Mechanical Tester for Bio-Medical Materials and Devices
The mechanical tester Mod. UMT-2 is developed for mechanical characterization of biomedical materials, tools and devices. It is based on the latest technological advances and is the most precision and unique instrument of its kind. This computerized tester has been used for medical sutures, needles, tubes, stents, balloons, joints, and dental material studies. The tester is capable of providing various motion modes. It measures friction force, wear/durability, micro-flexibility/hardness, scratch resistance and many other mechanical parameters, as well as electrical resistance or capacitance.
Figure 1. CETR Micro-Tribometer model UMT-2.
Figure 1 shows CETR Micro-Tribometer model UMT-2. This universal and highly precision instrument can easily accommodate biomedical assemblies like suture-on-suture, suture-on-stent, bio-film in physiological temperature liquid, artificial tubes, balloons, artificial foot, joints etc. It can provide any rotational or linear motion to the specimens, in both vertical and horizontal directions, thus simulating their dynamics in various applications. It can measure all important mechanical parameters, including:
- Friction force, torque and coefficient of friction,
- In-situ wear depth and wear rate,
- Elasticity and/or Micro hardness,
- Contact acoustic emission,
- Contact electric resistance or capacitance,
- Digital video, with magnifying optics,
- Temperature.
In this Applications Note, we demonstrate some tests performed on biomedical samples. These example tests are:
I. Medical surgical needle tests;
II. Abrasion tests on suture;
III. Suture on suture tests;
IV. Tube on Telflon tests;
V. Balloon flexibility tests.
I. Medical surgical needle tests
Samples:
- Medical surgical needles,
- Material for penetration: leather or pork skin.
Testing Procedure and Parameters:
The schematic of needle testing is shown in Figure 2.
Figure 2. Test Setup for Needle Testing. Measuring penetration depth at a constant load.
A needle was mounted on the upper sample holder and brought down to punch into leather, which was fixed on the lower holder. The penetration depth was then measured under a constant normal load, or the normal load can be measured for a predetermined penetration depth. A close-loop feedback loading mechanism was employed to ensure a constant normal force. The needle was then pulled out of the leather for next penetration. During the test, normal load (Fz) and penetration depth (Z) were continuously monitored and stored. Specific testing conditions are listed were:
Normal Load: 200 mN (20g)
Duration: 20 times of penetration
We have found that:
- The Micro-Tribometer (UMT) is fully capable of evaluating effectiveness of medical surgical needles.
- The penetration depths of the needles are shown in Figure 3 as a function of penetration times. The penetration depth decreased with penetration times, indicating that the needles became dull (less effective). It is clear that Needle 1 performed significantly better than Needle 2 over time.
Figure 3. Needle 1 and Needle 2, comparison of penetration depths (mm) under a constant force as a function of penetration times.
II. Abrasion tests on sutures
Samples:
- Medical surgical sutures,
- Stainless steel rod with CETR abrasion
Testing Procedure and Parameters:
The schematic of suture testing is shown in Figure 4.
Figure 4. Test Setup for Suture Testing. Measuring time of abrasion to suture failure/break.
A CETR Abrasion was mounted on the upper sample holder and moved on the suture back and forth for a reciprocating length of 5 mm at a frequency of 1 Hz under a constant normal load of 50 mN (5 grams). A close-loop feedback loading mechanism was employed to ensure a constant normal force. When the suture was worn to break, the normal force dropped back to zero, since there was no support for the abrasion. The time it took to abrade the suture to break was defined as time of abrasion to failure. Specific testing conditions are listed below:
Reciprocating Frequency 1 Hz
Reciprocating Length 5 mm
Normal Load 50 mN (5g)
Duration to failure/break
During the test the tester measured friction force, normal force and coefficient of friction, as well as time to failure.
We have observed that:
- The Micro-Tribometer (UMT) is fully capable of testing friction and durability properties of surgical sutures.
- As shown in Figure 5, Suture 1 exhibited better performance in both abrasion to failure (longer is better) and friction coefficient (smaller is better, not shown here) than suture 2.
Figure 5. Suture 1 and Suture 2, normal force (Fz) as a function of abrasion time. Time to failure was determined at the time when Fz dropped back to zero (suture broke).
III. Suture-on-suture test
Samples:
- Medical Surgical Sutures
Testing Procedure and Parameters:
The schematic of suture-on-suture testing is shown in Figure 6.
Figure 6. Test Setup for suture-on-suture. Measuring friction coefficient as a function of time.
A piece of suture was mounted on the upper sample holder; another piece cut from the same suture was mounted on the lower sample holder. The upper suture moved on the lower one back and forth with a reciprocating length of 5 mm at a frequency of 5 Hz under a constant normal load of 1 N (100 grams) for 600 sec. A close-loop feedback loading mechanism was employed to ensure a constant normal force. The tension of suture was adjustable using side screws to ensure a constant tension for each suture.
We observed that:
- The Micro-Tribometer (UMT) is fully capable of testing friction properties of
medical sutures.
Figure 7. Coefficient of friction of medical suture-on-suture under constant force.
IV. Tube on Teflon
Samples:
- Medical tube.
- Teflon material.
Testing Procedure and Parameters:
The schematic of suture testing is shown in Figure 8.
Figure 8. Test Setup for Tube Testing. Measuring friction coefficient at a constant force or variable forces.
A piece of Teflon was mounted on the upper sample holder and moved on the tube back and forth for a length of 5 mm at a frequency of 1 Hz under a constant normal load of 500 mN (50 grams) for 500 sec. A close-loop feedback loading mechanism was employed to ensure a constant normal force.
We have observed that:
- The Micro-Tribometer (UMT) is fully capable of testing friction/wear properties
of medical tubes.
Figure 9. Coefficient of friction of medical tube on Teflon under constant force as a function of time.
V. Balloon flexibility
Samples:
- Medical balloons
Testing Procedure and Parameters:
The schematic of balloon flexibility testing is shown in Figure 10. A balloon was mounted on the lower sample holder with three different methods; three point, cantilever, and flexible. A flat head screw was mounted on the upper holder and moved down to the surface of the balloon. This position was set as the zero position of Z carriage. Then, the displacement of screw was set to 0.6 mm and the screw with the stage was moved down and up twice at a speed of 0.05 mm/sec.
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Figure10. Test Setup for balloon flexibility: A-three point; B-cantilever; C-flexible basis
We have found that:
- The Micro-Tribometer (UMT) is fully capable of testing the flexibility of medical balloons.
- Balloon A is more flexible than Balloon B, since for a same force, Balloon A exhibited a larger deflection (Z1) .