Commit 65eebcd5 authored by Zach Fredin's avatar Zach Fredin
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parent ae1d7226
# Negative Pressure Covid Box
This to-be-better-named project is an isolation box that prevents aerosols from spreading beyond the patient's immediate area. It allows caregivers to use currently-prohibited respiratory aides, such as CPAP, BiPAP, and nebulizers. This rough CAD render shows the general idea:
This to-be-better-named project is an isolation box that prevents aerosols from spreading beyond the patient's immediate area. It allows caregivers to use currently-prohibited respiratory aides, such as CPAP, BiPAP, and nebulizers. The first physical prototype uses a modified version of our [Covid Isolation Box](, now equipped with a fancy cupola (images include protective film for clarity; in use, the box is transparent):
The box covers the patient from the torso up so they can be covered while prone or sitting up. The circular port at the back is connected to a HEPA filter and a blower inlet, while the cutout at the front has a flexible skirt to provide a reasonable seal.
## prototype details
This prototype is intended to validate simulation results and represents the first complete iterative spiral around the negative pressure box concept. Importantly, this model will answer a fundamental question about real-world airflow requirements; our initial simulations and expert design consultations (below) revealed a range of airflow specifications that spanned two orders of magnitude (~3 CFM - ~240 CFM). The fan used in this model splits the difference, topping out at ~90 CFM with substantial speed adjustment capability.
## status
Early development/exploration.
The design differs dramatically from other concepts in a few ways:
- the box itself is an innovative folded and latched design, which is lightweight, easy to ship and store, low cost, and disposable.
- the fan and filter assembly are integrated with the box to save cost and space.
- sophisticated electronic controls are used to monitor and control the system.
The upper section that holds the fan and electronics is cut and creased on our Zund using off-cuts from the larger Covid Isolation Box enclosure, then snapped together as shown here:
A [Bosch 6055C HEPA cabin air filter]( is adhered using a generous silicone bead to the top of the Covid Isolation Box, which now has a matching rectangular cutout to accommodate the filter:
The cupola then fits neatly around the filter:
During operation, the vacuum created by the fan securely holds the cupula assembly in place. Optionally, this piece can be taped or glued in place. Note that the critical leak path which leads around the HEPA filter is carefully sealed using silicone; other ingress paths, such as the snap assembly holes, caused negligible losses in efficiency and do not increase risk.
The holes next to the fan cutout will support a circuit board which will include a 24 VDC power jack, a status LED, a differential pressure sensor with extension tube and snubber, a buck converter, a USB debug/logging port, an on/off switch, and a SAMD11 microcontroller:
The differential pressure sensor will measure the vacuum inside the cupola, using the extension tube and snubber to reduce turbulence effects from the fan. This measurement will be used to validate simulation results at different fan speeds, and to quantify the effects of different shroud configurations around the patient's torso and caregiver access port. If necessary, this circuitry could remain in place to provide an alert if airflow conditions are no longer sufficient to evacuate aerosols from the interior of the box; ideally, the eventual version will simplify this portion to a simple fixed speed circuit to reduce cost and complexity.
Next steps:
- finish electronics design, construction, and programming.
- update simulation with detailed box model, fan flow rate, and filter.
- smoke wand, anemometer, and differential pressure testing to validate simulation results.
- iterate as needed.
## need
As discussed in an [issue I posted]( in mid-April, covid-19 has dramatically curtailed the support toolkit available to respiratory specialists. To recap:
......@@ -19,17 +52,6 @@ In short, a system that makes BiPAP, CPAP, and nebulizers viable again has great
- less need for PPE during non-physical patient interactions
- improved caregiver safety due to reduced aerosol contamination risk
## plans
While it is tempting to simply bolt a blower onto our [Covid Isolation Box](, one must remember the ubiquitous warning sticker present on most fume hoods:
In order to scavenge hazardous vapors, fume hoods need laminar flow and adequate velocity; the aforementioned sticker reminds users that raising the sash above a marked point disrupts airflow enough to prevent the system from functioning properly. Similarly, we must understand the airflow in our system and design the exhaust mechanism accordingly if we want it to protect caregivers. As such, we are starting with simulation using Dassault's [Simulia platform](, building on their work simulating sneezing, coughing, and breathing for our mask and face shield design efforts. The simulation results will feed into [blower sizing and selection](, at which point we will build our first physical prototype.
![initial simulation setup](img/Screen_Shot_2020-04-21_at_9.58.33_AM.png)
Due to the inherent uncertainty in sealing around the perimeter of the box, it is likely that we will need to implement some kind of active blower control to maintain sufficient aerosol scavenging. Since the exhaust will pass through a safety-critical filter, we should also monitor filter performance online and alert caregivers when replacement is needed. In both cases, we will likely instrument the box with low-cost PCB-mounted differential pressure sensors such as [these models]( from Amphenol (~$30 each single-lot from Digi-Key and in stock). Again, simulation results should point us to the needed pressure differential range we need to maintain to produce good flow conditions, eliminating the need for airflow sensors.
## design consultation
Gordon Sharp (chairman of Aircuity / visual artist / fume hood design veteran) and Willie Baker, MD (BMC) provided many useful comments they permitted me to reproduce here:
......@@ -111,17 +133,6 @@ Riley Kolus from BC helped consolidate design requirements from his discussions
- Access+visibility from the back and both sides for central line placement.
- Access+visibility from the right side for surgical airway procedures (may require two-handed access).
## BOM thoughts
Fan: [Sanyo San Ace 9GA0824P1S611](, $21.30 qty 1:
![San Ace](img/9GA0824P1S611_curve.png)
Differential pressure sensors: [Amphenol NPA-730B-05WD](, $27.96 qty 1, 5 inH2O range, PCB mount/SMT, 3 mm hose barb ports, I2C output
Filter: [Bosch HEPA cabin filter model 6055C](, common (many Toyota/Lexus models 2006-present), unknown specs
## other efforts
Harvard's GSD is working on a [similar concept]( a disposable folded box and a negative pressure system to reduce aerosol risk.
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