The product development process in the medical device field poses a demanding design environment for engineers and project managers. The design process often requires integrating the input from a multi-discipline team; in addition to engineers from various disciplines, team members might include medical doctors, life and health science practitioners, industrial designers, ergonomics specialists, and product marketers. Regulations and oversight typically involve government departments and various agency standards and approvals. Most critically, dealing with human life, well-being, and health requires a high level of quality, reliability, and effective product functioning not only to achieve satisfactory product performance, but also to avert product deficiencies and field failures that could cause safety issues and put users at risk.

As a result, engineers and technical managers in the medical device sector are constantly seeking out tools that will provide them with additional technical insights, a thorough exploration of the design space, alternative and redundant checks on reliability, and communication techniques that can turn volumes of data and complex findings into a clear, concise, and meaningful explanation for team members from different disciplines.

Engineering analysis and virtual test software are enjoying wider uptake in medical design because they offer solutions in all of these areas. The ability to present predictive visualizations and animations of engineering entities like stresses, deformation, temperature gradients, and fluid flow patterns in 3D product models communicates design issues in a comprehensible way and makes data easier to grasp. The ability to easily change material properties, product geometry, and applied loads in simulations enables an exploration of material tolerances, manufacturing variability, design options, and performance boundaries. Plus, simulations bring another perspective and an additional check on hand calculations, benchtop prototypes, and test data.

One indication of the growing implementation of simulation software in the medical field is the U.S. Food and Drug Administration’s (FDA) initiatives in issuing standards, guidelines, and instructions on the use of computational modeling for device submission. In addition, since 2008, the FDA, together with the National Science Foundation and the National Heart, Lung, and Blood Institute (NHLBI), have hosted an annual workshop on computational methods. The following small sample of Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) related topics addressed in this forum gives an idea of the breadth of applications receiving attention: computer-aided design of drug-eluting coatings, joint kinematics to predict prosthetic wear, modeling of fatigue behavior of Nitinol stents, and CFD analysis of coronary arteries. There are four data categories that device manufacturers submit and the FDA reviews in the pre-market setting to demonstrate required safety and performance: benchtop, computational, animal, and human.

Identifying Types of Mechanical Analysis Required for Medical Device Design

Fig. 1 – Left (a): Design changes are evident between two implantable bone screws. An FEA linear statics study enabled a design with less stress for similar loading in the product in the foreground. Right (b): An FEA contour plot of a bone screw with color gradations for different ranges of stress. Higher stress level areas are red, lower are blue. (Images courtesy of Trautwein Engineering and Schafer Micromed GmbH.)
The aim of this article is to provide a basic taxonomy of FEA and CFD software that will allow OEMs to match real-world mechanical design problems in medical devices with appropriate engineering analysis software. This exercise can also serve as a hierarchical framework for specifying software, as well as a survey of software solutions currently available for common engineering problems.

Identifying appropriate software is a matter of working through a hierarchy of criteria. For medical design, a sound system would be to consider criteria in the following order: analysis type, material property modeling capabilities, tools for achieving adequate fidelity, and finally, features needed for productivity and ease of use.

The software world groups solutions based on the engineering topic, the mathematical underpinnings used in setting up the problem, and the underlying numerical methods needed to arrive at a solution. Once the underlying physics of the problem is clear, the type of analysis can be defined. The types of mechanical analyses needed for medical device design are fairly extensive because these products serve the full range of medical disciplines and functions. Consider the diverse physical phenomena encountered in just a few medical disciplines — orthopedics, ophthalmology, dentistry, and cardiology. In addition to diversity in the medical disciplines, products fall into multiple functional categories: monitoring and diagnostics, replacement or augmentation of body functions, prevention of injury and disease, and tools and equipment for surgical and other procedures. A small random sampling of medical products illustrates this breadth: orthopedic bone screws, intraocular lenses, vascular stents, cochlear hearing implants, robotic surgical equipment, drug delivery devices, magnetic resonance imaging.

Fig. 2 – The capability of performing a nonlinear buckling analysis with FEA software allows modeling of the “snap-through” action in intraocular lenses (IOL) and the determination of values for various design parameters.
Nonetheless, mechanical design issues faced by medical product development teams can be sorted into a couple of main disciplines: Structural Mechanics, Heat Transfer, Fluid Mechanics, and Acoustics. We can further subdivide these main disciplines into more specialized analysis types. This article will limit its focus to Structural Mechanics and consider the following analyses under that heading: Statics, Buckling, Dynamics, Kinematics, Fatigue, and Impact. Several real-world examples will be used to illustrate the use of FEA software for several of these analysis types and show how applying criteria for material modeling, fidelity, and productivity can achieve product development success on a technical, project management, and business basis.

Medical Design Briefs Magazine

This article first appeared in the June, 2012 issue of Medical Design Briefs Magazine.

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