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Microfluidic Device for In-Vitro Studies
Spring 2020 MAE 156B Sponsored Project
University of California, San Diego
The executive summary can be found here and our Final Week's Presentation can be found here.
Background:
Children with congenital heart disease have or can potentially develop pulmonary hypertension which is a form of high blood pressure that puts excessive strain on their pulmonary arteries. These are the arteries that carry blood from the heart to the lungs to become oxygenated. Professor Juan Lasheras and his team in conjunction with researchers at the University of California, San Francisco are conducting investigations on how blood flow changes affect endothelial cells (ECs), the first layer of cells, Figure 1. Researchers have developed animal models to study these cells under different levels of pressure and wall shear stress. However, these models are very limited and don’t allow to fully study the effects of the different loads independently. For this reason, Professor Lasheras' group created a microfluidic device to study the effects of pressure, shear stresses and strain on ECs cultured in vitro.
Originally, we were tasked with designing a device that can accommodate ~10 channels in which conditions can be varied between channels. The final product should be able to create pressures and wall shear stresses across the entire range of interest and drastically increase the efficiency of tests being performed. However, due to the COVID-19 pandemic, this objective had to be slightly altered. The 10 channel device requirement was abandoned in favor of a 4 channel prototype that was fabricated by the engineering team prior to the pandemic. In addition, all tests can only be done using a centrifugal pump instead of the piston pump mentioned before. Besides this change, our task largely remained the same.
To aid in their research efforts, we have been tasked with updating the design of the current device being used to recreate conditions of pulmonary hypertension on endothelial cells. The current device features a one channel design that is the bottleneck in their research as tests are performed over 24 hour periods and with a single channel, only one test can be performed at a time. This device can be seen in figure 2 to the left.
Figure 1: Shows the Mechanical Forces applied to the ECs
The device contains two separated ECs monolayers cultured on two cover slips, one containing a control culture which is simultaneously subjected to the same flow and pressure conditions as the specific cell culture to be studied in the second cover slip. The device uses a ViVitro Pulsatile SuperPump, a digitally controlled hydraulic piston pump that creates physiological cardiac flows and allows to input any desired flow waveform as well as varying the systolic and diastolic pressures. A second version has been also created where the cover slips are replaced by stretchable membranes on which the ECs monolayer will be cultured. This second device allows for independently varying flow shear, pressure, and stretch.
Objective:
Figure 2: Current device that can vary flow shear,
pressure, and stretch but can only do one test at a time
Final Design (**for more information, please visit our Final Design Page):
The final design of the multiple channel system includes a deformation valve. The deformation valve is a 3D printed valve utilizing a servo motor with a cam and follower design electronically controlled with a potentiometer in order to minimize pressure drop as much as possible. In order achieve a distribution of pressures within a range of 50-70mmHg between the first and last channel, the cam and follower design allows for the deformation of super-soft tubing to vary the pressures within the channels. The 3D model of this design is shown in figure 3 and 4 below as well as the operation in video 1. The valve uses a 270o servo motor, snail cam, and follower design allows for the deformation of the tube up to 5mm. This design and functionality of the snail cam can be seen in figure 5 below.
Figure 3: Valve Design with a 270o servo motor
The servo motor drives the snail cam that has a 2mm change in radius from 0o to 270O. This cam drives the pusher deforming the tube.
Figure 5: Cam profile
Figure 4: Valve Design Side View showing flow
Video 1: Video of valve operation
This design successfully gives a minimal pressure drop when the valve is open (i.e. the tube is undeformed) while providing the user the ability to fine tune the pressure drop within each channel. In addition, this valve also provides the ability to create more pressure drop if the flow rate were to change. This valve is connected to an electronics box that reads the pressures within the channels and grants the user the ability to electronically vary the pressures of the channels using a knob while the device is inside the incubator. A temporary setup is shown in figure 6 below. Valve operation changing the pressure can be seen in video 2 below. Due to use of a centrifugal pump in the video, the pressure change is very minimal since the deformation also slows down the flow rate. This is not going to be a problem when the device is finally used with the piston pump.
Figure 6: Temporary setup of the electronics box
Video 2: Video of the valve changing pressure on the device
The valve is controlled by a potentiometer which gives the user the ability to deform the tube up to 5mm starting from 3mm. Deforming the tube increases resistance in the system which in turn increases the pressure drop.
With the valve design made, a complete setup was created. As shown in figure 7 below, each inlet and outlet was fitted with the 3D printed valve with an exception of the first inlet, which is connected to the pump, and the last outlet, which is connected to the reservoir. This final assembled device is connected back to the electronics box. A picture of the physical setup can be seen in figure 8 below. In addition, operation of the new setup can be seen in video 3 below.
Video 3: Video of the operation of the entire device
Each valve is controlled with its own individual potentiometer
Figure 7: Schematic of assembled device
Figure 8: Picture of the assembled device
This device can not be used as is since cells are involved. Thus this device will need to be placed into an incubator while the circuit box and LCD is placed in a user friendly position. However, due to the COVID-19 pandemic, we do not have access to the lab in order to make a physical setup. Thus, we made a schematic of the planned final system which can be seen in figure 9 below. Due to the cells being sensitive to changes in temperature, tubing outside of the incubator must be insulated. In addition, a valve will be placed after the device in order to have even better control of the pressure drop. The reservoir is placed inside the incubator both in order to maintain the temperature of the fluid as well as replenish the CO2 supply required by the cells to survive.
Figure 9: Schematic of the final system layout
Tubing outside the device is insulated for temperature control, electronics box is placed in a
user friendly position, a valve is placed after the device for better pressure control, and a
reservoir is placed inside the incubator for temperature control and CO2 supply.
To demonstrate these valves in action, the team was able to control the pressure in each channels in a demonstration akin to experiments that the sponsor would perform using the device. The device with a master valve to control the pressure in channel 1 is connected to a centrifugal pump. As seen in figure 10 below, the first channel was able to maintain pressure of around 50 mmHg with even pressure drops in between channels. However, to realize independent pressure control, a pulsatile pump must be used in lieu of a centrifgual pump .
Figure 10: Even Pressure Drops using Valve Control
