Medical equipment is expensive and not always accessible to a wide audience. In fact, in any biomedical measuring instrument there is nothing extraordinary – here you can find ADC, analog front-end with low noise op-amp, some controller. But the central part what makes it different from consumer electronics is safety. Because device is connected to your body through some sort of electrodes, there is always a risk of electric shock. So power supply has to be isolated if it is powered from mains. There might be other additional safety circuitry to prevent possible hazard. And of course there are many safety regulations and certificates to overcome before device can be used with patients. This is where main cost comes from.
It doesn’t mean that you cannot experiment with your body signals. You only need to know what are you doing. If you are building your circuit to acquire bio-signals, be sure to use a proper isolated power supply, even better – go with battery power. Once said, let’s look at great development of Arduino shield to be used for acquiring various bio-signals from your body.
The biomed shield is developed by Orlando Hoilett – a graduate student at Purdue University. IT packs a bunch of measuring capabilities allowing to monitor hearth rate, temperature, breathing rate, body impedance. It seems that the project is still in progress before fully complete, but it already shows promising results.
First of its features is heart rate, monitor. IT uses photoplethysmography technique to acquire blood flow pulses from a finger. The red and infrared light is passed through (or reflected) the tissue, and then photo detector in other side measures the amount of it. Depending on blood volume in tissue the amount of detected light varies. There is a specialized chip MAX30101 used which already comes with three internal LEDs and photodetector. MAX30101 works with reflected light.
The only problem, that detector works with 1.8V signals, this is why biomed shield has level shift circuit included.
Going further, MAX30101 is capable of detecting blood oxygenation level. This is achieved by a calculating ratio between absorbed IR RED and red LED. This feature is still being implemented in the software library.
The other notable feature is biopotential measurement capability. There is an instrumental amplifier used which is high pass filtered. Several types of biopotentials can be measured including Electrocardiogram (ECG), Electromyogram (EMG) and Electrooculogram (EOG).
The input circuit is straightforward and not very noise resistant, so don’t expect great results. To see proper signals, you may need to do some post-processing filtering. But this should be enough for tinkering tasks.
Bioimpedance measurement is another interesting signal to acquire. Bioimpedance carries lots of interesting and vital information like body fat amount, sympathetic nervous response. Bioimpedance is measured by producing constant current (1mA) with an AD5934 which is converted from constant voltage mode. The chip measures the resistance of the tissue. You can find this feature in modern bathroom scales. This feature on biomed shield also seems to be still in progress.
Body temperature measurement
There are many ways of measuring body temperature and even more possible sensors. In biomed shield, there are a non-contact infrared sensor MLX90614 used. It is convenient because it already has a signal conditioning circuit integrated into the sensor itself. It already has low noise amplifier, 17-bit ADC and DSP what allows achieving high accuracy. It has an I2C interface which is convenient for use with Arduino.
Other biomed shield features
Among biodata acquisition properties shield comes with CdS ambient light sensor. It can be assigned to health conditioning features as it may be used to measure environmental conditions like sunlight, which is an essential factor for well being. There are also plans to add a UV sensor which would help to prevent sunburns.
Another ambient factor is temperature. There is a simple thermistor used to measure temperature. It doesn’t produce high accuracy but is enough for general purpose monitoring.
Overall as Arduino shield, this device looks promising for many hobby level tasks. You cannot expect more from such an all-in-one device. If you decide to build it, be sure to check for the latest version in GitHub.
Keep in mind that this and other DIY devices cannot be used to replace medical equipment used in the clinic. It is a strictly educational tool that must be used cautiously and must be battery powered!