How are MEMS sensors impacting the medical market?

The advent of high performance sensors, enabled by micro-electro-mechanical system (MEMS) technology, are proving extremely advantageous for an array of industry sectors; developers of components of this kind are also experiencing considerable uptake within the healthcare arena. Through them it is possible to acquire data on various parameters, including position, acceleration, inertia and pressure that will enable both faster diagnosis and better treatment of a multitude of different medical conditions. They are in many cases far easier to implement than standard sensor technologies, furthermore they come in more compact package formats and generally have lower unit costs associated with them. Projections made by analyst firm IHS iSuppli suggest that the market for medical MEMS pressure sensors alone will be worth $186.7m by 2016.

The fact that MEMS is compatible with widely used CMOS process technology facilitates a higher degree of integration, allowing the sensing element and the signal conditioning electronics that support it to be placed together onto the same semiconductor die. This results in better signal integrity, as the detrimental effect electro-magnetic interference has on the sensor can be curbed to a considerable extent. It also means that overall size can be markedly reduced; a valuable attribute when developing many types of medical equipment, where the comfort of the patient becomes a factor. An additional benefit may be an overall reduction in the electrical energy needed by the sensor while it is functioning, which is typically much lower when MEMS is employed over older, conventional sensing technologies such as ceramic and metal substrates - although this should not be assumed to always be true, in some circumstances lower power consumption will not actually be witnessed.

Medical implants

Implantable medical devices may be introduced into the body of a patient to facilitate ongoing monitoring, they may also be used to execute certain sophisticated processes that will aid in maintaining the patient’s wellbeing. The compactness and lower power operation offered by MEMS technology can both prove extremely attractive to engineers designing medical implants, where long battery life and small form factors will be mandated. For serious conditions such as glaucoma, for instance, MEMS sensing ICs can be employed to ensure that acceptable pressure levels are not exceeded.

One of the issues that is closely related to the success of implantable devices is making certain that they are deemed to be biocompatible, which is vital if an implant is to remain in the patient’s body for a significant period of time. There are two different factors that need to be taken into account; exactly how the body’s defences will react to the implant’s presence,  which can be a major concern, and how well the implant deals with being placed in what can often be challenging operational settings. Blood and other bodily fluids are likely to have a corrosive effect on the implant over time. Some in-vitro environments will be more difficult to cope with than others; for example, if the implant is located in an area where it is exposed to constant blood flow (such as a cardiac pressure monitor) then the threat of corrosion will be particularly strong. In order to ensure biocompatibility between the implant and the patient, these devices will normally be housed within a titanium shell. As well as being capable of protecting against the onset of corrosion, titanium is also non-reactive to the human body. Research is now being done to find alternative materials capable of providing similar biocompatibility characteristics, but which are more cost effective in nature. Currently the expense involved in using titanium can be justified for implantable devices that will be in-situ long term (such as pacemakers), but for the implementation of implants that are present only for short periods of time the investment involved can often be simply too high.

To further complicate matters, other technical obstacles that implantable electronics face include the rigorous sterilisation procedures they have to endure, which comprise the application of electro-magnetic rays that can potentially effect the implants’ memory resources. Innovative new memory technologies that are not susceptible to electro-magnetic radiation are currently being investigated. Although there are hurdles to overcome here, it is widely acknowledged that MEMS could be a key enabler in further proliferation of implantable electronics.

Disposable implementations

If the medical device into which a MEMS component is integrated has to be disposed of after use then the cost aspect will be of a far higher priority than would be the case for a non-disposable implementation. Great effort must be made to keep the total bill of materials to a minimum, with each constituent part’s price tag being scrutinised thoroughly. The catheters employed during surgery need to incorporate pressure sensors that can obtain precise vascular data, but also have extremely low unit costs as they must be disposed of after every operation. MEMS is therefore a strong candidate for this kind of device design.

Conversely, MEMS based pressure sensors are now starting to be specified for inclusion in continuous positive air pressure monitoring systems, for example, where they will be able to combat respiratory conditions like asthma and obstructive sleep apnea, as well as the examination of various sleep disorders. Since such sensors are situated within the machine itself (as opposed to the mask worn by the patient) they can be utilised numerous times without disposal being necessary. Under such circumstances it is not the cost benefits presented by MEMS technology that are of prime importance, but the heightened degree of operational performance they deliver, as MEMS-based sensors are normally much more accurate than conventional sensors in low pressure applications..

If a medical device is categorised as being invasive then although it is not actually implanted inside the human body, it is still in direct contact with bodily fluids. The trauma of invasive medical device implementation should be avoided as much as possible. Invasive implementations rely on devices that are effectively disposable, so the cost considerations once again come into play and the economical manufacturing processes that MEMS components rely on give them the edge here. Even for non-invasive implementations, MEMS technology is likely to be favourable, since the packages that components can fit into allow greater patient comfort. One application where this is particularly helpful is in compiling positional data when studying sudden infant death syndrome.

Obtaining the relevant approvals with regard to medical devices is an incredibly difficult, laborious and long-winded endeavour. The validation cycles relating to such items are much longer than those found in other sectors, especially in life critical applications such as patient ventilation systems. Approval can take ten years or more to attain, which calls for a very large commitment by sensor manufacturers. That said, the rewards that can be received afterwards can make it worthwhile.

There are a host of possibilities for further exploitation of ultra-small, cost effective and highly integrated sensing devices within the medical sphere. Technological progression will mean that MEMS is certain to play a big part in making sure that exacting functional demands are met. In particular, MEMS pressure sensors (which already constitute the biggest proportion of the medical MEMS sensor market) seem certain to witness continued growth as all manner of new applications for them emerge. Semiconductor vendors who have developed their own advanced proprietary MEMS engineering resources will have access to the wealth of opportunity that the future holds.