Injection Moulding With a Water Soluble Material

6 Oct 08

A recently developed starch based, water-soluble, and injection-moldable material shows promise in several medical areas. Design goals for the material were to provide cost-effective capsules for an animal-free alternative to gelatine (to avoid mad-cow disease) and make them transparent or colored, and printable. An innovative capsule would also provide patent extensions, tamperproof sealing, and most importantly, timed drug release. Although capsules for drug delivery are the design focus here, the material lends itself to other ecologically friendly devices.

Materials

Design of the material began down two possible paths: with water-soluble or water-dispersible materials based on blends of starch and polyvinyl alcohol (PVOH), and another with a biodegradable material. In addition to being an animal-free composition, starch-based, and injection moldable, the materials had to be less susceptible to moisture absorption than gelatine. Production would be more controllable along with subsequent storage and handling.

An early goal was to produce capsules which meet requirements for human ingestion. This has led to different blends of PVOH and starch-based materials that combine the good molding characteristics of PVOH with the low cost of starch's, and it can be ingested.

Tooling

Conventional capsule manufacturing lets a gelatine film dry over a form. A more recent and patented method based on thin-wall injection molding allows automating production of the net form.

Multicavity tools allowed validating production techniques. A typical prototype, for example, would involve a ‘two plus two’ configuration that makes two caps and bases (connecting cylinder) in one tool, using hot runners. Multicavity molds have also been designed to accept inserts that make capsules with different shapes but similar overall sizes.

Starch-based materials in their elasticised state (hot and in a molding machine's barrel) resist flow so it's difficult getting a cavity to fill. This necessitates high machine screw speeds and mold pressures. Tools must withstand possible deflection caused by high fill pressures and speeds.

When molding polymers, deflection is often overcome by increasing temperatures. However, this is not possible with starch-based materials because they have a relatively narrow molding temperature range — too cold and the mold does not fill, too hot and the material degrades. While material degradation or the normal reduction of mechanical properties is always a concern, it is more acute for starch-based materials. Molding outside the range generates parts that become brittle and crumble. Hence, barrel-residence time and system volumes must be reduced to a minimum to stop degradation during processing.

Starch-based materials are also relatively sticky, so it takes work to ensure the capsule remains on the core and cleanly exits the tool cavity. Such work has been divided between developing appropriate surface finishes for the tooling, and adding mechanical features, such as ribs and drafts to parts. Draft must be added with reference to the original design to ensure retention of the expected filling volume.

Key program tests concern the time required for the capsule to initially release contents and then completely dissolve. Dissolve tests involved a heated stirrer and water at 37C. A certified lab using US Pharmacopoeia methods conducted further initial release and dissolve tests.
Timing content release

Release times are adjusted by geometry and materials. The end geometry of capsule body and cap have several panels or “windows” which are thinner that a nominal wall thickness. These thinned panels dissolve at the same rate as the rest of the capsule, but they dissolve sooner to release the contents. This effect is reproducible and demonstrates an ability to adjust an initial release time and the time to completely dissolve.

Capsule designers also found a couple ways to release multiple drugs. The MultiCap consists of a cylinder with a dividing wall and caps on each end. The caps dissolve at different rates than the cylinder.

SmartCap is a two-shot molded capsule with an inner biodegradable material and PVOH overmolding. This design features a base and cap but each of a different material. Components mate to a fast-dissolving panel with either a digestible or slower dissolving polymer. As the outer panel dissolves, several holes become visible releasing the dose. The holes also give the overmolded material more area to grip.

The material brings a few benefits to designs that need a dissolvable element. For example, it:

• Costs less than gelatin on a weight-for-weight basis
• Accepted in world markets, unlike HPMC (hydroxypropyl methylcellulose — a material for capsules) or Pullulan, a starch-based, edible polymer.
• Sourced from non-genetically modified crops and it's animal free.
• Carries text, colors, or can be transparent
• Brings patent extensions to pharmaceutical clients
• Tamperproof and contains anticounterfeiting concepts
• Capsule can dissolve at adjustable rates
• Suitable for existing filling lines
• Fills easily because it's nonstatic which helps it avoid particulate contamination
• Resists hot and humid conditions
• Has multi-compartments

Source: Michael Johnson Application Engineer Carclo Technical Plastics Latrobe, Pa www.carclo-usa.com

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