Final Design

The final design includes a 113kHz resonant frequency piezoelectric crystal, a housing unit for the piezoelectric crystal with a built-in water reservoir, a pump paired with a venturi mechanism, and tubing 4 feet long to interface with the patient's trachea collar from a mounting point around the patient’s waist.

Piezoelectric Crystal and Driver

Because the essential humidity in humans’ trach is 30 - 40 mg/L , and the inspiration rate is 10-20 L/min, the humidification device must provide an atomization rate of 0.8 mL/min. A 113kHz resonant frequency piezo crystal with its corresponding PCB driver is utilized to achieve a tested atomization rate of 1.12mL/min.

It is crucial to control the atomization rate with the PCB driver but many commercial PCB drivers do not have this capability. Initially, a circuit on a breadboard with one 18V power supply was used to drive the 113kHz resonant frequency piezo crystal. However, the maximum atomization rate produced (0.42 mL/min) was a fraction of its capability. Because the 555 timer has a maximum input voltage of 18V, two power supplies (24V and 15V) was used to bypass this specification from the 555 timer. The driving frequency is controlled with a panel mount potentiometer to achieve the desired atomization rate. The maximum achievable atomization rate is 0.85 ml/min when driving the piezo crystal with the customized circuit. Because this atomization rate is not as high as when the piezo crystal is driven by its corresponding PCB and barely over the required rate, the PCB was implemented into the final design.

Piezo Atomizer Housing Unit and Reservoir

The housing unit secures the micro aperature piezo atomizer in a vertical alignment. It interfaces with a larger feedstock reservoir via a threaded interface. Water travels from a smaller reservoir built into the housing unit, down a shoot directly behind one side of the atomizer. This supplies the back of the crystal with a controlled amount of water. When an oscillating voltage is applied to the crystal at its 113kHz resonance frequency, standing waves are creating on the surface of the water, atomizing it (i.e. creating a fine mist).

Pumping Mechanism

To not rely on the piezo crystal to drive the flow rate of the humified water, a pump interfaces with the tubing to increase the flow rate of the humidified water through the tubing. This allows the humidified water to travel larger distances through the tube and after exiting the tube.

It can be seen that the difference of speed of the fluid with and without the pump in effect is drastic. This demonstrates both the efficacy of the pump and venturi setup as well as the need for it in the first place. Additionally from the “Flow Rate” visualization, when the pump exceeds 50% of its power, the cubic feet per minute of flow exiting the tube outlet exceeds that of a human breath rate. This is a solution that we have come up with so as to transfer the humidified air through a 4ft tube.

Final Design: Agglomeration of Components

The final design solution includes two configurations. The first configuration is a device that reflects the visualized goal of this project: a device that is enclosed and capable of being inserted within a fanny pack while a tracheostomy patient is mobile. This configuration includes a micro aperture piezoelectric atomizer which is driven by a SEEU PCB driving module. The piezo atomizing crystal is housed and fed feedstock water from a Connex 3D-printed housing reservoir. This 3D printed water reservoir using a Connex printer allows a controlled amount of water to come into contact with the bottom of the piezo crystal that has a resonant frequency of 113kHz. The piezo crystal humidifies this water when connected to a PCB that outputs a voltage square wave with frequency of 113kHz. The piezo crystal converts the electrical energy to mechanical energy which vibrates the disk within it to humidify the water into mist. To transport atomized water through upwards of 3 feet of ½” diameter vinyl tubing, an air pump is integrated into the device by creating an interface between the pump and the tubing. This pump is used in conjunction with a Makerbot 3D-printed venturiesque mechanism that utilizes a jetstream of air to produce a negative pressure gradient to pull mist away from the atomizer and propel it through the tubing. Additionally, the pump has an adjustable knob that enables the user of the device to adjust the air flow rate to ensure that the atomized water reaches the patient’s trach collar. Without the pump and venturi device, the atomizer could not sufficiently transport the mist through the tubing. Finally, all parts are enclosed inside a Makerbot 3D-printed unit with access to insert tubing and mount a larger feedstock water reservoir. The enclosure also includes a sliding access panel for assembly, disassembly, or maintenance.

The configuration of the customized circuit breadboard set up serves as proof of an adjustable atomization rate from a single piezoelectric crystal. The driving circuit that enables this adjustability was not integrated into the first configuration due to insufficient time to manufacture a PCB that could fit inside of an enclosed unit sufficiently small as to fit inside a fanny pack. This configuration includes a customized circuit breadboard with an integration of an adjustable potentiometer having low sensitivity that is built from all electrical components based on the circuit diagram in Figure 7. This circuit breadboard is powered by two power supplies of 15 & 24V, and the piezo crystal is enclosed by a Connex 3D-printed housing reservoir with a water reservoir that can be placed on top.

Results and Performance

A positive correlation between pump power and relative humidity can be seen. While the prototype did not reach the desired 90% relative humidity threshold, it showed a strong difference between the nebulizer being attached with no pump active vs 100% power. Further optimization of the crystal’s atomization rate and the pump’s flow rate has the capability to reach a humidification surpassing 90%

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