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Elayne Gordonov
Market Manager, Bio Market
Top 5 Sources of Error in Biomedical Testing
2
1. Load Cell Measurement
2. System Bandwidth
3. Data Rate
4. Strain Measurement
5. Environmental Conditions
Top 5 Source...
3
Load Cell Measurement
• Within a load cell, a series of strain gauges measure deformation
• Deformation of strain gauges...
4
Load Cell Measurement
• Different load cells have different:
• Resolution
• Accuracy
• It’s important to know the operat...
5
• Resolution: the smallest increment that an instrument will measure or display
Note: Specified resolution of an instrum...
6
• Presented in 1 of 2 forms:
1. % of reading (relative and fixed accuracy)
2. % of full scale (fixed accuracy)
• Relativ...
7
Most Common Error: Low Force Measurement
Errors associated with load cell accuracy are common in biomedical testing
beca...
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....
9
Testing a specimen
to failure
Conversion of
force to electrical
signal
Capture
digital data
Sensors &
Electronics
Comput...
10
A
Signal In
 Low Bandwidth
A
A
 High Bandwidth
High Bandwidth
= very responsive
= noisy (jumpy displays,
scatter in d...
11
10 Hz Bandwidth
1 Hz Bandwidth
0.1 Hz Bandwidth
Compression testing on a syringe plunger to obtain break away force and...
12
• Understand the nature of the “event” you are trying to capture
• How long does it last?
• For example, I want to meas...
13
Data rate = frequency of data capture
Data rate (f) = 1/t (Hz)
t
• Data rate is critical to catching peaks
• Too low = ...
14
• Low data rate ruins output no matter what the input
• High data rate cannot salvage poor input provided by low bandwi...
15
• Consider the duration of the test
• Default data rate: 1 point every 100ms
• E.g., 60 inch/min, package failure in 5 ...
16
MeanMaxForce
Different Data Rate and Bandwidth Effect Results
Higher data rates do not yield any additional information...
17
Strain (ε) =
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑆𝑝𝑒𝑐𝑖𝑚𝑒𝑛 𝐿𝑒𝑛𝑔𝑡ℎ (∆𝐿)
Original Specimen Length (L0)
L0
ΔL
Example:
• L0 is 1 inch
• ΔL is 0.3 inc...
18
• Most common in tensile testing elastomers, films, biological tissues, hydrogels
• Specimen material is pulled out of ...
19
• Most common in testing hard tissues such as bone
• Where test setup does not include an extensometer and the material...
20
• Simulating physiological environmental conditions for implants and
biological tissue testing is a no-brainer
• Howeve...
21
• Testing at body temperature (37°C) can drastically effect results
• Especially common for catheters, medical gloves, ...
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....
23
Easy Ways to Test at Body Temperature
Example temperature cabinets
Designed for storage at 37°C
Instron’s BioBox
Design...
24
1. Load Cell Measurement
2. System Bandwidth
3. Data Rate
4. Strain Measurement
5. Environmental Conditions
Summary:
So...
25
Thank you for your time!
Please contact Instron® with any questions.
Visit www.instron.com
for more information.
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Top 5 Sources of Error in Biomedical Testing

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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.

Veröffentlicht in: Ingenieurwesen
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Top 5 Sources of Error in Biomedical Testing

  1. 1. Elayne Gordonov Market Manager, Bio Market Top 5 Sources of Error in Biomedical Testing
  2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 25 Thank you for your time! Please contact Instron® with any questions. Visit www.instron.com for more information.

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