How to take the right approach to micromolding

Lindsay Mann, MTD Micromolding, describes how to ensure you take the right approach when micromolding bioabsorbable polymers.

For micro medical components, there is a variety of materials to choose from. Most micromolding materials fall into two categories: thermoplastics and bioabsorbables.

Thermoplastics are polymers that become pliable and moldable above a specific temperature, and return to a rigid state upon cooling. They have long been used in the micromolding world.

Many new micro medical applications rely on bioabsorbable materials (also called resorbable, bioresorbable, or biodegradable) because the materials dissolve or absorb into the body, eliminating the need for additional surgeries and minimizing concerns about adverse effects. Devices made from these materials metabolize over time so secondary invasive procedures are not needed to remove them.

Across the board with plastics, the rules are different when you mold it so small. But when it comes to bioabsorbable resins, some molders assume that a thermoplastic material shares the same molding properties, so it is treated the same way. Bioabsorbables, however, require a much more extensive and specialized approach.

Specializing in bioabsorbables

If you have a complex bioabsorbable medical device design, you need a specialist to be successful.

Think of it this way. Who would you go to fix your complex medical issue, your general practitioner or a specialist?

The general practitioner can offer a broad spectrum of services, but cannot offer the same high-level expertise as a specialist. Because the specialist devotes all their effort on that one specialty, their level of expertise, specialized equipment, extensive knowledge and experience allows them to diagnose and fix your problem in the quickest and best way possible, whereas even a great general practitioner may not have the knowledge and tools needed to solve your problem at all.

For example, say you’re a runner and you have a recurring knee problem. Your general practitioner might tell you to take some pain medication and stop running. Since that’s likely not the right solution for you, you’d likely find a doctor who specializes in sports medicine. Having worked with numerous athletes and being equipped with specialized diagnostics tools, the sports medicine specialist could offer in-depth insight on how to manage your knee injury—and hopefully help you avoid surgery.

The same premise holds true for bioabsorbable micro molding. A molder who does not focus solely on medical micromolding and bioabsorbables will not have the necessary tools or knowledge to effectively find the optimal solutions for your bioabsorbable needs. Like the general practitioner who says ‘stop running,’ a non-specialized molder may tell you that your bioabsorbable design is impossible and can’t be done. By turning to a bioabsorbable specialist instead, you get access to the in-depth expertise and equipment to make your ideas possible.

Bioabsorbable materials

Designing a bioabsorbable medical device is expensive. First, these materials often cost more than conventional polymers.

Adding to the expense is the fact that bioabsorbable materials are much more difficult to mold and process than other polymers, due to their sensitivity. It’s important to understand the material capabilities—and limitations—to help avoid costly delays in the development process and material waste.

Working with a micro molder with an in-depth knowledge and experience working with both standard and custom-compounded bioabsorbable materials will help you to produce a bioabsorbable component with better speed-to-market and quality control.

Typical challenges faced with these novel materials include low glass transition temperatures (ie. 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.

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

For example, consider the striking differences between the more exotic bioabsorbable RESOMER X (polydioxanone) and the more common PURASORB PLG 8218. PLG 8218 is less challenging to mold. The material flows easier and can achieve crisp features and narrow geometries. RESOMER X is unique in that it is not stiff at room temperature, since that is below its glass transition. It never becomes a strong material and completely degrades in the body significantly faster than all other bioabsorbable polymers. It’s also extremely expensive. Despite its inherent molding difficulty, it opens some unique applications.

It’s critical to know what your component requires out of a material: strength, dimension, IV loss, and physical properties. It’s also important to select a material that will ensure premium quality and maximum cost-effectiveness.

Custom solution resorbable materials  

Boutique material suppliers are making special-recipe materials to meet the exact needs of up-and-coming next generation products. By adding pharmaceuticals, fillers, or lower molecular weight materials that dissolve quickly, the required material properties can be tailored to meet the specific needs of the application, both physically and chemically. For example, some medical applications require bioabsorbable materials with more flexibility, higher rigidity, or higher or lower rates of absorption. There are many possibilities and the various iterations of products that are available are wide-spectrum.

A successful bioabsorbable micromolding project will incorporate the criteria of premium part quality from a robust molding process window, less invasive devices with increased precision and capability, superior post-mold mechanical and functional properties, consistent and minimal post-mold IV loss, and highly capable critical dimensions. We help our strategic partners develop new solutions, exploring and producing tiny, delicate, and intricate components required for specialized surgeries, bioabsorbable devices, and implant devices.

MTD predicts that bioabsorbable applications will continue to grow with wider exposure and acceptance in marketplace. As exposure and interest continues to grow, novel and improved uses will follow suit. Resorbable materials have also become popular for products in the closure market and laparoscopic procedures and with the advent of new materials and engineers pioneering new solutions, the products have expanded into dynamic devices. Materials now need to bend and flex like an elastomer but provide high mechanical strengths, while also achieving the desired function and result.

It is also predicted that micromolding will also continue to be a solution for polymer-based drug delivery systems. Traditional molding practices would not allow a drug to be compounded into a material before molding, without severely degrading it through the injection molding process. But with specialized medical micro-molding it is, in fact, very possible. Products like drug-eluting bioabsorbable implants are becoming more prevalent in the market. These products consist of an active drug that is compounded with a bioabsorbable material that gets injection molded and then implanted inside the body. The bioabsorbable carrier dissolves, delivering the drug into the surrounding tissue over a controlled period of time.