Powered Air Purifying Respirator (PAPR)

Hospitals are urgently needing respiratory protection for medical staff. Within the Seattle area, masks are being rationed in hospitals. Providence hospital has asked the community to help sew masks for medical use from a kit provided by the hospital. As supplies continue to diminish and supply chains intermittent, medical workers will take on greater risk to meet the developing medical emergency.

Powered air purifying respirators (PAPR) are an alternative to masks. They are usually worn on a belt or pack and are a powered unit that pulls air through the filter media delivering filtered air into the hood. They are disinfected between use and can be used for many times before the filter needs to be changed. With increased availiability of PAPRs, hospitals and medical staff can mitigate the risk of supply chain interuptions in N95 masks.

Requirements

Based on this CDC document and other sources, the function requirements of our PAPR design are as follows:

  • Delivered the required amount of airflow to the user as measured at the level of their face.
    • 170 liters per minute or 6 CFM for loose fitting hoods.
  • Have a rated battery capacity to run the unit for at least 4 hours with the target rating of above 8 hours.
  • Give both an visual and audible alert to the user when the battery is low.
  • Be able to change batteries without interrupting the operation of the PAPR.
  • Can be easily wiped down and cleaned with disinfectant wipes.

In addition to these functional requirements, we have some operational requirements due to the current situation. Supply chains are still unreliable and restock times are longer than usual so we want to utilize commonly available parts, preferably immediately available parts. This is applicable to all components including the fan, filter, tubing, electronic components, and battery.

Components

Flow Generator

We are utilizing the readily available 120mm or 140mm computer fans as the flow generators. We are targeting the 12V fans and looking to determine which fans have the right power requirements to generate the requisite amount of airflow. This will vary depending on the resistance and surface area of the filter media attached. These fans are made for continuous use.

These fans are available in both 2 and 3 or 4 pin connectors. Two pin connectors are either on or off, operating it their full specified current. 3 and 4 pin connectors allow the use of a PWM signal to attenuate the RPM of the fan. These fans will operate at 100% if there is no PWM signal or if it is at 100% duty cycle.Power Supply

Filter Media

The filter media we are proposing to use is either the Honeywell HPA100 HEPA filters which are already in a small form factor or MERV 16 filters which will have to be repackaged in a smaller form factor. During repackaging it is important to ensure that there is no leakage around the edges of the filter, generally glue is used to adhere the filter to the cardboard enclosure.

The filter will provide resistance to the pressure being provided by the fan. Based on the NIH paper, MERV 16 filters provide less resistance and perform better with time and dust. MERV filters provide higher filtration but at the cost of higher resistance and degraded performance with particle buildup. This is the technical specifications for Lennox air filters.

We have not yet found a specification for the hose outlet for existing PAPR systems and expect it to be in the 1-2 inch range.

Mechanical Design

In a simplistic view, the mechanical parts of the PAPR consisits of three main components. The filter which stops particulate matter, the fan which pulls air through the filter and pushes it through the hose, and the hose which transfers the clean air to the headpiece. These are also the three parts which are the chief contributors on determining whether the unit can deliver the target airflow.

Package Design

The prototype will be a simple acrylic box. One side will hold the filter media with a fan holder behind it. There will be an outlet port on an opposing or adjacent side. There must be a place where the batteries either are held or can be attached to the unit. Finally, the entire unit must attach to a belt or similar apparatus.

Electrical Design

Battery Cells & Voltage Regulation

For power, we are using the standard 18650 lithium batteries. These can hold around 2000mah at 3.7V. The full range of voltages of the cell as it discharges is 4.2V to 2.8V. To stabilize this we are utilizing a buck boost converter to regulate the voltage to 12V. It is also possible to utilize LiPo battery packs.

Low Battery Alarm

For lithium cells it is important to ensure that the cells don’t get drained beyond their nominal voltage as that may decrease the lifespan of the cell. To do this and to give a low battery warning we are using a 8s low voltage alarm. These attach to the battery pack via a 8s balance cable which measures the voltage drop across each individual cell in the battery pack. Balance cables are to be wired as follows. The full voltage is drawn from power cables connected to the two end batteries.

Continuous Operation

We are using a three way switch (ON OFF ON) with two battery connectors in order to allow rapid switching of the active battery in order to have continuous operation. We could add a capacitor to even further reduce this gap. This switch is only attached to the primary pair of power cables for the battery pack and not the balance cable which will then have to be switched separately for low voltage warning.

Controls

If we want to create variable power utilization by the fans for different operating conditions we can use a circuit based on the 555IC to convert a potentiometer into a PWM signal to modulate the fan power. Hospitals with existing PAPRs should have a flow meter to determine if the flow generated meets the minimum requirements for usage. Otherwise the fans will work at their full operating current.

An alternative here is to connect a potentiometer directly to the circuit itself for simplicity. The fan speed is not directly proportional to voltage. This may generate heat, but since the voltage is low it may be okay also if the potentiometer is put inside the wind chamber it will be air cooled. We need to calculate the power lost through the potentiometer though.

If we are looking for a 4V drop over on a fan with 0.5A current, we would need 8 ohm resistance. This 10 ohm potentiometer is a candidate.

An alternative is to use a microcontroller. The schematic is shown below. It uses a 6 pin ATTINY 10 or 8 pin ATTINY 10 (10-12mA). It can deliver 40mA which can directly drive this 5V active buzzer at 32mA. This would detect overall low voltage (not by individual cell) as well as regulate voltage. We may want to move to a 8+ pin ATTiny platform in order to be able to give LED visual indicators of function as well.

Regulatory

With the COVID-19 pandemic, FDA has an Emergency Use Authorization (EUA) for NIOSH approved face masks, thus we would not require a 510k for the time period of the EUA if the device is NIOSH approved. See here for more information on how to obtain NIOSH approval.

FDA has provided an updated page on EUA for the COVID-19 pandemic with a section on personal protective devices (PPE).

Supply Chain & Manufacturing

Demand & Delivery

Appendix: Contributors

The research and development of this project could not have been possible without all of the contributors and under the organization of the 1 Million Ventilators project. Thanks to Help with COVID-19 for bringing all of us together.

  • Antalz
  • Paramonkreel
  • Yukon

Appendix: Further reading