Design and Validation of Creep Testing System for Hydrogels.
Team: Megan Toth& Kelly Williams, Advisor: Glennys Mensing, Client: Weiyuan John Kao,
University of Wisconsin-Madison, Dept. of Biomedical Engineering.
Grips
Abstract
Previously:
Legend:
Currently:
•G = Gelatin
•PEGdA = polyethylene
glycol diacrylate
Room Temperature
PBS Solution
•600 = 600 MW PEG
pH 7.4 +/- 0.1
•2k = 2k MW PEG
40G/60PEGdA 600
•All solution ratios are
given in weight %
40G/60PEGdA 2k
Figure : The previous top grip has
two components of force in the
longitudinal direction.
Figure : The current top grip has a
single force in the longitudinal
direction.
50G/50PEGdA 2k
Bottom and top grip clamp surfaces were sanded and lined with
adhesive rubber and sand paper to minimize slippage of samples.
Figure: Image of the current design. The current design is composed of: an acrylic
chamber, a pair of grips, counterbalance, granite block, a weight rack, an
extensometer (LVDT), an analog to digital converter, and a computer for data
acquisitioning.
LVDT (mm)
5.314
19.314
33.521
% Error
0.264
0.070
0.530
49.1
49.061
0.079
Creep Testing
8
7
6
40G/60PEGdA600
72.4
72.37
0.041
Average % Error Sd of % Error
0.197
0.210
Force Verification
y = 0.0348x + 0.0175
Force vs Displacement
LVDT Verification
Measured (mm)
5.3
19.3
33.7
2
R = 0.9869
•All formulations are 10%
Gelatin
50G/50PEGdA 600
Percent Strain
• Interpenetrating Networks (IPNs)
– Composed of cross-linked gelatin and
poly(ethylene glycol) diacrylate
– Used as a wound dressing
IPN Samples:
4 Formulations
Interpenetrating networks (IPNs) have many
biomedical applications such as wound healing.
The efficacy of the IPN as a wound dressing is
dependent on its mechanical properties. A creep
testing system coupled with an environmental
chamber was previously designed to evaluate the
tensile creep properties of the IPNs. The goal of this
project is to validate the pre-existing device
following ASTM standard D2990 while modifying the
device design and the test specimen fabrication
protocol for improvements. A protocol was written
to validate each component of the entire system.
The protocol was followed to test three latex
samples as standards and three IPN samples.
Background
The Samples
5
50G/50PEGdA600
50G/50PEGdA2k
40G/60PEGda2k
4
3
2
0.07
mg
0.06
Force (lb)
Motivation
0.05
T
0.04
0.03
-kx
0.02
• Creep testing of IPNs:
– Provides mechanical strength
characteristics
– Mimics tensile stress on IPNs due to
wound contraction
To verify the extensometer, the LVDT core was
marked and measured manually at 6 distances, while
being held in that position. These measurements
were compared with the output from the LVDT in
Microsoft Excel from the WinDaq software.
0.01
0
0.2
0.4
0.6
0.8
1
1.2
Displacement (inches)
Figure: Graph of Force (lb.) vs.
Displacement of the spring from 6
grams to 24 grams with 2 gram
increments. Spring constant of
0.0348 lb/in, 1.2% error.
Figure: Force body diagram
of the creep testing system
while undergoing force
verification with the spring.
Hooke’s law, F = -kx was used
to determine the spring
constant, k.
Vibration Control
Design Specifications
Protocol
A protocol was created for the validation of
the creep testing system to provide a
consistent method for the testing of
materials, following ASTM standards. The
protocol is divided into manageable sections
including Test Specimen Dimensions, Test
Specimen Fabrication,
Grip Maintenance,
Force Calibration, Vibration Control, Data
Collection, and Reporting of Data as seen in
greater detail below.
100
1000
Log[time] (Log[sec])
10000
100000
•Durable
Table 1: IPN Raw Testing Data.
•Can apply weight
slowly and evenly
•Fits onto existing
weight rack
Data for all three trials of each of four
formulations are given. This data
corresponds to the graph in Figure .
•Must not enter
LVDT core
LVDT
LVDT
•Add method to change temperature of chamber
Figure: Picture of the creep testing system with the
granite block beneath it. A 12”x12”x4” granite block
polished on both sides was purchased for absorbing
bench-top vibrations.
Temperature: +/- 2oC
pH: constant pH
Force: +/- 1% of applied load
Specimen Dimensions:
– 11 mm gage length
– 2 mm gage width
– 1 mm thickness
10
•Must maintain
constant weight
The Mold: Teflon® Reusable Mold
•
•
•
•
1
The average test results are shown for each
formulation tested with an n = 3. Tests were
conducted for three hours in PBS at room
temperature and pH of 7.4.
•Counter weight
compensation
• Modify existing protocol and creep testing
system to provide optimal experimental
conditions.
• Test different formulations of IPNs in an 11
week study.
0
Figure 8: IPN Testing Results.
1.4
•Light weight
Problem Statement
1
Counterweight
Previously:
Currently:
Aluminum Cylinder
Lead Slip Shots
•Did not account
for the buoyant
force of the top
grip
•Accounts for
the buoyant
force of the top
grip
•Meets ASTM dimensions
•Easy to fabricate
•Teflon® is inexpensive
and molds are reusable
•Vary stresses for each sample type
•Test formulations containing a modified gelatin
backbone
•Test formulations containing modified PEGdA
Figure 7: The Teflon®
mold is sandwiched
between glass slides to
keep the thickness
consistent. The IPN is
fabricated between them.
Marking Gauge Length
•IPNs containing 600MW PEGdA under
went more creep than 2k samples.
•Formulations containing 600MW
underwent more creep at lower
gelatin/PEGdA ratios
•The creep chamber is capable of testing
various formulations of IPNs consistently
•Mass is evenly
distributed
along wire
•Christy Palmer
•Kao Lab
At this step, food coloring is used to
mark a set gauge length on the IPN
•Glennys Mensing PhD
•WJ Kao PhD