Transcript Implantable Micro-Strain Gauge Arrays Lillian Shido Dr. William Tang

Implantable Micro-Strain
Gauge Arrays
Lillian Shido
Mentors:
Dr. William Tang
Gloria Yang
Image courtesy of Ken Otter
www.dctech.com/physics/humor/cartoons/math.php
What I’ll be talking about:
-
The Lab and Project
Purpose
Background
Fabrication
Tensile Testing
Specimen Design
Resistance Testing
Results
Conclusions
Painting by Domenico Fetti
http://math.nyu.edu/~crorres/Archimedes/Pictures/fetti_large.jpg
The Big Picture
Electrical Engineering
Biomedical Engineering
Strain Gauge Arrays for
Bones and Muscles
Mechanical Engineering
The Tang Lab
The Tang Lab is developing a micro-strain gauge array to be
implanted on bones, muscles, and other soft tissue.
The information that may potentially be uncovered by this device
will help early detection of osteoporosis, metastatic bone tumors,
sports related injuries, and development of prosthetic limbs and
organs.
Why Make a New Muscle Strain Gauge?
- Existing strain gauges are too
large to make sensitive
measurements.
- Need to monitor natural heart
valves.
- Performance of artificial heart
valves.
- No quantitative data on muscle
and tendon injury.
- Need to understand behavior of
soft tissue.
The human shoulder
Image from www.orthoassociates.com
A Background: Basic Strain Gauge
- As resistor feels the strain,
its resistance changes.
- Voltage change exists
across resistor.
Micro-Strain Gauge
Image from Yang, G.
Strain Gauge Array
Image from Yang, G.
How Do We Make It?
- Soft Tissue stretches up to
30%.
- Need material that is flexible.
- Polydimethylsiloxane (PDMS):
It’s bio-compatible and MEMScompatible.
- Methods: Spinning, use of
photoresist, deposition.
Fabrication Process
Image from Yang, G.
Why Tensile Testing?
-Need to understand mechanical behavior of PDMS.
-Need to be able to characterize Young’s Modulus.
-How does it reflect strain of tissue?
-Need to correlate change in strain with change in resistance.
Force Gauge
Straight!
Crank
Extensometer
Some Terms and Formulas
Hooke’s Law:
  E
Image Courtesy of
Dr. Ulrich H. Kurzweg
http://aemes.mae.ufl.edu/~uhk/strength.htm
P

A
Where Sigma = Engineering Stress,
P = Force Applied,
A = Cross-sectional Area
l

lo
Where Epsilon = Engineering Strain,
Delta L = Change in Length,
L = Original Length

E

Where E = Modulus of Elasticity,
which is a measure of stiffness. It is
calculated by taking the slope of line
until proportional limit.
Specimen Design
Problems:
1. ASTM Standard calls for “Dog-Bone” shaped
specimen which has a non-uniform cross sectional
area.
2.
Specimen needs to be able to withstand slippage.
3.
Uniform cross-sectional design is more robust and
therefore harder to rupture (if you need ultimate
strength).
4.
Need fairly high rate of production.
Specimen Types
Cross-sectional
Shape
Configuration
Dog Bone Shape
Tubular
(kept slipping)
Square
Rectangular
Mullin’s Effect in Rubber
65-100% Strain
-Curve for 1st time strain is never repeated.
-Curve stabilizes after 1st cycle to certain strain.
-Curve will change again significantly if taken past the first strain.
-Curve is different when increasing strain to when decreasing strain.
-After initial strain, does not return to zero strain at zero stress. There is some
permanent deformation.
Text Reference: Axel Products, Inc. “Using Slow Cyclic Loadings to Create Stress Strain Curves for Input
into Hyperelastic Curve Fitting Routines.” Testing and Analysis, Cyclic Loading RevC. Apr 2001.
38% Strain
76% Strain
57% Strain
65-100% Strain
What About the Resistance?
-Need to understand mechanical behavior of PDMS.
-Need to be able to characterize Young’s Modulus.
-How does it reflect strain of tissue?
-Need to correlate change in strain with change in resistance.
-To correlate change in strain with change in resistance, PDMS was
doped with a conductive powder called Carbon Black.
-There is a potential for this conductive polymer to be used as a direct strain
gauge.
Computer
Multimeter
26-27% Strain
Conductive Polymer 1
Conductive Polymer 2
39-40% Strain
Conductive Polymer 1
Conductive Polymer 2
52-54% Strain
Conductive Polymer 1
Conductive Polymer 2
What did we learn?
-Rectangular specimen configuration is preferred:
Uniform cross-section, large enough to record Mullin’s
effect, does not easily slip out, high rate of production.
-Doped and Non-doped PDMS has repeatable and
predictable mechanical behavior: Allows us to make
good assumptions later on.
-Carbon Black doped PDMS has repeatable
resistance behavior: Confirms CB PDMS can be used as
strain gauge.
-In general, the nature of PDMS allows us to make
correlations between resistance and strain, stress and strain
without testing every specimen. Compatible with our
application.
Future Work
-Create more uniform Carbon Black doped PDMS specimens.
-Create more dimensionally precise PDMS specimens.
-Calculate Young’s Modulus and compare to muscle.
-Get motor for hand crank!
-Build a strain gauge array using Carbon Black PDMS.
-More stress-strain/resistance-strain tests.
Acknowledgements
Dr. Adel Sharif (California State University, Los Angeles)
National Science Foundation (NSF)
Undergraduate Research Opportunities Program (UROP) and IM-SURE
Dr. William Tang
Gloria Yang
IM-SURE Fellows (Props for Anais Sahabian)
University of California, Irvine Faculty
Said Shokair