Soft Body Tensile Tester
Currently, my work focuses on soft tissue dynamics. Soft tissue generally meaning tendons and ligaments. Repairing injury in these mediums requires iteration and testing en masse. Though we have access to a large format tensile testing system, the available load cells and calibration were set for much stiffer materials… generally alloys like 304, 316L, 440 and various grades of titanium. The stress and strain modulii of soft tissue and ligament are obviously much different with their elasticity. The existing machine was also very large and difficult to use, often leaving us with low confidence in the resulting data set.
We wanted a device for rapid internal testing that was small is size, modular and very simple to use. Since most of our tests take only moments to run, we wanted a system that allowed for quick installation of the sample and one-click button operation. To provide this, a simple linear actuation mechanism was designed and built over a 12”x24” platform. The lead screw has two modes of operation - manual or motor-driven - which is based on simple arduino microcontroller.
Additionally it features a cryogenic clamping system, that allows for greater grip over the subject sample.
Cryo-Clamp Sample Holder
>>> The method of sample holding underwent several iterations primarily because tendon seems to be such a difficult material to constrain. It is highly elastic, malleable, and particularly slick. Everything from alligator clips to grooved clamps failed to hold the sample under cyclic loads.
Researchers at major institutions have resorted to freezing portions of the conformed samples so as to mechanically lock it into place. Having tried this, I can confirm that it is the most effective means of constraint I have found.
The clamp itself was custom designed entirely. The base was modeled and printed in Tough PLA while the arm was machined from 2024 Aluminum. A simple 1/4-20 thread provides the compression and, once secured, the cryogenic fluid can be released via the hose and port to lock it into place. The grooves in the clamp arm allow the sample relief under compressions that create the mechanical interference once frozen.
Cryostatic Fluid
<<< Initially I was planning to install a reusable canister that would circulate liquid nitrogen through the sample, but I found a disposable can for cryostatic sectioning that serves the same purpose without the need for a return flow. Since the area to be frozen is quite small, these canisters last quite a long time. While not as cool nor instant as liquid nitrogen, it still reaches -60 F, which is sufficient for testing.
DRO and Electronics
>>> Keeping the ease and speed of use design criteria for the tool in mind, the real-time digital readout was meant to add a degree of accuracy to manual operation of the lead screw. This allows for cyclic testing or pull-to-failure testing on a manually actuated machine. The benefit here is that informative data can be generated without all the fuss of programming and setup - just install the sample and go.
>>> The load cell mount is designed so that it can be swapped with cells of various ratings… 5kg, 10kg, 20kg, and so on. One end is mounted directed to the frame of the tester while the other is fixated on the free-floating sample slide. Deflection of the load cell transmits an impedance value directly to the microcontroller which is converted to a force value. This value is displayed on the DRO.
Primary Drive
<<< The primary drive uses straightforward linear actuation principles. A ‘fast-travel’ lead screw stabilized by two flanking linear shafts. The lead screw can be driven by hand with a crank (not pictured) or a high torque drill. In the future, a motor could be added for automated actuation.
The carriages that hold the sample clamps are designed to be expandable for wider or narrower samples and are aligned via ball-bearing housings.
Planned Upgrades
Initially this tester was designed to be almost fully automated, using a touch screen to enter testing schema parameters and a stepper motor to actuate the motion. I got about as far as programming the UX dashboard before the dire need to use the device arose. We were succeeding in other projects and we needed to be able to test the resulting samples. Pivoting quickly, I opted to use the manual drive system for the lead screw described above, ditch the touch screen for a simple digital read out (DRO) displaying the live load, and a cryogenic fluid port so as to freeze the sample in the clamp for strong fixturing. Below is the original design.
Original Hardware Schematic
Another late night testing the display and motor drive “Breadboard” prototype. Obviously here I am using a Raspberry Pi microcompter as it offered a number of benefits, the largest being that the operating system could run scripts for multiple features simultaneously. An arduino is capable of performing several actions at once but they must be objects embedded within the same script. I also could code using Python instead of C (which I am less skilled at). The Arduino is quicker and dirtier but that certainly has its value in the development world.
Another large benefit to using the Pi is that it has a touch screen display that is practically plug-n-play. The user only has to create a UX through which to interact with the hardware.
Below is the rough layup of the hardware and electronics as well as the prototype UX I created. It only needed a bit of code to connect the UX input with physical actuation of the motor and other hardware. You’ll notice there is also a camera connected to the system, which was intended to both record the tests and also perform Digital Image Correlation (DIC) to map deformation of the sample in two dimensions. Again, I was only a bit of code away from deploying this feature but time ran out and I also had a growing doubt that the stepper (NEMA 17 pictured here) would be powerful enough to really subject destructive loads to even soft materials. Tendon is a resilient material.
