Corrosion as the “Good Guy”

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Permanent Bio-Implantable Plates and Screws (Image courtesy of Praisaeng at FreeDigitalPhotos.net)

While plenty of industries abhor corrosion and its consequences, another sector has welcomed it as a step in the healing process: medical devices. Devices have evolved over the decades to be less-intrusive during (and after) implantation. The bio-inert nature of titanium (along with its weight and strength characteristics) has made it the go-to material for structural orthopedic implants (hip and knee joints, bone plates and screws, etc.). These implants are made to go into the patient’s body and remain there, hopefully performing well for an extended period of time without the need for replacement. But what about implantable devices that have a finite life of medical functionality, and afterwards can become detrimental to the patient’s quality of life?

Such is the case with attaching soft tissues to bone during ACL repairs, as described in a recent issue of Advanced Materials & Processes. Stainless steel or plastic attachments have been the accepted materials in the past because of their strength and biocompatibility behaviors. However, once these devices have done their job they can be hard to remove, or can (in the case of stainless steel) cause metal sensitivity in the patient. Implanted screws made of polymer-based biocomposites have been shown to degrade at a safe rate in living bone and tissue. This allows the repaired ligament to heal, while the tool itself is slowly absorbed by the body using its own metabolic conversion system (the Krebs cycle).

Another example is the performing of a balloon angioplasty to unblock clotted arteries. The device employed in this procedure is a balloon-tipped catheter, which widens the artery. A metallic mesh stent is placed in the area where the work was performed, to keep the artery open as it heals from the procedure. The mesh stent never goes away, which can have an unintended outcome as time progresses. In an ideal world, the stent would remain properly positioned in the artery and cause no further damage. In reality, the stent has the opportunity to create major issues in the body after the artery’s healing time (localized inflammation, or structural breakdown resulting in stent fracture and arterial wall damage). A research group at Michigan Tech is looking to take the bio-corrodible nature of zinc and use it to their advantage in stent design. An alloyed zinc stent would perform the necessary function of propping the blood vessel open as it heals, and then would break down into products that are harmless to the body after its function is complete. The degradation rate for zinc in the body has been shown to be approximately 0.015 millimeters/month for the first three months (the crucial timeframe for stent functionality), with an accelerated rate after that.

VEXTEC’s past success with modeling corrosion-induced damage propagation (previously used for corrosion mitigation purposes) provides an exciting opportunity to repurpose this methodology to model the corrosion state in materials and devices in which degradation is in fact encouraged. Whether seen as detrimental or beneficial, the processes of corrosion and fatigue are interrelated. The key to merging the two phenomena lies in reducing the size of the initial flaw (as described by traditional damage tolerance analysis) to better reflect the size ranges that are observed in corroded surfaces. In the realm of bioabsorbable medical devices, the ongoing degradation due to corrosion can be explicitly accounted-for during the service life of the implanted devices. The randomized load patterns of a given virtual patient (or a population of patients) can provide the external loads necessary to perform simulated damage progression. This analysis could provide insights into the reliability of a temporary implant and its effect on a patient’s wellbeing.

The Need for Speed

In the race to get products to market, does risk-mitigation get enough time in the winner’s circle?

indy-blog-cover-imageWhat do aerospace, medical device manufacturers, and auto racing all have in common?  Answer: the need to minimize risk of premature/unexpected component failure while crossing the finish line first.  While these industries each have vastly different stakeholders, goals, and success metrics, all look to avoid costly breakdowns in the field.  And speed is key.  Being the first across the finish line in auto racing gives you the largest share of the purse, not to mention first choice of lucrative endorsement deals.  Being the first to market with an innovative or more reliable medical implant or a lighter aircraft component helps in marketing, product launch success, or company profitability and growth.  However, pushing the design limits to gain this speed advantage must be weighed against the possible failure of the component in an unforeseen manner.

Speed in Design

Auto racing is one of the world’s most expensive sporting endeavors.  A recent USA Today article puts the price tag of prepping and running a car in the Indianapolis 500 at nearly $1 million, and that is already assuming that the car is owned outright.  While the bulk of this cost is sunk into parts, staffing, and off-track expenses, a not-insignificant 4.5% of that is spent on controlled testing.  For example, one day at a rolling wind tunnel costs $35,000…more than the MSRP of the average production vehicle on US roadways today.  While the finances of auto racing and the commercial automotive industry may differ, their goals are similar: to create lighter-weight components that will aid (or at least not hinder) aerodynamic performance.  Designers are constantly being tasked with pushing the envelopes of their designs, while still attempting to maintain reliability and risk targets.  These designs, in turn, lead to more expensive and detailed manufacturing/machining techniques and the use of more exotic material alloys.  The uncertainties in every design usually manifest themselves as restrictive knock-down or safety factors that inevitably detract from performance.  Governing bodies in the various auto racing categories (F1, NASCAR, drag racing, to name a few) place additional restrictions in the form of specification limits on components such as engines, body shapes, and spoilers to maintain competitive balance.  Regardless of the type of restriction, if a way to reduce the uncertainty in a design is found, it can be advantageous.  VEXTEC’s Virtual Life Management (VLM) simulation technology can be that solution.  Through rigorous computational analysis of design, load-induced stress, and material, VLM can efficiently identify and quantify those design uncertainties.  VEXTEC has provided VLM support to many of the industry’s leading manufacturers of heavy duty engine connecting rods, engine blocks, and turbochargers.  This new insight has offered engineers the ability to understand where they really are on their design envelope, and how far they can push certain parameters, even before the first test piece is built.

Speed in Optimizing Maintenance

Weight savings and aerodynamics are, arguably, even more critical in the aerospace industry, where the civilian maintenance repair & overhaul (MRO) market is expected to be $56 billion this year.  Engine maintenance alone will take up about 40% of this valuation.  The cost of an unexpected catastrophic failure is much higher here than in the auto world.  But in order to reduce this risk, aircraft must be maintained and repaired.  And while they’re being maintained, they are not in the air delivering passengers, hauling freight, or making money.  So minimizing the downtime is crucial to keeping viable profit margins. VEXTEC has partnered extensively with civil and military aircraft users, employing VLM on a multitude of issues including: unitized wing structures (US Air Force), certifying weld-repaired engine blades (EB Airfoils) and resolving premature bearing failure (American Airlines).  The bearing study, for example, saved American Airlines about $4 million per year by avoiding the repair/replacement of their APU bearings.  The results from these and other studies provide our clients with knowledge they would not have otherwise been able to acquire, and allow for sound financial decisions to be made on fielded components.

Speed in Reliability

One of the fastest-changing industries is the medical device industry.  Technology is racing forward, minimizing invasiveness is driving the miniaturization of implantable devices (especially in heart rhythm monitors), while manufacturing methods are still trying to catch-up.  It seems that no other industry is as heavily scrutinized in terms of reliability and risk, at least in public perception. Medical implants are exposed to harsh internal environments, unpredictable stress and strain cycles, and oftentimes difficult installation procedures.  Yet these devices are counted-on to reliably elevate our quality of life on a daily basis.  The variability observed in material, vendor supply, and manufacturing all play a part in the reliability of the components that make up a medical device.  Through industry-directed capability studies, the VLM technology pioneered by VEXTEC has effectively modeled these sources of variability, virtually tested millions of components, and delivered reliability answers to as many “what-if” scenarios as design and materials engineers saw fit to explore.  The VLM approach reduces the number of blind alleys (ineffective combinations of material, design, and operational limits) that companies would have to travel down through the traditional design-build-test method, and focuses internal R&D resources on the combinations most likely to succeed in both manufacturing cost and operational reliability.

These three high-risk/high-reward sectors are not the only sectors that have benefited from VLM technology.  Indeed, any company looking to speed-up their design phase, reduce their warranty reserves, or just wanting to make more-informed decisions on how their products can best be sourced, manufactured, and used would benefit from a conversation with us.  The green flag has dropped…where are you in the field?

VEXTEC: Meeting the Need for Speed.

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