Amid the global crisis caused by the corona virus pandemic, hospitals and healthcare facilities are reporting shortages of vital equipments. As makers it's our responsibility to combat the shortage by constructing makeshift-open-source substitute devices. Our country might be in a lock down but our ingenuity isn't !
One important device for which demand has ramped up is ventilators for patients who need assistance with their breathing due to the respiratory effects of COVID-19. Basically a ventilator is a machine that provides breathable air into and out of the lungs, to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. A DIY ventilator may not be efficient as that of a medical grade ventilator but it can act as a good substitute if it has control over the following key parameters
- Tidal volume: It's the volume of air delivered to the lungs with each breath by the ventilator - typically 500ml at rest.
- BPM(Breaths per minute): This is the set rate for delivering breaths.
- Inspiratory:Expiratory ratio (IE Ratio): refers to the ratio of inspiratory time:expiratory time.
- Flow rate: is the maximum flow at which a set tidal volume breath is delivered by the ventilator
- Peep (Positive end expiratory pressure): It is the pressure in the lungs above atmospheric pressure that exists at the end of expiration.
refer this link for more information on ventilation parameters https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6212352/
My design is based on the automation of the manual BVM (Ambu-bag),which you can find in any medical supply store. It is a hand-held device commonly used to provide positive pressure ventilation.
When the bag is squeezed, the air enters the lungs of the patient, while the nonreversible breathing valve prevents backfiring of the exhaled air. Then the AMBU bag self-dispenses by sucking air from the valve from its back side. Either ambient air can be used as "fuel", or an oxygen cylinder can be connected. In the latter case, it is possible to connect a tank to collect excess oxygen, which was not used by the patient.
Most of the DIY ventilator out there are based on ambu bags and a direct drive actuator. In my design, I've tried to simplify the actuator mechanism and I also made a better user interface for the unit.
The key element in my design is a linear actuator which is coupled with a lever mechanism that can compresses the ambu bag. A control panel is also provided for precisely controlling the ventilation parameters.
Use plywood or similar sturdy materials for making the housing and the lever mechanism. Make a cut under the lever arm so that the ambu bag can rest there without sliding. I am not suggesting you any particular hardware (nuts, bearings,..etc) because you may have a hard time in finding parts in this adverse situation. But make sure you follow the design. For those who own a CNC, I'll try to upload plans soon.Actuation Mechanism
In order to make a linear actuator, I used a bipolar stepper motor integrated with a lead screw(or a Nema 17+coupling) and coupled it with a traveling nut. As the shaft of the motor rotates the traveling nut translates along the screw so that linear motion is achieved.
In the end, control over ventilation parameters can be achieved by varying;
- No. of rotations (stroke) ----> Tidal volume
- Speed of rotation----->Flow rate
- Steps in clockwise : steps in anti-clockwise direction--->IE Ratio
- Frequency of direction change per minute---->BPM
I'm using an A4988 for controlling the stepper motor. The A4988 is a microstepping driver for controlling bipolar stepper motors which has built-in translator for easy operation. This means that we can control the stepper motor with just 2 pins from Arduino, ie one for controlling the direction of rotation and the other for controlling the steps.
For user interface I've made a control panel out of a 20X4 character display and 3 buttons (for up, down & ok functions). You could simply connect the display directly with arduino but I'd prefer using an I2c display adapter so that you can plug and play without a mess. 10K resistors are added for each individual buttons for pull down purpose.
Wire the components as shown in the diagram. Make sure that you had installed all the necessary libraries mentioned in code. Now, upload any of the sketches provided to your arduino. Use a 12v SMPS (3A min.) for powering the setup.
On running the test code, the motor executes cycles of clockwise& anti-clockwise rotations so that you can make sure that the actuator is running smoothly.
For testing the control panel and the device as a whole, upload the final_code.ino to the arduino. Now you can interface with the ventilator via control panel. By default, all the ventilation parameters are displayed on the screen. Use 'up'/'down' buttons to switch between parameters and press 'ok' button for selecting a parameter. Again use the up/down buttons to increase/decrease the value of the selected parameter. Finally press 'ok' to confirm the value.
Assemble the circuitry and the power supply unit inside the ventilator housing and make sure everything is fixed in place.
I had trouble finding resources for making the housing for mechanical parts. So I had to use what's available. But as you can see I had followed key features in my conceptual design while building this version and it works way better than I expected!
Feel free to ping me for any queries related to this project. Stay healthy, stay safe!