Lindsay Mann, director of sales and marketing, MTD Micro Molding
For micro medical components, there is a wide variety of materials to choose from. Most micromoulding materials fall into two categories, namely thermoplastics and bioabsorbables. Thermoplastics are polymers that become pliable and mouldable above a specific temperature, then return to a rigid state upon cooling. They have long been used in micromoulding.
The sharp points of this bioabsorbable tack design must be less than 0.0002" r. Bioabsorbable materials degrade with shear generated in small flow paths. Economical microprocessing of bioabsorbable materials requires minimum runner sizes to reduce waste. Manufacturing a solution that balances these conflicting needs is a challenge a moulder must conquer.
Many new micro medical applications rely on bioabsorbable materials (also called resorbable, bioresorbable or biodegradable) because they dissolve in or are absorbed by the body, eliminating the need for additional surgeries and minimising concerns about adverse effects. Devices made from these materials metabolise over time so secondary invasive procedures are not needed to remove them.
It can be assumed that bioabsorbable materials share the same moulding properties as thermoplastics, but the reality is that bioabsorbables require a much more extensive and specialised approach.
Key markets and applications for bioabsorbable micromouldingThe steady growth and interest in bioabsorbables mean that applications are always evolving and expanding.
This bioabsorbable device is made of polylactic-co-glycolic acid (PLG) and weighs only 0.025 g. The reduced inherent viscosity (IV) loss (less than 5 percent) is critical in ensuring optimal wound healing.
Historically, the market for bioabsorbables has been static fixation, e.g., rigid screws, staples, tacks and plugs. They have also become popular for wound/port closure, e.g., subcutaneous fasteners, anchors, staples, suture devices, fascia closures, femoral access closures. Furthermore, the advent of new materials and engineers pioneering new solutions has led to expansion into dynamic devices, such as drug-eluting implants. The materials now need to bend and flex like an elastomer but provide high mechanical strength to achieve the desired result.
Bioabsorbable drug-eluting implants are becoming more prevalent. An active drug and a bioabsorbable material are combined and moulded and once the implant has been placed in the body, the bioabsorbable carrier dissolves, delivering the drug over an extended period of time.
Bioabsorbable materials
Designing a bioabsorbable medical device is expensive. First, the materials often cost more than conventional polymers, costing around US$5,000/kg. Adding to the expense is the fact that bioabsorbable materials are much more difficult to mould and process than other polymers, due to their sensitivity. It is important to understand the material’s capabilities and limitations to help avoid costly delays in the development process and material waste.
This bioabsorbable fixation suture design required an extremely challenging tooling approach. Extremely tight tolerances were required throughout for part functionality.
A medical micromoulder possessing in-depth knowledge of both standard and custom-compounded bioabsorbable materials can help to produce a component design that affords better speed-to-market and quality control. Typical challenges faced in using these novel materials include low glass transition temperatures (i.e. distortion characteristics), ensuring the proper amount of crystallinity is present in the post-moulded product, and 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; however, a detailed documented starting point for micromoulding of bioabsorbable materials is difficult to come by. With limited processing data to start from, a micromoulder needs to employ a rigorous characterisation process for any new materials to assess and determine material behaviour at the microscale—before, during and after moulding.
It is critical to know what the component requires from a material in terms of, for example, strength, inherent viscosity (IV) loss and physical properties. It is also important to select a material that will ensure premium quality and maximum cost-effectiveness.
Custom bioabsorbable materials
Boutique material suppliers are producing 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 cover a wide spectrum.
Six steps to bioabsorbable success
As bioabsorbable polymers are so easily affected by slight processing variations, it is difficult to achieve repeatable results and consistency among different moulding runs for a given part. To ensure lot-to-lot consistency, a micromoulder must implement dedicated steps to maintain consistency in material handling and process consistency.
1. Runner optimisation
Given that bioabsorbable materials are so expensive, runner optimisation is extremely important. A micromoulder should have tools to determine the minimum runner size required to fill the volume of the part, and size a runner system to adequately mould a product without sacrificing material.
A typical runner (left) and MTD Micro Molding’s optimised runner (right). The typical runner is more than 10 times bigger than MTD’s optimised runner. With bioabsorbable resins costing around US$5 per gram and assuming an annual volume of 100,000 parts, this equates to an annual saving in material waste of over $100,000.
2. Robust validationValidating 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. With a collaborative approach, a micromoulder should fully document and customise the validation processes for each client and project. Each micro medical device part’s quality score and all the process data should be stored, thus providing a high level of traceability.
3. In-house testing
Testing is particularly important when processing bioabsorbable materials because some manufacturing steps can cause IV loss during processing. Many moulders outsource their testing for inherent viscosity, gas chromatography and differential scanning calorimetry, which can add weeks or months to the project timeframe. More importantly, without in-house testing and its immediate, real-time data, it is nearly impossible to efficiently evaluate the impact of the moulding process and create an optimised process for a given part. In-house analytical equipment allows the micromoulder to monitor, optimise and report things like IV loss throughout the development and validation processes of a product, as well as verify post-moulded outputs and critical dimensions in order to successfully release production lots to the customer.
4. Minimal and consistent IV loss
Post-mould IV loss is dependent on the micromoulder and the material. More specifically, it depends on whether the micromoulder has both the equipment and expertise to work with a particular material’s complexities. On-site IV, gas chromatography, and DSC testing capability enables a micromoulder to immediately adjust the impact of process variables on these outputs, allowing for a better optimised moulding process, minimal and consistent IV loss and improved capability. Without consistent IV loss, an OEM cannot be confident that its parts will achieve repeatable quality and functionality.
An MTD Micro Molding process engineer testing for IV.
5. Controlled handling and packaging
Storage of sensitive bioabsorbable materials and moulded inventory in temperature-controlled environments is critical. Temperature should be constantly monitored and logged with alert limits, with all manufacturing occurring in environmentally controlled cleanrooms. Micromoulders should also implement specialised shipping procedures to control and monitor temperature for sensitive materials.
6. Customised, specialised equipment
A micromoulder should invest in very specialised micromolding equipment to control critical bioabsorbable processing factors such as residence time, shear and degradation rate of material. This equipment can be further customised and optimised to best serve the needs of medical micromoulding.
A standard moulding cell at MTD Micro Molding, equipped with custom end-of-arm tooling (EOAT), robotics, in-line camera systems and automated part collection.
Moulders that do not specialise in processing bioabsorbable materials may not be as vigilant in enforcing these steps because they are not as critical for conventional materials. A case in point is outlined below.
Case study
An OEM designed a bioabsorbable suture fixation device but, over a span of five years, its moulding partner had limited success and could not produce the part represented in the drawing. There was 30 percent IV loss with consistency issues. To assess part functionality, a secondary operation was required to heat and bend the part into the final shape/orientation. This secondary process meant further IV loss and introduced stresses into the product, causing more inconsistency. In the end, the OEM turned to MTD Micro Molding (MTD).
The tool design for this part was so complex and challenging that it took approximately two months for MTD to design the mould, so significantly longer than the usual one week. Furthermore, typical side action clearance for steel movement is approximately .125" to eject the part out of the mould, but this design only allowed for .001". Creative side action techniques were utilised to accomplish proper part ejection from the tool. By guiding the customer through material characterisation and developing the unique tooling construction concept to reduce secondary operations, parts achieved minimal and consistent IV loss and were much more consistent shot to shot. This confidence in the repeatability of device function allowed reliable testing and successful proof of concept to take place.
Conclusion
Key recommendations for working with micromoulders to achieve bioabsorbable success include:
- involving the micromoulder as early in the project as possible and in material selection.
- exploiting the micromoulder’s knowledge of materials, processing, strategy, techniques and development requirements to achieve the best outcome possible, in the least amount of time; and
- understanding the requirements of the part in terms of consistency, since the percentage of IV loss is often less important than achieving consistent IV loss; and
- selecting a micromolder that has in-house equipment and measurement systems to support the development and validation of the bioabsorbable moulding process.
MTD Micro Molding