• Published November 1st, 2017

A.J. Griebel, Sr. R&D Engineer

J.E. Schaffer, PhD, Director of R&D

Wire cables are increasingly used in surgical procedures to provide mechanical fixation in applications like sternotomy repair and femoral cerclage. Current materials, including 316LVM, L605 and Titanium alloys, are stable in the body and are typically left behind indefinitely after healing. These implants deliver critical acute care and concurrently can lead to long-term complications such as bone stress shielding and imaging artifacts in later therapy. An emerging and more ideal solution may make use of implants which dissolve harmlessly and even beneficially after they are no longer needed. Recent efforts to develop these absorbable metals (Mg, Fe, or Zn) for medical implants have already resulted in two devices with promising outcomes and CE mark approval [1,2].

Fort Wayne Metals R&D is leading the way in absorbable metal wire technology, and has developed world-class capabilities in magnesium fine wire. By employing our precision stranding capabilities, we have also produced high-quality magnesium alloy cable (Figure 1), which could serve as an absorbable solution in cerclage and other hard and soft tissue applications.

Figure 1. 1.0 mm OD cables made of seven 7-wire strands (7x7 cable) of fine magnesium alloy wire could form the basis for an absorbable cerclage system with further degradation tuning by combination with polymers.

A key concern with absorbable devices is ensuring they do not lose functional strength prematurely. If general corrosion were too rapid, or localized corrosion led to fracture too soon, the implant could fail before adequate tissue healing. Proper alloy selection and tailored processing can largely mitigate this risk, and solve some applications, but a composite approach may provide a best-case solution in certain applications.

Degradable polymer materials are widely used in medicine for a variety of applications. While their strengths are typically low, they do offer very predictable degradation rates. When applied as a coating to magnesium alloys, they can predictably slow corrosion of the magnesium.

As an example, poly l-lactide (PLLA) or poly(lactide co-glycolide) (PLGA) undergo bulk degradation, wherein the body fluid infiltrates the entire cross-section of the polymer and it is uniformly broken down by hydrolysis [3]. This also means that the underlying magnesium is exposed to the physiological environment shortly after implantation. Corrosion will be slower because of slowed diffusion than without a coating, but it will still occur. Other polymers such as poly(glycerol sebacate) (PGS) degrade by surface erosion, meaning degradation only occurs from the outside in [4]. When applied over the magnesium substrate, this means that the magnesium will be completely protected from corrosion until metal is exposed during polymer erosion. These two types of polymer degradation, or a hybrid combination of them, allows for many different and tunable degradation cascades in absorbable multifilament composite cables and other filament-embedded polymer strip, wire, sheet and rope constructs useful in building a variety of medical devices (Figure 2).

Figure 2. Coating of absorbable metal cables with either bulk-degrading (A) or surface-eroding (B) polymers, or even a hybrid combination thereof, could enable very different degradation profiles over time.

Many combinations of absorbable metals, polymers and even ceramic conversion coatings are possible, allowing for a wide range of application-specific degradation tuning. If something like this interests you, talk to us. Engaging with our customers to enable new lifesaving devices is what we do. 

References:

1. [1] Windhagen, Henning, et al. "Biodegradable magnesium-based screw clinically equivalent to titanium screw in hallux valgus surgery: short term results of the first prospective, randomized, controlled clinical pilot study." Biomedical engineering online 12.1 (2013): 62.

2. [2] Waksman, Ron, et al. "Second-generation magnesium scaffold Magmaris: device design and preclinical evaluation in a porcine coronary artery model." EuroIntervention: journal of EuroPCR in collaboration with the Working Group on Interventional Cardiology of the European Society of Cardiology 13.4 (2017): 440.

3. [3] Nampoothiri, K. Madhavan, Nimisha Rajendran Nair, and Rojan Pappy John. "An overview of the recent developments in polylactide (PLA) research." Bioresource technology 101.22 (2010): 8493-8501.

4. [4] Pomerantseva, Irina, et al. "Degradation behavior of poly (glycerol sebacate)." Journal of biomedical materials research Part A 91.4 (2009): 1038-1047.

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Disclaimer: Our monthly highlights are sneak peeks of what our R & D department is working on. This does not mean we have what is referenced above ready for manufacturing.