The document outlines the top five sources of error in biomedical testing, including load cell measurement, system bandwidth, data rate, strain measurement, and environmental conditions. It emphasizes the importance of understanding the accuracy and resolution of load cells, the impact of bandwidth on signal clarity, and the significance of appropriate data rates for capturing peak forces. Additionally, it highlights common errors associated with environmental testing conditions and their effects on biomedical device performance.

1. Elayne Gordonov
Market Manager, Bio Market
Top 5 Sources of Error in Biomedical Testing

2. 2
1. Load Cell Measurement
2. System Bandwidth
3. Data Rate
4. Strain Measurement
5. Environmental Conditions
Top 5 Sources of Error in Biomedical Testing:

3. 3
Load Cell Measurement
• Within a load cell, a series of strain gauges measure deformation
• Deformation of strain gauges create a change in electrical resistance
• This electrical resistance is proportional to the mechanical force
Load cells transduce mechanical force into an electrical signal
• Key characteristics of a force transducer
•Resolution
•Accuracy

4. 4
Load Cell Measurement
• Different load cells have different:
• Resolution
• Accuracy
• It’s important to know the operating range
of your load cell
• What resolution and accuracy do I need?

5. 5
• Resolution: the smallest increment that an instrument will measure or display
Note: Specified resolution of an instrument has no relation to accuracy of measurement
• Resolution is not very important if it isn’t accurate!
• E.g., The resolution of a scale could display 3 decimal places, i.e. 50.000 lbs.
• If the accuracy is +/- 10 lbs., the resolution does not provide valuable information
Load Cell Resolution

6. 6
• Presented in 1 of 2 forms:
1. % of reading (relative and fixed accuracy)
2. % of full scale (fixed accuracy)
• Relative Accuracy
• This is the accuracy in the stated range
• E.g.1% of reading down to 1/1000th of full scale
• Fixed Accuracy
• Below the accuracy range, the error becomes fixed
• The percent error becomes larger as the readings
move further outside the relative range
• E.g., 1 kN load cell has 1% accuracy from 1N-1,000N
• Below 1N the error is fixed at ±0.01N
Load Cell Accuracy

7. 7
Most Common Error: Low Force Measurement
Errors associated with load cell accuracy are common in biomedical testing
because forces are usually very small

8. 8
Relative Error 0.5% of reading
Fixed Error at 0.051 gf
Your load cell needs to be verified
in the range that you use it.
How to Interpret Low Force Measurement

9. 9
Testing a specimen
to failure
Conversion of
force to electrical
signal
Capture
digital data
Sensors &
Electronics
Computer
Data
File
“Measured Event”
Bandwidth: Measurement of Force Data

10. 10
A
Signal In
 Low Bandwidth
A
A
 High Bandwidth
High Bandwidth
= very responsive
= noisy (jumpy displays,
scatter in data)
Low Bandwidth
= big range
= clean signal (low noise)
Error Due to Responsiveness of Electronics
• Responsiveness
of the electronics
defined by
“bandwidth”
• Bandwidth
implies a
“Time Constant”
… or Rise Time
• Events shorter
than the
Time Constant
will be missed!

11. 11
10 Hz Bandwidth
1 Hz Bandwidth
0.1 Hz Bandwidth
Compression testing on a syringe plunger to obtain break away force and
glide force. Different average glide force when using different bandwidths.
Most Common Error: Missing Peaks & Troughs

12. 12
• Understand the nature of the “event” you are trying to capture
• How long does it last?
• For example, I want to measure force at specimen break.
I estimate that my specimen breaks over 0.2 seconds.
• Understand the electronics being used
• Most “EM” systems between 1 and 10 Hz bandwidth
• 10 Hz BW: ~50 milliseconds rise time (time constant)
• If your event if faster than 50 ms, it will be clipped!
How to Set Your Bandwidth

13. 13
Data rate = frequency of data capture
Data rate (f) = 1/t (Hz)
t
• Data rate is critical to catching peaks
• Too low = missed peaks
• Too high = lots of data but no additional info
Data Rate: Sampling Rate of the Signal

14. 14
• Low data rate ruins output no matter what the input
• High data rate cannot salvage poor input provided by low bandwidth system
Data Rate Creates a Digital Reproduction

15. 15
• Consider the duration of the test
• Default data rate: 1 point every 100ms
• E.g., 60 inch/min, package failure in 5 seconds
• Only 50 data points
Most Common Error: High Testing Speed

16. 16
MeanMaxForce
Different Data Rate and Bandwidth Effect Results
Higher data rates do not yield any additional information: just bigger files
Bandwidth & Data Rate in Medical Packaging

17. 17
Strain (ε) =
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑆𝑝𝑒𝑐𝑖𝑚𝑒𝑛 𝐿𝑒𝑛𝑔𝑡ℎ (∆𝐿)
Original Specimen Length (L0)
L0
ΔL
Example:
• L0 is 1 inch
• ΔL is 0.3 inches
• Strain (ε)= ((0.3/1)*100) = 30%
L0
Measuring Specimen Strain

18. 18
• Most common in tensile testing elastomers, films, biological tissues, hydrogels
• Specimen material is pulled out of grip faces (different than slippage)
• How to fix it:
• Video extensometer
• Line contact faces
Note: Specimen extrusion can result in higher elongation results
Most Common Error: Specimen Extrusion

19. 19
• Most common in testing hard tissues such as bone
• Where test setup does not include an extensometer and the material is rigid
• Testing requires high force to achieve little specimen deflection
• So, machine deflection creates a larger error in the end result such as modulus
• How to fix it:
• Extensometer
• Compliance Correction
Most Common Error: Machine Compliance

20. 20
• Simulating physiological environmental conditions for implants and
biological tissue testing is a no-brainer
• However, environmental testing is often overlooked for testing medical
consumables such as medical tubing, catheters, bandages, gloves, etc.
Environmental Testing: Physiological Conditions

21. 21
• Testing at body temperature (37°C) can drastically effect results
• Especially common for catheters, medical gloves, medical tubing, and bandages
Most Common Error: Environmental Testing
of Medical Consumables

22. 22
Temperature at 28°C Temperature at 37°C
Mean result ±1 standard
deviation
28°C 37°C
Modulus (MPa) 26.34 ±0.45 19.58 ±0.48
Maximum Force (N) 16.62 ±0.54 15.02 ±0.42
Extension at Break (mm) 525.12 ±35.10 532.22 ±32.98
Example: Catheter Testing at 37°C

23. 23
Easy Ways to Test at Body Temperature
Example temperature cabinets
Designed for storage at 37°C
Instron’s BioBox
Designed for testing at 37°C
Photo source: ThermoFisher Scientific

24. 24
1. Load Cell Measurement
2. System Bandwidth
3. Data Rate
4. Strain Measurement
5. Environmental Conditions
Summary:
Sources of Error in Biomedical Testing

25. 25
Thank you for your time!
Please contact Instron® with any questions.
Visit www.instron.com
for more information.