In the medical device and pharmaceutical industries, data accuracy is incredibly important. In this presentation, Instron® Biomedical Market Manager Elayne Gordonov shares the most common areas overlooked in testing that could lead to inaccurate or misleading results.

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

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

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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

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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?

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• 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

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• 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

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

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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

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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

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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!

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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

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• 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

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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

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• 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

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• 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

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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

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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

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• 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

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• 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

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• 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

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• 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

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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

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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

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

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