DoF Analog Ventilator

Last crawled date: 1 year, 10 months ago

This is a concept for an analog ventilator to help address the urgent worldwide need for full-featured ventilators to fight the COVID-19 crisis. Patients in disparate locations and circumstances need similar levels of care, so this design attempts to decouple its essential functions from resources which may vary significantly between countries. Building on the Manley ventilator design from the 1950's, this design allows control of breathing rate, tidal volume, inspiratory pressure, maximum pressure, minimum pressure, and oxygen/air mix ratio without electricity or computer control.

Please note that this is purely a concept. This CAD does not represent a ventilator which can be used for a patient.

The intent for this design is to create a full-featured ventilator which can be successfully made and operated in a wide variety of environments with minimal compromises in functionality. This design attempts to avoid the assumption of extensive access to, or familiarity with, computer controls, 3D printing, programming, or other engineering disciplines. For example, this design allows production of critical components using 2D fabrication. The main structure, check valve assembly, bellows plates, and bellows can all be fabricated from sheet material, and often the same sheet. The seals and flaps of the check valve assembly are formed from single sheet of flexible material. This design does not preclude addition of electronics for telemetry, alarms, fault management, or patient-initiated cycling, but these aren't a predecessor to its successful operation.

Operating Sequence, starting with bellows fully extended and panel tilted upwards:
• When top panel is fully extended, slider on trigger bracket tidal volume gap contacts directional control valve toggle, pushing it up (stroke can be adjusted by moving and re-tightening slider on trigger bracket).
• Toggled directional control valve directs pressurized air away from air cylinder and to the PEEP cutoff valve chamber.
• Compressed air pushes PEEP valve cutoff flap against top of valve chamber, cutting off air flow from exhalation line to PEEP water volume. Meanwhile, ambient air flows through the needle valve into the air cylinder, with the retraction rate controlled by the degree of flow restriction through the needle valve.
• Panel falls under the weight of the assembly with rate controlled by ambient air flowing out of the cylinder and through the needle valve.
• Bellows are pressurized by the force applied by a fixed mass whose mechanical advantage is varied by moving the mass on top of the panel. Mix ratio of air and oxygen is set by the ratio of cross-sectional areas of air and oxygen bellows. Inspiration pressure can be verified by observing degree of movement of the outlet water column. Barotrauma is avoided by setting the total water column height in the outlet water column.
• Air and oxygen outlet check valves are pushed open by air exiting bellows. Inlet check valves are also pushed closed.
• Pressure, mix ratio, rate, and volume-regulated air is delivered to the patient.
• When top panel falls all the way down, top of edge of trigger bracket gap contacts directional control valve toggle, pushing it down.
• Toggled directional control valve directs pressurized away from PEEP cutoff valve chamber and into air cylinder.
• Compressed air flows to the air cylinder through the needle valve which controls extension speed through air flow restriction. The flap inside the PEEP cutoff valve chamber is pressed own by exhalation line pressure, opening the air channel to the PEEP water column.
• Bellows expand as panel is pushed up by air cylinder, pulling in both oxygen and air through check valves in the laminated valve assembly. On the exhalation line, air is permitted to flow through the PEEP close-off chamber and into the PEEP water column where it is vented at a controlled pressure.
• Pressure-regulated air is released from the patient. Air and oxygen bellows are refilled.
• Cycling rate is controlled by tuning flow in needle valve.
Benefits of the design include:
• Mechanism is driven by compressed air with common shop air pressures. Note that compressed air is only used to drive the mechanism. The unit draws ambient air for breathing from a separate shared line.
• Multiple ventilators can be run from shared manifolds of oxygen, air (for breathing), and compressed air. This can be done without compromising each unit's ability to individually control its volumes, rates, mix ratios, and pressures.
• The analog nature of device may help make it easier to verify functionality and settings at a glance. This also may make it more intuitive to people without extensive training or experience with computer controlled medical equipment.

Enhancements over the Manley ventilator include
• Using a-2D fabricated Designed Offset Joint to create deterministic 3D motion with a simpler structure.
• Use of folded bellows to allow creation of highly elastic, geometrically deterministic volumes using largely inelastic materials. This relates some of the constraints on bellows forming and material properties.
• Use of two bellows to control mix ratio rather than valve flow rate tuning to control mix ratio.
• Use of 2D stacked plates and a single sheet of flexible material with cuts and holes to create a valve assembly.
• Compressed air is toggled between the extension cylinder and PEEP closeoff valve, allowing breathing pressure to vary during inhale and exhale steps.

Some areas for improvement and forward work:
• Add locations for easier integration of electronics for telemetry, fault management, alarms, and patient-initiated breathing.
• PEEP cutoff valve should be closer to patient, reducing the amount of exhaled air which could be re-inhaled during next cycle
• Valve assembly needs screws for clamping shut. Plates likely have to be stiff to sufficiently distribute preload, or additional compliant material needs to be added to accommodate displacement of valve plates
• Origami bellows do not rigid fold, so some degree of compliance is necessary for folding behavior. It's likely possible to tune fold pattern such that it extends in an arc rather than linearly.
• If the PEEP cutoff valve seal isn't sufficiently tight under pressure, the compressed air may enter the breathing lines, pouring out the pressure relief bottle and potentially preventing continued operation of the unit. Pressure could be reduced to avoid this, but that also reduces pressure to the cylinder which could prevent it from cycling the bellows properly.

This was designed in Fusion 360 and the uploaded model is v118.
v101 eliminated two valve traces in the top plate of the valve assembly in v98.
v118 corrected label for directional control valve inhale/exhale assignment.

Thank you to the PEEP valve designer, Smart Water Bottle designer, and weight designer whose models I used in this design.