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For medical devices, rarely does “one size fit all”.  The human body is extremely variable and patient populations, especially those with disease present a wide range of differences that the medical device design engineer must consider.

Identifying the worst case size for a product family is an important part of the validation process for implantable medical devices.  Understanding your product and especially the in vivo loading conditions are essential for engineering the structure and material specifications for each size over the intended product range.

Typically, there are numerous sources of nonlinearity associated with implantable medical device design.  The materials we use, the geometries and especially the physiology we treat all respond in ways that are difficult to describe in simple terms.  It is tempting to design a product by considering an idealized patient population and then simply “scale” that design to smaller and larger sizes.  But this approach can can result in a poorly optimized product family.  Furthermore, when one considers device/lumen ineraction and teh resulting compliance under physiological loads, identifying the worst case loading condition is not a straightforward activity.

The figure above illustrates how the alternating fatigue strain for a stent-like product can vary for deployment to different diameters.  It is based on an analytic model of lumen compliance and finite element analysis models of the two device sizes.  Clearly, the results indicate a highly nonlinear system that precludes the selection of a single worst case device size and implant condition based on the “four corners” approach.  Assuming that the largest device put into the smallest lumen will result in the most challenged loading condition is niave.

When it is possible, it is preferred to model ALL device sizes to determine the worst case size. It is also advantageous to develop and validate a simulated model of the intended physiology for the implant and use this model to verify the performance of each size in a design family.  Parameters such as radial force, anchoringg, dynamic compliance, vessel tortuosity and fatigue loading conditions can then all be evaluated for each device size and safety established for the complete instructions for use (IFU) for the product.

There are many circumstances when it is necessary to model contact and contact interactions when analyzing medical implants.  Such circumstances arise when devices interact with tooling, catheters, vessels and when self-contact occurs.  This latter situation arises for example when a stent is crimped down tightly to catheter dimensions.

ABAQUS/Standard provides a range of methods for defining and managing contact in FEA simulations.  One first defines a master surface and a slave surface which can be based on node or elements sets and/or analytically defined surfaces.  Then one must select a contact interaction model and ascribe specific properties to that model.  The image on the left shows both node and element sets used in a typical contact definition for self-contact of a stent.

Considerable care must be used to define appropriate surfaces.  Contact interactions are computationally expensive and although ABAQUS efficiently manages their computational cost, considerations such as smoothness, consistency and other geometric factors need to be considered to maintain solution stability.

ABAQUS provides a variety of contact interaction models for mechanical contact.  These include “hard” contact, “softened contact”, “no separation” to name just a few.  Hard contact is the most simple as it simulates no reaction pressure until contact occurs then a quick ramp up as “penetration” increases.  Softened contact provides a small, exponentially increasing contact pressure just prior to penetration and increasing pressure thereafter.  The no separation model provides increasing pressure as penetration is increased and maintains contact with negative pressure if the surfaces attempt to move away from one another.  The behavior of softened contact and no separation contact models are illustrated in the plots below.

Each of these various models have utility in simulating implantable medical devices.  For example, the hard contact model and occasionally the no separation model are useful in simulating the interaction of devices with tooling during processing and manufacture and also modeling the interaction of the device with a catheter.  The softened contact model is useful for simulating a device when interacting with tissue.  Regardless of the model chosen, it is important to experiment with and evaluate the effects of the chosen parameters for the models.  The selection of these parameters will affect the solution and therefore it is important to study such effects and verify that the results make sense for the problem being considered.

The Table below summarizes one such study using different models and sets of parameters for typical stent analysis scenarios.  For this Table, a two-strut model is crimped to catheter dimensions using different contact interaction models.  As much as 0.2% strain difference occurs between the hardest and the softest of the various models considered.