Embedded Files
Project Objective
Quantify cartilage permeability and thus tissue health via pressure drop measurement
This project was supported by the Abrams Scholar Award 2021-2022 & 2022-2023 as a research project under the Diekman Lab through the Department of Biomedical Engineering.
Current Device Setup
Full system involving control electronics + GUI, motorized sample holder and syringe pump
Motorized Sample Holder in sealed position
Project Background
Cartilage acts like a sponge, releasing liquid when compressed and then reabsorbing. Certain compounds can increase the tissue's resistance to flow, and the research lab wants to quantify this impact. This hydraulic permeability can be calculated by measuring the pressure drop across the tissue, provided the flow rate and cross-sectional area are known.
V1 Device Overview
V2 Device Overview *In Progress*
Outline of system components
Design Inputs
Watertight
Hold Cartilage Sample
Interface with pressure sensors
Data Logging
Graphical User Interface (GUI) for real-time data visualization
Version 1
Sample Holder
For a watertight seal, I chose to use layered acrylic and rubber which was laser cut precisely to fit around brass fittings, all held together with four long bolts through the entire middle section, and four smaller bolts around each side to fasten the acrylic to the brass. Laser-cutting the acrylic to fit exactly was a difficult trial-and-error process that took me several weeks.
Center section CAD
Partially assembled
Assembled holder
Electronics: Hardware
I chose to use the BeagleBone Black microcontroller for this project over Arduino due to the need to create and display a GUI. RaspberryPi was another alternative but the BeagleBone offered more GPIO pins and greater affordability. I quickly learned how to diagnose hardware issues with this setup which helped in identifying that one of the pressure sensors was nonfunctional.
BeagleBone Black (BBB) Board
Wiring Diagram
Electronics: Software
Having never used Linux before, I quickly had to learn to navigate via command line to get the board up and running. I dove into Python and leveraged the PyQt5 library to make a custom GUI with live numerical and graphical visualization of the pressure sensor data along with the ability to log data for analysis in Excel. Building the GUI and getting the sensor data to graphically update were hurdles that took several months to overcome.
VSCode Python script screenshots
GUI displayed on LCD with live pressure display
Version 2
Though V1 works as intended, getting a cartilage sample in and out of the holder proved to be time-consuming for practical use, so I set out to radically improve and upgrade the system.
Redesign Goals
Easy sample replacement
Reproducible clamping force via motorized assembly
Full system control via GUI (including syringe pump)
Easy system setup
Soldered connections
More sensitive pressure sensors
Early redesign sketches
Sample Holder
I first sought to replace the center acrylic with 3D prints to reduce the number of layers needed to hold the sample. However, the acrylic-gasket-3D print interface kept leaking during testing, in addition to the center of the apparatus. I pivoted to heat set inserts to fix this issue and replaced the brass splitters with push-connect fixtures which took several hours of part identification and learning about tubing measurement systems.
Different internal rail + brass fitting designs
External rails + heat set inserts CAD design
Different assemblies and testing setups
After months of different configurations and prints, reverting back to acrylic proved to be the most reliable solution. A combination of acrylic cement, heat-set inserts, and a drill press were needed to fit the parts together.
Close-up of working quick-connect holder
Temporarily placed parts before bolting down
Electronics: Hardware
The Beaglebone Black unexpectedly died, so a quick switch to Raspberry Pi was needed. However, the Pi lacks analog pins so an analog-to-digital (ADC) converter was needed. Additionally, I added an RS232 to TTL board to interface with the syringe pump over a serial connection.
Updated Wiring Diagram
Components in laser-cut acrylic box
After hours of carefully taking measurements and countersinking holes, along with the addition of some custom 3D printed angle brackets for additional strength, the device was fully functional and controllable through the GUI.
Challenges
Problem: The brass fittings lacked a way to connect to the sample
Solution: Laser-cut many iterations of acrylic to get a water-tight seal around the brass but still have a spot to seat the sample
Problem: The pressure sensors were giving irregular results
Solution: Testing with the multimeter revealed that one sensor was broken and I had to order another
Problem: The BeagleBoneBlack Board errored when trying to use several critical packages
Solution: Many hours were spent debugging but only a full reset and clean image installation of the newest Linux version fixed the issue, which took weeks to get around
Current Problem: The 3D-printed sample holder continually leaks water
Solution: Implemented acrylic + gasket layers in combination with heated inserts and acrylic cement to meet the thickness requirements of the heat-set inserts
Project In Progress!
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