Week 3

Week 3

We finished developing a basic theoretical analysis on the shelf life of the VacuStor tube, and have started an experiment with the rise time of the tube. We first had to fabricate an experimental setup for this experiment, and now we have begun taking experimental data regarding the rise time, and we do so each day to monitor the internal pressure of the tube.

Theoretical Analysis

The shelf life of the tube is determined based on how long it can retain the vacuum. It is extremely difficult to create an impermeable tube and seal, especially one that can be mass manufactured. Hence, we want to be able to limit the permeability of gas to the best of our ability. There are two factors in determining the shelf life of the tube. One, which was already looked into, is the threshold or critical pressure. Since we know for sure the pressure in the tube will rise, it is a matter of not only what pressure that, once surpassed, makes the tube unusable, but also how quickly it reaches this pressure.

The diffusion of gas is described by Fick's First Law, and we used this equation to determine the mass transfer flux rate, which is the flux density times the surface area of the tube, or:

J is the mass transfer flux rate, which is the total rate of diffusion of gas across the entire tube. j is the flux density from Fick's First Law. A is the surface area, P is the permeability, p_out­ and p_in­ are the pressures outside and inside the tube respectively, A is the surface area of the tube, and δ is the thickness of the tube.This mass transfer flux rate is also the leaking rate, which we want to minimize. Hence, in order to do so, we want to have a tube with a small surface area and a large thickness with a small permeability constant, which is determined by the number of holes in the polymer structure of the tube. 

The models are helpful in understanding what occurs during the experiment and what variables to focus on most when attempting to test the tube, which is what we started doing this week.

Rise Time

The rise time is the time the tube takes to equilibriate from the low pressure vacuum to standard pressure. To test this quantitatively, we created the following setup:

Rise Time Experimental Setup

 The vacuum gauge was sealed to the VacuStor with an epoxy, as shown:

Vacuum Gauge and VacuStor Tube

This allows the vacuum gauge to monitor the pressure inside the tube. Then, having evacuated the tube, we added a leakage port so that the surrounding pressure would be equal to atmospheric pressure. We then monitor the pressure of the inside of the tube daily so that we can determine the rise time of the tube. This will obviously take multiple weeks until results can be analyzed, but hopefully this experiment will provide useful results.

Week 2

Week 2

For those of you who would like to see in depth the scope of this project, I have uploaded my proposal.

A vital step before beginning experimentation is to develop a theory behind the concepts that you wish to experiment on. This is crucial because it narrows the huge number of variables to a select few that will impact the outcome the most. For this project, the most crucial factors are assessing the vacuum needed for proper sample transfer, the shelf life and permeability of gas, or the leaking rate. These were modeled mathematically with certain assumptions. So far we have developed a theoretical analysis for determining the vacuum threshold for which there will be proper sample transfer of blood from finger to the VacuStor tube.

Theoretical Analysis

Figure 1

Figure 1 depicts the evacuated VacuStor tube with the reagent inside the tube. There is 200 microliters of reagent prior to blood collection. After blood collection, 20 microliters of blood are added to the 200 microliters of reagent in the tube. We have to factor in the occupied space by the reagent and the reagent and blood when determining the vacuum required for sample transfer. Hence, we used different models to develop an equality to determine the critical pressure to ensure sample transfer. This equation is:

where Pa,c is the critical pressure, above which the sample will not transfer. Vr, Vs, and Vt are the volume of reagent, sample, and the tube itself respectively. Pout is the outside pressure, Pc is the capillary pressure to be overcome for blood draw, and Pw is the water vapor pressure.

Table 1 tabulates these values for a glass tube and a plastic tube and shows the critical pressures at 10,000 ft and at sea level.

Table 1

Once again, these are just models to narrow our experimental focus. We need to confirm these theories through our experiments which we hope to start soon. We hope to develop a model for the permeability of gas as well, which can affect the shelf life of the tube. In addition, we are going to start testing the rise time, or the time it takes for the vacuum within the tube to equilibrate with the outside pressure.

Welcome to My Blog!

Week 1

This week I had developed some background for my project, which is to develop a device with an integrated vacutainer for blood collection. The vacutainer, known as a Vacustor, is like a miniature version of the commercially available vacutainer. The Vacustor is supposedly designed to store around 20 microliters of blood and have a shelf life of around a year. However, before divulging into how we will go about doing this, I want to share some background regarding the concept of vaccutainers. In addition, I have provided a schematic of the device we are attempting to fabricate.

Figure 1: Schematic of Concept Device

Background:

The introduction of evacuated blood collection systems have provided greater safety, while offering ease-of-use, speed, and accuracy in blood-to-additive ratios. Advancements in blood collection has accumulated in the current commercially available vacutainer. Vacutainers rely on a capillary tube so that the vacuum within the cylindrical chamber is maintained. That way, liquid can be inserted into the chamber without disrupting the established vacuum. However, investigations into different factors that influence the performance of evacuated tube performance, including factors such as tube material, additive stability, and environmental conditions impact the expiration dates of certain tubes. We need to consider these factors when developing our tube as well. In addition, we need to consider permeability and evaporation of liquid from tube, which is a major issue considering the small sample volume.

This week we considered the ends of the tube and its effects on the capillary action of the tube. We tested an angled cut versus a straight cut on how much liquid could be dispensed solely by gravity. We want to reduce the dead volume of liquid (liquid stuck in the tube), especially since when the tube is integrated, it will not be easily accessible. Using a liquid that mimics blood, we tested the capillary of the tube and how easily the blood would flow vertically, and how much liquid would come out of the tube without agitation. This was done by using the mass of the tube with the liquid before and after dispense, and using a density approximation of 1 g/mL to determine the volume of liquid dispensed. The data showed that straight cut is more consistent in dispensing around 30 microliters of liquid as compared to the angled cut.

Next week, we hope to test the evaporative behavior of the liquid in the tube and assemble the valves we will be using in our device.