What we learned developing biocompatible shape memory polymers

After solving several challenges for our implant, our team sees a promising future for shape memory polymers in other medical devices.

By Jim Kasic, Embolization Inc.

Embolization Inc. recently received 510(k) clearance from the FDA for a coil device using biocompatible shape memory polymers. The Nitinol Enhanced Device (NED) is intended for arterial and venous embolization in peripheral vasculature, specifically achieving better vascular occlusion while minimizing artifacts in CT and MR imaging that occur with traditional metal devices.

Shape memory polymers are key to the NED’s use and performance. While our efforts are focusing on applications within the venous system, what we learned from our work with these polymers could help medtech developers, others in the medical community, and ultimately, their patients.

The problem

Metals such as stainless steel and nitinol are commonly used biocompatible materials. Stainless steel offers excellent corrosion resistance in the body. It has been used in medical devices since the 1920s and in MRI applications since the 1980s. Nitinol, a nickel-titanium alloy, came to light in the medical device field in the early 1970s with the development of an intrauterine contraceptive device that could be deformed for insertion and then regain its shape. Nitinol is now widely used in medical devices due to its unique shape memory and elasticity properties.

These characteristics make nitinol (and to a lesser extent, stainless steel) ideal for many minimally invasive procedures. As a metal, nitinol also shows up well in fluoroscopy. However, it can have significant artifacts (shadows) when used in CT and MR imaging. The potential results – including compromised image quality and misdiagnosis – are significant. 

The solution

Shape memory polymers were developed in the 1980s, but it wasn’t until the 2010s that we started to see the first commercial devices incorporating them. These smart materials can “remember” a specific shape and return to it after being deformed, making them effective in a host of medical device applications.

Useful for many of the same procedures as nitinol, the advantage is that a shape memory polymer does not have the same type of imaging artifacts because it’s not metallic.

Lessons learned in using shape memory polymers

As we worked through design and development of our coil device, we encountered and addressed four main challenges:

1. Transition temperature: Maintaining shape memory at the appropriate temperature is critical. That means body temperature, so that the material remains flaccid at other temps, but takes its pre-determined shape when it reaches body temperature.

2. Biocompatibility: If a polymer reaction was an absolute known, you could usually come up with some very good biocompatible materials. But unique and novel polymers — specifically the crosslinkers and initiators — are not known biocompatible materials. To meet biocompatibility standards, we learned we needed to define and take careful steps to monitor the percent of polymerization and create ways to remove non-polymerized materials.

3. Radiopacity balance with strength: Along with defining the correct amount, we identified a tradeoff between radiopacity and structural soundness, or strength. To make the polymer more radiopaque, you must add a filler. This is basically adding certain atoms in the molecular chain. But that action decreases strength. In practice, we had to consider how strong or weak our polymer was at given diameters (our polymer diameter is .018 in.) and then see what effects changes in radiopacity had on polymer strength. It’s a meticulous back-and-forth process to achieve the right balance. Adding to the challenge is that anytime you start changing the polymer formulation, the transition temperature (that threshold where the material moves from flaccid to stiff) can easily change. As a result, we spent a great deal of time balancing all of these factors.

4. FDA criteria: Along the way, we encountered changes in FDA criteria, particularly in the primary way it looks at biocompatibility. Initially, the criteria was based on cell culture and animal testing, then moved to extractables and leachables testing. One of the issues we encountered was that the solvents used in extractables and leachables testing can actually attack the shape memory polymer. To fully meet all criteria, we conducted extensive animal testing as well as extractables and leachables testing. In doing so, we learned that we had to clean our product, leaching the product to remove any remains of unreacted materials.

Future applications

For our particular technology, other applications include more form factors for use in more areas of the body. For instance, we’ve already developed and tested a platform to help neurological embolisms clot.

The technology will lend itself to form factors for embolization beyond coils, such as with a plug, combination plug-and-coil system, a long, skinny device, or even something more like a sponge.

While Embolization’s work is concentrated on applications within the venous system, we believe the future of shape memory polymers is significant in many more areas of device development.

Jim Kasic is the CEO of Embolization Inc., chair of Boulder BioMed, and formerly served as president and CEO of Sophono (acquired by Medtronic) and president of OrthoWin (acquired by Zimmer Biomet). Kasic has more than 30 years of experience in the Class I, II, and III medical device industry and holds more than 40 U.S. and international patents. 

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The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design & Outsourcing or its employees.