MEMS implementation advances medical sensing technology
16 July 2014
The use of MEMS sensors in the medical environment is varied and wide ranging.
There are a broad range of opportunities for sensing devices within the healthcare sector; monitoring key dynamics or to enable the diagnosis of medical complaints. In the coming years, says Laurent Otte, Melexis, semiconductor innovations are likely to address these opportunities, nevertheless major technical challenges must still be overcome.
Conventional sensor technologies, due to large dimensions and relatively high unit costs, have limitations in the medical arena. Miniaturised sensor devices based on MEMS (micro-electro-mechanical system) technology are gaining greater traction. One report, published by Transparency, has predicted a CAGR in the medical MEMS market of over 20% between now and 2019 (when it is likely to be worth $6.5 billion annually).
Since MEMS technology is compatible with standard CMOS processes, it permits the production of sensing devices which are monolithic in form. The sensor element can thus be placed very close to the related signal conditioning electronics - something that differentiates MEMS devices from conventional sensors based on ceramic or metal substrates. As conventional sensors cannot incorporate their signal conditioning/processing onto the same substrate, there will be space between the sensing element and the signal conditioning/processing mechanism. This impacts the SNR (signal to noise ratio) of such sensors, as their capacity to deal with EMI is to some degree compromised. MEMS sensors may need to be placed in space constrained environments. Potentially they can offer lower power consumption too, though it should be noted that this is not always the case.
The cost optimised wafer batch fabrication achievable with MEMS is highly advantageous. It makes devices of this kind much better suited to medical deployment, especially given the increasing prevalence of home monitoring/telemedicine and personal care.
MEMS sensor implementation
There are different categories in which medical sensor implementations can be defined.
Disposable/non-disposable: There will be obvious implications in terms of the acceptable unit cost for the MEMS sensor being integrated. MEMS pressure sensors, for instance are currently designed into respiratory monitoring systems for chronic medical complaints like obstructive sleep apnea. As the sensors are located inside the machine, rather than the mask worn by the patient, they can be used many times, without the need for disposal. In this case it is not the cost, but the heightened degree of performance offered. (MEMS sensors are more accurate in low pressure applications than conventional sensors).
In contrast, the catheters used when surgical procedures are being conducted must incorporate pressure sensors that can provide accurate measurement data, but are cost effective, as they will only be used on one occasion then thrown away.
Implantable/non-implantable: Medical implants are introduced (either surgically or by some other method, such as ingestion) into the body and remain there for either the medium or long term, to either facilitate some process or permit continual monitoring. Among the areas where MEMS sensor implants could potentially be used is in the management of conditions like glaucoma. Here the pressure within the eye can be too high, leading to risk of blindness. This can be combatted through regular monitoring of the pressure via an implanted device (or alternatively use of a disposable device located in a contact lens). A major concern when employing implantable technology is bio-compatibility – which relates to the way the body’s defense mechanisms respond to the implant being introduced and conversely how the implant copes with the operational setting in which it is situated. Bodily fluids can have a corrosive effect on the implanted device, with some environments being more aggressive than others. If the device is positioned inside the heart, for example, it will be exposed to constant blood flow and the effect of its surroundings will be much greater over time than would be true for a device placed, say within the eye.
To combat issues of bio-compatibility, devices may need to be enclosed inside titanium. This material protects against corrosion, but is also non-reactive to the human body. As a result it is often employed in implantable medical devices such as pacemakers, where life cycles of 15+ years make the associated expense justifiable. New materials need to be developed however for less critical, shorter term implant deployments which are more price-sensitive.
Another technical challenge for implantable devices is that they must go through sterilisation procedures. This involves electro-magnetic radiation (normally gamma rays) being applied. The difficulty arising from this is ensuring that the radiation does not affect the electronics of the IC.
Invasive/non-invasive: An invasive medical device is not implanted in the human body, but is still in direct contact with bodily fluids. By nature they must also be disposable.
Even in non-invasive implementations, the smaller package formats of MEMS sensors can be favourable, as they permit a higher degree of comfort. This is particularly of value for applications like logging the position of babies to investigate sudden infant death syndrome occurrences. Other forms of wearable electronic device can benefit from the compactness of MEMS, including band aid-based sensors that can check on cardiac rhythms or wristband-based activity monitors for exercise (to treat chronic obesity or following on from hip/knee operations). Inertia data can be used to analyse posture and gait to help amputees when they are fitted with prosthetics. Keeping size to a minimum is of great value, especially if it will be worn long term.
Life support/non-life support orientated: In life support, the on-going operational performance of the sensor technology is, of course, critical. MEMS pressure sensors, for example can be used in ventricle assistance systems, where it is of paramount importance that the volume of air being administered to the patient is monitored exactly. MEMS sensor generally offer a far higher operational performance level than conventional electro-mechanical sensors, which are more prone to wear and tear over time.
The approval of medical devices can be a major obstacle. The validation cycles relating to disposable devices are much longer than for consumer electronics or even automotive sectors. Likewise non-invasive devices take time to attain compliancy. It is for implantable and life support implementations, however, that the demands are at their highest. The approval process here can be over a decade and the investment made by sensor manufacturers can, as a result, be very large. This must be weighed up against the revenue generation that can be derived once compliance has been received.
There are many areas where the treatment of patients can benefit from high performance sensing devices with very small dimensions. MEMS sensors can capture detailed data on dynamics such as pressure, acceleration, inertia and position, in order to assess the influence these have on the various biochemical processing that are taking place. They do so while meeting the exacting operational, financial and size demands of the medical sector.
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