GOALI: Modeling, analysis and numerical simulations of gels in the biomedical industry

Calderer

DMS-1009181

This GOALI project supports a collaboration between

investigators at the University of Minnesota and at Medtronic,

Inc. To improve the design of implantable medical devices, the

investigator and her colleagues develop fundamental understanding

of, and modeling tools for, polymer and solvent interactions that

result in swelling of the polymer. Many implantable devices

consist of plastic and metal components that are affixed to each

other by adhesives. When such a device is placed in the human

body, the plastic parts, made from polymer, interact with the

surrounding fluid and swell, while the metal parts are not

affected by the fluid. The consequent swelling mismatch causes a

high stress at the interface, which can lead to deformation of

the components and, if interfacial shear stresses exceed critical

values guaranteed by the adhesive manufacturer, to interface

delamination. Deformation and delamination can cause devices to

fail. The ability to model the behavior of the components is

critical in the design of implantable medical devices, such as

defibrillators, pacemakers, and neurological devices. This

project addresses the questions:

(1) How does polymer swelling proceed without polymer chain

relaxation? (Only elasticity is accounted for, and viscous

effects are neglected.)

(2) How does swelling proceed when polymer chains relax within

the same time frame?

(3) How does swelling proceed if there is mechanical strain

applied to the sample?

(4) How does swelling proceed if two samples made of the same

polymer but with different initial swelling are bonded together?

Many polymers can absorb an amount of water equivalent to

about 1 percent of their own weights; hydrogel can absorb water

many times its own volume. Swelling of polymers occurs in many

applications, particularly in implantable biomedical devices,

made of metal and plastic polymer components, that typically are

buried in wet tissue. Polymers can relax mechanical loading and

deformation over time due to viscoelastic properties.

Swelling-induced stress or size changes can relax as well. The

actual stress and geometric changes in polymers are determined by

the combined relaxation and swelling process. Each of these

processes is related to water diffusion, polymer chain motion,

and water-polymer interactions. Estimation of these processes

and interactions is critical for understanding swelling-induced

deformation, delamination, and stability, key factors in the

design and quality control of medical devices. Building on an

existing collaboration between Medtronic Inc and the University

of Minnesota, the investigator and her colleagues develop

mathematical models, analysis, and computational and

visualization tools to improve fundamental understanding of

polymer and solvent interactions; the tools can be used to

identify unsuitable polymers and so reduce the number of

laboratory experiments performed by polymer scientists at

Medtronic by more than half. Students and postdocs are part of

the project, which also includes seminars, workshops, and

outreach activities sponsored jointly with Medtronic.