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Resorbable Polymer Molding for Medical Devices 101

"Resorbable" is becoming an increasingly popular feature for the materials used in medical devices. Learn what it takes to work with this type of material.

"Resorbable" is becoming an increasingly popular feature for the materials used in medical devices. Learn what it takes to work with this type of material. 

The recent FDA approval of Abbott's Absorb GT1 Bioresorbable Vascular Scaffold (BVS) System is designed with a poly(L-lactide) polymer to fully dissolve in the patient's body over about three years. Another FDA-approved offering in the coronary stent market is Boston Scientific's Synergy bioabsorbable polymer drug-eluting stent, which has a drug coating and polymer used to elute the drug that are fully absorbed about three months after implant.

These products underscore the growing popularity of resorbable polymers in medical devices. What potential does this class of material hold for new medical devices and what do manufacturers need to know about designing with and molding it?

We asked experts from MTD Micro Molding to tell us more about the challenges, equipment, and new advances in molding resorbable polymers for medical devices. 

Editor's note: This Q&A has been edited slightly.

MD+DI: What are the most common types of resorbable polymer used in medical devices?

Dennis Tully, MTD Micro Molding president: Commercially available standard absorbable materials include PLA (polylactic acid), PLG (polylactide-co-glycolide), PLDL (polylactide/DL-lactide copolymer) and PCL (polycaprolactone), among others.

Unique polymers are created when additions are made to these materials to make them special like pharmaceuticals, fillers, and lower molecular weight materials that dissolve quicker. The recipe for a custom blend of resorbable material is determined based on the application of the molded part. Some medical applications require bioabsorbable materials with more flexibility, higher rigidity, higher or lower rates of absorption. Required material properties can be dialed in to create custom blends that are tailored to meet the needs of the specific application. There are many possibilities and the various iterations of products that are available are wide-spectrum.

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MD+DI: What are some typical medtech applications of this material?

Lindsay Mann, director of marketing: These materials are popular in many of the newest micro medical applications because the materials can dissolve/absorb into the body, minimizing concerns about any adverse effects. Devices made from these materials metabolize over time so secondary invasive procedures are not needed to remove them. They are absorbed into the body, leaving only benign by-products.

Brian Matachun, director of sales: Tissue Fixation is a popular MedTech application for resorbable materials. Implantable fixation products include anchors, staples, screws, and tacks for applications like hernia repair. Fixation may be in the form of tissue to tissue, tissue to bone, or tissue to biologic meshes, and the temporary fixation a bioabsorbable product provides leaves no foreign material in the body over time. Bioabsorbable materials are also popular for products in the closure market (i.e. fascia closure, femoral artery closure, etc.) and laparoscopic procedures.

Dennis Tully: In the near future, products like drug-eluting bioabsorbable implants will become more prevalent in the market. These products consist of an active drug compounded with a bioabsorbable material that gets molded, then implanted inside the body. The bioabsorbable carrier dissolves, delivering the drug over an extended period of time.

MD+DI: What are the main challenges in resorbable polymer molding? 

Dennis Tully: The beginning challenge is obtaining useful information for optimal processing of these types of resins. A detailed documented starting point for injection molding of bioabsorbable materials does not exist from any material manufacturer. With limited processing data to start from, MTD employs a rigorous characterization process for any new materials to assess and determine material behavior on the micro scale before, during, and after molding.

Peter Wojtas, senior process engineer: When you are working with such small quantities of resin to fill extremely small and exact geometries and details, you truly have to figure it out on your own. Real-time testing is required to understand realistic processing results and determine the effect on the molded product.

Maintaining consistent and minimal post-mold IV loss through highly controlled material handling and processing is crucial to achieving success in bioabsorbable molding. Specialized molding cell equipment in micro has to be much more capable to execute the job properly, and for bioabsorbable molding, specialized drying systems are critical to molding success. Incorrect equipment and technology can result in excessive heat history, which causes extreme IV loss.

IV testing is an important part of the resorbable polymer molding process.

MD+DI: How can these challenges be addressed?

Gary Hulecki, executive vice president: Internal testing to determine a baseline for the polymers is required to address these challenges. Procedures are developed to create a robust, repeatable molding process that challenges the critical parameters.

Process parameters need to be controlled at a much higher degree to create consistent molded product. Process development activities discover the nuances of a certain material as it relates to a part's specific geometry. Drying studies, temperature studies, inherent viscosity testing--all in addition to standard process development and OQ/PQ validation activities must be part of the process development plan.

Brian Matachun: Ask your molder the right questions to ensure they have the tools to meet the needs of your device by knowing what your component requires out of a material--strength, dimension, IV loss, physical properties.

Peter Wojtas: Understanding the chemical relationship from your process to the polymer is also critical.

Proper tools are needed to test inherent viscosity on the fly. Performing in-house IV testing as you are molding saves significant process development time with immediate confirmation that IV results are consistent and within specification. Not having real-time IV testing capability during bioabsorbable development activities limits the ability to optimize a molding process for minimal IV loss and maximum product strength.

MD+DI: What kind of specialized tools and equipment are needed in resorbable molding?

Peter Wojtas: Analytical equipment for bioabsorbable materials are needed, like capillary rheometers (IV) and differential scanning calorimetry instruments (DSC). Having the resources to fund, evaluate, research, test, and learn about resorbable micromolding phenomenon is also imperative.

The purpose of these specialized tools is to evaluate the impact the injection molding process has on the final component with immediate, real time data. Repeatable and capable molding and inspection equipment must be configured properly for bioabsorbable material molding.

MD+DI: What are trends or new specialties arising in resorbable molding?

Dennis Tully: Boutique material suppliers are making special-recipe materials to meet the exact needs of up-and-coming next generation products. These compounded materials are tailored to meet specific application needs, both psychically and chemically (i.e. degradation).

The options for resorbable materials has extensively broadened for product developers. Different material fillers are used for different purposes and include new types of minerals, and even drugs. Most molders would believe that putting a drug into a material before molding and not destroying it through the injection molding process is impossible. However, it is not impossible with micro molding. In fact, it is very possible.

Brian Matachun: Historically, the bioabsorbable marketplace for molded components has been in the static fixation market such as rigid screws, tacks, or staples. With the advent of new materials and engineers pioneering new solutions, the products have expanded into dynamic devices. The materials now need to bend and flex like an elastomer but provide high mechanical strengths to achieve the desired result. The challenges we, as molders, face with these novel materials include low glass transition temperatures (i.e. distortion characteristics), ensuring the proper amount of crystallinity within the product post- molded is present, as well as maintaining a consistent and acceptable molecular weight loss (IV loss) over long-term larger production lots.

MD+DI: At what stage does the molder get involved with the resorbable product? Are they involved at the design phase, or a later step? 

Gary Hulecki: As early as possible. Involve your molder at material selection.

Lean on your expert molder and their knowledge with materials, processing, strategy, techniques, and development requirements to get the best outcome possible, in the least amount of time.

MD+DI: Any advice to OEMs curious about resorbable polymer and molding process?

Dennis Tully: Understand the requirements of your part in terms of consistency. The two things we have found to be most important to our customers with bioresorbable products:

  1. Consistency--% IV loss is probably less important than consistent IV loss.
  2. Control--Select a molder with in-house processes and equipment to support the development and measurement of the bioabsorbable molding process. It is imperative for a molder to have a means for establishing a baseline and performing internal testing.

Lindsay Mann: Validating a bioabsorbable part requires more steps than a non-bioabsorbable part, but with good planning and exact execution, the timeline to get to production is far from daunting. Validate, then verify: Ensure your molder has the tools needed to be successful: real-time, continuous testing capability, special validation protocols and techniques, and vast experience in bioresorbable micromolding. 

[Images courtesy of MTD MICRO MOLDING]

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