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Which Polymer Is Best for My New Implantable Device?

Which Polymer Is Best for My New Implantable Device?
Here's how to take on the challenge of selecting the right materials for an implantable medical device. 

Here's how to take on the challenge of selecting the right materials for an implantable medical device.

Len Czuba

I don't think that there is any challenge more confusing to product development teams than selecting materials that can be used for long-term implantable medical devices.

Are there databases where implantable materials are featured? Can we go to our local supplier and ask them to recommend a material that can be used for implant applications? Usually the answer to both questions is "No." There are not databases that offer a variety of implantable materials like produce at a supermarket, nor would they be accurate if they did exist. 

As with most materials used in medical devices, the end use application dictates the selection of the material. The method of processing the component is critical, whether it is a molded part or an extruded tubing or a polymer used to coat a wire frame or filament. Each process has some effect on the material and factors into the choice of that material. The end product and how it is intended to be used helps define the requirements of the product and ultimately the materials selected for their construction. There are numerous possible products and each comes with significantly different material property requirements.

Consider these:

  • Artificial heart valve
  • Vascular graft
  • Hernia repair mesh
  • Pacemaker components, including (if used) the coating on wire leads
  • Bone screws, pins, and plates
  • Artificial joints: hips, knees, and shoulders
  • Sutures, tissue anchors, and resorbable adhesives

Different Products, Different Requirements

Implantable products that are within the bloodstream and vasculature will need to withstand constant contact with all the components of circulating blood, including the aqueous environment, blood lipids, enzymes, and a barrage of blood components that often tend to recognize the implanted product as a foreign body.   

Learn more from Czuba about "Controlling Materials-Related Costs in Implantable Devices" during the MD&M Minneapolis Conference, November 8-9, 2017.

Devices implanted in tissue, although often exposed to and surrounded by blood vessels, don't have the same challenges as materials in the bloodstream. But since they are still foreign bodies in the tissue, they must not be adversely affected by the normal body's response. For medical devices that are used with bone and joint surfaces, the materials used must also withstand the mechanical forces exerted on the implant, such as the moving surfaces of a joint replacement. If the implant surfaces result in any wear debris, the body will react and try to remove and or eliminate those particles. 

Implantable medical devices made from resorbable polymers are experiencing explosive growth as resorbable products are being developed. These implantable medical devices--whether they are surgical sutures, tissue anchors or clips, bone screws, resorbable electronics, or cardiac stents--are placed within the body for an intended purpose. After the healing or the purpose is achieved, the implant material is slowly eliminated from its implant site and the body. Some resorbable materials are digested, others simply dissolve and are cleared through the liver and kidney function, and others are metabolized. 

Product Function Dictates Material Selection

Having a complete product "requirements definition" or understanding of how the product will be made and used is essential before trying to select the implantable materials. The requirements definition must include all the intended processing of the materials, their assembly, and sterilization of the materials in the final product. The requirements for how the product will be used and what function it must perform must be considered during the material selection process. 

With a complete understanding of the requirements of the product and materials of construction, selection of the material can be conducted. However, keep in mind that every material selected will need to be thoroughly tested to show its acceptability for use in each particular application. All products and ultimately each material in that product will need to be thoroughly tested to show its suitability for use. It is often helpful to know what materials are used to construct similar medical products because if it is possible to select materials that have already been shown to be safe and effective, it can simplify the testing and evaluation process. 

Achieving the "Implantable" Designation

The important thing to remember is that the material becomes an implantable material only after it meets all the product and material requirements and is cleared for use by  FDA. Note that the designation "implantable" is very specific to that product in that intended use. It is not acceptable to take that material and consider other "approvals for use" a license to use the material in another application without conducting appropriate testing. Of course, it is clear that if a material is acceptable for use in one product made and used in a way that is similar to your application, then it is likely that the material will meet the requirements of the new product without resulting in test data surprises. There is less concern about finding unknown or questionable extractables or disappointing, unexpected biological test results.

FDA ultimately is the checkpoint and gatekeeper of the medical device industry. In the United States, all medical products, especially implantables, must pass a careful review of test results and be shown acceptable for use. 

Testing must be done in three primary areas:

  • Physical / Function
  • Chemical
  • Biological

The extent of testing in each area is determined by the intended use of the product. Contract labs as well as industry experts can assist in formulating a complete test plan to prepare for regulatory review. 

The Importance of Supplier Relationships

Once the implantable product is cleared for use--and often before that stage in the process--the material supplier will require some agreement from the user about how the material will be used. It has been common practice for suppliers to require legal agreements which indemnify the supplier from any litigation related to the use of their material in the implantable medical device. It is important to make a very careful assessment of the material's properties early in the development stage of the implantable medical device. What are the typical variations in property specifications of the material and are these variations acceptable in the intended application? Are there any property requirements unique to this new product that require adding additional property specifications that must be followed? If so, then this would be added to the final agreed upon materials specification. 

In addition, there is typically an agreement established that legally binds the material supplier and the user to the established specification of material. For the life of the medical implanted device, if the supplier maintains conformance to the agreed upon specification, they cannot be found liable for any problem with the device in the marketplace. Aside from the indemnification agreements usually required by material suppliers, the Biomaterial Access Assurance Act (BAAA) of 1998 in the United States also shields suppliers from litigation in the event a product is found defective or deficient after it is introduced in the marketplace. 

I believe that even with the protection of the BAAA, the best way to get cooperation from material suppliers is for device manufacturers to show a complete package of acceptable evaluation results of the material in use in the intended medical device. The supplier should be a partner in understanding the methods of fabrication, assembly, and use. They should understand the evaluations conducted to show acceptablility and communicate openly with the device manufacturer during the regulatory application, review, and clearance process. This will make it much more likely that the supplier will be satisfied with the decision to supply their material for the new implantable medical product.

Len Czuba is the president of Czuba Enterprises, a Chicago-based medical device product development consultancy.

[Image courtesy of RAMDLON/PIXABAY]

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