During Fall 2019, the team had both a functioning Heart Rate Variability Monitor and a functioning Pulse Oximeter.
HRV Team: Advay Mahajan, David Bat, Jayce Schwartz
Pulse Oximetry Team: Sara Guillén Fernández-Micheltorena, Javier García Baroja, María Lancho Lavilla, Patrice Gill
For the Heart Variability Monitor, we had the following goals for the semester
- Our first step this semester is to make the ECG from scratch by reverse-engineering the Bitalino. Part of this includes purchasing longer leads for the ECG so we can use the principles of Einthoven’s Triangle to place the leads on body parts other than the chest.
- After this, we want to carry out the statistical test to determine the accuracy of the ECG. Planning this out includes determining sample size, determining what devices we are comparing it against, and determining the kind of statistical test we are going to use.
- After ensuring the accuracy of our device, we would like to turn the breadboard and Arduino into a printed circuit board to eliminate all unnecessary wires. At this point, we would communicate with the Pulse-Ox groups.
Currently, we had planned to spend most of the semester designing the device and building it. The plan was to used all the time up to Spring Break to design and build the device and to use the remaining five weeks to carry out the statistical test, create the PCB, and create the case. However, the semester ended early due to COVID-19 and we were unable to construct our full device because multiple team members had various parts of the device, and were thus, unable to meet to put those parts together. However, we have a detailed plan on how to we will construct the device come Fall 2020 when we are all back together again. Once we are able to build the device, we can run the statistical test, make a PCB, and make a case for the device. Then, we would combine it with the Pulse Oximeter and make the combined device.
The first component of the device is the connection from humans to the circuit in the form of electrodes and cables. We will be using 3 leads total- 2 unipolar(body-bone) leads and 1 bipolar(body-body) leads. Thus, only 3 electrodes are required. For the cables, we will be using 3 RCA cables. To connect the electrodes to the RCA cable, we need to attach 3 snap buttons to the inner conductors of the RCA cable. We then, need to attach the other end of the cable to a 3.5mm audio jack.
The second step involves constructing the actual circuit, which involves various filters and amplifiers. The signal goes through an instrumental amplifier which will amplify the signals we want, thus increasing the contrast between desired signals and noise. The second component the ECG signal goes through is a notch filter (60 Hz) which filters out noise at 60 Hz. The third component that the ECG signal will go through is the low pass filter who’s job it is to filter out noise above a certain frequency. The low pass filter will have a cutoff at 80 Hz since the individual waveforms that make the ECG is made up of frequencies around 1-50 Hz. The final two components of the device are both inverting amplifiers which should magnify any potentially small signals that we will need to analyze.
This is what the breadboard will look like after we are done assembling it.
The image on the right is what the device will look like once it has been put in a case. However, since our goal is to make the device into a PCB prior to making the case, the case will be a lot smaller. As you can see there is a black knob at the bottom of the device. The audio jack connects to this, which connects the cables to the rest of our device.
The Arduino will connect to the computer and will output analog ECG signals in our computer. We have working Matlab code which can analyze ECG signals, detect R waves, and compute the time between adjacent R waves. This calculates Heart Rate Variability for us. For more information, visit the HRV final presentation here.
For the Pulse-Oximeter, we had the following goals for the semester
- We are to continue the work of the previous semester. Once we have the device, we will check which parts are missing (some of the op-amps were personal property of old team members) and find an appropriate substitution for them. In December we were able to obtain signal from our pulse oximeter. Therefore, the first logical step is to get to that stage again, where we would be able to read a signal that provides information about oxygen concentration in blood. After that, we will need to find a way to accurately establish the relation between the oxygen levels and the breathing rate of the patient with some signal processing.
- Even though our current design for the pulse oximeter is functional, it is not ideal but just a rough prototype. For this reason, it should be made lighter and truly wearable overall in order for the patient to be at ease while using the device to a point where it should be barely noticeable while executing a task. This is of utmost importance given that the device is evaluating stress and the slightest discomfort may alter the results.
- The pulse oximeter will not provide enough information to assess the levels of stress of the patient. Ever since we began developing this project, the goal was to combine the HRV device with our pulse oximeter to acquire a reliable stress evaluation. Subsequently, by the end of the semester we should have been able to put both together, that is, the aim of the semester is obtaining a first prototype for a wearable stress sensor for the patients that does not impair their ability to use our Aphasia and Alzheimer’s apps.
This is what the current device looks like:
At the beginning of the semester, we, in theory, had a fully functional device. However, between last semester and this semester, some parts of the device belonged to past members of the team who had left. We had to analyze what components of the device needed to be replaced, find out where we can obtain those components, and reassemble the device accordingly. We found that both the operational amplifiers we used and the LEDs we used needed to be replaced. We obtained both these parts from the Hive. After, obtaining these parts, we reassembled the device and tested it. We got the following signal, which implies the device still works
For the second semester goal, we tried to make improvements to the design of the current device to make the device more comfortable and “wearable” and more accurate. To improve the accuracy of the device, we wanted to implement both red and green LEDs. However, we faced an issue; the LEDs didn’t fit into the current existing 3D model. We tried two solutions to bypass this issue.
Straight Solution: make the hole bigger in the existing device.
- Pros: Compact ear clip (no extra pieces)
- Cons: It implies printing a whole new model (material and time cost) without being able to check whether it works or not in advance.
To try: additional piece at first to make sure that the device works with both LEDs.
- To keep the LEDs inside the added piece, we made a cover with a hole for joining the leads to the rest of the circuit
This is a temporal solution and a model similar to the existing one but with a bigger hole will be printed after making sure that everything works
After modeling this component, we met up with the heart rate variability team to figure out how we could combine our two devices once they finished their device and once we have the opportunity to come back to campus to combine the device. Any additional details on the device can be found here.
As a team, we also wanted to plan how the two devices would fit together, but from a software point of view and hardware point of view. Thus, two teams were created: a hardware team and a software team. The hardware team would research how the device would fit on the body. The software team would research metrics used to evaluate stress, how a threshold for stress can be calculated, how HRV data and Pulse Oximetry data would be summated, and how other research groups used multiple metrics for stress to quantify it. We began a document with all our research here.