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Over the summer of 2022, I worked for the University of Delaware's biomechanics research department to refurbish a custom microindenter device. The device in question was designed to measure the creep properties of cartilage tissue by applying a constant load over the duration of the test, which requires the indenter tip to "chase" the tissue as the tissue deforms. The device used by our lab had two sensors: a load cell which measured the force applied by the indenter, and an actuator which moved the indenter tip and measured displacement. Both of these devices were of high precision, measuring narrow ranges on scales of gram-force and microns.
Figure 1. (Left) A biomechanical creep test similar (but not identical) to those conducted by our lab's microindenter. A small cylindrical cartilage sample is compressed by an indenter tip. This particular device uses a button load cell, whose signal wire can be seen leading off to the right. (Right) An enlarged view of the contact between the indenter tip and the cartilage sample.
Figure adapted from:
Herzen, Julia & Karampinos, Dimitrios & Foehr, Peter & Birnbacher, Lorenz & Viermetz, Manuel & Burgkart, Rainer & Baum, Thomas & Lohoefer, Fabian & Wildgruber, Moritz & Schilling, Franz & Willner, Marian & Marschner, Mathias & Noël, Peter & Rummeny, Ernst & Pfeiffer, Franz & Jungmann, Pia. (2019). 3D grating-based X-ray phase-contrast computed tomography for high-resolution quantitative assessment of cartilage: An experimental feasibility study with 3T MRI, 7T MRI and biomechanical correlation. PLOS ONE. 14. e0212106. 10.1371/journal.pone.0212106.
My responsibilities included rewriting the non-functional code created by a previous lab member, troubleshooting the various amplifiers and controllers used in our workflow, designing adapter pieces to accommodate different sized tips, performing creep tests, and analyzing test data to extract material properties. I kept detailed logs of my activities, tasks, and ideas in a lab notebook, presented my progress to the lab during weekly meetings, and participated in our journal club by reading academic papers related to cartilage and creep.
The code I wrote for the indenter served purposes of both actuator control and data acquisition. As my first exposure to the LabVIEW programming language, my first weeks were spent learning the block code structure of my predecessor's program. Once I was able to get the computer and sensors communicating with one another, I focused on successfully recording data in .csv format, then implementing methods to dynamically adjust how many data points were saved. Considerations for data accuracy, including no-load zeroing of sensors and accounting for the stiffness of the indenter itself, were made. Steady revision and testing for the better part of two months paid off with a program that successfully recorded accurate data in a manageable format.
Figure 2. The relationship between different types of displacement in the microindentation experiments. At the small forces and displacements used in the creep experiments, compliance of the indenter system itself is non-negligible and must be quantified. System displacement/compliance is a function describing how the parts of the system themselves (indenter tip, load cell, sample dish, etc.) deform under input forces. For our experiment, the displacement read by the actuator can be considered a superposition of displacement of the sample and displacement of the system. We obtained a function for system displacement, and subtracted its value at each force to extract the displacement of the sample.
Figure 3. Qualitative graph with no numbers to show the importance of system compliance. Without accounting for system compliance (green line), we would mistakenly believe the absolute displacement (red line) to be the displacement of the sample (orange line).
With code taken care of, we replaced the amplifier and load cell of the indenter with more precise versions. The new load cell had different sized threading than the available tip holder, so I created a design to mate with both and act as a bridge between. Many tests were conducted with a variety of tip sizes to assess the performance of sensors, adapter, and code. Graphs of the output data were constructed and compared to those found in published literature to confirm expected results. A new protocol to replace the outdated version was also created.
By the end of three months, I had:
Learned a new programming language and how to troubleshoot my errors
Learned how to read unfamiliar, dense, technical literature (academic papers, electronics manuals)
Installed electronics equipment including sensors, signal filters, and amplifiers
Processed output data using curve fits to determine unknown material properties and produce graphs
Produced documentation (lab notebook, user manual, experiment protocol) to pass my expertise along to other members of the lab
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