Transcript Mechanical Testing System Coupled with an Environmental Chamber for Hydrogels

Mechanical Testing
System Coupled with an
Environmental Chamber
for Hydrogels
Team: Charlie Haggart, Gabriel J. Martínez-Díaz,
Darcée Nelson, and Michael Piché
Client: Weiyuan John Kao, Ph.D.
Advisor: Paul Thompson
BME 400 Midsemester Presentation
10/18/02
Overview
 Problem
Statement
 Project Background
 Design Specifications & Constraints

Environmental Chamber
• Prototype Testing

Creep Testing System
• Design Alternatives
 Future
Work
 Acknowledgements
Problem Statement
 Test
compatibility of environmental chamber
built during BME 301 with Instron 1000
 Test

hydrogel samples using Instron 1000
Compare results with Instron 5548 data
 Design

and build creep testing system
Study viscoelastic properties of hydrogels
Background - Hydrogels
Cross-linked polymeric structures
Ability to absorb water and/or biological
fluids
Experience physical changes




Dependent on pH and temperature
Applications:


Drug delivery vessels, bandages, and skin
adhesives for wound/burn care
Background - BME 301
 Drafted
procedure to fabricate tension
stencils (used in making hydrogel
samples)

In accordance with ASTM standards
 Designed
and built environmental
chamber


Used in tensile testing of hydrogel samples
For use with Instron 1000 in 1313 Eng. Hall
Environmental Chamber
Design
Specifications/Constraints
temperature (25 – 40˚C) and pH
(4 – 8) of solution
 Withstand
 Cause
no interference with tensile testing
of hydrogel samples
 Solution
must not contact Instron 1000
Environmental Chamber Testing
Temperature Test




4 L deionized H2O
Physiological 37 °C
ASTM tensile test
duration - 0.5 to 5.0
minutes
Results: 2.5 °C drop
over 30 minutes
duration
37.5
37
Temperature (C)

36.5
36
35.5
35
34.5
34
0
5
10
15
20
Time (s)
Trial 1
Trial 2
25
30
35
Environmental Chamber Testing
 Grip


Sensitivity
Instron 2711 Series - lever action grips
Will grips be sensitive enough for tensile
testing of hydrogels?
• YES
• 80% of hydrogel samples fractured at gauge length
• 20% fractured at point of contact with lower grip
Environmental Chamber Testing
 Compatibility

with Instron 1000
Issue: Top plate (obstruction)
• Solution: Removal of plate

Issue: Protective seal (moisture)
• Solution: Machine new adaptor
Background - Creep Testing


Tests effects of prolonged
loading of a material
Monitors strain of a
material under a constant
stress for a long period of
time.
ΔL
strain  ε 
Li
F
stress  σ 
A
Components of Creep Testing
Apparatus
•
Specimen Grips
To hold specimen during testing
•
Extensometer
To measure the change in length of the
specimen
•
•
Loading device (Weights)
Chamber
To keep specimen in conditions for which the
creep properties are of interest
Product Design Specifications

Chamber



Extensometer



Accurate to 1 mm and precise
Capable of logging data for up to 1 day
Grips



Material: transparent, durable, easy to manufacture, insulating, and
resistant to corrosion
Must maintain constant temperature
Must allow for fracture at gauge length
Must not slip
Loading System (Weights)

Must be variable between 0 and 50 g.
Design Alternatives:
Digital Camera
 Take
pictures of sample at periodic
intervals
 Measurement scale on sample or on
chamber
 Sample elongation calculated from each
image
Design Alternatives:
Ultrasonic Transducers

Ultrasonic waves are
emitted from position
on the sample and
received from a fixed
location
 Time difference is
proportional to
displacement
Picture from http://www.piezotechnologies.com/
Design Alternatives:
Linear Displacement Transducer

Output voltage varies
with changes in
displacement

Examples


Linear Potentiometers
(resistance)
LVDTs (inductance)
Picture taken from
http://www.novotechnik.com/linear.html
Final Design
Testing
 PEG-diacrylate hydrogels
bone shape


In air
In distilled water
 10
samples tested
with ASTM dog-
Experimental Set-up
Preliminary Results
 Room Temperature




25 g weight applied to sample
Immediately stretched ~13 mm
No further creep up to 1 hour
Other samples withstood maximum loads of 85 to
>195 g, however, samples were dry and brittle at
this time
Preliminary Results
 Distilled Water



@ 40 ºC
Samples with 5-10 g broke after 5 sec.
Samples with 1g broke after 5 sec – 2 min
6/7 samples broke at the gauge length
Preliminary Conclusions
 All
samples should be placed in aqueous
solution prior to testing
 Samples should be tested within 24 hours of
cross-linking to minimize degradation
 Samples should be more homogenous
Future Work
 Tensile Testing


Prototype Testing
Tensile testing
 Creep Testing

Additional design considerations:
• Data acquisition and logging
• Temperature control

Prototype
Acknowledgements
 Professor
Kao
 Paul Thompson
 BME Department
 Professor Webster
 Bill Hagquist, ME Shop
 John Dreger, 1313 Eng. Hall
 Ralph Wiggam, Springfield Elementary