3-Point Bending Device to Measure Transmural Strains forMultilayer Soft Tissue Composite

 Jennifer OlsonSarah RivestBrian SchmidtbergSponsor: Dr. Wei Sun

Overview•Background•Purpose•Objectives•Project Parts•Constraints•Budget•Conclusion

Background• The client, Dr. Wei Sun, researches the mechanical

properties of tissues with a focus on heart valves

• He currently uses biaxial testing to determine the stress strain relationship of soft tissues

• This testing is insufficient because it assumes the material is homogenous and most tissues are heterogeneous

• Flexure testing is a more effective method of evaluating the force-deformation relationship of different layers of soft tissues.

• Flexure testing is especially critical to Dr. Sun’s research of heart valves because it has been hypothesized that repetitive flexural stresses contribute to the fatigue-induced failure of heart valves

Purpose of the Project• Design and construct a three-point bending device capable of

flexural testing of soft tissues• Capable of calculating the flexure rigidity, bending stiffness,

transmural strain, transverse shear stiffness• Capable of tracking the tissue deformation through use of a CCD

camera and tissue markers

• Principle Goals:• Measure the stress-strain response in the low-strain region by

evaluating the instantaneous effective modulus• Identify the location of the neutral axis• Provide a suitable environment for testing

• Human body temperature and pH • Will produce repeatable results

Objectives•Controlled testing environment that

mimics in vivo conditions•Force application system•Neutral Axis Determination•Determination of flexural properties

▫Flexure rigidity, bending stiffness, and transverse shear stiffness

•Measure transmural strains

Previous Work Done by Others• Products

• ADMET Universal Testing Systems, Instron, and Tinius Olsen all produce 3-point bend fixture for their tensile testing devices

• These are focused on the testing of plastics, metals, alloys, and ceramics

• Do not meet all the project specifications• Patents

• No relevant patents were found• Soft-tissue bending devices found in the Bioengineering

Lab at the University of CA in San Diego, Tissue Mechanics Lab at the University of Miami, and at the University of Pittsburgh

• This project was previously attempted by a senior design group at the University of Connecticut in 2009

Project Parts•Outer/Inner Baths

•Sliding Mechanism

•Image Acquisition and Analysis

•Temperature Regulation

•LabVIEW

• Inner bath▫Provides a

surface for repeatable testing

• Outer bath▫Circulates

temperature controlled water

• Material: Lexan▫Cheap,

Transparent, Strong

Inner Bath

Outer Bath

Combined

Sliding Mechanism•Contains

bending bar, reference bar and CCD camera on movable cart

•Cart controlled by motor system

Bar/Camera Set-up

Top View of Sliding Mechanism

Bending Bar

Reference Bar

CCD Camera

Sliding Mechanism• Motor System

▫ Moves bending bar into tissue specimen

▫ Moves CCD camera during tissue deflection such that tissue never leaves frame of reference

PC equipped with LabVIEW and motion controller

Stepper drive

SH68-SH68 Cable

Stepper Motor

Linear Actuator

Sliding Mechanism

LabVIEW Program• Control all aspects of the

device▫ Integrate hardware▫ Perform necessary

calculations▫ Display results

Graphs Images Quantitative data

Instantaneous effective modulus

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Second moment of

inertia

Radius of curvature

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• LabVIEW 2010

Image Acquisition System• High resolution CCD camera

will be used to track the positions of the desired components in real time

• Positions on the tissue will be defined by sprayed on microdots

• Camera images will be acquired in LabVIEW

• CCD camera will move as the tissue is deformed

• Results will be calculated in LabVIEW using camera images to determine displacement of the tissue and positions of bending bar and reference bar

Controlled Environment▫ Simulate in vivo testing of

tissues Inner bath uses phosphate

buffered saline (PBS) solution

Provides a pH of 7.4

▫ Temperature controller used to constantly regulate temperature of outside bath 37°C ± 1°C Inner bath is protected from

current flow from the constantly regulated outer bath fluid

Project Constraints•Engineering - Errors from calculation,

image acquisition, and environmental changes will be minimized.

•Environmental –The temperature and pH of the test environment will be controlled

Safety Issues• Biological Hazards

▫The device will be routinely cleaned because the tissues used may expose the device to harmful bacteria.

▫Surface materials will be smooth and capable of being sterilized after each use to avoid bacterial growth.

• Equipment Hazards▫Wires will be insulated to avoid

electrocution/electrical fires▫Electrical components will be kept out of

contact with the bath solution

Budget•Most expensive

▫Integrated Stepper

▫Camera•With recycling

all parts, expected to spend only 13.7% of budget

Item / Part Projected Cost Purchased? Supplier

CCD Camera (Mightex)

-Computer Megapixel Lens -VM100 Extension Tube Kit

$749.00

$122 $53

Yes

Yes Yes

Sentech America, Inc

Tissue Bath - Bath Materials - Flow Pump - Bath Solution

$60 $40 $75

No Yes Yes

Hardware Store AquaLife Sigma-Aldrich

Surgical Steel Bending Bar(s) $2 x 10 No OnlineMetals /

UCHC Motor Parts

- Danaher Motion 2 Phase 1.8” Stepping Motor - 2-Axis Motion Controller PCI Card - 2-Axis Integrated Stepper Drive Power Unit (115 V) - 68 pin VHDCI to 68 pin VHDCI Cable - MID Connector Kit

$30 x 2

$765

$1800

$135

$44

Yes

Yes

Yes

Yes

Yes

National Instrument

Linear Actuator

-Model DL 15-25-SV $480 Yes Del-Tron Precision

Tissue Fixation Setup -Lexan for bath setups

$45 No US Plastic Corp

Steel Frame

1 ft x 1ft x 2 ft square tubes 0.5 in x 1 ft x 2ft sheet (Cart) .036 in x 1 ft x 2 ft (Rails)

$3.10 x 4

$5.30 $9.94

No No No

Metal Depot

Plastic Cart Wheels $30 No Home Depot Miscellaneous Parts $100 No Total Amount Needed: $282.64

Progress Through First Semester•Project design•Identified necessary components that can

be utilized from the Biomechanics Lab ▫CCD Camera▫Linear Actuator ▫Stepper Drive▫Stepper Motor▫Flow Regulator

•CCD camera installed and tested•LabVIEW Outline

Work to Be Completed•Program the components of the project in

LabVIEW•Consolidate VI’s•Machine device•Connect components of entire device•Debug device•Test multiple samples•Write user’s manual/test method

Division of Labor•Jennifer Olson

▫CCD Camera▫Image Acquisition System

•Sarah Rivest▫Environmental Control▫Flexural Properties

•Brian Schmidtberg▫Force System▫Motor System

Timeline

Timeline

Timeline

Timeline

Acknowledgements•Dr. Wei Sun•Eric Sirois•Dave Kaputa•Dr. John Enderle

Questions?