Alternative Designs

After the initial brainstorming period, several different concepts for sensors were developed. There were six original ideas discussed: acoustics, ultrasound, thermodynamics volume approach, optical sensor, infrared laser sensor, and a piezoelectric contact sensor. Once the initial design concepts were fully explored, issues of feasibility and examination of design criteria became the main factors narrowing the list of possibilities. This narrowed the choices and three designs remained for exploration: a piezoelectric contact sensor, an optical sensor, and a laser displacement sensor. Because of the complexity and bulk of the laser displacement sensor, the other two designs are more plausible.

Approach 1:

To determine end-systole, an optical sensor was used. This sensor relied on the optical properties of light as it passes through blood. When the pump is completely empty, light would pass completely through the blood sac as no blood would be present. Additionally, this sensor potentially had the capability to detect end-diastole. If the light source penetrate through the blood at full-empty, then the entire dynamic range of diaphragm displacement could be resolved.

The components of the sensor were be held flush to the PVAD surface by an acrylic housing and elastic strap combination that encompassed the whole device. This ensured that the light and sensor held constant placement and provided consistent readings unaffected by movement artifacts. In addition, because the components are external, the detector could be easily implemented and transported from one device to another. In terms of compliance with the original device, the sensor’s external property made it a high-quality option.

If the latter situation occured, there would be no need to attempt the second approach. However, the team investigated both simultaneously.

 

Approach 2:

In order to determine the end-diastole, a force sensing contact resistor was used. This sensor was placed inside the air side of the pump on the cap. When the diaphragm contacted this sensor, a voltage reading will indicate that the pump is full. The force that contacts the sensor, and therefore activates it, is actually a slight tensile force in the diaphragm as it pulls down on the sensor surface over a height difference of fractions of a millimeter.

To direct the force in the center of the FSR, a post was machined out of resin material and a useful force was applied to the active site on the sensor. The post consequently requires less contact with the diaphragm than if a post were not used. It has a domed top that was polished to prevent wear in the diaphragm. Additionally, using a post solved the issue of filling the gap between the inner surface of the PVAD housing and the fully drawn position of the diaphragm, simply by making it 2mm taller.

In the case that appraoch 1 did not resolve the entire dynamic range itself, the optical sensor would be used to detect at least end-systole. Then, the contact sensor from approach 2 would be used to detect the most important position, end-diastole. These two sensors would be used in conjunction to find the terminus points. The picture below shows the placement of these sensors on the VAD in this case.