MEMS sensors combined with the use of biosignals are enabling the development of more innovative and smarter ambulatory medical devices.
There probably has never been a time when the technology for harnessing biosignals has been as plentiful as now. Recent developments in microelectromechanical systems (MEMS) sensor technology have enabled truly innovative and adaptive solutions for real-life medical needs, opening up possibilities that were previously only achievable within the confines of a clinical laboratory.
For implementers of this technology, the focus has increasingly changed from attempting to find any way possible to realise a desired solution to selecting the best method from a range of possibilities. Having a proper understanding of the complexities these new opportunities present is what sets the real innovators apart.
The challenge now is to demonstrate ways of intelligently combining sensors to produce unprecedented subtleties of measurement. A simple example of this approach would be to combine an accelerometer with a pulse detection device; if a resting heart rate is the required information, the pulse would be measured only after a short period of relative stillness. The dividend of this method is that only a low-power accelerometer would need to be active, enabling the other parts of the circuit to remain inactive until required. This has obvious benefits for the battery life of an ambulatory system, its size and cosmetic appearance. There are similar examples of this type of sensor integration for other forms of activity monitoring and for the control of orthotic and movement aids. This article sets out to explore some of these.
The MEMS technology that most of us encounter on a daily basis through our cars, mobile phones, navigation equipment and other electronic consumer goods has paved the way for a plentiful supply of low-cost, highly integrated MEMS sensors that can be incorporated into the design of medical equipment. These sensing devices enable us to reliably measure acceleration, tilt, rotation, vibration and shock. They can be incorporated into systems by which medical professionals remotely monitor the condition of patients, secure in the knowledge that they will be instantly alerted in case of an emergency. The sensors also can be used to form intuitive interfaces for assistive medical devices, an example being the control of neuromuscular stimulation.
Developing intelligent neuromuscular stimulation systems
Neuromuscular stimulation, usually referred to as functional electrical stimulation or FES, is a well-established method for restoring movement following partial paralysis associated with stroke. In the past it has been difficult to achieve adequate user control that is intuitive and proportional to need. A good example is the forearm, where the mechanics of activating the muscles is well understood, but the subtlety of controlling patterns of movement—opening the hand to reach and grasp, for example—is still lacking. The progress made in consumer electronics with gesture recognition and haptic feedback can help remedy this medical need.
FIGURE 1: Gesture recognition using backward and forward movement, side to side motion and tilt and rotation of the arm.
Gesture recognition involves using simple ideas intelligently and building them into a system that can carry out complex tasks. In our medical example, gesture recognition can be used to determine the differences between backwards and forwards movement, side to side motion and tilt and rotation of the arm (Figure 1). The determination of these movements can be used to interpret the type of hand grip the patient wants to use. If the arm is presented with the hand facing down, the neuromuscular stimulation can provide a wide flat hand opening, enabling the user to pick up an object. Similarly if the arm is rotated and the hand is positioned sideways, then the thumb can be pulled back to make the sort of grip you would use to pick up a glass.
A further enhancement for the control would be to organise a biosignal feedback loop using the electromyogram (EMG) signals coming from the muscles. Electromyography is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG signals are routinely used in many clinical and biomedical applications and also can serve as control signals for prosthetic devices by sensing muscular activity even where no movement is produced. The action of muscles firing is electrically noisy and the signal generated by these actions can be used to measure a person’s intention to move. When linked with gesture recognition, the sensor technology within a device can detect the movement that a person is likely to want to make before facilitating it through neuromuscular stimulation (Figure 2).
FIGURE 2: Gesture recognition combined with electromyography detection.
Now suppose that the clinician wants to monitor a patient through a programme of rehabilitation, possibly using one of the systems described above. A simple way to get an indication of a patient’s fitness and improving health is to monitor his or her SpO2 levels. SpO2 (the saturation of peripheral oxygen) can be measured using a pulse oximeter device. Using sensors to detect the patient’s oxygen saturation levels and pulse measurement, we can also determine heart rate variability and hence respiration rate for an overall indication of general health. When combined with MEMS sensors to provide information relating to activity levels, SpO2 levels provide an indication of a person’s general fitness levels that can be tracked over time.
Optimising health intervention opportunities
When patients are discharged from the hospital with a rehabilitation programme as part of a care package, rehabilitation is usually administered through visits from community services or through regular hospital outpatient visits. Technology allows patients to link up to a PC to enable progress tracking and data logging of their recovery.
A benefit of this involvement by the patient in the recovery plan is that rather than being a recipient of a process, the patient has a sense of involvement and collaboration with the healthcare providers, which has positive effects on recovery. Long-term condition monitoring provides medical professionals with the ability to review a patient’s progress and compliance to the treatment plan.
For real-time monitoring and alerting benefits, these devices also can be linked to the cloud via a third-party application. Multiple business models can exist—companies can choose to sell or lease the devices on their own, or provide access via the cloud. Once set up, selected family members, friends and medical professionals have the ability to track, monitor and communicate with the user. In many cases, it is this smart integration of sensor technology and its wide range of applications that can give users the confidence to go out and live their lives as normally as possible.
Such capabilities provide patients with an improved quality of life and peace of mind because they know that there will be a quick response should an emergency situation arise.
For example, MEMS technology combined with GPS and general packet radio service (GPRS) could be used to trigger a targeted response in epileptics experiencing a seizure. In the case of the sometimes confused Alzheimer’s sufferers, the reassurance that somebody will be alerted if they wander out of a predefined area could give them the confidence they need to continue to go out without worrying their families.
Improving quality of life
Microelectromechanical systems technology fits within more than one business model. In addition to high-value specialist medical instruments, the technology is showing up in an increasing number of over-the-counter healthcare devices.
Plentiful and low-powered, MEMS sensors combined with the newest low-power wireless solutions are enabling a revolution in the integration of smart monitoring, tracking and recording systems. The reassurance and life-affirming qualities of next-generation ambulatory medical devices have the potential to improve the lives of many. For this reason, MEMS sensor technology combined with the use of biosignals has never been a more exciting field with which to be involved.