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 Withstand temperature (25 – 40˚C) and pH (4 – 8) of solution 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

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.

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 Material: transparent, durable, easy to manufacture, insulating, and resistant to corrosion Must maintain constant temperature Extensometer Accurate to 1 mm and precise Capable of logging data for up to 1 day Grips 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 What do we do? Wouldn’t you like to know….

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 with ASTM dog-bone shape In air In distilled water 10 samples tested

Experimental Set-up Nerds will be photo of our 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 Should this data be more organized, in chart form, perhaps?

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 Placing samples in aqueous solution will prevent them from drying out and becoming brittle, which changes the mechanical properties Samples should be tested immediately after being made to reduce degradation Heterogeneous samples account for the differences in time to failure. A suggestion is add the same amount of hydrogel solution to every stencil with a 1000 micron pipette.

Future Work Tensile Testing Creep Testing Prototype 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