WHAT IS A RESORBABLE MATERIAL?
Resorbable polymers have been on the market for over 20 years. These polymers are typically PLA (Poly Lactic Acid) based or milk based. They are commonly compounded with PGA, called PLA/PGA compounds (Lactide/Glycolide). These materials are used in implantable applications when the device is only needed in vitro short term. Bone screws, anchors, pharma-induced stents, and swallowable pills are larger scale resorbable moulding applications.

Bone screws typically made of titanium can be replaced with resorbable materials, so that patients are not “stuck” with that bone screw for the rest of their lives. With the resorbable bone screw — unlike the titanium one — after an approved amount of time, the bone fuses together and no longer requires the screw, then the resorbable material gets absorbed by the body, is turned into carbon dioxide and water and is flushed from the human system.

There are many different compounds of PLA/PGA. Most common is the 90/10 version (90% lactide, 10% glycolide). A very high concentration of glycolide creates material handling and feeding difficulties due to its gooey nature.

MATERIALS TESTING
Most polymer processing uses melt flow index as an indicator for processability. With resorbable polymers, an IV (Intrinsic Viscosity) test is used to determine the characterisation of the polymer as it relates to molecular weight, processability and in vitro stability.

IV is a measure of a polymer’s capability to enhance the viscosity of the solution it lives in. It is important to find the viscosity at different concentrations and extrapolate to zero concentration. For this reason, it is important to characterise the material during many different phases in the injection moulding process. For example, PLA/PGA pellets in their raw form are stored in nitrogen sealed pouches. Opening this pouch and exposing the polymer to a small degree of temperature and humidity starts to degrade it immediately (as if it were in the body already and starting to do its job).

Consequently, PLA/PGA compounds must be dried in nitrogen sealed hoppers in most cases and IVs must be validated throughout the injection moulding process. The temperature from shear in the injection moulding screw and the barrel temperatures also decrease the materials IV, as does additional shear from small mould gates. However, once the PLA/PGA component is moulded, it has a protective skin around the moulded part and can be left for a small period of time outside of a nitrogen-sealed environment. By the time a 4.0 IV material is processed, rapid deterioration of polymer properties can take place. If improperly processed, the material will act improperly in vitro and cause an implant to resorb prematurely. There are tools available to test the impact of processing conditions on PLA/PGA materials. One of these tools is a gate shear test and tensile bar test shown in Figure 1. The gate shear test is eight cavities with varying wall thickness from 0.002 inch to 0.009 inch (0.05-0.23 mm) and the gate is always 75% of the wall thickness. The varying gates will simulate varying shear on the PLA/PGA materials. These “coupons” can then be tested for IV loss and simulate what happens to a particular compound before an expensive shaped mould is built using a similar gate size. The tensile bar — an ASTM standard for micro moulding — can be used to test tensile properties of a given wall thickness.

MARKETS/APPLICATIONS FOR RESORBABLE IMPLANTS
Why are resorbable polymers good applications for micro moulding? The materials are between $3,000.00–$22,000.00/pound. For this reason, it is not desirable to have any sprue and/or runner scrap.

Figure 2 shows a Product Life Cycle chart for resorbable polymer implants. In the new product area (typically 1–100k parts annually), much work is being performed in clinical trials and research for pharma-induced devices. These devices have drugs compounded into the polymer and as such, when placed in vitro, they release the drug slowly or quickly according to polymer compounding design.

High volume resorbable products are facial implants from reconstructive surgery for aesthetic reasons or accident/disfiguring reasons. Additional high volume products (100k+) are seen in bone screws and sutureless devices, such as resorbable sutures, anchors and staples (see Figure 3).

Mature products, although not truly mature in time years, but mature in cost and depreciation, are seen when conventional injection moulding methods are used to create the mouded parts. This is typical when using 40–80 tonne presses that require larger surface area moulds to fit in them. The material loss due to large paths in sprues and runners is extremely costly for $3,000/lb materials. These mature products always return on their initial investment to transfer to a micro mouding sized mould and return even quicker with a runnerless mould.

Figure 4 shows a typical ROI for what is referred to as a “work around” conventional method to a micro moulding method. $4,000,000.00 in cost savings is realised when changing from a toothpick-sized runner and sprue to a runnerless resorbable mould. It would take just one moulding run to pay for the capital of a micro runnerless tooling/moulding machine. This is a controller’s dream and what is commonly referred to in the industry as a “no-brainer”.

PROCESSING RESORBABLE POLYMERS
There is an abundance of information that can be found using several of the resorbable polymer suppliers (BI, Purac and Lakeshore to name a few). When it comes to micro moulding, there is less information available due to proprietary processing techniques and lack of industry-specific testing for custom compounds. Here is an attempt at filling in some of the gaps.

In terms of material handling, this is the single largest area for error. PLA/PGA materials are highly susceptible to moisture and heat. They must be stored properly (usually in a freezer) at a specified temperature in nitrogen-sealed foil pouches. They must then be used according to the processing run quantities needed and material drying cycle. In many cases, a customised material hopper is used to handle very small amounts of material (test tube sized), so that the material is not sitting in an improperly sized hopper with over-drying occurring due to the prolonged temperature exposure.

A mould design with a properly sized gate and a very small runner and sprue (if any) is critical to product quality and cost. Mould venting is also important as clogged vents will also degrade the polymer prematurely and cause burning of the implant during processing. If gates and runners are used, a micro moulded part is typically easier to degate using an edge gate rather than a sub gate. If the resorbable implant is in direct contact with skin or arteries, even a very small gate vestige is detrimental to the implantable application as that picky piece can pierce an artery or vein. Degating methods, such as ultrasonic degating, tiny knives in a fixture, or in-mould degating, are used to avoid this effect. If the wall thickness allows, cutting into the part vs. leaving a small vestige is better than chancing the vestige. (+0.00/-0.05 mm is typical).

Micro moulding machines are also a key component in processing resorbable polymers. Because they are highly shear and heat sensitive, proper fit of the shot size to the screw and barrel is critical. The residence time (time the polymer sits in the barrel) can affect the IV of the material. Small shot sized machines are typical in the design of micro moulding machines. Some machines use reciprocating screws and some use screw over plunger technology. Other machines are being developed by processors of resorbable materials because even the smallest shot sizes available on the market are too large to properly process small amounts of resorbable polymers. These machines are typically proprietary and primarily used internally or through licensing agreements.

VALIDATION
Due to the nature of resorbable polymers and their use in implantable devices, they must be processed in a classified cleanroom and be validated using ISO 9001 and/or under ISO 13485 quality systems. Throughout the moulding process, Intrinsic Viscosity values should be validated. Samples should be taken from the bag, after the drying process, after the moulding cycle by testing both the runner and the part to compare shear effects through the gates, and after a period of time in the package, and through different temperatures for shelf life tests. This testing will ensure the validity of the implant throughout its living cycle in vitro for proper form, fit and function of the implantable device.

CONCLUSION
Direct gating of resorbable materials and other costly materials has extreme cost benefits over even toothpick sized micro runners and sprues. Material characterisation/computational analysis at the molecular level is necessary for validating micro moulded resorbable components.

Resorbable device manufacturers get to market faster with fewer development hiccups using experienced resorbable micro moulders. As shown in this article, there are critical steps required to handling, testing, processing, and validating resorbable moulded components. Direct gating can show significant cost savings over conventional gating and moulding. Material characterisation throughout the moulding process is critical to understand what happens to the Intrinsic Viscosity of resorbable polymers, such that when they are in the body, they will not prematurely resorb or stay too long for the implant to properly function.

As is usually the case with micro moulding, this type of processing requires specialised equipment, design and validation expertise. Choosing the right supplier, one that is experienced in resorbable processing and micro feature generation, can create a faster path to success.

Donna Bibber is President/CEO of Micro Engineering Solutions, a solutions-based company serving small to Fortune 50 companies. She is also technical partner for microPEP, a full service micro manufacturing company. She has written and lectured hundreds of technical papers on micro manufacturing associated topics worldwide and was recently voted onto the List of 100 Notable People in Medical Devices for 2008.