Today’s medical device manufacturers are facing changing and more challenging requirements for their products. Users are demanding less-invasive devices, and in some cases, wearable devices that are robust and long lasting. Regulations are becoming ever more stringent and costly, especially in terms of biodegradability. Yet at the same time, device manufacturers want to meet user demands by including the latest technologies, while keeping their costs under control.

As medical device manufacturers look for new ways to fabricate their products, one technology that is increasingly playing a key role is liquid silicone rubber (LSR) molding and multi-component manufacturing with varying substrates. Solutions can bring real advantages and benefits to both the manufacturer and the user.

Silicone is ideal for medical devices and equipment, not only because it is inert, biostable, and biocompatible with favorable physical and haptic attributes, but because it can be processed in many ways, including molding. In the liquid injection molding (LIM) process, liquid raw material is mixed from two separate components in a ratio of 1:1 and injected via a cold-runner system into a hot mold. Curing of the parts in the mold takes place within seconds, allowing fast cycling and production of large quantities quickly.

Multi-component LSR technology is the simultaneous injection of (LSR) in combination with engineered plastics and potentially other substrates. In what is also commonly referred to as 2K, 2shot, 2C LSR, or co-injection, it is used to develop innovative solutions, combining two or more individual materials into one fully bonded component in hard-soft and soft-soft combinations.

One of the primary advantages of this technology is the ability to produce, as a single component, complex geometries that typically require multiple separate elements to be assembled together. Two-shot molding can contribute significantly to lowering overall costs for the medical device manufacturer by eliminating secondary component assembly, as well as reducing the supply chain costs of stocking and handling of multiple parts. In terms of the integrity of the device, functional risks associated with assemblies, such as potential leak paths or undesirable spaces for bacterial growth, are removed.

One of the smallest pieces manufactured by LSR molding is a septum, the membrane in the cap of a medicine bottle through which a syringe is inserted and withdrawn. (Credit: Trelleborg)

Existing assemblies can be value engineered. This offers tangible benefits to the device manufacturer, resulting in improved performance, prevention of contamination, the opportunity for automating the manufacturer’s production lines, and the elimination of risk associated with mis-assembly.

For new multi-component LSR applications, it is important to involve the component manufacturer as early as possible in the development process; ideally from the concept stage so that applications can be optimized for market. Some molders take a black box approach in which the designer specifies what is wanted for the function and performance of the component, along with the available design window. The molder then develops a proposal that takes full advantage of all the benefits of LSR processing.

Design for Manufacturing

Part-function and maximizing performance of an application are primary considerations in the design. However, right from the earliest point possible, design for manufacturing (DfM) considerations should also be included with solutions for automation, creation of flash-less parts, waste-free production, in-process quality checks, batch by cavity, and packaging.

Importantly for miniaturization of devices, using the LSR molding process can produce micro- and nano-sized components below 10 mg in weight through needlepoint injection technology. One of the smallest pieces manufactured by LSR molding is a septum, the membrane in the cap of a medicine bottle through which a syringe is inserted and withdrawn. This typically weighs just 0.003 g. At that size, you can hardly pick the part up, and standard molding burrs are larger than the part itself.

One of the primary advantages of LSR technology is the ability to produce complex geometries. (Credit: Trelleborg)

Manufacturing a micro-component such as this requires extreme accuracy in tool construction, control of shot weight, and the molding process. Plus, automatic handling of the product after molding is performed by a specially developed robot gripper arm. The process ensures that levels of accuracy are maintained reliably for millions of shots.

As this example shows, automation is critical for medical device component manufacturers. It makes the production of extremely complex multi-component LSR components possible in the high volumes, which at full series demand could reach the tens of millions. It also helps meet cleanliness requirements with no human contact or potential contamination involved in production.

The quality and precision of tooling determines how effective automation can be, not just for molding tools but also for individually designed robotic grippers and handling units that guarantee the feeding of components and removal of finished parts from molds without damage.

Two-shot molding can help lower overall costs by eliminating secondary component assembly. (Credit: Trelleborg)

For medical devices, quality is paramount, and the holy grail of quality is to ensure quality in process rather than to conduct post-production quality checks. Quality is equally considered in a holistic approach, with certified quality systems and process controls built into the production process based on a mindset of producing 100 percent good quality.

The ability to segregate suspect product effectively with minimal disruption in the case of a quality concern, is key to a high-volume, fast paced production process. Ideally, in-line quality checks should be electronically recorded allowing full traceability, with products separated by cavity. Any issue can, therefore, be isolated to just a small number of components and delivered quality from the production line can be checked for that batch in detail.

Fundamental to the disciplines of any high-quality manufacturer involved in supplying “clean” product, whether from within or from outside of a classified cleanroom, is a Good Manufacturing Practice (GMP) discipline firmly rooted in the facility’s quality systems, including ISO 13485. The guidelines provide minimum requirements that a manufacturer must meet to ensure that products are of high quality and do not pose any risk to the consumer or public.

A 2C LSR part can eliminate leakage and friction issues in housings. (Credit: Trelleborg)

It is therefore critical for any manufacturer to apply due diligence in the establishment of GMP standards so that they are appropriately set to the specific application concerns and risks of parts produced. Standards, for example relating to medical devices, may vary depending on whether production relates to a low risk Class 1 device or a long-term implant.

Clean Manufacturing. Cleanliness is a vital element of quality. For some medical devices, production in an uncontrolled environment is of a high enough standard with regard to cleanliness. However, due to the nature and positioning of LSR moldings within a medical device, they may need to be manufactured and packed in a fully controlled cleanroom. In these cases, device manufacturers will require LSR molders that can provide automated production and packing in controlled environments of class 100,000, ISO 8 or class 10,000, ISO 7 cleanrooms.

While a cleanroom does generally not provide a sterile environment, microorganism control with constant monitoring of both environment and personnel takes place. Cleanrooms create an environment in which the level of environmental pollutants, such as dust, airborne microbes, aerosol particles, and chemical vapors, is controlled to a specified number of particles per cubic meter at a maximum specified particle size. The ambient air in a typical urban environment contains 35,000,000 particles per cubic meter on a size range of 0.5 μm or larger in diameter. This is in comparison to an ISO class 1 clean-room, where only 12 particles per cubic meter of 0.3 μm or less in diameter are permissible.

Current applications for LSR technologies range from drug delivery, such as primary drug packaging or wearable smart drug pump systems, to fluid management, diagnostics, and biotechnology.

Case Study: LSR in Action

A real-life example of the benefits that can be achieved with a multicomponent LSR solution is a valve within a medical device. A customer had a control valve with a spring-activated piston in its micro-pump system. The three-piece assembly, consisting of the piston sealed with two silicone O-rings, had a leakage problem and generated too much friction within the system.

A thorough analysis of existing components, mating environment, assembly process, and application were undertaken to identify the potential sources of leakage and friction. Finite element analysis (FEA) simulations provided a better understanding of how the part was reacting within the application.

As a result of poor quality tooling and a lack of process control, a key issue identified was misalignment of the plastic piston.

Silicone is ideal for wearables because it is inert, biostable, and biocompatible. (Credit: Trelleborg)

This created a potential leakage path. Automated O-ring assembly was not 100 percent guaranteed, while stack-up of tolerances between the piston, O-rings, and piston housing created increased friction. In addition, mating surfaces and materials were not friction optimized.

A new 2C LSR part was designed to address the root cause of the leakage and friction issues. It consisted of a single component with compressive inner sealing that was pressurized both sides and deflective outer sealing to reduce contact friction, creating a pressure-energized seal. The new design was subjected to FEA simulations to ensure optimum performance. In addition, a DfM analysis was performed, including material flow simulation, to ensure manufacturing feasibility and to prove out the intended tool concept.

Partnership during the design phase led to collaboration on product and component design, materials selection, manufacturing concept development, and validation. The new LSR part solved leakage and friction issues and provided a fully functional and reliable design.

The integration of three individual components into a single one eliminated steps in the manufacturer’s supply chain and production process, and thereby led to an increase in the company’s product quality and reliability, and a reduction in production risk and overall costs.

This article was written by Andrew Gaillard, Global Director, Healthcare & Medical Business Unit, Trelleborg Sealing Solutions, St. Louis Park, MN. For more information, visit here .


Medical Design Briefs Magazine

This article first appeared in the August, 2020 issue of Medical Design Briefs Magazine.

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