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 displayNote: 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 &

ElectronicsComputer

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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Mean M

ax F

orc

e

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

ΔLExample:

• 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

deviation28°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.