TIME-DEPENDENT DAMAGE ON BIODEGRADABLE DEVICES

André Vieira (1), Rui Guedes (1), António Marques, Volnei Tita (2)

1. Mechanical Engineering Department, Faculty of Engineering of University of Porto,

Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal;

2. Aeronautical Eng. Department, Engineering School of São Carlos, University of São Paulo, Av. Trabalhador São-carlense, 400,13566-590, São Carlos, SP, Brazil

Introduction

Only using biodegradable polymers can be possible

to develop new regenerative concepts for

implantable medical devices, in which

biomechanical functions are gradually transferred

to the neo-tissue formed over the device while it

degrades. However, new tolls must be developed

for the design and dimensioning of these devices,

and methods to simulate the mechanical behaviour

during hydrolytic degradation. In many

applications, this type of chemical damage,

common to biodegradable polymers, can be

combined with further mechanical damage, such as

fatigue or creep damage. The several types of

damage combined will stipulate the functional

compatibility, and must be correctly forecasted, to

guaranty the product reliability according to the

pre-established requirements. There are currently

no methods to predict damage evolution, and

consequently the long term mechanical behaviour

of biodegradable medical devices. In this study,

both damage types, chemical and mechanical, were

study as separated phenomena.

Hydrolytic damage

The mechanical behaviour during hydrolytic

degradation will discussed, and some results will be

presented for a blend of PLA-PCL. It has been

shown by Vieira et al. [2011] that the fracture

strength follows the same trend as the molecular

weight:

�t=�0*e-ut =�0* e-kEwt (1)

where u is the medium hydrolysis rate of the

material , k is the hydrolysis rate constant, and the

ester concentration (E) and the water concentration

(w) are constant in the early stages of the reaction.

In addition, water is spread out uniformly in the

sample volume (no diffusion control). Hydrolysis

rates are affected by the temperature or mechanical

stress, molecular structure, ester group density as

well as by the degradation media used. The

crystalline degree may be a crucial factor, since

water or enzymes attack mainly the amorphous

domains of a polymer. The hydrolytic damage can

be written, as Vieira et al. [2011], in the form:

dh=1- �t/�0=1- e-ut= 1- e-kEwt (2)

One method of long-term prediction of mechanical

behaviour, based on hyperelastic constitutive

models and associated to the hydrolytic damage,

was demonstrated. It consists on changing the first

material parameter of a hyperelastic constitutive

model with hydrolytic damage, μ1(d) , according to

the linear regression (see Fig. 1), and allows the 3D

modelling of the long-term mechanical behaviour

of the device.

Figure 1: Evolution during degradation of the

material parameter, μ1, of the constitutive models

[Vieira et al., 2010]

Mechanical damage

In many applications the materials are submitted to

large deformations, above the elastic limit, in

dynamic or static loading conditions, and therefore

they will progressively accumulate damage due to

fatigue or creep. Biodegradable devices can fail in

long term due laxity or by sudden failure. In the

case of polymers, creep and fatigue interactions

occur at low temperature and these two phenomena

are coupled. Some further fatigue and creep results,

for the same virgin material, will be presented and

discussed, based on the existing continuum damage

modelling methods.

References

Vieira et al, J Mech Behav Biomed Mater, 4:451-

60, 2011.

Presentation 1609 − Topic 05. Biomaterials S59

ESB2012: 18th Congress of the European Society of Biomechanics Journal of Biomechanics 45(S1)