Clinical Applications of BioMEMS

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MEMS have long been used in biomedical sensing applications around the patient,for monitoring things like blood pressure or activity levels in people or ?ow rates in bedside equipment. But now products based on the major progress in MEMS technology of the last few years are starting to make it through the long Biomedical development process to the market, applying that sensing information to intervene inside the body, in smarter implants and better minimally invasive procedures.

Inertial sensors help control chronic pain Medtronic’s neurostimulator implant to treat chronic pain, approved for use in the US in November, uses a MEMS accelerometer to help maintain stable pain control as the patient moves around. Development of the product started back when consumer motion sensors in the Wii and the iPhone ? rst attracted attention to the progress in accelerometers, and Medtronic researchers saw an opportunity to put these devices to use. But the sensors then used as much energy as the stimulation therapy did, so researchers adapted a robust commercial consumer 3-axis accelerometer to the more demanding medical application, customizing the ASIC and the algorithms to drastically reduce power usage and to offset drift to assure stability for the extended  lifetime of the device. By 2007 they’d designed the interface signal chain and algorithms with techniques like correlated double sampling to cut energy consumption to 100x what was available at the time, down to 100nanoamps per axis.Also challenging was ? nding the balance of robustness vs resolution vs energy use. The device has to survive in the unique biologic environment, which  has relatively stable temperatures and limited sensitivity range, but it also had to survive things like high- g drops on to stainless steel trays, in case the surgeon dropped it before implantation, and possible extreme temperatures during transport and storage. “There’s the hardware development cycle, and then there’s the clinical development cycle, and then you have to loop back to complete the engineering work, and it’s all serial,” notes Tim Denison, director of neuroengineering and technical fellow. “Medical devices just take a long time.”

The implant treats pain from nerve damage in the back and legs by inhibiting and  modifying pain signals from selected nerve ?bers in the spinal cord by stimulation with  an electrical ? eld. Conveniently the leads from the implanted stimulator can stimulate the  target nerve ? bers by this ? eld from outside the blood/brain barrier surrounding the spinal  cord. Inconveniently the target ? bers move in and out of the stimulation ? eld as the patient  bends and moves around, explains Mark Lent,  Senior Director of Technology, Medtronic.

Earlier generation implants had to be  continually adjusted by the patient. Now the  MEMS sensor enables automatic adjustments to the pain-blocking ? eld to keep the target  ?bers continually activated, once the patient trains and calibrates the device.
Neuromodulation and chronic pain are big business. Medtronic saw $1.6 billion in  revenues from neuromodulation in ?scal 2011, including both neurostimulation and  implantable drug delivery systems for speci?c sites to treat a variety of disorders.

The company cites studies that report some 116 million adults in the US suffer chronic pain,  with low back pain from nerve damage among the most common and hardest to manage types. Some 200,000 people around the world have used Medtronic neurostimulation  for chronic pain to date, and some 80%-90% of the patients in the recent clinical trial of the  MEMS position sensing version reported less pain, more convenience or clinical bene?t.

The motion sensor also tracks and records patient activity in onboard memory, giving the  patient and the doctor good data to track the effects of activity on pain, to help identify issues  such as disrupted sleep or too much or too little rehabilitation exercise that impact pain. “Our hope is that quanti? able information will lead to better treatment,” says Denison.

This First instance of building an arti?cial re?exive response system into the body does  take advantage of the unique spinal case, where stimulation works on nerve ? bers from  outside the blood brain barrier. “There’s a reason the RestoreSensor is our ? rst product,” jokes Denison. “Most MEMS is integrated with  hardware, but ours is integrated with the  body, so integration is much harder.” The next challenge for MEMS as a ?eld, he suggests, is how to make more intimate contact with the nervous system, developing micromachined electrodes with long term biocompatibility at the cellular level.  Smaller sensors enable portable support for arti?cial heart MEMS is also helping bring down the size of some medical support systems to small enough to be portable. SynCardia’s implanted arti? cial heart has been used for some years as  a bridge to transplant for patients waiting for
a heart transplant, but all the electronics and controls and vacuum pump and high pressure  air tanks required a 400 pound external console, keeping the patient tethered to the  machine in the hospital. The next generation  electronics and pneumatics, now approved in  Europe and under clinical trials in the US, use MEMS among other technologies to reduce  the external unit to under 14 pounds, so it can be carried around in a backpack or a shoulder  bag, allowing patients to return home and be
mobile while waiting for a donor.
One of the enablers allowing this more  compact unit to power the Total Arti? cial  Heart is a ?ow sensor from Omron Electronic
Components.The thermopile technology, where  the gas ? owing across a thermopile creates a  temperature differential to measure the ?ow,  helps reduce power consumption down to  15mA to help allow battery operation of the  unit. Etching the cavity under the thermopile from the top so the opening on top is larger  increases the sensitivity of the device, says  Donna Sandfox, Omron product manager, new business development.

Tweaking the  electronics reduced response time down to under 5ms for the arti? cial heart application. Omronis now starting to integrate the ASIC and the connector with the ? ow sensor to further reduce size and cost.

Sensors and actuators start to  enhance minimally invasive, robotic-assisted surgery  


More sensors are also starting to be used in minimally  invasive or robotic surgery, especially to give the  surgeon the tactile sense or force feedback that’s  missing when operating through a laparascope or  robotic console. Besides its technology for touch
screens, haptics supplier Immersion Corp. has also licensed technology and designed custom systems and software for biomedical applications,  to translate sensor input into tactile effects with actuators that provide vibration or resistance or a
physical stop. “We’re expecting there will be a lot of opportunity over the coming years as people see  more applications for sensors,” says Cheryl Shimek, Immersion director of product marketing, medical.Mako Surgical is one user, adding tactile feedback to its robotic-assisted surgery system for more accurate knee surgery. A map based on a CT scan of the knee is used to determine exactly how much bone to cut away, and that line is programmed into the robotic arm. When the surgeon then
uses the robotic arm to assist the surgery, it gives push back at the line to prevent going too far. The technology is also licensed to SOFAR for its new ALF-X robotic surgery system, developed in conjunction with the European Commission Joint Research Center. A SOFAR brochure out this fall advertises that the yet-to-be-released system will provide natural perception through haptic feedback of the consistency of soft tissues and of the forces exerted by the surgical instruments. Shimek notes that haptics added to current surgery tools that doctors are already used to using is typically limited to providing feedback for new information, often as a physical stop or vibration as an alarm or alert mechanism. Minimally invasive or robotic surgery is also
creating a market for MEMS devices that control the ? ow of gas through the laparoscope for argon beam cauterization, to seal the blood vessels to stop bleeding while cutting, or to remove irregular ells, in robotic or minimally invasive procedures.Omron is working with multiple medical customers on using its ? ow sensors to control the argon to do these procedures, reports Sandfox, though the ?ow sensor and controls remain outside of the patient in the robotic support system.

Haptics bring reality to medical simulators
Haptics are already well established in for giving physicians realistic force feedback as they learn to do minimally invasive laproscopy, endoscopy and catheter procedures on virtual reality simulators for the procedures, where the systems haven’t had to go through the longer approval process required of clinical equipment. CAE HealthCare in Canada acquired Immersion’s simulator business in early 2010, one of a round of acquisitions related to its $275 million investment in building up its medical simulation business. The company uses the haptics technology to give the realistic resistance of inserting a catheter in a person, or that mimics the feel of the endoscope bumping into the esophagus or intestinal wall, while the user views the procedure on a screen that’s like looking through the endoscope.

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