [40], and blood analyzers [41]
fall under their jurisdiction. Historically bioMEMS have had
design cycles between 5 and 15 years long. Of this time, one
to two years have been used for getting the necessary agency
approvals. Agencies require that all claims be verified for
effectiveness and that the product has proven to be reliable
in many sets of clinical trials before they allow a product
on the open market. The approval process for disruptive
technology can be substantially longer. These agencies also
have current good manufacturing practices (cGMP) on how
medical devices must be fabricated [42]. These procedures
establish a set of standards which aim to ensure that quality
products are produced.
Lengthy sets of clinical trials can be avoided ifMEMSsensors
are applied to existing surgical tools and do not claim to
alter the performance. Retrofitting existing surgical tools is
the preferred method of entry for MEMS companies because
it is the fastest path to market. Retrofitted tools have already
been accepted by surgeons who are familiar with their applications
and use. Another advantage for MEMS companies
is that they themselves do not have to pay for costly
clinical trials, which can be avoided by modifying existing
tools. If clinical trial cannot be avoided, MEMS companies
can partner with device manufacturers to reduce costs and
use their expertise in trials.
C. Design and Fabrication
Design tools are a challenging area for surgical MEMS.
MEMS computer-aided design (CAD) tools are constantly
improving, which is helping surgical MEMS to be designed
quicker and better, but there is still a disconnect between
medical simulation tools and MEMS CAD tools. Design
tools which straddle both the MEMS world and medical
world are needed to decrease the long time to market of
MEMS surgical tools. Recently Freker’s research group
at Penn State University, University Park, PA, has created
software which optimizes the design of MEMS-based
minimally invasive surgical tools [21], [22].
Once designed, all MEMS devices must overcome the
manufacturability problem. ICs use very similar if not the
same technology to make different devices. For example,
the same fabrication line that makes the latest microprocessor
will often also make graphics chips or digital signal
processors. The technology for building MEMS devices, on
the other hand, tends to be very application specific and,
as such, often has custom materials, fabrication processes,
REBELLO: APPLICATIONS OF MEMS IN SURGERY 45
and packaging which vary from device to device. This
tends to make the conventional MEMS fabrication process
more complex and expensive. The geometric constraints,
biocompatible material needs, and assembly complexities
of surgical MEMS make device fabrication even more
challenging.
MEMS devices have traditionally been fabricated on silicon
using surface and bulk micromachining technologies.
For in-depth coverage of micromachining technologies,
the reader is referred to excellent texts by Madou [43] and
Kovacs [44]. In surface micromachining, micromechanical
structures are fabricated on the surface of a substrate by
successively depositing, patterning, and etching selective
films. Bulk micromachining relies on wet chemistry or deep
reactive ion etching to etch deep structures into the substrate.
BioMEMS and in particular microfluidic devices have made
use of polymers and plastics as their structural and substrate
layers. Typical fabrication techniques for these materials include
micromolding, injection molding, and hot embossing.
Surgical MEMS devices use these technologies, but also are
incorporating newer nonplaner fabrication technologies to
better deal with the varied shapes and substrates of surgical
instruments.
1) Microelectrodischarge Machining: Microelectrodischarge
machining ( EDM) is a form of spark machining
used to shape conductive materials such as silicon and
metals. Electrodischarge machining erodes material by
creating a controlled electric discharge between an electrode
and the substrate. It is a noncontact process and there is no
direct mechanical cutting force applied to the substrate. Dielectric
fluid is used to remove the erosion particles as well
as to keep the substrate material from oxidizing. EDMs
can be used to make holes, channels gears, shafts, molds,
dies, and stents, as well as more complex three-dimensional
(3-D) parts such as accelerometers, motors, and propellers
[45].
2) Laser Micromachining: Lasers can be used to both deposit
and remove material. Laser ablation vaporizes material
through the thermal noncontact interaction of a laser beam
with the substrate. It allows for the micromachining of silicon
and metals as well as materials which are difficult to
machine using other techniques such as diamond, glass, soft
polymers, and ceramics. Laser direct writing and sintering
is a maskless process where a laser beam is used to directly
transfer metal materials onto a substrate. This can be used to
form metal traces on nonplaner surfaces, which reduces the
need for wires on surgical tools [46].
3) Stereolithography: This process generates 3-D structures
made out of UV-cured polymers. It is an additive
process where complex 3-D structures are made out of thin
two-dimensional (2-D) slices of polymer which have been
hardened from a liquid bath. Conventional systems were
limited in that they were a serial process where only one part
could be made at a time. MicroTEC has developed a batch
fabricated wafer level process called rapid material product
development (RMPD) which is capable of constructing
structures out of 100 different materials including plastics,
sol-gels, and ceramics [47].
D. Other Issues
Once the technology is developed, marketing, sales,
clinical trials, and litigation can require more money than
product development. It often makes sense for MEMS
companies to partner with more experienced medical device
companies to bring a product to market. Preferably earlier is
better, but care must be taken not to allow medical device
companies to steal the MEMS company’s intellectual property.
Patent litigation is a big concern in the medical field.
Medical device makers are notorious for targeting each other
with lawsuits. This is a much more aggressive environment
than most conventional MEMS companies have experienced.
Much money can be spent protecting, defending, and
circumventing intellectual property. Additionally, once a
product is on the market, companies must be prepared for
litigations from patients.