Capacitive sensors can be used to measure many different physical parameters that are important when monitoring the health of patients. In particular, the technical advances made by sensor manufacturers in using micro electro mechanical systems (MEMS), fabricated using silicon microchip manufacturing techniques, have opened up new possibilities for integration which improve the ease and adaptability of their use.
Capacitive sensors offer advantages over resistive or piezo-electric sensors because of their well-understood characteristics. In addition, they are free from the intrinsic noise found in all resistance (Johnson noise). The small changes in capacitance, however, place exacting requirements on the interface circuit.
MEMS accelerometers were among the first devices to be used extensively. They are manufactured using silicon lithography techniques of deposition, etching, and lithography. In principle, a capacitive sensor consists of two parallel electrodes. The following formula determines the total capacitance:
C=ε 0ε rA/d, where ε0 is the permittivity of free space, εr is the relative permittivity, A is the area of the electrodes, and d is the distance between them.
As a result, the capacitance changes with the distance between electrodes.
A common problem in the single electrode technique is that temperature will affect the result, along with other common mode effects. A much better approach is to use a differential input where a third electrode is used. Thus, any temperature effects will be cancelled out since both electrodes are affected in a similar way.
Accelerometer structures often consist of an array of movable fingers and a fixed mass. As the sensor accelerates, the cantilevers move and the capacitance between the fingers changes.
Typically, a change in capacitance of such a structure will be on the order of between 0.2pF/g to 1pF/g.
MEMS-based gyroscopes use the Coriolis Effect to determine angular velocity. The effect produces a coupling of energy between two vibration modes of a structure when it is rotated. The signal produced by the Coriolis Effect can be on the order of 4aF change in capacitance at the drive frequency, which is on the order of 10KHz. This type of readout requires advanced low-noise techniques such as synchronous demodulation.
MEMS Pressure Devices
Diaphragms are the simplest structure suitable for measurement of pressure and this structure has been used equally in MEMS and non-MEMS transducers.
One of the drawbacks of capacitive pressure sensors is their sensitivity to parasitic capacitance. This means that they should be positioned close to the interface IC. Also, they exhibit non-linearity and temperature dependence. Hence, the interface IC should contain linearization algorithms. e2v uses a high-performance, 32-bit processor core — the APS3 from CORTUS — within its designs. A 32- bit core is useful because of the high resolution (21bit) of the raw data.
Typical values for MEMs absolute pressure sensors are 8pF base with a change of less than 100fF/100 Pa.
MEMS Flow Measurement
Flow is an important parameter to measure for drug delivery and verification of respiratory function. The majority of flow sensors use resistive thermal techniques; however, flow can be calculated by measuring the differential pressure drop across an orifice or constriction.
True mass flow can use Coriolis Mass airflow sensors. These utilize a similar technique to gyroscope systems. This type of sensor is highly accurate and is currently used in operating theater drug delivery equipment. The parts are presently expensive but MEMS manufacture could bring costs down.
Chemical analysis can be performed using capacitive sensing techniques. The chemical is absorbed in the sensitive dielectric, causing it to expand, ultimately changing the structure's capacitance. Other techniques use a vibrating cantilever which changes mass as gas is absorbed onto it, changing the frequency of oscillation1. There are also other dielectric absorption principle techniques where the dielectric constant is changed and hence the capacitance.
There are many different methods of measuring capacitance. The measurement of the very small changes in capacitance of typical MEMs sensors requires high precision techniques. The capacitance to be measured can be part of an oscillator circuit, causing a change in frequency. However, for very precise measurements, there are temperature variation issues.
Charge amplifiers provide an output voltage proportional to the capacitance of the sensor. They have the advantage that the input is a virtual ground so stray capacitances do not cause a big error.
Synchronous demodulation can be used to detect very small changes in capacitance and is very often the technique used in gyroscopes and Coriolis Mass flow meters. The small changes are added to a larger modulation and the output is obtained by demodulating in phase with the original modulation signal.
Another approach is to use a sigma/ delta converter. Traditional sigma/delta converters use a varying input voltage and a fixed capacitance to produce a digital signal. They can be modified to switch a fixed reference voltage across a varying input capacitance, giving a digital representation of the capacitance value.
The sigma/delta approach removes the extra step in traditional capacitance to voltage to digital conversions and provides the inherent linearity and resolution of standard sigma/delta converters.
Accelerometers can be used to give a measure of inclination, for bed and chair positioning, by measuring the components of earth's gravity field. MEMs capacitive accelerometers produce only a small change in capacitance, which means that the readout device should have adequate low noise to be able to resolve angles to less than 1°. A resolution of 0.1° has been achieved with a MEMS accelerometer and one of e2v's readout components. In addition, the patient's bed can be fitted with accelerometers that detect signs of life – movement, heart rate, etc. to provide alarms in case of trouble.
Accelerometers can also be used to measure posture and gait. They have already been used to measure knee bend and other joint angles. In combination with other inertial sensors, they can be used to analyze movement and posture. Accelerometers have also been used to trigger muscle stimulation.
Pacemakers can now contain multiaxis accelerometers. They are used to measure the patient's position or motion, as well as heartbeat. The pacemaker's heart stimulations can be adjusted accordingly. Of course, many other measurements can be made by the pacemaker including the body's natural electrical stimulation, blood pH, blood temperature and also ventricular pressure — which may employ a capacitive sensor.
Gyroscopes or combination sensors (gyroscopes plus multi-axis accelerometers) can be used to measure motion and position of patients. Also, such sensors can be used to aide the guidance of surgical instruments as well as radiation therapy positioning. In one study, a miniature inertial sensor with wireless interface was implanted in a tumor to target the radiation with more accuracy2.
Pressure is measured in different forms in a medical situation. Blood pressure, respiratory pressure, inter-cranial pressure and airflow are important parameters. Accelerometers used in automatic blood pressure monitoring instruments enable more-accurate blood pressure measurement through cuff-positioning sensing. They are used to measure the angle or position of the arm. Invasive blood pressure sensors using MEMs capacitive sensors can also be fitted to catheters for direct measurement of blood pressure. Other interesting applications include a pressure-mat array that can detect very small changes in pressure, which can be used for early detection of breast cancer.
Coriolis Mass Flow meters can be very accurately used in the dispensing of drugs. Micro-needle MEMS structures and MEMS-based pumps can also be used for delivery of drugs.
Many of the implantable devices are new to the market or are at the research stage. Their use will soon become more apparent. In addition, non-invasive techniques will be improved and continual monitoring of parameters such as blood pressure, glucose, cholesterol and uric acid will be common.
In addition, "Lab on a Chip" applications will become available that will be able to provide DNA information using a number of different samples. Multisensor patches are already under development. These will contain an array of sensors targeted at a range of parameters and will integrate an RF link for wireless communication. Surgical guidance sensors have already been mentioned; in the future, invasive sensors will be implanted in surgical and neurological tools to report on a surgeon's progress and technique.
1. Zhao, Y.J.; Davidson, A.; Bain, J.; Li, S.Q.; Wang, Q; Lin, Q; A MEMS Viscometric Glucose Monitoring Device, Digest of Technical Papers, 13th International Conference on Transducers '05.
2. Bandala, M. and Joyce, M.J.; Wireless Inertial Sensor for Tumour Motion Tracking, Sensors and their Applications XIV, Liverpool 2007.